Membrane technologyMembrane technologycovers allengineeringapproaches for the transport of substances between two fractions with the help ofpermeablemembranes. In general, mechanical separation processes for separating gaseous or liquid streams use membrane technology.

Applications: Membrane separation processes operate without heating and therefore use less energy than conventional thermal separation processes such asdistillation,sublimationorcrystallization. The separation process is purely physical and both fractions (permeateandretentate) can be used. Cold separation using membrane technology is widely used in thefood technology,biotechnologyandpharmaceuticalindustries. Furthermore, using membranes enables separations to take place that would be impossible using thermal separation methods. For example, it is impossible to separate the constituents ofazeotropicsliquids or solutes which formisomorphicscrystals by distillation orrecrystallizationbut such separations can be achieved using membrane technology. Depending on the type of membrane, the selective separation of certain individual substances or substance mixtures is possible. Important technical applications include the production of drinking water byreverse osmosis(worldwide approximately 7 million cubic metres annually), filtrations in thefood industry, the recovery of organic vapours such as petro-chemical vapour recovery and theelectrolysisfor chlorine production. Inwaste watertreatment, membrane technology is becoming increasingly important. With the help of UF and MF (Ultra/Microfiltration) it is possible to remove particles, colloids and macromolecules, so that waste-water can be disinfected in this way. This is needed if waste-water is discharged into sensitive waters especially those designated for contact water-sports and recreation. About half of the market is in medical applications such as use in artificial kidneys to remove toxic substances byhemodialysisand asartificial lungfor bubble-free supply of oxygen in theblood. The importance of membrane technology is growing in the field of environmental protection (NanoMemPro IPPC Database). Even in modern energy recovery techniques membranes are increasingly used, for example infuel cellsand inosmotic power plants.

Current market and forecast : The global demand for membrane modules was estimated at approximately 15.6 billion USD in 2012. Driven by new developments and innovations in material science and process technologies, global increasing demands, new applications, and others, the market is expected to grow around 8% annually in the next years. It is forecast to increase to 21.22 billion USD in 2016 and reach 25 billion in 2018.

Membrane operationsAccording to driving force of the operation it is possible to distinguish: pressure driven operations microfiltration ultrafiltration nanofiltration reverse osmosis concentration driven operations dialysis pervaporation forward osmosis artificial lung gas separation operations in electric potential gradient electrodialysis membrane electrolysis e.g.chloralkali process electrodeionization electrofiltration fuel cell operations in temperature gradient membrane distillationMembrane separation processes: Membrane separation processes have very important role in separation industry. Nevertheless, they were not considered technically important until mid-1970. Membrane separation processes differ based on separation mechanisms and size of the separated particles. The widely used membrane processes includemicrofiltration,ultrafiltration,nanofiltration, reverse osmosis,electrolysis,dialysis,electrodialysis,gas separation, vapor permeation,pervaporation, membranedistillation, and membrane contactors. All processes except for pervaporation involve no phase change. All processes except (electro)dialysis are pressure driven. Microfltration and ultrafiltration is widely used in food and beverage processing (beer microfiltration, apple juice ultrafiltration), biotechnological applications andpharmaceutical industry(antibioticproduction, protein purification), water purification andwastewater treatment, microelectronics industry, and others. Nanofiltration and reverse osmosis membranes are mainly used for water purification purposes. Dense membranes are utilized for gas separations (removal of CO2from natural gas, separating N2from air, organic vapor removal from air or nitrogen stream) and sometimes in membrane distillation. The later process helps in separating of azeotropic compositions reducing the costs of distillation processes.

Pore size and selectivity The pore sizes of technical membranes are specified differently depending on the manufacturer. One common distinction is bynominal pore size. It describes the maximum pore size distributionand gives only vague information about the retention capacity of a membrane. The exclusion limit or "cut-off" of the membrane is usually specified in the form ofNMWC(nominal molecular weight cut-off, orMWCO,Molecular Weight Cut Off, with units inDalton). It is defined as the minimum molecular weightof a globular molecule that is retained to 90% by the membrane. The cut-off, depending on the method, can by converted to so-calledD90, which is then expressed in a metric unit. In practice the MWCO of the membrane should be at least 20% lower than the molecular weight of the molecule that is to be separated.Filter membranes are divided into four classes according to pore size:Pore sizeMolecular massProcessFiltrationRemoval of

>10"Classic"filter

>0.1m> 5000kDamicrofiltration