Q & A > Question Details
We have field small refinery to produce diesel ~10000 BFD API =36, BS&W=0.05 and salt= max 4 PTB in our plant already designed one stage deslater my question in such case need to operate this unit I mean mix crude with service water or no need. See below the water and crude desalter in/out result: (see attached file)
We faced problem with over head water accumulator with high chloride and iron content and ph fluctuating value out of range (5.5-6.5) as you can see from attached file results.
Note that we injecting neutralized amine which is diluted with service water 1:3 with total rate is 7 gpd ( naphtha reflux to crude tower injection point) and corrosion inhibitor with rate 8 gpd (upstream of overhead condenser exchanger).
Moreover, over head exchanger is experienced high corrosion, tubing crack at inlet ends no fouling problem has been reported for desalter downstream (Heat exchanger tubing) we used Antifoulant chemical (crude discharge pumps).
Using water wash is the root cause of corrosion and high chloride level at over head accumulator. What the source of high chloride level (our crude is very low PTB & BS &W).
It is worth to mention that we closed deslater operation for long time but we face same problem high chloride, iron and ph value fluctuating. What the source of chloride our crude feed low salt and BS&W values.
Your recommendation is highly appreciated
 
Answers
16/09/2016 A: Muhammad Faisal, OQ integrated energy company (Oman), engr.faisalfraz@gmail.com
you may check followings:
(1): metals in Feed, desalted crude (mostly ca, Mg,K can go upto main distillation after being hydrolysed in furnace outlet temp. desalter normally removes inorganic salts. organic salts i.e. metals can travel d/s of desalters. sodium can not go to column up, it will stay along with bottom heavy material (AR).
(2): how's desalting efficiency? check filterable solids in crude feed. due to oil well chemical treatments i.e. high pH excavangers (H2S), it can creates tight emulsion in desalter thus resulting lower desalting efficiency.
(3): for OVHD, mainain high pH of SW Boot sample i.e. 6-7 normally recommended ph 5.5-6.5 at ICP (initial condensation point which is probably at column top). also try to maintain top temp and reflux temp above dew point/desublimation temp. if SW boot water report high iron, then insted of circulating to cooler, dumped it to d/s i.e. SW Stripper or WWT. because recirculating SW with high salts and iron will accumulates in finfan tubes resulting plugging the tubes and erosion occur. further, arrange isolation valves to each finfan bundles sothat you may isolate any bundles and inspect/clean mechanically without shutting down the unit.
15/09/2016 A: Sudhakara Babu Marpudi, Dangote Oil Refinery Company, m_sudhakarababu@yahoo.com
Single stage desalting removes 90% of inlet salts at the best. Reminding salt will become Chlorides in overhead section. Heat exchanger Tube inlet cracks must be analysed to check whether the root cause is erosion or corrosion. Normally two phase flows (vapor + liquid water) can lead to erosion. Normally Anti foulant chemicals are used for to prevent heat exchanger fouling and so injected in hot preheat trains (not in feed pump discharge).
14/09/2016 A: Apoorv Gupta, IOCL Haldia, apoorv.work19@gmail.com
Operate the desalter at optimum range i.e. outlet salt content should remain in range of 1-1.5 ptb. If desalter is working fine then this is a problem of excessive organic chloride coming with the feed stock & dosage of caustic at downstream of the desalter can be carried out. Post caustic dosing, residue will become rich in sodium thus testing of vacuum gas oil & vacuum residue carried out for sodium within design limit.
11/09/2016 A: Peter Marsh, XBP Refining Consultants Ltd, peter.marsh@xbprefining.co.uk
Some crude oils are contaminated with inorganic chloride compounds (eg. dichloromethane) which are not detected by normal BS&W test methods and are not removed by the desalter. The contamination comes from use of chlorinated solvents in production wells. You can ask the producer to cease using such solvents or else buy the crude oil at a discount to offset increased corrosion control and equipment maintenance costs at the refinery.
07/09/2016 A: keith bowers, B and B Consulting, kebowers47@gmail.com
And where do you reckon the chloride ion is coming from?
It MUST be in the feedstock!! Test it!!
07/09/2016 A: Leonardo Leite Garcia de Souza, Petrobras SA / Duque de Caxias Refinery, llgs394041@yahoo.com.br
Do you really need a water wash at your overhead system? I don't see the need to use water wash at crude distillation overhead systems, unless one has amonium salts reverse sublimation problems, which are more common in FCC units. Depending on the quality of the water used it could explain the corrosion problems and also the high chloride levels (the high iron levels are probably caused by the low pH). I believe it's always a good practice to desalter the crude, since upsets at the crude production fields can raise the BSW and the salt levels (this is also the case with crudes from different origins), and, in doing so, try to operate your desalters at benchmark conditions: use adequate values for mixing valve dP, temperature and electrodes voltage.