20.2.9 Technical questions
1. What solvent can be used in the quenching process ?
Ans: HBr can be readily absorbed into water. Therefore, water can be used as a solvent in the quenching process. There is a patent available where it says that acetone can be used to recover HBr. More details can be obtained of this patent by furthering the search on the patent data base using the internet.
2. In the process flow sheet given the HBr recovery is not shown. Do you think the flow sheet is correct. Suggest any modifications to the flow sheet ?
Ans: Yes, the flowsheet has an error in that sense. The flowsheet should give a section for HBr recovery where HBr is recovered in the quenched solvent and the fresh HBr gas is allowed to enter the pyrolysis furnace along with the steam. In such case, one has to put an additional HBr stripper which can strip HBr from the HBr + solvent stream that is fed to this unit and the HBr can be sent to the pyrolysis unit. The fresh solvent can be then sent to a cooler and sent to the quench tower and the existing recycle stream to the quenching tower will not be there. Instead, this stream will go to the stripper as a feed stream.
3. Can we not do heat integration partially for the pyrolysis furnace with the vapour products ?
Ans: HBr is very corrosive and therefore, if we go for heat integration using the vapour product, then the shelf life of the pyrolysis furnace will be minimized. Therefore, better we don't go for heat integration in this way.
4. Is a purge stream required in the process flowsheet shown for the quench fluid + HBr mixture ?
Ans: Yes, and this is not shown in the process flow sheet. Some impurities such as some light ends that might dissolve with the solvent will have to be purged, if not these can cause coking problems and can cause unwanted organics in the pyrolysis furnace reactor.
5. What sort of regeneration would you recommend for the isomerisation catalyst ?
Ans: Heating the catalyst in oxygen or air to remove coke would be beneficial to regain the activity of the isomerisation catalytic bed.
6. Suggest what changes will be there in the process flow sheet if pure propylene instead of a mixture of propylene, propane and C1-C2 components is used ?
Ans: The C1-C2 tower and C3 towers can be replaced with a single tower after flash unit. This new unit will separate all light ends in trace amounts in a single tower with middle product being 2-methyl-1-pentene and heavy ends as bottom product. Therefore, a complex distillation unit arrangement would be at the maximum required replacing three distillation columns in series.