How does the quality of wash water affect the desalting of crude? what are the parameters based on which quality of wash water is decided for desalting?
Haris Djauhari, QP, firstname.lastname@example.org
In some references for crude desalter water wash, the ammonia level less than 50ppmwt and hydrogen sulfide less than 30 ppmwt have been determined
A delicate balance must be maintained while controlling mixing intensity, wash water quality, chemical demulsifier feed and other parameters that can provide optimal salt removal. On one hand the quality of the crude overflow must be within specific standards and on the other hand the under-carry must not be so potent that it compromises the system's dehydration abilities or fouls up downstream wastewater treatment. New legislative demands placed on effluent water quality present the operator with a difficult challenge. Optimizing the desalting process under constantly varying conditions is a key ingredient to success of the entire refinery operation.
The wash water is separated by electrostatic precipitation using de-emulsifiers and acid. The salts that are removed are mainly chlorides and carbonates. They can cause corrosion and fouling downstream in the heat exchangers, furnaces, and distillation units, if not removed.
Electrical desalting is the application of high-voltage electrostatic charges to concentrate suspended water globules in the bottom of the settling tank. Surfactants are added only when the crude has a large amount of suspended solids
Exceeding the H2S and ammonia limits causes increased corrosion in these areas which reduces equipment life and impacts unit reliability. The corrosion can be very aggressive and localized. Although neutralizing amines and filming inhibitors can help to minimize corrosion in the overhead condenser, the areas of corrosion can occur before the dew point and therefore not be addressed by the treatment program.
The other limits for ammonia and H2S in that area would be for the sour water stripper in reference to the mass balance for the composition of the desalter wash water with fresh water make up.
There is also sometimes a water wash just ahead of the overhead condenser(s). The ammonia and H2S limits should also be observed there for the same reasons.
A properly operating sour water stripper should not exceed those specifications. Ammonia and H2S loading can sometimes be addressed by draining any free water from the crude storage tanks instead of sending this water to the desalter. Increasing the wash water can dilute the ammonia and H2S and relieve the problem. Blending a very sour / high ammonia crude feedstock with better quality crude is another way to reduce impact to the unit.
Eric Vetters, ProCorr Consulting Services, email@example.com
You want a low oxygen water source with minimum other contaminants in it. Condensate from the overhead of the atmospheric tower and the vacuum tower are often used. If the ammonia content of either of these streams is high, they may need to be routed to a sour water stripper first. Stripped sour water is another good source of wash water. You want to keep the ammonia level in the stripped sour water <50 ppm. You also want to avoid hard water as the hardness will cause solids to form at desalter conditions. These solids can stabilize the rag layer in the desalter and/or form fouling deposits.
High pH in the wash water will cause NH3 or amines to partition to the oil phase, which can lead to ammonium or amine chloride salt depostion in the atmospheric column. This has been a source of a number of corrosion events reported at NACE. In addition high pH wash water promotes the formation of emulsions in the desalter when the basic compounds react with napththenic acids to form naphthenate salts e.g. sodium or ammonium naphthenate, which are essentially soap molecules.