Membrane-based processes

At present moment, no generally feasible method exists for the large-scale production of optically pure products. Although for the separation of virtually every racemic mixture an analytical method is available (gas chromatography, liquid chromatography or capillary electrophoresis), this is not the case for the separation of racemic mixtures on an industrial scale. The most widely applied method for the separation of racemic mixtures is diastereomeric salt crystallization [1]. However, this usually requires many steps, making the process complicated and inducing considerable losses of valuable product. In order to avoid the problems associated with diastereomeric salt crystallization, membrane-based processes may be considered as a viable alternative. 

In order to design a zeoHte membrane-based process a good model description of the multicomponent mass transport properties is required. Moreover, this will reduce the amount of practical work required in the development of zeolite membranes and MRs. Concerning intracrystaUine mass transport, a decent continuum approach is available within a Maxwell-Stefan framework for mass transport [98-100]. The well-defined geometry of zeoHtes, however, gives rise to microscopic effects, like specific adsorption sites and nonisotropic diffusion, which become manifested at the macroscale. It remains challenging to incorporate these microscopic effects into a generalized model and to obtain an accurate multicomponent prediction of a real membrane. 

Membrane-based processes Resting cells instead of growing ones Product-tolerant strains... 

RO is a membrane-based process that is used to remove solutes of relatively low molecular weight that are in solution. As an example, almost pure water can be obtained from sea water by using RO, which will filter out molecules of NaCl and other salts. The driving potential for water permeation is the difference in hydraulic pressure. A pressure that is higher than the osmotic pressure ofthe solution (which itself could be quite high if the molecular weights of the solutes are small) must be applied to the solution side of the membrane. RO also involves the concentration polarization of solute molecules. 

Pervaporation (PV) is a membrane-based process used to separate aqueous, azeotropic solvent mixtures. This is done using a hydrophihc, non-porous membrane that is highly selective to water. Figure 3.9 shows a typical PV system that produces a dehydrated solvent stream (retentate) from a solvent/water feed. 

Table 2.1 Overview of main polymer membrane characteristics and membrane-based processes for molecular separations in liquid phase.

In these systems, the interface between two phases is located at the high-throughput membrane porous matrix level. Physicochemical, structural and geometrical properties of porous meso- and microporous membranes are exploited to facilitate mass transfer between two contacting immiscible phases, e.g., gas-liquid, vapor-liquid, liquid-liquid, liquid-supercritical fluid, etc., without dispersing one phase in the other (except for membrane emulsification, where two phases are contacted and then dispersed drop by drop one into another under precise controlled conditions). Separation depends primarily on phase equilibrium. Membrane-based absorbers and strippers, extractors and back extractors, supported gas membrane-based processes and osmotic distillation are examples of such processes that have already been in some cases commercialized. Membrane distillation, membrane... 

The advantages and disadvantages of membrane based processes and pertraction through various types of liquid membranes are summarized in Table 23.5. HF contactors are supposed in these processes with the exception of pertraction into stable emulsions (ELM) where mixed column contactors or mixer-settlers are used. 

Table 23.5 Advantages and disadvantages of membrane-based processes and pertraction through various types of liquid membranes in two- and three-phase systems.

Schlosser, . (2000) Membrane based processes with immobilized interface, in Integration of Membrane Processes into Bioconversions (eds K. Bako, L. Gubicza and M. Mulder), Kluwer Academic, p. 55. 

To facilitate the discussion, conventional terminology used to refer to the most common types of membrane-based processes is presented in Table I along with typical driving forces used in each application. 

Membrane-based processes generally are highly sensitive to changes in temperature. These sensitivities may be much greater than those exhibited by processes occurring in the aqueous phase of the cell, and because of this membranes may play vital roles in establishing organisms thermal tolerance limits. Why are... 

Is the current separation the most suitable or should other methods of separation be considered For example, distillation is commonly the workhorse in the chemical industry and is often the immediate choice. Other separation methods, such as pervaporation, can be sometimes more effective especially if the volatility differences are small. Pervaporation is a membrane-based process with the difference that the permeate appears as a vapor, thus permitting solute recovery and recycle. For example, benzene can be recovered from hydrocarbon streams using this method in fairly high concentrations and in a usable form ready for recycle. Many alternative separation methods must be considered, and one should not simply bank on past experience or expertise. 

Ultrafiltration of whey is a major membrane-based process in the dairy industry however, the commercial availability of this application has been limited by membrane fouling, which has a concomitant influence on the permeation rate. Ultrasound cleaning of these fouled membranes has revealed that the effect of US energy is more significant in the absence of a surfactant, but is less markedly influenced by temperature and transmembrane pressure. The results suggest that US acts primarily by Increasing turbulence within the cleaning solution [91]. 

Electrodialysis is a membrane-based process which can be used for separation, removal, or concentration of ionic species present in aqueous solutions. These operations are accomplished by the selective transport of ions through an ion exchange membrane under the influence of a direct current. One of the earliest applications of electrodialysis was the desalting of brackish water. However, since the 1970s, extensive studies have been performed on the application of electrodialysis for waste-water treatment, especially in the electroplating and metal-finishing industries. 

Argonne National Laboratory received a 1998 Presidential Green Chemistry Challenge Award for the development of a novel membrane based process for producing lactate esters.The process uses pervaporation membranes and... 

Membrane chromatography proved to be a successful tool especially for separation of macromolecules. The large-size proteins cannot enter the small pores of the particles in the packed-bed columns, while in membrane-based processes they have access to a much higher binding surface due to the macroporous nature of the supports. One problem may nevertheless appear for membranes with large pore distribution. Suen [182] reported that a variation of 12% in porosity can be responsible for a loss of 50% of adsorption capacity at the breakthrough point. For variations in the membrane thickness a three times less-sensitive behavior was found. 

It is indeed impossible to capture aU current and future applications, and uses in any one publication. What follows here is a selective presentation from the large number of possible membrane-based processes of relevance to bioprocess industries. 

MEMBRANE-BASED PROCESSES AND THEIR USES 22.2.1 Microfiltration... 

The treatment and safe disposal of waste is equally important whether it be waste generated by the pulp and paper industry, by the leather industry or gaseous waste. Recently, in addition to other membrane-based processes, demand has grown in the wastewater treatment industry for a process that uses both a biological stage and a membrane module, known as the membrane bioreactor (MBR) process. The bioreactor and membrane module each have a specific function (1) the biological degradation... 

Evaluation of Membrane-Based Processing of Radioactive Nuclear Plant Waste Case Studies... 

When selecting between membrane and PSA purification processes, it is important to consider scale. Practice has shown that PSA scales up economically and for large stationary applications (e.g., petroleum refining, petrochemical production, coal gasification), this may be the best choice. By the same reasoning, PSA usually does not scale down economically, and here membrane-based processes may be strongly favored. Also, membrane processes are more likely to be selected for applications wherein the platform is moving or otherwise subjected to shock and vibration that would likely have detrimental effects on PSA adsorbent beds. 

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)... 

A. Frank Seibert, Ph.D., P.E. Technical Manager, Separations Research Program, The University of Texas at Austin Member, American Institute of Chemical Engineers (Liquid-Liquid ITispersion Fundamentals, Process Fundamentals and Basic Calculation Methods, Hydrodynamics of Column Extractors, Static Extraction Columns, Process Control Considerations, Membrane-Based Processes)... 

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