● At the point in time where substrate is not longer converted, there is equilibrium for the
product concentration between the medium phase and the organic phase.
The influence of the distribution coefficient (m) is shown in Figure 12.10a-c. Figure
12.10a and 12.10b show the concentrations in the medium phase, whereas Figure 12.10c
shows the product concentration in the dodecane phase. The maximum substrate
conversion rate was 0.01 mol/(m^3 s), and the toxic product concentration was 10 mol/m^3.
We took a medium flux of 1.29×10^2 m/s and a dodecane flux of 0.91×10−^2 m/s, which
resulted in gel bead hold-up of 0.32 and a dodecane hold-up of 0.093. Figure 12.10a
clearly shows that a larger m eventually results in a lower substrate concentration in the
medium phase, hence substrate is converted to a higher degree. Naturally, when substrate
is not converted totally, the product concentration in the medium phase is equal to the
toxic product concentration (Figure 12.10b). As there is equilibrium between the medium
and organic phase, the product concentration in the organic phase is higher for a higher
m, Figure 12.10c. If the m is high enough, than at the end of the batch operation the
product concentration in the medium phase will be lower than the toxic product
concentration. Consequently, substrate is totally converted. This is observed for a
distribution coefficient of 1000. The end of the batch operation is reached at an earlier
time for a lower m.
The influence of the toxic product concentration is shown in Figure 12.11.
Figures12.11a and 12.11b show the concentrations in the medium phase, whereas Figure
12.11c shows the product concentration in the dodecane phase. Obviously, a higher
results in a lower substrate concentration, hence there is a more completeconversion and the product concentration becomes. This concentration is reached
earlier for a lower. At equal to 100, substrate is completely converted, and
is not reached in the medium phase.
The influence of the organic solvent flux is shown in Figure 12.12. Figures 12.12a and
12.12b show the concentrations in the medium phase, whereas Figure 12.12c shows the
product concentration in the dodecane phase. We kept the medium flux constant and
changed the dodecane flux. The dodecane flux influences the hold-up of the different
phases. At a constant medium flux, a higher dodecane flux gives a higher dodecane
holdup, but a lower gel bead hold-up. Figure 12.12a shows that complete conversion of
substrate is reached for the highest Ud, although the gel-bead hold-up is the lowest (Ud=
0.75×10−^2 m/s, εs=0.26; Ud=0.14×10−^2 m/s, εs=0.30). So, at the highest dodecane flux, the
product concentration in the medium phase is always below the toxic product
concentration, Figure 12.12b. For the other dodecane fluxes applied in the simulations,
the product concentration in the medium phase is equal to the toxic product
concentration, Figure 12.12b. This concentration is reached at an earlier point of time for
the lowest Ud. The time course of product concentration in the organic phase follows the
time course for the product concentration in the medium phase.
COMPARISON BETWEEN A TWO-PHASE AND A THREE-PHASE
FLUIDISED-BED BIOREACTOR
Multiphase bioreactor design 370