Staged hydrocarbon synthesis process

Abstract: High conversion, substantially once-through hydrocarbon synthesis is achieved by reacting H 2 and CO in a first stage(s) in the presence of a non-shifting catalysts, separating liquid products and reacting the remaining gas streams in the presence of shifting catalysts.

Description: This invention relates to a hydrocarbon synthesis process. Hydrocarbon synthesis, also known as Fischer-Tropsch, processes involve the catalytic hydrogenation of synthesis gas, hydrogen and carbon monoxide, to form higher hydrocarbons, e.g., C 2 +, but preferably C 5 + materials. The synthesis gas feed can also contain small amounts of CH 4 and CO 2 as carry overs from the synthesis gas manufacturing step. The hydrocarbon synthesis reaction produces hydrocarbons and water, and at reaction conditions the water product can readily react with CO in the water gas shift reaction CO + H 2 O ⇄ CO 2 + H 2 to produce CO 2 , a deleterious side reaction since the objective of the process is the production of higher hydrocarbons from H 2 and CO. By the use of so-called "non-shifting" catalysts, e.g., cobalt on titania, silica, or alumina which have low selectivity for CO 2 ; the water gas shift reaction is largely suppressed. Iron based hydrocarbon synthesis catalysts usually promote the water gas shift reaction and have a high selectivity to CO 2 . Thus, by virtue of the relatively high CO concentration in the synthesis gas feed and the fact that water is produced in the synthesis process as hydrogen is reacted, the water gas shift reaction becomes important. The shift reaction can, however, be suppressed if the feed contains higher amounts of CO 2 relative to CO, and CO 2 . EP-A-0 153 781 describes a hydrocarbon synthesis process in which an H 2 - and CO-containing feed of low H 2 /CO molar ratio (0.75 to 1.75) is contacted with a defined cobalt-containing Fischer-Tropsch catalyst and, after water removal, with a catalyst composition comprising both hydrocarbon synthesis and activity for water gas shift reaction. Energy and Fuels, 1991, vol. 5, pages 847-855 describes the use of a mechanical mixture of a Co/MgO/SiO 2 Fischer-Tropsch catalyst with a Cu-ZnO/Al 2 O 3 water gas shift catalyst in a slurry reactor hydrocarbon synthesis process. Low H 2 /CO molar ratio (0.5 to 1.0) feeds are referred to. According to the present invention, there is provided a substantially once-through hydrocarbon synthesis process, which comprises reacting in a first stage or stages, a feed comprising hydrogen and carbon monoxide in a ratio from 1.9:1 to 2.5:1 and 1 to 10 mol% CO 2 , in the presence of a non-shifting hydrocarbon synthesis catalyst, recovering the product therefrom, condensing liquids from the product and reacting the remaining gases in a subsequent stage or stages in the presence of a hydrocarbon synthesis catalyst having shifting activity, and recovering the products therefrom. The process of the present invention employs non-shifting catalysts in the first stage or stages to convert a large portion of the CO in the synthesis gas to desired hydrocarbons, resulting in an enrichment of the gas phase CO 2 concentration, followed by the use in the subsequent stage or stages of a shifting catalyst wherein the presence of CO 2 suppresses further shifting and results in exceedingly high overall CO conversions in the order of 95+%, even 97% or greater. These CO conversion levels substantially eliminate the need for any recycling of hydrocarbon synthesis products back to the hydrocarbon synthesis reactors, thereby eliminating the need for costly recompression and treatment of recycled gases for removing oxygenated products, e.g., alcohols, aldehydes, that may poison or suppress the synthesis reaction. The feed to the first stage or stages contains hydrogen and carbon monoxide in a 1.9:1 to 2.5:1 mol ratio, preferably 1.9:1 to 2.3:1 mol ratio, and CO 2 in the range of 1.0-10 mol %, preferably 5-10 mol %. The product of the reaction over the non-shifting catalyst contains unreacted hydrogen and carbon monoxide, C 2 + hydrocarbons, oxygenated products such as alcohols, e.g., methanol, aldehydes, water, CO 2 , methane and some inerts such as nitrogen. After removing condensible products, the remaining vapor feed comprising hydrogen, carbon monoxide, uncondensed oxygenated products, CO 2 and water is reacted in a subsequent hydrocarbon synthesis stage or stages in the presence of a shifting catalyst and produces higher hydrocarbons, including olefins, in an olefin:paraffin mole ratio of about 1.5:1 to 4:1, the lower ratios being favored for higher hydrocarbons, the higher ratios being favored at lower carbon numbers. The hydrogen to carbon monoxide ratio entering the second or shifting stage(s) will have essentially the same mol ratio as that entering the first or non-shifting stage(s) since the stoichiometric reaction ratio for the Fischer-Tropsch synthesis is approximately 2.1:1. However, there will be less total gas entering the second stage(s) and, therefore, the relative concentration of CO 2 in the second stage(s) will be greater than that in the first stage(s). This once-through process, i.e., substantial absence of recycle of hydrocarbon synthesis products to the hydrocarbon synthesis reactions, results in overall CO conversion to hydrocarbon of at least 95+%, and preferably 97+%. DESCRIPTION OF THE DRAWING Figure 1 is a block schematic diagram, given as a non-limitative example only, depicting the process of this invention. DETAILED DESCRIPTION Details of the process can be easily obtained by reference to the drawing in which synthesis gas feed in line 10 enters reaction stages 12 in which a non-shifting catalyst is present. The hydrocarbon synthesis process is effected at temperatures ranging from 175°C to 450°C, preferably 190°C to 250°C and pressures of 1-100 bar, preferably 20-40 bar. The non-shifting catalyst can be any of the well known hydrocarbon synthesis catalysts such as cobalt or ruthenium supported on a Group IIIA, IVA, or VA inorganic refractory oxide material. Promoter materials can also be present such as Group IA, IB, or IIA metals. Preferred catalysts are cobalt on titania, silica, or alumina, preferably titania. Preferred promoters are ruthenium, rhenium, cerium, or hafnium, particularly ruthenium and rhenium, most particularly rhenium. These materials are prepared in well known ways, see for example US patents 4,637,993, 4,717,702, 4,477,595, 4,663,305, 4,822,824, 5,036,032, 5,140,050, 5,292,705. The reaction in the presence of the non-shifting catalyst can take place in one or more stages, normally resulting in at least about 80+% CO conversion, preferably at least about 85% conversion, more preferably at least about 90% CO conversion. The product of the non-shifting reaction is removed via line 14 and transferred to separator(s) 16 where condensible products are removed from the system by well known means, e.g., cooling and liquid phase separation. The liquid products separated contain C 5 + hydrocarbons, water, oxygenated compounds and small amounts of dissolved gases. The remaining vapor in line 20 is then transported to subsequent hydrocarbon synthesis stages 22 in which shifting type catalysts are present. The vapor stream 20 contains unreacted H 2 and CO as well as CO 2 , H 2 O, inerts, and uncondensed oxygenated products. This stream contains substantially less CO than the feed to the first stages (non-shifting catalysts) and the ratio of CO 2 to CO is greater than 2, preferably greater than 4. Sufficient hydrogen must also be present in the vapor stream to react with CO and CO 2 to form hydrocarbon products. Thus, the mole ratio of hydrogen to CO + CO 2 is at least 0.2:1, preferably at least 0.25:1. Additional or make up hydrogen, if necessary, may be added to stages 22 by means not shown. Reaction conditions in these subsequent shifting catalyst stages are similar to reactions conditions in the first, non-shifting catalyst stages, although in the absence of re-compression pressures will be at the lower end of the range, e.g., 10-35 bar, and will be reflective of the degree of CO conversion in the first stages, e.g., temperature of 220-350°C, SHSV 200-2000, dry feed, e.g., H 2 O < 5 vol%. Reaction conditions will, of course, favor maximum CO conversion with lowest selectivity to methane and CO 2 . Selectivity can be defined as moles methane or CO 2 produced per mole CO converted. Reaction product is withdrawn via line 24 and sent to separator(s) 26 where liquid products are withdrawn via line 28 and a tail gas is recovered in line 30. The tail gas is substantially devoid of reactive molecules and may be used as a fuel gas or flared. In one embodiment it may be recycled to the synthesis gas manufacturing step and used as a fuel. The shifting catalysts in stage(s) 22 are preferably iron based but also may be those disclosed in Catalysis Science & Technology; Anderson, J.R. Bondart, M. ed. Vol. 1, 1981, p. 175 et seq. Hydrocarbon synthesis processes can be carried out in fixed bed, fluid bed, and bubble column reactors -- all of which are well known to the art and fully described in available literature. It is not important to this invention whether reactors in the first stages are the same or different, although slurry type, bubble column reactors are now usually preferred. For the second (shifting) stage(s), due to the low concentration of reactant gases, substantially plug flow reactor(s) (e.g., fixed or fluid bed) are preferred. The process of this invention can be illustrated by the following example: A synthesis gas feed containing hydrogen and carbon monoxide in the ratio of 2.25:1 is reacted to convert 95% of the CO using a cobalt on titania catalyst. The tailgas from this reaction flows to a second stage reactor containing an iron based catalyst (shifting catalyst). Due to the shift reaction, there is a net disappearance of CO 2 in the second stage reactor, and an increase in hydrocarbon yield compared to a non-shifting reactor. Table 1 shows the material balances for both shifting and non-shifting second stage reactor simulations. Total (CO 2 + CO) conversion increases from 12% for the non-shifting case to 18.4% for the shifting second stage. Hydrocarbon and alcohol yield increases from 223 moles/hour to 344 moles/hour for the shifting case. Reactor Simulation Results Stage 2 Feed Stage 2 Product Shifting Catalyst Non-Shifting Component Rates, Moles/Hr H 2 11584 3021 6781 CO 2823 564 524 CO 2 16324 15065 16326 H 2 O 113 4887 2400 Hydrocarbon + Alcohol 13701 14045 13924 Inert 2434 2434 2434 Moles Converted CO 2259 2299 CO 2 1259 -2 Total 3518 2297 % (CO + CO 2 ) Converted 18.4 12.0 Hydrocarbon + Alcohol Yield, Moles/Hr 344 223

Claims: A substantially once-through hydrocarbon synthesis process, which comprises reacting in a first stage or stages, a feed comprising hydrogen and carbon monoxide in a ratio from 1.9:1 to 2.5:1 and 1 to 10 mol% CO 2 , in the presence of a non-shifting hydrocarbon synthesis catalyst, recovering the product therefrom, condensing liquids from the product and reacting the remaining gases in a subsequent stage or stages in the presence of a hydrocarbon synthesis catalyst having shifting activity, and recovering the products therefrom. The process of claim 1, wherein the feed gas contains 5 to 10 mol% CO 2 . The process of claim 1 or claim 2, wherein the H 2 :(CO + CO 2 ) mole ratio in the gases fed to the subsequent stage or stages is at least 0.2:1. The process of claim 3, wherein said ratio is at least 0.25:1. The process of any preceding claim, wherein the CO 2 :CO mole ratio in the gases entering the subsequent stage or stages is greater than 2:1. The process of claim 5, wherein the CO 2 :CO mole ratio is greater than 4:1. The process of any preceding claim, wherein the catalyst in the first stage or stages comprises cobalt. The process of any preceding claim, wherein the catalyst in the subsequent stage or stages comprises iron. The process of any preceding claim, wherein the reaction conditions in the first stage or stages and subsequent stage or stages include temperatures of from 175-450°C and pressures of from 1-100 bar.

Inventors: LONG DAVID CHESTER, LONG, DAVID CHESTER
Applicants: EXXON RESEARCH ENGINEERING CO, EXXON RESEARCH AND ENGINEERING COMPANY
Application year: 1995
Main classification: C07C1
IPC codes: C07C1, C10G2