Microfiltration system avoids clogged filters A new microfiltration system, DynaSep, offers the promise of improved efficiency and economy in a number of industrial filtration operations. The system is made and marketed by Ghia Corp., a subsidiary of Membrana Inc., a new joint venture established by Akzona Inc. and West Germany's Enka A.G. to commercialize both firms' proprietary membraneproducing technologies. With conventional microfiltration systems, Ghia notes, solids accumulate on the surface of the filter itself. As the solids layer thickens, the filter tends to become plugged and the flow rate decreases. Increasing the pressure to overcome this effect increases energy requirements and also brings the risk of rupturing the expensive filter. The Dyna-Sep system, in contrast, uses a technique called cross-flow filtration. The fluid suspension is pumped at high velocity but low pressure through the center of a microporous tube. A collector region outside the tube is at lower pressure, however, and the pressure differential forces the liquid through the pores of
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the tube. The suspended solids become concentrated in the feed stream. Because of the need to maintain a high stream velocity inside the porous tube, a recirculation loop is almost always required. For example, a 10 gal-per-minute feed stream containing 1% particles less than 1 microm-
eter in diameter obviously would need a filter large enough to pass 10 gal per minute of filtrate. But the feed stream would have to be recirculated through the filter at a flow rate of 30 to 100 gal per minute (depending on the nature of the particles). So a storage tank also is called for. As the fluid recirculates, solids accumulate
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C&ENOct. 20, 1980
in the tank and loop until they reach the desired concentration. At that point, the system also reaches equi librium in terms of the amount of solids being presented to the inside of the porous tube. The "mud" collects not on the surface of the filter but in the tank. The pasty solids are re moved periodically and, if necessary, further dewatered. Because the stream is moving fast, there is relatively little buildup of solids on the inside of the porous tube. Nevertheless, the flux can de crease with time, especially when fish-scale type or gelatinous particles are being filtered. So the system in cludes a built-in mechanism that applies a 0.5-second reverse-flow pulse at intervals of three minutes to three hours, depending on the con centration of the feed stream. The filter tubes are of microporous polypropylene, made by the Accurel process developed at Armak Co., an Akzona subsidiary (C&EN, Dec. 11, 1978, page 23). They represent one of the first commercial applications of that process, which is based upon the phase separation that occurs when hot solutions of polymers in selected solvents are cooled. The degree of porosity is controlled by varying the conditions of manufacture. The po rous polypropylene tubes are resis t a n t to chemical attack and are durable, the makers say. The Dyna-Sep system likely will be used mostly for separating solids from liquids. Ghia lists several applica tions, including separation of paint pigments and precipitated metals, dewatering of uranium yellow cake, and removal of yeast cells from wine. However, the system also can be used for liquid-liquid separations, partic ularly for oil-water emulsions. Cross-flow filtration is not intended for molecular separations, the makers note, but they point out that it can serve as a very useful prefilter to prevent plugging in reverse osmosis systems. Separation of solids from gases also is possible—for example, in a stack gas sampling filter. In that application, Ghia says, the dynamic separation approach results in low dead volume and fast response time. The properties of the feed stream, the flow parameters, and the required results will vary substantially from application to application, the com- j pany points out. Consequently, test ing usually is required to confirm the ! suitability and cost-effectiveness of I cross-flow filtration for a given ap- | plication and to design the system properly. To that end, Ghia has built | small portable Dyna-Sep systems I that can be taken to plants for onsite testing of process streams. α I
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