Process for removing hydrogen cyanide from synthesis gas

Abstract: The HCN concentration of HCN containing synthesis gas streams is reduced by treatment with a Group IVA metal oxide and optionally containing a Group IIB, Group VA, or Group VIA metal or metals, at reaction conditions preferably suppressing Fischer-Tropsch activity.

Description: Hydrogen cyanide, HCN, is a poison, some times reversibly so, for Fischer-Tropsch hydrocarbon synthesis processes. This invention relates to a process for the removal of HCN from synthesis gas feeds to Fischer-Tropsch processes. Synthesis gas, hydrogen and carbon monoxide, useful as feed to Fischer-Tropsch processes is often formed by steam reforming or partial oxidation of materials such as methane, natural gas, lower hydrocarbons, coal, and the like all of which often contain nitrogen in various forms. Oxygen, as such, or air can be used in either process can also lead to the presence of nitrogen in the feed to the synthesis gas plant. The presence of nitrogen in the feeds to a synthesis gas generation process often results in HCN in the synthesis gas product. (Air plants for the separation of oxygen and nitrogen are not 100% efficient.) HCN is a poison for Fischer-Tropsch processes, particularly those utilizing Group VIII metal catalysts. Catalyst activity is thus reduced, and while activity may be regained by hydrogen treatment, the interruption of the process and treatment step are economic debits for the process. While processes exist for HCN removal, these processes are not generally capable of removing the 90+% of HCN necessary for viable Fischer-Tropsch processing and do not operate at the relatively low Fischer-Tropsch reaction temperatures, also resulting in economic debits for the process. Also, HCN is only weakly soluble in water or normal solvents and water scrubbing or gas scrubbing of the HCN containing synthesis gas is not adequate for further use of the gas in Fischer-Tropsch synthesis. DE-A-4319234 discloses a process for removing HCN from a gas mixture comprising H 2 and CO. In the process, the gas mixture is passed in contact with a catalyst which comprises titiania or zirconia as a support material and chromia as a promoter. The present invention provides a process for decreasing the HCN concentration of an HCN-containing synthesis gas which comprises passing the synthesis gas in contact with a composition comprising a Group IVA metal oxide and a promoter metal component whereof the metal is selected from zinc, vanadium, niobium, molybdenum and mixtures thereof, wherein the composition has been activated with a hydrogen-containing gas at a temperature of at least 225°C. The process of the invention enables at least 90% HCN removal, and the HCN removal may be at least 95%, and may be at least 99%. The Group IVA metal oxides, titania, hafnia and zirconia, may be promoted by metals selected from the group consisting of Group IIB, Group VA, and Group VIA metals or mixtures thereof. Particularly preferred metals are those of Group IIB, most preferably zinc, and Group VIA, most preferably molybdenum. HCN removal from the synthesis gas is believed to be effected by hydrolysis in the case of unpromoted materials and by a combination of hydrolysis and hydrogenation in the case of promoted materials. While the process may be conducted at various operating conditions, temperatures should preferably be below those temperatures at which Fischer-Tropsch synthesis occurs, e.g., below 180°C, more preferably below 175°C, thereby substantially minimizing the Fischer-Tropsch reaction. DESCRIPTION OF PREFERRED EMBODIMENTS Decreasing the concentration of HCN in HCN containing synthesis gas can be effected with Group IVA metal oxides alone or composited with other essentially inert materials, e.g., alumina or silica. However, the Group IVA metal oxide, preferably titania should preferably be present in any composite to the extent of at least about 50 wt%, more preferably at least about 75 wt%, and still more preferably at least about 80 wt%. The surface area of the material should preferably be at least about 150 m 2 /gm (BET method), with a preferred bulk density of at least about 0.2 gm/cm 3 . Promoter metals can be added to the metal oxide by conventional methods, e.g., kneading, spray drying, impregnation, preferably by the incipient witness impregnation technique where the promoter metals are added as a soluble aqueous salt, e.g., ammonium heptomolybdate, zinc nitrate (preferably not halide salts). Following impregnation, the material is dried, e.g., at about 100-150°C, preferably under vacuum, followed by calcination at temperatures of about 350°C or higher, e.g., 350-550°C, to convert the salt to the oxide form. The oxide is then activated by treatment with a reducing gas, e.g., hydrogen or a hydrogen containing gas or a hydrogen containing synthesis gas (activation may occur in situ ), suitably at temperatures of at least 225°C, e.g., in a range of from 225°C to about 350°C; and pressures preferably in a range of from about 1-100 bar, e.g., 10-40 bar. The amount of promoter may vary widely and is necessary in amounts at least sufficient to provide an increase in HCN removal over and above that obtained with the Group IVA metal oxide alone. Preferably, promoters are present in amounts of about 1-30% by weight, preferably 2-30% by weight, more preferably about 5-30 wt%. Promoters, may be selected from Group IIB of the Periodic Chart of the Elements, e.g., zinc, cadmium, mercury; Group VA, e.g., vanadium, niobium, tantalum; Group VIA, e.g., chromium, molybdenum, tungsten, or mixtures of any one or more thereof. Preferred promoters are zinc and molybdenum, most preferably molybdenum. The HCN removal process can be effectively conducted over a wide variety of temperatures and pressures. However, it is preferred to operate at temperatures of at least about 65°C but below those temperatures at which the Fischer-Tropsch synthesis will be substantial, e.g., below about 180°C, preferably below about 175°C. Pressures may vary between about 1-100 bar, preferably 10-40 bar. Both temperatures and pressure are selected to maximize HCN removal and minimize reheating, and compression of the synthesis gas before entering the Fischer-Tropsch reaction. The hydrolyzed or hydrogenated HCN can be removed from the synthesis gas by any applicable method well known to those skilled in the art, e.g., water scrubbing or absorption onto a solid absorbent. The synthesis gas is then passed to a Fischer-Tropsch hydrocarbon synthesis process at temperatures of about 175°C, preferably 180°C to about 400°C and pressures of about 1-100 bar, preferably 10-40 bar. Fischer-Tropsch catalysts can be Group VIII metals either bulk or in supported form. The metals may be iron, nickel, cobalt, or ruthenium, preferably cobalt, and the support may be any carrier, e.g., silica, alumina, silica-alumina, titania, preferably silica or titania. Promoters may be employed, preferably zirconium or titanium with silica supports and rhenium or hafnium with titania supports. The Fischer-Tropsch process produces C 2 + materials, preferably C 5 + materials. The following examples will further serve to illustrate this invention but are not meant to be limiting in any way. A titania support material was used in each of the following examples, unless otherwise specified, and had the nominal composition and physical properties shown in Table I. TiO 2 81.6 wt% Na 2 O 6.7 MgO 4.7 Al 2 O 3 0.95 SO 3 3.3 surface area 200 m 2 /gm bulk density 50 lbs/ft 3 (0.8009 g/cm 3 ) A simulated HCN containing synthesis gas having the nominal composition shown in Table II was used for demonstrating HCN removal. Hydrogen 55.1 vol% Carbon Monoxide 22.3 Carbon Dioxide 3.2 Water Vapor 11.1 Argon 8.3 HCN 215 ppmv HCN removal tests were run at a GHSV of 11,500 and 22 bar unless otherwise specified. EXAMPLE 1 (comparative) Table III shows the results of several tests for HCN removal using various promoted and unpromoted material. Material 343°C (650°F) 288°C (550°F) 232°C (450°F) 177°C (350°F) 149°C (300°F) TiO 2 (from Table I) >99 >99 96 82 67 5 wt% Mo/TiO 2 >99 >99 >99 99 96 5 wt% Zn/TiO 2 99 99 99 95 79 5 wt% Mo/5 wt% Zn/TiO 2 >99 >99 >99 >99 94 The results of Table III show that all materials tested were adequate for removing 90+% HCN from the synthesis gas. However, as process temperature was reduced the ability of TiO 2 to remove HCN fell off significantly and at 300°F only Mo or Zn promoted TiO 2 could remove 90+% of the HCN. EXAMPLE 2 Activation of the promoted material can effect the HCN removal ability of the material. Table IV below shows activation temperatures from a 5% by weight Mo/TiO 2 material. Activation Temperature % HCN Removal At 177°C (350°F) 163°C (325°F) 149°C (300°F) 177°C (350°F) comparative 96.6 95.1 ---- 204°C (400°F) comparative 97.8 94.4 88.3 232°C (450°F) 99.9 99.4 97.1 288°C (550°F) 99.9 99.9 97.9 343°C (650°F) 99.9 100.0 98.7 Clearly, as processing temperature increases, activation temperature decreases for achieving a particular degree of HCN removal. To achieve 90+% HCN removal an activation temperature of about 232°C (450°F) is necessary. EXAMPLE 3 Experiments, Table V, were conducted to determine optimum loadings for Mo in HCN removal. Activation was conducted at 288°C (550°F), 39,000 GHSV, 22 bar (320 psig) for 5.5 hours with the synthesis gas blend with HCN. HCN removal was effected at the temperatures shown with 200 ppm HCN in the synthesis gas and pressure and GHSV the same as the activation conditions. Mo wt% 177°C (350°F) 163°C (325°F) 149°C (300°F) 30 99.03 94.8 84.8 20 99.6 95.8 86.4 15 98.1 93.8 82.4 10 97.9 90.5 NA 5 95.0 87.2 69.9 2.5 79.9 53.5 42.1 0 51.9 49.5 38.2 Optimum Mo loading for achieving 90+%, 95+%, or 99+% HCN removal depends on processing temperature. Nevertheless, loading is preferably at least about 5%, more preferably at least about 10%, still more preferably at least 20%, a range of 5-30% covering most optimum conditions. EXAMPLE 4 Table VI below shows the effect of surface and bulk density of various titania materials from different suppliers. Conditions were the same as for the molybdenum loading tests. % HCN Removal At Surface Area (m 2 /gm) Bulk Density (gm/cc) 177°C (350°F) 163°C (325°F) 149°C (300°F) Supplier A 57.9 51.6 NA 50 --- Supplier B 86.4 78.2 60.7 250 .16 Supplier C 95.0 87.2 69.9 250 .22-.25 Table VI shows that a slightly higher bulk density material removes a greater proportion of the HCN. In tests conducted with Mo/TiO 2 or Zn/TiO 2 , no hydrocarbon synthesis was effected. In the event Fischer-Tropsch synthesis may occur prematurely during the conduct of HCN removal, a Fischer-Tropsch synthesis suppressant may be added to the HCN removal process. Such suppressants, elements and compounds are well known and may include those containing Group IA metals, such as sodium; Group VIB elements, such as sulfur or oxygen; Group VB elements, such as nitrogen or phosphorous, or Group VIIB halides, such as chloride. Also, ammonia gas may be employed (a Group VB compound) as a Fischer-Tropsch suppressant. Of course, care must be taken to prevent carry over of the suppressant to the Fischer-Tropsch reaction where the suppressant would poison or otherwise deleteriously affect Fischer-Tropsch catalytic activity. Consequently, preferred suppressants are those that can be added to the Group IVA metal oxide and are not easily removed therefrom, e.g., sodium, or a gas, e.g., ammonia, which may be easily scrubbed from the ensuing synthesis gas product.

Claims: A process for decreasing the HCN concentration of an HCN-containing synthesis gas which comprises passing the synthesis gas in contact with a composition comprising a Group IVA metal oxide and a promoter metal component whereof the metal is selected from zinc, vanadium, niobium, molybdenum and mixtures thereof, wherein the composition has been activated with a hydrogen-containing gas at a temperature of at least 225°C. The process of claim 1 wherein the Group IVA metal oxide is or comprises titania. The process of claim 1 or claim 2 wherein the promoter metal component is present in amounts in a range of from 2 to 30% by weight. The process of any one of claims 1 to 3 wherein a suppressant for Fischer-Tropsch synthesis is present The process of claim 4 wherein the said Fischer-Tropsch suppressant is a material selected from the group consisting of Group IA, Group VIB, Group VB and Group VIIB elements and compounds, and/or ammonia. The process of any one of claims 1 to 5 conducted at a temperature of less than 180°C. The process of any one of claims 1 to 6 wherein at least 90% of the HCN is removed and a synthesis gas of reduced HCN concentration is recovered. The process of any one of claims 1 to 7 wherein the composition has been activated at a temperature in a range of from 225 to 350°C. The process of any one of claims 1 to 8 wherein a synthesis gas of reduced HCN concentration is recovered and is passed to a Fischer-Tropsch synthesis reaction step operating at Fischer-Tropsch synthesis conditions. The process of claim 9 wherein Fischer-Tropsch synthesis conditions include pressures in a range of from 1 to 100 bar and temperatures in a range of from 180 to 400°C.

Inventors: CORKERN KEITH EDWARD, KOVEAL RUSSELL JOHN JR, CORKERN, KEITH EDWARD, KOVEAL, RUSSELL JOHN, JR.
Applicants: EXXON RESEARCH ENGINEERING CO, EXXON RESEARCH AND ENGINEERING COMPANY
Application year: 1996
Main classification: C01B3
IPC codes: B01D53, C10K1, C01B3