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I have a query regarding distillation. How is it decided whether steam should be used or a re-boiler should be used in a distillation column. I am in DHDS unit. The finished diesel stripper uses direct steam injection as stripping media where as the stripper in Pre de-sulfurisation unit of Hydrogen generation unit (steam reforming) contains a reboiler. Both columns intention is to strip out light ends and also some amount of H2S.
 
Answers
09/09/2010 A: S Banik, Centre for High Technology, sbdr@rediffmail.com
Basic difference between reboiled stripper and steam stripper is that in the former energy is separating agent while later uses mass separating agent. Bottom temperature in the later would be lower. However comprehensive answers have already been given by Ragsdale and Vetters. Articles by Dr Suphanit of UMIST(1999) may be refered for better understanding.
08/09/2010 A: Eric Vetters, ProCorr Consulting Services, ewvetters@yahoo.com
There also some other practical considerations. The temperature in a reboiled stripper is limited to the temperature you can achieve with your steam supply. Something like diesel would be very difficult to reboil with a steam reboiler because the temperature is too high. To reboil something that heavy would require either hot process stream from another source or a fired heater - both of which would be quite expensive.

Steam stripping works differently though. It causes light ends to flash off from a saturated liquid by lowering the hydrocarbon partial pressure. It can work therefore even when the process fluid is hotter than the steam.
Another significant hidden cost factor that Mr. Ragsdale did not mention is that stripping steam generates sour water. In addition to the cost of generating the steam, there is capital and operating cost associated with treating the sour water produced by the steam stripping operation.
Need for dry feed is often the major determining factor for going with reboiled strippers. For instance the naphtha stripper on a naphtha desulfurizer making catalytic reformer feed needs to be reboiled because the reformer cannot tolerate a "wet" feed (moisture strips the chlorides off of the catalyst).
07/09/2010 A: Ralph Ragsdale, Ragsdale Refining Courses, ralph.ragsdale@att.net
It is not clear if your diesel stripper is a full column or a side stripper. Considering side strippers in general, and comparing “live steam” stripping vs. steam reboilers: In deciding which to use, we have the following factors:
• Capital cost
• Operating cost
• Metallurgy implications
• If the stripped product needs to be water-free
Your SMR pretreater product stripper could have been “live steam” stripped, because the product can be wet when steam is combined with the hydrocarbon feed to the SMR. For your diesel hydrotreater, the diesel product does not need to be as dry as jet/kero, for example, so live steam will work there as well. Before hydrotreaters began to be used for kerosene, it was common practice for the atmospheric crude column to have live steam strippers on all side strippers except for the kero stripper, which would have a steam reboiler, so that the product could be yielded dry. Alternatively, it, too, would have live steam with the stripped product sent to a salt drier. Kero must be dry, particularly if jet fuel.
The other three factors can be elaborated on as well if desired.