Page 83 - Analysis, Synthesis and Design of Chemical Processes, Third Edition

(2.3)

For the hydrodealkylation process introduced in Chapter 1, the following values are obtained for the most
costly reactant (toluene) from Table 1.5:

The single-pass conversion tells us how much of the toluene that enters the reactor is converted to
benzene. The lower the single-pass conversion, the greater the recycle must be, assuming that the
unreacted toluene can be separated and recycled. In terms of the overall economics of the process, the
single-pass conversion will affect equipment size and utility flows, because both of these are directly
affected by the amount of recycle. However, the raw material costs are not changed significantly,
assuming that the unreacted toluene is separated and recycled.

The overall conversion tells us what fraction of the toluene in the feed to the process (Stream 1) is
converted to products. For the hydrodealkylation process, it is seen that this fraction is high (99.3%). This
high overall conversion is typical for chemical processes and shows that unreacted raw materials are not
being lost from the process.

Finally the yield tells us what fraction of the reacted toluene ends up in our desired product: benzene. For
this case, the yield is unity (within round-off error), and this is to be expected because we have not
considered any competing or side reactions. In reality, there is at least one other significant reaction that
can take place, and this may reduce the yield of toluene. This case is considered in Problem 2.1 at the end
of the chapter. Nevertheless, yields for this process are generally very high. For example, Lummus [10]

quotes yields from 98% to 99% for their DETOL, hydrodealkylation process.

We can also look at the conversion of the other reactant, hydrogen. From the figures in Table 1.5 we get
the following: