Applications of Chemical Kinetics in Environmental Systems - Elements of Environmental Engineering: Thermodynamics and Kinetics - page 402

(CO 2 and NO x ) . Figure 6.75 is the overall process flow sheet. The stream that consists
of 200 kg/h of benzene and toluene is introduced first into an absorption column where
nonvolatile absorption oil is used to strip the benzene and toluene from the stream. The
oil is distilled to separate the benzene and toluene for reuse within the manufacturing
process. The oil is then recycled into the absorption column. The primary emissions
from the process occur through the vents, storage tanks, and other fugitive sources and
consist of traces of benzene, toluene, and utility gases (CO 2 ,NO x ) . These emissions
are dependent on the oil flow rate through the absorption column. We need to determine
the overall environmental impacts from this process and how they can be minimized
as we change the process parameter, namely, oil flow rate. The methodology described
here follows closely that given in Allen and Shonnard (2002).

Vent
(recovery
of Bz, Tol)

Vent

Oil

Products

Benzene,
toluene
200 kg/h

Absorption

Distillation

FIGURE 6.75 Overall process flow sheet for a typical chemical manufacturing
process.

Table 6.20 lists the various emission rates resulting from the process schematic given
in Figure 6.75. Ideally, one would use a commercial process simulator such as HYSIS,
ASPEN, or SimSci to carry out mass and energy balances on the various system com-
ponents and calculate the emission rates of different species generated in the process.
In doing so, one will have to use the standard EPA emission factors that are listed for
specific pollutants. We have circumvented that process to generate Table 6.20 using
hypothetical values for the process as an illustration of the calculation of environmen-
tal impact indices. The emission rate for benzene rapidly decreases with increasing oil
flowrate,whereasthatfortoluenedecreasesmuchmoreslowly.Withincreasingoilflow
rates, the utility systems emit more of CO 2 and NO x . Typical physicochemical proper-
ties required for the calculation will have to be obtained from thermodynamic arguments
presented in Chapters 2 and 3 or from standard tables such as given in Appendix 1.