Membrane processes liquid-phase separations

The first step in downstream processing is the separation of the product-rich phase from the second phase and the biocatalyst. This may be simplified if the enzyme is immobilized or if a membrane module is included in the experimental set-up. In the case of emulsion reactors, centrifugation for liquid phase separation is a likely separation process [58], although the small size of droplets, the possibility of stable emulsion formation during the reaction, particularly if surface-active... 

Membrane extraction encompasses a class of liquid-phase separations where the primary driving force for transport stems from the concentration difference between the feed and extractant liquids rather than a pressure gradient, as is the case with most of the other processes discussed above. A microporous membrane placed between the feed and the extractant liquids functions primarily as a phase separator. The degree of separation achievable is determined by the relative partition coefficients among individual solutes. This operationx is known as membrane solvent extraction. If a nonporous, permselective membrane is used instead, however, the selectivity of the membrane would be superimposed on the partitioning selectivity in this case the process may be referred to as perstraction. These process concepts are illustrated in Fig. 34. 

Young, T-H., Lin, D.-J., Gau, J.-J., Chuang, W.-Y., and Cheng, L.-P. (1999), Morphology of crystalline Nylon-610 membranes prepared by the immersion-precipitation process Competition between crystallization and liquid-liquid phase separation, Polymer, 40, 5011-5021. 

Lise Dahuron, Ph.D. Sr. Research Specialist, The Dow Chemical Company (Liquid Density, Viscosity, and Interfacial Tension Liquid-Liquid Dispersion Fundamentals Liquid-Liquid Phase Separation Equipment Membrane-Based Processes)... 

The authors of very many works on so-caUed membrane-based or nondispersive solvent extraction could not prove that the process reaches equilibrium. Therefore, we cannot confirm the processes, published in these works, as membrane-based solvent extraction, but can confirm them as liquid membrane processes. Liquid membrane separations are dynamic nonequilibrium processes, in which only local equilibrium at immiscible phases interface may be suggested. 

The Loeb-Sourirajan process often is referred to as diffusion induced phase separation (DIPS) to reflect the role of diffusion in forming the asymmetric structure. Liquid-liquid phase separation and the resulting asymmetric structure arise from diffusion of a solvent (acetone) out of the film and diffusion of a nonsolvent (water) into the film. This physical interpretation provided the basis for the development of asymmetric membrane manufacturing processes for other polymer - solvent - non-solvent systems. 

Cervera-Padrell, A. E., Morthensen, S. T., Lewandowski, D. J., Skovby, T., Kill, S., Gemaey, K. V. (2012). Continuous hydrolysis and liquid-liquid phase separation of an active pharmaceutical ingredient intermediate using a miniseale hydrophobic membrane separator. Organic Process Research Development, 16, 888-900. 

The field of membrane separations is radically different from processes based on vapor-liquid phase separation. Nevertheless, membrane separations share the same goal as the more traditional separation processes the separation and purification of products. The principles of membrane separation processes and their application to different types of operations are discussed in the last chapter. 

The use of di-(p-alkylphenyl)phosphoric acids containing butyl, hexyl, octyl and nonyl alkyl groups as carriers for separations of Co(II), Cu(II), Ni(II), and Zn(II) from aqueous sulfate solutions by bulk and emulsion liquid membrane processes has been explored. The organic phase was the di-(p-alkylphenyl)phosphoric acid in kerosene widi the inclusion of Span 80 as an emulsifier for the emulsion liquid membrane systems. Both single metal ion species and competitive transport of the transition metal cations were investigated. For comparison, the transport of these metal cations by commercially available Cyanex 272 and D2EHPA as carriers was studied also. To probe the mechanism of the liquid membrane transport processes, interfacial tension measurements were conducted. Multistage emulsion liquid membrane processes for the separation of the transition metal cation mixtures have been evaluated. 

O.M.Ilinich, Basic Principles of Action of Polymeric Membranes in Liquid Phase Catalytic Processes and Separations of Gaseous Mixtures, Boreskov Institute of Catalysis, Novosibirsk, 1997 (Russian). 

Process Description Microfiltration (MF) separates particles from true solutions, be they liquid or gas phase. Alone among the membrane processes, microfiltration may be accomplished without the use of a membrane. The usual materi s retained by a microfiltra-tion membrane range in size from several [Lm down to 0.2 [Lm. At the low end of this spectrum, very large soluble macromolecules are retained by a microfilter. Bacteria and other microorganisms are a particularly important class of particles retained by MF membranes. Among membrane processes, dead-end filtration is uniquely common to MF, but cross-flow configurations are often used. 

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). 

Liquid-liquid extraction is a basic process already applied as a large-scale method. Usually, it does not require highly sophisticated devices, being very attractive for the preparative-scale separation of enantiomers. In this case, a chiral selector must be added to one of the liquid phases. This principle is common to some of the separation techniques described previously, such as CCC, CPC or supported-liquid membranes. In all of these, partition of the enantiomers of a mixture takes place thanks to their different affinity for the chiral additive in a given system of solvents. 

Intelligent engineering can drastically improve process selectivity (see Sharma, 1988, 1990) as illustrated in Chapter 4 of this book. A combination of reaction with an appropriate separation operation is the first option if the reaction is limited by chemical equilibrium. In such combinations one product is removed from the reaction zone continuously, allowing for a higher conversion of raw materials. Extractive reactions involve the addition of a second liquid phase, in which the product is better soluble than the reactants, to the reaction zone. Thus, the product is withdrawn from the reactive phase shifting the reaction mixture to product(s). The same principle can be realized if an additive is introduced into the reaction zone that causes precipitation of the desired product. A combination of reaction with distillation in a single column allows the removal of volatile products from the reaction zone that is then realized in the (fractional) distillation zone. Finally, reaction can be combined with filtration. A typical example of the latter system is the application of catalytic membranes. In all these cases, withdrawal of the product shifts the equilibrium mixture to the product. 

Pervaporation. Pervaporation differs from the other membrane processes described so far in that the phase-state on one side of the membrane is different from that on the other side. The term pervaporation is a combination of the words permselective and evaporation. The feed to the membrane module is a mixture (e.g. ethanol-water mixture) at a pressure high enough to maintain it in the liquid phase. The liquid mixture is contacted with a dense membrane. The other side of the membrane is maintained at a pressure at or below the dew point of the permeate, thus maintaining it in the vapor phase. The permeate side is often held under vacuum conditions. Pervaporation is potentially useful when separating mixtures that form azeotropes (e.g. ethanol-water mixture). One of the ways to change the vapor-liquid equilibrium to overcome azeotropic behavior is to place a membrane between the vapor and liquid phases. Temperatures are restricted to below 100°C, and as with other liquid membrane processes, feed pretreatment and membrane cleaning are necessary. 

When a mixture in a reactor effluent contains components with a wide range of volatilities, then a partial condensation from the vapor phase followed by a simple phase split can often produce a good separation. If the vapor from such a phase split is difficult to condense, then further separation needs to be carried out in a vapor separation process such as a membrane. The liquid from the phase split can be sent to a liquid separation unit such as distillation. 

As for the former problem, the researchers of GA found that the mixed acid solution produced by the Bunsen reaction separates spontaneously into two liquid phases in the presence of excess amount of iodine [17]. The heavier phase is mainly composed of HI, I2, and H20, and is called "Hix" solution. The main components of the lighter phase are H2S04 and H20. The phenomenon (liquid-liquid (LL)-phase separation) offered an easy way of separating the hydriodic acid and the sulfuric acid. As for the HI processing, some ideas have been proposed by GA [17], RWTH Aachen [18], and JAEA. JAEA studied the utilization of membrane technologies for concentrating the Hix solution to facilitate the HI separation and also for enhancing the one-pass conversion of HI decomposition [19,20]. 

The unique ability of crown ethers to form stable complexes with various cations has been used to advantage in such diverse processes as isotope separations (Jepson and De Witt, 1976), the transport of ions through artificial and natural membranes (Tosteson, 1968) and the construction of ion-selective electrodes (Ryba and Petranek, 1973). On account of their lipophilic exterior, crown ether complexes are often soluble even in apolar solvents. This property has been successfully exploited in liquid-liquid and solid-liquid phase-transfer reactions. Extensive reviews deal with the synthetic aspects of the use of crown ethers as phase-transfer catalysts (Gokel and Dupont Durst, 1976 Liotta, 1978 Weber and Gokel, 1977 Starks and Liotta, 1978). Several studies have been devoted to the identification of the factors affecting the formation and stability of crown-ether complexes, and many aspects of this subject have been discussed in reviews (Christensen et al., 1971, 1974 Pedersen and Frensdorf, 1972 Izatt et al., 1973 Kappenstein, 1974). 

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