Biomedical Engineering Reference
In-Depth Information
or pores. The dead-end filtration is one of the simplest modes of operation
for filters and hence requires minimum accessories such as tubing/piping,
tanks, controls, and footprint.
Dead-end filters described using microporous membranes manufactured
out of synthetic polymers such as polyethersulfonate, polyamide, cyanoacry-
late, and polyvinylidene fluoride are used extensively for sterile processing.
They are used for adding media to the bioreactor, bioburden reduction in cell
harvest clarification, chromatography column protection, and final filtration
of the purified bulk drug substance. These filters often come attached to dis-
posable bags and are gamma sterilized.
The most common dead-end filtration devices are filter cartridges for reus-
able processes or capsules for fully disposable processes. They are used in
wide-ranging applications as pre- and sterilizing-grade filters in upstream
as well as downstream applications including media filtration, intermediate
product pool filtrations, and in form, fill, and finish for the sterilization of
drug substance. Dead-end filter devices are also used for sterilizing grade
air and vent filtration for cell harvest and clarification, and, most recently, for
viral clearance and membrane chromatography.
Cross-Flow Filtration
In chemical engineering, biochemical engineering, and protein purification,
cross-flow filtration (also known as
tangential flow filtration
) is a type of filtra-
tion, a particular unit operation. Cross-flow filtration is different from dead-
end filtration in which the feed is passed through a membrane or bed, the
solids being trapped in the filter, and the filtrate being released at the other
end. Cross-flow filtration gets its name because the majority of the feed flow
travels tangentially across the surface of the filter, rather than into the filter.
The principal advantage of this is that the filter cake (which can blind the fil-
ter) is substantially washed away during the filtration process, increasing the
length of time that a filter unit can be operational. It can be a continuous pro-
cess, unlike batch-wise dead-end filtration. This type of filtration is typically
selected for feeds containing a high proportion of small-particle-size solids
(where the permeate is of most value) because solid material can quickly
block (blind) the filter surface with dead-end filtration. Industrial examples
of this include the extraction of soluble antibiotics from fermentation liquors.
Since in cross-flow filtration the feed stream is led across or tangential to
the filter material surface and is recycled continuously around the filter, this
requires more complex equipment and controls, but the retentate is allowed to
pass through the filter device multiple times by recirculation. Thus, it is pos-
sible to perform concentration or buffer-exchange processes. Additionally,
for liquids with a heavy load of suspended particles, the filter is kept from
