Patent Application
20100000910
This invention relates generally to separating a trace element from a liquid hydrocarbon feed within a phase separation device, such as a desalting unit or oil-water separator.
Liquid hydrocarbon feeds generally contain an assortment of trace elements in amounts generally ranging from several parts per billion (ppb) to several thousand ppb depending on the feed source. These elements often cause corrosion within equipment and may deteriorate or poison a catalyst of a subsequent treatment process. For example, mercury may amalgamate with a surface metal, such as copper or aluminum, collecting with time in piping, valves and even in larger structures such as fractional distillation columns. Equipment replacement or abstraction of this deleterious metal from the equipment can be very expensive and potentially hazardous. Therefore, it may be preferable to remove the trace elements as early as possible during processing, such as removal prior to distillation of the feed or even while still at the hydrocarbon recovery site. However, due to the liquid hydrocarbon state of the feed prior to distillation being more chemically complex, current technologies for removing the trace elements prior to hydrocarbon distillation tend to be less developed.
Various successful methods for removal of trace metal contaminates within liquid hydrocarbon feed prior to fractional distillation have nonetheless been developed. For example, U.S. Pat. No. 6,350,372 B1 discloses utilizing a solubilized sulfur compound in combination with an absorbent carrier. In particular, a liquid hydrocarbon feed is mixed with a miscible sulfur compound and then placed in contact with a fixed bed absorbent, thus removing at least 85% of the mercury on an elemental basis. U.S. Pat. No. 4,474,896 claims the use of absorbent compositions, mainly polysulfide based, for removal of elemental mercury from gaseous and liquid hydrocarbon streams. Specifically, the absorbent compositions comprise a polysulfide, a support material and metal cation capable of forming an insoluble metal polysulfide. While the approach of using fixed bed absorbents to extract trace elements, including mercury, from a hydrocarbon feed have shown to be successful, they also include a number of less than desirable attributes. Absorbent beds tend to get clogged by solid particulates in the crude, thus impeding the flow of the feed. Absorbents can also be very costly due to the large quantity needed, especially if there is a high concentration of the trace element or elements being extracted. In addition, stripping the absorbent is generally necessary prior to disposal or recycling of the absorbent.
Another method to remove mercury from liquid hydrocarbon condensate is disclosed in U.S. Pat. No. 4,915,818. In this method, the use of absorbent carriers is eliminated by treating the liquid hydrocarbons with a dilute aqueous solution of alkali metal sulfide salt. Due to the high partition coefficient of the sulfur compounds in the aqueous phase, the risk of contaminating the liquid hydrocarbons with sulfur is limited. However, while this process minimizes the risk of sulfur contamination, mercury present in the organic phase may also be less likely to react to the alkali metal sulfide salt as its chemical dependency may be governed by the phase it resides in. In particular, the organic mercury compounds are soluble in the liquid hydrocarbon feed and typically are far less reactive than elemental mercury or inorganic mercury compounds.
In view of the foregoing, previous methods of trace element removal are considered less than desirable and new methods of overcoming the problems associated with trace element extraction from hydrocarbon feed would be extremely useful.
The present invention comprises removing a trace element from a liquid hydrocarbon, such as crude oil, natural gas, and other petroleum products. The liquid hydrocarbon is mixed or emulsified with water and a hydrocarbon-soluble additive. During mixing, the additive chemically reacts with the trace element forming a compound. This compound is typically an aqueous insoluble compound, such that the compound may easily be separated and removed in subsequent treatment processes. A phase separation device, such as a desalter or an oil-water separator, resolves, i.e., separates, the oil-water emulsion containing the compound. The resolved mixture produces the compound formed by mixing the additive with the trace element, effluent brine, and effluent liquid hydrocarbon with a reduced concentration of the trace element as compared to the liquid hydrocarbon feed. The compound may be dispensed from the phase separation device with the effluent brine or the effluent liquid hydrocarbon and may later be filtered out.
In some embodiments, the present invention is directed to removing elemental mercury from a liquid hydrocarbon feed. A sulfur-containing hydrocarbon-soluble additive is mixed with the liquid hydrocarbon feed and water to produce an emulsified solution. In some instances, the liquid hydrocarbon is already emulsified with the water prior to injection of the additive and in other scenarios the additive may be added directly to either the liquid hydrocarbon or water and then can all be mixed together. For instance, an organic polysulfide can be injected directly into the liquid hydrocarbon stream prior to being emulsified with water or it can be injected into an emulsified oil-water mixture. Regardless of the mixing strategy, the sulfur-containing additive reacts with the mercury, concentrated within the liquid hydrocarbon, rapidly forming an agglomeration of mercuric sulfide which is then dispensed with the effluent brine or the effluent liquid hydrocarbon for subsequent filtering.
According to one embodiment of the present invention, a system is employed to remove a trace element from a liquid hydrocarbon. The system includes first and second fluid lines fluidly communicating with a phase separation device. In a refinery setting, where the phase separation device may comprise a desalting unit, the first fluid line can contain a liquid hydrocarbon feed and the second fluid line can contain wash water. A hydrocarbon-soluble additive can be mixed with either the liquid hydrocarbon feed or the wash water, such that it chemically reacts with the trace element as the fluids are emulsified. The fluid mixture is then resolved within the phase separation device producing effluent liquid hydrocarbons with a reduced concentration of the trace element that can be dispensed through a first output line, effluent brine that can be dispensed through a second output line, and the compound formed by mixing the additive with the trace element, which can be dispensed from the phase separation device with either of the effluent brine or the effluent liquid hydrocarbon. If the trace element is removed at the hydrocarbon recovery site, such as an offshore platform, the phase separation device may comprise an oil-water separator. Here, the first fluid line can contain a contaminated oil-in-water mixture and the second fluid line can contain a hydrocarbon-soluble additive that can be directly injected into the first fluid line to treat the mixture. As the additive is mixed with the contaminated oil-in-water mixture, the additive chemically reacts with the contaminant or trace element forming a compound. As the mixture is separated, the liquid hydrocarbon is recovered such that it has a reduced concentration of the trace element.
The above mentioned and other features of this invention will become more apparent and better understood by reference to the following detailed description taken in conjunction with the accompanying drawings.
The figures are not necessarily to scale and certain features may be exaggerated in order to better illustrate and explain the present invention. Similarly, the figures have been simplified from a processing standpoint to exclude certain types of equipment, such as mixing devices, not essential for understanding the invention by one skilled in the art.
Hydrocarbon feeds, generally a conglomeration of hydrocarbon chains with approximate lengths ranging between C5H12 and C42H86, typically contain a variety of trace elements. The trace elements range from alkaline earth metals, transition metals, post-transition metals, and nonmetals and generally consist of calcium (Ca), vanadium (V), chromium (Cr), iron (Fe), cobalt (Co), nickel (Ni), copper (Cu), zinc (Zn), arsenic (As), selenium (Se), molybdenum (Mo), cadmium (Cd), indium (In), tin (Sn), antimony (Sb), tellurium (Te), barium (Ba), mercury (Hg), thallium (Tl), lead (Pb), and/or bismuth (Bi). For various reasons, including corrosion prevention and ensuring environmental sustainability, it is often desirable to extract one or more of these trace elements during initial treatment of the feed.
In certain embodiments, mercury is the trace element targeted for extraction and a hydrocarbon-soluble additive, such as an organic polysulfide such as Di-Tertiary-Nonyl Polysulfide (TNPS), is utilized to form a compound with the mercury. The hydrocarbon-soluble sulfur-based additive reacts with the mercury rapidly forming an agglomeration of mercuric sulfide through the following reaction:
R—S—Sx—S—R+XHg→R—S—S—R+XHgS
where R is any hydrocarbon or hydrogen, S is Sulfur, and X and x are the same whole number, typically between 3 and 8. As an inorganic salt, mercuric sulfide has essentially no vapor pressure and with the conversion to an ionic salt, makes the mercury more readily available for removal by various techniques already known in the art. In some instances, mercurous sulfide may also be formed from the reaction of the sulfur-based additive with the mercury in the liquid hydrocarbon feed.
Once separated, the effluent brine flows out of the desalter though a first output 114 and typically is filtered and recycled back through line 112 as wash water. The effluent liquid hydrocarbon is dispensed from the phase separation device 110 into piping 116 and is transported to fractional distillation column 120. Fractional distillation column 120 is comprised of a plurality of spaced plates 122 filled with multiple apertures 124. As the heated effluent hydrocarbon enters the fractional distillation column 120, it separates such that the hydrocarbon vapors continually ascend passing through the apertures 124 within the spaced plates 122. As the hydrocarbon vapors climb in the fractional distillation column 120, they cool down and begin to condense forming liquid fractions that are caught in the plurality of spaced plates 122. Vapors that pass all the way to the top of the fractional distillation column 120 exit through output 126. These vapors are typically very light hydrocarbons and are commonly called naphtha. Heavier hydrocarbons fractions such as gasoline, kerosene, diesel, lubricating oil and heavy gas oil are dispensed through outputs 128 each corresponding to the spaced plates 122 within the fractional distillation column 120. The heaviest hydrocarbon chains collect in the bottom of the fractional distillation column 120 and are dispensed through output 130. These hydrocarbons are commonly referred to as the residual. Depending on the respective output 126, 128, 130, the fractions may pass to subsequent condensers, which cool them further, and then go to storage tanks or be routed to other areas for further chemical processing. For instance, the naphtha dispensed from the top of the fractional distillation column may further be separated into light ends, such as liquefied natural gases, and heavier or denser ends. The compound formed by a reaction between the hydrocarbon-soluble additive with the trace element is dispensed along with the effluent brine or effluent liquid hydrocarbon. Conversion of the trace element to a compound makes it more available for subsequent removal through techniques such as filtration, coagulation, flotation, co-precipitation, ion exchange, reverse osmosis, ultra filtration and other typical treatment processes known in the art.
As previously mentioned, the phase separation device 110 may utilize various separation items, already known in the art, to assist in separating the mixture into phases. For example and as shown in
The following example shows how mercury content is reduced from a liquid hydrocarbon feed to minimal levels, according to the present invention. Test results were taken at a plurality of locations, each corresponding to a different stage within a liquid hydrocarbon treatment facility, over four hour intervals to measure the variation in the concentration of mercury within a contaminated liquid hydrocarbon feed. As shown in
The results above indicate that after the hydrocarbon-soluble additive was injected into the liquid hydrocarbon feed, the mercury concentration began to taper off significantly and stabilize by the twelfth hour of testing to a level of less than 15 ppb at point E, which is located downstream of the phase separation device 110 on output 126 of fractional distillation column 120. A more immediate drop at Point E may be realized through proper flushing of the equipment prior to commencing the injection of the additive. While no significant change may be seen at Point B in this example, a settling agent can be used, e.g., by injecting the settling agent at either Points 108 or 112, to promote an increase of mercury concentration in the effluent brine. Considering that a concentration of mercury is continually detected at Point C, it appears that the compound is carried by the effluent liquid hydrocarbon to the distillation chamber. Note that in this example, detection does not speciate and therefore, the readings include the total mercury concentration present in both an elemental and compound state. It is contemplated that the compound may have collected at the bottom of the distillation chamber, as an increased concentration was not detected at Point D, while a significant drop did occur at point E.
Certain terms are defined throughout this description as they are first used, while certain other terms used in this description are defined below:
The term “sulfur-based” as used herein means any compound containing one or more sulfur atoms.
The term “mercury salt” as used herein means any chemical compound formed by replacing all or part of the hydrogen ions of an acid with one or more mercury ions.
The term “mercury sulfide” as used herein means mercuric sulfide, mercurous sulfide, or a mixture thereof. Normally the mercury sulfide is present as mercuric sulfide and thus the stoichiometric equivalent would be one mole of sulfide ion per mole of mercury ion.
The term “organic polysulfide” as used herein means any chemical compound containing two or more sulfur atoms bonded to any hydrocarbon or hydrogen atom.
The unit “ppb” as used herein means parts per billion.
The term “oil-water” as used herein means any mixture comprising a liquid hydrocarbon with water. Therefore, it is to be understood that the term “oil-water” is inclusive of both oil-in-water emulsions and water-in-oil emulsions.
While this invention has been described as having an exemplary design, the present invention may be further modified within the spirit and scope of this disclosure. This application is therefore intended to cover any variations, uses, or adaptations of the invention using its general principles. Those skilled in the art will appreciate that the above described embodiments are merely illustrative of the present invention and that many variations of the above described embodiments can be devised without departing from the scope of the invention. For instance, it is contemplated that the hydrocarbon-soluble additive can be introduced into the liquid hydrocarbon or oil-water mixture through multiple injection points as compared to a single injection line. It is therefore intended that such departures from the present disclosure, that come within the known customary practice in the art to which this invention pertains, be included within the scope of the following appended claims and their equivalents.
This application is a continuation-in-part of U.S. patent application Ser. No. 12/132,475 filed Jun. 3, 2008, which is hereby incorporated herein in its entirety by reference.
