WSA-DC NEXT GENERATION TOPSØE WSA TECHNOLOGY FOR STRONGER SO 2 GASES AND VERY HIGH CONVERSION. Helge Rosenberg Haldor Topsoe

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1 WSA-DC NEXT GENERATION TOPSØE WSA TECHNOLOGY FOR STRONGER SO 2 GASES AND VERY HIGH CONVERSION Helge Rosenberg Haldor Topsoe Up to now, Topsøe WSA (Wet gas Sulphuric Acid) plants have been in operation for almost 30 years, and close to 80 plants have been sold. In spite of its unique features and ability to treat gases without any prior drying, the WSA technology has had certain restrictions with regards to SO2 concentration in the feed gas, and the overall conversion of SO 2 by catalytic means has been limited to approx. 99.7%. In commercial scale sulphuric acid production on the basis of elemental sulphur and metallurgical off-gases, there is a demand for higher SO 2 conversion and treatment of gases with a higher content of SO 2. Therefore Topsoe has further developed its WSA technology to cope with these demands. This paper will explain more about the principles of such new development and leave the delegate with a better understanding of the potentials of the new generation of the WSA technology from Topsoe. Background During the last 15 years the Topsøe WSA (Wet gas Sulphuric Acid) technology has gained a strong foothold for cleaning of gases with low to medium SO 2 contents (up to 6-7 vol%) by production of sulphuric acid. More than 80 plants have been contracted worldwide primarily in the following industries: Oil refining Coking and coal chemicals Coal gasification Viscose fibre production Metallurgical industry The plants range from very small units treating e.g. an H 2 S stream from a coking plant, to very big units treating flue gases from combustion of sulphur-rich fuel in a steam and power plant in an oil refinery. Process gas flows range from 2,000 to 1,200,000 Nm 3 /h and sulphuric acid productions from 4 to 1140 MTPD. The WSA technology is characterised by being a wet process, i.e. the process gas is not dried, and all the water vapour in the feed gas and the water vapour produced by chemical reactions remain in the gas. The SO 3 is not absorbed in sulphuric acid as in the conventional dry processes, but hydrated to H 2 SO 4 vapour that is condensed as Page 27

2 concentrated sulphuric acid in an air cooled tubular condenser. WSA process description A typical WSA plant treating a H 2 S gas from an oil refinery is shown in Figure 1. The process comprises three major steps: a. Combustion of the H 2 S gas with air and subsequent cooling of the combustion products to produce a process gas with around 6 vol% SO 2 at 400 C. b. Conversion of the SO 2 to SO 3 in a three-bed catalytic converter with Topsøe VK-W series catalyst, and cooling by steam generation after each bed. In the last cooling step, the majority of the SO 3 reacts with H 2 O in the gas to form H 2 SO 4 vapour. c. Cooling of the process gas in a condenser, whereby the acid vapour is condensed to form 98 wt% H 2 SO 4 which is cooled and pumped to storage. The cleaned gas passes to the stack. The condenser is a heat exchanger with vertical glass tubes. The glass tubes are cooled on the outside by atmospheric air. Figure 1 Advantages and limitations of the WSA technology Page 28

3 The WSA technology as known today has some inherent advantages and some inherent limitations when compared to conventional sulphuric acid technologies. The most pronounced advantages are described below: 1. Energy efficiency The WSA process has a very high energy efficiency because not only the heat of SO 2 oxidation but also the heat of reaction between gas phase SO 3 and H 2 O (to form H 2 SO 4 vapour), the heat of condensation of H 2 SO 4 vapour and the cooling duty of the process gas to approx. 100 C are made useful. These energi es are recovered partly in the form of high pressure steam and partly in the form of hot air that can be used e.g. as combustion air. Only the cooling duty of the produced sulphuric acid is lost with cooling water. 2. No by-products Since the process gas does not have to be dried in the WSA process, there is no loss of sulphuric acid and no generation of sour waste water. The limitations of the WSA technology are: A. Limited SO 2 content in feed Because of considerations concerning construction materials in the WSA condenser, is not possible to handle gases with sulphuric acid dew points higher than around 260 C. This corresponds to a content of SO 2 inlet to the SO 2 converter of some 6-7 vol%. This limitation of course can be overcome by dilution of the gas with atmospheric air, but this will increase the process gas volume and thereby the dimensions of the plant. B. Limited conversion efficiency Being a single-contact process, the SO 2 /SO 3 equilibrium curve limits the conversion to typically %. This limitation can be overcome by scrubbing the tail gas with caustic or hydrogen peroxide, but this means additional investment and operating costs. Introduction of WSA-DC In recent years the market has demanded the advantages of the WSA technology also in the fields where the limitations of the technology has hitherto made its use less obvious. That is why Topsøe now introduces the WSA-DC technology. DC means Double Condensation. The WSA-DC technology combines the advantages of the WSA technology, primarily the high energy efficiency, with the high conversion efficiency of the double-contact principle. This is illustrated in Figure 2. At the same time a modest change in the design of the intermediate WSA condenser makes it possible to accept feed gases with SO 2 concentrations of 13 vol% or higher. Page 29

4 The WSA-DC technology is not the most obvious choice for applications where the water vapour content of the gas is not controllable. In case of a H 2 S gas with a high content of water vapour, hydrogen and/or hydrocarbons, it is not possible to avoid a high H 2 O/SO 2 ratio, and that would result in production of acid of lower concentration in a WSA-DC plant. On the other hand, most gases are of such a nature that it is possible to adjust the H 2 O/SO 2 ratio to around This is also the case with sulphur-based plants, where H 2 O has to be added in any case. The WSA-DC technology is not intended to replace the WSA technology, but to supplement it. In many cases where the initial SO X content is low, the traditional WSA process may be better suited than WSA-DC, particularly when the H 2 O/SO 2 ratio is too high relative to the desired acid concentration. Figure 2 Advantages of WSA-DC The WSA-DC technology is characterised by the following advantages: High energy efficiency (even higher than that of today s WSA technology) In the WSA-DC technology, like in the WSA technology, the heat of SO 2 oxidation, the heat of SO 3 hydration and the heat of acid condensation are utilised. In WSA-DC the amount of cleaned gas will be smaller because the feed gas does not have to be diluted with air. This gives lower heat loss. Further, in the WSA-DC process most of the acid is produced at 150 C compared to 260 C in the WSA process, so even less heat is lost in cooling the product acid. Page 30

5 High inlet SO 2 concentration Since the flow pattern in the 1st WSA condenser of the WSA-DC plant is arranged in such a way that the tube plate in the inlet (upper) end of the condenser can be kept free of liquid sulphuric acid, the limitation of the gas dew point to 260 C has disappeared. Therefore gases with much higher SO 2 content can be accepted than in a WSA plant. High conversion of SO 2 Because the WSA-DC process is a double contact process, higher conversion can be reached in the two conversion stages than in the one stage of a WSA plant. High acid concentration Since the H 2 O/SO 2 is controlled, very strong acid can be produced. The WSA-DC technology has potential for production of oleum. Low cooling water consumption Since almost all process heat is recovered for steam production, there is only very little heat to be removed by cooling water, namely the enthalpy of cooling the product acid from 150 C to 40 C. Even this heat can in some cas es be utilised. WSA-DC process description A typical sulphur-based WSA-DC plant is illustrated in Figure 3. The process plant consists of four main steps: Sulphur burner with boiler Catalytic SO 2 converter in 3+1 configuration Intermediate WSA condenser Final WSA condenser Page 31

6 Figure 3 The sulphur burner uses preheated, humidified air for the combustion and gives a gas with approx. 11 vol% SO 2. The SO 2 gas is cooled in the high pressure steam boiler to around 400 C which is the optimal temperature for the SO 2 to SO 3 conversion catalyst of the Topsøe VK-W series. The VK-W catalysts are specially developed for use in humid process gases. The top layer of catalyst in the first bed consists of 25 mm daisy shaped particles which provide good capacity for accumulation of dust without creating excessive pressure drop. The remaining catalyst consists of either 9 mm or 12 mm daisies. Due to the exothermal reaction, the outlet temperature from the first bed is around 600 C. The gas is cooled by superheating of high pressure steam and the gas flows to the second bed. After the second bed the gas is cooled again by superheating of high pressure steam and the conversion is continued in the third bed. At the outlet of the third bed the conversion has reached 95%. The gas is then cooled to around C by production of high pressure steam, whe reby part of the SO 3 reacts with H 2 O to form H 2 SO 4 vapour. The partly converted gas is passed to the intermediate WSA condenser where it enters at the top and flows down through a number of vertical glass tubes which are cooled on the outside by humidified atmospheric air. During the cooling, the rest of the SO 3 reacts with H 2 O to form H 2 SO 4 and the H 2 SO 4 vapour is condensed. At the bottom outlet of the glass tubes, the gas and the produced acid have been cooled to 150 C. The H 2 O content of the gas is controlled carefully by injection of water in the cooling air circuit to achieve a H 2 O/SO 2 ratio of In this way the acid product does not absorb any significant amount of H 2 O and is therefore of high concentration, typically 99%. The cooling of the intermediate condenser takes place in a circulation loop of atmospheric air. Part of the hot air leaving the loop is used as combustion air in the sulphur burner, and part is added to the gas after the intermediate condenser to provide additional oxygen and water vapour. The remaining hot air is cooled by preheating of boiler feed water and further cooled by injection of water. The injection of water serves not only for cooling of the air but also Page 32

7 provides water vapour required for the formation of the sulphuric acid. The air taken out of the cooling loop is compensated for by adding hot cooling air leaving the final WSA condenser. The gas leaving the intermediate WSA condenser passes through a filter, removing droplets of acid. Hot, humid air is added, and the gas is preheated to approx. 380 C in two steps, first with boiler feed water and then by superheated steam, before it enters the fourth conversion step. The catalyst in the fourth conversion step is a special Cspromoted low temperature catalyst, ensuring that a conversion after the fourth catalyst bed of 99.95% is achievable. At the outlet of the fourth catalyst bed, the gas is cooled to 240 C by preheating of boiler feed water before it passes to the final WSA condenser. During the cooling, most of the SO 3 reacts with H 2 O to form H 2 SO 4 vapour. The gas enters the final WSA condenser at the bottom and flows up through vertical glass tubes which are cooled on the outside by atmospheric air. During the cooling, the remaining SO 3 reacts with H 2 O to form H 2 SO 4, and the H 2 SO 4 vapour is condensed to H 2 SO 4 liquid, which runs down the tubes counter-currently with the hot gas. The acid is collected at the bottom of the condenser as concentrated acid, and the gas leaving the top of the condenser can be sent directly to the stack. The acid product from the two WSA condensers is mixed, cooled in a cooling circuit, and pumped to storage. The concentration of the acid product is typically 99 wt%. The heat recovery system of the WSA-DC plant is designed in such a way that the steam is exported at approx. 80 bar g and 500 C, making it p erfectly suited for power generation. Performance, consumption and production figures of WSA-DC Table 1 shows typical performance figures of a WSA-DC plant based on molten sulphur in comparison with WSA and a typical conventional dry technology (DCDA): Sulphur-based plants WSA-DC WSA DCDA SO 2 conversion, % SO 2 emission, vol ppm SO 3 emission, vol ppm Acid concentration, wt% Table 1. Typical performance figures of WSA-DC, WSA and conventional dry sulphuric acid plants Table 2 shows typical consumption and production figures of a WSA-DC plant based on molten sulphur in comparison with a typical conventional dry technology (DCDA): Page 33

8 540 MTPD sulphur-based plants Unit cost Cons./ prod. per hour WSA-DC Annual costs, Cons. / prod. per hour DCDA Annual costs, Sulphur consumption 40 /t 7.36 t 2,355, t 2,355,000 Electricity 0.05 /kwh 1250 kwh 500, kwh 560,000 consumption HP steam production 8 /t 34 t -2,176, t -1,600,000 Cooling water consumption 0.03 /m 3 95 m 3 22, m 3 223,200 Process water consumption 0.50 /m m 3 16, m 3 13,200 Annual costs (8,000 h/y) 718,200 1,551,400 Cost per ton H 2 SO 4,, approx Table 2. Typical operation costs of WSA-DC and conventional dry sulphuric acid plants Costs not mentioned in the table, such as labour, maintenance and catalysts are largely the same for the two types of technologies. Investment cost for the WSA-DC plant is typically slightly less than for a corresponding DCDA plant. Applications of WSA-DC The WSA-DC technology offers a superior solution in cases where the SO 2 concentration in the gas is high and where the demands to SO 2 conversion and energy efficiency are high. These are predominantly metallurgical SO 2 gases and sulphur burning. Page 34

9 The Author Helge Rosenberg, Haldor Topsøe A/S, Sales Manager Helge Rosenberg holds a master degree in Mechanical Engineering and a bachelor degree in business administration. He has been responsible for sales and marketing of Topsoe s environmental technologies for the last eight years and before joining Topsoe, he worked for more than ten years within the power industry. Page 35

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