Continuous process and apparatus for purifying SO2-containing gases

Abstract

The invention relates to a continuous process for purifying a gas containing 60-99 percent SO.sub.2 (sulfur dioxide) by volume and 1 to 40 percent steam by volume, followed by synthesis of SO.sub.3 (sulfur trioxide) without first drying the gas, and to an apparatus for carrying out said method.

Claims

1. A method for the purification of SO.sub.2-containing exhaust gases, comprising the following steps: (a) introducing the SO.sub.2-containing exhaust gas into a gas scrubber and absorbing SO.sub.2 in the scrubbing liquid, (b) supplying the scrubbing liquid loaded with SO.sub.2 into the desorber, desorbing SO.sub.2 under generation of SO.sub.2-containing rich gas and returning the SO.sub.2 deplenished scrubbing liquid into the gas scrubber, (c) supplying the SO.sub.2-containing rich gas into a gas pre-heater, wherein air is added to the SO.sub.2-containing rich gas before it enters the gas pre-heater, (d) heating the SO.sub.2-containing rich gas with the added air in the gas preheater, (e) oxidizing the SO.sub.2 contained in the SO.sub.2-containing rich gas to SO.sub.3 in a tube contacting device, (f) absorption of the resulting SO.sub.3 in a SO.sub.3-absorber, (g) discharging gases that have not been absorbed in the SO.sub.3-absorber and supplying these gases into the SO.sub.2-containing exhaust gas before it enters the gas scrubber, wherein the air that is added to the SO.sub.2-containing rich gas before it enters the gas pre-heater in step (c) has a temperature that is higher than the temperature of the SO.sub.2-containing rich gas.

2. The method according to claim 1, wherein the SO.sub.2-containing rich gas formed in step (b) has a SO.sub.2-content of 60 to 99 vol.-% and a water content of 40 to 1 vol.-%.

3. The method according to claim 1, wherein the temperature of said air during the addition to the SO.sub.2-containing rich gas in step (c) is from 300 to 600 C.

4. The method according to claim 1, wherein a volume ratio of O.sub.2/SO.sub.2 is set to 0.5 to 1.2 by the supply of air in step (c).

5. The method according to claim 1, wherein the SO.sub.2-containing rich gas with the added air is heated in the gas pre-heater in step (d) to a temperature of 380 to 480 C.

6. The method according to claim 1, wherein the gas that has not been absorbed in the SO.sub.3-absorber in step (g) contains less than 10 vol.-% SO.sub.2.

7. The method according to claim 1, wherein the rich gas is not dried before its entry into the tube contacting device.

8. The method according to claim 1, wherein the air that is added to the SO.sub.2-containing rich gas in step (c) before its entry therein is withdrawn from the intermediate circuit for the supply and/or removal of heat to or from the oxidation of the SO.sub.2 to SO.sub.3 in the space between the inner tube and the outer tube of the tube contacting device.

9. The method according to claim 1, wherein the SO.sub.2 contained in the rich gas is converted to SO.sub.3 with a conversion of 70% to 99.9%.

10. A device for performing the method according to claim 1, wherein the device comprises the following components: a supply for the SO.sub.2-containing exhaust gas into the gas scrubber, a first absorber for absorbing the SO.sub.2 in a scrubbing liquid, a desorber for desorbing the SO.sub.2 from the scrubbing liquid, wherein the gas scrubber and the desorber are connected by a scrubbing agent circuit, a supply for the SO.sub.2-rich gases generated in the desorber into the tube contacting device, wherein this supply provides a feed of air, a tube contacting device, an SO.sub.3-absorber for the absorption of the SO.sub.3 formed in the tube contacting device, a recirculation line for recirculating the gas stream from the SO.sub.3-absorber into the supply for the SO.sub.2-containing exhaust gas to the gas scrubber.

11. The device according to claim 10, wherein the tube contacting device comprises an intermediate circuit for the supply and/or withdrawal of heat to or from the oxidation of SO.sub.2 to SO.sub.3.

12. The device according to claim 11, wherein the intermediate circuit comprises a cooling device, a fan or a pump and/or a heating device.

13. The device according to claim 10, wherein the device does not comprise an apparatus for condensation of the sulfuric acid resulting from the formed SO.sub.3.

14. The device according to claim 10, wherein the device does not comprise an apparatus for drying gases.

15. The device according to claim 10, wherein the device comprises a supply line for pre-heated air, which is added to the SO.sub.2-rich gas before it enters the gas pre-heater, wherein this supply line is connected to the circuit line of the intermediate circuit.

16. The method according to claim 1, wherein the SO.sub.2-containing rich gas formed in step (b) has a SO.sub.2-content of 80 to 98.5 vol.-% and a water content of 20 to 1.5 vol.-%.

17. The method according to claim 1, wherein the SO.sub.2-containing rich gas formed in step (b) has a SO.sub.2-content of 90 to 98 vol.-% and a water content of 10 to 2 vol.-%.

18. The method according to claim 1, wherein the temperature of said air during the addition to the SO.sub.2-containing rich gas in step (c) is from 380 to 560 C.

19. The method according to claim 1, wherein the temperature of said air during the addition to the SO.sub.2-containing rich gas in step (c) is from 400 to 520 C.

20. The method according to claim 1, wherein a volume ratio of O.sub.2/SO.sub.2 is set to 0.7 to 1.1 by the supply of air in step (c).

21. The method according to claim 1, wherein a volume ratio of O.sub.2/SO.sub.2 is set to 0.9 to 1.0 by the supply of air in step (c).

22. The method according to claim 1, wherein the SO.sub.2-containing rich gas with the added air is heated in the gas pre-heater in step (d) to a temperature of 400 to 450 C.

23. The method according to claim 1, wherein the gas that has not been absorbed in the SO.sub.3-absorber in step (g) contains less than 5 vol.-% SO.sub.2.

24. The method according to claim 1, wherein the gas that has not been absorbed in the SO.sub.3-absorber in step (g) contains less than 2 vol.-% SO.sub.2.

25. The method according to claim 1, wherein the SO.sub.2 contained in the rich gas is converted to SO.sub.3 with a conversion of 80% to 99.5%.

26. The method according to claim 1, wherein characterized in that the SO.sub.2 contained in the rich gas is converted to SO.sub.3 with a conversion of 90% to 99%.

27. The method according to claim 1, wherein characterized in that the SO.sub.2 contained in the rich gas is converted to SO.sub.3 with a conversion of 93 to 97%.

Description

(1) The invention is elucidated in more detail by the following connection-principle diagrams and the description thereof, without being confined thereto.

(2) FIG. 1 shows a schematic representation of a device for continuous catalytic oxidation of SO.sub.2 to SO.sub.3 from the prior art. This device and the method performed therein are already described comprehensively in example 4 and FIG. 5 of WO2008052649A1.

(3) FIG. 2 represents an exemplary embodiment of the method of the invention, wherein an SO.sub.2-rich gas is passed into a tube contacting device without gas drying beforehand.

(4) FIG. 3 represents an embodiment of the invention, wherein the SO.sub.2-containing exhaust gas is passed via a gas line into the gas scrubber.

LIST OF REFERENCE SYMBOLS

(5) 1 Gas dryer 2 Gas pre-heater 3 SO.sub.3-absorber 4 Outlet (chimney) 5 Tube contacting device 6 Heating device 7 Cooling device 8 Fan/pump 9 Gas scrubber 10 Desorber a Supply for the SO.sub.2-containing exhaust gas b Oxygen supply line c Air supply line d Air supply line into the intermediate circuit e Gas line to the SO.sub.3-absorber f Gas line to the outlet (chimney) g Gas line from the gas dryer to the tube contacting device h Gas line to the supply for the SO.sub.2-containing exhaust gas K Circuit line of intermediate circuit (cooling/heating) j Gas line from the SO.sub.3-absorber to the gas scrubber L Scrubbing liquid circuit m Gas line from the desorber to the tube contacting device

Example 1 (Prior Art)

(6) In the device according to FIG. 1, the SO.sub.2-containing exhaust gas is passed via the supply (a) into the gas dryer (1), after having been admixed with oxygen and/or air via the supply lines (b) and (d). The SO.sub.2-containing gas, now dried to a water content below 0.1 vol.-%, is guided via the gas line (g), after preheating via the gas pre-heater (2), completely into the tube contacting device (5). The gas is subsequently guided via the gas line (e) to the SO.sub.3-absorber (3). Via the gas line (j), the gas then passes for final purification into the gas scrubber (9), before being discharged to the surroundings via the gas line (f) and outlet (4).

(7) A particular disadvantage of this device is the gas dryer positioned upstream of the tube contacting device, this gas dryer entailing increased processing and apparatus cost and complexity relative to the method of the invention and the device of the invention, respectively.

Example 2 (Inventive)

(8) FIG. 2 represents an exemplary embodiment of the method of the invention. In this embodiment, the SO.sub.2-containing exhaust gas is passed via a gas line (a) into the gas scrubber (9). In this scrubber, the gas is purified of SO.sub.2 to an extent that it can be discharged to the environment via a gas line (f) and eventually an outlet (4).

(9) The purification of the SO.sub.2-containing exhaust gas in the gas scrubber (9) is accomplished by means of an aqueous scrubbing liquid. After becoming loaded with SO.sub.2, this liquid is passed via the scrubbing liquid circuit (L) into the desorber (10), where it is freed of SO.sub.2, preferably thermally, and is returned to the gas scrubber again.

(10) The water-containing SO.sub.2-rich gas liberated in the desorber is passed via the gas line (m) into the tube contacting device, and this SO.sub.2-rich gas is enriched with air (supply line (c)) and optionally oxygen (supply line (b)) and is heated in the gas pre-heater (2) to a temperature of 380 C. to 480 C., preferably 400 C. to 450 C.

(11) In the tube contacting device (5), which is a vertical heat exchanger comprising one or more jacketed tubes having an inner tube and an outer tube, with a catalyst having been introduced into the inner tubes, and the heat transfer taking place around these inner tubes by means of a medium which is passed in co-current or counter-current in the space formed by the inner tube and the outer tube, the SO.sub.2 is then oxidized to SO.sub.3. In order on the one hand to keep the catalyst active and on the other hand to prevent thermal damage to the catalyst, the tube contacting device is connected to an intermediate circuit, comprising the circuit line (K), the heating device (6), the fan/the pump (8) and the cooling device (7), which takes off excess heat from the tube contacting device or supplies heat to said device, according to requirements.

(12) The SO.sub.3 formed in the tube contacting device leaves this device via the gas line (e) and is guided by that line into the SO.sub.3-absorber (3), in which the SO.sub.3 is taken up in water or sulfuric acid in order to form sulfuric acid or oleum, respectively.

(13) The gas freed of SO.sub.3 is then fed via the gas line (h) into the supply (a) and is guided together with the SO.sub.2-containing exhaust gas into the gas scrubber (9) again. Residues of SO.sub.2 and/or SO.sub.3 that are possibly present are therefore supplied to the circuit again, and do not enter the environment.

Example 3 (Inventively Preferred)

(14) FIG. 3 represents an exemplary embodiment of the method of the invention. In this embodiment, the SO.sub.2-containing exhaust gas is passed via a gas line (a) into the gas scrubber (9). In this scrubber, the gas is purified of SO.sub.2 to an extent that it can be discharged to the environment via a gas line (f) and eventually an outlet (4).

(15) The purification of the SO.sub.2-containing exhaust gas in the gas scrubber (9) is accomplished by means of an aqueous scrubbing liquid. After becoming loaded with SO.sub.2, this liquid is passed via the scrubbing liquid circuit (L) into the desorber (10), where it is freed of SO.sub.2, preferably thermally, and is returned to the gas scrubber again.

(16) The water-containing SO.sub.2-rich gas liberated in the desorber is passed via the gas line (m) into the tube contacting device, and this SO.sub.2-rich gas is enriched with air (supply line (c)) and optionally oxygen (supply line (b)) and is heated in the gas pre-heater (2) to a temperature of 380 C. to 480 C., preferably 400 C. to 450 C.

(17) In the tube contacting device (5), which is a vertical heat exchanger comprising one or more jacketed tubes having an inner tube and an outer tube, with a catalyst having been introduced into the inner tubes, and the heat transfer taking place around these inner tubes by means of a medium which is passed in co-current or counter-current in the space formed by the inner tube and the outer tube, the SO.sub.2 is then oxidized to SO.sub.3. In order on the one hand to keep the catalyst active and on the other hand to prevent thermal damage to the catalyst, the tube contacting device is connected to an intermediate circuit, comprising the circuit line (K), the heating device (6), the fan/the pump (8) and the cooling device (7), which takes off excess heat from the tube contacting device or supplies heat to said device, according to requirements.

(18) The air which is added to the SO.sub.2-rich gas prior to entry into the gas pre-heater (2) is taken from the intermediate circuit line (k) by way of the supply (c) and therefore has a temperature that is considerably higher than that of the surroundings. The quantity of air taken from the intermediate circuit line (K) is supplied to the intermediate circuit line (k) again downstream of the first cooling device (7), in the flow direction, and upstream of the fan (8), by way of the supply line (d).

(19) The SO.sub.3 formed in the tube contacting device leaves this device via the gas line (e) and is guided by that line into the SO.sub.3-absorber (3), in which the SO.sub.3 is taken up in water or sulfuric acid in order to form sulfuric acid or oleum, respectively.

(20) The gas freed of SO.sub.3 is then fed via the gas line (h) into the supply (a) and is guided together with the SO.sub.2-containing exhaust gas into the gas scrubber (9) again. Residues of SO.sub.2 and/or SO.sub.3 that are possibly present are therefore supplied to the circuit again, and do not enter the environment.