Selectivity, membrane pervaporation

The selectivity of pervaporation membranes varies considerably and has a critical effect on the overall separation obtained. The range of results that can be obtained for the same solutions and different membranes is illustrated in Figure 41 for the separation of acetone from water using two types of membrane (89). The figure shows the concentration of acetone in the permeate as a function of the concentration in the feed. The two membranes shown have dramatically different properties. The siUcone mbber membrane removes acetone selectively, whereas the cross-linked poly(vinyl alcohol) (PVA) membrane removes water selectively. This difference occurs because siUcone mbber is hydrophobic and mbbery, thus permeates the acetone preferentially. PVA, on the other hand, is hydrophilic and glassy, thus permeates the small hydrophilic water molecules preferentially. 

Advantages to Membrane Separation This subsertion covers the commercially important membrane applications. AU except electrodialysis are pressure driven. All except pervaporation involve no phase change. All tend to be inherently low-energy consumers in the-oiy if not in practice. They operate by a different mechanism than do other separation methods, so they have a unique profile of strengths and weaknesses. In some cases they provide unusual sharpness of separation, but in most cases they perform a separation at lower cost, provide more valuable products, and do so with fewer undesirable side effects than older separations methods. The membrane interposes a new phase between feed and product. It controls the transfer of mass between feed and product. It is a kinetic, not an equihbrium process. In a separation, a membrane will be selective because it passes some components much more rapidly than others. Many membranes are veiy selective. Membrane separations are often simpler than the alternatives. 

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). 

The preferred choice of a water-selective membrane up to now has been hydrophilic membranes because of their high water affinity. However, recently Kuhn et al. reported an all-silica DDR membrane for dehydration of ethanol and methanol with high fluxes (up to 20kg m h ) and high selectivities (H20/ethanol 1500 and H20/methanol 70 at 373 K) in pervaporation operation. The separation is based on molecular sieving with water fluxes comparable to well-performing hydrophilic membranes [51]. 

Figure 8.21. Comparison of selectivity of pervaporation membranes and liquid-vapour equilibrium for...

The composite materials have been used to form selective membranes for the separation of liquid mixtures [181]. The membranes should consist of a polymer which is soluble in the liquid components) to be separated, as the dispersed phase-derived polymer, and a continuous phase-derived polymer which is insoluble in all components of the liquid mixture. Thus, membranes consisting of polystyrene in polyacrylamide will separate toluene from cyclohexane, and those comprising polyacrylamide in crosslinked polystyrene can be used for water removal from ethanol. Due to the very thin films of polymer which separate the polyhedral dispersed phase cells, the permeation rates, which are measured by pervaporation, are relatively high. 

Figure 4.3 Concentration gradients formed as a result of permeation through a selective membrane. By convention, concentration polarization is usually represented by the gradient of the minor component—salt in the reverse osmosis example and water in the pervaporation example (dehydration of an ethanol solution)...

In the case of pervaporation of dissolved volatile organic compounds (VOCs) from water, the magnitude of the concentration polarization effect is a function of the enrichment factor. The selectivity of pervaporation membranes to different VOCs varies widely, so the intrinsic enrichment and the magnitude of concentration polarization effects depend strongly on the solute. Table 4.2 shows experimentally measured enrichment values for a series of dilute VOC solutions treated with silicone rubber membranes in spiral-wound modules [15], When these values are superimposed on the Wijmans plot as shown in Figure 4.12, the concentration polarization modulus varies from 1.0, that is, no concentration polarization, for isopropanol, to 0.1 for trichloroethane, which has an enrichment of 5700. 

Figure 9.5 Pervaporation separation of acetone-water mixtures achieved with a water-selective membrane poly(vinyl alcohol) (PVA), and an acetone-selective membrane (silicone rubber) [21]. Reprinted from Hollein et al. [21], p. 1051 by courtesy of Marcel Dekker, Inc.

A third possibility, illustrated in Figure 9.7(c), is to sweep the permeate side of the membrane with a counter-current flow of carrier gas. In the example shown, the carrier gas is cooled to condense and recover the permeate vapor, and the gas is recirculated. This mode of operation has little to offer compared to temperature-gradient-driven pervaporation, because both require cooling water for the condenser. However, if the permeate has no value and can be discarded without condensation (for example, in the pervaporative dehydration of an organic solvent with an extremely water-selective membrane), this is the preferred mode of operation. In this case, the permeate would contain only water plus a trace of organic solvent and could be discharged or incinerated at low cost. No permeate refrigeration is required [36],... 

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. 

A good example of separation on the basis of affinity is the separation of alcohol/ water mixtures using a hydrophobic, silicalite membrane. Pervaporation of an ethanol/ water mixture through such a membrane resulted the removal of the alcohol from the mixture [16]. The separation selectivities achieved are between 10 and 60, depending on temperature and the alcohol content in the feed. In this way azeotropes can be broken. The reason for this is that the principle of separation, namely, differences in adsorptive behavior, is different from separation based on vapor pressure differences, used in distillation. Another example of such a separation is the pervaporation of an acetic acid/water mixture through a silicalite membrane, resulting in the removal of acetic acid [17]. 

Ray SK, Sawant SB, Joshi JB, and Pangarkar VG. Development of methanol selective membranes for separation of methanol-MTBE mixtures by pervaporation. J. App. Poly. Sci. 1999 74(ll) 2645-2559. 

N. Qureshi, Recovery of Alcohol fuels using selective membranes by pervaporation, Ph.D. Thesis, University of Nebraska, Lincoln, NE, 1997. 

In general, the selectivity of pervaporation is high, as demonstrated in Figure 6.41 for the system "benzene-cyclohexane, polyethylene membranes". The separation characteristic is shifted to much more favorable figures compared to the thermodynamic equilibrium curve the azeotropic point can be suppressed.28-30 Parameter in the diagram is the ratio of total pressure p at the permeate side to saturation pressure Pl = TjXjP + TjXjpj. 

Pervaporation (PV) is a membrane-separation process in which one or more components of a liquid mixture diffuse through a selective membrane, evaporate under low pressure on the downstream side, and are removed by a... 

Additional pressure losses caused by hydrodynamic resistances in the permeate pass from the permeate side of the membrane to the condenser or the vacuum pump will be even more detrimental to the performance of the pervaporation process. When an alcohol-water mixture has to be dehydrated to a final water content of 1000 ppm even at 100 °C the partial water vapor pressure at the feed side will be of the order of 10 mbar. Using a high-selective membrane the partial water vapor pressure at the permeate side of the membrane will have to be kept at a few millibar. As this pressure is determined by the temperature of the condensing liquid permeate there has to be an unobstructed flow of the permeate vapor from the membrane to the condenser. It is obvious from Eq. (24) that even a pressure drop of one or two millibar in the permeate channel of a module will have a severe effect on the ratio of the partial vapor pressure and thus on the performance of the system. 

The phenomenon of solvent transport through solid barriers has three aspects which discussed under the heading of permeability. These are the permeation of solvent through materials (films, containers, etc.) the use of pervaporation membranes to separate organic solvents from water or water from solvents the manufacture of permeate selective membranes. 

Buckley-Smith, M.K. 2006. The use of solubility parameters to select membrane materials for pervaporation of organic mixtures. Ph.D. Thesis, University of Waikato, Hamilton, New Zealand. 

Since both selective membrane permeation and evaporation occur, pervaporation both separates and concentrates. Evaporating the liquid at the downstream side of the membrane also increases the driving force. Assume that the driving force is adequately represented by the partial pressure difference across the membrane. The local partial pressure on the upstream side, labeled 1 in Figure 17-13. is... 

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. 

A general review about Rl-type PVMRs can be found elsewhere (Drioli Giomo, 2010). In this section, a review of Rl-type PVMRs for esterifications is given. The concept is simple instead of water, the main product (ester) pervaporates through a hydrophobic membrane. It is clear that this technology is less developed because each type of ester produced may need a specific membrane. In any case, to define continuous processes, hydrophobic ester-selective membranes may also need to be used. 

Finally, a concise overview of R1 -type configurations for esterification PVMRs was provided. This subject is less studied in the literature because ester-selective membranes are more difficult to be designed (also considering that theoretically for each ester an optimized membrane should be defined). Nevertheless, PDMS membranes showed to be a good reference for these types of applications, leading to good conversions. Moreover, it must be stressed that when hydrophilic membranes are not extremely selective toward water, in the case of a continuous setup, a hydrophobic membrane should be considered to obtain pure ester as product (e.g., in series with the water-selective membrane). This means that the development of reference membranes for ester pervaporation is an interesting research path. 

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