Gas and Liquid Diffusion in Membranes

Increasing industrial interest in the applications of permselective polymeric membranes has generated an abundance of research in the past 20 years. Industrial studies of permselective polymeric membranes are concentrated in two direct fields: gas separations and liquid separations. Gas separations, based on selective permeation through polymers, were first commercially utilized by Monsanto Co. in 1977 for the recovery of H2 gas from an industrial gas stream. 1 Liquid separations, through pervaporation techniques using polymer membranes, were not utilized until 1984 when Le Carbone-Lorraine/GFT Co. started using pervaporation plants for separations of alcohols, methyl ether ketone, and ethyl acetate from mixtures. 2 Since the first commercial developments in these separation fields, the use of polymeric membranes has been further exploited to affect the specific gas separations of H2, CO2, H2S, and N2 and to the specific liquid separations of organic acids, organic amines, and in esterification batch reactors, removing the water by-products directly from the reaction mix (See Figure 1). 2

Figure 1: Industrial Membrane Usage

Membrane separation processes offer numerous industrial advantages over distillation or disposal. Energy requirements are lower, providing for lower overhead costs. The equipment necessary for liquid and gas separations is significantly more compact, simple to build, and reasonably easy to operate. Handling various volumes of separated product is accomplished without having to utilized different equipment because the equipment can be scaled up or operated at partial capacity without problems occurring. Furthermore, permselective membranes have utility in not only industrial processes involving basic chemicals but also in commercial products. Polymer films, such as polyethylene, used in packaging meats, fruits, and vegetables need to have a certain amount of oxygen permeability and diffusion through the packaging while holding back water. This minimum diffusion, especially in meat packaging, allows the meat to retain a more desirable coloring for the consumer.

MEMBRANE PERMSELECTIVE PROCESSES

Permselectivity is the preferred permeation of one molecule through a polymer with respect to other diffusing molecules in a mixture. Permselectivity is defined as the ratio of the final concentration of molecules to initial concentration of molecules (see figure 2). The relationship between diffusion,

Figure 2: Permselective Diffusion Through a Polymer Membrane

flux, and permselectivity can be seen in the above derived equations. High permselectivity values may indicate complete separation in one stage of diffusion and if the diffusion rate is high through the membrane then a smaller surface area could be used. Most polymer membranes with high permselectivity have disappointingly low fluxes (See Figure 3). This indicates that although the polymer is efficient at separating molecules, diffusion

Figure 3: Membranes for Permselective Transport

through the membrane is slow, resulting in no economic benefit. In Figure 3, the graph shows an upper bound for selectivity vs. permeability properties. The table on the left gives some examples for two common polymers, giving an idea of overall selectivity and permeability compared to the most permeable polymer, poly(dimethylsiloxane).

Membranes utilized in separations need to possess both high selectivity and high permeation. The selectivity of the membrane to specific gas or liquid molecules is subject to the ability of the molecules to diffuse through the membrane. Diffusion of molecules through the polymer is dependent upon a number of polymer properties: crosslinking (if present), chain stiffness, or Tg, crystallinity (if present), crystallite size and distribution, and solubility of the molecules in the polymer membrane. In Figure 4, a gas or liquid molecule is depicted as moving through a membrane through the forced rearrangement of the polymer chains. Gas and liquid molecules might have the ability to swell certain polymeric membranes. Swelling can

Figure 4: Movement of Polymer Chains

alter the polymer structure by forcing rearrangement of the polymer chains 3. Disruption of the polymer matrix due to swelling usually causes loss of permselectivity due to increase in free volume. By increasing the amount of crosslinking in the polymer, the amount of swelling can be reduced. Chain stiffness and crystallinity affect the free volume of the polymer. Crystallites restrict the free volume, making diffusion more difficult. Diffusion of molecules, therefore, occurs to a great extent through the amorphous part of the polymer membrane. Increasing the chain stiffness in the amorphous regions essentially restricts the free volume. Having small, uniformly distributed crystallites in the polymer creates more tortuous pathways for the diffusing molecules. Polarity due to functional groups inside the membrane and van der Waals forces due to hydrocarbon fragments can also have a significant influence on separation processes, depending on the nature of the liquid and gas molecules. Both gas and liquid membrane separation cells usually utilize asymmetric or composite polymer membranes in the form of flat sheets/films or hollow fibers packed into a permeation cell. The success of permselective membrane technology is dependent upon selection of the appropriate polymeric membrane system. Considerable data on diffusion of gases and liquids through various polymeric systems exists. However, only the most recent research 1 has attempted to methodically analyze various polymer systems for structure/permeability/selectivity relationships.

Permselective polymeric membranes can be divided into two basic categories: glassy and rubbery. Glassy polymers have low chain intrasegmental mobility and long relaxation times, while rubbery polymers exhibit the opposite characteristics, namely high intrasegmental mobility and short relaxation times (See Figure 5).

Figure 5: Permeation Behavior in Polymers

Almost all industrial permselective membrane processes for gas separations utilize glassy polymeric membranes because of high gas selectivity and good mechanical properties. Polyimides are frequently used because they exhibit unusually high selectivity along with high permeability. For example, aromatic polyimides that contain -C(CF3)2- groups tend to have higher preference for CO2 relative to CH4. Introduction of -C(CF3)2- groups is believed to increase chain stiffness which reduces intrasegmental mobility, and reduce and limit the degree of chain packing by increasing the free volume, serving as molecular spacers and chain stiffeners in the polymer 1,4. Polysulfones have been used for years as permselective membranes, starting in 1977 when Monsanto utilized asymmetric hollow fiber coated with a thin layer of silicone rubber for H2 separations. Asymmetric cellulose acetate membranes are used for the removal of CO2 and H2S from natural gas. CO2 and H2S have high solubility in cellulose acetate which induces pseudo-plasticization, causing the polymer to swell with disruption of the polymer matrix which increases the mobility of the polymer chains.

In the area of rubbery polymers, the only systems currently under investigation are the poly(organosiloxanes). Poly(organosiloxanes) have been studied in detail because of the vast utility of polydimethylsiloxane (PDMS) as a pre-formed membrane that can then be used as a template for IPN formation in gas or liquid separation processes. PDMS possesses one of the greatest permeability coefficients of any polymer, due to its large free volume, and low selectivity. Through copolymerization, properties have the potential to be tailored to suit specific separation needs. Porosity control in materials used for separation processes is essential due to the potential variability of gases or liquids through the membrane. Sol-gel polymerizations can be manipulated to adjust the shrinkage of a network for the development of controlled porosity inorganic materials.

REFERENCES

1. Stern, S.A. J. Membrane Sci., 1994, 94, 1-65.
2. Le Carbone - Lorraine Product Bulletin Proc. Eng., 1994, Jan., 30-31.
3. Stancell, A.F. Diffusion Through Polymers, Chpt. 12, 247-273.
4. Kim, T.H.; Koros, W.J.; Husk, G.R.; O’Brien, K.C. J. Membrane Sci., 1988,37, 45-62.

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