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Free «The Process Of Separating Oil From Water: Dewater And Water Treatment: Desalting» Essay Sample

Introduction

Oil and water separating processes are well-known in the oil industry. The major differences in technology are not required when separating the oil and water mixture that has been recovered during an oil spill cleanup operation. The variation of influent is the major difference between the separation of oil and water connected to the handling and production of oil (American Petroleum Institute, 1979).

During a recovery action, the ratio of oil to water varies from the 90% oily phase during the boom containment and the collection in the calm water, to as much as 98% oil free water by use of the same equipment (Maelstrom, 1984). This is with three-meter waves. The lack of preparation for all the possible scenarios is another problem. The utility system, equipment’s, and the chemicals used should be flexible and general type that is capable of adapting all situations (American Petroleum Institute,1990).

The separation of the oil and water mixtures in chemical plants, electric power plants, refineries and other industrial facilities can cause problems. The inadequate understanding on how to take advantage of the mixtures properties to fulfil the separation required causes the problems, as well.

Treatment of the storm water from parking lots and other facilities where cars and trucks are present is necessary to prevent the entry of oil and fuel that may have leaked from vehicles into the water bodies (Banach, 1970). What is required for separation is the oil and water or the oily phase. The free water and the emulsified water are the two types of water. Most crude oil when spilled in the sea will undergo emulsification, the large amount forming a water-in-oil emulsion (9w/o). The stabilization of the w/o emulsions is by the natural emulsifier within the oil. This is due to the presence colloidal particles in the sea or the by-products of the biodegradation of oil (Au et al., 1992).

The Sources of Oil and Grease in the Water

One of the sources is from the petroleum refining and the used oil re-refining. The used oil, re-refining operation and refinery from the primary distillation through the final treatment contain certain amounts of the organo-sulfur compounds and oil in their wastewaters. Grease and oil appear as the free oil, emulsified oil - as a coating, suspended matter, or dispersed oil.

Another source is the crude oil producing facilities. Wastewaters from the oil field operation may contain various impurities, such as the free and emulsified oil, tank bottom sludge, drilling mud, brine, and natural gas. Brine bearing formations may be found in the oil-bearing strata (Banach, 1970).

The Types of the Oil Water Separation

There are four main types of the oil water separation that are used in the industrial plants, and numerous same situations do often exist. These types are emulsions, non-hydrocarbon oils, oil from water where main flow is mostly water, and water from oil where main flow is mostly oil.

Emulsions

An emulsion is not a solution; it is a mechanical mixture that consists of dispersion of droplets of one immiscible fluid in a continuous fluid. Two types of emulsion are common in the case of the oil and water separation, and this depends on the continuous phase. These types are water in the oil emulsion and oil in the water emulsion (Banach, 1970). The other type of emulsion is the water present in the oil; though, it is much rare.

Non-Hydrocarbons

Recently, the interest in renewable fuels has provoked the interest in the vegetable oils as fuels, especially as the biodiesel. Different problems are caused by the biodiesel system and the vegetable oil during the separation between the aqueous and non-aqueous phases (Hoffman, 1982). The high viscosity of most vegetable oils and the solubility issues in the facilities dealing with the biodiesel production causes an complication during the separation (Montgomery, 1985).

Oil from Water Where the Main Flow is Mostly Water

In the chemical plants and other industrial facilities for resource recovery and for the environmental conservation, the separation of oil from the continuous stream of water is commonly done. In some oil field situations where water is the main flow from the wells, this is also practiced. The application of the scientific principles to these separations began in 1948 when a study by the University of Wisconsin was commissioned by the American Petroleum Institute (API) in the preparation of a method for designing separators in recovery of oil from the main refinery waste water streams. The design was not worked out for the environmental purposes and generally produced discharge that is not suitable for release in the water bodies; though, it is still in use. This method is costly and bulky since it requires o lot of residence time, and therefore, the so modified design; ‘API Type’ are commonly used (Love, 1998).

Many designs have been used since 1948 in removal of the oil from water; some of them are represented in the following subchapters. It is possible to remove oil from water down to less than 10 mg/l by the use of the newer designs.

The Water from Oil where the Main Flow is mostly Oil

In the oil production applications, chemical plants, oil refineries and where it is a requirement that hydrocarbons should not be contaminated with water, the separation of water from the continuous flows of oil is a demand in such applications (Romano, 1990). During the last part of the World War II, the possible problems with the water contamination were first emphasized when it was discovered that the water freezes in the fuel lines by the airplanes flying high. This resulted in the inconveniences to the pilots since it caused the engines to stop, so the equipments were designed such that a small amount of water was allowed to remain in the aviation fuel. There was also a discovery that the refinery processes operated easier and better and the corrosion problems were prevented by the removal of water from the hydrocarbons (Romano, 1990).

A number of equipment has been designed so as to deal with the problems that arise when separating the water from the oil. These problems resulted in the removal of the water vary because of the variation of the viscosity of the hydrocarbons that need to be treated (Equipment manual,1980).

The Introduction to the Oil Water Separation Theory

Gravity and various enhanced gravity systems may separate oil and water. In removing water from oil, water droplets fall within oil (Romano, 1990).

It is advisable to apply some additional force so as to make the water separate in cases where the continuous phase is oil. An electrical field is applied in the electrostatic desalting, and tightly packed fiber beds are used in coalescing cartridge separators. Gravity and enhanced gravity are discussed below.

The Settling of Particles in a Gravity Separator

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The Stokes’ law governs the settling of the solid particles in a clarinet, and other settling devices. This function is stated as follows.

VP = {G/(18*U)}* (do- dc ) * D2

Where: Velocity of p = settling velocity of droplet in cm/sec.

 U = absolute viscosity of the continuous fluid in poise.

G = gravitational constant, 980 cm/sec2

Density of p = the density of the droplet in gm/cm3.

Dc = the density of the continuous fluid.

D = the diameter of the particle in cm.

A particle or the droplet rise velocity is a negative number since the equation was developed for falling solids. There are several assumptions that stoke made, and these are: particles have the same size, spherical, and the flow is laminar, both vertically and horizontally (Maelstrom, 1984).

The most significant variables from the above equation are the particle size, viscosity of the continuous liquid, as well as specific gravity difference between the particle and continuous liquid. The settling velocity and the size of the separator required can be calculated after these parameters are known (American Petroleum Institute, 1979).

Firstly, it is essential to calculate the size of an empty-vessel gravity separator and the rise velocity of the oil droplets by the use of the Stokes’ law. The calculation of the size of the separator is done in thought of the path of a droplet getting at the bottom of the separator and getting out from the other end of the device. The oil droplet entering the separator at its bottom will have time to rise to the surface before the water carrying the droplet leaves via the other end of the separator if the adequate volume is provided in the device (Au et al., 1992).

Since oil droplets are not of the same size and they tend to turn into larger droplets, the calculation of the rise rate using this method is a simplification of the real conditions that are necessary. The large droplets have trailed tails that are similar to the raindrops. These tails are caused by the distraction of the hydrodynamic drag.

If the laminar flow conditions prevail, then the droplet rise follows the Stokes’ law. The flow in the direction of the rise exists with the small droplets. Velocity of the laminar flow may be exceeded by the rate of the rise of the larger droplets, and in this case, the flow begins to be turbulent. The Stokes’ law expects the particles to rise fast, but this is not the case. When the particles come together, they become large droplets, and they do not form a flow. The droplets tend to assume the spherical shapes, due to effect of the surface tension since the spherical shape is the smallest possible shape for a given mass (Jackson, 1961).

When the particle size and the continuous liquid viscosity are accurately known, then the calculation of the oil drop rate and the solids settling is accurate. These calculations have problems, such as the size of the viscosity of the continuous liquid (American Petroleum Institute, 1990).

The viscosity of the continuous liquid is measurable in the case there is oil. It is found in the literature data in the case of water, such separators require design over a range of temperatures to be liable during the summer and winter situations, in addition to the possible process upsets; therefore, a number of viscosities may be put into consideration during the design (American Petroleum Institute, 1990).

The size of the droplet is difficult to determine. The particle size is fairly determined in the laboratory, but the information entailing the oil droplet size is hard to get. Taking a microscopic photograph of droplets in water and counting the number of the size droplets is one of the tiresome ways in the determination of the oil droplet sizes. Rommel notes that other methods have been used with the varying success (Rommel, 1992). These methods include the use of the particle counters, such as the electric sensing zone particle counters (Montgomery, 1985).

It is possible to generate the quantities of the oil droplets of the generally equal size with the use of the ultrasonic and dispersion methods. The droplets found in the normal field operation vary largely in size. From the particles that are less than 5 microns to the large quantities of oil found in the oil spills.

When the droplet size is unknown or a large range of the droplet is present, it is possible to determine the size separator required and the rate of rising of the droplets. The minute droplets contain extraordinary small quantities of oil - since the volume of oil in a droplet is proportional to the cube of the diameter. The oil droplets may be examined vastly, and the quantity of oil represented by them may lead to environmental problems. This happens when they are discharged into the surface of the waters.Oil should not be present in the vast quantities. This has effect on the oil withlittle particles to shrink. Standards have been enacted by many jurisdictions, and King County allowing the discharge of oil significantly lowers by the EPA limit of 15 pm oil and grease in the discharged water. It is wise to progress carefully and cautiously in designing the oil-water separator systems to reduce the possibility of such a discharge (Montgomery, 1985).

Non-Dissolved Versus Dissolved Oils

All the separators deal with the non-dissolved oils occurring in crude oil. In designing, the equipments, the limited solubility of the hydrocarbons in water and the considerable solubility of Benzene must be considered. . If these chemicals are present, then other methods of removing them must be put into consideration, such as distillation (Rommel, 1992).

Water from Oil Separators

Two and Three Phase Separators

In the oil production, as well as the refining systems and chemical plants, the two and three phase separators are frequently used. Mostly they are empty vessels, sized based on empirical relationships and provided frequently with rudimentary baffles and mesh pads for the elimination of mist and the heating arrangements that raise the oil temperature. This reduces the viscosity which, in turn, assists in the separation (Equipment manual,1980).

Where only oil and gas are present with no aqueous phase, two phase separators may be used. In conditions where a small amount of gas is present with the aqueous solution and the hydrocarbon, it may also be used. They are known as “free water knockout drums.” They are designed as either horizontal or vertical vessels. Sphere is the most economical shape while manufacturing in a high pressure design, and thus, it is the reason as to why it is used in the high pressure systems (Romano, 1990).

The three phase separators are the same as those of the two phases, but they are provided with the water connection, gas and the oil draw off. It is worth noting that a number of the general designs have been used. The one shown below is typical of the oil field practice. It is also heated, and burning of some of the gas in the incoming stream is the source of this heat. It is as shown below.

Electrostatic Des alters and Treaters

The electrostatic treater or des alters entails the generation of the high voltage electric field through which the crude flows to the exit header in the vessel. This is from entrance header that is below the electrodes. In the electric field, the small water droplets in the crude are coalesced into the large droplets that fall rapidly to the interface level, thus leading to the removal of the entrained salt and fastening the settling rate of the water phase (Montgomery, 1985).

High voltage is applied to one of two sets of steel electrodes grids in the vessel in the unit. The two sets are parallel to the horizontal center line of the vessel. The lower grid is charged with the secondary voltage or the transformer and is situated near the center line of the vessel. The grid is suspended from the insulted support frame. The upper grid is supported to the vessel wall. It serves as a ground grid.

The retention time in the electric field is determined by the flow rate. The coalesced droplets cannot settle when the flow rate is increased beyond the capacity of the unit, and some solid particles carry over into the product (Jackson, 1961).

Des alters differ from the treaters as they are provided with the additional water beyond the naturally entrained in the oil flow. This is so since the main function of the des alters is the removal of the water present. The main function of the des alters is removing salt present by dissolving them in the water, and then r the water is supposed to be removed. Corrosion is caused by the presence of the salts downstream in the refinery, and thus, they are removed (YE, 2010).

Coalescing Cartridge Separators

There are two types of coalescing cartridge separators. These are: packed separators or hay packed and Filter cartridge separators. The packed separators were first developed at the end of the Second World War so as to treat the aviation gasoline to remove water. The media are made up of fibrous materials, such as the excelsior, teflon shavings, stainless shavings, or the other fibrous material (Jackson, 1961).

These systems include one or more bales of the media in a horizontal pressure vessel.They function by the provision of the surface for the aqueous phase to build up on, and leading to the laminar flow that allows some settling time without channelling. In the vessel, the hydrocarbons flow horizontally. The occurrence of this causes an accumulation of the aqueous phase in the packs that lead to the formation of the large droplets that are removed at the bottom of the vessel after draining. This media are shown in the following figure.                                         

The Coalescing “Hay Pack” was constructed by the use of the excelsior wood shavings. Even though the excelsior type products are efficient to a good level in achieving the desired results, they still encounter some hindrances and tribulations. Sice it is organic it decomposes, and this proves that it has minimal duration. In many instances, the excelsior replacement is necessary since it has a tendency of clogging easily with debris, dirt and other materials that cause contamination. Clogging causes the fluid to channel, meaning that the fluid flows through the filter media in the small channels; thus, a small percentage of the filter media is used. The materials that are used to make the excelsior are randomly oriented, with no preferred direction of inclination of the elongated components, and it is also another difficulty. This makes the water droplets that coalesce on such a material not be preferred in the downward orientation in their flow path. This can make the droplets be disconnected from the horizontal portions of the filter medium, thus becoming resuspended in the fluid stream (Jackson, 1961).

The filter cartridge separators are mostly used in treating the refined petroleum products. This enhances the removal of the water acquired during the time in the tanks and pipelines. The cartridge used is composed of a dense fibreglass mat held together with a phenolic plastic binder. They are also used together with the screen separator cartridge of a hydrophobic character. In the separator cartridge, the flow goes from outside to inside; while in the coalescing cartridge, the flow runs from inside to outside. In the coalescer cartridge, the water droplets are coalesced into large droplets, and the hydrocarbons prevent them from leaving the filter vessel by the separator cartridge (Jackson, 1991). The coalescer cartridges are dense, and there are also very fine filter cartridges. They plug very easily as compared to other methods of separation or the packing method as described. It is the reason why they are used where the excellent water removal is necessary or where there are no solid particles. The treating jet fuel, kerosene or the gasoline are the general application for the removal of water. The schematic operation of a typical coalescer or the separator system is shown in Figure 4

A cartridge is commonly applicable in the treating of the aviation fuels.

Absorbent Separators

Absorbents are used as the final stage in the treatment of the hydrocarbons, such as the gasoline or the jet fuel. They are not separators since they do not separate and remove water, but they only confiscate it. This prevents it from passing downstream to the automobile or airplane. They are an expensive way of removing water from the hydrocarbons, and they are used where they are essential even though they are dense and are easily mounted on the fueling truck (Montgomery, 1985).

Legal Aspects

In an industrial setting, water is only separated from oil and the other hydrocarbons, which are not supposed to be released to the environment (Au et al.1992). Therefore, the safety aspects (OSHA) are the only legal aspects in the treatment of oil in the removal and discharging of the water. It is the reason why the following only deals with the legal aspects in the removal of oil from water.

A variety of the local , state and the federal laws governs the oil precence in the water discharges from the industrial and other facilities. The Oil Pollution Act of 1990, The Clean water Act (CWA) and its amendments, the Coastal Zone Management Act and others also govern these discharges (Banach, 1970).

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The Resource Conservation and Recovery Act of 1976 and its amendments do not cover most of the hydrocarbon wastes. It is also not covered by the Comprehensive Environmental Response, Compensation and the Liability Act also known as the Superfund Act.  These wastes are exempted from the regulation as “hazardous wastes”under any other law. The wastes are produced by the extraction, transportation, refining or processing of oil and the natural gas (Banach, 1970).

The Clean Water Act is the basic law that covers the discharges. It was amended at length in 1977 - but was enacted originally as the Federal Water Pollution Control Act of 1972. The Clean Water Act became known after the amendments added in 1977, in conjunction with the earlier legislation. The amended section 402 created the National Pollutant Discharge Elimination permit system under the terms of the Clean Water Act. The States with the EPA approved programs or the Environmental Protection Agency (EPA) gives the permit for the point sources under this system. Other discharges covered by the permit are illegal after this law has been enchanced. The Clean Water Act also controls the discharges of petroleum and other hydrocarbons. It is worth noting that it was enacted mainly to control the the discharges released from the Public Owned Treatment Works or the Sanitary Sewer Plants, and the harmful discharges from the industrial plants (American Petroleum Institute, 1990).

Considering the 24 hour composite sample, it is a requirement of most localities and the states for the discharges to contain 15 pm or minimal grease and oil. The animal and vegetable oils, as well as the petroleum are contained in oil and grease. Some localities have a minimum discharge limit. Counties like Washington and King county require the discharges to be less than 10 pm (American Petroleum Institute, 1990).

The new stormwater management rules published by the EPA in 1990 are also significant (Permit Application Regulations for Storm Water Discharges; final rule, 1990). The National Water Quality Inventory, 1988 Report to Congress consists of the reasoning behind the severe regulation of the stormwater. “Pollution from the diffuse sources, such as urban areas, land disposal, construction sites runoff from the agricultural and the resource extraction is the major cause of the water quality impairment as cited by the States” (Montgomery, 1985). This was the conclusion of the report. As the discharges of the industrial process wastewaters and the municipal sewage plants come under the increased control, the sources of the discharges tend to gain significance. The illegal discharges to the stormwater sewers at the rate of 60% in the businesses that are related to the auto-mobiles, such as auto dealerships, body shops, and the service stations, were detected by a study carried out by the Huron River Pollution Abatement Program (Federal Register, November 16, 1990).

The stormwater discharges did not need to have permits under the NPDES system up to the publication of the final rules in the Federal Register, November 16, 1990. The discharges from the snowmelt runoff, surface runoff, drainage and the rainwater runoff refer to the StormWater Discharges. This regulation does not cover the waters that do not meet this definition. Facilities with the stormwater discharges from the areas containing raw materials, by-products, intermediate products or the finished products located on site require a permit. This is specified in the new rule. These regulations exempt a number of categories of the facility. These are stormwater runoff from oil and gas exploration, mining operation, processing, and production parking lots, which have the rainwater sewers that are not unified with the manufacturing facility sewers (Montgomery, 1985).

It is important to note that the above legal aspects concern mostly the discharging waters directly to rivers and streams. Different rules must be applied when the discharge is in the sanitary sewers. 

Pretreatment/ to Discharge Sewer

The sanitary sewer authorities must meet the requirements of the Clean Water Act and the Clean Air Act. In order to meet their discharge permit requirements, the discharge to the sanitary sewer must have a large amount of the oil content. The amount varies, but it is frequently about 100mg/l (Montgomery, 1985).

Most industrial discharge their wastewater to the sanitary sewer since the paperwork requirements are very minimal. The local authorities have to be consulted before such a discharge is released. This is done so as to determine their requirements concerning the discharge quality and monitoring of the discharge. The discharge solutions to the surface water is the same as the solution to the problems of discharging to the sanitary sewer. It may be easier to implement because of the higher discharge limits (Love, 1998).

Practical Separation of Oil and Water Technology

The present state of technology used in the oil-water separation equipment has evolved logically from the industries dealing with the production of petroleum. It is suitable for the oil spill situations. The oil spill recovery separator that are appropriate for use in future designs should endeavor meeting the difficult set of operating requirements as shown in the following list. They were recommended by the Marine Response Corporation (MSRC) and the US Coast Guard after having carried out the joint test that was meant to evaluate the separators listed in the Oil Spill Response Products in the World Catalog (Pratt, 1996).

  1. A size that can fit within a space of 125 ftby 3.5 m.3
  2. The water discharge that is clean enough for the discharge into the sea directly. The discharge that is acceptable for the open sea is greater than 15 p.p.m.
  3. A suitable through put capacity for an oil water separator should be between 250-500 gm.
  4. The capability of processing oil in the viscosity range is of 1500-50000 St.
  5. The suitable unit size that is relatively light in weight, portable and compact. 4000-6000 pounds.

Oil from Water Separators

Separators of Pure Gravity

A pur gravity separator is the simplest type of separators. They are large holding tanks that are joined together in series. The debris laden oily water is pumped in and allowed to settle under the gravity action. The surface oily flotsam is then pumped off or skimmed. The water that is separated is then pumped off or drained just near the bottom of the tank. When these tanks are joined together in the series, the oil from the top of the first tank is taken into the second tank, and then the dispersed water that is remaining in the oil will have an opportunity to separate, and the water quality removed from the bottom of the second tank exceeds that from the first one (Romano, 1990). The rate of pumping determines the residence time in cases where pumping is nonstop, and this affects the degree of the separation. It will be doubted when under 15 p.p.m. at any rate of pumping (Romano, 1990).

Spill Control Separators

An empty chamber with an adequate volume so as to contain spills is the simplest possible separator. It is too small for it to cut off the small droplets, but it is significant for cutting off the spills of grease or oil. If the accumulated oil is removed from time to time, then the spill control separator will be effective. A storm may flush the accumulated oil out of the separator into the downstream if oil is not removed from time to time. This typical spill control separator is as shown in Figure 5.

The API systems are also similar to spill control separators. They are more sophisticated, larger, equipped with the oil removal facilities, and more effective. The design criteria for the oil water separators are provided by the American Petroleum Institute (API). Ther is a provision of the API manual on the disposal of refinery wastes.

In the oil refineries and the chemical processing facilities, the API separators are largely used where there is the presence for the large quantities of oil. They need to be processed so that they can meet the requirements of the NPDES permits. Below, there is a diagram of a typical API separator that is adapted from the API Publication 421, 1990. The same API publication contains a refinery API separators survey indicating them not to meet the requirement of the Clean Water Act (American Petroleum Institute, 1990).

It has successfully been used in the refineries for a long time. It is more effective, requires low cost, and has a simplicity of the design than the control separators.

The Coalescing Plate Separators

Depending on the application, the sludge and oily water are collected in a wash down pit where the heavier solids are trapped by a stainless steel basket or by using a silt tray. Using a system of pumps, oil and other remaining solids are then conveyed into the Cross Flow Interceptor Unit. The gravity separation between the corrugated multi-plate pack is used to remove oils and solids from the water where the oil droplets, upon rising in the flow, are caught when reaching the plate above just like in the same way solid particles are retained when they settle on the plate below. The corrugated plate packs are inclined at 60 degree to the horizontal. Where the pump out pits are discharging into the sewer, they must comply with the local water specifications. The separation units can be coupled with holding wells where water is eventually recovered and recycled into the clean water if desired (Mascot Engineering Group, 2012).

The Inclined Plate Separators

The removal and reduction of turbidity of thick suspended and flocculated solids from the waters and wastewaters are done using the Inclined Plate Separators in one complete system.  Basically, the process consists of rapid mixing and flocculation tanks that precede an inclined plate section with either a sludge hopper located directly underneath or an integral sludge thickener (Arnold & Stewart, 1999).

Horizontal Sinusoidal Plate Separators

The flat, also called horizontal, sinusoidal plate separators, are the corrugated plate separators with horizontal oleophilic polypropylene plates, which are held into position in packs with rods or wires. In this system, a combination of laminar flow coalescence and the oleophilic attraction is used. Turbulence is minimized by slowing the flow of water to low velocities to achieve the laminar flow regimes since the mixing of oil and water occurs due to turbulence, thus reducing oil droplet sizes. The larger droplets will raise faster and, thus, separate better in accordance with the Stokes’ law. Due to the oleophilic nature of the plates, oil droplets attach to them, thus encouraging them to coalesce into larger ones, which will then rise faster. This system is disadvantageous in that packs of solids can plug the plates damaging them and the vertical polypropylene plates are prone to attack by the solvents. Although the plates positioned vertically help to alleviate plugging by solids, they do not coalesce as effectively as it should be (Arnold & Stewart, 1999).

Multiple Angle Separators

The idea behind the development of multiple angle plate separators was to eliminate disadvantages of the horizontal sinusoidal separator plates. In a sort of the "egg-carton" shape, the plates create a sort of  the spacers built into the plates for two spacings nominal 0.25" and 0.5", or 8mm and 16 mm. The multiple angle system separators have a number of advantages. Firstly, since the plates shed solids to the bottom of the separator, it avoids plugging and directing the solids to where solids are collected. Secondly, as opposed to a case where solids must slide down the entire length of the plates like in the inclined plate systems, the solids slide for a few inches just before getting to where one multitude of solids removal holes in multiple angle systems (Stewart, 2009). Thirdly, the double corrugations in the multiple angle systems coalesced oil can easily migrate upward since the system provide surfaces that slope at least a forty-five (45) degree angle in all directions. Fourthly, convenient orifices for the insertion of cleaning wands are provided by the holes in the plates that constitute the oil rise paths and solids removal paths. Lastly, the above ground units are factory fabricated and, therefore, require minimum installation time. In addition, most large units are designed utilizing plates installed in the in-ground vaults.

Absorbents

Although it is expensive, absorbents are effective means of removing residual oil in water. In this method, activated carbon or other absorbents, such as sawdust, polypropylene fibers or polyester are used. Despite the fact that carbon is preferable at the polishing phase, it can be prohibitively expensive if the first stages are not effective. With the use of absorbents, non- detectable levels of hydrocarbons in the effluent water can be effectively removed by solvents. It is considered an advantage over other types of separation methods. Furthermore, no other equipment is usually required since absorbents are simply spread on the water to absorb the hydrocarbons.

However, the use of absorbents is that they are very expensive on every pound of hydrocarbon removed from the water and because they are readily used up. They are usually not changed frequently enough resulting in no system at all to remove the oil from the water after some time (Arnold & Stewart, 1999). 

1.0  Water Treatment: Desalting

Oil produced in most oil %uFB01elds is accompanied by water in the form of an emulsion that must be treated. Although processes for desalting the high-salinity water have been known for a very long time, their large-scale feasibility reached practical levels 50 years ago. The thermal-driven distillation systems (flash evaporators), electrically driven membrane systems (electro-dialysis) and pressure-driven membrane systems (reverse osmosis) are some of the widely used technologies. The criteria for selecting a desalting process include: product water goals, source water characteristics, concentrate disposal issues, contaminate removal requirements, as well as economics among other factors (Escobar & Schafer, 2010).

Reverse Osmosis (RO)

 
 
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In RO, pressure is used to separate water and salts by allowing some of the feed water to move through a membrane. The membrane blocks the passage of the dissolved salts. The desalted water pass through the membrane, and thus, the concentrate is trapped in the membrane. Water softening and other treatment applications requiring dissolved salts causing hardness, such as calcium and magnesium, or the minor salt removal is done using nanofiltartion (NF). NF is a membrane process similar to low-pressure reverse osmosis (Bergman, 2007).

Electro-dialysis (ED)

In ED, the electrical potential is used to move salts from the feed water through the membranes leaving dilute/desalted water as the product water. Microfiltration (MF) and ultrafiltration (UF) are sometimes used to pretreat the feed water before RO (Escobar & Schafer, 2010).

Thermal-driven (Flash evaporators)

It is the oldest method of desalination. This process involves heating of saline water to obtain virtually pure water vapour, which is then condensed to obtain the dilute water. A typical flash evaporator uses an external source of energy, such as steam from power generating plant to heat the incoming water to the boiling point. There are three commercially flash evaporators (thermal desalting processes): multiple effect distillation (MED), multistage-flash (MSF), vapor compression (VC) (El-Dessouky & Ettouney, 2002).

Disposal of Concentrate

Since all desalting processes produce a concentrate, it must be disposed accordingly. Concentrate disposal methods include the disposal to a disposal well, disposal to a surface water, disposal to a land surface, as well as disposal to a sanitary sewer and evaporation (Ladewig & Asquith, 2012).

2.1 Micro-organisms in Water

Microorganisms, such as algae, bacteria and other biological matter, can be found in water. These biological matters have the same effect in a desalting process as in form of scale-forming precipitants or suspended solids or both. An ideal environment for thebiological activity is concentrated, due to the nature of the desalting process,. In order to inactivate these biological activities, it has been the practice to use certain pretreatment techniques, which involve the application of a strong oxidant such, as ozone or chlorine. Although the use of chlorine or ozone is good for the thermal processes, it is not recommended for RO and ED membranes since the membranes are damaged by the presence of strong oxidants. However, the low concentrations of chlorine, can be used in the ED membranes since they tolerate the low levels of chlorine. Nevertheless, RO processes using a pretreatment of an oxidant require an additional step of deactivating any remaining oxidant that may be present in the feed water (AWWA, 2004, p.16).

2.2 Significance of Crude Oil Desalting

General Overview

When crude oil is processed in a re%uFB01nery in its raw form, the salt can cause various operating problems if it is left untreated. Consequently, it is usually a requirement that crude oil first undergo the desalting/dehydrating process before being refined to reduce its level of salinity. Desalters are the equipment used to remove salt from crude oil. Initially, salt is dissolved in the water but not in the crude oil. It should  be removed before refining because of a number of reasons. Firstly, since they are responsible for the high temperature downstream, it can cause hydrolysis of water, thereby enhancing the formation of hydrochloric acid, which is corrosive. Secondly, the presence of silt, sand and salts deposits in the oil refining system is responsible for fouling. Thirdly, arsenic, sodium and certain other metals usually compromise the efficiency of catalys. Fourthly, the removal of the suspended solids imply none is carried into the burner. Finally, there are environmental concerns that restrict certain levels of flue gas emissions (Al-Otaibi, 2003, p.65-82).

2.3 Effects of the Quality of Crude Oil on the Performance of the desalter

Crude oil usually contains water, the inorganic salts, the suspended solids, and the water-soluble trace metals. While starting the refining process, to reduce the corrosion, plugging, and the fouling of equipment not poisoning the catalysts in the processing units, these contaminants are eliminiated by desalting also referred to as dehydration.

Oil desalting or dehydration is the process of removing the water-soluble salts that exist in the oil stream.Crude oil is extracted in a number of oil fields, especialy those that are old (resulting in the wet crude oil production) contains traces of the salty water. Normally, the dissolved salts are principally chlorides of magnesium, sodium, and calcium. A desalter is used to remove them. In a desalting unit, when crude oil is heated as part of various desalting/dehydration or re%uFB01ning processes, the water may be driven off as steam. The salts in the water, however, do not leave with the steam. They crystallize and may either remain suspended in oil or form scale within the heat-exchange equipment (Al-Otaibi, 2003, p.65-82).

The two common techniques of crude-oil desalting, the chemical and electrostatic separation, make use of the hot water to be the extraction agent. In the chemical desalting, the water and the chemical surfactant called demulsifiers are added to crude, heated for salts and other impurities to dissolve in water or hold themselves to the water. Later, they are held by the tank where they are allowed to settle out. The electrical desalting process makes utilizes high-voltage electrostatic charges in order to cause the concentration suspended the water globules down in the settling tank. Then the surfactants are added when crude oil has large volumes of big suspended solids. The two methods of desalting are on a continuous basis. The third and the less-common process entail filtering the heated crude oil by the use of the diatomaceous earth. The inadequate desalting can lead to fouling of the heater tubes and even the heat exchangers in the refinery. The fouling process restricts the product flow and the heat transfer, and this causes failures, due to the presence of increased pressures, as well as temperatures. Corrosion that occurs since the hydrogen sulfide, hydrogen chloride and naphthenic acids and the other contaminants are present in the crude oil, thus leading to equipment failure. The neutralized salts after being moistened by the condensed water can lead to corrosion. Over-pressuring the unit is a great potential hazard that might cause failures (Crude oil pretreatment (desalting), 2008).

2.4 Fundamentals of Electrical Desalting

An electrical system that connects to electrodes within des altersgenerates an electrostatic field at potentials between 6,000 to 20,000 volts. It is to induce the di-pole attractive forces between the neighboring water droplets. Similarly, the electrostatic field leads to each droplet that has a positive charge on each side and a negative charge on the opposite side causing the droplets to coalesce, due to the attractiveness of the unlike charges. This leads to a larger water droplets called globules. The water settled is continuously removed from the desalter from the point above the des altersbottom. Such a bottom is called a brine as it has the inorganic salts which, originally, came in the des alterswith the water present in crude oil. Thus, the settled sediment is withdrawn as sludge at the intermittent intervals aimed at preventing the solids from penetrating the now settled water withdrawal outlet. Most of the refining processes found in a petroleum refining company gives the waste water streams that entail the dissolved hydrogen sulfide (H2S) and ammonia (NH3) gases that are in the form of ionic ammonium hydrosulfide(NH4HS). The refineries put together their sour waters and utilize the steam distillation towers called the sour water to strip virtually all hydrogen sulfide from the aggregated sour waters. Then, the striped sour water is recycled to be used later on as des alterswash water.;that is augmented by the fresh water if it is required. The part of the streams of the refinery sour water is composed of phenols. Thus, the water has phenols absorbed by crude oil and subsequently form part of naphtha and kerosene fractions that are distilled from crude oil (Desalter and dehydrator processes,2012).

Types of Desalting Systems

Desalting systems can be categorized as single, as well as two or three stage desalter depending on their layout ans operation performance. In the single stage, there is no dilution water injection. The method is usually recommended for the low salinity brine water. In a two stage desalter, the dilution water is injected into a mixer prior to the second stage desalter. This type is used for moderate brine salt considerations. Sometimes a two stage desalter has a brine cycle, and dilution water is injected prior to the final desalter. This system is used in situations where there are high salinity brine waters, such as those found in the Middle East. In applications where the salt content exceed 250,000pp, such as in Saudi Arabia, a single dehydrator vessel followed by two desalter vessels is used. In this arrangement, the dilution water is injected into the mixer head of the third stage desalter with the brine water being recycled back to the mixer head of the second stage desalter. Three phase separators are similar to two phase separators, except that they are provided with connections for water, oil and gas draw-off. (Kirby & Mohr, 2008, p.8; Desalter and dehydrator processes,2012).

Desalter components

A des alters is composed of components, such as process vessel, distribution system, electrodes and transactors, mud wash, and level control devices. A desalter vessel is a large containment where water, a high voltage electrified grid, and chemical additives, are used to remove corrosive salts from crude oil before the refining process. In the vessel, three liquid phases form: crude and salt water ( Refinery decreases usage of chemical additives with reliable desalter interface measurement, 2010).

A distribution system is a set of conductors that distribute electrical power to each electrode. Electrodes are to maximize the performance of the dual horizontal distribution system. The following system helps in aggressive mixing with a specialty valve of the crude oil and wash water so that in case coalescence occurs, more impurities are removed with water, and the cleaner treated product is realized. The strong electric field is also less dependent on chemicals for coalescence, so the lower chemical consumption is typical. Electrodes and transactors convey a high voltage electric field to the crude oil mixture for coalescence to take place. The mud wash heater heats the heavier accumulated water and sediment mixture. Then they are automatically withdrawn from the vessel by a pump for disposal. The level interface control valve regulates the rate of water in the vessel. Signals passed by the level interface controller regulate the amount of air passing to the control valve actuator. The level control valve is used to adjust the level of mixture in the vessel. It opens only when there is a sufficient water effluent that has accumulated and when the supply of crude oil is taking place (Dehydrators/Desalters, 2010).

Desalter Design Consideration

Deslater design considerations involve choosing the appropriate vessel to handle the expected output of the desalted crude oil. Based on the crude oil salinity, the desalter can be single, two or three stage. The higher the number of stages and the greater the vessel are, there is the more power in the consumption. The crude properties that need to be taken into account during the design of a desalter include: the nature of the selected crude oil, gravity, BS & W (basic sediment and water), salt content, viscosity at two temperatures, sulfur content, pour point, RVP (Reid Vapor Pressure), and delivery pressure at inlet of the mixing valve (Process design of crude oil electrostatic desalters, 2011).

Commercial Desalter Designs

(i)  Petreco Bilectric Dehydrators/Desalters

General

Petreco Bilectric Dehydrators/Desalters are state-of-the-art efficient systems. They are chnologically advanced electrostatic dehydrators/desalters for the treatment of crude oil prior to storage, transportation, or refining. These dehydrators/desalters consist of the electrostatic precipitators used to remove salt from crude oil. The Petreco Dielectric design makes use of the three-grid electrode system and the horizontal emulsion distribution for the purpose of the superior oil and water separation creation. The Petreco Dielectric des alters is identified by the refining industry leaders and is the first choice for big crude oil processing. In the two-stage installations, the process of the salt removal efficiencies, up to 99%, is routinely achievable (Dehydrators/Desalters , 2010).

Operation

In the process of electrostatic desalting, crude oil may be heated for viscosity to decrease. Crude oil is mixed with the fresh water, which is dispersed in crude oil in form of small droplets. Then the water-in-oil dispersion is introduced into the pressurized desalter vessel where the high voltage electrical field causes separation of the water laden from salt and other contaminants combined in oil.

The Benefits and Advantages

These units have been are reliable and applied in the whole world for about  20 years. The advantages include: throughput flexibility, high feed-type flexibilit,; good crude outle,; good water output; lowered operation and maintenance cost,; lowered chemical dependency, high quality flow distribution, the rapid coalescence with minimal electrical power requirements, and interface emulsion control (Dehydrators/Desalters, 2010).

The Petreco Dielectric desalter permits the maximum throughput, flexibility and reliability for any given vessel size. It handles a wide range of the crude oil gravities and viscosities and works with minimum operator supervision. The dual flow configuration provides up to twice the capacity per unit volume of vessel than that of the vertical flow desalters. The Petreco Dielectric design holds water level high, near mid-vessel, to allow the maximum residence time for oil to separate from the effluent water. The smaller size and weight are also particularly advantageous in the offshore applications (Dehydrators/Desalters, 2010).

(ii) NATCO Design

The electro-Dynamic Desalter was developed to replace the conventional multi-vessel, mechanical mixing, staged system. The NATO Electro- Dynamic Desalter provides multiple phases of electrostatic mixing, settling in a single vessel allowing dehydration and salt efficiencies to approach 100%. The NATO Electro-Dynamic Desalter provides the two-staged desalting efficiencies without the excessive capital investments or space requirements common to these systems (Kirby & Mohr, 2008).The patented process applies a high gradient, sustained DC field between pairs of electrodes, while maintaining an AC field between the oil/water interface and electrodes resulting in dramatic improvements over the AC dehydration. It was developed to obtain a degrading electrical field resulting in coalescence of the smallest water. Also, it was developed to regulate the flow of electrical current and to provide the electrical power levels that are self-adjusting for optimization of electrostatic coalescence, resulting in optimum salt removal over a wide range of feed stock materials. It was also constructed to increase the dilution water/ produced water and particulate contact resulting in the reduced dilution water rates while providing the maximum extraction efficiency. It is incorporated to provide multistage contact, resulting in mixing efficiencies approaching 100% while enabling the user to reduce the emulsion chemical consumption.

Factors that Affect the Performance and Operation of Desalters

The performance of a desultory is influenced by a number of factors. For example, the quality and federate of crude oil affect the choice of the des alters. Poor quality crude oil leads to poor performanace if the wrong type of des alters is selected. The temperature, viscosity and density of crude oil also affect the performance of the same des alters. The low temperature or highly viscous denser crude oil reduces the efficiency of the des alters. The higher the intensity of electrical field is, the greater the efficiency is and vice versa. The type of flow configuration, wash water rate and qualitrty affect the des alters. The water with lots of impurities and salinity will render the system inefficient. The water level control and emulsion layers are critical aspects of the desalter. The poor level control of crude/emulsion layer reduces the efficiencyof the des alter. Demulsifier technologies also affect. The highly effective demulsifier technologies ought to be chosen to improve the overall efficiency of the des alters. Other factors that affect the des alters performance include: crude properties, instrumentation and control systems, operating mechanical design conditions, such as operating pressure, operating temperature, allowable pressure drop, design flow rate of crude oil, and maximum anticipated system pressure and temperature, specified design throughput, desalted crude salt content, number of stages specified, and specified water types (Process design of crude oil electrostatic desalters,2011).

Greater part of performance of an oil-water separator is dependent on the influent conditions. As smaller droplets are not easy to separate, equipment or conditions that create the small droplets in the influent to the oil-water separator will make the separator be designed larger to accommodate the additional time needed for the smaller droplets to coalesce. The conditions necessary for small droplets cause shear in the incoming water. The following factors cause small droplet sizes: centrifugal pumps, valves, globe valves, other restrictions in flow such as tees, elbows , and the vertical piping. The ideal inlet requirements for an oil-water separator are the gravity flow in the inlet piping; inlet piping straight for ten pipe diameters upstream of the separator directed into the nozzle; and inlet piping containing valves, elbows, tees (Kirby & Mohr, 2008, p.30).

Desalting Types Application

The desalting processes depend on the type of crude oil. For example, for heavy crude desalting, it is recommended that Portico Dielectric process be chosen while for lighter crude, the mechanical process is desirable. Clean oil, freed of contaminants, continuously rises to the top of the vessel and flows out, while the accumulated water and sediment mixture is automatically withdrawn from the vessel for disposal. The dual horizontal distribution system gives the Portico Dielectric desultory a significantly better treating capacity than a conventional desultory and enhances the control of the interface emulsion (Dehydrators/Desalters, 2010).

Des Alters Troubleshooting

A desultory operating 100% efficiency must remove only water and other sediments as effluents. The presence of oil in the effluent indicates there must be a problem with the system. Thus, the control systems and power supply should be checked. The desultory should remove over 90% of the salts in crude oil. If it does not, the desultory temperature should be checked well (Lieberman, 2009, p.25).

Economic Effects

The aim of the desalting system is the elimination of the inorganic chlorides and the water-soluble compounds existing in crude oil. Instead of treating the wastewater after contamination, it is better to identify the stream and use more sophisticated control to avoid contamination at the source. Contamination contributors to wastewater are quantified: storage tanks 20%; desultory 40%; slop oil recovery 15%;  other processes 25%.

If the improved control methods can be applied to these processes, credits could be realized. An efficient desalting process is the most profitable. The improved control of crude oil desalting process, can facilitate more impressive returns such as longer run times the owner equipment fouling, and reduced the requirements of maintenance between and during shutdowns. The desalting system is a good example that demonstrates source reduction chances. The desultory has a critical impact on the wastewater treatment. The effects on waste treatment facilities from poor desultory reactions have increased hence become increasingly important as emissions limits are more stringently enforced by the government legislation. Many refiners try optimizing the desultory via the chemical addition (BAGGIE, 2012).

Conclusion

The primary function of the desalting system is  to remove the inorganic chlorides from crude oil. Crude oil desalting is accomplished in two fundamental steps: wash water injection and mixing and water/Oil Separation (Crude Dehydration). The salt removal takes place ahead of the vessel that has the name desultory. There are various kinds of desultory available, and the choice and of each depends on the nature and the expected quality of crude oil. The performance of desultory is a function parameters that should be keenly monitored for the maximum efficiency to be obtained.

As much as the des alters refined crude is of significant economic value, even the effluents have the same economic benefits. Effluents have to be treated in a manner that comform to the legal environmental concerns. Desultory ought to be optimized if the maximum economic benefits are to be realized. As such, th choice maintenance and troubleshooting of an appropriate des alter should be given the top priority to.

Example of mass calculation

C (%) = Ao/Ai * 100

C: Conversion factor

Ai: Amount of the process input material

Ao: Amount of output yielded by the internal process based on input Mi

The conversion factor is thus

C (%) =23/45 * 100= 51%

   

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