Page 87 - Analysis, Synthesis and Design of Chemical Processes, Third Edition

side reaction will occur. For this process, the yield of toluene for commercial processes is on the order of
98% to 99%. Although this is high, it is still lower than the 100% that was originally assumed. A very
small amount of toluene may react with the hydrogen to form small-molecule, saturated hydrocarbons,
such as ethane, propane, and butane. More important, a proportion of the benzene reacts to give a two ring
aromatic, diphenyl:

2C H C H + H
6 6 12 10 2

benzene diphenyl

The primary separation between the benzene and toluene in T-101 (see Figure 2.1) will remain
essentially unchanged, because the light ends (hydrogen, methane, and trace amounts of C – C 4
2
hydrocarbons) will leave in the flash separators (V-102 and V-103) or from the overhead reflux drum (V-
104). However, the bottoms product from T-101 will now contain toluene and essentially all the diphenyl
produced in the reactor, because it has a much higher boiling point than toluene. It is known that the
benzene/diphenyl reaction is equilibrium limited at the conditions used in the reactor. Therefore, if the
diphenyl is recycled with the toluene, it will simply build up in the recycle loop until it reaches its
equilibrium value. At steady state, the amount of diphenyl entering the reactor in Stream 6 will equal the
diphenyl in the reactor effluent, Stream 9. Because diphenyl reacts back to benzene, it can be recycled
without purging it from the system. The changes to the structure of the process that would be required if
diphenyl were produced are considered in Example 2.5.

Example 2.5

Consider the following two process alternatives for the toluene HDA process when the side reaction of
benzene to form diphenyl occurs.

Clearly for Alternative B, shown in Figure E2.5(b), we require an additional separator, shown here as a
second distillation column T-102, along with the associated equipment (not shown) and extra utilities to
carry out the separation. For Alternative A, shown in Figure E2.5(a), we avoid the cost of additional
equipment, but the recycle stream (Stream 11) will be larger because it now contains toluene and
diphenyl, and the utilities and equipment through which this stream passes (H-101, E-101, R-101, E-102,
V-102, V-103, T-101, E-106) will all be greater. Which is the economically preferable alternative?

Figure E2.5(a) PFD for Alternative A in Example 2.5—Recycle of Diphenyl without Separation (E-101
and H-101 Not Shown)