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We are facing an issue of LPG product off spec due to total sulfur being high. We have an amine treating unit followed by caustic treatment with CFC technology (EIL). Since the caustic regeneration system is not working frequently, caustic dumping and fresh caustic (25 Be) charging is being done. While the CFC unit is designed for RSH 4500 ppm and 100 ppm H2S, actual numbers are H2S NIL and RSH 1350 ppm in LPG.

The following activities have been done so far:
a) Line up checked for mixing feed and product to rule out short circuiting
b) Caustic strength reconfirmed and dumping / charging increased
c) Circulating caustic flow has been maximized and presently it is ~19.5 m3/hr.
d) Caustic levels in contactors have been increased to 70% to increase residence time.
e) Feed to CFC was reduced from 58 to 50 m3/hr.


Questions:
1. Is there anything which we are missing?
2. Is there any impact of Erha crude which we are processing in the CDUs?
3. What is the experience of troubleshooting in other Refineries?

 
Answers
01/10/2020 A: Sridhar Balakrishnan, Bharat Oman Refineries Limited , laksrid@yahoo.com
Erha is a low sulphur crude oil. The mercaptans content in LPG is low. The hydrogen sulphide value in LPG is not available from the crude assay data. I think this is not due to the effect of Erha crude oil. It may be due to processing of highly cracked LPG feedstock in your Merichem Unit. The reasons given by Mr. N.S. Murthy in his reply might meet the requirements for combating the problems you face.
29/09/2020 A: Nagarathinam S Murthy, Ashphil Consultancy, Chennai, nssvdvr@gmail.com
It appears to me that you need to set your amine system (ahead of caustic extraction) right. Typically when H2S is in excess in the sour LPG, which could be as high as 500ppm in cracked LPG, the amine system must work to bring down the H2S level as low as 10ppm. Thereafter the caustic prewash handles residual H2S making caustic extraction apply exclusively to RSH removal which will lower the final S level in product LPG. Check the following.

1. H2S and RSH level in incoming sour LPG
2. Look at the amine system for HSAS. If this is high then most amine systems will malfunction. The pH of lean amine should be more than say 10.2. Any less implies degraded amine.
3. Look at caustic prewash and set the level indication correctly.
4. Restart the caustic regen system to minimize caustic drain which is bad for d/s WWTP.
27/05/2020 A: Udit Nandi, Hindustan Petroleum Corporation Limited, uditnandi@hpcl.in
1. No. Our regeneration section is not in use. Caustic is dumped once strength drops to 15-16 Be.
2. Cracked LPG from FCCUs is being treated.
13/05/2020 A: Narendra Naidu, Bapco, nbpnnaidu@gmail.com
Need a couple of items of information:
1. Do you regenerate the caustic?
2. Where is the LPG from the Caustic wash settlers diverted to?