METHOD AND PLANT DESIGN FOR REDUCTION OF START-UP SULFUR OXIDE EMISSIONS IN SULFURIC ACID PRODUCTION

Abstract

The invention is a method and a sulfuric acid plant design for reduction of start-up SO.sub.2, SO.sub.3 and H.sub.2SO.sub.4 emissions in sulfuric acid production, in which SO2 is converted to SO.sub.3 in n successive catalyst beds, where n is an integer >1. The final catalytic beds are used as absorbents for SO.sub.2 to SO3 during the start-up procedure, and one or more of the m beds downstream the first bed are purged, either separately or simultaneously, with hot gas, where m is an integer >1 and m<n, during the previous shut-down. Also, one separate purge with hot gas is used on the final bed.

Claims

1. A method for reduction of start-up SO.sub.2, SO.sub.3 and H.sub.2SO.sub.4 emissions in sulfuric acid production, in which SO.sub.2 is converted to SO.sub.3 in n successive catalyst beds, where n is an integer >1, wherein the final catalyst beds are used as absorbents for SO.sub.2 and SO.sub.3 during the start-up procedure, one or more of the m beds downstream the first bed are purged, either separately or simultaneously, with hot gas, where m is an integer >1 and m<n, during the previous shutdown, and one separate purge with hot gas is used on the final bed.

2. A method according to claim 1, wherein one separate purge with hot gas is used on the bed prior to the final bed, and wherein the gas from the purged bed is fed to the final bed.

3. A method according to claim 1, wherein two or more beds downstream the first bed are purged separately with hot gas.

4. A method according to claim 1, wherein two or more beds downstream the first bed are purged simultaneously with hot gas.

5. A method according to claim 1, where the catalyst comprises a vanadium(V) compound, sulfur in the form of sulfate, pyrosulfate, tri- or tetrasulfate and alkali metals, such as Li, Na, K, Rb or Cs, on a porous carrier.

6. A method according to claim 5, wherein the porous carrier of the catalyst is silicon dioxide (SiO.sub.2).

7. A method according to claim 6, wherein the porous carrier of the catalyst is SiO.sub.2 with less than 10 wt %, preferably less than 5 wt %, of alumina.

8. A method according to claim 7, wherein the porous carrier of the catalyst is SiO.sub.2 with less than 2 wt %, preferably less than 1 wt %, of alumina.

9. A method according to claim 8, wherein the alkali metal content of the catalyst is 2-25 wt %, preferably 4-20 wt % and most preferably 8-16 wt %.

10. A method according to claim 5, wherein the catalyst contains 1-15 wt % of a vanadium(V) compound such as V.sub.2O.sub.5.

11. A method according to claim 10, wherein the catalyst contains 2-12 wt %, preferably 4-10 wt % of a vanadium(V) compound such as V.sub.2O.sub.5.

12. A method according to claim 5, wherein the catalyst contains 1-25 wt % sulfur in the form of sulfate, pyrosulfate, tri- or tetrasulfate.

13. A method according to claim 12, wherein the catalyst contains 2-20 wt % sulfur, preferably 3-18 wt % sulfur, in the form of sulfate, pyrosulfate, tri- or tetrasulfate.

14. A method according to claim 13, wherein the catalyst contains 4-16 wt % sulfur, preferably 4-10 wt % sulfur, in the form of sulfate, pyrosulfate, tri- or tetrasulfate.

15. A method according to claim 1, wherein the hot gas is air fed to the final bed at a temperature of 0-650? C., preferably 400-600? C.

16. A sulfuric acid plant design provided with means for securing reduced start-up emissions of SO.sub.2, SO.sub.3 and H.sub.2SO.sub.4, said plant design comprising n successive catalyst beds, where n is an integer >1, wherein the final catalytic beds are used as absorbents for SO.sub.2 and SO.sub.3 during the start-up procedure, one or more of the m beds downstream the first bed are purged, either separately or simultaneously, with hot gas, where m is an integer >1 and m<n, during the previous shutdown, and one separate purge with hot gas is used on the final bed.

Description

EXAMPLE

[0032] By using the method and the plant design according to the invention, the emissions of SO.sub.2 and SO.sub.3 are reduced during start-up, and the plant can be started up faster without violating SO.sub.2 and SO.sub.3 limits for transient operation. This reduction of emissions is illustrated in FIG. 2.

[0033] The basis of the example is a transient model for dynamic operation of an SO.sub.2 converter published by S?rensen et al. (Chemical Engineering Journal 278 (2015), 421-429). The mathematical model is capable of predicting the changes occurring in an SO.sub.2 converter due to changes in the operating conditions, because it can predict the dynamic changes in the temperature of the converter and the sulfur content of the catalyst.

[0034] In this example, a 3+1 double absorption plant is purged with 450? C. for 8 hours before the air supply is turned off. The plant is subsequently assumed to be shut down for a non-specific period of time and the beds re-heated to temperatures of 550? C., 460? C., 420? C. and 380? C., respectively, prior to introduction of the SO.sub.2 feed gas.

[0035] The curves in FIG. 2 show the SO.sub.2 emission in ppm as a function of the time passed (in hours) for both the straight-through purge and the separate purge embodiment. It appears clearly from the curves that the straight-through purge causes a substantial SO.sub.2 emission immediately after introducing the feed gas. Within minutes after the feed gas introduction, the SO.sub.2 emission increases to 300 ppm, whereas the separate purge according to the invention leads to a much lower SO.sub.2 emission, especially during the first half hour following the feed gas introduction. Only after around 1.5 hours from the feed gas introduction, the two curves approach the same SO.sub.2 emission level.