Pressure membrane processes
The pressure membrane processes utilize pressure as a driving force to transport substances through the membrane. From conventional filtration they differ by the size of particles being separated, applied pressures and material of the separation barrier. While cake filtration is performed on fabric or metal grids at low pressure, the pressure membrane processes utilize polymeric, ceramic or metal membranes for substance separation, usually at higher pressures.
The pressure membrane processes are widely applied in water industry, waste water treatment, food-processing industry, pharmacy, chemical industry, agriculture and production of electronics.
The company MemBrain s.r.o. offers the following services in the field of development activities:
- assessment of the possibility of separation of components from the medium by the pressure membrane processes
- laboratory tests to determine the basic parameters of the membrane process
- pilot tests at the place of application
- sales and lease of laboratory and pilot units
- model calculations to verify pilot tests
- proposal of the membrane separation technology
- technical and economic study of the process
Microfiltration
Closest to conventional filtration, microfiltration is performed at pressure up to 2 bar and it retains particles with the size of 5-0,03 µm, which corresponds to bacteria, colour pigments, dust micro-particles, oil emulsions etc. Application of microfiltration is determined by the size of separated particles. Most typically, it is applied in drinking water preparation, pre-processing of water for reverse osmosis or cold sterilization of drinks. Microfiltration membrane modules are usually flat or with hollow fibres produced of polymers, tubular or capillary, if they are produced of ceramic materials or sintered metal.
Ultrafiltration
The ultrafiltration membrane retains macromolecules with the molecular weight of 2-200kDa (the size of 100-3 nm), which corresponds e.g. to viruses, proteins, polysaccharides or colloid particles. Pressures are slightly higher than in microfiltration but remain in the order of bars. Most typically, ultrafiltration is applied in food industry for milk concentration, cheese production, obtaining proteins from whey or starch from potatoes. Ultrafiltration modules are similar to microfiltration ones; spirally wound membrane modules are also used.
Nanofiltration
Nanofiltration membranes are at the bounds of perceptible pores. The nanofiltration selectivity is determined by differences in speeds of substances dissolution in the polymer and differences in speeds of substances diffusion through the polymer. Nanofiltration is used for separation of low-molecular weight organic substances; the size of separated substances is state in molecular weight units rather than in size units. Nano-filtration separates substances approximately to 20 kDa, which corresponds e.g. to molecules of sugars, paints, pesticides and herbicides. Pressures are much higher than in ultrafiltration; they range in tens of bars and the osmotic pressure starts to develop. Most typically, nanofiltration is applied in water softening, removal of pesticides in drinking water production or desalination of paints after azo coupling. Spirally wound modules are applied almost exclusively, exceptionally those of hollow fibres but always made of polymeric materials.
Reverse osmosis
Reverse osmosis is the most frequently used membrane process. Sea water and brackish water desalination and preparation of demineralised water are the main applications of reverse osmosis. The ideal membrane for reverse osmosis only transmits water, while hydrated ions are retained by the membrane. Pressure driving force in the scale of tens of bars is needed to exceed osmotic pressure. Applications operated at 70 bar are no exception. Composite membranes are produced of several layers of different polymers. Spirally wound moduled are applied almost exclusively, exceptionally those of hollow fibres.
Gas separation
Gas separation is also performed by the pressure gradient on the membrane. There are several mechanisms of separation, always requiring high pressure in the order of MPa. Due to this high pressure, the same modules are used as in reverse osmosis, i.e. spirally wound or of hollow fibres. Typical applications are separations H2/NH3, CO2/H2S, N2/air, He/natural gas, dehydration of gases, recycling of organic vapours.
