METHOD FOR DESULFURIZING A GAS AND ABSORBENT SUITABLE FOR THIS METHOD

Abstract: The invention relates to a method for desulfurizing gases at increased temperature, in which the gas to be desulfurized is contacted with an absorbent based on two or more metal oxides, if desired provided on a support, which is characterized in that the gas to be desulfurized is contacted with an absorbent based on one or more molybdenum and/or tungsten oxides as a first metal component, with interaction occurring between said first metal oxide component and a second metal oxide component, and to an absorbent suitable for use in this method.

Description: Title: Method for desulfurizing a gas and absorbent suitable for this method. The present invention relates to a method for desulfurizing gases utilizing absorbents based on molybdenum and/or tungsten oxide, and to absorbents which are suitable for use in this method. It is generally known that it is necessary that gases which are further processed in chemical processes, are supplied to buyers or are released into the atmosphere, be purified of sulfur compounds, in particular hydrogen sulfide. A problem associated with the gasification of coal or heavy petroleum fractions is that hydrogen sulfide is formed. Hydrogen sulfide should be removed because of its corrosive character and its poisonousness to catalysts which are used to convert such gases into useful compounds . In the case where the sulfur-containing gas is used as fuel gas, for instance in power stations, the content of hydrogen sulfide, for environmental reasons and because of its corrosive ac-ion, should preferably be below 100 ppm. In the case where the gas is used as feed for catalytic conversions, the aim is to keep the H 2 S content below 25 ppm, preferably below 10 ppm. Accordingly, a number of methods are known which are directed to the removal of hydrogen sulfide from gas . There are a large number of methods known for the removal of hydrogen sulfide from gas streams by adsorption or absorption on solid substances. In the purification of gas streams coming from the gasification of coal or oil, for instance, involving particularly large amounts having a high temperature, often metal oxides are used for that purpose. Very typical are so-called bulk metal oxides, but the disintegration thereof during the absorption and regeneration results in dust formation, which renders these materials little attractive. The problem of disintegration can an least partly be overcome by the use of highly dispersed metal oxide on a stable suppor . For such purposes, it is known to use silicon dioxide as a support for iron oxide. In the literature (EP-A-71, 983) an iron oxide absorbent on silicon dioxide is described, which exhibits a good conversion degree and stability. The publication by Abel, Shultz and Langdon, Bur. Mines RI 7947 (1974) describes the results of an investigation into the usefulness of a large number of solid absorbents for the removal of hydrogen sulfide from hot gases . It appears from this publication that a large number of compounds, mainly metal compounds, exhibit an absorption of hydrogen sulfide. Examples include compounds of iron, magnesium, calcium, boron, copper, cerium, molybdenum, yttrium, lanthanum, cobalt and zinc. Substantially all of these compounds were tested as oxides, hydroxides or carbonates, in the form of granules sintered with fly ash. The best results were obtained with sintered granules comprising 75% fly ash with 25% iron oxide. At approx. 800°C, these yielded an absorption capacity of approximately 40 g sulfur per 100 g absorbent. These absorbents exhibit the same drawback as described hereinabove with respect to bulk metal oxides, viz. disintegration during absorption and during regeneration. The article by Furimsky et al, Solid adsorbents for removal of hydrogen sulfide from hot gases, issenschaft & Technik, Bd. 39, Heft 4, April 1986, pp. 163-172, gives an overview of absorbents described in the literature for the absorption of hydrogen sulfide from hot gases . Examples of absorbents described therein include compounds of calcium, magnesium, iron, manganese, zinc, molybdenum, tungsten, nickel, cobalt, barium, vanadium and copper. According to the authors, deactivated cracking catalysts based on molybdenum, cobalt and nickel on aluminum oxide, after the cokes are burnt off, are suitable as absorbent . The absorbents described in this article, too, exhibit the above-indicated drawbacks of disintegration and the associated dust formation. In the deactivated cracking catalysts referred to, this is caused by the reaction of the aluminum oxide to form sulfate. Further, a large number of the materials described have the disadvantage of a slight temperature resistance, which, after some time, leads to sintering and accordingly to loss of activity. A number of materials moreover exhibit the property of volatilizing at higher temperatures, which is undesired in view of the environment and the loading degree . GB-A 2,202,546 discloses absorbents for removing hydrogen sulfide from tail gases, which absorbents are based on a combination of molybdenum oxide (or tungsten oxide) and an oxide of manganese, cobalt or nickel. These absorbents are provided on titanium oxide as support. According to this publication, only these combinations of oxides can be used because other materials, during regeneration, give rise to the formation of sulfates, which sulfates decompose during absorption and give rise to S0 2 formation. Such S0 2 formation can only be suppressed by regeneration at a temperature above the decomposition temperature of the sulfate in question, which is realized by regeneration in the presence of an oxidative atmosphere. Because of this oxidative atmosphere, the temperature of the absorbent itself rises to a value far above the decomposition temperature of the sulfate, so that any sulfates are decomposed. Finally, a problem associated with a number of prior art materials is that they are very difficult to regenerate, if at all. The object of the invention is to provide a method of the nature described in the preamble, in which no problems are met with regard to the formation of metal sulfates which are difficult to regenerate, if at all, nor with regard to the formation of S0 2 when a regenerated absorbent is used. A further object of the present invention is to provide a method for removing H 2 S from gas streams, in which the loading degree is improved over the conventional materials . The present invention accordingly relates to a method for desulfurizing gases at increased temperature, in which the gas to be desulfurized is contacted with an absorbent based on two or more metal oxides, if desired provided on a support, which method is characterized in that the gas to be desulfurized is contacted with an absorbent based on one or more molybdenum and/or tungsten oxides as a first metal component, with interaction occurring said first metal oxide component and a second metal oxide component . By this method, a very good absorption of hydrogen sulfide is obtained, even at low temperatures, while no formation or substantially no formation of S0 2 or of metal sulfates that are hard to reduce occurs. The invention is based on the surprising insight that molybdenum and/or tungsten oxide in combination with a second metal oxide component, preferably provided on a support, absorb hydrogen sulfide very well at a high loading degree, provided an interaction occurs between the two metal components . This means that the absorption occurs up to a loading degree corresponding with a total conversion of the oxide to the sulfide, while no or substantially no sulfate is formed. Moreover, the absorbents can be properly regenerated, substantially up to their original capacity, without subsequent release of S0 2 in the gas. Further, through the interaction between the two metal components which are present on the support, no or considerably less volatilization of molybdenum oxide appears to occur. In the absorbents it appears to be of importance that the distribution of the active component on the support material is good. In particular well-dispersed oxides appear to possess a very high capacity while moreover the rate of absorption is high, which is of great importance for the prevention of H 2 S leakage . In the absorbents which are used in accordance with the invention, it is of importance that a good interaction occurs between the two metal components. In practice it has been found that interaction between the metal components on the support is involved if no or substantially no S0 2 is released during absorption after regeneration, since this means that no formation of stable sulfates has occurred. Another possibility for determining the presence of interaction is determining the decomposition temperature of the sulfates of the second metal component. In the case where interaction between the two metal components is in evidence, the decomposition temperature of the sulfate is lower than the decomposition temperature of the pure sulfate without interaction. In general, the occurrence of interaction can be determined in various manners. The method to be actually chosen depends on the nature of the metal oxide components. In all cases the above-described methods, S0 2 formation and reduction of the decomposition temperature, can be applied. Other suitable methods are the occurrence of a shift of the reduction temperature in Temperature Programmed Reduction, the occurrence of a shift in the XPS spectrum, and in the Mδssbauer spectrum; always in comparison with the situation where no interaction occurs. A number of alternatives for obtaining an interaction between the two metal components are open to the skilled worker. In the first place the metal components can be applied and/or aftertreated in a manner such that a mixed oxide is formed, for instance by a reaction between the metal components . It is also possible to provide a layer of the Mo/ component on a support with which they interact well, for instance a basic support. By subsequently providing the second metal component on the layer formed, an interaction of the two components occurs. The person of ordinary skill in the field of the preparation of absorbents can through simple experiments determine whether a method leads to a suitable absorbent. It is also possible to effect interaction through other suitable combinations of methods of application. By means of routine experiments the proper method can be determined. As active component, preferably an oxide is used selected from the group consisting of oxides of molybdenum and/or tungsten in combination with oxides of iron, vanadium, copper, manganese, cobalt, zinc, nickel and/or chromium, more particularly oxides of iron, vanadium, copper, zinc and/or chromium. In particular, the active component is chosen from iron molybdate, mixtures of iron (II) oxide and/or iron (III) oxide with molybdenum(VI) oxide and mixtures of iron molybdate with molybdenum(VI) oxide and/or iron (II) oxide and/or iron (III) oxide. As support material any suitable solid substance can be used which possesses sufficient thermal and chemical stability under the conditions of absorption and regeneration. In practice, this preferably means that the surface of the absorbent, and more particularly also that of the support, that is exposed to the gaseous phase during the regeneration does not form any sulfates that decompose during a subsequent absorption. The reason is that the formation of sulfates can lead to disintegration of the support if during the regeneration and absorption cycli the sulfate is formed and decomposed, respectively. The resultant changes in the structure of the substances often leads to disintegration and dust formation. Examples of applicable support materials include silicon dioxide, aluminum dioxide, silica-alumina, zeolites and aluminum phosphate. More particularly silicon dioxide and especially aluminum phosphate are preferred. It is also possible to use a support whose surface has undergone a treatment such that the problem of sulfate formation does not occur. An example of such a support is aluminum oxide whose surface has been converted to aluminum phosphate. When using an aluminum phosphate as support material, no problems appear to arise with regard to metal sulfate formation, not even if a major proportion of aluminum relative to the phosphate is present in the support. Accordingly, the absorbent is particularly easy to regenerate . In this connection, it is not necessary to make use of a multi-stage regeneration method. Also gained is the advantage of good regenerability and good stability at high temperatures, which improves the applicability of the absorbent. Another advantage of the present invention resides in the fact that the aluminum phosphate support, at the temperatures used in high-temperature desulfurization, is particularly stable and will retain its original structure completely or substantially completely, even in the case of prolonged use and frequent regeneration. The structure of the amorphous aluminum phosphate support can be adjusted particularly well by varying the conditions of preparation, such as pH, temperature and the aluminum to phosphate ratio. In this connectiom, it is for instance possible to work with a minor proportion of aluminum or with a major proportion of aluminum relative to the stoichiometric amount that is required. It is also possible to work with substantially stoichiometric amounts of aluminum and phosphate. Surprisingly, it has been found that with the invention, it is no problem if an aluminum phosphate is used comprising amounts of aluminum and phosphate deviating from the stoichiometric amount. Although it could be expected that in particular the use of a major proportion of aluminum would give rise to problems of sulfate and sulfide formation, it has been found that such is not the case at all. The aluminum phosphate support material can Be prepared in various manners. In general, the preparation will start from separate aqueous solutions of a soluble phosphate and a soluble aluminum compound. From the viewpoint of solubility, aluminum nitrate and ammonium phosphate are preferred. The solutions are put together, whereby aluminum phosphate precipitates . This can be effected by simultaneously injecting both solutions at preferably constant pH into a reactor in which, if desired, water or an aqueous solution may already be present . It is also possible to initially provide either solution in the reactor and inject the other solution. In general, this will not be done at constant pH. The distribution of the metal component over the support is of importance since a poor distribution detracts from the absorbing power. The active component is preferably present on the support as homogeneously as possible. This can be realized through accurate selection of the method of application and/or through the choice of the loading degree. The specific surface preferably amounts to 5 to 500 m /g, whilst the average pore diameter, based on cylindrical pores, is between 1 and 100 nm. Within the concept of the invention, specific surface is understood to mean the BET surface as defined by S. Brunauer et al. in J.A.C.S. ___), 309 (1938) . Use was made of a nitrogen adsorption at 77 K according to the so-called three-point measurement. For the purpose of the calculation, the surface area of a nitrogen molecule was assumed to be 16.2 A 2 . The average pore radius is determined starting from a cylindrical pore model using the following formula: 3 Average pore radius (A) = 2.. QQQ K p ore volume ( cm / g) BET surface (m 2 /g) The pore volume used herein is gravimetrically determined by impregnation with water under vacuum. The specific pore volume can further be determined ' using mercury po.rosimetry up to a pressure of 2000 bar. The values obtained with the two methods show a good correspondence. The content of support material in the absorbent used according to the invention is preferably between 25 and 99 wt . % . The invention also relates to an absorbent which is suitable for high-temperature desulfurization of gases, which absorbent is based on at least one oxide of molybdenum and/or tungsten provided on an aluminum phosphate support material, optionally in combination with a second component, as defined hereinabove. The preparation of such an absorbent and the application thereof correspond substantially with what has been indicated hereinabove with reference to the method for desulfurizing gases . The extent of removal of the hydrogen sulfide will be adjusted depending on the application contemplated. For the less critical applications, the typical aim is for the H 2 S content not to exceed 100 ppm. When using catalytic conversions of the treated gas with sulfur-sensitive catalysts, the hydrogen sulfide content should naturally be significantly lower. For such applications, the aim is for a H 2 S content of 25 ppm at most, more particularly 10 ppm at most . The adjustment of the H 2 S content in the treated gas can be effected by varying the time of contact between the gas and the absorbent . On the one hand, this can be effected by varying the gas rate and, on the other, by varying the amount and the height of the bed in which the absorbent is provided. Suitable contact times lie between 0.01 and 10 seconds. During the absorption of hydrogen sulfide, the greater part of the hydrogen sulfide will react with the metal oxide to form metal sulfide. The regeneration of the absorbent thus loaded with sulfur is accordingly effected through conversion of the metal sulfide with oxygen to form metal oxide. Depending on the amount of oxygen used, the sulfur can be obtained in the form of elemental sulfur, S0 2 , or mixtures thereof. In the case where relatively little oxygen is used in the regeneration, substantially elemental sulfur is obtained in the gas . Since sulfur can easily cause stoppages in the further equipment, it is preferred to condense this sulfur directly after the regeneration. When a large excess of oxygen is used, all of the sulfur is released in the form of S0 2 . If desired, this can be separated and/or further processed in known manner. The advantages of the present invention are especially evident in the desulfurization at very high temperatures . These temperatures generally lie above 150°C and more particularly between 400 and 625°C. At these temperatures, therefore, it is possible to regenerate the absorbent, the regeneration occurring through oxidation with oxygen and preferably at a temperature substantially comparable with that of the absorption. According to the invention, it is preferred that the contact takes place at a temperature of 150 to 625°C, preferably 400 to 625°C if the absorbent is based on iron (II)molybdate. The absorbents are preferably made by homogeneous deposition precipitation of the metal component or precursor therefor on the finished support. This method is described in J.W. Geus, Prep, of Cat. Ill, ed. G. Poncelet et al . (1983) pp. 1-25. In general, the amount of metal oxide provided on the support is not critical and can vary within wide limits . The suitable amounts are related to the specific surface of the support and will lie between 1 and 50 wt . % . The ratio of the amounts of the two or more metal oxide components is preferably such that in molar terms a minor proportion of molybdenum/tungsten is used with regard to the other metal component. In general the molar ratio of the second metal component, calculated as metal, to molybdenum/tungsten will be 0.1 to 20, preferably 1.25 to 20. Naturally, the aim is to have a highest possible content of metal component, finely distributed, present on the support, since this enables a highest possible loading thereof. It is preferred to utilize a particle size of 0.1 μm at a maximum, more particularly 0.05 μm at a maximum. The lower limit of the particle size is not very critical and is preferably at least 1 nm. The method according to the invention is especially suitable for treating large amounts of hot gases which may, for instance, originate from the gasification of coal or oil. Such gases are typically used as fuel gas in power stations and other gas-fueled installations. It is also possible to convert such gases catalytically to other compounds, such as ammonia, methanol or higher hydrocarbons, for instance via a Fischer Tropsch synthesis . The extent of removal of the hydrogen sulfide will be adjusted depending on the application contemplated. For the less critical applications, the typical aim is for the H 2 S content not to exceed 100 ppm. When using catalytic conversions of the treated gas with sulfur-sensitive catalysts, the hydrogen sulfide content should naturally be significantly lower. For such applications, the aim is for a H 2 S content of 25 ppm at most, more particularly 10 ppm at most. The adjustment of the content in the treated gas can be effected by varying the time of contact between the gas and the absorbent . On the one hand, this can be effected by varying the gas rate and, on the other, by varying the amount and the height of the bed in which the absorbent is provided. Suitable contact times lie between 0.01 and 10 seconds. The invention will now be illustrated in and by some examples without being limited thereto. EXAMPLES 1 Experimental procedures 1.1 Preparation of the absorbents All absorbents were prepared by homogeneous deposition precipitation, as described in J.W. Geus, Prep, of Cat. Ill, ed. G. Poncelet et al . (1983) pp. 1-25. The absorbents based on molybdenum on silicon dioxide were prepared by injection of an aqueous solution of molybdenum(III) chloride under the surface of a vigorously stirred suspension of silicon dioxide (Aerosil 200, Degussa) at a constant pH of 5. The molybdenum(III) chloride was prepared through electrochemical reduction of an aqueous solution of ammonium-hepta-molybdate (Merck p.a.) at pH 1. The preparation of well-dispersed molybdenum catalysts on silicon dioxide using molybdenum precursors of a higher valency than three is not possible on account of the low interaction of these substances with the support during precipitation. Iron molybdate on silicon dioxide was prepared in a number of ways. One method was the coprecipitation of an iron and molybdenum precursor. The use of a solution of iron (III) chloride was not possible because of the redox reaction between Fe 3+ and Mo 3+ ions. This resulted in reduction of the iron(III) ions and oxidation of the molybdenum(III) ions. Accordingly, a mixture of iron (II) chloride and molybdenum(III) chloride was used to prevent the formation of molybdenum with a higher valency. The aqueous solution of iron (II) was obtained by dissolving iron powder (Baker 99.99%) in concentrated hydrochloric acid (Merck p.a.) . A number of absorbents with varying iron/molybdenum ratios were prepared. The absorbents were also prepared by deposition precipitation of Fe 2+ and Mo 3 " 1 " precursors in succession onto the silicon dioxide support. The order in which the precipitation was carried out appeared to have a strong influence on the activity of the final absorbent. Utilizing aluminum phosphate as support, we used only the coprecipitation method. In this method Fe 3τ and Mo 5+ were used as precursor. The preparation of aluminum phosphate with an aluminum/phosphate ratio of 2 took place at pH 5. After the homogeneous deposition precipitation, the absorbents were filtered and dried overnight at 120°C. The absorbents were tabletted and broken again. A sieve fraction between 150 and 500 μm was separated and calcined in the air at 500°C at a heating rate of 5°C/minute . After reaching 50C°C it was maintained at that temperature for one hour. In the following table the various absorbents are include Absorbent Wt . % Wt . % Molar rat Fe2θ3 M0O3 Fe/Mo SM020 Si0 2 0 20 0 SM010 Si0 2 10 SFe5.8Mιo.6 Si0 2 5.8 10. β 0.5 SFei2.2M° ll Siθ2 12.2 11 SFei2.2Mθ5.9 Si0 2 12.2 5.9 1.9 SFe i 2Mθ6 1) Siθ2 12 SFeδMθi2 2) Siθ2 12 6 1) Precipitation of iron (II) followed by precipitation of molybdenum(III) 2) Precipitation of molybdenum (III) followed by precipitation 0 iron (II) 3) Coprecipitation of iron(II) and molybdenum (III) 4) Coprecipitation of iron (III) and molybdenum (IV) The test procedure which was followed in testing the diver absorbents was carried out with an appliance as described in Fig. 1. This appliance comprises a gas flow control system with mass-controlled feed of various gases, magnetic valves, and saturator and reactor oven and analyzing equipment . The gas was analyzed using a thermal conductivity detector (TCD) and a UV-V spectrophotometer, Varian Cary 1. With the last, the hydrogen sulfide concentration was measured at 232 nm and the sulfur dioxide concentration at 262 nm. The experiments consisted of a number of absorption and regeneration cycli, temperature-programmed reductions and oxidations, during which the hydrogen sulfide and sulfur dioxid concentrations were monitored. During the absorption procedure, use was made of a sample size of 1.5 ml, a reaction mixture of 1 vol.% hydrogen sulfide and 10 vol.% hydrogen in argon with a total amount of 100 ml/minute, a space velocity of 4000 hour -1 and a temperature of 400°C. The absorption step was generally preceded by a reduction step in whi-ch any iron sulfates formed were reduced i 10% hydrogen at the same temperature as the absorption step. After the absorption procedure the absorbent was regenerated wi a reaction mixture of 2 vol.% oxygen in argon under otherwise equal conditions. During the temperature-programmed reduction 10 vol.% hydrogen in argon was used and the temperature was increased by 4°C/minute . For the temperature-programmed oxidation a gas mixture of 2 vol.% oxygen in argon was used. During two experiments the reactor was flushed with argon for 10 minutes . The various absorbents described in the table were tested the manner as described hereinabove. The various figures append to this application indicate the absorption of hydrogen sulfide and any formation of S0 2 by the absorbent . Figure 2 indicates the absorption curve of 20 wt . % molybdenum oxide in silicon dioxide, SMo 2 o- At the beginning of the absorption no sulfur dioxide production occurs . Nor do any reduction treatments after regeneration give any sulfur dioxid formation. This indicates that during regeneration with oxygen sulfate formation occurs if the absorbent is based exclusively molybdenum oxide . During the absorption step itself nonetheless some S0 2 formation occurs. This is caused by reduction of Mo0 3 with H 2 S form molybdenum(IV) oxide and S0 2 . However, to prevent sulfur dioxide formation during the absorption step the hydrogen concentration can be raised, which is shown in Fig. 3. Figure 3 shows an absorption curve with 50 vol.% hydrogen in the feed. I the tail gas no S0 2 is found. The sulfur dioxide formation depends not only on the hydrogen concentration but also on the dispersion of the active compound over the support. Figure 4 shows an absorption curve o a 10 wt .% molybdenum oxide on silicon dioxide, SMθ:_. The sulfu dioxide formation is less than in the case of the 20 wt . % molybdenum oxide absorbent. The most striking difference, however, is the uptake capacity for hydrogen sulfide. The 10 wt molybdenum oxide absorbent exhibits a complete conversion to M0 sulfide. This is not the case with the SMO 20 . The difference in behaviour is undoubtedly attributable to the difference in dispersion between the two absorbents. On account of the high degree of dispersion of molybdenum oxide, the absolute capacity for hydrogen sulfide removal is higher for the 10 wt . % product than for the 20 wt . % product. Figure 5 shows a number of absorption curves for molybdenu oxide on silicon dioxide (SMoi . ) at different temperatures . At 235°C H 2 S is released after complete absorption. This is probab caused by the reaction of molybdenum(VI) sulfide to molybdenum(IV) sulfide . This can also be concluded from Figure 6 which shows the hydrogen sulfide uptake as a function of the temperature. The absorption capacity of the molybdenum oxide absorbent decreases at a temperature of approximately 525°C. There are three reasons for this decrease in capacity. In the first place there is a loss of molybdenum oxide due to evaporation. The second reason is the formation of larger molybdenum oxide particles which, as appeared from previous figures, have a lowe capacity. The third reason for the reduced capacity is the formation of molybdenum(VI) particles, which occurs at a high temperature through reduction of molybdenum(VI) to molybdenum(IV) . Figure 7 shows the absorption curve of an absorbent SFes.8 010. 6 • During the reduction of molybdenum(VI) to molybden (IV) by hydrogen sulfide, sulfur dioxide formation is observed again. However, this can be suppressed by increasing the hydrog partial pressure. Figure 8 shows the capacity of H 2 S per gram absorbent for iron-free and an iron-containing absorbent . It is evident that the inclusion of iron in the absorbent increases the capacity. The iron molybdate absorbent mentioned above still exhibit some sulfur dioxide production during reduction. This indicates that during regeneration iron sulfate has been formed. However, experiments at various temperatures demonstrate that the iron sulfates formed from iron molybdate are less stable than those formed with pure iron oxide. The decomposition temperatures of iron sulfate obtained from iron molybdate in oxygen appear to b a function of the iron molybdenum proportion. The following Table 2 includes the decomposition temperatures for various iro molybdenum proportions. Absorbent Wt .% Wt .% Molar ratio Decompositio Fe 0 3 M0O 3 Fe/Mo temp. °C SFe 5 . 8 Mo 10 .6 5 . 8 10 . 6 0 . 5 460 SFei2 .2Mo 5 . 12 . 2 5 . 9 1 . 9 500 SFe 2 5 25 0 625 Figure 9 shows the capacities of three absorbents with increasing iron oxide content after a number of absorption regeneration cycli at 400 and 600°C. The measurements were carried out at these temperatures without sulfate formation being observed. The results indicate that the addition of iron oxide can lead to an improvement of the capacity of molybdenum oxide-based absorbents. The optimum working temperature increases with the iron content . Since the absorbents based on iron molybdate are easier and cheaper to obtain than on molybdenum oxide, this circumstance can outweigh the additional costs involved. Another, particularly advantageous property of the iron molybdates is the higher thermal stability, which renders the absorbents less susceptible to undesired increases in temperature during regeneration. Experiments have also been carried out to determine the influence of the order of precipitation of iron and molybdenum. Figures 10 and 11 show the absorption curves for a material obtained, respectively, by first precipitating iron (II) followe by molybdenum (III) and by first precipitating molybdenum (III) followed by precipitation of iron (II) . The first figure has the same characteristic form as the absorbent obtained by coprecipitation. If the order of precipitation is reversed, the results are totally different. The figure in question shows that no iron sulfate formation occurs at 400°C. This is particularly remarkable result. Although it is highly likely that iron oxide is exposed to the feed, there is yet no iron sulfate formation. The reason for this is still unclear. Since the preparation of iron molybdate on silicon dioxide is rather cumbersome, tests were carried out with an absorbent on aluminum phosphate. Because of the positive interaction of the metal precursors with the phosphate groups on the surface o the support, the use of iron (III) and molybdenum(VI) ions is possible. Figure 12 shows a hydrogen sulfide absorption curve o 12 wt . % iron oxide and 10 wt . % molybdenum oxide on aluminum phosphate absorbent PFeι 2 Mθι 0 . The same characteristic curve is obtained as with absorbents based on silicon dioxide. However, the capacity is slightly smaller because of the interaction of part of the active compounds with the phosphate groups . The fac is the metal phosphates are not active in the absorption of hydrogen sulfide. The use of aluminum phosphate as support accordingly simplifies the preparation of iron molybdates considerably. The interaction of the metal precursors with the phosphate groups a the surface make it possible to use iron (III) and molybdenum(VI) as precursor during precipitation. As a result, a high degree o dispersion and thermal stability is obtained.

Claims: ■LAIMS 1. A method for desulfurizing gases at increased temperature, in which the gas to be desulfurized is contacted with an absorbent based on two or more metal oxides, if desired provided on a support, characterized in that the gas to be desulfurized is contacted with an absorbent based on one or more molybdenum and/or tungsten oxides as a first metal component, with interaction occurring between said first metal oxide component and a second metal oxide component . 2. A method according to claim 1, characterized in that, as second metal oxide component, oxides of iron, vanadium, copper, manganese, cobalt, zinc, nickel and/or chromium, more particularly oxides of iron, vanadium, copper, zinc and/or chromium, are used. 3. A method according to claim 1 or 2, characterized in that an absorbent is used which is based on iron (II)molybdate . 4. A method according to claims 1-3, characterized in that a support material is used whose surface which is exposed to the gaseous phase during regeneration does not enter into a reaction with sulfur compounds to form sulfates which can be decomposed during absorption. 5. A method according to claims 1-4, characterized in that the support material is selected from the group consisting of silicon dioxide, aluminum dioxide, silica-alumina, zeolites and aluminum phosphate, more particularly of silicon dioxide, zeolites and aluminum phosphate. 6. A method according to claims 1-5, characterized in that the content of support material is between 25 and 99 wt . % . 7. A method according to claim 5 or 6, characterized in that in the aluminum phosphate support material a major proportion of aluminum with regard to phosphate groups is present . 8. A method according to claim 5 or 6, characterized in that in the aluminum phosphate a minor proportion of aluminum with regard to phosphate groups is present . 9. A method according to claim 5 or 6, characterized in that in the aluminum phosphate a substantially stoichiometric amount of aluminum with regard to phosphate groups is present . 10. A method according to claims 1-9, characterized in that the gases are contacted with the absorbent at a temperature of from 150 to 800°C, preferably of from 150 to 625°C. 11. A method according to claim 10, characterized in that the contact occurs at a temperature of from 150 to 625°C, preferably from 400 to 625°C if the absorbent is based on a combination of molybdenum oxide and/or tungsten oxide with at least one iron oxide . 12. A method according to claims 1-11, characterized in that the absorbent has a specific surface of from 5 to 500 m 2 /g. 13. A method according to claims 1-12, characterized in that the absorbent has an average pore diameter of from 1 to 100 nm. 14. A method according to claims 1-13, characterized in that the molar ratio of the second metal component, calculated as metal, to molybdenum/tungsten is 0.1 to 20, preferably 1.25 to 20. 15. A method according to claim 14, wherein the absorbent comprises 5 to 25 wt . % iron oxide. 16. A method according to claims 1-15, wherein the gas to be desulfurized is selected from the group consisting of gases originating from gasification of coal and of oil. 17. A method according to claims 1-16, wherein the treatment with the absorbent occurs under conditions such that the content of H 2 S in the treated gas is 100 ppm at a maximum, more particularly 25 ppm at a maximum. 18. A method according to claims 1-17, wherein the time of contact between gas and absorbent is between 0.01 and 10 s. 19. An absorbent for sulfur compounds suitable for high- temperature desulfurization of gases, comprising an aluminum phosphate support material with one or more molybdenum and/or tungsten oxides provided thereon. 20. An absorbent according to claim IS, comprising one or more molybdenum and/or tungsten oxides as a first metal component, provided on a support, in which interaction occurs between said first metal oxide component and a second metal oxide component . 21. An absorbent according to claim 19 or 20, characterized in that an active component is used, selected from the group consisting of molybdenum(VI) oxide, iron (II)molybdate, mixtures of iron (II) oxide and/or iron (III) oxide with molybdenum(VI) oxide and mixtures of iron (II) molybdate with molybdenum(VI) oxide and/or iron (II) oxide and/or iron (III) oxide . 22. An absorbent according to claims 19-21, characterized in that the content of support material is between 25 and 99 wt . % . 23. An absorbent according to claims 19-22, characterized in that in the aluminum phosphate a major proportion, a minor proportion or a substantially stoichiometric amount of aluminum with regard to phosphate groups is present . 24. An absorbent according to claims 19-23, characterized in that the absorbent has a specific surface of from 5 to 500 m 2 /g. 25. An absorbent according to claims 19-24, characterized in that the absorbent has an average pore diameter between 1 and 100 nm. 26. An absorbent according to claims 19-25, characterized in that the molar ratio Fe/Mo is 0 to 20.

Inventors: GEUS JOHN WILHELM, VAN YPEREN RENEE, GEUS, JOHN, WILHELM, VAN YPEREN, RENEE
Applicants: GASTEC NV, GEUS JOHN WILHELM, YPEREN RENEE VAN, GASTEC N.V., GEUS, JOHN, WILHELM, VAN YPEREN, RENEE
Application year: 1993
Main classification: B01D53
IPC codes: B01D53, B01J20