Food Lipids: Chemistry, Nutrition, and Biotechnology

itations encountered in fixed bed reactors (Fig. 17). Stirring also prevents the formation
of temperature and concentration gradients between substrates or products.
A continuous stirred tank reactor can be in the form of a tank with stirring from the
top or bottom, or a column with stirring accomplished by propellers attached to the
sides of the column (68). The kinetics for a continuous stirred tank reactor, developed
by Lilly and Sharp (95), first encompass the substrate balance in the system as
dS
Q[S i] � Q[S 0]=
V
dt
where Q is the flow rate, [S i] is the initial substrate concentration entering the reactor,
[S 0] is the substrate concentration leaving the reactor, and V is the steady-state liquid
volume in the tank.
Rearrangement gives:
�
X k E
cat T
[S 0]X�K� m� =
1 � X Q
where [S0] is the initial substrate concentration, X is the amount of substrate converted
to product at any particular time (1 � [S]/[S0]), Q is the flow rate, ET is the
total number of moles of enzyme present in the reactor.
The main disadvantages of continuous stirred tank reactors are the higher power
costs associated with continuous stirring, the possibility of breaking up support particles
with agitation, and the requirement for a screen or filter at the outlet to prevent
losses of the immobilized lipase (7,80).
D. Membrane Reactors
Immobilization of enzymes onto semipermeable membranes is an attractive alternative
for lipase-catalyzed interesterification reactions. Membrane reactors involve
two-phase systems, where the interface of two phases is at a membrane. The advantages
of membrane systems are reduced pressure drops, reduced fluid channeling,
high effective diffusivity, high chemical stability, and a high membrane surface area
to volume ratio (90). Membranes are commonly produced in the form of a bundle
of hollow fibers and can be hydrophilic of hydrophobic in nature. Materials used in
membrane systems are polypropylene, polyethylene, nylon, acrylic resin, and poly-
Figure 17 Continuous stirred tank reactor for immobilized lipase–catalyzed interesterification.
Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.