CROSS REFERENCE TO RELATED APPLICATIONS
FEDERALLY SPONSORED RESEARCH
This application claims priority to U.S. Provisional Application No. 60/536,960, filed on Jan. 16, 2004.
- REFERENCE TO MICROFICHE APPENDIX
- FIELD OF THE INVENTION
- BACKGROUND OF THE INVENTION
The invention relates to a method for producing distillate fuels and lubricants in a single facility. The invention further relates to an integrated facility for practicing the method of the invention.
As the World's crude oil supply decreases, refiners must use poorer quality crude oils to produce high quality lubricant base oils. High quality lubricant base oils are those which have a high viscosity index (“VI”), i.e. VI>135, low volatility, and low pour points. Because of the presence of relatively high levels of aromatic compounds, severe hydroprocessing conditions are required to convert poorer quality crude oils into high quality lubricating oils. Less severe processing, such as distillation, result in lubricant base oils having high concentrations of naphthenic compounds which are known to lower the viscosity index.
In one known process, lubricant base oil and distillate fuels are simultaneously produced from low quality crude oil by means of hydrocracking a heavy hydrocarbon feed, separating a lubricant base oil boiling range material and a material which is fed to a second hydrocracker to produce distillate fuel. The heavy hydrocarbon is first cracked under mild conditions to produce the needed lubricant base oil feed, and the remainder is cracked under severe conditions to produce distillate fuels.
Synthetic feedstocks, such as the products of Fischer-Tropsch syntheses, are candidates for production of high quality lubricant base oils. Typically, large, high-severity hydrocrackers are required to upgrade the waxy Fischer-Tropsch product into high quality lubricant base oils.
Moreover, market demand for lubricant base oils does not generally justify the construction of a dedicated or nearly dedicated facility. Indeed, the market demand for distillate fuels far exceeds that for lubricant base oils. Consequently, facilities are generally constructed so as to maximize production of distillate fuels while simultaneously producing high quality lube oil base stocks.
Middle Distillate fuels need only minimal isomerization, i.e. 1 or 2 methyl branches with an iso to normal ration of between about 0.1:1 and about 5:1, in order to meet cloud point requirements. In contrast, lubricants, being more viscous, must meet more stringent pour point requirements for formulation into finished oils meeting stringent low temperature standards, such as that of SAE J300 published by the Society of Automotive Engineers, as well as engine manufacturer requirements for lubricating and hydraulic fluids. To meet such standards and requirements, lubricant base oils generally need an iso to normal ratio much greater than 10:1. Thus, while a feedstock normally must be subjected to relatively severe hydrocracking to achieve the isomerization required for lubricant base oils, there is no need for severe hydrocracking to produce middle distillate fuels.
Waxy streams are the ideal feeds for hydroprocessing reactors if high viscosity index Lubricant base oils are a the desired product. Waxes have been converted into high viscosity index base oils using catalysts as simple as aluminum trichloride which produces isoparaffinic base oils where the alkyl side chains formed during the reaction consist almost exclusively of methyl groups. Improvements to this batch process include a significant number of hydroprocessing schemes where various crude oil or synthetic hydrocarbon streams are processed by hydroconversion catalysts into methyl-branched isoparaffins and other products more highly saturated and branched than the feed stream. Such known processes almost exclusively employ linear schemes where the feed is hydroprocessed and distilled with some schemes utilizing a recycle loop to hydroprocess unconverted waxy residue from the distillation column. The equipment and catalysts used in such processing schemes is, however, expensive to purchase and operate. Minimizing the capital investment while increasing the production of desirable high viscosity lubricating base oils while maintaining or improving production of distillate fuels is a very desirable plant design feature. This invention describes such a plant design that maximizes distillate fuels production from waxy feeds, maximizes production of highly iso-paraffinic base oils, and minimizes capital investment costs by allowing plant equipment including reactors to be more closely sized to the minimum needed to produce the desired products.
- SUMMARY OF THE INVENTION
There remains a need for a process using high quality Fischer-Tropsch synthesis products as feed for concurrent production of distillate fuels and high quality lubricants. There remains a further need for a process wherein the size and severity of the hydrocrackers are matched to the need for isomerization in the product stream.
This invention relates to production of synthetic lubricant base oils and fuels concurrently from the same synthetic feedstock. The method includes an initial fractionation of the feed into fractions utilizing a separation unit, optional separate hydroprocessing of the lighter fraction and heavier fraction to remove contaminants, low severity hydroisomerization of the heavier fraction, a second fractionation of the entire stream into distillate fuels of several types and qualities along with a waxy lubricant feed, hydrodewaxing the waxy lubricant feed to low pour point, high quality base oil, optional hydrofinishing and final distillation to lubricating base oils and distillate fuels with recycle of the third fractionation bottoms stream, e.g. raffinate, to extinction.
In another embodiment of the invention, the recycle of partially converted and unconverted wax is accomplished in the first fractionator following the hydrocracking step, rather than in the third and final fractionator.
BRIEF DESCRIPTION OF THE DRAWINGS
The process of the invention which incorporates an initial low severity hydrocracking followed by high severity hydrocracking of only the heavy unconverted waxy recycle stream provides a simplified process for base oil production from waxy feed that maximizes yield of high quality lubricant base oil while minimizing equipment costs. Compared to conventional wax hydroconversion processes for fuels and lubricants production, this process scheme results in overall lower capital investment and higher yield of high value liquid products, increasing the overall capital efficiency of the facility.
FIG. 1 is a schematic diagram of a first embodiment of the method of the invention.
DESCRIPTION OF EMBODIMENTS OF THE INVENTION
FIG. 2 is a schematic diagram of a second embodiment of the method of the invention.
The process of the invention utilizes a low severity hydrocracker to initially process waxy feeds heavier than distillate fuels into distillate fuels of lower cloud and/or freeze point as well as a heavy isoparaffinic stream which is suitable for dewaxing, either catalytically or by solvent extraction, to low pour point isoparaffinic base oils of exceptionally high viscosity index and low volatility.
The feedstocks which can be employed in the process of the invention may be generally characterized as broad boiling range feeds of petroleum origin, including for example, heavy gas oil having about 50 wt % components with a boiling point above 700° F. or a vacuum gas oil having a boiling point range from between about 600° F. and about 1050° F. Alternatively, the feedstock may be of synthetic origin, such as all or part of the product of a Fischer-Tropsch synthesis.
In one embodiment of the invention, the process for upgrading the feedstock is integrated with a Fischer-Tropsch synthesis. Such embodiment includes processing of synthesis gas to produce a hydrocarbon stream via the Fischer-Tropsch reaction, and recovery of the Fischer-Tropsch product. The Fischer-Tropsch product is then processed utilizing the configuration of the low severity hydrocracker configuration as discussed herein.
Techniques for producing a synthetic gas, or syngas, which is the feed to a Fischer-Tropsch reactor are well known in the art and include oxidation, reforming and autothermal reforming. As an example, a Fischer-Tropsch conversion system for converting hydrocarbon gases to liquid or solid hydrocarbon products using autothermal reforming includes a synthesis gas unit, which includes a synthesis gas reactor in the form of an autothermal reforming reactor (“ATR”) containing a reforming catalyst, such as a nickel-containing catalyst. A stream of light hydrocarbons to be converted, which may include natural gas, is introduced into the reactor along with oxygen (O2). The oxygen may be provided from compressed air or other compressed oxygen-containing gas, or may be a pure oxygen stream. The ATR reaction may be adiabatic, with no heat being added or removed from the reactor other than from the feeds and the heat of reaction. The reaction is carried out under sub-stoichiometric conditions whereby the oxygen/steam/gas mixture is converted to syngas.
The Fischer-Tropsch reaction for converting syngas, which is composed primarily of Carbon Monoxide (CO) and Hydrogen (H2), may be characterized by the following general reaction:
2nH2 +nCO→(—CH2—)n +nH2O (1)
Non-reactive components, such as nitrogen, may also be included or mixed with the syngas. This may occur in those instances where air, enriched air, or some other non-pure oxygen source is used during the syngas formation.
The syngas is delivered to a synthesis unit, which includes a Fischer-Tropsch reactor (“FTR”) containing a Fischer-Tropsch catalyst. Numerous Fischer-Tropsch catalysts may be used in carrying out the reaction. These include cobalt, iron, ruthenium as well as other Group VIIIB transition metals or combination of such metals, to prepare both saturated and unsaturated hydrocarbons. The Fischer-Tropsch catalyst may include a support, such as a metal-oxide support, including silica, alumina, silica-alumina or titanium oxide. For example, a cobalt (Co) catalyst on transition alumina with a surface area of approximately 100-200 m2/g may be used in the form of spheres of 50-150 μm in diameter. The Co concentration on the support may also be 5-30 wt %. Certain catalyst promoters and stabilizers may be used. The stabilizers include Group IIA and Group IIIB metals, while the promoters may include elements from Group VIII or Group VIIB. The Fischer-Tropsch catalyst and reaction conditions may be selected to be optimal for desired reaction products, such as for hydrocarbons of certain chain lengths or number of carbon atoms. Any of the following reactor configurations may be employed for Fischer-Tropsch synthesis: fixed bed, slurry bed reactor, ebullating bed, fluidizing bed, or continuously stirred tank reactor (“CSTR”). The FTR may be operated at a pressure from about 100 to about 800 psia and a temperature from about 300° F. to about 600° F. The reactor gas hourly space velocity (“GHSV”) may be from about 1,000 to about 15,000 hr−1. Syngas useful in producing a Fischer-Tropsch product useful in the invention may contain gaseous hydrocarbons, hydrogen, carbon monoxide and nitrogen with H2/CO ratios from about 1.5 to about 3.0. The hydrocarbon products derived from the Fischer-Tropsch reaction may range from methane to high molecular weight paraffinic waxes containing more than 100 carbons.
The process of the invention utilizes a low severity hydrocracker to minimize the hydrocracking and hydroisomerization severity to that necessary to produce the desired product slate and product requirements. The process of the invention minimizes reactor size, catalyst content, and reactor severity while maximizing reactor utilization by increasing linear hourly space velocity. By matching these parameters to product needs and requirements, the equipment costs associated with building a processing facility are minimized along with minimization of operation expenses while maximizing catalyst life. The first hydrocracker is sized and conditions set to produce hydroisomerized and hydrocracked distillate products of proper paraffin and isoparaffin content to meet cloud point and Cold Filter Plugging Point (CFPP) specifications of the desired product slate. Such first hydrocracker size and conditions are also configured to minimize over-conversion of the heavier wax into lighter products or more highly isomerized products than necessary for conversion in subsequent processing units to high quality lubricant base oil products. Thus, the size of a downstream high severity hydrocracker may be significantly reduced so as to accommodate only that fraction of the process stream having boiling points greater than about 900° F. The high severity hydrocracker is used to hydro-convert unreacted and under converted waxy components from a second or third fractionator. The product of the high severity hydrocracker is then recycled into the second fractionator. In some embodiments of the invention, this recycle stream is reacted to extinction thereby maximizing utilization of the paraffinic feedstocks of the process.
By a middle distillate fraction having a boiling range of about 250° to about 700° F. is meant that at least 75 vol %, preferably 85 vol %, of the components of the middle distillate have a normal boiling point of greater than about 250° F. and furthermore that at least about 75 vol %, preferably 85 vol %, of the components of the middle distillate have a normal boiling point of less than 700° F. The term “middle distillate” is intended to include the diesel, jet fuel and kerosene boiling range fractions. The kerosene or jet fuel boiling point range is intended to refer to a temperature range of about 280° to about 525° F. and the term “diesel boiling range” is intended to refer to hydrocarbon boiling points of about 250° to about 700° F. Gasoline or naphtha is normally the C5 to 400° F. endpoint fraction of available hydrocarbons. The boiling point ranges of the various product fractions recovered in any particular refinery will vary with such factors as the characteristics of the crude oil source, refinery local markets, product prices, etc.
Referring to FIG. 1, the hydrocarbon feedstock 1 may first be distilled in a first fractionator 2 into three or more distillate streams. Distillate stream 3 contains material nominally boiling in the range from about C5 to about 320° F. Distillate stream 3 contains material boiling in the gasoline hydrocarbon range and can be further processed by methods known in the art into fuels and petrochemical feedstock products including, but not limited to, ethylene cracker feed or fuel cell fuel. Distillate stream 4 contains material boiling in the middle distillate range with an initial boiling point of approximately 320° F. and a final boiling ranging from approximately 550° F. to 700° F. where the final boiling point is chosen to meet finished product quality requirements, such as those set forth in ASTM 975-03. Distillate stream 4 may optionally be hydrotreated in a first hydrotreater 6 to saturate olefins and other unsaturated components. Distillate stream 5 includes hydrocarbons boiling above about 700° F. The upper boiling point of distillate stream 5 depends upon the feed composition, but generally ranges from between about 1050° F. and about 1250° F. Distillate stream 5 may optionally be hydrotreated in a second hydrotreater 7 to saturate olefins and other unsaturated components.
Hydrotreating catalysts useful in hydrotreaters 6 and 7 are well known in the art and consist of sulfided or non-sulfided metals which are active to hydrogenation transfer reactions, such as Cobalt, Nickel, Platinum, Palladium. Reactor flow rates of 0.1 to 10 LHSV are typical, and reactor temperatures of 200° F. to 750° F. are characteristic of this hydrotreating process. Hydrogen flow rates of 500 to 10,000 SCF/bbl and hydrogen pressure of 200 to 2500 psig are also typical of this hydrotreating process. Other acceptable hydrotreating conditions known in the art may also be used.
The effluent 8 from hydrotreater 7 is fed to a low severity hydrocracker 9. Conditions in the low severity hydrocracker 9 include the following: between about 400° F. and about 750° F.; between about 200 and about 3,000 psig; between about 1,000 and about 20,000 SCF/BBL; and between about 0.25 and about 10 LHSV. Hydrocracker 9 effects conversion of linear paraffins into branched paraffins and slightly branched paraffins into more highly branched paraffins. Some of the branched and more highly branched paraffins may also be hydrocracked into products boiling in the gasoline and middle distillate fuel boiling ranges.
Preferred catalysts for use in hydrocracker 9 include hydrogenation/dehydrogenation catalysts which have an acidic component sufficient to cause isomerization of nominally linear hydrocarbons to branched and isomerization of slightly branched hydrocarbons into more highly branched hydrocarbons where the branches consist predominantly of methyl groups. Examples of such catalysts are discussed in S. Tiong Sie, Ind. Eng. Chem. Res., 31(8) 1881(1992), incorporated herein by reference. The hydrocrackate 10 from hydrocracker 9 is passed into a second fractionator 11. Effluent 12 from hydrotreater 6 is also fed into fractionator 11.
Fractionator 11 separates hydrocrackate 10 into two or more fractions. One or more lighter fractions 13, containing hydrocarbons in the distillate fuel boiling range, are recovered from fractionator 11 and include distillate fuels containing normal, slightly methyl-branched, and more highly branched paraffins. A light lubricant feed stream 14 may also be recovered from fractionator 11. Light lubricant feed stream 14 generally contains paraffins boiling in the range of from about 500° F. to about 950° F. Lubricant feed stream 15 is recovered as a bottoms fraction from fractionator 11 and contains paraffins boiling in the range of from about 700° F. to about 1100° F. Lubricant feed stream 15 is then hydrodewaxed in hydrodewaxer 16 to remove normal paraffins and slightly branched paraffins. The severity of operation of hydrodewaxer 16 is controlled to maximize lubricant base oil production while meeting product quality requirements. Typical operating conditions of hydrodewaxer 16 are as follows: between about 400° F. and about 700° F.; between about 0.1 and about 5 LHSV; between about 1,000 and about 10,000 SCF/BBL; and between about 200 and about 2,000 psig. The effluent 17 from hydrodewaxer 16 is optionally hydrofinished in hydrofinisher 18 to saturate residual olefinic products. Lubricant base oil products 20 are recovered by distillation in a third fractionator 19. An overhead stream 21 from fractionator 19 containing paraffins in the middle distillate fuel range may also be recovered. A raffinate 22, containing unconverted wax with boiling points greater than the lubricant base oil distillation range (>1050° F.), may also be recovered. Raffinate 22 is comprised primarily of unconverted and under-converted waxy hydrocarbons. Raffinate 22 is processed in a high severity hydrocracker 23 into lower boiling range products, e.g. lower carbon number hydrocarbons, that are then fed into second fractionator 11. Typical hydrocracking conditions known in the art may be used in hydrocracker 23. Such conditions include, for example, those disclosed in U.S. Pat. No. 6,379,535 which is incorporated herein. Raffinate 22 may be recycled to extinction or alternatively, refractory components, when present, may be removed by a side bleed stream.
The term “hydrotreating” as used herein refers to processes wherein a hydrogen-containing treatment gas is used in the presence of suitable catalysts which are primarily active for the removal of heteroatoms, such as sulfur and nitrogen and for saturating olefins and aromatics. Suitable hydrotreating catalysts for use in the present invention are any known conventional hydrotreating catalysts. Examples of such hydrotreating catalyst include, for example, those comprised of at least one Group VIII metal, preferably iron, cobalt and nickel, more preferably cobalt and/or nickel on a high surface area support material, such as alumina. Other suitable hydrotreating catalysts include zeolitic catalysts, as well as noble metal catalysts where the noble metal is selected from palladium and platinum. More than one type of hydrotreating catalyst may be used in the present invention. Typical hydrotreating temperatures range from about 400° F. to about 900° F. with pressures from about 500 psig to about 2500 psig.
The term “hydrocracking” as used herein refers to a process having all or some of the reactions associated with hydrotreating, as well as cracking reactions, which result in molecular weight and boiling point reduction and molecular rearrangement, or isomerization.
Hydrocrackers 9 and 23 may contain one or more beds of the same or different catalyst. In some embodiments, when the preferred products are distillate fuels, the preferred hydrocracking catalysts utilize amorphous bases or low-level zeolite bases combined with one or more Group VIII or Group VIB metal hydrogenating components. In another embodiment, when the preferred products are in the gasoline boiling range, the hydrocracking zone contains a catalyst which comprises, in general, any crystalline zeolite cracking base upon which is deposited aminor proportion of a Group VIII metal hydrogenating component. Additional hydrogenating components may be selected from Group VIB for incorporation with the zeolite base. The zeolite cracking bases are sometimes referred to in the art as molecular sieves and are usually composed of silica, alumina and one or more exchangeable cations such as sodium, magnesium, calcium, rare earth metals, etc.
Referring now to FIG. 2, a second embodiment of the facility and method of the invention is shown. In this second embodiment, the process proceeds as described in reference to FIG. 1 to the feeds to a second fractionator 11.
Fractionator 11 separates hydrocrackate 10 into two or more fractions. One or more lighter fractions 13, containing hydrocarbons in the distillate fuel boiling range, are recovered from fractionator 11 and include distillate fuels containing normal, slightly methyl-branched, and more highly branched paraffins. A light lubricant feed stream 14 may also be recovered from fractionator 11. Light lubricant feed stream 14 generally contains paraffins boiling in the range of from about 500° F. to about 950° F. A heavy lubricant feed stream 24 is recovered from fractionator 11 and contains paraffins boiling in the range of from about 700° F. to about 1100° F. A raffinate stream 26 containing unconverted wax is also recovered from fractionator 11. That is, in this second embodiment, the unconverted wax is removed as a raffinate 26 prior to hydrodewaxing and hydrofininishing. Raffinate 26 is processed in a high severity hydrocracker 23 into lower boiling range products, e.g. lower carbon number hydrocarbons, that are then fed into second fractionator 11. Raffinate 26 may be recycled to extinction or alternatively, refractory components, when present, may be removed by a side bleed stream. Heavy lubricant feed stream 24 is then hydrodewaxed in hydrodewaxer 16 to remove normal paraffins and slightly branched paraffins. The severity of operation of hydrodewaxer 16 is controlled to maximize lubricant base oil production while meeting product quality requirements. Typical operating conditions of hydrodewaxer 16 are as described above. The effluent 25 from hydrodewaxer 16 is optionally hydrofinished 18 to saturate residual olefinic products. Lubricant base oil products 20 are recovered by distillation in a third fractionator 19. An overhead stream 21 from fractionator 19 containing paraffins in the middle distillate fuel range may also be recovered.
In a third embodiment, hydrotreaters 6 and 7 may be combined into a single hydrotreater placed upstream of first fractionator 2. Thereafter, subsequent processing may be conducted in accordance with either the first or second embodiments discussed above. In this third embodiment, the conditions of operation of the single hydrotreater may be as described with respect to hydrotreaters 6 and 7.