Q & A > Question Details
We have a very strange problem, it's that the desalter outlet crude has greater salt content than that of the inlet... the lab examinations proved that more than once...this always happens when the injection water is cut off-while switching from a tank to another. What could explain this?

Additional info/response:
1. We cut off water while switching between tanks because of the existing water accompanied with the crude from the new tank; I mean the first 30 minutes after switching to a tank, the crude has too much water to inject more.
2. How could the NaOH type could affect this situation?
 
Answers
12/12/2013 A: Virendra Kapoor, Petroleum Refining Consultants, vkkapoor9@yahoo.com
At the outset, I congratulate the person for asking a very good question which received such good response. My observations are as follows:
1. Some of the refiners temporarily stop make up wash water when they switch over the crude tank. It is to prevent water shot in the desalter and subsequently to CDU due to large amount of water coming with the crude.
2. Under such situations it becomes important to increase the dose of demulsifier appreciably very high. Monitoring and controlling of the interface layer and water drain valve becomes very important parameters to follow and act
3. Agar Corporation have invented very good design for such situation and control of desalter operation. It can be read at their site namely http://www.agarcorp.com/Animation/Desalter.html
4. Number of experts have expressed their opinion on use of caustic pre / post desalter. It is useful to overcome the issue of naphthenic acid. Sodium naphthenate formed in desalter make emulsion very tough to break.
05/12/2013 A: Prabu Natarajan, Natco Uk Ltd, prabunatarajanuk@yahoo.com
Well, for a stable desalter operation, its better to have continous injection of water. The reason is 1. if teh inlet water content is low, injection water will actually improve the dehydration to certain extent and thus will have better desalting as well. when injection water is cut off with low inlet water, dehyration/desalting may not be as efficient as it should be and also at this point of time the actual water carry over should be checked as it could be higher as well. The reason for having higher outlet salt than inlet could be because of doing the test method wrongly. for instance, if the inlet BS&W is 0.3% taken at 15.6C aand outlet sample is taken at 120C and cooeld it to 100C and not 15.6C will surely result in high water content & high salt content as water will be more soluble at higher temp.
12/07/2013 A: Prakash Pimparkar, Environmental Consulting Services , prakash.pimparkar@yahoo.in
The solution is to drain the water from crude tank and then only to send it for desalting, without cutting the wash water. This will ensure that the desalter functions properly.
When you feed crude without draining the water, you are loading your desalter with crude containing brine which is already saturated with salts and therefore desaled crude will have same salt contents as feed crude.
Adding caustic will not solve the problem,use of caustic in desalter is resortedfor high acid crudes and is done after a thorough analysis of the effects on down stream processing.
Also please check the quality of water you are using for desalting, clean and low TDS preferred if using Stripped sour water ensure proper pH as per recommendation of Desalter supplier.
If problem pesists consult desalter vendors
19/07/2012 A: Amr Amin, BSR , AMR@DQR.COM.VN
Just cool the crude sample before you put in the sample bottle. What is happening when the crude is hot and not enough cooling it vaporize and you are concentrating the salt in the bottle. you think that you collect a litter for example to fill in the bottle but actually, you collect 2 litters , one vaporize and one remain in liquid phase , at this moment the salt in the bottle is not for one litter it is doubled and finally you find the salt content in the outlet is higher than inlet ..
I experienced this issue myself and solved it by the proper cooling the sample before you put in the bottle.
08/03/2011 A: Sudhakara Babu Marpudi, Dangote Oil Refinery Company, m_sudhakarababu@yahoo.com
Cutting off water as part of preparing for the Crude tank switch can be avoided by proper mixing, settling and draining of Crude tank before the tank is lined up for unit feeding. Opportunity / heavy crudes call for longer times of mixing as the water at times remain as pockets in the startified layers of crude blends. To avoid this stratification one can store neat crudes in separate tanks (an expensive option) and carry out inline blending if the Refinery needs to process a blend of opportunity crudes so as to maximise profits.
NaOH injection will only help in improving the pH of Crudes when the pH is lower than the recommended range of 6.5 to 7.5. It can help in neutralising chlorides such as Mg Cl2, Na Cl2 that can lead to high Chloride corrosion. One needs to be very careful with NaOH injection as it can lead to a variety of extremely difficult to handle, downstream issues viz., poor desalting and poor dehydration when the crude pH goes beyond 7.5, preheat fouling due to poor dehydration, Catalyst poisoning in FCCs, Premature coking Cokers etc.
07/10/2010 A: mohamed ali, EPROM/Midor refinery, mhassan@midom.com.eg
First I would like to ask about NaOH injection point up stream or down stream the desalter
injection of NaOH up stream the desalter cause formation of Na Napthenes especially if you processed high napthenes crude which act as emulsion stablizer and this may be the cause of the problem
your problem also may be due to over mixing across the mixing valve so try to establish optimum pressure drop across mixing valve (as if pressure drop is higher than optimum it will cause water carry over and higher salt outlet)
01/08/2010 A: Jayaraj Jayam, Chennai Petroleum Corporation Limited, njayaraj@gmail.com
No situation will warrant to cut off desalting water while changing the feed tank. Once you are sure that the tank farm operations are taken care to prepare the feed tank well, that is makiing the tank free of water before feeding the unit, then you need not cut the desalting water. Even you feel water carryover in crude after feed tank is changing, you can cut off water after you realise water in crude. The trick is in preparing the feed tank. Once you cut off water it is obvious that salt carry over is natural.
NaOH may not affect the desalting process adversly, it helps only to maintain the pH of brine (desalter drain). But it will contribute to increase the metal content in VGO and this will be poison for the down stream units lile FCCu/HCU.
15/07/2010 A: Marcello Ferrara, ITW SrL, mferrara@itw.it
Another possible source of the problem could be the sludge carry-over, with related emulsion (which is far tighter than the rag layer at the interface). You could evaluate online cleaning the desalter. ITW has developed and patented a technology for doing the job in as low as 24 hours on a oil-to-oil basis.
12/07/2010 A: Roger Haglund, Eprom, r_haglund44@hotmail.com
If you inject NaOH that could contribute to the result you see. Are you using D3230 for analysing the salt?
Why do you cut the wash water, as I understand, completely, why not reduce it 40-50%.
If you could analyse the constituents by AAS/ICP I would assume this could help guiding you to what is contributing to your higher values
09/07/2010 A: Sam Lordo, Nalco Company, salordo@comcast.net
I guess my first question is why are you cutting off the desalter wash water when you switch tanks... this is not a common practice and probably contributing to your problem... one possible cause for what you are seeing is that when you cut the washwater off the desalting process is stopped and you may be losing some of the desalter emulsion into the outlet which could then increase the amount of salt coming in