WO2008151758A2

WO2008151758A2 - Process and apparatus for mixing gases

Info

Publication number: WO2008151758A2
Application number: PCT/EP2008/004510
Authority: WO
Inventors: Karl-Heinz Daum; Wolfram Schalk; David Cachero Ventosa
Original assignee: Outotec Oyj
Current assignee: Metso Corp
Priority date: 2007-06-13
Filing date: 2008-06-06
Publication date: 2008-12-18
Anticipated expiration: 2009-12-13
Also published as: WO2008151758A3; MX2009013478A; MX340510B; CN101679037A; DE102007027881A1; CL2008001718A1; AU2008261290A1; PE20090270A1; DE102007027881B4; AU2008261290B2; CN101679037B

Abstract

When mixing two gases of different temperature and/or composition in a converter for producing SO3 from an SO2-containing gas, a first SO2-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, which is traversed by the first gas flow from the bottom to the top. A second gas flow is supplied via a ring conduit arranged above 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 same. Then, the gas mixture obtained is supplied to a contact stage of the converter, in which the SO2 is at least partly converted to SO3 on a catalyst.

Description

Process and Apparatus for Mixing Gases

The present invention relates to a process for mixing two gases of different temperature and/or composition in a converter for producing SO₃ from an SO₂- containing gas, and to an apparatus for performing this process.

The present invention is concerned with the production of sulfuric acid. Conven- tionally, sulfuric acid mostly is produced by the so-called double absorption process, which is described in Ullmann's Encyclopedia of Industrial Chemistry, 5^th edition, 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

SO₂ + 1/2 O₂ → SO₃ + 98 KJ

to obtain sulfur trioxide. The produced gas containing sulfur trioxide then is supplied to an absorber and converted there to sulfuric acid. The oxidation of the sul- 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 below 380⁰C. As the process is strongly exothermal, the gas inlet temperature into the contact stage must be about 400⁰C. At a distinctly lower inlet temperature, the reaction is not initiated, whereas at a much higher iniet temperature the temperature 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 A1. To obtain a high yield, the reaction is performed in several stages, between which the process gas each 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 supplied to an intermediate absorption tower, in which the SO₃ is converted to sulfuric acid, oleum or liquid SO₃, and the SO₃ 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.

The process gas supplied to the converter suffers from frequent fluctuations of the quantity and SO₂ concentration. While in conventional converters the SO2 concentration 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 A1 provides for using higher SO₂ concentrations by recirculating SO₃-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 the inlet of the contact mass. This is effected by supplying cold SO₂-containing gas via a bypass conduit. In the above-mentioned recirculation of SO₃-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- cosities, which render mixing difficult. For the efficiency of the process it is, however, required to achieve a homogeneous gas mixture, if no sufficient homogeneity of the gas is achieved at the inlet of the contact mass, there are zones in which there is no conversion of SO₂ to SO₃ when passing through the contact stage, so that the efficiency of the converter is impaired. In zones with too high SO₂ content, overheating possibly can lead to a damage of the catalyst. It was found that mixing 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).

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.

Therefore, it is the object of the invention to increase the homogeneity of the mixture 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, SO₂- 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 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 SO₂ is at least partly converted to SO₃ 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 SO₂-containing gas of low tempera- ture, only partly passes through the apparatus provided upstream of the converter, a higher pressure with respect to the mixture 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 controlled. If the second gas is recirculated sulfur trioxide, its pressure can be in- creased by means of a blower or the like, so that here as well a higher pressure - A -

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 possible 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 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 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 Sθ₃-containing gas, as it is known from DE 102 49 782 A1 , the first gas flow includes more than 13 vol-% SO₂ and the second gas flow 3 to 40 vol-% SO₃, preferably 5 to 25 vol-% SO₃, in accordance with a development of the invention. The first gas stream also can contain SO₃, e. g. 1 to 10 vol-% SO₃, but preferably contains less than 1 % SO₃.

In a further embodiment of the invention the gas flow in the mixing chamber can 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 SO₃-containing gas, the temperature at the inlet of the first contact stage of the converter is regulated in accordance with the invention by means of a second gas flow, which includes up to 30 vol-% SO₂, preferably up to 12 vol-% SO₂- and has a temperature of O to 400 ⁰C, preferably 80 to 300⁰C, more preferably 100 to 12O⁰C. This gas flow can be passed around the usual heat exchanger 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- ing the pressure, e.g. blowers or throttle valves or flow resistors can be incorporated, whereby mixing can be controlled in addition.

In accordance with the invention, the volume flow rate of the second gas flow is smaller than that of the first gas flow and is 20 to 60%, preferably ≤ 50% and more preferably < 30 % of the first gas flow.

An inventive apparatus for mixing two gases in a converter for producing SO₃ from an SO₂-containing gas, which can be used in particular for performing the process 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 arranged 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- charge of second gas into the mixing chamber.

In a preferred embodiment of the invention, the first gas is introduced into the converter from above or below and upon deflection passed through the integrated heat exchanger, which preferably is arranged around the central supply conduit, and then enters the mixing chamber arranged above the heat exchanger. It₁ however, is possible that the gas is introduced from above or below into the heat exchanger 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 guidance of the flow, this results in a compact construction of the converter and the 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 gas flow should cocurrently be fed into the first gas flow, the holes in the ring con- duit are facing away from the heat exchanger, so that the second gas flow is discharged from the ring conduit in upward direction. It is of course possible that corresponding openings, which can be designed as bores, slots or the like, are provided both on the lower surface and on the upper surface or at the sides of the ring 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 60° with respect to the vertical. The openings can have a different size or shape, e.g. in dependence on the distance from the supply conduit. It is also possible to vary the 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 regulation or control means in particular in partial load operation.

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 discharged. The quantity of the second gas flow also can be increased thereby. The ring conduits may be located at different vertical levels.

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 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 and the uniform distribution in the converter, for which purpose it is possible to depart from the symmetry, if necessary.

In accordance with the invention, the connecting conduits are each connected with 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- 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- 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:

Fig. 1 shows a schematic sectional view of a converter for producing SO₃ from an SO₂-containing gas with an apparatus for mixing gases in accordance with an embodiment of the present invention with illustrated gas flows,

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- dance with the invention, and Fig. 4 shows a section through a ring conduit along line IV-IV of Fig. 3.

The converter 1 for converting SO₂ to SO₃ as shown in Fig. 1 includes a total of five contact stages K1 to K5, in which a catalyst, in particular a catalyst containing vanadium pentoxide is provided, in order to convert the SO₂ to SO₃. After passing through three contact stages K1 to K3, the SO₃-containing gas obtained is supplied 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 SO₂- 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 primarily 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 may also be realized in the second region of the converter (contact stages K4 and K5).

SO₂-containing gas, which flows through the supply conduit from the top to the bottom and is deflected at its lower end by 180° by means of a baffle plate 3, is 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.

A second gas, which for instance is recirculated SO₃-containing process gas or SO₂-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 conduits 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- duit 5 here is guided around the central supply conduit 2 e.g. outside the converter 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 apparatuses.

Via a conduit 9, additional e.g. cold or warm Sθ₂-containing gas can be supplied and mixed with the SO₃ containing gas before entering the converter 1.

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 temperature of the process gas increased as a result of the exothermal reaction in the contact stage K1 to a temperature of about 400⁰C suitable for entering a second contact stage K2 and at the same time heat the SO_∑-containing first gas supplied 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 withdrawn from the converter 1 via an outlet 10 and supplied to an intermediate ab- 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 converter comprising plural heat exchangers. Alternatively, it is possible to supply the gas from above or below where the mixing is performed at the exit of the heat exchanger.

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 and then is discharged into the mixing chamber 4. Via the connecting conduit 6 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 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 provided 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- 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 connecting conduits 6 and supply ports 7 uniformly distributed around the periphery of the 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.

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. List of Reference Numerals

1 converter

2 central supply conduit

3 baffle plate

4 mixing chamber

5 supply conduit

6 connecting conduits

7 supply port

8 ring conduits

9 bypass conduit

10 outlet

11 openings

K1-K5 contact stages

WT1-WT3 integrated heat exchangers

Claims

Claims:

1. A process for mixing two gases of different temperature and/or composition in a converter for producing SO₃ from an SO₂-containing gas, wherein a first, SO₂- 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 same, and wherein the gas mixture obtained then is supplied to a contact stage of the converter, in which the SO₂ is at least partly converted to SO₃ 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.

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 that the first gas flow contains more than 13 vol-% SO₂ and the second gas flow contains 5 to 25 vol-% SO₃.

5. The process according to any of claims 1 to 3, characterized in that the second gas flow contains up to 13 vol-% SO₂ and has a temperature of 80 to 300⁰C₁ preferably 10O 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.

7. An apparatus for mixing two gases in a converter (1 ) for producing SO₃ from an SO₂-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- changer (WT1 ), a ring conduit (8) for a second gas, which is arranged directly before 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).

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 exchanger (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 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).

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 (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.

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.

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 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 connecting conduits (6) connect the supply conduit (5) with the ring conduits (8).

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