Microfiltration cartridges

To extend its life, a microfiltration cartridge may contain two or more membrane filters in series, or as shown in Figure 7.13, a coarse prefilter cartridge... 

Microfiltration cartridges produced for this market are often sterilized directly after manufacture and again just prior to use. Live steam, autoclaving at 120 °C, or ethylene oxide sterilization may be used, depending on the applications. A flow schematic of an ampoule-filling station (after material by Schleicher and Schuell) is shown in Figure 7.18. 

Heavy wall tubing plain, coloured, striped tubing fabrications for instrumentation automotive push-pull cables industrial and process hydraulics and other fluids. .. Piping liners for glass-lined reactors, stainless steel reactors, glass equipment and mixers... Membranes, filter media, filter bags, cartridges, microfiltration membranes, vents and adsorbent products. .. 

Knitted, woven, braided or sewn fibres compression packing, sewing thread, membranes, filter media, filter bags, cartridges, microfiltration membranes, vents and adsorbent products... 

SPEC was essentially able to market their Zr02-based ultrafiltration membranes to an already existing market in the sense that these membranes replaced polymeric UF membranes in a number of applications. They also developed a certain number of new applications. For Ceraver, the situation was different. When the Membralox membranes were first developed, microfiltration was performed exclusively with dead-end polymeric cartridge filters. In parallel to the development of inorganic MF membranes, Ceraver initiated the development of cross-flow MF with backflushing as a new industrial process. 

Regarding water filtration procedures first, prefilters should be provided to prevent large particulates from entering the system and microfiltration to remove bacteria. Prefilters are usually the replaceable cartridge type with porosities ranging as high as 25 pm. Microfiltration follows and is generally accomplished with 0.2-pm absolute filters, which will remove most bacteria. 

The first major application of microfiltration membranes was for biological testing of water. This remains an important laboratory application in microbiology and biotechnology. For these applications the early cellulose acetate/cellulose nitrate phase separation membranes made by vapor-phase precipitation with water are still widely used. In the early 1960s and 1970s, a number of other membrane materials with improved mechanical properties and chemical stability were developed. These include polyacrylonitrile-poly(vinyl chloride) copolymers, poly(vinylidene fluoride), polysulfone, cellulose triacetate, and various nylons. Most cartridge filters use these membranes. More recently poly(tetrafluo-roethylene) membranes have come into use. 

However, the short lifetime of in-line cartridge filters makes them unsuitable for microfiltration of highly contaminated feed streams. Cross-flow filtration, which overlaps significantly with ultrafiltration technology, described in Chapter 6, is used in such applications. In cross-flow filtration, long filter life is achieved by sweeping the majority of the retained particles from the membrane surface before they enter the membrane. Screen filters are preferred for this application, and an ultrafiltration membrane can be used. The design of such membranes and modules is covered under ultrafiltration (Chapter 6) and will not be repeated here. 

Despite the limited volumes that can be treated before a filter must be replaced, microfiltration is economical because the cost of disposable cartridges is low. Currently, a lO-in.-long pleated cartridge costs between US 10 and US 20 and contains 0.3-0.5 m2 of active membrane area. The low cost reflects the large numbers that are produced. 

The primary market for the disposable cartridge is sterile filtration for the pharmaceutical industry and final point-of-use polishing of ultrapure water for the microelectronics industry. Both industries require very high-quality, particle-free water. The cost of microfiltration compared to the value of the products is small so these markets have driven the microfiltration industry for the past 15 years. 

Various techniques can be used to reduce the loading of suspended solids, organics and microbes in feed water. These include physical processes such as media filtration, cartridge microfiltration and chemical treatments. Chemical addition enhances the filter-ability of the solids such as the addition of coagulants (Table 2.2). Foulants and their control strategies are addressed in Table 2.8. Since any traces of soHds and organics get removed in the first membrane modules in RO and NF systems, these materials typically foul the first stages of an RO/NF system (Table 2.9). Once deposited on the membranes. 

Figure 3.32 A P ID of a dead-end cartridge microfiltration pre-treatment system for a seawater RO plant. 

The use of membranes in cartridge microfihration has be discussed already in Chapter 6, Section 6.6. That section also contained a number of test procedures enq)loyed for membrane characterisation which will not be repeated here. This chapter provides details of membrane configuration other than cartridges, mathematical models to assist in the understanding and control of the processes. Industrial applications or investigations of microfiltration and to a lesser extent ultrafiltration are also discussed. 

This chapter is concerned primarily with process scale membrane fihratinn and phenomena or effects that are relevant to such filtration. Cartridge filtration has already been discussed in Chapter 6, hence most of the following work refers to membrane filtration under crossflow conditions. Hiis technique is applicable to both microfiltration... 

Microfiltration membranes come in both in-line or cartridge filter arrangements and cross-flow arrangements. For cross-flow systems, the rejection rate is usually 5 to 10 percent, that is, 90 to 95 percent recovery. Higher recovery rates are feasible however, the overall flux through the membrane is reduced. 

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