On crude units, normally to control chlorides on top of the distillation column, we inject downstream of the dessalters, less than 12 ppm of caustic OHNa, This injection limit is to avoid caustic embrittlement downstream dessalters equipment. The charge to the visbreak unit comes from Vacuum bottom residuum unit, as the caustic precipitates on bottom cuts we have on the feed that goes to the Visbreaker unit about 45 ppm of sodium, we think that this sodium precipitation promotes cokefication on Visbreaker heater tubes. We have a HDS unit, which is running since 1996, and we never experience any kind of heater tube coking. Luis Marques

Your guess is CORRECT. The NaOH dosing in crude preheat train is done to convert hydrolysable chloride salts of Ca, Mg to more stable NaCl. However, unlimited NaOH dosing can not be done. It is generally limited to 3 PTB, i.e., approx 10-11 ppm. The reason is Na, like Vanadium etc., promotes coking. The coking of vis-breaker tubes depends on many variable. The most important factor is the operating temp of the heater. The operating temp varies depending on feed etc. and also process. Like our earlier vis-breaker had a COT of 505 C and used to need frequent decoking. However, after commissioning of a Soaker drum, the COT could be reduced to 405 C. All the coking now takes place in the soaker drum only. If your Process licensor has given restriction on 1 PTB on Na, I feel, that should be followed. However, the discussion will be more meaningful if you, LIMA & other participants could describe their coking experience in relation to Na in Feed, Heater COT etc. Regarding the reference to book/article, one that comes in mind is that of ASM Handbook Vol 13. There are too many ref. on that topic. Regards Chakra