Figure 13.4 Exploded view of the
membrane microreactor: A—feed
inlet; B—outlet; C—O—ring; D—
sealing holes (internal diameter 5 mm).
A membrane filter is placed between
the upper and the lower chamber; the
sealing is made with butterfly screws;
all dimensions are in mm.
value related to the importance of mass transfer limitations inside the yeast flocs. The
floc porosity was also determined by means of a thermogravimetric balance, obtaining a
value of 50.5%.
This value increased about 10% when the flocs were grown in the presence of a
polymeric flocculation additive (a cationic resin) causing a corresponding 10% increase
in the value of the effectiveness factor.
This is an example of one of many attempts that have been made in order to reduce
diffusional limitations in flocs through the use of polymeric additives (Salt et al., 1996;
Sousa and Teixeira, 1991; Weir et al., 1994). Those additives should enlarge the space
between adjacent cells, extending the bridges that link the cells in a floc (Lima et al.,
1992). In fact, reductions in diffusional limitations have been reported suggesting an
increase of the effective diffusion coefficients of the substrates in the floc, whenever
some flocculating additives are used.
Lima et al. (1992) studied the influence of several polymeric additives on specific
glucose uptake rate of flocs of S. cerevisiae, using the same system as in Figure 13.4. An
increase of glucose uptake rate by cells in the flocs grown in the presence of additive was
always observed when compared to those grown without additive: 19% for bis
[polyoxyethylene-bis(amine)] 20 000, more than 50% for BPA 1000 and two-fold for
Multiphase bioreactor design 402