Desalting of crude oil

Refining crude oil into useful petroleum products can be separated into two phases and a number of supporting operations. The first phase is desalting of crude oil and the subsequent distillation into its various components or "fractions." The second phase is made up of three different types of "downstream" processes combining, breaking, and reshaping. 

Petrollte Corporation, Petreco Division Houston, Texas "Electric Dehydration/ Desalting of Crude Oils in Oilfield Production" (Rev. I - March 1975). 

Desalting of crude oil streams does not always eliminate deposition of inorganic material. 

One of the most important operations in a refinery is the initial distillation of the crude oil into its various boiling point fractions. Distillation involves the heating, vaporization, fractionation, condensation, and cooling of feedstocks. This subsection discusses the atmospheric and vacuum distillation processes which when used in sequence result in lower costs and higher efficiencies. This subsection also discusses the important first step of desalting the crude oil prior to distillation. 

A modem petroleum refinery in the United States processes between 100,000 and 500,000 barrels/day of crude oil. The incoming cmde is first desalted and then passed through an atmospheric pressure distillation column that separates it into fractions, as shown in Figure 2-12. 

From the desalting unit, crude oil passes through a series of heat exchangers to increase the temperature to about 550°F (287,8°C). From the exchangers, oil passes... 

Salts form scale which can foul refining equipment. Under refining conditions, salts can break down to liberate acids during processing. If the NaCl expressed salt content of crude oil is greater than 30 ppm, the crude oil must be desalted before processing. 

Instead of filtration or centrifuging as a means of separating the crystallized wax from oil, an electrical precipitation method has been proposed, similar to that employed in desalting of crude or removal of entrained solids from gases. 

SGP T TWSEC vise VISCL Vt VTGOl VTWO wc wd specific gravity of production water at system temperature T system temperature, °F time for water drop in oil phase to fall, s viscosity of liquid, gas or oil at system temperature T, cP viscosity of liquid or oil at system temperature T, cP terminal velocity of particle, ft/s particle velocity in oil phase, ft/s terminal velocity of water particle in the oil phase, ft/s BS W of treated desalter outlet crude oil, percentage by volume Barrels of dilution water per day mixed with the crude oil fluid upstream of the desalter... 

The water knockout tank is designed to separate all free water from the incoming field production oil. It generally uses a 30- to 60-min storage time. In many field cases, the free water is only 30 to 60% of the total water content in the oil. This is due to oil-water emulsions. The oil fed to the downstream dehydration tank (see Fig. 4.15) is treated with an emulsion breaker chemical that is injected into the feed line to the dehydrator tank. As much as 70% of the production may therefore be separated in the downstream dehydrator tank. The KO tank may also receive fresh water for desalting the crude oil through its feed line. 

Wc = BS W of treated desalter outlet crude oil, percentage by volume... 

Within the refining environment, the field of crude-oil dehydration is viewed somewhat differently. The first process a crude oil (or blend of crude oils) is subjected to is the desalting process. This process was developed with the expectation that a crude oil will have a known water content (less than 0.5%) and a soluble inorganic chloride salts content associated with this water (formation waters from oil-field production may have salt contents approaching 300,000 rngl ). Salts may also occur in crystalline form dispersed within the oil. As these salts have considerable negative effects in the downstream processes of the refinery, it is desirable to remove them. 

This process may take several months. If initial specifications on product quality cannot be met, then changes may be made to mechanical equipment present, chemicals applied, rate of crude-oil charge, type of crude oil processed, or product quality specifications. The process of optimizing the desalting program then starts again. On particularly difficult crude oils this process may last many years, as is the case in refineries processing heavy crude oil. 

Desalted hot crude oil feed is sent to a preflash column where it is separated into gas, gasoline, naphtha, and topped crude as shown in Figure 9.5. Using column modules representation, determine the number of degrees of freedom for this operation. What specifications may be used to define the column performance The crude feed is of fixed composition and thermal conditions. The stripping steam thermal conditions are fixed, but its flow rate may be varied. The column pressure is independently fixed. 

The salt content of crude oil is highly variable and results principally from production practices used in the field and, to a lesser extent, from its handling aboard the tankers bringing it to terminals. The bulk of the salt present will be dissolved in coexisting water and can be removed in desalters, but small amounts of salt may be dissolved in the crude oil itself. Salt may be derived from reservoir or formation waters or from other waters used in secondary recovery operations. Aboard tankers, ballast water of varying salinity may also be a source of salt contamination. 

Sodium chloride is also present in most crude oils. When the quantity exceeds 15 to 25 pounds per 1,000 barrels of crude oil, desalting is required to remove the corrosive salt. Some of the salt is in the form of crystals in the oil and some is dissolved in the water which is normally produced along with the crude oil. 

Electrical charging methods may be used to remove aqueous materials and solids from organic fluids as in cmde oil desalting. Dispersed materials passed between electrodes are charged with either dipoles or net charges such that they are attracted either to each other or to an electrode surface where they coalesce with other dispersed-phase materials. The coalesced materials are readily separated by gravity settling. Some commercial units handle up to 125,000 bbl/day of crude oil. 

Desalting. As crude oil arrives at a refinery, it is generally desalted to remove suspended salt and solid contaminants. Crude oil is heated and mixed with hot water. Salt and contaminants are washed out either by adding chemicals or by application of an electric field. Another method is to filter heated crude oil through diatomaceous earth. 

Distillation. Desalted crude oil is heated for fractional distillation between 343 and 399 degrees Celsius (650 to 750 degrees Fahrenheit). The resulting vapor and liquid mix is sent to the first distillation tower that operates at atmospheric pressure. Because of the different boiling points of the different hydrocarbon molecules of crude oil, the hydrocarbons can be separated into different fractions (also called cuts). This occurs at the distillation tower, which can be as tail as 50 meters (164 feet). The heaviest hydrocarbons remain at the bottom, while the middle and lighter ones are extracted. 

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