Page 243 - Chiral Separation Techniques

9.2 The SMB Concept 221

9.2 The SMB Concept

The principle of SMB operation can be understood by analyzing the equivalent true
moving bed (TMB) process. In TMB, liquid and solid flow in opposite directions
(Fig. 9-1) and are continuously recycled: the liquid flowing out of section IV is recy-
cled to section I, while the solid coming out of section I is recycled to section IV.
The feed is continuously injected in the middle of the system and two product lines
can be collected: the extract, which is rich in the more retained species (B) and so is
carried preferentially with the solid phase, and the raffinate, rich in the less-retained
species (A) that move upwards with the liquid phase. Pure eluent is continuously
injected at the inlet of section I, with the liquid recycled from the end of section IV.
Because of the addition and withdrawal of the four streams (eluent, extract, feed, and
raffinate), the TMB unit is divided into four sections and the liquid flow rates differ
from section to section. This enables the four sections of the unit to perform differ-
ent functions. In sections II and III the two components must move in opposite direc-
tions. The less retained component A must be desorbed and carried with the liquid
phase, while the more retained species B must be adsorbed and carried with the solid
phase. Section II is the zone of desorption of the species A, while section III is the
zone of adsorption of the component B. In section IV, both components must be
adsorbed in order to regenerate the eluent that will be recycled to the first zone. Sec-
tion I is the zone of solid regeneration where both components must be desorbed in
order to obtain a clean solid phase at the beginning of this zone. The operation of a
TMB introduces problems concerning the movement of the solid phase. A uniform
flow of both solid and liquid is difficult to obtain, and mechanical erosion of the
adsorbent phase will also occur. In view of these difficulties, the SMB technique was
developed in order to retain the process advantages of continuous and countercurrent
flow without introducing the problems associated with the actual movement of the
solid phase (Fig. 9-2).
In the SMB system the solid phase is fixed and the positions of the inlet and out-
let streams move periodically. This shift, carried out in the same direction of the liq-
uid phase, simulates the movement of the solid phase in the opposite direction. In the
Sorbex SMB technology a rotary valve is used to change periodically the position of
the eluent, extract, feed, and raffinate lines along the adsorbent bed. However, there
are alternative techniques to perform the port switching. Scaling down of the Sorbex
flowsheet becomes less economical than using a set of individual on-off valves con-
necting the inlet and outlet streams to each node between columns [12]. Recently,
this technology has been applied in the pharmaceutical industry for the separation of
chiral drugs [13]. Generally, for pharmaceutical applications small-sized SMB units
are preferable, and a special emphasis is addressed to the versatility of this type of
unit to perform different chiral separations without major design modifications.