Concentration Polarization in Gas Separation

Concentration polarization is present in all membrane processes, even though it has been neglected for a long time in the gas separation field. As mentioned above, this phenomenon occurs because the permselectivity of the membrane itself consists in a decrease of driving force of the most permeable species owing to the presence of the least permeable ones. Its negative effect on membrane performances can be relevant when the selective layer is very thin ( 5pm approx.), as already shown in several works in the literature (see, for example, references 14-23). 

The first field for which concentration polarization was deeply investigated (since the 1960s ) was liquid separation by membrane processes, such as ultrafiltration and reverse osmosis. On the contrary, for a long time it had been generally accepted that concentration polarization had only a negligible effect on membrane performance in gas separation. This was justified by the fact that membranes were quite thick and permeating flux very low, and, moreover, that 

Polarization and Inhibition by Carbon Monoxide in Palladium-based Membranes 141 

Liidtke et al considered the effect of the concentration polarization on the separation of -butane/nitrogen by means of a composite (three layers) polymeric membrane. Under their operating conditions, they found that the relative resistance in the boundary layer was so significant as to exceed that in the membrane at a sufficiently high total pressure of feed. 

He et al used a binary mixture-based film model to perform a theoretical analysis on the concentration polarization in a generic membrane. They defined a concentration polarization coefficient for both the two species involved in the separation as the ratio of the actual flux to the ideal one (without polarization), quantifying the polarization effect by means of the ratio of the actual fluxes of the components. Although this is a simplified approach that cannot be generalized to multi-component systems, nevertheless, under some operating conditions, the authors predicted a significant influence of the external mass transfer on the process. 

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Concentration polarization in gas separation processes has not been widely studied, and the effect is often assumed to be small because of the high diffusion coefficients of gases. However, the volume flux of gas through the membrane is also high, so concentration polarization effects are important for several processes. 

Two other major factors determining module selection are concentration polarization control and resistance to fouling. Concentration polarization control is a particularly important issue in liquid separations such as reverse osmosis and ultrafiltration. In gas separation applications, concentration polarization is more easily controlled but is still a problem with high-flux, highly selective membranes. Hollow fine fiber modules are notoriously prone to fouling and concentration polarization and can be used in reverse osmosis applications only when extensive, costly feed solution pretreatment removes all particulates. These fibers cannot be used in ultrafiltration applications at all. 

Figure 4.13 A portion of the Wijmans plot shown in Figure 4.7 expanded to illustrate concentration polarization in some important gas separation applications...

Concentration polarization is a significant problem only in vapor separation. There, because the partial pressure of the penetrant is normally low and its solubihty in the membrane is high, there can be depletion in the gas phase at the membrane. In other applications it is usually safe to assume bulk gas concentration right up to the membrane. 

Picer and Picer [357] evaluated the application ofXAD-2, XAD-4, and Tenax macroreticular resins for concentrations of chlorinated insecticides and polychlorinated biphenyls in seawater prior to analysis by electron capture gas chromatography. The solvents that were used eluted not only the chlorinated hydrocarbons of interest but also other electron capture sensitive materials, so that eluates had to be purified. The eluates from the Tenax column were combined and the non-polar phase was separated from the polar phase in a glass separating funnel. Then the polar phase was extracted twice with n-pentane. The -pentane extract was dried over anhydrous sodium sulfate, concentrated to 1 ml and cleaned on an alumina column using a modification of the method described by Holden and Marsden. The eluates were placed on a silica gel column for the separation of PCBs from DDT, its metabolites, and dieldrin using a procedure described by Snyder and Reinert [359] and Picer and Abel [360]. 

This chapter will only deal with the possible gas transport mechanisms and their relevance for separation of gas mixtures. Beside the transport mechanisms, process parameters also have a marked influence on the separation efficiency. Effects like backdiffusion and concentration polarization are determined by the operating downstream and upstream pressure, the flow regime, etc. This can decrease the separation efficiency considerably. Since these effects are to some extent treated in literature (Hsieh, Bhave and Fleming 1988, Keizer et al. 1988), they will not be considered here, save for one example at the end of Section 6.2.1. It seemed more important to describe the possibilities of inorganic membranes for gas separation than to deal with optimization of the process. Therefore, this chapter will only describe the possibilities of the several transport mechanisms in inorganic membranes for selective gas separation with high permeability at variable temperature and pressure. 

Summarizing it can be stated that the separation by gas phase transport (Knudsen diffusion) has a limited selectivity, depending on the molecular masses of the gases. The theoretical separation factor is decreased by effects like concentration-polarization and backdiffusion. However, fluxes through the membrane are high and this separation mechanism can be applied in harsh chemical and thermal environments with currently available membranes (Uhlhorn 1990, Bhave, Gillot and Liu 1989). 

Derivatization After Desorption. Alkanolamines, highly polar basic compounds, present a difficult analytical problem. Although direct gas chromatographic separations can be achieved, this technique is not applicable to trace analysis due to sorption problems at trace concentrations. A derivatization/gas chromatographic procedure has been developed for the determination of alkanolamines in air as low as 100 ppb (54,55). The samples are collected on activated alumina and desorbed with an aqueous solution of 1-octanesulfonic acid. The... 

Also of interest is the maximum capacity of each phase for chemicals, i.e., the saturation concentration above which phase separation occurs. For water, this is obviously the solubility in water. For many polar substances, the chemical and water are miscible (e.g., ethanol) and no solubility limit exists. Similarly, a solubility limit in octanol may or may not exist. For air, the solubility corresponds to the saturation vapor pressure Ps. This can be converted to a solubility in units of mol / m3 by dividing by RT, the gas constant — absolute temperature product. Chapter 7 discusses solubility in water. Solubility in octanol is not by itself of comparable interest and is not treated. Vapor pressure and solubility in water are not only of fundamental interest, but their ratio H is essentially the Henry s Law constant or air-water partition coefficient, as Chapter 4 discusses. 

Gas separation Hollow fibers for high volume applications with low flux, low selectivity membranes in which concentration polarization is easily controlled (nitrogen from air)... 

Figure 4.1 shows the concentration gradients that form on either side of a dialysis membrane. However, dialysis differs from most membrane processes in that the volume flow across the membrane is usually small. In processes such as reverse osmosis, ultrafiltration, and gas separation, the volume flow through the membrane from the feed to the permeate side is significant. As a result the permeate concentration is typically determined by the ratio of the fluxes of the components that permeate the membrane. In these processes concentration polarization gradients form only on the feed side of the membrane, as shown in Figure 4.3. This simplifies the description of the phenomenon. The few membrane processes in which a fluid is used to sweep the permeate side of the membrane,...

In the discussion of concentration polarization to this point, the assumption is made that the volume flux through the membrane is large, so the concentration on the permeate side of the membrane is determined by the ratio of the component fluxes. This assumption is almost always true for liquid separation processes, such as ultrafiltration or reverse osmosis, but must be modified in a few gas separation and pervaporation processes. In these processes, a lateral flow of gas is sometimes used to change the composition of the gas on the permeate side of the membrane. Figure 4.14 illustrates a laboratory gas permeation experiment using this effect. As the pressurized feed gas mixture is passed over the membrane surface, certain components permeate the membrane. On the permeate side of the membrane, a lateral flow of helium or other inert gas sweeps the permeate from the membrane surface. In the absence of the sweep gas, the composition of the gas mixture on the permeate side of the membrane is determined by the flow of components from the feed. If a large flow of sweep gas is used, the partial... 

The membrane in a broad sense is a thin layer that separates two distinctively different phases, i.e., gas/gas, gas/liquid, or liquid/liquid. No characteristic requirement, such as polymer, solid, etc., applies to the nature of materials that function as a membrane. A liquid or a dynamically formed interface could also function as a membrane. Although the selective transport through a membrane is an important feature of membranes, it is not necessarily included in the broad definition of the membrane. The overall transport characteristics of a membrane depends on both the transport characteristics of the bulk phase of membrane and the interfacial characteristics between the bulk phase and the contacting phase or phases, including the concentration polarization at the interface. The term membrane is preferentially used for high-throughput membranes, and membranes with very low throughput are often expressed by the term barrier. ... 

The hollow fiber membranes are the optimum choice for gas separation modules due to their very high packing density (up to 30,000 m /m may be attained [1]). Figure 4.21 shows alternative configurations for such modules [108]. Modifications of this configuration exist, where possibility for introduction of sweep gas on permeate side is included, or fibers may be arranged transversal to the flow in order to minimize concentration polarization [109,110]. The hollow fiber membranes are usually asymmetric polymers, but composites also exist. Carbon molecular sieve membranes may easily be prepared as hollow fibers by pyrolysis. 

The performance of membranes often decreases over time due to effects such as fouling and concentration polarization. This is seen as a decrease in flux (Figure 9.15). This performance decline is a major concern for filtration processes, but less so for gas separation processes. 

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