24.2.4 Technical questions
1. Explain how azeotropic distillation unit functions to convert wet benzene to dry benzene ?
Ans: Wet benzene upon heating produces a heterogenous azeotrope at the top and dry benzene at the bottom. Therefore, the unit upon condensation of the top vapors produces two streams namely water and benzene rich wet stream which is recycled back as the reflux stream.
2. Why is the benzene + ethylbenzene stream returned back to the cooler but not the alkylation reactor ?
Ans: The alkylation reactor should be fed with very important chemicals only. It is possible that the temperatures prevailing in the dealkylator could enable the loss of polyalkylbenzenes to the vapour. Therefore, there is no point in feeding this stream to the alkylator. Instead the stream is sent to the cooler so that any polyalkylbenzenes could be condensed back and sent to the dealkylator again.
3. Why the AlCl3 complex is partially returned to the reactor ?
Ans: To maintain the required catalyst conditions. If not, then AlCl3 fresh has to be provided to the reactor as AlCl3 forms a complex with the hydrocarbons and would leave the alkylator along with the product streams.
4. Why do polyalkylbenzenes enter the ethylbenzene rich product ?
Ans: This is the basic problem of the equilibrium separation factors of polyalkylbenzenes between the AlCl3 complex rich product and the ethylbenzene rich product. Since sharp distribution of these compounds is not possible, polyalkylbenzenes get distributed between both these organic phases.
5. Why there are ethylbenzene + benzenes still available in the bottom product of the stripper ?
Ans: The answer is same as that of question 3 i.e., the phase equilibrium limitations enable the availability of both benzene + ethylbenzene in the polyalkylbenzene stream.
6. Why the benzene is wet from the benzene column ?
Ans: This is because caustic wash operation enabled the contact with water and some water will enter the organic phase due to phase equilibrium of water with the organic phase. This water therefore enters the benzene stream from the benzene column.
7. Why caustic wash followed by drying is carried out is carried out for the ethylbenzene stream ?
Ans: Caustic wash removes any undesired impurities where as dryer removes the water. Both water and acid are important compouds which if not eliminated contribute significantly to side reactions during the heating process with superheated steam. Therefore, totally dry and acid free conditions are targeted for ethylbenzene.
8. Why vacuum is used in the polyalkyl still unit ?
- 1. To enhance relative volatility of components that can be easily removed.
2. To reduce dissociation of polyalkyl components in the column.
9. Why steam quenching but not water quenching is adopted ?
Ans: Water quenching can give rise to additional reactions. Steam quenching does not allow condensation and hence is safe in that sense not to trigger any unwanted reactions of the styrene and ethylbenzene.
10. What gases are removed in the vent following quenching tower ?
Ans: Gases such as hydrocarbons that are resultant of undesired cracking are removed as non-condensibles in the partial condenser. These are vented out.
11. Why tar consist of styrene ?
Ans: Styrene being organic compound has affinity to get dissolved in the tar. Therefore, the batch still is used to extract styrene from the tar.
12. Why vacuum is used for the separation of benzene, toluene, ethylbenzene and styrene ?
Ans: All these compounds are closely boiling systems. By going for vacuum distillation, we are able to enhance the relative volatility of the components and hence better separation. The increasing order of higher vacuum levels is applied for these three columns in series i.e., benzene column, ethylbenzene column and finishing column.