MEMBRANE FILTRATION
 
Crossflow-Systems
The filtration input is directed tangentially to the membrane surface using a high flux velocity (v = 1 - 6 m/s) and a high pressure. The solvent part of the filtrate can pass unhindered through the membrane while residual components are retained. The build-up of a clogged layer on the membrane is prevented by turbulences that arise in the crossflow method. The part of the filtrate that was retained, the retentate, is then re-circulated several times over the membrane in order to achieve further concentration. The re-circulation process requires a relatively high energy input from about 3 to 7 KWh/m3. Typical flux performances are between 100 and 300 l/(m2h).
 
In order to maintain optimum flux performance, chemical cleaning processes are carried out regularly. The following figure compares schematically dead end and crossflow filtrations.
 

 
 
Crossflow membranes and modules
Cost effective polymer membranes are commonly used in crossflow filtration. High performance hydrophobic polymers such as polyethersulfone and polyvinylidenfluoride can be used as a membrane material. Ceramic membranes are employed if there are high demands regarding the chemical and thermal stability of the membrane.
 
The selectivity of porous membranes is based exclusively on the pore diameter. Substances having a pore size larger than the pore size of the membrane are retained and molecules that are smaller than the pore size of the membrane can pass unhindered through the membrane.
 
Crossflow membranes show an asymmetrical membrane structure that guarantees an effective cleaning of the membrane surface.
 
In order to increase the mechanical stability of porous membranes supportive materials are added onto the membranes.
 
Crossflow systems mainly employ tubular membranes because they exhibits some significant advantages over flat or capillary membranes. These are:

• Very small danger of blockage and good cleaning abilities
• No loss of pressure along the module
• Turbulences and laminar flux conditions can be adjusted in a defined way
• Insensitive to non-dissolved particles in the solution
• Treatment of high viscosity solutions possible
• High concentration rates can be achieved

To increase the packing density and thus the membrane surface within tubular crossflow modules a large number of mechanically stable and self-supporting tubular membranes with a small inner diameter are combined.
 
Potential applications
Crossflow systems are commonly used in a wide range of ultra- and microfiltration, processes that are of great economical interest. Typically, solutions with a high concentration of solid contaminants are treated.
 
For a large number of applications the crossflow process is more than merely an alternative to more conventional membrane technologies but the best method to reach excellent results while saving energy, time and costs.
 
The porous polymeric tubular membranes are pressure stable and very suitable for the filtration of liquids that arise in wastewater- beverage-, food-, and biotechnology industries, particularly for those that contain particles. This is demonstrated in the following examples of potential applications for crossflow processes.
 
Wastewater treatment

• Membrane bioreactors for the separation of biomass
  The combination of bioreactors with membrane technologies enables an innovative and effective cleaning technique. The biomass is not only separated with the aid of membrane technology but the quality of the so-cleaned water is enhanced considerably.
• Treatment of wastewater
  containing oil residues from metal processing industries
• Recovery of water-soluble lacquer
  from colour and lacquer processing industries
• Agricultural processes,
  such as the concentration of liquid manure
• Pre-filtration
  as relief of reverse osmosis processes

Food and beverage industries

• Clarification and concentration of fruit juices and vinegars
• Filtration of wine in order to completely remove yeast cells and other micro-organisms
• Production of cream cheese
• Production of beer from excess broth

Biotechnology, pharmaceutical and chemical industries

Concentration, filtration and recycling of resources
• Concentration and purification of cells and enzymes
• Continuous fermentation
• Media exchange
• Perfusion bioreactors
• Filtration und Klärung aller Essigarten