Reverse Osmosis and Ultrafiltration - American Chemical Society

34 Pervaporation Membranes Application in the Chemical Process Industry H. E. A. BRÜSCHKE1, G. F. TUSEL1, and R. RAUTENBACH2 1

Downloaded by COLUMBIA UNIV on July 1, 2012 | http://pubs.acs.org Publication Date: January 1, 1985 | doi: 10.1021/bk-1985-0281.ch034

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GFT Ingenieurbüro, Gerberstrasse 48, 6650 Homburg, Federal Republic of Germany Rhein Westfaren Technische Hochschule, Turmstrasse 46, 5100 Aachen, Federal Republic of Germany

Data for the separation of alcohol-water mixtures by means of a newly developed pervaporation membrane are reported. The membrane is of the composite type, comprising polyvinylalcohol as the main polymer of the separating layer. Critical process parameters and the application of the pervaporation process in an industrial scale are discussed. Membrane pervaporation is a very effective separation process, especially in the chemical industry. Various membranes of the composite type have been developed and tested for their pervaporation capabilities. These newly developed composite membranes exhibit excellent performance in the removal of water from mixtures with organic solvents, the organic solvent being a simple alcohol, such as ethanol or isopropanol; a ketone, such as acetone or methyl-ethyl-ketone; an ether, such as diethyl ether or dioxane or an ester, such as ethyl acetate. Starting with sub-azeotropic compositions, purities of the organic components of 99.8% or higher can be achieved. Especially feasible is the pervaporation process for multi-component mixtures. Since, even at high organic feed concentrations, the permeate streams contain approx. 90% water, evaporation heat for these permeate streams has to be supplied at a temperature level of only 50 - 100°C, making the use of waste heat possible. Different membranes still under development show extremely promising results with regard to the separation of organic-organic mixtures. Pervaporation and its potentials were studied fairly extensively by a small number of researchers 20 - 30 years ago (1-2). However, pervaporation processes did not find their way into industrial application because of the lack of suitable membranes and because interest in membrane research was shifted to more promising process such as R0 and UF. The energy crisis and environmental pollution, however, forced engineers and scientists to look into possibilities of reducing 0097-6156/85/0281-0467$06.00/0 © 1985 American Chemical Society

In Reverse Osmosis and Ultrafiltration; Sourirajan, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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energy consumption and avoiding environmental pollution, especially in the chemical industry. During recent years, scientists have developed a new and strong interest in pervaporation because of its high separation potential for such organic or aqueous-organic mixtures, which are difficult and costly to separate by conventional separation techniques (3-5). Pervaporation can - for example - successfully replace azeotropic distillation or extractive distillation; it is simpler and safer because addition of a third component is unnecessary and it is superior from an economical point of view.


Fundamentals of Pervaporation Processes The difference between pervaporation and all other membrane processes is the phase change of the substance permeating through the membrane from a liquid feed to the vaporous permeate. The liquid "pervaporates" when passing through the membrane. Driving force for this process is the difference in the chemical potential of the species on both sides of the membrane. As non-porous membranes have t* be used, the transport mechanism can be best described by a solution-diffusion model. In order to keep the difference in chemical potential sufficiently high, condensation on the permeate side of the membrane must be avoided. This is effected by continuous removal of the vapour by either sweeping with an inert gas or by a vacuum. Figure 1 shows the principal arrangement of a pervaporation system. Contrary to the well-known processes of RO and UF, the performance of the pervaporation system is practically not influenced by the feed-side pressure. However, both selectivity and flux of a pervaporation membrane are highly dependent on the ratio of the total pressure at the permeate side of the membrane to the saturation pressure of the permeating components. In Figure 2 the dependence of flux on this ratio is presented for the benzene-cyclohexan system. This figure indicates clearly that in the practical range of application at values of p/p° smaller than 0.4, flux and selectivity are fairly independent of this pressure ratio. This necessary range of pressure ratios corresponds to an absolute pressure between 2 and 50 mbar, which can easily be achieved by standard vacuum pumps. Like in vacuum distillation, it is only necessary to compress the inert gases from vacuum to atmospheric pressure if the vapours are condensed in the vacuum. A normal Arrhenius-type dependence of flux on the operation temperature is found with all types of pervaporation membranes [6)9 whereas no simple dependence of selectivity on temperature can be observed (9). Process Design Process design in pervaporation has obviously to concentrate on two problems: 1. The design of a system with low pressure losses on the permeate side, since the actual pressure at the membrane surface determines the process rather than the suction pressure of the pump. 2. The low cost supply of the necessary evaporation enthalpy for the permeating components. In Reverse Osmosis and Ultrafiltration; Sourirajan, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

BRUSCHKE ET AL.


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Pervaporation Membranes

Figure 1 .

Figure 2.

Schematic of Pervaporation.

Influence of permeate pressure on flux of benzene through a polyethylene membrane.


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In principle, the evaporation enthalpy can be supplied by the heating of one side of the feed channel. Such a design is expensive and it is a far better solution to draw the energy from the sensible heat of the feed. As a consequence, the temperature of the feed will decrease in the module, i.e. a marked difference exists between feedentrance and feed-exit temperature. Since the flux depends very much on temperature (8), the feed must be reheated and, therefore, the combination of membrane modules and heat exchangers in series (Figure 3) is typical for pervaporation.


Membranes Obviously membranes for pervaporation, which could be operated at elevated temperatures (100°C or more) would be highly desirable. This is a very demanding task for membrane development in a field where the systems to be separated are mostly solvents by nature and where the transport mechanisms in the membrane are sorption and diffusion (6). Recently new membranes have been developed for a certain group of separation problems - the dehydration of aqueous organic mixtures. These asymmetric composite membranes, comprising poly-vinyl-alcohol as the separating polymer layer, can be operated up to temperatures of 130°C. An important practical feature of these membranes is the simplicity of operation: shut-downs of the unit at weekends for example - and consequently a drying of the membranes - will not damage the membranes. The membranes have proven their reliability for the separation of water-ethanol, water-isopropanol, water-methanol mixtures and systems such as methanol-ethanol and methanol-acetone. Nevertheless, the development of membranes for pervaporation is still at an early stage. Promising results have been obtained in our laboratory with small membrane samples capable of separating olefines from paraffines, for example. Processes and Economics The basics of pervaporation will be explained using the ethanol-water system as an example. In Figure 4 a diagram is presented, which shows on the x-axis the composition of an ethanol/water mixture and on the y-axis the composition of the permeate, as well as the composition of the vapours in equilibrium with the liquid. The upper curve is the equilibrium curve, which is well-known in distillation techniques. As can be seen, ethanol is the more volatile component and, therefore, enriched in the vapour phase compared to its liquid concentration. However, the higher the concentration of the liquid mixture in ethanol, the lower the ratio of its volatility over water volatility. In the ethanol-rich range a point called the azeotropic point is found, where liquid and vapour have the same composition. The lower curve in Figure 4 shows the permeate composition versus feed composition, obtained with the new PVA-based membrane. As can be seen, water is by far the better permeating component and is, therefore, enriched in the permeate. It has to be stressed that


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Figure 3. Typical flow sheet for a pervaporation unit.


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the lower curve is no equilibrium curve, but obtained in a steadystate kinetic process. Starting at low water concentrations, water is constantly enriched in the permeate and it can be seen that over a broad concentration range, the composition of the permeate is nearly constant, independent of the feed composition, and only a very small portion of ethanol is present at all in the permeate. With increasing water concentration, the water content of the permeate goes through a minimum and in the water-rich range we observe a point where the membrane does not separate and which looks very similar to an azeotropic point. This diagram also clearly shows why and where pervaporation processes offer advantages over distillation. It shows as well that distillation and pervaporation possess optimal separation characteristics at different concentration ranges. A hybrid process, where each process is used in its most effective concentration range should give better results than each process alone. In the water-rich area ethanol can be effectively separated from the mixture by distillation. At a concentration range of 60 80% (b.w.) of ethanol, however, the selectivity of the membranes with the now minor quantities of water becomes more effective. At high ethanol concentration, where distillation is no longer effective, pervaporation shows the highest selectivity. Furthermore, mainly the water is removed from the mixture. Therefore, only the aqueous portion has to be evaporated. The necessary heat input is, therefore, several times smaller than in distillation, where a multiple of the whole mixture (including the reflux) has to be evaporated. It has to be pointed out that the characteristics shown in Figure 4 are only valid for a specific membrane, i.e. material of the active layer based on PVA, and for a specific range of temperature and permeate pressure. Figure 5 shows the separation behaviour of membranes with cellulose triacetate as the main polymer of the active layer. Figures 4 and 5 also clearly demonstrate that the interactions between feed and permeate and active layer polymer play a more important role in pervaporation than in other membrane processes. Figure 6 shows a schematic diagram of a hybrid process for the production of dehydrated ethanol from a fermentation broth (7). In a simple distillation column, ethanol with a concentration oT approx. 80% b.w. is produced and is further dehydrated in a pervaporation system. Waste heat from the distillation column is used to supply the necessary energy for pervaporation. The permeate containing a small amount of ethanol is condensed and recycled to the distillation column. Figure 7 shows a similar system, but for mixtures available at higher concentrations. This might be the case at central collection points collecting the product from smaller distilleries. The permeate is fed to a stripping column, which is used at the same time for preheating the main feed stream. In this way, ethanol losses are close to zero and environmental pollution, occurring in conventional dehydration units, does not exist. Depending on the plant size, the investment costs of systems described above, including vacuum system and stripping column, are between 40 and 80% of those of a conventional (azeotropic distillation) system. Savings in operation costs are in an even higher range. (For details in investment and operation costs refer to {8} and (£)). Figure 8 shows a unit supplied to the



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Figure 4.


Permeate composition versus feed composition (composite membrane). Feed temperature 90 - 100°C P /P sat. >0.1.

Figure 5. Permeate composition versus feed composition. Asymmetric cellulose triacetate membrane Permeate pressure 20 mbar. In Reverse Osmosis and Ultrafiltration; Sourirajan, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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Figure 6.

Combination distillation/pervaporation for EtOH dehydration.


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Figure 7.

Combination of a separate pervaporation plant with a stripping column.


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Figure 8.

Pervaporation unit for the pharmaceutical industry.


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Pervaporation

Membranes

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pharmaceutical industries for the dehydration of ethanol for medical purposes. Very similar results have been obtained for the following systems, using the same membranes: a) Isopronal/water b) Acetone/water c) Methyl-ethyl-ketone/water d) Dioxan/water e) Ethyl-acetate/water f) Diethylen glycol/water The main differences between these systems and the ethanol-water mixture are found in slightly increased selectivities and fluxes in the organic-rich concentration ranges and shifting of the pseudoazeotropic points. All these systems have been examined in the laboratory and partly on small technical scales. Small-size production units for the dehydration of several of the above-mentioned systems are under construction and will be in operation in early 1985. Pervaporation processes exhibit extreme advantages in the dehydration of azeotrope-forming multi-component mixtures of organic solvents with water. In many cases, water removal from these systems is very difficult, if not impossible, irrespective of the type of thermal process. As the amount of water to be removed from such a mixture is normally not more than 10%, pervaporation offers a yery simple and inexpensive solution. The membranes under discussion can even be used to separate mixtures of methanol/ethanol and methanol/acetone, as methanol permeation rates are high compared to those of other organic solvents. In several cases, these separations are already applicable on a large scale and are more economical than distillation processes. Future Trends Development of pervaporation membranes is still in its early stages. Promising results have been obtained in the laboratory with special membranes, developed not for the removal of water, but for the separation of organic fluids. For different types of separation, different types of membranes are needed, as the solubilities of the permeating components seem to govern the overall separation characteristics. It will be possible to separate chemicals, such as d e fines from paraffines by pervaporation processes. Furthermore, it is possible as well to remove small amounts of organic substances (concentration below 1%) from water, down to levels below 1 ppm, using especially developed membranes. Literature Cited 1. 2. 3.

Heisler, E.G. "Solute and Temperature Effects in the Pervaporation of Aqueous Alcoholic Solutions"; Science 124, 1956, 77/78. Binning, R.C.; James, F.E. "Membrane Permeation"; Petroleum Refiner 37, 1958, 5, 214-216. Aptel, P.; Cuny, J.; Challard, N.; Neel, J. "Application of the Pervaporation Process to Separate Azeotropic Mixtures"; Journal of Membrane Science 1, 1976, 3, 271-287.


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Nagy, E.; Borlai, O.; Kjhidy, A. "Membrane Permeation of Water Alcohol Binary Mixtures"; Journal of Membrane Science 7, 1980, 109/118. Rautenbach, R.; Albrecht, R. "Separation of Organic Binary Mixtures by Pervaporation"; Journal of Membrane Science 7, 1980, 203-223. Albrecht, R. "Pervaporation - Beiträge zur Verfahrensentwicklung"; Ph.D Thesis, RWTH Aachen (FRG), 1983. Ballweg, A.H.; Brüschke, H.E.A.; Schneider, W.; Tusel, G.F. etal. "Pervaporation Membranes - An Economical Method to Replace Conventional Distillation and Rectification Columns in Ethanol Distilleries", 5th Int. Symp. on Alcohol Fuel Technology, Auckland, New Zealand, 1982. Mokhtari-Nejad, E.; Schneider, W. Europe-Japan Congress on Membrane and Membrane Processes, 1984, Stresa, Italy. Soukup, P.B. Diploma Thesis, University of Munich, 1983.

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R E C E I V E D February 22, 1985


Reverse Osmosis and Ultrafiltration - American Chemical Society

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