Permeation separations, defined

Besides some measures of separation efficiency such as the separation factor and extent of separation defined above, some quantity indicative of the throughput rate of a membrane system is needed to compliment the permselectivity of the membrane. It is quite common and practical in the membrane technology to use a phenomenological expression to relate the permeate flux (Ja in the unit of cm (STP)/s-cm7) of a given gas (A) through the membrane to the driving force, the transmembrane pressure difference (Ap) as follows ... 

Common practical applications of permeation separation phenomena include gas permeation, dialysis, and reverse osmosis. A variety of equipment modules for carrying out these processes are in use. The membrane type and the way it is incorporated in the device define the different separator assemblies that are commonly available. 

When a complete separation of a solution takes place by a reverse osmosis membrane. only one component of the solution (usually the solvent) goes through the membrane and is enriched in the permeate. The other component (usually the solute), on the other hand, is left behind on the feed solution side of the membrane. Unless the solute diffuses back to the main body of the feed solu> tion quickly, the solute concentration in the vicinity of the membrane increases until a steady state is reached where the diffusion rate, enhanced by the high concentration near the membrane surface, counterbalances the rate of the solute accumulation near the membrane surface. This phenomenon is called concentration polarization and may occur even when the separation of the solute from the solvent is not perfect. It is known that the concentration polarization exerts an unfavorable effect on the performance of reverse osmosis membranes. For example, the experimentally observable solute separation defined later by Equation 5.46 is lowered, since the concentration in the permeate, c>i3, is governed not by the concentration of the solute in the main body of the feed solution, but... 

Van Reis and Saksena (1997) have identified the product yield and the product purification factor as the two dimensions of selective protein separation with, for example, y/ or (DP) AS as parameters. The product purification factor Pi for solute 1 in the permeate was defined as... 

The most convenient mathematical method of describing pervaporation is to divide the overall separation processes into two steps, as shown in Figure 40. The first is evaporation of the feed Hquid to form a (hypothetical) saturated vapor phase on the feed side of the membrane. The second is permeation of this vapor through the membrane to the low pressure permeate side of the membrane. Although no evaporation actually takes place on the feed side of the membrane during pervaporation, this approach is mathematically simple and is thermodynamically completely equivalent to the physical process. The evaporation step from the feed hquid to the saturated vapor phase produces a separation, which can be defined (eq. 13) as the ratio of... 

The second step, permeation of components / andy through the membrane, is related directiy to conventional gas permeation. The separation achieved in this step, can be defined as the ratio of components in the permeate vapor to the ratio of components in the feed vapor (eq. 14). 

An enrichment is defined as a separation process that results in the increase in concentration of one or mote species in one product stream and the depletion of the same species in the other product stream. Neither high purity not high recovery of any components is achieved. Gas enrichment can be accompHshed with a wide variety of separation methods including, for example, physical absorption, molecular sieve adsorption, equiHbrium adsorption, cryogenic distillation, condensation, and membrane permeation. 

Multiple-Component Separation Separation Factor Consistent with the characterization of different separation methods, one can define a separation factor a,j (also called selectivity) for components i andj that compares their relative concentrations in the permeate stream to those in the feed stream ... 

The membrane performance for separations is characterized by the flux of a feed component across the membrane. This flux can be expressed as a quantity called the permeability (P), which is a pressure- and thickness-normalized flux of a given component. The separation of a feed mixture is achieved by a membrane material that permits a faster permeation rate for one component (i.e., higher permeability) over that of another component. The efficiency of the membrane in enriching a component over another component in the permeate stream can be expressed as a quantity called selectivity or separation factor. Selectivity (0 can be defined as the ratio of the permeabilities of the feed components across the membrane (i.e., a/b = Ta/Tb, where A and B are the two components). The permeability and selectivity of a membrane are material properties of the membrane material itself, and thus these properties are ideally constant with feed pressure, flow rate and other process conditions. However, permeability and selectivity are both temperature-dependent... 

We define a membrane reactor as one in which two phases are separated by a wall through which only one species can permeate. It is also common to have catalyst within the... 

Pervaporation separation index (PSI), which is a measure of the separation ability of a membrane was defined by Huang and Yeom [7] and expressed as the product of separation factor and permeation rate. 

The three factors that determine the performance of a membrane gas separation system are illustrated in Figure 8.12. The role of membrane selectivity is obvious not so obvious are the importance of the ratio of feed pressure (p ) to permeate pressure (pt) across the membrane, usually called the pressure ratio, [Pg.317]

Having said this, the bulk of the pervaporation literature continues to report membrane performance in terms of the total flux through the membrane and a separation factor, /3pervap, defined for a two-component fluid as the ratio of the two components on the permeate side of the membrane divided by the ratio of the two components on the feed side of the membrane. The term /3pervap can be written in several ways. 

When the gas or vapor feed stream contains a component that is highly soluble in the polymer membrane and causes plasticization, then the selectivity as defined by Equation 4.6 will depend on the partial pressure or the amount of the plasticizing component sorbed into the membrane. Furthermore, pure-gas permeation measurements are generally not a good indicator of the separation performance, and mixed-gas permeation measurements will be needed [21-23]. Often, the mixed-gas selectivity is less than predicted from pure-gas measurements [8] however, the opposite has been observed [24], Competitive sorption effects can also compromise the prediction of mixed-gas behavior from pure-gas measurements [25], For gas pairs where each component is less condensable than C02, like 02/N2, it is generally safe to conclude that the selectivity characteristics can be accurately judged from pure-gas permeabilities at all reasonable pressures. When the gas pair involves a component more condensable than C02, plasticization is likely to be a factor and pure-gas data may not adequately reflect mixed-gas selectivity. When C02 is a component, the situation depends on the partial pressures and the nature of the polymer. 

Some of the variables that are important for the subsequent discussion are recalled here. The membrane properties are related to the mass transport of the different chemical species through the membrane itself or its separating layer (for an asymmetric or multilayer membrane). Permeability and selectivity were defined for the mass transport by permeation both depend on the membrane nature and morphology that impose the specific transport mechanism driving the permeation of which it is characteristic. Table 13.2 reports the permeability coefficient, selectivity and permeating driving force of some permeation mechanisms. 

A crucial characteristic of a membrane is the molecular weight cutoff (MWCO) value, which is defined as the molecular weight at which 90% of the solutes are retained by the membrane. The retention factor R of solute A to be separated by the membrane is defined by the ratio of the concentration of A in the permeate to that in the retentate, as expressed in the following equation ... 

Gel permeation chromatography (GPC) on styrene-divinylbenzene copolymer (PLgel, Polymer Laboratories) columns has been found to separate PCN from e.g., PCB. The gel has a defined pore size of 50 A and may be used also at high pressures. The method was first applied to the isolation of PCNs in commercial PCB products [3]. 

The mass flow is effected by keeping the downstream side of the membrane at reduced pressure. The performance of membranes for the pervaporation of ethanol-water mixtures is evaluated by the separation factor a H and the specific permeation rate R. is defined as follows ... 

Ceramic membrane is the nanoporous membrane which has the comparatively higher permeability and lower separation fector. And in the case of mixed gases, separation mechanism is mainly concerned with the permeate velocity. The velocity properties of gas flow in nanoporous membranes depend on the ratio of the number of molecule-molecule collisions to that of the molecule-wall collision. The Knudsen number Kn Xydp is characteristic parameter defining different permeate mechanisms. The value of the mean free path depends on the length of the gas molecule and the characteristic pore diameter. The diffusion of inert and adsorbable gases through porous membrane is concerned with the contributions of gas phase diffusion and sur u e diffusion. 

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