Method for removing sulphur dioxide from gas streams, using titanium dioxide as catalyst

Abstract

The present invention relates to a method for removing sulphur dioxide from gaseous effluent, wherein a mixture of gaseous outlet gasses or gaseous effluent includes sulphur dioxide and carbon monoxide, and wherein, to perform a catalytic reduction, a catalyst is used to catalyze a reaction between carbon monoxide and sulphur dioxide to produce carbon dioxide and sulphur.

Claims

1. A method comprising removing sulphur dioxide from gaseous effluent using a Claus catalyst, wherein the gaseous effluent includes sulphur dioxide, hydrogen and carbon monoxide, wherein the Claus catalyst comprises at least 90% by weight of titanium dioxide to catalyse a catalytic reaction between carbon monoxide and sulphur dioxide to produce carbon dioxide and sulphur to remove sulphur dioxide from the gaseous effluent, wherein the catalytic reaction is performed at a gauge pressure of from 0.1 bar(g) to 1 bar(g), and wherein the catalytic reaction occurs at a temperature from about 350? C. to about 450? C.

2. The method according to claim 1 wherein the Claus catalyst simultaneously catalyses a reaction between sulphur dioxide and carbon monoxide to produce carbon dioxide and sulphur and a reaction between hydrogen sulphide and sulphur dioxide to produce sulphur and water.

3. The method according to claim 1 wherein the Claus catalyst comprises at least 95% by weight of titanium dioxide.

4. The method according to claim 1 wherein the Claus catalyst consists essentially of titanium dioxide.

5. The method according to claim 1 wherein the gaseous effluent comprises hydrogen sulphide.

6. The method according to claim 1, wherein the catalytic reaction is performed at a temperature from about 350? C. to a temperature below 420? C.

7. The method according to claim 1, wherein the catalytic reaction is performed at a temperature from about 350? C. to a temperature below 390? C.

8. A method for removing sulphur dioxide from gaseous effluent by performing a catalytic reduction using a Claus catalyst: wherein the gaseous effluent includes sulphur dioxide, hydrogen and carbon monoxide, wherein the catalytic reduction is performed at a gauge pressure of from 0.1 bar(g) to 1 bar(g), wherein the catalytic reduction is performed using titanium dioxide to catalyse a reaction between carbon monoxide and sulphur dioxide to produce carbon dioxide and sulphur such that a majority of the carbon monoxide reacts, and wherein the catalytic reduction is performed at a temperature from about 350? C. to about 450? C.

9. The method according to claim 8 further comprising the steps of: a. performing a thermal reduction step on the gaseous effluent to produce sulphur and a mixture of gaseous outlet gasses; b. optionally, separating the mixture of gaseous outlet gasses from the sulphur; and c. subsequently performing the catalytic reduction on the mixture of gaseous outlet gasses.

10. The method according to claim 9 wherein the catalytic reduction is performed in a reactor having a reactor inlet for receiving the mixture of gaseous outlet gases and a reactor outlet wherein the temperature of the mixture of gaseous outlet gases at the inlet is less than about 250? C.

11. The method according to claim 9, wherein the thermal reduction step comprises the step of reacting sulphur dioxide and a fuel gas in a furnace.

12. The method according to claim 11 wherein the fuel gas comprises methane.

13. The method according to claim 11, wherein the sulphur dioxide and fuel gas are heated sufficiently so that they react by combusting the fuel gas and oxygen in the furnace.

14. The method according to claim 8 further comprising the steps of: a. performing a fuel supported Claus reaction on the gaseous effluent to produce sulphur and a mixture of gaseous outlet gasses; b. optionally, separating the mixture of gaseous outlet gasses from the sulphur; and c. subsequently performing the catalytic reduction on the mixture of gaseous outlet gasses.

15. The method according to claim 14, wherein the fuel supported Claus reaction comprises the step of reacting hydrogen sulphide and oxygen in a furnace.

16. The method according to claim 15 wherein the fuel gas comprises methane.

17. The method according to claim 15, wherein the hydrogen sulphide and oxygen are heated by combusting a fuel gas and oxygen in the furnace.

18. The method according to claim 8 wherein the catalyst has one or more of a surface area of at least about 120 m2/g, a bulk density of from about 650 to about 920 kg/m3, and a total pore volume (Hg) of from about 0.45 to about 0.60 cm3/g.

19. The method according to claim 8 wherein the catalyst also catalyses the reaction between hydrogen sulphide and sulphur dioxide to make sulphur and/or also the hydrolysis of carbon-sulphur compounds, including carbonyl sulphide and/or carbon bisulphide.

20. The method according to claim 8, wherein the catalytic reduction is performed at a temperature from about 350? C. to a temperature below 420? C.

21. The method according to claim 8, wherein the catalytic reduction is performed at a temperature from about 350? C. to a temperature below 390? C.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) The above-mentioned and other features and objects of this invention, and the manner of obtaining them, will become more apparent and the invention itself will be better understood by reference to the following description of embodiments of the invention taken in conjunction with the accompanying drawings, wherein:

(2) FIG. 1 is a schematic of an exemplary process according to the invention;

(3) FIG. 2 is a schematic of an alternative exemplary process according to the invention; and

(4) FIG. 3 shows a catalytic reactor according the sixth aspect of the invention.

(5) Although the drawings represent exemplary embodiments of the present invention, the drawings are not necessarily to scale and certain features may be exaggerated to better illustrate and explain the invention. The exemplification set out herein illustrates exemplary embodiments of the invention only.

DETAILED DESCRIPTION

(6) The present invention provides a method for removing sulphur dioxide from gaseous effluent or a mixture of gaseous outlet gasses, preferably the gaseous effluent of a smelter furnace, by performing a catalytic reduction reaction, wherein the mixture of gaseous outlet gasses or effluent includes sulphur dioxide and carbon monoxide, and wherein the catalytic reduction is performed using a catalyst which catalyses a reaction between carbon monoxide and sulphur dioxide to produce carbon dioxide and sulphur.

(7) Typically, the method comprises the steps of: a. performing a thermal reduction step on the effluent to produce sulphur and a mixture of gaseous outlet gasses; b. separating the mixture of gaseous outlet gasses from the sulphur; and c. subsequently performing a catalytic reduction on the mixture of gaseous outlet gasses.

(8) Alternatively, the method comprises the steps of: a. performing a fuel supported Claus reaction to produce sulphur and a mixture of gaseous outlet gasses; b. optionally, separating the mixture of gaseous outlet gasses from the sulphur; and c. subsequently performing the catalytic reduction on the mixture of gaseous outlet gasses.

(9) FIG. 1 shows a schematic representation of an exemplary process according to the invention.

(10) Thermal reduction of sulphur dioxide is performed in the furnace. The feed gasses for the furnace comprise a fuel gas, sulphur dioxide and air. Typically, substantially no other gasses are present in the feed gases.

(11) Typically, the sulphur dioxide is collected from the effluent of a smelter furnace. Preferably, the effluent undergoes an absorption and regeneration process in order to provide sulphur dioxide for thermal reduction. Preferably, separating the sulphur dioxide from the remainder of the smelter effluent provides concentrated sulphur dioxide and effluent suitable for discharge into the atmosphere.

(12) Typical absorption and regeneration processes include, but are not limited to, carbon bed, solvent and chemical base processes, including amine gas treatment. Such processes and equipment for performing sulphur dioxide absorption and regeneration process are known in the art.

(13) The air is collected from the surrounding environment. For the avoidance of doubt, it contains oxygen. Alternatively, pure oxygen or oxygen enriched air can be used.

(14) The fuel gas is preferably natural gas, although it may also be selected from the group consisting of methane, ethane, propane, carbon monoxide or mixtures thereof.

(15) The fuel gas for reducing the sulphur dioxide is heated by combusting the fuel gas with oxygen. Typically, the fuel gas and sulphur dioxide are heated to a temperature of at least about 1000? C., preferably at least about 1100? C., more preferably at least about 1250? C., preferably from about 1200? C. to about 1400? C., preferably from about 1000? C. to about 1500? C.

(16) The products from the thermal reduction step flow from the furnace to waste heat boiler or condenser where they are cooled such that the sulphur formed in the reduction step condenses. The condensed sulphur is preferably removed.

(17) The products of the thermal reduction step are then transferred to a catalytic reactor to undergo the method according to the invention. FIG. 3 illustrates a catalytic reactor (1) according to the invention.

(18) The catalytic reactor removes sulphur dioxide from effluent gas mixture. In the exemplified case, the effluent gas mixture is the mixture of gaseous products from the thermal reduction step with the sulphur removed, although, in alternative arrangements, the sulphur may be present.

(19) The effluent gas mixture entering the reaction chamber comprises sulphur dioxide and carbon monoxide, and the catalyst catalyses a reaction between carbon monoxide and sulphur dioxide to produce carbon dioxide and sulphur, preferably as well as promoting the Claus reaction and hydrolysis of carbon-sulphur compounds contained in the effluent gases from the furnace.

(20) Following the reaction in the reactor, the sulphur can be removed using a separate condenser. Preferably, the sulphur remains gaseous while present in the reactor, so as to prevent fouling of the catalyst.

(21) The remaining hydrogen sulphide and sulphur dioxide in the outlet gasses are treated using the catalytic stages of a typical Claus process. The Claus process is well known to those skilled in the art, as are the catalytic steps conducted therein. The general formula for such catalysis is 2H.sub.2S+SO.sub.2.fwdarw.3S+2H.sub.2O. In the case of the invention, the Claus process catalytic steps are used to increase sulphur recovery.

(22) In the Claus process, the catalytic recovery of sulphur consists of three sub-steps: heating, catalytic reaction and cooling plus condensation. These three steps are normally repeated a maximum of three times. The more Claus reactors that are used, the better the recovery of sulphur from the process. In the embodiment shown in FIG. 1, incineration and a tail-gas treatment unit (TGTU) are used downstream of the Claus catalytic stages.

(23) Returning to FIG. 3, the catalytic reactor (1) comprises a reactor chamber (2) and a catalyst bed (3). The reactor chamber (2) is lined with refractory cement. The catalyst bed (3) comprises a layer of catalyst (5) supported by inert refractory balls in turn supported by a stainless steel mesh (4, 8) resting on a support structure (not shown). Typically, the layer of catalyst (5) will be between about 500 mm and about 2000 mm deep, preferably between about 750 mm and 1500 mm deep, preferably between about 1000 mm and about 1200 mm deep. Baffles (6, 7) are used to improve the distribution of gases within the reaction chamber.

(24) The catalyst typically consists of titanium dioxide. The catalyst may be in the form of balls, pellets or extrudate.

(25) Typically, the catalyst has a surface area of at least about 200 m.sup.2/g, preferably at least about 240 m.sup.2/g. Typically, the catalyst has a bulk density of from about 650 kg/m.sup.3 to about 1000 kg/m.sup.3, preferably from about 750 to about 800 kg/m.sup.3. Preferably, the catalyst has a total pore volume (Hg) of from about 0.3 to about 0.65 cm.sup.3/g, preferably from about 0.50-0.6 cm.sup.3/g. Suitable catalyst is sold under the trade name S-7001 from Euro Support B.V.

(26) The reactor chamber (1) comprises an inlet (9) for receiving the effluent gas mixture and an outlet (10) for expelling a gaseous mixture including the products of the catalytic reduction step. The outlet (10) will typically be in fluid communication with a subsequent catalytic reactor chamber which performs the first subsequent catalytic step of the Claus process. Alternatively, and preferentially, a condenser may be positioned between the reactor chamber (1) of the invention and the subsequent Claus catalytic reactor which in use removes sulphur from the gaseous mixture including the products of the catalytic reduction step and Claus reaction occurring in the reactor of the invention. The inlet (9) will typically be in fluid communication with either the furnace for performing a sulphur dioxide thermal reduction or the furnace for a fuel supported Claus reaction or a condenser for removing sulphur from the gaseous products of the thermal reduction or fuel supported Claus reaction.

(27) FIG. 2 show an alternative exemplary embodiment according to the second embodiment of the first aspect of the invention, including performing a fuel supported Claus reaction to produce sulphur and a mixture of gaseous outlet gasses. The catalytic reactor (1) shown in FIG. 3 may be used in the process of FIG. 2.

Examples

(28) Pilot trials of a thermal process using natural gas to reduce sulphur dioxide to sulphur in an industrial scale Claus plant were carried out.

(29) A first catalytic reactor following the thermal stage contained titanium dioxide catalyst. The catalyst did not strongly catalyse hydrogenation or shift reactions which were demonstrated by the co-existence of sulphur dioxide and hydrogen in the reactor outlet gasses.

(30) However significant amounts of carbon monoxide did react and this was explained by the direct reaction of carbon monoxide and sulphur dioxide.

(31) This unexpected reaction on a hydrolysis catalyst has significant advantages over the use of a hydrogenation catalyst.

(32) The analysis of gases to and from the CO removal reactor was as follows in Table 1.

(33) TABLE-US-00001 TABLE 1 REACTOR INLET REACTOR OUTLET Mol % Mol % H2 1.747 1.771 O2 0.000 0.000 N2 27.660 31.173 CO 4.640 0.757 CO2 40.363 53.892 H2S 14.617 11.173 COS 4.040 0.469 SO2 6.903 0.762 CS2 0.030 0.003 TOTAL 100.000 100.000

(34) The analysis was on a dry basis and does not show elemental sulphur.

(35) The removal of SO.sub.2 and creation of CO.sub.2 is consistent with the overall reactions: SO.sub.2 removal: (Overall reactions shown)
SO.sub.2+2H.sub.2S=3S+2H.sub.2O
SO.sub.2+2CO=2CO.sub.2+S
SO.sub.2+2COS=2CO.sub.2+3S
SO.sub.2+CS.sub.2=CO.sub.2+3S
SO.sub.2+2H.sub.2=S+2H.sub.2O
CO.sub.2 creation: (overall reactions shown)
SO.sub.2+2CO=2CO.sub.2+S
SO.sub.2+2COS=2CO.sub.2+3S
SO.sub.2+CS.sub.2=CO.sub.2+3S
NoteS is shown for conveniencethe sulphur formed is S.sub.6 or S.sub.8.

(36) From the measured results the above overall reactions are consistent with the sulphur dioxide used and carbon dioxide formed and negligible amounts of sulphur were reconverted to hydrogen sulphide.

(37) Laboratory trails were carried out to test the action of the titanium dioxide catalyst with a mixture of gases predicted from the thermal stage of a Claus type sulphur recovery process or a sulphur dioxide reduction process.

(38) The gas mixture fed to the catalytic reactor contained hydrogen sulphide, sulphur dioxide, carbon dioxide, nitrogen, carbon monoxide, hydrogen, carbonyl sulphide, carbon bisulphide, sulphur vapour and water vapour.

(39) The gas composition was such that there was an excess of H.sub.2S over SO.sub.2 in the effluent gases from the catalyst, which is typical for industrial Claus plant operations.

(40) There was a sharp rise in carbon monoxide conversion to practically 100% at around 350? C., and the hydrogen conversion increased slowly only after 420? C. was reached.

(41) Most of the carbonyl sulphide and carbon bisulphide were destroyed. (i.e. a normal reaction for the first catalytic stage of a Claus plant).

(42) The results show that for this catalyst carbon monoxide reacts with sulphur dioxide at a lower temperature than hydrogen reacts with sulphur dioxide. This means that the catalyst temperature can be adjusted to avoid reaction of hydrogen while allowing the reaction of carbon monoxide.

(43) It will be appreciated by those skilled in the art that the foregoing is a description of a preferred embodiment of the present invention and that variations in design and construction may be made to the preferred embodiment without departing from the scope of the invention as defined by the appended claims.