Page 130 - Materials Chemistry, Second Edition

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Mass-Balance Concept and Reactor Design

4.1 Introduction
Various treatment processes are employed in groundwater and soil remedia-
tion. Treatment processes are generally classified into physical, chemical, bio-
logical, and thermal processes. A treatment system often consists of a series
of unit operations/processes, which form a process train. Each unit opera-
tion/process contains one or more reactors. A reactor can be considered as
a vessel in which the processes occur. Environmental engineers are often in
charge of, or at least participate in, conceptual and preliminary design of the
treatment system. The conceptual and preliminary design typically includes
selection of treatment processes, determination of reactor types, and sizing
of the reactors.
For system design, treatment processes should be chosen first by screening
the alternatives. Many factors should be considered in selection of treatment
processes. Common selection criteria are implementability, effectiveness, cost,
and regulatory consideration. In other words, an optimal process would be
the one that is implementable, effective in removal of compounds of concern
(COCs), cost efficient, and in compliance with the regulatory requirements.
Once the treatment processes are selected for a remediation project, engi-
neers will then design the reactors. Preliminary reactor design usually
includes selecting appropriate reactor types, sizing reactors, and determin-
ing the number of reactors needed and their optimal configuration. To size
the reactors, engineers first need to know if the desirable reactions or activi-
ties would occur in the reactors and what the optimal operating conditions,
such as temperature and pressure, would be. Information from chemical
thermodynamics and, more practically, a bench- and/or pilot-scale study
would provide answers to these questions. If the desired reactions are feasi-
ble, the engineers then need to determine the rates of these reactions, which
is a subject of chemical kinetics. The reactor size is then determined, based
on mass loading to the reactor, reaction rate, type of the reactor, and target
effluent levels.
This chapter introduces the mass-balance concept, which is the basis
for process design. Then it presents reaction kinetics, as well as types,

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