Sat and Unsat LPG

Dear Norm, Why have two different LPG merox units – sat and unsat in refineries?

(1) In our configuration the straight run LPG from the crude unit is going to SAT MEROX UNIT – and then to product tanks.

(2) LPG from FCCU and COKER is going to UNSAT MEROX UNIT – and then to C3/C4 splitter – then to propylene/propane splitter.

Reason is that only the LPG from FCCU and COKER is having propylene in it (which is formed during cracking).

Is this right?

--Prabhakar, India

            Dear Prabhakar, I assume you are talking about a Merox Extraction plant, not a sweetening plant. You can split cracked C3-C4 first and then mix sat and un-sat C3’s to extract mercaptans. Propylene is irrelevant. Obviously, cracked P-P’s are 50% propane anyway.

            I’m coming to India later in the year. I’ll let you know when and where.

--Norm