Sustainability and Stability - Bioprocess Engineering: Kinetics, Biosystems, Sustainability and Reactor Design - page 785

Stability for a bioprocess system refers to the ability to dampen the “movements”
effected by external disturbances to the bioprocess system. For example, there are
two steady states for a normal person: either standing up or lying down. Clearly, lying
down is a more stable state than standing up. When a person is lying down, the body
center of mass is very close to the surface and there is more contacting area with the
surface. At standing position, the center of mass is farther away from the surface and
there are only two contacting spots (feet) with the surface. To maintain standing,
a person must be constantly adjusting the center of mass to be directly vertical above
and between the two feet. If one is weak, he/she may fall as ones' body is not able to
adjust against sudden pushes (such as wind) exhibited fluctuations. For a steady
reactor operation, one must maintain the reactor inlet (feed) conditions, surroundings
(for example, temperature and air movement), and effluent rates constant. However,
strict constant is not practical as fluctuations always exist. Fluctuation is stochastic
and unexpected. If a reactor is able to self-adjust to return all the system parameters back
to the steady state before fluctuation disturbances exerted, the reactor can maintain the
set steady-state operations. In this case, the reactor is stable. If a reactor runs away from
a set steady-state operation point when experiences fluctuations, then the reactor is not
stable. For example, explosive gaseous reaction and genetic transformation caused by envi-
ronmental fluctuations are two unstable bioprocess systems. Referring to Fig. 16.1 ,theloca-
tion of the ball is not a stable steady state as any fluctuation would force the ball change the
location and not able to return. Commonly, stability issues occur when multiple steady
states (MSS) exist. Yet, there are cases where “instability” is associated with a single desir-
able or possible steady state.
One of the important aspects of reactor design is to seek desirable stable steady-state reac-
tion conditions as reactor set point. Only stable steady state can be operated such that the bio-
process system will not runaway when fluctuations are encountered. As such, the quality of
the product can be maintained. Normally, a bioprocess system is operated such that the
output fluctuations are small, and it gradually returns to the desired set point when a fluctu-
ation is expired. Therefore, bioprocess system stability is an important topic for bioprocess
system (reactor) design and operations.

16.1. FEED STABILITY OF A CSTR

Let us first consider the simplest case of steady-state operations of a continuous-flow
stirred tank reactor (commonly known as CSTR for chemical transformations, or chemostat
for biotransformations) in which a single reaction is taking place under isothermal conditions:

A / Products

We do not eliminate the possibility of multiple reactants and/or multiple products.
Fig. 16.2 shows a schematic of jacketed CSTR where the heat generated in the reactor is
removed via a saturated liquid e vapor mixture, so that the reactor is maintained at a constant
temperature T. The variability in operation is controlled so that only fluctuations can come in
from the feed.