The present invention relates to the use of gas-solid sepa ^¬ ration unit operations for the removal of sulfur trioxide (SO ₃ ) formed by catalytic oxidation of hydrogen sulfide (¾S) with the purpose of removing ¾S from a gas. More specifically, this catalytic oxidation of ¾S first yields sulfur dioxide (S0 ₂ ) and then SO ₃ through the use of known catalysts, and the subsequent recovery of SO ₃ takes place in a separation unit, such as a filter or scrubbing unit, using a sorbent, which converts SO ₃ to a sulfate to be sep ^¬ arated in said unit.

The process of removing ¾S from a gas can be summarized schematically as follows: A potentially pre-heated ¾S- containing gas is mixed with air or oxygen, and then the mixture is fed to a first catalyst-containing reactor via a heat exchanger. In this first reactor, ¾S is oxidized to sulfur dioxide (S0 ₂ ) · The effluent from the first reactor is passed to a second catalyst-containing reactor, where the SO ₂ is oxidized to SO ₃ . Then the S03-containing effluent is fed to a separation unit, into which a sorbent slurry or powder is injected. The purpose of this is to remove SO ₃ by converting it to a sulfate that can be separated from the gas.

The energy efficiency of the plant can be improved signifi ^¬ cantly by means of heat integration, i.e. the cold feed gas is heated to a suitable temperature by the hot effluent gas from the last reactor or the separation unit in a heat exchanger. Alternatively, the hot effluent gas can be used to produce steam. The first oxidation reactor contains a monolith type cata ^¬ lyst, and the second oxidation reactor contains a supported liquid phase (SLP) catalyst, more specifically a VK cata- lyst.

The ¾S can also, on purpose, be oxidized directly to SO ₃ in said first reactor by proper choice of oxidation cata ^¬ lyst and reaction conditions. In this case, the effluent from the first reactor is fed to the particle separation unit for removal of SO ₃ in the form of a sulfate. As oxida ^¬ tion catalyst for this direct oxidation to SO ₃ , a noble metal catalyst, such as a Pt/Pd catalyst, is used.

With the exception of the case of sulfuric acid plants, SO ₃ is generally undesired in process gas streams due to the risk of condensation of sulfuric acid and subsequent corro ^¬ sion of the equipment operating below the acid dew point. Therefore, the SO ₃ content in process gas streams is nor ^¬ mally kept at a minimum. In the method of the present in ^¬ vention, ¾S is first oxidized to SO ₂ and subsequently to SO ₃ on purpose, because this approach offers the most cost efficient method for the removal of ¾S according to the present invention. Especially capital investment costs are reduced .

Usual routes to abatement of sulfur are solutions of absor ^¬ bent type for low concentrations of ¾S, whereas higher concentrations of ¾S can be used for production of chemi- cals, e.g. elemental sulfur or sulfuric acid. For a variety of concentrations, combustion (understood as thermal non- catalytic oxidation) can also be used. The use of gas-solid separation unit operations for the removal of SO ₃ according to the present invention can be seen as an alternative measure to reduce the chemical consumption cost and reduce the water consumption with a minimal need for installed equipment, said measure especially being usable for ¾S levels between a few hundred ppm and a few percent.

The present invention utilizes catalytic oxidation of ¾S to SO ₂ at temperatures between 150 and 500°C, preferably between 180 and 450°C and most preferably between 200 and 400°C. In comparison with combustion, which takes place at temperatures above 800°C, catalytic oxidation therefore of ^¬ fers the possibility of reducing the use of supplemental fuel in order to increase the temperature, thereby lowering the operating costs. Furthermore, the catalytic oxidation of ¾S can be performed at an oxygen concentration of below 2 vol%, measured at the outlet of the ¾S oxidation reac ^¬ tor, whereas combustion of ¾S typically requires an oxygen concentration of more than 3 vol% at the outlet of the fur- nace . This means that the process gas flow is reduced com ^¬ pared to combustion, thereby reducing both investment and operating costs.

The sulfate separated from the gas in the method according to the invention can easily be handled and sold, e.g. for the construction industry, or it can be landfilled. For certain applications, these are cost efficient options for the removal of ¾S compared to the production of elemental sulfur or sulfuric acid.

In the process of removing ¾S from a gas, a monolithic type catalyst is preferably used in the reactor converting ¾S to SO ₂ . This catalyst is a corrugated fibrous monolith substrate coated with a supporting oxide. It is preferably coated with T1O ₂ and subsequently impregnated with V ₂ O ₅ and/or WO ₃ . The channel diameter of the corrugated monolith is between 1 and 8 mm, and the wall thickness of the corru ^¬ gated monolith is between 0.1 and 0.8 mm.

The monolith type catalyst is preferably manufactured from a support material comprising one or more oxides of metals selected from aluminium, silicon and titanium, and the active catalytic components preferably comprise one or more oxides of a metal selected from vanadium, chromium, tungsten, molybdenum, cerium, niobium, manganese and copper. Said materials are effective in the catalytic oxidation of hydrogen sulfide at low temperatures.

The VK catalysts are specifically designed by the applicant to be used for converting SO ₂ to SO ₃ in any sulfuric acid plant. They are generally vanadium-based and may contain cesium as an additional catalyst promoter to enhance the action of the vanadium and activate the catalyst at a much lower temperature than conventional non-cesium catalysts. A major leap in activity has been obtained with VK catalysts containing a high fraction of vanadium in the active oxida- tion state V ⁵⁺ .

Monoliths are increasingly being used, developed, and eval ^¬ uated as catalyst supports in many new reactor applications such as chemical and refining processes, catalytic oxida- tion, ozone abatement etc. When the active catalyst has a monolithic structure, it displays a low pressure drop. According to the invention, a number of gas-solid separation units and unit operations can be used for the removal of SO ₃ formed by catalytic oxidation of ¾S. The separation units include metal filters, electrofilters , bag filters, ceramic filters, mist filters, cyclones and scrubbers, and unit operations for the gas-solid separation include use of low velocity filter, wet ESP (electrostatic precipitator) and other kinds of polishing technologies. Wet and dry scrubbing processes are widely used to remove

502 from gas coming from combustion processes, such as in power plants. Typically, the SO2 levels from power plants are in the range from 100 to 2000 ppm, with corresponding

50 ₃ levels from the combustion depending on the combustion temperature. The amount of SO ₃ in the tail gas is within the range from 0.5% to 5% of the total SO _x ( S0 ₂ + S0 ₃ ) amount, giving an SO2 to SO ₃ ratio in the range of 20 to 200. The present invention utilizes catalytic oxidation of SO2 to SO ₃ . Typically, more than 80%, preferably more than 90% and most preferably more than 95% of the SO2 is converted to SO ₃ , resulting in an SO2 to SO ₃ ratio below 0.25, prefer ^¬ ably below 0.11 and most preferably below 0.05. The high content of SO ₃ makes the dry scrubbing process more effi ^¬ cient, resulting in lower scrubbing costs.

It would be possible to oxidize ¾S contained in a gas to SO2 catalytically and subsequently remove this SO2 using a wet or a dry scrubber. It would likewise be possible to catalytically oxidize the SO ₂ in a gas stream from a combustion process, such as a power plant, to SO ₃ and subsequently remove this SO ₃ using a dry scrubber.

A dry scrubber system is described in US 2013/0294992, which concerns an air quality control system useful for processing a gas stream, such as a flue gas stream emitted from a fossil fuel fired boiler, for at least partial re- moval of acidic and other polluting species, such as SO ₂ ,

SO ₃ , HC1, HF, fly ash particulates and/or other acidic pol ^¬ luting species, therefrom.

US 4.314.983 describes a process for converting H ₂ S to SO ₂ with a solid catalyst comprising at least 5 wt% of bismuth. Essentially no SO ₃ is formed in the catalytic process. In this patent it is stated that the bismuth content is neces ^¬ sary to stabilize the catalyst. US 2014/020399 describes a method for generating current from an exhaust gas containing ¾S. The exhaust gas is com ^¬ busted, possibly under addition of supplementary fuel, and the heat released is used for current generation. The SO ₂ and the SO ₃ in the gas after combustion of the ¾S are de- livered for desulfurization .

US 2004/0109807 describes a method for removing SO ₃ from flue gases, where a calcium hydroxide slurry is injected into the off-gases in the exhaust duct of an industrial plant, wherein sulfur-containing fuels are combusted. The calcium hydroxide slurry reacts with SO ₃ produced as a re ^¬ sult of the combustion process and forms a primary solid calcium sulfate reaction product. The industrial plant in ^¬ cludes a wet scrubbing system which utilizes wet slaking of calcium oxide for the removal of sulfur oxides from off- gases .

US 6,143,263 describes a method and a system for removing SO ₃ and SO ₂ from a flue gas produced by combusting of a fossil fuel. A calcium-based, sodium-based or magnesium- based dry sorbent is injected into the flue gas to react with and remove substantially all of the SO ₃ from the flue gas, thereby producing a substantially S03-free flue gas containing both reacted dry sorbent and unreacted dry sorbent. The flue gas with reacted and unreacted dry sorbent is then fed to a wet scrubber to remove both the reacted and the unreacted dry sorbent, thereby making the unreacted dry sorbent available as a wet reagent for SO ₂ removal .

In US 2005/0201914, a method for treating a flue gas stream to remove strong acid compounds, selected from HF, HC1, H ₂ SO ₄ and SO ₃ , is described. The compounds are removed by injecting a sodium sorbent, selected from sodium sesquicarbonate, sodium carbonate-bicarbonate and various forms of ^x trona' (trisodium hydrogen dicarbonate-dihydrate, a non- marine evaporite mineral also called sodium sesquicarbonate dihydrate; Na ₃ (C0 ₃ ) (HC0 ₃ ) · 2H ₂ 0) into the flue gas stream. Substantially all of the sodium sorbent is calcined in the presence of the flue gas stream to form a soda ash, and the concentration of the strong acid compounds in the flue gas is reduced by reaction with the soda ash to form a sodium- based by-product. US 2010/0096594 describes a process for decontaminating syngas by contacting the syngas with one or more sorbents upstream of a catalytic candle filter containing a mixed cracking catalyst, wherein ammonia and tars are removed, leaving a purified syngas.

US 2009/0277325 discloses an emission treatment system, in which particles entrained in an emission stream can be removed using various separation devices, such as electro- static precipitators, cyclone collectors and filter devic ^¬ es, e.g. candle filters.

Finally, WO 2004/037369 describes a system and a method for removing ¾S and SOx from a gas. The system comprises a sorbent slurry feeder and at least one reaction zone con ^¬ figured for introduction of a sorbent and the gas, and it also comprises fabric filter bags, wet scrubbing and elec ^¬ trostatic precipitator technologies. However, it does not comprise a mist filter in combination with the separation unit for removal of acid mist and also not a quench scrub ^¬ ber for the removal of residual SO ₃ , which is not present as a mist.

The treatment method for the removal of sulfur trioxide un- derlying the present invention differs from the prior art techniques in that a pre-heated gas containing ¾S is mixed with air, and the mixture is fed to a first catalyst- containing reactor via a heat exchanger. In this first reactor, ¾S is oxidized to sulfur dioxide (S0 ₂ ) according to the reaction

1.5 0 ₂ + H ₂ S → S0 ₂ + H ₂ 0 (1) The catalyst in the first reactor is a monolith type cata ^¬ lyst as described earlier. The catalyst can be manufactured from various ceramic mate ^¬ rials used as a carrier, such as titanium oxide, and active catalytic components are usually either oxides of base met ^¬ als (such as vanadium, molybdenum and tungsten) , zeolites, or various precious metals. Catalysts of monolithic struc- ture are known to provide favourable performance with re ^¬ spect to selectivity when the desired reaction is fast and the undesired reaction is slow. This is also the case in the present invention, where the conversion of ¾S to SO ₂ is a fast reaction that benefits from the high surface area whereas the low load of active material per volume in a monolithic structure restricts the rate of oxidation of SO ₂ to SO ₃ , thereby enabling full control of the catalytic re ^¬ actions and subsequently reducing the risk of corrosion or fouling of the equipment.

Then the effluent from the first reactor is passed to a second catalyst-containing reactor, where the SO ₂ is oxi ^¬ dized to SO ₃ according to the reaction 2 S0 ₂ + 0 ₂ → 2 S0 ₃ (2)

The catalyst used in this reaction is selected among the applicant's VK catalysts, which are so-called supported liquid phase (SLP) catalysts. With SLP catalysts, the oxi- dation of SO ₂ takes place as a homogeneous reaction in a liquid film consisting of V ₂ O ₅ dissolved in alkali-metal pyrosulfates on an inactive porous silica support made from diatomaceous earth. This second catalytic step results in an SO ₂ to SO ₃ ratio which is much lower than what is seen in desulfurization processes utilizing sorbents for direct removal of SO ₂ . The introduction of this catalytic step is deemed to be particularly inventive because it makes the desulfurisation of the gas stream, here in the form of adsorbing SO ₃ , sulfate formation and separation, very cost effective compared to known wet scrubbing solutions for S0 ₂ .

Finally a sorbent, preferably an alkaline sorbent, is added to the SO ₃ containing gas and S03 is absorbed as sulfate. Subsequently, the gas and solid is fed to a gas-solid sepa ^¬ ration unit, where the spent sorbent with sulfur is sepa- rated from the gas. The solid discharge of sulfate and par ^¬ tially spent sorbent can be mixed with water and re ^¬ injected in the system.

A preferred alkaline sorbent to be injected is calcium hy- droxide (Ca(OH)2), but instead of calcium hydroxide, calci ^¬ um carbonate may be used. Other alkaline sorbents may be used as well. For example it is possible to use a magnesi ^¬ um-based sorbent, such as magnesium oxide or magnesium hydroxide, or a sodium-based sorbent, such as sodium car- bonate .

Further it has turned out that certain sodium-based alka ^¬ line sorbents, such as sodium bicarbonate ( aHCOs) and Tro- na (trisodium hydrogendicarbonate dihydrate, also known as sodium sesquicarbonate dihydrate; Na ₃ (C0 ₃ ) (HC0 ₃ ) ·2Η ₂ 0) , are more reactive with SO ₂ than calcium-based sorbents in the temperature range from 135 to 500°C. Thus, the present invention concerns a method for removing sulfur trioxide from a gas obtained by oxidizing hydrogen sulfide to sulfur trioxide in at least one catalyst- containing reactor, said method comprising feeding the effluent from the last reactor to a separation unit for sul ^¬ fur trioxide removal, wherein the effluent from the last reactor is mixed with a slurry or powder of a sorbent or a combination of sorbents, injected into the mixture through one or more injection points in the separation unit or upstream of it, to form a mixture of a sulfate, unused sorbent and a hot clean gas, from which the sulfate and unused sorbent is subsequently separated by any means selected among gas-solid separation unit operations, and wherein a mist filter in combination with the separation unit is used for removal of acid mist, while residual SO ₃ , which is not present as a mist, is removed in a quench scrubber .

The sorbents are preferably alkaline sorbents. The separation unit, which constitutes the crux of the pre ^¬ sent invention, can be selected from a range of devices based on gas-solid separation unit operations and suitable for the removal of absorbed SO ₃ formed by catalytic oxida ^¬ tion of ¾S. As already mentioned, a number of gas-solid separation units and unit operations can be used for said removal of SO ₃ . These separation units include (without be ^¬ ing limited thereto) various metal filters, electrofilters , bag filters, ceramic filters, mist filters, cyclones and dry scrubbers. The preferred choices are bag filters, metal filters and ceramic filters because of build-up of filter cake, which enhances the scrubbing efficiency.

The filter unit may be a catalytic filter unit.

The unit operations for the gas-solid separation include use of polishing technologies such as low velocity filter, wet ESP (electrostatic precipitator) and other kinds of technologies. Use of these technologies in combination with a dry scrubber makes it possible to go from 10-30 ppm SO ₃ at the outlet of the separation unit down to practically 0 ppm emissions.

More specifically, the sorbent can be injected into the ducting or an absorption unit, such as a fluidized bed or entrained flow reactor, placed between the last reactor, in which the catalytic oxidation of SO ₂ to SO ₃ takes place, and a separation unit, e.g. a bag filter or a ceramic filter, to lower the acid dew point by removing SO ₃ from the gas, and enable cooling of the gas to a temperature that is suitable for bag filter operation. This temperature will typically be in the range from 150 to 500°C.

According to the invention it is possible to use one or more sorbent injection points. In case of more sorbent in ^¬ jection points, these can be either in the separation unit, upstream of it or a combination of both. For example, one injection point may be placed in the ducting between the last reactor and the separation unit and a second injection point may be placed in the housing of the separation unit. Different modes of sorbent injection can be used. These in ^¬ clude (without being limited thereto) dry injection, spray drying injection and in-duct injection.

In addition, a mist filter in combination with the separation unit is used for removal of any acid mist. Residual SO ₃ , which is not present as a mist, is removed in a quench scrubber, e.g. a venturi scrubber. A venturi scrubber is designed to effectively use the energy from an inlet gas stream to atomize the liquid being used to scrub the gas stream.

For gases with a high ¾S concentration, recovery of ele- mentary sulfur is considered to be a cost effective method for desulfurization . For lean gases, oxidation to SO ₂ and subsequent wet scrubbing of SO ₂ or absorption of ¾S in a disposable sorbent is considered to be a cost effective method for desulfurization . The present invention is con- sidered to be a cost effective method for desulfurization in a range between lean and strong ¾S gases, specifically in the range from 25 ppm to 50000 ppm ¾S, preferably from 100 ppm to 20000 ppm ¾S and most preferably from 200 ppm to 15000 ppm ¾S. An especially preferred interval is from 300 ppm to 10000 ppm ¾S. Especially the investment costs are considered to be lower than those of competitive tech ^¬ nologies in these intervals.

The catalyst for oxidizing the ¾S to SO ₂ is preferably im- pregnated with V ₂ O ₅ and one or more oxides of a metal se ^¬ lected from chromium, tungsten, molybdenum, cerium, niobium, manganese and copper. Preferably a part of the effluent from the first or the second reactor is recycled to the ¾S containing feed gas. The overall method according to the invention can be carried out in a plant for oxidizing hydrogen sulfide to sul ^¬ fur trioxide. Such plant, which is depicted on the appended figure, mainly consists of two oxidation reactors Rl and R2 for the above oxidation reactions (1) and (2), respective- ly, and a separation unit for the removal of absorbed sul ^¬ fur trioxide from the process gas. The plant further com ^¬ prises a unit for pre-heating the H ₂ S-containing gas, and a heat exchanger (Hex) . In the heat exchanger, the gas is heated to a temperature of 150-500°C before entering the first reactor Rl . Following the reaction (1) in Rl the effluent gas is either fed to the reactor R2 at a temperature of 300-500°C or fed directly to the separation unit (as shown by the dotted line in the figure) . After the reaction (2) in R2 the resulting S0 ₃ -containing gas is led to the separation unit, where a sorbent, preferably an alkaline sorbent, or a combination of two or more such sorbents is injected to remove SO ₃ .

The SO ₃ ends up as sulfate in the filter cake, possibly to- gether with an excess of oxide. The cleaned gas with a tem ^¬ perature around 150-500°C is passed through the heat ex ^¬ changer for heating up the feed gas, and it leaves the heat exchanger as a cleaned gas with a temperature around 100 °C. In the above plant design, all oxidation catalysts can fit into the reactors, and the separation unit is replacing similar technologies where wet caustic scrubber systems are used. A major advantage in this respect is that the caustic chemicals cost will be markedly reduced, and a hot clean gas is produced, which can be used in the heat exchanger of the plant as mentioned above.