Treatment of Produced Fluids: Crude Oil and Water: Desalting of Crude Oil By Pn Khairunissa Syairah Credit to Pn Azduwin

INTRODUCTION • The removal of salt from crude oil for refinery feed stocks has been and still is a mandatory step. • This is particularly true if the salt content exceeds 20 PTB (pounds of salt, expressed as equivalent sodium chloride, per thousand barrels of oil). • The remnant brine is that part of the salty water that cannot be further reduced by any of the dehydration methods. • It is commonly reported as basic sediments and water (B.S.&W.).

REMNANT BRINE • This remnant water exists in the crude oil as a dispersion of very fine droplets highly emulsified in the bulk of oil. • The mineral salts of this brine consist mainly of chlorides of sodium, calcium and magnesium.

• The amount of salt in the crude oil is a function of the amount of the brine that remains in the oil, WR (% B.S.&W.) and of its salinity, SR in parts per million (ppm); γBrine = the density of the saline water SG=Spesific gravity

RELATIONSHIP AMONG VOLUME OF REMNANT WATER, ITS SALINITY AND SALT CONTENT • Taking one-tenth of 1% by volume of water to remain in the crude as a basis for our calculation, the relationship between the salt content of the remnant water expressed in pounds of salt per thousand barrels of oil (PTB) and its concentration or salinity expressed in ppm is presented graphically in Figure 1. • For other volumes, simple multiples of the numbers given by this graph are used.

FIGURE 1 oil (PTB) as a function of salinity of its remnantwater 0.1% Salt content of crude oil (PTB) as a function of salinity of its remnantwater 0.1% (1/1000) by volume remnant water.

Solutions [Sodium Chloride (NaCl)] Densities of Aqueous Inorganic Solutions [Sodium Chloride (NaCl)] Unit = g/cm3

EXAMPLE 1 • Find the PTB of a crude oil having 10% by volume remnant water if its concentration is estimated to be 40,000 ppm at 25°C.

EXAMPLE 2 Assuming that the water salinity is 40,000 ppm instead of 20,000 ppm and dehydration is done to two-tenths of 1%. Calculate the PTB of the oil.

DESALTING PROCESS • Single-stage desalting system • A two-stage desalting system

Single-stage desalting system Wash water, also called dilution water, is mixed with the crude oil The mixing results in the formation of water– oil emulsion. The oil (and emulsion) is then dehydrated. The separated water is disposed of through the field-produced water treatment and disposal system.

Two-stage desalting system In the two-stage desalting system, dilution water is added in the second stage The disposed water in the second stage is recycled and used as the dilution water for the first stage. Two-stage desalting systems are normally used to minimize the wash water requirements.

Mixing device • The usual mixing device is simply a throttling valve. • A useful device for such a purpose is the application of multiple-orifice-plate mixers (MOMs) • Accomplished by pumping the crude oil (which is the continuous phase) and wash water (which is the dispersed phase) separately through a mixing device.

 The chemical desalting process involves adding chemical agents and wash water to the preheated oil followed by settling.  The settling time varies from a few minutes to 2 h.  Some of the commonly used chemical agents are sulfonates, long-chain alcohols, and fatty acids.

ELECTROSTATIC DESALTING • An external electric field is applied to coalesce the small water droplets and thus promote settling of the water droplets out of the oil.

Chemelectric Desalting System The chemielectric concept utilizing both chemical agents and electrical field

DETERMINING DILUTION WATER REQUIREMENT • The amount of water of dilution WD added in the desalting of crude oils is in the range of 5–10% by volume, based on the amount of remnant water and its salinity. WR, SR, SD :(salinity of the dilution water) E: The efficiency of mixing between the two phases. SD and SR are in parts per million To set the acceptable limits on the salt content in crude oil; SB refers to some average salinity in the bulk of the homogeneous phase as a result of mixing the remnant water with the fresh water

EFFECT OF OPERATING PARAMETERS The efficiency of desalting is dependent on the following parameters ; 1. Water–crude interface level. This level should be kept constant; any changes will change electrical field and perturbs electrical coalescence. 2. Desalting temperature. Temperature affects water droplet settling through its effect on oil viscosity; therefore, heavier crude oils require higher desalting temperatures. 3. Wash water ratio. Heavy crudes require a high wash water ratio to increase electrical coalescence. A high wash ratio acts similarly to raise temperatures, as illustrated in Table 4.

EFFECT OF OPERATING PARAMETERS 4. Pressure drop in the mixing valve. A high-pressure- drop operation results in the formation of a fine stable emulsion and better washing. However, if the pressure drop is excessive, the emulsion might be difficult to break. The optimum pressure drop is 1.5 bar for light crudes and 0.5 bar for heavy crudes. 5. Type of demulsifiers. Demulsifiers are added to aid in complete electrostatic coalescence and desalting. They are quite important when heavy crudes are handled. Levels ranging between 3 and 10 ppm of the crude are used.

Problems, Causes, and Solutions

Problems, Causes, and Solutions

DESIGN CONSIDERATION The following major parameters are considered when designing the desalting system: 1. Flow scheme arrangements (conventional one-stage or countercurrent contact desalters) 2. Number of desalting stages 3. Dehydration levels achieved 4. Salinity of the brine in the crude 5. Efficiency of valve mixing 6. Salinity of dilution water 7. Target PTB specification

Calculation Methods Single-stage Desalting System

• Basic equations for calculating required dilution water are as follows

Calculation Methods Two-stage Desalting System

CLASS EXERCISE An oil field produces 200,000 bbl/day of net oil with a salt content of 10 PTB. The oil out of the emulsion treater contains 0.3% water (salinity=250,000 ppm and specific gravity=1.07). In the desalting– dehydration process, the oil is mixed with dilution water (3000 ppm salinity and 1.02 specific gravity) and dehydrated down to 0.1% BS&W. Assuming that the mixing efficiency is 80%, determine the following: (a) The salt content in PTB of the oil out of the emulsion treater (b) The amount of dilution water required (in bbl/day)

THE END