The invention deals with a process and its relating plant for producing sulfuric acid by catalytic oxidation of SO ₂ to SO3 and subsequent absorption of the SO3 in concentrated sulfuric acid in an intermediate and a final absorber stage, wherein the intermediate absorber stage features a pre-absorber and a post-absorber, which are arranged such that the not-absorbed SO ₃ leaving the pre-absorber is supplied to a post-absorber and such that the sulfuric acid leaving the post-ab sorber is split into two streams of which the first stream is recirculated back in the pre-absorber and the second stream is at least partly passed into the post-ab sorber.

Typically, sulfur dioxide (SO ₂ ) is formed in a combustion of elemental sulfur. Al ternatively, sulfur dioxide can be obtained from off gases out of metallurgical pro cesses, e.g. from the pyrometallurgical production of non-ferrous metals, from the roasting of sulfide ores, from the thermal decomposition of metal sulfates or alkali sulfates or from the processing of contaminated waste sulfuric acid by thermal decomposition. First, impurities are removed from this off gas, which impair or could impair the quality of the sulfuric acid or the catalytic conversion to sulfur trioxide. In a downward second cleaning step, the purified exhaust gas is dried in a drying tower with concentrated sulfuric acid of e.g. 94 - 96 wt.-% H ₂ SO ₄ , i.e. quantitatively freed from water moisture, whereby this sulfuric acid is diluted ac cordingly by the water absorption.

Independent from the source of the sulfur dioxide, sulfuric acid is usually produced using the so-called double absorption process. Thereby, the SO ₂ is converted into sulfur trioxide (SO ₃ ) in a multistage converter with the aid of a solid catalyst, e.g. with vanadium pentoxide as an active component. The obtained SO ₃ is drawn off after the first contact stages of the converter and fed to an intermediate absorber. SO3 being drawn off after the last contact stage of the converter is fed to a final absorber. In each absorber, the SO3 containing gas is traditionally fed counter currently to concentrated sulfuric acid and absorbed in at least one of these two absorbers.

In both absorbers, the absorption takes place in concentrated sulfuric acid with a feed concentration of e.g. 98.5 wt.-% H ₂ SO ₄ , whereby its concentration increases. The water required to form sulfuric acid from SC and H ₂ O and to dilute it to ap prox. 98.5 wt.-% H ₂ SO ₄ comes partly from the gas/air humidity absorbed in the drying tower. The remaining water is supplied as process water to the intermedi ate absorber and/or the final absorber. The concentration of sulfuric acid is kept constant by suitable control.

From WO 2009/065485 it is known to use acid withdrawn from the drying tower in the intermediate absorber. Figure 1 illustrates this well-known process.

Air or S0 ₂ -containing gas containing water moisture is fed via line 11 into a drying tower 10. Dried gas is withdrawn by line 17, while acid is withdrawn via line 12 and pump 13. Parts of the acid are recirculated via line 14 and its related heat- exchanger 15. The recirculated acid is used as drying medium in said drying tower 10.

The other part of the acid is added via line 16 into line 37, combining in line 38. Line 38 is the supply line for the absorbent. This absorbent is used in the inter- mediate absorber 20 to absorb sulfur trioxide, whereby sulfur trioxide is fed into the intermediate absorber 20 via line 31. To adjust the appropriate concentration, process water is also fed into the intermediate absorber 20 via line 39. The use of sulfuric acid coming from the drying tower via line 16 as an absorbent in the intermediate absorber is a so-called crossflow. Concentration of this acid is typically 94 - 96 wt.-%. After mixing of this acid with the recirculation flow line 37, this results in a concentration at line 38 between 97.8 and 98.2% H2SO4.

The SO3 enriched acid is discharged via line 32 and transferred to the heat ex- changer 35 via pump 33. From there, the acid is fed in a first partial flow into line 34. The flow in line 34 is divided again and one of the resulting partial flows is fed into line 37. This flows into the already discussed line 16. The total flow formed from these two partial flows forms the absorbent, which is introduced into the in termediate absorber 20 via line 38.

The second partial flow from line 32 is returned to drying tower 10 via line 41. The third part is discharged via line 42 and a further heat exchanger 43.

The second partial flow of line 35 is introduced via line 36 as an absorbent into the final absorber 50, which is fed with SO3 containing gas via line 52. Sulfuric acid from this absorber 50 is recirculated back into the intermediate absorber 20 via line 53.

Due to the high solubility of sulfur dioxide in the circulating sulfuric acid of the drying tower, this circuit is particularly suitable for off gases from pyrometallurgical processes containing sulfur dioxide. The concentration of the SO2 dissolved in the above-mentioned crossflow line 16 is lowered in the acid circulation of the inter mediate absorber 20. The partially stripping within this intermediate absorber 20 is favoured by the increased temperature of the acid. The resulting stripped gas- eous SO2 is leaving the intermediate absorber 20 via line 18 and fed with the residual gas to the not-shown subsequent catalytic stage and converted into SO3, where it is finally recovered as sulfuric acid in the final absorber 50. As a conse quence, the crossflow acid from the drying tower 10 does not enter the acid cir culation of the final absorber 50 and thus cannot be stripped there from the dissolved SO2 either, which would ultimately lead to the SO2 leaving the plant in the chimney gas via line 54 and thus to inadmissible emissions.

A disadvantage of this arrangement, however, is that the acid concentration on top of the intermediate absorber is below the optimum of 98.5 wt.-% H2SO4. This optimal value results from the fact that the minimum acid vapour pressure (azeo trope) is at ~98.5 wt.-% H2SO4, which, therefore, enables a quantitative absorp tion of the sulfur trioxide at said concentration. The increase in concentration due to the absorption of SO3 within the packing in the absorber is limited to a typical acid outlet concentration of 99.3 to 99.4 wt.-% H2SO4. The acid outlet concentra tion must not significantly exceed 99.3-99.4 wt.-% H2SO4, since the SO3 partial pressure increases noticeably and would impede complete absorption.

However, the discussed lower acid concentration during the entry into the inter mediate absorber caused by the addition of the crossflow acid from the drying tower according to the above-mentioned process known from WO 2009/065485 is no longer correlated to the explained azeotropic concentration. Consequently, the process is also no longer has the lowest total vapour pressure and at acid concentrations below 98.5 wt.-% H ₂ SO ₄ , both the H ₂ SO ₄ vapour pressure and the SO ₃ partial pressure decrease, while the H ₂ O partial pressure increases. As a result, the lower acid concentration does not lead to a reduction in SO ₃ absorption, but leads to increased mist formation, which in turn requires the installation of highly efficient mist filters (candle filters) in the head of the intermediate absorber.

Moreover, excessive acid mist would also form within the hot SO ₃ -containing gas entering the tower. This formed mist has the disadvantage that it could not be separated in the subsequent packing and would require additional extensive measures to remove it in order to protect the downstream apparatus from corro sion. The concentration difference of typical (outlet - inlet) 99.3 - 98.5 = 0.8 wt.-% H ₂ SO ₄ together with the amount of sulfur trioxide to be absorbed determines the required acid circulation quantity at the intermediate absorber. With increasing SO ₂ content of the input gas and thus a higher SO ₃ content of the gas entering the intermediate absorber, the acid circulation quantity must be increased practi cally proportionally. As a result, the flooding limit of conventional ceramic packing would be exceeded. In such cases, it is therefore necessary to increase the diam eter of the tower considerably for hydraulic reasons, even if the mass transfer does not require this.

A splitting of the intermediate absorption stage in at least two devices is also known from US 7,837,970 B2. This ensures the possibility of cheap device, like a venturi as a first pre-absorber and smaller packed bed absorber as a post ab sorber. As a result, investment costs can slightly be reduced. However, it does not have any impact on mist formation.

Therefore, it is the subject of the current invention, to provide a process and a correlating plant with improved stripping of dissolved SC and, therefore prevent ing stripped SO2 vanishing the plant towards the stack, and also a reduction of acid mist formation without a hydraulic over-dimensioning of the intermediate ab sorber.

This aim is achieved with a process according with the features of claim 1.

Such a process for producing sulfuric acid by catalytic oxidation of SO2 to SO3 and subsequent absorption of the SO3 in concentrated sulfuric acid requires in termediate and a final absorber, wherein the absorption takes place. In both ab sorbers, the absorption takes place in concentrated sulfuric acid with a feed con centration of e.g. 98.5 wt.-% H2SO4, whereby its concentration increases. The intermediate absorber features a pre-absorber and a post-absorber. Both ab sorbers, which build together the intermediate absorber, are arranged such that the not-absorbed SO3 leaving the pre-absorber is supplied to the post-absorber. The sulfuric acid withdrawn from the post-absorber is split into two streams of which the first stream is recirculated back in the pre-absorber and the second stream is at least partly passed into the post-absorber.

It is the basic idea underlying the invention that the first stream is uncooled directly injected into the pre-absorber with a temperature between 80 and 200 °C. Prefer- ably, the temperature of the sulfuric acid being injected into the pre-absorber is between 100 and 150 °C. Uncooled in the sense of the invention means that it does not pass any heat exchanger, so heat losses via line losses are the only heat losses. It is possible to admix the first stream with sulfuric acid from another source, like the drying tower.

The second stream passes a cooling and is at least partly fed into the post-ab sorber with a temperature between 60 and 90 °C. Further streams for the post absorber or a complete withdrawing can be branched off. This design has the technical effect that mist formation in the pre-absorber can be reduced significantly or completely suppressed. This due to the low tempera ture difference between the entering gas and acid fed to the pre-absorber.

Moreover, the heat exchanger is smaller which leads to lower investment costs.

In a preferred embodiment, the pre-absorber is designed as an empty tube or venturi, which leads to very low investment costs due to the simple design. Also, controlling and maintenance can be done economically and easily. Additionally or alternatively, the post-absorber is designed as a packed-bed ab sorber. This ensures a maximum efficiency of the absorption. Due to the additional pre-absorber, the packed bed can be smaller without a risk of hydraulic flooding, which further lowers investment costs.

Moreover, the first stream features a flow rate between 20 and 80 %, preferably between 25 and 40 % of the sulfuric acid withdrawn from the post-absorber. This leads to the most efficient heat distribution. Particular the preferred range leads to significantly reduced dimensions of and, therefore, costs for the post-absorber

While the pre-absorber is operated with a feed acid concentration below 98.5% H2SO4 (because of the addition of “weaker” acid from drying tower crossflow), the post-absorber is operated with conventional 98.5% acid at the inlet and thus op erates under ideal absorption conditions.

It is also preferred that the first acid stream entering the pre-absorber features a temperature difference to the entering SO3 containing gas of less than 50 °C, preferably less than 30 °C. Thus, the temperature of both streams are nearly equal which increases absorption rates. In detail, the elevated temperature of this acid thus favors the stripping of the SO2. As a result, the inventive process significantly reduces mist formation, which means less efficient mist filters are required and thus lower gas pressure loss therein due to the lower temperature difference be tween the incoming gas and the feed liquid / acid at the pre-absorber.

As a further advantage of the invention, the SO2 containing feed gas to the plant has a concentration of more than 9.0 vol.-% prior to the conversion, preferably more than 11.5 vol.-%. These are relatively high concentrations, which are difficult to handle in formally known processes due to the flooding limit of conventional ceramic packing. It has to be understood that the higher the concentration of the SO2 content in the input gas, the more advantageous the process according to the invention. Accord ing to the invention, the input gas into the converter for the conversion of the sulfur dioxide has 6.5 to 30 vol.% SO2, preferably > 11 vol.% SO2. The process is there fore particularly suitable for use in conjunction with a process for the catalytic conversion of gases with a high SO2 content, as described in DE 10249782 A1.

Furthermore, air or SO2 containing gas is dried in at least one drying tower by means of sulfuric acid for removing water moisture.

It is preferred to admix the sulfuric acid supplied into the pre-absorber with sulfuric acid from another source. In this context, it is particularly preferred that the sulfuric acid from the other source is sulfuric acid from the drying tower. Consequently, a closed loop of sulfuric acid as absorbent is possible. Preferably, the temperature of the acid from the other source, particularly the drying tower is between 80 and 180 °C, preferably above 90 °C.

It is also preferred that the second stream is mixed with sulfuric acid from another source upstream to the cooling. This reduces the number of acid circuits.

It has further been recognized that the second stream can be further split in at least two streams downstream of the cooling. So, this third stream can be fed in the final absorber after the cooling. As a result, an additional cooling device for the final absorber is omitted.

In addition, it is necessary to add process water to the sulfuric acid for adjusting the acid concentration. In the invention, it is preferred to feed this into the lower vestibule of the pre-absorber and/or the lower vestibule of the post-absorber and/or the lower vestibule of the final absorber. So, no separate devices are nec essary. However, it is also preferred that process water is added to the sulfuric acid sec ond stream as discharged from the acid cooler, at a separate device. As a result, concentration of sulfuric acid is higher when passing the cooler, which avoids or substantially reduces corrosion. It is also possible that process water is added to the sulfuric acid first stream entering the pre-absorber, at a separate device.

The invention further belongs to a plant according to claim 12, particular a plant for performing a process according to claims 1 to 11.

Such a plant for the production of sulfuric acid for producing sulfuric acid by cat alytic oxidation of SO2 to SO3 and subsequent absorption of the SO3 in concen trated sulfuric acid in an intermediate and a final absorber. The intermediate ab sorber features a pre-absorber and a post-absorber which are arranged such that the not-absorbed SO3 leaving the pre-absorber is supplied to a post-absorber. A discharge conduit for sulfuric acid from the post-absorber is split into a recircula tion conduit feeding sulfuric acid back into the pre-absorber and a supply conduit feeding sulfuric acid into the post-absorber.

It is essential for the invention that a cooler is only foreseen in the supply conduit to the post-absorber. The plant does not feature a cooling of the acid directed to the pre-absorber, particular no heat exchanger is placed in the acid feeding line of the pre-absorber.

This design has the benefit that only part of the conventionally required amount of cooled circulating acid is transferred to the said post-absorber, hydraulic flood ing is prevented. Simultaneously, size and cost of this post absorption tower are reduced. This is due to the fact that the total amount of acid is determined by increasing the concentration of circulating acid by typically 99.3 to 98.5 wt,-% H2SO4 based on the total SO3 supplied to the intermediate absorber, while the amount of acid supplied to the packed bed post-absorber is approximately 20-90 wt.-%, preferably 50-70 wt.-% of the conventionally calculated total amount of acid.

The inventive design splits the intermediate absorption tower into two separate vessels. Preferably, the pre-absorber is an empty tube or venturi type and/or the post-absorber is designed as a packed bed counter-current tower. While the post absorber is fed with cooled acid, the pre-absorber is fed with non-cooled acid. So, only one cooler is foreseen, whereby the term "cooler" also include the possibility of at least two cooling devices.

In a preferred embodiment, an air-cooled heat exchanger is used as the cooling device for the post-absorber, which is of particular importance. The increasing scarcity of suitable cooling water is widespread and increasingly noticeable, es pecially in areas with limited / non-existent availability of cooling water, as well as e.g. with regulated limited withdrawal possibilities from rivers or oceans. For this reason, fin-fan air coolers have already been used in the past to dissipate excess energy from acid cooling. The available materials for this air coolers were e.g. 316L/304 with acid temperatures up to 80 °C or anodic protected air coolers from 304 up to 110 °C, whereby the latter have very limited proven record in practice.

The use of air-cooled heat exchangers for cooling the circulating acid is advanta geous for the invention-based process with acid temperatures of > 80 °C, better >100 °C, much better >120 °C, and the use of suitable material, such as Si-alloyed stainless-steel material e.g. SX™, both in terms of investment and operating costs, as well as space required. The higher temperature level and the lower vol ume of circulating acid result in a considerably smaller cooler (dimensions, weights, cost) and lower power consumption for the cooling air fans. In addition, in the case of air coolers, the risk potential due to leakage of acid in cooling water (and vice versa) is eliminated, so that, for example, the possibility of the formation of hydrogen as a reaction product of corrosion (and the associated risk of explo sion) can be largely prevented.

Moreover, it is preferred that the pre-absorber is made of carbon steel with internal lining and the post-absorber is made of stainless steel or carbon steel with an internal brick lining. Conventional absorbers are usually made of carbon steel with an internal acid-resistant brick lining. Although the materials used are relatively inexpensive, the installation is costly due to the manual work involved in the lining. Alternatively, absorption towers are built from stainless steel, resulting in reduced costs mostly caused by faster installation.

However, these stainless steels are susceptible to corrosion if the operating con ditions deviate from the specifications. Preferably such towers are made of Si- alloyed stainless-steel material, e.g. SX™, or austenitic alloys, e.g. 31 OS, whereas Si-alloyed stainless steel offers lower resistant to oleum (with free SO3 = > 100 wt.-% H2SO4). Due to the high SO3 concentration of the inlet gas, oleum can form locally and lead to significant corrosion. Therefore, the acceptance of this design in industry is somewhat sceptical. The inventive process eases this situation in the post-absorber, wherein the incoming SO3 is already partly ab sorbed in the pre-absorber and consequently the gas at the inlet to the post-ab sorber has a much lower SO3 concentration compared to the conventional process and hence a lower risk of oleum formation. According to the inventive process, the pre-absorber is made of carbon steel with internal lining, and the post-ab sorber offers the opportunity of comfortable design as "brick-lined as well as stain less", which can, therefore, be used for cost optimization. This leads to reduced investment costs.

In addition, a drying tower for drying a gas containing air or SO2 containing gas with sulfuric acid is foreseen. The drying tower features preferably a pre-drying tower and a post-drying tower, which are arranged such that moisture containing air or SO2 containing gas is supplied to a pre-drying tower, wherein a discharge conduit for acid from the post-drying tower is split into a recirculation conduit feed ing sulfuric acid back into the pre-drying tower and a supply conduit feeding sul furic acid into the post-drying tower. This enables an improved drying similar as the splitting of the intermediate absorber into two devices.

Further developments, advantages and possible applications can also be taken from the following description of exemplary embodiments and the drawings. All features described and/or illustrated from the subject matter of the invention per se or in any combination, independent of their inclusion in the claims or their back reference.

In the drawings:

Fig. 1 shows schematically a process known from the state of the art

Fig. 2 shows schematically a first process according to the invention whereby the post-absorber and the final absorber feature a common acid supply conduit,

Fig. 3 shows schematically a second process according to the invention whereby the post-absorber and the final absorber feature separate acid circuit,

Fig. 4 shows schematically a third process according to the invention whereby drying tower is separated in a pre-dryer and a post-dryer,

Fig. 5 shows schematically a fourth process according to the invention whereby the dryer, the post-absorber and the final absorber feature a common acid supply conduit, Fig. 6 shows schematically a fifth process according to the invention whereby only one source for process water is used in a common acid circuit and

Fig. 7 shows schematically a sixth process according to the invention whereby only one source for process water is used in the pre-ab sorber.

Figure 1 has already been discussed in the introduction discussing the state of the art in detail.

Figure 2 shows an installation according to the invention method for processing. A sulfur dioxide containing gas, e.g. a pyrometallurgical off gas containing SO2, is introduced via conduit 19 into pre-absorber 30, which represents together with post-absorber 40 the intermediate absorber. Preferably, the pre-absorber is de signed as an empty tower or as a venturi. Sulfuric acid is added via conduit 46 is absorption agent.

Liquid and gaseous parts are withdrawn via conduit 31 and 47 and introduced in the post-absorber 40, which is preferably designed as a packed-bed absorber. Therein, remaining SO3 is absorbed, preferably counter-current, with liquid acid fed in via conduit 37 and withdrawn via conduit 32 and pump 33. Parts of the withdrawn acid stream are recycled via conduit 44 to mix it with fresh sulfuric acid from conduit 45 and recirculated via conduit 46 into the pre-absorber 30.

Water is added into the sump of the post-absorber 40 via conduit 39. Alternatively, the process water can be added to the sump of the pre-absorber 30 or to a not- shown pump tank. The other part of the sulfuric acid stream is passed from conduit 32 in conduit 34 and cooler 35. Parts of the cooled acid are fed via conduit 36 and 37 back into the post-absorber 40. So, the absorption agent of the post-absorber 40 is cooled.

Talking about the sulfuric acid stream of conduit 36, it is further possible to send a part of this stream via conduit 38 into a final absorber 50, which is also prefera bly designed as a packed bed. Therein, sulphur trioxide containing gas is fed in via conduit 52. Gas is blown out via conduit 54 while acid from the sump of the final absorber 50 is recirculated back via line 53 into the sump of post-absorber 40.

Another part of the stream from conduit 34 is withdrawn via conduit 42 and op tionally an additional cooler 43. The last part is recycled via conduit 41 into a drying tower 10.

Sulfuric acid is fed via line 14 into this drying tower 10. Dried acid is withdrawn via line 12 and pump 13. Parts of the acid are recirculated via line 14 and its related heat-exchanger 15. The recirculated acid is used as drying medium in said drying tower 10. Parts of the sulfuric acid are withdrawn from conduit 12 and fed into the pre-absorber 30 via conduits 45 and 46.

This embodiment of the invention shows a common acid circulation with the post absorber 40 and the final absorber 50. It features the common acid cooler 35. The distribution of the circulating acid required for the intermediate absorption typically amounts to typically 65% cooled for the post-absorber 40 via lines 36 and 37 and the residual 35% uncooled via line 44 to the pre-absorber 30 but can vary de pending on the SO2 content of the gases to be processed.

Figure 3 shows another embodiment which differs from figure 2 in the acid circuits. The cooled sulfuric acid from conduit 34 and cooler 35 is split into a stream recirculated via conduit 41 in the drying tower 10 and a cooled stream fed into the post-absorber 30 via conduit 37. The remaining part is passed into the sump of the final absorber 50 via conduit 42.

SO3 containing gas is injected into this final absorber via conduit 52. An independ ent acid circuit feeds acid into the final absorber 50 via conduit 58. The acid is withdrawn via conduit 53, pump 51 and a separate cooler 55. Parts of the acid are withdrawn via conduit 56 and cooler 57 while the other part is recirculated via conduit 58. Water is added separately via conduit 59.

Figure 4 shows an embodiment with an arrangement of the absorbers as it is known from figure 2. The difference is the arrangement of the drying towers, where the drying tower 10 is split into a pre-drying tower 1 and a post-drying tower 6. Gas is inserted via conduit 11 into the pre-drying tower 1 and dried with acid from conduit 9. Acid from the common acid circuit of post-absorber and final ab sorber is recycled in the sump of pre-drying tower 1 via conduit 41. Naturally, all other shown possibilities of separated acid circuits and water adding are also pos sible.

Acid and gas are withdrawn from the sump of pre-drying tower 1 via conduits 2 and 3 and fed into post-drying tower 6, preferably counter-current. Acid from its sump is withdrawn via conduit 4 and pump 5. A first stream from conduit 4 is recirculated via conduit 9 into the pre-drying tower 1.

Another stream from conduit 4 is recirculated in the post-drying tower 6 via line 14 and cooler 15 while the last part is transported to the pre-absorber via conduits 45 and 46. A third stream is passed via line 9 back into the pre-drying tower 1.

Figure 5 shows another alternative of the current invention, using only one drying tower 10. In contrast to figure 2, figure 5 shows a common acid circuit of the absorbers 30, 40 and 50 as well as the drying tower 10. This leads to a further reduction of the coolers since only the two coolers 35 and 43 remain in the pro cess. In detail, the drying tower 10 does no longer feature a separate acid circuit, but its absorbent if fed in via conduit 41 , which is branched off conduit 34 after passing cooler 35. So, it is derived from acid from the sump of post-absorber 40.

All acid from the sump of the drying tower 10 is withdrawn via conduit 12 and 49 and mixed with acid from the sump of the pre-absorber 30.

Figure 6 shows another embodiment of the invention directed to an additional admixing of process water. Therein, the common acid circuit of post-absorber 40 and final absorber 50 features an additional water diluter 60. Therein, water is admixed via conduit 61 to the stream in conduit 34, which is used as absorbent in both absorber 40 and 50. Alternatively, said diluter 60 can be located in-line 34 upstream the common acid cooler 35.

Figure 7 depicts another possibility for adding process water to regulate and con- trol the acid concentration. A diluter 62 is positioned in the supplying conduit 46 for the acid used as absorbent in the pre-absorber 30.

Reference numbers

1 pre-drying tower 2-4 conduit

5 pump

6 post-drying tower 7-9 conduit

10 drying tower 11, 12 conduit

13 pump

14 conduit

15 cooler 16-19 conduit 20 intermediate absorber

30 pre-absorber

31, 32 conduit

33 pump

34 cooler 35 - 39 conduit

40 post-absorber

41,42 conduit

43 cooler

44 - 49 conduit 50 final absorber

51 - 59 conduit

60 diluter conduit diluter conduit