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I am working as production Engineer in FCC block which consists of FCC and LPG Treater unit.
LPG treater treats cracked LPG from FCC and Delayed coker Unit.IT has capacity of treating 404 m3/hr.We are experiencing a peculiar problem these days. Our LPG is getting contaminated with light yellow colour at the outlet of LPG treater unit. LPG ex FCC and coker unit is clear and having no signs of yellowish colour but at outlet it is failing. Copper strip corrosion and weathering tests are absolutely fine.
The following are the observations --
1) These units are commissioned 6 months ago and we didn't come across DSO formation in DSO settler but LPG quality is ok (Probably due to low mercaptans load in LPG, DSO is not forming in the process)
2) The lean amine H2s loading in FCC & DCU LPG washing is around 1200 -2400 ppm. May be noted here that on earlier occasions with this loading of amine,the downstream treater unit was giving good LPG wrt all lab tests.
3)The only problem is with colouring with which we are struggling.
4) sometimes we experience yellowish colour at the unit inlet of LPG treater.Even at the outlet of FCC & DCU also.But the colour is not that much remarkable.But in treater outlet the pale yellow/dark yellow colour is visible upon complete evaporation of LPG.Sometimes 0.5 to 0.1 ml of yellow residue is left and sometimes not.
5) We took complete shutdown of unit, depressurised, all caustic drained from system and vessels and fibre film contactors were water washed and again took the system inline but the yellowish colour in the system LPG still coming.The custic regenration is good and quality is good and copper corrosion test is ok.
The reason for yellowishness we unable to find out.
Can anyone share their experiences in dealing with this type of typical problem in MERICHEM section of caustic wash of LPG?
 
Answers
12/10/2019 A: Mainak Sarkar, Indianoil Corporation Ltd, sarkarm@indianoil.in
May be due to chloride contamination in CRU LPG
10/07/2018 A: Sudhakara Babu Marpudi, Dangote Oil Refinery Company, m_sudhakarababu@yahoo.com
yellowish color in treated LPG could be a resultant of Caustic carry over. In case the system has a water wash vessel and sand filter after the Disulphide separator, check the performance. If the sand is saturated with the caustic laced moisture, water wash the sand or try replacing with fresh sand.
27/08/2016 A: Leonardo Leite Garcia de Souza, Petrobras SA / Duque de Caxias Refinery, llgs394041@yahoo.com.br
Have you tried to analyse the yellow residue that remains after the weathering test? I would bet in a persistent mercaptan with a boiling point higher than for C4's. Try to decrease the content of C5's in your LPG and see if there is a difference. Sometimes, higher molecular weight mercaptans are harder to neutralize and separate than the lighter ones. Also, notice that Cu tests positive for H2S and terminal mercaptans, but secondary sulphur may be present even in a negative Cu test.
30/07/2016 A: Rajkumar Yadav, Indianoil Corporation Limited, rajkumar.chem@gmail.com
The LPG after treater passing the cu strip corrosion test and thiolss are within limit.LPG Treater treats LPG ex FCC and coker.LPG is being treated here after amine wash in cover as well as FCC.Amine wash is located at respective units.a after amine wash (MEA) LpG colour is normal not yellowish in both units.But sometimes LPG from comers is yellowish long back.Presently LPG colour after FCC and Come are normal but after treater yellowish colour.if mercaptans present there are probable chances of failure of cu corrosion and total sulphur high at exit of treater but this is not the case here
28/07/2016 A: Egbert van Hoorn, Hocon B V, Egbertvh@hotmail.com
Treating FCC LPG regularly causes problems.
Not clearly defined reaction can occur in the initial amine extractor. It is not quite clear if you have such an amine extractor, but it is quite likely.
The yellow colour needs to be investigated further. A first guess would be oxygen ingress forming sulphur.
These LPG issues are not simple. There is no one liner to resolve this issue. As said many reactions can occur.
Could you perhaps elaborate on the FCC operation?
Do you operate the FCC at high severity?
Do you have a full combustion FCC regenerator?
28/07/2016 A: Sridhar Balakrishnan, Bharat Oman Refineries Limited , laksrid@yahoo.com
The yellowish colour in LPG is due to presence of methyl mercaptans. This phenomenon can occur when processing certain type of crude oils .