Page 174 - Analysis, Synthesis and Design of Chemical Processes, Third Edition

1. Toluene is the limiting reactant.
2. Hydrogen is an excess reactant (more than 400% excess).
3. Methane, a reaction product, is present in significant amounts.

Reaction Products (Methane) in Feed. The presence of reaction product in the feed results in a
reduction in the equilibrium conversion (see Table 6.3). However, Example 6.3 shows that at the
conditions selected for the reactor, the equilibrium conversion remained high despite the presence of the
methane in the feed.

Excess Reactant (Hydrogen) in Feed. The presence of excess reactants in the feed results in an
increase in equilibrium conversion (see Table 6.3) . Example 6.4 explores the effect of this excess
hydrogen on conversion.

Example 6.4

(Reference Example 6.3). Reduce the amount of hydrogen in the feed to the reactor to the stoichiometric
amount—that is, 144 kmol/h—and determine the effect on the equilibrium conversion at 600°C.

The calculations are not shown. They are similar to those in Example 6.3(b). The total moles of hydrogen
in the feed were changed from 735.4 kmol/h to the stoichiometric value of 144 kmol/h.

The results obtained were N = 128.8 kmol/h, equilibrium conversion = 0.895 (89.5%).

Example 6.4 reveals that the presence of the large excess of hydrogen had a noticeable effect on the

equilibrium conversion.

We conclude that thermodynamic considerations do not explain the selection of the high temperature, the

high pressure, and the presence of reaction products in the feed. The presence of a large excess of
hydrogen is the only positive effect predicted by thermodynamics.

Consideration of Reaction Kinetics. The information on reaction kinetics is limited in this chapter. We

will present a more detailed description of the kinetics rate expression in a case study in Chapter 20, and
we investigate the predictions made in this chapter with this limited information. However, you will find
that a great deal of understanding can be extracted from the limited information presented here.

From the information provided in Table 6.6 and Chapter 1 we know that
1. The reaction takes place in the gas phase.
2. The reaction is kinetically controlled.
3. There are no significant side reactions.

High-Temperature Concern (see Table 6.1). In a region where the reaction kinetics control, the
reaction rate increases rapidly with temperature, as Example 6.5 illustrates.

Example 6.5

The activation energy for the rate of reaction for the hydrodealkylation of toluene is equal to 148.1 kJ/mol
(Tarhan [2]). What is the reaction rate at 600°C relative to that at 400°C?