Page 51 - Introduction to chemical reaction engineering and kinetics

2.4 Plug-Flow Reactor (PFR) 33

2.4 PLUG-FLOW REACTOR (PFR)

2.4.1 General Features

A plug-flow reactor (PFR) may be used for both liquid-phase and gas-phase reactions,
and for both laboratory-scale investigations of kinetics and large-scale production. The
reactor itself may consist of an empty tube or vessel, or it may contain packing or a
tied bed of particles (e.g., catalyst particles). The former is illustrated in Figure 2.4, in
which concentration profiles are also shown with respect to position in the vessel.
A PFR is similar to a CSTR in being a flow reactor, but is different in its mixing
characteristics. It is different from a BR in being a flow reactor, but is similar in the pro-
gressive change of properties, with position replacing time. These features are explored
further in this section, but first we elaborate the characteristics of a PFR, as follows:
(1) The flow through the vessel, both input and output streams, is continuous, but
not necessarily at constant rate; the flow in the vessel is PF.
(2) The system mass inside the vessel is not necessarily fixed.
(3) There is no axial mixing of fluid inside the vessel (i.e., in the direction of flow).
(4) There is complete radial mixing of fluid inside the vessel (i.e., in the plane per-
pendicular to the direction of flow); thus, the properties of the fluid, including its
velocity, are uniform in this plane.
(5) The density of the flowing system may vary in the direction of flow.
(6) The system may operate at steady-state or at unsteady-state.
(7) There may be heat transfer through the walls of the vessel between the system
and the surroundings.
Some consequences of the model described in the seven points above are as follows:
[l] Each element of fluid has the same residence time t as any other; that is, there is
IZO spread in t.

Figure 2.4 Property profile (e.g., CA for A+. . . +
products) in a PFR (at steady-state)