Page 43 - Introduction to chemical reaction engineering and kinetics

Chapter 2

Kinetics and Ideal

Reactor Models

In this chapter, we describe several ideal types of reactors based on two modes of op-
eration (batch and continuous), and ideal flow patterns (backmix and tubular) for the
continuous mode. From a kinetics point of view, these reactor types illustrate different
ways in which rate of reaction can be measured experimentally and interpreted opera-
tionally. From a reactor point of view, the treatment also serves to introduce important
concepts and terminology of CRE (developed further in Chapters 12 to 18). Such ideal
reactor models serve as points of departure or first approximations for actual reactors.
For illustration at this stage, we use only simple systems.
Ideal flow, unlike nonideal flow, can be described exactly mathematically (Chapter
13). Backmix flow (BMF) and tubular flow (TF) are the two extremes representing
mixing. In backmix flow, there is complete mixing; it is most closely approached by flow
through a vessel equipped with an efficient stirrer. In tubular flow, there is no mixing in
the direction of flow; it is most closely approached by flow through an open tube. We
consider two types of tubular flow and reactors based on them: plug flow (PF) charac-
terized by a flat velocity profile at relatively high Reynolds number (Re), and laminar
flow (LF) characterized by a parabolic velocity profile at relatively low Re.
In this chapter, we thus focus on four types of ideal reactors:
(1) Batch reactor (BR), based on complete mixing;
(2) Continuous-flow stirred tank reactor (CSTR), based on backmix flow;
(3) Plug-flow reactor (PFR), based on plug flow; and
(4) Laminar-flow reactor (LFR), based on laminar flow.

We describe each of these in more detail in turn, with particular emphasis on the
material-balance equation in each of the first three cases, since this provides an in-
terpretation of rate of reaction; for the last case, LFR, we consider only the general
features at this stage. Before doing this, we first consider various ways in which time is
represented.

2.1 TIME QUANTITIES

Time is an important variable in kinetics, and its measurement, whether direct or indi-
rect, is a primary consideration. Several time quantities can be defined.
(1) Residence time (t) of an element of fluid is the time spent by the element of fluid
in a vessel. In some situations, it is the same for all elements of fluid, and in others

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