Membranes vapor permeation

Figure 8.18 Flow schemes ofthe separation train of a 50-million gallon/y bioethanol plant. Current technology is illustrated in (a). Pervaporation membranes can be used to replace the molecular-sieve drier of the plant (b) or vapor-permeation membranes can be used to replace the rectifier column and molecular-sieve units (c).

Separation of liqui(J mixture Application of PV and VP General review, examples of polymers for membrane preparation, as well as performance parameters of pervaporation and vapor permeation membranes, are described [159]... 

Fig. 22 shows a reaction scheme enhanced by continuously removing water directly from the reactor. In this case, the water is removed from the vapor phase. A vapor stream is sparged from the reactor and circulated through a vapor permeation membrane module, where water is selectively permeated through the membranes. The membrane unit is sized, such that all the reaction water can be removed with the water/ alcohol ratio just below the azeotropic composition. 

A membrane reactor using a H-ZSM5 membrane was used by Bernal et al. [3.42] to carry out the esterification reaction of acetic acid with ethanol. An equimolar etha-nol/acetic acid liquid mixture was fed in the membrane interior, while He gas was used as an inert sweep on the shell-side. In this particular application the membrane, itself, provides the catalysis for the reaction. NaA and T-type zeolite membranes have been utilized for esterification reactions in a PVMR and in a vapor permeation membrane reactor (VPMR) by Tanaka et al [3.43, 3.44]. Both membranes are hydrophilic and show good separation characteristics towards a number of alcohols. The NaA membrane was used to study the oleic/acid esterification in a vapor permeation membrane reactor (VPMR) at 383... 

Cranford, RJ Darmstadt, H Yang, J Roy, Ch. Polyetherimide/ polyvinylpyrrohdone vapor permeation membranes. Physical and chemical characterization. Journal of Membrane Science, 1999, 155, 231-240. 

A crystalline or semicrystalline state in polymers can be induced by thermal changes from a melt or from a glass, by strain, by organic vapors, or by Hquid solvents (40). Polymer crystallization can also be induced by compressed (or supercritical) gases, such as CO2 (41). The plasticization of a polymer by CO2 can increase the polymer segmental motions so that crystallization is kinetically possible. Because the amount of gas (or fluid) sorbed into the polymer is a dkect function of the pressure, the rate and extent of crystallization may be controUed by controlling the supercritical fluid pressure. As a result of this abiHty to induce crystallization, a history effect may be introduced into polymers. This can be an important consideration for polymer processing and gas permeation membranes. 

FIG. 25-19 Pervaporation of gas from liquid feed across membrane to vaporous permeate. (SOURCE Redrawn from Ref. 24.)... 

The most common membrane systems are driven by pressure. The essence of a pressure-driven membrane process is to selectively permeate one or more species through the membrane. The stream retained at the high pressure side is called the retentate while that transported to the low pressure side is denoted by the permeate (Fig. 11.1). Pressure-driven membrane systems include microfiltration, ultrafiltration, reverse osmosis, pervaporation and gas/vapor permeation. Table ll.l summarizes the main features and applications of these systems. 

Zeolite A is a very successful membrane for separation of water from alcohols, but it suffers from stability issues under acid conditions [23]. Usually, a Hquid phase should be avoided and, for this reason, vapor permeation is preferred. Recent developments show that the hydrophilic MOR [23] and PHI [50] membranes are more stable under acidic conditions in combination with a good membrane performance. 

Figure 12.37 shows a flowsheet for the separation of an azeotropic mixture using a membrane, but this time using vapor permeation. The mixture is first distilled to approach the azeotrope using a distillation column with... 

K., Kondo, M., and NaA, Z. (2001) membrane preparation, single-gas permeation, and pervaporation and vapor permeation of water/organic liquid mixtures. Ind. Eng. Chem. Res., 40, 163-175. 

Gas Dehydration. It has been found that water vapor permeates cellulose acetate membranes at a rate approximately 500 times that of methane (Ref. 2). This exceptionally high selectivity for water vapor make cellulose acetate membrane systems attractive for dehydration of hydrocarbon gas streams to pipeline specifications on either a pure gas stream or while simultaneously removing contaminating acid gases. For these applications the small size, low weight and low maintenance of the SEPAREX system is particularly advantageous for offshore installations. 

B. Will, R.N. Lichtenthaler, Comparison of the separation of mixtures by vapor permeation and pervaporation using PVA composite membranes. Part I. Binary alcohol—water systems, J. Membr. Sci. 68 (1992) 119-125. 

T. Kataoka, T. Tsuro, S.-I. Nakao and S. Kimura, Membrane Transport Properties of Pervaporation and Vapor Permeation in an Ethanol-Water System Using Polyacrylonitrile and Cellulose Acetate Membranes, J. Chem. Eng. Jpn 24, 326 (1991). 

Figure 4.19(b) shows an equivalent figure for a counter-flow module in which 5 % of the residue gas containing 100 ppm water vapor is expanded to 50 psia and introduced as a sweep gas. The water vapor concentration in the permeate gas at the end of the membrane then falls from 1900 ppm to 100 ppm, producing a dramatic increase in water vapor permeation through the membrane at the residue end of the module. The result is a two-thirds reduction in the size of the module. 

Both Mitsui [26] and Sulzer [27] have commercialized these membranes for dehydration of alcohols by pervaporation or vapor/vapor permeation. The membranes are made in tubular form. Extraordinarily high selectivities have been reported for these membranes, and their ceramic nature allows operation at high temperatures, so fluxes are high. These advantages are, however, offset by the costs of the membrane modules, currently in excess of US 3000/m2 of membrane. 

Figure 8.13 Calculated permeate vapor concentration for a vapor-permeable membrane with a vapor/nitrogen selectivity of 30 as a function of pressure ratio. The feed vapor concentration is 1 %. Below pressure ratios of about 10, separation is limited by the pressure ratio across the membrane. At pressure ratios above about 100, separation is limited by the membrane selectivity [36]...

Figure 9.12 Isothermal vapor permeation and multistage pervaporation with intermediate heating. GFT poly (vinyl alcohol) membranes [45]...

U.H.F. Sander, Development of Vapor Permeation for Industrial Applications, in Pervaporation Membrane Separation Processes, R.Y.M. Huang (ed.), Elsevier, Amsterdam, pp. 509-534 (1991). 

Vane, L.M. and Alvarez, F.R. (2008) Membrane-assisted vapor stripping energy-efficient hybrid distillation vapor permeation processes for alcohol-water separation. Journal of Chemical Technology and Biotechnology, 83(9), 1275-1287. 

This form stresses that part of the separation in the perva-poration process occurs independent of the presence of the membrane, /9evap. Equation (15) also stresses that part of the separation relies strictly on the identity of the membrane material being used, /9mem. In this context, the membrane is seen as separating a hypothetical vapor feed (in equilibrium with the actual liquid feed) and the downstream vapor permeate product. 

A vapor permeation process is considered that is, the membrane is placed above the vapor-liquid interface. 

In a partial pressure-driven vapor permeation process, the NC-dimensional vector of mass fluxes through the porous membrane can be expressed as [33] ... 

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