Continuous Cultivation - Bioprocess Engineering: Kinetics, Biosystems, Sustainability and Reactor Design - page 646

In a stagnant biological film, nutrients diffuse into the biofilm and products diffuse out
into liquid nutrient medium. Nutrient and product profiles within the biofilm are important
factors affecting cellular physiology and metabolism. Biofilm cultures have almost the same
advantages as those of the immobilized cell systems over suspension cultures.
The thickness of a biofilm is an important factor affecting the performance of the biotic
phase. Thin biofilms will have low rates of conversion due to low biomass concentration,
and thick biofilms may experience diffusionally limited growth, which may or may not be
beneficial depending on the cellular system and objectives. Nutrient-depleted regions may
also develop within the biofilm for thick biofilms. In many cases, an optimal biofilm thickness
resulting in the maximum rate of bioconversion exists and can be determined. In some cases,
growth under diffusion limitations may result in higher yields of products as a result of
changes in cell physiology and cell e cell interactions. In this case, improvement in reaction
stoichiometry (e.g. high yield) may overcome the reduction in reaction rate, and it may be
more beneficial to operate the system under diffusion limitations. Usually, the most sparingly
soluble nutrient, such as DO, is the rate-limiting nutrient within the biofilm.
Immobilized cell culture offersmore freedom in terms of methods of cultivation. When cells
are immobilized, the fermentation can be carried out in batch and flow reactors. Fluidized bed
(close to CSTR or chemostat) and packed bed (close to PFR) reactors can be employed such that
cells are retained in the reactor. The cost to cell immobilization and some loss of efficiency
(effectiveness) can be justified by the saving from cell recycle (or with minor loss only).

Example 12.6. Xylose (S) is converted to ethanol (P) in a packed bed by a genetically
altered S. cerevisiae (X) cells immobilized via entrapment in Ca-alginate beads. Ethanol
production rate follows Monod/Michaelis e Menten equation. The maximum specific rate
of formation of ethanol

0.25 g-P/(g-X $ h) and the saturation constant is 0.0018 g/L.
The average cell loading achieved in the bed is

m Pmax ¼

20 g/L. Assume that cell growth is negli-
gible and the bead size is small enough and the flow condition is turbulent enough that the
overall effectiveness factor is 1. That is, the reaction rate is not affected by the immobilization.
The xylose feed concentration is 120 g/L and the ethanol yield factor YF P/S ¼

X ¼

0.48. If the

desired xylose conversion is 99%, determine
(a) Required dilution rate.
(b) Ethanol concentration in the effluent.

Solution. (a) A packed bed reactor can be approximated by a PFR. Mass balance of the
substrate (xylose) in a differential volume of the reactor leads to

d ( SdV )

QS − QS + r S dV =

(E12-6.1)

+

V V dV

dt

At steady-state operation, nothing changes with time: 0
Equation (E12-6.1) is reduced to

Q d S ¼r S d V (E12-6.2)

From stoichiometry,

r P
YF P = S

r S ¼ (E12-6.3)