Crude Oil Desalting:Salts are distributed in crude oil as brine (Nacl) Defects of Salts are: -Cause plugging of heat exchangers tubes. -Cause deposits in pipe still tubes (furnaces) and consequently cause . -Decrease in heat transfer coefficient. -Create pressure drop which affects on pumps. Salts may reach up to 260 PTb each 100 PTb cause deposits of about 3 ft³/day . Salts should be removed before distillation tower where these salts liberate HCL gas which cause many corrosion problems . DR.Hemdan
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Problems of oleophobics -Corrosion due to hydrolysis of salts . -Increase consumption of NH3 to neutralize HCL -Erosion . -Plugging of Equipment and fouling . -Trace metals in distillates which acts as catalyst poisoning
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Crude desalting is the first processing step in a refinery . The objectives of crude desalting are : 1-the removal of salts and solids 2- the formation water from unrefined crude oil before the crude is introduced in the crude distillation unit . 3-removal of suspended solids from the crude oil. 4-Reduced equipment fouling and corrosion and 5-longer catalyst life DR.Hemdan
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SINGLE-STAGE DESALTER
Demulsifier
Desalted Crude
Heat Exchangers
Desalter Feed Oil
Mix Valve
Water Effluent Water ..
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Two-Stage Desalter Example Desalted Crude (0.5 PTB, 0.2% BS&W)
1st Stage Crude Outlet (5.0 PTB, 0.2% BS&W)
2nd Stage Desalter
1st Stage Desalter
Mix Valve
Mix Valve
Crude Charge (100 PTB, 1% BS&W)
5.8% Effluent Water 5.0%
Figure 2
2nd Stage Effluent Water is 1st Stage Wash Water DR.Hemdan
Some deemulsifier may be added to break forces between salts and the oil
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In electrostatic precipitator applied voltage up to 30KV
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Salt drops coalesce together and form larger drops which separated down to the bottom of desalter
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Salt removal reach to about 90% DR.Hemdan
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It is necessary to adjust the pH of the brine to obtain a value of 7 or less.
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If the pH of the brine exceeds 7, emulsions are formed because of the presence of sodium naphthenate and sodium sulfide.
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For most crude oils, it is desirable to keep the pH below 8.
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Better dehydration is obtained in electrical desalters when they are operated at a pH of 6.
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The pH is controlled by the addition of acid to the incoming or recycle water. DR.Hemdan
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The trend toward running heavier crude oils has increased the importance of efficient desalting of crudes. Until recently, the criterion for desalting crude oils was 10 lb salt/1000 bbl (expressed as NaCl) or more, but now many companies desalt all crude oils.
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Salt in the crude oil is in the form of dissolved or suspended salt crystals in water emulsified with the crude oil.
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The basic process of desalting is to wash the salt from crude oil with water. Problems occur are: 1-inefficient and economical water/oil mixing, 2-water wetting of suspended solids, and 3-separation of oil from wash water
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The separation of oil and washwater i.e desalter performance is affected by : 1-the gravity 2-viscosity 3-pH of the crude oil 4-the ratio of water/crude used for washing. 5-temperature 6-pressure drop 7-chemicals additions DR.Hemdan
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Desalting is carried out by emulsifying the crude oil with 3 to 10 vol% water at a temperature of 200-300F. Both the ratio of water to oil and the operating temperature are functions of the gravity of the crude oil. Typical operating conditions are :
