The present invention relates to a catalyst for use in the selective oxidation of hydrogen sulfide (¾S) to elemental sulfur.

It is known from the refinery industry that the selective reaction of ¾S to elementary sulfur can be catalyzed, and this reaction is used as a final catalytic reaction step connected to the well-known Claus technology under the trade names SuperClaus and EuroClaus, where EuroClaus (a trade mark of Jacobs Nederland B.V.) is a process including an upstream hydrogenation of all sulfur components followed by the selective ¾S to S conversion step. The Applicant has developed the POC (preferential oxidation catalyst) and a POC process to treat PhS-containing off-gases from the viscose industry, where the special POC catalyst is capable of catalyzing the ¾S to S reaction but displays a very low activity for the Claus reaction, and thus the reaction of ¾S to yield S is not reversed, even at the very high oxy gen levels present in a viscose off-gas. Furthermore, due to the fact that the viscose off-gases contain particulates and due to the advantages of a high surface area and a low diffusion resistance, the POC catalyst is in monolithic form. The advantageous result is that plugging is avoided, and a high reaction rate is achieved along with a low degree of by-product formation. A patent application directed to the POC process was filed by the Applicant in 2016. Said patent application, now pub lished as WO 2016/065173 A1 , concerns a method for cleaning an off-gas from viscose production, essentially containing ¾S and CS2, which comprises passing the gas through a cat- alytic reactor to convert ¾S in the gas to elemental sul fur, SO2 or mixtures thereof, either via the oxygen present in the gas or via oxygen added to the gas stream. Elemental sulfur and SO2 are removed from the effluent gas from the catalytic reactor, and the unconverted CS2 is recycled to the viscose production process.

US 2005/0147554 A1 describes the use of a wet oxidation process to control the concentration of reduced sulfur com pounds from a viscose process. The wet oxidation process oxidizes the reduced sulfur compounds to convert them to an alkali sulfate and/or thiosulfate species. The alkali sul fate stream is recycled to the viscose process. The oxida tion is performed under elevated temperature and pressure conditions, such that the off-gas has an oxygen concentra tion between about 2% and about 18%, and the alkali sulfate stream has a pH of about 2 to 10.

In US 6.416.725 Bl, sulfurous gas streams comprising ¾S and CS2, such as produced as a by-product of the process of rayon-forming, are processed to recover the components in a usable form. First the gas stream is contacted with aqueous NaOH to dissolve out ¾S and some of the CS2. The dissolved CS2 is driven off from the solution and condensed out as a liquid concentrate. Any CS2 remaining in the gas stream is recovered, such as by condensation. The aqueous sodium sul fide solution, which remains from removal of CS2, is con centrated and the pH is adjusted, as necessary, to a value at which the sodium sulfide is predominantly in the form of sodium bisulfide. The concentrated sodium bisulfide solu tion is capable of being re-used in the rayon-forming pro cess along with the liquid concentrate of CS2.

WO 97/32813 A1 describes a catalyst for the selective oxi- dation of sulfur compounds, in particular ¾S to elemental S. In an example of preparing the catalyst, cylindrical S1O2 (silica) extrusions of a diameter of 2.1 mm and a length of 10 mm (monoliths) were impregnated with iron cit rate and zinc citrate followed by drying and calcination. The loading of the S1O2 is described to be 5 wt% of iron oxide and 10 wt% of zinc oxide. A method for the selective oxidation of ¾S to elemental S, where a H2S-containing gas together with an oxygen-containing gas is passed over the catalyst, is also described. Furthermore, it is disclosed that the temperature of the catalyst bed must be above the dew point temperature of the sulfur formed.

Various embodiments of waste gas recovery and adsorption systems for waste gases from viscose fiber production are described in Chinese utility model and patent applications CN 204107303 U, CN 204134460 U, CN 201115810 Y and CN

101219319 A, generally employing alkali cleaning/adsorption and condensation methods. While relevant, however, these CN applications do not anticipate the present invention.

At present, the general standard solution for removing sul- fur from viscose off-gases is to use LoCat®-type technology for enrichment of ¾S combined with a standard Claus reac tion situated downstream. This technology makes use of a proprietary liquid redox catalyst that converts ¾S to solid elemental sulfur through a direct oxidation of ¾S (¾S + ½ O ₂ H ₂ O + S°) . This reaction consists of five se quential steps: Absorption of ¾S, ionization of ¾S, cata lytic oxidation of sulfide, absorption of oxygen and iron oxidation to get Fe ²⁺ back to the Fe ³⁺ state.

It has now been found that Applicant's POC catalyst is ca pable of selectively converting ¾S to elemental S in a gas containing up to more than 20 wt% oxygen, as is the case in off-gas in the viscose industry, by using the oxygen that is sometimes present in the gas and/or by adding oxygen.

The sulfur can then be removed by condensation or by using a liquid contactor. This is very attractive in the applica tion of the POC catalyst to remove sulfur from coke oven gas and synthesis gas from coal gasification, biomass gasi fication and waste gasification.

More specifically, the present invention relates to a cata lyst for use in the selective conversion of hydrogen sul fide (¾S) to elemental sulfur (S) and SO ₂ in a gas that is passed through a catalytic reactor containing the catalyst, where the selective conversion of ¾S in the gas is estab lished either via the oxygen present in the gas or via oxy gen added to the gas stream, and where removal of the ele mental sulfur from the effluent gas from the catalytic re actor is done by condensation or by contact with a liquid contactor .

In the most preferred embodiment, the monolithic catalyst of the invention comprises a silica carrier impregnated with a material that is catalytically active in oxidizing ¾S to elemental sulfur and SO2, said material being embed ded in the walls of a corrugated monolith made from a fi- berglass matrix. A fibrous monolith provides a unique fea ture in that it provides a very open internal pore struc ture in the wall and simultaneously a high surface area which, uniquely, provides a benefit for the reaction versus side reactions. This cannot be accomplished with a standard extruded coated monolith or a coated corrugated metal mono lith.

Preferably, the material that is catalytically active in oxidizing ¾S to elemental sulfur and SO2 comprises an iron compound . In a preferred catalyst according to the invention, the iron compound is combined with zinc oxide. It is preferred that the iron compound in the catalyst according to the in vention contains from 1 to 15 wt% iron.

Laboratory experiments have demonstrated that Applicant's POC monolith, which is a monolithic catalyst where a silica carrier is embedded in the walls of a corrugated monolith made from a fiberglass sheet matrix, selectively oxidizes the ¾S present in a gas into elemental sulfur. The silica carrier is impregnated with an iron catalyst, optionally in combination with zinc oxide. The catalyst has very specific measurements in terms of height, width and depth as well as the distance between the corrugated glass fiber sheets.

Thus, the catalyst of the invention is a monolithic cata lyst embedded in the walls of a corrugated monolith made from a fiberglass matrix. It comprises a silica carrier that is preferably impregnated with a catalytic iron com pound. Advantageously, the iron compound can be combined with zinc oxide. The invention is further illustrated with reference to the attached figures, where

Fig. 1 and Fig. 2 are schematic drawings of a catalytic monolith of the present invention seen from the end and in cross-section, respectively, and

Fig. 3 is an enlarged view of the cross-sectional view of the catalytic monolith of the invention.

Fig. 1 shows a catalytic monolith 1 in the shape of a box, whilst Fig. 2 shows a spirally wound monolith 1' in cylin drical shape. The monoliths 1, 1' comprise full body porous material in the form of porous substrates coated and im pregnated with a catalytic substance. The substrates of the monolith of figures 1 and 2 are corrugated sheets of full body porous material 2, 2', also denoted "corrugated sheets", which are supported and separated from each other by substrates 3, 3' of substantially flat sheets of full body porous material, also denoted "liners" or "plates".

The corrugated and flat sheets 2,2’ ,3,3’ are full body po- rous material, i.e. they each comprise a porous substrate and a catalyst coating homogeneously distributed throughout the substrate, and each of the sheets 2,2' ,3,3' are suita bly impregnated.

The effects and advantages of the invention are as follows: The catalyst of the invention enables a simpler and more effective way to remove sulfur from viscose off-gas. The method is suitable for low and medium high sulfur contents. The technology is low in CAPEX and has a low OPEX due to the fact that no additional chemicals are needed to remove the sulfur compounds.

The technology can be combined with ¾S sorbent technology and further combined with hydrogenation of any slip of sul- fur components.