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3.2 Heterogeneous Reactors 73

involved reacting phases —

• homogeneous reactors, where one phase such as liquid or gas exists in the reactor
• heterogeneous reactors, where two or three distinct phases coexist in the reactor .

A further classification of homogeneous and heterogeneous reactors is based on the nature
of the inolved reacting phases— v

• homogeneous reactors
• liquid-phase reactors
• gas-phase reactors

• heterogeneous reactors
• liquid–solid (L–S) reactors
• gas–solid (G–S) reactors
• liquid–gas (L–G) reactors
• liquid–gas–solid (L–G–S) reactors

Finally, classification could be based on the contacting pattern of the ined reacting v olv
phases, as it is described in the follo wing sections.

3.2.2 Homogeneous reactors

Plug-flow tubular reactor (PFTR): This reactor is operated under steady-state condition.
The reactor is of tubular shape, the reactants enter at the inlet and the composition is a
function of the distance from the inlet. However, the composition is not a function of time.
The ideal plug-flow reactor is characterized by the absence of mixing in the direction of
flow and complete mixing in the transverse direction.
Continuous-stirred tank reactor (CSTR): This reactor is operated under steady-state
condition. The reactants flow continuously in and out of the vessel at a constant flo w rate
and are perfectly mixed by mechanical means, and thus the composition is the same
throughout the reactor. The result is that the exit concentration is the same as the one in the
reactor. The concentration is constant, i.e. is not time-dependent.
Batch-stirred tank reactor (BSTR): In this type of reactor the reactants are fed into the ,
container, the ed by means of mechanical agitation, y are well mix and left to react for a cer-
tain period of time. This is an unsteady-state operation, where composition changes with
time. However, the composition at any instant is uniform throughout the reactor .

3.2.3 Heterogeneous reactors

Gas–liquid heterogeneous reactors

Gas–liquid continuous-stirred tank r eactor: This is a CSTR, where the liquid and gas
phases are mechanically agitated (Figure 3.4).