Sustainability and Stability - Bioprocess Engineering: Kinetics, Biosystems, Sustainability and Reactor Design - page 784

operation without fluctuation in reaction conditions. In practice, especially in mass-
production facilities, steady continuous operations are preferred. The steady continuous
operation may also be termed sustainable operation, although a sustainable system need
not be (strictly) stable. During steady operations, the feed conditions, reactor tempera-
ture, pressure, the extent of reaction, and yield are all invariant with time or vary only
slightly with time within a narrow margin of fluctuation. This “narrow” margin of fluc-
tuation is ideally very small in most chemical and biochemical process operations.
However, in ecosystems (nature) and waste treatment facilities, the margin of fluctua-
tions could be “big.” To study the formation and maintenance of steady (sustainable)
operation conditions in a reactor, the steady state and the stability are two important
concepts.
Steady state refers to the equilibrium state of a bioprocess system, where all the parame-
ters are invariant with time. One can imagine that the steady states are the positions that
the ball can be placed along the path shown in Fig. 16.1 . Of course, the strict steady state
does not exist in reality due to fluctuations external to the system. We usually refer steady
state to a bioprocess system where all the measurable parameters of interest are invariant
on a given timescale and a given space volume scale (i.e. on an average sense). More
precisely, steady state is a sustainable state. This is much the same as the continuum scale
concept for a fluid. For example, we say a fluid is continuous, which is only valid if the time-
scale is “long” enough and the volume scale is “large” enough. When timescale is too short
and/or volume scale is too small, molecules are individual objects that move about all the
time, whereby continuum concept is void. For a continuous flow reactor, the steady state
refers to the state where the material (including individual species) flow and energy flow
are constant. When a catalyst is involved, the activity of the catalyst remains constant under
the timescale of observation.
External luctuation
Gravity pull
FIGURE 16.1 Equilibrium positions (steady states) of a ball on a curvy path.