PROCESS AND PLANT FOR IMPROVED ENERGY-EFFICIENT PRODUCTION OF SULFURIC ACID

Abstract

The invention describes a process for producing sulfuric acid by catalytic oxidation of SO.sub.2 to SO.sub.3 and subsequent absorption of the SO.sub.3 in sulfuric acid, wherein the SO.sub.3 is introduced into a first absorption stage (primary absorber) and at least partly absorbed there in concentrated sulfuric acid, wherein the SO.sub.3 not absorbed in the first absorption stage is supplied to a second absorption stage (secondary absorber) for the further absorption in concentrated sulfuric acid, and wherein the sulfuric acid is cooled after passing through the two absorption stages. The cooling of the sulfuric acid is effected in at least two heat exchangers connected in parallel, wherein one of the at least two heat exchangers is operated as partial evaporator and is cooled with boiler feed water/steam and the other one is cooled with cooling water and operated as pure acid cooler.

Claims

1. A process for producing sulfuric acid by catalytic oxidation of SO.sub.2 to SO.sub.3 and subsequent absorption of the SO.sub.3 in sulfuric acid, wherein the SO.sub.3 is introduced into a first absorption stage (primary absorber) and at least partly absorbed there in concentrated sulfuric acid, wherein the SO.sub.3 not absorbed in the first absorption stage is supplied to a second absorption stage (secondary absorber) for the further absorption in concentrated sulfuric acid, and wherein the sulfuric acid is cooled after passing through the two absorption stages, wherein cooling of the sulfuric acid is effected in at least two heat exchangers connected in parallel, wherein one of the at least two heat exchangers is operated as partial evaporator and is cooled with boiler feed water/steam and the other one is cooled with cooling water and operated as pure acid cooler, wherein the outflowing sulfuric acid is guided from the first absorption stage to the sump of the second absorption stage or that the outflowing sulfuric acid is guided from the second absorption stage to the sump of the first absorption stage or that the sulfuric acid runs into a common sump and/or a common pump tank of the two absorbers, characterized in that in the primary absorber the sulfuric acid is guided in co-current flow to the SO.sub.3 containing gas and that in the second absorber the sulfuric acid is guided in counter-current flow to the SO.sub.3 containing gas.

2. The process according to claim 1, characterized in that in the heat exchanger cooled with boiler feed water/steam a first part (A) of 0-100% of the entire heat quantity is cooled, and in the heat exchanger cooled with cooling water a second remaining part (B) of 100-0% is cooled.

3. (canceled)

4. (canceled)

5. The process according to claim 1, characterized in that the concentration of the sulfuric acid after cooling in the two parallel arranged heat exchangers is adjusted to a value between 98.0 and 99.4 wt %, preferably between 98.5 and 99.2 wt-%, by admixing process water.

6. The process according to claim 1, characterized in that the concentration of the sulfuric acid which is fed to the primary absorber is higher than the concentration of the sulfuric acid which is fed to the secondary absorber.

7. The process according to claim 1, characterized in that the exit temperature of the sulfuric acid downstream the heat exchanger cooled with cooling water lies between 60 and 90° C., preferably between 70 and 80° C.

8. The process according to claim 1, characterized in that the exit temperature of the sulfuric acid downstream the heat exchanger cooled with boiler feed water/steam lies between 150 and 230° C., preferably between 180 and 210° C.

9. A plant for producing sulfuric acid by catalytic oxidation of SO.sub.2 to SO.sub.3 and subsequent absorption of the SO.sub.3 in sulfuric acid, comprising a primary absorber to which gaseous SO.sub.3 and concentrated sulfuric acid are supplied, in order to absorb SO.sub.3 in the sulfuric acid, a secondary absorber to which the SO.sub.3 not absorbed in the primary absorber is supplied for the further absorption in sulfuric acid, and at least one recirculation conduit for recirculating the sulfuric acid from a sump of the two absorbers into the head of the primary absorber, at least two heat exchangers connected in parallel in the at least one recirculation conduit, one of which is designed for cooling with boiler feed water/steam and the other one is designed for cooling with cooling water and by a connection connecting the sumps of the primary and the secondary absorber or a common sump and/or a common pump tank of the primary absorber and the secondary absorber, characterized in that in the primary absorber is designed for guiding the sulfuric acid in co-current flow to the SO.sub.3 containing gas and that in the second absorber is designed for guiding the sulfuric acid in counter-current flow to the SO.sub.3 containing gas.

10. The plant according to claim 9, characterized in that the primary absorber and/or the secondary absorber is/are designed as fixed-bed absorber(s).

11. The plant according to claim 9, characterized in that the primary absorber and/or the secondary absorber is/are designed as Venturi absorber(s).

12. The plant according to claim 9, characterized in that in the at least one recirculation conduit at least one process water mixing device is arranged for the addition of water.

13. The plant according to claim 12, characterized in that the at least one process water mixing device is arranged in flow direction downstream the two heat exchangers.

14. The plant according to claim 12, characterized in that at least one conduit is branched off after the at least one process water mixing device, through which the sulphuric acid is passed in the secondary absorber directly.

Description

[0053] In the drawings:

[0054] FIG. 1 shows the plant according to the invention with two process water mixing devices for the addition of process water,

[0055] FIG. 2 shows the plant according to the invention with direct addition of acid from the first into the second absorber,

[0056] FIG. 3 shows the plant according to the invention with a single process water mixing device,

[0057] FIG. 4 shows the plant according to the invention with the primary absorber designed as packed bed absorber, and

[0058] FIG. 5 shows the plant according to the invention in the entire sulfuric acid plant, i.e. with representation of the drying tower and final absorber and their acid circuits.

[0059] FIG. 1 shows the configuration according to the invention. Via conduit 1, gaseous sulfur trioxide is fed into the primary absorber 42 designed as Venturi absorber. Via conduit 3, the absorbent sulfuric acid likewise is added at the head of the primary absorber 42, so that SO.sub.3 and sulfuric acid co-currently pass through the primary absorber 42. Via the connection 61, the sulfuric acid concentrated by the absorption flows into the sump 62 of the secondary absorber 41 together with the gas.

[0060] Advantageously, the secondary absorber 41 is designed as fixed-bed absorber. The gaseous SO.sub.3 introduced via the connection 61 escapes upwards and is absorbed virtually completely in the secondary absorber 41. Remaining SO.sub.2 is withdrawn via conduit 2 together with inert gases. Via conduit 11, sulfuric acid additionally is introduced into the head of the secondary absorber 41. The same trickles down into the secondary absorber 41 preferably designed as fixed-bed reactor, so that here SO.sub.3 and H.sub.2SO.sub.4 are guided in counter-current flow.

[0061] Via conduit 10 acid originated from the sump 62, which is composed of the acid discharged in the primary absorber 42 and the acid discharged in the secondary absorber 41, can be discharged into the common pump tank 46. From this pump tank 46, the sulfuric acid is supplied by means of the pump 45 via conduit 9 to two heat exchangers 43 and 44 connected in parallel.

[0062] The supply of boiler feed water to the heat exchanger 43 is effected from the steam drum 57 via the circulation pump 58 and conduit 30 into the evaporator heat exchanger 43 cooled with water/steam, with the outlet conduit for the steam/water mixture 31 back to a steam drum 57 in which the steam is separated from water. The steam produced thereby is exported from the plant via conduit 32.

[0063] Fresh boiler feed water is supplied to the steam drum 57 via conduit 29. Via conduit 7, the cooled sulfuric acid can then be withdrawn from the heat exchanger 43.

[0064] From conduit 7, the cooled acid is delivered via conduit 5 into a mixing device 49, in which via conduit 12 and the flow control valve 52 contained therein, process water is admixed to the acid, in order to adjust the concentration of the acid to a range between 98.0 and 99.4 wt-%. Via conduit 3, the acid diluted in this way then gets back into the primary absorber 42.

[0065] Via conduit 6 and the control valve 59 parts of the cooled acid can be supplied to the further heat exchanger 47 via conduit 19 or to the heat exchanger 48 via conduit 20. The cooled acid exiting from the heat exchanger 47 is discharged via conduit 21. The cooled acid exiting from the heat exchanger 48 is discharged via conduit 22. Acid from conduits 21 and 22 is combined and discharged as product via conduit 23.

[0066] Alternatively or parallel to the cooling of the circulating acid in the heat exchanger 43, circulating acid to be cooled can be fed via conduit 17 into a second heat exchanger 44 cooled with cooling water with a shut-off or flow control valve 56 provided therein. This heat exchanger 44 includes a corresponding feed conduit 101 and an outlet conduit 102 for the water used as coolant. Via conduit 16, the cooled acid is withdrawn.

[0067] Via conduit 15, parts of the hot acid get into a mixing device 50. In this mixing device 50, process water is added to the acid via conduit 13 and the flow control valve 51 provided therein, so that the concentration of the acid is adjusted to values between 98.0 and 99.4%. Acid diluted in this way then is withdrawn via conduit 14.

[0068] The conduits 9, 8, 7, 5, 4 and 3 thus form a recirculation line in the heat recovery mode, and the conduits 9, 17, 16, 15, 14 and 3 form a recirculation line in the cooling mode.

[0069] Via conduit 27, the heat exchanger 47 is cooled with water originating from conduit 26, which is withdrawn via conduit 28. This water preferably is demineralized water, which ultimately is utilized for steam generation. Via conduit 28, this water preferably is supplied to a non-illustrated thermal water deaerator. In the heat recovery mode, the energy transmitted in the heat exchanger 47 ultimately is taken over as increased steam production for high- and low-pressure steam.

[0070] As heat-exchanging medium, sulfuric acid from the drying tower circuit is used in the heat exchanger 48. The same is introduced via conduit 24 and fed into the pump receiver 46 via conduit 25, whereby in the heat recovery mode heat losses due to hot acid flowing off via conduit 6 are minimized and thus an increase in the low-pressure steam quantity is achieved.

[0071] The circulating sulfuric acid for the primary absorption can be cooled completely or in part in each of the heat exchangers 43 or 44. For distributing the acid on the two heat exchangers, shut-off or control valves 54 and 53 or 56 and 55 respectively are provided before and after these heat exchangers. The adaptation to the respective demands, such as e.g. reduced export of low-pressure steam, can be performed during the operation.

[0072] In the pure acid cooling mode, a part of the circulating acid is withdrawn via conduit 18 by means of the control valve 60 and discharged as product.

[0073] FIG. 2 likewise shows a configuration according to the invention. However, a conduit 11a is branched off here from conduit 3, which likewise feeds sulfuric acid as absorbent into the secondary absorber 41, so that the fresh sulfuric acid stream 11 can be reduced or entirely be set to zero.

[0074] This offers the advantage that in such configuration the supply of sulfuric acid from the final absorber circuit can be avoided and thus a decoupling of intermediate and final absorption takes place.

[0075] FIG. 3 furthermore shows a configuration of the plant according to the invention in which a single process water mixing device 49 is used. From both, the first heat exchanger 43 operated as evaporator the correspondingly cooled sulfuric acid is supplied via conduit 7 and the flow control valve 53 contained therein as well as the sulfuric acid from the second heat exchanger 44 operated with cooling water is supplied via conduit 14 to the single mixing device 49, from which the correspondingly diluted sulfuric acid is directly introduced into the primary absorber 42 via conduit 3. The required process water is supplied to the process water mixing device 49 via conduit 12 and the control valve 52.

[0076] FIG. 4 shows the design of the primary absorber 42 as packed bed absorber, wherein the mode of function basically is identical to the representation of FIG. 3. The collected acid from the outlets of the primary and secondary absorbers can be collected both in the sump 63 of the primary absorber as well as in the sump 62 of the secondary absorber. The outflow via conduit 10 into the pump tank 46 can be effected both from the sump 63 and in a manner not shown here, or from the sump 62. The channel 61 serves as level equalizer. This allows an optimum adaptation of the pump tank 46 to possibly existing restrictions regarding space conditions.

[0077] FIG. 5 finally shows the process according to the invention in a particular configuration in connection with the entire acid plant process, wherein the gas entering the drying tower 72 must not exceed a particular water content/moisture. Via conduit 1, SO.sub.3 to be absorbed is introduced into the primary absorber 42, where it is absorbed by the sulfuric acid supplied via conduit 3. Via a gas conduit 61, a mixture of gas and sulfuric acid is guided into the sump 62 of the secondary absorber 41. Via conduit 11, this secondary absorber is supplied with sulfuric acid from the circuit of the final absorber 71.

[0078] Via conduit 81, the residual SO.sub.2 converted to SO.sub.3 in the second catalytic stage is supplied to the final absorber 71 designed as packed bed absorber, and absorbed there in sulfuric acid. Contained inert gases escape from the final absorber 71 to a stack via conduit 82.

[0079] Via conduit 79, ambient air or SO.sub.2-containing process gas is guided into a drying tower 72 designed as packed absorber and leaves the same via conduit 80 to the blower 100 which conveys the gas through the entire plant. The moisture contained in this gas stream of conduit 79 is absorbed in circulating sulfuric acid.

[0080] Via conduit 21, the production from the intermediate absorber system, consisting of primary absorber 42 and secondary absorber 41 gets into a pump tank 76 of the common acid circuit for the final absorber 71 and the drying tower 72. Via conduit 98 the concentrated acid flowing off from the final absorber 71 and via conduit 99 the diluted acid flowing off from the drying tower 72 are also fed to the pump tank 76. The mixture of these two acid streams in conduit 97 still has a concentration which lies above 98.5 wt-% and therefore must again be brought to the required concentration by means of process water addition.

[0081] For this purpose, the acid from the pump tank 76 is fed to an acid cooler 73 by means of the pump 77 via conduit 97 and after cooling enters into a process water mixing device 75 via conduit 93, in which mixing device the acid concentration is adjusted to 98.5 wt. % of H.sub.2SO.sub.4. For this purpose, process water is supplied to the process water mixing device 75 via conduit 66 with the control valve 78.

[0082] Here as well, the increased acid concentration to which the acid cooler 73 is exposed has an advantageous effect on the corrosion behavior of the cooler 73 as well as of the pump 77 and the connected acid conduits 93, 26, 27 and 12.

[0083] Preferably demineralized water for steam generation is introduced into the plant via conduit 64 and split up into the streams of conduit 65 and conduit 66. Via conduit 65, this cold water flows to the acid cooler 73, where it absorbs energy from the acid cooling. Via conduit 26, the water heated in this way is then introduced into the heat exchanger 47 for further heating.

[0084] The acid from conduit 92, which exits from a process water mixing device 75, is split up into three partial streams 90, 91 and 94. One partial stream is guided onto the head of the end absorber 71 via conduit 90, another partial stream is guided onto the head of the drying tower 72 via conduit 91, and via conduit 94 a third partial stream is guided as production with conduit 95 and as cross-flow to the secondary absorber with conduit 11.

[0085] The acid produced in the primary and secondary absorber is supplied to the circuit of end absorber and drying tower via conduit 21 and combined with the acid formed in the system of end absorber and drying tower and ultimately via conduit 95 is jointly introduced into a product cooler 74 as export product acid of the plant. After cooling by means of cooling water, the product acid is exported from the plant via conduit 96. Cooling water enters into the product cooler 74 via conduit 103 and leaves the cooler via conduit 104.

LIST OF REFERENCE NUMERALS

[0086] 1-2 conduit [0087] 3-32 conduit [0088] 41 secondary absorber [0089] 42 primary absorber [0090] 43 heat exchanger operated with steam/water [0091] 44 heat exchanger operated with cooling water [0092] 45 pump [0093] 46 pump tank [0094] 47 heat exchanger [0095] 48 heat exchanger [0096] 49 process water mixing device [0097] 50 process water mixing device [0098] 51-52 flow control valve [0099] 53-56 flow control valve [0100] 57 steam drum [0101] 58 pump [0102] 61 connection [0103] 62 sump [0104] 64-66 conduit [0105] 71 final absorber [0106] 72 drying tower [0107] 73-74 heat exchanger [0108] 75 process water mixing device [0109] 78 flow control valve [0110] 76 pump tank [0111] 77 pump [0112] 79-82 conduit [0113] 90-99 conduit [0114] 100 blower [0115] 101-104 conduit