Q & A > Question Details
I am working in hydrocracker unit. Since two month LPG is getting failed due to positive H2S and in copper corrosion test. We are maintaining debutanizer bottom and top temperature 186 & 78 degree C and pressure 16.5 kg/cm2g. Our LPG r/d flow is10-12 m3/hr while lean amine flow is 20-25 m3/hr. When we check caustic strength found 20-22% which is quite normal. Generally we change caustic in 5-7 days.
Our system is like this LPG comes at top of debutanizer and goes in amine absorber at bottom and lean amine mix at top of absorber after amine wash goes to water wash amine settler where water circulation is done by pump before going to water wash LPG and water is mixed and mixed in mixer. After water wash LPG goes to caustic wash tank through nozzle but no circulation pump is there (Tank). After caustic wash LPG goes to sand filter and then goes to Deethaniser for FG removal and cooled in Water cooler and goes to LPG storage tank. Please suggest the solution.



 
Answers
13/07/2016 A: Lindsay McRae, Pall Corporation, Lindsay_McRae@pall.com
Sand filters are not particularly effective at removing water/ caustic from LPG. Sand filters are effective when IFT is >20 dynes/cm however due to presence of caustic traces the Interfacial Tension (IFT) may be in 3-5 dynes/cm range and even as low as 0.5 dynes/cm in some cases.
High efficiency Liquid Liquid coalescers such as Pall's PhaseSep have been proven to be able to remove entrained water/caustic to <15 ppmw in LPG ( and reach Na+ to <1ppb levels).
You may also want to check that there is no amine carryover from amine tower to the caustic treater.
12/07/2016 A: Sudhakara Babu Marpudi, Dangote Oil Refinery Company, m_sudhakarababu@yahoo.com
Is the system description right. LPG goes to Amine wash for 95% H2S removal. The amine traces that entrains in to LPG at the outlet of Amine absorber are rinsed off before routing the LPG to Caustic prewash for absorbing the remaining H2S. The caustic traces entrained into LPG gets washed in a water wash column (normally) and then the water traces are removed in the sand filters. from the description in the question, it appears that there is not water wash between Caustic prewash and sand filter. In such cases, sand filter gets saturated with caustic entrained with LPG. When LPG is passed thru the sand filter which is saturated with Caustic traces (which has absorbed H2S) Copper strip will fail. The root cause appears to be entrainment with LPG (Amine from Amine absorber, Caustic from Prewash) which could be due to the higher LPG load. Replacing the sand with fresh sand should give temporary relief. But sand filters mainly remove water traces from LPG (not Caustic) and so gets saturated with caustic quickly calling for frequent sand replacements.
09/07/2016 A: Raghu Raj Singh, IOCL Haldia Refinery, singhrr@indianoil.in
We are using MDEA for H2S in amine absorber.
08/07/2016 A: Raghu Raj Singh, IOCL Haldia Refinery, singhrr@indianoil.in
MD EA is used in amine absorber.
08/07/2016 A: Sabapathi RATHINA, KNPC, k.sabapathi@gmail.com
Check the all the other sulphur species like cos , cs2.rsh etc
08/07/2016 A: NS Murthy, Suez, murthy.ns.ext@suez.com
The processing sequence looks some what unique. Typically the de-ethanizer is placed ahead of LPG column (de-butanizer) so that all C2s and H2S gets knocked out before LPG from debut column is taken to amine wash followed by caustic wash. Even 0.5ppm pf H2S can cause such failure as there will be practically no other sulphur species in hydro-cracker LPG. It looks to be one or more of the following that have caused the Cu strip failure.
1. Check for amine / caustic presence in the LPG ex de-ethanizer column.
2. Check the de-ethanizer operation to strip off H2S totally. Lower pr, higher reflux can assist effective stripping,
3. Check whether any other stream is joining LPG stream prior to sample point. We had one such experience and refiners blamed Merox treated LPG while the actual culprit was from CCR unit which did nor have any caustic treatment. In this case there was Sulphur ingress into CCR Platformer.
Alt, simply use H2S scavenger which can act as a masking agent and get the Cu corrosion test pass. As such there is no issue with such when LPG is used in domestic use. GE will be willing to supply H2S masking agent.
08/07/2016 A: utkarsh mishra, Ingenero Technologies, utkarsh1612mishra@gmail.com
What type of amine are you using? It plays a key role in determining the extent to which absobtion of H2S from LPG will take place??
Generally speaking,
Monoethanolamine: About 20 % for removing H2S and CO2, and about 32 % for removing only CO2.
Diethanolamine: About 20 to 25 % for removing H2S and CO2
Methyldiethanolamine: About 30 to 55% % for removing H2S and CO2
Diglycolamine: About 50 % for removing H2S and CO2.
08/07/2016 A: utkarsh mishra, Ingenero Technologies, utkarsh1612mishra@gmail.com
What type of amine are you using? Type of amine plays a key role in determining the extent to which H2S will get absorbed from LPG (i.e.
Monoethanolamine: About 20 % for removing H2S and CO2, and about 32 % for removing nly CO2.
Diethanolamine: About 20 to 25 % for removing H2S and CO2
Methyldiethanolamine: About 30 to 55% % for removing H2S and CO2
Diglycolamine: About 50 % for removing H2S and CO2.
07/07/2016 A: Ralph Ragsdale, Ragsdale Refining Courses, ralph.ragsdale@att.net
The caustic wash, which is normally before the circulating amine treater, should include a circulating pump, orifice mixers and settler.