Process for production of sulfuric acid

Abstract

A process plant for production of sulfuric acid from a process gas comprising SO2, including a process gas inlet, a first SO2 converter having an inlet and an outlet, a first condenser having a gas inlet, a gas outlet and a liquid outlet, a gas mixing device having a first inlet, a second inlet and an outlet, a process gas heater having an inlet and an outlet, a second SO2 converter having an inlet and an outlet, a second condenser having a gas inlet, a gas outlet and a liquid outlet, one or more means for cooling and storage of sulfuric acid and a purified process gas outlet.

Claims

1. A process plant for production of sulfuric acid from a process gas comprising SO.sub.2, comprising a process gas inlet, a first SO.sub.2 converter having an inlet and an outlet, a first condenser having a gas inlet, a gas outlet and a liquid outlet, a gas mixing device having a first inlet, a second inlet and an outlet, a heat exchanger having an inlet and an outlet, a process gas heater having an inlet and an outlet, a second SO.sub.2 converter having an inlet and an outlet, a second condenser having a gas inlet, a gas outlet and a liquid outlet, one or more means for cooling and storage of sulfuric acid and a purified process gas outlet, in which said process gas inlet is connected to the inlet of the first SO.sub.2 converter, the outlet of the first SO.sub.2 converter is connected to the gas inlet of the first condenser, the liquid outlet of the first condenser is connected to one of said means for cooling and storage of sulfuric acid, the gas outlet of said first condenser is connected to the first inlet of a mixing device, the outlet of said mixing device is connected to the inlet of the heat exchanger, the outlet of the heat exchanger is connected to inlet of the process gas heater, the outlet of said process gas heater is connected to the inlet to said second SO.sub.2 converter and the second inlet of said gas mixing device, the outlet of the second SO.sub.2 converter is connected to the gas inlet of the second condenser, the liquid outlet of the second condenser is connected to one of said means for cooling and storage of sulfuric acid, the gas outlet of said second condenser is connected to the purified process gas outlet, wherein the first inlet of said gas mixing device being connected to the outlet of said first condenser, without intermediate heat exchange, and wherein the heat exchanger is in thermal communication with the outlet of the second SO.sub.2 converter.

2. A process plant according to claim 1 in which said mixing device comprises one or more elements for enhancing mixing.

3. A process plant according to claim 1 in which one or both of said first and second condenser comprises a cooling medium enclosure, having a cooling medium inlet and a cooling medium outlet.

4. A process plant according to claim 3, in which said cooling medium enclosure is a pressure shell, and in which said condenser comprises a number of tubes made from corrosion resistant material.

5. A process plant according to claim 3, in which said cooling medium enclosure comprises a number of tubes made from corrosion resistant materials and in which said condenser comprises a shell made from corrosion resistant materials.

6. A process plant for production of sulfuric acid from a feedstock comprising sulfur in any oxidation state according to claim 1, further comprising an incinerator having a feedstock inlet, an oxidant inlet, an optional support fuel inlet and an outlet, in which said feedstock comprising sulfur is connected to said feedstock inlet, an oxidant, is connected to said oxidant inlet, an optional support fuel feed is connected to said support fuel inlet and the incinerator outlet is connected to said process gas inlet.

7. A process plant according to claim 1, wherein said mixing device mixes a partially desulfurized gas and a recycled hot intermediate process gas.

8. A process plant according to claim 1, wherein a ratio of the partially desulfurized gas to the recycled hot intermediate process gas is from 12:1 to 4:1.

Description

FIGURES

(1) FIG. 1 shows a process layout according to the prior art, with either acid gas (H.sub.2S) or elemental sulfur as feedstock.

(2) FIG. 2 shows a process layout which is a detail of the prior art.

(3) FIG. 3 shows a process layout of the present disclosure, with either acid gas (H.sub.2S) or elemental sulfur as feed stockfeedstock.

(4) FIG. 4 shows a process layout of the present disclosure, with so-called spent sulfuric acid as feedstock.

ELEMENTS USED IN THE FIGURES

(5) (1) Sulfur containing feedstock (2) Compressed atomizing air (3) Support fuel (4) Combustion air (5) Combustion chamber (6) Hot incinerated gas (8) Heat exchanger (12) Process gas cooler (16) Filtration device (17) Solids line (18) Solids-free process gas (20) Diluted process gas (22) SCR catalytic reactor (24) Hot SO.sub.2 containing process gas (26) First SO.sub.2 converter (28) Catalyst layer (30) Interbed cooler(s) (32) Partially converted process gas cooler (34) Cooled SO.sub.3 containing process gas (36) First sulfuric acid condenser (38) Partially desulfurized gas (39) First process gas heater (40) Liquid outlet of the first condenser (41) Mixing point (43) Reheated partially desulfurized gas (44) Secondary process gas (46) Process gas blower (48) Pressurized process gas (50) Second process gas reheater (52) Pre-heated process gas (54) Third process gas reheater (58) Hot partly desulfurized process gas (59) Recycled hot process gas (60) Second SO.sub.2 converter (62) Catalyst layer (64) Fully converted process gas (66) Cooled process gas (68) Second sulfuric acid condenser (70) Liquid outlet of the second condenser (72) Fully converted desulfurized process gas (74) Cooling air for the second sulfuric acid condenser (76) Second cooling air blower (78) Pressurized cooling air for second sulfuric acid condenser (80) Hot cooling air from second condenser (82) Stack air heat exchanger (84) Hot air (86) Stack gas (88) Stack (90) Cooling air for first sulfuric acid condenser (92) First cooling air blower (94) Pressurized cooling air for first sulfuric acid condenser (96) Hot cooling air (97) Cooling air heater (98) Hot air to the first process gas reheater (100) Air leaving first process gas reheater (102) Combustor fraction of the cooling air (104) Combustion air blower (108) Compressed dilution air (110) Hot dilution air (112) NH.sub.3 source (114) Dilution air mixture (116) Excess cooling air (118) Cooling air heat recuperator (120) Cold cooling air

(6) In FIG. 1, an overall process layout of prior art for a so-called double conversion double condensation sulfuric acid plant is shown. A sulfur containing feedstock (1), such as an H.sub.2S containing gas and/or elemental sulfur, is fed into a combustion chamber (5), where any sulfur compound is converted into SO.sub.2 in the hot flame zone of the combustor. Oxygen for the oxidation of feedstock is added to the combustion chamber (5) as preheated atmospheric air (4) from the 1.sup.st sulfuric acid condensation step. If support fuel is needed, it is also added to the combustion chamber (5). The hot incinerated gas (6) leaves the combustion chamber (5) at 800-1,200 C. and it is cooled to 380-420 C. in heat exchanger (8) forming a hot SO.sub.2 containing process gas (24). Typically this heat exchanger is a so-called waste heat boiler, producing saturated high pressure steam from the duty transferred from the hot incinerated gas (6). The hot SO.sub.2 containing process gas (24) enters the first SO.sub.2 converter (26), in which one or more layers of catalyst (28), suitable for the oxidation of SO.sub.2 to SO.sub.3, are installed. The number of layers of catalyst (28) is typically between 1 and 3, depending on the desired SO.sub.2 conversion efficiency. The oxidation of SO.sub.2 is an exothermal reaction, which increases the temperature of the catalyst and process gas and in order to provide beneficial thermodynamic conditions for the SO.sub.2 conversion, the heat of reaction is typically removed in one or more interbed cooler(s) (30), installed between the catalyst layers. Usually high pressure steam is used to cool the process gas to the optimal temperature for the next catalyst layer. After the final catalyst layer in the first SO.sub.2 converter (26), typically 95% of the SO.sub.2 has been oxidized and the partially converted process gas is cooled to around 280-300 C. in the partially converted process gas cooler (32), producing high pressure saturated steam. The cooled SO.sub.3 containing process gas (34) is directed to the first sulfuric acid condenser (36) in which the process gas is cooled to around 100 C. by heat exchange with atmospheric air (94). The SO.sub.3 reacts with water in the gas phase to form H.sub.2SO.sub.4 and upon cooling the H.sub.2SO.sub.4 is condensed from the gas phase and is withdrawn from the liquid outlet of the first condenser (40) at the bottom of the first sulfuric acid condenser. The cooled partially desulfurized gas (38) leaving the condenser is practically free of sulfuric acid vapor, but a small amount of sulfuric acid aerosol in unavoidable. To evaporate this sulfuric acid aerosol, the partially desulfurized gas (38) is directed to the first process gas reheater (39), which is made of corrosion resistant material and uses the hot air from the first sulfuric acid condenser (36) and cooling air heater (97) to bring the temperature of the reheated process gas (43) to around 180 C., i.e. above the sulfuric acid dew point temperature. To increase the secondary process gas (44) temperature to around 210 C., recycled hot process gas (59) is mixed with the reheated process gas (43) and afterwards compressed in the process gas blower (46). In the second process gas reheater (50) the pressurized process gas (48) exchanges heat with the fully converted process gas (64) from the second SO.sub.2 converter (60) and the final reheating of the pre-heated process gas (52) may be carried out in the third process gas reheater (54), ensuring the optimal temperature of the process gas entering the second SO.sub.2 converter (60), typically 370-410 C. A fraction of the hot process gas from the third process gas reheater (54) is recycled (59) to a position upstream the process gas blower (46). The process gas for recycle could also have been withdrawn from position (52), but would require a higher flow rate due to the lower temperature of the process gas. The process gas (58) entering the second SO.sub.2 converter has low SO.sub.2 and SO.sub.3 concentration and thus it is possible to achieve high SO.sub.2 conversion efficiency with only a single catalyst layer (62), but in principle the second SO.sub.2 converter could also consist of two catalyst layers separated by an interbed cooler, just as depicted in the first SO.sub.2 converter (26). The fully converted process gas (64) leaving second SO.sub.2 converter is cooled in second process gas reheater (50) and the cooled process gas (66) is directed to the second sulfuric acid condenser (68), which works in the same manner as the first sulfuric acid condenser (36). The sulfuric acid withdrawn from the liquid outlet of the first condenser (40) and the liquid outlet of the second condenser (70) are mixed and cooled before sent to a sulfuric acid storage tank.

(7) The fully converted desulfurized process gas (72) leaving the second sulfuric acid condenser at around 100 C. contains minimal amounts of SO.sub.2 and sulfuric acid aerosol and can be sent to the stack (88) without further treatment.

(8) The cooling air for the second sulfuric acid condenser (78) may be ambient air which is compressed in second cooling air blower (76) before entering the second sulfuric acid condenser (68). The hot cooling air from the second condenser (80) is heated in a stack air heat exchanger (82) to increase the temperature of hot air (84) and mixed directly with the fully converted desulfurized process gas (72) in order to ensure complete evaporation of the sulfuric acid aerosol and provide a dry stack gas (86), such that the stack (88) can be designed for dry conditions. In some cases it may not be necessary to increase the temperature of the hot cooling air from the second condenser (80) and thus the stack air heat exchanger (82) can be omitted. If the stack is designed for wet conditions by use of corrosion resistant materials a stack air heat exchanger (82) may alternatively be used to cool the hot cooling air, thus increasing the heat recovery of the plant.

(9) In FIG. 2, corresponding to a detail of FIG. 1, the process layout of prior art around the first sulfuric acid condenser and first reheating of the partially desulfurized gas is shown. The cooled SO.sub.3 containing process gas (34) enters at the bottom of the first sulfuric acid condenser (36), which consists of a tube bank of vertical glass tubes in which the process gas enters the tubes from the bottom. As the process gas is cooled on its way up through the tube, sulfuric acid is formed and condenses on the glass tube inner surface and/or the internal coil used for enhancement of heat transfer. By gravity the condensed sulfuric acid flows to the bottom of the tubes and is withdrawn at the liquid outlet of the first condenser (40). At the 100 C. process gas outlet temperature practically no sulfuric acid can exist in the gas phase of the partially desulfurized gas (38), but small amounts of sulfuric acid aerosol has been formed, of which the majority is captured in demisters at the top of the glass tubes. The process gas line containing the partially desulfurized gas (38) is thus considered wet and must be made of a material suitable to withstand the corrosive nature of the sulfuric acid. In first process gas reheater (39) the temperature of the process gas is increased to between 160 C. and 200 C., which is sufficient to ensure evaporation of the sulfuric acid aerosol and thus provide a dry process gas for further reheating to the 370-410 C. required for the final SO.sub.2 conversion in the second SO.sub.2 converter (60). First process gas reheater (39) must be constructed of sulfuric acid resistant material on the process gas side and glass is usually selected due to its very high corrosion resistance and relatively low cost.

(10) Atmospheric air (90) is used as the cooling media in the first sulfuric acid condenser. The cooling air is compressed in first cooling air blower (92) and send to the cold end of first sulfuric acid condenser at a temperature in the typical range 20-50 C. In the first sulfuric acid condenser (36) the cooling air is heated and leaves the condenser as heated cooling air (96) at a temperature in the typical range 200-260 C., which is suitable for reheating the partially desulfurized gas (38) leaving the first sulfuric acid condenser (36) while not being too hot to exceed the design temperature for the construction material of the first process gas reheater (39).

(11) The heated cooling air (96) may pass an optional cooling air heater (97), which increases the temperature of the hot cooling air (98) to the process gas reheater (39) to the desired 230-260 C., should the heated cooling air (96) not already have this temperature.

(12) The cooling air leaving the first process gas reheater (100) is typically 180-220 C. and to increase heat recovery of the plant, this remaining thermal energy can be used. In FIG. 1 is shown a layout in which a combustor fraction of the cooling air (102) is compressed in combustion air blower (104) and used for combustion air (4) in the combustion chamber (5). In this way all the thermal energy of the cooling air is recovered in the plant. The cooling air flow is typically twice the amount of combustion air and thus an excess fraction of cooling air (116) must be directed via another route. If economically viable, heat can be taken out of the cooling air in a cooling air heat recuperator (118), before the cold cooling air (120) is vented. The heat from the cooling air can be used for e.g. boiler feed water preheating, demineralized water preheating, low pressure steam production and/or drying purposes.

(13) One drawback of this process gas cooling and reheating layout is that much cooling duty is required to cool the process gas from 270-300 C. to 100 C. and reheat it from 100 C. to 180 C. again. This requires high cooling air flow and large heat exchanger areas, as gas/gas heat exchangers have relatively low heat transfer coefficients. Furthermore during off-set conditions, such as low load operation (both with regard to flow and/or sulfuric acid production) it can be difficult to maintain a high temperature of the heated cooling air (96) leaving the first sulfuric acid condenser and thus it is necessary to add a cooling air heater (97), increasing cost and adding complexity to the plant. Electric heaters, steam and hot oil heaters are applicable for the cooling air heating. Also during start-ups and shut-downs it can be difficult to control the temperature of the air to the first reheater (39), due to heating of equipment and due to chemical variations from variations in the amount of sulfur, and thus the amount of energy released may vary in such situations.

(14) In FIGS. 3 and 4 improved layouts are proposed, which mitigates some drawbacks of the prior layout as described above.

(15) In FIG. 3, an example of an overall process layout of prior art for a so-called double conversion double condensation sulfuric acid plant is shown. The majority of the process layout correspond to the layout shown in FIG. 1 in which a sulfur containing feedstock (1), such as an H.sub.2S containing gas and/or elemental sulfur, is fed into a combustion chamber (5), where any sulfur compound is converted into SO.sub.2 in the hot flame zone of the combustor. Oxygen for the oxidation of feedstock is added to the combustion chamber (5) as preheated atmospheric air (4) from the 1.sup.st sulfuric acid condensation step. If support fuel is needed, it is also added to the combustion chamber (5). The hot incinerated gas (6) leaves the combustion chamber (5) at 800-1,200 C. and it is cooled to 380-420 C. in heat exchanger (8) forming a hot SO.sub.2 containing process gas (24). Typically this heat exchanger is a so-called waste heat boiler, producing saturated high pressure steam from the duty transferred from the hot sulfuric acid gas (6). The hot SO.sub.2 containing process gas (24) enters the first SO.sub.2 converter (26), in which one or more layers of catalyst (28), suitable for the oxidation of SO.sub.2 to SO.sub.3, are installed. The number of layers of catalyst (28) is typically between 1 and 3, depending on the desired SO.sub.2 conversion efficiency. The oxidation of SO.sub.2 is an exothermal reaction, which increases the temperature of the catalyst and process gas and in order to provide beneficial thermodynamic conditions for the SO.sub.2 conversion, the heat of reaction is typically removed in one or more interbed cooler(s) (30), installed between the catalyst layers. Usually high pressure steam is used to cool the process gas to the optimal temperature for the next catalyst layer. After the final catalyst layer in the first SO.sub.2 converter (26), typically 95% of the SO.sub.2 has been oxidized and the partially converted process gas is cooled to around 270-300 C. in the partially converted process gas cooler (32), producing high pressure saturated steam.

(16) The cooled SO.sub.3 containing process gas (34) is directed to the first sulfuric acid condenser (36) which is similar to the one described in the FIG. 1, but the main difference is that the partially desulfurized gas leaves the first sulfuric acid condenser at 180 C., thus significantly reduces the cooling duty in the first sulfuric acid condenser and eliminates the first process gas reheating step. The partially desulfurized gas (38) having a temperature in the range 140-190 C. is combined with recycled hot process gas (59) to evaporate the small amounts of sulfuric acid aerosol in the partially desulfurized gas (38) and provide a dry gas for the downstream process gas blower (46). Between the mixing point (41) and process gas blower, a gas mixer (42) comprising elements for enhancing the mixing, such as impingement plates or packing elements can be installed to ensure that the two gases (38 and 59) are sufficiently mixed to ensure that aerosol evaporation is completed in the secondary process gas (44) before the process gas blower (46)

(17) The flow of heated cooling air (96) from the first sulfuric acid condenser (36) is reduced compared to prior art, but there is still surplus compared to the need for combustion air (4) and still only a fraction (but a larger fraction) of the hot cooling air is directed to the combustion air blower (104). The smaller fraction of excess cooling air (116) can be used to heating purposes as described in the prior art.

(18) To increase the secondary process gas (44) temperature to around 210 C., recycled hot process gas (59) is mixed with the partially desulfurized process gas (38) and afterwards compressed in the process gas blower (46). In the second process gas reheater (50) the pressurized process gas (48) exchanges heat with the fully converted process gas (64) from the second SO.sub.2 converter (60) and the final reheating of the preheated process gas (52) may be carried out in the third process gas reheater (54), ensuring the optimal temperature of the process gas entering the second SO.sub.2 converter (60), typically 370-410 C. A fraction of the hot process gas is recycled (59) to a position upstream the process gas blower (46). The process gas for recycle could also have been withdrawn from position (52), but would require a higher flow rate due to the lower temperature of the process gas.

(19) The process gas (58) entering the second SO.sub.2 converter has low SO.sub.2 and SO.sub.3 concentration and thus it is possible to achieve high SO.sub.2 conversion efficiency with only a single catalyst layer (62), but in principle the second SO.sub.2 converter could also consist of two catalyst layers separated by an interbed cooler, just as depicted in the first SO.sub.2 converter (26). The fully converted process gas (64) leaving second SO.sub.2 converter is cooled in second process gas reheater (50) and the cooled process gas (66) is directed to the second sulfuric acid condenser (68), which works in the same manner as the first sulfuric acid condenser (36). The sulfuric acid withdrawn from the liquid outlet of the first condenser (40) and the liquid outlet of the second condenser (70) are mixed and cooled before sent to a sulfuric acid storage tank.

(20) The fully converted desulfurized process gas (72) leaving the second sulfuric acid condenser at around 100 C. contains minimal amounts of SO.sub.2 and sulfuric acid aerosol and can be sent to the stack (88) without further treatment.

(21) The cooling air for the second sulfuric acid condenser (78) may be ambient air which is compressed in second cooling air blower (76) before entering the second sulfuric acid condenser (68). The hot cooling air from second condenser (80) is heated in a stack air heat exchanger (82) to increase the temperature of hot air (84) and mixed directly with the fully converted desulfurized process gas (72) in order to ensure complete evaporation of the sulfuric acid aerosol and provide a dry stack gas (86), such that the stack (88) can be designed for dry conditions. In some cases it may not be necessary to increase the temperature of the hot cooling air from second condenser (80) and thus the stack air heat exchanger (82) can be omitted. If the stack is designed for wet conditions by use of corrosion resistant materials a stack air heat exchanger (82) may alternatively be used to cool the hot cooling air, thus increasing the heat recovery of the plant.

(22) In an alternative embodiment not shown the heat exchange medium used for cooling the condenser may be process gas instead of atmospheric air. This has the benefit of providing at least partial pre-heating of the process gas prior to reaction, but it may require a more careful design of the condenser, with respect to gas leakage.

(23) In a further alternative embodiment not shown, the oxidant directed to the incinerator may be pure oxygen or another oxygen enriched gas instead of atmospheric air. This has the benefit of higher combustion efficiency and lower volumes of process gas, but the drawback may be that the cost of oxygen is too high and that the reduced volume also means a reduced thermal dilution of the released heat.

(24) In FIG. 4, an alternative double conversion double condensation process layout is shown for regeneration of so-called spent sulfuric acid from e.g. an alkylation unit. It is primarily in the front end of the sulfuric acid plant that the layout differs from the process layout as shown in FIG. 3.

(25) Spend sulfuric acid from an alkylation unit is roughly 90% w/w H.sub.2SO.sub.4, 5% w/w H.sub.2O and 5% w/w hydrocarbons, which must be regenerated to at least 98% w/w H.sub.2SO.sub.4 before recycled back to the alkylation unit. The process of regenerating the spent acid is to combust the hydrocarbons at high temperature (>1000 C.) at which the hydrocarbons are oxidized to CO.sub.2 and H.sub.2O. At that temperature, sulfuric acid is decomposed into SO.sub.2, O.sub.2 and H.sub.2O. The SO.sub.2 must then be oxidized to SO.sub.3, react with water to form H.sub.2SO.sub.4 and condensed to produce the desired sulfuric acid product.

(26) The spent sulfuric acid, which is the sulfur containing feedstock (1) in this example, is atomized into the flame of the combustion chamber (5) by means of compressed atomizing air (2). Support fuel (3) is needed to sustain a high combustion temperature and to reduce support fuel consumption, hot combustion air (4) is used as the O.sub.2 source. If higher acid production is desired, other sulfur containing feeds can be added, e.g. H.sub.2S gas and/or elemental sulfur. The hot process gas from the combustion chamber is cooled to 450-550 C. in heat exchanger (8) which may be a waste heat boiler, producing high pressure saturated steam, and further cooled in process gas cooler (12) to a temperature in the range 380-420 C. As the spent acid feed contains (minor) amounts of dissolved metals, the metals will form oxides and sulfates during combustion and the process gas (6, 10 and 14) will contain minor amounts of solids, which are removed in a filtration device (16). The filter can be either an electrostatic precipitator or a ceramic filter. The solids are separated from the process gas and are withdrawn from the filter in line (17). The solids-free process gas (18) is combined with a hot air stream (114), optionally containing NH.sub.3. Compressed cooling air (108) from the first sulfuric acid condenser (36) is heated from the 240-280 C. at the outlet of the combustion air blower (104) to 380-420 C. in process gas cooler (12). The hot dilution air (110) is optionally mixed with an NH.sub.3 source (112), such as anhydrous NH.sub.3, aqueous NH.sub.3 or urea, before the dilution air (114) optionally comprising NH.sub.3 is combined with the SO.sub.2 containing process gas. The combustion chamber (5) is operated with moderate excess of O.sub.2, to minimize the process gas flow and thus the volume and cost of combustion chamber (5), heat exchanger (8) (waste heat boiler) and filtration device (16). Therefore the SO.sub.2 laden process gas (18) does not contain sufficient O.sub.2 for the complete oxidation of the SO.sub.2 to SO.sub.3 in the first SO.sub.2 converter (26) and thus the dilution air (114) is required. Alternatively the excess of O.sub.2 in the combustion chamber (5) could be higher, at the expense of increased process gas volumes and equipment costs.

(27) The diluted process gas (20) optionally passes through a SCR catalytic reactor (22), in which NO and NO.sub.2 in the process gas reacts with the NH.sub.3 (112) supplied via the dilution air (114) to form harmless N.sub.2 and H.sub.2O, in the so-called selective catalytic reduction (SCR) process.

(28) The process gas then enters the first SO.sub.2 converter (26) in which the SO.sub.2 is catalytically oxidized to SO.sub.3 and the first sulfuric acid condensation step, process gas reheat, second SO.sub.2 oxidation step and second sulfuric acid condensation step is as described previouslysee also FIG. 3.

(29) An alternative layout, when a SCR reactor is required, is to split the hot dilution air (110) into two fractions: one NH.sub.3 containing stream (114) as shown in FIG. 4 and a NH.sub.3-free stream that is mixed with stream 24 just upstream the SO.sub.2 converter (26). By using the minimum carrier air for NH.sub.3, the major fraction of the dilution air (110) is bypassed the SCR catalytic reactor (22), thus minimizing the size of the reactor.

(30) In a further embodiment not illustrated, an equivalent SCR system comprising an SCR reactor and an NH.sub.3 containing stream could be added to the process layout shown in FIG. 3.

Example 1: First Sulfuric Acid Condenser and Process Gas Reheating for a 900 MTPD Sulfuric Acid Plant

(31) In this example process calculations for a 900 MTPD (Metric Tons Per Day) have been calculated for two layouts of the double conversion double condensation process as described above. In the prior art the process gas in the first sulfuric acid condenser is cooled from 290 C. to 100 C. and reheated to 180 C. as sketched in FIG. 1. In the proposed new layout the same process gas is cooled from 290 C. to 180 C. as depicted in FIG. 3.

(32) The flow and composition of the process gas entering the first sulfuric acid condenser are similar, as the upstream processes are similar for the two layouts. The plant layout downstream first reheating section (i.e. from stream 44 and to the stack (88) is also similar for the two layouts. Due to the 80 C. increase in temperature the process gas composition in stream 38 has a slightly higher concentration of H.sub.2SO.sub.4.

(33) In Table 1, the effect of size and duty of the heat exchangers (36 and 39) are compared, for this H.sub.2SO.sub.4 plant and it is seen that the total heat exchange duty is reduced from 20.9 Gcal/h to 16.0 Gcal/h, i.e. a 23% decrease. The decreased duty to be transferred combined with larger temperature differences between the heat exchange medias result in an almost 40% decrease in the required heat exchange area.

(34) The lower duty required in the first sulfuric acid condenser also result in a lower cooling air flow (stream 90) and thus a significantly lower power consumption in cooling air blower (92).

(35) TABLE-US-00001 TABLE 1 Description Prior layout New layout Duty in 1.sup.st sulfuric acid 18.4 Gcal/h 16.0 Gcal/h condenser (36) Duty in 1.sup.st process gas re- 2.5 Gcal/h N/A heater (39) Heat exchange area (36) + (39) 12,000 m.sup.2 7,300 m.sup.2 Power in 1.sup.st cooling air blower 1,150 kW 860 kW (92)

Example 2. Sulfuric Acid Concentration in Process Gas Leaving First Sulfuric Acid Condenser

(36) In this example the effect of increasing the temperature of the process gas (38) leaving the first sulfuric acid condenser (36) is calculated. At the process gas outlet of the first sulfuric acid condenser, a demister is installed and it can be assumed that the process gas leaving the condenser is in thermodynamic equilibrium with the sulfuric acid detained in the demister filament.

(37) In Table 2, the vapor concentration of H.sub.2SO.sub.4 in the process gas (38) leaving the first sulfuric acid condenser (36) is shown as a function of process gas temperature, at a process gas pressure of 1.013 bar. The process gas entering the first sulfuric acid condenser contains 6 vol % SO.sub.3 (unhydrated) and 10 vol % H.sub.2O (unhydrated). The major part of the sulfuric acid is condensed and withdrawn in the bottom of the sulfuric acid condenser. The vapor phase sulfuric acid leaving with the process gas will be condensed in the second sulfuric acid condenser (68).

(38) As seen in the table, the H.sub.2SO.sub.4 vapor concentration increases with increasing temperature, from practically zero concentration at 100 C. to as much as 0.9 vol % at 220 C. At 100 C. practically 100% of the acid is withdrawn as condensed product, which decreases to 86.7% at 220 C.

(39) In principle the first sulfuric acid condenser could operate at 220 C., further reducing the duty in first sulfuric acid condenser and need for process gas reheating. But the large fraction of H.sub.2SO.sub.4 vapor leaving the first sulfuric acid condenser will negatively influence the thermodynamic equilibrium of reaction SO.sub.2+0.5 O.sub.2.Math.SO.sub.3, taking place in the second SO.sub.2 converter, as H.sub.2SO.sub.4 decomposes to SO.sub.3 and H.sub.2O at the high temperature in the SO.sub.2 converter. The result will be a lower SO.sub.2 conversion efficiency and/or a larger catalyst volume required.

(40) Also a higher H.sub.2SO.sub.4 concentration in the process gas to the second sulfuric acid condenser will increase the size and duty of this unit, reducing the cost savings gained for the first sulfuric acid condenser.

(41) The 160-190 C. temperature range of the process gas leaving the first sulfuric acid condenser represent the optimal trade-of between capital and operation cost and SO.sub.2 conversion efficiency

(42) A further benefit of the higher process gas outlet temperature is that more water vapor is stripped from the demister acid and carried away with the process gas, slightly increasing the concentration of the condensed product acid.

(43) TABLE-US-00002 TABLE 2 Temperature of the partially desulfurized gas C. 100 120 140 160 180 200 220 H.sub.2SO.sub.4 vol % 0.00 0.00 0.01 0.03 0.12 0.37 0.90 vapor con- centration Fraction % 100 100 99.1 99.5 98.2 94.6 86.7 of H.sub.2SO.sub.4 removed as liquid