Page 462 - Academic Press Encyclopedia of Physical Science and Technology 3rd Chemical Engineering

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Encyclopedia of Physical Science and Technology EN014A-654 July 28, 2001 16:35

24 Reactors in Process Engineering

function of temperature only and is generally modeled A. Operation Type
by an exponential relationship such as the Arrhenius
The operational configuration for the reactor can be a
equation.
primary method of classification.
Rate equation Mathematical expression that is a func-
tion of both concentration of reactants or products, and
temperature. 1. Batch
Reaction mechanism Series of elementary reaction steps
Batch reactors are operated with all the material placed in
that when combined, gives the overall rate of reaction.
the reactor prior to the start of reaction, and all the material
Space time Time to process one reactor volume based on
is removed after the reaction has been completed. There is
inlet conditions.
no addition or withdrawal of material during the reaction
Yield Moles of a desired product divided by moles of a
process.
limiting reactant.
2. Semibatch
A CHEMICAL REACTOR is any type of vessel used
The semibatch reactor combines attributes of the batch
in transforming raw materials to desired products. The
and the continuous-stirred tank. The reactor is essentially
vessels themselves can be simple mixing tanks or complex
batch but has either a continuous input or output stream
flow reactors. In all cases, a reactor must provide enough
during operation.
time for chemical reaction to take place.
The design of chemical reactors encompasses at least
3. Continuous Flow Reactors
three fields of chemical engineering: thermodynamics, ki-
netics, and heat transfer. For example, if a reaction is run Continuous flow reactors represent the largest group of
in a typical batch reactor, a simple mixing vessel, what reactor types by operational classification. Several con-
is the maximum conversion expected? This is a thermo- tinuous flow reactors are used industrially.
dynamic question answered with knowledge of chemical
equilibrium. Also, we might like to know how long the
a. The continuous-stirred tank reactor (CSTR) involves
reaction should proceed to achieve a desired conversion.
feeding reactants into a well-mixed tank with simultane-
This is a kinetic question. We must know not only the sto-
ous product removal.
ichiometry of the reaction but also the rates of the forward
b. The plug flow reactor (PFR) consists of a long pipe or
and the reverse reactions. We might also wish to know
tube. The reacting mixture moves down the tube resulting
how much heat must be transferred to or from the reactor
in a change in concentration down the length of the reactor.
to maintain isothermal conditions. This is a heat transfer
c. In the recycle reactor part of the outlet stream is
problem in combination with a thermodynamic problem.
returned to the inlet of the reactor. Although not a typical
We must know whether the reaction is endothermic or
reactor classification by type, the recycle reactor allows
exothermic.
for continuous operation in regimes between CSTR and
After chemical reaction a series of physical treatment
PFR conditions.
steps is usually required to purify the product and per-
haps recycle unreacted material back to the reactor. The
quantity of material to be processed is a key factor in deter- B. Number of Phases
mining what type of reactor should be used. For small-lot
Reactors can also be classified by the number of phases
quantities, a batch reactor is commonly used in industry.
present in the reactor at any time.
For large, high-volume reactions, such as in the petroleum
industry, flow reactors are common.
1. Homogeneous
Homogeneous reactors contain only one phase throughout
I. REACTOR CLASSIFICATIONS the reactor.
Reactors may be classified by several different methods
2. Heterogeneous
depending on the variables of interest. There is no single
clear cut procedure for reactor classification. As a result, Heterogeneous reactors contain more than one phase.
several of the more common classification schemes are Several heterogeneous reactor types are available due to
presented here. various combinations of phases.