Page 176 - Analysis, Synthesis and Design of Chemical Processes, Third Edition

It has been proposed that we handle the hydrogen/methane stream in the same manner that we handled the
toluene/benzene stream. We recall that the unreacted toluene was separated from the benzene product and
then recycled. It is proposed that the methane be separated from the hydrogen. The methane would then
become a process by-product and the hydrogen would be recycled. Discuss this proposal using the
arguments provided in Tables 6.1 and 6.2.

To use distillation for the separation of methane from hydrogen, as was used with the toluene/benzene,
requires a liquid phase. For methane/hydrogen systems, this requires extremely high pressures together
with cryogenic temperatures.

If the hydrogen could be separated from the methane and recycled, then the reactor feed would not contain
significant quantities of methane, and the large excess of hydrogen could be maintained without the steep
cost of excess hydrogen feed. Note that the overall conversion of hydrogen in the process is only 37%,
whereas for toluene it is 99%.

Alternative separation schemes that do not require a liquid phase (e.g., a membrane separator) should be
considered. The use of alternative separation technologies is addressed further in Chapter 12.

6.4.2 Evaluation of High-Pressure Phase Separator V-102

This vessel separates toluene and benzene as a liquid from the noncondensable gases hydrogen and
methane. The reactor product is cooled and forms a vapor and a liquid stream that are in equilibrium. The
vapor-liquid equilibrium is that at the temperature and pressure of the stream entering V-102. From
Tables 6.1 and 6.2, we conclude that the lower temperature (38°C) was provided to obtain a liquid phase
for the vapor-liquid equilibrium. The pressure was maintained to support the formation of the liquid
phase. Because the separation can be affected relatively easily at high pressure, it is worthwhile
maintaining V-102 at this high pressure.

6.4.3 Evaluation of Large Temperature Driving Force in Exchanger E-101

There is a large temperature driving force in this exchanger, because the heating medium is at a
temperature of approximately 250°C, and the inlet to the exchanger is only 30°C. This is greater than the
100°C suggested in Table 6.4. This is an example of poor heat integration, and we will take a closer look

at improving this in Chapter 15 (also see the case study presented in Chapter 28).

6.4.4 Evaluation of Exchanger E-102

Stream 9 is cooled from 654°C to 40°C using cooling water at approximately 35°C. Again this is greater

than the 100°C suggested in Table 6.4, and the process stream has a lot of valuable energy that is being
wasted. Again, we can save a lot of money by using heat integration (see Chapter 15).

6.4.5 Pressure Control Valve on Stream 8