Q & A > Question Details
We are processing a heavy crude of API 18. Salt content of the crude is 80-100ptb. BS&W is 0.8 to 1.2%.
We are using stripped sour water as wash water, made up by BFW. pH of wash water is between 7-7.5.
We are maintaining a desalter temperature of 150 deg C.
We are having two desalters in series, which is supposed to bring down the salt by 99%. but we reached up to 90% earlier.
Last two months we are having a high emulsion band. The BS&W of desalted crude is 2-3 and the oil in brine is 1-2% even at minimum delta P (0.3kg/cm2). Water injection rate is 3% to individual desalters (circulation not done as oil carryover with brine observed.
The voltage across the grid is as low as 6-10 KV (Tapping at 22KV). We have changed the secondary tapping to 18V and the deslater is showing an improvement in Voltage (10-12 KV)
1.what can be the reasons for this upset?
2. Can you explain the effect of change in secondary voltage?

Additional info:

We are not adding any scavenger to the crude.
Stripped water pH is 6-6.5 and brine water pH is in 7.5 range.
What parameters of crude oil should I check?
06/11/2015 A: Vijay Beleri, Rateau energy systems, v_beleri@hotmail.com
Is this desalter the low velocity design?. Crude being distributed below the grids. If so you can look at low cost revamp to upgrade to edge 2 design of forum energy technologies. Especially for 1st stage. You get a much higher emulsion as well as water resolution time. Have known quiet a few refineries upgrading to edge-2 and getting very good results. Higher voltage is generally better. However the current drawn could be quite high considering that you are running quite high temperatures too. In which case you have to reduce secondary voltage.
18/05/2014 A: Sunderam Nagar, ALBEMARLE CATALYST CO BV, snagar31@gmail.com
With such a heavy crude, this problem is common, as the crude change over is frequent and demuslifier dosage is also not adjusted. I can recommend a few things:
1) Use of BFW was wash water can cause the corrosion in the crude overhead as the chemicals used in Boiler Water treatment like TSP and hydrazine are added in the demulsifier.
2) How many times do you have desludging of desalter ? Increase the frequency to twice a week or alternate days.
3) Increase the wash water rate to 6% and tweak the Mix valve deltaP to get optimum BS&W, you might have to check BS&W on shiftly basis to get accurate results, when BS&W is constant for continuous 3 readings then you can keep the Mix valve constant.
4) Increase the voltage a bit, see tht the current should not increase, else the transformer might trip.
5) Check the trycocks for better understanding of Rag layer.
6) Increase demulsifier dosage, check the pH of Boiler water and stripped sour water, high pH can tend to cause a tight emulsion thus increasing oil in brine.
7) With so much of salt in the crude, i am surprised to see the pH of brine water being 7.5. If the transfer of salt is more from the crude to wash water the pH of brine should decrease and should be in range of 4 to 5.5. I also want to know the salt content in crude ex desalter, is it fluctuating or giving similar results ?
Hope this will help you in solving the problem.
15/05/2014 A: Sunderam Nagar, ALBEMARLE CATALYST CO BV, snagar31@gmail.com
With such a heavy crude, your demulsifier should be good enough to perform well. As we say its Mechanical,Operational and Chemical approach in proving a solution to a problem. Start with the increase in the wash water injection to 6%, tweak the Mix valve deltaP to get the best separation, BS&W and Salt levels should be done every shift when you are doing this exercise. Check the trycocks where the emulsion band is lying, it can happen that the parcel of crude is containing some different species than the previous parcels, higher the electric field more is the separation of Oil and crude in the desalter. you have to check whether all grids are working or not.
04/05/2014 A: Eric Vetters, ProCorr Consulting Services, ewvetters@yahoo.com
The thick emulsion band is driving a lot of your problems. It sounds like you either have high solids in your crude and/or don't have an effective emulsion breaker. This is leading you to use inadequate wash water which has big negative impact on desalter operation. If you operate on too high of voltage tap, especially with a big rag layer, you will overload the transformers. When the load gets too high they go into reversal and automatically drop the voltage to protect the transformer. By going to a lower voltage tap, you may be keeping the electrical load in a range that prevents going into reversal and actually ends up with a higher voltage as you have experienced.
In the near term you might consider bringing in a centrifuge to treat desalter brine. That would allow you to let some oil and rag layer to go out with the water. You would then be able to shrink your rag layer and return to a normal water wash rate. You might also check the pH of your desalter brine. If you have some H2S scavengers in your crude, they can raise the desalter pH, which can cause a bad rag layer.
I have done desalter audits in the past and could possibly help your site. Desalting can be challenging when running crudes as heavy as you are.