25.2.4 Technical questions

1. Why is benzene separately vaporized in excess air ?

Ans: To avoid the formation of explosive compositions, benzene is separately vaporized and then mixed with air. The lower flammability limit of benzene-air mixture is 1.35% and the upper flammability limit of benzene is 6.35%. Therefore, with such low levels of flammability limits, heating the benzene in hot process air can keep the process safe. Directly generating benzene vapors and mixing it with air is more dangerous.

2.  Explain in brief how azeotropic distillation of maleic acid is carried out to generate maleic anhydride ?

Ans: The aqueous maleic acid is fed to the azeotropic distillation column which is fed with an azeotropic agent such as xylene. The water is removed along with xylene as overhead vapors as a minimum boiling heterogenous azeotrope as the top product and the bottom product is the maleic anhydride. The heterogeneous azeotrope can be easily separated from the xylene with gravity settling principle and the recovered xylene is fed back to the distillation column as the reflux stream.

3.  Explain why waste steam recovery boiler is kept before heat integrated exchanger from the vapour products perspective ?

Ans: The reaction is highly exothermic and heating the feed to a high temperature is not desired. Had it been so, the waste steam recovery boiler would be kept after the heat integrated exchanger. This way, we generate good quality process steam from waste heat recovery boiler.

4.  Explain how the usage of centrifuge is justified from process technology perspective ?

Ans: From physical property data, the solubility of maleic acid and fumaric acids are 68 g/100 ml water and 0.63 g/100 ml water respectively. This indicates that while maleic acid is soluble in water, fumaric acid is not. Therefore, after isomerisation reaction, the fumaric acid solids rich solution is sent to a centrifuge so as to separate the solids from the water. This clearly indicates how physical properties play an important role in choosing the separation process.

5.  Can the process steam generated from the waste heat recovery boiler be also used for reboiler requirements in the azeotropic distillation unit, benzene vaporizer, vacuum distillation unit etc ?

Ans: Yes, this way we reduce the total utility requirements in the process. In fact the exact steam requirements (fresh) cannot be estimated unless we have a good idea of the total energy balances for the system.

6.  Why is vacuum distillation required for maleic anhydride refining ?

Ans: Maleic anhydride tends to polymerize upon aging and heating. Therefore, heating the maleic anhydride in vacuum conditions reduces the boiling point. Also, maleic anhydride is very corrosive and corrosiveness of any compound enhances with temperature. With all these limitations, vacuum distillation would suite the requirement.