DE69810607T2 - SLURRY CARBON SYNTHESIS METHOD RENTAL EXTENDED LIFE - Google Patents

Description

Background of the revelation Scope of the invention

The invention relates to a hydrocarbon synthesis process with increased catalyst life. More particularly, the invention relates to a catalytic slurry hydrocarbon synthesis process using a supported cobalt metal catalyst, wherein the catalyst half-life is increased by using a synthesis gas feed containing less than 50 ppb of nitrogen-containing catalyst deactivating species.

Background of the invention

Slurry hydrocarbon synthesis (HCS) processes are known. In a slurry HCS process, a synthesis gas (syngas) comprising a mixture of H2 and CO is passed as a third phase through a suspension in a reactor, the slurry liquid comprising hydrocarbon products of the synthesis reaction and the dispersed suspended solids comprising a suitable Fischer-Tropsch type hydrocarbon synthesis catalyst. Reactors containing such a three-phase suspension are sometimes referred to as "bubble columns" as disclosed in U.S. Patent 5,348,982. Regardless of whether the slurry reactor is operated as a dispersed or slumped bed, the mixing conditions in the suspension will typically be somewhere between the two theoretical conditions of plug flow and backmixing. It is also known that Fischer-Tropsch type catalysts useful for the formation of hydrocarbons from a synthesis gas are rapidly but reversibly deactivated by certain nitrogen-containing species in the syngas feedstock, particularly HCN and NH3. Syngas produced from hydrocarbon feedstocks containing nitrogen (i.e. natural gas) or nitrogen-containing compounds (i.e. residues, coal, shale, coke, tar sands, etc.) inevitably contain HCN and NH3 which degrade the reactive suspension contaminate and deactivate the catalyst. Certain oxygen and carbon containing compounds formed in the suspension as byproducts of the HCS reaction are believed to also cause rapid deactivation. Deactivation of such catalysts by HCN and NH3 can be reversed and catalytic activity restored (rejuvenation) by contacting the deactivated catalyst with hydrogen or a hydrogen-containing gas (rejuvenation gas). Deactivation of such catalysts by these species is reversible and catalytic activity is restored by either continuously or intermittently contacting the catalyst with hydrogen (the catalyst is rejuvenated), as disclosed, for example, in U.S. Patents 5,260,239; 5,268,344 and 5,283,216. While methods have been proposed to reduce the HCN and NH3 content of syngas to about 0.1 ppm (100 ppb) by catalytic hydrolysis (US 4,769,224) and chemical scrubbing (US 5,068,254), it has now been found that even as little as 100 vol.ppb of the total combined amount of HCN and NH3 in the syngas results in a catalyst half-life of only four days in the case of a supported cobalt metal catalyst in an HCS suspension. It has now been found that lowering the level of HCN and NH3 catalyst poisons in the syngas to below 50 ppb results in increased catalyst lifetime and less need for catalyst rejuvenation. A method to achieve such low levels has also been discovered and is disclosed in pending US patent applications Application No. 08/512,734 (corresponding to EP patent application 96111013.7), 08/636,425 (corresponding to EP patent application No. 97922432.6) and 08/797,368 (corresponding to EP patent application No. 98920021.7), filed August 8, 1995, April 23, 1996 and February 7, 1997, respectively.

Summary of the invention

The present invention relates to a slurry hydrocarbon synthesis (HCS) process using a supported cobalt metal catalyst in which the short term catalyst half-life is at least 10 days, preferably at least 30, and most preferably at least 40 or more days. By short term half-life it is meant that the catalytic activity caused by reversible deactivation of the catalyst is 50% of that of the fresh catalyst and that this loss can be substantially restored (the catalyst rejuvenated) by contacting the deactivated catalyst with a H₂-containing rejuvenation gas. Catalyst activity is defined by the CO conversion to hydrocarbons. Thus, if under a given set of HCS conditions fresh catalyst achieves a CO conversion of 80 mol%, the catalyst half-life is reached when the conversion falls to 40% as a result of contact with the reversibly deactivating nitrogen-containing species in the synthesis gas (syngas) feedstream. By reversibly deactivating nitrogen-containing species is meant HCN, NH₃ and mixtures thereof. It is also an embodiment of the process according to the invention that the catalyst has a long-term half-life of at least 100 days and preferably of at least 200 days. It has been found that there is also a non-refreshable catalyst activity loss which occurs over time and cannot be reversed by contacting the catalyst with H₂, but which can be reversed by regeneration. The non-refreshable but regenerable loss of catalyst activity falls continuously so that the catalyst eventually has a CO conversion activity in terms of its long-term half-life of only half or 50% of the fresh catalyst and this long-term activity loss cannot be reversed by contacting the deactivated catalyst with H₂ or a rejuvenating gas containing H₂ (the catalytic activity cannot be restored). Therefore, long-term Half-life is the time required for the catalyst to have only half the activity of the fresh catalyst, and that this loss of activity, although reversible, is not restored by a rejuvenation process in which the deactivated catalyst is contacted with H₂ or a gas containing H₂. Instead, the catalyst must be separated from the suspension and regenerated by processes involving oxidation or firing, re-reduction of the catalytic metal(s), and optionally passivation in CO and/or syngas. Therefore, the long-term loss of catalyst activity in the context of the present invention is regenerable but not rejuvenable with hydrogen. Furthermore, the regenerable loss of activity differs from the irreversible loss of catalyst activity due to, for example, sulfur poisoning, which requires catalyst replacement. The relatively long short-term and long-term catalyst lifetimes of the present invention are achieved by using a syngas feedstock in which the total level of the catalyst deactivating nitrogen-containing species HCN, NH3 and mixtures thereof is less than 50 vol.ppb (parts per billion by volume), preferably less than 20 vol.ppb and most preferably less than 10 vol.ppb. A suspension HCS catalyst useful in the present invention comprises a catalytically active cobalt component dispersed and supported on a refractory inorganic particulate oxide support or carrier material and preferably present as a thin catalytically active surface layer ranging in thickness from about 5-200 microns. It is also preferred that the catalyst has a productivity of at least 150 h⁻¹ at 200°C, preferably at least 500 h⁻¹ and particularly preferably at least 1000 h⁻¹. Productivity means the standard volume of CO converted per volume of catalyst per hour. In a further embodiment, the catalyst used in the process according to the invention has a methane selectivity of less than 10 mol% and preferably less than 5 mol%. This means that less than 10% of the CO is converted to methane. In one embodiment, the catalyst comprises catalytically effective amounts of cobalt and one or more of Re, Ru, Fe, Ni, Th, Zr, Hf, U, Mg and La on a suitable inorganic support material, and preferably one comprising one or more refractory metal oxides. Preferred supports for CO-containing catalysts include titania and titania-silica composites, particularly when a slurry HCS process is used in which higher molecular weight, predominantly paraffinic liquid hydrocarbon products are desired. Useful catalysts and their preparation are known and described; non-limiting examples can be found, for example, in U.S. Patents 4,568,663; 4,663,305; 4,542,122; 4,621,072 and 5,545,674, with those disclosed in U.S. Patent 5,545,674 being particularly preferred.

An HCS suspension process according to the invention comprises reacting a syngas containing HCN, NH₃ or mixtures thereof in the presence of a solid particulate HCS catalyst in a suspension comprising the catalyst and gas bubbles in a hydrocarbon suspension liquid at reaction conditions suitable to produce hydrocarbons from the syngas, wherein the total amount of HCN, NH₃ or mixtures thereof in the syngas is less than 50 vol.ppb, preferably less than 20 vol.ppb and more preferably less than 10 vol.ppb, to achieve a short term catalyst half-life greater than 10 days, preferably greater than 30 days and more preferably greater than 40 days and with a long term catalyst half-life greater than 100 and preferably greater than 200 days. Those skilled in the art will appreciate the unusually high difference in catalyst half-life as a result of a relatively small difference in the concentration of HCN and NH3 catalyst deactivating species in the syngas feed. It was not known that the relatively small differences and extremely low concentrations of HCN and NH3 in the syngas feed would cause such a large difference in catalyst half-life. The extended catalyst half-life reduces catalyst rejuvenation requirements. and related hydrogen consumption, while maintaining good productivity and selectivity to liquid hydrocarbon products. The process of the invention was demonstrated with a suspension HCS process in which the syngas was passed upwardly through a three-phase HCS suspension comprising the particulate catalyst and gas bubbles in a hydrocarbon suspension liquid, and in which the catalyst comprised a catalytically active cobalt component dispersed and supported on a particulate inorganic refractory oxide support or support material as a thin catalytically active surface layer which satisfied the above-mentioned conditions regarding productivity and methane production. The catalyst was of the type disclosed and claimed in the '674 patent referenced above.

A number of methods have been found to achieve the low concentrations of HCN and NH3 in syngas useful for use in the present invention. These include catalytic hydrolysis of HCN to NH3 followed by water washes to dissolve out the NH3 and optionally the use of guard beds containing one or more solid adsorbent materials, preferably acidic, to adsorb any HCN and NH3 that might break through. This method is disclosed in copending U.S. application Ser. No. 08/797,368, referred to above. Another method involves cryogenically separating nitrogen from natural gas used as syngas feedstock so that there is not enough nitrogen in the natural gas to produce the catalyst deactivating species in the syngas generation unit. Also in this process, in case of nitrogen breakthrough after the syngas production unit and the result of an increase in the concentration of the catalyst deactivating species in the syngas, solid adsorber beds are placed between the syngas production and the HCS reactor(s). In a more specific embodiment of a suspension HCS process, the invention comprises reacting a synthesis gas comprising a mixture of H₂ and CO containing HCN, NH₃ or mixtures thereof in the presence of a hydrocarbon synthesis catalyst in a suspension comprising the catalyst and gas bubbles in a hydrocarbon suspension liquid under reaction conditions suitable to form hydrocarbons from the syngas, the catalyst comprising a catalytically active cobalt component present on a particulate inorganic refractory oxide support or assist material as a thin catalytically active surface layer, the catalyst has a productivity of at least 150 h⁻¹ and produces less than 5 mol% methane from the synthesis gas and in which the amount of HCN, NH₃ or mixtures thereof present in the gas is less than 50 vol.ppb to achieve a short term catalyst half-life of at least 10 days. The hydrocarbon slurry liquid comprises hydrocarbon products of the HCS reaction which are liquid at the reaction conditions, and a portion is continuously or intermittently removed from the slurry HCS reactor as the hydrocarbons are produced. The hydrocarbon liquid removed from the reactor comprises C5+ , mainly paraffinic hydrocarbons, and is typically upgraded to more valuable products through one or more conversion steps, or sold neat. As the HCS reaction proceeds, the catalyst loses activity due to the presence of HCN, NH3 or mixtures thereof in the synthesis gas and must be refreshed either continuously or intermittently by passing H2 or a gas containing H2 through the slurry where it contacts the catalyst and at least partially and preferably substantially completely restores catalytic activity as disclosed in the prior art described above and most preferably after all or at least a portion of the CO has been removed from the slurry.

Detailed description

In a Fischer-Tropsch slurry HCS process, a syngas comprising a mixture of H2 and CO is passed into a reactive slurry where it is catalytically converted to hydrocarbons, and preferably to liquid hydrocarbons. The molar ratio of hydrogen to carbon monoxide can range roughly from about 0.5 to 4, but is typically in the range of about 0.7 to 2.75, and preferably 0.7 to 2.5. The stoichiometric molar ratio for a Fischer-Tropsch HCS reaction is 2.0, but there are many reasons for using non-stoichiometric ratios that are known to those skilled in the art and discussion of which is outside the scope of the present invention. In a slurry HCS process, the molar ratio of H2 to CO is typically about 2.1:1. Slurry HCS process conditions vary somewhat depending on the catalyst and desired products. Typical conditions suitable for producing hydrocarbons comprising predominantly C5+ paraffins and preferably C10+ paraffins (e.g., C5+ -C200) in a slurry HCS process using a catalyst comprising a supported cobalt component include, for example, temperatures, pressures and gas hourly volume velocities in the range of about 320-600°F, 80-600 psi and 100-40,000 V/hr/V, expressed as standard volume of gaseous CO and H2 mixture (0°C, 1 atm) per hour per volume of catalyst. The slurry catalyst rejuvenation conditions are those of hydrocarbon synthesis in terms of temperature and pressure and are disclosed in the prior art. The syngas can be produced in a variety of ways such as by contacting hot carbonaceous materials such as coke or coal with steam or from a feedstock containing methane. A feedstock containing methane is preferred for reasons of convenience and cleanliness and because it does not leave large amounts of ash that must be handled and disposed of. The methane-containing gas feedstock is produced from natural gas or by the combustion of coal, tar, liquid hydrocarbons, etc. and is passed to the syngas generator. The production of syngas from methane by either partial oxidation, steam reforming, or a combination thereof is well known and disclosed, for example, in US Patent 4,888,131. In many cases it is preferred to catalytically partially oxidize and reform the methane in a fluidized bed syngas production unit (FBSG), such as disclosed, for example, in US Patents 4,888,131 and 5,160,456. Regardless of the origin of the methane, nitrogen or nitrogen-containing compounds are present in the methane-containing gas feedstock passed to the syngas generator, some of which are converted to NH3 and HCN during syngas formation. These will deactivate a Fischer-Tropsch HCS catalyst, particularly those comprising Co as the catalytic metal. As is apparent from the prior art, deactivation by these species is reversible and the catalyst can be rejuvenated by contacting it with hydrogen. This restoration of catalytic activity of a reversibly deactivated catalyst is referred to as catalyst rejuvenation and is disclosed, for example, in the above-cited patents 5,260,239; 5,268,344 and 5,283,216. It has also been found that both the short term and long term catalyst half-life of a Co-containing slurry HCS catalyst are unacceptably short so long as the combined amounts of HCN and NH3 present in the syngas fed to the HCS reactor are not less than 50 vol ppb, preferably less than 20 vol ppb, and most preferably less than 10 vol ppb, such that the short term or H2 deactivation is not less than 10 vol ppb. refreshable catalyst half-life is at least 10 days, preferably at least 30 days, and most preferably at least 40 days, and the long-term catalyst half-life is at least 100 days, and preferably 200 days. As mentioned above, with a Co-metal-containing HCS catalyst of a type as described and claimed in US Patent 5,545,674, in a reactive HCS suspension 100 vol.ppb of combined total amount of HCN and NH₃ present in the syngas results in the catalyst having a half-life of only 4 days. With half-life is meant that the overall activity of the catalyst body decreases by 50% in 4 days. An activity level of 50% is absolutely unacceptable. It means that the productivity of the catalyst (and in relation to it the reactor), measured as CO conversion, is only 50% of what it should be after 4 days. A productivity level of at least 90% is desired. This means that the reactor is shut down for cyclic or intermittent rejuvenation for a quarter of each day to maintain an activity level of not less than about 90%, during which time the catalyst is rejuvenated with hydrogen in the reactor. For practical reasons, the reactor is shut down for more than a quarter of each day due to the time required to purge the syngas, pass the hydrogen or hydrogen-containing catalyst rejuvenation gas and restart the HCS reaction. This results in a continuous average 25% loss of hydrocarbon production from the reactor, even with rejuvenation. In addition, when the catalyst is deactivated, with all other conditions constant, the conversion level drops, resulting in a reduction in liquid hydrocarbon production and a slight increase in methane production. Alternatively, conversion can be kept relatively constant despite catalyst deactivation by increasing the reactor temperature, but this results in a relatively large increase in methane production and consequently a drop in liquid product production. At a combined HCN and NH3 level of about 20 vol.ppb in the syngas, the catalyst half-life is 20 days. This means that approximately every fourth day the catalyst must be rejuvenated, using the same amount of time and hydrogenation for rejuvenation as in the case described above, giving an average production loss of only about 6%. At about 40-50 vol.ppb it is about 15%. At a combined level of about 10-12 Vol.ppb, the catalyst half-life is about 40 days and the catalyst only needs to be refreshed every eighth day for a quarter of a day, resulting in a productivity loss of only about 3%. The catalyst half-life is about 30 days when the combined amounts of HCN and NH₃ are about 13-17 vol. ppb. In the case of a slurry HCS process, the catalyst in the slurry can either be continuously reconstituted with the reactor remaining in operation using the methods disclosed in US Patents 5,260,239 and 5,268,344. However, in the case of a catalyst half-life of only 4 days, five times more hydrogen make-up gas is still consumed than if the half-life were 20 days and ten times the amount needed for a 40 day half-life.

While known processes involve catalyst hydrogenation and chemical scrubbing to reduce syngas HCN content to 0.01 vol% or 100 vol ppb, even 100 vol ppb of HCN in syngas represents an unacceptably high level. Furthermore, HCN removal by alkaline scrubbing and scrubbing with alkaline ferrous sulfate solutions is hampered by the presence of other acidic materials in the syngas, particularly CO₂. Scrubbing with water containing chemicals is further disadvantaged by process complexity, costly chemical consumption and residue disposal requirements. In addition, while NH₃ is water soluble, HCN is not soluble enough in water to allow its removal to levels of less than 50 vol ppb, preferably less than 20 vol ppb and most preferably less than 10 vol ppb as needed to achieve favorable levels of catalyst half-life. Chemical scrubbing processes are not selective enough to remove HCN to these levels. Some previous catalytic conversion processes have used relatively low activity catalysts, requiring excessive catalyst volume and/or high process temperatures. Other processes have used sulfidic catalysts that release sulfur and irreversibly deactivate a downstream HCS catalyst. Processes that rely primarily or exclusively on adsorption to remove HCN and NH₃ require impractically large amounts of adsorbent to achieve adequate run times to remove the combined HCN and NH₃. NH₃ concentrations to the desired levels. The methods disclosed in the above-mentioned copending patent applications are preferred to achieve the low levels of HCN and NH₃ required for acceptable catalyst half-life.

In a suspension HCS process according to the invention, liquid and gaseous hydrocarbon products are produced by contacting syngas comprising a mixture of H₂ and CO with a Fischer-Tropsch type HCS catalyst under equilibrium shifting or non-shifting conditions, and preferably under non-shifting conditions in which little or no water gas equilibrium shift reaction occurs, particularly when the catalytic metal comprises Co, Ru or mixtures thereof. Suitable Fischer-Tropsch reaction type catalysts comprise, for example, one or more Group VIII catalytic metals such as Fe, Ni, Co, Ru and Re. Particularly preferred according to the invention is a catalyst in which the catalytic metal comprises a catalytically active cobalt component dispersed and supported on a particulate inorganic refractory oxide support or support material having a total catalytically active layer thickness in the range of about 5-200 microns. For support particles having a diameter greater than these values, the metal is impregnated as a thin surface layer not thicker than this range. For support materials smaller than the upper limit of this range, the catalytic metal can either be uniformly impregnated throughout the particles or applied as a thin(er) surface layer. Paraffinic C5+ hydrocarbon products are preferred and preferably more than 50% of the C5+ hydrocarbons are paraffins. Preferably, the catalyst has a productivity above 150 hr-1 at 200°C and has a methane selectivity of less than 10%. More specifically, and as described above, the catalyst comprises catalytically effective amounts of cobalt and one or more of Re, Ru, Fe, Ni, Th, Zr, Hf, U, Mg and La on a suitable inorganic support material, and preferably one comprising one or more refractory metal oxides. Preferred supports for Co-containing catalysts include titania and titania-silica composites, particularly when using a slurry HCS process where higher molecular weight, predominantly C5+ paraffinic liquid hydrocarbon products are desired. Suitable catalysts and their preparation are known and described; non-limiting examples can be found, for example, in US Patents 4,568,663; 4,663,305; 4,542,122; 4,621,072 and 5,545,674, with those disclosed in US 5,545,674 being particularly preferred.

The hydrocarbons produced by an HCS process of the invention are typically upgraded to more valuable products by subjecting all or a portion of the C5+ hydrocarbons to fractionation and/or conversion. By conversion is meant one or more steps in which the molecular structure of at least a portion of the hydrocarbon is altered and includes both non-catalytic processing (e.g., steam cracking) and catalytic processing (e.g., catalytic cracking) in which a fraction is contacted with a suitable catalyst. When hydrogen is present as a reactant, such processing steps are typically referred to as hydrogen conversion and include, for example, hydroisomerization, hydrocracking, hydrodewaxing, hydrofining, and the more vigorous hydrofining referred to as hydrotreating, all conducted under conditions known in the literature for hydroconversion of hydrocarbon feedstocks, including hydrocarbon feedstocks rich in paraffins. Illustrative but non-limiting examples of more valuable products formed by conversion include one or more of synthetic crude oil, liquid fuel, olefin, solvent, lubricant, industrial or medicinal oil, waxy hydrocarbons, nitrogen and oxygen containing compounds and the like. Liquid fuel includes one or more of motor fuel, diesel fuel, jet fuel, and kerosene, while lubricating oil includes, for example, automotive, aircraft, turbine, and metalworking oils. Industrial oil includes drilling fluids, agricultural oils, heat transfer fluids, and the like.

It should be understood that various other embodiments and modifications in the practice of this invention will be apparent to and can be readily made by those skilled in the art without departing from the scope and spirit of the invention described above. Accordingly, it is not intended that the scope of the appended claims be limited to the precise description set forth above, but that the claims be considered to encompass all the forms of patentable innovation inherent in the present invention, including all such aspects and embodiments as would be treated by those skilled in the art as equivalents of the present invention.

Claims (10)

1. A suspension hydrocarbon synthesis process for producing liquid hydrocarbons from a synthesis gas mixture comprising H₂ and CO containing HCN, NH₃ or mixtures thereof, in which the gas is reacted in the presence of a hydrocarbon synthesis catalyst in a suspension comprising the catalyst and gas bubbles in a hydrocarbon suspension liquid under reaction conditions suitable for forming hydrocarbons from the synthesis gas, at least a part of which is liquid under the reaction conditions, wherein the suspension liquid comprises the liquid hydrocarbons, the catalyst has a productivity of at least 150 h⁻¹ and comprises a catalytically active cobalt component on a particulate, inorganic, refractory oxide support and wherein the total amount of HCN, NH₃ and mixtures thereof present in the synthesis gas is less than 50 vol.ppb. 2. The process of claim 1, wherein the hydrocarbon liquid produced by the reaction comprises C5+ hydrocarbons. 3. A process according to claim 2, wherein the C5+ hydrocarbons comprise mainly paraffins. 4. A process according to claim 2 or claim 3, wherein at least a portion of the C5+ hydrocarbons is upgraded to more valuable products by one or more conversion steps. 5. A process according to any one of claims 1 to 4, wherein the catalytically active cobalt component of the catalyst is dispersed on an inorganic refractory oxide support and is supported. 6. Process according to one of claims 1 to 5, in which the catalytically active cobalt component is present on the carrier as a layer with a thickness of 2 to 200 µm. 7. A process according to any one of claims 1 to 6, wherein the catalyst has a methane selectivity of less than 5 mol%. 8. A process according to any one of claims 1 to 7, wherein the total amount of HCN, NH₃ or mixtures thereof present in synthesis gas is less than 20 vol.ppb. 9. A process according to any one of claims 1 to 8, wherein the total amount of HCN, NH3 or mixtures thereof present in the gas is less than 10 vol.ppb to achieve a short term catalyst half-life of at least 40 days. 10. A process according to any one of claims 1 to 9, wherein the catalyst has a long-term half-life of at least 100 days and a short-term half-life of at least 30 days.