Patent Application
20090114569
The present invention relates generally to processes for reducing impurity levels in organic compositions, such as oils. More particularly, the invention relates to the removal of metallic and non-metallic impurities from fuel oils, e.g., from crude oils.
Hydrocarbon oils represent a type of crude oil (petroleum) found throughout the world, consisting of a complex mixture of hydrocarbons (mostly alkanes) of various lengths. In most cases, the hydrocarbon oils (e.g., the heavy oils) are processed and refined into other useful petroleum products, such as diesel fuel, gasoline, heating oil, kerosene, and liquefied petroleum gas.
It is well known in the art that hydrocarbon oils, like other organic compositions derived prehistorically from nature, contain at least small amounts of contaminating metals, sulfur, and other elements and compounds, such as nitrogen. (Crude oil from regions such as Saudi Arabia often contains relatively high levels of many of these contaminants). The contaminants are detrimental to the direct use of the crude oil as a fuel (e.g., use of the oil with minimal processing), as well as being detrimental to the processing of the oil to produce other commercially valuable products.
As an illustration in the case of gas turbine engines which may use this type of oil as a fuel, the impurities can cause serious corrosion problems on the turbine blades and other components. More specifically, vanadium compounds which form hard deposits on turbine blades are known to promote serious corrosion. In addition to causing material degradation and processing problems in refineries, the presence of contaminants like sulfur (usually in organic form) can also result in serious environmental and regulatory problems. For these reasons, gas turbine engines and other equipment may not always be capable of efficient operation when running on this type of fuel oil.
The metal contaminants in heavy oil, such as nickel and vanadium, are usually present in the form of one or more organo-metallic compounds. Examples include various porphyrinic compounds. As one specific example, J. Janssens et al discuss treatment of heavy oils which contain vanadium in the form of vanadyl-tetraphenyl-porphyrin (“VO-TPP”). (See “Competitive Effects of Hetero-Atom Containing Compounds in the Hydrodemetallisation of Vanadyl-Tetraphenyl-Porphyrin”, Fuel 1998, Volume 77, No. 12, pp. 1367-1374). The metallic compounds can be present in the form of non-porphyrin metal species as well, e.g., as metal salts. Moreover, other elements which may be present in crude oil include potassium, lead, sodium, and iron.
A number of techniques have been employed to remove impurities from crude oil, or to minimize their harmful effects. In terms of general processes, distillation techniques commonly used in oil refining remove some of the contaminants, as various oil fractions are boiled off in traditional distillation columns. However, distillation techniques can be very energy-intensive; and may not be suitable for removing contaminants from the heavy, “bottom” fractions of crude oil.
Catalytic hydrodesulfurization techniques have been used to remove sulfur from the crude product. However, vanadium and nickel impurities which are also present in the oil tend to adhere to the catalysts and thereby block the active site, diminishing the efficiency of the desulfurization reactions.
Techniques specific to metal removal are also available in the art. As an example, J. Reynolds describes the treatment of heavy crude oil compositions containing vanadium or nickel in the form of petroporphyrin fractions. (See “Removal of Nickel and Vanadium from Heavy Crude Oils by Exchange Reactions”, J. Reynolds, Prepr. Pap. Am. Chem. Soc., Div. Fuel Chem. 2004, 49(1), xxxx (2 pages; UCRL-ONF-201920). Exemplary treatment agents which are described therein include maleic acid in dimethylformamide; and montmorillonite in 1-methyl naphthalene. Following treatment with the chemical agents, the fractions are washed in aqueous solutions to effect final removal of the metals.
Other techniques for removing nitrogen, sulfur, and metal contaminants (e.g., vanadium) from heavy oils are described in U.S. Pat. No. 4,465,589, issued to Kukes et al. The heavy oils are treated by contact with a methylating agent, such as dimethyl sulfate, resulting in the formation of precipitates which include the contaminants. The precipitate can then be separated from the heavy oil by filtration or other techniques.
Magnesium compounds are sometimes used to address the problems of metal contamination. (See U.S. Pat. No. 6,632,257, issued to Feitelberg et al, as an illustration in the case of turbine equipment). For example, magnesium is capable of forming relatively low-melting alloys with contaminant metals such as vanadium. These low-melting compounds can be removed more easily (e.g., by washing) from the surface of turbine blades, as compared to the harder, higher-melting contaminants themselves.
While the use of the magnesium compounds may be suitable in some situations, there are limitations in other situations. For example, the compounds may form excessively hard deposits if the underlying part (for example, a gas turbine) is exposed to higher temperatures, e.g., greater than about 2,000° F. (1093° C.). In that case, the problem of deposits adhering to the metal surface remains. Thus, use of the gas turbine component may have to be restricted to lower operating temperatures, and this limitation can also represent a significant drawback.
Moreover, combustion of the lower-melting magnesium-vanadium alloys can result in the generation of significant amounts of ash, which also forms a residue on an underlying substrate, e.g., the turbine blades. These deposits can adversely affect the gas flow path over the turbine blades. Furthermore, cleaning steps to remove the deposits may require temporarily shutting down the turbine, which will also lower power-generation efficiency.
In view of these concerns, it should be clear that new techniques for reducing the level of metallic or non-metallic impurities in heavy oils would be welcome in the art. The processes should be capable of substantially reducing the level of at least some of the impurities in the heavy oils. They should also be relatively cost-effective and energy-conservative. Moreover, these techniques should not adversely affect other treatment processes to which the heavy oil is subjected.
An embodiment of the invention is directed to a treatment method for reducing the level of metallic and nonmetallic impurities in an oil. The method comprises the step of contacting the oil with a porous silica adsorbent material. The adsorbent material is characterized by a Brunauer-Emmett-Teller (BET) surface area value (total) of at least about 15 m2/g; and a Barrett-Joyner-Halenda (BJH) pore volume (total) of at least about 0.5 cc/g.
Other embodiments, features and advantages of the invention will become more apparent from the following detailed description.
As mentioned above, this invention relates to the treatment of organic compositions, e.g., oils such as the various fuels used in gas turbines. Non-limiting examples of the fuels include fossil fuels, such as crude oils and bituminous, processed/distilled residues. More specific examples include coker oils, coker gas oils, atmospheric and vacuum residual oil, fluid catalytic cracker feeds; deasphalted oils and resins, processed residual oil, and heavy oils (e.g., heavy crude oils). Other examples include light crude oil (e.g., “light distillates” such as liquefied petroleum gas (LPG), gasoline, and naphtha); middle distillates such as kerosene and diesel fuel; and other heavy distillates and residuum, such as lubricating oils, wax, and tar. Combinations of any of these materials may also be treated. (Those familiar with the art understand that the terminology for these fuels is not always precise, so there may be some overlap in describing the materials).
As also described above, the primary metal contaminants are typically organo-metallic compounds of nickel and/or vanadium, e.g., porphyrin and non-porphyrin metal complexes. Other metallic contaminants may also be present, e.g., compounds of potassium, lead, sodium and iron. The amount of metallic contaminants in the oil being treated may vary considerably. As an example, fuel oil may contain up to about 500 ppm by weight vanadium, and up to about 200 ppm nickel. (These amounts are expressed in terms of the metal itself, based on the total weight of the oil). Frequently (though not always), untreated fuel oil may contain about 0.5 ppm to about 500 ppm vanadium; about 0.8 ppm to about 200 ppm nickel; and up to about 500 ppm by weight of other metals.
The fuel oil may also contain varying amounts of non-metallic contaminants, such as asphaltenes, other resins and resin-like materials, and organic sulfur compounds, which may be treated according to this invention. Non-limiting examples of the organic sulfur compounds include thiophenes, benzothiophenes, dibenzothiophenes, mercaptans, sulfides, disulfides, and combinations thereof. Alkyalted derivatives of some of these compounds may also be present, e.g., 2-methylthiophene, 3-methylthiophene, 2-ethylthiophene and dimethylbenzothiophene. The organic sulfur compounds may be present at levels of up to about 6% by weight sulfur, based on the total weight of the oil. Frequently (though not always), untreated fuel oil may contain about 0.5% by weight to about 4% by weight sulfur.
The oil is treated by contact with an inorganic silica adsorbent material—usually in particle form. As used herein, the term “silica” can refer to silica gels, fumed silicas, or precipitated silicas, as described in U.S. Pat. No. 5,391,385 (Seybold), which is incorporated herein by reference. The silica may be in a hydrogel or a xerogel form. While the distinction between these two forms is not always clear in the art, U.S. Pat. No. 3,617,301 (Barby et al) provides some useful guidelines, and is incorporated herein by reference. Moreover, product literature from PQ Corporation, Valley Forge, Pa. also provides some guidance. (See, for example, product release “BRITESORB_A100.doc” for BRITESORB® A100, Jul. 8, 2004, which is incorporated herein by reference). The product literature briefly describes a typical silica gel production process which involves the combination of sodium silicate and sulfuric acid. A subsequent acidic wash yields a hydrogel after a milling step; while an alkaline wash, followed by milling and drying, results in the formation of a xerogel. The average particle size (diameter) of the silica particles is usually in the range of about 5 microns to about 100 microns (depending on the commercial grade), and more often, in the range of about 10 microns to about 50 microns.
While various silica adsorbent materials are known in the art, the present inventors have discovered that silica materials with specific physical characteristics are very effective for removing contaminants from the oils. The primary characteristics relate to surface area, pore volume, and average pore size. Another important characteristic is known as the “cumulative pore volume distribution” of the adsorbent particles.
In most instances, the total surface area of the silica adsorbent particles is at least about 15 m2/g, based on the Brunauer-Emmett-Teller (BET) measurement technique. In some specific embodiments, the total surface area of the particles is at least about 200 m2/g; and more often, at least about 400 m2/g.
The pore volume of the silica adsorbent particles represents the total interior volume of the particles. The pore volume (total) is usually at least about 0.5 cc/g, based on the Barrett-Joyner-Halenda (BJH) pore volume measurement technique. In some specific instances, the total pore volume is at least about 0.8 cc/g, and in some preferred embodiments, at least about 1.0 cc/g. As described in copending patent application Ser. No. 11/603,764 (A. Stella and D. Hall, filed on Nov. 22, 2006), the total pore volume is given by the sum of the pore volumes of all adsorbent particles over the entire pore size range present in the adsorbent sample. (application Ser. No. 11/603,764 is incorporated herein by reference).
The silica adsorbent particles can be further characterized by pore size, i.e., pore diameter. Usually, adsorbent silica materials that provide a mesoporous surface, or a combination of mesoporous and microporous surfaces, can be used. The selected pore size will depend on various factors, such as the particular type of silica used; the specific type of oil being treated; the impurities which are initially present in the oil; the size and wetting characteristics of the impurities; and the chemical nature of the impurities, e.g., whether they are polar or non-polar. In some cases, the adsorbent materials have an average pore size of less than about 60 nm, while in other instances, the average pore size is less than about 10 nm. In still other embodiments, the average pore size is less than about 3 nm. In terms of ranges, the average pore size of the adsorbent materials is often in the range of about 3 nm to about 100 nm, and in some specific embodiments, in the range of about 10 nm to about 20 nm.
As described in application Ser. No. 11/063,764, the cumulative pore volume distribution can be obtained from the total cumulative pore volume, which in turn can be obtained from the total pore volume. The total cumulative pore volume is usually expressed as a percentage of the total pore volume. It should also be understood that in preferred embodiments, substantially all of the silica particles in a typical batch contain at least some pores). In some preferred embodiments, the silica adsorbent material has a cumulative pore volume distribution of at least about 20% of particles having a pore diameter in the range of about 3 nm to about 20 nm. In other words, at least about 20% of the pores, i.e., the total number of pores in all of the particles, preferably have a pore diameter within this range. (Other factors may sometimes influence the selection of a desired pore volume distribution for the silica, e.g., the type and shape of a metallic impurity such as vanadium).
Silica adsorbent materials meeting the requirements of the present invention are commercially available. These materials are sufficiently robust, so that the pore structure of the silica will not collapse or degenerate when contacted with various polar and nonpolar solvents, e.g., during a regeneration step. Some examples of suitable adsorbents include those available from PQ Corporation, such as the Britesorb® C930, C935, D350 EL, D300 CE, A100, and R100 silicas; as well as CBV901 zeolite.
In some instances, it may be desirable to include a magnesium silicate compound along with the silica adsorbent material. Such a combination is described, for example, in U.S. Pat. No. 4,508,742 (John McLaughlin et al), which is incorporated herein by reference. In some embodiments, the magnesium silicate material comprises an amorphous, hydrous-precipitated, synthetic magnesium silicate. Some of the magnesium silicate compounds which are useful have a mole ratio (MgO:SiO2) in the range of about 1:1.6 to 4.7; a surface area of about 30 m2/g to about 600 m2/g); and a bulk density (tamped) of about 0.25 to about 0.75 g/cm3. In these embodiments, the silica adsorbent material may preferably be in the form of a hydrogel, as described in the McLaughlin reference. Moreover, in some cases, the magnesium silicate is present at a level of about 0.2 to about 6 pbw (parts-by-weight), per 100 pbw of the silica adsorbent.
In some embodiments, the silica adsorbent material is dried, prior to contact with the oil being treated. Drying can be carried out by various techniques. As an example, the adsorbent can be heated at temperatures between about 80° C. to about 250° C., under vacuum, to remove surface moisture.
The manner in which the oil is contacted with the silica adsorbent material is not generally critical to this invention. In general, the selected method should involve relatively intimate mixing of the adsorbent with the oil, e.g., by stirring, shaking, static mixing, flow techniques (such as those described below), or similar techniques. The method may be carried out as a batch process, or by way of a continuous operation. (Simulated, continuous counter-current techniques could also be used, as well as semi-batch or “semicontinuous” processes). Those of ordinary skill in the art, e.g., chemical engineers, will be able to select the most appropriate technique for a given situation.
In some preferred embodiments, contact between the oil and the silica adsorbent material is carried out in the presence of at least one organic solvent. Usually, the solvent should be substantially compatible with the oil. Some of the factors which determine the most appropriate solvent for a given situation may include the respective viscosities of the solvent and the oil being treated; as well as their miscibility and their respective densities.
Some general examples of solvents which can be used include non-polar, branched or linear alkanes containing at least about 3 carbon atoms. In some instances (though not all), the solvent has a viscosity of less than about 0.35 centipoise, at 25° C. Non-limiting examples of the solvents include benzene, naphthalene, alkyl oxalate, decaline, tetraline, xylene, decane triethylene glycol dimethyl ether, tetraethylene glycol dialkyl ether, anisol, dimethoxy benzene, dimethoxy toluene, propane, butane, pentane, petroleum ether; and various combinations of any of the foregoing. The proportion of solvent to oil will depend in part on some of the solvent factors noted above. In many instances, the ratio of solvent to oil is in the range of about 1:1 to about 6:1.
In some embodiments, the preferred solvent is petroleum ether, or a petroleum ether-based solvent mixture. Those skilled in the art understand that some types of petroleum ether are referred to as “benzine” or “X4”, and are used as a mixture of hydrocarbon, non-polar solvents. Petroleum ether can be obtained from petroleum refineries as the portion of the distillate which is intermediate between the lighter naphtha and the heavier kerosene. It usually has a specific gravity between about 0.6 and 0.8 and a boiling range of about 30° C. to about 60° C., depending on the particular grade.
In some instances, contact between the oil and the porous silica material is carried out in at least one stirred or agitated mixing vessel, such as those known in the art, and commercially available. The mixing vessel would preferably also contain at least one organic solvent, as described above. The reaction is usually carried out at a temperature in the range of about 0° C. to about 60° C. However, this temperature range can vary, depending on a number of factors, e.g., the grade of oil being treated; the type of adsorbent employed; the type of mixing vessel, pressure conditions; and the like. A preferred range in some cases is about 15° C. to about 30° C., with room temperature often being most preferred.
It should be noted that in some preferred embodiments, the oil is deasphalted, prior to full treatment with the silica adsorbent. This step results in the removal of substantial amounts of asphaltene and other heavy oil fractions. (These materials may have accumulated as precipitates when the oil being treated is first brought into contact with an organic solvent). Removal of the asphaltene-type materials is especially beneficial when a continuous process is being employed to treat the oil, so as to minimize the build-up of pressure in treatment columns, which can otherwise result from the presence of the heavy fractions.
Deasphalting techniques and deasphalting units are known in the art, e.g., for upgrading various types of crude oil. Non-limiting examples of the processes include those described in U.S. Pat. Nos. 6,106,701 (Hart) and 5,124,027 (Beaton et al), which are incorporated herein by reference. As an illustration, deasphalting may be carried out by any of the separation techniques discussed below, e.g., filtration or centrifugation.
Contact between the oil and the silica adsorbent in the mixing vessel results in a mixture which comprises organic solvent; treated heavy oil; and silica particles which contain impurities from the heavy oil. The impurities are adsorbed on the surface of the silica particles; within the pores of the silica particles, or in both regions.
The mixture can then be subjected to a separation step, so as to separate the treated heavy oil from the remainder of the system. A variety of separation techniques can be employed. Some are described in the referenced application Ser. No. 11/063,764, and others are known in the art as well. Usually, the separation procedure involves a technique such as settling, centrifugation, filtering, decantation, hydrocycloning, and the like, or various combinations of such techniques.
The treated oil may then be stored or used immediately, e.g., in a gas turbine. The adsorbent and remaining solvent can be further treated by other techniques, e.g., to separate the solvent from the adsorbent; and/or to regenerate the adsorbent for additional use in the treatment of oil.
In some embodiments of this invention, the level of each of vanadium and nickel in the oil, after treatment, is reduced to less than about 0.2 ppm (based on the amount of metal itself). In preferred embodiments, the level of each of these metals is reduced to less than about 0.1 ppm. Moreover, in some embodiments, the level of sulfur (elemental sulfur) present in the oil, after treatment, is reduced to less than about 1 wt %, based on total weight of the oil.
Variations regarding oil-adsorbent contact are possible. For example, in some embodiments, contact between the oil and the porous silica material can be carried out by passing the oil through (or over) at least one bed which is packed with the silica material. (Multiple beds can be employed, e.g., arranged in series or parallel). Those skilled in the chemical engineering arts are familiar with details regarding these type of procedures, and are familiar with different variations which may be suitable, e.g., the use of fluidized-bed reactor technology or ebullated bed technology.
The present invention provides for the economical and highly effective removal of various organic and inorganic impurities from hydrocarbon oil. For this reason, those involved in the use of hydrocarbon fuels, e.g., fuel suppliers, energy producers, and related equipment manufacturers, can benefit from much greater “fuel flexibility”. As one example, conventional gas turbine engines can be employed to combust a wider spectrum of crude oil, resulting in better utilization of petroleum resources. Moreover, turbines using fuel treated as described herein should be able to operate at higher temperatures, which often improves overall turbine efficiency.
Another aspect of this invention relates to an apparatus or system for reducing the level of metallic and nonmetallic impurities in oil. The system would include at least one vessel which can accommodate the silica adsorbent material and the oil (and usually at least one organic solvent), along with mixing means for ensuring intimate contact between the adsorbent and the oil. The system could also include other features as well, e.g., means for separating the treated oil from the silica particles and solvent, such as settling tanks, piping systems, separation columns, and the like. As mentioned above, the overall system can also include reactor beds which can incorporate the silica adsorbent material. This apparatus can be adapted for batch operations, semi-batch operations, or continuous operations.
The examples which follow are merely illustrative, and should not be construed to be any sort of limitation on the scope of the claimed invention.
Britesorb® C930 silica adsorbent was used in this example. The material had a surface area (BET) of 452 m2/g; and a pore volume (Vp) of 1.27 cc/g. Approximately 0.11 g of the C930 material was weighed into a vial. 5.06 g of a petroleum ether solution of vanadium etioporpyrin (15 ppm V in the solvent) was added to the vial. The mixture was stirred for 2 minutes, at room temperature. (The approximate composition of this grade of petroleum ether, as determined by gas chromatograph (GC) analysis, was as follows: 50 wt % pentane; 5 wt % 2,2-dimethylbutane; 5 wt % 2,3-dimethylbutane; 25 wt % 2-methylpentane; and 15 wt % 3-methylpentane). After stirring, the mixture was centrifuged, and the clear liquid which resulted was decanted. All of the color which was characteristic of the vanadium compound had been completely absorbed by the silica adsorbent. The petroleum ether solution was thus free of vanadium.
Approximately 0.11 g Britesorb® R100 silica adsorbent was used in this example. The material had a surface area (BET) of 277 m2/g; and a pore volume (Vp) of 0.55 cc/g. The R100 material was weighed into a vial, and 5.07 g of a petroleum ether solution of vanadium etioporpyrin (15 ppm V) was added. The mixture was stirred for 2 minutes at room temperature. It was centrifuged, and the slightly-pinkish liquid was decanted. In this instance, the R100 did not completely absorb the color which is characteristic of the vanadium compound. This observation provides an indication that the R100 grade of silica adsorbent may not have the optimal pore structure for some applications, and more of the adsorber may be required to treat a given volume of oil. (However, use of the R100 would be suitable in other situations).
The experiment outlined in Example 2 was repeated, using a different silica adsorbent material. In this instance, a silica xerogel was employed, designated as Britesorb® D350EL. The material had a surface area (BET) of 680 m2/g; and a pore volume (Vp) of 1.4 cc/g. 3.5 g of the gel was weighed into the Waring blender. 10 g of crude oil was added, along with 40 g of petroleum ether. The resulting slurry was mixed for 2 minutes, and poured into centrifuge tubes. Each tube was centrifuged for 10 minutes, at 2100 rpm. The resulting liquid fractions were decanted into vials, and the vanadium and nickel contents were measured by the ICP-MS (Inductively Coupled Plasma Mass Spectrometry) technique. The results indicated that residual vanadium and nickel levels were below detection limits.
7 g of Britesorb® C935 silica adsorbent was used in this example. This material was a hydrogel silica, having a surface area (BET) of 500 m2/g; and a pore volume (Vp) of 1.3 cc/g. The adsorbent was weighed into a Waring blender. 20 g of crude oil (a Saudi Heavy grade) was added. The crude oil contained 3 wt % sulfur, 58 ppm vanadium, and 18 ppm nickel. 80 g of petroleum ether was added as a diluent, in order to reduce the viscosity of the oil. The slurry was mixed for 2 minutes in the blender, and then poured into centrifuge tubes. The tubes were centrifuged at 2100 rpm for 10 minutes.
The resulting liquid fractions were decanted into vials. The vanadium and nickel contents were then measured by ICP-MS. The results showed the residual vanadium to be less than 0.1 ppm; and the residual nickel to be less than about 0.1 ppm. Sulfur was measured by XRF (x-ray fluorescence), and was found to be present at a level of less than 0.3 wt %. This value corresponds to about 1.0 wt % with respect to the starting oil, i.e., correcting for dilution.
The adsorbent was washed with cyclohexane to remove excess oil, and then dried in a vacuum oven at 80° C., for 4 hours. The XRF spectrum clearly indicated that substantially all of the sulfur was retained on the adsorbent.
Batch tests were carried out, using crude oil (Saudi Heavy grade, “Shuquiaq”, 8 grams); petroleum ether (16 grams); and 4 grams of the adsorbent indicated in Table 1. The crude oil initially contained about 16 ppm vanadium (based on metallic vanadium). The table also indicates the surface area, pore diameter, and average particle size for each adsorbent. “C-935” is the Britesorb® material described previously. Mixtures of the components listed above were shaken on an auto-shaker for 10 minutes, and then centrifuged for 10 minutes at 2100 rpm. The product oil was decanted, and then analyzed by ICP/MS for vanadium content.
The data of Table 1 relates to some embodiments of this invention, providing an indication of the effect of various silica gel adsorbent parameters in this particular treatment regimen. In this instance, both sample 3 and the C-935 sample contained extremely low vanadium content, after treatment. The other samples also exhibited a reduction in vanadium, with various adsorbent parameters affecting final metal content.
The present invention has been described in terms of some specific embodiments. They are intended for illustration only, and should not be construed as being limiting in any way. Thus, it should be understood that modifications can be made thereto, which are within the scope of the invention and the appended claims. Moreover, as used throughout this disclosure, the terms “a” and “an” do not denote a limitation of quantity, but rather denote the presence of at least one of the referenced items. The suffix “(s)” as used herein is intended to include both the singular and the plural of the term that it modifies, thereby including one or more of that term (e.g., the “solvent” includes one or more solvents). Furthermore, all of the patents, patent applications, articles, and texts which are mentioned above are incorporated herein by reference.
