Desalter Emulsion Separation By Emulsion Recycle (United States Patent Application 20150152340 - ExxonMobil Research and Engineering)

Desalter Emulsion Separation By Emulsion Recycle (United States Patent Application 20150152340 - ExxonMobil Research and Engineering)
June 4, 2015
Abstract
A petroleum desalting process in which fluid from interfacial boundary layer between the settled water layer and the settled oil layer or emulsion-water layer in the vessel is withdrawn from the desalter and recycled to the crude oil inlet of the desalter to improve separation of the oil and water phases.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims priority to U.S. Provisional Application Ser. No. 61/911,153 filed Dec. 3, 2013, herein incorporated by reference in its entirety.
FIELD OF THE INVENTION
[0002] This invention relates to petroleum desalters and their operation.
BACKGROUND OF THE INVENTION
[0003] Crude petroleum normally contains salts that may corrode refinery units; salt is removed from the crude oil by a process known as "desalting", in which hot crude oil is mixed with water and a suitable demulsifying agent to form a water-in-oil emulsion which provides intimate contact between the oil and water, transferring salt into the water. The salty emulsion is then passed into a high voltage electric field inside a closed separator vessel. The electric field forces water droplets to coalesce, forming larger water droplets. As the water droplet volumes increase, they settle to the bottom of the tank under gravitation. The desalted oil forms at the upper layer in the desalter from where it is continuously drawn off for distillation. The salty water is withdrawn from the bottom of the desalter.
[0004] During operation of desalter units, a stable emulsion phase (also known as a "rag layer") of variable composition and thickness forms above the interface between the oil-continuous phase and the water-continuous phase at the bottom of the desalter. Certain crude oils contain natural surfactants (e.g. asphaltenes and resins) which tend to form a barrier around the water droplets in the emulsion, preventing coalescence and stabilize the emulsion in the desalting vessel. Finely-divided solid particles in the crude may also act to stabilize the emulsion, and it has been found that solids-stabilized emulsions present particular difficulties. Solids coated with crude oil components, such as those found in oil sands, are thought to be particularly effective in forming stable emulsions. This emulsion phase may become stable and persistent in the desalting vessel. The growth of stable emulsion layer reduces workable volume and may short the electric circuit and force unplanned and costly desalter shut down. To mitigate the rag layer buildup in some cases, emulsion is withdrawn from the unit; alternatively or in addition, costly demulsifiers may be added to the oil phase upstream of the desalter although with limited success.
[0005] Additionally, processing crudes with high rag layer formation tendencies in current desalter configurations may cause poor desalting (salt removal) efficiency due to solids build up at the bottom of the vessel, and/or a solids-stabilized rag layer leading to erratic level control and insufficient residence time for proper water/oil separation. Solids-stabilized emulsion layers have become a major desalter operating concern, generating desalter upsets, increased preheat train fouling, and deteriorating quality of the brine effluent and disruption of the operation of the downstream wastewater treatment facilities.
[0006] Refinery sites which process high solids-content crudes (characterized as containing more than 150 ppm inorganic solids) have the most pervasive problems with emulsion formation. Heavy crude oils and bitumens from Western Canada which contain elevated levels of small clay fines and other small solids are particularly prone to forming large volumes of highly stable emulsion and with such feeds, growth of the rag layer is more prevalent. These feeds are, however, being introduced to refineries in greater quantities despite two main disadvantages related to the efficacy of desalting. First, the viscosity of these crudes can be quite high, so transport of water through the feed is slower than in high API gravity crude. Second, the density mismatch between water and oil is lower, so the gravitational energy gradient is reduced compared to higher API gravity crudes. Growth of the rag layer in the desalter requires either the amount of crude passed through the desalter is reduced or removal of the rag layer from the desalting vessel for external treatment.
[0007] The water content of the rag layer could range from 20 to 95% water with the balance being hydrocarbon (normally full range crude oil) and up to 5 weight percent inorganic solids. Precipitated asphaltenes, waxes, and paraffins may also be found at elevated levels in the rag layer (compared to the incoming crude oil) which combine with particulates (solids), to bind the mixture together to form a complex structure that is highly stable. Intractable emulsions of this kind comprising oil, water and solids make adequate separation and oil recovery difficult. Often, these stable emulsions arising from the desalter are periodically discarded as slop streams. This results in expensive treating or handling procedures or pollution problems as well as the fact that crude oil is also lost with these emulsions and slop streams.
[0008] Emulsions must be separated into well-defined oil and water phases before they can be reintroduced to refinery process units (e.g. crude distillation, coker, etc.) or a waste water treatment plant. These stable emulsions may not be completely separated by heating and conventional gravity settling and require specialized separation equipment.
[0009] One of the most common industry practices is to separate the stable emulsion into separate water, oil and solids phases using 3-phase centrifuges (decanter centrifuges). The centrifuge separation is often enhanced with the use of chemical emulsion breakers, heating and/or depressurizing the emulsion to facilitate the process. US 2012/0024758 (Love) proposes a technique in which the emulsion "rag" layer is withdrawn from the separator vessel at a rate that maintains the height of the emulsion layer approximately constant so as to permit withdrawal of the rag layer at a fixed level from the vessel. The withdrawn emulsion is then processed outside the vessel through a stacked disk centrifuge. Currently practiced centrifuge separation approach has, however, numerous reliability and cost drawbacks centering on the separation of the oil and water phases before they can be reintroduced to refinery process units (e.g. crude distillation, coker, etc.) or the waste water treatment plant.
[0010] As one problem area arises from the relatively high viscosity of the emulsions formed from heavy oil feeds, various proposals for dilution of the emulsions to reduce their viscosity have been made. U.S. Pat. No. 3,396,100 (Pettefer), for example, proposes the separation of oil and water in the rag layer by the direct addition of naphtha or kerosene to the rag layer in the separation tank without withdrawal of the rag layer, the injection of the diluent into the interfacial zone acts to release the solids otherwise retained in the layer so that they settle out into the water layer as a residue.
[0011] U.S. Pat. No. 4,200,550 (Scherrer) discloses withdrawal of the rag layer that is tested to determine the amount of stable emulsion, water, and oil. If there is more than a certain amount of rag layer in the withdrawn sample, then a demulsifier is injected directly into the desalter.
[0012] U.S. Pat. No. 5,219,471 (Goyal) discloses dilution of the rag layer with an aromatic hydrocarbon followed by centrifugation to separate the phases.
[0013] U.S. Pat. No. 4,824,555 (Paspek) discloses a method in which the stable emulsion is removed from the desalter and a hydrocarbon with a high vapor pressure is added under high pressure in a separate vessel.
[0014] U.S. Pat. No. 5,882,506 (Ohsol) describes method for treating desalter rag layer emulsions, for the recovery of processable oil values by adding a sufficient amount of a light hydrocarbon diluent to the emulsion to lower its overall viscosity and to reduce the specific gravity of the oil phase. The diluted emulsions are subjected to flashing at emulsion-breaking conditions after which the oil is recovered from the various streams created in the flashing steps.
[0015] A related process is described in U.S. Pat. No. 4,938,876 (Ohsol) in which emulsions are rendered more amenable to gravitational and cyclonic separation by causing a portion of the normally water-dispersed phase to flash into vapor by suddenly reducing pressure on the emulsion which has been heated by direct contact with superheated water and/or steam. The envelope around each droplet is thus shattered so the dispersed phase can be coalesced and separated by gravity, or enhanced gravity forces, when there is a sufficient divergence of specific gravity and a low viscosity. Suitable anti-emulsion chemicals are often added to prevent re-emulsification. These processes, however, are energy-intensive requiring significant amounts of heat in the flash vaporization.
[0016] A different approach is suggested in U.S. Pat. No. 4,722,781(Swartz). A portion of the rag layer is returned to the crude oil feed stream, upstream of water and chemical addition, and without any additional processing. A slip stream comprising the portion which is not returned to the crude oil feed is drawn off to avoid solids contaminant buildup. This slip stream is diluted with a light hydrocarbon material to break the emulsion and the oil reduced in solids content is then recovered by settling and decantation. Oil is removed from a tank after the water and oil layers have settled. The oil phase and rag layer may be removed together from the desalter and taken to a second vessel in which they are separated.
[0017] Co-pending U.S. Provisional Patent Application Ser. No. 61/774,957, filed 8 Mar. 2013 (EM Family No. 2013EM063), describes an improved mode of desalter operation in which for withdrawal of a portion of the emulsion layer is withdrawn from the desalter vessel through one or more external withdrawal headers according to the thickness and position of the emulsion layer with the selected withdrawal header(s) being controlled by sensors monitoring the position and thickness of the emulsion layer. The withdrawn emulsion layer is then routed as such or with the desalter water effluent to a settling tank or directly to another unit for separation and reprocessing.
[0018] Co-pending U.S. Provisional Patent Application Ser. No. 61/828,963, filed 30 May 2013 (EM Family No. 2013EM170), describes an improved mode of desalter operation in which a portion of the emulsion layer from the desalter vessel is withdrawn for treating the emulsion layer withdrawn from the desalter vessel in order to separate it into its oil and water components along with any solids brought along with it. This treatment comprises diluting the withdrawn emulsion with added water or oil to destabilize the emulsion and permit its subsequent separation. The desalting method is operated by forming a settled water layer containing the dissolved salts with a settled supernatant, desalted oil layer and an intervening emulsion layer formed from the oil and the water. A portion of the emulsion is withdrawn through one or more withdrawal ports or headers and diluted with an added fluid, typically water or an added hydrocarbon feedstock, to destabilize the emulsion which is then separated, optionally with the aid of an electrostatic precipitator in a separator vessel which itself may be a desalter type vessel operating with a high voltage electric filed to facilitate the separation.
[0019] Co-pending U.S. Provisional Patent Application Ser. No. 61/882,358, filed 25 Sep. 2013 (EM Family No. 2013EM259, Brian D. Albert et al.), describes a desalting process in which the oil/water emulsion layer which forms in the desalter vessel between the settled oil and water layers is separated into the oil and water components by direct contact with a heated, high boiling hydrocarbon acting as a heating medium to transfer heat from the heating medium to the emulsion. This has the effect of breaking the emulsion and then at least partly vaporizing the water content of the emulsion in a flash drum downstream from the desalter vessel. The preferred heating medium is an atmospheric or vacuum resid, both of which have the advantage of being readily available and of not introducing additional light hydrocarbon vapors into the flash drum along with the water vaporized from the emulsion.
SUMMARY OF THE INVENTION
[0020] According to the present invention, a fluid stream that includes the interface between the effluent water and fluid above is removed from the desalter. Additionally, this fluid can be recycled back to the crude feed into the desalter, preferably to be returned upstream of the mix valve that creates the water-in-oil emulsion before it is sent to the desalting vessel. The fluid that is withdrawn and taken to recycle includes fluid extracted from the boundary layer at the interface of the water and the supernatant emulsion including oil-wet solids which stabilize the emulsion layer. The addition of a small amount of rag layer to crude oil and water before emulsification not only does not inhibit performance, it enhances water separation by reusing the trapped demulsifier for better water resolution. In one variant of the process, a hydrocarbon diluent is used to aid the separation of the oil from the solids in the settler.
[0021] In operation, the desalting is carried out by mixing a crude oil to be desalted with water and passing the mixture of oil and water to the desalter vessel. The emulsion enters the desalting vessel between electrodes at a high voltage. Water droplets coalesce in the electric field and settle towards the bottom of the tank under gravitational forces. An emulsion layer formed from the oil and the water and emulsion-stabilizing solids forms between the settled water layer and the settled oil layer; water is removed from the water layer through a water outlet conduit at the bottom of the vessel and desalted oil is removed from the oil layer through an oil outlet conduit at the top of the vessel. An emulsion outlet for removing an emulsion stream from the emulsion layer is provided in the vessel and this is connected to an optional settling drum interposed between the emulsion outlet of the desalter vessel and the recycle conduit to reduce the amount of non-emulsified water returned to the process.
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