21.3.4 Technical questions
1. Despite removing H2 and CH4 from the phase separator, why again we remove them from the gas stripper unit ?
Ans: This is due to the basic problem in the difficulty of sharp equilibrium factors which do not exist for these components. The equilibrium separation factors in a phase separator unit where the governing pressure and temperature dictate the distribution of components in the liquid and gas stream. Mostly methane and H2 are removed as vapour stream in the unit. But still at the pertaining pressure and temperature of the phase separator, some lower order alkanes stay back in the liquid stream. Therefore, gas stripping is carried out to remove these.
2. Comment upon fuel efficiency in the process and possibilities to enhance it ?
Ans: The process should indicate maximum fuel efficiency. This can be achieved by
- 1. Heat integration of feed and product streams to the reactor.
2. Re use of fuel gas streams as a fuel in the pre-heating furnace.
If these two options are followed, then the process has maximum fuel efficiency and can be regarded to be very energy efficient.
3. Do you suggest to put one more distillation column for the heavy end compounds. If so why ?
Ans: Yes, putting one more distillation column will separate toluene + diphenyl from other heavy end compounds. While diphenyl acts towards favouring forward reaction, the other heavy end compounds could tend to produce more coke during the reaction. Therefore, one more distillation unit to separate toluene+diphenyl from the heavy end compounds is recommendable.
4. Do you suggest any other alternative to eliminate the water cooler and even enhance the energy efficiency of the process ?
Ans: Yes, it is possible, but existing temperatures of the gas stripper bottom sections need to be carefully analyzed. This is also due to the fact that phase separator operates at 450 psig and 100oF. Therefore, heat integration with gas stripper or fractionator reboilers can be beneficial and this way the process can be made even more energy efficient.
5. What is the basic problem of H2 in the reactor ?
Ans: Hydrogen causes embrittlement due to severe adsorption on the metal surface at higher pressures. Therefore, chrome steel is used to avoid these embrittlement problems as material of construction for the reactor.
6. Why is methane purged using the gas purge stream from the phase separator ?
Ans: The reaction kinetics dictate the maximum concentration of methane in the gas stream entering the reactor. Therefore, to achieve maximum conversion, methane is purged.
References:
1. Dryden C. E., Outlines of Chemical Technology, East-West Press, 2008
2. Kirk R. E., Othmer D. F., Encyclopedia of Chemical Technology, John Wiley and Sons, 1999-2012