Applications of Chemical Kinetics in Environmental Systems - Elements of Environmental Engineering: Thermodynamics and Kinetics - page 403

TABLE 6.20
Typical Air Emission Rates from the Absorption/Distillation Process
Described in Figure 6.75

Air Emission Rate (kg/h)

Absorption Oil Flow Rate (kg mol/h) Benzene Toluene CO 2 NO x

0 200 200 0 0

50 1 150 150 0.2

100 0.05 120 300 0.5

250 0.05 10 700 1.0

500 0.05 0.2 1500 2.0

Table 6.21 lists the various RRIs for individual chemicals needed for the calculation
of overall environmental risk index for each category. Note that the values of GWP,
SFP, and ARP for chemicals are listed in various sources (Allen and Shonnard, 2002).
Similarly, the RfD and RfC values for benzene and toluene are listed by the Agency
for Toxic Substance Disease Registry (http://www.atsdr.cdc.gov/). We can now utilize
Equations 6.282 and 6.283 for the calculation of I GWP , I SFP , and I ARP using the above
values for various values of the oil flow rate given in Table 6.20. For the calculation
of I INHTP and I INGTP , using Equations 6.282 and 6.283, the benchmark chemical used
is benzene in Table 6.21. To do so, we have to calculate the concentrations in air and
water compartments for benzene and toluene in the general environment resulting from
releases from the process. This is done using a fugacity model that is described in
Chapter 3. Typically, a Level III model is required; however, we use a Level II model
here as an illustration. We consider a three-compartment model (air, water, and soil) as
given in Example 3.6. A total emission rate of 0.5 mol/h each of benzene and toluene
to air is assumed so that E = 1 mol/h. All other values are same as given in Example
3.6. Note that the value of E will not affect the calculation of environmental indices in
Equations 6.282 and 6.283 since the concentration ratios of benzene and toluene are
used. The outputs from Level II fugacity calculation are given in Table 6.22 along with
the calculated values of I INGTP i and I INHTP i for benzene and toluene.

TABLE 6.21
Relative Risk Index for Each Chemical in the Process Described in
Table 6.20

Category Benzene Toluene CO 2 NO x

Global warming potential, I GWP i 3.4 3.3 1.0 40

Smog-forming potential, I SFP i 0.42 2.70 — —

Acid rain potential, I ARP i — — — 1.07

RfD for ingestion (mg/kg/d) 0.004 0.2 — —

RfC for inhalation (mg/m 3 )

0.03 0.4 — —

continued