Industrial Biotransformations

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5 Basics of Bioreaction Engineering
solution is negligible. UF membranes are usually asymmetrical in structure and the
pores have a diameter of 1–10 nm. In nanofiltration, the cut-off size is about 0.001 µm,
so nanofiltration can remove larger molecules, particulates and divalent species. In
reverse osmosis, the cut-off size is about 0.0001 µm. Particles, macromolecules and low
molecular mass compounds such as salts and sugars can be separated from a solvent,
usually water. The feed solution often has high osmotic pressure and this must be overcome
by the hydrostatic pressure that is applied as the driving force. Thus, microfiltration,
ultrafiltration, nanofiltration and reverse osmosis differ from each other in the size
of the particles being separated (Fig. 5.2).
type of filtration: retention:
microfiltrationfiltration
ultrafiltrationfiltration
nanonanofiltration reverse
osmosis
Fig. 5.2 Classification of membrane filtration processes.
suspendedparticles
microorganisms
macromolecules
(e.g. enzymes)
cofactors
salts2+/ cofactors
salts
salts +/-
salts
solvents
pore sizes,
cutoff:
0.2 - 2 µm
0.02 – 0.2 µm
2,000-100,000 2,000- g mol
0.002 – 0.02 µm
200 - 2.000 g mol mol-1 0.002 – 0.02 µm
0.0002 0.0002 – 0.002 µm
Apart from being used as separation devices, membranes can also be used to decouple
the residence time of the biocatalyst and reactants, which leads to continuously operated
reactors [8–10].
5.1.2.2 Crystallization
Crystallization is a heat- and mass-transfer process that is strongly influenced by fluid
and particle mechanics. Nucleation and growth kinetics are the key processes in this unit
of operation. Even small traces of impurities may influence the process dramatically.
5.1.2.3 Precipitation
A very rapid process of crystallization could be considered to be precipitation; however,
precipitation is often an irreversible process. Many precipitates are virtually insoluble
substances produced by a chemical reaction. On the other hand, crystallized products
can often be redissolved when the original conditions of the solvent, temperature and
concentration are restored.
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