WO2025140972A1 - Process and plant for producing a sulfur trioxide-containing gas - Google Patents

Abstract

The invention is directed to a process for producing a sulfur trioxide-containing gas stream. A sulfur dioxide and oxygen containing gas are introduced in at least one converter stage to produce a sulfur trioxide-containing gas stream in a heterogeneous catalysation. In doing so a partial stream of a sulfur trioxide containing gas stream produced in the first or a subsequent converter stage is branched off as a recycling stream and recycled to be part of a feed gas of the first converter stage. It is essential that he sulfur dioxide containing gas and added oxygen are introduced as a motive medium in an ejector via an motive medium inlet such that it sucks in and accelerates the recycling stream through a sucking medium inlet as a sucking medium to produce a feed gas streaming through a discharge outlet for the converter stage at a set pressure range.

Description

Process and plant for producing a sulfur trioxide-containing gas

The present invention relates to a process and a corresponding plant for producing a sulfur trioxide-containing gas stream, wherein a sulfur dioxide and oxygen containing gas are introduced in at least one converter stage to produce a sulfur trioxide-containing gas stream in a heterogeneous catalysation, wherein a partial stream of a sulfur trioxide containing gas stream produced in the first or a subsequent converter stage is branched off as a recycling stream and recycled to be part of a feed gas of the first converter stage.

Sulfuric acid is a mineral acid composed of the elements sulfur, oxygen, and hydrogen, with the molecular formula H2SO4. It is a colorless, odorless, and viscous liquid that is miscible with water. It is one of the most important chemical and, therefore, often used as an indicator of a country's industrial strength. As a key substance in the chemical industry, it is most commonly used in fertilizer manufacture, but is important in mineral processing, oil refining, wastewater processing, and chemical synthesis, too. Moreover, it has a wide range of end applications, including in domestic acidic drain cleaners, as an electrolyte in lead-acid batteries, in dehydrating a compound, and in various cleaning agents.

One of the primary methods for manufacturing sulfuric acid is the contact process, which involves a double absorption process as elaborated in Ullmann's Encyclopaedia of Industrial Chemistry, 5th edition, Vol. A 25, pp. 635-700. Sulfur dioxide (SO2) is obtained through sulfur combustion or as waste gas from metallurgical plants. It is then converted to sulfur trioxide (SO3) in a typically four or five-stage converter utilizing vanadium pentoxide as a solid catalyst. Sulfur trioxide is produced and retrieved after the contact stages of the converter and is conveyed to an intermediate absorber. Conversely, it can also be supplied to a final absorber after the final converter stage. In the ultimate absorber, the gas containing sulfur

O 1 P 378 WO trioxide is fed in a counter-current manner to concentrated sulfuric acid and simultaneously absorbed.

However, the conversion from sulfur dioxide to sulfur trioxide is one of the main steps in this process, whereby the reaction in the converter stages proceeds according to the following equation:

2 SO2(g) + 02(g) 2 SOs(g) : AH = -197 kJ-

As it is obvious from the given reaction parameters, the reaction is highly exothermic which involves a high risk for so called hotspots in the catalyst bed. Therein, temperature increases locally, which in turn leads to even higher reaction rates and thus even stronger local temperature increases. If the temperature exceeds certain critical values locally, the catalyst is irreversibly damaged. This problem occurs especially in the first converter stage, due to the excessively high levels of sulfur dioxide during gas intake, and the absence of sulfur trioxide in the gas stream, which would shift the reaction rate in the equilibrium.

The most important measure is therefore to divide the converter into individual stages, each of which is filled with a catalyst. This allows the gas flow exiting one stage to be cooled before it enters the next stage. In addition, measures such as different catalyst concentrations, catalyst types, bed sizes, etc. can influence the reaction in the individual stages in such a way that hot spots are avoided. However, also in these converters feed gases can only be applied with a maximum sulfur dioxide content between 11 and 13 vol.-% to avoid hot spots reliably.

For a converter being able to handle a feed gas with a higher sulfur dioxide content the so called LUREC® (described in WO 2004/037719 A1 ) has been developed. this process uses the understanding of the reaction as an equilibrium reaction by recycling at least parts of an intermediate or final product stream

O 1 P 378 WO containing sulfur trioxide back into the first contact stage. Thereby, the contained sulfur trioxide influence the equilibrium between sulfur dioxide and sulfur trioxide in the reaction equation such that the reaction rate for producing sulfur trioxide is lowered. Consequently, the lowered turnover rate also produces less energy, which prevents the formation of hot spots.

However, the recycled gas stream features a low gas pressure of typically between 90 and 130 kPa due to the pressure drop resulting from overcoming the flow resistance of the contact stage(s) of the converter. Therefore, it is necessary to increase the pressure of the partial gas stream before recycling it back to the feed gas of the initial contact stage to match the inlet conditions. As a result, the feed gas obtained must have sufficient pressure to overcome at least the flow resistance of the first contact stage.

In conventional plants being able to handle the increase sulfur dioxide contents, this is achieved by means of a hot gas blower. However, hot sulfur trioxide containing gases are highly corrosive and thus, cause damage to sensitive parts of the blower such as the fan or the seals. Such damages will result in dangerous leaks of corrosive gases over the time and thus, renders the hot gas blower a costly high maintenance device.

Therefore, the recycled partial gas stream is typically cooled from a temperature of between 420 and 630 °C to a temperature of between 200 and 250 °C prior to passing the hot gas blower. However, at least if certain recycling rates are to be achieved, this temperature would lead to the gas mixture entering the first stage of the converter being so cold that no reaction would occur. Therefore, the pressurized partial gas stream is often reheated to temperatures of typically between 380 and 420 °C before being recycled into the feed gas to ensure the necessary temperature of the feed gas for catalytic converter ignition, which is approximately 400 °C. While this partially mitigates the corrosion issue, the cooling and

O 1 P 378 WO subsequent reheating stages requires additional heat exchangers. Moreover, the heat exchangers cause a further pressure drop which means that the hot gas blower has to be even more powerful. By avoiding cooling and heating of recycling stream the heat stays at a higher temperature level and can thus be utilized for e.g. higher pressure steam production or reducing overall heat exchanger surfaces.

Moreover, for feeding a homogeneous stream into the converter, a mixing of such a recycling stream into a sulfur dioxide and oxygen containing gas an additional gas mixer is necessary. All these devices lead to additional capex and opex.

Therefore, it is the object of the present invention to provide a process and a corresponding plant for producing a sulfur trioxide-containing gas stream which produces a homogenous sulfur dioxide-containing feed gas for a first converter stage more efficiently.

This object is solved by a process with the features of claim 1. In this process, a sulfur dioxide and oxygen containing gas are introduced in at least one converter stage to produce a sulfur trioxide-containing gas stream in a heterogeneous cata- lysation. A partial stream of a sulfur trioxide containing gas stream produced in the first or a subsequent converter stage is branched off as a recycling stream and recycled to be part of a feed gas of the first converter stage.

According to the invention, the sulfur dioxide and oxygen containing gas are thereby introduced as a motive medium in an ejector such that it sucks in, accelerates and compresses the recycling stream working as a suction medium to produce a feed gas for the converter stage at a set pressure range.

The principle underlying said ejector is, that a high-pressure gas stream, i.e. , the motive medium, enters the ejector via a nozzle. This causes the (static) pressure

O 1 P 378 WO energy of the high-pressure gas stream corresponding to the sulfur dioxide and oxygen containing gas to be converted into a higher dynamic pressure such that the velocity of the sulfur dioxide and oxygen containing gas increases.

As a result, around the nozzle tip the velocity is highest, which is why a low-pressure region is generated as this high gas velocity entrains gases from the adjacent sub-area. The resulting low pressure sucks in the recycling stream via a sucking medium inlet of the ejector, also called suction branch. Thereby, the recycling stream is accelerated and compressed. The two gas streams then travel through the so-called diffuser section of the ejector, in which the streams are mixed to form a homogeneous feed gas for the first converter stage. As a result of the diverging geometry of the diffuser section, the gas velocity decreases and a set pressure is gained, which is between the initial pressure of the sulfur dioxide and oxygen containing gas and the initial pressure of the recycling stream.

In contrast to the conventionally used hot gas blowers, the simple yet robust design of the ejector does not feature sensitive moving parts, such as fans, or corrosion-sensitive seals. Thus, the risk of leakage caused by sulfur trioxide is reduced and no cooling and re-heating is necessary.

Furthermore, despite the differences between the sulfur dioxide and oxygen containing gas and the recycling stream in terms of pressure, temperature and chemical composition, a homogenous feed gas for the first converter stage is produced without an additional mixing device.

It is preferred that the feed gas leaving the ejector has a pressure range between 110 and 160 kPa. This pressure range corresponds to a pressure sufficient to overcome the flow resistance of the at least one converter stage. In dependency of the number of converter stages, the pressure is set either towards the lower or

O 1 P 378 WO towards the higher limit of the aforementioned range. Said pressure range can be adjusted by the inlet pressure of the motive medium.

In another or a supplemental embodiment the differential pressure between the sucking medium inlet and the discharge outlet is between 5 and 30 kPa. This pressure range is particularly preferred for a constant flow of the recycling stream and a complete mixture in the ejector.

In a further embodiment, the concentration of sulfur dioxide in the sulfur dioxidecontaining feed gas is at least 14 vol.-%, preferably between 18 and 66 vol.- whereby the overall pant capacity can be increased.

It is further preferred if the recycling stream is neither actively heated nor cooled. As described above, the ejector is less sensitive than the conventional hot gas blower due to a lack of parts being particular sensitive towards a hot sulfur trioxide containing gas stream. Therefore, the gas stream needs not to be cooled and subsequently reheated, which improves the energy balance of the process.

Considering that no active lossy heat transfer to another medium takes place, this embodiment also corresponds to the best possible utilization of the reaction heat in the recycling stream in terms of efficiency. In particular, a feed gas with a temperature above the minimum working temperature of the at least one converter stage may be obtained despite a comparatively low temperature of the sulfur dioxide and oxygen containing gas. In turn, this allows a better utilization of the heat content of a gas stream elsewhere in the process. For example, the reaction heat from the initial generation of the sulfur dioxide for the process may be used to a greater extent in, e.g., a firetube boiler for producing superheated steam.

O 1 P 378 WO Alternatively, at least one heat exchanger may be foreseen to actively heat or cool the recycling stream. This allows a precise control of the recycling stream's temperature.

In a further embodiment, the pressure of the sulfur dioxide and oxygen containing gas is used as a control variable for the pressure of the feed gas. As described above, the pressure of the feed gas leaving the ejector depends on the pressures of the motive medium as well as the suction medium. Considering that means for adjusting the pressure of the sulfur dioxide and oxygen containing gas, such as blowers, are inevitably present to convey this gas stream, the pressure of the sulfur dioxide and oxygen containing gas can thus be adjusted. However, the pressure of this gas stream working as the motive medium directly influence the sucking of the recycle stream as well as the outlet pressure of the mixed feed gas. Therefore, the pressure of the sulfur dioxide and oxygen containing gas can be used as a control variable for pressure of the feed gas. In this context, a pressure between 120 and 200 kPa when entering the ejector as motive medium is particularly preferred for said gas stream.

For covering a broader range of operating conditions an array of ejectors of different sizes can be used. These ejectors are arranged in series or parallel or in combination thereof.

According to another embodiment, a further sulfur trioxide containing gas stream from a subsequent converter stage is also recycled into the ejector or separately mixed in to form the feed gas for the first converter stage. Thereby, a better control of the amount of sulfur trioxide recycled to be part of the feed gas for the first converter stage is achieved.

Preferably, the partial stream of the sulfur trioxide containing gas stream amounts to a volume fraction of between 15 and 80 % of the sulfur trioxide containing gas

O 1 P 378 WO stream produced in the first or the subsequent converter stage. It is particularly preferred, that recycling of the partial stream(s) is conducted such that the resulting feed gas has a sulfur trioxide content of between 2 and 50 vol.-%.

According to another embodiment, a further gas stream is also conveyed to the ejector to form the feed gas for the first converter stage. Thereby, utilizing the excellent mixing characteristic of the ejector to better control the amount of sulfur trioxide or the temperature of the feed gas for the first converter stage.

According to one of the most preferred embodiments, the first converter stage takes place in a pre-converter featuring at least one pre-converter stage. Parts of the sulfur trioxide containing stream from this first stage is recycled into the feed for this first converter stage to limit the reaction by shifting the equilibrium. Naturally, the admixing of the sulfur trioxide containing stream with the feed gas for the first pre-converter stage takes place in an ejector according to the invention.

The remaining part of the sulfur trioxide containing gas stream, i.e. the part of the sulfur trioxide containing gas stream which is not recycled, is passed to further converter stages in the pre-converter and/or the main converter.

Further, it is possible to foresee subsequent pre-converter absorber between preconverter and main converter for an absorption with sulfuric acid to form sulfuric acid. In the subsequent pre-converter absorber, the remaining part of the sulfur trioxide containing gas stream is absorbed with sulfuric acid as absorption medium such that at least part of the sulfuric trioxide is processed to sulfuric acid at an early process stage, i.e., before entering a conventional sulfuric acid plant.

A conventional sulfuric acid plant comprises at least two contact stages of a main converter which are arranged in series to react sulfur dioxide with oxygen to produce sulfur trioxide. The generated sulfur trioxide-containing gas is fed to at least

O 1 P 378 WO one further absorber, wherein the produced sulfur trioxide is absorbed with sulfuric acid as absorption medium. Capacity c of such a sulfuric acid plant is limited by the overall gas volume \/ which can be handled in this plant as well as the amount of sulfur dioxide a which can be reacted in main converter. By using the pre-converter with a recycling stream of sulfur trioxide, it is possible to increase the amount of sulfur dioxide which can be handled. Moreover, this arrangement allows a reaction control with increased oxygen amount, preferably an oxygen concentration in the added gas stream between 75 and 100 vol.-%. In contrast to the classic reaction control with air, the proportion of inter gases is dramatically reduced, which means that the volume flow can be significantly reduced. So, the range of the limiting factors of the gas volume \/ and the amount of sulfur dioxide a is broadened extremely.

Such an arrangement is particularly useful if the capacity c of a conventional plant has to be enhanced either because demand has increased or more sulfur dioxide needs to be processed. An increase of the demand can be particular taken into account if the source of the sulfur dioxide is a burning of elemental sulfur with oxygen. This can be easily scaled up with an additional parallel burner, whereby the wording "burner" covers all kinds of reactions between sulfur and oxygen independent whether a flame occur or not. On the other hand, the amount of sulfur dioxide generated in metallurgical processes can be increased due to a decrease in the quality of ores. This allows operating a conventional sulfuric acid plant featuring a capacity c to convert sulfur dioxide in combination with the above-described sources generating a sulfur dioxide-containing gas in an amount a, that exceeds the plant's capacity c.

The invention further relates to a plant with the features of claim 9, in particular a plant for carrying out the claimed process. Such a plant comprises at least one converter stage for reacting a sulfur dioxide and oxygen containing gas to produce a sulfur trioxide-containing gas stream. Furthermore, it features a recycling

O 1 P 378 WO conduit for branching off a partial stream of the sulfur trioxide-containing gas stream and recycling it to a first converter stage. For admixing this recycling stream with the sulfur dioxide and oxygen containing gas, an ejector is foreseen. This ejector features an inlet for a motive medium and an inlet for a suction medium, whereby the inlets are arranged such that the sulfur dioxide and oxygen containing gas is introduced as the motive medium and the recycling stream is introduced as the suction medium.

As a source for sulfur dioxide, the plant may comprise at least one elemental sulfur burning unit for generating a sulfur dioxide-containing gas as an educt to produce sulfur trioxide. In this context, the term "burning unit" covers all catalytic and non-catalytic processes reacting sulfur and oxygen to sulphur dioxide, irrespective of whether flame formation occurs or not. In particular, sulfur guns, lances and any other kind of furnaces wherein droplets of molten sulfur are converted are disclosed in this wording.

It is especially preferred that the burning unit is designed such that it works with relatively pure oxygen, e.g. an amount of inert gases below 20 vol.-%, particularly with an oxygen content between 85 and 99 vol.-%. This allows a reduction of the gas volumes passed through all downwards reaction stages.

It is further preferred that the recycling conduit does not feature a heat exchanger, which is directly coupled to the basic idea underlying the invention, namely to use an ejector for mixing the sulfur trioxide containing gas stream with the primary feed gas containing sulfur dioxide and oxygen. By eliminating the traditional blower, it is no longer necessary to adjust the temperature of the recycling stream to the temperature range of the blower. Beside the reduced equipment this has also the advantage that energy losses are reduced.

O 1 P 378 WO According to a further embodiment the first converter stage takes place in a preconverter with a subsequent pre-converter absorber, wherein a remaining part of the sulfur trioxide containing gas stream, which is not recycled, is passed to an absorption with sulfuric acid as absorption medium to form sulfuric acid. Therein, the sulfur trioxide contained in the gas stream is absorbed in the sulfuric acid forming a remaining sulfur dioxide containing gas stream for further processing. The sulfuric acid withdrawn from the pre-converter absorber is supplied to a pump tank, which in a particular preferred embodiment also acts a reservoir for the sulfuric acid to be fed to the pre-converter absorber as absorption medium.

In this aspect, it is particularly preferred that the plant comprises a downwards arranged conventional sulfuric acid plant with a capacity c. Such a plant features a main converter with at least two main converter stages arranged in series being able to handle sulfur dioxide in an amount a. Further, it contains at least one further absorber for absorbing the produced sulfur trioxide in sulfuric acid. In this embodiment, the remaining sulfur dioxide containing gas stream is fed the main converter, which is, therefore, able to handle this gas stream even if the original feed gas has as very high amount a of sulfur dioxide. So, the combination of a pe- converter with sulfuric trioxide recycling via an ejector is particularly advantageous for increasing the capacity c of existing conventional sulfuric acid plants.

In this context it is preferred that the conventional plant and the pre-converter are linked with each other. One possibility can be a common pump tank shared by all absorbers of the conventional plant as well as the absorber of the pre-converter. In addition or alternatively, a sulfur trioxide stream can be recycled from any stage of main converter back into the pre-converter.

Moreover, the plant according to the invention can show each design which is linked to previously described embodiments of the process and its advantages previously described for the process and vice versa.

O 1 P 378 WO 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 plant for producing a sulfur trioxide-contain- ing stream featuring the inventive ejector,

Fig. 2: shows a detailed drawing of an ejector according to the invention,

Fig. 3: shows schematically a plant for producing a sulfur trioxide-contain- ing stream featuring a hot gas blower as known from prior art and

Fig. 4: shows schematically a combination of a plant for converting sulfur trioxide with a conventional sulfuric acid plant.

Figure 1 shows the basic principle underlying the invention. Thereby, a sulfur dioxide and oxygen containing gas are reacted in a converter 10 to produce a sulfur trioxide containing stream.

The sulfur dioxide and oxygen containing gas is produced in at least one not- shown source and afterwards fed into the converter 10. This converter 10 can be designed as a pre-converter or a main converter. In any case, it features at least one converter stage 11 , whereby a main converter typically has 4 to 8 converter

O 1 P 378 WO stages. In any case, these converter stages are filled with a solid catalyst where the reaction takes place as a heterogeneous catalyzation.

A produced sulfur trioxide containing stream is withdrawn from the converter 10 via conduit 14. From said conduit 14, a conduit 15 is branched off for recycling a partial stream of the sulfur trioxide-containing stream as a recycling stream. The remaining sulfur trioxide-containing gas stream, i.e. which is not recycled, is passed on via conduit 18.

This recycling stream in conduit 15 is mixed with the sulfur dioxide and oxygen containing gas from a conduit 11 such that a mixed feed gas results, which is transported into the converter 10 via conduit 13. For mixing the streams from conduits 11 and 15 and simultaneously adjusting the pressure of the resulting mixed feed gas in conduit 13, an ejector 20 is foreseen which is sucking in the recycling stream.

The underlying principle of the ejector 20 is depicted in figure 2 showing a detailed view.

The ejector 20 comprises an inlet 21 for a motive medium from which a nozzle 22 extends. The inlet 21 is arranged such that the sulfur dioxide and oxygen containing gas transported via conduit 12 is introduced as a motive medium. By passing through the converging nozzle 22, the sulfur dioxide and oxygen containing gas, which has an initial pressure of between 90 and 170 kPa, is accelerated. The increasing gas velocity entrains gas from the surrounding, non-flowing region. Thereby, a low-pressure region 23 is created. Upstream or in this low-pressure region an inlet for a sucking medium 24 is foreseen. In the current application, this inlet for a sucking medium 24 is connected to conduit 15 through which the sulfur trioxide containing recycling stream is sucked,

O 1 P 378 WO The two gas streams of the motive and the sucking medium mix subsequently in a diverging diffuser 25 of the ejector 20 whereby a homogeneous feed gas is formed. Preferably, the ejector 20 is configured to generate a feed gas passed into converter 10 via conduit 13, which preferably has a pressure of between 140 and 160 kPa. This pressure can be significantly lower for a preconverter setup, because the overall pressure drop of a preconverter is much lower than a conventional plant.

On the contrary, Figure 3 displays a conventionally designed system that utilizes a hot gas blower 16 for transporting the recycling stream in conduit 15 back to the mixing with the sulfur dioxide and oxygen containing gas. However, this blower's usage is restricted to a certain temperature range of the recycling gas stream to prevent damage from corrosion caused by hot sulfur trioxide.

In order to avoid an overly strong wear of the sensitive components of hot gas blower 16, the recycling stream, which exits the converter with a temperature of between 600 and 640 °C is cooled. This cooling to a typically temperature range 250 and 300 °C takes place in heat exchanger 17. After passing the blower 16, the recycling gas stream has often to be reheated to a temperature to ensure a sufficient mixing of the sulfur dioxide and oxygen containing gas and to meet the necessary inlet temperature of the converter between 380 and 420 °C. Moreover, in most cases a homogeneous mixing can only be achieved with an additional mixing device 18 when the two streams meet.

A further embodiment according to the invention is shown in Figure 4. Therein, the source for generating a sulfur dioxide-containing gas in an amount a is a sulfur burner unit 31 arranged upwards of the first converter stage 11 . Therein, the sulfur dioxide-containing gas is produced in the sulfur burner unit 31 , wherein this unit is preferably an elemental sulfur burner. Sulfur-containing material, is introduced into reactor 31 via conduit 32. The oxygen required at least to form sulfur dioxide

O 1 P 378 WO is introduced into reactor 31 via conduit 33 and is conveyed by means of a sufficiently dimensioned blower 34. It is preferred that the oxygen is already added in a high concentration, preferably above 90 vol.-%. Alternatively, a metallurgical process can replace the sulfur burner unit as a source for the required sulfur dioxide.

In any case, produced sulfur dioxide-containing gas stream is withdrawn from reactor 31 via conduit 35. In a not shown way, energy efficiency of the plant can be increased by cooling the hot sulfur dioxide stream to a temperature between 200 and 420 °C and to use the obtained energy, e.g. by the use of a firetube boiler.

It is possible to add oxygen to the sulfur dioxide-containing gas via conduit 36 to form the sulfur dioxide and oxygen containing gas. However, if the sulfur burner unit is operated with an excess of oxygen, this is not mandatory.

In any case, a gas stream containing sulfur dioxide and oxygen is passed via conduit 12 into the ejector 20 as motive gas where it is mixed with a recycling stream from conduit 15 as described with regard to figure 1.

The resulting feed gas is passed into first converter stage 11 arranged in a preconverter 10' which may feature only this first converter stage 11 as well as several stages. Therein, the remaining part of the sulfur trioxide-containing gas which is not recycled is passed via conduit 18 a pre-converter absorber 40. Optionally, a not depicted heat exchanger for cooling this remaining part of the sulfur trioxide- containing is foreseen prior to pre-converter absorber 40 for further energy utilization.

In the pre-converter absorber 40, concentrated sulfuric acid as absorption medium is introduced via conduit 41 from a pump tank (not shown) to produce a liquid sulfuric acid stream and a remaining sulfur dioxide-containing gas. The

O 1 P 378 WO former is withdrawn and recirculated to the pump tank, the latter passed on to a conventional sulfuric acid plant 50, having a capacity c to convert sulfur dioxide, via conduit 43. Such a conventional sulfuric acid plant 50 comprises of several converter stages arranged in series to react sulfur dioxide with oxygen to produce sulfur trioxide. The sulfur trioxide-containing gas obtained from the converters is passed to at least one absorber, whereby an arrangement using two absorbers, often called intermediate and final absorbers, for the product gas stream of different converter stages is the usual arrangement. Independent from the specific embodiment, concentrated sulfuric acid, preferably having a concentration between 93 and 99.5 %, is supplied as absorption medium to the intermediate absorber in a typically countercurrent manner to the sulfur trioxide-containing gas to produce sulfuric acid.

O 1 P 378 WO Reference numbers

10 converter

10' pre-converter

11 , 11 ' converter stage

12-15 conduit

16 hot gas blower

17, 17' heat exchanger

18 conduit

19 mixing device

20 ejector

21 inlet for a motive medium

22 nozzle

23 low-pressure region

24 inlet for a sucking medium

26 diffuser

31 sulfur burning unit

32, 33 conduit

34 blower

35, 36 conduit

40 pre-converter absorber

41 -43 conduit

50 conventional sulfuric acid plant

O 1 P 378 WO

Claims

Claims:

1 . A process for producing a sulfur trioxide-containing gas stream, wherein a sulfur dioxide and oxygen containing gas are introduced in at least one converter stage to produce a sulfur trioxide-containing gas stream in a heterogeneous cata- lysation, wherein a partial stream of a sulfur trioxide containing gas stream produced in the first or a subsequent converter stage is branched off as a recycling stream and recycled to be part of a feed gas of the first converter stage, characterized in that the sulfur dioxide containing gas and added oxygen are introduced as a motive medium in an ejector via an motive medium inlet such that it sucks in and accelerates the recycling stream through a sucking medium inlet as a sucking medium to produce a feed gas streaming through a discharge outlet for the converter stage at a set pressure range.

2. A process according to claim 1 , characterized in that a gas mixture leaving the ejector has a higher pressure of between 110 and 200 kPa, than the pressure in the downwards unit.

3. A process according to claim 1 or 2, characterized in that a differential pressure between the sucking medium inlet and the discharge outlet is between 1 and 30 kPa.

4. A process according to any of claims 1 to 3, characterized in that the concentration of sulfur dioxide in the sulfur dioxide-containing feed gas is at least 14 vol.-%.

5. A process according to any of the previous claims, characterized in that the recycling stream is neither actively cooled and/or heated, and thus is designed to be pressure-drop minimized.

O 1 P 378 WO

6. A process according to any of the preceding claims, characterized in that the pressure of the sulfur dioxide and oxygen containing gas is used as a control variable for the pressure of the sulfur trioxide gas.

7. A process according to any of the preceding claims, characterized in that at least two ejectors are used in an array.

8. A process according to any of the preceding claims, characterized in that another gas stream is also recycled into the ejector or separately mixed in to form the feed gas for the first converter stage.

9. / process according to claim 8, characterized in that the first converter stage takes place in a pre-converter with a subsequent absorber stage and that the remaining part of the sulfur trioxide containing gas stream, which is not recycled, is passed to an absorption with sulfuric acid to form sulfuric acid.

10. A process according to claim 8 or 9, characterized in that a gas stream comprising sulfur dioxide obtained from the at least one absorber is fed into a downwards arranged conventional sulfuric acid plant, wherein the conventional sulfuric acid plant comprises at least two contact stages of main contacts arranged in series to react sulfur dioxide with oxygen to produce sulfur trioxide, and wherein the generated sulfur trioxide-containing gas is fed to at least one further absorber, wherein the produced sulfur trioxide is absorbed with sulfuric acid as absorption medium.

11. A plant for producing a sulfur trioxide-containing gas stream, in particular, for carrying out a process according to any one of claims 1 through 9, comprising at least one converter stage (11 ) for reacting a sulfur dioxide and oxygen containing gas with oxygen to produce a sulfur trioxide-containing gas stream, a recycling

O 1 P 378 WO conduit (15) for branching off a partial stream of the sulfur trioxide-containing gas stream as a recycling stream and for recycling the recycling stream to be part of a feed gas of the first converter stage (11 ), characterized by an ejector (20) with an inlet for a motive medium (23) and an inlet for a sucking medium (24) , whereby the inlets are arranged such that the sulfur dioxide containing gas and added oxygen is inserted as the motive medium and the recycling stream is inserted as the sucking medium.

12. A plant according to claim 11 , characterized in that the plant comprises at least one elemental sulfur burning unit (31 ) for generating the sulfur dioxide-con- taining feed gas.

13. A plant according to any of claims 11 or 12, characterized in that no heat exchanger is foreseen for the recycling stream via conduit (15).

14. A plant according to any of claims 11 to 13, characterized in that the first converter stage (11 ) takes place in a pre-converter (10') with a subsequent preconverter absorber (40), wherein a remaining part of the sulfur trioxide containing gas stream, which is not recycled, is passed to an absorption with sulfuric acid to form sulfuric acid.

15. A plant according to any of claims 11 to 14, characterized in that the plant comprises a downwards arranged conventional sulfuric acid plant (50) for processing a gas stream comprising sulfur dioxide obtained from the at least one absorber, wherein the conventional sulfuric acid plant comprising at least two further converter stages of main contacts arranged in series to react sulfur dioxide with oxygen to produce sulfur trioxide and at least one further absorber for absorbing the produced sulfur trioxide in sulfuric acid.

O 1 P 378 WO