Design and Construction of a Permeation Measurement Equipment
1999:185
MASTER'S THESIS
Design and construction of a permeation measurement equipment
Fredrik Jareman
Civilingenjörsprogrammet
Institutionen för Kemi och metallurgi Avdelningen för Kemisk teknologi
1999:185 • ISSN: 1402-1617 • ISRN: LTU-EX--99/185--SE Design and construction of a permeation measurement equipment
Fredrik Jareman
DEPARTMENT OF CHEMISTRY AND METALLURGICAL ENGINEERING DIVISION OF CHEMICAL TECHNOLOGY
July 1999 Acknowledgements
First of all I would sincerely thank my two supervisors PhD Derek Creaser and PhD Jonas Hedlund for their support when I was in need for help and inspiring discussions during the project. I also thank my examiner and future supervisor professor Johan Sterte for his acceptance of my total failure in keeping my budget.
I am also grateful to PhD Derek Creaser for his help with the noble art of English writing, Tekn. lic. Peter Sedin for letting me borrow his private drilling machine during such a long time, M.Sc. Martin Kwasniewski for his cheerful words “Hey you got nothing left, this is done in fifteen minutes” when I was in a bad mood and for his friendship.
Finally, my deepest thanks to my family for encouraging me to move up here to Luleå and start studying chemical engineering and their support during the years, to my love Pernilla for your support during this project. Summary
This project started due to the need for a permeation test facility at the division of chemical technology at Luleå University of technology. The project covers design, equipment selection and construction of the test facility.
A permeation measurement testing system was constructed consisting of several components, which is capable of making advanced measurements of the permeation characteristics on different gases and liquids. The system will be automated by a computer system once routine tests are started.
The system contains three main components; one gas chromatograph one tubular split type furnace and a heated zone. The heated zone contains evaporating units and pressure regulating equipment. Quantitative gas analysis of the retentate and permeate will be performed with the gas chromatograph, in order to evaluate separation characteristics of the membrane.
Several limitations had to be made regarding temperature, pressure and other operating parameters in order to find appropriate equipment and keep costs manageable. Within these limitations, all equipment was ordered and mounted together into the permeation test facility. Contents
1 Introduction...... 1 1.1 Purpose...... 1 1.2 Background...... 1 1.2.1 General description of zeolites, their structure and applications ...... 1 1.2.2 Thin zeolite films as membranes ...... 2 1.2.3 Previous work at the division ...... 3 1.2.4 Permeation measurement experiments...... 3 2. System description...... 5 2.1 Introduction...... 5 2.2 Detailed descriptions of some of the components...... 7 2.2.1 Furnace ...... 7 2.2.2 Oven, heated zone ...... 7 2.2.3 Gas chromatograph...... 8 2.2.4 Back pressure regulators...... 11 2.2.5 Permeation cell...... 12 2.2.6 Computer system ...... 12 3 Design and construction ...... 14 3.1 General considerations...... 14 3.2 Explanation regarding choices of equipment ...... 15 3.2.1 Mass flow controllers...... 15 3.2.2 Magnetic valves...... 16 3.2.3 Gas Chromatograph ...... 16 3.2.4 Furnace ...... 17 3.2.5 Heated zone...... 18 3.2.6 Pressure regulators...... 18 3.2.7 Fluid pump ...... 19 3.3 Calculations of necessary tubing lengths...... 20 3.3.1 Equations and approximations ...... 20 3.3.2 Results of calculated lengths ...... 23 4 Recommendations for future development and expansion ...... 27 5 Conclusions...... 28 References ...... 29 1 Introduction
1.1 Purpose
The purpose of this master thesis was to design and construct an automated laboratory system for permeation evaluation of zeolite membranes. I will share my experiences from this work and hopefully explain some of the pitfalls that occurred during the different stages in the design and construction of the laboratory system.
1.2 Background
1.2.1 General description of zeolites, their structure and applications
Zeolites are a highly crystalline microporous material, with pore sizes of about
0.3 to 1.0 nm [1]. They consist of a network of connected SiO4 and AlO4 tetrahedra [2] with the silicon or aluminia atom in the centre. The connection between different tetrahedra is by shared oxygen atoms. This leads to the following general structural formula [1]:
[]()()⋅ M x'/ n AlO2 x' SiO2 y' wH 2O where the bracketed term is the crystallographic unit cell of the zeolite. The n denotes the valence of the cation present in order to make the unit cell electrically neutral, since the aluminia tetrahedron has a net charge of –1.
The connected tetrahedra results in rings of oxygen atoms. These rings form the apertures to the channels inside the zeolite. Due to the fact that there may be 4, 5, 6, 8, 10 or 12 [1] oxygen atoms in the rings, there is a large range in possible aperture sizes.
The ratio of Si/Al content present in a zeolite varies between different types of zeolites. The more Al present in the structure increases the acidic and hydrophilic properties of the zeolite. A zeolite with acidic sites is capable of interchanging protons and therefore it is suitable for catalytic applications. Due to this property zeolites have been used as catalyst in different process applications.
1 One of the mostly widely spread is the ZSM-5 cracking catalyst in the oil refining industry [1].
Zeolites have other different interesting properties. One of them is their molecular sieve property. A molecular sieve is a porous material capable of sorting different compounds by size and shape.
1.2.2 Thin zeolite films as membranes
Due to their small pores, molecular sieve property and temperature stability, zeolites are of interest in several industrial applications where gas separation at an elevated temperature is desired. A practical example is production of styrene from ethyl-benzene, by catalytic dehydrogenation. With existing technology the process stream must be cooled down in order to separate the styrene from the unreacted ethylbenzene and hydrogen.
With a thin zeolite film this would be done in one step by combining the membrane and the reactor i.e., hydrogen would be removed in situ from the reaction zone [3]. This combination would not only reduce the separation costs of the process, but it should also be possible to drive the reaction beyond its thermodynamic equilibrium and thus improve productivity.
To obtain a high flux through the membrane, which increases its potential for use in industry, it is important that the film is ultra thin, i.e. less that 100 nm [4]. These ultra thin films have to be supported by another porous material such as alumina oxide, having pore sizes of 50 to 100 nm, to improve their mechanical strength. With these types of supported films a broad range of applications are possible. One was mentioned earlier, another is pure gas separation.
Due to the broad range of crystal structures of zeolites there is a broad range of separation possibilities between molecules. This characteristic makes zeolite membranes interesting from an industrial point of view. The biggest challenge is to find a synthesis method, which is capable of producing membranes of sufficiently high quality. A high quality membrane would have a very thin zeolite film for high fluxes and be free of defects such as pinholes and cracks.
2 1.2.3 Previous work at the division
The research group at the division of chemical technology at Luleå University of technology has developed the ‘seed-film method’ for the synthesis of ultra-thin films of zeolites on various support materials including porous alumina [4]. High quality ZSM-5 films have been synthesised on α-alumina supports and the permeation characteristics of these composite membranes have been evaluated in cooperation with the Institute of Applied Chemistry, Berlin Germany [5]
Work is ongoing at the division of chemical technology to further develop the seed-film method for the synthesis of membranes with different zeolites with varying properties (i.e. different film thickness, Si/Al ratios, cation species).
With this situation as a background the need for an advanced permeation test facility in Luleå arose. The planned system will be able to perform advanced permeation measurements on the synthesised membranes at the division. The system should also be able to be expanded in the future to accommodate future anticipated catalytic experiments and work with membrane reactors.
1.2.4 Permeation measurement experiments
In order to measure membrane permeation characteristics there are some basic tests that are done.
One test is to measure the flow rate of a single gas through the membrane at a given pressure difference over the membrane, and compare the flow rates between different gases. This comparison results in an estimate of the selectivity of the membrane. The flow rates are dependent on the thickness of the zeolite film and the amount of defects.
Another experiment that provides the same information as the first experiment is to evacuate the permeation cell and then apply a single gas feed at a certain pressure on the feed side of the membrane while the permeate side is closed. Due to permeation of the gas through the membrane the pressure will increase on the permeate side. From the rate of pressure increase molar fluxes are calculated and are used as measurements of the permeation characteristics.
3 The other main type of test is to make a mixture of two or more gases and flow these gases past the feed side of the membrane. The partial pressure difference and the absolute pressure difference will be the driving force for flow through the membrane. By analysing the outlet gas mixture composition from each side of the membrane and then calculating separation factors, a measurement of the separation characteristics of the membrane is obtained.
Separation factors can also be calculated from single gas permeation measurements, but they can differ significantly from those based on multi gas permeation measurements, due to various factors that effect mass transport that can be important with gas mixtures, such as competitive adsorption and single file transport.
The two latter experiments will be easily performed in the system as presently designed. With an additional flow meter connected to the permeate side of the membrane cell, the first type of experiment can also be performed without moving the membrane between different laboratory systems.
4 2. System description
2.1 Introduction
The permeation testing system consists of three large components; a tubular furnace, a heated zone and a gas chromatograph.
The furnace is used to control the temperature at which the permeation characteristics of the membrane will be evaluated. In order to control the temperature with high precision a thermocouple is mounted directly in the measurement cell. The furnace, a vertical split type tubular furnace, is used in order to make membrane changes easier since the membrane cell is horizontally positioned.
The heated zone is an oven that prevents condensation of higher molecular weight compounds in the system. Its maximum operating temperature is 200 °C, the anticipated actual temperature in the system is 150 °C.
A gas chromatograph (GC) is used for quantitative gas analysis. These analyses make calculations of the mass or molar fluxes across the membrane possible for dynamic permeation experiments and for measurements of the separation factor of mixtures.
Three mass flow controllers regulate the amount of gas that is fed to the membrane, and two backpressure regulators control the pressure. This set-up allows for great flexibility in variation of process parameters such as flow, pressure drop over the membrane and gas mixture composition. A regulating valve combined with a pressure transmitter and a PID control unit, regulates the total pressure of the feed and permeate streams.
5 Feed G G G G G G G G A A A A A A A A Permeat S S S S S S S S Regulating valve L C Sweep-gas On/off valve LPT: Low pressure transducer
FC FC FC HPT: High pressure transducer Ev Fluid pump EV: Evaporator Membrane cell Heated zone GC Furnace
HPT T
Exhaust HPT
LPT Signals
Vacuum pump Computer
Figure 1: Principal drawing of the permeation equipment
Figure 1 above is a schematic of the permeation equipment. The number of gas tubes and liquid containers connected to the system can vary. Additional parts in the system include a fluid pump, an evaporator (EV), several magnetic on/off valves and the permeation cell. The fluid pump is a syringe pump that is capable of providing liquid flow rates as low as 1 µl/min. The evaporator is a helix of 1/8” stainless steel tubing. All of the system components are connected with 1/8-inch stainless steel tubing and appropriate Swagelok couplings.
Before permeation measurements a vacuum pump is used to degas the membrane at an elevated temperature, in order to remove adsorbed components from the membrane. A commercial software package controls the system and logs physical data from the measurements. This allows for the greatest possible automated operation of the system within economic and practical limitations.
6 2.2 Detailed descriptions of some of the components
2.2.1 Furnace
When choosing a furnace, there where several factors that had to be considered. The furnace must be able to be controlled by a computer, preferably by a serial interface such as RS 232, which is a standard for serial communications between computers and laboratory equipment. With a computer connected to a PID- regulator it is possible to make temperature variations with slow and steady ramps. This is an important feature because exposing the membrane to rapid temperature variations increases the risk for crack formation.
In order to allow easy access to the permeation cell the cylindrical furnace has a split design so that it can be opened. Additionally, to simplify the procedure for changing membranes, the furnace is designed to be oriented vertically. The photograph (figure 2) shows the general type of furnace except that it will be positioned vertically.
Figure 2
2.2.2 Oven, heated zone
Unlike the furnace, computer control of the heated zone was considered unnecessary. It will be operated only at a constant temperature in order to prevent higher molecular weight compounds from condensing.
When selecting the operating temperature for the heated zone a compromise had to be made between cost and the range of molecular species that could be studied. A higher operating temperature allows for studies with higher molecular
7 weight species, but this puts increasing demands on the maximum operating temperatures of the instrumentation inside the heating zone. A reasonable optimum temperature was found to be 150°C since that temperature exceeds the boiling point of several important compounds, such as styrene, xylenes, and ethylbenzene.
2.2.3 Gas chromatograph
Since the system is automated it was desired that the controlling software would be able to command the GC to take gas samples online. The GC should also be capable of analysing different types of gases, like N2, H2, O2, H2O, CO, CO2,
SF6, alcohols up to butanol and different hydrocarbons with boiling points less than 150 °C. If any number of these gases are going to be mixed with varying proportions, there is no single GC-column that would suffice for all possible separations.
An alternative is to choose a GC-system that is equipped for column switching. The method is based on the use of different columns for different ranges of compounds, such as a capillary column for hydrocarbons and a molecular sieve column for inert light gases. Carrier
Sample out
Sample in
V2
V1
1079 w R EFC
make up Vent. Chromosorb 107 50 m DB-1 m 50
MS 13X
V3
TCD
Restrictor
FID Figure 3: A schematic diagram of the GC.
8 In the actual gas chromatograph schematically, shown above (figure 3) there are two different detectors; one FID (Flame Ionisation Detector) and one TCD (Thermal Conductivity Detector). These will be described later in this chapter. The different automated valves are used to automate the analysis and make sample injection, and back flushing the molecular sieve columns possible. The GC system contains three different columns that perform different tasks. In the drawing the columns are named, DB-1, Chromosorb 107 and MS 13X. The DB-1 is an ordinary nonpolar capillary column with a 0.25-µm inner diameter with a length of 50 m. Chromosorb 107 is a packed column, which separates light inert gases and the MS 13X is a molecular sieve packed column and it separates lighter hydrocarbons such as ethane, methane and inorganic gases such as carbon dioxide.
The analysis begins with dividing the sample into two samples and feeding the first sample to a packed column system in order to separate the light inert gases. This packed column system consists of two columns, the Chromosorb 107 and the MS 13X. The first sample is fed to the Chromosorb column in order to separate light inert gases. When the separation is completed and the light gases have been detected by the TCD, the carrier gas flow is reversed in order to backflush the column. This is done in order to prevent the column from being plugged with the higher molecular weight compounds. The backflushed sample is now fed to the second column (MS 13X) in order to separate gases such as carbon dioxide and ethane. When these gases have been detected by the TCD, then the MS 13X column is also backflushed, for the same reason mentioned previously. The second sample is now fed to the capillary column in order to separate the higher molecular weight compounds and they are detected by both the TCD and the FID.
In some permeation measurements it is desirable to have a fast quantitative analyse of the permeate. If the gas only contains one component except the carrier gas, which is in our case is helium, it is possible to disconnect the columns in the GC and lead the gas stream directly to the detector. This can easily be done with an additional four port valve between V1 and V2 in the wiring diagram.
9 The FID is the most widely used and general detector, with a sensitivity down to PPB (parts per billion) for certain compounds. It pyrolyzes the effluent from the column with hydrogen and air. The pyrolyzation process produces ions that are capable of conducting current through the gas if an electrical potential is applied between the burner tip and the collector electrode. This electrical potential generates a weak current that is amplified and measured. It is insensitive to non- combustible gases such as water, carbon dioxide, sulphur dioxide and nitrous gases. The greatest disadvantage with this detector is the destruction of the sample. Figure 4 below shows a typical FID-detector.
Figure 4: Drawing of a typical FID-detector.
The TCD is one of the most common and earliest used detectors for gas chromatography. Detection is based on changes in the thermal conductivity of the gas stream.
When the analyte molecule is mixed with the carrier gas it changes the thermal conductivity which is measured by a katharometer. The sensing element of a katharometer is an electrically heated element whose temperature at constant electrical power depends upon the thermal conductivity of the surrounding gas. The temperature is easily measured from the resistance in the wire according to
10 Ohms law. The element may be a fine platinum, gold or tungsten wire. Figure 5 below is a schematic drawing of a TCD.
Flow out
Flow in Figure 5: Principal drawing of a TCD
2.2.4 Back pressure regulators
At an early stage of the project there where two different alternatives for backpressure regulators. One with all parts put together i.e. the control valve, pressure transmitter and PID regulator in one unit or one with all of these three parts separated. Due to the high temperature in the heated zone, there where problems in finding a unit consisting of the three earlier described parts with sufficient temperature resistance and the second alternative was chosen. Since the separation process of the membrane is highly dependent on the pressure difference across the membrane it is important that the pressure is accurately regulated. The regulator recieves a setpoint from the control system and sends back the actual value of the pressure. Figure 6 below shows the main connections in a PID-regulating system.
Actual Setpoint value
PID-regulator
Signals to Pressure adjust valve signal
Tubing Pressure transmitter
Regulating valve Figure 6: PID-regulating system
11 2.2.5 Permeation cell
The permeation cell is of a Wicke Kallenbach design. It allows gas flow in and out, on both sides of the membrane, making it possible to have a flow of gas along the membrane. This can be useful when the membrane permeation characteristics are studied since membranes are often used in a continuous system for gas separations. Two o-rings made of graphite tighten the membrane and make the installation free from leaks.
Membrane ø 25 mm
Feed in Sweep gas in Retentate out Permeate out
Flange ø 70 mm Flange ø 70 mm
Graphite o-rings Outer ø 32 mm Inner ø 19 mm . Figure 7: Principal drawing of the Wicke Kallenbach permeation cell
When the cell was constructed a T-connection was added at the feed outlet tubing so that a thermocouple could be added to the measurement cell. This gives the opportunity to measure the true temperature for the separation process instead of making the approximation that the temperature inside the permeation cell is equal to the operating temperature inside the furnace.
2.2.6 Computer system
The computer system contains several parts. Excluding the computer, the following components comprise the control system: one DAQ-board (Data AcQuisition), one RS232 serial interface computer board that allows for up to four RS232 ports and a SCXI (Signal Conditioning eXtensions for Instrumentation) chassis with two signal conditioning modules. All of these parts are operated by the commercial control software package called LABVIEW.
The DAQ-board is the heart of the system, it is the bridge between the instruments such as pressure transmitters, flow controllers etc. and the controlling software. It contains a DA (Digital to Analogue) and AD (Analogue to Digital)
12 converter. These converters sample and transform the signals to and from the computer into a readable format and thus provides for communication between the computer and the instruments.
When using complete systems with built in PID-regulators such as the furnace its common to use a serial interface for communication between the instrument and the computer. There exist several types of serial interface and one of the most widely spread is the RS232 standard.
The DAQ-board is connected to a SCXI –chassis containing the following signal conditioning modules. • Analogue module with 16 input channels • Analogue module with 6 output channels • Digital output module with 32 channels. The digital out module is capable of providing up to 240 VAc or VDc as signal to the magnetic valves thus making an external power supply and switching relays unnecessary.
13 3 Design and construction
3.1 General considerations
In the first phase of construction decisions had to be made regarding the scope of the system. Since funding is always an important limiting factor, the following considerations had to be made. What are the desired maximum operating conditions, such as pressure and temperature for the system. Higher pressures and temperatures result in more costly equipment and in some cases larger equipment due to the lack of availability of laboratory scale equipment with the desired specifications. This can lead to design problems, since larger equipment often requires larger tubing, membranes, samples etc. Since a laboratory scale system was required the use of process or pilot scale equipment, was avoided.
Another limiting factor is what type of experiments you want to perform with the system. The greater number of possible types of experiments, puts increasing demands on the flexibility of the system. Increasing the flexibility of the apparatus increases its complexity and of course cost. The scope of the system was adjusted to fit the total budget as more exact cost estimations for different components were gathered.
The next step was to find suitable suppliers of the different kinds of equipment. It is always desirable to have at least two different suppliers of the desired equipment in order to get a comparison between different suppliers regarding price, quality and delivery time. And then choose the supplier that best fits your needs. Since time was also a limiting factor it was desirable to have a supplier with a short delivery time. In this stage some time could be saved since if the equipment with the likely longest delivery time was ordered first. In the actual case it was the furnace and the GC that required the longest delivery times.
When all equipment was ordered it was time to check connections of the equipment so that the appropriate tube couplings could be ordered. Since this type of material had a short delivery time it was not crucial to get all of the connections at the start. Although it was preferable to make as exact predictions as possible in order to have all material when the ordered equipment arrived. In the actual system the use of Swagelock brand couplings was chosen, since a very
14 broad assortment was available and they are well known to have good quality and few installation problems.
With all the material present it became time to mechanically put all the parts together. During this phase it was preferable to test for leaks during the assembly of the equipment, especially connections with threads that connect tubing to the different components. It was difficult to get the connections leak proof without the use of a seal or threading tape.
The pressure regulating equipment and a number of on/off valves was mounted inside the heated zone oven were the door of the oven serves as a mounting platform for the equipment. All equipment was mounted with the electronics on the outside of the heated zone in order to reduce the temperature of the electronic parts. The tubing and couplings are kept inside the heated zone in order to prevent higher molecular weight compounds from condensing.
The final part of construction was to connect all the different equipment to the control system. This involved making all the electrical connections, configuring the control system and finally creating a control program. The control program makes the experimental system automated to a degree so that it is possible to operate all valves, mass flow controllers etc. with the computer.
3.2 Explanation regarding choices of equipment
3.2.1 Mass flow controllers
When choosing mass flow controllers there are some operating aspects that have to be considered, such as the type of controlling that would be required.
There exists two main types of control interface for mass flow controllers. The more advanced is the RS232 interface with its capabilities to do advanced flow programming such as flow ramps etc. The analogue set point is the other main type of controlling interface. It is used for processes where a single set point control on the flow is sufficient.
15 I chose mass flow controllers with analogue communication mainly because it was not required in the planned system to be able to program flowramps or connect the flow controller to a printer etc. A secondary reason is the price difference between the two different control interfaces for the flow controllers.
This price difference is not always a criterion for choosing a type of control interface. There are cases where a solution with the serial interface might be less expensive than the analogue interface. Such a situation is if only a few components build the system and there would be no need for a separate data acquisition board in the computer if components with RS232 interface are chosen. The disadvantage with this type of system is its poorer flexibility compared with the constructed control system.
3.2.2 Magnetic valves
On/off valves comes in many different types but they can be separated into three main groups. Magnetically- pneumatically- or manually controlled. Manually controlled valves were not suitable in this case since an automated experimental system was desired. Pneumatically controlled valves are capable of withstanding higher pressure differences than magnetically controlled valves but require externally supplied compressed air and control valves for the compressed air in order to be functional. This need for compressed air and additional valves is the pneumatic valves biggest disadvantages since it complicates the installation of the valves and increases costs.
Therefore the use of magnetically controlled valves were chosen because of their lower cost and the fact that they are easier to install.
3.2.3 Gas Chromatograph
Due to the demands that the gas chromatograph should be able to separate light gases and different hydrocarbons in the same sample and since there is no single column that is capable of achieving the separation between such compounds, a solution was a system with three different columns interconnected between each other with three multiport valves. This configuration is able for example to
16 separate n-butane, i-butane hydrogen and nitrogen from each other, when they coexist in the same gas stream.
Another way to achieve the desired separation is to cool the sample with for example liquid nitrogen so that the less volatile species in the sample condense and then apply the technique of temperature programming over the column to achieve the separation. The biggest disadvantage with this method is the possibility that higher molecular weight compounds such as organic solvents such as hexane, would freeze and therefore plug the column.
Another alternative was to have several columns and switch them manually depending on the type of analysis. This would be rather time consuming and the gas chromatograph would not be automated as desired. Therefore this solution was never considered feasible.
Due to the separation performance of the gas chromatograph it is well suited for analysis of reaction mixtures from catalytic experiments. An illustrative example, where an advanced high performance gas chromatograph is useful is the formation of xylenes from toluene and methanol, which produce a rather complex reaction mixture.
3.2.4 Furnace
When choosing the type of furnace there were a number of practical operating aspects that had to be considered. Of primary importance was that the permeation cell had to be easy to access in order to make membrane changes easier.
There were two main designs of furnaces available, an ordinary furnace with a door on the front side and a tubular furnace. An ordinary furnace with a door on the front side was not a feasible solution for this system due to the difficulty in placing the permeation cell within it and reducing accessibility to it.
A split type tubular furnace was a more feasible solution since it can be opened to allow easy access to the permeation cell. In order to further increase the accessibility of the cell a vertical option on the furnace was chosen.
17 Since the membranes are sensitive to rapid temperature changes a programmable temperature control unit was desired with the ability to create temperature ramps. These temperature ramps reduce the risk of crack formation in the thin films that can result from rapid temperature increase.
A PID regulating temperature controller with serial communication interface was chosen in order to simplify the temperature control of the furnace. Analogue control of the furnace would require a great deal of programming in order to provide the capability of temperature ramping.
3.2.5 Heated zone
The heated zone was one of the most difficult parts of the system to design and construct. In the beginning the plan was to build it ourselves. The problem was to find a suitable heating element, a fan for circulation to give a homogenous temperature inside the heated zone, a temperature controller with a thermocouple and a well insulated container. All of these parts together would have been more expensive and probably less functional than the eventual solution, which was to use a conventional oven capable of heating up to approximate 250 °C. It contains all the earlier described parts except the fan.
3.2.6 Pressure regulators
There is a large variety of pressure regulators available. In the oldest designs the regulating valve and pressure transmitter are separate units and controlled by a PID-regulator. In recent year these three parts have been built together into one unit for simplicity and ease of installation.
Due to the fact that the regulators would be placed inside the heated zone there were some problems that had to be solved. Most of the regulators that are built into one unit were not able to withstand the temperature of at least 150 °C inside the heated zone. Therefore the solution was to have the components separated into three different parts, despite the fact that the use of a regulator with all components in one unit would have been less costly.
18 The chosen design may also be preferable since it is more flexible with these three separate parts. For instance, if we want to increase the heated zone temperature or we want to use a different range of pressures we might only need to change one of the components.
3.2.7 Fluid pump
There were a number of operating criteria that the pump had to fulfil. First of all it had to be able to deliver very low flows down to 10 µl/min without any pulses and with a high accuracy over rather long time periods. An additional criteria was the ability to pump at a few bars pressure.
Peristaltic pumps were considered unsuitable, since flow rates can change slightly after long usage because of changes in tubing diameter due to mechanical wear. The peristaltic pumps also have problems with producing a non-pulsating flow at the desired low flow rates. The biggest advantage of the peristaltic pump was its low cost and simple design.
The next type of pump is an HPLC-pump (High Performance Liquid Chromatography). It has all the desired properties but the high cost made it impossible to fit it in the project’s budget. In a later upgrade of the system the purchase of such a pump could be considered.
The third alternative is the one that is installed into the system. A syringe pump has almost all the desired properties. However the time of continuous flow is limited to the amount of liquid inside the syringe, although a semi continuous flow can be established with an automated refill of the syringe. Another disadvantage is that it is limited to operating at moderate pressures due to its design. This disadvantage is not crucial since permeation and separation characteristics may be examined with only a partial pressure difference as driving force for the flow. Considering the budget constraints of this project this pump was considered most suitable.
There were numerous other types of fluid pumps that were rejected from considerations mostly due to their inability to provide the required low flow rates.
19 3.3 Calculations of necessary tubing lengths
3.3.1 Equations and approximations
In order to have sufficiently long tubing inside the heated zone and the furnace and thereby fully the evaporate liquids or sufficiently heat gases, an approximate calculation of the required length of the tubing had to be made. As a safety precaution, I added at least 30% of the calculated length in order to be certain that the gases would reach the required temperatures. The following approximations where made. The non moving ambient air inside the furnace and the heated zone result in low Nusselt numbers, which leads to low heat transfer coefficients. It is thus reasonable to make the approximation that all heat transfer resistance is on the outside of the tubing. Calculations in one case will show that this is a reasonable approximation. This means that the wall temperature on the outside of the tubing is equal to the temperature in the gas stream. The following correlation for the Nusselt number was chosen [6] p-358
æ ö2 ç ÷ ç ÷ h ⋅d 0.387⋅ Ra 1/6 Nu = = ç0.60 + D ÷ ç 8/27 ÷ k æ æ ö9/16 ö ç ç + ç 0.559÷ ÷ ÷ ç ç1 ÷ ÷ è è è Pr ø ø ø æ ⋅ β ⋅ ρ 2 ö = ç g ÷ ⋅ ∆ ⋅ 3 Ra D ç ÷ T d è µ 2 ø
Equation 1: Correlation of the Nusselt number for natural convection
Where • h is the heat transfer coefficient • d is the diameter of the tubing • k is the thermal conductivity of the gas
• RaD is the Rayliegh number • Pr is the Prandtl number
20 • (gβρ2/µ2) is tabulated and evaluated at the film temperature in [6] p-756 • ∆T is the temperature difference between the wall and the surrounding
The k, RaD and Pr entries are properties of the surrounding gas. All properties are evaluated at the film temperature, i.e. the average temperature between the wall of the tubing and the surrounding temperature. The correlation together with the heat transfer equation for a heat exchanger [6] p-407 is used to calculate the appropriate length of the tubing.
()()T − T − T − T Q = h ⋅ 2π ⋅ r ⋅ L ⋅ Surr out Surr in æ T − T ö lnç Surr out ÷ ç − ÷ è TSurr Tin ø
Equation 2: Heat transfer equation
Where
• Tout is the temperature of the outgoing stream from the tubing.
• Tin is the temperature of the incoming stream
• TSurr is the surrounding temperature • h is the heat transfer coefficient calculated from equation 1 • r is the radius on the tubing • L is the length of the tubing
The amount of heat that is transferred for gases equals
= ⋅ ⋅ ()− Q m Cpgas Tout Tin
Equation 3
In the case of evaporating liquids it equals
= ⋅ ( ⋅ ( − )+ ∆ + ⋅ ( − )) Q m Cpliq Tvap Tin H vap Cp vap Tout Tvap
Equation 4
21 Where • m is the mass flow
• ∆Hvap is the evaporating enthalpy
• Tvap is the boiling point
• Cpliq is heat capacity of the liquid
• Cpvap is the heat capacity of the vaporised liquid
In order to verify the approximation with the temperature on the outside of the tubing, I compared the heat transfer resistances on the inside of the tubing, in the tubing wall and on the outside of the tubing. The three different resistances can be written as follows.
æ D ö lnç o ÷ 1 è D ø 1 R = R = i R = i ⋅ steel ⋅ o ⋅ "" D"!i "h i "" 2"!k"steel "" D"!o "h o Inside Tubing Outside
Equation 5: Heat transfer resistances Where
• Di is the inner diameter of the tubing
• Do is the outer diameter of the tubing
• hi is the heat transfer coefficient on the inside of the tubing
• ho is the heat transfer coefficient on the outside of the tubing
The heat transfer coefficient on the outside of the tubing has been calculated previously. The calculation of hi follows the principle used for the outside of the tubing. First one needs to calculate the Reynolds number, determine the Prandtl number and calculate from them the Nusselt number and finally from that the heat transfer coefficient. The appropriate equations are as follows[6] p-366.
v ⋅ D Re = i ν h ⋅ D Nu = i i = 0.023⋅ Re0.8 ⋅ Pr 0.4 k i
Equation 6: Nusselt number on the inside of the tubing
22 Where • v is the gas velocity inside the tubing
• ki is the heat condictivity of the gas inside the tube • Pr is the Prandtl number
All gas properties in the equations above are evaluated at the film temperature, which is equal to the average of the bulk temperature and the wall temperature of the tubing.
As a source of physical and chemical data of the gases and liquids I have used references [6],[7] and [8].
3.3.2 Results of calculated lengths
With the following data I calculated the required length of the heat exchange helixes inside the furnace to be approximately 30 cm.
Value Unit
Do 3.23E-03 m
Tin 150 °C
Tout 395 °C
Tsurr 400 °C ∆T 127.5 °C Film temp 269 °C tab 9.96E+06 1/(K m3) Pr 0.68 Ra 2.91E+01 Nu 1.35 k 4.305 W/m K h 1.80E+03 W/m2 K
The required length of the evaporator helix was found to be slightly less than 6 m. For convenience I have separated the heating and evaporating lengths. This results in a heating length of 33 cm and a evaporating length of 5.6 m. The data used is as follows.
23 Heating Value Unit
Do 3.23E-03 m
Tin 20 °C
Tout 145 °C
Tsurr 150 °C ∆T 67.5 °C Film temp 97 °C tab 5.29E+07 1/(K m3) Pr 0.694 Ra 8.34E+01 Nu 1.61 k 3.1497 W/m K h 1.57E+03 W/m2 K
Evaporating Value Unit
Do 3.23E-03 m
Tin 145 °C
Tout 145 °C
Tsurr 150 °C ∆T 5°C Film temp 147.5 °C tab 3.03E+07 1/(K m3) Pr 0.687 Ra 3.50E+00 Nu 0.99 k 3.504 W/m K h 1.07E+03 W/m2 K
∆Hvap 347085 J/kg
∆Hvap is for o-xylene from [7] p-3:125
When comparing the heat transfer resistances between the inside of the tubing, tubing wall and outside of the tubing, I choose to do the comparison for the heat exchangers inside the furnace, since this is the only location with gas on the both
24 sides of the tubing wall. In the case of heating and evaporating the liquids there will be a much higher heat transfer coefficient than that with only gas streams. Another choice I made was to calculate with a gas stream with a rather low heat conductivity and high heat capacity. This will result in a lower heat transfer coefficient and will therefore be the worst case, i.e. the case where both heat transfer coefficients are most equal.
Inside the tubing Value Unit
Di 1.50E-03 m V 1.00E-03 m3/s ρ 4.54E-02 kg/m3 m 4.54E-05 kg/s Cp 14.521 J/(kg K) ki 0.2914 W/m K ν 2.99E-04 m2/s Re 2.84E+03 Pr 0.667 Nu 11.3 hi 1.43E+06 W/m2 K
Heat resistances Value Unit ksteel 45 W/m K hi 1.4348E+06 W/m2 K ho 1.7982E+03 W/m2 K
Di 1.5000E-03 m
Do 3.2300E-03 m
Ri 4.6464E-04 m K / W
Rsteel 8.5224E-03 m K / W
Ro 1.7217E-01 m K / W ksteel is from [8] p-72
From this table we can see that the heat transfer coefficients differ by a factor 20 between the outside resistance and that in the tubing wall which has the second
25 highest resistance. These calculations verify the approximation earlier made that the outside tubing wall temperature can be considered equal to the inside stream temperature.
26 4 Recommendations for future development and expansion
An experimental system like the one constructed in this project is probably never finished. There are inevitably some improvements that can be made to both hardware and software.
An improvement in the hardware that may be considered is replacing the syringe pump with a HPLC pump. The two biggest advantages is the convenience in changing the liquid since the HPLC pump has a much smaller dead volume containing the previous liquids. Its smaller dead volume compared to that of the syringe pump can be very important since a small amount of a large molecule can plug the membrane and thereby disturb the test. The other big advantage as mentioned previously is its capability to produce low steady flows during a long time interval. Ultimately, the need for an HPLC pump depends on the number of liquids that will eventually be used.
If the system is entirely automated it would be desirable to be able to switch between liquids automatically. But this capability would increase the importance of the separation between the liquids which is influenced by the pump dead volume.
Additional gases are the most likely area for expansion in the nearest future, since it is easy to add gases to the magnetic valve system. Some other gases of particular interest are methane, oxygen, i-butane, n-butane etc.
The software will be adjusted so that a higher degree of automation is achieved. It is desirable to be able to do each type of test and membrane drying automatically. This automation will reduce operator time for each membrane test and provide more time for evaluation of experimental data instead of data collection.
Since the system probably will be used for studying catalytic reactions in the future it could be advantageous to connect a mass spectrometer to the gas chromatograph in order to ease product species identification.
27 5 Conclusions
The conclusions that can be drawn from this project are the importance to decide on the limitations of an experimental system before its construction. If this is not done there is a great risk that time and funding is used unnecessarily. Even though, these limitations can change during the project and often become narrower. The reason for further reductions in the project scope is often difficulties in finding the appropriate equipment at a manageable cost or that the required equipment is not available in laboratory scale. Another problem with finding the appropriate equipment is to find dealers of such equipment. In this case the Internet had been a good source to obtain contact with dealers. Often is it possible and desirable to find local dealers for most of the equipment. This is always an advantage due to linguistic problems that might otherwise occur.
If a detailed system description is made in the early stage of the project and all of its components are studied carefully then both funding and time may be saved. This description will also reduce the amount of problems that may occur later in the project.
The automation of the experimental system is at a computer/manual control level. Currently, the operator can control all parts of the system through the computer. This level of automation is satisfactory in this early stage because of the need of developing the desired experiments in more detail, such as choosing appropriate flow rates, pressures, treatment duration etc. As long as the experimental details are not fully known it is premature and thereby not time effective to produce programs that perform the experiments.
28 References
[1]. Charles N. Satterfield, Heterogenous Catalysis in industrial practice second edition, Krieger Publishing company, Florida, 1996, p 226-258 [2]. Lubomira Tosheva, Zeolite macrostructures, Luleå university of technology, 1999. [3]. Andrzej Cybulski et al, Structured catalysts and rectors, Marcel Dekker Inc, 1998, p 543-573. [4]. Jonas Hedlund. Thin films of molecular sieves, Luleå university of technology 1998 [5]. Jonas Hedlund et al, Evaluation of ZSM-5 membranes synthesized in the absence of organic template molecules, Journal of Membrane Science 159 (1999), p 263-273. [6]. James R. Welty et al, Fundamentals of momentum, heat and mass transfer thhird edition, Wiley, 1984 [7]. Robert H. Perry et al, Perry’s chemical engineers handbook sixth edition, Mc Graw Hill, 1984 [8]. Sten-Erik Mörtstedt Gunnar Hellsten, Data och diagram, Liber utbildning, 1994.
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