Page 67 - The engineering of chemical reactions

Chemical Reactors 51

Figure 2-8 A “flow sheet” of possi- batch
ble reactor configurations and modes
of operation.
semibatch
/ CSTR H steady state
flow - partially mixed 1
- PFTR b transient

CHEMICAL REACTORS

The chemical reactor is the “unit” in which chemical reactions occur. Reactors can be
operated in batch (no mass flow into or out of the reactor) or flow modes. Flow reactors
operate between limits of completely unmixed contents (the plug-flow tubular reactor or
PFTR) and completely mixed contents (the continuous stirred tank reactor or CSTR). A flow
reactor may be operated in steady state (no variables vary with time) or transient modes.
The properties of continuous flow reactors will be the main subject of this course, and an
alternate title of this book could be “Continuous Chemical Reactors.” The next two chapters
will deal with the characteristics of these reactors operated isothermally. We can categorize
chemical reactors as shown in Figure 2-8.
We will define these descriptions of reactors later, with the steady-state PFTR and CSTR
being the most considered reactors in this course.

Example 2-6 Consider the situation where the reactants at constant density are fed
continuously into a pipe of length L instead of a tank of volume V as in the batch
reactor. The reactants react as they flow down the tube with a speed u, and we assume
that they flow as a plug without mixing or developing the laminar flow profile. Show
that the conversion of the reactants is exactly the same in these very different reactor
configurations.

A molecule flowing with a speed u has traveled a distance z after it has flowed in
the reactor for time

and the time t it requires to flow down the reactor is
L
t=--
u
as sketched in Figure 2-9. Therefore, the increment of time dt to travel a distance
dz is given by