degree of acyl migration because some reactants may have shorter reaction time and
some may have longer time, which has close correlation with both the product yield
and the degree of acyl migration.
Flow rate orresidence timeis the most crucial factor for the reactions in PBR.
When enzyme activity is gradually reduced, the flow rate must be regulated to main-
tain the desired reaction progress. The experimental relationship between flow rate
and activity was observed in previous studies (Hansen and Eigtved, 1987; Posorske
et al., 1988) (Figure 8). Other factors such as temperature, water content, and sub-
strate molar ratio are also important. Temperature not only affects the lipase activity
but also affects the mass transfer. Water content was not crucial in the short-term
running of the reactor (Mu et al., 1998), but for long-term operations it affected
the enzyme stability (Xu et al., 1998b).
Theoretical and practical comparisons have been conducted widely in literature
between STR and PBR in terms of productivity, efficiency, and other characteristics
(Gacesa and Hubble, 1987; Prenosil et al., 1987; Chaplin and Bucke, 1990). For the
production of regiospecific products, such as Betapol, CBE, SSL, etc., one more
merit attributed to PBR can be added. Acyl migration in the PBR was found to
be extremely low compared to that in batch STR (Xu et al., 1998b); as a conse-
quence, acyl migration can be neglected in the design of PBR and process optimiza-
tion.
206 11 Modification of Oils and Fats by Lipase-Catalyzed Interesterification
Figure 8. The relationship between flow rate and operation time for a constant incorporation of lauric
acid into soybean oil in a packed-bed reactor with Lipozyme IM as the biocatalyst. The experiment was
conducted at 60 8 C and the flow rate was adjusted to maintain the incorporation of 20 % under the sub-
strate weight ratio of 5 : 2 (soybean oil: lauric acid). (From Posorske et al., 1988.)