CA1305188C - Catalytic hydrocarbon synthesis from co and h _over metallic cobalt - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

The addition of water into a Fischer-Tropsch hydrocarbon synthesis reaction zone employing a catalyst comprising cobalt in a reduced, metallic form results in increased CO conversion and C5+ hydrocarbon production, and a decrease in methane production.

Description

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BACKGROt~ID OE THE INVENTION
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Field of the Invention _ . _ . ~ _ . _ This invention relates to a water.addition proces~ for increasing the activity of a Fischer-Tropsch, fixed bed, hydrocarbon synthesis reaction over a catalyst comprising metallic cobalt. Mo~e particularly, this invention relates to decreasing the methane make and increasing the C0 conversion activity and C5+ hydrocarbon selectivity of a Fischer-Tropsch hydrocarbon synthesis process which comprises co-feeding a mixture of H2, C0 and H20 into a reaction zone containing a catalyst comprising cobalt in the reduced, metallic for~ wherein said mixture contacts said catalyst at elevated temperature for a time sufficient to convert at least a portion of said feed to C5+ hydrocarbons.
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Background of the Disclosure _ _ _ _ _ _ ;

The production of hydrocarbons from mixtures of H2 and C0 via the Fischer-Tropsch process is well known to those skilled in the art. As opposed to the well-known "methaniza~ion" proce-cs which produces methane as synthetic natural gas from mixtures of H2 and C0, the Fischer-Tropsch process is more generally aimed at producing higher value products such as chemical feeds~ocks and liquid fuels. ~hus, high methane make is undesirable in Fischer-Tropsch syn-thesis processes because it is a relatively low value product which is formed at the expense of more de-.~

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sirable products. It is also uneconomical to try toconvert the so formed methane back into a CO and H2 mixture and recycle it back into the reactor.

Methane make in Fischer-Tropsch reactions is often expressed by a ~erm known as methane selec-tivity. Methane selectivity can be defined by either of two methods. They are, (a) mole % CH4 produced based on the amount of CO consumed or (b) weight 3 of CH4 produced based on total hydrocarbon products formed.

Many different catalysts and processes have been disclosed for Fischer-Tropsch synthesis, some of which have extremely high methane make. Thus, U.S.
Patent 4,077,995 discloses synthesis of Cl-C4 aliphatic hydrocarbons over a catalyst comprising a sulfided mixture of CoO, A12O3 and ZnO while U.S. Patent 4,039,302 discloses Cl-C3 hydrocar~on production using a mixture of the oxides of Co, Al, Zn and Mo. UOS.
Patent 4,151,190 discloses producing C2-C4 hydrocarbons from mixtures of CO and H2 using a supported catalyst comprising a metal oxide or sulfide of Mo, W. Re, Ru, Ni or Pt plus an alkali or alkaline earth metal, with Mo-~ on carbon being preferred. ~.S. Patent Nos.
4,243,553 and 4,243,554 disclose MoS2 as a Fischer-Trop~ch catalyst. Many other catalysts are known to be useful for Fischer-Tropsch synthesis employing metals such as iron, copper, titania, etc. These are known to those skill@d in the art.

The type of catalyst used and process conditions employed have an important bearing on CH4 selectivity. Eor example, nickel gives a high CH4 selectivity and is used mainly as a methanization catalyst. Methane selectivity usually increases with increasing temperature, decreasing pressure and ~3~5~

increasing the H2/CO ratio of the feed. Accordingly, process conditionS are selected so as to minimi~e CH4 selectivity while maintaining a relatively high reaction rate as is well known to those skilled in the art.

It is known that CH4 selectivity is in-fluenced by the choice of promoter and support, such as alkali metal promoters reducing CH4 selectivities of iron catalysts. It is also known in the art that noble metals such as ruthenium supportQd on inorganic refractory oxide supports exhibit superior hydrocarbon synthesis characteristics with relatively low methane production. Thus, U.S. Patent No. 4iOB8,671 suggests minimizing methane production by using a small amount of Ru on a cobalt catalyst. Examples of supported ruthenium catalysts suitable for hydrocarbon synthesis via Fischer-Tropsch reactions are disclosed in U.S.
Patent Nos. 4,042,614 and 4,171,320. It is also known that the type of support used also influences methane production. In the case of supported ruthenium catalysts, the use of a ti~tania or titania containing support will result in Iower methane production than, for example a silica, alumina or manganese oxide support.
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European published patent applica~on 0109702 relates to a Fischer-Tropsch hydrocarbon synthesis proc ss em~loying, as a catalyst, cobalt supported on silica promoted with zirconium, titanium or chromium wherein the cobalt is deposited on the silica carrier by a kneading process. In this process, 10 to 40 volume percent steam is added to the feed, based on the H2/CO/H20 mixture. The amount of cobalt present in the catalyst is 10-40 pbw based on 100 pbw of silica. In tne Examples, very little enhancement in either Co conversion or C3~ selectivity is shown as a result of .

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the water addition to the feed. U.K.~patent applica-tion 2146350A relates to a Fischer-Tropsch hydrocarbon synthesis process employing a similar catalyst, followed by a hydrocracking process wherein the Fische~-Tropsch products are hydrocracked over a noble metal catalyst. The same type of catalyst is also used in the~ Fischer-Tropsch processes described in U.K.
p~b~,she patent application nos. 2,149,812A and 2,149,813A.

U.S. Patent 3,927,399 relates to a process for producing a methane rich f~el gas containing 70 to 98 mole % methane by contacting a mixture of H~, CO and H2O with a suitable catalyst at elevated temperature.
This invention is based ~n the discovery that the methane content of the product gas from the ~ethanator is maximized by adjusting the mole % H2O in the synthesis gas feed to a critical value in the range of 1 to 3, while maintaining the H2/CO mole ratio of t'ne synthesis gas feed to a critical value in the range of about 1 to l.lS. The feed gas may also contain methane. Group VIII transition metals such as iron, nickel and cobalt are suggested as suitable catalysts, but only nickel on alumina is actually used. Example 1 shows the production of a fuel gas containing 96.4 mole ~ CH4 from a feed gas containing 48.1 mole ~ CH4 over a catalyst containing nickel, thoria, magnesia and kieselguhr.

UOS. Patent 2,479,439 relates to a process for alternately increasing the activi~y of an alkali metal promoted, powdered iron catalys~ or decreasing the coke buildup thereon, or accomplishing both simultaneously by passing an aqueous solution of alkali metal salt, such as KF, into the hydrocarbon synthesis reaction zone to contact the iron catalyst. The presence of the water is said to remove carbon from the iron catalyst. U.S. Patent 2,539,847 relates to a ~3~
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Fischer-Tropsch hydrocarbon synthesis process employing a catalyst consisting of thoria promoted cobalt supported on bentonite. U.S. Patent 4,568,663 relates to a Fischer-Tropsch hydrocarbon synthesis process employing a rhenium cobalt on titania catalyst.

The Fischer-Tropsch ~FT) process, the process to which this invention relates, must also be distinguished from the well known Kolbel-Engelhardt (KE~ process (described, for example, in ~anadian Patent ~!o. 530,932). The FT process may be described by equation (1), the KE process by equation (2).

2nH2 + nCO -~CH2)n~ + nH20 nH20 + 3nCO -(CH2)n- ~ 2nC2 (2) The two processes may appear similar in that they yield similar products and can employ Group VIII
metals as catalysts. The reactions are not similar because their reaction paths differ in the formation of hydrocarbons and because FT yields H20 as a product while KE consumes H20 as a reactant. The reactions can be distinguished in that in FT there is no net consumption of H20, whereas in KE there is net consumption of H20. Even if hydrogen is added to the reaction mixture for KE, water will be consumed.

There exists a need in the art for Fischer-Tropsch processes useful for the conversion of mixtures of CO and hydrogen to C5~ hydrocarbons at high CO
conversion levels, and high hydrocarbon yields, with relatively low me~h~ne make.

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SUMMARY OF THE INVE~iTIO~!

It has now been discovered that in a fixed bed, Fischer-TrOpsch hydrocarbon synthesis reaction for producing Cs~ hydrocarbons from a gaseous feed mixture of H2 and CO in the presence of a catalyst comprising cobalt, one can increase both the CO
conversion activity and the Cs+ hydrocarbon selectivity and, at the same time, decrease the methane make by adding H20 to the reaction zone. ~hus, the present invention relates to a Fischer-Tropsch process for synthesizing Cs~ hydrocarbons by introducinq into a catalytic reaction zone a feed mixture of CO, H2, and H20 wherein said feed contacts a catalyst comprising cobalt at elevated temperature and ~or a time sufficient to convert at least a portion of said feed to C5+ hydrocarbons. By cofeeding H20 into the reaction zone, along with the H2 and CO, it has been found that Cs+ hydrocar~on production is increased, CO
conversion is increased and CH4 production is decreased. By cobalt is meant cobalt in the reduced, metallic form, preferably a high surface area cobalt like cobalt black.

The H20 that is added to the reaction zone may be in the form of steam or moisture or a suitable H20 precur~or, such as Cl-C6 alcohols, for forming H20 in-situ in the reaction zone. It is essential to the understanding of the process of this invention that the H20 introduced into the reaction zone is external H20 and not that H20 which is formed in-situ in the reaction zone as a cons quence of the Fischer-~ropsch hydrocarbon synthesis reaCtiQn from the H2 and CO. It has also been found that the process of this invention improves with increasing pressure in the reaction zone and with decreasing CO conversion.

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In a pref~rred embodiment of the invention, the synthesis ~as feed, carbon monoxide and hydrogen is substantially free of and preferably completely (except for trace a~ounts) free of methane or other light hydrocarbons. The hydrocarbon free synthesis gas feed is easily accomplished in a once-through system where there is no recycle of unconverted hydrogen and carbon monoxide talong with undesirable light hydrocarbons~
directly to the hydrocarbon synthesis reaction zone.
Preferably, the synthesis gas feed contains only H2, CO
and water and is substantially free of light hydro-carbons, e.g., methane. Allowable limits on light hydrocarbons in the synthesis gas feed are no more than about 1 vol.% and preferably less than about 0.5 vol.%.

_ETAILED DESCRIPTION

The process of the present invention resides in adding, to the Fischer-Tropsch reaction zone, H2O or a suitable H2O precursor such as an alcohol. The amount of H2O added to the reaction zone will range from about 1-70 volume ~ of the total feed mixture of H2O, CO and H2 and, preferably, from about 5-30 volume ~. As the extent of CO conversion increases, the amount o water produced in-situ in the reaction zone increases and, concomitantly the beneficial effect of introducing additional water into the reaction zone to i~rease CO conversion, reduce C~4 selectivity and increase Cs~ hydrocarbons selectivity decreases. Thus, in some cases, it may be advantageous to practice the process of this invention CO conversion levels below about 60 percent. That i , below about 60S per pass, reaction zone or stage. As the pressure in the reaction zone increases, the beneficial e~fect of the process of this invention of adding water to the reaction zone increases with respect to increasing CO
conversion activity, decreased C~4 selectivity and increased Cs+ hydrocarbon selectivity. At relatively low pressures in the reaetion zone (i.e.; less than about l atmosphere), little effect will be seen in increased conversion activity, etc. by adding H2O to the reaction zone. Thus, the process of this invention will be operated at a pressure above about one atmos-phere. In general the pressure will range from about 1-50 atmospheres, and more preferably 5-30 atmospher~s.

In the process of this invention, the addition of ~2 enhances the formation of C5+ hydro carbons and reduces CHg make in a uni-directional or linear fashion. It has also been found that the rate of the Fischer-Tropsch reaction employing the process of this invention increases with increasing par~ial pressure of H2O at any fixed total pressure in the reaction zone, up to a point, after which point the rate slowly goes down with continually increasing H2O
par~ial pressure.

In general, the Fischer-Tropsch hydrocarbon synthesis reaction pro~ess of this invention is carried out at a H2:CO mole ratio of greater than about 0.5, and preferably ~he H2:CO mole ratio range~ from about O.S to about 6, more preferably from about O.S to about 3, at gas hourly space velocities ranging from about lO0 V/Hr/V to about 5000 V/Hr/V, preferably from about 300 V/Hr/V to about 1500 V/Hr/V, at temperat~res ranging from about 150C to about 300C, preferably from about 180C to about 240C, and pressures above abou~ l atm., preferably ranging from about l atm. to about 50 atm., more preferably from about 5 atm. to about 40 atm. and s~ill more preferably from about 5-30 atmospheres.

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g As previously stated, the hydrocarbon synthesis process of this invention employs a catalyst comprising cobalt in its reduced, metallic form, preferably a high surface area cobalt like cobalt black. The cobalt may be in the form of a finely divided powder, or granules which may be mixed ~ith a suitable diluent to aid in heat transfer and removal from the reaction zone. The cobalt may be supported on a suitable support material, such as cobalt plated on carrier metal. However, this is not meant to include cobalt dispersed on inorganic refractory oxide sup-ports. The cobalt in bulk form, or plated, explosion-coated, etc. may also be in the form of various high surface area shapes such as spirals, metal wool, honeycomb configurations, etc., the choice being left to the practitioner.

When supported catalysts are employed, for example, cobalt on a support such as titania, silica, alumina, or silica-alumina, the improvement in the process from the addition of water is apparent only with ~upports of a relatively low surface area. Thus, supported cobalt catalyst preferably have a surface area of less than about 40 square meters per gra~ of catalyst ~BET).

The invention will be more readily under-stood by reference to the examples below.

EXAMPLES

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A bulk cobalt catalyst, designated as a cobalt black catalyst, was prepared by a conventional method known in the art, i.aO, adding a stoichiometric amount of ammonium carbonate (as an aqueous solution) ~3~

to an aqueous solution of cobalt nitrate, filtering the precipitate, ~ashing the filtered solid with deionized water, drying at 120C, and calcining at 500C in air for 5 hours. The resulting ~aterial showed an X-ray diffraction pattern of Co3O4 which, when reduced at 450C in a H2 stream, produced a cobalt black catalyst having a surface area of 7.7 m2/gr.

The performance test of this catalyst was carried out by charging an intimate mixture of the Co3O4 powder (6.8 grams) and a diluent (quartæ powder, 80-1~0 mesh, 18 grams) into a down-flow fixed bed reactor made of 3/81' OD stainless steel tube with a concentric 1/8" OD thermocouple well. When reduced, 6.8 grams of Co3O4 will yield 5.0 grams of cobalt b].ack.
The use of diluent and an aluminum block jacket fitted tightly around the reactor minimized uneven temperature profile along the bed. The Co3O4 was then re-reduced in-situ in the reactor in a flowing H2 stream ~200 cc/m, 1 atm) overnight at 450C, cooled to 175C and then pressurized to 20 atm using a pre-mixed gas composed of 63.9% ~2~ 32.1% CO and 4-0% ~2 At pressure and at a preset flow rat~, the temper~ture was raised to 200C over a period of one hour and then data acquisition ~as initiated. The rate of CO conversion ahd the rates of various hydrocarbon products formation were moni~ored using two on-line GC's. The data taken ater 70 hour on-strea~-time are shown in Table 1.

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TABLE l -ADDITION O~ ACTIVITY

__ _ __ Temperature = 200C
Feed Gas Composition = 63.1~ H2/33.0% CO/3.9~ N2 SV = 3600 Scm3/g hr ~calcined for H2 and CO)*l Run 1 Run 2 Run 3 Run % H2O Added~2 012.5 27.7 50~0 Total Pressure, atm20. 7 21. 624 . 9 28 . 6 PH2 + Pco, atm20.018.7 18.9 18.6 PH2O, atm 0 2.3 5.2 9.3 % CO Conv. 12.127.3 28.718 . 8 CH4 Select*3 10.57.1 5.8 4.0 C2 Select~3 0.270.21 0.34 Hydrocarbon Product Distribution, wt.%
CH4 11.58.1 6.7 4.6 C2-C4 15.68.7 7.8 8.6 C5-C9 17.~~.5 9.0 9.7 Clo+ 55.573.7 7~.5 77.1 C10-~20 21.717.0 C31_C30 21 713 8 C41-C50 4.78.1 Csl+ 11.823.7 *1 cc of H2 and CO measured at 1 atm, 22C per gram of catalyst per hour.

*2 Moles of H2O added per 100 ~oles of CO and H2 *3 Moles of CHg or C2 per 100 moles of CO converted Comparison of the data with and wi~hout the ~xternal H2O addition clearly establishes the advan-tages of H2O addition; ~1) the CO conversion increases markedly ~up to 2.5 fold increase), (2) the ~ethane ~3~

make i5 greatly reduced and, (3) the desired heavy hydrocarbon product yields, which may be gauged by the Clo+ selectivity, increase dramatically.

Thus~ this example clearly demonstrates the benefits of conducting a Fischer-Tropsch hydrocarbon synthesis reaction with external H2O addition in the presence of a catalyst comprising metal cobalt.

Example 2 Another batch of a bulk cobalt catalyst was prepared using highly purified Co(NO3)2 6H~O ~99.99~
pure Puratronic grade) according to a procedure similar to that described in Example 1. The evaluation procedure was also identical to that used in Example l.
After an extended reduction in a flowing H2/He mixture at 500C, the material was passivated and found to have about 1.5 m2 surface area per gram. This catalyst (S
grams) and 10 grams of quartz powder were mixed and charged into the reactor and pretreated as described in Example 1. At 201~5C and 20.7 atm., a gas mixture composed of 63.1~ H2, 33.0% CO and 3.9~ N2 was passed at a rate of 105 Scm3/m. After one day on stream the data in Table 2 were obtained.

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EFFECTS O~ ADDED H2O ON THE ACTIVITY
AND SELECTIVITIES OF CO HYDROGENATION
OVER A HIGH-PURITY BULK CO CATALYST
Temperature = 202C
Feed Gas Composition - 63.13 H2/33% CO/3.9~ ~12 Pressure = 20 atm.
SV = 1200 Scm3/g hr Run 5 Run 6 % H2O Added 0 21 % CO Conversion 7.2 15.6 CH4 5electivity 8.2 4.2 l-olefin/paraffin Ratio C2 0~13 0.36 C3 2.47 3.68 c4 1.87 2.52 Hydrocarbon Product Distribution, wt.~
CH4 9.4 4.8 C2-C4 21.1 2~.4 ~5-Cg ll.0 11~6 Clo+ 58.5 63.

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Claims (6)

1. A fixed bed catalytic process for synthesizing C5+ hydrocarbons which comprises reacting, in a reaction zone at elevated temperatures, hydrogen and carbon monoxide substantially free of light hydrocarbons in the presence of added water, wherein the catalyst is comprised of cobalt in bulk form or as cobalt on titania support, and wherein there is no net comsumption of water.

2. The process of claim 1 wherein the water is added to the feed prior to entering the reaction zone.

3. The process of claim 1 wherein the water is added to the reaction zone.

4. The process of claim 1 wherein the amount of water added ranges from about 1 to 70 vol.% of the total amount of H2O, CO and H2.

5. The process of claim 1 wherein the catalyst is comprised of cobalt on a support and the support has a surface area of less than about 40 m/gm.

6. The process of claim 1 wherein the pressure in the reaction zone is greater than 1 atmosphere.