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In my unit, Main fractionator is running steady all parameters are normal, suddenly one day, CLO Flash point came very low compared to earlier it was high(75-89 C) and used to fluctuate by 10 C. Now flash is always coming below 60 C. We have increased HCO Stripper stripping steam and Main Column bottom stripping(Agitation) steam to maximum but still CLO flash is not improving. Checked for FLO to bottom circuit, all locations blinded. Main Column Flash zone temp & bottom temperature are 356 C and 349 C respectively. Kindly suggest best ways to improve CLO flash and how to find out the problem?
 
Answers
13/11/2017 A: Eric Hennings, Technip Stone & Webster, Eric.Hennings@technipFMC.com
Flash in CLO is an important spec. Our Main Fractionator design incorporates a degree of separation using internal baffle and stripping steam in the tower bottom. If CLO flash has decreased from normal, it is likely from lighter flush oil such as LCO. LCO can enter CLO section through FT purges, pump seal and wear-ring flush, or maintenance flush (heat exchanger, pumps, isolation valve seats, etc.). A symptom of excess flush is lower density of CLO.
08/11/2017 A: William Collings, Jacobs Consultancy, dave.collings@jacobs.com
Have you ruled out cross-contamination? Pretty difficult to get that low of a flash on clarified oil without contamination.
07/11/2017 A: Bruce Carr, Suncor Inc, bcarr@suncor.com
Stripping steam works on the principle of partial pressure. The steam is cooler than the product but reduces the partial pressure allowing lighter components of the oil to boil off.... The latent heat required to boil these comes from the oil itself not the steam.... I would guess the problem started with wet steam... The wet steam contacted the hot oil and that latent heat was used instead to boil the moisture in the steam instead of the light ends.... Increasing the stripping steam has created another problem of too much vapour traffic and compounded the problem by increasing entrainment...possibly even flooding of the upper trays....(If that were the case I would expect to see pressure surges within the main tower as the steam would slug liquid... but I'm guessing its just entrainment)
I would suggest you cut stripping steam back to where it normally runs and ensure your steam is kept dry.... With stripping steam Less is sometimes better...