Gas Separation Using Membranes. 2. Developing a New Membrane

Jan 1, 1997 - Kenneth E. Porter,* Anthony B. Hinchliffe, and Brian J. Tighe. Department of Chemical Engineering and Applied Chemistry, Aston Universit...
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Ind. Eng. Chem. Res. 1997, 36, 830-837

Gas Separation Using Membranes. 2. Developing a New Membrane for the Separation of Hydrogen and Carbon Monoxide Using the Targeting Approach Kenneth E. Porter,* Anthony B. Hinchliffe, and Brian J. Tighe Department of Chemical Engineering and Applied Chemistry, Aston University, Aston Triangle, Birmingham B4 7ET, England

This paper describes the concept of “targeting” the development of new membranes such that the resulting membrane separator will be economically viable in a particular commercial process. The separation of carbon monoxide and hydrogen for the production of acetic acid is used to illustrate the development of a new membrane material using the targeting procedure. It is shown by economic evaluation of the complete process that the cost of separation can be related to the permeability and selectivity of the membrane. Costs upstream and downstream of the separator are taken into account. The same cost may be achieved by many different combinations of membrane permeability and selectivity; therefore, iso-cost lines can be drawn on a plot of permeability and selectivity which provides a guide for the development of new membranes. A new membrane material has been selected for the separation of carbon monoxide and hydrogen for the acetic acid process. The copolymer was synthesized and dense homogeneous membranes were manufactured for the measurement of gas transport and physical properties. The membrane has substantially different gas transport properties from currently used commercial membranes. If this membrane was developed, it would be expected to result in a reduction in the cost of carbon monoxide separation to 40% of the cost of the currently used method, cryogenic distillation. Introduction The separation of gases by a membrane is one of several separation processes which depend on the properties of a separating material. The other methods are adsorption, which depends on the properties of the solid adsorbent, and absorption, which depends on the properties of the liquid solvent. Traditionally, the development of a new separating material starts in the laboratory and may be followed by pilot-plant studies of promising candidate materials before evaluating the new material for a proposed application by an economic analysis. A more efficient way of managing the research is to start with an economic evaluation of the separator and process before developing the new material, that is, to use economic evaluation to set targets for material performance which may be used to guide the research. This approach has been used previously by Porter (1987), Porter et al. (1990), and Bhide et al. (1991). In this paper we describe a method for setting targets for the development of a new membrane for separating hydrogen from carbon monoxide for the manufacture of acetic acid. However, the targeting approach may be used for other processes and for other separation methods such as adsorption and absorption mentioned above. The targeting analysis shows that substantial economic benefits would result from increases in membrane permeability. To illustrate the advantages of membrane development using the targeting approach, a polymer material has been chosen for this separation which has significantly different gas transport properties from those of polymers currently used in commercial membrane systems. To complete the development cycle, the chosen polymer has been synthesized and dense, homogeneous membranes have been manufactured. Transport propS0888-5885(96)00328-4 CCC: $14.00

erties were measured, and these results have been used in an evaluation procedure to complete the targeting analysis. The selection of this particular polymer serves to illustrate the development process using the targeting analysis, and while significant improvements in membrane permeability are obtained, further improvements could be made. Targeting Membranes for Gas Separation in a Process To Manufacture Acetic Acid In part 1 it was shown that, for the separation of carbon monoxide and hydrogen by a membrane separator for the manufacture of acetic acid, there was an optimum separation at which the overall cost of separation is a minimum. The cost of the membrane separator depends on the rate at which gases permeate through the membrane and on the membrane selectivity. As shown, the contribution of the permeator design to the cost of the separator may be evaluated by the new concepts of cost permeability and effective selectivity, which are related to the permeability and selectivity of the membrane material. Here we show how the concepts of cost permeability and effective selectivity, in combination with the cost of separation (as defined by the acetic acid process), may be used to set targets for the development of new membrane materials. In general, the cost permeability of a membrane separator determines the capital cost of the separator, and the effective selectivity determines the gas recycle and optimum recovery and thus the running cost. The total cost of the separation depends on both capital and running costs, so it follows that there are many different combinations of cost permeability and effective selectivity which will result in the same total cost. Whereas in part 1 we used the values of cost permeability and effective selectivity for real commercial membrane © 1997 American Chemical Society

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separators to find the minimum total cost of separation, here we let cost permeability and effective selectivity be hypothetical quantities which are used to explore how the separation cost will vary as they are changed. The results of the targeting analysis are presented in Figure 6 as iso-cost lines, each the locus of those cost permeability and effective selectivity combinations which would result in the same cost of separation. Costs are presented in terms of the cost of separation by cryogenic distillation both to maintain confidentiality and to present results which are likely to be unchanged by inflation. The calculation of results in Figure 6 was done as follows. Method of Economic Evaluation This is described in detail in part 1. For each hypothetical membrane, defined by its cost permeability and effective selectivity, there is an optimum combination of CO product purity and CO recovery at which its separation cost is a minimum, and these minimum cost values are used in constructing the targeting curves. The results of the optimum separation calculations are shown in Figures 1-4. Figure 1 shows how the separation cost varies with product recovery for a range of cost permeabilities at effective selectivities of 30 and 50. At an effective selectivity of 30 the minimum cost is at a recovery of 90-91% for the range of permeabilities. The optimum increases very slightly as the cost permeability increases, but at the optimum recovery the curves are very shallow and the cost of separation increases by less than 0.5% for a product recovery of 1% on either side of the optimum. A similar picture is observed at an effective selectivity of 50; there is a slight increase in optimum recovery with cost permeability, but the change in cost over the range of cost permeabilities is negligible. The optimum recovery in this case is 93%. Because the cost change close to the optimum recovery (1% on either side) is so small, the optimum recovery has been taken to be independent of cost permeability. Carrying out the cost analysis at various effective selectivities, an optimum recovery profile can be drawn for the range of effective selectivities of interest as shown in Figure 2. Figure 3 shows curves of relative separation cost against carbon monoxide purity for different values of cost permeability, and Figure 4 shows the optimum carbon monoxide purity profile for the range of cost permeabilities under consideration.

Figure 1. Comparative cost of separation against product recovery for various membrane cost permeabilities, at effective selectivities of 30 and 50.

6. This shows a separation cost of below 60% of that of the cryogenic process, which agrees with the conclusion in part 1.

Construction of Targeting Graphs To construct the targeting graphs, the relative cost of separation is calculated at the optimum purity and recovery for the particular combination of cost permeability and effective selectivity. The cost is plotted against effective selectivity for many values of cost permeability, as shown in Figure 5. These curves are used to construct the targeting graph by drawing isocost lines on a plot of effective selectivity versus cost permeability, as shown in Figure 6. Note that Figure 6 applies only to the carbon monoxide-hydrogen separation in the context of the manufacture of acetic acid. Existing membrane permeators (identified by the values of their cost permeability and effective selectivity in part 1: supplier A, Rc ) 1.73 sm3/(£k‚h‚bar), Re ) 31; supplier B, Rc ) 2.37 sm3/(£k‚h‚bar), Re ) 22) may be located on the targeting graphs, as shown in Figure

Limiting Values of Cost Permeability and Effective Selectivity The targeting graphs of Figure 6 have this in common: at low values of cost permeability they are almost vertical, and at high values of cost permeability they are almost horizontal. It was noted in the economic evaluation that the capital cost of the membrane separator depends on the cost permeability and the running cost depends on the effective selectivity. Thus, at low values of cost permeability the near vertical lines indicate that the total cost is dominated by the capital cost and can be best reduced by increasing cost permeability, and at large values of cost permeability the total cost is dominated by the running cost and can be best reduced by increasing the effective selectivity.

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Figure 2. Optimum product recovery profile for membrane separation of hydrogen and carbon monoxide.

Targeting Considerations The targeting graphs show the economic gains to be made by improving the permeability or selectivity of the membrane. Two commercially available membranes are pinpointed on the graph (labeled A and B) by their cost permeability and effective selectivity values calculated in part 1. By locating the existing membranes on the targeting graphs in Figure 6, it is shown that significant cost reductions for this separation will be gained by increasing the cost permeability of the membranes at this level of effective selectivity (∼20). Increases in effective selectivity, however, would yield little economic advantage at the cost permeabilities of currently available commercial systems. The other considerations for the membrane are that it must be a glassy polymer at the temperature of operation in order to maintain good selectivity and it must be mechanically strong enough to be used as a membrane. Membrane Design and Selection Membrane development at Aston University is well established. The membrane described here, selected for the separation of hydrogen and carbon monoxide, was partly the result of complementary work carried out by Kishi (1987) for the development of high-permeability materials for oxygen. These materials are based on copolymers of methacrylates and highly branched siloxy compounds. The polymer selected to illustrate the development procedure, from targeting to the evaluation of the new membrane, was a copolymer synthesized from a 50:50 (mole ratio) mix of methyl methacrylate (MMA) and tris(trimethylsiloxy)[γ-(methacryloxy)propyl]silane (TRIS). See Figure 7. The polymer was expected to show a considerable increase in hydrogen permeability over currently used membranes because of the very bulky nature of the siloxy group, but keeping the flexible Si-O groups in the side chains, the backbone of the polymer remains rigid enough to ensure that the polymer is in the glassy state at the temperature of operation. The

Figure 3. Comparative cost of separation against product purity for various membrane cost permeabilities, at effective selectivities of 30 and 50.

membrane should therefore maintain a reasonable selectivity of hydrogen to carbon monoxide. Experimental Section 1. Synthesis. To manufacture a thin sheet of membrane which is suitable for the measurement of gas transport properties, a prepolymerization technique was used before synthesis by free-radical polymerization. The comonomer formulation was prepolymerized before being synthesized by free-radical polymerization. The reason for this is that the monomers are very volatile and excessive loss can occur if polymerization is attempted without prepolymerization, and thinning of the polymerized sheet will occur due to shrinkage in the mold because of the large difference in the polymer and monomer densities. By using cyclohexyl-tert-butyl perdicarbonate as the initiator, it was possible to bulk polymerize the mixture at a low temperature of 40 °C. This initiator was used because of its high reactivity, allowing the polymerization to be carried out at low temperature, minimizing the potential for loss of volatile monomers during the

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Figure 4. Optimum product purity profile for membrane separation of hydrogen and carbon monoxide.

Figure 6. Targeting graph for membrane separation of hydrogen and carbon monoxide.

Figure 5. Comparative cost of membrane separation of hydrogen and carbon monoxide against effective selectivity for many values of cost permeability.

polymerization, which would result in irregularities in the polymer mixture. Nitrogen was bubbled through the filtered comonomer mixture, and the flask was heated in a water bath to 40 °C for a period of up to 7 h; a fairly viscous solution was obtained. Figure 8 shows the experimental equipment. 2. Membrane Preparation. Thin sheets of polymer were prepared after the polymerization. Polymer sheets processed in plastic molding produce a membrane of up to approximately 1 mm thick. The viscous comonomer solution is injected into the mold cavity. The apparatus, shown in Figure 9, consists of two glass plates covered with Melinex sheets separated by a gasket made of polyethylene. The Melinex

helps to release the polymer sheets from the glass plates and the gasket, also giving a smooth finish. The glass plates and gasket were tightly clamped on three sides using spring clips to hold the polymer solution and to prevent any air bubbles from entering the system. A total of 3-4 mL of the prepolymerized comonomer mixture was injected from the unclamped side of the glass plate into the mold cavity. This side was clamped after the introduction of the monomer to form a tight seal. The glass plates with the monomer were placed in an oven at 50-60 °C for 3 days and then postcured for 2 h at 90 °C to complete the polymerization. A uniform sheet of polymer was produced using this preparation. The glass plates were placed at an angle from the horizontal in the oven to displace any air bubbles from the center to the sides. After the postcuring, the polymer sheets were removed from the glass plates and polyethylene gasket. The polymer membrane produced was a clear plastic with tough characteristics. The glass transition temperature of the polymer was measured to be 55 °C and had a density of 1.14 g/cm3. A boring tool was used to cut disks of the polymer to size for permeability measurements. 3. Transport Measurements. The measurement of gas permeability coefficients of the polymer membrane was carried out using gas permeability apparatus supplied by Davenport Instruments (U.K.), shown diagramatically in Figure 10. Figure 11 shows the layout of the experimental apparatus. Permeability measurements were taken of hydrogen, supplied by BOC Ltd. in high-pressure cylinders at a purity of 99.99%, and of carbon monoxide, supplied, similarly, at high pressure by Air Products at a purity of 99.9%. The gas permeability apparatus and experimental procedure are well documented and are described in detail in Davenport’s handbook (1989) and will therefore not be described in detail here. 4. Calculation of Pure Gas Permeability Coefficients. The gas transmission rate, TR, can be calcu-

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Figure 8. Experimental equipment used for prepolymerization.

Figure 7. Monomers used to synthesize a new polymer membrane.

lated from the equation:

TR ) 273PV/(PATA(P1 - P2))

(1)

where P ) rate of pressure change in the manometer (cmHg/s), V ) total volume between the test specimen and the mercury level in the capillary tube (cm3). This is the sum of the volume of the insert (5, 10, 15, 20 cm3), the volume of the capillary above the mercury level halfway through the test (cross-sectional area of capillary is 0.018 cm2), and the free volume in the filter paper, PA ) atmospheric pressure (cmHg), T ) temperature of the test (K), A ) area of the test specimen ) 23.77 cm2, and P1 - P2 ) pressure difference across the specimen (cmHg). The permeability coefficient, R, can then be calculated:

R ) TR × t × 1010 (barrer)

(2)

Figure 9. Thin film apparatus.

before the permeation rate comes to steady state. This is because of the way the membrane is allowed to equilibrate in the test gas prior to the start of the experiment. From these measurements the pure gas permeabilities have been calculated.

where t ) membrane thickness.

hydrogen at 20 °C

Results

carbon monoxide at 20 °C

1. Hydrogen and Carbon Monoxide Permeability. Figure 12 shows the pressure versus time plots for hydrogen and carbon monoxide through the membrane on the Davenport apparatus. It can be seen from Figure 12 that there is an initial rapid desorption of CO

ideal separation factor

RH2 ) 70.3 barrer RCO ) 3.2 barrer R* ) RH2/RCO ) 22

2. Evaluation of Membrane Performance. At present, most of the open literature permeator design methods, for example, the methods used by Weller and

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Figure 10. Davenport gas permeability apparatus.

Figure 12. Davenport permeability plots for carbon monoxide and hydrogen through the new membrane. Table 1. Effective Selectivity and Cost Permeability of Commercial Membrane Plant Studied in Part 1 Figure 11. Experimental layout for permeability measurements.

Steiner (1951), Stern and Walawender (1969), and Pan (1983), are similar to that of the Saltonstall (1987) method used in this work. That is, for a given separation, they provide a method of calculating the required membrane area, assuming that the membrane permeability and selectivity are both constant and independent of flowrate and composition. This implies that the expected performance of a permeator containing a new membrane material may be calculated from that of a permeator containing another known membrane material and assuming that the permeator module would be modified using geometry similar to that of existing modules. Thus, the cost permeability of a new membrane can be related to the cost permeability of a known membrane, the pure gas permeabilities of the new and known membranes, and a module design factor (MDF) which takes into account changes in module design. Module Design Factor. The cost permeability of a membrane separation plant depends on many factors: (1) permeability of the membrane, (2) cost of manufacturing the membrane, (3) cost and design of the vessels and pipes, (4) valving, (5) instrumentation, etc. The design of the membrane element will be affected by the permeability of the membrane to the gases being separated; an increase in the permeability will result in a decrease in the specific area of the membrane in each module, i.e., the area per unit volume will decrease. Hence, for a unit area of membrane the cost of the associated vessels and pipes, etc., will increase. The effect of developing a new membrane with increased permeability is thus reduced by the need to

supplier

effective selectivity, Re

cost permeability, Rc (sm3/£k‚h‚bar)

A B

31 22

1.73 2.37

modify the design of the membrane element. The cost permeability of a new membrane plant can be related to the cost permeability of a known membrane plant and pure gas permeabilities using the module design factor, M. Thus

R2 Rc2 ) Rc1M R1

(3)

where R is the material permeability of the pure gas, Rc is the cost permeability, and subscript 1 refers to the known membrane and subscript 2 to the new membrane. While the authors recognize that they have only limited knowledge of permeator module design, previous work on targeting membrane properties for gas separation, for example, Bhide et al. (1991), has assumed a constant cost per unit area of membrane. The use of the concept of cost permeability takes into account changes in module design as well as membrane properties. To evaluate the performance of the new polymer material for the separation of hydrogen and carbon monoxide for the acetic acid process, the calculated values of cost permeability and effective selectivity of commercially used cellulose acetate spiral wound membrane systems (Table 1), which were studied in part 1, are related to pure gas permeability data for cellulose

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40% of the cryogenic cost line. This represents a further 15% improvement over currently available membrane systems. Discussion

Figure 13. Evaluation of new MMA-TRIS copolymer for carbon monoxide/hydrogen separation.

acetate published by McCandless (1972). A range of values for the cost permeability and effective selectivity, based on the limited amount of data available to us, can therefore be calculated from the following inequalities:

0.15MR > Rc > 0.11MR

(4)

0.9R* > Re > 0.63R*

(5)

where M is the MDF, R is the permeability of hydrogen through the membrane expressed in barrer, and R* is the ideal selectivity of the membrane calculated from pure gas permeabilities. Rc and Re are the cost permeability (sm3/£k‚h‚bar) and effective selectivity, respectively. An estimate of the module design factor has been made for the MMA-TRIS membrane illustrated here. R2/R1 ) 4.48, and the module is spirally wound. This calculation has assumed that the pressure drops on the feed and permeate sides of the new membrane are the same as that for the spirally wound separation plants studied in part 1 and also that the new membrane could be produced at the same unit cost as the existing membranes and that the thickness of the active layer will be the same in the new membrane as in the existing membranes. Based on these assumptions, the MDF, M, ) 0.85. Using the above inequalities, the following ranges of cost permeability and effective selectivities have been calculated for the MMA-TRIS copolymer:

6.80 > Rc > 9.05 (sm3/£k‚h‚bar)

(6)

19.8 > Re > 13.9

(7)

The targeting graphs shown in Figure 8 were derived from only the costs of the process and membrane design theory. No real values of cost permeability and effective selectivity are involved in this calculation. They show, for a particular process, either the separation cost for a known membrane permeator or how the permeator must be changed to match a required cost (e.g., the cost of separation by another method). The lines are based on the optimized separation at each point on them and provide a means of evaluating alternative membrane permeators. This would require membrane plant suppliers to provide values of cost permeability and effective selectivity or alternatively to provide enough plant cost data for these parameters to be estimated. The use of the new concepts of cost permeability and effective selectivity is potentially useful because they are defined in terms of a simple theory of membrane design, e.g., that of Saltonstall (1987) used here. Like the concept of tray efficiency and load factor used in distillation practice, they can provide a useful way of evaluating the cost of membrane separators in different processes. However, this work highlights the need for more research and design studies on permeator performance and how it is related to the separating properties of the membrane material. The membrane material selected shows enhanced performance over existing membranes used commercially. The cost of CO separation in the acetic acid process would be reduced to about 50% of the cost of using cryogenic separation, and this is an improvement over membranes currently in use. Further improvements in the membrane properties may be possible by adjusting the copolymer blend to optimize the resulting permeability and selectivity for the acetic acid process. The work serves to illustrate the concept of the targeting procedure in the development of new membrane materials. Using the new membrane parameters, cost permeability, and effective selectivity and optimizing the process under consideration by taking into account the processes upstream and downstream of the separator, targets can be set for the properties required by the membrane so that it will be economically viable. The targets can guide polymer scientists to develop materials with different transport properties from those currently in use to increase permeability and/or selectivity. The targeting analysis is then used again to evaluate the new membrane material. Nomenclature Rc ) cost permeability (sm3/£k‚h‚bar) R ) pure gas permeability (barrer) Re ) effective selectivity M ) module design factor Subscript 1 refers to known membrane material, and subscript 2 refers to new membrane material.

Literature Cited These values can be shown on the targeting graph, indicating the range of economic performance to be expected if the membrane was to be developed. Figure 13 shows that the new MMA-TRIS membrane lies at

Bhide, B. D.; Stern, S. A. A New Evaluation of Membrane Processes for the Oxygen-enrichment of air. 11. Effects of economic parameters and membrane properties. J. Membr. Sci. 1991, 62, 37.

Ind. Eng. Chem. Res., Vol. 36, No. 3, 1997 837 Davenport Permeability Apparatus, Serial No. GP 468/44, Handbook, Davenport (London) Ltd., Welwyn Garden City, Hertfordshire, England, 1989. Kishi, M. Synthetic Polymers for Ophthalmic Applications. Ph.D. Thesis, Aston University, Birmingham, U.K., 1987. McCandless, F. P. Separation of Binary Mixtures of CO and H2 by Permeation Through Polymeric Films. Ind. Eng. Chem. Process Des. Dev. 1972, 11 (No. 4), 470-478. Pan, C. Y. Gas Separation by Permeators with High Flux Asymmetric Membranes. AIChE J. 1983, 29 (No. 4), 545. Porter, K. E.; Jenkins, J. D. Alternative Separation Processes. TCE Distill. Suppl. 1987, Sept, 23. Porter, K. E.; Hinchliffe, A. B.; Pardoe, J. Targeting Membranes for Gas Separation. Gas Sep. Purif. 1990, 4, 185.

Saltonstall, C. W. Calculation of the Membrane Area Required for Gas Separation. J. Membr. Sci. 1987, 32, 185. Stern, S. A.; Walawender, W. P. Analysis of Membrane Separation Parameters. Sep. Sci. 1969, 4 (No. 2), 129. Weller, S.; Steiner, W. A. Separation of Gases by Fractional Permeation through Membranes. J. Appl. Phys. 1951, 21, 279.

Resubmitted for review June 10, 1996 Accepted August 14, 1996X IE960328U Abstract published in Advance ACS Abstracts, January 1, 1997. X