Crude oil desalter dehydration vessel

Abstract

A system for removing salt from oil includes a vertically oriented desalting vessel, a crude oil entry located at a bottom of the desalting vessel in fluid communication with the top crude oil layer, a pair of electrode plates positioned vertically in the top crude oil layer, at least one ground plate inside the desalting vessel, a water outlet located on the bottom of the desalting vessel, and a treated oil outlet located at a top of the desalting vessel. A method for removing salt from crude oil includes providing a crude oil mixture into a vertically oriented desalting vessel, applying an electric field through a pair of electrode plates, collecting treated oil from a treated oil outlet located at a top of the desalting vessel, and collecting water from a water outlet located on the bottom of the desalting vessel in fluid communication with the bottom water layer.

Description

BACKGROUND

[0001] Removing salt from crude oil is an important oil processing technique that suffers from several deficiencies in the incumbent technology. One prevalent technique used to remove salt from water is electrocoalescence. Electrocoalescence is the process by which coalescence of water droplets is accelerated by the application of an electrical field. Coalesced water droplets can then be separated from crude oil by the effect of gravity as they fall to the bottom of the vessel as they grow larger and heavier.

[0002] Current electrocoalescence oil processing methods typically utilize rod electrodes that form an electrical grid that suffers from a small surface area and gaps between electrodes. In addition, the rod electrodes used to form an electrical grid are susceptible to corrosion, which reduces overall efficiency. Therefore, there exists a need for improved efficiency and durability in electrocoalescence desalting vessels.SUMMARY

[0003] This summary is provided to introduce a selection of concepts that are further described below in the detailed description. This summary is not intended to identify key or essential features of the claimed subject matter, nor is it intended to be used as an aid in limiting the scope of the claimed subject matter.

[0004] In one aspect, embodiments disclosed herein relate to a system for removing salt from oil which includes a vertically oriented desalting vessel configured to contain a fluid, a crude oil entry located at a bottom of the desalting vessel and in fluid communication with the top crude oil layer, a pair of electrode plates positioned vertically in the top crude oil layer, at least one ground plate inside the desalting vessel, a water outlet located on the bottom of the desalting vessel and in fluid communication with the bottom water layer, and a treated oil outlet located at a top of the desalting vessel and in fluid communication with the top crude oil layer. The fluid includes a top crude oil layer, a middle emulsion layer, and a bottom water layer. Each electrode plate is in electrical communication with an electrical transformer.

[0005] In another aspect, embodiments disclosed herein relate to a method for removing salt from crude oil, which includes providing a crude oil mixture into a vertically oriented desalting vessel via a crude oil entry located at a bottom of the desalting vessel, applying an electric field through a pair of electrode plates such that water droplets in the crude oil mixture coalesce and fall to a bottom of the desalting vessel, collecting treated oil from a treated oil outlet located at a top of the desalting vessel and in fluid communication with the top crude oil layer, and collecting water from a water outlet located on the bottom of the desalting vessel and in fluid communication with the bottom water layer. The desalting vessel comprises a pair of vertically oriented electrode plates. A top crude oil layer, a middle emulsion layer and a bottom water layer are formed from the crude oil mixture in the desalting vessel.

[0006] Other aspects and advantages of the claimed subject matter will be apparent from the following description and the appended claims.BRIEF DESCRIPTION OF DRAWINGS

[0007] FIG. 1 is a diagram of a desalting vessel in accordance with one or more embodiments of the present disclosure.

[0008] FIG. 2 is a diagram of a desalting vessel and electrical components in accordance with one or more embodiments of the present disclosure.

[0009] FIG. 3 is a diagram showing a top view of a desalting vessel and electrical components in accordance with one or more embodiments of the present disclosure.

[0010] FIG. 4 is a flow diagram of a method of removing salt from oil in accordance with one or more embodiments of the present disclosure.DETAILED DESCRIPTION

[0011] Crude oil is a complex mixture of components produced from an oil well that generally includes various salts and additives that can be detrimental to crude oil processing systems. These salts need to be removed to prevent corrosion and other potentially hazardous issues in oil and gas processing. In the desalting process, crude oil is vigorously mixed with water to form an emulsion. The emulsion is then broken and separated into oil and water phases, where the salts are dissolved in the water phase and can, therefore, be separated from the oil. Breaking the emulsion can be facilitated by applying an electric field. This process of removing the salts from oil in a water phase is referred to as desalting.

[0012] One of the most commonly used desalting methods in the industry employs desalting vessels with rod electrodes forming an electrified grid, in a typical arrangement of three treatment stages. These treatment stages are conducted in three vessels connected in series, a dehydrator, a 1st desalting vessel, and a 2nd desalting vessel. The electrified grid produces an electrical field that causes water droplets to form dipoles, which elongates emulsified droplets. Adjacent water droplets coalesce, forming larger and heavier droplets, which fall to the bottom of the vessel through the effect of gravity.

[0013] Methods utilizing an electrified grid suffer from gaps between the electrodes and small electrode surface area. In addition, rod electrodes used to form the electrified grid are susceptible to corrosion, which leads to a reduction in overall efficiency. Thus, one or more embodiments disclosed herein relate to an oil desalting system that does not suffer from the deficiencies of the rod electrodes employed as an electrified grid. The disclosed desalting system is vertically oriented and uses large, rectangular electrode plates to improve the overall performance of the vessel.

[0014] The present disclosure generally relates to a system and method for removing salt from oil, which includes using a vertically oriented desalting vessel with vertically oriented electrode plates. The desalting vessel is filled with a mixture of crude oil and water (that may contain well defined water and oil layers and an emulsion of oil and water phases), and then subjected to an electric field by electrode plates. The fluid used is typically a mixture of crude oil and formation water. Additionally wash water may be added before, during, or after filling the vessel. The fluid may already have formed water and oil layers before filling, or may contain a proportion of emulsion, but will typically start to form separate oil and water layers in the vessel after filling. Generally, the vessel is filled until the fluid covers the electrode plates, and then electric field is applied. The application of the electric field begins a process of electrocoalescence in the crude oil, which results in the formation of large water droplets that settle to the bottom of the vessel and form or add to a water layer. Thus, an oil layer is thereby formed at the top of the vessel. Between the water layer and crude oil layer an emulsion layer is formed containing an emulsion of both phases. When a water layer is present, water may be removed from the vessel, bringing dissolved salt and other water-soluble compounds with the water, thus desalting the oil. Water is removed from the bottom of the vessel in a manner that does not contaminate the removed water with the emulsion or oil phases. Also, the water layer cannot contact the electrode plates, as such contact would cause short circuiting. Treated oil, which has had salt and water removed, may be retrieved from the top of the vessel. With this system, water and salt are extracted from crude oil, yielding a desalted treated oil and a brine byproduct.

[0015] FIG. 1 illustrates a desalting vessel 100 according to one or more embodiments of the present disclosure. As shown in FIG. 1, the desalting vessel 100 includes a vessel body 101. The vessel body 101 is configured to contain the fluid of the electrocoalescence process. The fluid includes a top crude oil layer 103, a middle emulsion layer 105, and a bottom water layer 107. A treated oil outlet 109 is located at the top of the vessel body 101, in fluid communication with the top crude oil layer 103. The treated oil outlet 109 conveys treated oil out of the desalting vessel 100. A water outlet 111 is located at the bottom of the vessel body 101, in fluid communication with the bottom water layer 107, and conveys water out of the desalting vessel 100. A crude oil entry 113 is located at the bottom of the vessel body 101, and conveys crude oil into the desalting vessel 100. In some embodiments, the crude oil entry 113 is connected to a pipe that conveys the crude oil to the top crude oil layer 103 without passing through the bottom water layer 107. In such embodiments, a crude oil outlet 115 is in fluid communication with the top crude oil layer 103. While treated oil outlet 109, water outlet 111, and crude oil entry 113 are depicted in their respective locations, it should be understood that these may be located in a different position, such as located on a side of vessel body 101, as long as they are still in fluid communication with the appropriate fluid layer. For example, in one or more embodiments, the crude oil entry 113 may be located on one of the vessel body 101 side walls, provided the crude oil outlet 115 is still in fluid communication with the top crude oil layer 103.

[0016] FIG. 1 also illustrates typical relative levels for the fluid layers in relation to the height of the vessel body 101. The top crude oil layer 103 is shown as occupying 60% of the height of the vessel body 101. The middle emulsion layer 105 is depicted as occupying 1% of the height of the vessel body 101. The bottom water layer 107 is depicted as occupying 39% of the height of the vessel body 101. These are general values which may fluctuate, and the height of the fluid layer relative to the height of the vessel body 101 only applies when the vessel body 101 is filled with fluid. In operation, the emulsion layer is monitored, and process variables are controlled to maintain the emulsion layer near 40% of the height of the vessel body. The emulsion layer is restricted to fluctuating between 15% and 42.5% of the height of the vessel body, ensuring the level does not drop too low and contaminated removed water, or rise too high and contact the electrode plates and lead to short circuiting.

[0017] FIG. 2 illustrates a desalting vessel 200 and associated electrical components with their positions in the fluid layers according to one or more embodiments of the present disclosure. The desalting vessel components are as described with reference to FIG. 1 above. Electrode plates 203 are located in the top crude oil layer 103, held in place by insulated supports 207 which secure the electrode plates 203 to the vessel body 101. Electrode plates 203 are in electrical communication with electrical transformers 219 through transformer leads 221, which supply electric potential to the electrode plates. One electrode plate functions as the cathode while the other functions as the anode. Ground plates 205 are also contained in the top crude oil layer 103, held in place by insulated supports 207 which secure the ground plates 205 to the vessel body 101. The insulated supports 207 which secure the ground plates 205 are also in electrical communication with a ground. A treated oil outlet 109 is located at the top of the vessel body 101, in fluid communication with the top crude oil layer 103, and conveys treated oil out of the desalting vessel 200. A water outlet 111 is located at the bottom of the vessel body 101, in fluid communication with the bottom water layer 107, and conveys water out of the desalting vessel 200. An interface level device 223 is configured to float in the middle emulsion layer 105, tracking the interface between the top crude oil layer 103 and a bottom water layer 107. The interface level device 223 is in communication with an interface level transmitter 225, located at the top of the vessel body 101.

[0018] FIG. 2 shows the same fluid layers as described with reference to FIG. 1. Also, FIG. 2 illustrates typical lengths for the electrode plates 203 and ground plates 205 relative to the height of the vessel body 101. These lengths are only examples, and may vary. For example, in one or more embodiments, the lengths of electrode plates 203 may be in a range from a lower limit of any of 35%, 40%, and 45% of the height of the vessel to an upper limit of any of 40%, 45%, and 50% of the height of the vessel, where any lower limit can be used in combination with any mathematically compatible upper limit. The lengths of ground plates 205 may be in a range from a lower limit of any of 40%, 42%, and 45% of the height of the vessel to an upper limit of any of 45%, 48%, and 50% of the height of the vessel where any lower limit can be used in combination with any mathematically compatible upper limit.

[0019] Example dimensions and positioning of the electrode plates and ground plates are as follows. It should be understood that these are exemplary embodiments, and the exact values may change and the system will perform a similar function. A typical arrangement is depicted in FIG. 2. For illustrative purposes, an exemplary vessel body has a height of 10 meters (m) and a diameter of 4 meters. In such embodiments, the pair of electrode plates 203 are rectangular, with dimensions of 4.5 m (height)×3.3 m (width)×2.5 cm (thickness). These exemplary dimensions may vary based on the overall dimensions of the vessel and the particular performance needs. The dimensions may vary by approximately 20% in any given direction. The plates are positioned such that the top of each plate is about 7.5% of the height of the vessel from the top of the vessel.

[0020] In one or more embodiments, there may be more than two ground plates, and the ground plates may have different sizes and positioning. A typical arrangement is depicted in FIG. 2. For example, a typical arrangement would have a single ground plate positioned between the electrode plates, equidistant from each of the electrode plates. In the same exemplary embodiment described above (i.e., a 10m tall vessel) this ground plate may have dimensions of 5 m×3.9 m×2.5 cm, with the top edge positioned 5% of the height of the vessel from the top of the vessel and the face of the plate being 50% of the diameter from the walls. Additionally, a typical arrangement would have two additional ground plates, each positioned between one of the electrode plates and the vessel wall, with dimensions of 4 m×0.7 m×2.5 cm, and positioned 10% of the height of the vessel from the top of the vessel. Also, FIG. 2 provides example spacings of the electrode plates 203 and ground plates 205 relative to the diameter of the vessel body 101. For example, the outer ground plates are shown as being positioned 1% of the diameter away from the vessel wall, with the nearest electrode plate being positioned 25% of the diameter away from the vessel wall, and the middle ground plate being 50% of the diameter away from the vessel wall.

[0021] FIG. 3 illustrates a top-down view of the vessel 300 showing the position of the electrode plates 203 and ground plates 205 inside the vessel wall 101. Electrode plates 203 and ground plates 205 are held in place by insulated supports 207 which secure the electrode plates 203 and ground plates 205 to the vessel body 101. The insulated supports 207 which secure the ground plates 205 are also in electrical communication with a ground. In one or more embodiments, the width of electrode plates 203 may be in a range from a lower limit of any of 50%, 55%, 60%, 65%, 70%, 75%, and 80% of the diameter of the vessel to an upper limit of any of 70%, 75%, 80% 85%, 90%, and 95% of the diameter of the vessel, where any lower limit can be used in combination with any mathematically compatible upper limit. The width of ground plates 205 may be in a range from a lower limit of any of 15%, 20%, 25%, 30%, 35%, 40%%, and 45% of the diameter of the vessel to an upper limit of any of 50%, 55%, 60%, 65% 70%, 75%, 80%, 85%, 90%, 95% or 97.5% of the diameter of the vessel where any lower limit can be used in combination with any mathematically compatible upper limit. Referring back to FIG. 2, in one or more embodiments, the level of the fluid layers is monitored by an interface level device 223. The interface level device 223 is configured to float within the middle emulsion layer 105, and is in communication with the interface level transmitter 225, which transmits the interface level to a control and monitoring system and allows an operator to track the position of the middle emulsion layer, and thus monitor the position of the fluid layers. The interface level device 223 and interface level transmitter 225 are commercially available. Examples include a guided wave radar, a displacer type level transmitter, or nuclear level type measurements. By tracking the layers' position, the operator can make appropriate decisions about applying electric field, when and how much oil should be added through crude oil entry 113, when and how much water should be removed through water outlet 111, and when and how much oil should be removed through treated oil outlet 109. This monitoring of the fluid layers and appropriate operation adjustments will ensure that the electrode plates 203 and ground plates 205 are contained within the top crude oil layer 103. It is important to keep the electrode plates 203 and ground plates 205 contained within the top crude oil layer 103 while the electric field is applied to avoid any short circuiting that could occur if the electrode plates 203 touch the bottom water layer 107.

[0022] In one or more embodiments, the electrode plates 203 and ground plates 205 are metal. In particular embodiments, carbon steel is used. The shape, size, and positioning of the electrode plates 203 results in a majority of the top crude oil layer 103 being exposed to the applied electric field.

[0023] In another aspect, embodiments disclosed herein relate to a method 400 for removing salt from crude oil. An exemplary method is described with reference to FIG. 4. In one or more embodiments, a crude oil mixture 401 is initially introduced into the vessel as described above to a level that fills the vessel and covers the electrode plates 203 and ground plates 205, while the electric field is off. After the vessel is filled, the electric field is applied 403. The electric field can be applied once the vessel is filled and there is minimal gas or air present inside the vessel. In one or more embodiments, the applied voltage may be in a range from a lower limit of any of 120, 125, and 130 kV to an upper limit of any of 135, 140, 145, and 150 kV, where any lower limit can be used in combination with any mathematically compatible upper limit.

[0024] Once the vessel is filled, the electric field is applied and the process of electrocoalescence begins. As described above, the fluid may contain emulsion and / or may have already started forming separate oil and water layers, with the formation of the bottom water layer being accelerated by electrocoalescence. Under electrocoalescence, the applied electric field causes water droplets to form dipoles, which elongates emulsified droplets. Adjacent water droplets coalesce, forming larger and heavier droplets. Coalesced water droplets can then be separated by the effect of gravity as they fall to the bottom of the vessel as they grow larger and heavier. This settling of electrocoalesced water droplets causes the formation of the top crude oil layer 103, the middle emulsion layer 105, and the bottom water layer 107. Monitoring the position of the interface level device 223, which is configured to float in the middle emulsion layer 105 in communication with the interface level transmitter 225, allows an operator to track the level of the fluid layers. This is critical for proper water and treated oil collection, and ensures the electrode plates 203 and ground plates 205 remain contained in the top crude oil layer 103. This is ensured by tracking the middle emulsion layer 105 and comparing to set minimum and maximum threshold heights. In one or more embodiments, the collecting treated oil 405 from the treated oil outlet occurs when the interface level device 223 is between the maximum and minimum threshold heights, and the rate of water collection 407 and fluid addition may be adjusted based off the position of the interface level device 223. If the interface level device 223 is approaching the minimum threshold height, the rate of water collection may be reduced, or water collection may be halted altogether, so that the collected water does not become contaminated with oil or emulsion. If the interface level device 223 is approaching the maximum threshold height, the rate of fluid addition may be lowered and / or the rate of water collection may be increased. Treated oil may be collected whenever there is treated oil present to collect, but in this configuration that will only happen when the vessel is filled and there is oil in fluid communication with treated oil outlet 109. By monitoring the interface level device 223 and adjusting the rates of water collection and fluid addition, continuous or near-continuous collection of treated oil may be achieved. If there is sufficient water in the fluid, continuous collection of water and treated oil is possible with continuous fluid addition into the desalting vessel.

[0025] In one or more embodiments, the maximum threshold height may be in a range from a lower limit of any of 32.5%, 35%, and 37.5% of the height of the vessel body to an upper limit of any of 40%, 42.5%, 45%, and 47.5% of the height of the vessel body where any lower limit can be used in combination with any mathematically compatible upper limit. In one or more embodiments, the minimum threshold height may be in a range from a lower limit of any of 10%, 11%, 12%, 13%, and 14% of the height of the vessel body to an upper limit of any of 15%, 16%, 17%, 18%, 19%, and 20% of the height of the vessel body, where any lower limit can be used in combination with any mathematically compatible upper limit. The maximum threshold height is set to prevent the electrode plates from contacting the water layer, and the minimum threshold height is set to prevent oil or emulsion contamination in the collected water.

[0026] In one or more embodiments, when the interface level device 223 is at a position between the maximum and minimum threshold heights, treated oil and water may be collected simultaneously (through the treated oil outlet 109 and water outlet 111, respectively) while additional crude oil is added to the vessel. This provides continuous operation of the desalting vessel with continuous production of treated oil and water as long as crude oil is supplied and the interface level device 223 remains between the maximum and minimum threshold heights. If the interface level device 223 reaches the maximum threshold height, the electric field application is halted to avoid short circuiting. As described above, continuous collection of oil will stop if the fluid level in the desalting vessel drops too low and treated oil outlet 109 is no longer in fluid communication with the top treated oil layer 103. Additionally, if the interface level device 223 is at or approaching the maximum threshold height, the rate of fluid addition may be slowed or halted and the rate of water collection may be increased.

[0027] Alternatively, if the if the interface level device 223 is at or approaching the minimum threshold height, the collection of water 407 may be slowed or halted and the level of fluid addition may be adjusted so that the collected water is not contaminated with emulsion or oil.

[0028] Embodiments of this method may provide at least one of the following advantages. The shape, size, and positioning of the electrode plates 203 used in this method provide several advantages over methods using rod electrodes to form electrified grids. The rectangular shape combined with dimensions of the electrode plates 203 maximizes the exposure of the top crude oil layer 103 to the applied electric field, whereas the electrified grid setup suffers from gaps between the rod electrodes and small electrode surface area. Additionally, large rectangular metal plate electrodes will be less susceptible to corrosion during use, which can reduce overall efficiency. Also, the size of the vessel and the vertical arrangement of the plates and vessel maximizes the potential fluid pressure of the oil and water column that acts on forming water droplets under electrocoalescence. This maximized fluid pressure due to the arrangement and orientation of the vessel and electrodes accelerates settling of water drops during electrocoalescence by maximizing the potential effect of gravity.

[0029] Although only a few example embodiments have been described in detail above, those skilled in the art will readily appreciate that many modifications are possible in the example embodiments without materially departing from this invention. Accordingly, all such modifications are intended to be included within the scope of this disclosure as defined in the following claims.

Claims

1. A system for removing salt from oil, the system comprising:a vertically oriented desalting vessel configured to contain a fluid, the fluid comprising a top crude oil layer, a middle emulsion layer, and a bottom water layer;a crude oil entry located at a bottom of the desalting vessel and in fluid communication with the top crude oil layer;a pair of electrode plates positioned vertically in the top crude oil layer, each plate being in electrical communication with an electrical transformer;at least one ground plate inside the desalting vessel;a water outlet located on the bottom of the desalting vessel and in fluid communication with the bottom water layer; anda treated oil outlet located at a top of the desalting vessel and in fluid communication with the top crude oil layer.

2. The system of claim 1, further comprising an interface level device positioned in the middle emulsion layer, wherein the interface level device is in communication with an interface level transmitter.

3. The system of claim 1, wherein a ground plate of the at least one ground plate is positioned between the pair of electrode plates in the top crude oil layer.

4. The system of claim 1, wherein the pair of electrode plates are comprised of metal.

5. The system of claim 1, wherein each electrode plate of the pair of electrode plates has a length ranging from 35% to 50% of a height of the desalting vessel.

6. The system of claim 1, wherein a length of at least one of the at least one ground plate ranges from 40% to 50% of a height of the desalting vessel.

7. The system of claim 1, wherein each electrode plate of the pair of electrode plates are rectangular.

8. The system of claim 7, wherein dimensions of the pair of electrode plates 4.5 m×3.3 m×2.5 cm.

9. The system of claim 8, wherein the pair of electrode plates are 7.5% of a height of the desalting vessel from the top of the desalting vessel.

10. The system of claim 8, wherein a face of each member of the pair is 25% of a diameter of the desalting vessel from a closest wall of the desalting vessel.

11. A method for removing salt from crude oil, the method comprising:providing a crude oil mixture into a vertically oriented desalting vessel via a crude oil entry located at a bottom of the desalting vessel, wherein the desalting vessel comprises a pair of vertically oriented electrode plates;applying an electric field through the pair of electrode plates such that water droplets in the crude oil mixture coalesce and fall to a bottom of the desalting vessel, thereby forming a top crude oil layer, a middle emulsion layer and a bottom water layer;collecting treated oil from a treated oil outlet located at a top of the desalting vessel and in fluid communication with the top crude oil layer; andcollecting water from a water outlet located on the bottom of the desalting vessel and in fluid communication with the bottom water layer.

12. The method of claim 11, wherein the collecting treated oil and the collecting water are conducted continuously.

13. The method of claim 11, further comprising:monitoring a level of the middle emulsion layer via an interface level transmitter, wherein the interface level transmitter is in communication with an interface level device positioned in the middle emulsion layer.

14. The method of claim 11 further comprising:introducing additional crude oil mixture to the vessel to maintain a fluid level in the vessel above a top of the pair of electrode plates.

15. The method of claim 11, wherein the pair of electrode plates are rectangular.

16. The method of claim 11, wherein the pair of electrode plates have a length that is in a range of 35% to % of a height of the desalting vessel.

17. The method of claim 11, wherein the pair of electrode plates are comprised of metal.

18. The method of claim 11, wherein the electric field is applied at a voltage ranging from 120 kV to 150 kV.

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