REACTORS: IDEALIZED CHEMICAL REACTORS - Biomass as a Sustainable Energy Source for the Future - page 161

the design of the supports, construction materials, wall thickness, internals, etc., is
out of the scope of this topic.
A reactor is nothing else than a vessel to carry out a reaction; a pan or a pot could be
considered a chemical reactor. The typical steps involved in the functional design of a
chemical reactor are:

1. Defining the rate law of the reactions taking place. In this chapter, only single
reactions will be considered. If the rate law depends on the concentration of
more than one species, all these concentrations must be related to the concen-
tration of a key species using stoichiometric relations.
2. Selecting a type of operation, batch, or continuous (see Table 6.1).
3. Selecting a mixing mode. In this chapter, two extreme cases will be considered,
plug flow and perfect mixing.
4. Setting up the mass and energy balances for the reactor.

In the subsequent sections, we will see that the kinetic term of the molar balance can be
handled either in terms of concentration or conversion. Concentration has the advan-
tage of being a directly measurable property, which is related to the molar balances
in the reactor. Conversion is a better descriptor of the evolution of the reaction, but
it must be ultimately related to the concentration to get information about the molar
balances in the reactor. It is defined as

n i 0 − n i

X i ≡ ð Eq : 6 : 1 Þ

n i 0

TABLE 6.1 Comparison between batch and continuous-flow reactors:
(+) advantage and (−) disadvantage
Batch reactor (BR) Continuous-flow reactor

(+) Suitable for small productions volume,
such as in fine chemistry

(+) Suitable for large production volumes

(+) High flexibility. Different products can
be produced in the same reactor

(−) Lower flexibility

(+) Easy startup and shutdown (−) Difficult startup and shutdown. The

startup process to steady-state
operation can take weeks

(+) Capital cost relatively low (−) Capital cost usually high. Need of a

flowing system, such as pumps and pipes

(−) Downtime between batches (+) No downtime between batches

(−) Maintaining product uniformity between
batches is difficult due to unsteady-state
operation

(+) Uniformity of product is easily maintained
working in steady state

(−) If the BR is part of a continuous process,
buffering tanks are required

(+) Steady-state operation allows easy
coupling with continuous downstream
operations