PERFORMANCE OF DESALTING UNITS

FIELD [0001] The present invention relates to methods and systems for desalting oil. 5 More particularly, the present invention relates to methods and systems for desalting oil with integrated hydrodynamic cavitation. BACKGROUND [0002] Generally desalting units are one of the first processing units in an oil refinery and are employed to reduce salts that are dissolved in the water in the 10 crude oil. Most desalting units in use today operate by mixing wash water with an oil feed and then subjecting the mixture to electrostatic desalting, inducing attractive dipole forces between neighboring water molecules promoting the formation of larger water droplets or globules which settle to the bottom of the desalter with water-insoluble solids. 15 [0003] Increasingly, heavy crude sources pose challenges for existing desalter units. For example, large molecules in the oil such as asphaltenes and resins may contribute to the stabilization of oil/water interfaces in desalter units leading to the formation of emulsions, often referred to as“rag layers”. In such an event, chemical additives such as surfactants are generally added to reduce the emulsions20 to facilitate the separation of the water from the oil. [0004] There remains a need for better or alternative ways to reduce the formation of or treat such emulsions. SUMMARY [0005] The present invention addresses these and other problems by providing new methods and systems for desalting utilizing hydrodynamic cavitation to crack molecules that have a tendency to form emulsions. 5 [0006] In one aspect, a method is provided for improving the performance of a desalting unit. The method comprises subjecting a feed oil stream to hydrodynamic cavitation to produce a cavitated oil stream, and thereafter desalting the cavitated oil stream to remove at least a portion of salt contained in the cavitated oil stream. 10 [0007] In another aspect, a system is provided for desalting a feed oil. The system includes a feed oil stream containing salt; a hydrodynamic cavitation unit receiving the feed oil stream and adapted to subject the feed oil stream to hydrodynamic cavitation and thereby produce a cavitated feed oil stream; and a desalting unit downstream of the hydrodynamic cavitation unit, the desalting unit15 adapted to remove at least a portion of the salt from the cavitated feed oil stream. BRIEF DESCRIPTION OF THE DRAWINGS [0008] FIG. 1 is a cross section view of an exemplary hydrodynamic cavitation unit, which may be employed in one or more embodiments of the present invention. 20 [0009] FIG. 2 is a flow diagram of a system for improving the performance of a desalter unit according to one or more embodiments of the present invention. DETAILED DESCRIPTION [0010] Methods and systems are provided for improving the performance of a desalting unit. Such improvements are provided by the integration of a25 hydrodynamic cavitation unit with the desalting unit to crack larger hydrocarbon molecules that contribute to the formation of stable water/oil interfaces into lower molecular weight hydrocarbon molecules that are less promotive to the formation of such stable water/oil interfaces. Advantageously, in some embodiments the methods and systems can increase desalter loading and throughput, can reduce 5 water carryover from heavy crudes to atmospheric distillation units, can promote faster settling time of droplets, can reduce the amount of dilution required for processing some of the heavier crudes, can reduce desalter operating temperatures, and can allow for the use of smaller desalters reducing the required plot space for the desalting unit. 10 [0011] Applicable oil feeds include any hydrocarbon oil feed; however, the methods and systems may be most advantageous with hydrocarbon oil feeds that have the tendency to produce stable water/oil interfaces when mixed with wash water. Thus, the methods and systems disclosed herein have particular applicability to heavy crude feeds. Benefits of the present invention may be15 particularly realized with feeds having an API gravity of 20° or less, or 18° or less, or 15° or less. Furthermore, such methods may employed with particular advantage to feeds having an asphaltene content greater than 5 wt%, and/or a resin content of greater than 5 wt%, as such feeds may have an increased tendency to produce stable emulsions. Asphaltenes and resins are defined in and can be20 measured by ASTM D4124. [0012] In an exemplary embodiment, a method for improving the performance of a desalting unit is provided that comprises subjecting a feed oil stream to hydrodynamic cavitation to produce a cavitated oil stream and thereafter desalting the cavitated oil stream. The cavitated oil stream may be desalted by various 25 desalting processes, such as electrostatic desalting. [0013] In any embodiment, hydrodynamic cavitation of the oil feed may be performed in such a way to convert at least a portion of the feed oil stream to lower molecular weight hydrocarbons. Specific aspects of the hydrodynamic cavitation process and suitable hydrodynamic cavitation units is described in greater detail subsequently. In general, the hydrodynamic cavitation process may advantageously target larger asphaltene and resin molecules in the oil feed that contribute to stable water/oil interfaces and crack these molecules into lower 5 molecular weight hydrocarbons that are less promotive of stable water/oil interfaces. [0014] In any embodiment, at least a portion of the asphaltene or resin molecules are“cracked” or reduced in molecular weight when the feed oil stream is subjected to hydrodynamic cavitation. In doing so, the resulting cavitated feed10 stream may have one or more of the following characteristics: lowered asphaltene and/or resin content, reduced viscosity, reduced sulfide content, reduced polar molecule content, and combinations thereof. [0015] In some embodiments, after subjected the oil to hydrodynamic cavitation it is thereafter mixed with wash water, e.g., at a mixing valve, prior to 15 being fed to a desalter, such as an electrostatic desalter. In other embodiments, the feed oil may be mixed with wash water prior to being subjected to hydrodynamic cavitation. In any embodiment, a solvent oil having a higher API gravity than the feed oil may be mixed with the feed oil prior to hydrodynamic cavitation. Alternatively, some of the lighter products from the cavitation may be 20 recycled upstream of the cavitation unit to further reduce viscosity prior to entering the cavitation unit. In some embodiments, the heavy oil may be cavitated without the use of a diluent. [0016] To improve performance of the hydrodynamic cavitation unit, the feed oil may be preheated by a heat exchanger prior to being fed to the hydrodynamic 25 cavitation unit. Advantageously, the feed may be desalted at commercial throughputs at a temperature of 200 to 325°F, or preferably from 240 to 300°F. [0017] In any embodiment, the cavitated oil stream may be subjected to serial desalting whereby the oil stream is fed to sequential desalting units. In the first desalting unit, the oil feed may be subjected to electrostatic desalting to reduce the salt content by 80-95%. In a subsequent second desalting unit, the oil feed may be further subjected to electrostatic desalting to reduce the salt content of the feed oil by up to 99%. Preferably, the desalted oil stream leaving the desalter should 5 have a salt content of less than 3-10 pounds per thousand barrels (ptb), more preferably less than 1 ptb to reduce the poisoning effect salts may have on downstream catalytic units. [0018] In the illustrative embodiment of FIG. 2, a feed oil stream 100, such as crude oil, is fed to a heat exchanger 106 via a pump 104. The heated oil feed 10 stream may then be fed to a hydrodynamic cavitation unit 110 under conditions that subject the feed oil to hydrodynamic cavitation, thereby cracking at least a portion of the hydrocarbons in the feed oil stream 100 to lower molecular weight hydrocarbons. The cavitated feed oil stream 112 may then be mixed with a wash water stream 129, such as by a mixing valve prior to being fed to a first desalting15 unit 114. [0019] Optionally, one or more desalting aids102, may be added to the oil feed stream 100 before the oil feed stream is subjected to hydrodynamic cavitation. Various types of desalting aids may be used, and may be used in varying concentrations depending upon the properties of the oil and the type of desalting 20 units. In an exemplary embodiment, one more desalting aids are provided in the range of 3 to 10 parts per million in the oil. In other embodiments, the desalting aids 102 may be mixed with the cavitated feed oil stream 112 after the feed oil has been hydrodynamically cavitated. Still in other embodiments, the oil feed may be desalted without the use of desalting aids such as demulsifiers. 25 [0020] In the first desalting unit 114, the cavitated feed oil stream 112 and wash water are subjected to electrostatic desalting, whereby attractive dipole forces are induced in water droplets in the mixed stream causing brine water and non-soluble contaminants to separate from the oil feed. The brine water and contaminants may be evacuated from the first desalting unit 114 by effluent stream 120. [0021] The intermediate desalted stream 118 from the first desalting unit 114 is then mixed (such as via a mixing valve) with a stream of fresh water 126 pumped 5 from a water source via pump 128 and the mixed stream is then fed to the second desalting unit 122. In the second desalting unit 122, the mixed intermediate desalted stream 118 and fresh water stream 126 are subjected to electrostatic desalting, whereby water and salt are separated from the desalted product stream 124 as wash water stream 129, which is pumped via pump 130 upstream of the 10 first desalting unit 114 to mix with the cavitated oil feed oil stream 112. Prior to being fed to the second desalting unit 122, a deemulsifier 116, which may be same or different than the deemulsifier used in the desalting aids 102 (if used), may be added to the intermediate desalted stream 118. [0022] In any embodiment, after the oil is subjected to hydrodynamic 15 cavitaion, it may thereafter be mixed with wash water at rates of 4-8 vol% of the crude throughput, e.g., at a mixing valve, prior to being fed to the desalter. Hydrodynamic Cavitation Unit [0023] The term“hydrodynamic cavitation”, as used herein refers to a process whereby fluid undergoes convective acceleration, followed by pressure drop and 20 bubble formation, and then convective deceleration and bubble implosion. The implosion occurs faster than mass in the vapor bubble can transfer to the surrounding liquid, resulting in a near adiabatic collapse. This generates extremely high localized energy densities (temperature, pressure) capable of dealkylation of side chains from large hydrocarbon molecules, creating free25 radicals and other sonochemical reactions. [0024] The term “hydrodynamic cavitation unit” refers to one or more processing units that receive a fluid and subject the fluid to hydrodynamic cavitation. In any embodiment, the hydrodynamic cavitation unit may receive a continuous flow of the fluid and subject the flow to continuous cavitation within a cavitation region of the unit. An exemplary hydrodynamic cavitation unit is illustrated in FIG. 1. Referring to FIG. 1 , there is a diagrammatically shown view 5 of a device consisting of a housing I having inlet opening 2 and outlet opening 3, and internally accommodating a contractor 4, a flow channel 5 and a diffuser 6 which are arranged in succession on the side of the opening 2 and are connected with one another. A cavitation region defined at least in part by channel 5 accommodates a baffle body 7 comprising three elements in the form of hollow 10 truncated cones 8, 9, 10 arranged in succession in the direction of the flow and their smaller bases are oriented toward the contractor 4. The baffle body 7 and a wall 11 of the flow channel 5 form sections 12, 13, 14 of the local contraction of the flow arranged in succession in the direction of the flow and shaving the cross- section of an annular profile. The cone 8, being the first in the direction of the 15 flow, has the diameter of a larger base 15 which exceeds the diameter of a larger base 16 of the subsequent cone 9. The diameter of the larger base 16 of the cone 9 exceeds the diameter of a larger base 17 of the subsequent cone 10. The taper angle of the cones 8, 9, 10 decreases from each preceding cone to each subsequent cone. 20 [0025] The cones may be made specifically with equal taper angles in an alternative embodiment of the device. The cones 8, 9, 10 are secured respectively on rods 18, 19, 20 coaxially installed in the flow channel 5. The rods 18, 19 are made hollow and are arranged coaxially with each other, and the rod 20 is accommodated in the space of the rod 19 along the axis. The rods 19 and 20 are 25 connected with individual mechanisms (not shown in FIG. 1 ) for axial movement relative to each other and to the rod 18. In an alternative embodiment of the device, the rod 18 may also be provided with a mechanism for movement along the axis of the flow channel 5. Axial movement of the cones 8, 9, 10 makes it possible to change the geometry of the baffle body 7 and hence to change the profile of the cross-section of the sections 12, 13, 14 and the distance between them throughout the length of the flow channel 5 which in turn makes it possible to regulate the degree of cavitation of the hydrodynamic cavitation fields downstream of each of the cones 8, 9, 10 and the multiplicity of treating the 5 components. For adjusting the cavitation fields, the subsequent cones 9, 10 may be advantageously partly arranged in the space of the preceding cones 8, 9; however, the minimum distance between their smaller bases should be at least equal to 0.3 of the larger diameter of the preceding cones 8, 9, respectively. If required, one of the subsequent cones 9, 10 may be completely arranged in the 10 space of the preceding cone on condition of maintaining two working elements in the baffle body 7. The flow of the fluid under treatment is show by the direction of arrow A. [0026] Hydrodynamic cavitation units of other designs are known and may be employed in the context of the inventive systems and processes disclosed herein. 15 For example, hydrodynamic cavitation units having other geometric profiles are illustrated and described in U.S. Patent No. 5,492,654, which is incorporated by reference herein in its entirety. Other designs of hydrodynamic cavitation units are described in the published literature, including but not limited to U.S. Patent Nos. 5,937, 906; 5,969,207; 6,502,979; 7,086,777; and 7,357,566, all of which are 20 incorporated by reference herein in their entirety. [0027] In an exemplary embodiment, conversion of hydrocarbon fluid is achieved by establishing a hydrodynamic flow of the hydrodynamic fluid through a flow-through passage having a portion that ensures the local constriction for the hydrodynamic flow, and by establishing a hydrodynamic cavitation field (e.g., 25 within a cavitation region of the cavitation unit) of collapsing vapor bubbles in the hydrodynamic field that facilitates the conversion of at least a part of the hydrocarbon components of the hydrocarbon fluid. [0028] For example, a hydrocarbon fluid may be fed to a flow-through passage at a first velocity, and may be accelerated through a continuous flow-through passage (such as due to constriction or taper of the passage) to a second velocity that may be 3 to 50 times faster than the first velocity. As a result, in this location 5 the static pressure in the flow decreases, for example from 1 -20 kPa. This induces the origin of cavitation in the flow to have the appearance of vapor-filled cavities and bubbles. In the flow-through passage, the pressure of the vapor hydrocarbons inside the cavitation bubbles is 1 -20 kPa. When the cavitation bubbles are carried away in the flow beyond the boundary of the narrowed flow-through passage, the10 pressure in the fluid increases. [0029] This increase in the static pressure drives the near instantaneous adiabatic collapsing of the cavitation bubbles. For example, the bubble collapse time duration may be on the magnitude of 10 ^-6 to 10 ^-8 second. The precise duration of the collapse is dependent upon the size of the bubbles and the static 15 pressure of the flow. The flow velocities reached during the collapse of the vacuum may be 100-1000 times faster than the first velocity or 6-100 times faster than the second velocity. In this final stage of bubble collapse, the elevated temperatures in the bubbles are realized with a velocity of 10 ¹⁰ -10 ¹² K/sec. The vaporous/gaseous mixture of hydrocarbons found inside the bubbles may reach 20 temperatures in the range of 1500-15,000K at a pressure of 100-1500 MPa.

Under these physical conditions inside of the cavitation bubbles, thermal disintegration of hydrocarbon molecules occurs, such that the pressure and the temperature in the bubbles surpasses the magnitude of the analogous parameters of other cracking processes. In addition to the high temperatures formed in the 25 vapor bubble, a thin liquid film surrounding the bubbles is subjected to high temperatures where additional chemistry (ie, thermal cracking of hydrocarbons and dealkylation of side chains) occurs. The rapid velocities achieved during the implosion generate a shockwave that can: mechanically disrupt agglomerates (such as asphaltene agglomerates or agglomerated particulates), create emulsions with small mean droplet diameters, and reduce mean particulate size in a slurry. Specific Embodiments [0030] To further illustrate different aspects of the present invention, the 5 following specific embodiments are provided: [0031] Paragraph A– A method for improving the performance of a desalting unit comprising: subjecting a feed oil stream to hydrodynamic cavitation to produce a cavitated oil stream, and thereafter desalting the cavitated oil stream to remove at least a portion of salt contained in the cavitated oil stream. 10 [0032] Paragraph B– The method of Paragraph A, wherein at least a portion of hydrocarbon molecules in the feed oil stream are thermally cracked to lower molecular weight hydrocarbons when subjected to hydrodynamic cavitation. [0033] Paragraph C– The method of any of Paragraphs A-B, where the cavitated oil stream is desalted at a temperature between 200°F and 325°F,15 preferably 240-300°F. [0034] Paragraph D– The method of any of Paragraphs A-C, wherein the feed oil stream has an API gravity of 20° or less. [0035] Paragraph E– The method of any of Paragraphs A-D, wherein the desalting is performed in the absence of a demulsifier. 20 [0036] Paragraph F– The method of any of Paragraphs A-E, wherein at least a portion of the asphaltene molecules are cracked to lower molecular weight hydrocarbons when the feed oil stream is subjected to hydrodynamic cavitation. [0037] Paragraph G – The method of any of Paragraphs A-F, further comprising mixing the cavitated oil stream with a wash water stream prior to25 desalting the cavitated oil stream. [0038] Paragraph H – The method of any of Paragraphs A-G, further comprising mixing the feed oil stream with a wash water stream prior to subjecting the feed oil stream to hydrodynamic cavitation. [0039] Paragraph I – The method of any of Paragraphs A-H, further 5 comprising heating the feed oil stream prior to subjecting the feed oil stream to hydrodynamic cavitation. [0040] Paragraph J– The method of any of Paragraphs A-I, wherein the feed oil stream is subjected to hydrodynamic cavitation in the absence of a catalyst. [0041] Paragraph K– The method of any of Paragraphs A-J, wherein the feed10 oil stream is subjected to hydrodynamic cavitation in the absence of a diluent. [0042] Paragraph L– The method of any of Paragraphs A-K, wherein the feed oil stream is subjected to hydrodynamic cavitation in the absence of a hydrogen containing gas or with a hydrogen containing gas content of less than 50 standard cubic feet per barrel. 15 [0043] Paragraph M– The method of any of Paragraphs A-L, wherein the feed oil stream is hydrodynamically cavitated at a temperature between 200°F and 325°F. [0044] Paragraph N– The method of any of Paragraphs A-M, wherein the feed oil stream is hydrodynamically cavitated at a pressure drop greater than 400 psig, 20 or more preferably greater than 1000 psig, or more preferably greater than 2000 psig. [0045] Paragraph O – The method of any of Paragraphs A-N, further comprising adding a solvent or oil having an API gravity greater than 30° to the feed oil stream prior to subjecting the feed oil stream to hydrodynamic cavitation. 25 [0046] Paragraph P– A system for desalting a feed oil comprising: a feed oil stream containing salt; a hydrodynamic cavitation unit receiving the feed oil stream and adapted to subject the feed oil stream to hydrodynamic cavitation and thereby produce a cavitated feed oil stream; and a desalting unit downstream of the hydrodynamic cavitation unit, the desalting unit adapted to remove at least a portion of the salt from the cavitated feed oil stream. 5 [0047] Paragraph Q– The system of Paragraph P, adapted to perform the method of any of Paragraphs A-O.