-An ideal method for separation
would be a barrier that allows the desired substance
to pass while restraining other materials.
Semipermeable membranes sometimes meet this ideal by having pores through
which one substance can pass while others cannot. In nature such membranes
enclose living cells; nutrients enter while wastes escape, and some substances
are transported against their concentration gradients by energetic mechanisms.
Synthetic membranes are available with a wide range of pore sizes. Very
minute pores allow only molecules of gases or water to pass, intermediate
sizes of pores discriminate between molecules such as different proteins,
and larger pores may be just fine enough to prevent passage of particles
and cells such as microorganisms.
It must be remembered that membrane
separations are not complete; as a solution becomes
concentrated the driving forces to
achieve acceptable rates become too difficult to maintain. In other words,
as material passes through the membrane and reaches the product compartment,
the concentration difference between compartments declines. At some point
the concentration driving force becomes too small to keep transfer rate
in a practical range. This means that there will be a product stream and
a waste stream that may present disposal problems.
WHAT ARE THEY USED FOR?
Gas separation (helium from underground
gas, recycle unreacted hydrogen from the synthesis of ammonia)
Convert saline water to potable water
Sterilize air or water by filtering
out microorganisms
Concentration of solutions by expelling
water
Recovery of industrial products that
do not pass the membrane
Collection of materials for analysis,
e.g., using a membrane filter instead of the MPN test for estimating coliforms
in water. Usually a membrane filter test is preferred in which the organisms
are captured on a membrane and cultured on selective growth media that
accentuates the differences for coliform colonies for counting.
Membrane bioreactors
in which products or inhibitors can be removed during the bioprocess.
PAGE DEVELOPED BY:
DAN COPELAND
INTRO. BIOCHEM. ENGR.
SPRING 1997
REFERENCES: Bungay, Henry, Basic Biochem. Engr., BiLine
Assoc., 1993, p. 11.1-11.5.
Silvestri, John, Mixbed Ion Exchange Animation. LAST UPDATED 1 MAY 97