The comparison between both bioreactors was made for different combinations of
medium and dodecane fluxes. Two different toxic product concentrations were used, i.e.
1 and 100 mol/m^3 with an initial substrate concentration of 1 kmol/m^3.
Introducing an organic solvent in a three-phase fluidised bed, will in most cases result
in a lower gel-bead hold-up. This means loss of biocatalytic activity per unit volume of
bioreactor. This loss of activity has to be compensated for by lowering the product
concentration in the gel beads, resulting in a higher substrate conversion rate. This is
achieved by extracting the product into the organic solvent. The extraction rate is
influenced by the distribution coefficient. As can be deduced from the equation in Table
12.2, a higher distribution coefficient gives a higher transfer rate.
So, at a given maximum substrate conversion rate, the time to reach a certain degree of
conversion can be manipulated by the distribution coefficient (m): a higher m results in a
shorter time, see also Figure 12.10. Thus, to conclude whether a three-phase fluidised bed
performs better than a two-phase fluidised bed, at a given maximum substrate conversion
rate, a distribution coefficient is determined, that yields the same time to reach 99% of
the total amount of moles converted in a two-phase fluidised bed. A higher distribution
coefficient results in better performance, whereas a lower distribution coefficient gives a
worse performance.
We chose to evaluate the performance of the bioreactor with two parameters, i.e. the
maximum substrate conversion rate (X·vmax) and the distribution coefficient (m), and to
keep the other parameters constant. The maximum substrate conversion rate can be
influenced by using more or less biocatalyst, and the distribution coefficient might be
influenced by adding specific compounds to the organic solvent for enlarging the
distribution.
The distribution coefficient at a given maximum substrate conversion rate is thus
calculated for which a three-phase fluidised bed performs the same as two-phase fluidised
bed. This was done for a number of organic solvent fluxes and medium fluxes, and for
different toxic product concentrations. The results are shown in Figures 12.13a-c. A
three-phase fluidised bed performs better for combinations of the maximum substrate
conversion rate (X·vmax) and distribution coefficient in the area above the lines in these
figures.
The distribution coefficient at each X·vmax in favour of the three-phase fluidised bed is
relatively low, see Figures 12.13a-c, particularly when we consider the examples in Table
12.4 for distribution coefficients for different solutes for different liquid-liquid two-phase
systems. Based on this table one might suggest that at any pre-set medium flux, a three-
phase fluidised bed performs better than a conventional two-phase fluidised bed.
Looking in more detail at Figures 12.13a-c, the results in Figure 12.13a and Figure
12.13b—a lower water flux applied—show that a lower dodecane flux requires a higher
distribution coefficient, whereas the results in Figure 12.13c—the highest water flux
applied—show that a higher dodecane flux requires also a higher distribution coefficient.
Although the latter observation might seem unexpected, both observations can be easily
explained. In most cases the dodecane flux decreases the gel-bead hold-up and, as
explained above, this decrease in gel-bead hold-up has to be compensated for with a high
enough transfer rate to the dodecane phase. The transfer rate itself is influenced by the
dodecane flux, as the dodecane flux determines largely the dodecane hold-up and the
mass transfer coefficient, see also the equation in Table 12.2. A higher dodecane flux
Design of liquid-liquid-solid fluidised-bed bioreactors 373