Nov 18, 2014 in Blog Articles

The following has been lifted from IHS Engineering 360:The chemicals, pharmaceuticals, semiconductor, food and beverage industries consume large amounts of highly purified water. These industries use filtration to purify their process water and in other aspects of production. Increasingly, they also require finer porosities to increase efficiency by using smaller particle sizes, and higher operating temperatures to handle hot liquid suspensions and exhaust gases, according to filtration consultant Ken Sutherland writing in the April 2013 issue of Filtration + Separation magazine.Membrane filtration is becoming the technology of choice for these applications. It is based on semipermeable membranes designed to selectively separate unwanted particles or molecules from an incoming stream. The substances that are retained are called the retentate and those that pass through are known as the permeate.A differential pressure applied across the membrane barrier drives the process. Membrane media are split into four categories based on the sizes of the substances that can pass through them. In decreasing size order they are microfiltration, ultrafiltration, nanofiltration and reverse osmosis.Microfiltration and ultrafiltration operate on the sieve principle, which holds that only some particles are small enough to fit through the membrane’s pores. They differ from traditional filters, however, in the extremely small sizes of their pores, the largest of which is 10 micrometers.Nanofiltration and reverse osmosis do not physically block particles. Instead, they operate on the principle of diffusion through the molecule-sized spaces in the membrane material. An important difference is that microfiltration and ultrafiltration require relatively low pressure (1–30psi) to drive the process, whereas nanofiltration and reverse osmosis require up to 1,200 psi. Reverse osmosis and nanofiltration, therefore, use significantly more energy.Reverse osmosis is unique among the filters in that it is based upon the natural principle of osmosis. This holds that if two fluids containing different concentrations of dissolved solids are separated by a semi-permeable membrane, the fluid containing the lower concentration will move through the membrane and into the fluid containing the higher concentration of dissolved solids.