AU2008261290A1

AU2008261290A1 - Process and apparatus for mixing gases

Info

Publication number: AU2008261290A1
Application number: AU2008261290A
Authority: AU
Inventors: David Cachero Ventosa; Karl-Heinz Daum; Wolfram Schalk
Original assignee: Outotec Oyj
Current assignee: Metso Metals Oy
Priority date: 2007-06-13
Filing date: 2008-06-06
Publication date: 2008-12-18
Anticipated expiration: 2028-06-06
Also published as: DE102007027881A1; AU2008261290B2; MX340510B; WO2008151758A3; DE102007027881B4; WO2008151758A2; CN101679037B; CL2008001718A1; CN101679037A; PE20090270A1; MX2009013478A

Description

WO 2008/151758 PCT/EP2008/004510 Process and Apparatus for Mixing Gases 5 The present invention relates to a process for mixing two gases of different tem perature and/or composition in a converter for producing SO 3 from an S02 containing gas, and to an apparatus for performing this process. The present invention is concerned with the production of sulfuric acid. Conven 10 tionally, sulfuric acid mostly is produced by the so-called double absorption proc ess, which is described in Ullmann's Encyclopedia of Industrial Chemistry, 5 t edi tion, Vol. A25, pp. 635 to 700. First of all, a starting gas containing sulfur dioxide is at least partly reacted with oxygen in a plurality of successive contact stages of a converter corresponding to the formula 15 S02 + 1/2 0 2 -+ S03 + 98 KJ to obtain sulfur trioxide. The produced gas containing sulfur trioxide then is sup plied to an absorber and converted there to sulfuric acid. The oxidation of the sul 20 fur dioxide to sulfur trioxide is effected in the presence of a catalyst, which usually contains vanadium pentoxide as active component and has a working range of about 380 to 6400C. While at temperatures above 6400C an irreversible damage of the vanadium pentoxide catalyst occurs, the same is inactive at temperatures be low 3800C. As the process is strongly exothermal, the gas inlet temperature into 25 the contact stage must be about 400 0 C. At a distinctly lower inlet temperature, the reaction is not initiated, wheas at a mnuch higher inlet temperature the tempera ture rises so much during the process that the catalyst is damaged. It is, however, possible to also use other catalysts which allow a higher working temperature, as is known e.g. from EP 1 047 497 B1 or DE 100 23 178 Al. To obtain a high yield, 30 the reaction is performed in several stages, between which the process gas each WO 2008/151758 PCT/EP2008/004510 -2 is cooled by means of integrated heat exchangers, in order to achieve a suitable gas inlet temperature for the next contact stage. Usually, such converter includes four to five contact stages, and in the above-mentioned double absorption process the process gas having passed through a number of, e.g. three, contact stages is 5 supplied to an intermediate absorption tower, in which the SO 3 is converted to sul furic acid, oleum or liquid SO 3 , and the SO 3 concentration in the process gas thereby is decreased again. Upon heating to the required process temperature, the process gas then is supplied to the next contact stages of the converter and thereafter to the final absorption. 10 The process gas supplied to the converter suffers from frequent fluctuations of the quantity and S02 concentration. While in conventional converters the SO 2 concen tration usually is restricted to about 12 vol-% due to the high temperatures achieved in the first step of catalysis, the process described in DE 102 49 782 Al 15 provides for using higher S02 concentrations by recirculating S0 3 -containing gas. This recirculation limits the reaction in the first contact stage and as a result the heat generated there. Due to the fluctuations of the inlet gas, it is required to control the temperature at 20 the inlet of the contact mass. This is effected by supplying cold S0 2 -containing gas via a bypass conduit. In the above-mentioned recirculation of S0 3 -containing gas, the mixing ratio must also be adjusted. Therefore, gases of different temperature and/or composition must be mixed in the converter at different points. Even with gases of the same composition, the temperature difference leads to different vis 25 cosities, which render mixing difficult. For the efficiency of the process it is, how ever, required to achieve a homogeneous gas mixture. if no sufficient homogene ity of the gas is achieved at the inlet of the contact mass, there are zones in which there is no conversion of SO 2 to SO 3 when passing through the contact stage, so that the efficiency of the converter is impaired. In zones with too high SO 2 content, 30 overheating possibly can lead to a damage of the catalyst. It was found that mixing WO 2008/151758 PCT/EP2008/004510 -3 gases of different temperature in the pipe conduits of sulfuric acid plants is not a fast, spontaneous process. Due to the different viscosities, the gases flow parallel to each other without mixing (so-called streaming). 5 For the solution of this problem, it is state of the art to provide local pressure losses, which lead to turbulences with a high degree of fluidization. However, this solution is not sufficient in many cases, as the entire pressure loss achievable or allowed in the system is limited or must be limited for reasons of plant technology. 10 Therefore, it is the object of the invention to increase the homogeneity of the mix ture of two gases of different temperature and/or composition in a converter and reduce or prevent the so-called streaming. This object substantially is solved by means of the invention in that a first, SO 2 15 containing gas flow is introduced into the converter through a central supply pipe and passed through an integrated heat exchanger arranged around the central supply pipe, that a second gas flow is supplied via a ring conduit arranged in the converter directly before or after the integrated heat exchanger, from which the second gas flow is discharged through a plurality of openings and is fed into the 20 first gas flow, so that it mixes with the same, and that the gas mixture obtained then is supplied to a contact stage of the converter, in which the SO 2 is at least partly converted to S03 on a catalyst. Via the ring conduit, the second gas thus is fed into the first gas at a plurality of points, so that an excellent mixing is achieved. As the second gas flow, which for instance is S0 2 -containing gas of low tempera 25 ture, only partly passes through the apparatus provided upstream of the converter, a higher pressure with respect to the mivture is available, whereby the pressure loss can also be increased and/or controlled. With this adjustment or control of pressure or pressure loss, the mixing and the mixing quality can be adjust or con trolled. If the second gas is recirculated sulfur trioxide, its pressure can be in 30 creased by means of a blower or the like, so that here as well a higher pressure WO 2008/151758 PCT/EP2008/004510 -4 with respect to the mixture is available as compared to conventional processes, and the pressure loss can be increased. By means of such blower it is also possi ble to regulate or selectively adjust and control the pressure or pressure loss. With this adjustment or control of the pressure or pressure loss the mixing quality can 5 also adjusted and controlled. It is preferred, that the pressure loss in the mixing device is optimised, to get sufficient turbulence and mixing quality without much pressure loss. In a preferred aspect of the invention, particularly good mixing results are obtained 10 when the second gas flow is countercurrently fed into the first gas flow. In this way, additional turbulences are generated, which promote mixing. However, it is also or additionally possible to cocurrently feed the second gas flow into the first gas flow. In the recirculation of S0 3 -containing gas, as it is known from DE 102 49 782 Al, the first gas flow includes more than 13 vol-% SO 2 and the second gas flow 3 to 40 15 vol-% SO 3 , preferably 5 to 25 vol-% SO 3 , in accordance with a development of the invention. The first gas stream also can contain S03, e. g. 1 to 10 vol-% SO 3 , but preferably contains less than 1 % S03. In a further embodiment of the invention the gas flow in the mixing chamber can 20 be guided e. g. by guiding plates or by the gas inlet of the ring conduit, so that e. g. a stream like a swirl in the mixing chamber results. If there is no recirculation of SO 3 -containing gas, the temperature at the inlet of the first contact stage of the converter is regulated in accordance with the invention by 25 means of a second gas flow, which includes up to 30 vol-% SO 2 , preferably up to 12 vol-% SO 2 - and has a temperature of 0 to 400 *C, preferably 80 to 300 0 C, more preferably 100 to 1200C. This gas flow can be passed around the usual heat ex changer as a bypass for increasing the temperature of the first gas, so that it has a higher pressure than the first gas. In the bypass, devices for controlling or regulat- WO 2008/151758 PCT/EP2008/004510 ing the pressure, e.g. blowers or throttle valves or flow resistors can be incorpo rated, whereby mixing can be controlled in addition. In accordance with the invention, the volume flow rate of the second gas flow is 5 smaller than that of the first gas flow and is 20 to 60%, preferably s 50% and more preferably s 30 % of the first gas flow. An inventive apparatus for mixing two gases in a converter for producing SO 3 from an S0 2 -containing gas, which can be used in particular for performing the process 10 described above, includes a supply conduit for a first gas which is introduced into the converter and is passed through an integrated heat exchanger, which is ar ranged around the central supply conduit, a ring conduit for a second gas, which is arranged in the converter directly before or after the heat exchanger in a mixing chamber, wherein the ring conduit includes a plurality of openings for the dis 15 charge of second gas into the mixing chamber. In a preferred embodiment of the invention, the first gas is introduced into the con verter from above or below and upon deflection passed through the integrated heat exchanger, which preferably is arranged around the central supply conduit, 20 and then enters the mixing chamber arranged above the heat exchanger. It, how ever, is possible that the gas is introduced from above or below into the heat ex changer in a straight way without deflection. The ring conduit also preferably is arranged around the central supply conduit for the first gas. Beside a simple guid ance of the flow, this results in a compact construction of the converter and the 25 mixing means. For countercurrently introducing the second gas flow into the first gas flow, the holes in the ring conduit are facing the heat exchanger. Thus, the second gas flow preferably is discharged from the ring conduit in downward direction. If the second 30 gas flow should cocurrently be fed into the first gas flow, the holes in the ring con- WO 2008/151758 PCT/EP2008/004510 -6 duit are facing away from the heat exchanger, so that the second gas flow is dis charged from the ring conduit in upward direction. It is of course possible that cor responding openings, which can be designed as bores, slots or the like, are pro vided both on the lower surface and on the upper surface or at the sides of the ring 5 conduit. The openings can be provided in several rows one beside the other, in order to increase the quantity of the second gas flow to be supplied. Preferably, the openings have an angle of 20 to 70*, particularly preferably 30 to 600 with re spect to the vertical. The openings can have a different size or shape, e.g. in de pendence on the distance from the supply conduit. It is also possible to vary the 10 openings or the gas flow around or from the openings by further installations, e.g. baffle plates, welding seams, etc. This variation can also concern only individual openings or individual groups of openings. It is furthermore possible to provide a closing device for individual openings or groups of openings, so as to have a regu lation or control means in particular in partial load operation. 15 In accordance with a preferred aspect of the invention, two to seven, in particular three concentric ring conduits are provided, from which the second gas is dis charged. The quantity of the second gas flow also can be increased thereby. The ring conduits may be located at different vertical levels. 20 To facilitate switching of the plant, the ring conduits are connected with a common supply conduit for the second gas in accordance with the invention. It was found to be advantageous that the common supply conduit for the second gas is at least partly guided around the central supply conduit for the first gas and that a plurality 25 of downwardly guided connecting conduits connect the supply conduit with the ring conduits. As a result, the second gas can simultaneously be introduced into the converter at several points, wherein a preferred symmetric configuration of the converter and hence a preferred symmetric guidance of the flow is achieved. What is essential for the invention, however, is the rather uniform mixing of the gases WO 2008/151758 PCT/EP2008/004510 -7 and the uniform distribution in the converter, for which purpose it is possible to de part from the symmetry, if necessary. In accordance with the invention, the connecting conduits are each connected with 5 all ring conduits via supply ports, in order to provide for an easier supply. In accordance with a further aspect of the invention, the plant has conduits for supplying gases to the mixer, which can be adjusted or controlled for the volume flow rate of the gas stream and/or pressure and/or pressure loss inside the con 10 duit. This control can be done with blowers or the like and/or devices for control pressure loss, e. g. buffles, nozzles or flow resistors. Developments, advantages and possible applications of the invention can also be taken from the following description of embodiments and from the drawing. All fea 15 tures described and/or illustrated per se or in any combination form the subject matter of the invention, independent of their inclusion in the claims or their back reference. In the drawing: 20 Fig. 1 shows a schematic sectional view of a converter for producing SO 3 from an S0 2 -containing gas with an apparatus for mixing gases in accordance with an embodiment of the present invention with illus trated gas flows, 25 Fig. 2 shows the region of introduction of the second gas flow into the first gas flow, Fig. 3 shows a partial top view of ring conduits of the apparatus in accor 30 dance with the invention, and WO 2008/151758 PCT/EP2008/004510 -8 Fig. 4 shows a section through a ring conduit along line IV-IV of Fig. 3. The converter 1 for converting S02 to S03 as shown in Fig. 1 includes a total of 5 five contact stages K1 to K5, in which a catalyst, in particular a catalyst containing vanadium pentoxide is provided, in order to convert the SO 2 to S03. After passing through three contact stages K1 to K3, the S0 3 -containing gas obtained is sup plied to a non-illustrated heat recovery plant and the intermediate absorption, in order to at least partly remove the sulfur trioxide from the process gas. The S02 10 containing process gas then is again supplied to the converter at the bottom, in order to pass through the contact stages K4 and K5. The present invention primar ily is intended to be realized in the first region of the converter 1 with the contact stages K1 to K3, so that the following description is restricted to this region. In an alternative embodiment (not shown) the mixing method of the present invention 15 may also be realized in the second region of the converter (contact stages K4 and K5). S0 2 -containing gas, which flows through the supply conduit from the top to the bottom and is deflected at its lower end by 1800 by means of a baffle plate 3, is 20 supplied to the converter 1 via a central supply conduit 2. Thereupon, the first gas traverses a first heat exchanger WT1, which in particular constitutes a tubular heat exchanger, from the bottom to the top and is discharged into a mixing chamber 4 arranged above the heat exchanger WT1. 25 A second gas, which for instance is recirculated S0 3 -containing process gas or S0 2 -containg gas of lower temperature than the first gas, likewise is introduced into the mixing chamber 4 via a supply conduit 5 and adjoining connecting con duits 6, which via supply ports 7 are connected with ring conduits 8 extending around the central supply conduit 2, and mixes with the first gas. The supply con 30 duit 5 here is guided around the central supply conduit 2 e.g. outside the converter WO 2008/151758 PCT/EP2008/004510 -9 1, for instance as a three-quarter circle, wherein the connecting conduits are downwardly branched from the supply conduit 5 and enter the converter 1. The ring conduits 8, from which the second gas is discharged, are provided directly after the integrated heat exchanger WT1, i.e. without interconnection of further 5 apparatuses. Via a conduit 9, additional e.g. cold or warm S0 2 -containing gas can be supplied and mixed with the SO 3 containing gas before entering the converter 1. 10 The gas mixture then flows through the first contact K1 and subsequently is passed through the integrated heat exchanger WT1, in order to cool the tempera ture of the process gas increased as a result of the exothermal reaction in the con tact stage K1 to a temperature of about 4000C suitable for entering a second con tact stage K2 and at the same time heat the S0 2 -containing first gas supplied 15 through the central supply conduit 2. Upon traversing the second contact stage K2, the process gas in turn is passed through an integrated heat exchanger WT2, in order to be cooled to an inlet temperature of about 4000C suitable for the third contact stage K3, and upon passing through this third contact stage K3 is with drawn from the converter 1 via an outlet 10 and supplied to an intermediate ab 20 sorption plant. It should be noted that the above described structure including the deflection of the first gas flow prior to entering the first heat exchanger WT1 is preferred for a con verter comprising plural heat exchangers. Alternatively, it is possible to supply the 25 gas from above or below where the mixing is performed at the exit of the heat ex changer. In Figures 2 to 4, the apparatus for mixing the first and second gases is shown in detail. In Figure 2, the first gas enters the first heat exchanger WT1 from below 30 and then is discharged into the mixing chamber 4. Via the connecting conduit 6 WO 2008/151758 PCT/EP2008/004510 -10 and the supply port 7, the second gas is fed into the ring conduits 8, from which it is discharged into the mixing chamber 4 in downward direction via a plurality of openings 11 (cf. Fig. 4). The openings 11 here are arranged in several rows of holes possibly arranged offset with respect to each other, so that the second gas 5 is fed into the first gas as a plurality of small flows and can easily mix with the first gas due to the resulting turbulences promoted by the countercurrent feeding. In another embodiment not shown here, corresponding openings can also be pro vided on the upper surface of the ring conduits 8 in addition or alternatively to the openings 11 provided on the lower surface of the ring conduits 8, in order to cocur 10 rently feed the second gas into the first gas. As can be taken from Fig. 3, three concentric ring conduits 8 are arranged around the central supply conduit 2, which are supplied with the first gas via four connect ing conduits 6 and supply ports 7 uniformly distributed around the periphery of the 15 converter 1. By means of the invention, a homogeneous mixing of two gas flows can be achieved, so that the composition and temperature of the process gas at the inlet of the first contact stage K1 can easily be adjusted. 20 It is within the meaning of the present invention that the mixing of the two gas flows is performed before the gas enters the converter. In this case the structure of the mixing chamber 4 and the ring conduits 8 is provided prior to the mixed gas entering the converter 1. 25 WO 2008/151758 PCT/EP2008/004510 - 11 List of Reference Numerals 1 converter 5 2 central supply conduit 3 baffle plate 4 mixing chamber 5 supply conduit 6 connecting conduits 10 7 supply port 8 ring conduits 9 bypass conduit 10 outlet 11 openings 15 K1-K5 contact stages WT1-WT3 integrated heat exchangers

Claims

1. A process for mixing two gases of different temperature and/or composition in a converter for producing SO 3 from an S0 2 -containing gas, wherein a first, S02 5 containing gas flow is introduced into the converter through a central supply pipe and passed through an integrated heat exchanger, wherein a second gas flow is supplied via a ring conduit arranged in the converter directly before or after the integrated heat exchanger, from which the second gas flow is discharged through a plurality of openings and is fed into the first gas flow, so that it mixes with the 10 same, and wherein the gas mixture obtained then is supplied to a contact stage of the converter, in which the S02 is at least partly converted to S03 on a catalyst.

2. The process according to claim 1, characterized in that the second gas flow is countercurrently fed into the first gas flow. 15

3. The process according to claim 1 or 2, characterized in that the second gas flow is cocurrently fed into the first gas flow.

4. The process according to any of the preceding claims, characterized in 20 that the first gas flow contains more than 13 vol-% SO 2 and the second gas flow contains 5 to 25 vol-% SO 3 .

5. The process according to any of claims 1 to 3, characterized in that the second gas flow contains up to 13 vol-% SO 2 and has a temperature of 80 to 25 3000C, preferably 100 to 1200C.

6. The process according to any of the preceding claims, characterized in that the second gas flow is smaller than the first gas flow. WO 2008/151758 PCT/EP2008/004510 -13

7. An apparatus for mixing two gases in a converter (1) for producing SO 3 from an S0 2 -containing gas, in particular for performing a process according to any of the preceding claims, comprising a supply conduit (2) for a first gas, which is introduced into the converter (1) and is passed through an integrated heat ex 5 changer (WT1), a ring conduit (8) for a second gas, which is arranged directly be fore or after the heat exchanger (WT1) in a mixing chamber (4), wherein the ring conduit (8) includes a plurality of openings (11) for the discharge of second gas into the mixing chamber (4). 10

8. The apparatus according to claim 7, characterized in that the first gas is introduced into the converter (1) and is passed through the integrated heat ex changer (WT1), which preferably is arranged around the central supply conduit (2).

9. The apparatus according to claim 7 or 8, characterized in that the ring 15 conduit (8) is arranged around the central supply conduit (2) for the first gas.

10. The apparatus according to any of claims 7 to 9, characterized in that the mixing chamber (4) is arranged above or below the heat exchanger (WT1). 20

11. The apparatus according to any of claims 7 to 10, characterized in that the openings (11) in the ring conduit (8) are facing the heat exchanger (WT1).

12. The apparatus according to any of claims 7 to 11, characterized in that the openings (11) in the ring conduit (8) are facing away from the heat exchanger 25 (WT1).

13. The apparatus according to any of claims 7 to 12, characterized in that on the lower surface and/or upper surface of the ring conduit (8) one or several rows of holes are provided, through which the second gas is discharged. 30 WO 2008/151758 PCT/EP2008/004510 - 14

14. The apparatus according to any of claims 7 to 13, characterized in that several, in particular three concentric ring conduits (8) are provided, from which the second gas is discharged. 5

15. The apparatus according to any of claims 7 to 14, characterized in that the ring conduits (8) are connected with a common supply conduit (5) for the second gas.

16. The apparatus according to claim 15, characterized in that the common 10 supply conduit (5) for the second gas is at least partly guided around the central supply conduit (2) for the first gas and that several downwardly extending connect ing conduits (6) connect the supply conduit (5) with the ring conduits (8).

17. The apparatus according to claim 16, characterized in that the connecting 15 conduits (6) are each connected with all ring conduits (8) via supply ports (7).