WO2009065485A2

WO2009065485A2 - Process and plant for producing sulfuric acid

Info

Publication number: WO2009065485A2
Application number: PCT/EP2008/009193
Authority: WO
Inventors: Karl-Heinz Daum; Wolfram Schalk; Wolfgang Götz
Original assignee: Outotec Oyj
Current assignee: Metso Corp
Priority date: 2007-11-23
Filing date: 2008-10-31
Publication date: 2009-05-28
Anticipated expiration: 2010-05-23
Also published as: MX2010005600A; AU2008328289A1; AU2008328289B2; CN101848857A; PE20091297A1; WO2009065485A3; CL2008003420A1

Abstract

When producing sulfuric acid from a gas containing sulfur dioxide, the sulfur dioxide is catalytically oxidized in a converter to obtain sulfur trioxide, the sulfur trioxide produced thereby is absorbed in concentrated sulfuric acid in an intermediate absorber, and the residual gas possibly is again supplied to a catalytic conversion stage. The sulfur trioxide produced then can be absorbed in concentrated sulfuric acid in a final absorber. To simplify the plant configuration and control also for feed gases with high SO2 contents, it is provided in accordance with the invention that the inlet concentration of the acid to the intermediate absorber is about 97.3 to 98.4 % H2SO4.

Description

PROCESS AND PLANT FOR PRODUCING SULFURIC ACID

Field of the Invention

This invention relates to a process and a plant for producing sulfuric acid from a gas containing sulfur dioxide, wherein the sulfur dioxide is catalytically oxidized in a converter to obtain sulfur trioxide, wherein the sulfur trioxide produced thereby preferably is absorbed in concentrated sulfuric acid in an intermediate absorber and the residual gas preferably is again supplied to a catalytic conver- sion stage, and wherein the sulfur trioxide produced then is absorbed in concentrated sulfuric acid in a final absorber.

The production of sulfuric acid usually is effected by the so-called double absorption process as it is described in Winnacker/Kϋchler, Chemische Technik: Prozesse und Produkte, Vol. 3: Anorganische Grundstoffe, Zwischenprodukte, pp. 64 to 135. Sulfur dioxide (SO₂) obtained as waste gas of metallurgical plants or by combustion of sulfur is converted to sulfur trioxide (SO₃) in a multistage converter by means of a solid catalyst, e.g. with vanadium pentoxide as active component. The SO₃ obtained is withdrawn after the contact stages of the con- verter and supplied to an intermediate absorber or after the last contact stage of the converter to a final absorber, in which the gas containing SO₃ is guided in counterflow with concentrated sulfuric acid and is absorbed in the same.

When the sulfur dioxide is obtained from waste gases of metallurgical plants, e.g. from the pyrometallurgical production of non-ferrous metals, e.g. from the calcination of sulfidic ores, the thermal decomposition of metal sulfates or alkali sulfates or from the processing of contaminated waste sulfuric acid by thermal decomposition, the gases initially are cleaned from impurities which might impair the quality of the sulfuric acid or impede the catalytic conversion to sulfur triox- ide. The waste gas cleaned in this way then is dried in a drying tower with con- centrated sulfuric acid of e.g. 94-96 % H₂SO₄ (the sulfuric acid concentration each is indicated in percent by weight), i.e. quantitatively liberated from steam, with this sulfuric acid then being diluted correspondingly by absorbing water. The sulfur trioxide, which in the subsequent converter is produced from sulfur dioxide and oxygen by catalytic oxidation, is absorbed in absorbers into concentrated sulfuric acid of e.g. 98.5 % H₂SO₄, with its concentration increasing in the process. After the intermediate absorption of the sulfur trioxide, the residual gas is again supplied to a catalytic conversion stage, and in a final absorber the sulfur trioxide produced is liberated from the remaining SO₃. The final absorber is operated with concentrated sulfuric acid in the same way as the intermediate absorber, with the concentration of the sulfuric acid increasing here as well. The water required for forming sulfuric acid from SO₃ and H₂O and for dilution to about 98.5 % H₂SO₄ is in part obtained from the gas/air humidity absorbed in the drying tower. The rest is supplied to the intermediate absorber and/or the final absorber as process water. By means of a suitable control, the concentration of the sulfuric acid is kept constant.

A similar arrangement is used in plants for producing sulfuric acid on the basis of elementary sulfur. Here, air is dried in the drying tower and the oxygen con- tained therein is used for the combustion/oxidation of elementary sulfur to SO₂- containing gas. In the further course, this sulfur dioxide then is catalytically converted to sulfur trioxide as described above and subsequently absorbed in the intermediate and final absorbers and converted to sulfuric acid.

Fig. 1 shows the acid-side circuitry of a typical conventional arrangement of the drying and absorption towers. The gas-side circuitry is not shown. The enclosed Tables indicate the process parameters in conduits 1 to 14. In so far, Table 1 indicates the process parameters for a circuitry as shown in Fig. 1. In Fig. 1 , SO₂-containing gas is supplied to the drying tower TT via a non- illustrated conduit, which gas is guided in counterflow with the sulfuric acid supplied via conduit 1 , which has a concentration of 96 % H₂SO₄. The SO₂- containing gas is dried thereby, and the steam contained therein leads to a dilution of the sulfuric acid which is withdrawn from the bottom of the drying tower TT via conduit 2 and supplied to an acid circulation tank T1. From the acid circulation tank T1 , the sulfuric acid is supplied by means of the pump P1 via conduit 3 to an acid cooler C1 , in which the acid temperature is decreased from 81⁰C to 65°C. With this temperature, the sulfuric acid then is again supplied to the drying tower TT via conduit 1. The acid circuit of the drying tower TT is defined thereby. A partial stream of the sulfuric acid is supplied as so-called crossflow acid via conduit 4 to an acid circulation tank T2 of the intermediate absorber circuit. The quantity of the partial stream is controlled via a control valve V (LIC) on the basis of a level measurement in the acid circulation tank T1.

Sulfuric acid is supplied to the intermediate absorber ZA via conduit 5, which in the intermediate absorber ZA is guided in counterflow with SO₃-containing gas from the non-illustrated converter in which the sulfur dioxide was converted to sulfur trioxide. The sulfur trioxide is absorbed in the sulfuric acid and increases its concentration to about 99 %. Via conduit 6, the sulfuric acid is withdrawn from the bottom of the intermediate absorber and supplied to the acid circulation tank T2 of the intermediate absorber circuit. Via a pump P2, the sulfuric acid, which for level control was diluted with the partial stream from the drying tower circuit supplied via conduit 4, is delivered via conduit 7 through the acid cooler C2 and into conduit 5. A partial stream of the acid is branched off via conduit 8 into the pump receiver of the drying tower circuit, in order to adjust the concentration of the drying tower acid. The quantity of the partial stream is adjusted via a control valve V (QIC) on the basis of a concentration measurement in conduit 1 . Sulfuric acid with a concentration of 98.5 %, which is guided in counterflow with SO₃-containing gas and absorbs the SO₃, is supplied to the final absorber EA via conduit 9. Via conduit 14, process water is supplied to the bottom of the intermediate absorber ZA and of the final absorber EA₁ in order to again dilute the sulfuric acid to the desired value of 98.5 %. This is effected on the basis of concentration measurements in conduits 5 and 9. From the bottom of the final absorber EA, 98.5 % sulfuric acid is withdrawn via conduit 10 and supplied to an acid circulation tank T3 of the final absorber circuit. From the same, the sulfuric acid is guided by means of the pump P3 via conduit 11 through an acid cooler C3, before it is again supplied to the final absorber EA via conduit 9. Via conduit 13, the product sulfuric acid is withdrawn from the plant and supplied to a non- illustrated product cooler.

Between the acid circulation tanks T2 and T3 of the intermediate absorber circuit and of the final absorber circuit, acid can be shifted via conduit 12.

In the circuit shown in Fig. 1 , the performances and capacities of the recirculat- ing pumps P1 to P3 are adapted to the requirements of the associated tower circuit and in general are very different. The same is true for the acid coolers C1 to C3, which must be adapted to the different cooling requirements of the individual acid circuits. Both the throughput and the temperature levels are different, which leads to different dimensions.

A simplification of the acid-side circuitry can be effected by combining several tower circuits. Such a conventional arrangement of the drying and absorption towers and the acid-side circuitry thereof is shown in Fig. 2. The gas-side circuitry in turn is not shown. O ^

In this arrangement, all three circuits are combined, so that the drying tower TT, the intermediate absorber tower ZA and the final absorber tower EA are each operated with the same acid concentration of 98.5 % H₂SO₄. The control effort thereby is reduced considerably. Another advantage with respect to the circuitry as shown in Fig. 1 consists in that both the recirculation pumps P1 and P2 and the acid coolers C1 and C2 can be kept identical. Not only the maintenance and warehousing costs are reduced thereby, but also the installation costs.

The essential structural elements of the plant of Fig. 2 correspond to those of Fig. 1 , so that in so far the same reference numerals are used and reference is made to the above description. Therefore, merely the differences will be explained separately below. Table 2 indicates the process parameters for the circuitry as shown in Fig. 2.

In the plant as shown in Fig. 2, sulfuric acid with a concentration of about 98.5 % H₂SO₄ is supplied to the drying tower TT via conduit 1. Via conduit 2, the sulfuric acid is withdrawn from the bottom of the drying tower TT and supplied to the acid circulation tank T1 provided for all tower circuits in common. By means of the pumps P1 and P2, the sulfuric acid is supplied via conduits 3, 4.1 and 4.2 to the acid coolers C1 and C2, from which it is supplied via conduits 1 , 6 and 9 to the drying tower TT, the intermediate absorber ZA and the final absorber EA. The sulfuric acid withdrawn from the towers, which at the bottom of the intermediate absorber ZA and of the final absorber EA can be diluted to the desired concentration of 98.5 % with process water via conduit 14, is supplied to the common acid circulation tank T1 via conduits 2, 6 and 10. The product is discharged via conduit 13 and supplied to a non-illustrated product cooler.

Such circuitry is particularly useful for plants which can also be used for the combustion of elementary sulfur, since the drying tower here is operated with air. For waste gases from metallurgical processes, which contain sulfur dioxide, this circuitry cannot be used due to the high solubility of sulfur dioxide in the circulating sulfuric acid of the drying tower. The concentration of the dissolved SO₂ then is reduced by mixing with the acids from the intermediate and final absorbers, but is in part expelled (stripped) again in the further course at the final absorber, so that the SO₂ then leaves the plant in the chimney gas and thus leads to inadmissible emissions.

For SO2-containing waste gases, various alternative circuitries therefore were proposed in practice, which reduce the expulsion of dissolved SO₂ from the acid and the resulting increase in emissions. For instance, the acid circuits of drying tower and intermediate absorber were combined, whereas the final absorber still had a separate circuit similar to the circuitry shown in Fig. 1. In accordance with this solution, dissolved sulfur dioxide still is expelled, but this is only effected in the intermediate absorber, so that in the second catalytic stage it is converted to sulfur trioxide and then removed in the final absorber. Thus, stripped sulfur dioxide cannot get into the chimney. This circuitry has the advantage that the pumps and acid coolers for the common circuit of the drying tower and of the intermediate absorber can be identical. However, pump and cooler of the final absorber still are different. In addition, this circuitry impairs the water balance, as in particular when processing gases with low SO₂ contents the amount of water introduced into the drying tower is too high as compared with the absorbed amount of SO₃ in the intermediate absorber. This can lead to the fact that the desired concentration of the common circuit of 98.5 % H₂SO₄ can no longer be maintained. Therefore, this circuitry is hardly employed in practice.

Another alternative is the combination of the acid circuits of intermediate absorber and final absorber, as it is shown in Fig. 3. Here, the drying tower TT forms a separate circuit. As in the preceding plants, merely the acid-side circuitry is shown. The main components of the plant again are the same as in Figures 1 and 2, so that in so far the same reference numerals are used. Table 3 indicates the process parameters for the circuitry as shown in Fig. 3.

In the plant of Fig. 3, the circuit of the drying tower TT is the same as in the plant of Fig. 1 , so that in so far reference is made to the above description.

Upon dilution with process water, which is supplied via conduit 14, the sulfuric acid formed in the intermediate absorber ZA is supplied via conduit 6 to a common acid circulation tank T2 for the absorber towers ZA, EA. By means of the pumps P2 and P3, the sulfuric acid is supplied via conduits 7, 11 to the acid coolers C2 and C3 and then via conduits 5, 9 to the intermediate absorber ZA and to the final absorber EA, respectively. Via conduit 13, product sulfuric acid is withdrawn from the plant and supplied to a non-illustrated product cooler.

One advantage of this circuitry consists in that the pump and acid coolers for the common absorber circuit can be identical. However, the drying tower pump P1 and the associated cooler C1 still are different. Like in the circuitry shown in Fig. 1 , the crossflow acid from the drying tower TT is supplied to the acid circulation tank T2 of the absorbers ZA, EA as 94 to 96 % sulfuric acid. Even if this cross- flow quantity is relatively small, there still exists the above-described problem of the sulfur dioxide dissolved therein. This SO₂ is partly expelled in the final absorber EA and hence reduces the turnover and increases the emission. With increasing SO₂ content of the feed gas, the solubility thereof increases and intensifies the described effect. Due to the stricter requirements for a minimization of emissions, this actually advantageous circuitry therefore can no longer be employed. Summary of the Invention

Therefore, it is the object of the invention to simplify the plant circuitry and control when producing sulfuric acid, in particular when using feed gases with a high SO2 concentration.

This object substantially is solved with the invention in that the inlet concentration of the acid in an absorber is about 97.3 to 98.4 % H₂SO₄. In a commonly used double absorption plant, as it is predominantly configured worldwide, this first absorber is the intermediate absorber of such a plant. It is, however, possible here that the intermediate absorber also is divided into several individual absorbers, as is the case e.g. in DE 59 701 328. In the case of such a division, intermediate absorber each is understood to be the entire aggregate, constructed of several individual partial absorbers.

The fundamental difference of the present invention with respect to the circuitry shown in Fig. 3 consists in that the feed acid to the intermediate absorber no longer has the conventional acid concentration of typically 98.5 % H₂SO₄, but that the intermediate absorber is operated with a lower concentration of in par- ticular 97.5 to 98.2 % H₂SO₄.

In the conventional SO₃ absorption, a 98.5 % acid typically is charged at the top of the intermediate absorber. The increase in concentration due to the absorption of SO₃ inside the filler packing in the absorber is limited to an acid outlet concentration of typically 99.3 % H₂SO₄. Both of the aforementioned concentrations are due to the physical properties of the sulfuric acid, namely on the one hand due to the minimum of the acid vapor pressure (azeotrope) at 98.5 % H₂SO₄ and the necessity for a quantative absorption of the sulfur trioxide. On the other hand, the acid outlet concentration should not lie much above 99.3 % H₂SO₄, since the SO₃ partial pressure is increasing here considerably and would impede the absorption. Under such circumstances, acid mists would also be formed with the hot Sθ₃-containing gas entering the tower, which cannot be separated in the subsequent filler packing and would require additional extensive measures, in order to protect the downstream apparatus against corrosion.

The concentration difference of typically 99.3 - 98.5 = 0.8 % H₂SO₄ together with the amount of sulfur trioxide to be absorbed determines the required amount of circulating acid at the intermediate absorber. With increasing SO₂ content of the feed gas and hence also with a higher SO₃ content at the inter- mediate absorber, the amount of circulating acid must virtually be increased linearly. This leads to the fact that the flooding point would be exceeded when using conventional ceramic fillers. In such cases, one is therefore forced to considerably increase the towers for hydraulic reasons, even if the mass transfer does not require the same.

With the present invention, said concentration difference of the circulating acid now is increased from typically 0.8 % H₂SO₄ to preferably 99.3 - 98.0 = 1.3 % H₂SO₄. The amount of circulating acid thus can be reduced considerably by maintaining the required outlet concentration. Due to the lower circulating amount, the intermediate absorber can be configured with a substantially reduced diameter, since the distance to the flooding point is increased. At the same time, the lower circulating amount leads to the fact that the pump capacity can be reduced considerably. In turn, this leads to the fact that the capacity of all circulating pumps can be adapted to that of the drying tower.

For adjusting the lower inlet concentration at the top of the intermediate absorber, a partial stream of the sulfuric acid withdrawn from the drying tower is branched off and admixed only to the inflow of the intermediate absorber as crossflow acid, in accordance with a preferred aspect of the invention, inde- pendent of whether starting gas containing sulfur dioxide or air with concen- trated sulfuric acid is dried in the drying tower. In accordance with the invention, the crossflow acid from the drying tower has a concentration of 93 to 97 % H₂SO₄, preferably 95.5 to 96.5 % H₂SO₄ and in particular about 96 % H₂SO₄. In accordance with a development of the invention, the concentration of the cross- flow acid is adjusted in that process water is admixed to the bottom of the drying tower or to the sulfuric acid withdrawn from the drying tower.

Due to the relatively low concentration of the crossflow acid, a correspondingly lower mixing concentration is obtained depending on the moisture content of the gas at the inlet of the drying tower and hence on the amount of crossflow acid. To keep this mixing concentration supplied to the top of the intermediate absorber substantially constant, process water is admixed to the inflow of the intermediate absorber in accordance with a preferred aspect of the invention. In accordance with the invention, the amount of said process water is adjusted by a concentration control.

At the same time, the strongly SO₂-containing crossflow acid from the drying tower thereby is guided to the top of the intermediate absorber together with the circulating acid. This drying tower acid thus is prevented from mixing with the circulating acid of the final absorber, and an additional SO₂ emission, as it would occur with a circuitry as shown in Fig. 3, thus is avoided. The SO₂ dissolved in the crossflow acid of the drying tower is expelled at the top of the intermediate absorber and supplied with the gas to the second catalytic conversion stage and converted to SO₃.

In accordance with a development of the invention, the acid circuits of the intermediate absorber and of the final absorber are combined. In accordance with the invention, the acid outflow of the intermediate absorber and of the final absorber correspondingly is supplied to a common acid circulation tank. For controlling the acid concentration in the inflow of the final absorber, process water preferably is also admixed to the bottom of the intermediate absorber.

Alternatively, however, it is always possible to use dilute acid, e.g. from plants corresponding to DE 10 2007 047 319, Fl 2007 0054 or EP 177 839, instead of the process water. This dilute acid can originate both from an alternative process step to the final absorber, from a further gas cleaning stage downstream of the final absorber, or from a plant completely independent of the described sulfuric acid plant.

The process of the invention is the more advantageous for the operation of the plant the higher the concentration of the SO₂ content in the feed gas. In accordance with the invention, the feed gas in the converter for converting the sulfur dioxide includes 6.5 to 30 vol-% SO₂, preferably > 12 vol-% SO₂. Thus, the process is particularly suitable for application in connection with a process for the catalytic conversion of high-percentage SO₂-containing gases, as it is described in DE 102 49 782 A1.

In general, the amount of heat to be dissipated at the acid coolers remains constant. With a reduced amount of circulating acid, this means that the temperature difference must increase. With a constant temperature of the acid charged to the towers, the inlet temperature of the acid into the acid coolers therefore is increased and is above 90⁰C in accordance with the invention. The acid coolers can easily process acids even in this temperature range, but just as the associated acid conduits they can be reduced in size due to the reduced amount of circulating acid.

This invention also relates to a plant for producing sulfuric acid from a gas containing sulfur dioxide, which can be used for the process described above and includes a drying tower for drying the SO₂-containing gas or air, a converter for the catalytic conversion of the sulfur dioxide to sulfur trioxide, an absorber and preferably a further stage for cleaning the gas from SO₂, preferably a further absorber (final absorber) for the absorption of the sulfur trioxide in concentrated sulfuric acid, wherein the acid is circulated in the drying tower and in the ab- sorber(s). In accordance with the invention, the absorbers (if several absorbers are present) include a common acid circuit, whereas the drying tower has a separate acid circuit, from which a crossflow conduit is branched off, which is connected with the acid supply conduit of the one absorber.

For adjusting the concentration of the sulfuric acid supplied to the intermediate absorber, a process water supply conduit is connected with the acid supply conduit of the intermediate absorber in accordance with a development of the invention. In accordance with the invention, mixing is effected in a mixing tank, preferably with a static mixer, e.g. in a mixing line of the pipe conduit.

Since static mixers are particularly reliable when mixing acids, whereas problems can occur when mixing acid and water, it is provided in accordance with a particularly preferred aspect that the process water supply conduit is connected with the bottom of the drying tower. Adjusting the acid concentration at the inlet of the intermediate absorber thus is effected indirectly by adjusting the concentration of the crossflow acid and by the amount thereof, which is admixed to the circulating acid of the intermediate absorber. Alternatively, the amount of water can also be supplied in the form of dilute acid, whereby a dilute acid supply conduit completes or replaces the process water supply conduit.

In accordance with the invention, the control of the process water feed stream is effected by means of a control means on the basis of the acid concentration in the acid supply conduit of the intermediate absorber. The flow control of the crossflow acid is effected on the basis of the level in the drying tower. The low acid inlet concentration in the intermediate absorber no longer represents the azeotropic concentration and consequently no longer has the lowest total vapor pressure. While both the H₂SO₄ vapor pressure and the SO₃ partial pressure are decreasing with a concentration lower than the azeotropic concen- tration, the H₂O partial pressure is increasing. This does not lead to a reduction of the SO₃ absorption, but can possibly lead to a slightly increased formation of mist. In accordance with a development of the invention, mist filters therefore are provided downstream of the intermediate absorber, which prevent the apparatus from being affected by acid condensation and hence corrosion.

The acid circuits of the drying tower, the intermediate absorber and the final absorber are operated by means of acid pumps, which due to the circuitry of the invention can each have the same capacity (delivery rate), although the acid flow rates to the towers are very different. At the same time, the sum of all deliv- ery rates to the towers is smaller than in the prior art.

Because of the usual configuration of sulfuric acid plants as double absorption plants, reference was made in this description especially to such double absorption plant. In the individual case, however, it is also known that a further catalytic conversion and a final absorption of the residual gas after the first absorber is omitted and instead the waste gas is directly discharged into the atmosphere or supplied to another method of SO₂ conversion, cf. for instance EP 0 177 839 or DE 37 44 031 , or an absorption of alkaline substances, e.g. solid CaCO₃ or ammonia solution. Furthermore, it is sometimes necessary to provide a further gas cleaning stage after the final absorption, which can produce e.g. dilute acid or with NaOH solution sodium sulfite or sulfate. The invention is also applicable in these cases, and possibly produced dilute acid can be recirculated into the process. Developments, advantages and possible applications of the invention can be taken from the following description of embodiments and the drawing. All features described and/or illustrated form the subject-matter of the invention per se or in any combination, independent of their inclusion in the claims or their back- reference.

Brief Description of the Drawings

Fig. 1 schematically shows a conventional plant for producing sulfuric acid with an illustration of the acid circuit (double absorption plant).

Fig. 2 schematically shows another conventional plant for producing sulfuric acid with an illustration of the acid circuit.

Fig. 3 schematically shows a further conventional plant for producing sulfuric acid with an illustration of the acid circuit.

Fig. 4 schematically shows a preferred embodiment of a plant of the invention for producing sulfuric acid with an illustration of the acid circuit in a double absorption plant, and

Fig. 5 schematically shows a plant for producing sulfuric acid in accordance with another preferred embodiment of the invention with an illustration of the acid circuit.

Detailed Description of the Preferred Embodiments

Fig. 4 shows a first embodiment of the present invention. In this embodiment, the same main components as used in the description of the prior art shown in Figures 1 to 3 are designated with the same reference numerals. In so far, ref- erence is also made to the above description. As mentioned already, the invention does, however, not only relate to the preferred embodiment in a double absorption plant, which is only cited as an example generally known to one of skill in the art.

In the first embodiment of the present invention, sulfuric acid with a concentration of 96 wt-% is supplied to the drying tower TT via conduit 1. The sulfuric acid is guided in counterflow with non-illustrated Sθ₂-containing gas or air, in order to dry the same by absorbing water. Via conduit 2, the sulfuric acid diluted in this way is supplied to the acid circulation tank T1 of the drying tower circuit. By means of the pump P1 , the sulfuric acid is guided via conduit 3 through the acid cooler C1 and again supplied to the top of the drying tower TT via conduit 1. Part of the sulfuric acid is branched off from conduit 3 and via conduit 4 supplied as crossflow acid to a mixing tank M, in which it is mixed with sulfuric acid of the absorber circuit, which is supplied via conduit 8, and with process water supplied via conduit 14.1 or alternatively with dilute acid. The flow rates of the crossflow acid supplied from the drying tower TT via conduit 4 to the circulating acid of the absorber circuit, and of the stream of process water or of dilute acid are controlled on the basis of a concentration measurement in the acid supply conduit 5 of the intermediate absorber ZA such that in the inflow of the intermediate absorber ZA an acid concentration of 98±0.2 % H₂SO₄ is obtained.

In principle, it is also possible to premix the process water with the acid to obtain a weaker acid and mix this dilute acid into the absorber circuit stream, so as to avoid the problems of water mixing with acid in this stream.

In the intermediate absorber ZA, the sulfuric acid is guided in counterflow with gas containing sulfur trioxide, which was produced by converting the SO₂- containing gas from the drying tower TT in a non-illustrated converter. The crossflow acid from the drying tower TT can have a relatively high content of sulfur dioxide, which then gasses out in the intermediate absorber ZA and is supplied from the same to a further catalytic conversion stage and converted into sulfur trioxide, before it is supplied to the final absorber EA.

Via conduit 14.2, process water can be introduced into the bottom of the intermediate absorber ZA, in order to adjust the acid, which is withdrawn from the intermediate absorber ZA via conduit 6, to a desired value of e.g. 98.4 % H₂SO₄. Via conduit 6, the acid is supplied to an acid circulation tank T2 common to the two absorber towers ZA and EA and supplied from the same by means of two pumps P2 and P3 of the same capacity via the conduits 7 and 11 to the acid coolers C2 and C3, which in turn have the same capacity. Of course, it is also possible to use only one pump P2 and one acid cooler C2 with a correspondingly higher capacity. Via conduit 9, the acid cooled in this way is supplied to the top of the final absorber EA and via conduit 8 to the pump receiver of the drying tower TT. Via conduit 5, another partial stream is supplied to the mixing tank M and then to the top of the intermediate absorber ZA. Via the product conduit 13, the acid not required for circulation is drawn in as product from the plant and supplied to a non-illustrated product cooler.

Subsequent to the intermediate absorber and the final absorber, non-illustrated mist filters are provided, which prevent the apparatus from being affected by acid condensation.

Fig. 5 shows a variant of the plant circuitry in accordance with the invention, which largely corresponds to the plant as shown in Fig. 4. However, in the embodiment of Fig. 5, the process water is not directly introduced via conduit 14.1 into the mixing tank M, but into the bottom of the drying tower TT, in order to adjust the concentration of the acid withdrawn from the drying tower TT and hence of the crossflow acid, which is mixed with the acid of the absorber circuit via the mixing chamber M. This circuitry has the advantage that a static mixer can be used in the mixing chamber M₁ or the mixing chamber can be configured as a mixing line in the pipe conduit, where, however, only acids are suitably mixed with each other and admixing water can lead to problems.

In the following Tables, the mass flow rates, concentrations and temperatures in the conduits shown in Figures 1 to 5 are indicated.

Table 1 refers to the plant circuitry as shown in Fig. 1 , where in the converter for producing SO₃, which is not shown in the drawing, a metallurgical gas with a content of 10 vol-% SO₂ was supplied, which was previously dried in the drying tower

Table 2 refers to the plant circuitry as shown in Fig. 2, wherein sulfur dioxide was produced by combustion of elementary sulfur and a feed gas with a concen- tration of 11.8 vol-% SO₂ was supplied to the converter not illustrated in the drawing. In the drying tower TT, air is dried with sulfuric acid.

Table 3 refers to a plant circuitry as shown in Fig. 3, wherein a metallurgical gas with an SO₂ concentration of 12 vol-%, which was previously dried in the drying tower TT, was supplied to the converter not shown in the drawing.

Tables 4.1 to 4.3 refer to the plant circuitry as shown in Fig. 4, where in Table 4.1 a metallurgical gas with an SO₂ concentration of 8 vol-%, which was previously dried in the drying tower TT, was supplied to the converter not shown in the drawing. In Table 4.2, a metallurgical gas with an SO₂ concentration of 12 vol-% was supplied to the converter, whereas in Table 4.3 a metallurgical gas with an SO₂ concentration of 18 vol-% was supplied to the converter. Table 5 refers to a plant circuitry as shown in Fig. 5, wherein a metallurgical gas with an SO₂ concentration of 18 vol-%, which was previously dried in the drying tower TT, was supplied to the converter not shown in the drawing.

List of Reference Numerals:

1 acid supply conduit of the drying tower

2 discharge conduit of the drying tower 3 conduit

4 crossflow conduit

5 acid supply conduit of the intermediate absorber

6 discharge conduit of the intermediate absorber

7 conduit 8 conduit

9 acid supply conduit of the final absorber

10 discharge conduit of the final absorber

11 conduit

12 conduit 13 product conduit

14 process water supply conduit

C1 to C3 acid mixers

EA final absorber M mixing tank

P1 to P3 acid pumps

T1 to T3 acid circulation tanks

TT drying tower

V valve ZA intermediate absorber

LIC level control

QIC concentration control

Claims

Claims:

1. A process for producing sulfuric acid from a gas containing sulfur dioxide, wherein the sulfur dioxide is catalytically oxidized in a converter to obtain sulfur trioxide, wherein the sulfur trioxide produced is absorbed in concentrated sulfuric acid in an absorber (ZA), characterized in that the inlet concentration of the acid to the absorber (ZA) is about 97.3 to 98.4 % H₂SO₄.

2. The process according to claim 1 , characterized in that the inlet concen- tration of the acid to the absorber (ZA) is about 97.5 to 98.2 % H₂SO₄.

3. The process according to claim 1 or 2, characterized in that a starting gas containing sulfur dioxide or air is dried with concentrated sulfuric acid in a drying tower (TT), and that a partial stream of the sulfuric acid withdrawn from the drying tower (TT) is branched off and admixed to the inflow of the absorber (ZA) as crossflow acid.

4. The process according to claim 3, characterized in that the crossflow acid from the drying tower (TT) has a concentration of 93 to 97 % H₂SO₄, pref- erably 94 to 96.5 % H₂SO₄, and in particular about 96 % H₂SO₄.

5. The process according to any of the preceding claims, characterized in that process water or dilute acid is admixed to the bottom of the drying tower (TT) or to the sulfuric acid withdrawn from the drying tower (TT).

6. The process according to any of the preceding claims, characterized in that process water or dilute acid is admixed to the circulation of the absorber (ZA).

7. The process according to any of the preceding claims, characterized in that the residual gas from the absorber (ZA) is again supplied to a catalytic conversion stage and that the sulfur trioxide produced thereby then is absorbed in concentrated sulfuric acid in a further absorber (final absorber EA).

8. The process according to any of the preceding claims, characterized in that the acid outflow of the absorber (ZA) and of the final absorber (EA) is supplied to a common acid circulation tank (T2).

9. The process according to any of the preceding claims, characterized in that process water is admixed to the bottom of the intermediate absorber (ZA).

10. The process according to any of the preceding claims, characterized in that the feed gas into the converter includes 6.5 to 30 vol-% SO₂, preferably >12 vol-% SO₂.

11. A plant for producing sulfuric acid from a gas containing sulfur dioxide, in particular for performing a process according to any of the preceding claims, comprising a drying tower (TT) for drying the SO₂-containing gas or air, a con- verier for the catalytic conversion of sulfur dioxide to sulfur trioxide, an absorber (ZA) for the absorption of sulfur trioxide in concentrated sulfuric acid, wherein the acid is circulated in the drying tower (TT) and in the absorber (ZA), characterized in that the drying tower (TT) includes an acid circuit separate from the absorber (ZA) and that a crossflow conduit (4) is branched off from the acid circuit of the drying tower (TT), which is connected with the acid supply conduit (5) of the absorber (ZA).

12. The plant according to claim 11 , characterized by a process water supply conduit (14i) which is connected with the acid supply conduit (5) of the ab- sorber (ZA).

13. The plant according to claim 11 or 12, characterized by a process water supply conduit (14i) which is connected with the bottom of the drying tower (TT).

14. The plant according to claim 12 or 13, characterized by a control means for controlling the process water feed stream into the bottom of the drying tower (TT) and/or the acid supply conduit (5) of the absorber (ZA) on the basis of the acid concentration in the acid supply conduit (5) of the absorber (ZA).

15. The plant according to any of claims 11 to 14, characterized by a control means for controlling the acid stream in the crossflow conduit (4) on the basis of the level in the acid circulation tank (T1 ) of the drying tower (TT).

16. The plant according to any of claims 11 to 15, characterized in that mist filters are provided downstream of the absorber (ZA).

17. The plant according to any of claims 11 to 16, characterized in that a further absorber (final absorber EA) for the absorption of sulfur trioxide in concentrated sulfuric acid or another gas cleaning plant is provided downstream of the absorber (ZA).

18. The plant according to any of claims 11 to 17, characterized in that the intermediate absorber (ZA) and the final absorber (EA) have a common acid circuit.

19. The plant according to any of claims 11 to 18, characterized in that the acid circuits of the drying tower (TT), of the intermediate absorber (ZA), and of the final absorber (EA) are operated by means of acid pumps (P1 , P2, P3), and that all these acid pumps (P1 , P2, P3) have the same capacity.