Patent Grant
7282138
The invention relates to a multistage process for removing heteroatoms and wax from distillate fuel. In an embodiment, the process involves hydrotreating a distillate fuel feed to remove heteroatoms, separating the treated feed into light and heavy fractions, with the heavy fraction catalytically dewaxed.
Middle distillate fuel stocks such as diesel, kerosene, jet fuel and home heating oil, are produced from distillate hydrocarbon feeds that contain undesirable components including aromatics and heteroatom compounds containing sulfur and nitrogen. Therefore, the distillate fuel feed is typically hydrotreated by reacting it with hydrogen in the presence of a hydrotreating catalyst, to remove the heteroatoms as H2S and NH3, and remove some aromatics by saturation. These feeds also contain waxy hydrocarbon molecules. There are increasing requirements for distillate fuels to have better low temperature properties, including lower pour, cloud, freeze and fuel filter plugging temperatures and cold filter plugging point (CFPP). To obtain fuel stocks that will meet more severe cold temperature requirements, distillate fuel fractions must be dewaxed in addition to being hydrotreated. Various process schemes have been proposed and used for hydrotreating distillate fuel stocks, some of which incorporate catalytic dewaxing into the process, and sometimes into the same reactor vessel used for hydrotreating. Illustrative examples may be found, for example, in U.S. Pat. Nos. 4,358,362; 4,436,614; 4,597,854; 4,846,959; 4,913,797; 5,720,872; 5,705,052; and 6,103,104; and U.S. Patent Application No. 20020074262 A1. Since existing fuel hydrotreating facilities have neither dewaxing capability nor ground space available on which to add new units to provide it, there is a need for a process that will remove both heteroatoms and wax from distillate fuel feeds. Desirably, such a process could readily be adapted for use with existing hydrotreating facilities, with minimal investment in dewaxing equipment and facilities.
The present invention relates to a process for removing heteroatoms and wax from a distillate fuel feed which comprises (i) hydrotreating the feed in one or more hydrotreating reaction stages to produce a hydrotreated fuel reduced in heteroatoms, (ii) separating the treated fuel into a light and a heavy fraction, and (iii) dewaxing the heavy fraction in one or more dewaxing reaction stages to improve one or more low temperature properties. The heavy fraction comprises less than about 80 and preferably less than 60 vol. % of the feed. Separating and dewaxing only the hydrotreated heavy fraction, as compared to the total hydrotreated feed, enables the use of one or more of (a) less catalyst for dewaxing, (b) lower space velocity of the liquid through the dewaxing catalyst bed, with concomitant deeper dewaxing due to greater residence time, and (c) lower dewaxing temperature and pressure. In an embodiment, the hydrotreating conditions result in the vaporization of most, and preferably all of the light fraction, but not the waxy heavy fraction. In this embodiment the hydrotreating reaction products comprise the hydrotreated liquid heavy fraction and a gaseous effluent comprising the hydrotreated and vaporized light fraction, along with gaseous reaction products which include unreacted hydrogen, H2S and NH3. The hydrotreated liquid heavy fraction is separated from the gaseous effluent. The gaseous effluent is cooled to condense the hydrotreated light fraction to liquid, which is then separated from the gaseous reaction products. If desired, all or a portion of the hydrotreated light fraction may be recombined with all or a portion the hydrotreated and dewaxed heavy fraction.
Dewaxing catalysts are known to be sensitive to organic heteroatom-containing compounds, NH3 and H2S. Catalysts that dewax mostly by isomerization with minimal cracking of the feed to lower boiling hydrocarbons are typically particularly sensitive. In an embodiment, therefore, the hydrotreated heavy fraction liquid be stripped to remove dissolved H2S and NH3 before it is dewaxed. Following dewaxing, the hydrotreated and dewaxed heavy fraction, and the hydrotreated light fraction, are typically stripped to remove residual and dissolved heteroatoms, gas and other impurity species, either separately or as a recombined stream. A single stripping vessel with separate stripping stages may be used to strip (a) the hydrotreated heavy fraction liquid prior to and after dewaxing, (b) the hydrotreated and condensed light fraction liquid, and/or (c) the recombined stream comprising the hydrotreated and dewaxed heavy fraction and hydrotreated light fraction. In another embodiment, any of these three streams may be hydrofinished, with or without prior stripping, to form a fuel stock. In a preferred embodiment, fresh hydrogen treat gas is introduced into the one or more dewaxing stages, with unreacted hydrogen from the dewaxing used for hydrotreating.
A more detailed embodiment of the invention comprises (a) passing hydrogen and a wax and heteroatom-containing distillate fuel feed into one or more hydrotreating stages, at reaction conditions effective for the feed and hydrogen to react in the presence of a catalytically effective amount of hydrotreating catalyst, to produce a feed reduced in heteroatoms, (b) separating the heteroatom-reduced feed into a light fraction and a heavy fraction liquid, and (c) passing the separated heavy fraction liquid and hydrogen into one or more dewaxing reaction stages, at reaction conditions effective for the hydrogen to react with the heavy fraction in the presence of a catalytically effective amount of a dewaxing catalyst, to improve one or more of the fuel's low temperature properties. The preferred embodiment in which the hydrotreating reaction vaporizes the light fraction, eliminates the need for distillation or fractionation external of the hydrotreating reactor. In this embodiment the process comprises (a) passing hydrogen and a wax and heteroatom-containing distillate fuel feed into one or more hydrotreating stages, at reaction conditions effective for the feed and hydrogen to react in the presence of a hydrotreating catalyst, to (i) produce a feed reduced in heteroatoms and (ii) vaporize at least a portion of the lighter feed components to produce a light fraction vapor and a heavy fraction liquid, (b) separating the heavy fraction liquid from the light fraction vapor, and (c) passing the heavy fraction liquid and hydrogen into one or more dewaxing reaction stages, at reaction conditions effective for the hydrogen to react with the heavy fraction in the presence of a catalytically effective amount of a dewaxing catalyst, in order to improve one or more of the feed's low temperature properties.
The process can be retrofitted into an existing distillate fuel hydrotreating unit, which typically operates at a similar, but sometimes lower, temperature and pressure than a typical catalytic dewaxing unit. This is because hydrotreating, and preferably hydrotreating combined with stripping the waxy heavy fraction to remove the heteroatom impurities prior to dewaxing, permits the use of lower dewaxing temperatures and pressures. Lowering the dewaxing temperature and pressure, and particularly the pressure, makes it easier for both hydrotreating and dewaxing to be achieved in the same reaction vessel at the same time. Thus, another embodiment relates to retrofitting or adding catalytic dewaxing capability to an existing distillate fuel hydrotreating facility. In this embodiment, (a) one or more catalytic dewaxing stages are added to a distillate fuel hydrotreating facility comprising one or more hydrotreating stages and (b) employing the process steps comprising hydrotreating, separation and dewaxing only the hydrotreated heavy fraction, etc., including any or all the various embodiments set forth above. The one or more dewaxing stages can be in a separate reactor added to the facility, but in at least some cases they may be added to an existing hydrotreating reactor, either internally in the reactor or as an extension welded to the top of the reactor and more preferably interior of the reactor with gas communication, but not with liquid communication, between the one or more dewaxing and hydrotreating stages. In an embodiment, one or more hydrotreating stages in a hydrotreating reactor are converted to one or more dewaxing stages. If the hydrotreating reactor has interstage gas-liquid separation trays, then hydrotreating catalyst in one or more hydrotreating stages may be replaced with dewaxing catalyst.
The invention relates to a method for upgrading a hydrocarbon by hydrotreating and dewaxing. In an embodiment, the hydrocarbon feed is a distillate fuel feed comprising a hydrocarbon fraction boiling generally in the diesel and jet fuels range, which may broadly range between about 300 to about 700° F. (about 149 to about 371° C.) and more typically about 400 to about 650° F. (about 204 to about 343° C.). In an embodiment, the cut point separating the heavy fraction from the lighter fraction is typically in the range of from about 450 to about 580° F. (about 232 to about 304° C.). Most of the wax is concentrated in the heavy fraction; consequently, only the heavy grade need be dewaxed in order to obtain improved low temperature properties. This heavy fraction is typically less than about 80 and preferably less than about 60 vol. % of the total liquid feed. Major benefits are achieved by hydrotreating to remove heteroatom impurities prior to dewaxing and by dewaxing only the separated heavy fraction. For a given dewaxing reaction stage volume, reducing the volume of waxy feed being dewaxed results in an increased residence time for the waxy liquid and a concomitant increased hydrogen treat gas to waxy hydrocarbon ratio in the dewaxing stage(s). Alternately, less dewaxing catalyst can be used to achieve the same level of dewaxing and, therefore, a smaller dewaxing stage can be used, resulting in a desirable decrease in the dewaxing reaction residence time. Removal of the heteroatom impurities prior to dewaxing results in greater catalyst dewaxing activity and this too enables the use of less catalyst and a smaller stage. In a combined hydrotreater/dewaxer reactor retrofit, a smaller dewaxing stage would make more space available for hydrotreating catalyst. Moreover, employing a smaller dewaxing stage enables the addition of a smaller dewaxing reactor or combined dewaxing and hydrotreating reactor to an existing hydrotreating facility, if it is not possible to add a dewaxing stage to an existing hydrotreating reactor. Another benefit of heteroatom removal prior to dewaxing is that the dewaxing reaction can be operated at milder conditions of lower pressure and temperature than would otherwise be possible if the heteroatoms had not been removed. In an enbodiment shown in
By heteroatoms is meant primarily sulfur and nitrogen, which are present in the feed as sulfur and nitrogen containing compounds, but the term also includes oxygen in oxygen containing compounds. In the one or more hydrotreating reaction stages, the feed reacts with hydrogen in the presence of a catalytically effective amount of a hydrotreating catalyst under catalytic hydrotreating conditions, to produce a hydrotreated fuel having fewer heteroatoms. Sulfur and nitrogen in organic heteroatom compounds in the feed are removed as H2S and NH3, with oxygen removed as H2O. The hydrotreating also converts at least a portion of aromatics and other unsaturates that may be present by hydrogenating them. The sulfur content of the feed may vary, but will typically be from about 0.5 to about 2.0 wt. % sulfur in the form of various sulfur-bearing compounds. If previously hydrotreated, the feed sulfur could be lower than about 0.5 wt. % (e.g., about 500 wppm). The nitrogen content of the feed will range from about 20 to about 2000 wppm and preferably no more than about 300 wppm. By way of an illustrative, but nonlimiting example, these feeds are hydrotreated to reduce the respective sulfur and nitrogen content to from about 5 to about 100 wppm and about 10 to about 100 wppm, depending on the impurity levels in the feed. Improved low temperature properties, include one or more of lower pour, cloud, freeze and CFPP temperatures. Low temperature property requirements will vary depending on the fuel and some depend on the geographical location in which the fuel will be used. For example, jet fuel should have a freeze point of no higher than about −47° C. Diesel fuel has both summer and winter cloud point specifications, varying by region, from about −15 to about +5° C. and about −35 to about −5° C. Both fuels have fuel filter plugging requirements. Heating oils typically have low pour point requirements. The feed may be derived from light and heavy whole and reduced crude oils, as straight run distillates, from vacuum tower resids, cycle oils, FCC tower bottoms, gas oils, vacuum gas oils, deasphalted residua, tar sands, shale oil and the like. The heavier sources tend to have more heteroatom impurities and therefore require more severe processing.
As discussed, the invention relates to a fuel upgrading process involving hydrotreating followed by dewaxing a portion of the hydrotretaed feed. The hydrotreating will be described first, followed by a description of the dewaxing. As used herein, hydrotreating refers to a process in which a feed to be hydrotreated and a hydrogen-containing treat gas react in the presence of one or more suitable catalysts primarily active (selective) for the removal of heteroatoms, such as sulfur, and nitrogen, and for the saturation of aromatics and other unsaturates with hydrogen. Conventional hydrotreating catalysts can be used including, for example, catalysts comprising one or more Group VIII metal catalytic components, preferably Fe, Co and Ni, more preferably Co and/or Ni, and most preferably Co; and one or more Group VI metal catalytic components, preferably Mo and W, more preferably Mo, on a high surface area support material, such as alumina. The Groups referred to herein refer to Groups as found in the Sargent-Welch Periodic Table of the Elements copyrighted in 1968 by the Sargent-Welch Scientific Company. Other suitable hydrotreating catalysts include zeolitic catalysts, as well as noble metal catalysts, wherein the noble metal is selected from Pd and Pt. It is within the scope of the present invention that more than one type of hydrotreating catalyst may be used in the same reaction stage or zone. Catalysts useful for saturating aromatics include nickel, cobalt-molybdenum, nickel-molybdenum, nickel-tungsten and noble metal (e.g., platinum and/or palladium) catalysts, with the noble metal catalysts being sulfur sensitive, but more selective for aromatics removal. Typical non-noble metal hydrotreating catalysts include, for example, Ni/Mo on alumina, Co/Mo on alumina, Co/Ni/Mo on alumina, and the like. Hydrotreating conditions typically include temperatures in the range of from about 530 to about 750° F. (about 277 to about 400° C.), preferably about 600 to about 725° F. (about 316 to about 385° C.), most preferably about 600 to about 700° F. (about 316 to about 371° C.), at a total pressure in the range of about 400 to about 2000 psi, at a hydrogen treat gas rate in the range from about 300 to about 3000 SCF/B (about 53 to about 534 S m3 of H2/m3 of oil), and a feed space velocity of about 0.1 to about 2.0 LHSV. In an embodiment, the hydrotreating conditions are selected so as to be sufficient to vaporize at least a portion of the lighter feed fraction, but not the wax-containing heavy fraction, thereby eliminating the need for a separate fractionation or distillation zone for separating the two fractions. However, if desired and/or if distillation capacity is available, separation of the light fraction may be achieved using fractional distillation. It will be understood by those skilled in the art that, unlike fractional distillation, reaction conditions effective to vaporize the light fraction in one or more hydrotreating stages may result in some of the heavy fraction being vaporized and some of the lighter fraction remaining in the heavy liquid. This is acceptable for the hydrotreating of this embodiment. Having described the hydrotreating, the dewaxing can now be more fully described.
By dewaxing herein is meant catalytic dewaxing in which the waxy, heavy fraction reacts with hydrogen in the presence of a dewaxing catalyst at reaction conditions effective to reduce its pour and cloud points, and increase the cold cranking performance of the dewaxed fuel. While some hydrotreating catalyst compositions may be used to dewax the heavy fraction (e.g., those which include one or more of Co, Ni and Fe and which will typically also include one or more of Mo or W, as well as Pt and Pd noble metals on an acidic support such as alumina, as is known), in some cases it will be preferred to employ a dewaxing catalyst that dewaxes mostly by isomerization and not by cracking, to maximize yield of the dewaxed fuel. However, this may not always be a viable option. The dewaxing is conducted at reaction conditions which include a temperature ranging from about 300 to about 900° F. (about 149 to about 482° C.), preferably about 550 to about 800° F. (about 289 to about 427° C.) and pressures in the range of from about 400-2000 psig. The hydrogen containing treat gas rate will range between about 300 to about 5000 SCF/B (about 53 to about 890 S m3/m3) with a preferred range of about 2000 to about 4000 SCF/B (about 356 to about 712 S m3/m3), while the liquid hourly space velocity, in volumes/volume/hour (V/V/Hr), will range between about 0.1 to about 10 and preferably about 1 to about 5. The acidic oxide support or carrier may include silica, alumina, silica-alumina, shape selective molecular sieves which, when combined with at least one catalytic metal component, have been demonstrated as useful for dewaxing such as silica-alumina-phosphates, titania, zirconia, vanadia, and other Group II, IV, V or VI oxides, ferrierite, mordenite, ZSM-5, ZSM-11, ZSM-23, ZSM-35, ZSM-22 also known as theta one or TON, ZSM-48 and the silicoaluminophosphates known as SAPO's, including SAPO-11, 36, 37 and 40 as well as Y sieves, such as ultra stable Y sieves and like, as is known. If stripping is not available prior to dewaxing and/or if the sulfur content of the hydrotreated and separated heavy fraction is high enough to result in dewaxing catalyst activity reduction or loss, zeolites containing framework transition metals having improved sulfur resistance (c.f., U.S. Pat. Nos. 5,185,136; 5,185,137 and 5,185,138) may be employed.
A treat gas is used in the hydrotreating and dewaxing. The terms “hydrogen”, “hydrogen treat gas” and “treat gas” are used synonymously herein, and may be either pure hydrogen or a hydrogen-containing treat gas which is a treat gas stream containing hydrogen in an amount at least sufficient for the intended reaction(s), plus other gas or gasses (e.g., nitrogen and light hydrocarbons such as methane) which will not adversely interfere with or affect either the reactions or the products. Impurities, such as H2S and NH3 are undesirable and would typically be removed from the treat gas before it is conducted to the reactor. The treat gas stream introduced into a reaction stage will preferably contain at least about 50 vol. % and more preferably at least about 75 vol. % hydrogen.
A distillate fuel base stock produced by this process may be hydrofinished at mild conditions, to improve color and stability, to form a finished fuel base stock. Hydrofinishing is a very mild, relatively cold hydrogenating process, which employs a catalyst, hydrogen and mild reaction conditions to remove trace amounts of heteroatom compounds, aromatics and olefins, to improve oxidation stability and color. Hydrofinishing reaction conditions typically include a temperature of from about 300 to about 660° F. (about 150 to about 350° C.) and preferably from about 300 to about 480° F. (about 150 to about 250° C.), a total pressure of from about 400 to about 2000 psig. (about 2859 to about 20786 kPa), a liquid hourly space velocity ranging from about 0.1 to about 10 LHSV (hr−1) and preferably about 0.5 to about 5 hr−1. The hydrogen treat gas rate will range from about 2550 to about 10000 scf/B (about 44.5 to about 1780 m3/m3). The catalyst will comprise a support component and one or catalytic metal components of metal from Groups VIB (Mo, W, Cr) and/or iron group (Ni, Co) and noble metals (Pt, Pd) of Group VIII. The metal or metals may be present from as little as about 0.1 wt. % for noble metals, to as high as about 30 wt. % of the catalyst composition for non-noble metals. Preferred support materials are low in acid and include, for example, amorphous or crystalline metal oxides such as alumina, silica, silica alumina and ultra large pore crystalline materials known as mesoporous crystalline materials, of which MCM-41, available from the ExxonMobil Company, is a preferred support component. The preparation and use of MCM-41 is disclosed, for example, in U.S. Pat. Nos. 5,098,604, 5,227,353 and 5,573,657.
Two related embodiments will be described with reference to the Figures. For the sake of simplicity, not all process reaction vessel internals, valves, pumps, heat transfer devices etc. are shown. Also, units and streams common to the embodiments of both Figures have the same numbers and features. Thus, what is described for a common unit with regard to
In the stripper (14), the hydrotreated, waxy heavy fuel fraction liquid contacts a steam stripping gas flowing up through the gas-liquid separation tray 36, from the dewaxed fuel stripping stage 34 below. The steam strips dissolved and entrained heteroatom compounds (H2S, NH3 and H2O) out of the heavy liquid. In addition to resulting in less dewaxing catalyst activity loss downstream, stripping out the dissolved heteroatom compounds enables the use of a more heteroatom sensitive dewaxing catalyst, such as those that dewax mostly by isomerization and not by cracking. A catalyst that dewaxes mostly by isomerization produces a greater yield of dewaxed fuel, because less of it is cracked into hydrocarbons, including methane, boiling below the desired fuel range. The stripped heavy liquid collects on tray 36 and is withdrawn from the stripper via line 52, with the steam and stripped components passing up and out the top of the stripper via line 50. Line 52 passes the stripped heavy liquid into line 56 and then down into the dewaxing reaction stage 24 in vessel 12. At the same time, a hydrogen treat gas is passed, via lines 54 and 56, down into the dewaxing stage. Flow distributor 30 distributes the downflowing hydrogen treat gas and the liquid, waxy, stripped and hydrotreated heavy diesel fraction across the top of the one or more dewaxing catalyst beds 24. The dewaxing catalyst may comprise one or more separate and sequential beds of the same or different dewaxing catalyst, as a plurality of dewaxing zones, into each of which the entire effluent from a preceding zone passes. In dewaxing reaction stage 24, the hydrogen reacts with the waxy components in the hydrotreated and stripped heavy diesel fraction to reduce its pour and cloud points, and improve its low temperature properties. The dewaxing reaction is operated at milder conditions than would otherwise be possible if dissolved H2S and NH3 had not been removed from the heavy fraction and/or if the entire feed, instead of only the heavy fraction, was being dewaxed. The smaller volume of waxy feed being dewaxed results in an increased liquid residence time and a concomitant increased hydrogen treat gas to waxy hydrocarbon ratio in the dewaxing stage. The stripping prior to dewaxing reduces the H2S and NH3 partial pressures in the dewaxing stage, and the higher treat gas to liquid ratio further decreases them. This means the dewaxing catalyst activity will be higher and the dewaxing temperature and pressure can be lower. The hydrogen treat gas introduced into 24 preferably contains enough hydrogen for both the dewaxing and hydrotreating reactions. The hydrotreated and dewaxed liquid collects on tray 26, from which it is removed via line 58.
In this particular illustration, the condensed, hydrotreated light fuel fraction is separated from the heteroatom-containing, gaseous hydrotreating reaction products in drum 18, and passed via line 60, into line 58, where it recombines with the hydrotreated and dewaxed heavy diesel fraction. The gaseous reaction products from drum 18 are conducted away from the process via line 62 for storage or further processing, e.g., H2S and NH3 clean up. The cleaned gas may be used as fuel or, if it contains sufficient unreacted hydrogen, it may be passed into one of the reaction stages as a source of hydrogen. Line 58 passes the combined fractions into the lower stage 34 of the stripper. In 34, the combined fractions are stripped with steam entering the bottom of the stripper via line 48. In both stripping stages 32 and 34, the stripping removes dissolved H2S, NH3, H2O, hydrogen and light, normally gaseous (e.g., C1-C4) hydrocarbons. A hydrotreated, dewaxed and stripped diesel stock is removed from 14 via line 49. If needed, and irrespective of whether or not the diesel stock comprises only the heavy fraction or has been recombined with the light fraction, the diesel stock can be mildly hydrofinished either before or after stripping.
While only two stages are shown in this illustration of an embodiment, more than two stages may be used for either or all of the hydrotreating, dewaxing and stripping. For example, the disclosure of U.S. Pat. No. 5,705,052, which is incorporated herein by reference, illustrates the use of three reaction stages in a single vessel, in combination with three stripping stages in a single stripper. Those skilled in the art will appreciate that these configurations can also be applied to four or more stages, if desired. Further, while cocurrent gas and liquid flow is shown in the hydrotreating and catalytic dewaxing stages above, one or more stages could have countercurrent gas and liquid flow.
The hydrotreated heavy liquid is removed via line 43 and passed into the top stripping stage 32, of the stripper 14. The separated gaseous effluent comprising the hydrotreated light faction vapor and gaseous reaction products is removed from gas separation space 88 via line 47 and passed through heat exchanger 20, which cools and condenses the hydrotreated vapor to liquid. As in
This application claims benefit of U.S. Provisional Patent Application Ser. No. 60/517,471 filed Nov. 5, 2003.
| Number | Name | Date | Kind |
|---|---|---|---|
| 4358362 | Smith et al. | Nov 1982 | A |
| 4394249 | Shen | Jul 1983 | A |
| 4436614 | Olbrich et al. | Mar 1984 | A |
| 4597854 | Penick | Jul 1986 | A |
| 4648957 | Graziani et al. | Mar 1987 | A |
| 4846959 | Kennedy et al. | Jul 1989 | A |
| 4913797 | Albinson et al. | Apr 1990 | A |
| 4917789 | Butler et al. | Apr 1990 | A |
| 5015359 | Harandi et al. | May 1991 | A |
| 5670116 | Gupta et al. | Sep 1997 | A |
| 5705052 | Gupta | Jan 1998 | A |
| 5720872 | Gupta | Feb 1998 | A |
| 6103104 | Gupta | Aug 2000 | A |
| 6623622 | Gupta | Sep 2003 | B2 |
| Number | Date | Country |
|---|---|---|
| WO9718278 | May 1997 | WO |
| WO9723584 | Jul 1997 | WO |
| WO0231087 | Apr 2002 | WO |
| WO0238704 | May 2002 | WO |
| Number | Date | Country | |
|---|---|---|---|
| 20050092654 A1 | May 2005 | US |
| Number | Date | Country | |
|---|---|---|---|
| 60517471 | Nov 2003 | US |
