Industrial membrane processing

Industrial membrane processes may be classified according to the size range of materials that they are to separate and the driving force used in separation. There is always a degree of arbitrariness about such classifications, and the distinctions that are typically drawn. Table 16.1 presents classification of membrane separation processes for liquid systems. 

Turner, M. K. (1991). Effective Industrial Membrane Processes Benefits and Opportunities, Elsevier Applied Science, London. 

The work described in this chapter is especially concerned with three of the most widely used pressure driven membrane processes microfiltration, ultrafiltration and nanofiltration. These are usually classified in terms of the size of materials which they separate, with ranges typically given as 10.0-0.1 xm for microfiltration, 0.1 p.m-5 nm for ultrafiltration, and 1 nm for nanofiltration. The membranes used have pore sizes in these ranges. Such pores are best visualised by means of atomic force microscopy (AFM) [3]. Figure 14.1 shows an example of a single pore in each of these three types of membrane. An industrial membrane process may use several hundred square meters of membrane area containing billions of such pores. 

Brown, R.G. et al., A Comparative Evaluation of Cross-flow Microfiltration Membranes for Radwaste Dewatering, in Effective Industrial Membrane Processes Benefits and Opportunities, M.K. Turner, ed., published hy Elsevier AppUed Science, London and New York, 1991. 

The candidate technologies for purification are many. Distillation, the work-horse of the chemical processes, leads the pack. Most of the synthesis effort to date has concentrated on the product purification step. This step is often the last step for liquid products especially in the chemical and petrochemical industries. The biochemical industry utilizes membrane and chromatographic processes more than the other industries due to the thermal stability and purity requirements. In the electronic industry, membrane processes are more prevalent due the ultra-purities necessary. Supercritical fractionation of alcohol water systems with the aid of a dense gas is an example of a purification step. 

The product flux obtained is determined by the applied pressure and the membrane resistance (or permeability). Typical values for applied pressures and fluxes are given in table 1.9. Present da industrial membrane processes involve microfiltration, ultrafiltration, nanofiltraiion and reverse osmosis. Other commercial membrane processes are elcctrodialysis. membrane electrolysis, diffusion dialysis, per aporation, vapour... 

Sajima Y., Sato K., Ukihashi H. Industrial Membrane Process Symposium. 1985 Spring National AIChE Meeting, Houston, Texas pp 108-113. 

For each application, membranes for a desired performance are expected to be designed and manufactured. We still remain in the infant stage in this respect. Even though chemical and physical properties of the membrane, especially those of the membrane surface, are known to govern the membrane performance, they depend on the many parameters involved in membrane fabrication. Because of the complexity arising among those parameters, to make a desirable membrane for a particular aim is still considered to be an art, even several decades after the emergence of industrial membrane processes. 

Gurr, W.R., An operators view on gas membranes, in Effective Industrial Membrane Processes — Benefits and Opportunities, Turner, M.K., Ed., Elsevier Seience Publishers, Barking, Essex, 1991, 329. 

A special t5q>e of driving force arises in Item 4 of Table 1.2. The process here is the selective transport of water through a semipermeable mernbrane from a dilute solution (high water concentration) to a more concentrated solution (low water concentration), introduced in Illustration 1.3. The driving force is in this case the difference of the so-called osmotic pressure n, which makes its appearance in transport through cell membranes as well as in industrial membrane processes. We will take a closer look at osmotic-pressure-driven processes in Chapter 8. 

A dimensionless group not listed in Table 5.1 is the so-called Wall Sherwood Number that represents the resistance to mass transfer through a fluid in laminar flow divided by the resistance within the tubular wall. It is used to gauge the relative importance of these resistances in industrial membrane processes as well as those occurring in living organisms (see Chapter 8). 

Hamaker, R. J., 1991, Evolution of a Gas Separation Membrane 1983-1990, Presented at the International Conference on Effective Industrial Membrane Processes—Benefits and Opportunities, Edinburgh, Scotland, March 19-21. 

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