Membrane Filters

Membrane Filters

Membrane filters are thin microporous films employed to trap or retain particulate impurities from an air source that is drawn through them. With efficiency rates of over 99% particulate removal, these semi-permeable materials rival even ULPA and HEPA air filters. As a result, these compact filtration devices are seen in a number of industries. Electronics, research and science laboratory, sanitation, medical, ventilation and biopharmaceutical industries all make use of membrane filters as a means of air purification. As these mechanisms do not require the use of chemicals or additives as with some filtration systems, they are safe to use and cost effective. Additionally, membrane filters require minimal energy as they can be installed as inline filters within pre-existing flow tubes. As with all filters, membranes must be cleaned regularly to ensure optimal functionality. Backwashing, process flow disruption and diafiltration are all effective means of automatic in place cleaning. Alternatively, filters can be removed, washed and sterilized in an autoclave. Successive membrane filtration significantly reduces maintenance needs and is often used to improve productivity. In this arrangement, a series of membranes are put in place with progressively smaller pore size to first remove large particulates allowing secondary filters to trap micro particles and microorganisms. In some instances the trapped slurry can be recycled and drawn back into a given system to further cut operating costs while improving productivity. Successive membrane filtration allows for very specific slurry separation and retrieval.

Nanofiltration, ultrafiltration and microfiltration techniques all employ membrane filters. The basic function of each is that an air stream is drawn through a thin material which allows the air, but not particulates to pass. The composition of the membrane must be carefully calculated to exclude undesirable particles while remaining semi-permeable. The air stream and particulates must both be taken into account during selection. The continuous matrix of pores can be classified as either polymeric or ceramic. While ceramics are used in air filters, they are more frequently used in liquid membrane filtration. Polymeric and other advanced synthetic materials vary in thickness, a significant factor in choosing the correct membrane for a given application. Nitrocellulose, cellulose acetate, coated PTFE, nylon, polycarbonate and mixed cellulose ester materials are common membrane materials. Considerations beyond porosity and thickness include effectiveness, absorption, flow and binding requirements. The frame or holder should also be suited to the specific application. Vacuum type glass holders, sintered glass or stainless steel supports and polyester drain discs are often used to hold the membrane taught and in place. Membrane filters are becoming more popular as they remove the possibility or filter contamination due to shedding particles or out-gassing which occurs in other fibrous filters.