Crude oil desalting method

In one embodiment, the invention is related to a process for desalting crude oil that requires less wash water than conventional desalting methods. In the preferred embodiment of the invention, a chemical demulsifier formulation comprising an emulsion-breaking chemical and a solvent carrier is added to the crude oil. Wash water is then added to the crude oil until the volume of water in the oil ranges from about 0.1 to about 3 vol. %. Subsequently, the mixture of crude oil, wash water, and chemical demulsifier formulation is subjected to opposed-flow mixing. Chemical emulsion-breakers useful in the invention have a hydrophobic tail group and a hydrophilic head group. Preferably, the emulsion breaker has the formula: ##STR1## x ranges from 1 to 5, y ranges from 0 to 2, and R is an alkyl group having 4-9 carbon atoms, and n ranges from 3 to 9.

FIELD OF THE INVENTION
 The invention is related to chemical demulsifier formulations useful in
 desalting heavy or waxy crude oils. The invention is also related to
 methods for mixing crude oil and chemical demulsifier formulations.
 BACKGROUND
 Crude oil contains varying amounts of inorganic salts. The presence of such
 salts presents difficulties during crude oil processing such as corrosion
 of the oil processing equipment. In order to mitigate the effects of
 corrosion resulting from the presence of salts, it is advantageous to
 reduce the salt concentration to the range of 3 to 5 ppm by weight of the
 crude oil. This concentration corresponds to approximately 2 pounds of
 inorganic salts per 1,000 barrels of crude oil.
 Among the crude oil desalting methods in use today, electrostatic desalting
 is frequently used with crudes containing 0.5 to 2% water. Wash water is
 added until the crude's water content is in the range of 4 to 8 vol. %,
 and a chemical emulsion breaker is added so that the oil and the aqueous
 phases can be separated and diverted for storage or further processing. As
 used herein, a crude oil emulsion is a mixture of crude oil and a
 dispersed aqueous phase, which may be in the form of droplets stabilized
 by naturally occurring surface active compounds in the crude oil.
 Additionally, inorganic fines such as clay particles can contribute to
 emulsion stabilization. Dispersing added wash water into the crude
 increases both the average droplet number density and the droplet surface
 area available for binding the surface active components. Increasing
 droplet surface area results in a reduction in droplet coverage by the
 surface active components; this results in a decrease in emulsion
 stability and an increase in droplet coalescence.
 In electrostatic separation, brine droplets in the mixture of crude oil,
 wash water, and chemical emulsion breaker coalesce in between electrodes
 located in the oil phase. The coalesced aqueous droplets then settle below
 the oleaginous crude oil phase. The separation may occur in a separator
 where an effluent brine may be removed. Treated crude containing 3-5 ppm
 inorganic salts is removed from the upper part of the separator.
 Intermediate between the oil phase and the brine phase is an undesirable
 "rag" layer comprising a complex mixture of oil-in-water emulsion,
 water-in-oil emulsion, and solids. The rag layer remains in the desalter
 vessel or it may be removed therefrom for storage or further processing.
 Electrostatic desalting may undesirably require adding a substantial amount
 of wash water to the crude prior to desalting. Frequently, water must be
 purchased for this purpose. Another difficulty in electrostatic desalting
 results from the quantity and quality of effluent brine, which itself may
 require further processing before discharge.
 Other problems associated with electrostatic desalting include crude
 incompatibility and the formation of undesirable emulsions. For example,
 electrostatic desalting becomes more difficult as a crude's concentration
 of asphaltenes, resins, waxes, and napthenic acids (typically found in
 "heavy" or "waxy" crudes) increases. Rag layers at the water-oil phase
 boundary also result in processing difficulties that become more serious
 as the emulsion becomes more stable, the rag layer increases in volume, or
 both.
 Consequently, there is a need for a crude oil desalting method that limits
 the formation of undesirable emulsions, is effective with heavy and waxy
 crudes, that minimizes the quantity of water added prior to crude
 treatment, and that minimizes the quantity of effluent brine.
 SUMMARY OF THE INVENTION
 In one embodiment, the invention is a crude oil desalting process
 comprising:
 (a) adding to the crude oil a chemical demulsifier formulation, the
 chemical demulsifier formulation being present in an amount ranging from
 about 1 ppm to about 1,000 ppm based on the weight of the crude oil;
 (b) adding wash water to the crude oil and chemical demulsifier formulation
 in an amount ranging from about 0.5 vol. % to about 3.0 vol. %, provided
 that no wash water is added when the concentration of the brine in the
 crude oil is greater than about 3.0 vol. % , all vol. % being based on the
 total volume of the crude oil; and
 (c) separating the brine from the crude oil and chemical demulsifier
 formulation.
 In another embodiment, the invention is a method for removing a brine of
 salt and water from a crude oil, the method comprising:
 (a) mixing the crude oil under opposed-flow conditions at a temperature
 ranging from about 20.degree. C. to 150.degree. C., for a time ranging
 from about 1 minute to about 50 hours, and at a viscosity ranging from
 about 1 cP to about 250 cP in order to coalesce the brine droplets, and
 then
 (b) separating the brine from the crude oil.
 DETAILED DESCRIPTION OF THE INVENTION
 The invention is based on the discovery that brine droplet coalescence in
 crude oil can be enhanced by adding chemical emulsion breakers to the
 crude oil emulsion, subjecting the crude oil and brine to opposed-flow
 mixing, or both. Typically, brine droplets in crude oil are stabilized by
 a mixture of surface active components such as waxes, asphaltenes, resins,
 and naphthenic acids that are electrostatically bound to the droplets'
 surface. Such components provide an interfacial film over the brine
 droplet resulting in highly elastic collisions between droplets during
 processing, resulting in diminished droplet coalescence.
 While the invention can be practiced with any crude oil containing a brine,
 it is preferably practiced with heavy or waxy crude oils. Heavy or waxy
 crude oils have one or more of the following characteristics:
 (a) The crude oil has an API gravity ranging from about 5 to about 30.
 (b) The crude oil has a high naphthenic acid concentration, characterized
 by a high "TAN" number (the TAN number represents the number of
 milliequivalents of potassium hydroxide required to neutralize 1 gram of
 crude oil).
 (c) The fraction of the crude oil insoluble in n-heptane ranges from about
 0.5 wt. % to about 15 wt. %.
 Adding water to the crude can decrease the concentration of the surface
 active components on the surface of each droplet because the number of
 droplets is increased without increasing component concentration. It has
 been discovered that the amount of added water needed for desalting may be
 minimized by adding a chemical emulsion-breaker to the crude that is
 capable of displacing the surface active components from the brine
 droplets and then subjecting the crude oil to controlled mixing.
 Chemical emulsion-breakers useful in the invention have a hydrophobic tail
 group and a hydrophilic head group. Preferably, the emulsion breaker has
 the formula:
 ##STR2##
 x ranges from 1 to 5, y ranges from 0 to 2, and R is an alkyl group having
 4 to 9 carbon atoms, and n ranges from 3 to 9.
 Preferably, the chemical emulsion-breaker is used in combination with a
 delivery solvent. Delivery solvents useful in the practice of this
 invention include a high aromaticity solvent such as toluene, xylene, and
 high aromatic condensates such as heavy aromatic naphtha in combination
 with an oxygenated solvent such as diethylene monobutyl ether or benzyl
 alcohol. The preferred formulation comprises about 10 wt. % to about 60
 wt. % chemical emulsion breaker, about 35 wt. % to about 75 wt. %
 diethylene glycol mono butyl ether, and about 5 wt. % to about 15 wt. %
 heavy aromatic naphtha. Particularly preferred is a formulation of 45%
 chemical emulsion-breaker, 50 wt. % diethylene glycol mono butyl ether,
 and 5 wt. % heavy aromatic naphtha ("HAN").
 An effective amount of the chemical emulsion-breaker-delivery solvent
 formulation ("chemical demulsifier formulation") is combined with the
 crude oil. An effective amount of the formulation is the amount necessary
 to displace the surface active component from the brine droplets and
 render the brine droplets more amenable to coalescence. The effective
 amount ranges from about 1 ppm to about 1,000 ppm based on the weight of
 the crude oil, with about 20 to about 40 ppm being preferred.
 In a preferred embodiment, a crude oil and a chemical demulsifier
 formulation are combined and then desalted under electrostatic desalting
 conditions. Electrostatic desalting is known to those skilled in the art
 of crude oil processing. Accordingly, the crude is desalted in a vessel
 having electrodes at potentials ranging from about 10,000 volts to about
 40,000 volts, A.C. or D.C. Voltage gradients present in the vessel range
 from about 500 volts per inch to about 5,000 volts per inch, preferably at
 a potential ranging from about 500 to about 1,000 volts per inch. Crude
 oil temperature ranges 220.degree. F. to about 300.degree. F., and
 residence times range upwards from about one minute, preferably from about
 1 to about 60 minutes, and more preferably from about 1 to about 15
 minutes.
 Advantageously, mixing energy may be applied to the mixture of crude oil
 emulsion and chemical demulsifier formulation in order to increase brine
 droplet coalescence rate. When mixing is used, it is important to
 carefully control mixing geometry and mixing energy. The mixing may be
 conventional ("static") or opposed-flow, and may occur in the same vessel
 as electrostatic desalting.
 In opposed-flow mixing, two or more counter-currents of the mixture of
 crude oil emulsion and chemical demulsifier impact and intermingle.
 Opposed propeller(or impeller) and opposed jet (or nozzle) configurations
 are nonlimiting examples of opposed-flow mixing.
 In the opposed-propeller geometry, at least two counter-rotating propellers
 are immersed in the crude oil-brine mixture in order to form opposed
 streams within the mixture. The streams of the mixture impact and
 intermingle in the volume between the propellers. The propellers may be in
 close proximity in the same reservoir or vessel, in different regions of
 the same vessel, or in connected vessels or reservoirs with baffles or
 pipes providing conducting means for directing the streams to a region
 where opposed-flow mixing can occur. Parameters such as propeller spacing,
 propeller angular speed, and the nature of any conducting means may be
 determined by those skilled in the art of mixing from mixture properties
 such as viscosity and the desired mixing energy.
 In the opposed jet geometry, the crude oil-brine mixture is separated into
 at least two streams. Conducting means such as pipes are used to direct
 the streams into an opposed-flow configuration. Accordingly, the
 longitudinal axes (the axes in the direction of flow) and the outlets of
 the pipes are oriented so that the streams impact and intermix in a region
 between the outlets. Preferably, two opposed pipes are employed and the
 angle subtended by the longitudinal axes of the pipes is about
 180.degree.. The outlets may be in the form of nozzles or jets. As in the
 opposed propeller geometry, parameters such as the surface area of the
 conduits, the flow rate of the mixture in the conduits, the size and shape
 of any nozzle or jet employed, and the distance between the outlets may be
 determined by those skilled in the art of mixing from mixture properties
 such as mixture viscosity and the desired mixing energy.
 Importantly, when mixing is used, the mixing energy rate is controlled in a
 range where brine droplet coalescence occurs. Too great a mixing energy
 results in brine droplet break-up, and too low a mixing energy results in
 too few brine droplet collisions. Wile the exact range of mixing energy
 rate will depend, for example, on the crude oil's viscosity, mixing energy
 rate (mixing power) will typically range from about 0.1 hp per 1000
 gallons of the mixture of crude oil emulsion and chemical demulsifier to
 about 3 hp per 1000 gallons, with about 0.2 hp per 1000 gallons to about
 0.5 hp per 1000 gallons being the preferred range. The invention can be
 practiced when the mixture's temperature ranges from about 20.degree. C.
 to about 150.degree. C. and viscosity ranges from about 1 to about 250 cP.
 Preferably, mixture temperature ranges from about 80.degree. C. to about
 130.degree. C. and viscosity ranges from about 1 to about 75 cP. Care
 should also be taken to prevent undesirable water vaporization during
 mixing. Water vaporization can be substantially reduced or prevented by
 increasing mixing pressure. Mixing times are preferably greater than about
 1 minute, and more preferably range from about 1 to about 10 hours.
 In some cases, it may be desirable to add a very small amount of wash water
 to the crude oil-brine mixture in order to optimize the coalescence rate
 and to extract salt that is not present in a brine phase. When used, the
 amount of added wash water ranges from about 0.5 to about 3.0 vol. % water
 based on the total volume of the crude oil, i.e., far less than is used in
 conventional desalting. Generally, no added wash water is used when brine
 is at least 3.0 vol. %.
 While not wishing to be bound by any theory, it is believed that efficient
 brine droplet coalescence occurs when droplet collision frequency is
 increased and when individual droplets can be made to collide with an
 energy great enough to overcome the droplets' interfacial or surface
 tension so that a larger droplet is formed upon collision. Importantly,
 mixing energy should not exceed the point at which two droplets collide to
 produce three or more droplets. Furthermore, mixing energy should be
 sufficient so that the droplets do not merely collide and recoil away from
 each other without coalescing, as would happen in cases of insufficient
 mixing energy. The presence of surface or interfacially active species on
 the droplets' surface may result in raising or lowering the droplets'
 interfacial energy and interfacial elasticity. The presence of treatment
 solutions affecting such species may further alter the droplets'
 interfacial energy and interfacial film elasticity. Accordingly, mixing
 energy under opposed-flow conditions may vary in the practice of the
 invention, depending on the presence of treatment solutions or stabilizing
 species.
 Conventional static mixing is not as effective as opposed-flow mixing in
 the practice of the invention because, it is believed, droplet collisions
 occur too infrequently and at too low an energy to cause coalescence. In
 conventional mixing, the neighboring droplets are at rest or move at small
 velocities with respect to each other, the energy of mixing being directed
 towards macroscopic fluid motion only.
 It should be noted that opposed-flow mixing under the conditions set forth
 above results in some brine droplet coalescence even in cases where the
 crude oil-brine mixture does not contain a demulsifier or any other
 treatment solution. Accordingly, opposed-flow mixing can be used to remove
 droplets of any undesirable liquid impurity suspended in a continuous
 phase of a second liquid. In addition to crude oil-brine mixtures, such
 mixtures include crude oil products that contain process-water impurities,
 droplets in crude oil products resulting from the use of liquid
 hydrophilic catalysts, mixtures derived from the neutralization of acidic
 crude oil or products derived from crude oil, and mixtures derived from
 the caustic treatment of crude oil products and polyurea. It is
 advantageous to use opposed-flow mixing to enhance droplet coalescence in
 mixtures that do not contain a demulsifier or treatment solution when the
 presence of such a demulsifier or treatment solution would be incompatible
 with or would otherwise undesirably affect the mixture.
 As set forth above, chemical demulsifier formulations and opposed-flow
 mixing, whether used above or in combination, are useful in improving
 electrostatic desalting processes. In addition, it has been discovered
 that such mixing and formulations, alone or in combination, are useful in
 improving other common forms of brine-crude oil separation, such as
 gravitational (settling) and centrifugal separation. In gravitational
 separation, for example, the increase brine droplet size resulting from
 the use of chemical demulsifier formulations, opposed-flow mixing, or
 both, shortens the retention time necessary for desalting.
 The invention is further set forth in the following non-limiting examples.

EXAMPLES
 Example 1
 The Effect of Opposed-flow Mixing on Final Salt Concentration.
 Two identical crude oils containing 0.5 vol. % water were combined with 40
 ppm of a chemical demulsifier formulation of alkoxylated nonyl phenol
 resin. The formulation comprised 45 wt. % of a chemical emulsion breaker
 having the formula
 ##STR3##
 5 wt. % heavy aromatic naphtha; and 50 wt. % diethylene glycol monobutyl
 ether. Water was added until the total water concentration in the crude
 was 2.1 vol. %. One mixture (Case A) was subjected to opposed-flow mixing
 for 30 minutes at 80.degree. C. and 200 psi, using two laboratory marine
 blade propellers configured so that the top blade's pitch was the reverse
 of the pitch of the bottom blade. This mixing geometry and configuration
 promotes axial flows in directly opposing manner that increases collision
 frequency. Impeller rotation was 400 rpm. This mixture was then directed
 to an electrostatic desalter, where the mixture was subjected to an 830
 volts/inch potential at 80.degree. C. for one hour.
 The mixture of Case B was directed to the electrostatic desalter for
 identical treatment without opposed-flow mixing. The results are
 summarized in Table 1.
 TABLE 1
 Case A Case B
 Water Concentration in Crude 2.1% (volume) 2.1% (volume)
 % Dehydration 90 84
 Salt Concentration in lbs/1,000 barrel
 Initial 16 16
 Final &lt;3 14
 The table shows that opposed-flow mixing with electrostatic desalting
 resulted in greater crude dehydration and lower salt concentration than
 electrostatic desalting alone.
 Example 2
 Opposed-flow Mixing Results in a Reduced Emulsion Rag Volume.
 As discussed in the Background, an undesirable rag layer forms in
 electrostatic desalter between the oil phase and the water phase. In this
 example, two crude mixtures were prepared and combined with 40 ppm of the
 demulsifier formulation of Example 1.
 TABLE 2
 Case A Case B
 Total Water Concentration 1.8 Vol. % 4.5 Vol. %
 Rag Volume 0.2% (Vol.) 0.5% (Vol.)
 One mixture, Case A, was subjected to opposed-flow mixing under the
 conditions set forth in Example 1. This mixture was then directed to an
 electrostatic desalter operated under conditions set forth in Example 1.
 The other mixture, Case B using the same starting crude oil as Case A, was
 electrostatically desalted under the same conditions, but without
 opposed-flow mixing. The results are summarized in Table 2.
 Example 3
 The Invention is Compatible with Crudes of Widely Varying Viscosity and
 Salt Concentration.
 Three crudes were each combined with 40 ppm of the de-emulsifier
 formulation of Example 1, subjected to opposed-flow mixing as in Example
 1, and subjected to electrostatic desalting also as set forth in Example
 1. The results are set forth in Table 3.
 TABLE 3
 Case A Case B Case C
 Total Vol. % H.sub.2 O 2.07 2.07 1.80
 Viscosity (Cp@ 80.degree. C./1 sec.sup.-1 8 19 3
 Salt Concentration
 lbs/1,000 bbl
 Initial 16 7 113
 Final &lt;3 &lt;3 13
 Example 4
 Optimizing the Water Concentration in the Crude for Most Effective
 Desalting.
 In this Example, 3 samples of the same crude were tested, each having an
 initial water concentration of 0.5 vol. %. The mixtures were combined with
 a demulsifier formulation, and subjected to opposed-flow mixing and
 electrostatic desalting as set forth in Example 1. The results are set
 forth in Table 4.
 TABLE 4
 Case A Case B Case C
 Total Vol. % H.sub.2 O 0.81 1.35 2.07
 % Dehydration 60 87 90
 Total Salt Concentration
 165/1,000 bbl
 Initial 15 14 16
 Final 6 3 &lt;3
 Example 5 and 6
 Opposed-flow Mixing Results in Brine Droplet Coalescence Even When No
 Chemical Demulsifier Formulation is Employed.
 Example 5
 A homogeneous crude oil blend comprising 200 gms of San Joaquin Valley (SJV
 crude oil and 200 gms of Alaskan North Slope (ANS) crude oil was prepared
 in a 500 ml polyethylene bottle. This starting blend had a moisture
 content of about 1.0% and a volume mean diameter of 26.3 microns.
 About 260 gms of the blend was decanted into a 300 ml autoclave equipped
 with two laboratory marine propeller mixers (1" blade) affixed to a common
 shaft. To create opposing liquid flows, the top propeller's pitch was
 reversed compared to the pitch of the bottom blade. This arrangement
 directs the top blade's liquid flow downward opposite the upward liquid
 flow of the bottom blade. The distance between the blades was about 2
 inches. The mixture was pressurized to about 700 kPa with nitrogen to
 minimize vaporization of water. The blend was mixed at about 400 rpm,
 80.degree. C. at a pressure of about 1100 kPa for 30 minutes. The mixture
 was immediately cooled to room temperature with ice cold water surrounding
 the autoclave, while the mixer speed was at 200 rpm and the heater turned
 off. Then the mixture was decanted into a 500 ml polyethylene bottle. The
 resulting crude blend was found to have a moisture content of 1.0% and a
 volume mean particle diameter of 49.4 microns.
 Example 6
 The procedure in Example 5 was repeated, except that Arab Heavy crude oil
 was used instead of the SJV-ANS crude oil blend. The Arab Heavy crude
 sample was found to contain less than 0.1% of moisture. To match the about
 1% moisture content of the 1:1 SJV-ANS blend, about 4 gms of deionized
 water was homogenized in 400 gms of Arab Heavy in a laboratory blender for
 5 minutes at low speed. The resulting crude (Crude B) was found to have a
 moisture content of about 1% and a volume mean diameter of about 54
 microns.
 240 grams of crude-water mixture were subjected to the mixing procedure of
 Example 5, except the mixer speed was 100 rpm and the mixing time was 3
 hours.
 The resulting crude was found to have a moisture content of about 1% and a
 volume mean diameter of about 77 microns.