Page 173 - Analysis, Synthesis and Design of Chemical Processes, Third Edition
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stoichiometric feed conditions separately.
High-Temperature Concern (see Table 6.1). Figure 6.1 provided the important information that the
reaction is exothermic. Table 6.1 notes that for an exothermic reaction, the result of increasing
temperature is a reduction in equilibrium conversion. This is confirmed by the plot of the equilibrium
constant versus temperature given in Figure 6.1. The decrease in the equilibrium conversion is
undesirable. The actual conversion for the HDA process is compared with the equilibrium conversion in
Example 6.3.
Example 6.3
For the PFD presented in Figure 1.5,
a. Calculate the actual conversion.
b. Evaluate the equilibrium conversion at 600°C.
Assuming ideal gas behavior: K = (N benzene N methane )/(N toluene N hydrogen )
p
where N represents the moles of each species at equilibrium.
Information on the feed stream to the reactor from Table 1.5 (Stream 6 on Figure 1.5):
Hydrogen
735.4 kmol/h
Methane
317.3
Benzene
7.6
Toluene
144.0
Total
1204.3
c. Actual Conversion: Toluene in exit stream (Stream 9) = 36 kmol/h
Conversion = (144 – 36)/144 = 0.75 (75%)
d. Equilibrium Conversion at 600°C. From Table 6.6 @600°C K = 265
p
Let N = kmol/h of benzene formed
265 = [(N + 7.6)(N+ 317.3)]/[(735.4 –N)(144 –N)]
N = 143.6
Equilibrium Conversion = 143.6/144 = 0.997 (99.7%)
The equilibrium conversion for the hydrodealkylation reaction remained high in spite of the high
temperature. Although there is no real problem with using the elevated temperature in the reactor, it
cannot be justified from a thermodynamic point of view.
High-Pressure Concern (see Table 6.2). From the reaction stoichiometry, we see that there are equal
numbers of reactant and product moles in the hydrodealkylation reaction. For this case, there is no effect
of pressure on equilibrium conversion. From a thermodynamic point of view there is no reason for the
high pressure in the reactor.
Non-stoichiometric Feed (see Table 6.3). The component feed rates to the reactor (see Example 6.3)
show that
