Ultrafiltration systems

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Ultrafiltration is a separation process using membranes with pore sizes in the range of 0.1 to 0.001 micron. Flux of a membrane is defined as the amount of permeate produced per unit area of membrane surface per unit time. Generally flux is expressed as gallons per square foot per day (GFD) or as cubic meters per square meters per day.

   1.Description

   2.Why

   3.How

   4.Future Trends

   5.Related Links

Useful Links Ultrafiltration, permeate 

Description 

Ultrafiltration is an excellent membrane technology for removing very fine particles, microorganisms and organics as small as 1000 molecular weight. 

  • Ultrafiltration, like reverse osmosis, is a cross-flow separation process. 
  • Liquid stream to be treated (feed) flows tangentially along the membrane surface, thereby producing two streams. 
  • The stream of liquid that comes through the membrane is called permeate. (The type and amount of species left in the permeate will depend on the characteristics of the membrane, the operating conditions, and the quality of feed. )
  • The other liquid stream is called concentrate and gets progressively concentrated in those species removed by the membrane. 
  • In cross-flow separation, therefore, the membrane itself does not act as a collector of ions, molecules, or colloids but merely as a barrier to these species.

Why

The Ultrafilter used in the study had a molecular-weight cutoff of 100,000- (pore size 0.006 micron).  As the requirements for the quality of high purity water become more stringent, an increasing use of ultrafiltration as a final filter is seen.Conventional filters such as media filters or cartridge filters, on the other hand, only remove suspended solids by trapping these in the pores of the filter-media. These filters therefore act as depositories of suspended solids and have to be cleaned or replaced frequently. Conventional filters are used upstream from the membrane system to remove relatively large suspended solids and to let the membrane do the job of removing fine particles and dissolved solids. In ultrafiltration, for many applications, no pre-filters are used and ultrafiltration modules concentrate all of the suspended and emulsified materials. 

How 

The work is based on the supposition that the application of the pressure difference across the colloidal suspension in the ultrafiltration process creates a volume fraction profile within the system.

  • Typically, ultrafiltration will remove high molecular-weight substances, colloidal materials, and organic and inorganic polymeric molecules. 
  • Low molecular-weight organics and ions such as sodium, calcium, magnesium chloride, and sulfate are not removed. 
  • Because only high-molecular weight species are removed, the osmotic pressure differential across the membrane surface is negligible.
  • Low applied pressures are therefore sufficient to achieve high flux rates from an ultrafiltration membrane. 
  • When removed from the applied pressure, the system returns to a uniform state, but with an equivalent bulk electrolyte concentration that depends on the total particle concentration and the composition of the filtrate.

Ultrafiltration Membrane modules come in plate-and-frame, spiral-wound, and tubular configurations. The configuration selected depends on the type and concentration of colloidal material or emulsion.  

  • For high purity water, spiral-wound and capillary configurations are generally used. 
  • For more concentrated solutions, more open configurations like plate-and-frame and tubular are used. 

In all configurations the optimum system design must take into consideration the flow velocity, pressure drop, power consumption, membrane fouling and module cost.Attempting to use ultrafiltration as a method of preparing colloidal suspensions of known composition for use in verifying electrokinetic theories requires care in the determination of the bulk suspending electrolyte concentration even at low particle volume fraction.Factors that can affect the performance of an ultrafiltration system

Flow Across the Membrane Surface

  • The permeate rate increases with the flow velocity of the liquid across the membrane surface. 
  • Flow velocity if especially critical for liquids containing emulsions or suspensions. 
  • Higher flow also means higher energy consumption and larger pumps. 

Operating Pressure:Permeate rate is directly proportional to the applied pressure across the membrane surface

Operating Temperature:  Permeate rates increase with increasing temperature

When an ultrafiltration membrane is used instead of a 0.2-micron cartridge filter, particle removal efficiency is greatly improved.  In addition, ultrafiltration membranes are not susceptible to the problem of bacteria growing through them, as is the case with 0.2-micron filters.

Future Trends 

An ultrafiltration membrane with a molecular-weight cutoff of 10,000 has a nominal pore size of 0.003 micron. Ultrafiltration membranes can have extremely high fluxes but in most practical applications the flux varies between 50 and 200 GFD at an operating pressure of about 50 psig in contrast, reverse osmosis membranes only produce between 10 to 30 GFD at 200 to 400 psig.In high purity water systems, ultrafiltration is slowly replacing the traditional 0.2-micron cartridge filters.   In Japan, practically all of the semiconductor industry follows this practice. Ultrafiltration is used for a pretreatment for reverse osmosis, as a oil/water separator, to remove organic contaminants and to provide high quality filtration for industrial applications. 

Keywords

Ultrafiltration systems, Flow velocity, membrane technology, permeate, separation process, Nanofiltration, reverse osmosis

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