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Reactors in Process Engineering 41

L. Lagoon the rate of recycle must be much greater than the rate of
product withdrawal.
1. Description
Lagoons are used for the deposition and degradation of in-
3. Applications
dustrial and human wastes. The waste, in water, is pumped
into a holding lagoon. Water in the lagoon usually evap- An example for the loop reactor is the oxidation of normal
orates but may be pumped out under some conditions. butane.
The advantage of a biological lagoon is long holding times
for the degradation of compounds that have extremely
N. Packed Bed
slow reaction rates.
There are three modes of operation for lagoons. They 1. Description
may be either anaerobic, aerobic, or facultative (which is a
Thepackedbedreactorisusedtocontactfluidswithsolids.
combination of aerobic and anaerobic). Aerobic lagoons
It is one of the most widely used industrial reactors and
require the additional cost of aerators and compressors
may or may not be catalytic. The bed is usually a column
for continuous bubbling of air, oxygen, or ozone into the
with the actual dimensions influenced by temperature and
lagoon.
pressure drop in addition to the reaction kinetics. Heat
limitations may require a small diameter tube, in which
2. Classification case total through-put requirements are maintained by the
use of multiple tubes. This reduces the effect of hot spots
The biological lagoon is difficult to categorize since a re-
in the reactor. For catalytic packed beds, regeneration is
action and a separation process are occurring simultane-
a problem for continuous operation. If a catalyst with a
ously. Water flow through the system should ideally be at
short life is required, then shifting between two columns
steady state; however, variable input, climatic conditions,
may be necessary to maintain continuous operation.
and rain all affect the water in the system as a function
of time. Chemical concentrations are similar to semibatch
operation but may be at a relatively steady state. 2. Classification
A packed bed reactor is a continuous heterogeneous reac-
3. Applications tor. The gas or liquid phase obeys plug flow theory. The
Lagoons are a simple, low-cost reaction system for solids are considered batch, with even long-life catalyst
wastewater treatment. Anaerobic lagoons are capable of beds losing activity over time.
handling high-concentration wastes but then require an
aeration lagoon to treat the water effluent. Effluent from 3. Applications
aerobic lagoons with low-concentration feed usually re-
quires no additional treatment to meet water quality Noncatalyticpackedbedreactorshavebeendiscussedsep-
standards. arately in other sections of this article. They include blast
furnaces, convertors, roasting furnaces, rotary kilns, and
gasifiers.
M. Loop Reactor
1. Description
O. Recycle
For reactions where high-pressure requirements do not
1. Description
allow large diameter tanks for homogeneous reaction ki-
netics, a loop reactor can be used. The loop is a recycle A recycle reactor is a mode of operation for the plug flow
reactor made of small diameter tubes. Feed can be sup- reactor in reaction engineering terms. Recycle may also be
plied continuously at one location in the loop and product used in other configurations involving a separation step. In
withdrawal at another. plug flow some percentage of the effluent from the reactor
is mixed back into the feed stream. The reason for this
is to control certain desirable reaction kinetics. The more
2. Classification
recycle in a plug flow reactor, the closer the operation is to
Despite its complex construction, the loop is essentially a a stirred-tank reactor. Therefore, with recycle it is possible
stirred-tank reactor. By recirculating fast enough the sys- to operate at any condition between the values predicted
tem can be considered well mixed. For this to be the case, by either CSTR or PFR. There is no advantage in operating