Page 175 - Analysis, Synthesis and Design of Chemical Processes, Third Edition

Ratio of Reaction Rates = exp[-E/R{1/T – 1/T }]
1
2
= exp[148100/8.314{1/673 – 1/873}] = 430

The size of a reactor would increase by nearly three orders of magnitude if the reaction were carried out
at 400°C (the critical temperature for materials selection, Table 6.1) rather than 600°C. Clearly the effect
of temperature is significant.

Most reactions are not kinetically controlled as is the case here. In most cases the rate is controlled by
heat or mass transfer considerations. These are not as sensitive to temperature changes as chemical
reaction rates. For more detail, see Chapter 20.

High-Pressure Concern (see Table 6.2). For gas phase reactions, the concentration of reactants is
proportional to the pressure. For a situation where the reaction rate is directly proportional to the
concentration, operation at 25 bar rather than at 1 bar would increase the reaction rate by a factor of 25
(assuming ideal gas behavior). Although we do not know that the rate is directly proportional to the
concentration, we can predict that the effect of pressure is likely to be substantial, and the reactor size
will be substantially reduced.

Non-stoichiometric Feed (see Table 6.3). The reactor feed contains both excess hydrogen and the
reaction product methane.

Methane in the Feed. The effect of methane is to reduce the reactant concentrations. This decreases the
reaction rate and represents a negative impact. The methane could possibly reduce the formation of side
products, but we have no information to suggest that this is the case.

Excess Hydrogen in the Feed. The large amount of excess hydrogen in the feed ensures that the
concentration of hydrogen will remain large throughout the reactor. This increases the reaction rate.
Although there is no information provided regarding the decision to maintain the high hydrogen levels, it
may be linked to reducing the formation of side products.

With the exception of the presence of methane product in the feed, the high-temperature operation, the
excess hydrogen, and the elevated pressure all support an increase in reaction rate and a reduction in
reactor volume. This suggests that the catalyst is not “hot”—that is, the catalyst is still operating in the
reaction-controlled regime and mass transfer effects have not started to intrude. For these conditions, the
manipulation of temperatures and pressures is essential to limit the reactor size.

There is a significant economic penalty for using more than 400% excess hydrogen in the reactor feed.
The raw material cost of hydrogen would be reduced significantly if excess hydrogen were not used. The
fact that this large excess is used in spite of the economic penalty involved suggests that the hydrogen
plays an important role in the prevention of side products. The concept of selectivity is discussed further
in Chapter 20.

The presence of methane in the feed has not yet been resolved. At best it behaves as an inert and occupies
volume that must be handled downstream of the reactor, thus making all the equipment larger and more
expensive. This question is considered in more detail in Example 6.6.

Example 6.6