Patent Application
20090242384
The typical process for desalting crude oil involves mixing fresh water into a crude oil stream and taking a pressure drop across a mixing valve. In this way the fresh water “washes” the salt out of the oil. Once the oil and water are mixed, the water is extracted from the oil by flowing the mixture through an electrostatic dehydrator. To avoid creating an emulsion that an electrostatic field cannot process, the pressure drop across the mixing valve is typically limited to less than 15 psi.
As production and processing techniques for crude oil have evolved, it has become common for the techniques to create salt crystals in the crude oil. Because these crystals cannot be removed directly by the electrostatic process, the crystals must first be dissolved or wetted by the fresh water. However, the crystals are difficult to dissolve because they are oil-coated. Use of pressure drops higher than 15 psi, therefore, is required to dissolve the salt out of the oil prior to extraction by the electrostatic field.
A second, more common problem occurs in refineries where salt levels must be reduced to very low levels in order to avoid corrosion and catalyst fouling. Although the oil arriving at a refinery has been previously processed by a production company to meet a refinery acceptance specification, residual water containing salt remains as a very fine dispersion and is very difficult to remove. These oils, therefore, also benefit from a higher pressure drop mix system.
Last, taking a higher pressure drop across the mixing valve imposes a higher back pressure on a crude charge pump. This back pressure reduces the capacity of the pump, thereby affecting the crude charge rate. A high pressure mix system, therefore, is needed to achieve mixing requirements but the system must be designed to overcome its own pressure drop to avoid any reduction in crude charge rate.
As shown in Table 1, tests of a crude containing crystalline salt revealed that pressure drops higher than 45 psi can be required to meet a salt limit of 1 ptb (pounds of salt per thousand barrels). Assuming no crystalline salt present in a typical crude oil, and using one stage of mixing—equating to a pressure drop of 15 psi—a 0.3% BSW content in the crude should result in a salt level of 1 ptb. Testing has determined that three mixing stages, equating to a pressure drop of 45 psi, and passing the crude oil through a dual polarity electrostatic field failed to meet the 1 ptb level. Higher mix energy required a more aggressive electrostatic dehydration technology, such as a dual frequency process. Dual frequency is a new electrostatic technology; dual polarity is an older technology. The dual frequency process coupled to a three-stage mix system easily met the salt limit, thus showing the superiority of using the dual frequency process. The data support a finding that higher mixer energy is required to dissolve salt crystals but at the same time it creates an emulsion that is more difficult to resolve. This test suggests that some portion of the mix energy can be provided by a pump.
A method and system for reducing the salt content of a crude oil stream includes using a quill to disperse a fresh water stream into the crude oil stream and then routing the mixed oil/water stream through four mixing stages. Each mixing stage increases the homogeneity of the mixed oil/water stream. The first mixing stage produces the only backpressure the crude charge pump needs to overcome. Upon exiting the fourth stage, the mixed oil/water stream is electrostatically treated in a separator vessel. The separator vessel may be a dual frequency separator vessel or a dual polarity separator vessel (desalter). The desalted oil is removed from an upper portion of the vessel and the effluent water is extracted from a lower portion of the vessel.
The first and third mixing stages include static mixers and are lower pressure mixing stages relative to the second mixing stage. The pressure drop across the first and third mixing stages may be in the range of 3 to 5 psi. The second mixing stage provides a pressure increase effective for flowing the mixed oil/water stream through the third and fourth mixing stages. This second stage preferably includes a boost pump and provides a pressure increase of about 25 psi. The fourth mixing stage includes a mixing valve and is capable of providing higher mix energy than the third stage. The pressure drop across the valve may be in the range of 5 to 20 psi. Depending on desalting requirements, one or more four-stage mixing systems and separator vessels in series may be required. Similarly, the first and second mixing stages may be bypassed.
The water stream may include a wash water that has been preconditioned with an effluent water. The effluent water that is extracted from the separator vessel may be recycled and used in the preconditioning step. A static mixer may be used to precondition the wash water by mixing the wash water with the effluent water. A portion of the recycled effluent water may also be routed to a second four-stage mixing system and separator vessel.
A better understanding of the method and system will be obtained from the following detailed description of the preferred embodiments taken in conjunction with the drawings and the attached claims.
The invention described below is not limited in its application to the details illustrated in the accompanying drawings. The invention is capable of other embodiments and of being practiced or carried out in a variety of ways. The phraseology and terminology employed herein are for purposes of description and not limitation. Elements illustrated in the drawings are identified by the following numbers:
10 Oil/water mix system
12 Crude charge pump
16 Heat Exchanger
18 Quill
20 Wash water
22 Recycled water
24 Water static mixer
26 Isolation valve
28 Oil/water static mixer
30 Pressure gauge
32 Bypass valve
34 Pressure gauge
36 API boost pump
38 Oil/water static mixer
40 Isolation valve
42 Pressure gauge
46 Mix valve
48 Separator vessel
50 Isolation valve
52 Isolation valve
58 Separator vessel
62 Recycle pump
64 Bypass piping
Referring to the drawings and first to
Water mixer 24 mixes recycled water 22 with wash water 20 prior to waters 20, 22 being injected into the crude stream. Water mixer 24 is preferably configured so that a substantially homogeneous water stream is produced. Recycled water 22 is preferably drawn from a bottom portion of a desalting vessel (see
Because wash water 20 is fresh water, it fails to disperse as rapidly in crude and contact the crystallized salt as does recycled water 22. Recycled water 22 may disperse more readily because it has been previously contacted by the crude, making it more compatible with the crude. Mixing the two water sources 20, 22 in mixer 24 prior to injection has the advantage of pre-conditioning the wash water 20, making wash water 20 easier to disperse into the crude and contact the crystalline salt. A wetting agent may be added to wash water 20 to improve the efficiency of droplet-crystal contact. The pressure drop across mixer 24 is preferably in a range of 3 to 5 psi.
The water stream exiting water mixer 24 is routed through injection quill 18 to oil/water mixer 28. Mixer 28 is of a type well-known in the art and preferably comprises several short stationary vanes arranged in series. Each vane rotates the oil/water emulsion stream 90 degrees and subsequent vanes are set at a 90 degree angle to split the flow of the stream. Mixer 28 provides a first stage of mixing to increase homogeneity of the oil/water emulsion. The pressure drop across mixer 28 is preferably in a range of 3 to 5 psi. This pressure drop represents the only pressure drop that crude charge pump 12 must overcome.
The oil/water emulsion stream exiting mixer 28 is routed to a centrifugal boost pump 36. Pump 36 is of a type well-known in the art and typically used to increase pressure in a pipeline. Pump 36 is preferably a variable frequency drive pump and the differential pressure across pump 36 is preferably about 25 psi. Pump 36 provides two primary functions for mix system 10. First, pump 36 provides a second stage of mixing between the crude oil and the substantially homogenous mix of waters 20, 22. To avoid excessive shearing, pump 36 is a closed impeller type pump, but because pump 36 is mixing as well as pumping an open impeller might prove better in certain applications. Second, pump 36 increases pressure of the flowing oil/water emulsion. This increase in pressure pushes the emulsion to pass through a second mixer 38 and into a mix valve 46. Mixer 38, which is preferably similar to mixer 28, further homogenizes the oil/water emulsion. Mixer 38 is required in case pump 36 should promote centrifugal separation of the waters 20, 22 from the crude oil. Mixer 38 represents a third stage of mixing.
The oil/water emulsion exiting from mixer 38 is routed to mix valve 46. Mix valve 46 is of a type well-known in the art and typically is a single or double port globe valve or a ball valve. The style of valve used is not critical to the process but preferably mix valve 46 is suitable for creating pressure drops ranging from 5 to 20 psi. Mix valve 46 represents a fourth and final stage of mixing.
Referring now to
The second stage includes a separator vessel 58 in communication with a second mix valve 46. Separator vessel 58 is preferably similar to vessel 48. Because the second stage typically uses wash water 20 and recycled water 22, water mixer 24 (not shown) is included in the second mix system 10. Desalted oil is then discharged from a top portion of vessel 58.
Referring now to
Referring now to
The foregoing description details certain preferred embodiments of the present invention and describes the best mode contemplated. It will be appreciated, however, that changes may be made in the details of construction and the configuration of components without departing from the spirit and scope of the disclosure. Therefore, the description provided herein is to be considered exemplary, rather than limiting, and the true scope of the invention is that defined by the following claims and the full range of equivalency to which each element thereof is entitled.
This application claims priority to, and the benefit of, U.S. Provisional Patent Application No. 61/039,897, filed on Mar. 27, 2008.
| Number | Date | Country | |
|---|---|---|---|
| 61039897 | Mar 2008 | US |
