Crude oil desalting proccess in Crude oil desalting proccess in refineryrefinery

Slides Prepared by YASIR KHAN

Crude Oil DesaltingCrude Oil DesaltingPrinciples of Crude Oil DesaltingPrinciples of Crude Oil Desalting

Desalting is an integral part of refinery crude oil Desalting is an integral part of refinery crude oil processing and can be the key to controlling processing and can be the key to controlling pipestill corrosion, heat exchanger fouling, furnace pipestill corrosion, heat exchanger fouling, furnace tube coking, and process water disposal.tube coking, and process water disposal.

Salts, which normally occur in the form of brine Salts, which normally occur in the form of brine suspended in the crude, promote corrosion, fouling, suspended in the crude, promote corrosion, fouling, and coking. The primary function of a desalter is to and coking. The primary function of a desalter is to remove this salt from the oil.remove this salt from the oil.

Other contaminants, such as sediment, which can Other contaminants, such as sediment, which can promote heat exchanger fouling and plugging, promote heat exchanger fouling and plugging, erosion, and residual product contamination, can erosion, and residual product contamination, can also be removed in a desalter.also be removed in a desalter.

Electrostatic desalting is used to remove salts and Electrostatic desalting is used to remove salts and particulates from crude oil. The crude oilbrine particulates from crude oil. The crude oilbrine mixture is contacted with wash water using a mix mixture is contacted with wash water using a mix valve just upstream of the desalter vessel. Salt is valve just upstream of the desalter vessel. Salt is extracted from the brine into the wash water extracted from the brine into the wash water droplets. The electric field in the desalter enhances droplets. The electric field in the desalter enhances water droplet coalescence so that water/oil water droplet coalescence so that water/oil separation requires much less residence time, and separation requires much less residence time, and hence a smaller vessel, than is needed for hence a smaller vessel, than is needed for unenhanced settling. Small quantities of desalting unenhanced settling. Small quantities of desalting aids are often added to enhance contacting aids are often added to enhance contacting effectiveness, droplet coalescence, and water effectiveness, droplet coalescence, and water separation. Desalted oil is removed from the top of separation. Desalted oil is removed from the top of the desalter vessel and the briny water from the the desalter vessel and the briny water from the bottom.bottom.

As indicated by the schematic presented in Figure 1 As indicated by the schematic presented in Figure 1

for a single stage desalting operation, chemical for a single stage desalting operation, chemical

desalting aid (demulsified) is typically injected at the desalting aid (demulsified) is typically injected at the

suction side of the crude charge pump, and wash suction side of the crude charge pump, and wash

water (fresh water) is added at the mix valve water (fresh water) is added at the mix valve

immediately upstream of the desalter. The treated oil immediately upstream of the desalter. The treated oil

from the desalter (desalted product) is fed through from the desalter (desalted product) is fed through

the remaining crude preheat exchangers before the remaining crude preheat exchangers before

entering the atmospheric pipestille.entering the atmospheric pipestille.

FIGURE 1:SINGLE-STAGE DESALTING FLOW DIAGRAM

A typical two-stage electrostatic desalting operation is A typical two-stage electrostatic desalting operation is

shown schematically in Figure 2. In such an shown schematically in Figure 2. In such an

operation, fresh water is added at the mix valve for operation, fresh water is added at the mix valve for

the second stage desalter and the effluent water the second stage desalter and the effluent water

from the second stage is used as wash water for the from the second stage is used as wash water for the

first stage. Demulsifier injection is required first stage. Demulsifier injection is required

upstream of the first stage, and depending on the upstream of the first stage, and depending on the

nature of the demulsifier, may also be needed nature of the demulsifier, may also be needed

upstream of the second stage as well.upstream of the second stage as well.

FIGURE 2:TWO-STAGE DESALTING FLOW DIAGRAM

Oil feed qualityDesalter operating temperature and pressure

Wash water amountand qualityPressure drop

across the mixing valve

The electric field

Oil and water residence times in the vessel

Type and amount of chemicaladditive used.

Process Variables

Process Variables and Operating Guidelines

Oil Feed QualityOil Feed QualityOil feed type and quality have a significant influence Oil feed type and quality have a significant influence

on desalter performance. on desalter performance. Light Light (high API gravity) oils are relatively easy to (high API gravity) oils are relatively easy to

desalt. desalt. HeavierHeavier oils are more difficult to desalt for several oils are more difficult to desalt for several

reasons:reasons: 1-The density difference between the oil and water is 1-The density difference between the oil and water is

small and the oil viscosity is relatively high so that small and the oil viscosity is relatively high so that the rate of water droplet settling in the desalter is the rate of water droplet settling in the desalter is low.low.

2-Heavier oils also tend to contain more naturally 2-Heavier oils also tend to contain more naturally occurring emulsifiers than lighter crudes. These occurring emulsifiers than lighter crudes. These tend to inhibit water droplet coalescence and tend to inhibit water droplet coalescence and promote the formation of stable emulsions in the promote the formation of stable emulsions in the desalter.desalter.

TemperatureTemperature

For every desalter installation and crude blend processed, For every desalter installation and crude blend processed, there is an optimum desalter operating temperature.there is an optimum desalter operating temperature.

High temperature is required for several reasons:High temperature is required for several reasons:

The primary purpose is to lower the oil viscosity to increase The primary purpose is to lower the oil viscosity to increase the settling rate of water droplets in the desalter. the settling rate of water droplets in the desalter.

2-In addition, higher temperature tends to promote 2-In addition, higher temperature tends to promote coalescence of the water droplets by enhancing the coalescence of the water droplets by enhancing the drainage of the oil-surfactant layer surrounding the water drainage of the oil-surfactant layer surrounding the water droplets. Larger water droplets thus formed settle more droplets. Larger water droplets thus formed settle more rapidly in the lower viscosity oil.rapidly in the lower viscosity oil.

Production field desalters typically operate at temperatures Production field desalters typically operate at temperatures between 1000F and 2000F. The operating temperature range between 1000F and 2000F. The operating temperature range is typically 200- 3000F for refinery desalters. This is typically 200- 3000F for refinery desalters. This temperature range is high enough to melt waxes that could temperature range is high enough to melt waxes that could hinder coalescence and water separation from the oil.hinder coalescence and water separation from the oil.

Excessively high desalter operating temperatures can Excessively high desalter operating temperatures can cause significant operating problems. High desalting cause significant operating problems. High desalting temperatures may increase crude conductivity, temperatures may increase crude conductivity, causing high current draw and low desalting voltage causing high current draw and low desalting voltage that could result in poor water droplet coalescence and that could result in poor water droplet coalescence and desalting.desalting.

Since water solubility in the crude increases with Since water solubility in the crude increases with increasing temperature, high desalter operating increasing temperature, high desalter operating temperatures can also lead to higher water content in temperatures can also lead to higher water content in the crude from the desalter.the crude from the desalter.

PressurePressure

Desalter operating pressure must be maintained at a Desalter operating pressure must be maintained at a

sufficiently high level for vaporization not to occur. If sufficiently high level for vaporization not to occur. If

a vapor space develops in the vessel, a safety float a vapor space develops in the vessel, a safety float

switch or low level switch will automatically de-switch or low level switch will automatically de-

energize the electrodes and effectively shut down energize the electrodes and effectively shut down

the desalter. The required pressure depends on the the desalter. The required pressure depends on the

desalter operating temperature and crude type. desalter operating temperature and crude type.

Desalters typically operate at pressures between 65 Desalters typically operate at pressures between 65

and 300 psig.and 300 psig.

Wash Water Rates, Quality, Injection Points, and Wash Water Rates, Quality, Injection Points, and

SourcesSources

Wash water rates between 4 and 8 vol% (10 to 12 vol% Wash water rates between 4 and 8 vol% (10 to 12 vol%

maximum) of the crude throughput are required to maximum) of the crude throughput are required to

maintain effective desalter performance. The wash maintain effective desalter performance. The wash

water is normally injected just upstream of the mixing water is normally injected just upstream of the mixing

valve. Wash water addition provides the water droplet valve. Wash water addition provides the water droplet

concentration needed to contact and rupture the concentration needed to contact and rupture the

protective coating surrounding the brine and promote protective coating surrounding the brine and promote

coalescence to form larger, more easily separated coalescence to form larger, more easily separated

droplets with reduced salt concentration. This water is droplets with reduced salt concentration. This water is

essential for the desalting process.essential for the desalting process.

Insufficient wash water leads to poor contacting with Insufficient wash water leads to poor contacting with

brine droplets in the oil, reduces the dilution effect on brine droplets in the oil, reduces the dilution effect on

the saltthe salt

The wash water quality for refinery desalters is a key The wash water quality for refinery desalters is a key

process consideration that not only affects the desalting process consideration that not only affects the desalting

POSSIBLE CAUSESCORRECTIVE ACTION

Desalter capacity exceeded by handling heavieroil than design basis.

Decrease throughput.Increase operating temperature.Blend heavy oil with lighter oil.

Insufficient wash water rate.

Increase wash water rate to between 4% and 8% of oil flowrate.

Inadequate mixing.Increase mix valve DP in 1-2 psi increments to establish optimum.

Low operating temperature.

Increase temperature of untreated oil, close all unnecessary heat exchanger bypasses.

Low electrode voltage.Check electrical system for operating problems.

Insufficient demulsifier dosage or ineffectivedemulsifier.

Increase demulsifier chemical injection rate and/or changetype.

Troubleshooting Desalters Operating Inadequate Salt Removal

POSSIBLE CAUSESCORRECTIVE ACTION

Water level in desalter too high.

Check water level using interface samples; decrease to lowest level

that gives good quality effluent and clear water at 30 in. level. Check interface level controller and valve for proper operation;

check sensor calibration if necessary.

Stable emulsion formed in desalter.

Increase injection rate and/or change type of demulsifier chemical.

Excessive water injection.

Check that wash water rate is between 4% and 6% of oil flow

rate; stop wash water injection if controller or water flow meter

operation is questionable.

Gas forming in desalter vessel.

Operating temperature too high or back pressure insufficient.

Check backpressure valve operation.

Voltmeter and/or Ammeter Readings Vary Widely and Continuously

Failed entrance

bushing.

Check bushing and replace if

necessary. Ascertain that transformer

connected to bushing is not source of

problem before checking bushing.

Failed insulator inside

desalter.

Take desalter out of service. Empty

and purge the vessel. When entry is

permitted, enter vessel, determine

which insulator has failed by visual

inspection and/or electrical resistance

test, and replace it.

Energized electrode

has become grounded.

Shut down system, empty and purge

vessel. When safe entry permitted,

inspect vessel interior and

ungrounded electrode.

POSSIBLE CAUSESCORRECTIVE ACTION

Water slug entering with crude.

Reduce wash water injection rate and check off sites crude handling

procedures.

High water level in desalter.

Check level controller setting by using interface sampling system. Lower level while retaining good effluent water quality and clear

water at 30 in. level.

Sharp Increase in Current Draw (Amperage)

POSSIBLE CAUSESCORRECTIVE ACTION

Oil feed properties - highBS&W, low gravity, waxy

constituents, high particulate

loading, emulsifiers from oil

field recovery.

Slug feed chemical (e.g., 2 to 4 x normal rate) for a maximum

of 2 to 3 hours - then lower injection rate to less than 10 ppm to stabilize operation.

Investigate offsite crude handling procedures. Check for alternative chemical additive package with more effective

solids wetting agent.

Excessive mixing valve DP.

Open mixing valve completely, allow amperage to stabilize and slowly increase DP to

optimum value.

Wide Emulsion Band