Page 131 - Materials Chemistry, Second Edition

114 Practical Design Calculations for Groundwater and Soil Remediation

configuration, and sizing of reactors. This chapter covers the topics on how
to determine the rate constant, removal efficiency, optimal arrangement of
reactors, required residence time, and reactor size for specific applications.

4.2 Mass-Balance Concept

The mass-balance (or material balance) concept serves as a basis for design-
ing environmental engineering systems (reactors). The mass-balance con-
cept is nothing but conservation of mass. Matter can neither be created
nor destroyed (a nuclear process is one of the few exceptions), but it can be
changed in form. The fundamental approach is to show the changes occur-
ring in the reactor by the mass-balance analysis. The following is a general
form of a mass-balance equation:
 Rate of mass 
 Rate of mass   Rate of mass  Rate of mass  
  =   −   ±  GENERATEDor
  ACCUMULATED    IN   OUT     DESTROYED   

 
(4.1)
Performing a mass balance on an environmental engineering system is
just like balancing a checking account. The rate of mass accumulated (or
depleted) in a reactor can be viewed as the rate that money is accumulated
in (or depleted from) the checking account. How fast the balance changes
depends on how much and how often the money is deposited and with-
drawn (rate of mass input and output), interest incurred (rate of mass gener-
ated), and bank charges for service and ATM fees (rate of mass destroyed).
In using the mass-balance concept to analyze an environmental engi-
neering system, we usually begin by drawing a process flow diagram and
employing the following procedure:

Step 1: Draw system boundaries or boxes around the unit processes/
operations or flow junctions to facilitate calculations.
Step 2: Place known flow rates and concentrations of all streams, sizes
and types of reactors, as well as operating conditions such as
temperature and pressure on the diagram.
Step 3: Calculate and convert all known mass inputs, outputs, and
accumulation/disappearance to the same units and place them
on the diagram.
Step 4: Mark unknown (or the ones to be found) inputs, outputs, and
accumulation/disappearance on the diagram.