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we face a big problem. We have a caustic unit to sweetened propane and butane in the gas refinery. For caustic regeneration, after oxidation of rich caustic, it converts to De sulfide oil and lean caustic. Wash oil is used to separate De sulfide oil and caustic because of close density of these two product. Sometimes we don't have any wash oil to use and total sulfur at both products increasing awfully. It means that no mercaptane was separated from propane or butane. Do you have any suggestion to separate De sulfide oil from Caustic without wash oil? Do you know any alternative besides wash oil?
 
Answers
10/02/2011 A: Lindsay McRae, Pall Corporation, Lindsay_McRae@pall.com
Pall Corporation's high efficiency PhaseSep Liquid Liquid coalescer can be used to separate even very stable emulsions where IFT can be as low as 0.5 dynes/cm. We have very good experience removing caustic from disulphide oil using our high efficiency Liquid Liquid coalescers. (used on extractive Merox units sometimes to prevent any caustic carried over in disulphide oil causing issues with HDS catalyst activity when wash oil is sent to HDS).
It make take a small pilot unit to confirm the sizing requirements, and pre-filter size however it would be quite likely that we could get to <20 ppmv disulphide oil in caustic with a PhaseSep Liquid liquid coalescer and pre-filter. It's more normal to use rise with naptha or polygasolene though as disulphide oil is more soluable in wash oil than it is in caustic so it helps in removing disulphide oil to meet residue & corrosion test specs.
We do similar things around caustic treaters routinely for removal of water/caustic from LPG /Kero/ FCC gasolene & even GO. Or vice versa too ( to remove red oil from spent caustic for instance).
31/01/2011 A: Ralph Ragsdale, Ragsdale Refining Courses, ralph.ragsdale@att.net
With all due respect, I do not agree that it means that the caustic wash is not doing its job. Naphtha wash will not remove mercaptans. If the product is off-spec only when the naphtha wash is out of service, it means that you have disulfide re-entry all of the time and that the naphtha wash is necessary for your operation. Can you provide with piping more than one source of naphtha for the wash, for more operating flexibility? I presume there is no problem with the spent naphtha wash that is yielded from the treater. It can be sent to any hydrotreater feed tank.