Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
  • B01J35/00—Catalysts, in general, characterised by their form or physical properties
  • B01J35/60—Catalysts, in general, characterised by their form or physical properties characterised by their surface properties or porosity
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01D—SEPARATION
  • B01D53/00—Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols
  • B01D53/34—Chemical or biological purification of waste gases
  • B01D53/74—General processes for purification of waste gases; Apparatus or devices specially adapted therefor
  • B01D53/86—Catalytic processes
  • B01D53/8603—Removing sulfur compounds
  • B01D53/8612—Hydrogen sulfide
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
  • B01J23/00—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00
  • B01J23/70—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00 of the iron group metals or copper
  • B01J23/74—Iron group metals
  • B01J23/745—Iron
• • C—CHEMISTRY; METALLURGY
  • C01—INORGANIC CHEMISTRY
  • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
  • C01B17/00—Sulfur; Compounds thereof
  • C01B17/02—Preparation of sulfur; Purification
  • C01B17/04—Preparation of sulfur; Purification from gaseous sulfur compounds including gaseous sulfides
• • C—CHEMISTRY; METALLURGY
  • C01—INORGANIC CHEMISTRY
  • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
  • C01B17/00—Sulfur; Compounds thereof
  • C01B17/02—Preparation of sulfur; Purification
  • C01B17/04—Preparation of sulfur; Purification from gaseous sulfur compounds including gaseous sulfides
  • C01B17/0404—Preparation of sulfur; Purification from gaseous sulfur compounds including gaseous sulfides by processes comprising a dry catalytic conversion of hydrogen sulfide-containing gases, e.g. the Claus process
  • C01B17/046—Preparation of sulfur; Purification from gaseous sulfur compounds including gaseous sulfides by processes comprising a dry catalytic conversion of hydrogen sulfide-containing gases, e.g. the Claus process without intermediate formation of sulfur dioxide
  • C01B17/0465—Catalyst compositions
• • C—CHEMISTRY; METALLURGY
  • C01—INORGANIC CHEMISTRY
  • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
  • C01B17/00—Sulfur; Compounds thereof
  • C01B17/02—Preparation of sulfur; Purification
  • C01B17/04—Preparation of sulfur; Purification from gaseous sulfur compounds including gaseous sulfides
  • C01B17/0404—Preparation of sulfur; Purification from gaseous sulfur compounds including gaseous sulfides by processes comprising a dry catalytic conversion of hydrogen sulfide-containing gases, e.g. the Claus process
  • C01B17/046—Preparation of sulfur; Purification from gaseous sulfur compounds including gaseous sulfides by processes comprising a dry catalytic conversion of hydrogen sulfide-containing gases, e.g. the Claus process without intermediate formation of sulfur dioxide
  • C01B17/0469—Preparation of sulfur; Purification from gaseous sulfur compounds including gaseous sulfides by processes comprising a dry catalytic conversion of hydrogen sulfide-containing gases, e.g. the Claus process without intermediate formation of sulfur dioxide at least one catalyst bed operating below the dew-point of sulfur
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
  • B01J21/00—Catalysts comprising the elements, oxides, or hydroxides of magnesium, boron, aluminium, carbon, silicon, titanium, zirconium, or hafnium
  • B01J21/06—Silicon, titanium, zirconium or hafnium; Oxides or hydroxides thereof
  • B01J21/08—Silica
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
  • B01J35/00—Catalysts, in general, characterised by their form or physical properties
  • B01J35/60—Catalysts, in general, characterised by their form or physical properties characterised by their surface properties or porosity
  • B01J35/64—Pore diameter
  • B01J35/651—50-500 nm
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
  • B01J35/00—Catalysts, in general, characterised by their form or physical properties
  • B01J35/60—Catalysts, in general, characterised by their form or physical properties characterised by their surface properties or porosity
  • B01J35/66—Pore distribution

Definitions

• the invention is in the field of the removal of sulfur compounds. More in particular, the present invention is directed to supported catalyst systems, as well as to the use of these catalyst systems in the removal of sulfur compounds, such as H2S and to processes for de-sulfurization of gases containing sulfur compounds, such as H 2 S.
• Dry-bed sorption processes are commonly applied for removal of relatively small quantities of sulfur compounds, such as H2S, i.e., up to 10 to 20 tons of sulfur equivalents per day. Since most of the time the sulfur compounds to be removed contain or are H 2 S, this is the market of the so- called H 2 S scavenging processes.
• the applications include purification of natural gas, landfill gas, biomass digester gas and a variety of industrial (off) gases.
• H2S scavenging operations are of growing importance to natural gas producers, as production is becoming increasingly sour.
• the exploration of renewable energy sources and increasingly stringent legal emission requirements also add to a growing need for low temperature desulfurization processes, such as H2S scavenging processes.
• Hydrogen sulfide scavengers have been used for many years.
• One example is iron sponge.
• Another well-known example is caustic soda, which covers a large share of the market. Nevertheless, the technology to cope with H2S at a relatively low temperature, in particular when only small quantities of H2S are to be removed, is still suffering from economic and/or environmental problems.
• Liquid sorption comprises washing of the hydrogen sulfide containing gas with chemical or physical solvents, such as solutions of an alkaline solution (such as caustic soda), methanol or alkanolamines.
• the non-re generable solvent types have to be disposed of after the sorption process, which is a major drawback in this age of environmental awareness.
• Another disadvantage of the "once through” principle is the high and economically unattractive consumption of chemicals.
• regenerable solvents may be used.
• the re gener ability of, for example, alkanolamines allows the solvent to be reused, thus reducing the chemical costs and waste disposal problems.
• the regeneration process generates a stream of concentrated H2S, which has to be processed further. This is usually done in a Glaus plant, which converts the extremely hazardous hydrogen sulfide into safe to handle elemental sulfur, which can be sold to the sulfur processing industry.
• the amine-Claus-technology is widely used.
• the capital cost involved in an additional sulfur recovery plant puts a lower limit on the sulfur production rate.
• the sulfur production of the amine and Glaus plant combination should be more than 10 to 20 tons per day.
• a common problem of basic solvents is the interfering absorption of other components in the gas, in particular CO2, which reduces the H2S uptake considerably.
• Another disadvantage is that wash towers have to be designed to cope with the peak values in the H2S content of the feed gas. As a result, the installations are operating at almost constant turn down, when purifying gases with a widely varying H2S content. This is often the case with, for instance, small industrial off gas flows and gases from bio mass conversion processes.
• Dry-bed sorption processes are easier to operate and cope with varying H2S concentrations in the feed gas more economically, compared to liquid sorption processes. There is no need for an over-designed installation. Dry-bed sorption is therefore particularly interesting for the scavenging market.
• H2S Hydrophilicity
• a charged absorber has to be taken from the process gas hne to be replenished with a fresh absorbent, either by replacement with new material or by regeneration of the sulfur-laden absorbent.
• the process economics benefits therefore strongly from long absorption times and low sulfur production rates, in particular less than 10 to 20 tons per day.
• Well-known dry-bed scavengers are activated carbon and certain types of zeolite, in which H2S is adsorbed on the adsorbent surface and/or retained in the zeolite cages. These materials are commonly used in non- regenerative applications, having the same economical and environmental disadvantages as the above mentioned liquid equivalents. Moreover, they suffer from a limited H2S uptake capacity, due to interfering absorption of other components in the feed gas, e.g. CO2, hydrocarbons and water vapor.
• Metal oxides form a second group of low temperature dry bed scavengers. They react with H2S to form metal sulfides. To be effective, in terms of H 2 S uptake activity and capacity, the metal oxide surface area accessible to H2S should be large. Therefore, these absorbents are highly porous materials with a high metal oxide content, typically over 50 wt%.
• the material may be the pure metal oxide itself, or the oxide may be present on an inert support material.
• a wide variety of metal oxides suites the purpose, inter alia oxides of Ni, Fe, Co, Cu, Cr, Mo, W, V, Zn. Single oxides as well as mixtures of different metal oxides are described in the patent literature.
• the standard commercial products within this group are zinc oxide and iron oxide. In non-regenerative use they present the problems mentioned above. In principle, zinc oxide can be regenerated in oxygen containing gas at elevated temperatures. The SO2 evolved, however, presents a disposal problem and the regenerability is hampered by degradation of the absorbent.
• Hydrated iron oxide is in use as a scavenger for well over a century. Hydrogen sulfide in coal gas from nineteenth century town gas plants, for example, was already removed by reaction with moist iron oxide at a temperature below 100 °C.
• the iron oxide absorbents currently available have a high absorption capacity and H2S removal activity in humid (or humidified) process gases. Regeneration below 150 °C in oxygen containing gas is possible. However, the H 2 S absorption capacity drops considerably after regeneration (up to 40%).
• Regeneration of the charged absorbent with oxygen proceeds according to:
• a disadvantage of this regeneration method besides the limited recovery of H2S uptake capacity, is the fact that the oxygen flow has to be controlled very carefully. Excess oxygen leads to combustion of iron sulfide to iron oxide and SO2 formation. The heat evolved accelerates the combustion reaction, resulting in ignition and destruction of the absorbent and the evolution of large quantities of SO2, which is not easily disposed of.
• a next step is to add some oxygen to the absorption feed gas and to carry out reaction (1) and (2) simultaneously, at temperatures at which sulfur is retained in the catalyst. Indeed, it has been a long time practice to add a small amount of oxygen — if not already present in the feed gas - to enhance the H2S removal capacity of iron oxide absorbents.
• the overall reaction that results from (1) and (2) is:
• F ⁇ 2 ⁇ 3 drops out of the equation, it now acts as a catalyst, together with F ⁇ 2S3.
• F ⁇ 2 ⁇ 3 only functions as an absorbent as far as it needs to take up H2S to reach an equilibrium state with F ⁇ 2S3 in the reduction- oxidation cycle, which switches back and forth between reaction (1) and (2).
• the present inventors believe that the actual catalytically active species in the process, once stabilized, is probably not a mixture of F ⁇ 2 ⁇ 3 and F ⁇ 2S3, but a non stoichiometric iron oxy-sulfide FeS x O y in the surface of iron oxide and/or iron sulfide particles.
• H2S removal by selective oxidation to sulfur, according to reaction (3) is not restricted to the iron oxide/sulfide system.
• Several other metals among them Cu, Mn, Cr, Ni, W, Mo, Co and V, are known to catalyze the same reaction.
• Other suitable materials are activated alumina, titania and activated carbon.
• Particularly effective is regenerative catalytic selective oxidation of H 2 S with free oxygen to sub dew point sulfur, which is retained in the pores of the catalyst, at temperatures between ambient and 180 °C, preferably between ambient and 100 °C.
• the sulfur is removed from the charged absorbent by evaporation in non oxidizing gas, at temperatures between 200 °C and 500 °C.
• Dry bed sorption processes represent an attractive technology for H2S removal, particularly from gas streams with a low sulfur production rate, such as landfill gas, bio-mass digester gas and industrial waste gases.
• hydrated iron oxide/sulfide For de-sulfurization below 100 °C there are two catalysts in standard commercial use: hydrated iron oxide/sulfide and activated carbon. Activated carbon may be used as such or in combination with a promoter, e.g. potassium iodide.
• the catalysts can be regenerated by vaporization of the sulfur retained in the catalyst and removal of the sulfur from the catalyst bed with an oxygen free sweeping gas, at a temperature between 200 to 500 °C.
• Activated carbon suffers from co-absorption of other feed gas components than H2S, thus reducing the H2S and sulfur uptake capacity.
• sulfuric acid is formed as an oxidation bj ⁇ roduct, which has to be removed or reduced before sulfur evaporation.
• US-A-5 256 384 describes H2S oxidation, in an oxygen containing gas, at 50 to 180 °C over activated carbon, in which the regeneration procedure consists of reduction of the sulfuric acid by H2 or H2S, at 100 to 180 °C, before evaporation of the absorbed sulfur at 200 °C to 400 °C.
• Glaus off gas contains H2S, SO2, COS, CS2 and a small amount of sulfur (Sx).
• Sx sulfur
• these types of feed gases contain more than 30 vol. % of water vapor, it is advantageous to operate above the water dew point, i.e., between 80 °C and 180 °C.
• the temperature of gases such as bio-mass digester gas, industrial waste gases, etc., is typically between ambient and 100 °C. Energetically, it is advantageous to remove H2S in the same temperature range.
• Metal oxides / sulfides are perfectly capable of doing the job, but kinetic limitations have a considerable effect on the catalytic activity at the lower temperature end of sub sulfur dew point de-sulfurization. Therefore, measures have to be taken to keep up an acceptable reaction rate.
• a way to enhance catalytic activity is to apply the active component in very finely divided form to a catalyst support material, to maximize the catalyst surface area exposed to the reacting gas phase molecules.
• Catalysts with highly dispersed metal oxides run a very high risk of creating locally high activities, in such a way that H2S is no longer selectively oxidized to sulfur but completely converted into SO2.
• Catalytically active materials described in the patent literature are oxides and or sulfides of Ni, Co, Fe, Cu, Ag, Mn, Mo, Cr, Ti, W, V, on support materials such as bauxite, activated and/or stabilized alumina, titania, zirconia, zeolites, silica/alumina mixtures, silica/titania mixtures, silica, activated carbon, silicon carbide.
• support materials such as bauxite, activated and/or stabilized alumina, titania, zirconia, zeolites, silica/alumina mixtures, silica/titania mixtures, silica, activated carbon, silicon carbide.
• support materials such as bauxite, activated and/or stabilized alumina, titania, zirconia, zeolites, silica/alumina mixtures, silica/titania mixtures, silica, activated carbon, silicon carbide.
• US-A-6 083 471 and US-A-6 235 259 describe an elaborate pre-sulfiding procedure using sulfur or H 2 S containing inert gas, at 250 °C to 400 °C.
• US-A-6 235 259 states that metal oxy-sulfides are the actual active species and prescribes a second activation step in which the pre-sulfided catalyst is partially re-oxidized in inert gas containing a small amount of free oxygen.
• a drawback of these procedures is that they require additional chemicals and carefully controlled procedures. In practice there is usually no sulfur available for pre-sulfiding at the start up of a new plant.
• H2S Pre-sulfiding in H2S is also a problem, particularly in small plants, which are often at remote locations, for instance, landfill and bio-mass digester sites and near a diversity of factories and installations producing small sulfur containing waste gas streams.
• Chemicals like H2S are not readily available there, unless the gas to be purified can be used for the purpose. Often this is not the case, because, for instance, gas from landfills and waste gases very often contain oxygen.
• a potential problem of the selective oxidation process is that some catalytically inactive metal sulfates are formed in the absorption stage. If the metal sulfate is not reduced, at least partly, after every absorption run, the catalyst will deactivate in successive absorption / regeneration cycles.
• Several patent documents, such as GB-A 1 504 059 and US-A-5 607 657 try to cope with the deactivation by sulfation.
• the deactivation by catalyst sulfation is counteracted by reduction to metal sulfides with H2S, H2 or CO, which is added to the regeneration gas. This procedure, however, has the same disadvantages as mentioned above in connection to the pre-sulfiding of the fresh metal oxide catalyst.
• H2S removal by oxidation is the exothermicity of the reaction. This prohibits the removal of high H2S concentrations in a single absorption step in a (near) adiabatic reactor.
• the oxidation of each vol.% of H2S to sulfur is associated with an adiabatic temperature rise of 60 °C to 75 °C, depending on the feed gas composition. If the temperature rises to a level were the catalyst starts to catalyze the oxidation of sulfur to SO2, an additional 60 °C to 75 °C adiabatic temperature rise is generated for every vol. % of SO2 produced. In the worst case, the result is a temperature runaway, igniting the metal sulfide and leading to destruction of the absorbent.
• a further object is to provide a process for the removal of sulfur compounds, such as H2S, which solves at least in part the above-mentioned prior art problems.
• the present invention is aimed at providing a regenerative process for quantitative removal (up to vol.% levels) of sulfur compounds, such as H2S from gases by catalytic selective oxidation of sulfur compounds to elemental sulfur at a moderate temperature (typically between ambient and 80 °C).
• a moderate temperature typically between ambient and 80 °C.
• the present invention should in particular find use in the so-called H2S scavenger processes, without being limited to H 2 S. It has been found that these and other objects can be solved by a process according to claim 1.
• the sulfur generated in the absorption stage can effectively be used as a reducing agent for the conversion and regeneration of catalytically inactive metal sulfates to active metal sulfides.
• the reduction process can be carried out at temperatures as low as 200 °C to 350 °G As sulfation is a determining factor in the catalyst de-activation and the regenerability of the process, the reduction with "process-own" sulfur provides a cost effective and simple way to obtain the required extended regenerability.
• the pore structure of the catalyst has an influence on the properties.
• a very important characteristic is the specific pore volume of the catalyst system.
• the specific pore volume in the micropore and mesopore range is very low.
• the sum of the pore volume present in micro-pores (i.e., pores with diameters less than 4 nm) and in mesopores (i.e., pores with diameters between 4 and 100 nm) is preferably less than 0.20 ml per gram of catalyst, more preferably less than 0.15 ml/g and even more preferably less than 0.10 ml/g.
• V-N2 N2 pore volume
• the lower limit of V-N2 is, according to the present invention, in principle zero, but in practical catalyst systems it is usually larger than 0.01 ml/g.
• catalyst systems with a micropore and mesopore volume (N-N2) of 0.20 ml/g or more tend to convert H2S quantitatively to SO2 instead of sulfur, in particular with relatively high Fe loadings of the support.
• a high total pore volume is advantageous for a high sulfur uptake capacity.
• the pore volume V-Hg representing pore diameters ranging from 8 nm to ca. 20000 nm (i.e., sum of macropores and mesopores), as measured by mercury intrusion, is preferably at least 0.30 ml/g, more preferably more than 0.40 ml g and even more preferably more than 0.45 ml/g.
• the maximum of V-Hg of the catalyst system according to the present invention is about 1 ml/g, since for larger values it appeared to be very difficult to maintain the above-mentioned requirement for the maximum of V-N2, in particular the contribution of mesopores will become considerable.
• the dry -bed sorption process consists, if required, of a process gas make up section and one adiabatic absorber filled with a catalyst according to the invention.
• the gas containing sulfur compounds, such as H2S is passed to a make up section.
• the feed gas is too low in oxygen, it is mixed with a gas containing free oxygen. This may be pure oxygen, air, or any other mixture of free oxygen and inert gas.
• the molar ratio of oxygen to sulfur compounds will further be described as oxygen to hydrogen sulfide ratio.
• the molar O2/H2S ratio in the resulting mixture may vary within a wide range. In general, the lower hmit is determined by the content of sulfur compounds allowed in the product gas. Theoretically, the ratio could be as low as 0.01, without any effect on the process chemistry.
• the upper value is determined by mainly three factors: 1) the degree to which the sulfur compounds containing gas can be diluted with excess oxygen containing gas without serious consequences for the plant size and the associated capital costs, 2) the tendency of the catalyst, once it is in (partially) sulfided form, to form inactive sulfates through slow oxidation of the metal (oxy-)sulfide, and 3) the acceptable 02-content in the treated gas.
• O2/H2S ratios as high as 500 may occur.
• a more suitable range is 0.01 to 20.
• Preferred is 0.5 to 10, more preferred 1 to 5.
• the relative humidity of the sulfur compounds and free oxygen containing gas is adjusted, e.g. by steam injection or temperature control of the gas stream.
• Another possibility of adjusting the relative humidity is by feeding the gas stream to a water wash tower.
• the tower consists of a packing (which may be a structured packing or any other packing), which is sprayed with water, coming from a spray nozzle above the packing.
• Particularly suitable packings include valve trays, sieve trays and random packing like pall rings.
• the water trickling through the contact area is collected in a reservoir in the bottom of the wash tower. The water is recycled from the reservoir to the spray nozzle.
• the H2S-O2 containing feed gas enters the tower at the bottom (above the water reservoir) and is contacted with the downward trickling water in the contact area in counter current.
• the water saturated gas leaves the tower at the top.
• the water reservoir contains a cooling and/or heating element.
• the water temperature in the reservoir is adjusted in such a way that the gas leaving the wash tower is at the desired temperature.
• the reservoir is supplied with a water drain and a water supply, to compensate for water condensation from the process gas, or water uptake by the gas.
• the water saturated gas is passed to the absorber through a hne supplied with a heat tracing.
• the tracing heats up the process gas to a temperature slightly above the temperature at the outlet of the wash tower.
• the absorber is supplied with a gas having the desired relative humidity.
• the relative humidity may be 60% to 80%, for very low sulfur compounds feed levels (in the order of 10 ppmv), up to 80% to 95% for higher sulfur compounds levels if very low levels of sulfur compounds in the product gas are required.
• lower relative humidities can be chosen depending on the concentration of sulfur compounds in the gas stream to be desulfurized and the maximum allowed concentration in the product gas, e.g. for prime movers.
• the gas conditioning can be skipped if enough oxygen and water vapor is present. This is typically the case for off gases like biogas off gas streams and other waste gases.
• the absorber is constructed in such a way that the temperature at every point in the absorbent bed is at least equal to the feed gas temperature. This is the case, for instance, with well insulated adiabatic absorbers. Particularly suited for the purpose are double walled absorbers, with a fluid, having at least the feed gas temperature, flowing between the walls. However, from economical point of view electrical tracing combined with good insulation is often preferred.
• the sulfur compounds such as H2S are converted with the oxygen present in the gas to sulfur.
• the sulfur deposits in the absorbent pores.
• the H 2 S is also taken up by the iron oxide in the form of iron sulfide.
• some or all sulfur compounds can first be converted to H2S prior to the selective oxidation to elemental sulfur.
• the gas hourly space velocities employed in the process may vary over a wide range, from the order of 10 Nm 3 gas/m 3 catalyst/h up to 5000 Nm 3 gas/m 3 catalyst/h, preferably up to 1000 Nm 3 gas/m 3 catalyst/h and more preferably up to 500 Nm 3 gas/m 3 catalyst/h, depending on the product gas specifications and the desired standing time.
• the catalyst is repeatedly regenerated at elevated temperatures, say at 200 to 350 °C, by evaporation of sulfur, which is swept from the catalyst bed using an inert gas at a high space velocity to remove the sulfur effectively, it appears that the catalyst slowly looses sulfur uptake capacity in successive absorption / regeneration cycles, due to formation of inactive iron sulfate.
• Pre-sulfiding of metal oxide catalysts using sulfur is a known technique.
• US-A-6 235 259 and US-A-6 083 471 describe pre-sulfiding of metal oxides, in particular nickel oxide, with sulfur at 250 °C to 400 °C.
• Pre-sulfiding of hydro -treating catalysts, based on metal oxides, by impregnation with sulfur or sulfur derivates, followed by heating of the catalysts in a reducing gas is also well documented in the patent literature.
• US-A-5 786 293 discloses a method in which hydro-treating catalysts, in metal oxide form, are impregnated with an inorganic polysulfide solution.
• the polysulfide laden material is treated in inert gas at preferably 230 °C to 350 °C. It is claimed that at least part of the metal oxide is converted to metal sulfide in the process, sufficient to avoid the usually required pre-activation of the catalyst in a reducing gas, before it is used in hydrotreating.
• the reaction of metal oxides with sulfur is a relatively easy reaction.
• the less reactive metal sulfates in particular iron sulfate
• Reduction of iron sulfate by sulfur formed in the absorption process can be achieved in several ways. The first is by "once through” regeneration. This is particularly applicable for catalyst beds which are small enough to be effectively heated through the absorber walls.
• the sulfur laden catalyst is taken from the absorption process gas line by system of valves and switched in an inert gas hne, on site or at another location.
• the inert gas is passed over the catalyst.
• the gas may be any suitable inert gas: nitrogen, CO2, methane, etc., as long as it has no oxidizing properties under the employed conditions.
• the catalyst is heated up from absorption temperature to ca. 105 - 110 °C, to dry the catalyst.
• the drying takes place at a relatively high space velocity - preferably at > 250 Nm 3 gas/m 3 catalyst/h, more preferably at > 500 Nm 3 gas/m 3 catalyst/h - to remove the water effectively.
• the space velocity according to the present invention is reduced after drying to maintain a high sulfur vapor pressure in the catalyst bed, in order to reduce possibly formed sulfates effectively.
• the space velocity is reduced to preferably less than 100 Nm 3 gas/m 3 catalyst/h, more preferably less than 50 Nm 3 gas/m 3 catalyst/h and in particular less than 10 Nm 3 gas/m 3 catalyst/h.
• the catalyst temperature is raised from 105-110 °C to a level between 200 °C and 500 °C, preferably 225 °C to 450 °C and more preferably 250 °C to 400 °C. Suitable heating rates vary between 1 and 30 °C/min.
• the catalyst is kept at this temperature for 1 to 20 hours, preferably 5 to 15 hours and more preferably 5 to 10 hours.
• This regeneration procedure has proven very effective, in the sense that it allows for subsequent (at least 25) absorption / regeneration cycles without de-activation by progressive sulfation. It has also been found that the catalyst according to the invention does not require re-hydration, such as in EP 0 324 091 A, before the absorber is switched back to the process gas line. The catalyst is immediately hydrated and activated once the process gas is passed over the catalyst bed. Large catalyst beds and absorbers require heating by the regeneration gas itself, which is more economically done in a closed regeneration loop.
• the loop consists of the absorber, foUowed by a water and sulfur condenser - usually integrated in one condenser/heat exchanger - a recirculation blower and a heat exchanger to heat up the gas that is recirculating in the loop.
• the loop is connected to a bleed valve and an inert gas supply.
• the regeneration starts with drying, under comparable conditions.
• the recirculating gas is heated to a temperature of 100/110 °C by the heater between the recirculation blower and the absorber.
• the heat required to evaporate the water keeps the catalyst temperature at 80°C to 100 °C.
• the evaporated water is condensed in the water annex sulfur condenser.
• the condenser is a heat exchanger which is cooled or heated by a fluid in which no phase transitions occur in its operating temperature range. In the water removal stage, the condenser operates at ca. 40 °C.
• the condensed water is drained from the condenser.
• the recirculating inert gas entering the absorber is heated to the desired sulfur evaporation temperature: 200 °C to 500 °C, preferably 225 °C to 450 °C and more preferably 250 °C to 400 °C.
• the gas hourly space velocity with which the gas is passed over the bed is less crucial than in the "once through" case.
• An indicative range is 100 to 1000 Nm 3 gas/m 3 catalyst/h. The optimum value is predominantly determined by the effectiveness of the sulfur condenser, the power required by the recirculation blower and the available pressure difference over the absorbent.
• the sulfur condenser is set at a temperature at which, on the one hand, most of the sulfur is condensed, but at which, on the other hand, there is still a significant quantity of sulfur left in the vapor phase to be recirculated to the catalyst, in order to reduce the metal sulfates.
• Suitable condenser temperatures range from 125 °C to 230 °C, preferably from 130 to 200 °C, more preferably up to 160 °C.
• the catalyst is kept at the sulfur evaporation temperature until the pressure rise in the system, due to SO2 evolution as a result of sulfate reduction, drops to zero. Then the condenser is cooled to a temperature just above the sulfur solidifying point and the liquid sulfur is drained.
• the condenser is cooled to ca. 40 °C, to remove the last of the sulfur vapor from the loop.
• the gas and the catalyst are cooled to absorption temperature while the SO2 is removed through the bleed valve and the system is purged with inert gas or (cleaned) process gas. After this procedure the catalyst is ready for reuse.
• Some metal sulfides may again form sulfates with the S0 2 remaining in the loop.
• the sulfur is stored in the condenser while the S0 2 is bled from the system.
• the condenser is reheated and the sulfur vapor generated is again passed over the catalyst to reduce the remaining sulfate.
• metal sulfate By the methods described above it is possible to reduce the metal sulfate completely. However, it is also possible to reduce the metal sulfate partially to a mixture of metal sulfide with excess sulfur, e.g. FeS2, and metal sulfate, e.g. FeS0 . This can occur, for instance, if sulfur is removed too quickly from the catalyst bed, as happens when sulfur is simply removed by evaporation into a fast flowing sweeping gas. By heating the catalyst to a temperature of 400 °C or more the remaining sulfate is reduced to sulfide by the excess sulfur in the metal sulfide in the starting mixture.
• metal sulfate By heating the catalyst to a temperature of 400 °C or more the remaining sulfate is reduced to sulfide by the excess sulfur in the metal sulfide in the starting mixture.
• the catalyst can cope with a temperature rise in the order of 10 to 15°C, after which sulfur compounds, such as H2S are no longer converted, causing significant (H2S) slip in the product gas. If the product gas specification accept such levels there is no problem.
• this basic property of the system opens the way to cope with relatively high concentrations of sulfur compounds, such as H2S in the feed gas while remaining high removal percentages sulfur compounds, using multistage absorption with inter-cooling without the use of expensive heat exchange equipment. As the process operates within a narrow temperature band of 10 °C to 15 °C the gas can be re-conditioned in a simple inexpensive water wash tower, of the type as described above.
• a feed gas made up as described, containing 4000 ppmv H2S and free oxygen, at an O2/H2S ratio of at least 1.
• the gas conditioned to a relative gas humidity of 95%, is passed to a double walled absorber having a temperature of 40 °C.
• Approximately 2500 ppmv H 2 S is converted to sulfur.
• the associated temperature rise of ca. 15 °C de-activates the catalyst and automatically prevents temperature runaways which would generate SO2.
• the product gas can be cooled to 40 °C and re-humidified to 95% in a simple inexpensive water wash tower, identical to the one in the feed gas make up section.
• the water temperature is adjusted in such a way that the desired gas outlet temperature is obtained. This is basically the only process control necessary, apart from a check on H2S breakthrough.
• the re-conditioned gas is passed to a second absorber in which the rest of the H2S is converted to sulfur.
• first or “lead” absorber gets saturated with sulfur it is switched to the regeneration mode.
• the second or “lag” absorber becomes the “lead” absorber and the freshly regenerated absorber is switched in to the "lag” position.
• the operating range of the process can be extended to gases containing vol.% levels of sulfur compounds, such as H2S by adding water-wash-absorber systems in series.
• gases containing vol.% levels of sulfur compounds such as H2S
• water-wash-absorber systems in series.
• the catalyst system and process of the present invention can very suitable be applied for the removal of sulfur compounds, such as H2S from gas sources such as landfill biogases, digester gas, sour natural gas, industrial gases and/or flare gases.
• the present invention provides an economical and environmentally viable process for sulfur compounds removal, such as H2S, from these and other types of gases.
• the clean gases produced by the process of the present invention, as well as by the use of the catalyst system of the present invention are of sufficient quality to be used in prime movers for the production of electricity and/or heat and if required of sufficient quahty to be used for the production of gas of natural gas quality.
• the invention provides for a high flexibility for the application of desulfurization units.
• the standardization of absorbers gives the opportunity to exchange absorbers between users, hence reducing costs.
• the choice between one or more absorbers depends of the gas flow to be treated (limitation of gas hourly space velocity) and/or the concentration of sulfur compounds in the gas and the level to which this concentration has to be reduced (limitation in maximum concentration of sulfur compounds to be treated in one step to ensure a complete removal).
• an integrated desulfurization unit is installed. In this case the regeneration procedure is performed on site. Often, but not by definition, such desulfurization units consist of three absorbers, one in regeneration mode and two in absorption mode (one in the lag position and one in the lead position).
• the regeneration of catalyst from sites without integrated regeneration can also be performed using (the limited) excess regeneration capacity of desulfurization units with integrated regeneration.
• extra investments in centrally located stand alone regeneration installations is hmited, the coverage of central regeneration installations in a certain area is higher, and shorter transportation hnes for absorbers (catalyst) from desulfurization units with only absorbers to the central regeneration unit can be realized.
• desulfurization units with central regeneration to units with integrated regeneration lies between daily sulfur production rates of 2 to 40 kg, preferably between 3 and 20 kg and more preferably between 6 and 10 kg.
• the fresh catalyst system used in accordance with the present invention preferably comprises a metal compound (in particular a metal oxide, sulfide and/or metal oxysulfide) which is present on a support material in dispersed form.
• the metal can be an element or a combination of elements taken from the group Ni, Co, Fe, Cu, Ag, Mn, Mo, Cr, Ti, W, V, precious metals (such as Au, Ag, Pt, etc.), etc.
• the properties of the corresponding metal oxides and metal sulfides, and the way and quantity in which they are apphed to the support, and the structural characteristics of the catalyst, are such that the resulting H2S oxidation activity of the catalyst is strongly dependent on the state of hydration of the active species and, in this connection, on the relative humidity of the process gas.
• the support may be made of refractory materials, or combinations thereof. Examples are natural or synthetic aluminas, natural or synthetic silica-aluminas, silica, titania, zirconia, carbides, etc.
• the support shaped in the desired form (e.g., extrudates or pellets of various shapes and sizes or monoliths), should have sufficient strength and resistance toward thermal stresses, to avoid mechanical degradation in successive absorption / regeneration cycles.
• the support surface should be inert in the process gases. For instance, the support material should not be prone to sulfation.
• the catalyst for the process according to the invention is catalytically active for the selective oxidation of sulfur compounds, such as H2S to sulfur at temperatures ranging from just above the water freezing point to 100 °C, with feed gases having a relative humidity of up to 100%, more in particular ranging from 40 % to 99%, preferably from 60 % to 97%, more preferably from 80 to 95%.
• sulfur compounds such as H2S to sulfur at temperatures ranging from just above the water freezing point to 100 °C
• feed gases having a relative humidity of up to 100%, more in particular ranging from 40 % to 99%, preferably from 60 % to 97%, more preferably from 80 to 95%.
• a suitable catalyst consists of iron oxide on a silica support.
• the amount of iron is 0.1 % to 20 %, preferably 2 % to 10 % by weight, based on the weight of the catalyst. For metals other than iron, similar amounts will apply.
• the specific surface area, as determined by the BET multipoint nitrogen absorption method is within the range of 2 to 200 r ⁇ 2 /g, preferably 5 to 100 m 2 /g, more preferably 10 to 60 m 3 /g.
• the catalyst may be prepared by any known method. As an example, one may impregnate pre-shaped support bodies by incipient wetness with a metal complex or salt, dry the impregnated support at 100 - 120 °C and calcine the resulting material at 400 - 500°C.
• the catalyst particles may be in the shape of cylindrical extrusions, tri- and quadrulobes, spheres, rings, etc.; whatever may be appropriate for a given application.
• catalyst coatings on pre-shaped substrates such as ceramic or metal honeycomb structures, sintered metal, heat exchanger tubes or plates, etc. The use of honeycombs is advantageous in case of pressure drop limitations. Coatings on a metal substrate with a high thermal conductivity are useful to disperse the reaction heat more effectively, increasing the catalysts' sulfur uptake per unit volume.
• the catalyst coating may be apphed to the substrate by any known method.
• the metal oxide should be present in high dispersion on the support. More importantly, the active species should have a high dispersion, i.e. a small average particle size, to ensure a good and stabilizing interaction with the support, in order to survive numerous absorption / regeneration cycles without deactivation by sintering of the active material.
• the metal oxide dispersion can be determined using techniques known to the skilled person, in particular techniques, such as electron microscopy and X-ray diffraction. Once it has been ascertained that a material has a suitable metal oxide dispersion, it may be desirable to record its reduction behavior in a thermo gravimetric analysis (TGA), as is illustrated for iron oxide/silica in the paragraphs below, since TGA measurements are less elaborate than techniques such as the ones mentioned above.
• TGA thermo gravimetric analysis
• Iron oxide on silica is a particularly suitable material in this respect. At the interface between the iron oxide and the silica support FeSi0 is formed, which acts as a stabilizing layer between the support and the active particles, particularly for the smaller, highly dispersed particles. It can easily be checked if a catalyst system according to the present invention, e.g. an iron oxide on silica catalyst, has sufficient dispersion and interaction with the support by means of thermo-gravimetric analysis (TGA).
• TGA thermo-gravimetric analysis
• a sample of a fresh catalyst is crushed to a particle size between approximately 0.5 and 1 mm. 50 to 100 mg of the crushed sample is placed a quartz crucible. The crucible is suspended in an oven tube. The tube is flushed with a mixture of 20 vol. % O2 in N2.
• the sample is heated to 250 °C in this mixture, at a rate of 10 °C/min. After 1 hour at 250 °C, to remove water from the sample, the sample is cooled to 35 °C. The system is than flushed for one hour with nitrogen, after which the gas supply is switched to 20 vol. % H2 in N2. In this mixture the sample is heated from 35 °C to 950 °C, at a rate of 5 °C/min, and the sample weight is monitored. Highly dispersed iron oxide particles, with strong support interaction, are reduced at temperatures between 250 °C and 450 °C, and between 600 °C and 950 °C.
• Weight loss between 450 °C and 600 °C is associated with reduction of larger, sinter sensitive, iron oxide particles to iron metal.
• metals other than iron and carriers other than silica the desired degree of metal dispersion can be assessed by the above-mentioned known techniques, such as electron microscopy and X-ray diffraction and subsequently linked to their own TGA temperature windows.
• the weight loss between 450 °C and 600 °C in the TGA as described is less than 25% and preferably less than 20% of the total weight loss between 250 °C and 950 °C.
• the weight loss is less than 35% and preferably less than 25% of the total weight loss between 450 °C and 600 °C.
• metals other than iron similar results will be obtained.
• thermocouples were placed in the central longitudinal axis of the catalyst bed to monitor the temperature profile in the bed. The feed gas inlet temperature was recorded by a thermocouple 10 mm above the catalyst bed.
• the reactor was equipped with a heating ribbon, which was uniformly wound around the reactor tube.
• the ribbon ensured a flat temperature profile in the catalyst bed when an inert gas was passed through the bed.
• the reactor behaved as an intermediate between an isothermal and adiabatic type of reactor.
• the reactor was placed in two concentric quartz tubes, with an inner diameter of 80 mm and 100 mm respectively.
• the catalysts were aged in a flow of 1 vol.% H 2 S in nitrogen, at a space velocity (GHSV) of 700 Nm 3 gas/ m 3 catalyst/h, for 16 hours at 350 °C. After ageing, the catalysts were cooled to test temperature. The catalysts were not pre-hydrated before testing.
• GHSV space velocity
• Moist feed gas containing 2000 ppmv H2S and 4000 ppmv O2 in nitrogen, was passed through the catalyst bed at an inlet temperature of 40 °C and a GHSV of 1000 Nm 3 gas/ m 3 catalyst/h.
• the * relative feed gas humidity was 95%.
• the test results after 20 hours absorption time are summarized in Table 2.
• catalysts according to the invention show clearly improved results when compared with the reference catalysts (A and
• the catalysts according to the invention maintain a high sulfur selectivity, even under relatively severe reaction conditions, i.e. in feed gases with high H 2 S and O2 levels. This is illustrated in the following Example.
• Example 1 Two 100 ml samples of catalyst C from Example 1 were tested for oxidative H2S absorption in a reactor as described in Example 1 and pre-aged by the method also described in Example 1.
• Table 4 shows the results of these runs with different feed gases after 20 hours absorption time.
• This example demonstrates the effectiveness of sulfur as a reducing agent in the regeneration of the catalyst.
• a 100 ml sample of catalyst C was tested in successive absorption/regeneration cycles in a reactor as described in Example 1 and pre- aged by the method also described in Example 1.

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