Membrane sepration processes

Membrane separation is a process in which a membrane is used to separate the components in a solution, rejecting unwanted substances while allowing the others to pass through. Membranes are semi-permeable barriers that separate two phases — permeate and retentate — on the basis of particle size and electric charge. The driving force is the difference in concentration or pressure between the two sides of the membrane, which is why most food and beverage duties are configured as pressure-driven systems where the membrane separates the solvent (generally water) from the other components of a solution.

Four methods cover most food plant applications. Reverse osmosis (RO) concentrates a solution by removing water, working in the particle size range of 5×10−3 μm to 10−4 μm, and is applied to water purification, recovery of sugar from candy, recovery of oil-seed protein and concentration of dilute solutions such as fruit and vegetable juices and milk. Nanofiltration (NF) concentrates organic components by removing part of the monovalent ions such as sodium and chlorine — partial demineralization — over 5×10−2 μm to 5×10−3 μm, and is used for the removal of two-valued ions and larger mono-valued ions such as heavy metals. Ultrafiltration (UF) concentrates large and macromolecules over 0.15 μm to 5×10−2 μm. Microfiltration (MF) removes bacteria and separates macromolecules, with a particle size range of 0.15 μm to 0.15 μm.

The application map is specific. UF retains suspended solids and solutes of high molecular weight while water and low molecular weight solutes pass through, and is applied in the dairy industry mainly for two purposes: production of protein concentrates from whey, and cheese from milk. NF is applied to pre-concentration of thin juices in the production of sugar from sugarcane and beet, desalting of lactose, production of protein concentrates, and the demineralization and de-acidification of whey obtained from ultrafiltration. MF, a pressure-driven process widely used in concentrating, purifying or separating macromolecules, colloids and suspended particles from solution, serves clarification of beer and wine — removal of yeasts and microorganisms — pretreatment of cheese whey, removal of oil droplets and fat globules, and wastewater treatment.

For a plant owner, the attraction is that membranes can recover minor but valuable components from a main stream without substantial energy costs, on a very simple flowsheet, with extremely high selectivities and potentially better environmental outcomes. The constraints must be designed around: membranes cannot be easily staged compared with processes such as distillation, can have chemical incompatibilities with process solutions, can be saddled with major problems of fouling, and carry a temperature limitation.