Page 140 - Materials Chemistry, Second Edition

Mass-Balance Concept and Reactor Design 123

4.4 Types of Reactors
Reactors are typically classified based on their flow characteristics and the
mixing conditions within the reactor. Reactors may be operated in either
a batchwise or a continuous-flow mode. In a batch reactor, the reactor is
charged with the reactants, and the content is well mixed and left to react.
At the end of a specified time period, the resulting mixture is discharged.
A batch reactor is an unsteady-state reactor, because the composition of the
reactor content changes with time. The capital cost of a batch reactor is usu-
ally less than that of a continuous-flow reactor, but it is very labor intensive,
and the operating costs are higher. It is usually limited to small installations
and to the cases when raw materials are expensive.
In a continuous-flow reactor, the feed to the reactor and the discharge from
it are continuous. In most of the cases, the flow reactors are operated under
steady-state conditions in which the feed stream flow rate, its composition,
the reaction condition in the reactor, and the withdrawal rate are constant
with respect to time. Frequently, reaction kinetics is studied in a laboratory
to determine the rate constant by using a batch reactor. The application of the
obtained rate constant to the design of a continuous-flow reactor, however,
involves no changes in the kinetics principles; thus, it is valid. In general,
there are two ideal types of flow reactors: continuous-flow stirred tank reac-
tor (CFSTR) and plug-flow reactor (PFR). They are classified mainly by the
mixing conditions within the reactors.
The CFSTR consists of a stirred tank that has feed stream(s) of the reactants
and discharge stream(s) of reacted materials. The CFSTR is usually round,
square, or slightly rectangular in a plan view, and it is necessary to provide
sufficient mixing. The stirring of a CFSTR is extremely important, and it is
assumed that the fluid in the reactor is perfectly mixed (i.e., the content is
uniform throughout the entire reactor volume). As the result of mixing, the
composition of the discharge stream(s) is the same as that of the reactor con-
tent. Therefore, it is also called a completely stirred tank reactor (CSTR) or
completely mixed flow reactor (CMF). Under the steady-state conditions, the
concentration of the effluent and that at any location within the reactor are
the same and should not change with time.
The PFR ideally has the geometric shape of a long tube or tank and has a
continuous flow in which the fluid particles pass through the reactor in series.
The reactants enter at the upstream end of the reactor, and the products leave
at the downstream end. Ideally, there is no induced mixing between ele-
ments of fluid along the direction of flow. Those fluid particles that enter the
reactor first will leave first. The composition of the reacting fluid changes in
the direction of flow. For the case of COC removal or destruction, the con-
centration will be the highest at the entrance and dropped continuously to