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
15/12/2020 A: Jayaraj Jayam, Chennai Petroleum Corporation Limited, njayaraj@gmail.com
This is not uncommon to many refiners.
Following could be the probable reasons:
1 Prepare crude feed tank and ensure it is free of water and sludge. Water draining is a routine part whereas sludge removal is not that much easy. Either you should have facility to gravitate the bottom stock of the tank to the other low level tank or heating coil should be proper. While receipt of crude, the side mixer of the tank should be kept online. In spite of doing all these, if sludge carryover is still there, possibility of sludge breaking into water and oil occurs at desalter when crude is subjected to the desalting temperature. This separation takes place throughout the desalter and when this water meets the electric grid it causes high current and dip in voltage causing grid fluctuation.
2 Make an attempt to reduce mix valve dp and observe during tank change over and slop injection.
3 Ensure only dry slop is spiked into the system at very slow rate, that is around 2-3% of the feed rate.
4 Confirm DMF dosing is at its appropriate rate.
5 Possibility is also there, that any one of the transformer grid may not be working/ short circuit in the grid might have occurred.
As the desalters are in parallel, bypassing a desalter may limit unit production. If the problem is not getting addressed, open and clean the desalter and check the healthiness of electric grade.
If it happens in both desalters during crude switch or slop injection, we can attribute the problem to poor preparation of the tank but in this case it appears to be local problem to the particular desalter.
Pl confirm that slop and crude are fed proportionate to the desalters' capacity.
Take longer duration for crude change over.
N. Jayaraj.