Q & A > Question Details
Here are a few points regarding the desalter issue in our refinery:
1. There are two desalter trains installed in parallel.
2. When the desalter current fluctuates, the brine water coming out of the desalter has an opaque black colour.
3. The events occur only at one train. In this case, the rest of the trains are fine with the same crude. 4. The events tend to occur at crude switch or introduction of slop oil, but this is uncertain.
What are the possible causes? Please advise what we should investigate and analyze to clarify the causes.
 
Answers
26/12/2020 A: Sridhar Balakrishnan, Bharat Oman Refineries Limited , laksrid@yahoo.com
This type of problem can occur in desalter operations. This is due to sludge formation in the desalter which causes brine water colour change to black or brown. Also, processing certain heavy crudes can cause this type of phenomenon to occur. If you are processing heavy slop oil with crude oil this type of problem can occur.
15/12/2020 A: Morgan Rodwell, Fluor Canada Limited, morgan.rodwell@fluor.com
It sounds like the "troubled" train may have issues with the emulsion it is forming (overshearing in the mixing valve), or damage to electrodes or other internals, or the interface level is not at the same place. All of these could cause what is described. If the "troubled" train is operating with larger rag layer buildup, or damage to the electrodes, the switches in crude charge or addition of slop oils could push it over the edge in terms of stability.

I would check:
1. Mixing valve position - are all the trains running the same flow rates (or if the trains are not the same size, the same volumetric fluxes)?
2. Interface level measurement performance and historical data
3. Compare electrical draw performance between the trains during transitions
4. Take interface samples to see if your monitoring of rag layer is accurate.
5. Analyze the "black brine" to see what is making it black (is it oil, organics, solid fines?)
15/12/2020 A: Nagarathinam S Murthy, Ashphil Consultancy, Chennai, nssvdvr@gmail.com
Couple of points: it is not clear whether both trains have identical desalter design. Invariably there are variances in performance wrt designs like crude oil water mix injection below grid or between grids. Also, the electrical intensity of a grid makes a difference wrt coalescence of water and effective separation thereof. Suggest looking at the following key process indicators between the two desalters and minimize deviations.

1. Electrical intensity KV/Cm. Check / calibrate voltage so that there is no error.
2. Mixing of emulsion breaker. It is more effective to have split dosing than mere injection into crude pump suction alone.
3. Wash water added into crude u/s of preheat and near mix valve
4. Frequency of mud washing
5. Mixed valve DP
6. Desalting temp. Typically, it shall be high enough so that viscosity of crude oil at desalter conditions shall be less than say 5 cst.
7. Check crude / slop compatibility as even the interphase layer between crude mix change over need to be compatible