METHOD AND SYSTEM FOR CATALYTIC OXIDATION OF A LEAN H2S STREAM

Abstract

The present invention relates to a method and a system for catalytic oxidation of a lean H.sub.2S stream. More specifically, the invention concerns a novel way of removing sulfur dioxide (SO.sub.2) formed by catalytic oxidation of hydrogen sulfide (H.sub.2S) with the purpose of removing H.sub.2S from a gas. This catalytic oxidation of H.sub.2S yields sulfur dioxide (SO.sub.2) through the use of known catalysts, so-called SMC catalysts.

Claims

1. A method for the oxidation of hydrogen sulfide, carbonyl sulfide or carbon disulfide to sulfur dioxide, said method comprising the steps of: providing a feed gas lean in H.sub.2S, adding air or O.sub.2 to the feed gas, heating the feed gas to the desired inlet temperature, feeding the gas mixture to an oxidation reactor, which contains an oxidation catalyst, and recovering a cleaned gas, wherein syngas is added to the gas mixture prior to feeding the gas mixture to the oxidation reactor.

2. Method according to claim 1, wherein the oxidation catalyst is a sulfur monolith catalyst.

3. Method according to claim 1, wherein the gas mixture is fed to the oxidation reactor via a heat exchanger.

4. Method according to claim 1, wherein the oxidation of H.sub.2S proceeds at an inlet temperature to the catalyst between 200 and 450 C.

5. Method according to claim 4, wherein the inlet temperature to the catalyst is between 200 and 400 C.

6. A system for carrying out the method according to claim 1 for the oxidation of hydrogen sulfide to sulfur dioxide, said system comprising a gas blower, a heat exchanger and an oxidation reactor, which contains a catalyst.

Description

[0016] FIG. 1 illustrates a typical design as used in the conventional way of gas treatment. In addition to the catalytic oxidation reactor, the design includes two gas blowers, a heat exchanger and a burner to raise the temperature of the feed gas before entering the catalytic reactor.

[0017] FIG. 2 illustrates the design of an SMC unit to be used when carrying out the process of the invention. This design comprises only one gas blower, and use of a gas burner to raise the temperature of the feed gas is not needed.

[0018] By adding syngas to a lean H.sub.2S stream, which is to be treated by catalytic oxidation, the heating value of the gas will increase. Further, the amount of equipment needed is minimized, which also holds true for the size of the individual pieces of equipment, especially the size of the feed/effluent heat exchanger(s) constituting the most expensive part of the SMC unit. In addition, the OPEX (the operating expense) decreases because expensive feed gases or natural gas can be replaced by cheaper fuel gases, which also are oxidized at a lower air surplus. This decreases the air requirement and the duty of the blowers.

[0019] Basically the SMC technology is the catalytic oxidation of H.sub.2S at temperatures between 200 C. and 450 C. The oxidation reactions can increase the temperature across the reactor by 10 to 150 C. depending on the gas composition. The feed gas is often delivered at ambient temperatures, and heating of the feed gas can be accomplished by a feed/effluent heat exchanger. However, if the gas is lean (low temperature increase), such heat exchangers will become uneconomically large due to a low driving force for the heat exchange process. The heat exchanger itself will often constitute a large fraction of the overall CAPEX (capital expenditure) of an SMC unit. Therefore, a fired heater using fuel gas or natural gas is sometimes used to raise the temperature. This adds to both the CAPEX and the OPEX, since a burner as well as a combustion air blower is needed along with continuous consumption of fuel gas or natural gas. Furthermore, NO.sub.x (i.e. NO and/or NO.sub.2) may be formed in the burner. As mentioned above, use of a burner is not needed in the process of the invention.

[0020] The method according to the invention, which is especially relevant for coal gasification units, includes adding a small fraction of syngas (containing H.sub.2, CO and CO.sub.2) to increase the temperature raise across the reactor, since both CO and H.sub.2 can be oxidized over the SMC catalyst. This approach has the benefits of decreasing the size of the heat exchangers without adding more equipment to the plant. In addition, the oxygen consumption as well as the air blower power consumption will be lower since the SMC unit operates at a lower air surplus compared with a burner, and formation of NO.sub.x is avoided. Syngas is most often cheaper than fuel gas or natural gas, whereby the OPEX is reduced.

[0021] Methods as well as catalysts for oxidative conversion of H.sub.2S to SO.sub.2 are well-known in the art. Thus, EP 2 878 367 A1, belonging to the applicant, relates to materials consisting of V.sub.2O.sub.3 on a porous support, that are catalytically active in the oxidation of sulfur compounds, such as oxidation of H.sub.2S to SO.sub.2, at temperatures between 180 and 450 C.

[0022] U.S. Pat. No. 4,314,983 describes a process for converting H.sub.2S to SO.sub.2 with a solid catalyst comprising at least 5 wt % of bismuth. Essentially no SO.sub.3 is formed in the catalytic process. In this patent it is stated that the bismuth content of at least 5 wt % is necessary to stabilize the catalyst.

[0023] US 2014/0020399 describes a method for generating current from an exhaust gas containing H.sub.2S. The exhaust gas is combusted, possibly under addition of supplementary fuel, and the heat released is used for current generation. The SO.sub.2 and the SO.sub.3 in the gas after combustion of the H.sub.2S are delivered for desulfurization.

[0024] In RU 2.276.097 C, a process for selective catalytic oxidation of H.sub.2S is disclosed. The catalyst is iron oxide supported by a porous oxide. A similar process, in which a different catalyst is used, is disclosed in RU 2.533.140 C.

[0025] In none of these documents the possibility of adding syngas to the feed gas prior to entering the catalyst-containing oxidation reactor is mentioned.

[0026] Usual routes to abatement of sulfur are solutions of absorbent type for low concentrations of H.sub.2S, whereas higher concentrations of H.sub.2S can be used for production of chemicals, e.g. elemental sulfur or sulfuric acid.

[0027] The present invention utilizes catalytic oxidation of H.sub.2S to SO.sub.2 at temperatures between 200 and 450 C., preferably between 250 and 400 C. and most preferably between 250 and 300 C. In comparison with combustion, which takes place at temperatures above 800 C., catalytic oxidation therefore offers 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 H.sub.2S can be performed at an oxygen concentration of below 2 vol %, measured at the outlet of the H.sub.2S oxidation reactor, whereas combustion of H.sub.2S typically requires an oxygen concentration of more than 3 vol % at the outlet of the furnace. This means that the process gas flow is reduced compared to combustion, thereby reducing both investment and operating costs.

[0028] In the process of removing H.sub.2S from a gas, an SMC catalyst, i.e. a monolithic type catalyst, is used in the reactor converting H.sub.2S to SO.sub.2. This catalyst is a corrugated fibrous monolith substrate coated with a supporting oxide. It is preferably coated with TiO.sub.2 and subsequently impregnated with V.sub.2O.sub.3 and/or WO.sub.3. The channel diameter of the corrugated monolith is between 1 and 8 mm, and the wall thickness of the corrugated monolith is between 0.1 and 0.8 mm.

[0029] 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.

[0030] Monoliths are increasingly being used, developed, and evaluated as catalyst supports in many new reactor applications such as chemical and refining processes, catalytic oxidation, ozone abatement etc. When the active catalyst has a monolithic structure, it displays a low pressure drop as already mentioned.

[0031] The invention is illustrated further in the examples which follow.

EXAMPLES

[0032] The addition of syngas is interesting in situations where the temperature increase over an SMC catalyst is less than 40 C., which corresponds to a concentration of H.sub.2S below 2000-3000 ppm H.sub.2S (dependent on the remaining constituents and whether other combustible compounds, like CO or H.sub.2, are present). The lower heating value (LHV) should be less than roughly 22 kCal/h.

[0033] Two situations are investigated: One according to the prior art and one according to the invention, illustrated in FIG. 1 and FIG. 2, respectively.

Example 1

[0034] This example illustrates the prior art as shown in FIG. 1. In this prior art design, a burner is utilized to increase the temperature approach in the feed/effluent heat exchanger (HEX). The basis of the examples is the feed gas, which has the properties and the composition given in Table 1 below:

TABLE-US-00001 TABLE 1 Properties and composition of feed gas properties Temperature [ C.] 10 Pressure [mbar g] 150 Flow [Nm.sup.3/h] 100,000 composition N.sub.2 [mole %] 14 CO.sub.2 [mole %] 84.7 H.sub.2O [mole %] 1 H.sub.2S [ppm] 3000

[0035] In this example, the consumption of fuel gas amounts to 541 Nm.sup.3/h.

Example 2

[0036] This example illustrates the present invention as shown in FIG. 2. In this inventive design, it is not required to use a burner to increase the temperature approach in the heat exchanger (HEX).

[0037] Syngas is added to raise the temperature increase in the SMC reactor, hereby also improving the temperature approach in the feed/effluent heat exchanger.

[0038] The consumption of fuel gas and syngas, as well as an estimated OPEX based on a fuel gas price of 3 RMB/Nm.sup.3 and a syngas price of 0.5 RMB/Nm.sup.3 can be seen in Table 2 below. More syngas will be required since the heating value is lower. However, the syngas cost will be much lower compared to Example 1.

TABLE-US-00002 TABLE 2 Comparison of examples 1 and 2 Gas consumption Price [Nm.sup.3/h] [mRMB/yr] Relative [%] Example 1 541 13 100 Example 2 1380 5.5 44

[0039] Regarding the equipment, a blower is saved as well as a burner in the design according to the invention, as can be seen in FIG. 1 and FIG. 2.