Biomedical Engineering Reference
In-Depth Information
There is considerable confusion in the open literature as to the distinction
between a few membrane separation processes, that is, the microfiltration
(MF), ultrafiltration (UF), and reverse osmosis (RO). Occasionally, one will
see it referred to by other names such as “hyperfiltration (HF).” In order to
distinguish these separation processes clearly that RO has, the separation
range of 0.0001 to 0.001 m (i.e., 1 to 10 Å ) or < 300 mol wt. RO is a liquid-
driven membrane process, with the RO membranes capable of passing water
while rejecting microsolutes, such as salts or low-molecular-weight organics
(<1000 Da). A pressure driving force (1 to 10 MPa) is needed to overcome the
force of osmosis that cause the water to flow from dilute permeate to con-
centrated feed. The principle use of this membrane process is desalination,
which shows its great advantage over the conventional technique of desali-
nation, that is, ion exchange.
The biotechnology industry, which originated in the late 1970s, has become
one of the emerging industries that draw the attention of the world, especially
with the emergence of genetic engineering as a means of producing medically
important proteins during the 1980s. Two of the major interest applications
of membrane technology in the biotechnology industry is the separation and
purification of the biochemical product, as is often known as downstream
processing, and the membrane bioreactor, which was developed for the trans-
formation of certain substrates by enzymes (i.e., biological catalysts).
Since its introduction in the 1970s, the membrane bioreactor has gained a
lot of attention over the other conventional production processes concerning
the possibility of high enzyme density and hence high space-time yields.
Whereas downstream processing is usually based on discontinuously oper-
ated microfiltration, the membrane bioreactor is operated continuously and
is equipped with UF membranes. Two types of bioreactor designs are pos-
sible: dissolved enzymes, (as in used with the production of l-alanine from
pyrurate) or immobilized enzymes membrane.
Membrane science began emerging as an independent technology only in
the mid-1970s, and its engineering concepts still are being defined. Many
developments that initially evolved from government-sponsored funda-
mental studies are now successfully gaining the interest of the industries as
membrane separation has emerged as a feasible technology.
Today, membrane polymers used in pharmaceutical processes include
polyvinylidene fluoride (PVDF), expanded polytetrafluorethylene (ePTFE),
polyethersulfone (PESu), polyamide (PA), cellulose acetate (CA), regener-
ated cellulose (RC), and mixed cellulose ester (MCE), a mixture of cellulose
nitrate (CN) and CA. Membranes provide the highest retention efficiency or
the smallest pore sizes; the microfiltration membrane pore sizes range from
10 to 0.1 μm; the ultrafiltration membranes have pore sizes from 0.1 μm to a
few nanometers, making them even more suitable for virus filtration. The
nanofilters are in the range of 50 nm and smaller and rated for the molecular
weight they can retain in kilodaltons. Microfiltration membranes are suitable
for prefiltration, others for sterilization.
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