Title of Invention

A PROCESS FOR ACTIVATION OF A FISCHER-TROPSCH CATALYST

Abstract

(57) Abstract: The invention provides a process for activation of a Fischer-Tropsch catalyst packed in a bed or for re-activation of an at least partially deactivated Fischer-Tropsch catalyst packed in a bed by contacting the catalyst prior to operation with a reducing gas such as herein described at a temperature below 500°C, wherein the reducing gas is passed through the catalyst bed in a direction reversed to the direction of the flow of reactants during operation. PRICE: THIRTY RUPEES

Full Text

The present invention relates to a process for activation of a Fischer-Tropsch catalyst which is used for the conversion of a mixture of carbon monoxide and hydrogen into hydro¬carbons. The present invention also relates to a process for reactivating such catalyst after it has been used and has been at least partially deactivated. The preparation of hydrocarbons from a mixture of carbon monoxide and hydrogen at elevated temperature and pressure in the presence of a suitable catalyst is generally known as the Fischer-Tropsch hydrocarbon synthesis. Catalysts used in this hydrocarbon synthesis are normally referred to as Fischer-Tropsch catalysts and usually comprise one or more metals from Group VIII of the Periodic Table of Elements, optionally together with one or more promoters, and, typically a carrier material. In order to be suitable in the conversion of a H2/CO mixture into hydrocarbons the Fischer-Tropsch catalyst is normally first subjected to an activation treatment. Activation generally is carried out by contacting the catalyst with a reducing gas, such as a hydrogen-containing gas. For instance, in US-A-4,413,064 .a process for preparing a Fischer-Tropsch catalyst is disclosed, whereby after the last impregnation step the catalyst is activated by slowly reducing it in the presence of hydrogen at a temperature from about 250 °C to 400 °C. The hydrogen source may be pure hydrogen or a mixture of hydrogen and nitrogen. Similarly, in EP-A-O,168,894 a method for activating a Fischer-Tropsch catalyst is disclosed which involves contacting the catalyst with a hydrogen-containing gas at elevated temperature, whereby the hydrogen partial pressure is gradually or step-wise increased from an initial value to an ultimate value which is at least 5 times as high as said initial value. The activation procedures according to both US-A-4,413,064 and EP-A-0,168,894 are conducted on the catalyst before loading it into a fixed bed. In situ activation of catalyst, i.e. activation of catalyst after it has been loaded into the reactor where the Fischer-Tropsch synthesis will take place, is a known procedure. It involves passing a stream of reducing gas, usually a hydrogen-containing gas, through the catalyst bed in the same direction as the flow of reactant gas during operation, thereby activating the catalyst particles. One of the disadvantages of this method is that the catalyst activity decreases in the direction of the gas flow. This is mainly caused by the fact that water is formed as a reaction product in the reduction of the inactive Group VIII metal compound into its catalytically active form. For this reason the off-gas stream, i.e. the gas stream leaving the reactor after the activation, contains water. The formed water is passed with the reducing gas through the catalyst bed. However, without wishing to be bound by a particular theory, it would appear that water inhibits the reduction of Group VIII metal compound(s) and as a result the degree of reduction, and thus the activity of the catalyst, decreases in the direction of the reducing gas flow. During normal operation, therefore, the level of conversion also decreases in the direction of the reactant gas flow. This can be directly measured by determining the temperature profile along the catalyst bed. The Fischer-Tropsch synthesis reaction is strongly exothermic and accordingly a lot of heat is generated. The temperature profile shows that the temperature is the highest in that part of the catalyst bed first contacted with the reactant gas flow and decreases as the reactant gas flow passes further through the catalyst bed. This shows that the conversion level indeed decreases as the reactant gas flow passes through the catalyst bed. In US-A-4,778, 826 this problem was also recognised. It relates to a process for converting a feedstock of C1-C3 alkane into higher molecular weight hydrocarbons by first reacting the C1-C3 alkane with air to form a gas mixture comprising carbon monoxide, hydrogen and nitrogen and subsequently converting this gas mixture into the said higher molecular weight hydrocarbons via a Fischer-Tropsch synthesis reaction. As a solution to the above mentioned problem occurring with in situ activation, it is proposed to perform the Fischer-Tropsch synthesis reaction by passing the gas mixture through an elongated reactor packed with a bed of Fischer-Tropsch catalyst, of which the activity increases from the inlet to the outlet of the reactor. According to US-A-4,778,826 such activity gradient can be achieved in three ways. Firstly, by dilution of the Fischer-Tropsch catalyst particles with inert particles, whereby the degree of dilution decreases from the inlet to the outlet of the reactor. Secondly, by applying a temperature gradient along the catalyst bed, whereby the temperature increases from the inlet to the outlet of the reactor. Finally, by increasing the concentration of the catalytically active component in the catalyst particles from the inlet to the outlet of the reactor. Actual activation of the catalyst particles is suitably carried out by treatment with a reducing agent, such as hydrogen, at a temperature of from 320 to 440 °C. This is implemented in practice by heating the reactor to the appropriate temperature and passing a stream of hydrogen through the catalyst bed in a top to bottom direction, i.e. in the same direction as the flow of reactants during actual operation. The method according to US-A-4,778,826 evidently requires additional measures, e.g. careful loading of the catalyst bed, in order to ensure the desired activity gradient to occur. It is one of the main objectives of the present invention to avoid such additional measures and still accomplish a more constant conversion level throughout the entire catalyst bed. More specifically, the present invention aims to provide an activation process whereby an activity gradient along the catalyst bed is obtained with increasing activity in the direction of the reactant gas flow. Furthermore, the present invention aims to keep the activation process as simple as possible and to avoid large capital expenditures for adapting existing equipment. It will be clear that this is desirable from both an efficiency and cost perspective point of view. In fact, it is an objective of the present invention to provide an activation method, which requires hardly any adaptation of existing equipment in order to still achieve the desired activity gradient along the catalyst bed. Accordingly, in a first aspect the present invention relates to a process for activation, preferably in-situ activation, of a Fischer-Tropsch catalyst packed in a bed by contacting the catalyst prior to operation, i.e. prior to operating the catalytic Fischer-Tropsch hydrocarbon synthesis process, with a reducing gas at a temperature below 500 °C, characterised in that the reducing gas is passed through the catalyst bed in a direction reversed to the direction of the flow of reactants during operation. Fischer-Tropsch catalysts and methods to prepare them are known in the art. Usually such catalysts comprise one or more metals from Group VIII of the Periodic Table of Elements on a suitable carrier, optionally together with one or more promoters. Examples of such catalysts and methods for preparing them are disclosed in EP-A-0,428,223 and EP-A-0,510,771. Also in the patent specifications discussed above, examples of suitable catalysts are described. A preferred catalyst to be activated or reactivated according to the process of the present invention comprises a cobalt, iron, nickel or ruthenium metal compound or mixtures thereof. Most preferably, the catalyst comprises a cobalt metal compound, in particular a cobalt oxide. The metal compound is typically supported on a catalyst carrier. A suitable catalyst carrier may be chosen from the group of refractory oxides, preferably, alumina, silica, titania, zirconia or mixtures thereof, more preferably, silica, silica-zirconia mixtures, titania or zirconia. The amount of catalytically active metal present on the carrier is typically in the range of from 1 to 100 parts by weight, preferably 10 to 50 parts by weight, per 100 parts by weight of carrier material. The catalytically active metal may be present in the catalyst together with one or more metal promoters or co-catalysts. The promoters may be present as metals or as the metal oxide, depending upon the particular promoter concerned. Suitable promoters include oxides of metals from Groups IIA, IIIB, IVB, VB, VIB and/or VIIB of the Periodic Table, oxides of the lanthanides and/or the actinides. Preferably, the catalyst comprises at least one oxide of an element in Group IVB, VB and/or VIIB of the Periodic Table, in particular titanium, zirconium, manganese and/or vanadium. As an alternative or in addition to the metal oxide promoter, the catalyst may comprise a metal promoter selected from Groups VIIB and/or VIII of the Periodic Table. Preferred metal promoters include rhenium, platinum and palladium. A most suitable catalyst comprises cobalt as the catalytically active metal and zirconium as a promoter. Another most suitable catalyst comprises cobalt as the catalytically active metal and manganese and/or vanadium as a promoter. The promoter, if present in the catalyst, is typically present in an amount of from 0.1 to 60 parts by weight, preferably from 0.5 to 40 parts by weight, per 100 parts by weight of carrier material. It will however be appreciated that the optimum amount of promoter may vary for the respective elements which act as promoter. If the catalyst comprises cobalt as the catalytically active metal and manganese and/or vanadium as promoter, the cobalt : (manganese + vanadium) molar ratio is advantageously at least 12:1. A particularly preferred Fischer-Tropsch catalyst is a Co/Zr02/Si02 catalyst, i.e. cobalt as the catalytically active metal on a carrier comprising silica admixed with zirconium oxide. Prior to activation the cobalt is usually present as cobalt oxide. By reducing the cobalt oxide the catalytically active cobalt is obtained. The reducing gas employed in principle may be any gas having reducing properties. It is, however, preferred to use a hydrogen-containing gas. For the purpose of this specification, a hydrogen-containing gas is a gas containing hydrogen and, optionally, one or more inert gas components like nitrogen. A synthesis gas mixture, comprising hydrogen and carbon monoxide, is not included in the term hydrogen-containing gas as used herein. It will, however, be appreciated that a synthesis gas mixture is a reducing gas in its own right and may be used as such in the process of the present invention. If the catalyst to be activated comprises iron, it is preferred to use a synthesis gas mixture. When the catalyst is activated by contacting it with a hydrogen containing gas, water is formed in the reduction reaction and this water flows along with the flow of hydrogen-containing gas through the catalyst bed. Accordingly, the water content of the reducing gas stream increases as the reducing gas passes through the catalyst bed. Since water inhibits the reduction reaction, the activation of catalyst may increasingly be hampered in the direction of the flow of reducing gas, if gas rate and hydrogen content of the reducing gas are kept constant. In order to minimise this effect, it is preferred to increase the amount of hydrogen passing through the catalyst bed during activation in such way that the water content in the gas stream leaving the catalyst bed after activation, i.e. the off-gas, is kept below a certain level. This level may depend on the catalyst being (re-)activated and can be determined by routine experimentation. In general, the water content in the off-gas is preferably kept below or at 60 mbar. More preferably, the water content of the off-gas is kept below 50 mbar. However, catalysts comprising a silica-containing carrier tend to be sensitive to too high quantities of steam present during (re-)activation. Thus, if a catalyst is to be (re-)activated comprising a silica-containing carrier, the quantity of steam present in the hydrogen-containing offgas is preferably less than 40 mbar, more preferably less than 30 mbar. For some titania or zirconia-containing catalysts, the quantity of steam in the hydrogen-containing offgas may suitably be higher, for example in the range from 40-1000 mbar, preferably from 40-100 mbar. The amount of hydrogen passing through the catalyst bed can be increased either by increasing the gas rate of the reducing gas during activation, or the total pressure, while keeping the hydrogen content of the reducing gas at a constant level, or by increasing the hydrogen content of the reducing gas gradually or stepwise during activation. It will be clear that a combination of both may also be applied. Alternatively, or in combination with one or more of the above methods to control the water content in the off-gas, the temperature of the catalyst bed may be decreased or any temperature increase temporarily stopped by decreasing the cooling medium temperature. The catalytic Fischer-Tropsch hydrocarbon synthesis process is suitably carried out in a fixed bed operation and therefore the activation process described above is also suitably carried out via a fixed bed operation. However, it will be appreciated that also catalysts for use in catalyst beds different from fixed beds may be activated by the process of the present invention. The activation process itself is most suitably carried out in a fixed bed of catalysts. However, other catalyst beds, like moving beds, may also be applied in the activation process. The activation process is preferably carried out at a temperature below 450 °C, more preferably below 400 °C, more preferably below 300 °C. Typically, the activation process is carried out at a temperature above 150 °C, preferably above 200 °C. The pressure at which the process is carried out typically may range from 1 to 150 bar abs., preferably from 1 to 60 bar abs., more preferably from 1 to 20 bar abs. The gas rate, that is the Gas Hourly Space Velocity may typically range from 100 to 3000 Nl/l/h, preferably from 200 to 1500 Nl/l/h. The activation process is typically carried out for a period sufficient to substantially activate the catalyst. It will be appreciated that this period may vary, depending on the composition of the catalyst, the average reaction temperature, the gas rate, and the reducing gas partial pressure. Typically, the catalyst is contacted with the reducing gas for 0.5 to 150 hours, preferably for 8 to 120 hours, more preferably for 16 to 96 hours. According to a preferred embodiment, the catalyst is contacted with the reducing gas until at least 25% by weight, preferably at least 50 % by weight, more preferably at least 80% by weight, of the Group VIII metal compound is reduced to the metallic state. The guantity of the Group VIII metal compound that has been reduced can suitably be monitored by measurement of the cumulative water production during the process. Other methods known to those skilled in the art include Thermogravimetric Analysis and Temperature Programmed Reduction. As set out above, the temperature, gas rate (GHSV) and the content (partial pressure) of the reducing gas may be varied in order to control the activation process. It will be appreciated that it belongs to the skill of the skilled person to select the most appropriate way to control the activation process for a particular catalyst by routine experimentation. According to one typical activation process scenario, the temperature, total pressure and total gas rate are kept constant and the reducing gas content, preferably the hydrogen content, is gradually or step-wise increased from 1% up to e.g. 85% by volume or higher, preferably up to 100% by volume. According to another embodiment, the temperature is continuously or step-wise increased from at least 150 °C up to e.g. at most 400 °C at a rate in the range from 0.5 to 5 °C/min. In US-A-4,605,676 and US-A-4,670,414 methods for activating a Fischer-Tropsch catalyst are disclosed involving the successive steps of reduction in hydrogen, oxidation in an oxygen-containing gas and activation of the catalyst by reduction in hydrogen. All steps are typically performed at temperatures between about 100 and 450 °C. The activation method is referred to as the "ROR treatment". The activation process according to the present invention as described above can very suitably be applied as the activation step and/or the first reduction step in the ROR treatment. Thus, according to a further aspect, the present invention relates to a process for activation of a Fischer-Tropsch process by contacting the catalyst successively with (a) a reducing gas; {b) an oxidising gas; and wherein step (a) and/or step (c), preferably step (c), is carried out as described hereinbefore. Preferably, at least step (c) is carried out in-situ, more preferably steps (a) to (c). It will be appreciated that it is also possible to conduct step (b) such that the direction of flow of oxidising gas is reversed to the direction of flow of reactants during operation. After the Fischer-Tropsch catalyst has at least partially been deactivated after operation, it may be re¬activated for repeated use. Suitably, the activation process of the present invention is used to re-activate the catalyst. Thus, according to a further aspect the present invention relates to a process for re-activation of an at least partially deactivated Fischer-Tropsch catalyst packed in a bed by contacting the catalyst with a reducing gas at a temperature below 500 °C, wherein the reducing gas is passed through the catalyst bed in a direction reversed to the direction of the flow of reactants during operation. It has been found that the ROR treatment is also very suitable for re-activation of at least partially deactivated Fischer-Tropsch catalyst. Without wishing to be bound by a particular theory, it would appear that the ROR treatment, when used for re-activation, works as follows. The first step of the ROR treatment basically involves stripping with hydrogen to remove heavy wax and/or carbonaceous particles, which have precipitated onto the catalyst particles during operation, and slow reduction of the catalyst. In the subsequent oxidation step any carbonaceous particles still present on the catalyst are oxidised into carbon dioxide and water and the catalytically active metal is oxidised- Finally, in the activation step, the oxidised catalyst is converted into its active form by reduction and hence is ready again for operation. The activation process according to the present invention as described above can very suitably be applied as the activation step in the ROR treatment. Accordingly, the present invention also relates to a process for the reactivation of an at least partially deactivated Fischer-Tropsch catalyst packed in a bed comprising the successive steps of: (a) contacting the catalyst with a reducing gas, in particular a hydrogen-containing gas; (b) contacting the catalyst with an oxidising gas; and (c) contacting the catalyst with a reducing gas (that is, reducing the catalyst) , characterised in that step (c) is performed according to the activation process described above as an aspect of the present invention. It will be appreciated that it is also possible to conduct step (b) such that the direction of flow of oxidising gas is reversed to the direction of flow of reactants during operation. In fact in the process for re-activation of at least partly deactivated Fischer-Tropsch catalysts this may be preferred if for example relatively high amounts of water are produced as a result of oxidation of carbonaceous particles. The presence of high amounts of water may e.g. induce formation of metal-support compoi the above ROR process is further characterised in that the oxidising gas of step (b) is passed through the catalyst bed in a direction reversed to the direction of the flow of reactants during operation. According to one embodiment of the invention the oxidation step is carried out such that the amount of water present in the off-gas is kept within the limits as discussed above, but higher or lower amounts may also be preferred. In principle it is also possible to conduct step (a) such that the direction of flow of reducing gas is reversed to the direction of flow of reactants during operation. However, as step (a) in the ROR process for re-activation of an at least partly deactivated Fischer-Tropsch catalyst, mainly comprises removal of carbonaceous particles and heavy wax, no substantial improvement will be encountered when operating step (a) in this way. All steps are preferably carried out at a temperature between 150 and 400 °C, more preferably between 200 and 300 °C. As regards steps (a) and (b) the conditions as described in the before-mentioned two U.S. patents are applicable. It is preferred to use a hydrogen-containing gas as the reducing gas in step (a) and to use an oxygen-containing gas as the oxidising gas in step (b). An example of a suitable oxygen-containing gas is diluted air, i.e. air diluted with an inert gas such as nitrogen. Preferably, the oxygen-containing gas contains from 0.1 to 10% by volume of oxygen, more preferably from 0.2 to 5% by volume. The amount of oxygen is preferably kept within the above range as a way to control the oxidation step. The operating conditions for conducting step (b) are preferably in the same range as set out hereinbefore with respect to the activation (reduction) process. In principle, it is also possible to use an oxygen- containing gas containing a higher amount of oxygen, such as air. It will be appreciated by those skilled in the art that in order to control the oxidation reaction, operating conditions may then have to be adapted. As regards step (a), preferably the operating conditions are within the same range as set out hereinbefore with respect to the activation (reduction) process, being step (c) of the ROR treatment. It will however be appreciated that if the ROR treatment is used for re-activation of an at least partially deactivated catalyst, a high percentage of Group VIII metal on the catalyst will already be in the metallic state. Nevertheless, it is preferred to contact the catalyst with the reducing gas for 0.5 to 150 hours, more preferably for 8 to 120 hours, most preferably for 16 to 96 hours. Further/ according to one preferred embodiment, the hydrogen content in the hydrogen-containing gas, and other operating conditions, like the temperature, are kept constant during step (a) of the ROR treatment, when used for re-activation of an at least partially deactivated catalyst. The hydrogen partial pressure in step (a) of the treatment is preferably less than 15 bar abs., more preferably less than 10 bar abs. Accordingly the present invention provides a process for activation of a Fischer-Tropsch catalyst packed in a bed or for re-activation of an at least partially deactivated Fischer-Tropsch catalyst packed in a bed by contacting the catalyst prior to operation with a reducing gas such as herein described at a temperature below 500°C, wherein the reducing gas is passed through the catalyst bed in a direction reversed to the direction of the flow of reactants during operation. The invention is farther illustrated by the following examples. Example 1 The catalyst used was an 1.7 mm trilobe Fischer-Tropsch catalyst comprising 23% by weight Co, 10% by weight of Zr02 and 56% by weight of SiO2 based on fully oxidised catalyst. The experiments were carried out in a single tube pilot plant equipped with two reactors connected in series. Each reactor had a length of 4 meters. The volume of each catalyst bed was 1950 ml. The reversed flow activation was conducted as follows. A reducing gas was passed through the catalyst beds at 250°C and 4 bar in the direction reversed compared to the direction of the gas flow during normal operation. The reducing gas was a nitrogen/hydrogen mixture and during activation the hydrogen partial pressure was raised in such a way that the water content in the off-gas stayed below 5000 ppmv. The maximum hydrogen content of the reducing gas stream was 75% volume. The gaseous hourly space velocity (GHSV) was 600 Nl/1/hour. Total reduction time was 48 hours. Exact conditions are listed in table 1. A CO/H2 containing gas was subsequently passed over the activated catalyst obtained in the way described above. The conditions applied and the results as determined after 50 hours of operation are listed in table 3, while the temperature profile along the catalyst bed as measured after 50 hours of operation is depicted in figure 1. Comparative Example 1 The same catalyst as used in Example 1 was activated by passing the reducing gas through the catalyst bed in the same direction as the gas flow during normal operation (normal flow activation). The conditions are listed in table 2. A CO/H2 containing gas was subsequently passed over the activated catalyst obtained in the way described above. The conditions applied and the results as determined after 50 hours of operation are listed in table 3, while the temperature profile along the catalyst bed as measured after 50 hours of operation is depicted in figure 1. From Table 3 it can be seen that the reversed flow activation according to the present invention results in a better catalyst performance. At an even lower weight average bed temperature (WABT), namely, both C1+ yield and C5+ selectivity of the catalyst activated via reversed flow activation are higher than the catalyst activated via normal activation. C1+ yield is indicated as Space Time Yield (STY), that is the amount of hydrocarbons containing two or more carbon atoms in grams produced per litre catalyst per hour. In figure 1 the temperature profiles along the two 4 m reactors as measured after 50 hours of operation is given for both reversed activated catalyst and normally activated catalyst. Along the vertical axis the difference between the temperature of the coolant and the temperature within the reactor during the hydrocarbon synthesis reaction is indicated, while along the horizontal axis the distance from the top of the upper reactor is indicated. From figure 1 it can be seen that tne temperature profile along the reactors containing reversed activated catalyst (RAC reactors) is more flat than the temperature profile along the reactors containing normally activated catalyst (NAC reactors) during operation, which implies that the temperature within the RAC reactors during the hydrocarbon synthesis reaction is more constant than the temperature within the NAC reactors. This in return indicates that the activity of the reversed activated catalyst is more constant along the entire length of the reactors, as a result of which the conversion level also fluctuates less. As can be seen from table 3, this also positively influences the overall conversion level: the final space time yield of the reaction in the RAC reactor is higher than that of reaction in the NAC reactor, at a WE CLAIM : A process for activation of a Fischer-Tropsch catalyst packed in a bed or for re-activation of an at least partially deactivated Fischer-Tropsch catalyst packed in a bed by contacting the catalyst prior to operation with a reducing gas such as herein described at a temperature below 500°C, wherein the reducing gas is passed through the catalyst bed in a direction reversed to the direction of the flow of reactants during operation. The process according to claim 1, wherein the reducing gas is a hydrogen-containing gas. The process according to claim 2, wherein the reducing gas is a mixture of nitrogen and hydrogen. The process according to claim 2 or 3, wherein the amount of hydrogen passing through the catalyst bed during activation is increased, preferably in such way that the water content in the gas stream leaving the catalyst bed after activation does not exceed 60 mbar. The process according to any one of the preceding claims, wherein the Fischer-Tropsch catalyst comprises cobalt. The process as claimed in any one of the preceding claims, wherein the activation or reactivation is carried out in-situ. A process for activation of a Fischer-Tropsch catalyst substantially as herein described and exemplified with reference to the accompanying drawings.

Documents:

1563-mas-95 abstract.pdf

1563-mas-95 claims.pdf

1563-mas-95 correspondence-others.pdf

1563-mas-95 correspondence-po.pdf

1563-mas-95 description(complete).pdf

1563-mas-95 drawings.pdf

1563-mas-95 form-1.pdf

1563-mas-95 form-26.pdf

1563-mas-95 form-4.pdf

1563-mas-95 petition.pdf