The Use of Ammonium Carbamate As a High Specific Thermal Energy Density Material for Thermal Management of Low Grade Heat

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THE USE OF AMMONIUM CARBAMATE AS A HIGH SPECIFIC THERMAL ENERGY DENSITY MATERIAL FOR THERMAL MANAGEMENT OF LOW GRADE HEAT

Thesis Submitted to The School of Engineering of the University of Dayton

In Partial Fulfillment of the Requirements for The Degree of Master of Science in Chemical Engineering

By Joel Edward Schmidt Dayton, OH August 2011

THE USE OF AMMONIUM CARBAMATE AS A HIGH SPECIFIC THERMAL

ENERGY DENSITY MATERIAL FOR THERMAL MANAGEMENT OF LOW

GRADE HEAT

Name: Schmidt, Joel Edward

APPROVED BY:

Kevin J. Myers, D.Sc., P.E. Douglas S. Dudis, Ph.D. Advisory Committee Chairman Research Advisor Professor, Chemical and Materials Principal Research Chemist Engineering Department Air Force Research Laboratory

Robert J. Wilkens, Ph.D., P.E. Committee Member Associate Professor, Chemical and Materials Engineering Department

John G. Weber, Ph.D. Tony E. Saliba, Ph.D. Associate Dean Dean, School of Engineering School of Engineering & Wilke Distinguished Professor

ABSTRACT

THE USE OF AMMONIUM CARBAMATE AS A HIGH SPECIFIC THERMAL

ENERGY DENSITY MATERIAL FOR THERMAL MANAGEMENT OF LOW

GRADE HEAT

Name: Schmidt, Joel Edward University of Dayton

Research Advisor: Dr. Douglas Dudis

The specific energy storage capacities of phase change materials (PCMs) increase

with temperature, leading to a lack of thermal management (TM) systems capable of

handling high heat fluxes in the temperature range from 20°C to 100°C. State of the art

PCMs in this temperature range are usually paraffin waxes with energy densities on the order of a few hundred kJ/kg or ice slurries with energy densities of the same magnitude.

However, for applications where system weight and size are limited, it is necessary to

improve this energy density by at least an order of magnitude. The compound ammonium

carbamate (AC), [NH4][H2NCOO], is a solid formed from the reaction of ammonia and

carbon dioxide which endothermically decomposes back to ammonia and carbon dioxide

in the temperature range of 20°C to 100°C with an enthalpy of decomposition of 2,010

kJ/kg. Various methods to use this material for TM of low-grade, high-flux heat have

iii

been evaluated including: bare powder, thermally conductive carbon foams, thermally

conductive metal foams, hydrocarbon based slurries, and a slurry in ethylene glycol or

propylene glycol. A slurry in glycol is a promising system medium for enhancing heat and mass transfer for TM. Small-scale system level characterizations of AC in glycol

have been performed and results indicate that AC is indeed a promising material for TM

of low-grade heat. It has been shown that pressures on the order of 200 torr will achieve

rapid decomposition and thermal powers of over 300 W at 60°C have been found,

demonstrating the capability of AC.

I would like to dedicate my work to my father and mother and thank them for all of the support they have always given me and for serving as an example to follow. I know that

without them I would not have completed this process.

ACKNOWLEDGEMENTS

I would first like to thank the Air Force Research Laboratory for providing the funding

for this work as well as extensive technical expertise. This work was performed at the

Thermal Sciences and Materials Branch in the Materials and Manufacturing Directorate of the Air Force Research Laboratory at Wright Patterson Air Force Base. I would

specifically like to thank Dr. Douglas Dudis (AFRL/RXBT) for serving as the research

advisor for the thesis work and Dr. Karla Strong (AFRL/RXBT) for assuring the funding

was in place for the project. Additionally, I would like to thank Dr. Douglas Miller

(AFRL/RXBT) for all of his assistance with the effort. Dr. Soumya Patnaik

(AFRL/RZPS) and Stephen Emo (AFRL/RZPS) provided great collaborations and advice

for the project, especially in scale-up efforts. I would like to thank Dr. Kevin Myers for

serving on my committee as well as for serving as my academic advisor and I would like

to thank Dr. Robert Wilkens for serving on my committee.

TABLE OF CONTENTS

ABSTRACT ...... iii

LIST OF FIGURES ...... x

LIST OF TABLES ...... xv

LIST OF ABBREVIATIONS AND SYMBOLS ...... xviii

CHAPTER I: INTRODUCTION ...... 1

1.1. Overview of Current Thermal Management Solutions ...... 2

1.1.1. Phase Change Materials ...... 2

1.1.1.1. Water ...... 2

1.1.1.2. Ammonia...... 3

1.1.1.3. Carbon Dioxide ...... 3

1.1.1.4. Paraffin Waxes ...... 4

1.1.2. Chemical Reaction Systems ...... 5

1.1.2.1. Gas-Gas Reaction Systems ...... 6

1.1.2.2. Gas to Solid Reactions ...... 8

1.2. Ammonium Carbamate Background ...... 11

1.2.1. Kinetics of Ammonium Carbamate Formation and Decomposition ...... 14

1.2.2. Previous Uses of Ammonium Carbamate for Thermal Management ...... 15

vii

1.2.3. Ammonium Carbamate for Thermal Management ...... 18

1.2.4. Activation Energy of Decomposition ...... 19

CHAPTER II: EXPERIMENTAL METHODOLOGY AND ANALYSIS ...... 21

2.1. Thermal Management Proof of Concept ...... 21

2.2. Thermal Conductivity ...... 22

2.3. System Concept Evaluation ...... 24

2.3.1. Metal and Carbon Foams ...... 24

2.3.2. Liquid Ammonia Evaluation ...... 26

2.3.3. Additional Solvents ...... 29

2.3.4. Aluminum Foam ...... 32

2.3.5. Foam Impregnation Summary ...... 36

2.3.6. Heat Transfer Fluid Evaluation ...... 36

2.3.6.1. Experimental Procedure ...... 38

2.3.6.2. Heat Transfer Fluid Analysis ...... 39

2.3.7. Ammonium Carbamate Materials Compatibility...... 49

2.3.8. Hysteresis ...... 50

2.3.9. Heat of Solution ...... 52

2.3.10. Born-Harber Cycle for Ammonium Carbamate ...... 56

2.3.11. Specific Heat of Ammonium Carbamate ...... 57

2.4. Decomposition Pressure...... 59

2.5. Ammonium Carbamate Decomposition Test System ...... 62

2.5.1. Reactor Sizing ...... 63

2.5.2. Vacuum Pump Selection ...... 65

viii

2.5.3. Vacuum Controller...... 66

2.5.4. Vacuum Gauge Selection ...... 67

2.5.5. Temperature Measurement ...... 67

2.5.6. Tubing Selection ...... 68

2.5.7. Simulated Thermal Load ...... 68

2.5.8. Completed Experimental Apparatus ...... 68

2.6. Ammonium Carbamate Decomposition Test System Experimental Work ...... 69

2.6.1. Decomposition Test System Experimental Procedure ...... 71

2.6.2. Experimental Data Analysis ...... 72

2.6.3. Discussion of Experimental Results ...... 73

2.6.4. Conclusions from Ammonium Carbamate Decomposition System Tests .. 78

CHAPTER III: CONCLUSIONS AND FUTURE WORK ...... 80

REFERENCE LIST ...... 81

APPENDIX ...... 93

LIST OF FIGURES

Figure 1. Dissociation pressure of ammonium carbamate ...... 13

Figure 2. Thermal camera image showing thermal management potential of ammonium

carbamate ...... 22

Figure 3. Vessel used to impregnate carbon foam with methanol and ammonium

carbamate ...... 31

Figure 4. Duocel aluminum foam ...... 33

Figure 5. Ammonium carbamate in aluminum foam ...... 35

Figure 6. Schematic of system used to measure the rate of decomposition of ammonium

carbamate ...... 38

Figure 7. Comparison of ammonium carbamate conversion in ethylene glycol, tetraglyme

and bare powder at two times solubility and 30°C ...... 40

Figure 8. Comparison of ammonium carbamate conversion in ethylene glycol and

tetraglyme at four times solubility and 30°C ...... 40

Figure 9. Conversion as a function of time for ammonium carbamate in ethylene glycol at

two times its solubility limit ...... 42

Figure 10. Conversion as a function of time for ammonium carbamate powder and

ammonium carbamate in ethylene glycol solution at 40°C ...... 44

Figure 11. Conversion as a function of time for ammonium carbamate powder and

ammonium carbamate in ethylene glycol solution at 30°C ...... 44

Figure 12. Conversion as a function of time for ammonium carbamate powder and

ammonium carbamate in ethylene glycol solution at 20°C ...... 45

Figure 13. Conversion as a function of time for ammonium carbamate powder and

ammonium carbamate in ethylene glycol solution at 10°C ...... 45

Figure 14. Comparison of different initial loadings of ammonium carbamate in ethylene

glycol at 30°C ...... 47

Figure 15. Rate of thermal power versus conversion for the decomposition of ammonium

carbamate in ethylene glycol at 30°C ...... 48

Figure 16. Comparison of ammonium carbamate in ethylene glycol and propylene glycol

at 30°C ...... 49

Figure 17. Conversion as a function of time for the hysteresis measurements of

ammonium carbamate in ethylene glycol at 30°C ...... 52

Figure 18. Heat of solution of ammonium carbamate in ethylene glycol as a function of

molality ...... 55

Figure 19. Born-Harber cycle for the decomposition of ammonium carbamate ...... 57

Figure 20. Temperature dependent specific heat of ammonium carbamate ...... 59

Figure 21. Experimental decomposition pressure of ammonium carbamate powder ...... 60

Figure 22. Decomposition pressure of ammonium carbamate in ethylene glycol ...... 62

Figure 23. Ammonium carbamate decomposition test system ...... 63

Figure 24. Experimental test apparatus ...... 69

Figure 25. Decomposition of ammonium carbamate in reactor ...... 71

Figure 26. Temperatures and heat rejected for ammonium carbamate in propylene glycol

at 30°C ...... 95

Figure 27. Pressure and ΔT for ammonium carbamate in propylene glycol at 30°C ...... 95

Figure 28. Temperatures and heat rejected for ammonium carbamate in ethylene glycol at

30°C ...... 96

Figure 29. Pressure and ΔT for ammonium carbamate in ethylene glycol at 30°C ...... 96

Figure 30. Temperatures and heat rejected for ammonium carbamate in propylene glycol

at 45°C ...... 97

Figure 31. Pressure and ΔT for ammonium carbamate in propylene glycol at 45°C ...... 97

Figure 32. Temperatures and heat rejected for ammonium carbamate in ethylene glycol at

45°C ...... 98

Figure 33. Pressure and ΔT for ammonium carbamate in ethylene glycol at 45°C ...... 98

Figure 34. Temperatures and heat rejected for ammonium carbamate in propylene glycol

at 50°C ...... 99

Figure 35. Pressure and ΔT for ammonium carbamate in propylene glycol at 50°C ...... 99

Figure 36. Temperatures and heat rejected for ammonium carbamate in propylene glycol

at 55°C ...... 100

Figure 37. Pressure and ΔT for ammonium carbamate in propylene glycol at 55°C ..... 100

Figure 38. Temperatures and heat rejected for ammonium carbamate in propylene glycol

at 60°C ...... 101

Figure 39. Pressure and ΔT for ammonium carbamate in propylene glycol at 60°C ..... 101

Figure 40. Temperatures and heat rejected for ammonium carbamate in propylene glycol

at 60°C with two pressure settings ...... 102

xii

Figure 41. Pressure and ΔT for ammonium carbamate in propylene glycol at 60°C at two

pressures ...... 102

Figure 42. Temperatures and heat rejected for ammonium carbamate in ethylene glycol at

60°C ...... 103

Figure 43. Pressure and ΔT for ammonium carbamate in ethylene glycol at 60°C ...... 103

Figure 44. Temperatures and heat rejected for ammonium carbamate in ethylene glycol at

60°C at two pressures ...... 104

Figure 45. Pressure and ΔT for ammonium carbamate in ethylene glycol at 60°C at two

pressures ...... 104

Figure 46. Temperatures and heat rejected for ammonium carbamate in propylene glycol

at 60°C at three pressures ...... 105

Figure 47. Pressure and ΔT for ammonium carbamate in propylene glycol at 60°C at

three pressures ...... 105

Figure 48. Temperatures and heat rejected for ammonium carbamate in ethylene glycol at

60°C at three pressures ...... 106

Figure 49. Pressure and ΔT for ammonium carbamate in ethylene glycol at 60°C at three

pressures ...... 106

Figure 50. Temperatures and heat rejected for ammonium carbamate in propylene glycol

at 60°C with double ammonium carbamate loading ...... 107

Figure 51. Pressure and ΔT for ammonium carbamate in propylene glycol at 60°C with

double ammonium carbamate loading ...... 107

Figure 52. Temperatures and heat rejected for ammonium carbamate in propylene glycol

at 60°C with reused propylene glycol ...... 108

xiii

Figure 53. Pressure and ΔT for ammonium carbamate in propylene glycol at 60°C with

reused propylene glycol ...... 108

Figure 54. Temperatures and heat rejected for ammonium carbamate in propylene glycol

at 60°C with twice reused propylene glycol ...... 109

Figure 55. Pressure and ΔT for ammonium carbamate in propylene glycol at 60°C with

twice reused propylene glycol ...... 109

xiv

LIST OF TABLES

Table 1. Chemical reaction heat storage working pairs ...... 7

Table 2. Heat of reaction of ammonium carbamate ...... 18

Table 3. Energy densities of common thermal management materials ...... 19

Table 4. Activation energy of decomposition of ammonium carbamate ...... 20

Table 5. Thermal conductivity of ammonium salts ...... 23

Table 6. Solubilities in liquid ammonia at 25°C (g/100 g ammonia)85 ...... 27

Table 7. Solubility of ammonium carbamate in methanol ...... 30

Table 8. Dimensions and masses of aluminum foam samples ...... 34

Table 9. Temperature-time records for ammonium carbamate in aluminum foam ...... 35

Table 10. Solubility of ammonium carbamate in ethylene glycol ...... 37

Table 11. Amounts of ammonium carbamate and tetraglyme added for decomposition

studies ...... 41

Table 12. Error analysis for ammonium carbamate decomposition ...... 43

Table 13. Initial amounts of ethylene glycol and ammonium carbamate to test rate of

reaction at different initial loadings of ammonium carbamate in ethylene glycol

...... 47

Table 14. Hysteresis measurements of ammonium carbamate in ethylene glycol at 30°C

...... 51

Table 15. Experimental details and results for the heat of solution of ammonium chloride

in water ...... 53

Table 16. Uncertainty in the heat of solution calculation ...... 54

Table 17. Experimental data for determination of heat of solution of ammonium

carbamate in ethylene glycol ...... 56

Table 18. Specific heat of ammonium carbamate96 ...... 58

Table 19. Test matrix for ammonium carbamate system (see key and text for explanation)

...... 65

Table 20. Summary of ammonium carbamate test system experimental data ...... 73

Table 21. Experimental data for ammonium carbamate in propylene glycol at 30°C ...... 95

Table 22. Experimental data for ammonium carbamate in ethylene glycol at 30°C ...... 96

Table 23. Experimental data for ammonium carbamate in propylene glycol at 45°C ...... 97

Table 24. Experimental data for ammonium carbamate in ethylene glycol at 45°C ...... 98

Table 25. Experimental data for ammonium carbamate in propylene glycol at 50°C ...... 99

Table 26. Experimental data for ammonium carbamate in propylene glycol at 55°C .... 100

Table 27. Experimental data for ammonium carbamate in propylene glycol at 60°C .... 101

Table 28. Experimental data for ammonium carbamate in propylene glycol at 60°C with

two pressure settings ...... 102

Table 29. Experimental data for ammonium carbamate in ethylene glycol at 60°C ...... 103

Table 30. Experimental data for ammonium carbamate in ethylene glycol at 60°C at two

pressures ...... 104

xvi

Table 31. Experimental data for ammonium carbamate in propylene glycol at 60°C at

three pressures ...... 105

Table 32. Experimental data for ammonium carbamate in ethylene glycol at 60°C at three

pressures ...... 106

Table 33. Experimental data for ammonium carbamate in propylene glycol at 60°C with

double AC loading ...... 107

Table 34. Experimental data for ammonium carbamate in propylene glycol at 60°C with

reused propylene glycol ...... 108

Table 35. Experimental data for ammonium carbamate in propylene glycol at 60°C with

twice used propylene glycol ...... 109

xvii

LIST OF ABBREVIATIONS AND SYMBOLS

AC Ammonium carbamate CFCs Chlorofluorocarbons Cp Specific heat EG Ethylene glycol HCFCs Hydrochlorofluorocarbons k Reaction rate constant m Mass flow rate mt Total mass of solvent n Number of moles P Pressure PCMs Phase change materials Peq Equilibrium pressure PG Propylene glycol Q Rate of heat flow rnet Net rate of reaction T Temperature TES Thermal energy storage TM Thermal management wx Error of a measurement ΔH Change in enthalpy ΔHm Heat of solution at a known molality ∆H° Standard heat of reaction ° ΔH∞ Standard heat of solution at infinite dilution ΔT Temperature change ΦC Apparent molal heat capacity of a solute ΦLm Correction factor for the heat of solution at infinite dilution

xviii

CHAPTER I: INTRODUCTION

Ever increasing power loads for various electronic devices have created a demand

for novel thermal management (TM) technologies to allow these devices to operate in

their ideal temperature ranges to ensure efficiency and lifetime. Most electronic devices

need to operate between 20°C and 100°C. Cooling these devices with air or conventional

liquid coolants can be energy intensive, require a large TM system, or even be impossible

with high thermal fluxes.1 One class of materials being explored to stabilize these devices

is graphitic foams impregnated with paraffin wax phase change materials (PCMs).1-6

However, the growth in heat fluxes and thermal loads is outpacing the capability of conventional cooling systems.7,8 This TM problem becomes further complicated when

these high-flux loads are confined to weight and volume limited environments.

Therefore, the need exists to investigate novel methods for the TM of high heat fluxes

near room temperature.

The TM challenge which will be explored in this thesis, herein referred to as this

work, is one which has the following characteristics. The system of this work will be

capable of maintaining temperature in the range of 30°C to 60°C and amenable to volume

and weight limited applications. It will also operate on a reasonable time scale, defined

by a high thermal power, and will scale well. For this work all materials and proposed

system architectures will be evaluated based on these benchmark characteristics.

1.1. Overview of Current Thermal Management Solutions

Current material options for TM include traditional fluids such as water, ammonia

(NH3), or carbon dioxide (CO2), commercially available PCMs such as paraffin waxes, alternatives like metal hydride decomposition reactions, and other chemical reaction and sorption technologies that are in the experimental phase. This section gives a brief overview of these various TM techniques.

1.1.1. Phase Change Materials

The following materials rely on either a solid-liquid phase change, solid-gas phase

change or liquid-gas phase change for TM.

1.1.1.1. Water

Water is the best known and most widely used PCM and TM material. For TM

purposes water has exceptional properties for its phase changes. At atmospheric pressure

the liquid-gas phase change has a latent heat of 2452 MJ/m3 or 2450 kJ/kg while its latent

heat of melting is 306 MJ/m3 or 330 kJ/kg.9 The usable temperature range can be extended beyond the normal liquid range of 0°C to 100°C by using pressure to elevate the vaporization temperature or creating water-salt solutions to lower the melting point.

Water, and to a greater extent salt solutions, suffer from corrosion issues in application.

Additionally, water is difficult to use for this work since keeping a device at room

temperature relies only on the specific heat of water and not the much higher energy

associated with vaporization.9

1.1.1.2. Ammonia

Ammonia has been used as a PCM in many industrial refrigeration applications.

Its low boiling point at one atmosphere of pressure of -33.4°C makes it suitable for low

temperature refrigeration. It has a high latent heat of vaporization of 1371 kJ/kg or 935

MJ/m3 of liquid, which is an order of magnitude increase over conventional refrigerants,

10,11 making it one of the most efficient refrigerants. The major drawbacks of NH3 for use in refrigeration are that it is both flammable and toxic in small concentrations. Toxicity is the primary reason NH3 has not been applied to domestic cooling and only to industrial and secondary cooling systems where some non-toxic fluid, such as water, serves as a

12,13 heat transfer medium between NH3 and the air to be cooled. However, over 100

years of use have led to numerous strategies and a history of experience in dealing with

NH3. The flammability of NH3 is also of concern, as it has a flammable range of 15% to

28% by volume in air. However, NH3 is not considered highly flammable since its flame

temperature is usually below its ignition temperature, so another fuel source must be

present for any fire to propagate.14

1.1.1.3. Carbon Dioxide

The most common use of CO2 as a phase change material is to use its solid-gas

15 phase change for portable cooling. The enthalpy of sublimation for CO2 is 512 kJ/kg.

Additionally, CO2 has a long history as a refrigerant which began in 1930s and 1940s as a

refrigerant on ships. However, it fell out of use with the advent of CFCs

(chlorofluorocarbons) and HCFCs (hydrochlorofluorocarbons). Carbon dioxide is being

reexamined as a refrigerant now that the detrimental environmental impacts of CFCs and

HCFCs are better understood. It can also be used as a pressurized refrigerant in the

16,17 temperature range of -30°C to 0°C by employing the evaporation of liquid CO2. An alternative method for using CO2 as a refrigerant is to use liquid CO2 to generate a two-

phase flow stream of solid and gaseous CO2, which can be used to achieve cryogenic

temperatures below -56.6°C on a continuous basis as opposed to using dry ice for

portable refrigeration. This system was only proposed in 2008 but carries the possibility

18 of using CO2 as a cryogenic refrigerant.

For the scope of this work, the use of CO2 as a TM material is interesting since it

can be used as a solid to gas transformation but the temperatures of use are too low to be

practical.

1.1.1.4. Paraffin Waxes

Various paraffins hold potential as solid-liquid PCMs for use in electronics

because their melting temperatures can be tuned over a range for which water is not

applicable, mainly between 5°C and 95°C. They also have gravimetric melting enthalpies

around 200 kJ/kg, and densities of 750-800 kg/m3, which lead to volumetric melting

enthalpies of ~150 MJ/m3 for the pure materials.9 The main drawback to using paraffins

as PCMs is that they have very low thermal conductivities of around 0.2 W/m•K.9 In

practice paraffins also suffer from reversibility issues which stem from a density change

associated with the solid-liquid transformation. The change in density can lead to a loss

of thermal contact with heat transfer surfaces and lower the thermal power rating.

Furthermore, paraffins require encapsulation to prevent leakage, which is an ever present

possibility.

The low thermal conductivity of paraffins inhibits heat transfer and thus leads to

long thermal charging and discharging times, making them difficult to use as PCMs.

However, work has been done with impregnating various types of thermally conductive

graphitic foams with paraffins to increase the thermal conductivity. A study conducted in

2000 found that impregnating compressed expanded natural graphite with paraffin

loadings from 65 wt% to 95 wt% led to thermal conductivities of 4 W/m•K to 70 W/m•K, equivalent to those of the foam matrix itself. This tremendous, near two order of magnitude increase from 0.24 W/m•K for the pure material, demonstrates how this system level approach can make a low thermal conductivity material into a system with a good thermal power rating by increasing the thermal conductivity, something necessary for this work.19 Other studies have found similar results, that using thermally conductive

matrices can greatly enhance the thermal conductivity of paraffin PCM systems.1,2,4 A drawback with using a heat transfer enhancing medium is that it increases system size and weight, which lowers the system specific properties. Additionally, as the system size is increased, the system properties will scale linearly as the same amount of thermally conductive medium must be used, so there is no advantage to make a larger system.

Overall, paraffins are promising PCMs. However, they do suffer from long cycle times, reversibility issues which lead to long-term performance degradation, the liquid phase introduces the possibility of a liquid leak in the system and paraffins are not suitable for applications requiring high energy densities. Therefore, they are not suitable for this work.

1.1.2. Chemical Reaction Systems

Chemical reaction systems are promising for transporting and storing heat

because the enthalpies of reactions are fundamentally higher than those of phase changes

since chemical bonds are being broken and formed. These types of materials generally fit

into two categories: those with a gas-gas reaction and those with a solid-gas reaction.

1.1.2.1. Gas-Gas Reaction Systems

There have been many chemical reaction working pairs suggested which are difficult

to use for this work because of the high operation temperatures, bulky containment

systems required for high pressures, and the corrosive or toxic nature of some of the

species.

One proposed chemical reaction system is the conversion of methane in a steam

reforming plant powered by heat from a nuclear reactor.20 The methane is reacted with

steam at 950°C by the endothermic reactions in Equations 1 and 2.

CO 3H ΔH = 205.2 kJ/mol (1)

2 CO 4H ΔH = 163.3 kJ/mol (2)

The methane can then be catalytically reformed in an exothermic process by the reverse reactions which take place at 450°C and the heat used to make steam. The purpose of this

process is to react the methane at a central location to store the heat in the reaction

products and then be able to distribute the product gases and reform them in locations

where steam is needed. While impractical for the type of problem which is the focus of

this work, it shows that chemical reaction systems have a very high energy change and

are serious contenders for thermal energy storage (TES).

A similar process is the CO2 reforming-methanation cycle shown in Equation 3.

22 (3)

This process has been proposed to use a catalyzed reaction at 700°C to 900°C to form

the CO and H2 which are then transported to the place of use where they recombine

exothermically at 350°C to 450°C to form steam.21 The main advantage of this reaction over methane-steam reformation is that there are no condensable species. This feature makes the system operation much simpler and does not require the gases to be transported at high temperature to prevent condensation.

Other possible reaction pairs proposed in the literature are listed in Table 1.22-24

All operate at too high of a temperature to be practical for this work (300°C to 700°C) and many are hazardous, but collectively they provide background to the motivation for using a chemical reaction heat transformer rather than a phase change TES material.

Table 1. Chemical reaction heat storage working pairs Reaction Equation 2 3 66.5 / (4) 2 2 98 / (5) 2 108 / (6) 2 3 129 / (7)

At lower temperatures from 150°C to 200°C the decomposition of methanol is a

candidate chemical reaction system. The reaction and enthalpy are shown in Equation 8.

2 95/ (8)

This reaction is promising for large scale applications due to the low cost of methanol and the relatively low temperatures required, which are still much higher than those required by this work.

In the above reaction pairs all species are gaseous provided that condensation is prevented. This is ideal for large scale processes since it allows for easy transportation of the reactants and products over relatively large distances. It also presents difficulties since the high pressure gases require expensive support equipment. While these systems

are possible for large scale operations, they are not viable for a small system or where the

system energy density needs to be maximized. Additionally, they are not practical for the

temperature range of this work. Overall, these pairs demonstrate the difficulty of

identifying a suitable system for low-grade, high-flux waste heat. The toxicity, flammability and reactivity of the materials shown attest to the difficulty of finding a better system and underscore the need for this work.

1.1.2.2. Gas to Solid Reactions

Gas-solid reactions are promising for energy storage and TM because of their

high specific enthalpy changes. In addition to using the high energy change from a

reaction they also benefit from a large entropy change for the solid to gas transformation which increases their specific properties. However, many have discounted these types of

TES materials because the large change in volume makes them difficult to handle and also makes cycling problematic.25-27 Three types of solid-gas reactions are considered:

metal hydrides, ammoniates and complete decomposition reactions.

Many solid-gas reactions are of the form of sorption with reaction. Metal hydrides

are one example of this type of system and can have very high gravimetric thermal

energy densities. There are many possible metal-hydrogen storage material systems

which have been reported elsewhere.25,28 These materials are candidates for thermal energy storage because hydrogen is able to chemically bind to the metal, with a high binding energy, in a process which is controlled by temperature and pressure. Much of the research into metal hydrides is being conducted for hydrogen storage for fuel cells, especially those on vehicles. Hydrogen storage requires minimization of the thermal load associated with the sorption process; however, metal hydride systems with a high thermal

load may be good candidates for TM. The Mg/MgH2 system is well characterized with a

thermal energy storage density of 1850 kJ/kg while LiAlH4 has been reported to have an extremely high thermal density of 4200 kJ/kg.22,29

Metal hydrides appear to be promising materials for TM applications as well as

other energy applications but they suffer from many shortcomings that need to be

addressed before they can be used. The first is that of safety. Hydrogen is dangerous to handle because of its extreme flammability and danger of explosions. Additionally, metal hydrides are often violently reactive to water, including atmospheric water, making their storage a serious concern. Finally, these materials suffer from slow kinetics, poor reversibility, a very high desorption temperature and little familiarity outside of laboratories. For these reasons metal hydrides are still a far off goal for thermal energy storage.

A second popular class of sorption working pairs is ammoniates which utilize

NH3 and various metal salts such as LiCl, MgCl2, PbCl2, ZnCl2, CuCl2, FeCl2 and other

period 4, 5 and 6 metal chlorides.30-32 These systems take advantage of the exothermic reaction of the NH3 with metal salts and the endothermic desorption reaction to upgrade

waste heat to usable temperatures. While these can operate at lower temperatures than the

metal hydrides, the systems require high pressures and large masses of adsorbent

materials, which make them impractical for any application requiring a high specific energy density at the system level. The low thermal conductivity of the sorbent salts also

presents cycling challenges. These systems generally operate above 150°C and at high

pressures.32

The final type of gas-solid reaction considered is that of complete decomposition

reactions in which the precursors are solids and the only products are gases. These are

different from the various adsorption reactions since no solid is left behind, which can

present cycling challenges. This type of reaction can also present extremely high energy

densities for TM, especially at temperatures from 30°C to 60°C, which are the focus of this work. Three candidate, complete decomposition reaction materials proposed here are ammonium carbonate ((NH4)2CO3), ammonium bicarbonate ((NH4)HCO3), and

ammonium carbamate (NH2COONH4), (AC). Thermogravimetric analysis of their

decomposition (ramp rate 0.4°C/minute) showed complete decomposition of AC by 60°C

while ammonium carbonate and ammonium bicarbonate showed complete decomposition

by 85°C.33 The heat of decomposition of ammonium carbamate is 2.01 MJ/kg while it is

1.97 MJ/kg for ammonium carbonate, and estimated to be 2.12 MJ/kg for ammonium

bicarbonate from standard heats of formation.34-36 Ammonium carbamate is clearly the

preferred material among these three for two important reasons. First, the decomposition

temperature of AC is lower than that of ammonium carbonate and ammonium

bicarbonate by 25°C, which makes AC the only viable material for this work. Second AC decomposition does not form water, which would readily condense below 100°C and pose serious problems for system operation.

Ammonium carbamate is formed from NH3 and CO2 by the reversible reaction in

Equation 9.

2 (9)

This reaction can be controlled by temperature and pressure. The reaction to form AC

and its disassociation are well documented. It is a simple reaction and fairly safe with

NH3 gas being the only toxic product. It is even used in a Physical Chemistry laboratory

textbook as an undergraduate student experiment to study the kinetics and

thermodynamics of a heterogeneous gas phase reaction by studying the decomposition

pressure of the material as a function of temperature and is a suggested experiment in the

Journal of Chemical Education.34,37

1.2. Ammonium Carbamate Background

Ammonium carbamate is formed by mixing NH3 and CO2. The normal

stoichiometric equation for the formation of AC was shown in Equation 9. AC has been

described in literature for over 140 years, normally as a mixture with ammonium

38-41 carbonate. It was first characterized as a mixture of salts formed with NH3, CO2 and water. The first patent in the United States that discusses AC was granted in 1907. It describes the use of AC as an intermediate product for making either ammonium carbonate by treating AC with superheated steam or heated to produce ammonium carbonate and urea.42 The next patent to reference AC, issued in 1915, claims the use of

AC to neutralize lactic acid produced in acid fermentation.43 In 1918 AC was mentioned

in a patent for petroleum refining where it was used to neutralize acid.44 By 1920 AC was

a well known industrial chemical. After this point AC was primarily used as an

intermediate in urea synthesis for fertilizers.45-47 Another industrial use of AC is in the

manufacture of metal carbamates, such as sodium carbamate (NaNH2CO2), potassium

carbamate (KCO2NH2) or calcium carbamate (Ca(NH2CO2)2) which can then be

converted into valuable products such as calcium cyanamide or potassium carbonate.48-50

AC has also found use as an agent to expand tobacco, in deicing compositions, and in the production of melamine.51-53 In more modern patents the most prevalent use of AC is as

11 an intermediate in urea synthesis.54-56 Today the global urea production is over

100,000,000 tons annually, and AC is used as an intermediate in the process; additionally, as of 1983 (the most recent figure available) over 4,000 tons of AC were produced annually for other purposes.57,58 This makes AC particularly attractive for TM applications because of its low cost and broad experience in its use and handling.

Therefore, the scope of this project is not in material development, but in the development of a material system for a new application.

Industrially, several methods to produce AC exist. If high purity is not necessary, the AC is normally produced in an aqueous stream. This is accomplished in a supersaturated aqueous solution of NH3 and CO2, with NH3 content greater than 30%, and the AC is precipitated out of the solution by cooling.59 However, this method only has a yield of about 60% and produces a mixture of AC, ammonium carbonate and ammonium bicarbonate; this mixture is acceptable for urea production for use in fertilizers. A second method to produce high purity AC is to pass anhydrous CO2 and

NH3 gases through cooled vertical columns where AC forms on cooled walls. This method has the disadvantage of requiring large cooling surfaces, due to the poor thermal conductivity of the AC and its high heat of formation, and requires mechanical removal of the carbamate, but does produce high purity AC.59 A third method to produce AC industrially is to use a fluidized bed cooled below 25°C with CO2 circulated as the

59 fluidizing agent and NH3 introduced to form AC. A fourth method to produce AC is to introduce liquid NH3 and liquid CO2 together in the tip of a spray nozzle so that the AC forms as a fine powder just outside of the nozzle. This method is reported to produce high purity AC at nearly 100% yield.51

The dissociation pressure of AC as a function of temperature has been published

several times; in addition its determination is an experiment in college undergraduate

chemistry laboratories. The first study of AC dates back to 1838 as an experiment in the

60,61 equilibrium between NH3, CO2 and AC. The dissociation pressure as a function of temperature is shown for four literature sources in Figure 1 and there is excellent agreement among the data.34,60,62,63 These data give a good indication of the pressures

necessary to cause AC to decompose as a function of temperature. A pressure below atmospheric is necessary for rapid decomposition since the desired temperature range for this work is from 30°C to 60°C. The overhead pressure is important since it will

determine the necessary system pressure in order to cause a rapid decomposition.

1000 Egan, Potts, Potts 900 Joncich, Solka, Bower 800 Briggs and Migrdichian 700 Bennett, Rithie, Roxburgh, Thomson 600

500

400 Pressure (Torr) 300

200

100

0 10 20 30 40 50 60 70 Temperature (°C)

Figure 1. Dissociation pressure of ammonium carbamate

1.2.1. Kinetics of Ammonium Carbamate Formation and Decomposition

At least three different mechanisms have been proposed for the formation and

disassociation of AC. The two most recent reports were made in 1988 and 1998 and are summarized below.64,65

Claudel and Boulamri studied the reaction forming AC as well as its decomposition and found the rate expression of the form in Equation 10.

(10)

In this equation P is the total pressure of the stoichiometric mixture of the gases, Peq is the pressure at equilibrium conditions, and the rate constant k has the units of torr-1min-1. The

constant k depended primarily on the surface state and surface area of the reactive species

and was only a weak function of temperature. As this is a second order rate law, the

authors proposed that it is governed by the rate of crystal growth or decrease. They point out that this is normally seen in crystal growth kinetics in solution but has also been proposed for gas-solid reactions in the case of HCl and NH3 forming ammonium

chloride. The authors proposed that this rate is governed by Equations 11 and 12.

(11)

(12)

They suggest that the formation of carbamic acid (NH2CO2H) in the first reaction is the

limiting step. Theoretical studies have pointed to the presence of the carbamic acid

intermediate, but no direct evidence of its existence has been found.64,66-68 While this

does not invalidate this mechanism, it does allow for other possible explanations.

The most recent study on AC kinetics was published in 1998 by Ramachandran,

Halpern, and Glendening65 and summarized prior mechanistic studies. They critiqued the

Claudel and Boulamri64 reaction mechanism as being implausible since it breaks down

for P > Peq as it predicts that for this case the pressure would increase in time, not

decrease towards Peq. Additionally Ramachandran, Halpern, and Glendening point out that the correlation between kobs and 1/T was poor. The authors considered two different scenarios for the kinetic mechanism. The first mechanism involves NH3, CO2 and

carbamic acid in the gas phase. However, they found this type of mechanism to be

improbable and to lack evidence since IR analysis of the gas phase did not show any

evidence of carbamic acid. The second mechanism proposed still involved a carbamic

acid intermediate, but the carbamic acid did not leave the surface of the AC. The authors found this mechanism to be the most promising for the reaction. In their publication they alluded to future work on determining rate constants, but that was apparently never published.

For the purpose of this work, knowing the exact mechanism of reaction is not critical. Rather, establishing pressure as the primary driving force of the reaction as well as understanding the impact that surface area will have on reaction rate is key. This conclusion will need to be considered in any system design since a large pressure gradient resulting from a low pressure system will be desirable to achieve a maximum rate of reaction. It will also be desirable to process the AC to a small particle size to maximize the surface area available for reaction.

1.2.2. Previous Uses of Ammonium Carbamate for Thermal Management

The use of AC to upgrade heat and store thermal energy has been proposed in two

previous, but unimplemented systems. Both concepts use heat from solar energy or some

other low grade heat source, such as geothermal, to decompose the AC and then store the

CO2 and NH3 separately. These stored gases can then be brought back together to reform the AC, an exothermic process, to provide heat for space heating and generating steam.

Ammonium carbamate is good for this system since the gases can be stored indefinitely and then reacted to provide heat as needed, creating an on-demand, regenerable heat source.

The first system was presented in a patent from 1979 and was intended to use solar energy to decompose the AC, then reform it later to take advantage of its exothermic formation.69 The patent presents a few different operation modes. In the first

the carrier fluid is water. However, this is undesirable to the system because of the

formation of urea, which in a mixture of water, is corrosive. In the second operational

method an anhydrous heat transfer medium such as Therminol or Dowtherm is used

instead of water, avoiding urea formation and corrosion problems. The principle of

operation of the system is to dissociate AC at a temperature of 100°C to 200°C under

pressure using solar energy. The NH3 and CO2 are then used immediately to provide heat

at a different location or are separated using compression and stored indefinitely. The

separation is simple in principle because at a pressure of 10 atmospheres at ambient

temperature the NH3 is a liquid while the CO2 remains a gas. They can be separated

easily, the NH3 stored as a liquid, and the CO2 stored as a compressed gas. When

recombined, the heat can be used for a wide range of purposes. The authors suggest a

radiator, air heater or hot water heater. They also claim this system is advantageous

because of the low operating temperatures and the relatively low hazards associated with

AC as compared with other systems such as phosgene and carbon monoxide. However,

using a system similar to this is not practical for this work because the operating

temperatures of 100°C to 200°C are too high. Importantly, this paper elaborates a simple

method of separating the CO2 and NH3, as the patent suggests a compression to ~10

atmospheres is sufficient to separate the species.

The second system is similar to the first since it uses solar energy but is

specifically designed to use solar energy to disassociate the AC and then store the gases, and finally use them to provide space and water heating.70 The proposed system was also

very large, enough to provide heating to a Greek settlement of 500 people for 15 winter

days. Such a large system would be used to heat 11,800 kg/hr of water from 20°C to

90°C by reacting 820 kg/hr of NH3 and 1060 kg/hr of CO2, the water would then be used

as the heat transfer medium. This system runs based on the same principles of the

previous patent but relies totally upon an anhydrous system to avoid urea formation.

Another distinction is that this analysis also included the idea of using CO2 that was

produced as a byproduct of fermentation or combustion processes. In this mode of

operation the NH3 was recycled and was separated using ethanol. This is a simple

separation since the solubility of NH3 in ethanol at 0°C is over 20% by weight at 1

atmosphere of pressure. The NH3 can then be reacted with the CO2 in the ethanol

solution, the heat of reaction will cause all components to volatilize, then the ethanol can

be condensed and reused. The study found that using CO2 obtained from fermentation to

be the most practical since it avoided costly compression, the closed loop cycle involving

CO2 compression to be the second most desirable and that obtaining CO2 from

combustion would be the least economical due to the expensive purification required of the effluent gas. Like the previous patent, the operating temperatures and pressures are

well above those required for this work, but the system does suggest an additional

separation method using inexpensive alcohols at ambient pressure.

The two aforementioned systems demonstrate that AC has been considered for

large scale, terrestrial-based thermal energy storage. However, the operating temperatures

are significantly above room temperature. Important insights that can be drawn from

these proposed systems are methods for separating the NH3 as well as the idea of using a

carrier fluid to enhance heat transfer and aid in mass transfer.

1.2.3. Ammonium Carbamate for Thermal Management

The decomposition reaction of AC is remarkable from a TM perspective since it

occurs in the desired temperature range of 30°C to 60°C and with a high heat of reaction

on a gravimetric basis. Table 2 gives literature values for the heat of reaction.

Table 2. Heat of reaction of ammonium carbamate Source ∆° (MJ/kg) Joncich et al34 2.00 Janjic71 2.07 Bennett63 1.96 Egan, Potts and Potts62 2.01 Clark and Hetherington72 1.94 Claudel, Brousse, Shehadeh73 2.05 Patent 4,169,49969 1.99

The average of these values is 2.01 MJ/kg which was used for the purpose of this work as the accepted heat of reaction. As a comparison the heat of vaporization of water is 2.45

MJ/kg and the heat of vaporization of NH3 is 1.3 MJ/kg. To put this on a volumetric basis

the density of the AC or of the system must be known. A single crystal analysis was performed by Adams and Small in 1973 and gave a calculated density of 1.38 g/cm3, and

they measured the density to be 1.36 g/cm3.74 Bulk AC available from BASF chemicals

in 25 kg bags has a density range of 780-850 kg/m3.75 According to patent 4,567,294 the

bulk density of AC is 780 kg/m3.59 A density of 780 kg/m3 corresponds to a volumetric energy density 1.57 GJ/m3. A comparison to other commonly used materials for TM is

given in Table 3, all volumetric measurements are based on the solid bulk density. Thus

the energy density of decomposition for the material is quite competitive and at a lower

temperature than the liquid to gas phase change of water but higher than that of NH3.

Even though the phase change of NH3 occurs at a lower temperature, the use of NH3

requires bulky pressure containment systems and presents a significant toxicity risk,

which is not encountered with solid AC.

Table 3. Energy densities of common thermal management materials Gravimetric Volumetric System Enthalpy Enthalpy Temperature (°C) Ammonium carbamate 2010 MJ/kg 1570 MJ/m3 25-60 Water freezing 330 kJ/kg 306 MJ/m3 0 Water vaporization 2450 kJ/kg 2452 MJ/m3 100 Ammonia 1371.2 kJ/kg 935 MJ/m3 -33.5 Dry Ice 571 kJ/kg 856 MJ/m3 -78.5 Paraffins 200 kJ/kg 150 MJ/m3 5-95 Metal Hydrides Up to 4200 kJ/kg Varies by material

1.2.4. Activation Energy of Decomposition

The activation energy of the decomposition of solid AC has been measured by

two different groups as shown in Table 4. These values are significantly different,

however the important conclusion is that it may be possible to lower the activation energy

of decomposition through a catalyst which could increase the rate of reaction. This will be important to maximize the rate of reaction to give the system a high power rating. No measurements of the activation energy in solution have been reported.

Table 4. Activation energy of decomposition of ammonium carbamate Source (kJ/mol) Janjic71 79 Claudel, Brousse, Shehadeh73 42

CHAPTER II: EXPERIMENTAL METHODOLOGY AND ANALYSIS

2.1. Thermal Management Proof of Concept

In order to demonstrate the tremendous potential of AC for TM, a thermal camera was used to show how AC can quickly reduce the temperature of ethylene glycol (EG).

Two Erlenmeyer flasks were filled with 75 mL of EG and placed on a hot plate. Each was heated to ~100°C. Ammonium carbamate was then added to the flask on the right through a funnel. Once the AC came in contact with the EG it vigorously decomposed and lowered the temperature of the fluid. The images in Figure 2 show the time progression of the temperature of the flasks with time increasing from left to right. The first image was taken before the AC was added. There is a 10 second delay between each image. A color of yellow corresponds to a temperature of ~100°C and a color of green corresponds to a color of ~60°C. As the image progression shows the addition of the AC lowered the temperature by over 40°C within 40 seconds. It should be noted that what the thermal camera detects is the infrared emission of the glass and not of the fluid inside the glass. This underscores the rapid cooling of the EG since the thermal transfer between the

EG and the inner surface of the glass and then to the outer surface of the glass will delay the visual appearance of the temperature drop. The dark red at the bottom of the images is from the hot plate.

t = 0 s t = 10 s t = 20 s t = 30 s t = 40 s

Figure 2. Thermal camera image showing thermal management potential of ammonium carbamate

2.2. Thermal Conductivity

An inherent problem in the use of AC as a TES system is that it has a low thermal

conductivity, though no exact value can be found in literature. A patent gives a

description for a prior process to produce AC which involves its formation from NH3 and

59 CO2 gases on a chilled metal surface. The patent states that “the poor thermal

conductivity of the salt necessitates large cooling surfaces,” which gives evidence to the

idea that the thermal conductivity could be problematic. The thermal conductivities of

other NH3 salts are listed in Table 5. From this table it can be assumed that the thermal conductivity of AC is low. All thermal conductivities are reported as close to room temperature and pressure as possible

Table 5. Thermal conductivity of ammonium salts Compound Source Thermal Conductivity Measurement (W/m•K) Technique NH4Br Ross and ~1 Hot wire using Anderson76 pressed sample NH4I Ross and ~0.8 pellets 7mm Anderson76 thick NH4Cl Ross and ~1.5 Anderson76 77 CaCl2 (powder) Wang et al. 0.31 77 CaCl2 (pellet) Wang et al. 0.11 77 CaCl2 • 2NH3 (powder) Wang et al. 0.37 Hot wire method 77 CaCl2 • 4NH3 (powder) Wang et al. 0.42 77 CaCl2 • 6NH3 (powder) Wang et al. 0.48 78 Mg(NO3)26H2O– Frusteri et al. 0.47 Hot wire on bulk MgCl26H2O– material NH4NO3 79 NH4SCN Osman et al. 2.9 79 NH4HCO2 Osman et al. 2.7 79 Hot wire CH3COONH4 Osman et al. 2.4 79 NH4NO3 Osman et al. 2.0

As part of this work the thermal conductivity of a bulk AC sample was

experimentally determined by the guarded hot plate method (ASTM E1530). The sample

used was a pressed pellet of AC made by pressing powdered AC in a pellet die in a hydraulic jack to 10 tons of pressure, the pellet was held at this pressure for 60 seconds.

The resulting pellet was 28.82 mm in diameter and 7.15 mm thick with a mass of 5.557 g

and a density of 1.19 g/cm3. The thermal conductivity of the pellet, which was measured

at 12.9°C to avoid decomposition, was determined to be 0.531 W/m•K. This result is in

good agreement with the data available on other NH3 compounds.

The low thermal conductivity of the pellet demonstrates that it will be necessary to

provide some mechanism to enhance the heat transfer of the system. This will be even

more important since in practice the AC used will be in powder form and will have an

even lower thermal conductivity.

2.3. System Concept Evaluation

A number of possible methods exist for enhancing the heat transfer in an AC system. Some of the possible solutions are:

 Metal foam to enhance heat transfer

 Carbon foam to enhance heat transfer

 Using a heat exchanger with AC dissolved in liquid NH3

 Using a heat exchanger with AC suspended in a heat transfer liquid like

Dowtherm or Therminol in which it is insoluble

 Using a heat exchanger with AC in a heat transfer liquid in which it is soluble

such as ethylene glycol

2.3.1. Metal and Carbon Foams

Because of the low thermal conductivity, it will be inherently difficult to use neat AC

for TES. The low conductivity would lead to exceptionally long cycling times, or an

excessively large system, and be impractical. Therefore, the heat transfer will need to be

enhanced for any practical device to be made.

The problem of low thermal conductivity has been encountered in other PCM

TES systems. The use of thermally conductive carbon foams has been described in the literature and has been shown to greatly reduce cycling times in wax systems. The use of carbon foams is analogous to the use of extended surface area in any heat exchanger. For instance, in shell and tube heat exchangers a large number of tubes are used to increase

the heat transfer area. Therefore, impregnating a foam with AC would enhance heat

transfer and lead to improved cycling times in a TES system.

The difficulty in impregnating a foam lies in the fact that AC exists only as a solid. The method used to impregnate carbon foams with paraffin PCMs is to melt them into the foam in a vacuum oven to degas the foam and draw the wax into the pores. This method has been shown to fill around 80% of the available pore space.2 A similar method

has been used to impregnate foams with salts such as CaCl2•2H2O or MnCl2. In this

process the salt is dissolved in water and then put in a vacuum chamber to allow the salt

solution to fill the pores. Finally the water is removed at high temperature, leaving the

salt in the foam.80,81

Based on this previous research, there exists the possibility of dissolving the AC

in a liquid and using a vacuum technique to impregnate it in a foam and then removing

the liquid. The difficulty for doing this with AC is its limited solubility and the fact that it

cannot be taken above ~20°C while the liquid is removed. This restriction eliminates

using many liquids, such as water, since they could not be effectively removed without

also decomposing the AC. Liquid NH3 has a good possibility of being used to

impregnate AC by two different methods. One method would be to use a low temperature

vacuum technique to impregnate the foam and then slightly raise the temperature to

remove the NH3 but leave the carbamate in the foam pores. A second method would be to

use the same procedure to impregnate the foam but not remove the NH3. The TES device

could then rely on the heat of vaporization of NH3 and the heat of decomposition of the

AC, possibly enhancing the overall thermal energy density of the system. The second

design would necessitate a pressure containment system to prevent the NH3 from

evaporating during storage. It may also be possible to use other solvents in which AC

may be soluble to impregnate the foam.

2.3.2. Liquid Ammonia Evaluation

A literature survey was conducted to determine if liquid NH3 would be a possible

solvent for AC impregnation in the foams. The solubility of NH3 salts in liquid NH3

varies considerably. Table 6 details the solubilities of many different NH3 salts in liquid

NH3. One reference states that AC is “slightly soluble” in liquid NH3 while a patent states

that it is “very readily soluble in liquid NH3 which is anhydrous or contains very little

water.”49,82 A 1926 reference gives an approximate solubility of 1.5 g/L.83 The

conditions of this solubility are not clear but it is assumed that the solubility experiment

was performed at room temperature in sealed glass tubes. Additionally, as is shown in

Table 6, the solubility of NH3 salts varies considerably. Another literature source

supported the idea that the addition of AC to NH3 could possibly lower the vapor pressure of the liquid NH3, by reporting this effect with other ammonium salts in liquid

84 NH3. One example of this is that a saturated solution of ammonium nitrate in liquid

84 NH3 is stable at room temperature. If this occurred with AC it would reduce the

pressure containment system needed, and lead to higher system level specific properties.

Table 6. Solubilities in liquid ammonia at 25°C (g/100 g ammonia)85 Compound Solubility Compound Solubility NH4I 368.4 AgCl 0.83 NH4ClO4 137.9 AgBr 5.92 (NH4)2S 120 AgI 206.84 Na2SO4 0 AgNO3 86.04 KNH2 3.6 Ca(NO3)2 80.22 KClO3 2.52 Sr(NO3)2 87.08 (NH4)2HPO4 0 Ba(NO3)2 97.22 (NH4)HCO3 0 BaCl2 0 KBrO3 0.002 MnI2 0.02 (NH4)(CH3COO) 253.2 ZnI2 0.1 LiNO3 243.66 ZnO 0 Li2SO4 0 H3BO3 1.92 NaNH2 0.004

The solubility of AC in liquid NH3 was determined in this work at two temperatures as follows. Gaseous NH3 (Aldrich, 99.99%) was passed from a lecture

bottle and condensed into a flame dried 100 mL round bottom flask in a dry ice/acetone

bath. Approximately 50 mL of liquid NH3 was obtained to which 0.206 g of AC were

added. The solubility at -78.5°C was determined by stirring the mixture with a magnetic

stir bar in a dry ice/acetone bath. From visual observation it did not appear that a

significant amount of the AC went into solution, leading to the conclusion that AC is

virtually insoluble at that temperature in liquid NH3. The solubility at -41°C was then evaluated using an acetonitrile slush bath. Once the NH3/AC mixture was warmed to this

temperature the solubility did not appear to change appreciably.

The room temperature solubility of AC in liquid NH3 was also determined. At this temperature the vapor pressure of NH3 is ~8 atmospheres. A sealed, flame dried heavy

walled Schlenk tube was filled with ~25 mL of liquid NH3. To this 0.5 g of AC were

added. The tube was allowed to slowly heat to room temperature while stirring using a

magnetic stir bar. As the mixture heated, it first turned a violet color, this color went

away after 1 hour. After the mixture reached room temperature (20°C) the entire solid

was not dissolved. The source of the violet color is unknown. However, it is known that

when metals dissolve in liquid NH3 they form similar colored solutions as the result of

free electrons, so it is possible that the color came from the metal cannula that was used to put the NH3 inside the tube.

Since AC was not readily soluble in liquid NH3, additional methods were

explored to increase its solubility. According to a 1935 patent, adding ammonium

48 chloride (NH4Cl) will increase the solubility of AC in liquid NH3. This patent claims

the solubility of ammonium chloride to be 134.0 g/(100 g NH3) at 30°C. The solution

made previously to find the solubility of AC in liquid NH3 was used to find the effect on

solubility of adding ammonium chloride. Since there were ~15 g of NH3, it should be

possible to dissolve ~19.5 g NH4Cl. For the first attempt 5 g of NH4Cl were added while

the NH3/AC was in a dry ice/acetone bath to prevent evaporation. Once the solution had

warmed to room temperature all the solids dissolved, confirming the patent’s assertion

that the NH4Cl would increase the solubility of AC in liquid NH3. To find the maximum

amount of AC that would dissolve with the NH4Cl, additional AC was added in the

following increments: 0.5 g, 0.5 g, 1 g, 1 g, 1 g. The procedure at each addition was to

cool the solution in dry ice/acetone, add the solid and then allow the mixture to heat up to

room temperature slowly. Each time the solution was cooled a good amount of solid

came out of solution and had a “fluffy” white appearance but when the mixture was

brought up to room temperature all the solid redissolved. Eventually 4.5 g AC were

added to the 5 g NH4Cl and at that point no more AC would go into solution, giving a mass ratio of AC:NH4Cl of 1:1.1 and a molar ratio of 1:1.6. Although this result shows

one way to increase the solubility of AC in liquid NH3, it is not expected to be useful in

an actual system because the melting point of NH4Cl is 340°C and the effect of adding

such a large amount of ammonium chloride would lower the energy density by mass of

the system by more than a factor of two and will still require pressure to maintain the

NH3 as a liquid at room temperature.

All of these experiments show that NH3 is impractical in a system involving AC since the solubility of AC in liquid NH3 is so small that for any practical system it can be

considered negligible.

2.3.3. Additional Solvents

There also exists the possibility of using other solvents besides NH3. For example,

a synthetic procedure for metal carbamates states that AC can make a “solution” in

methanol and a “dilute solution” in ethanol.50 This motivated an experiment to determine

the solubility of AC in anhydrous methanol and ethanol. The experimental apparatus

consisted of an Erlenmeyer flask on a stir plate. A titration burette was used to measure

the amount of solvent added to the flasks. The amount of 100 mL of ethanol and

methanol were used in the experiment, and both were put in an Erlenmeyer flask with a

stir bar and an argon purge to prevent water accumulation. The AC was slowly added

until no more would dissolve in the liquids. At ambient temperature and pressure the

methanol dissolved ~7 g AC/100 mL methanol and the ethanol dissolved ~2 g AC/100

mL ethanol. Since methanol dissolved more AC it was used as the solvent of choice for

the impregnation of carbon foam.

To more accurately determine the solubility of AC in methanol, the experiment

was repeated three times with the following procedure. Approximately 1 g of AC was put

in a 50 mL Erlenmeyer flask. Methanol was added to a titration burette and slowly added to the stirring methanol/AC solution until all of the AC had just dissolved. Exact amounts

used in each experimental run are listed in Table 7.

Table 7. Solubility of ammonium carbamate in methanol AC Added (g) Methanol Added (mL) Solubility (g/mL) 1.031 20.71 0.0498 1.061 19.59 0.0542 1.188 20.94 0.0567 Average: 0.0536 Standard Deviation: 0.0035

The first method used to impregnate the foam was the vacuum method. This is similar to techniques used to impregnate carbon foams with paraffin wax. The carbon foam used was not thermally conductive due to the expense of conductive foams. The experimental setup was a small, skinny polymer vessel equipped with a stopcock valve shown in Figure 3. A small piece of carbon foam was put in the vessel and a Teflon coated stir bar placed on top of it to prevent it from floating. The saturated methanol/AC

solution was poured over the foam. The vessel was immersed in an acetone/dry ice bath

to minimize the amount of methanol that evaporated during the impregnation. A Schlenk line was used to create a vacuum and remove the air from the foam and replace it with the

AC solution. The excess solvent was removed under vacuum in an ice/water bath to ideally allow the methanol to escape but not the AC.

Vacuum Vessel

Methanol with AC Weight

Carbon Foam

Figure 3. Vessel used to impregnate carbon foam with methanol and ammonium carbamate

After the liquid evaporated, the carbon foam was removed. It was cut in half using

a razor blade and examined visually under a microscope. No AC was observed in the

pores although the odor of NH3 was noticeable. Because no AC could be seen it was

apparent that most of the AC had been removed along with the methanol. A series of

experiments are subsequently detailed to test this.

Approximately 2 g of AC powder were added to the previously used polymer

reactor. The vessel was immersed in an ice/water bath. The system was connected to the

Schlenk line and the system was exposed to a vacuum of 10 mTorr for three hours. After this time it did not appear that any significant amount of AC had evaporated. Next a saturated solution of AC/methanol was placed in the vessel and the same vacuum was applied with the vessel in ice water. Under these conditions all of the methanol evaporated and no AC was left in the vessel. These experiments show that it is not possible to remove the methanol without the AC.

To test if the reason the AC was being removed with the methanol was its

solubility, an experiment was performed with the AC in pentane, in which it is insoluble.

The boiling point of pentane at one atmosphere of pressure is 36.1°C and that of

methanol is 64.7°C, so the pentane should be much easier to remove using vacuum. A

slurry of pentane and AC was placed in the previously used vessel and immersed in ice

water. When the vessel was evacuated the pentane was removed, leaving the AC behind.

This experiment demonstrates that using a liquid medium in which the AC is insoluble will not cause the AC to decompose while the liquid evaporates, but one in which it is soluble will cause the AC to decompose as the liquid evaporates. The practical implication is that using a solvent will not allow the impregnation of foams followed by solvent removal in which the AC is left in the foams. Additionally, a liquid in which AC

is insoluble will not allow the pores to be infiltrated with AC, so the foams cannot be

impregnated with this type of fluid either. These limitations show that it will not be

possible to impregnate a foam with AC through a vacuum technique.

2.3.4. Aluminum Foam

Metal foams can be made in what is known as an open cell foam, making them

much easier to impregnate. Open cell versions of carbon foam are available but are

thermally non-conductive unless some sort of surface treatment is applied and these are

prohibitively expensive. Therefore, as an initial trial aluminum foam was used to test an

open cell foam.

Open cell Duocel Aluminum Foam samples were obtained from ERG Aerospace.

The samples are shown in Figure 4. Three different porosities were obtained and the

pores are rated by pores per inch (PPI). As would be expected, the higher the volume percent aluminum, the higher the thermal conductivity will be. The tradeoff between additional weight and volume the aluminum occupies versus the gain in system thermal

32 conductivity will have to be evaluated for any use of a foam to maximize energy density and minimize cycling time. Surface area increases with both increasing density and pores per inch. The amount of surface area will be the amount of heat transfer area for the AC, so increasing surface area will increase the rate of heat transfer, and this will lead to a tradeoff similar to that for thermal conductivity.

Figure 4. Duocel aluminum foam

The dimensions and masses of the foam samples received are given in Table 8. A simple experiment was conducted to demonstrate how well the bulk AC powder would pack into the foam. The procedure was to first ball-mill the AC powder to reduce particle size so that it could easily fit inside the foam pores. Next the foam samples were encased in aluminum foil. The powder was then manually packed into the foam. The density of the powder inside the foam is given below. Note these densities are only approximate but

33 show that it is possible to pack the AC inside metal foam close to its bulk powder density of approximately 780 kg/m3.

Table 8. Dimensions and masses of aluminum foam samples Sample Dimensions Volume Mass AC AC (mm) (cm3) (g) Mass Density (g) (g/cm3) 10 PPI 38.20 x 12.45 x 88.26 41.98 10.236 26.784 0.638 20 PPI 37.60 x 12.73 x 88.50 42.36 7.356 26.030 0.614 40 PPI 38.14 x 12.93 x 89.35 44.06 9.879 22.583 0.512

To quickly examine the effect that AC would have on the heat transfer in an aluminum foam, a simple experiment was run with the foam on a hot plate. The 10 PPI foam was filled with 26.784 g of AC powder and placed on a hot plate. The 20 PPI foam had no AC in it and served as the control for temperature. A thermocouple was placed inside each foam sample as well as on the surface of the hot plate. The experimental setup is shown in Figure 5. The temperatures of the hot plate surface, bare metal foam and impregnated metal foam are given in Table 9. The experiment clearly demonstrates that the AC is effective at removing heat from the foam through its decomposition. It is apparent from this that a thermally conductive open celled foam would be able to enhance the heat transfer to the AC.

Hot place surface temperature Weight to ensure measurement good contact

Hot plate surface Foam with AC

Thermocouple wires Control Foam

Figure 5. Ammonium carbamate in aluminum foam

Table 9. Temperature-time records for ammonium carbamate in aluminum foam Time (hr) Hot Plate (°C) 20 PPI (°C) 10 PPI/AC (°C) 0 57.2 51.9 40.8 0.8 58.2 50.1 40.3 1.3 58 50.2 40.4 17 53.9 45.8 54.1 (AC gone)

It may be difficult to impregnate the foam at the device level and may also lead to

a difficulty in mass transfer. This same problem has been observed in other sorption

systems where foams are used to increase thermal conductivity: that the small pore size

can have a detrimental effect on mass transfer which lowers the overall rate of reaction

and thus heat transfer.31,32,86-88 Additionally, using foams could lead to an explosion hazard at high thermal power ratings because of the large volume change associated with a solid to gas reaction. Also there is no pragmatic way to pack the foams and using a foam does not offer any scalability advantage since the system specific properties would scale linearly.

2.3.5. Foam Impregnation Summary

The small, closed cell pores of carbon foam make it difficult to impregnate. The

previous experiments show that the vacuum methods, which have been used to impregnate the foam with other materials, will not work for AC since it readily decomposes. Because of this, carbon foams were abandoned as a method for increasing the heat transfer of bulk AC.

It was possible to mechanically pack open cell aluminum foams and initial testing

showed that the AC did lower the foam temperature through its decomposition. Although

this was a success the foams in general suffer from several problems which will inhibit

their use for this application. The first is that the nature of the foams leads to inherent

mass transfer limitations, which would lead to an upper limit for a power rating.

Associated with this, mass transfer is a safety problem since high thermal power ratings

would lead to a high decomposition rate and could cause the foams to explode. Also, a

system utilizing foams to enhance thermal conductivity will scale linearly, offering no

advantage at a larger system size which is undesirable for this work. For all of these

reasons foams will not be a viable system level solution to increasing the thermal power

rating.

2.3.6. Heat Transfer Fluid Evaluation

Heat transfer fluids as a system medium will provide good heat transfer to the AC

and could also lead to simple system operation as well as good scalability. The good

scalability should exist since the AC could be stored as a solid and then injected into the

system, so the specific properties should increase for a larger system needing a greater

amount of TES reserves. Two general classes of heat transfer fluids exist, those in which

AC is soluble and those in which it is insoluble. A heat transfer fluid in which it is

soluble may well be advantageous compared to one in which it is insoluble. A reason for

this is that in any flow system a soluble heat transfer fluid should prevent clogging.

Additionally, in aforementioned experiments in section 2.3.3, the use of methanol, in

which AC is soluble, led to complete decomposition of the AC under vacuum, while in the pentane, in which AC is insoluble, the AC remained behind when the pentane was removed using vacuum. Both classes of heat transfer fluids will need to be evaluated to determine which leads to the best performance. As a selection criteria for heat transfer fluids, they should have a low vapor pressure to ensure that the fluid will not be consumed through evaporative losses. It may also be possible to use a vacuum to greatly increase the rate of AC decomposition through Le Chatlier’s principle and therefore increase the rate of cooling. In order to establish a baseline for the maximum rate of decomposition as a function of temperature several experiments were conducted.

The first solvent chosen was EG. Ethylene glycol has a high boiling point at one atmosphere of pressure (197.3°C) and a very low vapor pressure in the temperature range of interest. The solubility of AC in EG was determined using the same experimental technique which was used to determine the solubility of AC in methanol and is shown in

Table 10.

Table 10. Solubility of ammonium carbamate in ethylene glycol AC Added (g) EG (mL) Solubility (g/mL) 3.364 18.09 0.186 2.387 13.15 0.182 1.601 8.52 0.188 Average: 0.185

The schematic of the experimental apparatus used to measure the maximum rate

of decomposition of AC as a function of temperature is shown in Figure 6. The lowest pressure that the system could achieve was lower than the gauge resolution of 1 x 10-5

torr. However, given that the gauge was located much closer to the diffusion pump than the reaction vessel, it is likely that the pressure was higher at the reaction vessel. For the temperature range for the experiment the water bath could be regulated between 5 and

50°C.

Figure 6. Schematic of system used to measure the rate of decomposition of ammonium carbamate

2.3.6.1. Experimental Procedure

The EG (Sigma-Aldrich, ≥99%) was dried using 3Å molecular sieves to remove any residual water. The glassware was dried to remove water prior to use. All decomposition reactions were conducted using heavy wall Schlenk flasks. The EG was degassed using a freeze-thaw procedure. All measurements of the EG during the experiment were gravimetric and the reaction vessel mass was determined before the EG was added. The

AC was added to the flask, its mass determined, and then the Schlenk flask was

immersed in the temperature controlled water bath for at least twenty minutes to allow

thermal equilibrium. The air was then evacuated from the system to a pressure no greater

than 5 x 10-4 Torr on the pressure transducer. Next the valve between the liquid nitrogen

trap and the vacuum pump was closed. In this way the liquid nitrogen worked to keep the

pressure low in the vacuum manifold as well as to trap any decomposed AC. This setup

prevented AC from being pulled through the system into the pump. The valve on the

reaction vessel was opened and the AC was allowed to decompose for five minutes. After

this time the vessel was backfilled with nitrogen and weighed to find the mass change.

The procedure to evacuate the system, allow it to come to thermal equilibrium and then

run the decomposition for five minutes was repeated for a total decomposition time of at

least thirty minutes. At each temperature an initial run with just EG was conducted to find

its baseline rate of evaporation and loss.

2.3.6.2. Heat Transfer Fluid Analysis

The first experiment which was conducted was to determine whether EG, in which

AC is soluble, or a heat transfer fluid in which it is insoluble, would lead to a greater rate of decomposition. Tetraglyme (boiling point at one atmosphere pressure is 275°C) was selected as the heat transfer fluid in which AC is insoluble due to its high boiling point.

Both tetraglyme and EG were evaluated at 30°C using equal fluid volumes and amounts

of AC using the aforementioned experimental procedure. They are also compared to the

bare powder at twice the solubility multiple in Figure 7 and at four times the solubility

multiple in Figure 8. The amounts used are shown in Table 11 and the solubility

multiples correspond to the solubility of AC in EG.

1.00 0.90 Ethylene Glycol 0.80 Tetraglyme 0.70 Bare Powder 0.60 0.50

Conversion 0.40 0.30 0.20 0.10 0.00 0 102030405060 Time (min)

Figure 7. Comparison of ammonium carbamate conversion in ethylene glycol, tetraglyme and bare powder at two times solubility and 30°C

1.00 Ethylene 0.90 Glycol 0.80 Tetraglyme 0.70 0.60 0.50

Conversion 0.40 0.30 0.20 0.10 0.00 0 1020304050 Time (min)

Figure 8. Comparison of ammonium carbamate conversion in ethylene glycol and tetraglyme at four times solubility and 30°C

Table 11. Amounts of ammonium carbamate and tetraglyme added for decomposition studies Solubility Multiple Tetraglyme (mL) AC (g) 2x 49.8 18.79 4x 51.1 37.51

As these profiles show, the reaction rate is greatly depressed in tetraglyme at the

same experimental conditions of temperature, pressure, amount of solvent and amount of carbamate. Additionally, the conversion rate is nearly identical in tetraglyme as it is in bare powder showing that merely having a heat transfer fluid is not sufficient to increase the conversion rate. This leads to the conclusion that the fact that AC is soluble in EG must play a role in the rapid decomposition under vacuum conditions. This was suspected with the initial experiments in methanol where the AC was removed when methanol was the fluid but not when pentane was used. Therefore, EG will be further evaluated and the study of heat transfer fluids in which AC is insoluble will be discontinued.

Since EG was demonstrated to be the superior heat transfer fluid, the experiment in EG was repeated at 50°C, 40°C, 30°C, 20°C and 10°C, and the results are shown in

Figure 9. For a first run at each temperature, the AC was added to the vessel at twice its solubility limit at room temperature (18.5g/100mL EG). As expected, as temperature increased the rate of reaction increased as well. The 40°C and 50°C conversion profiles are nearly identical. This is most likely because the rate of reaction was limited in these cases by how quickly the gases could be removed. The rapid rate of decomposition limited how far the valve on the Schlenk tube could be opened because with the valve fully open, the rapid gas removal rate led to liquid entrainment into the vacuum system.

This meant the rate of decomposition was effectively “throttled” to keep the EG inside the tube. At lower temperatures this problem was not as pronounced since the

decomposition was slower. Because the 40°C and 50°C profiles were nearly identical it

was decided to not run further trials at 50°C on this system. A limitation of the system is

the inherently low rate of mass transfer found in high vacuum systems, which are

designed to achieve low pressure but not high flow rates. This means that the rate of reaction may have been limited at all temperatures by mass transfer.

0.9

0.8

0.7

0.6

0.5

Conversion 0.4

0.3 50° 40° 0.2 30° 0.1 20° 10° 0 0 5 10 15 20 25 30 35 40 Time (min)

Figure 9. Conversion as a function of time for ammonium carbamate in ethylene glycol at two times its solubility limit

As there are an extensive number of variables in the procedure, many of which

cannot be measured, the uncertainty of the conversion was estimated by repeating the

procedure several times at each temperature to find the standard deviation in the conversion as a function of time calculation. These standard deviations were then

averaged at each temperature and the results are shown in Table 12 and are reflected as

the error bars on the plots.

Table 12. Error analysis for ammonium carbamate decomposition Temperature (°C) System Average standard deviation in conversion 10 Slurry 0.01 10 Powder 0.02 20 Slurry 0.06 20 Powder 0.06 30 Slurry 0.07 30 Powder 0.04 40 Slurry 0.05 40 Powder 0.03

Results of the decomposition experiments at 40°C, 30°C, 20°C and 10°C for a

slurry in EG and the bare powder are shown in Figure 10 through 13. A sharp difference can be seen between the powder and slurry at 40°C and 30°C, while at the lower temperatures this difference is not as evident due to the low reaction rates. These profiles demonstrate the advantage of using some sort of heat transfer medium in which AC is

soluble to increase the rate of reaction.

1.00 Run 1 Powder 0.90 Run 2 Powder 0.80 Run 3 Powder 0.70 Run 1 Slurry 0.60 Run 2 Slurry 0.50 Run 3 Slurry

Conversion 0.40 0.30 0.20 0.10 0.00 0 5 10 15 20 25 30 35 40 Time (min)

Figure 10. Conversion as a function of time for ammonium carbamate powder and ammonium carbamate in ethylene glycol solution at 40°C

1.00 0.90 0.80 0.70 0.60 0.50 Run 1 Powder Run 2 Powder

Conversion 0.40 Run 3 Powder 0.30 Run 1 Slurry 0.20 Run 2 Slurry 0.10 Run 3 Slurry 0.00 0 1020304050 Time (min)

Figure 11. Conversion as a function of time for ammonium carbamate powder and ammonium carbamate in ethylene glycol solution at 30°C

1.00 0.90 0.80 0.70 0.60

0.50 Run 1 Slurry

Conversion 0.40 Run 2 Slurry 0.30 Run 3 Slurry Run 1 Powder 0.20 Run 2 Powder 0.10 Run 3 Powder 0.00 0 5 10 15 20 25 30 35 40 45 Time (min)

Figure 12. Conversion as a function of time for ammonium carbamate powder and ammonium carbamate in ethylene glycol solution at 20°C

1.00 Run 1 Slurry 0.90 Run 2 Slurry 0.80 Run 3 Slurry 0.70 Run 1 Powder Run 2 Powder 0.60 Run 3 Powder 0.50

Conversion 0.40 0.30 0.20 0.10 0.00 0 5 10 15 20 25 30 Time (min)

Figure 13. Conversion as a function of time for ammonium carbamate powder and ammonium carbamate in ethylene glycol solution at 10°C

As this work developed, it was determined that the optimum desired operating temperature for the system is ~30°C. Therefore, attempts were undertaken to run

additional trials at 30°C with different loadings of AC in the EG. Trials were run at 2, 3,

4, 5 and 6 times the solubility limit of AC in EG. A larger Schlenk tube had to be used for

these trials because the decomposition was so rapid. Use of the smaller tube resulted in material being pulled out of the small tube. Because of this, the valve could not be fully

opened, which would lower the reaction rate, forcing a change to a larger tube. The conversion as a function of time is shown in Figure 14 and the bare powder conversion is included for comparison. Table 13 shows the initial amounts of solvent and AC.

1.00 2x Solubility 0.90 3x Solubility 4x Solubility 0.80 5x Solubility 0.70 6x Solubility Powder 0.60

0.50

Conversion 0.40

0.30

0.20

0.10

0.00 0 5 10 15 20 25 30 Time (min)

1.00 2x Solubility 0.90 3x solubility 0.80 4x solubility 0.70 5x Solubility 6x Solubility 0.60 Powder 0.50

Conversion 0.40

0.30

0.20

0.10

0.00 0 5 10 15 20 25 30 Time (min)

Figure 14. Comparison of different initial loadings of ammonium carbamate in ethylene glycol at 30°C

Table 13. Initial amounts of ethylene glycol and ammonium carbamate to test rate of reaction at different initial loadings of ammonium carbamate in ethylene glycol Solubility Multiple EG (mL) AC (g) 2x 50.3 19.45 3x 51.0 28.17 4x 49.2 35.25 5x 49.3 46.18 6x 49.2 54.27

In each trial approximately the same amount of solvent was used, but as the amount of AC was increased the conversion as a function of time decreased only slightly, meaning a greater rate of decomposition occurred at increasing solubility multiples. The amounts of AC were increased to determine the maximum rate of decomposition that 47

could be accomplished in this system. In order to determine this, a plot of the average thermal energy consumed in the decomposition (in kW) was plotted as a function of

conversion. The power needed was determined from the reaction enthalpy change. The

plot is shown in Figure 15 which shows that a maximum rate of ~0.065 kW is reached for

the three systems with 4x, 5x and 6x loadings of AC.

0.07

0.06

0.05

0.04

0.03 2x Solubility 3x solubility Power Rating (kW) 0.02 4x solubility 0.01 5x Solubility 6x Solubility 0 0.00 0.20 0.40 0.60 0.80 1.00 Conversion

Figure 15. Rate of thermal power versus conversion for the decomposition of ammonium carbamate in ethylene glycol at 30°C

Propylene glycol (PG) was evaluated as an alternative heat transfer fluid in which

AC is soluble. This is because EG is toxic and bad for the environment while PG is food-

safe and considered to be environmentally friendly. Propylene glycol was tested with AC

at 30°C with 18.58 g of AC in 50 mL of PG and is graphically compared to AC in EG at

30°C with 19.45 g of AC in 50 mL of EG in Figure 16. In the figure the run in PG shows

a conversion of greater than 1 which occurred since a larger amount of PG was entrained

than with EG. The experiment was also repeated with the same result. While it is not

possible to find the amount of PG entrained the fact that conversion as a function of time

is comparable to that of AC in EG lends support to using PG instead of EG. The greater

amount of liquid entrainment can be explained by comparing the viscosity of the two

liquids. The viscosity of PG at room temperature is 50 centipoise and the viscosity of EG

at room temperature is 11 centipoise.89,90 The greater viscosity would lead to a greater amount of PG being entrained which explains a conversion greater than one.

1.20

1.00

0.80

0.60 Conversion 0.40

0.20 PG EG 0.00 0 5 10 15 20 25 30 35 40 45 Time (min)

Figure 16. Comparison of ammonium carbamate in ethylene glycol and propylene glycol at 30°C

2.3.7. Ammonium Carbamate Materials Compatibility

No data on corrosion specific to AC exists, so corrosion considerations will focus on issues arising from CO2 and NH3. The Handbook of Corrosion Data presents

information on the corrosion of a wide variety of compounds with many metals.91 Only

NH3 is considered since it is generally more corrosive than CO2 or the heat transfer fluids

used in this work. Metals suitable for use with NH3 are stainless steels, aluminum and

titanium. Copper and copper alloys must be avoided when working with NH3 as NH3 and

ammonium compounds can lead to stress corrosion cracking in copper. Ammonium

bicarbonate is a related ammonium compound, and it is not corrosive to stainless steel.

Ammonium carbonate is similar to AC but it contains water as well and it is not corrosive

to aluminum or stainless steel.

Based on the data available for corrosion, both stainless steel and aluminum

should be good materials to work with AC, especially in heat transfer applications.

Aluminum will probably prove to be a more advantageous material since its thermal

conductivity is 250 W/m•K at room temperature and that of stainless steel is only 16

W/m•K and it is also considerably less dense than stainless steel which should lead to higher system level specific properties.

2.3.8. Hysteresis

As this has been envisioned as a system where AC could be continuously loaded

into a glycol slurry and the glycol not recycled or treated in any way, it is desirable to

have an idea of the effect of reusing the glycol. The solubility of CO2 in EG has been

reported to be 39.4 mmol/L at 25°C and a partial pressure of 760 Torr.92 However, the

solubility of CO2 in EG increases with increasing pressure, so at the vacuum conditions of

93 the system the solubility will be lower. The solubility of NH3 in EG has been reported

in terms of mole fraction to be very high. At atmospheric pressure and 25°C it has been

reported to be a mole fraction of 0.406.94 It is been conjectured that this extremely high

solubility is due to a reversible chemical reaction between the NH3 gas and solvent

forming ammonium radicals at the hydroxyl groups.94 It is possible that this high

solubility of NH3 will affect the system of AC and EG. However, under lower pressure

the solubility will decrease which should help avoid any effects due to a large amount of dissolved NH3 on the reaction equilibrium.

The possibility of hysteresis on the EG was tested by repeating the previous

experiment to remove the AC from the EG at 30°C under vacuum. The same EG was

used for four consecutive experimental runs. Initially, 50.0 mL of EG were loaded into

the Schlenk tube. For each run ~28 g of AC was added. Table 14 shows the initial and

final amounts of solvent for reach run and how much AC was added. The amount of

solvent was found using the baseline solvent evaporation rate of 0.05 g/min, which was found by subjecting the Schlenk tube with only EG to vaccum to find its rate of evaporation. It should be noted that the mass of material remaining in the flask after the final run was 55.21 g. This can be attributed to NH3 or CO2 dissolved in the EG.

However, even though the reaction products are soluble in the EG it is evident from the

conversion time profiles in Figure 17 that the rate of decomposition is relatively

unaffected by the reuse of EG, so the EG could be successfully reused in a system level

application.

Table 14. Hysteresis measurements of ammonium carbamate in ethylene glycol at 30°C Run Initial Solvent (g) Final Solvent (g) AC added (g) 1 55.62 54.12 28.12 2 54.12 52.62 30.72 3 52.62 51.12 28.85 4 51.12 49.62 27.87

1.00

0.90

0.80

0.70

0.60

0.50

Conversion 0.40

0.30 Run 1 0.20 Run 2 Run 3 0.10 Run 4 0.00 0 5 10 15 20 25 30 Time (min)

Figure 17. Conversion as a function of time for the hysteresis measurements of ammonium carbamate in ethylene glycol at 30°C

2.3.9. Heat of Solution

Upon dissolution in a solvent, salts normally have either endothermic or

exothermic heats of solution. During the course of the experiments conducted it was

observed that AC has an endothermic heat of solution in EG. This could be valuable to the project since it could provide additional cooling to a slurry based system. The experiment to find the heat of solution of AC is based on a procedure by NIST which gives the heats of solution for a wide variety of salts at infinite dilution.95 In this work the

° heat of solution at infinite dilution, , is given by Equation 13.

° (13)

where is the heat of solution at a known molality and is a correction for

the heat of dilution to infinite dilution. It is also necessary to know the apparent molal

heat capacity of the solute, . It is possible to estimate to both and . In the

literature the normal way to estimate is to take the measurements of at a

variety of concentrations and to estimate by the differences between points and then

° to extrapolate to . The value of is calculated from Equation 14.

∆T (14)

where mt is the mass of solvent and solute, Cp is the specific heat, ΔT is the temperature change, and n is the moles of solute added. This value is determined experimentally by adding a known quantity of solute to a known amount of solvent and finding the maximum temperature change in an adiabatic vessel.

The experimental methodology was validated by finding the heat of solution of

NH4Cl in water. Data were collected for three trials of NH4Cl in water and the

experimental details and results are shown in Table 15. The heat of solution must be

adjusted for the dilution using Equation 13 where at 1.5 molarity from NIST is

560 J/mol, giving an average value of 14,840 J/mol.

Table 15. Experimental details and results for the heat of solution of ammonium chloride in water Mass H2O (g) Mass NH4Cl (g) ΔT(°C) Heat of Solution (kJ/mol) Trial 1 103.73 8.47 -5.2 15.4 Trial 2 103.79 8.35 -5.1 15.3 Trial 3 104.97 8.49 -5.2 15.2

The experimental uncertainty was evaluated using the method in Equation 15 and

the variables are defined along with their values in Table 16. All error values are from the

instrument manufacturers.

(15) …

Table 16. Uncertainty in the heat of solution calculation Variable Meaning Value wy Error of heat of solution Unknown y Heat of solution 15,400 J/mol

Error of water mass measurement ±0.05g x1 Water mass measurement 103.73 g

Error of solid mass measurement ±0.003g x2 Solid mass measurement 8.470 g

Error of thermocouple measurement 1 ± 0.31°C x3 Thermocouple measurement 1 20.6°C

Error of thermocouple measurement 2 ±0.31°C x4 Thermocouple measurement 2 15.4°C

The resulting uncertainty is ±310 J/mol or ±2.0%. Therefore, the final value for the

experiment is 14,840±310 J/mol. Comparison to the value of 14,780 ± 60 J/mole from

literature validates the experimental procedure since the percent difference is only 0.4%

and lies well within the uncertainty estimation.95

The same procedure was used to find the heat of solution of AC in EG. The

uncertainty was evaluated using the same method as used for the NH4Cl in water. The

plot in Figure 18 gives the heat of solution as a function of molality with error bars and

gives the line used to extrapolate the heat of solution to infinite dilution. Multiple measurements were conducted at each concentration and the details of each experiment

54 can be found in Table 17. The heat of solution of AC in EG was determined to be 9,700

J/mol. This value is lower than that of ammonium chloride in water and quantifies the cooling effect observed when the AC was dissolved in EG; it also shows that merely adding AC to EG will lower the system temperature.

12000

10000

8000

6000

4000 Heat of Solution (J/mol) of Heat Solution

2000 y = -2379.3x + 9686.4 R² = 0.9647 0 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 Molality

Figure 18. Heat of solution of ammonium carbamate in ethylene glycol as a function of molality

Table 17. Experimental data for determination of heat of solution of ammonium carbamate in ethylene glycol Molality Trial ΔT EG Mass AC Mass Heat of Solution Uncertainty (°C) (g) (g) (J/mole) (J/mol) 0.25 1 1 100 1.952 9787 1370 0.25 2 0.9 100 1.954 8809 1370 0.25 3 0.9 100 1.952 8809 1370 0.5 1 1.7 100 3.908 8471 698 0.5 2 1.7 100 3.905 8478 698 0.5 3 1.8 100 3.905 8977 698 0.75 1 2.3 100 5.867 7792 474 0.75 2 2.2 100 5.874 7453 474 0.75 3 2.2 100 5.87 7453 474 1 1 2.9 100 7.809 7504 362 1.25 1 3.1 100 9.675 6528 295 1.5 1 3.5 100 11.721 6252 250

2.3.10. Born-Harber Cycle for Ammonium Carbamate

The Born-Harber cycle for the decomposition of AC in EG is presented

graphically in Figure 19, and represented in Equations 16 through 19. There are three

main conclusions which apply to this work which can be drawn from this cycle. The first

is that the heat of solution could be used to provide subambient cooling at a system level

which could be important for achieving the desired temperature range. The second point is that although the endothermic heat of solution could be used to provide subambient cooling, it does not have an overall effect, since as Figure 19 illustrates, the total amount of energy consumed by the decomposition is the same whether or not the AC goes into solution. The final point is that this heat of solution could even be detrimental depending on the mixing point. If the mixing takes place away from the heat exchange vessel then it is possible that the solution could warm again to room temperature, which would result in a loss of 6.2% of the total heat rejection capability of the AC.

COg 2NHg NHCOONHsolid 157 kJ (16)

NHCOONHsolid 9.7kJNHCOONHsolution (17)

NHCOONHsolution 147.3 kJ COg 2NHg (18)

NHCOONHsolid 157 kJ COg 2NHg (19)

Figure 19. Born-Harber cycle for the decomposition of ammonium carbamate

2.3.11. Specific Heat of Ammonium Carbamate

It is essential to know the specific heat of AC for any calculations requiring a temperature change of the AC. The specific heat of AC from 20°C to 180°C, given in

Table 18, have been reported in the Kirk-Othmer Encyclopedia of Chemical Technology but no reference to primary literature or experimental technique is available.96 57

Table 18. Specific heat of ammonium carbamate96 T (°C) Cp (J/gK) 20 1.67 60 1.92 100 2.18 140 2.43 180 2.59

To validate the literature data, DSC was used to find a single point measurement

of the specific heat of AC at 0°C. Two values of 1.39 J/g•K and 1.56 J/g•K were found

and the average of 1.48 J/g•K was used. This point was plotted with the literature data

from 20°C to 100°C to verify the data. Only the points from 20°C to 100°C were used

since the others were well outside the operating parameters of the experimental work.

The R2 of 0.99 for the linear fit suggests that the data from the literature is valid

compared to the experimental work, so the literature value for the specific heat was used.

The data used to find the linear trendline is below in Figure 20 and shows the relationship in Equation 20.

Cp (J/gK) = 0.0069T(°C) + 1.5018 (20)

2.5

1.5

1 y = 0.0069x + 1.5018 R² = 0.9937 Specific Heat (J/gK)

0.5

0 0 20 40 60 80 100 120 Temperature (°C)

Figure 20. Temperature dependent specific heat of ammonium carbamate

2.4. Decomposition Pressure

The decomposition pressure of AC has been reported in literature. Since the focus

is now a system using AC for TM that will have the AC in a liquid heat transfer medium

such as EG, it is advantageous to know the decomposition pressure of AC in a liquid

medium. For this study the decomposition pressure of the solid was first measured and

compared to literature to validate the experimental procedure. Next, the decomposition

pressure of AC in EG was measured from 10°C to 50°C.

The experimental setup consisted of a glass Schlenk vessel 100 mL in volume. It

was connected to a VacuumBrand VSK 3000 absolute pressure diaphragm gauge which

is capable of measuring pressure from 795 torr to 1 torr, independent of gas species

present. To measure the pressure, the vessel was filled with AC powder, solution or

slurry. The non-condensable gases were removed from the vessel by evacuating the system and then allowing it to come up to its equilibrium pressure several times. Then the

vessel was immersed in a thermostated water bath and the material in the vessel was allowed to equilibrate for at least 30 minutes at each temperature before a reading was taken.

The first experiment conducted was to measure the decomposition pressure of the neat powder to compare to published results. The results of the experiment are shown in

Figure 21. The black squares are experimental data and all other points are from literature sources. As the data shows the experimental setup gives accurate data so it can be used for decomposition pressure experiments.

1000 Egan, Potts, Potts (62) 900 Joncich, Solka, Bower (34) 800 Briggs and Migrdichian (60) 700 Bennett, Rithie, Roxburgh, Thomson (63) 600 Experimental Bare Powder 500 400 Pressure (Torr) 300 200 100 0 10 20 30 40 50 60 70 Temperature (°C)

Figure 21. Experimental decomposition pressure of ammonium carbamate powder

The vapor pressure of pure EG was measured as a baseline over the temperature

range from 10°C to 50°C. It was found to be less than 1 torr throughout the measurement

range, which was the lower limit of the sensor. Therefore, the vapor pressure of the

solvent is well below the decomposition pressure of the solid, so it should not play a

significant role in the experiment.

The vapor pressure of the AC in EG was measured by making a slurry of AC in

EG. The same experimental procedure was used as with the neat powder. The results are

shown in Figure 22, the literature references in this figure are the same as in Figure 21.

Up to 40°C, the results for the slurry are identical to the bare power and show a slightly

higher pressure above 40°C. This can be explained by examining the components that

contribute to the total pressure in the vessel. Equation 21 shows that the total pressure in

the vessel will be a combination of the partial pressures of the EG, CO2 and NH3. It is likely that the sum of these terms will be greater than the sum of the partial pressures of

CO2 and NH3 above the neat solid for two reasons. The first is that the partial pressures above the solution will likely be higher due to the entropy dependence of the decomposition pressure which will show a dependence on concentration and temperature.

This will not exist for the neat powder and the temperature dependence explains why the

deviation was only noticed at higher temperatures. The second reason is that the addition

of the EG term should also slightly raise the equilibrium pressure. Although the

experiment shows a slight elevation in the decomposition pressure for the EG slurry, it is

unlikely that this small deviation explains the markedly different decomposition rates

observed.

(21)

1000 Literature Bare Powder 900 Experimental Bare Powder 800 AC in EG 700

600

500

400 Pressure (Torr) 300

200

100

0 10 20 30 40 50 60 70 Temperature (°C)

Figure 22. Decomposition pressure of ammonium carbamate in ethylene glycol

2.5. Ammonium Carbamate Decomposition Test System

In order to evaluate the use of AC for heat rejection, a test system was constructed.

This system needed to be capable of quantitatively determining the amount of thermal energy rejected by the AC as a function of temperature as well as control the overhead pressure on the AC. A schematic of the proposed system is in Figure 23. The principle behind this system is that the amount of heat rejected by the AC can be quantified using an energy balance over the simulated thermal load. The steps in the design process are detailed in the subsequent sections.

Figure 23. Ammonium carbamate decomposition test system

2.5.1. Reactor Sizing

The first consideration in the design of this system was determining the reactor size. A test matrix was developed and is shown in Table 19. The basis for this matrix is the rejection of 20 MJ of thermal energy for the full scale system, which would require

11.1 kg of AC. The solubility multiple number is based on 18.5 g AC per 100 mL of EG.

For the purposes of a laboratory scale demonstration unit, a system size of less than 1 liter is necessary and less than 500 mL is desirable to operate in practical limits. It was assumed that the system size would be two times the liquid volume used. In the matrix below those combinations that are possible with a 1 liter system are shown in yellow and those possible with a 500 mL system are shown in orange. From this test matrix it was determined that a 500 mL reactor would be sufficient for laboratory scale experiments as

it would be capable of running measurements at a scale of 1:50 and below, depending on

the solubility multiple. A 500 mL jacked reactor from Chemglass (CG-1926-01) was selected, and the jacket evacuated with a rotary vein vacuum pump to ensure that the vessel is well insulated so the assumption of an adiabatic reactor can be made.

On a system level the reactor size will need to be more carefully selected to match the peak thermal load required to avoid unnecessary dead space in the reactor. This is to limit oversizing the reactor which will lower the specific properties and to avoid a large gas volume to fill with decomposed AC as bringing this volume to equilibrium would cause unnecessary decomposition.

Table 19. Test matrix for ammonium carbamate system (see key and text for explanation) Solubility 1:1 1:5 1:10 1:25 1:50 1:100 1:200 Multiple Scale Scale Scale Scale Scale Scale Scale 11.1 2.22 1.11 0.444 0.222 0.111 0.0555 0.5 120 24 12 4.8 2.4 1.2 0.6 240 48 24 9.6 4.8 2.4 1.2 11.1 2.22 1.11 0.444 0.222 0.111 0.0555 1 60 12 6 2.4 1.2 0.6 0.3 120 24 12 4.8 2.4 1.2 0.6 11.1 2.22 1.11 0.444 0.222 0.111 0.0555 2 30 6 3 1.2 0.6 0.3 0.15 60 12 6 2.4 1.2 0.6 0.3 11.1 2.22 1.11 0.444 0.222 0.111 0.0555 3 20 4 2 0.8 0.4 0.2 0.1 40 8 4 1.6 0.8 0.4 0.2 11.1 2.22 1.11 0.444 0.222 0.111 0.0555 4 15 3 1.5 0.6 0.3 0.15 0.075 30 6 3 1.2 0.6 0.3 0.15 11.1 2.22 1.11 0.444 0.222 0.111 0.0555 5 12 2.4 1.2 0.48 0.24 0.12 0.06 24 4.8 2.4 0.96 0.48 0.24 0.12 11.1 2.22 1.11 0.444 0.222 0.111 0.0555 6 10 2 1 0.4 0.2 0.1 0.05 20 4 2 0.8 0.4 0.2 0.1 Key AC Needed (kg) Liquid Volume (liters) System Size (liters)

2.5.2. Vacuum Pump Selection

One of the most common laboratory vacuum pumps is the rotary vein pump.

These are generally able to obtain pressures less than 10-3 torr, but they rely on the pumping mechanism being immersed in vacuum pump oil. This presents a problem for use with this system since the AC would likely accumulate in the vacuum pump oil and at worst destroy the pump or at least necessitate frequent oil changes. Diaphragm pumps present a good option for this experimental application. Although they are only capable of an ultimate vacuum of ~1 torr, this pressure should be sufficient for the AC based on

the decomposition pressure as a function of temperature curves. The advantages of

diaphragm pumps are that they are oil free and the only surfaces that the removed gases

come into contact with are polymer membranes which are resistant to chemical attack.

Additionally, if AC were to condense inside of the pump, the pump can be quickly

disassembled and cleaned. The diaphragm vacuum pump chosen for this application is

the Vacuubrand MD1C three stage chemistry diaphragm pump. It is capable of obtaining

an ultimate vacuum of 1.5 torr and a pumping speed of 1.5 m3/hr. This pump speed

corresponds to a maxium rate of decomposition of AC of 1.6 kg/hr which sets an upper limit to the power obtainable by the system. It would be possible to achieve a higher

pump speed with a larger pump but every vacuum pump has an upper limit to its

pumping speed so at the system level the pump speed will need to be selected to meet the

thermal power needs.

2.5.3. Vacuum Controller

A vacuum controller is necessary so that the effect of various overhead pressures

on the rate of decomposition can be measured. Although the lowest possible pressure will

always lead to the most rapid rate of decomposition, operating at the lowest possible

pressure may not be optimal at the system level. Accordingly it is important for a system

design to know if achieving a 1 torr pressure is necessary or if a higher pressure such as

200 torr or 400 torr would be sufficient to achieve rapid decomposition and yield good

system performance. The necessary pressure will drive pump selection, so it will be

important to fully understand the pressure dependence of the rate of decomposition.

Vacuubrand sells a vacuum controller which, when coupled with a fast-acting solenoid

valve between the vacuum pump and system, is able to control the system pressure. The

Vacuubrand CVC 3000 vacuum controller was selected for the process. The controller

measures pressure using an external vacuum gauge on the reaction vessel and regulates the vacuum through the Vacuubrand VV-B 6C solenoid vacuum valve. The valve is made with a corrosion resistant flowpath which prevents the NH3 from causing any problems to

the valve.

2.5.4. Vacuum Gauge Selection

The CVC 3000 vacuum controller comes with an integrated vacuum gauge inside

the controller unit. However, it was determined that an external gauge should be used that

is connected directly to the reactor to prevent any delay in pressure reading caused by the

tubing. The Vaccubrand VSK 3000 ceramic diaphragm gauge was chosen for this

application. It is corrosion resistant to almost any environment since the only part of the

gauge that comes in contact with the process is a ceramic. Additionally, a diaphragm type

gauge was chosen since it is capable of measuring pressure independent of gas species.

This feature is important since the two gas species which will be present in the vessel are

NH3 and CO2, and vacuum gauges are not normally calibrated to these gases. The gauge

is capable of measuring pressures from 0.1 to 810 torr.

2.5.5. Temperature Measurement

The underlying principle to the system is to perform an energy balance around the

reactor vessel to determine the amount of heat rejected to the AC. In order to do this the

temperatures of the water inlet and outlet as well as the internal reactor temperature must

be accurately measured and recorded. The Omega Portable Handheld Data Logger (OM-

DAQPRO-5300) was selected for this purpose along with thermistor temperature probes.

The data logger is capable of up to 8 temperature input channels and can store up to

512,000 temperature points.

2.5.6. Tubing Selection

For all the areas under vacuum exposed to AC, clear Tygon vacuum tubing (3/8”

ID, 7/8” OD, ¼” wall) was selected so that any accumulation of AC could be visually

observed and the lines cleaned if necessary.

2.5.7. Simulated Thermal Load

For the purposes of this project it is important to have flexibility in a simulated

thermal load. A general purpose laboratory circulator (Polyscience Model 9005) was

chosen to represent any generic thermal load placed on the system. This circulator is

capable of operation from -20°C to 150°C depending on the circulating fluid selected. It

has a reservoir capacity of 6 liters and has two flow rates of 15 liters per minute or 7 liters

per minute. The recirculator was filled with distilled water as the working fluid.

For the energy balance on the water circulator, the water flow rate must be

known. The liquid flow rate through the system was determined by measuring the mass of water that passed into the circulator at its inlet in a certain amount of time. This procedure was repeated at each temperature to ensure accuracy.

2.5.8. Completed Experimental Apparatus

The completed experimental apparatus is shown in Figure 24. The water

circulator and the reactor jacket vacuum pump are in the bottom of the cart. The

circulator fluid loop is equipped with two valves so the reactor can be bypassed if

necessary. The entire system was assembled according to the schematic in Figure 23. The exhaust from the diaphragm pump was vented into a laboratory hood.

Solenoid valve Pressure Transducer

Reaction vessel Diaphragm vacuum pump

Magnetic stir plate Temperature logger

Pressure controller Thermal load

Figure 24. Experimental test apparatus

2.6. Ammonium Carbamate Decomposition Test System Experimental Work

The decomposition system allows for the thermal power rating to be calculated

for the system by using the time dependent temperature data. This is a measurement

based on an energy balance around a recirculator and is different than the earlier

measurements which calculated the energy based on the amount of AC decomposed. For

each experiment conducted with the system, 650 mL of heat transfer fluid was used as it

was determined that this was the amount necessary to cover the heat transfer coils.

Although it was initially planned to use less heat transfer fluid in the system design

process, it was necessary to use this amount of fluid to completely cover the heat transfer

coils. The fluid was brought into thermal equilibrium at the test temperature before the

addition of the AC. Before the AC was added the temperature logger was started. The amount of AC that was added was recorded as well as the time when the AC was added and the time when the vacuum pump was started. The overhead pressure could be controlled for each experiment. By using the time dependent temperature data, circulator flow rate and the specific heat of water, the thermal power for the system could be found by Equation 22.

∆ (22)

where is the rate of heat flow (W), is the mass flow rate of the water (g/s), Cp is the specific heat of water, and ΔT is the water temperature change between the inlet and

outlet of the reactor. The time dependent power rating was then integrated with respect to

time using the trapezoid rule to find the total amount of energy consumed by the AC decomposition. This result was compared with the amount of energy consumed by the decomposition and the amount of energy necessary to heat the AC to the reactor temperature. The amount of energy consumed by the decomposition was determined from the mass of the AC added and the literature value of the heat of decomposition of

2.01 kJ/g. The amount of energy necessary to bring the AC from room temperature to the reactor temperature was calculated using the heat capacity of the AC, which was assumed to be constant. The heat loss through the reactor walls was neglected and a vacuum insulated reactor was used to assure that this was a reasonable assumption.

Both EG and PG were used as heat transfer fluids in the decomposition experiments so that their performance could be compared to give a recommendation of

the optimal heat transfer fluid for the system. Tests were conducted at the following

temperatures: 30°C, 45°C, 50°C, 55°C and 60°C. The greatest number of experiments

was conducted at 60°C since this is the highest temperature that would be of practical value for this work and as the highest temperature should give the best thermal power rating by providing the most rapid decomposition. For an initial run at each temperature, the pressure controller was set to 0 torr so that the vacuum pump would run continuously and result in the maximum rate of decomposition available from the system. At 60°C higher pressures of 200 and 400 torr were tested as well. A representative picture of the decomposition reaction occurring is shown in Figure 25. Bubbles of the product gases can be seen in the image.

Figure 25. Decomposition of ammonium carbamate in reactor

2.6.1. Decomposition Test System Experimental Procedure

Heat transfer fluid (650 mL) was added to the reactor. The fluid was agitated with

a magnetic stir bar to optimize the heat transfer. The circulator was set to the test

temperature, and the reactor contents were allowed to come to the desired test

temperature as measured by the reactor thermistor probe. The vacuum pump to the

reactor jacket was turned on. The approximate amount of AC was measured; the exact

amount added for the reaction was determined by weighing the jar with the AC and then

the empty jar after addition. The temperature logger was started at least one minute prior

to the test so that the initial temperatures could be recorded. The pressure controller was

set to the appropriate pressure for the test. Next, the AC was quickly added to the reactor,

the reactor was closed and the vacuum pump was turned on using the pressure controller.

The time when the AC was added to the reactor and the time when the vacuum pump was

started were recorded. The pressure displayed on the vacuum controller was manually

recorded at appropriate intervals throughout the experiment. The decomposition was

allowed to continue until the reactor contents, water inlet and water outlet were at steady

state.

2.6.2. Experimental Data Analysis

The experimentally collected parameters for each test were the time dependent temperature of the reactor contents, the time dependent circulator fluid temperature at the

reactor inlet and outlet, the time dependent pressure in the reactor, the amount of AC

added and the room temperature. Specific information for each run including

experimental parameters and graphs can be found in the appendix. Since the AC was

stored on the laboratory bench, its initial temperature was assumed to be that of the room.

For each run the lower bound of integration is the time when the AC was added to the

reactor and the upper bound of integration is the time when thermal equilibrium was

reached. For runs where the pressure was subsequently lowered once thermal equilibrium

was reached, the lower bound of integration is when the pressure controller was changed.

The bounds of integration are shown for each run in the form of vertical lines on the graphs which can be found in the appendix. For each run the percent difference between energy found from the integration and AC mass value is shown. The ΔT shown is that between the reactor inlet and outlet for the circulator. Each experimental run is summarized in Table 20. In this table the difference is negative when the predicted energy value is higher than amount of energy found using numerical integration.

Table 20. Summary of ammonium carbamate test system experimental data Temp Solvent AC Set Decomp. Cp (kJ) Integration Difference Peak (°C) (g) Pressure (kJ) (kJ) Power (W) (torr) 30 PG 105.2 0 211.53 1.88 161.32 -24.4% 39.3 30 EG 104.0 0 208.95 1.86 107.99 -48.8% 52.2 45 PG 104.6 0 210.31 6.01 255.98 18.3% 138 45 EG 104.3 0 209.59 5.92 225.05 4.4% 145.5 50 PG 102.4 0 205.73 7.40 190.67 -10.5% 144.7 55 PG 101.4 0 203.76 6.96 206.31 -2.1% 203.8 60 PG 100.9 0 202.89 7.05 186.82 -11.0% 223.9 60 PG 102.6 200, 0 206.27 7.18 195.37 -8.5% 223.3, 42.0 60 EG 102.0 0 204.99 6.58 206.64 -2.3% 241.5 60 EG 101.7 200, 0 204.34 6.30 219.50 4.2% 238.8, 74.2 60 PG 102.2 400, 200, 205.39 7.38 236.12 11.0% 174.2, 0 101.3, 44.0 60 EG 103.0 400, 200, 207.02 7.41 208.73 -2.7% 159.4, 0 134.1, 87.7 60 PG 204.4 0 410.75 14.77 530.85 24.8% 319.2 60 PG, reused 100.3 0 201.53 7.16 248.90 19.3% 266.9 60 PG, reused 102.3 0 205.59 7.27 256.73 20.6% 269.8

2.6.3. Discussion of Experimental Results

An analysis of the results shows several trends that were expected from the early decomposition experiments conducted under high vacuum. Experimental trends that will

be discussed are the effect of temperature, pressure, solvent, and the amount of AC added on the amount of heat removed by the AC and the peak power rating that was achieved.

The early decomposition experiments conducted for this work indicated that temperature would have a strong effect on the rate of decomposition and therefore on the thermal power achieved. This trend is clearly demonstrated since the peak power

increases as the temperature increases from a minimum of 39.3 W at 30°C to a maximum

319.2 W at 60°C. Hence, for situations in which it is desired to have a high thermal

power rating, it is shown that the decomposition reaction vessel should operate at the

highest possible temperature. The current system has mass transfer limitations because

the vacuum pump being used limits mass transfer capability. Therefore, one way to

overcome the temperature limitation would be to install a more powerful vacuum pump,

or multiple pumps, to achieve a greater rate of mass transfer.

The pressure set by the vacuum pump also has an effect on the peak power

achieved by the system. The data collected only allows analysis at 60°C, but it is

expected that similar trends would also be found at other temperatures. At 60°C, setting

the pressure controller to either 200 torr or 0 torr did not have an appreciable effect on the

peak power. With EG the peak powers were observed to be 241.5 and 238.8 W at 0 and

200 torr, respectively, and with PG the peak powers were 223.9 and 223.3 W at 0 and

200 torr respectively. A way to explain this observation is that the equilibrium pressure of

AC at 60°C is approximately 850 torr which is considerably higher than 200 torr.

Additionally, examining the pressure data for the experiments in Figure 39 and Figure 43

shows that even when the pressure controller was set to 0 torr it remained considerably

above this pressure for much of the experiment. This means that while the controller was

set low, the pressure in the reactor was much higher due to rapid decomposition.

However, a more marked difference is noted when the pressure was set to 400 torr. With

EG the peak power was 159.4 and 241.5 W for 400 torr and 0 torr, respectively, and for

PG the peak power was 174.2 W and 223.9 W for 400 torr and 0 torr respectively. The

difference observed in changing the set overhead pressure from 200 to 400 torr shows

that the pressure certainly has a strong effect on the system thermal power and therefore must be considered in any system design.

An important result of the pressure tests is that while it is necessary to lower the pressure of the system for high power ratings a “high” vacuum will not be needed. In the early decomposition testing, all runs were conducted with a high vacuum system. It was suspected that a system capable of that level of vacuum was not needed for rapid

decomposition. The fact that there was no difference in peak power at both a system set

to 0 and 200 torr makes the idea of using a system at reduced pressure more realistic. In

an application, it is foreseeable to lower system pressure to this level, but it would be in

no way reasonable to lower the system pressure to 1 x 10-5 torr as was used in the early

decomposition experiments.

It was noted when the system was set to different pressures that the amount of AC

which decomposed until the system reached steady state changed. This is evaluated using

numerical integration to find the amount of heat rejected to the thermal load. For the case

of AC in PG at 400 torr, 200 torr and 0 torr, as seen in Figure 47 and Table 31, the

amount of energy that was rejected to the AC at each pressure was 173 kJ, 44.6 kJ and

18.4 kJ, respectively, for a total of 236.1 kJ, compared to a predicted total of 212.8 kJ.

The reason that steady state was reached at each pressure is that some amount of AC, or

its decomposition products, will be soluble in the PG at each pressure. This is evident

since as the pressure was reduced from a value where the system was at steady state, the

vessel temperature fell as the AC decomposed and caused the system to reject more heat to the simulated thermal load. Visually, it was evident in the system since when the set

pressure was changed from 400 torr to 200 torr or from 200 torr to 0 torr, the heat transfer

fluid began to bubble. The source of this bubbling could either be dissolved AC

decomposing or decomposition products being removed from solution in an endothermic

process which lowered the vessel temperature. It is likely that ammonia could be the

decomposition product removed since it is extremely soluble in EG, and presumably in

PG, as discussed in Section 2.3.8. Therefore, it is plausible that once the pressure was

lowered, additional ammonia was removed from the solution, lowering the temperature.

On a full scale system this steady state limitation can be overcome by continuously

adding AC to the decomposition system to keep the reactor contents far from equilibrium

conditions to maintain a high power rating.

Early experiments showed that using a heat transfer fluid in which AC is soluble

showed a drastic increase in the rate of decomposition as compared to a heat transfer

fluid in which it is insoluble. Ethylene glycol was the first heat transfer fluid which was

found to satisfy both the solubility criteria and have a very high boiling point so that it

would not be consumed during the process. Propylene glycol was also considered since it

is similar to EG but is much less toxic, to the point where it is considered to be food safe.

Both EG and PG were considered in testing to determine if EG had an appreciable

advantage over the preferred PG. It was found at each temperature that EG had a slightly

higher peak power than PG. The comparisons can be found in Table 20 and showed

76 percent differences of 28%, 5% and 8% for 30°C, 45°C and 60°C, respectively. The larger percent difference at 30°C can be explained since the peak powers were relatively low so a small difference of only 12.9 W leads to a difference of 28% while at 60°C a difference of 17.6 W leads to a difference of 8%. Since, especially at higher temperatures, the two solvents offer comparable performance it can be concluded that using nontoxic

PG should not lead to performance degradation compared to EG.

The amount of AC initially added to the system has an effect on the peak power.

This can be shown by comparing the experiments conducted in PG at 60°C where initial charges of 100.9 g and 204.4 g of AC were added. The peak power found at 100.9 g was

223.9 W, while that for the 204.4 g charge was 319.2 W. Part of this increase in peak power arises from the heat capacity of the AC. Adding more AC will cause a greater initial temperature depression in the reactor contents since it is at a lower temperature than the PG and a greater amount of AC will further lower the temperature of the PG.

The heat capacity of AC at 60°C is 1.92 kJ/kg·°C and the heat capacity of PG at 60°C is

2.73 kJ/kg·°C.90,96,97 Therefore, adding 100.0 g of AC at 20°C to 650 mL PG at 60°C should cause the temperatures to stabilize at approximately 56°C while adding 204.4 g of

AC at 20°C should cause the temperature to stabilize at 53°C. It was observed that adding

100.9 g of AC caused the reactor contents to reach a temperature minimum of 47.6°C, while adding 204.4 g of AC caused the reactor contents to reach a minimum of 45.6°C.

These temperature differences are approximately the same as would be expected from the specific heats alone. However, this is not sufficient to account for a peak power difference of 35%. A second explanation for the marked increase in peak power is that adding a greater amount of AC kept the reactor contents farther from equilibrium

conditions and therefore increased the rate of decomposition which increases the power

rating. This makes sense in light of the aforementioned discussion of the effect of equilibrium in the system. For system design recommendations this shows that keeping a greater amount of AC in the slurry will increase the system power rating by maximizing the rate of decomposition.

2.6.4. Conclusions from Ammonium Carbamate Decomposition System Tests

The tests conducted in this work have demonstrated several concepts that will be

necessary to incorporate into the next generation test system. The first result is that running the system at higher temperatures will lead to much higher thermal power ratings. Although this was initially expected and was shown with the early decomposition tests using a high vacuum system, it reinforces that in any larger system design, every effort should be made to use a high temperature for maximum thermal power. The second important conclusion, which was not previously demonstrated, is the effect of pressure on the rate of decomposition and therefore on thermal power. Many tests were conducted with the system pressure set to 0 torr, but as the results in the appendix show, the pressure in the reactor during the experiment was far from this value since rapid decomposition kept the system pressure higher than the set pressure. Experiments were then conducted using the pressure controller to maintain a higher pressure in the system. It was found that a higher pressure could be set and still maintain a rapid rate of decomposition.

Specifically, the tests conducted at 60°C showed essentially no difference in the peak power between the system set at 0 torr and the system set at 200 torr but did show a difference in the total amount of AC that would be decomposed before equilibrium is reached at a pressure, a limitation which can be overcome by continuously adding AC.

The conclusion from the pressure effect is positive from a real system perspective since it

is much more realistic to obtain a pressure on the order of 200 torr than 10-5 torr. The

choice of either EG or PG led to comparable results, so PG seems preferable as the

system heat transfer medium, since it is non-toxic and environmentally friendly. Finally, the amount of AC in the reactor does have an effect on the thermal power and adding a

greater amount of AC to the system should function to increase the thermal power rating.

This can be tested by using a method to continuously add AC to the system and then evaluate the optimal amount of AC to maintain in the reactor.

CHAPTER III: CONCLUSIONS AND FUTURE WORK

Ammonium carbamate shows promise as a high energy density thermal energy

storage material with the ability to effectively manage high-flux low-grade heat. This

project has demonstrated that AC will best function when integrated with a heat transfer

fluid. Testing has shown that both EG and PG are effective heat transfer fluids for AC

with PG the preferred fluid since it does not have the toxicity problems that are

associated with EG. It has also been found that reducing the overhead pressure on the

system will greatly enhance the power rating through improving the rate of mass transfer.

This had been expected from the equilibrium decomposition pressure and was reinforced

with all experiments. It was also shown that it is not necessary to reduce the pressure to very low vacuum levels (10-5 torr), but that a vacuum on the order of 200 torr (0.25 atm)

will be effective. The small scale test system has shown peak thermal power ratings of

over 300 W or over 340 W/kg of AC and PG, which demonstrates the capability of AC.

Future work should focus on developing more temperature-pressure-power rating

relationships for the decomposition test system. Additionally, a method should be

developed to continuously add AC to the reactor during an experiment without the need

to bring the system to atmospheric pressure and remove the lid.

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APPENDIX

The data presented in the appendix has a data table and two graphs for each experimental run. In the data tables the initial bulk temperature is the temperature of the heat transfer fluid prior to the addition of the AC. The initial AC temperature is the room temperature, which is assumed to be the AC temperature since it was stored on the laboratory bench. The heat from decomposition is the amount of energy required to completely decompose the mass of AC added. The heat from the temperature change of the AC accounts for the temperature change of the AC using the heat capacity. The bounds of integration were chosen based on the time when the AC was added or the pressure changed and the time when steady state was reached. The percent difference is negative when the energy from the integration is less than that from the calculated energy based on the AC mass. In the first figure for each run the bulk temperature is the temperature of the heat transfer fluid. The temperature in is the water temperature measured from the recirculator immediately before it enters the heat transfer coil into the reactor. The temperature out is the water temperature measured from the recirculator immediately after its leaves the heat transfer coil in the reactor. The heat rejected is the thermal power calculated from the energy balance around the recirculator heat exchanger in the reactor. In the second figure the pressure was manually recorded as displayed on the pressure controller. The ΔT is the reactor inlet and outlet recirculator water temperature difference. The vertical lines on each graph represent the bounds of

integration. Where there is more than one area of integration different colors were used for each zone of integration.

Table 21. Experimental data for ammonium carbamate in propylene glycol at 30°C Initial Bulk Temperature (°C) 30.0 Initial AC Temperature (°C) 19.3 AC Added (g) 105.241 Water Flow Rate (g/s) 12.00 Heat from Decomposition (kJ) 211.5 Heat from Temperature Change of AC (kJ) 1.88 Lower Bound of Integration (s) 100 Upper bound of integration (s) 9500 Sum of Integration (kJ) 161.3 Difference (integration and theoretical) -24.41% Solvent PG 300 35 Heat Rejected 34 250 Bulk Temperature Temp In 33 200 Temp Out 32 31 150 30 29 100 28 Temperature (°C) Temperature

Heat Rejected (Watts) 50 27 26 0 25 0 2000 4000 6000 8000 Time (s)

Figure 26. Temperatures and heat rejected for ammonium carbamate in propylene glycol at 30°C 50 2.00 45 Pressure 1.80 40 ΔT 1.60 35 1.40 30 1.20 25 1.00 T (°C)

20 0.80 Δ

Pressure (torr) 15 0.60 10 0.40 5 0.20 0 0.00 0 2000 4000 6000 8000 10000 Time (s)

Figure 27. Pressure and ΔT for ammonium carbamate in propylene glycol at 30°C

Table 22. Experimental data for ammonium carbamate in ethylene glycol at 30°C Initial Bulk Temperature (°C) 30.0 Initial AC Temperature (°C) 19.3 AC Added (g) 103.957 Water Flow Rate (g/s) 12.00 Heat from Decomposition (kJ) 208.9 Heat from Temperature Change of AC (kJ) 1.86 Lower Bound of Integration (s) 180 Upper Bound of Integration (s) 10000 Sum of Integration (kJ) 108.0 Difference (integration and theoretical) -48.77% Solvent EG 300.00 35 Heat Rejected 34 Bulk Temperature 250.00 33 Temp In 200.00 Temp Out 32 31 150.00 30 29 100.00 28 Temp (°C) Temp Heat Rejected (Watts) 50.00 27 26 0.00 25 0 2000 4000 6000 8000 10000 Time (s)

Figure 28. Temperatures and heat rejected for ammonium carbamate in ethylene glycol at 30°C 40 2 Pressure 35 1.8 ΔT 1.6 30 1.4 25 1.2 20 1 T (°C) 15 0.8 Δ

Pressure (torr) 0.6 10 0.4 5 0.2 0 0 0 2000 4000 6000 8000 10000 Time (s)

Figure 29. Pressure and ΔT for ammonium carbamate in ethylene glycol at 30°C

Table 23. Experimental data for ammonium carbamate in propylene glycol at 45°C Initial Bulk Temperature (°C) 45.2 Initial AC Temperature (°C) 28.2 AC Added (g) 104.632 Water Flow Rate (g/s) 12.79 Heat from Decomposition (kJ) 210.3 Heat from Temperature Change of AC (kJ) 6.00 Lower Bound of Integration (s) 60 Upper Bound of Integration (s) 3700 Sum of Integration (kJ) 256.0 Difference (integration and theoretical) 18.34% Solvent PG 200 50 180 48 160 46 140 44 120 42 Heat Rejected 100 40 Bulk Temperature 80 Temp In 38 60 Temp Out 36 Temperature (°C) Temperature

Heat Rejected (Watts) 40 34 20 32 0 30 0 1000 2000 3000 4000 5000 6000 Time (s)

Figure 30. Temperatures and heat rejected for ammonium carbamate in propylene glycol at 45°C 140 5 4.5 120 Pressure ΔT 4 100 3.5 80 3 2.5

60 T (°C)

2 Δ

Pressure (torr) 40 1.5 1 20 0.5 0 0 0 1000 2000 3000 4000 5000 6000 7000 Time (s)

Figure 31. Pressure and ΔT for ammonium carbamate in propylene glycol at 45°C

Table 24. Experimental data for ammonium carbamate in ethylene glycol at 45°C Initial Bulk Temperature (°C) 45.3 Initial AC Temperature (°C) 28.6 AC Added (g) 104.274 Water Flow Rate (g/s) 12.79 Heat from Decomposition (kJ) 209.6 Heat from Temperature Change of AC (kJ) 5.92 Lower Bound of Integration (s) 20 Upper Bound of Integration (s) 5500 Sum of Integration (kJ) 225.1 Difference (integration and theoretical) 4.43% Solvent EG

200 50 180 48 160 46 140 44 120 42 Heat Rejected 100 40 Bulk Temperature 80 Temp In 38 60 Temp Out 36 Temperature (°C) Temperature

Heat Rejected (Watts) 40 34 20 32 0 30 0 1000 2000 3000 4000 5000 6000 Time (s)

Figure 32. Temperatures and heat rejected for ammonium carbamate in ethylene glycol at 45°C

120 5 Pressure 4.5 100 ΔT 4 80 3.5 3 60 2.5 T (°C)

2 Δ 40 Pressure (torr) 1.5 20 1 0.5 0 0 0 1000 2000 3000 4000 5000 6000 7000 Time (s)

Figure 33. Pressure and ΔT for ammonium carbamate in ethylene glycol at 45°C

Table 25. Experimental data for ammonium carbamate in propylene glycol at 50°C Initial Bulk Temperature (°C) 49.8 Initial AC Temperature (°C) 19.3 AC Added (g) 102.353 Water Flow Rate (g/s) 12.71 Heat from Decomposition (kJ) 205.7 Heat from Temperature Change of AC (kJ) 7.40 Lower Bound of Integration (s) 50 Upper Bound of Integration (s) 2500 Sum of Integration (kJ) 190.7 Difference (integration and theoretical) -10.54% Solvent PG

300 55

250 50 200 Heat Rejected 45 150 Bulk Temperature Temp In 40 100 Temp Out Temperature (°C) Temperature

Heat Rejected (Watts) 50 35

0 30 0 1000 2000 3000 4000 Time (s)

Figure 34. Temperatures and heat rejected for ammonium carbamate in propylene glycol at 50°C

160 5.00 140 Pressure 4.50 4.00 120 ΔT 3.50 100 3.00 80 2.50 T (°C) 60 2.00 Δ

Pressure (torr) 1.50 40 1.00 20 0.50 0 0.00 0 1000 2000 3000 4000 Time (s)

Figure 35. Pressure and ΔT for ammonium carbamate in propylene glycol at 50°C

Table 26. Experimental data for ammonium carbamate in propylene glycol at 55°C Initial Bulk Temperature (°C) 55.2 Initial AC Temperature (°C) 21.5 AC Added (g) 101.372 Water Flow Rate (g/s) 12.18 Heat from Decomposition (kJ) 203.8 Heat from Temperature Change of AC (kJ) 6.95 Lower Bound of Integration (s) 90 Upper Bound of Integration (s) 2200 Sum of Integration (kJ) 206.3 Difference (integration and theoretical) -2.09% Solvent PG

300 60 58 250 56 200 54 52 Heat Rejected 150 50 Bulk Temperature Temp In 48 100 46

Temp Out (°C) Temperature

Heat Rejected (Watts) 50 44 42 0 40 0 500 1000 1500 2000 2500 3000 Time (s)

Figure 36. Temperatures and heat rejected for ammonium carbamate in propylene glycol at 55°C 300 5.00 Pressure 4.50 250 ΔT 4.00 200 3.50 3.00 150 2.50 T (°C)

2.00 Δ 100 Pressure (torr) 1.50 50 1.00 0.50 0 0.00 0 500 1000 1500 2000 2500 3000 Time (s)

Figure 37. Pressure and ΔT for ammonium carbamate in propylene glycol at 55°C

100

Table 27. Experimental data for ammonium carbamate in propylene glycol at 60°C Initial Bulk Temperature (°C) 60.1 Initial AC Temperature (°C) 20.8 AC Added (g) 100.94 Water Flow Rate (g/s) 11.57 Theoretical Heat from Decomposition (kJ) 202.9 Heat from Temperature Change of AC (kJ) 7.05 Lower Bound of Integration (s) 39 Upper Bound of Integration (s) 2000 Sum of Integration (kJ) 186.8 Difference (integration and theoretical) -11.01% Solvent PG

300 70

250 65 200 60 150 Heat Rejected Bulk Temperature 55 100 Temp In

Heat Rejected (Watts) 50 Temp Out 50

0 45 (°C) Temperature 0 500 1000 1500 2000 2500 Time (s)

Figure 38. Temperatures and heat rejected for ammonium carbamate in propylene glycol at 60°C

300 7 Pressure 250 6 ΔT 5 200 4 150

3 T (°C) Δ 100 Pressure (torr) 2 50 1 0 0 0 500 1000 1500 2000 2500 Time (s)

Figure 39. Pressure and ΔT for ammonium carbamate in propylene glycol at 60°C

101

Table 28. Experimental data for ammonium carbamate in propylene glycol at 60°C with two pressure settings Initial Bulk Temperature (°C) 60.2 Initial AC Temperature (°C) 20.7 AC Added (g) 102.624 Water Flow Rate (g/s) 11.57 Heat from Decomposition (kJ) 206.3 Heat from Temperature Change of AC (kJ) 7.18 Lower Bound of Integration (s) 60 Upper Bound of Integration (s) 1700 Sum of Integration (kJ) 195.4 Difference (integration and theoretical) -8.47% Solvent PG

300 70 Heat Rejected 250 Bulk Temperature Temp In 65 200 Temp Out 60 150 55 100 Temperature (°C) Temperature

Heat Rejected (Watts) 50 50

0 45 0 1000 2000 3000 Time (s)

Figure 40. Temperatures and heat rejected for ammonium carbamate in propylene glycol at 60°C with two pressure settings

350 7 Pressure 300 6 ΔT 250 5 200 4

150 3 T (°C) Δ

Pressure (torr) 100 2 50 1 0 0 0 1000 2000 3000 Time (s)

Figure 41. Pressure and ΔT for ammonium carbamate in propylene glycol at 60°C at two pressures 102

Table 29. Experimental data for ammonium carbamate in ethylene glycol at 60°C Initial Bulk Temperature (°C) 60.3 Initial AC Temperature (°C) 24 AC Added (g) 101.986 Water Flow Rate (g/s) 11.57 Heat from Decomposition (kJ) 205.0 Heat from Temperature Change of AC (kJ) 6.58 Lower Bound of Integration (s) 50 Upper Bound of Integration (s) 1900 Sum of Integration (kJ) 206.6 Difference (integration and theoretical) -2.33% Solvent EG

300 70

250 65 200 60 150 Heat Rejected Bulk Temperature 55 100 Temp In Temp Out (°C) Temperature Heat Rejected (Watts) 50 50

0 45 0 500 1000 1500 2000 2500 3000 Time (s)

Figure 42. Temperatures and heat rejected for ammonium carbamate in ethylene glycol at 60°C

300 10 Pressure 9 250 ΔT 8 200 7 6 150 5 T (°C)

4 Δ 100 Pressure (torr) 3 50 2 1 0 0 0 500 1000 1500 2000 2500 3000 Time (s)

Figure 43. Pressure and ΔT for ammonium carbamate in ethylene glycol at 60°C 103

Table 30. Experimental data for ammonium carbamate in ethylene glycol at 60°C at two pressures Initial Bulk Temperature (°C) 60.3 Initial AC Temperature (°C) 22 AC Added (g) 101.661 Water Flow Rate (g/s) 11.57 Heat from Decomposition (kJ) 204.3 Heat from Temperature Change of AC (kJ) 6.30 Lower Bound of Integration 1 (s) 121 Upper Bound of Integration 1 (s) 2200 Lower Bound of Integration 2 (s) 2800 Upper Bound of Integration 2 (s) 4200 Sum of Integration (kJ) 219.5 Difference (integration and theoretical) 4.21% Solvent EG 300 70 Heat Rejected 250 Bulk Temperature Temp In 65 Temp Out 200 60 150 55 100

Heat Rejected (Watts) 50 50

0 45 (°C) Temperature 0 1000 2000 3000 4000 5000 Time (s)

Figure 44. Temperatures and heat rejected for ammonium carbamate in ethylene glycol at 60°C at two pressures 300 10 Pressure 9 250 ΔT 8 200 7 6 150 5 T (°C)

4 Δ 100 Pressure (torr) 3 50 2 1 0 0 0 1000 2000 3000 4000 5000 Time (s)

Figure 45. Pressure and ΔT for ammonium carbamate in ethylene glycol at 60°C at two pressures

104

Table 31. Experimental data for ammonium carbamate in propylene glycol at 60°C at three pressures Initial Bulk Temperature (°C) 59.7 Initial AC Temperature (°C) 19.3 AC Added (g) 102.184 Water Flow Rate (g/s) 11.57 Heat from Decomposition (kJ) 205.4 Heat from Temperature Change of AC (kJ) 7.38 Lower Bound of Integration 1 (s) 60 Upper Bound of Integration 1 (s) 2300 Lower Bound of Integration 2 (s) 2380 Upper Bound of Integration 2 (s) 3200 Lower Bound of Integration 3 (s) 3580 Upper Bound of Integration 3 (s) 4100 Sum of integration (kJ) 236.1 Difference (integration and theoretical) 10.97% Solvent PG 300 70 Heat Rejected 250 Bulk Temperature Temp In 65 200 Temp Out 60 150 55 100 Temperature (°C) Temperature

Heat Rejected (Watts) 50 50

0 45 0 1000 2000 3000 4000 5000 Time (s)

Figure 46. Temperatures and heat rejected for ammonium carbamate in propylene glycol at 60°C at three pressures 450 4.00 400 Pressure 3.50 ΔT 350 3.00 300 2.50 250 2.00

200 T (°C) 1.50 Δ 150 Pressure (Torr) 100 1.00 50 0.50 0 0.00 0 1000 2000 3000 4000 5000 Time (s)

Figure 47. Pressure and ΔT for ammonium carbamate in propylene glycol at 60°C at three pressures

105

Table 32. Experimental data for ammonium carbamate in ethylene glycol at 60°C at three pressures Initial Bulk Temperature (°C) 59.7 Initial AC Temperature (°C) 19.5 AC Added (g) 102.997 Water Flow Rate (g/s) 11.57 Heat from Decomposition (kJ) 207.0 Heat from Temperature Change of AC (kJ) 7.41 Lower Bound of Integration 1 (s) 60 Upper Bound of Integration 1 (s) 900 Lower Bound of Integration 2 (s) 1400 Upper Bound of Integration 2 (s) 2500 Lower Bound of Integration 3 (s) 3150 Upper Bound of Integration 3 (s) 3900 Sum of Integration (kJ) 208.7 Difference (integration and theoretical) -2.66% Solvent EG 300 Heat Rejected 64 Bulk Temperature 250 Temp In 62 Temp Out 200 60

150 58 56 100 54 Temperature (°C) Temperature

Heat Rejected (Watts) 50 52 0 50 0 1000 2000 3000 4000 5000 Time (s)

Figure 48. Temperatures and heat rejected for ammonium carbamate in ethylene glycol at 60°C at three pressures 450 3.5 Pressure 400 3 350 ΔT 2.5 300 250 2

200 1.5 T (°C) Δ 150 Pressure (Torr) 1 100 50 0.5 0 0 0 500 1000 1500 2000 2500 3000 3500 Time (s)

Figure 49. Pressure and ΔT for ammonium carbamate in ethylene glycol at 60°C at three pressures

106

Table 33. Experimental data for ammonium carbamate in propylene glycol at 60°C with double AC loading Initial Bulk Temperature (°C) 59.8 Initial AC Temperature (°C) 19.1 AC Added (g) 204.355 Water Flow Rate 11.57 Heat from Decomposition (kJ) 410.7 Heat from Temperature Change of AC (kJ) 14.8 Lower Bound of Integration (s) 110 Upper Bound of Integration (s) 2700 Sum of Integration (kJ) 530.8 Difference (integration and theoretical) 24.75% Solvent PG 500 65 450 63 400 61 350 59 300 57 250 Heat Rejected 55 200 Bulk Temperature 53 Temp In 150 51

Temp Out (°C) Temperature

Heat Rejected (Watts) 100 49 50 47 0 45 0 1000 2000 3000 4000 Time (s)

Figure 50. Temperatures and heat rejected for ammonium carbamate in propylene glycol at 60°C with double ammonium carbamate loading

300 10 Pressure 9 250 ΔT 8 200 7 6 150 5 T (°C)

4 Δ 100 Pressure (torr) 3 50 2 1 0 0 0 500 1000 1500 2000 2500 3000 3500 Time (s)

Figure 51. Pressure and ΔT for ammonium carbamate in propylene glycol at 60°C with double ammonium carbamate loading

107

Table 34. Experimental data for ammonium carbamate in propylene glycol at 60°C with reused propylene glycol Initial Bulk Temperature (°C) 59.8 Initial AC Temperature (°C) 19.6 AC Added (g) 100.264 Water Flow Rate (g/s) 11.57 Heat from Decomposition (kJ) 201.5 Heat from Temperature Change of AC (kJ) 7.16 Lower Bound of Integration (s) 70 Upper Bound of Integration (s) 1500 Sum of integration (kJ) 248.9 Difference (integration and theoretical) 19.27% Solvent PG Reused 300 65 63 250 61 200 59 57 150 Heat Rejected 55 Bulk Temperature 53 Temp In 100 51

Temp Out (°C) Temperature

Heat Rejected (Watts) 50 49 47 0 45 0 500 1000 1500 2000 2500 3000 Time (s)

Figure 52. Temperatures and heat rejected for ammonium carbamate in propylene glycol at 60°C with reused propylene glycol

300 10 Pressure 9 250 ΔT 8 200 7 6 150 5 T (°C)

4 Δ 100 Pressure (torr) 3 50 2 1 0 0 0 500 1000 1500 2000 2500 3000 Time (s)

Figure 53. Pressure and ΔT for ammonium carbamate in propylene glycol at 60°C with reused propylene glycol

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Table 35. Experimental data for ammonium carbamate in propylene glycol at 60°C with twice used propylene glycol Initial Bulk Temperature (°C) 59.9 Initial AC Temperature (°C) 19.9 AC Added (g) 102.285 Water Flow Rate (g/s) 11.57 Heat from Decomposition (kJ) 205.6 Heat from Temperature Change of AC (kJ) 7.27 Lower Bound of Integration (s) 100 Upper Bound of Integration (s) 1600 Sum of Integration (kJ) 256.7 Difference (integration and theoretical) 20.61% Solvent PG Reused 300 65 63 250 61 200 59 57 150 Heat Rejected 55 Bulk Temperature 53 Temp In 100 51

Temp Out (°C) Temperature Heat Rejected (Watts) 50 49 47 0 45 0 500 1000 1500 2000 2500 3000 Time (s)

Figure 54. Temperatures and heat rejected for ammonium carbamate in propylene glycol at 60°C with twice reused propylene glycol 300 10 Pressure 9 250 ΔT 8 200 7 6 150 5 T(°C) 4 Δ 100 Pressure (torr) 3 50 2 1 0 0 0 500 1000 1500 2000 2500 3000 Time (s)

Figure 55. Pressure and ΔT for ammonium carbamate in propylene glycol at 60°C with twice reused propylene glycol 109