Microfiltration application examples

Polymeric membranes are prepared from a variety of materials using several different production techniques. Table 5 summarizes a partial list of the various polymer materials used in the manufacture of cross-flow filters for both MF and UF applications. For microfiltration applications, typically symmetric membranes are used. Examples include polyethylene, polyvinylidene fluoride (PVDF) and polytetrafluoroethylene (PTFE) membrane. These can be produced by stretching, molding and sintering finegrained and partially crystalline polymers. Polyester and polycarbonate membranes are made using irradiation and etching processes and polymers such as polypropylene, polyamide, cellulose acetate and polysulfone membranes are produced by the phase inversion process.f Jf f ... 

Brief Examples Microfiltration is the oldest and largest membrane field. It was important economically when other disciphnes were struggling for acceptance, yet because of its incredible diversity and lack of large apphcations, it is the most difficult to categorize. Nonetheless, it has had greater membrane sales than all other membrane applications combined throughout most of its histoiy. The early... 

When planning an industrial-scale bioprocess, the main requirement is to scale up each of the process steps. As the principles of the unit operations used in these downstream processes have been outlined in previous chapters, at this point we discuss only examples of practical applications and scaling-up methods of two unit operations that are frequently used in downstream processes (i) cell separation by filtration and microfiltration and (ii) chromatography for fine purification of the target products. 

Laboratory Microfiltration membranes have countless laboratory uses, such as recovering biomass, measuring particulates in water, clarifying and sterilizing protein solutions, and so on. There are countless examples for both general chemistry and biology, especialty for analytical procedures. Most of these applications are run in dead.-end flow, with the membrane replacing a more conventional medium such as filter paper. 

A good example of the filtration of hard, abrasive materials is the application of ceramic membranes in the cleaning of waste water of the ceramic industry [33]. Waste water in this industry typically contains clay, sand, glazes, etc. The use of microfiltration allows for the return of solids to the production... 

In the clarification of beer by cross-flow microfiltration the paper by Trag rdh and Wahlgren [58] seems to be one of the sporadic examples of this application. Here the use of 0.5 xm membranes (Membralox) is necessary to maintain the taste of the beer 0.2 pm shows an unacceptable retention of proteins and colour. Bacteria were retained by the 0.5 pm membrane. 

Membrane technology used in water reclamation includes five major membrane types reverse osmosis, nanofiltration, ultrafiltration, microfiltration, and liquid membranes. These five types of membranes are discussed briefly, and examples of their applications in municipal and industrial wastewater reclamation is also described. 

Table 12 shows the typical LRV values obtained using a polymeric and ceramic microfilter. Sterile filtration requires 100% bacteria retention by the membrane, whereas in many industrial bacteria removal applications the presence of a small quantity of bacteria in the filtrate may be acceptable. For example, drinking water obtained by microfiltration may contain nominal counts of bacteria in the filtrate which is then treated with a disinfectant such as chlorine or ozone. The use of ceramic filters may allow the user to combine the sterile filtration with steam sterilization in a single operation. This process can be repeated many times without changing filters due to their long service life (5 years or longer). 

The study of gas transport in membranes has been actively pursued for over 100 years. This extensive research resulted in the development of good theories on single gas transport in polymers and other membranes. The practical use of membranes to separate gas mixtures is, however, much more recent. One well-known application has been the separation of uranium isotopes for nuclear weapon production. With few exceptions, no new, large scale applications were introduced until the late 1970 s when polymer membranes were developed of sufficient permeability and selectivity to enable their economical industrial use. Since this development is so recent, gas separations by membranes are still less well-known and their use less widespread than other membrane applications such as reverse osmosis, ultrafiltration and microfiltration. In excellent reviews on gas transport in polymers as recent as 1983, no mention was made of the important developments of the last few years. For this reason, this chapter will concentrate on the more recent aspects of gas separation by membranes. Naturally, many of the examples cited will be from our own experience, but the general underlying principles are applicable to many membrane based gas separating systems. 

Vapor permeation and pervaporation are membrane separation processes that employ dense, non-porous membranes for the selective separation of dilute solutes from a vapor or liquid bulk, respectively, into a solute-enriched vapor phase. The separation concept of vapor permeation and pervaporation is based on the molecular interaction between the feed components and the dense membrane, unlike some pressure-driven membrane processes such as microfiltration, whose general separation mechanism is primarily based on size-exclusion. Hence, the membrane serves as a selective transport barrier during the permeation of solutes from the feed (upstream) phase to the downstream phase and, in this way, possesses an additional selectivity (permselectivity) compared to evaporative techniques, such as distillation (see Chapter 3.1). This is an advantage when, for example, a feed stream consists of an azeotrope that, by definition, caimot be further separated by distillation. Introducing a permselective membrane barrier through which separation is controlled by solute-membrane interactions rather than those dominating the vapor-liquid equilibrium, such an evaporative separation problem can be overcome without the need for external aids such as entrainers. The most common example for such an application is the dehydration of ethanol. 

The use of reverse ATRP enlarges the scope of substrate polymers for controlled polymerizations even further because the required radical initiators can in many cases be provided more easily than ATRP initiators on surfaces of inert polymers. For example, poly(vinylidene fluoride) microfiltration membranes were irradiated with UV light and then exposed to air to create hydro(peroxide) species [11]. These were then used to initiate the reverse ATRP of methyl methacrylate in the presence of copper(I) chloride, 2,2 -bipyridine, and benzoylperoxide (Figure 3.5). Similar reaction schemes are applicable to (hydro)perox-ide patterns created directly on polymer surfaces using the lithographic methods discussed in Chapter 2. 

A range of membrane processes are used to separate fine particles and colloids, macromolecules such as proteins, low-molecular-weight organics, and dissolved salts. These processes include the pressure-driven liquid-phase processes, microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), and reverse osmosis (RO), and the thermal processes, pervaporation (PV) and membrane distillation (MD), all of which operate with solvent (usually water) transmission. Processes that are solute transport are electrodialysis (ED) and dialysis (D), as well as applications of PV where the trace species is transmitted. In all of these applications, the conditions in the liquid boundary layer have a strong influence on membrane performance. For example, for the pressure-driven processes, the separation of solutes takes place at the membrane surface where the solvent passes through the membrane and the retained solutes cause the local concentration to increase. Membrane performance is usually compromised by concentration polarization and fouling. This section discusses the process limitations caused by the concentration polarization and the strategies available to limit their impact. 

Let us take polysulfone as an example. This is a polymer which is frequently used as a membrane material, both for microfiltration/ultrafiltration as well as a sublayer in composite membranes. These applications require an open porous structure, but in addition also asymmetric membranes with a dense nonporous top layer can also be obtained which are useful for pervaporation or gas separation applications. Some examples are given in table ni.S which clearly demonstrate the influence of various parameters on the membrane structure when the same system, DMAc/polysulfone(PSf), is employed in each case. How is it possible to obtain such different structures with one and the same system To understand this it is necessary to consider how each of the variables affects the phase inversion process. The ultimate structure arises through two mechanisms i) diffusion... 

---