Pervaporation permeability selectivity

A similar example of a promising application of solar heat for intensified process systems is pervaporation. In pervaporation, a selective membrane is used as barrier between two phases, the liquid feed and the vapour permeate. The process depends on the sorption equilibrium and the mobility of the components through the membrane and is rather independent of the vapour liquid equilibrium. The desired component, which is in liquid form in the feed, permeates through the membrane and evaporates while passing the membrane, because the partial pressure of the permeating component is kept lower than the equilibrium vapour pressure [21]. Permeabilities depend on the solubility and diffusion rates through the membrane. 

An important characteristic of pervaporation that distinguishes it from distillation is that it is a rate process, not an equilibrium process. The more permeable component may be the less-volatile component. Perv oration has its greatest iitihty in the resolution of azeotropes, as an acqiinct to distillation. Selecting a membrane permeable to the minor corTiponent is important, since the membrane area required is roughly proportional to the mass of permeate. Thus pervaporation devices for the purification of the ethanol-water azeotrope (95 percent ethanol) are always based on a hydrophihc membrane. 

Membrane Pervaporation Since 1987, membrane pei vapora-tion has become widely accepted in the CPI as an effective means of separation and recovery of liquid-phase process streams. It is most commonly used to dehydrate hquid hydrocarbons to yield a high-purity ethanol, isopropanol, and ethylene glycol product. The method basically consists of a selec tively-permeable membrane layer separating a liquid feed stream and a gas phase permeate stream as shown in Fig. 25-19. The permeation rate and selectivity is governed bv the physicochemical composition of the membrane. Pei vaporation differs From reverse osmosis systems in that the permeate rate is not a function of osmotic pressure, since the permeate is maintained at saturation pressure (Ref. 24). 

Membranes of PVA/PAcr.Ac blends evidence a selective permeability against different components of a liquid mixture. So, they may be used for the ethanol dehydration by pervaporation technique. 

Table 1.1 shows two developing industrial membrane separation processes gas separation with polymer membranes (Chapter 8) and pervaporation (Chapter 9). Gas separation with membranes is the more advanced of the two techniques at least 20 companies worldwide offer industrial, membrane-based gas separation systems for a variety of applications. Only a handful of companies currently offer industrial pervaporation systems. In gas separation, a gas mixture at an elevated pressure is passed across the surface of a membrane that is selectively permeable to one component of the feed mixture the membrane permeate is enriched in this species. The basic process is illustrated in Figure 1.4. Major current applications... 

Depending on the enrichment term (E0) of the membrane, the modulus can be larger or smaller than 1.0. For reverse osmosis E0 is less than 1.0, and the concentration polarization modulus is normally between 1.1 and 1.5 that is, the concentration of salt at the membrane surface is 1.1 to 1.5 times larger than it would be in the absence of concentration polarization. The salt leakage through the membrane and the osmotic pressure that must be overcome to produce a flow of water are increased proportionately. Fortunately, modem reverse osmosis membranes are extremely selective and permeable, and can still produce useful desalted water under these conditions. In other membrane processes, such as pervaporation or ultrafiltration, the concentration polarization modulus may be as large as 5 to 10 or as small as 0.2 to 0.1, and may seriously affect the performance of the membrane. 

The first application of pervaporation was the removal of water from an azeotropic mixture of water and ethanol. By definition, the evaporative separation term /3evap for an azeotropic mixture is 1 because, at the azeotropic concentration, the vapor and the liquid phases have the same composition. Thus, the 200- to 500-fold separation achieved by pervaporation membranes in ethanol dehydration is due entirely to the selectivity of the membrane, which is much more permeable to water than to ethanol. This ability to achieve a large separation where distillation fails is why pervaporation is also being considered for the separation of aromatic/aliphatic mixtures in oil refinery applications. The evaporation separation term in these closely boiling mixtures is again close to 1, but a substantial separation is achieved due to the greater permeability of the membrane to the aromatic components. 

Equation (9.11) identifies the three factors that determine the performance of a pervaporation system. The first factor, pevAp, is the vapor-liquid equilibrium, determined mainly by the feed liquid composition and temperature the second is the membrane selectivity, G-men, an intrinsic permeability property of the membrane material and the third includes the feed and permeate vapor pressures, reflecting the effect of operating parameters on membrane performance. This equation is, in fact, the pervaporation equivalent of Equation (8.19) that describes gas separation in Chapter 8. 

The selectivity (amcm) of pervaporation membranes critically affects the overall separation obtained and depends on the membrane material. Therefore, membrane materials are tailored for particular separation problems. As with other solution-diffusion membranes, the permeability of a component is the product of the membrane sorption coefficient and the diffusion coefficient (mobility). The membrane selectivity term amem in Equation (9.11) can be written as... 

At low permeate pressures typical of ideal pervaporation, the same form for membrane selectivity applies as for gas or vapor separation of components A and B, which will be discussed later. This simple selectivity equals the inverse ratio of the resistances of the membrane to permeation of components A and B [S2B / 2A from Eq. (1)]. This ratio can be shown to simply equal to the ratio of permeabilities of the two components in the material comprising the selective layer of the membrane. 

Process Descriptions Selectively permeable membranes have an increasingly wide range of uses and configurations as the need for more advanced pollution control systems are required. There are four major types of membrane systems (1) pervaporation (2) reverse osmosis (RO) (3) gas absorption and (4) gas adsorption. Only membrane pervaporation is currently commercialized. 

Separation from mixtures is achieved because the membrane transports one component more readily than the others, even if the driving forces are equal. The effectiveness of pervaporation is measured by two parameters, namely flux, which determines the rate of permeation and selectivity, which measures the separation efficiency of the membrane (controlled by the intrinsic properties of the polymer used to construct it). The coupling of fluxes affecting the permeability of a mixture component can be divided into two parts, namely a thermodynamic part expressed as solubility, and a kinetic part expressed as diffusivity. In the thermodynamic part, the concentration change of one component in the membrane due to the presence of another is caused by mutual interactions between the permeates in the membrane in addition to interactions between the individual components and the membrane material. On the other hand, kinetic coupling arises from the dependence of the concentration on the diffusion coefficients of the permeates in the polymers [155]. 

As a rule, permeability in glassy polymers (e.g. cellulose) is lower than in rubbery polymers (e.g. polydimethylsiloxane, PDMS) on the other hand, selectivity is dictated by the molecular dimensions of the permeating species [167]. The polymers used as membranes in analytical pervaporation are similar to those employed for gas separation and possess a dense, non-porous macroscopic structure. The difference between the two lies in the transport mechanism and arises mainly from a large affinity difference between the permeating molecules and the polymer membrane. 

The permeability for pervaporation depends on the concentrations of permeants in the polymer, which can cause swelling and solute-interaction effects in polymers. Inorganic membranes have recently been used in this application to overcome some of these limitations. Because of these non-ideal effects, the selectivity can be a strong function of feed concentration and permeate pressure, causing inversion of selectivity in some cases. 

Semi-permeable membranes are generally used to select the chemical species to be diffused towards the acceptor stream [253]. Alternatively, other restrictive media can be used in a similar way to gas diffusion, e.g., in pervaporation, membraneless gas diffusion and distillation, as discussed below. 

Example 26.3. Laboratory tests of a pervaporation membrane exposed to liquid with 90 weight percent ethanol and 10 percent water at 60°C showed a flux of 0,20 kg/ra -h and a permeate compostion of 7.1 percent ethanol when the downstream pressure was 15 mmHg. (a) Calculate the permeability of the membrane to ethanol and to water at the test conditions and the selectivity for water. (6) Predict the local permeate composition for 90 percent ethanol and 60°C if the downstream pressure is kept at 30 mm Hg by a water-cooled condenser. What is the condensing temperature (c) Calculate the local permeate composition for 95 percent, 99 percent, and 99.9 percent ethanol at 60°C and 30 ram Hg assuming the permeabilities are the same as for part... 

This application is based on the hydrophilicity of the ion exchange membrane. Though it is not essential to use an ion exchange membrane,219 they show excellent performance in pervaporation for dehydration of organic solvents. Pervaporation is the separation of solvents on the basis of their different affinities for the membrane and different permeation speeds through the membrane phase. The system consists of a liquid mixture to be separated, which contacts one side of the membrane, and a gas phase to permeate under reduced pressure, which is on the other side of the membrane (Figure 6.37). Membrane performance is evaluated by a permeability coefficient (flux) and separation factor (selectivity coefficient). The permeability coefficient, Q, is the permeated solvent through the membrane per unit area and unit time (kgm 2 h1). When a mixed solvent composed of components A and B is separated, the separation factor, a, is defined as... 

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