Desalting crude oil[edit]

The salts that are most frequently present in crude oil are calcium, sodium and magnesium chlorides. If these compounds are not removed from the oil several problems arise in the refining process. The high temperatures that occur downstream in the process could cause water hydrolysis, which in turn allows the formation of corrosive hydrochloric acid. Sand, silts and salt cause deposits and foul heat exchangers. The need to supply heat to vaporize water reduces crude pre-heat capacity. Sodium, arsenic and other metals can poison catalysts. By removing the suspended solids, they are not carried into the burner and eventually flue gas, where they would cause problems with environmental compliance such as flue gas opacity norms.

DesalterFrom Wikipedia, the free encyclopedia

A desalter is a process unit in an oil refinery that removes salt from the crude oil. The salt

is dissolved in the water in the crude oil, not in the crude oil itself. The desalting is usually

the first process in crude oil refining. The salt content after the desalter is usually

measured in PTB - pounds of salt per thousand barrels of crude oil.[1] Another

specification is Basic sediment and water

The term desalter may also refer to a water desalination facility used to treat brackish

water from agricultural runoff. This may be done either to produce potable water for

human or animal consumption, or to reduce the salinity of river water prior to its crossing

an international border, usually to comply with the terms of a treaty. Desalters are also

used to treatgroundwater reservoirs in areas impacted by cattle feedlots and dairies.

Desalting of crude oil in refinery

Purpose of crude oil desaltingCrude oil introduced to refinery processing contains many undesirable impurities, such as sand, inorganic salts, drilling mud, polymer, corrosion byproduct, etc. The salt content in the crude oil varies depending on source of the crude oil.  When a mixture from many crude oil sources is processed in refinery, the salt content can vary greatly.

The purpose of desalting is to remove these undesirable impurities, especially salts and water, from the crude oil prior to distillation.

The most concerns of the impurities in crude oil:

The Inorganic salts can be decomposed in the crude oil pre-heat exchangers and heaters. As a result, hydrogen chloride gas is formed which condenses to liquid hydrochloric acid at overhead system of distillation column, that may causes serious corrosion of equipment.

To avoid corrosion due to salts in the crude oil, corrosion control can be used. But the byproduct from the corrosion control of oil field equipment consists of particulate iron sulfide and oxide. Precipitation of these materials can cause plugging of heat exchanger trains, tower trays, heater tubes, etc. In addition, these materials can cause corrosion to any surface they are precipitated on.

The sand or silt can cause significant damage due to abrasion or erosion to pumps, pipelines, etc.

The calcium naphthanate compound in the crude unit residue stream, if not removed can result in the production of lower grade coke and deactivation of catalyst of FCC unit

Benefits of Crude Oil Desalting Increase crude throughput Less plugging, scaling, coking of heat exchanger and furnace tubes Less corrosion in exchanger, fractionators, pipelines, etc. Better corrosion control in CDU overhead Less erosion by solids in control valves, exchanger, furnace, pumps Saving of oil from slops from waste oil

Desalting process

The desalting process is completed in following steps:

Dillution water injection and dispersion Emusification of diluted water in oil Distribution of the emulsion in the electrostatic field Electrostatic coalescence Water droplet settlingCrude oil passes through the cold preheat train and is then pumped to the Desalters by crude charge pumps. The recycled water from the desalters is injected in the crude oil containing sediments and produced salty water. This fluid enters in the static mixer which is a crude/water disperser, maximizing the interfacial surface area for optimal contact between both liquids.

The wash water shall be injected as near as possible emulsifying device to avoid a first separation with crude oil. Wash water can come from various sources including relatively high salt sea water, stripping water, etc. The static mixers are installed upstream the emulsifying devices to improve the contact between the salt in the crude oil and the wash water injected in the line.

The oil/water mixture is homogenously emulsified in the emulsifying device. The emulsifying device (as a valve) is used to emulsify the dilution water injected upstream in the oil. The emulsification is important for contact between the salty production water contained in the oil and the wash water. Then the emulsion enters the Desalters where it separates into two phases by electrostatic coalescence.

The electrostatic coalescence is induced by the polarization effect resulting from an external electric source. Polarization of water droplets pulls them out from oil-water emulsion phase. Salt being dissolved in these water droplets, is also separated along the way.

The produced water is discharged to the water treatment system (effluent water). It can also be used as wash water for mud washing process during operation.

A desalting unit can be designed with single stage or two stages.  In the refineries, the two stages desalting system is normally applied, that consists of 2 electrostatic Coalescers (Desalter).

 

Crude Oil Pretreatment (Desalting)

Description Desalting Process Safety Considerations Corrosion Considerations

 

Description

Crude oil often contains water, inorganic salts, suspended solids, and water-soluble trace metals. As a first step in the refining process, to reduce corrosion, plugging, and fouling of equipment and to prevent poisoning the catalysts in processing units, these contaminants must be removed by desalting (dehydration).

The two most typical methods of crude-oil desalting, chemical and electrostatic separation, use hot water as the extraction agent. In chemical desalting, water and chemical surfactant (demulsifiers) are added to the crude, heated so that salts and other impurities dissolve into the water or attach to the water, and then held in a tank where they settle out. 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. Both methods of desalting are continuous. A third and less-common process involves filtering heated crude using diatomaceous earth.

 

Electrostatic Desalting Flow Chart

The feedstock crude oil is heated to between 150° and 350°F to reduce viscosity and surface tension for easier mixing and separation of the water. The temperature is limited by the vapor pressure of the crude-oil feedstock. In both methods other chemicals may be added. Ammonia is often used to reduce corrosion. Caustic or acid may be added to adjust the pH of the water wash. Wastewater and contaminants are discharged from the bottom of the settling tank to the wastewater treatment facility. The desalted crude is continuously drawn from the top of the settling tanks and sent to the crude distillation (fractionating) tower.

Desalting Process

Feedstock From Process Typical products - to - unit

Crude oil Storage Treating Desalted crude to Atmospheric

distillation tower

Waste water to Treatment

Safety Considerations

The potential exists for a fire due to a leak or release of crude from heaters in the crude-desalting unit. Low boiling point components of crude may also be released if a leak occurs.

Because this is a closed process, there is little potential for exposure to crude oil unless a leak or release occurs. Where elevated operating temperatures are used when desalting sour crudes, hydrogen sulfide will be present. There is the possibility of exposure to ammonia, dry chemical demulsifiers, caustics, and/or acids during this operation.

Depending on the crude feedstock and the treatment chemicals used, the wastewater will contain varying amounts of chlorides, sulfides, bicarbonates, ammonia, hydrocarbons, phenol, and suspended solids. If diatomaceous earth is used in filtration, exposures should be minimized or controlled. Diatomaceous earth can contain silica in very fine particle size, making this a potential respiratory hazard.

Corrosion Considerations

Inadequate desalting can cause fouling of heater tubes and heat exchangers throughout the refinery. Fouling restricts product flow and heat transfer and leads to failures due to increased pressures and temperatures. Corrosion, which occurs due to the presence of hydrogen sulfide, hydrogen chloride, naphthenic (organic) acids, and other contaminants in the crude oil, also causes equipment failure. Neutralized salts (ammonium chlorides and sulfides), when moistened by condensed water, can cause corrosion. Over-pressuring the unit is another potential hazard that causes failures.

Crude oil desaltingAs many refineries around the world are increasingly processing “dirty sour crude” rather than “light sweet crude”, desalting is often a bottleneck, since the originally planned desalting capacity is insufficient to process the same volume of “dirtier” crude.  In addition, desalting is increasingly being carried out at wellhead sites (particularly offshore).

Desalting is commonly performed using electrostatic separating tanks, 50-100m3, operating at relatively high pressures of about 20 Bar, and temperatures up to 160oC.  The crude oil is heated and mixed with sweet water (and often demulsifiers) in order to dilute the salt, and the emulsion is separated in the electro-separators.

 

Crude Oil Desalting Process Flow 

The two-stage desalting process eliminates about 90% of the salt; 10% of the salt is present in the oil in the smallest droplets (10 microns and less), which do not coalesce with the introduced sweet water and pass untouched through the electro-separation dehydrators. This salt, which remains in the oil, is the major cause of corrosion in refinery distillation equipment, and a major driver of catalyst replacement cycles.

Our turbulent mixer/coalescer, with a volume of approximately 5m³, and driven by a 70kw electric motor, is placed in-line before the electrostatic separator, either replacing the mixing valve used to introduce sweet water and the demulsifier optionally following it. Throughput for this size device will be approximately 35,000 barrels per day, so for a 100,000 barrels per day desalting line (a refinery may have several lines), 3 mixers would be required.

The controlled turbulence in the mixer will reduce the final salt concentration in the desalter output of crude oil by more than half, which will result in a significant reduction of corrosion and catalyst damage and the associated expenses.

Alternatively, Turbulent Mixers can be used to approximately double the throughput of an existing desalting line without increased chloride content in the desalted crude output. A third option is to replace the first stage of a conventional two-stage electro-separation desalting line, with a Turbulent mixer.

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=======Crude Oil Dehydrators and DesaltersElectrostatic desalting is employed for both oil field and refinery applications, to facilitate the removal of inorganic chlorides and water-soluble contaminants from crude oil. In refinery applications, the removal of these water-soluble compounds

prevents or minimizes damage to downstream distillation equipment. Click here to learn more.

Conventional AC Electrostatic Dehydrator/DesalterThe conventional AC (Alternating Current) electrostatic dehydration system is an efficient method to remove high salinity formation water from the crude oil stream. This process relies on establishing a high voltage AC electrical field in the oil phase of dehydrator/desalter vessels. The electrical field imposes an electrical charge on water droplets entrained in the oil stream, thus causing them to oscillate as they pass through the electrodes.

During this oscillation the droplets are stretched or elongated and then contracted during reversal of the imposing AC electrical field. During this agitation the water droplets co-mingle and coalesce into droplets of sufficient size to migrate, by gravity, back into the lower water phase of the vessel for disposal.

CONVENTIONAL AC ELECTRODE FIELD

Several modifications are available to enhance the conventional AC electrostatic system. These modifications include:

Double-hot electrode system: apply power to the two electrodes Triple-hot electrode system: apply power to the three electrodes Systems tailored to the project specific requirements

Electrostatic AC/DC Dual Wave Dehydrator/DesalterGasTech's Dual Wave design utilizes both alternating and direct current through multiple plates to promote enhanced water extraction and chloride level reduction.

DUAL WAVE AC/DC PLATE TYPE ELECTRODE

GasTech's Dual Wave AC/DC system provides the client with the following benefits:

Increased flow rates, resulting in:o Smaller vessel footprints

o Less capital investments

Optimized water removal:o Lower quantities of water to the pipeline or storage.

o Less demulsifier chemical

o Less internal corrosion

Lower operating temperatures:o Conserves oil gravity and volume

STANDARD FEATURES OPTIONS

Inlet diffuser manifold Anti-vortex water collector Anti-vortex drains Oil collection weir baffle Open bottom distributor

troughs Dry oil collector piping AC/DC Plate type electrodes

Sand-jetting system Inverted sand troughs Interface sludge draw-offs Matrix packing AC electrodes Steam-out system Electrostatic

Heater/Treater

The GasTech horizontal heater-treater combines the advantages of emulsion breaking heat with electrostatic dehydration in a single vessel package.

The unique thermal section is designed with an inlet shroud for liberating both gas and free water from the emulsion. This flow path allows the free water to separate for disposal, without further heating, resulting in fuel savings for the client.

The open bottom distributor troughs provide even dispersal of the heated oil across the entire electrode section for optimum dehydration before being collected for discharge to the pipeline or storage.

Desalting

The first process unit in an oil refinery is the desalter. Salts in crude are generally contained in residual water suspended in the oil phase. The chemical composition of these salts varies, but the major portion is nearly always sodium chloride with lesser amounts of calcium and magnesium chlorides. Crudes also contain impurities such as silt, iron oxides, sand and crystalline salt as mechanical suspensions. Removal of these contaminants is known as “desalting”.

Process Description

Since the operating range of the process is usually 100° to 150°C,the desalter is located in the pipestill preheat train. The desired temperature is obtained by heat exchange between pipe still products or recirculated reflux, in the crude charge. To dissolve the salts and/or wet the impurities, fresh water of around 3-8% by volume is added to the raw crude then mixed with the crude via a mixing device, such as a mixing valve and/or a static mixer. The resulting oil/water mixture is then resolved by electrostatic coalescence through a high voltage electrical field inside the desalting vessel. Water droplets coalesce under the influence of the electric field, and sink to the bottom of the vessel. Electrodes maintaining the electric field are spaced to produce a voltage intensity of between 2,000 to 4,000 volts per inch. Applied voltage ranges from 12,000 to 35,000 volts. In cases of very high initial salt contents, desalting may be a two stage, or even a three stage operation.

Design and Operating Features

Mackenzie Hydrocarbon desalters and dehydrators have a number of proven design features that ensure process criteria will be met. Designs typically include provision for addition of demulsifier chemicals and usually include a hydraulic wash header to enable periodic removal of solids on the bottom of the vessel (mud washing). A number of power and connection options can be offered for the electrostatic grid of the desalter to suit specific operating conditions. Proper selection of mixing devices is also crucial to the success of the operation.

Transformers

High voltage transformers are essential in electrostatic treating. Usually the transformers are oil-filled, designed for hazardous location, and located close to the vessel. Power is introduced to the vessel through the entrance busing. Usually the transformers offer a number of voltage options, between 12,000 to 35,000 volts. Usually the internal voltage is AC, but DC is used in certain circumstances

IntroductionCrude oil contains mineral salts in various forms such as dispersed or emulsified drops of injection water, solid crystals of water-soluble salts, etc.

This salt is primarily dissolved NaCl (associated brines), which are produced

with crude oil. 

Crude oil desalting is a process by which a dilute water source is used to reduce the salt content by mixing salted crude oil and water, followed by coalescing of the resulting water for settling and removal of water droplets. Crude oil salt content must typically be reduced to 10 to 20 PTB (pounds per thousand barrel) to meet pipeline or tanker specifications, and to prevent salt deposition, corrosion, or refinery catalyst deactivation. Crude oil typically has to meet B.S.& W. specifications that may vary from 0.10 to 0.50 volume percent.

Coalescing is typically accomplished in electrostatic dehydrator vessels, occasionally in mechanical coalescors, or in field Heater-treaters or Chem-electric units. Special mixing devices are used for mixing dilution water with desalter brine water.

In rare situations, crystalline salt can exist in oil fields. Although crystalline salts is not common, it can occur in early production when very little water is present. Crude oil emulsions or Regular emulsions are very small water droplets suspended in a crude oil phase.

This article addresses the causes of crude oil emulsions and presents methods of treating and dehydrating crude oils. 

Causes of Emulsions Agitation and turbulence across a Choke Turbulence in Riser piping Shearing of Centrifugal Pumps Presence of Emulsifing Agents (Organic acids, paraffins, small solids,

chemicals or contaminants) Agitation caused by Gas Lift valves in Wells Valves or Fittings with excessive velocity

Factors that affect Emulsions Density difference between brine and crude oil

Viscosity - Heating reduces viscosity and improves coalescing Interfacial tension Water droplet size: Water settling velocity increases with Diameter

squared (Stokes Law) Water Salinity Volume percent B.S.& W. Emulsifying Agents Age of emulsions: Emulsions stabilize (become worse with time)

Desalter designUnder construction

TerminologyFree water – The percent of water that will settle out of an oil-water mixture in five minutes. This test is performed in a 100 milliliter laboratory cylinder.

B.S.&W. – Basic sediment and water. A standard test method is used along with solvent (typically toluene) and Demulsifier chemical in a 100 milliliter sample cylinder and centrifuged for 15 minutes.

Regular emulsion – This consists of small water droplets suspended in a crude oil phase. Various contaminants form a film around the water droplet preventing combining or coalescing of the water droplets.

Reverse emulsion – This consists of small oil droplets suspended in a water phase, which typically occurs where the percent water content or B.S. & W. exceeds 50 to 60 percent.

Loose emulsion – This consists of varying water droplet sizes with many droplets of a relatively large size, which separates readily upon settling.

Tight emulsion – This consists of many very small water droplets that do not separate readily due to high surface tension, or due to emulsifying agents which prevent coalescing.

Salt content – A measured salt content of a crude oil reported in PTB (pounds per thousand barrel). Salt is normally dissolved in the water or brine phase. On rare occasion salt can exist as crystalline salt.

Transformers – Device mounted on top of a Desalter or Dehydrator to step up electric power up from 480 or 4800 volts to 16,000 volts to supply to electric grids internal to the vessels.

Crude oil desaltingElectrostatic desalting, whether employed for oil field production dehydration and desalting or at oil refineries, is used to facilitate the removal of inorganic chlorides and water-soluble contaminants from crude oil. In refinery applications, the

removal of these water-soluble compounds takes place to prevent or minimise long term damage to downstream distillation processes.

The electrostatic desalting process consists of two important factors:

Emulsion preparation by increasing the population density of small water droplets suspended in the crude oil via wash water injection

Creating a uniform droplet size distribution by imparting mechanical shearing and dispersion of the dispersed aqueous phase