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Flow refers to the time it takes for a particular flow stream to pass through the filter. The flow rate of a filter is important in determining how rapidly filtration can be completed. Although flow rate generally decreases with pore size among membranes of a single type, membranes with the same pore rating, but made from different materials or by different methods, can have very different flow rates. Flow rate differences can be caused by differences in thickness, porosity, and pore architecture.
After a microporous membrane is produced, flow rate or flow time is measured using an ideal liquid or gas. Hydrophilic filters are usually tested with water. Hydrophobic membranes are usually tested with an alcohol. Membranes to be used for air filtration can be tested with dry air or dry nitrogen. By using ideal liquids and gases, the flow properties of the filter can be assessed independently of particulates or other contaminants that could clog the pores. If there is nothing in the sample stream to clog the pores, the flow rate should remain constant. For ultrafiltration, there are special considerations on flow rate.
Flow Rate and Ultrafiltration
During ultrafiltration, it is important to balance flow rate with retention to obtain optimal performance. With ultrafiltration membranes the term more commonly used is flux. A membrane’s flux is defined as flow divided by the membrane area. The reason that flux is used with ultrafiltration membranes is the need for scalability. Ultrafiltration membranes are commonly used in the purification of expensive biomolecules. Separations are investigated on a small scale in the laboratory before being scaled up to larger volumes in a production setting. Characterizing separations on the basis of flux makes is easier to convert a lab scale investigation to a production scale process.
Using membranes with higher NMWL ratings will increase the flow rate, but at the same time lower the retention. A membrane should be selected for the required retention, coupled with the desired flow rate. This is determined by:
Concentration and diffusivity
Temperature effects on viscosity
When concentration polarization is rate-controlling, flux is affected by solute concentration, fluid velocity, flow channel dimensions, and temperature.
Air and Gases
Since sterility is a common requirement of vent membranes, pore rating is an important consideration. Please note that the mechanism of bacterial retention by hydrophobic membranes in a gas stream differs from that for hydrophilic membranes. Bacteria and other pathogens float in air attached to particles (aerosol or dust). Consequently, in air filtration, pathogens can be rejected by membranes with pore sizes larger than the pathogen alone. Membranes with pore sizes up to 5.0 µm are claimed to exhibit >99.99% bacterial retention efficiency by some suppliers. Similar claims exist for viral retention on 0.2 µm membranes. Therefore, membranes with larger pore sizes are used in less critical applications, yielding the benefits of higher flow rate.
When comparing the air flow rates of different membranes, it is important to note the units in which flow rate is reported and any differences in the conditions under which testing was done. Small changes in pressure and temperature can dramatically affect reported air flow rates.
Liquid flow is measured by placing the filter into an appropriate holder, adding a defined volume of liquid to the holder, and then pulling the liquid through the filter with a constant vacuum. Flow thus depends on the nature of the liquid, the surface area of the membrane, and the vacuum level. To compare different membranes, the same liquid and vacuum level should be used.
While water and alcohols can be used to test flow rate in large scale testing, they may not provide enough discrimination in predicting membrane performance when a more complex solution such as serum or cell culture medium is to be processed. For specific applications, it is appropriate to use other test solutions. Since complex solutions, such as cell culture media, are considerably more expensive than water or alcohols, sampling plans and test protocols should balance the amount of extra data required against the additional cost.