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I work on a vacuum gas oil hydrotreater(Axens licensed) unit. We have a water injection system at the upstream of AK-001 to absorb the ammonium salts, along with water injection. We also add hydrocarbon drawn from a cold LP separator at the upstream of AK-001. I just want to know the purpose of hydrocarbon washing? What happens if hydrocarbon washing is not implemented? We don't have a hydrocarbon washing system in the diesel hydrotreater unit.
 
Answers
15/09/2020 A: Nagarathinam S Murthy, Ashphil Consultancy, Chennai, nssvdvr@gmail.com
It is not clear from the question whether the AK - 001 is on the HP loop or in the LP loop of the VGO HT. On the assumption that AK - 001 is on the HP loop prior to routing hydrogen rich recycle gas into the amine system via the cold flash drum, recycle of liquid HC ( predominantly naphtha and diesel fraction) from the cold LP separator can be for the following reasons:
1. During SOR conditions, the production of light ends in VGO HT will be low and any retention of C1+ HC from recycle gas (which is H2 rich gas) prior to the amine absorber will be an advantage to minimize C4+ from entering the amine column.
2. Also, recycling of light HC will help to wash off heavier HC that might have been entrained from the HP flash drum which, during peak winter months, could precipitate wax in Hy HC. As such, wash water can not remove the wax formed when its melting temp is more than say 45 degC. Further, wax on deposition leads to back pressure issues which will hurt the recycle gas performance and overall hydrogen availability in the reactor.
By the way, why not ask the licensor?
14/09/2020 A: Ed Ouwerkerk, Catalyst Intelligence Sarl, ouwerkerk@catalyst-intelligence.com
You correctly state this is a washing stream, because it absorbs C1-C4 and H2S from the recycle gas improving the H2 pressure in your reactor (and thus reduces the catalyst volume required for a given conversion and cycle length). In a smart design such a wash oil loop does not need an extra (high pressure) pump, because it would use the spare feed pump. It does however increase your physical losses of H2.
11/09/2020 A: Jake Gotham, InSite Technical Services, jake.gotham@insitetechnical.com
Some units use this approach to ‘wash’ H2S out of the high-pressure loop. Increasing the liquid : volume ratio in the high pressure separator increases the amount of H2S dissolved in the liquid. Most of this H2S flashes off in the low-pressure separator and will be captured in the low-pressure offgas amine scrubber. This approach avoids the cost and pressure drop of an amine scrubber in the high-pressure loop, though it is less effective and adds an additional high-head pump to the flowsheet.

If the wash recycle is stopped, H2S will build up in the high-pressure loop which will reduce sulphur removal to some extent. If necessary, this could be counteracted by bleeding some off-gas from the high pressure loop.