Page 81 - Analysis, Synthesis and Design of Chemical Processes, Third Edition

Profit Margin

Profit Margin = (51.25 + 4.83) – (59.62 + 2.00) = –$ 5.54 or –$ 0.060/kg toluene

Based on this result, we conclude that further investigation of this process is definitely not warranted.

Despite the results illustrated in Example 2.1, benzene has been produced for the last 50 years and is a
viable starting material for a host of petrochemical products. Therefore, how is this possible? We must
conclude that benzene can be produced via at least one other route, which is less sensitive to changes in
the price of toluene, benzene, and natural gas. One such commercial process is the disproportionation or
transalkylation of toluene to produce benzene and a mixture of para-, ortho-, and meta-xylene by the
following reaction.

2C H → C H + C H
7 8
6 6
8 10
toluene benzene xylene
The profit margin for this process is given in Example 2.2.

Example 2.2

Evaluate the profit margin for the toluene disproportionation process.

From Table 6.4, we have

Mixed Xylenes = 0.608 $/kg

Using 2 kmols of toluene feed as a basis

Cost of Raw Materials

184 kg of Toluene = (184 kg)($0.648/kg) = $ 119.23

Value of Products

78 kg of Benzene = (78 kg)($0.657/kg) = $ 51.25

106 kg of xylene = (106 kg)($0.608/kg) = $ 64.45

Margin

Profit Margin = 64.45 + 51.25 – 119.23 = –$3.53 or –$0.019/kg toluene feed

Based on the results of Example 2.2, the production of benzene via the disproportionation of toluene is
better than the toluene HDA process but is still unprofitable. However, a closer look at the cost of
purified xylenes (from Table 8.4) shows that these purified xylenes are considerably more valuable
(ranging from $0.805 to $2.91/kg) than the mixed xylene stream ($0.608/kg). Therefore, the addition of a
xylene purification section to the disproportionation process might well yield a potentially profitable
process—namely, a process that is worth further, more-detailed analysis. Historically, the prices of