Membrane separation involves separating components from liquid, fluid or gaseous streams by means of forcing the stream to flow under pressure over the surface of a membrane. The technology is actually a family of processes that include reverse osmosis, nanofiltration, ultrafiltration and microfiltration. Components smaller than the membrane pore size pass through the membrane while larger components are retained.
Components can be selectively separated over a wide range of particle sizes.
Examples are bacteria and pyrogen; macromolecular materials, such as starch and protein; and small soluble molecules such as amino acid, sugar and inorganic acids.
Separation technology enables processors to simultaneously concentrate, fractionate, and purify their products. Large volumes can be treated with remarkable energy efficiency since the technology does not require a phase change to effect water removal, nor do the processes require a steam source or ancillary equipment such as heat generators, evaporators, or condensers.
Membrane processes separate molecules on the basis of size and molecular weight. The following is a description of reverse osmosis, nanofiltration, ultrafiltration and microfiltration.
RO is the most complex technique in membrane separation. RO membranes concentrate low molecular weight organic materials and salts while allowing water and solvents to pass. High pressures of about 35-100 bar are required in order to overcome the high osmotic pressures across the membrane. This permits water to flow from the concentrated feed stream to the dilute permeates, a direction that is just the reverse of what would occur naturally during osmosis. RO has been most widely used for desalinating sea water and reclaiming brackish well water.
NF separates liquid in a region between reverse osmosis (RO) and ultrafiltration (UF). The NF membrane displays excellent rejection of divalent ions while allowing a majority of monovalent ions to pass. Organic molecules in the 200-300 molecular weight ranges are also highly rejected. The unique separation capability of NF provides the opportunity to selectively concentrate either valuable or undesirable substances from a process stream with greater effectiveness, consistancy, reliability and economy. NF applications include demineralisation, color removal and desalting.
UF is a low pressure fractionation of selected components by size. Ultrafiltration separates dissolved solutes of .005-.1 microns. This corresponds to a molecular weight cut off (MWCO) of about 1,000 to 300,000. Depending on the molecular weight cut off selected, the membrane will concentrate high molecular weight species while allowing dissolved salts and lower molecular weight materials to pass. UF membranes are used in numerous industries for concentration, clarification and diafiltration of large process streams.
MF is the most open membrane. MF separates macro materials and suspended solids in the size range of about 0.05-2.0 microns. Typical materials removed by MF are starch, bacteria, fat, molds, yeast and emulsified oils. MF offers unlimited new process opportunities in the industrial market.