Ideal Flow Reactors - Bioprocess Engineering: Kinetics, Biosystems, Sustainability and Reactor Design - page 226

formation of B, V is the volume of reactive mixture volume, and J B is the mass transfer flux
of B out of reactive stream. Mole balance of species B in the stream over the other side of the
membrane (nonreactive stream) leads to

d F OB þ aJ B d V ¼ 0 (5.93)

where F OB is the molar flow rate of species B in the nonreactive stream. The mass transfer flux
between the two streams can be expressed as
J B ¼ k cB p B p OB

(5.94)

where k cB is the permeability of species B through the membrane, p B is the partial pressure of
species B in the reactive stream and p OB is the partial of species B in the nonreactive stream.
Therefore, a membrane reactor problem can be solved in a similar fashion as one would for
a plug flow reactor. However, there is an added term for mass transfer.

5.8. PFR OR CSTR?

In this chapter, we have discussed how to solve flow reactor problems. It helps up in
designing and/or performing performance evaluations for a reactor. Before we get into the
whether a PFR or a CSTR should be chosen, we should look at why we use a flow reactor
in the first place. Flow reactors can produce products continuously at a specified quality
with a consistent feed. The continuous nature makes it qualify for mass production. It comes
also with it less labor cost as automation can be effectively put in place. Therefore, flow reac-
tors are used when mass production of a product is desired and a consistent feed stream is
guaranteed for the quality of the product.
We next examining whether a PFR or a CSTR should be chosen. Examples 5-1 and 5-3
showed that a PFR can achieve the same task as that of a CSTR with a smaller volume (and
thus less operating as well as capital costs) if the reactors are run under isothermal conditions
and for reactions where the products do not affect the reaction rate. What will happen if one or
more products are catalyzing the reaction (affecting the reaction rate)? For example, autocat-
alytic reactionwill only occur if there is the product in the reactionmixture. In this case, a CSTR
might be preferred as the product is being recirculated inside the reactor. If a PFR is chosen,
a recycle stream will improve the production. One other example is exothermic reaction. At
low-temperature, the reaction rate is very low. At the end of the reaction, the heat released
by the reaction can raise the reaction mixture temperature and thus speedup the reaction.
This is similar to an autocatalytic reaction as the heat is also a by-product in the reaction. There-
fore, the selection of CSTR or PFR depends strongly on the type of reactions.
While in general, the reactor selection can be achieved by economic analysis (see Bio-
process System Optimization, in Chapter 3). There are chemical and/or physical restrictions
for bioprocess systems. Choice of the type of reactors is then dependent on physical and
chemical properties of the reaction mixture. In most bioprocess systems, multiple phases:
solid, liquid, and/gases are involved as reactants and/or products. For example, a soluble
solid is to react with a liquid stream, it might be better off to choose a CSTR as stirring action
is need to mix the reaction mixture.