Q & A > Question Details
Recently we carried out liquid phase sulfiding instead of gas phase sulfiding of freshly loaded hydrotreating & hydrocracker catalyst in the hydrocracker unit. Liquid phase sulfiding done with DMDS in light diesel oil & 50: 50 hydrogen/nitrogen pressure. After sulfiding phase over & unit feed cut-in with vacuum gas oil ex. vacuum distillation unit we encountered severe problem with sulfur in H2S form detected in light naphtha product (C5-135 deg C range) coming out from stabilizer column. Pl note fractionator column ovhd goes to D-ethanizer & stabilizer column after ovhd gas compression in three stg compressor to separate out fuel gas, LPG fraction & Light naphtha product.What may be the probable reason for H2S sulfur in high concentration (> 700 ppm) in light naphtha product? Is there any possibility of sulfur stripped out from liquid phase sufided catalyst?
13/07/2010 A: Randy Alexander, EURECAT, randy.alexander@eurecat.com
As the previous post stated, the excess H2S is probably coming from labile sulfur that was adsorbed on the catalyst during sulfiding but not converted into active metal sulfides.
A solution to prevent this phenomena on future startups is the use of ex-situ preactivation such as Eurecat's Totsucat process. With preactivation, very little H2S will evolve from the catalyst during startup since the sulfided catalyst is stripped with nitrogen after the activation step to remove any unconverted sulfur. The active metal sulfide phase that is formed when the catalyst's metal oxides are converted is very stable during startup conditions and will not be "stripped" from the catalyst.
07/07/2010 A: keith bowers, B and B Consulting, kebowers47@gmail.com
Yes--there may be (is likely) some DMDS adsorbed on the catalyst that was not stripped out by a hot hydrogen circulation step. However, that will diminish quickly.
What is the sulfur balance? How much is going where? Is the H2S primarily distributed to the LPG? or?
What is the yield of light naphtha vs balance of feed. There IS ALWAYS some H2S in the light naphtha, even when the 'stabilizer' is appropriately re-boiled. The main fractionator may not be performing to spec and be leaving a lot of H2S in the top end liquid rather that stripping it overhead with the LPG (it behaves much like C3) .
You MAY have to 'undercut' the light naphtha to a lower vapor pressure to better strip out the H2S. Without a good material balance and stream compositions it is not possible to better judge where the mal-operation(s) are.