Membrane processes multicomponent separation

Figure 2 Hybrid membrane process to separate a) close boiling, b) binary azeotropic and c) multicomponent mixtures (Hommerich, 1998a).

For crossflow membrane processes (Figure 7.0.1(e)-(g)), the species which encounters the least resistance from the membrane (when subjected to its chemical potential based driving force across tbe membrane) will have the highest velocity of movement through the membrane. Correspondingly, its rate of disappearance from the feed stream will be highest, and its concentration in the bulk flow of the reject stream will be lowest Current practice in membrane separation processes does not exploit the differences in individual species velocities through the membrane for multicomponent separation. 

Distillation is still the most common unit operation to separate liquid mixtures in chemical and petroleum industry because the treatment of large product streams and high purities with a simple process design is possible. Despite of this the separation of azeotropic mixtures into pure components requires complex distillation steps and/or the use of an entrainer. Industrial applied processes are azeotropic, extractive or pressure swing distillation (Stichlmair and Fair, 1998). Another sophisticated method for the separation of binary or multicomponent azeotropic mixtures is the hybrid membrane process, consisting of a distillation column and a membrane unit. 

In the following part of this section, we provide simple mathematical descriptions of a few common features of two-phase/two-region countercurrent devices, specifically some general considerations on equations of change, operating lines and multicomponent separation capability. Sections 8.1.2, 8.1.3, 8.1.4, 8.1.5 and 8.1.6 cover two-phase systems of gas-Uquid absorption, distillation, solvent extraction, melt crystallization and adsorption/SMB. Sections 8.1.7, 8.1.8 and 8.1.9 consider the countercurrent membrane processes of dialysis (and electrodialysis), liquid membrane separation and gas permeation. Tbe subsequent sections cover very briefly the processes in gas centrifuge and thermal diffusion. 

A final example of the use of dithiolenes comes from the work of Grimaldi and Lehn,222 and Ohki, Tagaki and Ueno.223 These groups used tetraphenylnickeldithiolene as a redox potential driven electron carrier and cation carrier through artificial membranes. Lipophilic cocarriers were employed to generate a multicomponent carrier system, in which charge equalization occurs. Applications to biomembranes, ion separation and related processes were suggested. 

The chemical and petrochemical industries have utilized distillation, freezing, ion exchange, electrodialysis, selective membrane, and hydrate processes for a number of years to separate certain species or components from a multicomponent solution in their refining operations. Recent emphasis has been placed on developing and modifying these basic processes to obtain fresh water from brackish and sea water supplies. 

Pervaporation is a separation process in which a multicomponent liquid is passed across a membrane that preferentially permeates one or more of the components. A partial vacuum is maintained on the permeate side of the membrane, so that the permeating components are removed as a vapor mixture. Transport through the membrane is induced by maintaining the vapor pressure of the gas on the permeate side of the membrane at a lower vapor pressure than the feed liquid. The gradients in chemical potential, pressure, and activity across the membrane are illustrated in Figure 2.12. 

Many current protein separation operations involve exposure of a protein to interfaces, sometimes as the primary purpose of the process step and sometimes as a secondary consequence of that step. In either case, the extent to which a protein partitions between bulk solution and the interface greatly affects the process, and how multicomponent mixtures partition is even more important and even less understood and less predictable. Protein transport processes are also significant and not well understood, especially in confined or highly concentrated domains such as interstices in porous media and faces of membranes. 

Ultrafiltration — This process has been successful with mixtures difficult to separate, such as oily machining wastes and oily wastewater. A pressure-driven filtration membrane separates multicomponent solutes from solvents, according to molecular size, shape and chemical bonding. Substances below a preselected molecular size are driven through the membrane by hydraulic pressure, while larger molecules, such as oil droplets, are held back. Effluent oil concentration depends on influent concentration, but properly operated ultrafiltration units can produce oilfree water (less than 0.1 ppm for all practical purposes). 

Bausa, J. and W. Marquardt, 2000, Shortcut Design Methods for Hybrid Membrane/Distillation Processes for the Separation of Nonideal Multicomponent Mixtures, Ind. Eng. Chem. Res., 39, 1658-1672. 

In mass transfer apparatus one of two processes can take place. Multicomponent mixtures can either be separated into their individual substances or in reverse can be produced from these individual components. This happens in mass transfer apparatus by bringing the components into contact with each other and using the different solubilities of the individual components in the phases to separate or bind them together. An example, which we have already discussed, was the transfer of a component from a liquid mixture into a gas by evaporation. In the following section we will limit ourselves to mass transfer devices in which physical processes take place. Apparatus where a chemical reaction also influences the mass transfer will be discussed in section 2.5. Mass will be transferred between two phases which are in direct contact with each other and are not separated by a membrane which is only permeable for certain components. The individual phases will mostly flow countercurrent to each other, in order to get the best mass transfer. The separation processes most frequently implemented are absorption, extraction and rectification. 

For many years polymeric membranes have been utilized widely for material separation without detailed characterization of the pore size and the pore size distribution. Most of the commercially available membranes are prepared by either a dry or a wet phase-inversion process. These membranes are formed by the phase separation of multicomponent polymer-solvent systems, the underlying principle being phase separation of the polymer solution. 

Results from two studies involving high volume recovery of multicomponent process effluents are presented here as illustrations of recent applications of hyperfiltration membranes in a tubular configuration supported by porous stainless steel. The first is a laboratory separation of dyes frcm a saline dye manufactiaring process effluent and the second a pilot renovation of wash water from a dye range for reuse. 

Bausa, J. and W. Marquardt, Shortcut design methods for hybrid membrane/distillation processes for the separation of nonideal multicomponent mixtures. Industrial and Engineering Chemistry Research, 2000, 39(6) 1658 1672. 

Psrvaporation. In this separation process, illustrated schematically in Figure 42, a multicomponent liquid stream is passed across a membrane that preferentially permeates one or more of the components. As the feed liquid flows across... 

The description of the separation of multicomponent mixtures requires a more complex approach, for example by using Maxwell-Stefan methodology. However, the real membrane often assumes a more complex structure, in which, beside the microporous zeolite layer, the mesoporosity of the intra-crystalline-defects and of the underlying support can play an important role, especially when the capillary condensation phenomenon can occur, as in the case of the permeation of vapour. Kondo and Kita (Kondo and Kita, 2010) attempted an interpretation of the dehydration process by including narrow non-zeolitic pores into the support. The water molecules in the feed selectively adsorbed in zeolite pores are then transported to the non-zeolitic pore, where they are released in the permeate side of the membrane. 

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