<<

METHANOL PRODUCTION BY DIRECT OXIDATION

OF METHANE IN A PLASMA REACTOR

by

RICK MOODAY, B.S.

A DISSERTATION

IN

CHEMICAL ENGINEERING

Submitted to the Graduate Faculty of Texas Tech University in Partial Fulfillment of the Requirements for the Degree of DOCTOR OF PHILOSOPHY

Approved

Accepted

August, 1998 ACKNOWLEDGEMENTS

I would like to thank Phillips Company for their constant support during the course of my work. This research project was made possible by their generous

financial assistance. I would like to extend my gratitude to Dr. Uzi Mann, for his

guidance, creative ideas, encouragement, and unlimited patience. Dr. Mann ensured that

the project never strayed off the correct path and often contributed beyond the call of

duty as my committee chairman. Sincere thanks are also extended to Dr. Richard W.

Tock, Dr. Dominick J. Casadonte, Jr., and Dr. Lynn L. Hatfield, for their active

participation in the project and technical guidance while serving on my committee. Dr.

Raghu S. Narayan was extremely kind and made me feel more than welcome in the

Chemical Engineering Department at Texas Tech.

I would like to extend special thanks to Dr. Robert M. Bethea. My experience

working for Dr. Bethea in the Unit Operations Lab re-introduced me to education and

chemical engineering after being away for a time. His example and guidance have been

invaluable and will continue to serve me well throughout my career.

Many people contributed to making this time a success for me. Dr. James Riggs

is a highly respected chemical engineer and faculty member at Texas Tech, but I

acknowledge him here for his ability to play the sport of golf I must admit that he taught

me much about course etiquette, scoring, foreign golf traditions ("Aussie Rules"), and

temperament. I will miss our morning outings but I will not forget the significance of a

"tainted par" or a "fully legitimate birdie."

11 Thanks must go out to a few of the people who helped me through the difficult times during my pursuh of this degree. My good friends Ravishankar Sethuraman,

Mahesh Rege, Aashish Ahuja, Robert Ellis, Steve Tsai, Johnson Fung, Siva Natarajan,

Scott Hurowitz, Coby Crawford, and Mike Barham deserve special mention. They are

responsible for many good times at the office, on the golf course/football field, or at some

other establishment. Everyone should be so lucky to have friends like these.

Many thanks go out to Bob Spruill, Marybeth Abemathy, Tammy Low, and

Kathy Womble for their unfailing support. It was a pleasure to work with such a group of

professionals who always got the job done well, and on time.

I must express my deepest thanks to my family and friends for their unconditional

love and support throughout my . Distance, no matter how great, has never cracked

the foimdation that their love gives me. When I am reunited wdth them, it is always as if

we never parted.

Lastly, and mostly, I would like to thank my parents. Loving, selfless, and

capable parents are and have always been my greatest earthly gift. They taught by

example and never discouraged my dreams or desires. They made my education their

priority and selflessly gave of themselves to that end. Only after I became a man did I

begin to understand the sacrifices that they made for me and my sisters. I hope to be as

strong as they one day, and I will consider myself to be successful only when I have

given to my children what they have given to me.

Ill TABLE OF CONTENTS

ACKNOWLEDGEMENTS ii

ABSTRACT viii

LIST OF TABLES x

LIST OF FIGURES xi

CHAPTER

I. INTRODUCTION 1

n. TECHNICAL BACKGROUND .... 6

2.1 Background of Natural and 6

2.2 ABrief History of Methanol Synthesis 7

2.3 Commercial Methanol Synthesis ... 8

2.3.1 Synthesis Gas Preparation Techniques 8

2.3.2 Methanol Synthesis from Synthesis Gas 14

2.4 Potential Advances in Methanol Use . .20

2.4.1 Methanol as Transportation 20

2.4.2 Methanol to Facilitate the use of Methane 22

HI. LITERATURE REVIEW 24

3.1 Homogeneous Partial Oxidation ... 24

3.1.1 Effect of Reactor Walls, Additives, and Promoters 2 5

3.1.2 Kinetics and Kinetic Modeling 27

IV 3.2 Heterogeneous Catalytic Partial Oxidation 29

3.3 Methane Oxidation in the Liquid Phase 32

3.4 Methane Oxidation in Plasma Reactors 34

IV. PLASMA 36

4.1 Background ...... 36

4.1.1 Plasma in .... 37

4.1.2 Potential Applications for Plasma 38

4.2 Plasma Characteristics, Generation and Uses. 39

4.2.1 Plasma States ..... 39

4.2.2 Plasma Generation .... 41

4.3 ABrief Description of the Physics of Plasmas 45

4.3.1 Criteria for Plasma Occurrence 47

V. EXPERIMENTAL SYSTEM AND PROCEDURES 50

5.1 Experimental Approach 50

5.2 Experimental Apparatus .... 52

5.2.1 Feed System ..... 52

5.2.2 Plasma Generation System ... 56

5.2.3 Reactor System . 61

5.2.4 Product Collection System ... 62

5.2.5 Product Analysis .... 63

5.3 Experimental Procedures .... 65 5.3.1 System Preparation and Warm-up 66

5.3.2 Experimental Run Procedures and Checklist. 68

5.3.3 Hazards and Emergency Procedures . 70

VI. RESULTS AND DISCUSSION 73

6.1 Preliminary Phase Experiments ... 73

6.1.1 Injection Distance .... 77

6.1.2 Concentration .... 80

6.1.3 Methanol Selectivity Dependence on Methane Conversion .... 80

6.2 Phase I Experiments ..... 82

6.2.1 Injection Distance 83

6.2.2 Water Concentration .... 83

6.2.3 Concentration ... 85

6.2.4 Mixing Effects .... 88

6.2.5 Overall Performance of Phase I Experiments. 92

6.3 Phase II Experiments ..... 92

6.3.1 Injection Distance .... 94

6.3.2 OveraH Performance of Phase II Experiments 94

6.4 Phase III Experiments..... 97

6.4.1 Oxygen with Methane Stream 99

6.4.2 Oxygen Divided into Plasma and Methane Streams 100

6.5 Overall Performance Evaluation . 102

VI 6.5.1 Mixing and Flow Analysis 104

6.5.2 Material Balances 104

6.6 Sources of Error . 105

Vn. CONCLUSIONS AND RECOMMENDATIONS . 108

BIBLIOGRAPHY Ill

APPENDICIES

A. CALIBRATION OF EXPERIMENTAL EQUIPMENT AND INFORMATION ON SYSTEM HARDWARE 117

B. SAMPLE CALCULATIONS 123

C. RAW EXPERIMENTAL DATA 128

VII ABSTRACT

Methanol is one of the most widely-produced chemicals in the world. It is a key

raw material in the production of many chemicals in the industry.

Methanol also has vast potential for expanded applications as a fuel. It is currently

produced by an energy intensive and expensive two step process. An economically

feasible one step process could significantly reduce methanol production cost, saving

millions of dollars. A methane-to-methanol process, built at remotely located methane

reserves, would convert methane into a different energy form that is much easier to

transport. This would make methane a much more attractive and valuable energy source.

The purpose of this investigation was to evaluate the feasibility of producing

methanol by direct oxidation of methane using a plasma reactor. The of

methane oxidation is well understood and free radicals play a central role in methane

oxidation reactions. Low pressure experiments by other researchers indicated that

methanol can be produced by direct oxidation of methane in plasma reactors. However,

the viability of a plasma-based methanol production process depends on its ability to

convert large quantities of methane. This work was directed at plasma reactor operation

near atmospheric pressure to increase the amount of material processed.

The focus of this mvestigation was the design and construction of an experimental apparatus which could achieve methanol synthesis in a plasma reactor by direct oxidation of methane at atmospheric pressure. A microwave source provided the energy to generate the plasma. The system was designed to study the effects of reactant

viii concentration and flow configurations on methanol production. Since high levels of methanol selectivity are the primary consideration in direct synthesis of methanol from methane, improvements in methanol selectivity were desired. The objective of the four

experimental phases was to investigate reactor operating conditions and improve

methanol production and selectivity.

Methanol production at atmospheric pressure was demonstrated in this plasma

system and steady improvements in methanol selectivity were achieved as the

investigation proceeded. Experiments showed that high concentrations of water and low

concentrations of oxygen improved methanol selectivity. In the last experimental phase,

oxygen was divided into both reactant streams, but this approach did not improve

methanol production. It was observed that higher methanol selectivities were obtained

only at low methane conversions. As in other plasma studies, methanol production did

not approach what would be required for commercial feasibility.

IX LIST OF TABLES

5.1 Technical Characteristics of the Microwave Generation System 57

6.1 Preliminary Phase Experimental Parameters . ... 74

6.2 Phase 1 Experimental Parameters ..... 82

6.3 Phase II Experimental Parameters ..... 94

6.4 Phase III Experimental Parameters ..... 98

6.5 Reynolds Number Calculation Results (run 0213) 104

C.l Preliminary Phase and Phase I Raw Experimental Data 129

C.2 Phase II and Phase III Raw Experimental Data 133 LIST OF FIGURES

2.1 M. W. Kellogg Methanol Synthesis Loop 19

4.1 Electron Avalanche ...... 43

5.1 Schematic of General Approach 51

5.2 Schematic Diagram of Reactor Configuration and Methane Injection 53

5.3 Schematic Diagram of Experimental Apparatus ... 54

5.4 Schematic of Waveguide and Quartz Reactor Tube Configuration . 60

6.1 Schematic of Injection Distance ..... 75

6.2 Preliminary Phase-Methane Conversion Versus Injection Distance . 78

6.3 Preliminary Phase-Methanol Selectivity Versus Methane Conversion 81

6.4 Phase I-Methane Conversion Versus Injection Distance 84

6.5 Phase I-Methane Conversion Versus Oxygen Concentration. 86

6.6 Phase I-Methanol Selectivity Versus Oxygen Concentration. 87

6.7 Schematic of Mixing Region ...... 89

6.8 Preliminary Phase and Phase I-Methanol Selectivity Versus

Methane Conversion ...... 93

6.9 Preliminary Phase, Phase I, and Phase II-Methanol Selectivity Versus

Methane Conversion . 95

6.10 Phase II and III-Methanol Selectivity Versus Methane Conversion . 101

XI 6.11 Preliminary Phase, Phase I, II, and III-Methanol Selectivity Versus

Methane Conversion . 103

A.l Rotameter Calibration Plot 119

A.2 Methane Rotameter Calibration Plot . 120

A. 3 Oxygen Rotameter Calibration Plot . 121

A.4 Methanol Calibration Plot 122

Xll CHAPTER I

INTRODUCTION

Methanol is one of the most widely produced (21 million tons/year) chemicals in the world. It used to be called "wood alcohol." That name refers to the destructive distillation of wood, the first wide-spread method of producing methanol. Methanol is produced in such large volume because it has many uses. For example, methanol is used as a solvent, a gasoline additive, and a chemical feedstock for the production of hundreds of other chemicals.

Methanol is widely regarded as the most promising candidate for use as an alternate automobile fuel. Development of automobiles and other transportation vehicles that are able to bum pure, or nearly pure, methanol would be highly beneficial. Some of the positive effects of developing a world-wide market for methanol as a fuel are: (a) decreased dependence on foreign energy sources, (b) conservation of existing petroleum- based energy reserves, (c) increased competition among energy providers, resulting in lower cost, and (d) environmental benefits from improved emissions inherent in the of methanol, as compared to petroleum-based . Adoption of methanol as a primary automobile, or transportation, fuel would necessitate a large increase in the world's methanol production capability. In addition, a methane-to-methanol process, built at remotely located methane reserves, would convert methane into a different energy form that is much easier to transport. This would make methane a much more attractive and valuable energy source. Methanol has tremendous potential to positively affect changes in energy infra-structure, but methanol production is very expensive.

Presently, the dominant method of producing methanol is a two-step process. The

first step is conversion of methane () into a gas mixture of monoxide

and known as synthesis gas. This step is very energy intensive and represents

the major expense in methanol production. The second step in methanol production is

reaction of the synthesis gas over a catalyst that selectively produces methanol. There is

significant interest in developing a direct method of converting methane to methanol, as

this would eliminate the need for expensive and save millions of dollars.

In addition, methanol production by a direct method would lower the price of methanol

making it more attractive as a potential fuel source.

Currently, the most utilized feedstock for methanol production is methane. It

plays a major role in the world's energy infra-structure and is the primary constituent in

natural gas. Natural gas is an abundant, inexpensive, clean-burning fuel that is used

throughout the world. Reserves of natural gas stand at approximately 5,000 trillion cubic

feet and those reserves equal roughly 47% of the world's petroleum reserves. Despite the

benefits and abundance of natural gas, petroleum remains the world's main source of

energy. This is because natural gas is often located in isolated reserves, difficult and

expensive to transport, and inexpensive (i.e., making it difficult to gamer a profit). The

low cost, low profit margin, and hazardous characteristics of natural gas make it

unattractive and economically unfeasible to transport over long distances. For these

reasons, huge amounts of natural gas remain locked away in distant reserves, mostly located in the former Soviet Union and the Middle East. If a simple and effective process could be developed to convert methane directly to methanol, methanol could be produced from methane, on-site. A direct methane-to-methanol process could make remotely located natural gas reserves more usable and economically attractive.

The search for a successfial direct process to achieve this goal has been underway for more than 20 years. Most of the research into this area has fallen under two main approaches, homogeneous gas-phase partial oxidation and heterogeneous catalytic partial oxidation. The homogeneous studies achieved methane conversions of approximately

10%, but the corresponding methanol selectivity was very low. High selectivity to methanol occurred only at very low methane conversion. Heterogeneous catalytic investigations yielded lower methane conversions and methanol selectivities than homogeneous systems. Homogeneous and heterogeneous investigations into direct oxidation of methane to methanol have failed to achieve acceptable methanol production.

Recently, several new approaches have been investigated to produce methanol directly from methane. Liquid phase studies and studies into reactors achieved methane conversion into methanol. Liquid phase and supercritical reactor systems also failed to produce methanol at levels required for commercialization.

It is known that free radicals play an important role in oxidation reactions.

Plasma technology is being adopted increasingly in the processing industry and plasmas are known to generate abundant levels of free radicals. Plasma reactors allow the generation of free radicals followed by the addition of a stream of methane downstream of the plasma region. This isolates the methane from the high energy region in the plasma and allows the radicals to react at less oxidizing conditions. Reaction at under less oxidizing conditions should promote higher selectivity towards methanol. During the last few years, plasma reactors have been investigated to produce methanol by the direct oxidation of methane. Low-pressure studies have demonstrated the ability to produce methanol directly from methane and water has been shown to play a role in methanol selectivity.

Partial oxidation of methane to three primary products. The desired product, methanol, is more reactive than methane. The other two important products are carbon oxides (, CO, and , CO2). Of particular concem is the oxidation of methanol in the presence of oxygen to produce carbon oxides.

Conditions that will induce oxidation of methane will certainly induce oxidation of methanol. The oxidation of methanol to carbon oxides is favored thermodynamically and methanol that is produced will react further to yield carbon oxides. Methanol must be removed from reactive conditions for it to survive the reaction process. The commercial methanol production route overcomes this problem by dividing the process into two steps.

The goal of this research is to investigate the production of methanol by direct oxidation of methane in a plasma reactor. Since the viability of any process depends on its ability to convert large quantities, the plasma reactor system should operate at atmospheric pressure. The plasma generates free radicals in the plasma stream and mixes them with a methane-rich stream. When the streams are mixed, the free reactions commence. Water is included in this study and its effects are investigated. The plasma reactor isolates the methane from highly oxidizing conditions and allows reactions to occur at

lower temperature (relative to the plasma region) and more selective conditions. As

noted above, high pressure is required to achieve feasible production rates. Higher

pressure creates higher temperatures in the plasma region (relative to low pressure

studies) which may be detrimental to selective conversion of methane. The high

temperature should help to speed removal of methanol product from the reactive region

because of higher stream velocities. The plasma and methane streams will be contacted

in such a way as to accelerate mixing, allowing the reactions to proceed at desirable

conditions. This is one of the first high pressure studies of its kind to be conducted and it

is hoped that the information gained from this study proves to be useful to future

researchers. CHAPTER II

TECHNICAL BACKGROUND

2.1 Background of Natural Gas and Methanol

Natural gas is an intemational commodity that consists primarily of methane. As noted above, current estimations of natural gas reserves stand at about 5,000 trillion cubic feet, which is equivalent to approximately 47% of world petroleum reserves. Over 75% of this natural gas is located in remote locations in the former USSR and in the Arabian

Gulf countries of the Middle East. It is a clean-buming fuel source, and the use of natural gas is steadily increasing and promises continued aggressive growth in the immediate future. Methane, along with methanol, is an integral part of the chemical industry.

Methanol is a vitally important chemical used in a variety of areas. It is a primary

CI building block in the chemical synthesis of many compounds via esterification, addition, carbonylation, and dehydration reactions. Some of the major derivatives of methanol are , , methyl-tert-butyl-ether (MTBE, an important fuel booster), dimethyl terephthalate, and methyl . It is also used as a solvent, antifreeze, reaction inhibitor, carbon-rich substrate used in the growth of , and has been shown to improve yield in some crop varieties. Most of the methanol produced world wide is used as a chemical feedstock, fijel, or fiiel additive.

Methanol holds a unique position in the chemical industry, due to its highly favorable physical and chemical properties. 2.2 A Brief History of Methanol Synthesis

Robert Boyle is believed to have discovered methanol in 1661, though no written record of any domestic or industrial use exists before the 19th century. Dumas and

Peligot first established the chemical and molecular identity of methanol in 1834. The

original method of producing methanol was by the destmctive distillation of wood.

"Wood spirit" or "wood alcohol" was the name given to methanol for many years,

referring to this original synthesis method. French chemist isolated the first

synthetic methanol route in 1905 and BASF commercialized the first synthetic methanol

plant in 1934. The process used a zinc/chromium oxide catalyst operating at 300°C and

200 atmospheres. Methanol produced by the BASF method was of much higher purity

than wood-derived methanol and this initiated European domination in methanol

synthesis technology. High-pressure methanol synthesis produced great contributions to

chemical reaction engineering, catalyst technology, process instrumentation, and high-

pressure technology. This highly successful process was used for many years until a

more efficient low-pressure synthesis route was discovered in 1966 (Lee, 1990). •

Methanol production in the U.S. was started in 1927 by Commercial Solvents

Corporation and DuPont. In the Commercial Solvents Corporation process, CO2

produced in the company's fermentation unit was hydrogenated to methanol at 300

atmospheres over oxide catalysts. The DuPont process used to produce a

gaseous feedstock, known as synthesis gas (), consisting primarily of carbon

monoxide and hydrogen. The syngas was purified and passed over a methanol synthesis

catalyst which yielded the final methanol product. The plant produced both and methanol until the late 1940's. At that time, plentiful supplies of natural gas became available and natural gas replaced coal as the preferred feedstock for the production of methanol.

In 1966, Imperial Chemical Industries, Ltd. introduced copper/zinc oxide (Cu/Zn oxide) catalysts that were far more active in the synthesis of methanol. These catalysts spelled the end of the high pressure methanol synthesis route. This new low pressure methanol synthesis technology operated at temperature ranges of 250-300°C and pressures of 50-100 atmospheres. These new Cu/Zn oxide catalysts were highly susceptible to poisoning and deactivation (from overheating) making close control of methanol reactors very important. Advances in syngas production and purification removed most of the catalyst poisons and made use of the new catalysts possible. These low pressure methanol synthesis catalysts have a lifetime of up to 4 years.

2.3 Commercial Methanol Synthesis

Today, commercial synthesis of methanol is based exclusively on heterogeneous synthesis. As presented earlier, heterogeneous methanol synthesis consists of a two step process. The initial step is to convert a carbon rich fuel source, most often natural gas, into syngas. The second step is to convert the syngas into methanol.

2.3.1 Synthesis Gas Preparation Techniques

Syngas can be produced in a number of ways. The method of choice depends on the desired characteristics of the syngas product. The principal routes of syngas production are through steam reforming of natural gas (or other feedstock), coal , partial oxidation of heavy oils, combined or oxygen-enhanced reforming, and heat-exchange reforming (Cheng and Kung, 1994). These syngas production routes have different operating costs and complexities. In general, there is no overall best route and project/site specifics will determine which syngas generation route is preferred.

A measure of synthesis is given by the stoichiometric ratio R.

moles H. R = (2 1) 2 X moles CO + 3 X moles COj

Syngas which is hydrogen-rich has R values greater than 1.0. Hydrogen-lean mixtures have R values less than 1.0. The methanol synthesis plant performance characteristics will define the optimum R value for peak methanol production. A brief review of the different syngas preparation techniques is presented below.

2.3.1.1 Steam Reforming of Natural Gas

This method of syngas generation has been the preferred method for many years.

As of 1990, 75% of the world's methanol production capacity was based on natural gas feedstock. Natural gas is an appropriate feedstock for a synthesis plant containing Cu-Zn catalyst because it commonly possesses low content.

Steam reforming is a heterogeneous process and takes place over a -on- alumina catalyst. The reformers are large process fumaces in which catalyst-filled tubes are heated by direct firing, which induces the reactions to proceed. Steam reforming generates syngas through the following simultaneous reactions. CH4 + H2O <^ CO + 3H2 (2.2)

CO + H20<:::>C02+H2 (2.3)

Reaction (2.3) is known as the water-gas shift reaction. Common steam-to-methane

ratios for steam reforming are in the range of 3 to 1. It is apparent from the above

reactions that a methane-rich feed gas generates a hydrogen-rich syngas. Commonly,

syngas with an R value of 1.3-1.4 is generated using typical natural gas feedstock.

Simply stated, the natural gas feed is preheated, desulfurized, mixed with steam, reformed

over catalyst, and then cooled. Typically, the effluent stream from a reforming plant will

be at ~850°C and 20 atmospheres. The only treatment it receives before being fed to the

methanol synthesis loop is compression.

Steam reforming is highly endothermic and high temperature, low pressure, and

high steam-to-carbon ratios enhance performance. It is very energy intensive, expensive,

and represents the major expenditure in methanol production. Elimination of this step

would provide a significant economic advantage in methanol synthesis (Cheng and Kung,

1994).

2.3.1.2

In coal gasification, syngas is generated by a combination of partial oxidation and

hydrogasification of coal feedstock through the following simultaneous reactions.

1 C + -O2OCO (2.4)

C + H2O0CO + H2 (2.5)

10 Equilibrium of carbon monoxide and carbon dioxide is maintained through the water-gas shift reaction (2.3), and the following reaction.

CO2 + C o 2C0 (2.6)

Many different types of coal gasification units exist, including moving and fixed-bed

gasifiers, fluidized-bed gasifiers, entrained-flow gasifiers, and gasifiers based on the

molten-batch process.

Gasification equipment must be selected and designed around the properties of the

coal to be processed. Important coal properties include ash content, moisture content,

caking behavior, reactivity, particle size distribution, impurities, and fixed carbon

availability (Cheng and Kung, 1994). Very low levels of some impurities (potassium,

iron, etc.) can create a catalytic environment, modifying reaction rates. Often, effluent

gas from a coal gasification unit requires additional treatment before it can be used for

methanol synthesis. Impurities, especially sulfur and sulfur derivatives, must be removed

to preclude poisoning of the catalyst. Coal gasification yields a raw gas that is very

carbon-rich (R<1.0) and composition adjustments are frequently necessary.

This coal-based method has been expected to become the preferred syngas

generation method in the U.S. for many years. Large indigenous coal reserves were

predicted to create a boom in the use of coal as a chemical feedstock. However, the

expense of coal conversion, variation in coal characteristics, and continued availability of

natural gas feedstock have prevented large-scale conversion to coal.

11 2.3.1.3 Partial Oxidation of Heavy Oils

This process is accomplished by incomplete combustion of heavy feedstocks according to the following reactions.

n m C„H^ +^^2 ^ nCO + —H2 (2.7) -O2 <^ nCO + — 2 ^ 2

C„H^ + nH20 o nCO + (y + n)H2 (2.8)

C„H, + n02«nC02+YH2 (2.9)

A minimum amoimt of oxygen (1/2 O2 per mole of carbon) is added to achieve complete conversion of the hydrocarbon feedstock. Steam is added to control the reaction temperature, which affects via reaction (2.3). The composition of the effluent gas is govemed by reaction (2.3) and the following chemical reactions.

CH4 + H2O <=> CO + 3H2 (2.10)

H2S + CO2 <=> H2O + COS . (2.11)

CO + -02<»C02 (2.12)

CH4 + CO2 o 2C0 + 2H2 (2.13)

Reactions take place at temperatures of 1350-1600°C and pressures of up to 15MPa (150 atmospheres). A major advantage of this process is that h makes use of heavy feedstock not usable in other, vapor-only processes. Disadvantages of the process include

12 formation, high effluent levels of sulfur and sulfur derivatives, and the requirement that pure oxygen must be provided for the reaction, instead of air.

Several large refining companies (including Shell Oil Co. and Texaco Inc.) have

achieved successful commercialization of this type of partial oxidation process. The raw

effluent gas from this process is highly carbon-rich and not suitable for methanol

synthesis. Along with sulfur removal, the composition of the gas must be adjusted and

excess CO2 removed before the syngas can be compressed and processed in a

conventional methanol synthesis loop (Cheng and Kung, 1994). The next two methods of

syngas production do not make use of unique feedstock, but the methods under which the

conversion takes place differentiate them from other approaches.

2.3.1.4 Combined Reforming

Combined reforming (also known as combination reforming and oxygen-

enhanced reforming) makes use of two reformers in series for the production of syngas.

The primary reformer is operated similarly to the natural gas reformer above, but the

secondary (or autothermal) reformer is injected with pure (99.5%) oxygen. Pure oxygen

in the second reformer precludes the burden of compressing large amounts of .

The oxygen also consumes excess hydrogen, making it possible to produce a nearly

stoichiometric syngas from the natural gas feedstock.

The advantage of this approach is brought about by shifting a portion of the reformer duty from the primary reformer to the secondary reformer. The partial combustion that occurs in the secondary reformer heats the process stream and allows

13 reduction of the fired duty in the primary reformer. In general, this approach is more costly than the steam reforming of natural gas approach but is justified in cases where energy costs are extremely high. Combined reforming does offer significant environmental benefits over other approaches, including reductions in CO2 and NO^

emissions. (Cheng and Kung, 1994).

2.3.1.5 Heat-Exchange Reforming

In heat-exchange reforming, a heat-exchange reformer is operated in series with

an autothermal reformer. The central concept in this approach is that heat generated in

the secondary or autothermal reformer is used to heat the process gas reacting within the

heat-exchange reformer. This approach is simple, effective, and can provide syngas of

nearly stoichiometric composition. Heat-exchange reforming provides the following

advantages: increased operational flexibility, high reliability, reduced maintenance and

energy costs, physically compact units, and reduced hazardous emissions.

Although both combined reforming and heat-exchange reforming are more

efficient than steam reforming, steam reforming still provides the most economical means

of syngas generation for methanol production. Readers interested in more information on

syngas generation are referred to Cheng and Kung (1994).

2.3.2 Commercial Methanol Synthesis from Syngas

Methanol synthesis reactions, shown below, are exothermic and experience a

decrease in volume as the reactions proceed towards methanol production. From these

14 considerations, methanol synthesis is favored by low temperature and high pressure.

Methanol generation from syngas occurs in the gas phase over a heterogeneous catalyst.

2.3.2.1 Methanol Synthesis Equilibrium

The synthesis of methanol occurs through the following reactions.

CO + 2H2 <^CH30H (2.14)

CO2 + 3H2 <^ CH3OH + H2O (2.15)

Reaction (2.3) is also induced over the catalyst. In fact, reaction (2.15) is the sum of

(2.14) and the reverse of reaction (2.3), as can be observed below.

CO + 2H2 <:^ CH3OH (2.14)

CO2 + H2 <^ CO + H2O (in reverse) (2.3)

CO2 + 3H2 « CH3OH + H2O (2.15)

Reactor effluent conditions and compositions are govemed by thermodynamic kinetics and equilibrium. Equilibrium compositions are calculated from simultaneous solution of the equilibrium constant expressions. The equilibrium constant expressions for a set of independent reactions (reaction 2.14 and the reverse of reaction 2.3) depend on the component partial pressures and are given below.

[P*co][p*H:o] ^rev2.3 ~ r« * 1 r« * 1 V^-^"/ LP COJLP HJ 1

K [P*CH,OH] LP COJLP HJ

15 If we take into the account the non-ideal behavior of the high pressure , the concept of fugacity must be considered. The governing fugacity relation is

fi = P:

Pi = partial-pressure of the i-th component, and

^, = fugacity coefficient of the i-th component.

Incorporating these fugacity relations into the equilibrium expressions results in the following expressions.

^ _ LP'CO]LP*H,O] JCOJ[<^^HJ ^ •

_ LP*CH30H ] L^CH30H ] [P COJLP H J [^CO ][<^HJ

Fugacity coefficients can be calculated or approximated in a variety of ways (Smith and

Van Ness, 1959). Many expressions exist for these temperature-dependent expressions.

For K^ev2 3' Bisset (1977) proposes the relationship

5639 5 49170 hi(K,^2.3) = 13.15-——-1.08*hi(T)-5.44*10-'*T-1.13*10-'*T'+—:^ (2.21) where T is in degrees Kelvin.

Thomas and Portalski (1958) derived the following expression for K2,4.

3921 log(K2,4) = -Y--'7.971*log(T) + 0.002499*T-2.953*10-'*T'+10.2 (2.22) where T is in degrees Kelvin.

16 2.3.2.2 Methanol Synthesis Kinetics

Commercial methanol synthesis processes are offered under license by process designers and catalyst providers. Most of these providers have developed their own proprietary kinetic rate model of the process. The literature contains many different

proposed models and most are based on consideration of the rate-limiting step in the

catalytic processes. There is continuing discussion about rate model stmcture and what

factors have the most influence on methanol synthesis. Examples of some of these rate

models are given below to illustrate the variety of the expressions.

Natta proposed that methanol is formed over specific catalysts according to the

following expression (Satterfield, 1980) given in terms of fiigacities.

••....H = fcofH. (^p^)(A + Bf,o + Cf„^ + Dfc„,o„ ) (2.23) eq

Terms A, B, C, and D are experimentally determined rate parameters. Notice that there is

no term involving CO2 in the above expression. To date, it is generally accepted that CO2

plays an important role in methanol synthesis. Seyfert (1984) derived the following

expression for methanol synthesis by the low pressure method.

r„.,. = fcHfH, - %^(A + Bfco + Cf„_ + Efeof„, + Ffeo,)' (2.24) eq

The terms A, B, C, E, and F are experimentally determined rate parameters. Dybkjaer

(1981) claimed that water had a profound inhibiting effect on methanol synthesis and

postulated the following expression for the rate of methanol production. The expression

17 is in terms of component equilibrium constants (K^), component activites (a,), and a derived equilibrium term (B,).

, .^w ^co.^co, ,., K:H a^ (1- B,) Tmeth = 1^.(T)(T—^ )*[ K 1 1 (2.25) ^^^(^-^-^ ^K,a, )-^

The kinetic models presented here illustrate the variety of forms that methanol

rate models can take. Models exist that account for catalyst sites in both the reduced and

oxidized state. It is evident that rate expressions for heterogeneous catalytic methanol

synthesis can take many forms and be very complicated.

2.3.2.3 Methanol Synthesis Loop Designs

Several methods and designs exist to convert syngas into methanol. These

"converter" designs are available from a few key technology providers. Figure 2.1 shows

a methanol synthesis loop from the M. W. Kellogg Company. It utilizes a series of

adiabatic, intercooled, spherical reactors. In the loop, fresh feed gas (containing H2, CO,

CO2 and inert material) is mixed with recycle gas on the discharge side of a single-stage

recycle compressor. This fresh stream is preheated to reaction temperature in a shell-and-

tube feed effluent exchanger before passing into the first reactor vessel.

Reaction proceeds over the first reactor bed adiabatically, the effluent being

cooled indirectly by an intercooler that raises intermediate-pressure steam. The second,

third, and fourth reactors operate in a similar manner. The final reactor effluent is cooled

18 Start-up Heater Feed

Recycle Converter Compressor Feedgas V) \

Spherical Converter Purge >

V^J Intercooler Catchpot

> i Crude Methanol \ Loop Feed/Effluent Condenser Exchanger

Figure 2.1. M. W. Kellogg Methanol Synthesis Loop (Cheng and Kung, 1994)

19 in the feed effluent exchanger. The effluent from reactor 4 is typically about 5% methanol. Once the effluent is cooled, it passes to a cmde condenser and is separated from the circulating gas. The product is then distilled where methanol is purified to

specification.

A variety of other methanol synthesis converters are used to produce methanol

commercially, including the ICI tube-cooled converter, the ICI quench converter, and the

Lurgi tubular converter. For more information on methanol synthesis loop design, the

reader is referred to Cheng and Kung (1994).

2.4 Potential Advances in Methanol Use

Many uses of methanol have been outlined above. Some mention should be made

of the potential that methanol has for increased use. Current methanol production sources

could not meet demand if potential applications of methanol were to come to fmition.

The two areas that have potential for increased methanol use are as an altemative, clean-

buming, automobile fiiel and as an intermediate form in the use and transport of methane.

2.4.1 Methanol as Transportation Fuel

The use of methanol as a neat ftiel for automobiles has been the subject of

substantial research for many years. It is currently used as a fiiel additive and in the

manufacture of other important fiiel additives. The purpose of these additives is to offer

economic advantages in the use of methanol in automobile fuel and to decrease pollutant

emissions from automobiles. Research into methanol applications as a fiiel and ftiel

20 additive can be found in the literature (Brinkman, Ecklund, and Nichols, 1990). For practical reasons, this discussion will briefly consider world-wide impact of increased methanol use as a neat automobile fiael.

Assuming that the use of methanol as a fuel becomes economically and

technically feasible, world-wide increases in use will cause modest but measurable

decreases in the cost of oil. OPEC suppliers have incentives to maintain a steady market,

but even modest competition for their energy product could result in price decreases

(Brinkman, Ecklund, and Nichols, 1990). OPEC suppliers have a practical monopoly on

the world's energy market and increased use of methanol would threaten the monopoly

and decrease OPEC's "markup."

If and when ftiel methanol approaches economic feasibility, OPEC suppliers will

probably be less likely to raise oil prices. Increases in oil prices would prompt methanol

proponents to speed introduction of methanol fiiel technology. This pressure would be

placed on the entire OPEC organization and coordinated OPEC actions to raise prices

would be less probable. Creation of a transportation infrastmcture capable of using

methanol or gasoline (i.e., ftiel-flexible vehicles) would have the overall effect of

stabilizing world-wide oil prices (Brinkman, Ecklund, and Nichols, 1990).

When and if fiiel methanol is introduced, the producers would probably be a

highly diverse group of countries, much more diverse than the existing group of world oil

suppliers. An increased level of diversity among energy suppliers would create a more

secure supply of energy and decrease the likelihood that world events, political or

otherwise, could adversely affect world energy supplies. More stability in world-wide

21 energy supplies lowers the chances of energy-related economic hardship throughout the world.

Increased dependence on methanol would create utilization of relatively inexpensive and underdeveloped natural gas resources. Use of methanol in the area of transportation would reduce the demand for petroleum products and measurably extend that supply into the future. Highly developed and exploited petroleum resources would be able to share the world's energy burden far into the future and a stable, long-lasting energy infrastmcture will result.

Methanol is very clean-buming. Significant improvements in environmental conditions will result if methanol replaces gasoline in substantial amounts. Areas that will improve are public health, , and visibility. These benefits could be doubly important in developing countries, where short-term industrialization benefits can often overpower environmental concems (Brinkman, Ecklund, and Nichols, 1990).

2.4.2 Methanol to Facilitate the Use of Methane

As outlined above, much of the 5,000 trillion cubic feet of methane in reserve exists in remotely located gas fields. Even though methane is abundantly available and inexpensive, its low price makes h impractical to transport very far. Compressing and transporting (pumping) large quantities of natural gas is very expensive and rarely economically practical.

Methane has physical properties that make it very hazardous and problematic to handle. It exists as a gas, even at high pressure, though liquefaction can be performed.

22 Hazards exist in handling and is extremely hazardous. Compression and liquefaction significantly improve the economics associated with transportation, but the hazards remain. Safety concems are paramount in moving natural gas and, although immense amounts are pumped throughout the world, it is not considered the best economic or energy management approach. As a result, huge

amounts of natural gas stay locked in remote fields.

If conversion of methane to methanol could be accomplished on-site, in a single

step, the economic and safety factors that hinder the transport and use of natural gas could

be virtually eliminated. Problems pertaining to the handling and transport of methane

would be transformed into ones conceming methanol handling and transport. Handling

methanol, while not without hazard, is much easier and safer than handling methane.

Transportation of liquid methanol is relatively simple, safe, and economical. Conversion

of natural gas to methanol, were it less involved, could make remotely located natural gas

fields a usable and highly economical energy resource.

23 CHAPTER III

LITERATURE REVIEW

For many years, efforts have been underway to discover an economical methanol

production route directly from methane. For reasons presented above, research into this

area has been steady. Several different approaches have been investigated to achieve this

goal. The two areas that have received the most attention are homogeneous and

heterogeneous gas phase partial oxidation of methane. Other approaches, including

liquid phase and plasma reactor studies, have also received significant attention,

especially during the last decade. This brief review will focus on the most significant

research that has been conducted on the subject of methanol production directly from

methane.

3.1 Homogeneous Partial Oxidation

As mentioned previously (Chapter 1), direct oxidation of methane to methanol is

attractive because it eliminates a very energy intensive step currently required in

methanol production, steam-reforming methane into synthesis gas. Helton (1991)

reviewed and summarized economic evaluations of current methanol production reported

in the literature. He concluded that methanol production via a direct, single step method

would be competitive with existing methanol production methods (steam reformation of methane followed by oxidation of syngas) if it achieved a single pass methane conversion of at least 5.5 %, with selectivity to methanol of 80%. This information is presented here because it serves as a performance standard that any prospective direct methanol

24 production process must meet or exceed to be economically feasible. The low magnitude of the conversion requirement is indicative of the high expense associated with existing methanol production methods. Thermodynamically, the most favorable reaction products of methane oxidation are carbon oxides (carbon monoxide and carbon dioxide). For this reason, the dominating consideration for methane oxidation is high selectivity to methanol.

3.1.1 Effect of Reactor Walls, Additives, and Promoters

Yarlagadda, Morton, Hunter, and Cesser (1988) studied Pyrex tubular reactors

and reported methane conversions of 8-10% and methanol selectivities of 75-80%. The

reactions occurred at 65 atmospheres and 723 °K, with residence times of about 2

minutes. Higher selectivities were favored by low (<5%) oxygen concentrations and high

pressures (>50 atmospheres). Burch, Squire, and Tsang (1989) and Helton (1991)

attempted to reproduce the experiments of Yarlagadda et al. (1988) but they were unable

to do so. These results have yet to be reproduced by researchers.

Hunter, Cesser, Morton, Yarlagadda, and Fung (1990) studied the effects of

reactor wall composition on the homogeneous reaction of methane. Pyrex, Teflon, and

metal reactor tubes were investigated as reactor wall materials. It was concluded that, for

most of the materials tested, reactor wall composition had no effect on partial oxidation

of methane to methanol. These investigators also examined the effects of 31 promoters

(mostly C2-C4 ) on the partial oxidation of methane to formaldehyde. High

yields of formaldehyde were produced at the expense of methanol. In general, C3

25 hydrocarbons (and higher) were more effective promoters than the C2 hydrocarbons present in natural gas.

Three investigators studied the effects of additives ( and ) on the homogeneous reaction of methane. These hydrocarbon additives enhance reaction initiation by generating free radicals. Burch et al. (1989) investigated a feed composition of 5% ethane and 95% methane. This feed composition did not alter methanol selectivity but did produce a reduction in reaction temperature by 50K. Helton (1991) also investigated ethane as an addhive and found that the selectivities for carbon monoxide,

formaldehyde, and carbon dioxide were similarly unaffected by the presence of ethane.

Fukuoka, Omata, and Fujimoto (1989) investigated the effects of propane on the homogeneous reaction of methane with oxygen. It was found that the reaction temperature was reduced by 40K. Methanol yield is reported to have increased with the addition of propane, though specific methane conversion and methanol selectivity were not given.

Conclusions of the above studies are summarized as follows:

a. Increasing reaction pressure in a metal reactor decreases the occurrence of

complete oxidation of methane and methanol.

b. Vycor and quartz reactors do not contribute to oxidation processes.

Introduction of beads to increase surface-to-volume ratios in the reactor

inhibits primary reactions and promotes secondary reactions. The influence of

Pyrex on oxidation is an area deserving of fiiture study.

c. Feed streams that contain hydrocarbon additives react at temperatures

approximately 50K lower than feed streams consisting of pure methane.

26 For more information on homogeneous oxidation of methane to methanol, the reader is referred to Chou and Albright (1978), Rytz and Baker (1991), Chun (1992), Chun and

Anthony (1993), Feng (1993), and Casey and Folger (1994).

3.1.2 Kinetics and Kinetic Modeling

Chou and Albright (1978) compared experimental data from methane oxidation to

methanol over a wide range of experimental conditions to a model based on 27 gas-phase

reactions and 3 surface reactions. They concluded that the data correlated sufficiently

well (i.e., to within experimental error) to the model. The experimental data for oxidation

in aluminum, copper, and packed glass reactors differed significantly from the data for

glass reactors due to the contribution of surface reactions. Their model clarified the

contributions of the gas-phase reactions that were occurring and provided information on

the extent to which surface reactions participated.

Vardanyan and Yan (1981) modeled free-radical reactions and calculated

conversions and selectivities for methane oxidation at 738K and 1 atmosphere.

Comparison of the model with experimental data revealed that the concentration profiles

of principle intermediates (CH20«, H2O2, CH3OOH, and H02») showed satisfactory

agreement.

Onsager, Soraker, and Lodeng (1989) suggested a model that consisted of 116

elementary free-radical reactions. These researchers concluded that most of the methane

conversion occurred at low temperatures, and their kinetic simulation was effective in

explaining methane oxidation phenomena at low methane conversions. Low temperature

conversion of methane is of interest since h is where highly selective oxidation occurs.

27 Helton (1991) investigated 179 free-radical reactions dealing with the homogeneous oxidation of a mixture of methane and ethane. The model accounted for formation of many components not considered previously. His proposed kinetic

mechanism was found to be capable of satisfactory prediction of the reaction of

molecular oxygen with methane and ethane.

Conclusions of the above investigations are as follows:

a. Homogeneous oxidation of methane can be predicted to within experimental

error by many of the proposed free-radical kinetic models.

b. Investigations into kinetic modeling of homogeneous oxidation of methane

applied to oxygen feed concentrations of less than 10%.

c. Although oxidation products can be predicted, predictions relating to reaction

temperature, residence time, and variations in oxygen feed require ftjrther

study. They should be conducted at high methanol selectivities.

For more information on kinetic modeling of homogeneous oxidation of methane

to methanol, the reader is referred to Droege, Hair, Pitz, and Westbrook (1989), Durante,

Walker, Seitzer, and Lyons (1989), Chun (1992), Gray, Griffiths, Foulds, Charleton and

Walker (1994), and Lodeng, Lindvag, Soraker, Roterund, and Onsager (1995).

While significant research has been conducted with some promising resuhs, none

of the homogeneous partial oxidation approaches has been found to be commercially

feasible for methanol production. High selectivities have been achieved, but they occur

at very low methane conversion levels. As methane conversion is increased, methanol

selectivity decreases rapidly. No single set of reaction conditions comes close to the

conversion and selectivity requirements for commercialization.

28 3.2 ' Heterogeneous Catalytic Partial Oxidation

Since the desired product, methanol, is more reactive than methane, some researchers believe that successftil direct synthesis of methanol from methane will require the use of a highly selective catalyst. Methanol selective catalysts are relatively well understood in the realm of methanol synthesis from syngas. The challenge for direct synthesis is to develop catalysts, which selectively convert methane directly to methanol.

Lunsford (1988) concluded that the formation of methyl radicals was the initial step in the production of oxygenated products and higher hydrocarbons from methane over metal oxide catalysts. Experimental evidence was uncovered indicating an

important role of O , although several types of surface oxygen were effective in

abstracting hydrogen from methane. Methyl radicals reacted to form methoxide ions or

desorbed into the gas phase. In all cases, homogeneous and heterogeneous secondary

reactions limited the yields of the desired partial oxidation products.

Chung, Miranda, and Bennett (1988) conducted low temperature studies of the

partial oxidation of methane using molybdenum oxide catalysts. Small amounts of

, dimethoxymethane, and were formed. Several

techniques were used to observe the electronic and geometric states of the catalyst. A

mechanism was proposed involving methoxy intermediates chemisorbed on vacant

oxygen sites. The mechanism explains which types of catalyst sites account for product

formation.

Sazonov and Popovskii (1968) correlated the catalytic activity of metal oxides in relation to certain oxygen-catalyst interactions. They found that the reaction energy of activation was proportional to the energy of the oxygen bond. The catalysts were also

29 ranked as to the of absorbed surface oxygen and activation energy of methane oxidation.

Helton (1991) showed that closure of the oxygen balance was cmcial in minimizing error in calculating methanol selectivities. Large deviations in product selectivity could occur even though carbon and overall material balances were ±2%.

Durante et al. (1989) operated a quartz-lined reactor with iron-sodalite catalyst at

55 atmospheres and reported methanol selectivity of 44% at 5% methane conversion.

Methanol selectivity in the quartz reactor with by-pass improved to 70% at 7% methane

conversion. High space velocities, such as those required for commercial processing,

decreased methanol selectivity to 15-28%.

Walker, Lapszewicz, and Foulds (1994) tested various catalysts to see if they

performed as claimed by the manufacturers. Manufacturers had claimed that some

catalysts convert methane to methanol at selectivities in excess of 75%. The authors

tested some of these catalysts (molybdenum oxide/ uranium oxide impregnated on

aluminum silica, iron sodalite, y-alumina, stannic oxide, and palladium on magnesium

oxide) and compared their performance to published claims and to homogeneous gas phase results. They found that none of the catalysts performed up to the levels cited in patents and literature. None of the catalysts showed any improvement over the performance of the homogeneous gas phase selectivities.

Chun and Anthony (1993) reported that temperatures of heterogeneous reactions were higher than those of homogeneous reactions under similar conditions. They report that the cause of the higher temperature may be due to variations in the residence time of reactants in the reaction zone and inhibition effects of the oxide catalysts. It was also

30 noted that a significant amount of homogeneous oxidation occurred in the void space of the catalyst bed and that catalyst surfaces inhibit free-radical homogeneous reaction.

Several researchers (Liu, Liu, Liew, Johnson, and Lungsford (1984), Zhen, Khan,

Mak, Lewis, and Somorjai (1985), and Durante et al. (1989)) report that molybdenum

and vanadium oxide based catalysts actively promote methane oxidation to methanol and

formaldehyde. Using oxygen feed ratios of 3-7%, methanol selectivities of 30-70% were

achieved under a wide range of reaction conditions.

The above investigations can be summarized as follows:

a. Spectroscopic studies indicate that important catalyst surface intermediates are

methoxide ions.

b. Methane is oxidized to CO2 over oxide catalysts with a high excess of oxygen

at 1 atmosphere.

c. Oxygen balance closure is essential for accurate selectivity calculations.

d. Molybdenum and vanadium oxide based catalysts are active for methane

oxidation to methanol and formaldehyde.

e. High pressure favors methanol production and low pressure (1 atmosphere)

favors formaldehyde production.

f High selectivities for methanol are possible, but only at low (-3%) methane

conversions. High methane conversion resuhs in lower methanol selectivity,

g. Heterogeneous methane oxidation reactions occur at higher temperatures (by

40-50°C) than homogeneous methane oxidation reactions.

For more information on the catalytic partial oxidation of methane to methanol,

the reader is referred to Kaliaguine, Shelimov, and Kazansky (1978), Andmshkevich,

31 Popovskii, and Boreskov (1965), Dowden, Schnell, and Walker (1968), Spencer, (1988),

Serafin and Friend (1989), and Hargreaves and Hutchings (1990).

The aforementioned research has yielded some promising results, but none of the heterogeneous catalytic partial oxidation approaches has been found to be commercially feasible for methanol production. High selectivities are achievable, but again, they occur

at low methane conversions. Continued research is required to develop a catalyst that is

highly selective for direct synthesis of methanol from methane.

3.3 Methane Oxidation in the Liquid Phase

Olah, Klopman, and Schlosberg (1969) studied methane reaction at room

temperature and 1 atmosphere in the presence of FSOsH-SbFs (magic acid). They

suggested that methane behaved as a super acid (CHs^) in this solution and produced

many highly reactive ionic species. These species reacted with other ions and to

give t-butyl, t-hexyl, and t-octyl cations or higher molecular weight species.

Olah, Yoneda, and Paka (1977) studied the reaction of hydrogen with

in magic acid solutions. They concluded that peroxide and magic acid reacted at

temperatures above 273K to produce methanol.

Konig (1982) used palladium catalysts in aqueous ferric sulfate solution to

oxidize methane to methanol. The reaction was allowed to proceed at 293-303K and 30-

60 atmospheres. Methane conversion was not provided, but a methanol selectivity of

92% was reported.

Geletii and Shilov (1983) studied oxidation of methane in solutions of Pt(II) and

Pt(IV) salts in Na8HPMo6V604o (HPA-6) at 393K and 60-100 atmospheres. Methanol

32 and methyl chloride were formed in roughly equal amounts and comprised the main reaction products. It was found that methanol underwent ftirther oxidation while the methyl chloride product did not react further.

Periana et al. (1994) reported a novel liquid phase methane to methanol synthesis via methyl bisulfate intermediate. Mercury ions catalyze a reaction in which

concentrated sulfuric acid oxidizes methane to give methyl bisulfate, water, and sulfur

dioxide. This process is reported to produce the highest single-pass yield (-43%) of

methanol of any catalytic methane oxidation to date.

Savage, Li, and Santini (1994) used supercritical water for the homogeneous

partial oxidation of methane to methanol. Water was chosen as the solvent in this and

other studies because of its excellent supercritical properties. They reported methanol

selectivities of 4-75%, with high selectivity occurring only at low conversions (.04%).

The major products of the experiments were carbon monoxide and carbon dioxide.

Dixon and Abraham (1992) investigated conversion of methane to methanol in

supercritical water over Cr203. They concluded that high concentrations of water inhibit

methane conversion but promote the yield of methanol. A consistent set of reaction

pathways was proposed and rate constants were calculated which accurately modeled the

experimental results.

For more information of the partial oxidation of methane to methanol in the liquid

phase, the reader is referred to Kao, Houston, and Sen (1991), McHugh and

Occhiogrosso (1987), and Webley and Tester (1991).

33 3.4' Methane Oxidation in Plasma Reactors

The chemistry of methane oxidation is well-known, although various mechanistic kinetic models are debated. It is clear that free-radicals play a very important role in the gas phase oxidation of methane. Plasmas offer an attractive method of generating free- radicals for chemical synthesis. The National Research Council (NRC, 1992) has identified plasma technology as a "future promising technology" and plasma is being adopted in process industry (Zanetti, 1983). Research relating to methanol synthesis from methane in plasma reactors is presented below. An introduction to the principles of plasmas is presented in the next chapter.

Huang, Badani, Suib, Harrison, and Kablaoui (1994) used a plasma generated by microwave energy to oxidize methane to methanol. The study focused on plasma reactor design to control free-radical reactions and maximize selective conversion of methane to methanol. From their reactor configuration studies, they concluded that isolation of the methane reactant from the plasma zone minimized methane dimerization and preserved intermediates that led to the formation of the desired product, methanol. High methanol selectivity was obtained only at low methane conversion. All of their experiments were conducted under vacuum (approximately 0.02 atmospheres).

Oumghar, Legrand, Diamy, and Turillon (1996) studied methane conversion into useful products by an air microwave plasma. They concluded that the location of methane addition from the end of the plasma discharge plays an important role in the product distribution. The air plasma produced higher levels of methane conversion than a nitrogen plasma. The high conversion of this plasma system suggests that it is suited to the conversion of methane for the purpose of synthesizing syngas.

34 Badani, Huang, Suib, Harrison, and Kablaoui (1995) investigated a variety of oxygen sources for improving partial oxidation of methane to methanol in microwave plasma reactors. The best methanol production was achieved when both H2O and O2 are present in the plasma stream. Two different pathways for methanol synthesis are postulated and it was concluded that -OH radicals play an important role. Methane conversions and methanol selectivities were found to be too low for commercialization.

Reactor configurations affect conversion and selectivity performance and oxygen sources are important because they alter available pathways to methanol.

35 CHAPTER IV

PLASMA

To aid in the understanding of this work, plasma characteristics and uses will be reviewed briefly. Section 4.1 provides a description and background information about plasma and gives examples of plasma occurrences in nature. Section 4.2 discusses how plasma is generated and sustained. It also discusses plasma use in a variety of processes.

In section 4.3, a brief description of the physics of plasmas is presented. Section 4.4 gives specific information on the microwave unit that was used to produce the plasma for this research.

4.1 Background

The term plasma is commonly used to describe a variety of electrically conducting, but globally neutral, materials, usually gases, that contain many interacting free electrons, ionized or , and neutral particles, which exhibit collective behavior due to long-range Coulomb forces (Bittencourt, 1986). In 1928, American physical chemist Irving Langmuir first used the word plasma to describe this fourth state of matter (Bova, 1971). The word is derived from an old Greek root, plassein, which means "to shape or mold."

The study of electrical discharge in gas has been ongoing for well over 100 years.

Siemens experimented with generation by silent discharge as early as the I850's.

In 1879, Sir William Crookes proposed that gases with the ability to discharge

36 should be considered a fourth state of matter (Hellund, 1961). Although the boundary between gases and plasmas overlap, the plasma state warrants this classification because

plasmas can display a vast array of physical and chemical properties depending on the

level of ionization.

It is interesting to note that ancient civilizations thought all substance existed in

the form of four elements: , water, wind, and fire. This idea parallels today's

accepted states of matter: solid, liquid, gas, and plasma. Aristotle even placed an order

on the ancient elements. He reasoned that earth resided at the bottom of his ordering

hierarchy, due to its substance and immobility. Liquid was next in his order because it

was much more mobile than any solid but much more defined than the wind. Wind was

next because it was considered smooth and ephemeral, yet considerably more substantive

than fire. Fire was last, due to incredible mobility, and transitory behavior. Aristotle had

correctly arranged these "elements" according to energy content 2,000 years before

anyone knew anything about the physics of gases or of electromagnetic forces (Bova,

1971). Understanding in these two fields was required before could really

begin to address plasma phenomena.

4.1.1 Plasma in Nature

By all accounts, plasma is uncommon on the surface of the earth under normal

circumstances. It does occur with frequent regularity if you know where to look.

Examples of plasma in nature include; lightening, aurora borealis, aurora australis, the

ionosphere, and even an ordinary fire, since it exhibits some electrical conductivity.

37 Examples of plasma produced from man-made sources are; neon signs, electrical discharges (i.e., from a light switch), plasmas that form along the leading edge of supersonic or hypersonic objects in air, and all types of man-made energy releases, from heaters to thermonuclear explosions.

If the universe were examined as a whole, the cold and solid environment here on earth is rare. The vast majority of known matter exists in the plasma state (Frank-

Kamenetskii, 1972). In stars, very high temperatures (on the order of 10,000,000°K) produce the ionization required to maintain the plasma state. In nebulae and interstellar gases, the ionization is produced by the radiation given off by the stars. Plasma may be considered unusual here, but it is the mle throughout the universe.

4.1.2 Potential Applications for Plasma

Plasma has tremendous potential for applications in many areas. Foremost under research is hamessing the power effusion in confined fusion reactors. This approach uses a plasma confined in a magnetic field to generate power by ftision. Intricacies of handling very high energy plasma are the subject of intense research. Many applications for plasma exist in the area of propulsion and space flight. Plasma has been "fired" from a "plasma gun" at velochies of 100 kilometers/sec (Frank-Kamenetskii, 1972). This

"plasma gun" could be used as an engine that uses small amounts of ftiel and produces high levels of thmst. Potential plasma applications as electrical conductors and high- temperature media are numerous. Plasma is many thousands (even millions) of times

38 lighter than , giving plasma a significant advantage in potential electrical applications.

4.2 Plasma Characteristics, Generation, and Uses

The properties of a plasma are defined primarily by the state of the ionized gas.

As noted before, the total number of positively and negatively charged particles must be

roughly equal, if global neutrality is to be maintained. The degree of ionization is also

very important in describing a plasma. Plasmas with a low degree of ionization possess a

number of charged particles that is much less than the number density of neutral

particles. As the degree of ionization goes up, the population of charged particles

increases relative to the population of neutral species. At the upper extreme of ionization,

a fusion plasma is completely ionized and few neutral particles exist.

4.2.1 Plasma States

Plasmas exist under one of three conditions: (i) break down, (ii) equilibrium, and

(iii) non-equilibrium. These conditions define important characteristics of the particles

that make up the plasma. They relate to how energy is distributed in plasma.

Breakdown is when the plasma forms and it occurs as the electrical conductivity

of a gas goes up sharply. This increase of electrical current, from lO"*'' amps to more than

10-^ amps, often causes light emissions (Razier, 1991). It is brought about by the energy

that is accumulating in the gas. The increase in electrical conductivity results from the

rapid increase in the population of charged species (i.e., electrons and ions).

39 Equilibrium conditions of the plasma relate to the energy levels of the particles.

When an equal partition of energy exists between charged and neutral species in a plasma, it is said to be an equilibrium plasma. If a plasma exists at high pressure and charged particles do not travel great distances between collisions, the kinetic energy of the particles is well-distributed, or equally partitioned, throughout the system (Eliasson and Kogelschatz, 1991). Very low electric field also contributes to evenly distributed kinetic energy between charged and neutral species. Often, a plasma at equilibrium exists at high temperature. For this reason, these equilibrium plasmas are known as hot plasmas. High temperatures cause large increases in the frequency of collisions, inducing more equal distribution of energy throughout the system. The electronic and molecular temperatures are equal in equilibrium plasmas.

Non-equilibrium plasmas exist under conditions contrary to those of equilibrium plasmas (i.e., energy is not equally partitioned throughout the system). Low pressure or high electric field causes fewer collisions to occur. This lack of frequent interaction means that electrons, and to a certain extent ions, can maintain higher energy states than the surrounding neutral species. Infrequent collisions enable high energy particles to maintain high energy because they cannot distribute the energy to the other particles in the system. In non-equilibrium plasma, the electronic temperature is higher than the molecular temperature, often by a factor of 10 or more (Baddour and Timmins, 1967).

These plasmas are known as cold plasmas. Chemical processing applications utilize, for the most part, non-equilibrium plasmas. This type of plasma is used in this research and

40 it will be the subject of the remainder of this discussion on plasma, unless noted otherwise.

4.2.2 P lasma Generation

Several different methods are employed to generate plasma, each generates plasmas with specific properties that make them attractive for a specific application. In general, these plasma production methods can be divided into three groups (LaDue,

1993): (i) constant DC potential field, (ii) pulsed DC or altemating current (AC) potential, and (iii) high frequency radiation.

4.2.2.1 Plasma From Constant DC Potential

Plasma generation from constant DC potential is accomplished by applying DC voltage across two electrodes. The electrodes are maintained at high enough potential difference to ionize gas molecules that are located between the electrodes. Some examples of this type of plasma are glow discharges, arc discharges, and corona discharges.

Glow discharges are low-pressure (<10 mbar) processes that occur between flat electrodes m a tube or reactor. They are popular because they operate at low current (10" to 10'' amps) and moderate voltages. The low pressure can result in high field and high energy electrons that produce intense glows, hence the name. These are the discharges used in fluorescent tubes, neon signs, and new high-efficiency light bulbs. The low

41 pressures under which these discharges occur makes them inappropriate for large-scale

chemical processing.

Arc discharges operate at higher current (>1 amp) and low voltage (several to tens

of volts). The gas is highly ionized and forms an equilibrium plasma. Arc discharges

have been investigated as a means of improving combustion (Lee, 1988).

Corona discharges are operated at or near atmospheric pressure and produce

insufficient field for complete breakdown to occur. The geometry of corona discharges

often makes use of a pointed electrode and a flat electrode. This results in a cone shaped

discharge volume. Corona discharges are used in high-speed printout devices, dry-ore

separation systems, and radiation detectors (Eliasson and Kogelschatz, 1991).

4.2.2.2 Plasma From Pulsed DC or AC Potential

The plasma from a pulsed DC or AC potential occurs from application of the

electrical potential across two electrodes. One of the electrodes is coated with a dielectric

material which accumulates charge once breakdown occurs. This buildup of charge alters

the field and intermpts the current for a time. Discharge eventually occurs and current

resumes in concert with the oscillating potential. The duration of the current pulses

depend on the pressure, ionization characteristics of the plasma, and the dielectric

properties (Eliasson and Kogelschatz, 1991). These "micro-discharges" are randomly

distributed in space and time. Examples of this type of plasma generation are known as

silent-barrier, dielectric-barrier, or simply, barrier discharges. These plasmas are ideally

suited for applications in volume plasma chemistry processing and off-gas treatment.

42 4.2.2.3 Plasma From High-Frequency Radiation

Plasma is generated by high-frequency radiation through inductive coupling of the

gas to the impinging radiation. Radio frequency (RF) radiation (10^ to 10* Hz) interacts

with gas molecules within an induction coil. Microwave frequency (10^ to 10'° Hz)

radiation is applied to gas molecules within a waveguide arrangement.

High-frequency plasmas often require the introduction of "seed" electrons to

initiate breakdown. These "seed" electrons can be introduced by a high frequency spark

(i.e., from a Tesla Coil) or a conductor placed in the plasma zone. These electrons

oscillate with the high-frequency applied field and experience inelastic collisions.

Electron collisions knock them out of phase with the field, allowing them to gain energy.

The electrons gain energy until they are more energetic than the outermost electrons in

the gas molecules. Collisions of high energy electrons with gas molecules releases more

high energy electrons in an electron avalanche (Razier, 1991). This electron avalanche is

shown in Figure 4.1.

Figure 4.1: Electron Avalanche in which electrons, represented as black dots, are emitted from the atoms or molecules, represented as open circles.

43 This avalanche of electrons rapidly increases the number of charged particles in the plasma, in other words, induces breakdown.

Creating and maintaining high energy electrons depends on the ability of the gas molecules to absorb energy. Electron energy is dissipated through excited states of gas

molecules and elastic collisions. Molecules without rotational and vibrational excited

states have less ability to absorb energy from high energy electrons than molecules with

rotational and vibrational excited states. Monatomic gases (i.e., Noble Gases) do not

have any rotational or vibrational excited states available to them. Diatomic gas ,

molecules have 2 rotational and 1 vibrational excited states available to them. Diatomic

gases can collide with high energy electrons and absorb some energy, placing them in

excited rotational or vibrational states. The result of this is that gases possessing

rotational and vibrational excited states naturally quench high energy electrons and,

consequently, higher power input is required to maintain the plasma state. For example, a

microwave-induced argon plasma requires 150 W of power to maintain the plasma state,

whereas, a microwave-induced nitrogen plasma requires 450 W of power to maintain the

plasma state (Krause and Heh, 1993). High energy electrons are also lost by an number

of processes, including drifting out of the plasma region and contacting reactor walls.

RF induced plasmas have the advantage of being able to isolate the electrode from

the discharge region, preventing electrode erosions and contamination of the plasma with

electrode ions. The wavelength of RF radiation is usually much larger than the plasma

dimensions. This normally creates a relatively homogeneous field inside the plasma

region. RF plasmas can be generated at pressures on the order of 1 atmosphere.

44 The high frequency of microwave radiation makes it impossible for heavier ions to follow field oscillations. High energy electrons can follow the field oscillations, but the result is that microwave plasmas normally exist far from thermodynamic equilibrium.

Microwave plasmas can be operated from 1 mbar to several atmospheres. Operation of microwave plasmas over this large range of pressures makes it possible to create electron from 10* to 10'^ cm'l The ease of operation, incorporation of flow through the plasma region, and relatively simple modification of plasma parameters makes microwave-induced plasmas attractive for plasma reaction chemical investigations.

4.3 A Brief Description of the Physics of Plasmas

The multitude of phenomena that are exhibited by plasmas is a consequence of charged particles. The electrically charged particles are capable of interacting with electromagnetic fields, as well as, generating their own electromagnetic fields.

Researchers often study plasma behavior in the presence of both magnetic and electric fields. Plasmas are generally very good electrical conductors and good thermal conductors. The ability of plasmas to conduct electricity and heat is due to the high mobility of electrons.

Plasmas exhibit a variety of difftision characteristics because of charge and mobility factors. When particle density gradients exist, particles difftise from areas of high density to areas of lower density. The much smaller particles, electrons, can difftise much faster and generate an electrical polarization effect. This field enhances the difftision of the ions and tends to make both electrons and ions difftise at approximately

45 the same rate. This type of diffusion is called ambipolar diffusion (Bittencourt, 1986).

Other types of difftision, classical diffusion and Bohm diffusion, describe movement of particles across magnetic fields.

An important characteristic of plasmas is their ability to sustain many different

types of waves. Dispersion relations characterize wave propagation modes in plasmas,

examples of these modes are called Alfven waves and magnetosonic waves. Wave

propagation in plasmas is studied to provide information on plasma properties and is

useful in plasma diagnostics.

Dissipative processes dampen wave amplitudes as energy is absorbed from the

wave by plasma particles. Some of these processes have been addressed above. One

mechanism not mentioned earlier does not involve collisions and is called Landau

damping. This mechanism for energy transfer happens when particles become trapped in

the "energy well" of an electromagnetic wave. If a particle in a plasma has roughly the

same velocity as a wave propagating through the plasma, the particle can move in concert

with the wave and the net result is that energy is transferred from the wave to the particle.

Some processes cause instabilities which result in increasing wave amplitudes. These

instability phenomena play a central role in plasma dynamics, especially in hot plasmas.

Instabilities greatly complicate confinement of hot plasmas, like those being examined

for thermonuclear energy research.

The emission of radiation is another phenomena used to characterize plasma

properties. Radiation emission in plasma results from two processes: (i) radiation emitted

by atoms or molecules, and (ii) radiation from accelerated charges. As the process of

46 ionization occurs, the opposite process of recombination also proceeds. As excited particles combine and decay to the ground state, radiation is emitted and can be observed in the plasma line spectra. When a charged particle traverses an electric field, it will be acted upon by forces produced by its own charge and the charge of the particle whose field it is traversing. Classical mechanics theory states that charged particles accelerated by interactions with either magnetic or electric fields must radiate energy. This emission of radiation will decrease the energy of the particle that is emitting it. This type of radiation emission occurs in many ways and is known as bremsstrahlung.

4.3.1 Criteria for Plasma Occurrence

Without external disturbances, a plasma will be maintained in a macroscopically neutral state. Since charged particles exist as free-moving entities, there must be some characteristic distance over which this balance is maintained. Departures from electrical neutrality only occur over distances that allow balance to be maintained between thermal particle energy and electrostatic particle energy. This distance is called the Debye length and departures from neutrality do not occur over larger distances than this. This Debye length is expressed by

n,e where, 8^ is the permittivity of free space, k is Boltzmann's constant, T is the absolute temperature, n^ is electron density, and e is the charge. Cmdely stated, a charged particle in a plasma interacts only with particles located at distances less than one Debye

47 length away. The charged particle has a negligible effect on any particles farther than one

Debye length away.

It is helpful to define a Debye sphere as a sphere inside of a plasma that has a radius of X^. Electric fields that originate outside the sphere are screened by the charged particles in the sphere and shield the electric field located at the center. If there is not enough space for this shielding, a plasma will not exist. The obvious requirement for a plasma is that the physical dimensions of the plasma system be large in comparison to XQ.

If L is a characteristic length in the plasma, this first criterion of a plasma is written as

L » ^ . (4.2)

Since particle shielding is a cumulative effect, it is necessary that the number of particles be very large. This second criterion of a plasma is expressed as

He^D » 1 • (4.3)

The third criterion for a plasma is that of macroscopic neutrality, although this criterion is not independent of the other criterion already mentioned. This aspect has been incorporated into the analysis previously and is expressed as

He = ni,„,. (4.4)

Consider a plasma with a charge separation or polarization induced from an extemal means. Overall, the plasma maintains neutrality, but inside the plasma volume excess charge has accumulated at each "end." If the extemal disturbance is removed, the field generated by the charge separation accelerates the particles back towards an equilibrium poshion. Inertial effects cause the electrons to continue moving beyond the

48 equilibrium position resulting in a charge separation in the opposite direction. This oscillatory behavior will continue and the frequency of oscillation is called the electron plasma frequency. The fourth criterion for a plasma is that the electron-neutral collision frequency be smaller than the electron plasma frequency. It is expressed as

t^pc > t^en (4.5)

This condition must be satisfied because collisions of electrons with neutral particles tend to dampen the oscillations described above. The electrons in the plasma must be able to act independently, frequent collision with neutrals forces the elec'^-ons to be in equilibrium with the neutrals. Readers interested in more detailed and complete plasma information should consult Bittencourt, 1986.

49 CHAPTER V

EXPERIMENTAL APPROACH, APPARATUS, AND PROCEDURES

Details of the experimental aspects of this research will be presented below.

Section 5.1 outlines the considerations of the experimental approach and the design objectives of the system. Section 5.2 presents a detailed description of the experimental system and how products are analyzed. Section 5.3 describes the experimental procedures that are executed during experimentation.

5.1 Experimental Approach

The objectives of the experimental system are:

• generate a microwave-induced plasma at atmospheric pressure;

• generate and maintain plasma in an argon, , oxygen environment;

• isolate the methane-rich stream from severely oxidizing plasma conditions;

• configure the system so reactants can be introduced both upstream and

downstream of the plasma zone;

• ensure streams are well-mixed before entering plasma or reaction zone;

• rapidly and efficiently mix the streams below the plasma zone;

• reliably separate condensable and non-condensable products;

• use available microwave energy source; and

• sample effluent streams for analysis.

50 A viable plasma-based process to convert methane directly to methanol depends on its ability to process large amounts of gas. Thus, operation at or above atmospheric pressure is required. Microwave-induced plasmas are one of the few types of plasmas that can be operated simply, safely, and reliably at higher pressures (~l atmosphere). To our knowledge, this type of high pressure plasma investigation has not been conducted.

Incorporating reactants (like HjO and Oj) in an Argon plasma operated at atmospheric pressure could provide essential information. Argon would be used as the plasma medium as it will easily maintain a plasma at relatively low power. In addition, argon will not participate in the chemical reactions to the extent that air or nitrogen would. A schematic diagram of the approach is shown in Figure 5.1.

ca

Argon O2 > Plasma Oxidation H2O Reactor Reactor Products

Figure 5.1. Schematic Diagram of General Approach

Plasma reactors give us the ability to maintain more close control over reactant mixing and reactant exposure to strongly oxidizing conditions. The reactor will generate free radicals and then mix that free-radical rich stream with a stream containing methane.

This approach isolates methane and methanol product from the high energy plasma enviroimient.

51 Figure 5.2 shows schematically how the methane is injected downstream of the plasma zone and reactions leading to methanol occur at lower temperature and less oxidizing conditions. The lower temperature of the reaction environment, as compared to the plasma environment, allows more selective conversion of methane to methanol.

5.2 Experimental Apparatus

A schematic diagram of the experimental apparatus is shown in Figure 5.3. It consists of four major components; (i) the feed system, (ii) the plasma generation system,

(iii) the reactor system, and (iv) the product collection system. Most of the experimental apparatus is located inside a walk-in vent hood for safety. Detailed descriptions of each part of the apparatus are self-explanatory and outlined in the schematic. The product analysis system is also part of the experimental apparatus but it is not physically attached to the apparatus and not shown on the diagram. Figure 5.3. The analysis system will be described in more detail below.

5.2.1 Feed System

The purpose of the feed system is to control the flow of reactants, ensure reactants are well-mixed, and prepare them for introduction into the reactor system. Gaseous reactants flow from cylinders through flow control devices (rotameters) and are mixed into feed streams. One reactant stream (subsequently called the plasma stream) passed through the high energy plasma generation zone and a second reactant stream (containing methane and subsequently called the methane stream) is added downstream of the plasma

52 Argon

Waveguide Adjustable Short Microwave Energy

Products

CH4 rich stream

Figure 5.2. Schematic Diagram of Reactor Configuration and Methane Injection

53 o E ^-*

O >l '•4-J C/5 ^ a O > Cfl 1 = cd o

O D O cd •f-o> U o ^—» ^ 3 <2u 4-» o g 0^ vo^ G o o (U cd o *3 a a s o U <

> •«—> 03 c

i-i D a, X w

CO a

on

D 00

54 zone. The methane stream may contain other reactants, but it is the only methane source.

As discussed above, methane must be isolated from the highly oxidizing conditions in the plasma zone to prevent the occurrence of undesirable reactions.

Needle valves and pressure gauges are placed after the rotameters so that

atmospheric conditions caimot bias flowrates. Rotameters were calibrated under constant

pressure conditions. As a result, rotameter feed and exit pressure is monitored and

maintained at these conditions during experimental runs. In this way, the high and low

pressure sides of the rotameters are maintained and the pressure drop over the rotameters

is constant and at the same level as during calibration.

Water is introduced to the plasma stream via a water saturation flask through

which the stream is bubbled. The temperature of the water and the head-space above the

water is monitored. These temperatures determine the amount of water that is introduced

into the plasma stream. It was decided that 1.5 ml was the minimum acceptable amount

of liquid product to be collected during an experimental run. This would assure that

sufficient product would be available for weighing, multiple GC injections, and would fill

the GC injection vials. The maximum amount of liquid product that could be collected

was determined by the size of the collection vessel in the apparatus. When more than 5

ml was collected, the level of product rose to a height that interfered with the smooth

flow of the reactor effluent stream (gas and liquid product). This interference caused

small pressure fluctuations in the apparatus and resulted in displacement of liquid

product, which was unacceptable. Liquid product limits were set at 1.5 ml and 5 ml.

55 Product collection limitations must be weighed against requirement conceming the duration of the experimental runs. Very low water concentrations would extend the run times beyond manageable limits and high concentrations would shorten runs making multiple GC product injections impossible. All of these considerations prove to be important later, when experimental parameters are modified to improve system operation.

Immediately downstream of the water saturation flask is a manometer to measure the pressure inside the reactor.

When water is added to the plasma stream, the tubing carrying the stream is heated to well above 100°C to prevent condensation of water. Silicone coated heating tapes are used to prevent premature condensation of condensable components in the system.

Vents are placed on both reactant streams for practicality and safety. The operation of these vents is addressed in the section describing experimental procedures and emergency procedures.

5.2.2 Plasma Generation System

The microwave unit used in this research was originally designed to direct the fire of anti-afrcraft artillery. This unit possesses characteristics that are unlike those of common commercial microwave generation systems. Detailed information on the unit can be obtained in War Department Technical Manual TMl 1-1524, (WDTM 11-1524.

1946). The characteristics of the microwave energy that this unit produces and how those

56 characteristics impact this research will be presented in this section. The technical characteristics of the system used in this research are listed below in Table 5.1.

Table 5.1. Technical Characteristics of the Microwave Generation System

Microwave frequency 2.700-2.900 GHz Wavelength 10.3-11.1 cm Average power input 600 Watts Average power output 300 Watts Peak power output 210kWatts Pulse width 7 |is/pulse Pulse repetition frequency (prf) 1700 pulses/second

The unit is comparable to commercial microwave ovens in some areas. The

frequency of the MW radiation is only slightly higher than that of commercial microwave

ovens (2.45 GHz). The power output of commercial microwave units is now up to 900

W, three times larger than the average power output of this unit.

The product of the pulse width and pulse repetition frequency is the total time that

the unit emits energy per unit time. This unit emits microwave energy only 1.36 % of the

time. In other words, microwaves are emitted for only 0.0136 seconds during a one

second time . The unit is OFF more than 73 times longer than h is ON. (This may

seem unusual, but an energy source designed for a RADAR system must "look" for

aircraft at extended ranges and then must "listen" for the reflected energy to return to the

source. This is the primary reason for extended off time in this and other types of

RADAR units.)

57 One characteristic of this unit that definitely is not similar to commercial microwave ovens is the peak power output. The peak power output of this unit is extremely high (210kW). The high peak power of this unit has important implications on the ability of this unh to breakdown and sustain the plasma state. Because of this high power, contacting the magnetron located atop the modulator can be fatal. Hence, the magnetron is covered by a protective metal mesh cage.

Often, in plasma studies, there are issues with initiating the plasma and maintaining it. Variations in concentration of reactants or unsteady flow conditions have been known to extinguish the plasma. The plasma generation units used in many other studies do not possess the high power of this unit. (This particular unit did not ever allow the plasma to extinguish under any flow condition or set of reactant concentrations.)

Sometimes, the plasma self-ignited as the energy was increased to operational levels.

This unit was capable of maintaining a plasma in air reliably, even though it has been reported that air plasmas require up to 450 W of energy. This unit overcame potential problems associated with maintaining the plasma state during experimental runs. The high power of this unit is probably very inefficient due to losses, but plasma operations at atmospheric pressure will demand higher peak and average power output.

We were unable to measure forward and reflected power levels due to lack of equipment. The requirement for liquid cooling and pressurization inside the waveguide indicates a significant amount of energy is being consumed by processes other than the reaction inside the quartz reactor. It is assumed that only a fraction of the microwave

58 energy generated is absorbed by the reacting species. The energy efficiency of this system was not an area of investigation.

The microwave generation unit is physically located outside of the vent hood and

apart from the bulk of the experimental system. The magnetron produces microwaves

that are carried in the waveguide to the reactor system. Figure 5.4 shows the waveguide

and quartz reactor tube configuration. Downstream of the reactor is a tunable "short"

which reflects the energy back through the waveguide. This allows the microwave

energy to resonate on the quartz reactor in the waveguide. The microwave energy.forms

what is known as a "standing wave" which imparts the maximum amount of energy into

the plasma stream inside the quartz tubing. The "short" is manipulated to adjust the

"standing wave" and maintain the plasma in its most vigorous state (i.e., brightest and

loudest conditions).

Reflected microwave energy is prevented from re-entering the magnetron with a

waveguide coupler. (This coupler blocks energy moving back towards the unit but allows

energy moving in the other direction to pass.) The waveguide also has "sleeves" attached

to the waveguide to prevent radiation from leaking out the gaps through which the quartz

tube passes. These "sleeves" are long enough (>7 inches) to dissipate the magnitude of

microwaves that propagate down the "sleeves." The quartz reactor tube is held in place

by fittings on the sleeves.

59 Quartz Reactor Tube "CAJON" Fittings Brass Sleeve

Brass sleeve CAJON' Fittings

Figure 5.4. Schematic of Waveguide and Quartz Reactor Tube Configuration

60 5.2.3 Reactor System

The reactor system is the region in which the reactant streams and high energy microwaves are combined. The reactor system really consists of two reaction regions as depicted in Figure 5.2. A 24 inch, 17 mm OD, 15 mm ID quartz tube is the heart of the reactor system. The quartz tubing is connected to the metal "sleeves" via Cajon"^^ fittings. These fittings enable airtight connection of glass or quartz with metal fittings by using tough, flexible o-rings. The o-rings also maintain an airtight seal at the high temperatures of the quartz tubing. The connections are well separated from the microwave radiation and the highest temperature experienced by the o-rings is approximately 150°C.

The plasma stream enters the top of the tube at elevated temperature (~140°C). A smaller quartz tube (24 inch, 6mm OD, 4mm ID) is placed inside the larger tube downstream the plasma zone (i.e., from the bottom). The methane stream is introduced from the bottom through this smaller tube. It flows vertically upward into the plasma stream that is flowing in the opposite direction. After mixing, the combined stream flows downward in the annulus of the two tubes. The point of methane injection is adjustable and can be moved closer or farther from the high energy radiation area (i.e., plasma zone). All lines downstream of the reactor system are maintained at elevated temperatures (>120°C) to prevent premature condensation of reactants or products.

Quartz is used exclusively to confine the reaction streams inside the waveguide.

Due to its excellent thermal properties, quartz remains a solid at temperatures above

61 1500°C. Disadvantages of using quartz are that it is very brittle and glassblowers often have difficulty fashioning quartz into shapes.

One feature of the reactor system deserves special mention at this point. The waveguide is designed to direct energy into the quartz reactor tube but microwave energy is present everywhere inside the waveguide. The close proximity of the outside of the quartz tube to the inside of the waveguide wall creates a favorable environment for breakdown to occur. This breakdown degrades both the waveguide surface and the outside of the quartz reactor tube. To preclude this from occurring, the waveguide is pressurized with COj (-30 psi). As discussed above, the high pressure diatomic CO2 creates an environment that the microwave radiation cannot breakdown. Water flows continuously through cooling coils mounted on the outside of the waveguide to prevent hazardous waveguide temperatures.

5.2.4 Product Collection System

The methane stream is mixed with the plasma stream in close proximity to the plasma zone (<1.5 inches in all cases). The plasma stream cools very rapidly. Mixing with the cold (i.e., 120°C) methane stream ftirther cools the final product stream. This product stream passes through a short stainless steel tube into an unheated section of tubing oriented vertically downward that leads to the condenser and collector section of the apparatus. The vertical orientation or this tube is important here because the first drops of condensation must be prevented from accumulating anywhere in the apparatus.

62 Drops that do condense in this section fall down the tube into the condenser and collection vessel.

The condenser consists of the tube passing through a water/ice mixture. The

subsequent cooling of the product stream, inside the tube, forces condensable products

out of the vapor phase. The product stream then passes into a collection vessel that holds

all liquid products (mostly water). Condensed liquid product accumulates in the chilled

collector for the duration of the experiment. Upon completion of the experiment, the

product is weighed and placed in an airtight sample vial for subsequent analysis by gas

chromatography.

The now cool (~5°C) gas product leaves the collection vessel and passes through

Tygon'^'^ tubing into a flow measuring device (bubble-meter). The flow rate of the exit

gas is recorded throughout the experiment with the bubble-meter and an electronic timer.

The noxious gas product exits the top of the bubble-meter and is vented.

5.2.5 Product Analysis

Both, gas and liquid, products were analyzed using a Hewlett-Packard (HP) 5890

series II Gas Chromatograph (GC). Gas products were analyzed during experimental

runs by taking gas samples from the exit stream with a gas-tight syringe and injecting into

the GC. Gas sample analysis required approximately 30 minutes between runs and at

least 2 samples were taken during each run. Liquid samples were collected, weighed and

transferred to amber sample vials to prevent breakdown of products sensitive to UV light.

63 These samples were stored in a until the GC was reconfigured for the liquid analysis at a later date.

5.2.5.1 Gas Product Analysis

Since the analysis system was not arranged for on-line measurement, the GC

operation and calibration had to be coordinated with operation of the experimental

system. This coordination will be evident when the experimental procedures are

presented in section 5.3.

Before gas samples can be analyzed, the GC must be initialized and operating

satisfactorily with a steady baseline. The gas samples were analyzed using a Supelco

packed column (15ft x 1/8 inch stainless, 60/80 mesh, Carboxen™ 1000). Proper GC

operation was checked before each experimental run with two calibration runs that

analyzed two different calibration gases (see appendix A for calibration gas specifics).

Gas product was injected into a gas sampling loop in the side of the GC. An

automatic switching valve injected the V2 ml sample into the column. Data collection was

automatic via HP Chemstation Data Analysis Software. Data files were stored in digital

form and analyzed at a later time. Gas sample components were detected using an HP

Thermal Conductivity Detector (TCD). This was the most appropriate detector for the

variety of components of interest to us. The gases that were detectable included all (even

trace compounds) of the hydrocarbon reactants (methane, ethane, ethylene, ) and

the permanent gas products (carbon monoxide, carbon dioxide, hydrogen). Oxygen and

argon were not reliably separated or detected by this system.

64 5.2.5.2 Liquid Product Analysis

Liquid samples were analyzed after a series of experiments had been completed so that many samples could be automatically injected and analyzed without supervision.

Since changing columns was fairly involved and column conditioning was slow, the ability of the GC to rapidly analyze liquid products significantly lessened the intervals between experiments. As with gas samples, data was collected digitally by HP

Chemstation Data Analysis Software and analyzed at a later time.

Before experimentation began, liquid calibration samples were analyzed to determine how long they could be stored before product degradation occurred. The first signs of detectable breakdown appeared somewhere between 14 and 21 days. The longest interval between collection and analysis for any experimental sample was 8 days.

This is well within the 14 day limit that marks the earliest possible onset of breakdown.

Liquid samples were analyzed using a Supelco Supelcowax 10'^'^ capillary column

(30 m X .25 mm, .25 fim film thickness). The liquid sample components were detected using a Ionization Detector (FID) that was installed on the same GC. Liquid products that were detectable included methanol and other compounds present in the product (e.g., trace amounts of acetic and ). The product consisted mostly

(99.9+%) of water.

5.3 Experimental Procedures

This section presents and discusses all procedures used during experiments. It also examines the operational checklists which guided system operation. Consistent

65 procedures and checklists are required in any experimental undertaking because they increase experimental reliability, increase safety, and decrease researcher workload.

5.3.1 System Preparation and Warm-up

The first step in the preparation phase of this experiment was to calculate the desired flowrates and temperatures for the experiment. These flows and temperatures were converted into rotameter readings and heater settings on the apparatus. The conversions were performed using calibration information. Since microwave power was not changed from run to run, it was not necessary to make modifications to the plasma generation system.

Preparation of the experimental apparatus consists of many activities. The experimental preparation checklist is presented below for reference and involves obtaining supplies required for the run, cleaning and connecting components, placing valves and switches in the correct positions, and safety. The steps in the preparation checklist are self-explanatory.

5.3.1.1 Experimental Apparatus Preparation

Before beginning experimentation, ensure the following tasks are completed.

1. Vent hood fan and light ON (door closed as far as possible, unless entering/leaving).

2. Ensure condenser and collector are prepared vdth sufficient ice.

3. Clean saturation vessel and refill with water then re-connect to system.

4. Ensure scale is available and operational (for weighing glassware and product).

66 5. Ensure sufficient supplies of reactant gases and pressurization gas is available.

6. Ensure quartz reactor and injection tubes are clean and ready for connection.

7. Ensure appropriate bubble-meter is connected and ready for use.

8. Place vent valves (VI and V2) in the VENT position.

9. Ensure that lower CAJON"^^ fitting is disconnected so plasma can be ignited.

5.3.1.2 Experimental Apparatus Warm-up Procedure

The purpose of the warm-up phase procedures is to bring the system to steady-

state operation at the specified flows, temperatures, and reaction conditions. The name,

warm-up phase, may be misleading because the system is not only "warmed-up" but also

brought to a fijlly operational state during this phase. A primary consideration in this

phase is that the condenser section must be fully "wet" with product before the actual

experimental run is started. This is important because only a few drops of product

accumulation in the system can create large errors. This phase of the experiment may last

longer than the actual experimental run as the system settles into steady operation and

"wets" appropriate sections. Most of the steps in the experimental apparatus warm-up

procedure are self-explanatory. For other steps, ftirther explanation is provided.

1. Apply power to heating elements.

2. Prepare and install condenser, collector, and preliminary collection vessel.

3. Set reactant gas flowrates to desired levels.

4. Adjust pressure of streams exhing rotameters to appropriate level using needle valves.

5. Close valve VI to supply plasma stream to reactor section.

67 6. Ensure waveguide pressurization is connected and tight.

7. Pressurize waveguide (30 psi max), check for leaks.

8. Check cooling water connections and set water flow to appropriate level.

9. Complete the Microwave Generator Operating Procedure (operations manual).

(Note: This procedure outlines the proper way to energize the MW generation unit.)

10. Ensure plasma is lit

(Note: If plasma is not lit, refer to the operations manual)

11. Connect quartz reactor outlet to product collection section with CAJON"^^ fitting.

12. Connect methane injector quartz tubing to reactor system.

13. Check reactant flows and adjust, if necessary.

14. Close valve V2 slowly.

15. Continuously monitor system for leaks, overheating, failures, or malftmctions.

16. Monitor system for adherence to desired conditions and allow it to run until

significant liquid product is collected.

5.3.2 Experimental Run Procedures and Checklist

During the preparation and warm-up phase of the experiment, the GC calibration should be underway. Both procedures (experimental preparation/warm-up and GC calibration) take approximately one hour. The GC must be ftilly prepared for injection when the experiment is started. A gas sample be injected immediately, or very soon

(within 10 minutes), after the experimental run commences because 2 samples must be injected before the end of the experimental run. Depending on the level of water vapor

68 present, a complete experimental run may require anywhere from 35 minutes to 75 minutes. Since the analysis of gas samples takes approximately 30 minutes, slight delays in gas injection can significantly extend the time required for an experimental run.

Data that should be recorded or calculated before an experimental run includes; (i) atmospheric temperature and pressure, (ii) temperatures of water saturator unit, and (iii) rotameter readings. When all experimental settings are steady at desired levels, the experimental run can begin.

5.3.2.1 Experimental Run Operational Procedure

For simplicity, the experimental run procedures contain all information through the final disposition of the liquid product. The experimental procedures contain items that comprise the shutdown of the apparatus. This is because removal of liquid product occurs after some of the apparatus has been shutdown. Shutdown is not separated because it only takes about 5 minutes and is easily incorporated with the operational procedure.

1. Note and record time.

2. Exchange collection tube with a clean, labeled, and pre-weighed collection tube.

3. Inject sample of reactor effluent into GC for analysis.

4. Check, adjust (if necessary) and record all temperatures, pressures, rotameter flows

(at least 4 times during experiment).

5. Record gas product flow-rate (5 readings) with bubble-meter every 10-15 minutes (at

least 4 times during experiment).

69 6. Continuously monitor system for leaks, overheating, failures, or malftmctions.

7. When GC has completed first gas sample analysis, collect and inject second sample.

8. When sufficient liquid product collected, record all system data, denote time, and

place VI and V2 in VENT positions (to discontinue reactant flows to reactor).

9. Close supply valves on all reactant cylinder regulators.

10. Complete Microwave Generator Shutdown Procedure (operations manual).

11. Unplug heating elements.

12. Disconnect collector tube, weigh tube and sample, place liquid sample in amber

sample vial, seal and label vial, then refrigerate.

13. Place condenser and collector units into refrigerator (to prepare for next run).

14. Turn off supply of waveguide pressurization gas at cylinder.

15. Turn off waveguide cooling water.

16. Do not touch quartz parts for at least Vi hour.

17. Monitor GC for completion of analysis, follow GC shutdown procedures.

18. When the system is cool, disconnect the reactor quartz parts and clean all

components in preparation for next run.

5.3.3 Safety and Emergency Procedures

Specific mention of the dangers and emergency procedures is usually exiled to the appendix in studies such as this. The safety considerations that deal with operation of this unit are numerous. Most deal with electrical current and radiation. The reader is referred to WDTM TMl 1-1524 for specific safety information, especially that conceming

70 hazardous components and grounding. In the case of plasma reactors, the potential for injury or death is real and safety considerations have to be addressed.

Primary dangers associated with the microwave system have been presented above. One consideration that has not been addressed is radiation leaks from the waveguide. In the case of this experiment, the waveguide was constructed, assembled, and tested for leaks by the Department of Electrical Engineering at Texas Tech

University. Leak tests were conducted before experimentation began and after it was completed. Radiation leaks are a potential hazard in almost any radiation system.

Another area of concem with plasma systems is high energy or temperature. As addressed above, quartz and metal parts have the potential of reaching very high temperatures. High temperature metals often radiate significant heat when at elevated temperatures. Quartz components will not give such indications when they are hot. A piece of quartz at room temperature appears much like a piece of quartz at 300°C. In fact, quartz at 800°C may only look "warm." Quartz must be respected as a hazardous item in these systems. It is not hazardous in a chemical or biological way, but it has physical characteristics that make it capable of collecting extremely high amounts of energy.

Operation of this system in a vent hood with the door closed is vital for safety.

This system produces significant carbon oxide products (CO and CO2). These and other products can produce a hazardous or deadly environment if they are not handled correctly. Entry into the hood is required but, time spent inside the hood was minimized.

71 Safe operation of this system demands the use of personal protective equipment.

Anytime this system is in operation, several components can severely bum the skin, instantly. Protective gloves and long sleeves are required for safety.

5.3.3.1 Emergency Shutdown Procedure

An emergency can take many forms and it is always difficult to formulate a general procedure to account for all situations. Since many components of this system are isolated inside a vent hood, formulation is doubly difficult. The primary concem is to stop the generation of all microwave energy. This is simple because the controls for this system are outside of the hood. Securing the flow of reactants is a problem because the cylinders are inside the hood. The vent valves are also inside of the hood. The final emergency procedure must be short and this one contains only four steps. Complete as many of the steps as allowed by the emergency and exit quickly.

1. Depress Contactor Control Trip Button on MW Rectifier (identified by red arrow).

2. Place valve VI and V2 in Vent position.

3. Tum off all gas flows at cylinder regulators.

4. Close door to vent hood.

72 CHAPTER VI

RESULTS AND DISCUSSION

The experimental portion of this work was conducted in four phases. Section 6.1 presents results from the Preliminary Phase of experimentation. Sections 6.2 through 6.4 outline the resuhs from Phases I, II, and III. Section 6.5 presents a summation of the overall performance of the experimental system. A discussion of the sources of error present in this system is presented in Section 6.6.

6.1 Preliminary Phase Experiments

The initial phase of experiments was conducted to hone system operation and test process parameters that would serve as starting points in later experiments. The experiments conducted during this phase used water as the only oxygen source for methanol production. Earlier researchers have indicated water to be important in the production of methanol from methane and it is a vital component in the experimental system used in this research. Methane reaction with water is very unfavorable compared to methane reaction with molecular oxygen. The later experimental phases of this research (Phases I, II, and III) also include water but, molecular oxygen serves as the primary oxygen source for methanol production. Raw experimental data for all of the runs can be found in Appendix C.

Two vital factors conceming system operation were examined during the

Preliminary Phase. The first was the injection distance. This is the distance from the edge of the waveguide to the end of the methane injection tube. It was important to

73 locate the appropriate injection distance for the initial experiments. The second factor was the amount of water present in the plasma stream.

The experiments that were conducted during the Preliminary Phase

involved injection of methane at different distances. These distances ranged from -0.25

inches (0.25 inches inside the waveguide) to 1.5 inches (1.5 inches from the edge of the

waveguide). Table 6.1 shows the Preliminary Phase experimental parameters.

Table 6.1. Preliminary Phase Experimental Parameters

Experiment Injection Ar/CH4 CH4/H2O Methane Methanol Distance Ratio Ratio Conversion Selectivity (inches) (%) (%) 0925 1.50 6.65 2.24 0.09 0.0906 0930 0.75 6.65 1.93 0.26 0.5546 1008 0.25 6.65 2.15 1.82 0.2500 1009 -0.25 6.65 2.13 24.65 0.0046 1013 0.00 6.65 1.97 10.84 0.0411 1014 0.13 6.65 2.03 6.47 0.0995 1030 0.50 10.94 1.25 1.96 0.3620 1103 0.13 10.83 1.16 13.27 0.0741 1104 0.25 11.11 1.09 7.38 0.2840 1106 0.31 10.83 1.22 5.02 0.2737 1106-2 0.00 10.83 1.18 14.14 . 0.0549

Figure 6.1 shows how the injection distance is defined. It was reasoned that the

distance of methane injection would affect the extent of methane conversion. Injection

too close to the plasma zone would greatly increase methane conversion but would

probably produce excessive carbon oxides and C2 hydrocarbons, as outlined by prior

researchers.

74 Quartz Reactor Tube

Waveguide Plasma Zone \

Mw Energy ^

Methan Injection Distance

Methane Injection Tube

Figure 6.1. Schematic of Injection Distance, depicted above, is the distance from waveguide edge to end of methane injection rod.

75 Injection too far from the plasma zone would isolate the methane from oxidizing

conditions and prevent significant conversion of methane.

The other factor vital to system operations is the amoum of water presem in the

plasma stream. Water is the primary componem in the condensed product. If only a very

small amount of water is used during the experimental run (i.e., 0.01 ml in a 1 hr

experiment), the tiny drop can easily remain in the product collection section of the

apparatus. Water must be present in sufficient quantity to obtain samples for injection

into the GC. Water concentration requirements will be addressed in more detail when the

results of this phase are presented.

At this point, it is necessary to define, explicitly, the terms conversion and

selectivity, due to the variety of accepted definitions in use. For this research, conversion

will be defined as Methane Conversion = "^^thane reacted methane fed to reactor Methanol selectivity will be defined as

, , , , ^ , . . methanol produced Methanol Selectivity = . methane reacted It is noted that the product of methane conversion and methanol selectivity is methanol

methanol produced yield. Methanol Yield = . methane fed to reactor

Yield is often used as a performance measure. Yield will not be discussed in this

research because it obscures information about vital performance issues, namely methane

conversion and methanol selectivity. For example, a yield of 5% can mean that methane conversion is 100% and methanol selectivity is 5%, or that methane conversion is 5% and methanol selectivity is 100%, or that conversion and selectivity levels exist somewhere in

76 between. Yield obscures these vital performance parameters and is not an appropriate measure to evaluate reactor performance in this case.

6.1.1 Injection Distance

Figure 6.2 shows the conversion of methane plotted against the injection distance.

Methane conversion increases as the methane stream is injected closer to the waveguide.

In fact, examination of the data shows that conversion increases sharply as the injection point is moved closer to the waveguide. This result confirms the assumptions conceming injection distance effects on conversion that were made in section 6.1.

The primary aim of the Preliminary Phase was to select a range of injection distances that would result in satisfactory conversion for the next experimental phase

(Phase I experiments). Since high methanol selectivity is only observed at low methane conversion and high selectivity is a primary consideration for methane conversion to methanol, it will probably be advantageous for this research to focus on lower conversion levels (less than 10%). Low conversion also minimizes the waste of methane because unconverted methane can be recovered for later use.

It is important to note that the preliminary experiments used only water and methane as reactants (in an argon carrier gas). As noted above, the reaction between methane and water is much less thermodynamically favorable than the reaction between methane and molecular oxygen. The next phase of experiments (Phase I experiments) was designed to include molecular oxygen and probably exhibhs higher conversions than the methane/water experiments, at comparable injection distances.

77 Methane Conversion . 0.25 0.15 \J.\JJ i n s 0.2 0.1 n - Figure 6.2.PreliminaryPhase-Methan e ConversionVersusInjectioDistance. -0.5 -0.20 ; 1 i : I PreliminaryPhaseExperiments(11runs,nooxygen)[ + ^ 1 i i — - • • - — 0.25 0.0.71 Injection Distance(inches) ^ - 1 • k 78 1 1 1.25 1.1.7 J i j • 1 Figure 6.2 shows that conversion levels of 3-6% are obtained at an injection distance of roughly VA inch. An even more highly reactive componem mixture, due to the presence of molecular oxygen, was used in the next phase of experiments. Since conversions were expected to increase in the more reactive environment, it was reasonable to assume that injection at 'A inch would result in conversions higher than those desired (-10%). For this reason, a lower injection distance limh of 3/8 inches (i.e., injection no closer than 3/8 inches) was adopted for the next phase of study. Since detectable conversion occurred at 1.5 inches, this distance was adopted as the upper injection limh (i.e., most distant) for the next phase of study.

Two injection distances have been selected as the injection limits for investigation in the next phase. A third injection location, between the endpoints, was tested. This point was selected to help cover the range of injection more completely. The conversion obtained at these three points is expected to exhibit similar behavior as the that shown in

Figure 6.2, higher conversions at closer injection distances. Since conversion increases as the point of injection approaches the waveguide, the intermediate point will be selected nearer to the least distant endpoint, where the conversion should be changing at a higher rate. The third injection point was selected to be 5/8 of an inch. So, for the next phase of experiments (Phase I experiments), 3/8 inches, 5/8 inches, and 1.5 inches will be the injection locations investigated. A precise analysis or calculation of the ideal injection distance is not possible as reaction conditions will change drastically when molecular oxygen is added.

79 6.1.2 Water Concentration

The experimental apparatus has limitations on the amount of water that can be used in each experiment. During the preliminary phase, the water concentration was altered by changing the methane flowrate. The methane-to-water ratio varied from -1.1 to 2.3 during the 11 preliminary phase experimental mns. The water concentrations tested in the preliminary phase produced sufficient liquid product and resulted in satisfactory experimental mn times. A more detailed investigation conceming water concentration (i.e., numerous experiments with significant variation in water concentration) was not warranted until molecular oxygen was introduced as a reactant.

At this point, it was important to discover which water concentrations would enable satisfactory system operation and to investigate limits on water vapor concentration.

6.1.3 Methanol Selectivity Dependence on Methane Conversion

A recurring phenomenon in methanol synthesis by direct oxidation of methane is an apparent dependence of methanol selectivity on methane conversion. An inability to achieve high levels of methane conversion and methanol selectivity, simultaneously, is often referred to in the literature and represents the limiting factor in direct methanol synthesis from methane. Figure 6.3 shows methanol selectivity plotted against methane conversion for the preliminary phase experimental runs. Although the preliminary phase experiments were conducted over a wide range of conditions, the results from these experimental runs illustrate the dependence of methane conversion on methanol selectivity. The data are very poorly correlated due to the wide range of conditions. It is evident that higher selectivhies occur only at lower conversion levels. As more methane

80 93 0.006 93 u Z 0.005 B J3 'o i.I »0.004

B 0.002

0.001 >

C/3 0 0 0.05 0.1 0.15 0.2 0.25 0.3 J3 Methane Conversion (mole methane reacted/ mole methane fed)

Preliminary Phase Experiments (11 runs, no oxygen)

Figure 6.3. Preliminary Phase-Methanol Selectivity Versus Methane Conversion

81 is converted, less and less converted methane leads to methanol formation. The optimum progression would be to obtain high methanol selectivity at high methane conversion.

Methanol selectivity dependence on methane conversion will be used to illustrate system performance improvements in ftiture experimental phases.

6.2 Phase I Experiments

Phase I experiments were designed to investigate the impact of three different levels of oxygen on methanol production. Table 6.2 shows the experimental parameters from Phase I runs. These mns were the first to contain molecular oxygen (O2), which served as the primary oxygen source for methane oxidation to methanol.

Table 6.2. Phase I Experimental Parameters

Experiment Injection Ar/CH4 CH4/O2 CH4/H2O Methane Methanol Distance Ratio Ratio Ratio Conversion Selectivity (inches) (%) (%) 1118 3/8 6.56 3.88 1.76 13.500 0.269 1124 3/8 6.56 4.44 1.88 7.850 0.424 1125 3/8 6.56 2.97 1.92 7.613 0.728 1125-2* 5/8 6.56 2.93 1.55 0.167 8.042 1126 5/8 6.56 3.82 1.72 2.430 0.301 1129 5/8 6.56 4.44 2.02 1.705 0.652 0124 5/8 6.56 2.93 1.59 1.029 1.242 1201 1.5 6.56 2.88 1.91 0.054 0.927 1201-2 1.5 6.56 3.82 1.91 0.024 0.901 1202 1.5 6.56 4.44 1.84 0.027 1.060

Experimental run 1125-2 is denoted by an asterisk because it produced highly erroneous results. The primary reason it was discarded is that the run produced virtually no CO or CO2 product. Run 0124 was conducted to replicate the conditions of 1125-2.

82 The results of run 0124 were significantly differem than mn 1125-2 as can be observed in the data. Run 0124 achieved results that compared favorably with similar mns and carbon oxide products were detected in appreciable quantities. It was surmised that a serious experimemal error occurred during mn 1125-2 that to the unbelievable results.

6.2.1 Injection Distance

The effect of injection distance on conversion for Phase I experiments is shown i m Figure 6.4. Increases in methane conversion are evident at closer injection distances.

Moreover, the conversion levels at the closer injection distances are in the preferred range for this research (<10%). Since the reaction of methane and oxygen is many orders of magnitude more favorable, thermodynamically, than the reaction of methane and water, it is surprising that the increase in methane conversion is not more dramatic. Injection at

3/8 inches resulted in satisfactory conversion (<10%) of the reactant mixture. The 3/8 inch injection distance was, initially, chosen to be the sole location for the Phase II experiments. It was discovered, during Phase II experiments, that injection at 3/8 inches did not result in sufficient conversion of the reactant gases. To obtain an improved range of conversion, a second injection distance had to be tested. Injection at V* inch was also tested in Phase II experiments.

6.2.2 Water Concentration

Although variations in water concentration were not planned for this experimental phase, small but measurable variations in water concentration occurred, nonetheless.

83 1 I i !

; 1

0 14 1 1 • ! i X

; i I 0 12 9i \ \ I 0 1 i 1 1 0 08 i 1 1 1 006 -

I 0 04 - § Ui X 0 01 I \l.\3^ X O X U 0 - K— 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 Injection Distance (inches)

X Phase I Experiments (9 runs, 3 inj. locations, runs include oxygen)

Figure 6.4. Phase I-Methane Conversion Versus Injection Distance

84 These variations were caused by imprecise heating controllers used in the apparatus. The changes in water content, although slight, were examined to identify the effects of water content on system performance. Upon examination of the limited (10 experiments) experimental data, it was impossible to draw a conclusion about water content influences on performance. The data failed to indicate a trend that would guide reaction conditions for future experimental runs. Based on this, it was decided to test two levels of water concentration in the next phase of experiments (Phase II experiments). One level would be at or near the maximum amount allowed by experimental constraints, another at an intermediate level.

6.2.3 Oxygen Concentration

Figures 6.5 and 6.6 show methane conversion and methanol selectivity, respectively, plotted against oxygen concentration. The graphs do not indicate that higher oxygen concentrations increase conversion and decrease selectivity. In fact, though the data are not correlated strongly, there is some indication that higher oxygen concentration actually increases selectivity. The effects of oxygen on this chemical

system are well-known. The most highly oxygenated products of methane oxidation (CO and CO2) are the most favored, thermodynamically, but the least desirable. The desired partial oxidation product (methanol) is highly reactive to molecular oxygen. Any

methanol that is formed during reaction should be isolated from oxygen in reactive

conditions. Less oxygen should favor less oxygenated products and minimize

consumption of methanol through oxidation reactions. From these considerations alone,

lower oxygen concentrations should favor the production of methanol. Less oxygen has

85 0.16

T3 0.14

93 0.12

93 ••-» 0.1

S^ 0.08 c o Ui Wi U 0.06 > C o U u 0.04

93 ;s 0.02

0 0.025 0.027 0.029 0.031 0.033 0.035 0.037 0.039 0.041 0.043 Oxygen Concentration (mole fraction)

I Injection Distance at 3/8 inches •Injection Distance at 5/8 inches • Injection Distance at 1.5 inches

Figure 6.5. Phase I-Methane Conversion Versus Oxygen Concentration

86 0.014

G 0.012 4 •

A 0.01 • A 0.008 1 • • 0.006

I 0.004 • .• > 0.002

13

I 0.025 0.03 0.035 0.04 0.045 Oxygen Concentration (mole fi-action)

I Injection Distance at 3/8 inches •Injection Distance at 5/8 inches A Injection Distance at 1.5 inches

Figure 6.6. Phase I-Methanol Selectivity Versus Oxygen Concentration

87 the added benefit of decreasing methane conversion, which has been found to improve selectivity and will conserve unconverted methane for recovery and use. Consequently, the experimental data was expected to show that lower oxygen concentrations improve system performance. This obvious inconsistency indicates that other factors are contributing to system performance.

6.2.4 Mixing Effects

The impinging flow pattem that was designed into this system (see Figure 5.2,

"head-on" mixing of counter-current flows in concentric tubes, followed by exh of the mixture through the annulus formed by the two tubes) is unique when compared to other plasma experimental setups. The aim of this mixing design is to completely mix the two streams (plasma stream and methane stream) as rapidly as possible. Mixing two streams that are flowing in opposite directions has a more dismptive effect than mixing of co- current flows, under similar flow condhions. A co-current flow configuration in concentric tubes allows stream mixing to occur more slowly, allowing the reaction conditions to change, or "decay" from a favorable state to a different or less favorable state. Rapid mixing is doubly important in plasma systems where temperature quench rates can be extremely high (>10^ K/minute) (La Due, 1993). This conceptual evaluation of mixing makes the assumption that condhions at the mixing point are favorable for methanol production. If this is tme, accelerated mixing of the streams would enable more production of methanol.

Figure 6.7 shows an expanded view of the mixing region of the plasma stream and the methane stream. The mixing point occurs directly above the methane injection

88 Plasma Region

Highly Turbulent • T y i i >r Mixing Region

Oxidation Zone

AAA

Quartz Reactor Tube n j< A

Methane Injection Tube

Figure 6.7. Schematic of the Mixing Region (formed when the plasma stream and methane stream collide inside the quartz reactor mbe.)

89 tube as the methane stream encounters the hot plasma stream flowing toward it. The methane stream must completely change direction and move toward the outer tube wall so that it may exit down the annulus formed by the two quartz tubes. This flow configuration was incorporated to improve and accelerate mixing, but had a more profound impact than expected on overall system performance.

As the plasma stream passes through the plasma region, it experiences a tremendous temperature increase. A thermocouple inserted into the plasma region from above indicated a temperature increase from ~150°C to over 1400°C, over just a few centimeters distance. The temperature indicated by the thermocouple rapidly fell to approximately 400°C when the thermocouple was moved to only 1 inch below the plasma region. The non-equilibrium nature of the plasma indicates that the energy of the charged species (called the electronic temperature) corresponds to a temperature higher than that indicated by the thermocouple. This rapid heating and cooling is accompanied by turbulence and gas velocity fluctuations. These conditions are expected in species in a highly energetic or plasma state.

It is logical for changes in the plasma stream or methane stream to affect the size and shape of the mixing region. An increase in plasma stream flowrate necessitates a higher velocity through the tubing, especially as the stream experiences rapid heating.

The plasma stream collides with the methane stream at a higher velocity, reducing the vertical penetration of the mixing region into the plasma stream. Effectively, this increases the injection distance. It has already been established that more distant injection results in decreased conversion and, often, increased selectivity.

90 Phase I experiments involved investigation of the effects of changing the amount of oxygen in the plasma stream. Any increase in conversion and decrease in selectivity

resulting fi-omth e thermodynamics of higher oxygen concentrations is in direct

competition with the decrease in conversion and increase in selectivity resulting from the

physical effects of mixing described above. The mixing effects seem to offset the

anticipated effects of higher oxygen concentration.

For investigations conducted at low pressure, temperature increases are much

more modest and mixing effects such as the ones discussed here may have less impact.

These and other mixing effects probably apply and should be considered in other plasma

studies conducted at atmospheric pressure. Other significant mixing phenomena may

arise from the rapid heating, cooling, and mixing of plasma streams. These effects will

be completely dependent on flow pattems and reactor configuration. Again, these effects

were not considered in the design of this experiment but do explain the unusual behavior

that was observed.

It is desired to find operational conditions under which this system will produce

methanol in the most plentifiil and efficient manner. The primary aim of Phase I

experiments was to determine the best oxygen concentration (of 3 levels tested) for

optimum methanol production. The data are inconclusive as to what oxygen level tested

is best. The primary consideration in direct production of methanol is high selectivity for

methanol, for reasons presented earlier. Accepted chemical principles indicate that lower

oxygen concentrations should favor high selectivity for methanol. Experimental data are

inconclusive, possibly due to conditions created by mixing phenomena that were not

anticipated. A decision must be made regarding oxygen concentration. It was decided to

91 operate with low oxygen concentrations for the next experimental phase. This decision was reached because it is aligned with accepted chemical principles and a potential explanation exists for the inconclusive experimental data. If this decision is incorrect and oxygen concentrations should be high for increased selectivity, this will be indicated by ftiture experimentation.

62J—Overall Performance of Phase I Experiment*;

Phase I experiments included molecular oxygen and improved methanol production was expected. Figure 6.8 shows the overall performance of Phase I data compared to Preliminary Phase data. The trendlines are included to show how the separate data sets compare and do not imply linear or ftmctional dependencies. The expected improvements in methanol selectivity are apparent and methane conversions are at acceptable levels(=10%). Performance is improved, albeit modestly. The data also exhibh characteristic methanol selectivity dependence on methane conversion, with higher methanol selectivity observed at lower methane conversion.

6.3 Phase II Experiments

The goal of Phase II experiments was to improve system performance by attaining higher levels of selectivity for methanol. The injection distance and oxygen concentration levels were adjusted, enabling the system to reach higher levels of methanol selectivity at desired methane conversion levels. Phase II experiments involved testing 2 injection distances, 2 levels of oxygen concentration, and two levels of water concentration. The oxygen concentrations tested were at the lower limit of what was reliably measurable by

92 0.014 1 1 r—^—— A i 0.012

T3 1 ' 93 1 0.01 ! 1 y. \ 1 1 o « 0.008

i 1 ^ 1 ^ o 1 0.006 I 1 J3 a S "^ 2 • Tv a 0.004 a ij^ • X 0.002

1 i • ^^^"^-^^ > • 0 " 0 0.05 0.1 0.15 0.2 0.25 0.3 Methane Conversion (mole reacted/mole fed)

• Preliminary Phase Experiments • Phase I Experiments (3/8 inch Injection) A Phase I Experiements (5/8 inch Injection) X Phase I Experiments (1.5 inch Injection)

Figure 6.8. Preliminary Phase and Phase I-Methanol Selectivity Versus Methane Conversion

93 the apparatus. The upper water level tested was near the upper limit of experimental

feasibility (i.e., more water would have made h impossible to complete the experiment

effectively). Table 6.3 shows the experimental parameters from Phase II runs.

Table 6.3. Phase II Experimental Parameters

Experiment Injection Ar/CH4 CH4/O2 CH4/H2O Methane Methanol Distance Ratio Ratio Ratio Conversion Selectivity (inches) (%) (%) 0213 0.375 7.22 4.75 1.04 0.453 3.715 0215 0.375 7.02 4.75 0.89 0.756 1.826 0217 0.375 8.02 4.75 1.90 2.035 1.301 0218 0.375 7.98 4.44 1.93 3.161 0.854 0225 0.250 8.02 4.75 1.67 4.769 0.785 0226 0.250 7.98 4.44 1.96 6.062 0.578 0226-2 0.250 7.78 4.44 0.96 2.450 0.990 0226-3 0.250 7.82 4.75 0.95 4.505 0.596

6.3.1 Injection Distance

From inspection of the methane conversion attained in Phase II runs, injection

location behavior parallels the behavior observed in earlier phases. More distant

injection resulted in lower methane conversions and conversion levels were in the desired

range (<10%). The locations selected for methane injection were appropriate for this

experimental apparatus and these experimental conditions.

6.3.2 Overall Performance of Phase II Experiments

The selectivity levels obtained in Phase II experiments are clearly higher than

those of earlier phases. Figure 6.9 shows the comparison of Phase II data with both the

94 0.04

T3 0.035

93 0.03 §

0.025 J3

13 C/2 u 0.02 (J 3 2 0.015

J3 •«-• ^(L ) 0.01 s "o B 0.005

0 0.05 0.1 0.15 0.2 0.25 0.3 Methane Conversion (mole reacted/mole fed)

• Preliminary Phase Experiments • Phase I Experiments (injection at 3/8 inches) A Phase I Experiments (injection at 5/8 inches) X Phase I Experiments (injection at 1.5 inches) D Phase II Experiments (injection at 1/4 inches) • Phase II Experiments (injection at 3/8 inches)

Figure 6.9. Preliminary Phase, Phase I, and Phase II-Methanol Selectivity Versus Methane Conversion

95 Phase I data and Preliminary Phase data. Progression of the experimental data away from the origin is desired. The optimum trend in data would be high conversion and high selectivity (i.e., progress up and to the right on the Figure 6.9). The data from Phase II experiments shows a definite increase in the methanol selectivity over Preliminary Phase and Phase I data. The data exhibh considerable scatter, but the Phase II data confirm system performance improvements, especially at the 3/8 inch injection distance.

The data from this research indicate that the best methane-to-oxygen ratio for methanol production from methane for this apparatus is approximately 4.5 to 1. A higher methane- to-oxygen ratio may result in performance improvements but this apparatus is unable to operate at higher ratios.

The data from this research indicate that the best water-to-oxygen ratio for improved methanol selectivity is approximately 5 to 1. This is a surprisingly high concentration of water. Methane conversion is favored by lower water levels because water dilutes the system and interferes with the reactions between oxygen and methane.

High water content has been shown to favor methanol selectivity, while low water content favors methane conversion. Data from investigations into the effects of water content also illustrate the competition between methanol selectivity and methane conversion.

Water has been shown to participate favorably in the production of methanol from methane by increasing the number of pathways towards methanol production (Badani et al., 1995). This research supports the claim that high levels of water in the plasma stream, relative to oxygen, improve methanol selectivity. Little has been written about

96 how water participates in other important reactions occurring in this type of plasma reactor.

For Phase II experiments, oxygen and water levels are at or near the experimental apparatus limhs. It is not possible to make significant changes in these two parameters without system redesign and reconstmction. Further refinement of reactant levels could continue, resulting in slight improvements in system performance. Small refinements will not solve this synthesis problem. Large-scale (i.e., order of magnitude) improvements must occur for this approach to become feasible and it is unlikely that ftirther modification of these parameters will produce significant improvement unless changes in system design incorporated. For this reason. Phase III experiments will not attempt further refinement of reactant concentration levels but concentrate on investigation of a novel approach aimed at improving system performance.

6.4 Phase III Experiments

It has been theorized by other researchers that water contributes to methanol production by increasing the number of pathways that lead to its production. Apparently, the presence of water and radicals generated from water improved the likelihood that methanol would result from free-radical reactions. This idea of increasing the number of methanol synthesis pathways led to a new approach for this research. For Phase I and II experiments, oxygen (along with argon and water) passed through the plasma region and then reacted with the methane stream. The methane stream has always consisted of pure methane. If oxygen were present in the methane stream, it would not be "activated" by the plasma and could react at a lower energy state than the more energetic oxygen in the

97 plasma stream. It seems plausible that oxygen introduced by way of the cooler methane stream would exhibit different reactivity and could potemially improve the pathways to methanol. Phase III experiments divided the oxygen into the two reactant streams to discover if this created measurable improvemems in methanol production.

A single experimem was conducted in which all of the oxygen was placed into the methane stream and isolated from the plasma conditions. In light of the other planned experiments, an experiment of this type was warranted to obtain some perspective about system reactivity when oxygen was only presem in the methane stream. This approach was not expected to produce significant conversion of methane since oxygen would not be energized in the plasma stream. The oxygen was well-mixed with the methane and the low energy state of the oxygen should have prevented appreciable reaction with methane.

This particular experiment produced some interesting results. Table 6.4 shows experimental parameters from Phase III mns.

Table 6.4. Phase III Experimental Parameters

Experiment Injection Ar/CH4 CH4/O2 CH4/H2O Methane Methanol Distance Ratio Ratio Ratio Conversion Selectivity (inches) (%) (%) 0330 0.250 7.22 4.75 1.01 1.679 1.349 0331 0.250 6.88 4.71 1.13 1.036 1.534 0403 0.250 6.88 4.05 1.54 2.189 0.345 0403-2 0.250 6.88 4.05 1.02 1.614 0.744 0405 0.250 7.22 3.48 0.95 1.142 0.766 0406 0.250 6.19 5.53 1.64 2.311 0.198 0406-2 0.250 6.88 5.53 1.07 2.056 0.225

98 6.4.1 Oxygen with Methane Stream

The first experimental mn of Phase III (mn 0330) was conducted whh all of the oxygen placed in the methane stream. Appreciable conversion of methane was not anticipated in this mn. This flow configuration did indeed achieve measurable methane conversion and respectable methanol selectivity. The run replicated the experimental conditions of a Phase II experimental mn (Phase II, mn 0213), with the noted exception that oxygen was included with the methane stream. That particular mn (0213) had achieved the highest methanol selectivity of any experimental mn.

Including oxygen in the methane stream reduced methane conversion, relative to the earlier run of Phase II. This reduction in conversion was expected because a primary reactant has been isolated from the highly reactive conditions generated in the plasma. (It should be noted that placement of oxygen in the methane stream induces greater penetration of the methane stream into the plasma zone because of the mixing effects discussed above). The high conversion and selectivity of this run implies that one, or a combination, of the following must be tme.

1. The argon and water in the plasma stream must, to a certain extent, be capable of

producing species that (a) survive long enough to contact the methane stream and (b)

are effective in oxidizing methane.

2. The methane stream, now well-mixed with oxygen and more reactive, is significantly

easier to convert.

3. The high energy of this microwave generation system "leaks" down the waveguide

"sleeve" (which houses the quartz tube) and contributes to methane conversion.

99 4. The methane stream penetrates considerably ftirther into the plasma zone than

expected. (This factor probably has limited impact since oxygen makes up less than

2.5% of the total gas flow.)

The implications of this particular experimental run were not investigated ftirther. This

experimental mn reinforces the ideas discussed earlier about extremely complicated

energy and mixing/flow considerations involved in this high pressure (~ 1 atmosphere)

plasma reaction system.

6.4.2 Oxygen Divided into Plasma and Methane Streams

There were 6 experimental runs in which oxygen was divided equally into both

the plasma stream and the methane stream. A variety of experimental conditions were

tested in the ranges that have been identified as optimum for this apparatus. The overall

performance results of this approach are shown in Figure 6.10. It is apparent that this

approach did not result in the order of magnitude performance improvements that were

desired. For reference, this figure includes the results from Phase II.

In general, Phase III experiments do not approach the performance of Phase II

experiments. As discussed above, quicker mixing should resuh from the impinging flow

approach, enabling reactions to proceed farther before conditions degrade. The theory of

improving the pathways to methanol by placing oxygen in both streams has not been

supported. Data from this research indicate that the presence of oxygen in both streams is

counterproductive to methanol production.

Several factors may contribute to the decreased performance of Phase III

experiments.

100 0.04

0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 Methane Conversion (mole reacted/mole fed)

• Phase II Experiments (injection at 1/4 inches) • Phase II Experiments (injection at 3/8 inches) A Phase III Experiments (injection at 1/4 inches;pt O oxygen in methane stream)

Figure 6.10. Phase II and Phase III-Methanol Selectivity Versus Methane Conversion

101 1. The presence of oxygen with methane means that reactive species (intermediates)

have a higher probability of being consumed by oxygen at the mixing point.

2. Any methanol that is produced will contact oxygen with greater frequency because

oxygen is present throughout the system. Methanol is easily oxidized to carbon oxide

products in the presence of oxygen.

3. The presence of oxygen mixed with methane anywhere in a highly oxidizing

environment should contribute to greater amounts of CO and CO2 produced because

these products are favored by thermodynamics.

These and other factors may make it imperative that a pure methane stream be mixed with a plasma activated stream, or combination of streams, to maximize methanol production in this and other plasma reactors.

6.5 Overall Performance Evaluation

Figure 6.11 shows the selectivity/conversion performance of all of the experimental phases of this research. The progression towards higher methanol selectivity is illustrated and performance improvements are apparent from the

Preliminary Phase through Phase II. A new approach to methanol synthesis is attempted

in Phase III, but fails to result in performance improvements. The ability of this system to produce methanol directly from methane has been demonstrated. This system compares favorably with other more advanced, low-pressure plasma systems.

102 0 0.05 0.1 0.15 0.2 0.25 0.3 Methane Conversion (mole reacted/mole fed)

• Preliminary Phase Experiments • Phase I Experiments (injection at 3/8 inches) A Phase I Experiments (injection at 5/8 inches) X Phase I Experiments (injection at 1.5 inches) D Phase II Experiments (injection at 1/4 inches) • Phase II Experiments (injectionat 3/8 inches) A Phase III Experiments (injection at 1/4 inches, pt O oxygen in methane stream)

Figure 6.11. Preliminary Phase, Phase I, II, and III-Methanol Selectivity Versus Methane Conversion (all phases)

103 6.5.1 Mixing and Flow Analysis

The rationale behind the impinging flow design of this system was to improve the mixing characteristics in the oxidation region with the goal of improving reactor performance. A simple Reynolds Number calculation was performed to gain some

insight into typical flow conditions existing in the three streams (plasma stream, methane

stream, combined stream) during experimentation. Reynolds Number calculations for

streams during run 0213 are shown in Table 6.5. The calculation assumes the lowest

possible viscosity for the streams.

Table 6.5. Reynolds Numbers for streams during Run 0213

Reynolds Number of the Reynolds Number of the Reynolds Number of the Plasma Stream (0213) Methane Stream (0213) Combined Stream (0213) (assumed to be 400°C) (assumed to be 250°C) (assumed to be 250°C) 60 35 16

It is apparent that the flow conditions present in each stream are well within the

laminar regime. The benefits of the impinging flow design used in this reactor system

should be accentuated when streams in laminar flow are mixed. Although, it is not

known whether other researchers operated under laminar or turbulent flow conditions, an

impinging flow design will always improve mixing of the plasma and methane streams at

the mixing point.

6.5.2 Material Balances

The credibility of this experimental research demands that overall material

balances are presented. It is appropriate that balance closure be presented here to support

104 findings. Although significant experimental error are present, carbon and hydrogen

element balances closed to ±25% for all experimental runs. Only 1 experimental run

exceeded 20% error (20.3%) in the carbon balance, and none exceeded 20% error in the

hydrogen balance. Of the 36 mns, 7 exceeded ±10% carbon element balance error and 5

exceeded ±10% hydrogen element balance error. All other experimental mns were

within ±10% of closure for carbon and hydrogen balances. The inability to address

oxygen balance closure is a major concem in direct methanol synthesis from methane.

This point is addressed in greater detail below. This research was conducted using

existing department equipment which necessitated some important design and

experimental limitations. The major and minor sources of error will be discussed briefly

at the end of this chapter.

6.6 Sources of Error

In any experimental research project, serious consideration of the sources of error

must occur. It is not uncommon for experimental results to contain error margins of 50-

100%. Minimizing sources of error can be an expensive proposition as more accurate

and precise equipment is purchased, experiments become more numerous, and project

duration is extended. The limited budget available for this project made it impossible to

remove some considerable error sources that were present. The major sources of error in

this experiment are listed below.

1. Small errors, or non-closure, in the oxygen balance of this chemical system (methanol

production by direct oxidation of methane) have been shown to produce large errors

105 in system performance parameters, namely methanol selectivity (Helton, 1991). The

inability of this system to even measure oxygen balance closures must be considered

the major source of error in this experiment.

2. The analysis method used in this research required that columns be interchanged

between gas and liquid analysis. Removing, replacing, and re-conditioning analytical

GC columns can create major inconsistencies in performance. Optimally, two

different gas chromatographs, with the appropriate detectors, should be used in the

analysis. This would maximize the reliability of the columns and the analysis.

3. Although every effort was made to minimize the effects of transporting samples to

the analysis system, off-line analysis must be considered another major source of

error. All activities having to do with sample handling (see Chapter V) will introduce

errors. On-line analysis would minimize these sample handling sources of error.

4. Inability to maintain constant mixing and flow characteristics must be considered a

potentially major source of error. From run to run, slight changes in flow might

create significant changes in the mixing region. As discussed above, small changes in

this mixing region may impact reaction characteristics to a greater extent than the

other experimental parameters under investigation.

Other sources of error are listed below. These are not considered "minor" but probably have less impact than those presented above.

1. The apparatus was vented to the atmosphere for simplicity. The pressure in the

reactor was approximately 0.9 atmospheres. This corresponds to the normal local

atmospheric pressure where the research was conducted. Changes in atmospheric

pressure did occur between experiments and must have affected system performance.

106 2. Accumulation of liquid product in the system. Efforts were made to minimize this

factor but, no matter how long the system "warmed-up," water balances exhibited

significant error.

3. Inability to precisely control water temperature in the water saturation unit made it

very difficult to closely control the amount of water placed into the plasma stream. It

was possible to maintain water temperature within a few degrees, but significant

changes in water content occur within a few degrees. Consequently, it was possible

to maintain water content within a certain range, but not at a precise point.

4. The errors inherent in regulators, flow controllers (rotameters), pressure gauges,

thermometers, timers, and GC calibration cannot be disregarded or ignored.

Additionally, the equipment used to constmct this apparatus was, by no means, new.

5. Concentration levels in the GC calibration gases are advertised to be ±5%.

6. Reactor/Feed gases have appreciable concentration tolerances (±5%).

107 CHAPTER VII

CONCLUSIONS AND RECOMMENDATIONS

This investigation into methanol production via an experimental plasma-based reactor operated at atmospheric pressure leads to the following conclusions:

1. Methanol production by direct oxidation of methane in a plasma reactor operated at

high pressure (approximately I atmosphere) has been demonstrated.

2. Low oxygen concentrations, relative to the concentration of methane, contribute to

increased selectivity for methanol in this system.

3. High water concentrations, relative to the concentration of oxygen, contribute to

increased selectivity for methanol in this system.

4. High pressure plasma reactor operation creates high temperature gradients, inducing

significant velocity fluctuations in plasma streams. Complicated flow and mixing

phenomena result from the substantial temperature increases in these systems.

Reactor configuration and stream mixing techniques had significant impact on system

performance. These should be primary considerations when designing a high

pressure (~1 atmosphere) plasma reactor system.

5. The high peak power microwave system used in this research was able to ignite and

sustain a plasma in an argon environment without difficulty. Problems sustaining a

plasma experienced by other researchers could be alleviated by microwave systems

with higher peak power. This type of system is ideal for plasma studies using air.

because of the high peak power of the system and its ability to maintain the plasma.

108 6. Placing oxygen in both streams (plasma and methane) had a detrimental effect on

methanol production, relative to methanol production with oxygen present in the

plasma stream only.

It is not possible to make a conclusion about the effects of higher pressure on methanol production. The higher temperatures associated with plasma at high pressure should induce less selective oxidation of methane. Methanol production in this system is somewhat less than other low pressure systems, but still comparable. This unit was not operated at low pressure. Detailed low and high pressure investigations will have to occur before a conclusion about pressure effects can be made.

Significant improvements in plasma reactor performance must occur before this approach can become useful for methanol production by direct oxidation of methane.

Investigation into reactor configuration, mixing techniques, plasma generation techniques, and high pressure operation are critical to progress in this area.

Based on the experimental and operational insight gained from this study, the following recommendations are made:

1. High pressure plasma studies should be continue in this area. High pressure

conversion is cmcial if this type of plasma reactor technology is to operate

commercially. High Pressure plasma reactor systems should be developed that can

accurately evaluate many factors, including power input, effects of variable rates of

quench, effects of using air as a reactant, effects of impurities on system performance,

pressure effects, effects of reactant concentration levels, etc. The systems should

109 possess high reliability and minimize contributions from experimental and analysis

errors.

2. Studies into innovative reactor configurations and novel mixing techniques should be

undertaken. The physical and chemical interactions that occur in high pressure

plasma reactors of this type are very complicated. Optimizing performance in high

pressure plasma reactors may require complicated reactors and creative flow systems.

For example, combination of a plasma energized stream with another oxygen

containing stream(s), followed by subsequent mixing with the methane stream, or

incorporation of a methanol selective catalyst into the plasma system.

3. Serious consideration should be given to the energy source for the generation of

plasma. This and other approaches use pulsed systems to create the plasma. It is

recommended that a continuous source be considered for plasma generation.

Conditions created by pulsed systems are highly variable because energy is added to

the system in short, but powerful bursts, after which, the energy is completely shut off

(hence the term "pulsed"). If a set of condhions exists, under which methanol

production is favorable, then those conditions should be established and maintained.

Continuous sources will create a more "steady-state" energy environment. (Note:

Considerable difficuhies exist in igniting and maintaining a plasma, from a

continuous energy source, in any environment at atmospheric pressure.)

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16 APPENDIX A

CALIBRATION OF EXPERIMENTAL

EQUIPMENT AND INFORMATION ON SYSTEM HARDWARE

The apparatus consisted of many different mechanical devices and analytical instmments which required calibration for reliable operation. This consists primarily of rotameters that controlled the flow rate of reactant stream and GC calibration.

Calibration of the GC for analysis of the gas product took place before each experimental mn and details are presented in Appendix B, which consists of sample calculations.

Specific equipment and supply information is also presented in this section.

A. 1 Calibration of Gas Rotameters

Three gas rotameters were used to control the flow of reactant gases (argon, methane, and oxygen). Methane and oxygen flows were controlled by Brooks rotameters

(Brooks R-2-15-AAA, and Brooks Flow Controller 8744A). Argon was controlled by

Brooks rotameter (Brooks 2-65). Back pressure on the rotameter was set at the regulators on the cylinders to approximately 40 psi. Downstream of the rotameters, pressure was maintained constant at 938.6 torr. This level was selected to me approximately 5 psi above normal local atmospheric pressure. The temperature was not controlled in the vent hood, but did not ever vary by more than 1.5° from 22°C. Several flow measurements

(50) were collected with a bubblemeter and stopwatch for each flow rate represented on each calibration plot. Volumetric flows were converted to molar flows assuming the

Ideal Gas Law applies. The average of the flows was plotted and a trendline applied.

117 Calibration curves for argon, methane, and oxygen are shown in Figures A.l, A.2, and

A.3.

Calibration for methanol product was obtained by preparation of standard solutions for multiple injection into the GC. As expected, large variations were observed.

It was decided to make a large number of injections to minimize instmment error. Four different concentrations were tested. The calibration for methanol is shown in Figure

A.4.

As mentioned above, calibration for the gas samples occurred before every experiment. Two different calibration gases (Scott Specialty Gases Mix 234 and Mix

216) were injected prior to experimental runs. The output from these calibration runs was used to analyze the data for that particular experiment. Frequent changing of the GC columns and re-conditioning necessitated this daily re-calibration. This method should

have minimized errors. The concentration of components present in the reactor effluent

gas stream was calculated using the output from the daily calibration injections. Details

of how this was accomplished are presented in the sample calculation section (Appendix

B).

118 40 1

o o o X! C I J3

93 a

Flowmeter reading

Figure A. 1. Argon Rotameter Calibration Plot (back pressure of 40 psig, downstream pressure of 5 psig, and 22°C).

119 Flowmeter readmg

Figure A.2. Methane Rotameter Calibration Plot (back pressure of 40 psig, downstream pressure of 5 psig, and 22°C).

120 o o o X c I

93

30 40 50 60 Flowmeter Reading

Figure A.3. Oxygen Rotameter Calibration Plot (back pressure of 40 psig, downstream pressure of 5 psig, and 22°C).

121 10000000 1 i i 1 ; i ! 1 1 i 1 I ' i Mil 1 J III'' <1 T 1 I'll! 1000000 i \X\ \^^ • M Ii Jr' 1 1 j II I 1 1 1 in42^\ !

1

1 ! 1 \ \ Jr 1 1—1- : ; 1 100000 , i y) 1- 1 1 1 1 1 i i 1 i/ 1 1 i 1 o i ! j 1 i 1 1 93 j

10000 - 1 h- ! u 1

1 1

1 i

1 -U— —h- 1000 l/r - 1 1 ' 1 ! i' B 1—^— ! ! I i 1 ill 1 1 1 1 i : 1 1 ' '1

; 1 1 1 ^" 1 1 100 — 10000 100000 10 100 1000 Concentration (ppm)

Figure A.4. Methanol Calibration Plot (multiple injections at four different concentrations).

122 APPENDIX B

SAMPLE CALCULATIONS

This section describes how quantities were calculated from the measured variables. All of the experimental data that was measured during each experimental mn is given in Appendix C. All of the calculations that are required to obtain the methane conversion and methanol selectivity will be outlined. These two quantities are the most important for this research.

B. 1 Molar Reactor Feed Rates

The total molar feed rate is obtained by simply summing the molar feed rates of all components. Streams are assumed to behave as ideal gases throughout the analysis.

total moles into reactor moles argon moles methane 1 r + S time time time J (B.l) f moles oxygen 1 moles water 1 + [ time J 1 time J

The amount of water being evaporated into the feed stream deserves special attention. Multiple experiments were conducted without plasma to discover the amount of water that was being evaporated into the system. Early estimations were based on assuming that feed gas leaving the water sattiration unit was sattirated with water the temperature in the headspace of the unit. Experiments revealed that gas leaving the unit was, on the average 90% saturated. It was assumed that this was due to the relatively short exposure of the stream to the hot liquid. The molar flow rate of the water into the

123 reactor was calculated from the molar flow rate of the plasma stream that was passing through the saturation unit and the temperature of the headspace above the unit. Since the mole fraction of water in the stream is known (because of our assumption of 90% stream saturation at the headspace temperature), a simple mass balance is used to calculate the amount of water fed to the reactor. The results of that mass balance are below. Let Y be the mole fraction of water in the stream,

moles water [ moles gas through unit X S (B.2) time 1 Ume 1(1-Y) J

B.2 Reactor Effluent

The amount of material leaving the reactor was calculated from the two effluent

streams, gas and liquid. The volumetric flowrate of gas leaving the reactor was measured

with a bubblemeter and stopwatch, as described in Chapter V. The ideal gas law was

used to obtain the molar flowrate of gas. This effluent molar flowrate is vital to the

results of this study.

moles gas out volume of effluent atmospheric pressure ^ X S (B.3) = i time R X Temperature time where R is the gas constant.

The liquid product flowrate is simply measured at the end of the experimem and

is applied to the entire duration of the mn. The liquid product consists of many products,

but is essentially water. All non-water components added together do not appmach 1%.

124 It is important to maintain units in moles because this is how the analysis system measures concentration. Both detectors measure a response that is based on molar amount (i.e., a flame ionization detector or a thermal conductivity detector will not

indicate triple the response for a compound that is three times heavier.)

moles water out f mass collected 1 _ J s X < (B.4) time I time J [molecular weight

B.3 Calculation of Effluent Component Concentrations

The GC analysis of the liquid and gas products creates a response for each

detected component. The primary components of interest are CO, CO2, and methanol.

These three components contain most of the carbon that is converted in the reactor. Since

this system is unable to track oxygen, analysis of carbon-containing products must

provide us with the required information.

The carbon-containing components present in the gas phase (primarily CO, CO^,

and methane) produce a response from the GC. This response is compared to the

response from the known concentration of the standard calibration gas injected prior to

the experimental mn. Componem concentration (cone.) was calculated like this.

. f cone, (ppm) of standard | ^^^^ cone, (ppm) = {component area counts) x | j.^^ ^rea counts of standard J

Liquid concentrations of methanol and other products were calculated in a similar

mamier. Other than methanol, there were few products in the liquid phase. Traces of

fomiic and acetic acid were detected and quantified to the highest extent possible.

125 standard solutions of these components were mixed and injected in the GC. Liquid concentration was calculated in the following way.

liquid cone, (ppm) = {area coums} x j cone, of standard 1 [ area counts of standard J ^ ^^

Knowing the total molar flowrate of the liquid stream and the concentration of the components in the stream, the molar flowrate of the componems is obtained.

moles of component /total molar flowrate! tJm^ ~ component cone, (ppm) x < ^""^ [ time (B.7)

This consideration does not take into account the contribution of the non-water constituents in the liquid. These components will not significantly affect the calculation of the total molar flows since their concentration is so small (<1000 ppm or 0.1 molar %).

B.4 Material Balances

Since all of the flowrate data is known, it is a simple matter to compare the amounts entering and exiting the system. To compute a carbon balance, the amount entering was calculated from the rotameter reading and the known feed composition. The amount leaving was calculated from the effluent flowrate and the concentration. The amount entering was compared to the amount exiting, revealing the mass balance closure.

Carbon balances were closed to within ±10% for more than 80% of the experimental runs. Only 7 of the 36 experimental mns exceeded ±10% error in the carbon balance.

Only 5 of the 36 runs exceeded ±10% error in the hydrogen balance. One run exceeded

126 ±20% in carbon balance error (20.4%) and no mns exceeded ±20% error in the hydrogen balance error.

B.5 Calculation of the Conversion and Selectivity

These are the two most important quantities for this research. The high degree of variation in the GC analysis made it impossible to use differences in methane concentration as a basis for conversion. With conversion levels in the 1% regime and low

GC reproducibility, small errors in methane concentration measurement (±5%) can easily obscure the results. It was necessary to use the observed carbon-containing products as the basis for conversion. Methane conversion was calculated by

S all carbon - containing products not present in feed time (B.S) conversion = i methane fed to reactor/ /time

The calculation for methanol selectivity was dependent of this definition of conversion that was used out of necessity. In short, methanol selectivity is an expression for the amount of methane that reacts to produce methanol divided by the total amount of methane that reacts. It is defined in this research as

methanol produced time (B.9) selectivity = I all carbon containing products not present in feed 'time J

These two quantities contain most of the valuable information about the performance of this system and are the focal points of the analysis and the performance.

127 APPENDIX C

RAW EXPERIMENTAL DATA

This section contain all of the information that was recorder during the experimental mns. All of the data used in the analysis of this research was generated from the information in this section.

128 Table C.l. Preliminary Phase and Phase I Raw Experimental Data

Experiment Experiment Experiment Temperature Argon Methane Oxygen Atmospheric Phase Name Duration Ambient Flowrate Flowrate Flowrate Pressure (min) (C) (reading) (reading) (reading) (torr) Prelim 925 62.75 21 1.8 6.6 0 685.7 Prelim 930 60 23 1.8 6.6 0 681.9 Prelim 1008 77 24 1.8 6.6 0 679 Prelim 1009 75 21 1.8 6.6 0 684 Prelim 1013 63 21 1.8 6.6 0 686.9 Prelim 1014 48.5 23.5 1.8 6.6 0 690 Prelim 1030 63 23.6 1.8 3.96 0 678.8 Prelim 1103 87 24.1 1.8 4 0 685 Prelim 1104 65 23.4 1.8 3.9 0 681.4 Prelim 1106-1 67 23.7 1.8 4 0 689.1 Prelim 1106-2 60 23.9 1.8 4 0 686.5

1118 50 24 1.8 6.6 19.5 685.6 1124 59 24.3 1.8 6.6 15 685.3 1125-1 60 24 1.8 6.6 29.5 683.7 1125-2 48 24.3 1.8 6.6 30 680.7 1126 60 24 1.8 6.6 20 683.3 1129 58 24.1 1.8 6.6 15 679 124 60 21.8 1.8 6.6 30 679.4 1201-1 60 24.2 1.8 6.6 30.65 680.5 1201-2 64 24.4 1.8 6.6 20 677.7 1202 59 24 1.8 6.6 15 677.5

129 Table c.l. (com.)

Experiment Experiment Injection Temperature Product Gas Prod. Time req. Methanol Phase Name Distance Water Sat. mass vol. basis to collect Liq. Prod. (inches) Headspace (collected) (ml) basis (raw (C) (sec) area cts.) Preli m 925 1.5 310.71 1.9791 90 6.476 185 Prelim 930 0.75 313.15 2.3289 90 6.382 4003 Preli m 1008 0.25 311.21 2.319 90 6.338 20101 Prel m 1009 -0.25 311.483 1.3531 90 5.924 5829 Prel im 1013 0 312.94 1.8817 90 6.196 17056 Prel im 1014 0.125 312.5 1.7141 90 6.164 22319 Prel im 1030 0.5 311.76 2.0407 90 6.4547 17205 Prel im 1103 0.125 313.44 2.7722 90 6.4284 23415 Prel im 1104 0.25 314.02 2.2198 90 6.6355 49855 Prel im 1106-1 0.3125 312.72 2.1068 90 6.648 36121 Prelim 1106-2 0 313.15 1.9835 90 6.4635 17543

I 1118 0.375 314.21 3.0099 90 6.2947 96590 I 1124 0.375 313.18 3.0631 90 6.3095 105666 I 1125-1 0.375 312.47 3.0522 90 6.221 193948 I 1125-2 0.625 316.18 2.425 90 6.0453 38766 I 1126 0.625 314.62 2.9808 90 6.164 19571 I 1129 0.625 311.69 2.5454 90 6.1953 36487 I 124 0.625 315.65 2.2691 90 6.0055 50781 I 1201-1 1.5 312.41 2.8375 90 6.056 962 I 1201-2 1.5 312.59 3.1111 90 6.0987 353 I 1202 1.5 313.32 2.8208 90 6.164 491

130 Table c.l. (com.)

Experimental Experiment Hydrogen Carbon Methane Carbon Acetylene Ethylene Phase Name Vap. Prod, Monoxide Effluent Dioxide Effluent Effluent (raw (raw (raw (raw (raw (raw area cts.) area cts.) area cts.) area cts.) area cts.) area cts.) Preli m 925 0 0 599694 824 0 836 Preli m 930 0 700 738742 2033 0 1081 Prelii m 1008 1400 9303 641752 7185 0 1252 Prel m 1009 8000 112565 372466 12685 10202 2814 Prel im 1013 5411 52912 588956 15985 3605 2768 Prel im 1014 3347 31834 563546 14107 1732 2111 Prel im 1030 512.5 7112 583390 8771 0 ^1368.5 Prel im 1103 2607.5 49673 489567 23184 1920.5 2120 Prel im 1104 1871.5 31886 601273 19259 1117 2282 Prel im 1106-1 1445 23546 527686 14542 487 1490 Prelim 1106-2 3650 87888 507878 27693 3362 2691

I 1118 577 58506 711122 81520 1572 3105 I 1124 489 23312 631391 57806 0 1986.5 I 1125-1 253 18354 688873 49721 0 1891.5 I 1125-2 0 505.5 729281 964.5 0 1773 I 1126 0 4749 692523 18164 0 1205 I 1129 0 3630.5 660906 11949 0 1680 I 124 0 2800 759315 7292 0 2171 I 1201-1 0 0 705438 422 0 1441 I 1201-2 0 0 711898 154 0 1610 I 1202 0 0 664591 171 0 1767

131 Table c.l. (com.)

Experiment Experiment Ethane Formic Acid Acetic Acid Phase Name Effluent Liq. Prod Liq. prod (raw (raw (raw area cts.) area cts.) area cts.) Prelim 925 4223 0 0 Prelim 930 6610 3082 0 Prelim 1008 6048 2201 0 Prelim 1009 4545 1975 0 Prelim 1013 6926 0 0 Prelim 1014 6634 0 0 Prelim 1030 5457 10788 496 Prelim 1103 5609.5 19329 4804 Prelim 1104 6839.5 20215 5344 Prelim 1106-1 5672 13169 3956 Prelim 1106-2 5635.5 8601 2081

I 1118 8038 25653 11019 I 1124 7012 23780 13372 I 1125-1 7094.5 24542 17894 I 1125-2 6270 16694 1985 I 1126 6204 9613 864 I 1129 5712 10389 1423 I 124 6683 47800 5276 I 1201-1 5863 10641 0 I 1201-2 6060 12069 0 I 1202 5826 12890 0

132 Table C.2. Phase II and Phase III Raw Experimental Data

Experiment Experiment Experiment Temperature Argon Methane Oxygen Atmospheric Phase Name Duration Ambient Flowrate Flowrate Flowrate Pressure (min) (C) (reading) (reading) (reading) (torr) II 213 50 23.7 1.8 6 10 682.1 II 215 53 23.9 1.75 6 10 670.1 II 217 61 22.9 2 6 10 672.5 II 218 70 23.4 1.99 6 12 676 II 225 75 22.9 2 6 10 670.7 II 2261 60 23 1.99 6 12 673 II 2262 60 23.4 1.94 6 12 673.1 II 2263 58 22.8 1.95 6 10 673.7

III 330 60 23.2 1.8 6 10 674.5 III 331 52 23.4 2 7 15 675.5 III 403-1 57 23.1 2 7 20 678.3 III 403-2 55 23.4 2 7 20 681.8 III 405 52 22.6 1.8 6 20 675 III 406-1 66 23 1.8 7 10 671.1 III 406-2 60 22.6 2 7 10 670.4

133 Table C.2. (cont.)

Experiment Experiment Injection Temperature Product Gas prod. Time req. Methanol Phase Name Distance Water Sat. mass vol. basis to collect Liq. Prod. (inches) Headspace collected (ml) basis (raw (C) grams (sec) area cts.) II 213 0.375 321.94 3.6714 90 6.342 29184 II 215 0.375 324.82 4.1285 90 6.337 21714 II 217 0.375 309.21 2.4591 90 5.644 97427 II 218 0.375 309.13 2.7689 90 5.729 101706 II 225 0.25 311.4 3.6509 90 5.5913 116601 II 2261 0.25 308.75 2.5316 90 5.704 127289 II 2262 0.25 321.71 4.6587 90 5.776 41577 11 2263 0.25 322.02 4.649 90 5.746 45068

III 330 0.25 322.27 3.9546 90 6.146 46398 III 331 0.25 321.19 3.4089 90 5.4413 37108 III 403-1 0.25 315.68 2.859 90 5.543 21485 III 403-2 0.25 323.04 4.3849 90 5.5265 21517 III 405 0.25 323.15 3.5977 90 6.2225 14746 III 406-1 0.25 316.31 3.0756 90 6.012 13152 III 406-2 0.25 321.99 4.6427 90 5.494 7464

134 Table C.2. (com.)

Experiment Experiment Hydrogen Carbon Methane Carbon Acetylene Ethylene Phase Name Vap. Prod. Monoxide Effluent Dioxide Effluent Effluent (raw (raw (raw (raw (raw (raw area cts.) area cts.) area cts.) area cts.) area cts.) area cts.) II 213 0 1350 747385 2999.5 0 2205 II 215 0 1544 633088 5333 0 2153 II 217 0 5283 627575 13743 0 2084.5 II 218 0 8632.5 574030 23192 0 2049 II 225 0 13074 684214 29844 0 2662 11 2261 0 16636 646539 40172 0 2738 II 2262 0 6368 833603 16791 0 2898 II 2263 0 13674 692444 34328 0 2935

III 330 0 7946 771093 10511 0 2881 III 331 0 4720 713577 6523 0 2567 III 403-1 0 13127 764574 13266 0 3233 III 403-2 0 7267 803205 9560 0 3313 III 405 0 5147 719941 7734 0 2914 III 406-1 0 14434 818502 14453 0 3760 III 406-2 0 11366 755515 12153 0 3436

135 Table C.2. (cont.)

Experiment Experiment Ethane Formic Acid Acetic Acid Phase Name Effluent Liq. prod Liq. prod (raw (raw (raw area cts.) area cts.) area cts.) 11 213 6674.5 957.2 0 II 215 5605 783.6 0 II 217 5886 287.8 728.4 II 218 5539 1470.2 1354.2 II 225 7023 13254 7713 II 2261 6939 15483 11973 II 2262 7961 9798 1986 II 2263 6977 7079 2784

III 330 7435 5990 3264 III 331 6647 6432 1182 III 403-1 7485 3121 4264 III 403-2 7750 4974 1880 III 405 6847 2869 1472 III 406-1 8114 2151 4648 III 406-2 7540 310 4789

136