Page 262 - Academic Press Encyclopedia of Physical Science and Technology 3rd BioTechnology

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Encyclopedia of Physical Science and Technology EN014J-683 July 30, 2001 20:3

670 Separation and Purification of Biochemicals

from the point of sample introduction it will appear at the exclusion and ion exchange chromatography mainly in
bottom of the bed. A relation between a fixed bed process the petrochemistry or in the fine chemicals industry. More
and the same process applied to the CAC is given by recently, the simulated moving bed (SMB) has also a been
proposed as a very elegant solution for the efficient sepa-
α = ω · t, (18)
ration of racemic mixtures into enantiopure drugs at large
where α is the elution angle from the feeding point, ω is scale.
the rotation speed, and t is the elution time in fixed bed
chromatography. An alternative design has been reported
where the sample inlet and the collection ports are rotated III. STATIONARY PHASES FOR
while the annular bed is kept fixed in order to overcome BIOCHROMATOGRAPHY
mechanical difficulties.
Initially, mostly isocratic separations were carried out, Traditionally, chromatographic separations of bio-
and SEC mode was common. However, modern CAC sys- (macro)molecules were fairly slow processes. The typi-
tems also allow for step gradient elution, which is usually cal columns packed with porous particles could not be
done by injecting the respective eluents at fixed positions operated at high speed, due to considerable diffusional
around the circumference of the bed. Continuous separa- constraints. The mass transfer within the pores of the par-
tions of amino acids, sugars, proteins, and DNA have been ticles, where most of the adsorptive surface is located,
reported with such systems. occurs mainly by molecular diffusion. The inherently low
diffusionrateofmacromoleculesgivesrisetoconsiderable
band broadening and a rapid reduction in resolution when
4. Simulated Moving Bed
increasing eluent velocities are used (see van Deemter
In the true moving bed system, feed inlet and collection curve in Fig. 4). As a consequence, the target molecules
outlets are fixed while the adsorbent moves in the oppo- are exposed for some time to potentially harmful condi-
site direction of the eluent (countercurrent flow). The sys- tions and significant degradation may occur.
tem is extremely difficult to operate because it involves Besides the mass transfer phenomena, some other more
the circulation of a solid adsorbent. However, the mov- technicalfactorssuchaslimitedmechanicalstabilityofthe
ing bed can be simulated by a number of columns con- column and the stationary phase itself have been known
nected so that by using the appropriate valving system, to limit the speed of chromatographic separations. Dif-
the operation of a countercurrent is approached by the ap- ferent approaches to overcome this problem include the
propriate shift of the injection and collection points of the improvement of the conventional porous particles (non-
columns as shown in Fig. 14. The adsorbent is still moving porous, tentacle, gigaporous and hyperdiffusive particles)
with respect of the inlet/outlets points if they are moved as well as the introduction of new nonparticle-based sta-
step by step between a given number of fixed columns. tionary phases (membrane, disk, and continuous bed ad-
Whereas such systems are not capable of resolving mul- sorbers). These modifications are directed at speeding up
ticomponent mixtures, they offer a promising continuous the separations, which have as consequences, e.g., the re-
high throughput alternative for the processing of binary duction of losses and degradation by shorter contact with
mixtures. Such systems have been used for the purifica- certain solvents as well as the reduction of solvent con-
tion of dextrans, carbohydrates, and sugars using size- sumption and manufacturing costs.

FIGURE 14 Schematic representation of a simulated moving bed (SMB) system with sequential operation.