PROCESS FOR THE PREPARATION OF HYDROCARBONS

Abstract: In the preparation of hydrocarbons from syngas over a Co/Zr/SiO₂ catalyst which has been prepared by impregna­ tion and/or kneading, the catalyst is applied in the form of a fixed bed which meets a specified relation between its external (S e ) and internal (S i ) surface. In order to show a high C ⁺ / ₅ selectivity the catalyst bed should have an S e (between 5 and 70 cm²/ml) and an S i (between 10 and 400 m²/ml) such that the following relation is satisfied: 10⁶ > S e ² x S i > 2.5 x 10⁴.

Description: PRDCESS FOR THE PRPARATION OF HYDR0CARBONS The invention relates to a process for the preparation of hydrocarbons by catalytic reaction of carbon monoxide with hydrogen. The preparation of hydrocarbons from an H2/CO mixture by contacting this mixture at elevated temperature and pressure with a catalyst is knawn in the literature as the Fischer-Tropsch hydrocarbon synthesis process. Catalysts frequently used for this purpose contain one or more metals of the iron group together with one or more promoters and a carrier. For the preparation of these catalysts the known preparation techniques such as precipitation, impregnation, kneading and smelting are suitable. The products which can be prepared with these catalysts usually possess a very wide molecular weight distribution and in addition to branched and unbranched paraffins often contain considerable quantities of olefins and oxygen-containIng organic compounds. As a rule only a minor portion of the resultant products consists of middle distillates. Besides the yield, the pour point of these middle distillates also leaves something to be desired. In view of this, the direct conversion of H2/CO mixtures by the Fischer-Tropsch process is not a particularly attractive route for the preparation of middle distillates on the technical scale. In this patent application, the term "middle distillates" is understood as hydrocarbon mixtures having a boiling range principally corresponding to that of the kerosine and gas oil fractions which are obtained in the conventional atmospheric distillation of crude petroleum. The middle distillate range extends principally between approx. 150 and 360 OC. Recently a class of Fischer-Tropsch catalysts was discovered which possess the property of yielding a product containing only very few olefins and cxygen-contaIning organic compounds and consisting virtually completely of unbranched paraffins a substantial proportion of which have a boiling point above the middle distillate range. It has been found that the high-boiling portion of this product can be converted at high yield into middle distillates by means of hydrocracking. The feedstock chosen for the hydrocracking treatment is at least that portion of the product of which the initial boiling point is above the final boiling point of the heaviest middle distillate desired as end product. The hydrocracking treatment, which features a very low hydrogen consumption, yields middle distillates with a considerably better poor point than those obtained in the direct conversion of an H2lCO mixture by the Fischer-Tropsch process. The Fischer-Tropsch catalysts belonging to the abovementioned class contain silica, alumina or silica-alumina as carrier and cobalt together with zirconium, titanium and/or chrcmium as catalytically active metals in such proportions that the catalysts contain 3-60 parts by weight of cobalt and 0.1-100 parts by weight of zirconium, titanium and/or chrcnium per 100 parts by weight of carrier. The catalysts are prepared by applying the metals concerned to the carrier by means of kneading and/or impregnation. For further information concerning the preparation of these catalysts by kneading and/or impregnation reference is made to the Netherlands patent application No. 8301922 recently filed in the name of the Applicant. A further investigation into the use of above-rrntioned cobalt catalysts in the form of a fixed bed has now disclosed that + their performance in terms of C5 selectivity is in large measure dependent on the external and internal surface area of the catalyst bed. It has been found that an optimum performance in terms of C5 selectivity can be obtained if the catalyst has an external surface area (Se) between 5 and 70 cm2/ml and an internal surface area (Si) between 10 and 400 m2/ml such as to satisfy the relationship 106 > 5e2 x Si > 2.5 x 104. The present patent application therefore relates to a process for the preparation of hydrocarons by catalytic reaction of carbon monoxide with hydrogen, in which a mixture of carbon monoxide and hydrogen is contacted at elevated temperature and pressure with a catalyst which contains 3-60 parts by weight of cobalt and 0.1-100 parts by weight of at least one other metal selected fran zirconium, titanium and cranium per 100 parts by weight of silica, alumina or silica-alumina, and which catalyst is prepared by kneading and/or impregnation, the catalyst being present in the form of a fixed bed and the catalyst bed having an external surface area (Se) between 5 and 70 cm2/ml and an internal suface area (Si) between 10 and 400 mê/ml such as to satisfy the relation ship 106 > Seê x Si > 2.5 x 104. The external surface area (S ) of the catalyst bed can be found for a representative sample with a given volume expressed in ml by determining the external surface area expressed in aQ2 of each of the catalyst particles present therein, Suing the external surface areas found and dividing the sum by the volume of the sample. The internal surface area Si of the catalyst bed is determined by means of nitrogen adsorption. In the process according to the invention, use is preferentially made of the cobalt catalysts which are the subject of the Netherlands patent application No. 8301922. These are catalysts which satisfy the relationship L (3 + 4 R) > - > (0.3 + 0.4 R), wherein S. L = the total quantity of cobalt present on the catalyst, expressed in mg of Co/ml, the the internal surface area of the catalyst, expressed in m2/ml, and R = the weight ratio between the quantity of cobalt applied to the catalyst by kneading and the total quantity of cobalt present on the catalyst. In the process according to the invention, preferential use is further made of cobalt catalysts prepared by any one of the three following procedures: a) cobalt is first applied by impregnation in one or more steps and subsequently the other metal is likewise applied by impregnation in one or more steps b) the other metal is first applied by impregnation in one or more steps and subsequently cobalt is likewise applied by imoregnation in one or mare steps, and c) cobalt is first applied by kneading in one or more steps and subsequently the other metal is applied by impregnation in one or more steps. In the process according to the invention, preferential use is further made of cobalt catalysts containing 15-50 parts by weight of cobalt per 100 parts by weight of carrier. The quantity of the other metal preferentially present in the cobalt catalysts depends on the manner in which this metal is applied. In the case of catalysts where first the cobalt and then the other metal is applied to the carrier, preference is given to catalysts containing 0.1-5 parts by weight of the other metal per 100 parts by weight of carrier. In the case of catalysts where first the other metal and then the cobalt is applied to the carrier, preference is given to catalysts containing from 5 to 40 parts by weight of the other metal per 100 parts by weight of carrier. For the other metal it is preferred to use zirconium and for the carrier it is preferred to use silica. According to the invention, in order to achieve optimum performance in terms of C5+ selectivity, the Se and Si of the catalyst bed must satisfy the following requirewents: Se = 5¯70 cm2/ml S. = 10-400 m2/ml, and 16 2 > 2.5 x 10 106 > Seê x Si > 2.5 x 104. It is preferred to use a catalyst bed which satisfies: S = 10-50 om2/ml, e Si = 15-200 mê/ml, and 2.5 x 105 > Seê x Si > 3 x 104. Before beccming suitable for utilization in the preparation of hydrocarbons from an H2/CO mixture, the cobalt catalysts have to be activated. This activation can suitably be carried out by contacting the catalysts at a temperature between 200 and 350 C with hydrogen or a hydrogen-containing gas. The conversion of the H2/CO mixture into hydrocarbons according to the invention is preferentially carried out at a temperature of 125 to 350 OC, and in particular of 175 to 275 OC, and a pressure of 5 to 500 bar, in particular of 10 to 75 bar. H2/CO mixtures which are suitable for conversion according to the invention into hydrocarbons can be very suitably obtained starting fran light hydrocarbons such as methane by means of steam reforming or partial oxidation. Special preference is given to the use of natural gas as feedstock for the preparation of the H2/CO mixture. The H2/CO mixture converted according to the invention into hydrocarbons preferentially possesses an H2/CO molar ratio of more than 1.5. If the feedstock possesses an H2/CO molar ratio of less than 1.5, it is preferentially raised to a value between 1.5 and 2.5 and in particular to a value between 1.75 and 2.25 before being contacted with the cobalt catalyst. Raising of the H2/CO molar ratio of hydrogen-lean H2/CO mixtures can be performed, inter alia, by the addition of hydrogen, removal of carbon monoxide, admixture of a hydrogen-rich H2/Co mixture or by applying the CO shift reaction to the hydrogen-lean H2/CO mixture. As has already been mentioned above, when used for the conversion of an H2/CO mixture the present cobalt catalysts yield a substantially paraffinic product of which the highboiling portion can be converted at high yield into middle distillates by means of a hydrocracking treatment. The feedstock chosen for the hydrocracking treatment is at least that portion of the product of which the initial boiling point is above the final boiling point of the heaviest middle distillate desired as end product. Although in the preparation of middle distillates the product obtained over the cobalt catalyst it is sufficient to use as feedstock for the hydrocracking treatment that portion of the product of which the initial boiling point is above the final boiling point of the heaviest middle distillate desired as end product, for this purpose it is preferred to use the total Cg fraction of the product prepared over the cobalt catalyst because it has been found that under the influence of the catalytic hydrogen treatment a quality improvement takes place in the gasoline, kerosine and gas oil fractions present therein. The hydrocracking treatment is carried out by contacting the fraction to be treated, at elevated teeperature and pressure and in the presence of hydrogen, with a catalyst containing one or more noble metals of Group VIII on a carrier. The hydrocracking catalyst used is preferentially a catalyst containing 0.1-2% by weight and in particular 0.2-1% by weight of one or more noble metals of Group VIII on a carrier. Preference is given to catalysts containing as Group VIII noble metal platinum or palladium, and silica-alumina as carrier. The hydrocracking treatment is preferentially carried out at a temperature of 200-4000C and in particular of 250-350 OC and a pressure of 5-200 bar and in particular of 10-75 bar. The invention will now be elucidated with reference to the following Example. MP1E Nine Co/Zr/SiO2 catalysts (catalysts 1-9) were prepared by impregnation of six spherical silica carriers (silicas A-F) with solutions of cobalt and zirconium compounds. At each impregnation step a quantity of solution was used of which the volume substantially corresponded to the pore volume of the carrier concerned. After each impregnation step the solvent was removed by heating and the material was calcined at 500 OC, After the final calcination the compositions we e activated in hydrogen as follows: catalysts 1-3, 6, 8 and 9 at 260 C and catalysts 4, 5 and 7 at 250 OC. Catalysts 1-9 were prepared as follows. Catalysts 1, 2, 3, 6, 8 and 9 One-step or multi-step impregnation of a silica carrier with a solution of zirconium tetra-n-prcpcxide in a mixture of n-propanol, toluene and acetyl acetone, followed by one-step impregnation of the zirconium-loaded carrier with an aqueous solution of cobalt nitrate. In the preparation of catalysts 1 and 3, the zirconium impregnation was carried out in three steps; catalysts 6, 8 and 9 were prepared via two-step zirconium impregnation; in the preparation of catalyst 2, zirconium impregnation took place in one step. Catalysts 4, 5 and 7 One-step impregnation of a silica carrier with an aqueous solution of cobalt nitrate, followed by one-step impregnation of the cobalt-loaded carrier with an aqueous solution of zirconium nitrate. Further particulars of catalysts 1-9 are shown in Table I. Catalysts 1-9 were used in nine experiments (experiments 1-9) in the preparation of hydrocarbons from a mixture of carbon monoxide and hydrogen having an H2/CO molar ratio of 2. The e experiments were carried out at a pressure of 20 bar in a reactor in which there was a fixed catalyst bed. The other conditions under which these experiments were carried out, together with the results of the experiments, are shown in Table II. Of the experIments shown in Table II, only experiments 7-9 are experiments according to the invention. In these experiments the catalyst bed satisfied the relationship 106 > Seê xSi > 2.5 x 104 and a high C5+ selectivity was achieved.Experiments 1-6 come outside the scope of the invention. They are included in the patent application for purposes of comparison. In these experiments using a catalyst bed satisfying Seê x Si < 2.5 x 104, a relatively low C5+ selectivity was observed. TABLE I Catalyst. No. 1 2 3 4 5 6 7 8 9 Silica carrier A B B A C C D E F Zr load, ppw Zr/100 ppw SiO2 18 6 18 0.9 0.9 12 0.9 12 12 Co load, mg Co/ml catalyst 41 33 101 36 93 102 97 94 40 Si, mê/ml 28 32 30 29 107 104 100 97 31 Se, cmê/ml 15 26 26 15 14 14 26 24 35 Seê x Si 6300 21632 20280 6525 20972 20384 67600 55872 40176 TABLE II Experiment No. 1 2 3 4 5 6 7 8 9 Catalyst. No. 1 2 3 4 5 6 7 8 9 Temperature, C 240 227 220 240 225 215 215 210 220 Space velocity, N1.1-1. hour-1 1000 900 1000 1000 900 900 900 900 900 Conversion of the H2/CO mixture, % by volume 57 60 53 52 64 65 61 58 63 C1+ production, g.1-1. hour-1 108 101 103 100 109 110 103 101 110 C5+ selectivity, % by weight 60 72 64 52 64 70 76 79 78

Claims: CLAIMS 1. Process for the preparation of hydrocarbons by catalytic reaction of carbon monoxide with hydrogen, characterized in that a mixture of carbon monoxide and hydrogen is contacted at elevated temperature and pressure with a catalyst which contains 3-60 parts by weight of cobalt and 0.1-100 parts by weight of at least one other metal selected frcm zirconium, titanium and chromium per 100 parts by weight of silica, alumina or silica-alumina, the catalyst being prepared by kneading and/or impregnaticn, in that the catalyst is present in the form of a fixed bed and in that the catalyst bed has an external surface area (Se) between 5 and 70 cmê/ml and an internal surface (Si) between 10 and 400 mê/ml such as to satisfy the relation ship 106 > Seê x Si > 2.5 x 104. 2. Process as claimed in claim 1, characterized in that the catalyst bed has an 5e between 10 and 50 cm2/ml and an S. between 15 and 200 m2/ml such as to satisfy the relationship 2.5 x 105 > Seê x Si > 3 x 104. 3. Process as claimed in claim 1 or 2, characterized in that the catalyst satisfies the relationship L (3 + 4 R) > - > (0.3 + 0.4 R), wherein Si L = the total quantity of cobalt present on the catalyst, expressed in mg of Co/ml, the the internal surface area of the catalyst, expressed in m2/ml, and R = the weight ratio between the quantity of cobalt applied to the catalyst by kneading and the total quantity of cobalt present on the catalyst. 4. Process as claimed in any one of claims 1-3, characterized in that the catalyst contains 15-50 parts by weight of cobalt per 100 parts by weight of carrier and either 0.1-5 parts by weight of the other metal if in the preparation first cobalt and then the other metal is applied or 5-40 parts by weight of the other metal if in the preparation first the other metal and then cobalt is applied. 5. Process as claimed in any one of claims 1-4, characterized in that the catalyst contains zirconium as the other metal and silica as carrier. 6. Process as claimed in any one of claims 1-5, characterized in that it is carried out at a temperature of 125-350 OC and a pressure of 5-100 bar. 7. Process as claimed in any one of claims 1-6, characterized in that the H2/CO mixture has an H2/CO molar ratio between 1.75 and 2.25. 8. Process as claimed in any one of claims 1-7, characterized in that in order to prepare middle distillates from the product prepared over the cobalt catalyst, at least that portion of which the initial boiling point is above the final boiling point of the heaviest middle distillate desired as end product is subjected to a hydrocracking treatment by contacting it at elevated temperature and pressure with a catalyst containing one or more noble metals of Group VIII on a carrier. 9. Process as claimed in claim 8, characterized in that in the hydrocracking treatment a catalyst is used containing 0.1-2% by weight of one or more noble metals of Group VIII. 10. Process as claimed in any one of claims 8-9, characterized in that in the hydrocracking treatment a catalyst is used containing platinum or palladium as noble metal of Group VIII and silicaalumina as carrier.

Inventors: POST, MARTIN FRANCISCUS MARIA, SIE, SWAN TIONG
Applicants: SHELL INTERNATIONALE RESEARCH MAATSCHAPPIJ B.V.
Application year: 1985
Main classification: C07C1
IPC codes: C10G47, C10G3, C07B61, B01J23, C10G2, C07C67, C07C1