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 a set of successive catalyst beds connected in series, the method comprising: flowing a waste gas stream comprising SO.sub.2 through the set of catalyst beds; shutting down the catalyst beds, the shutting down comprising: isolating at least one final downstream catalyst bed from the set of catalyst beds; leaving all other catalyst beds of the set connected in series; flowing a first heated purge gas stream through the at least one final downstream catalyst bed; flowing a second heated purge gas stream through the other catalyst beds; starting up the set of catalyst beds, the starting up comprising: placing the at least one final catalyst beds back in series with the other catalyst beds; and absorbing SO.sub.2 and SO.sub.3 into a catalyst of the at least one final downstream catalyst bed during the starting up.

2. A method according to claim 1, wherein the shutting down comprises: isolating at least two final downstream catalyst beds from the set of catalyst beds; configuring the penultimate catalyst bed and final downstream catalyst beds in series; and flowing the first hot purge gas stream through the penultimate catalyst bed prior to the final downstream catalyst bed.

3. A method according to claim 1, wherein the shutting down comprises: isolating two or more beds from the other catalyst beds, downstream of the first catalyst bed; and purging each isolated catalyst bed separately with hot purge gas.

4. A method according to claim 1, wherein the shutting down comprises: isolating two or more beds from the other catalyst beds, downstream of the first catalyst bed; and purging each isolated catalyst bed simultaneously with hot purge 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 comprises silicon dioxide (SiO.sub.2).

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

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

9. A method according to claim 8, wherein an alkali metal content of the catalyst is 2-25 wt %.

10. A method according to claim 9, wherein the alkali metal content of the catalyst is 8-16 wt %.

11. A method according to claim 1, wherein the first heated purge gas stream is air fed to the final catalyst bed at a temperature of 400-600 C.

12. 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.

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

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

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

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

17. A method according to claim 16, wherein the catalyst contains 4-10 wt % of a vanadium(V) compound such as V.sub.2O.sub.5.

18. A method according to claim 7, wherein the porous carrier of the catalyst comprises SiO.sub.2 with not greater than 5 wt % of alumina.

19. A method according to claim 8, wherein the porous carrier of the catalyst comprises SiO.sub.2 with not greater than 1 wt % of alumina.

20. 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 a set of successive catalyst beds connnected in series, wherein: a waste gas stream comprising SO.sub.2 is flowed through the set of catalyst beds; at least one final downstream catalyst bed is isolated from the set of catalyst beds during shut down, leaving all other catalyst beds of the set connected in series a first heated purge gas stream is flowed through the at least one final downstream catalyst bed during shut down; a second heated purge gas stream is flowed through the other catalyst beds during shut down; the at least one final catalyst beds are placed back in series with the other catalyst beds; and SO.sub.2 and SO.sub.3 is absorbed into a catalyst of the final downstream catalyst bed during start up.

Description

(1) FIG. 1 illustrates, from left to right, the normal operation (prior art), the normal shut-down purging (prior art) and the separate purging according to the invention, all illustrated for a plant design comprising four catalyst beds (i.e. n=4), and

(2) FIG. 2 shows a comparison of the separate purge according to the invention and the straight-through purge of the prior art as regards SO2 emission [ppm] as a function of time passed after introduction of feed gas [h].

(3) Normally, SO.sub.3 is purged from the catalyst during shut-down by passing hot air to the converter inlet and through all catalyst beds connected in series. The heat is primarily supplied by residual heat accumulated in the front-end of the plant (e.g. sulfur burner, boiler(s), ducting etc.). However, due to the above reaction (2), the SO.sub.3 released from the upper beds will accumulate in the final bed. If purging is not long enough, or if the temperatures are too low for reaction (2) to proceed to the left, then the SO.sub.3 desorption will cease. As a consequence, the concentration of free SO.sub.3 in the final bed is high at the next start-up, which will lead to SO.sub.3 emissions as described above.

(4) In the process and the plant according to the invention, the shut-down procedure is changed. The hot air used for purging the upper catalyst beds is not sent to the final catalyst bed, but rather to an SO.sub.3 absorption tower before going to the stack. The final catalyst bed is purged separately with hot air, and as a result, the final bed desorbs SO.sub.3 and shifts reaction (2) to the left. During the next start-up, the sulfur-deficient final catalyst bed will act as a sulfur oxide filter and absorb both SO.sub.2 and SO.sub.3 due to reaction (2) and the reaction
SO.sub.2+1/2O.sub.2+A<->A.SO.sub.3+heat (3)
where A is a species in the melt which is able to chemically bind SO.sub.3 as mentioned earlier.

(5) In this way 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.

(6) The rate of reaction (1) is very low for vanadium-based catalysts at temperatures below 370-400 C. depending on the specific catalyst type and gas composition. Now it has surprisingly been found that the rate of reaction (3) is high, even at low temperatures, for a sulfur-deficient catalyst which can remove SO.sub.2 at temperatures well below 350 C.

(7) Regarding the catalyst, a preferred catalyst comprises a vanadium(V) compound such as V.sub.2O.sub.5, 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. The porous carrier of the catalyst is preferably silicon dioxide (SiO.sub.2) with less than 10 wt %, preferably less than 5 wt %, more preferably less than 2 wt % and most preferably less than 1 wt % of alumina.

(8) It is preferred that the alkali metal content of the catalyst is 2-25 wt %, more preferably 4-20 wt % and most preferably 8-16 wt %.

(9) A preferred catalyst contains 1-15 wt %, preferably 2-12 wt % and most preferably 4-10 wt % of a vanadium(V) compound such as V.sub.2O.sub.5.

(10) Further it is preferred that the catalyst contains 1-25 wt %, more preferably 2-20 wt % and most preferably 3-18 wt % sulfur in the form of sulfate, pyrosulfate, tri- or tetrasulfate. It is even more preferred that the catalyst contains 4-16 wt % sulfur, especially 4-10 wt % sulfur, in the form of sulfate, pyrosulfate, tri- or tetrasulfate.

(11) It is preferred that the hot gas is air fed to the final bed at a temperature of 0-650 C., preferably 400-600 C.

(12) The invention is illustrated further in the following example.

EXAMPLE

(13) 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.

(14) The basis of the example is a transient model for dynamic operation of an SO.sub.2 converter published by Srensen 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.

(15) 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.

(16) 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.