The present invention relates to a process and its associated plant for producing sulphuric acid, in which a sulphur dioxide containing gas at least partly reacts with oxygen contained in this same gas in a converter featuring at least two sequenced catalyst beds to form sulphur trioxide, and in which the produced sulphur trioxide containing gas introduced into an absorber wherein in its further converted to sul phuric acid.

The production of sulphuric acid is well-known for decades. The so-called double absorption process is detailed described in Ullmann's Encyclopaedia of Industrial Chemistry, 5th edition, Vol. A25, pages 635 to 700. Therefore, elementary sulphur is burnt with oxygen contained in ambient air, to form sulphur dioxide. Then the sulphur dioxide is catalytically converted to sulphur trioxide. While an irreversible damage of the catalyst occurs at temperatures above ~640 °C, the same is inac tive at temperatures below ~380 °C. To avoid damages of the catalyst, typically sulphur dioxide contents are limited to maximally 13 vol.-% since otherwise the exotherm icity of the oxidation reaction would lead to hotspots above the critical temperature, when gases of a higher concentration are used.

On the other hand, while increasing S0 ₂ -concentration by burning sulphur with air, the residual 02-concentration ex combustion diminishes accordingly and hence the thus reduced 0 ₂ /S0 ₂ -ratio would cause a reduced conversion of SO2 and thus an increase of SO2 emissions to the stack, possibly being beyond stat utory limits. Therefore, in practice, a typical limit of the S0 ₂ -concentration is around 11.5 vol.-%, -with corresponding residual 02-concentration of ~9.2 vol.-% for gas being fed to the catalytic oxidation, i.e. a molar ratio of O2/SO2 of 0.8. As a result, very large gas volumes must be passed through the converter and other related equipment. This leads to large capital expenditure (CAPEX) and op erating expenditure (OPEX) of the sulphuric acid plant.

To overcome these disadvantages, processes for producing sulphuric acid, based on sulphur combustion gas, have already been proposed, in which starting gases with a sulphur dioxide content of more than 13 vol.-% can be supplied to the cat alyst. However, these processes fail to match the required emission standards as a result of the influence of the reduced partial pressure of oxygen.

As explained above, conventional technology is characterized by a feed gas to the catalytic SO2 converter, typically containing a maximum of 11.5 - 12.0 vol.-% of SO2. As an oxygen source, ambient dried air is fed into the sulphur combustion furnace. The initial oxygen content of said air amounts to ~ 20.9 vol.-%. The sul phur combustion leads to a consumption of oxygen according to the reaction

S + 0 ₂ ® S0 ₂ AH = -297 kj /mol

The residual O2 content of the gas originating from the combustion with air is correlated to the desired SO2 concentration of said gas, as it can be seen from fig. 1.

Therefore, the resulting ratio of O2/SO2 shows the course as presented in fig. 2.

In case such gas is fed directly to a catalytic oxidation converter to process the SO2 gas into SO3, more oxygen is required, according to

1

S0 ₂ + - 0 ₂ ® ^S °s ^H = -99 kj/mol Consequently, the theoretical stoichiometric molar ratio of O2/SO2 of this feed gas must be >= 0.5 to enable a complete conversion. It is obvious that the thermody namic equilibrium of the technical reaction requires a larger ratio, i.e. stoichio metric excess of oxygen, - subject to the overall desired degree of conversion. The overall achievable maximum conversion of said catalytic reaction is shown qualitatively in fig. 3 for sulphur combustion gas with air and resulting SO2 con centrations of 11-19 vol.-% (with O2 content according to fig.1. ).

Obviously, an O2/SO2 ratio of the gas fed to the catalytic converter below 0.5 does not allow to target full quantitative conversion and current plants are thus de signed at a minimum molar 0 ₂ /S0 ₂ -ratio of ~ 0.75 to 0.85, which still provides sufficient excess oxygen to achieve an acceptable overall SO2 conversion of typ ically 99.8% with double-absorption technology. The lower the SO2 concentration in the feed gas (and thus the higher the O2/SO2 ratio), the better is the conversion efficiency, up to and beyond 99.9 %.

It is well-known in the sulphuric acid manufacturing industry that processing high SO2 concentrated gas, such excessively high SO2 concentrations will affect the adiabatic oxidation temperature at the exit of the first catalytic bed, i.e. may lead to inacceptable high temperatures, which would irreversibly deteriorate / destroy the function of the catalyst. As described in detail in DE 102004022 506, higher SO2 concentration fed to the first catalyst bed will result in excessive gas temper atures, based on adiabatic operation. For gas of metallurgical origin, this is pre sented in fig. 5 based on a ratio of O2/SO2 = ~0.8.

A typical process schematic of a conventional sulphuric acid plant based on sul phur combustion with air is presented in fig.11. It has been proposed to add various amounts or ratios of technical (bulk tonnage) oxygen or oxygen enriched air, both for the combustion of sulphur and/or as an additional feed to the catalytic converter, with the aim to increase the overall plant efficiency, reduce cost of equipment or minimize gaseous emissions from such plants. W02005/105666 A2 describes a process applying a gas with >20 vol.-% SO2 with an SC /C -ratio of >2.67 and provides examples of SC -concentrations up to 95 vol.-%, - with and without various recirculation of process gas.

Similarly, DE 2 223 131 presents a process with addition of technical oxygen to the sulphur combustion and partial recirculation of process gas for the purpose of managing temperatures and emissions and of course, reduction of the cost of equipment.

The use or addition of oxygen or oxygen enriched air, compared to the traditional process using air only as an oxygen source, does lead to the processing of less gas volumes and hence lower plant capital expenditure, as is the purpose of the two above mentioned patents.

Summing up, an operation of an acid plant based on sulphur combustion with air or oxygen enriched air resulting in significantly higher SO ₂ concentrations of the gas fed to the catalytic conversion to SO ₃ , would offer huge advantages as the overall capital and operating costs would be reduced.

Therefore, it is the object of the current invention to provide an alternative process for producing SO ₃ from SO ₂ containing gas, generated from combustion of ele mental sulphur with air, and separate oxygen or oxygen enriched air addition for enhanced SO ₂ conversion to the catalytic converter, with increased economic ef ficiency and overall profitability of sulphuric acid production. Technical grade bulk oxygen can be added to this combustion gas prior or past the first catalytic bed to provide a suitable O2/SO2 molar ratio, sufficient to achieve a reasonable low SO2 stack emission rate and being in line with prevailing statu tory requirements.

Said problem is further solved also with a process according to claim 1.

It is the basic idea of this invention that via a line an amount A of the admixed oxygen is added into one catalyst bed whereby the amount A is calculated such that the equilibrium at the given temperature in the relative catalyst bed depending on the sulphur dioxide content is reached.

This is illustrated in fig. 12. According to said fig. 12., a typical sulphur combustion gas with 18 vol.-% SO ₂ using solely air as raw material is fed to a first catalytic bed at typically ~470°C, thereafter partially oxidized by a catalyst to SC to an extend determined by its O ₂ concentration, adiabatically arriving at a temperature of approximately 580°C, can achieve an overall conversion of the inlet amount of SO ₂ to approximately 28%. The related temperature-conversion diagram is pre sented at fig. 9. Adding an appropriate amount of bulk oxygen to said exit from the first bed, will achieve a virtual overall gas composition of e.g. 18 vol.-% SO ₂ and 14.7 vol.-% O _2. Feeding said gas with a conventional temperature to the sec ond catalytic bed, and in continuation to further subsequent catalytic beds with appropriate intermediate cooling between beds, will in this example arrive at a total degree of conversion of approximately 96% prior to the intermediate absorp tion of the SO _3. At this 4+1 arrangement, a total desired conversion of >99.8 % can be achieved.

In detail, in such a process for producing sulphuric acid a feed gas containing sulphur dioxide and oxygen at least partly reacts in a converter featuring at least one catalyst bed to form sulphur trioxide at a given temperature and the produced sulphur trioxide containing gas introduced into an absorber wherein it is further converted to sulphuric acid. The feed gas to the first catalyst bed is generated by combustion of elemental sulphur with air or oxygen enriched air. Thereby, said feed gas featuring a sulphur dioxide content between 14.0 and 20.0 vol-%, and said feed gas featuring a molar ratio of oxygen to sulphur dioxide of 0.05 < R < 1.0 of O2/SO2. Oxygen or oxygen enriched air is admixed to the gas leaving the first catalyst bed, prior to entering a subsequent catalyst bed, to an amount to enable the overall desired conversion of SO2 considering all catalyst beds of the plant. No oxygen addition or oxygen enriched air addition is required when the plant is operated below 65 % of name plant capacity and/or below a value of 11 ,5 for a ratio of the content of not-converted SO2 to the SO2 content in the feed, while the required amount of oxygen is gradually reduced from a nominal figure at name plate capacity down to zero at said 65 % load and/or said ratio.

In this context it is preferred, that the gas leaving the first catalyst bed is cooled and mixed with bulk oxygen or oxygen enriched air prior to entering the subse quent bed, with an amount sufficient to achieve an overall ratio of C /SC of 0.4 < R < 1.1, relative to the initial feed gas SO2 concentration to the first catalyst bed.

In addition or alternatively, the gas leaving the second or further subsequent cat alyst beds is cooled and mixed with bulk oxygen or oxygen enriched air to achieve an overall ratio of O2/SO2 of 0.7 < R < 1.2, relative to the initial feed gas SO2 concentration to the first catalyst bed.

It shall be noted that the above plant load ratio of typically ~65% is specific to a feed SO2 of 18 vol.-% to a “nominal” concentration of 11.5 vol.-% SO2 (= 11.5 / 18 = ~0.65) is valid for said specific condition. More general, this ratio shall be regarded as being (11.5 % SO2 / feed SO2 %), thus generalized for all feed con centrations between 14 and 20 vol.-%. SO2. Said problem is further solved also with a process according to claim 10.

The process contains the following steps:

(i) Combustion of elemental sulphur with dried ambient air (only) and thereby producing a gas containing an SC -concentration above the conventional level of typically 11.5 vol.-%, preferably with ~ 18-20 vol.-% with a respec tive residual oxygen content,

(ii) feeding this SO2 containing gas to a first catalytic bed and reacting at least a portion of the said sulphur dioxide with at least part of the oxygen therein to form sulphur trioxide,

(iii) operating said first catalytic bed with a stoichiometric lack of oxygen, i.e. below a molar ratio of O2/SO2 = 0.5,

In order to achieve a sustainable operation and activity of the catalyst, it has been found that for a sulphur dioxide content between 14.0 and 19.0 vol-% the range of gas inlet temperature Ti _n to the first catalytic bed has to be between Ti _n-min and Tin-max, which are calculated as

Ti _n-min = - 0.066 x c ³ + 3.3 x c ² - 44.2 x c + 585, preferably Ti _n-min = - 0.066 x c ³ + 3.3 x c ² - 44.2 x c + 590 and Ti _n-max = - 0.066 x c ³ + 3.3 x c ² - 44.2 x c + 615, preferably Ti _n-max = - 0.066 x c ³ + 3.3 x c ² - 44.2 x c + 610 with c [%-vol.] defined as the SO2 concentration of the feed gas. For a sulphur dioxide content above 19.0 vol-% and below 20.0 vol-%, the range of gas inlet temperature Ti _n is fixed to 490 °C +/- 15 °C, preferably +/- 10 °C. This temperature leads to a sustainable operation and activity of the catalyst, whereby it is possible to achieve the adiabatically exit temperature of the catalytic bed and, according to the thermodynamic equilibrium the maximum turnover.

The following designs are preferred designs of the two independent claims 1 and 10:

The method leads to particular good results if the gas feed to the first catalyst bed has a sulphur dioxide content between 17 and 19 vol.% and/or a molar ratio of 0 ₂ /S0 ₂ of 0.15 < R < 0.22.

Summing up, the combustion gas of the sulphur with air, containing a typical over all ~18 vol.-% S0 ₂ will have a residual oxygen content of 0 ₂ of ~ 2,6 vol.-%, thus not suitable to directly feeding a catalyst bed for maximized conversion of S0 ₂ to SO3 (see fig. 6).

Based on sulphur-burning with air, fig. 7 illustrates the thermodynamic equilibrium lines vs the operating lines at various S0 ₂ feed concentrations at the first bed catalyst. Therein, the achievable bed exit temperatures and relating conversions can be seen, as a result of the limits set by the thermodynamic equilibrium. The operating line hits the equilibrium line for e.g. 18 vol.-% gas at typically ~580 °C (starting with 475°C) and corresponding ~28 % conversion. Obviously, the ther modynamic equilibrium prevents any higher temperature and conversion despite the high S0 ₂ feed concentration. Similarly, an 11 %-vol. S0 ₂ gas crosses the equilibrium line at ~620 °C (starting with 420°C), at close to 70 vol.-% conversion.

While a conventional plant offers a typical turn-down ratio of 40%, this oxygen enhanced process offers a turn-down ratio to 25%. As a result, the exit temperature of said catalyst bed does not exceed the maximum tolerable temper ature of say 640°C. In this context, it is particularly preferred that oxygen is ad mixed only to a contact stage downstream of a first contact stage, thus with al ready reduced content of sulphur dioxide and presence of sulphur trioxide, the oxygen content is increased. For a typical arrangement of a 4+1 configuration, i.e. 4 catalytic beds prior and 1 catalytic bed post the intermediate absorption of SO3, but can also be applied for any other configuration, e.g. 2+1 , 2+2, 3+1 , 3+2 and the like.

As an option to the sulphur combustion with air only, oxygen enrichment can also be used for the combustion of sulphur. The adiabatic combustion of sulphur with air or oxygen enriched air is presented in fig. 4. The resulting combustion tem peratures are shown therein.

If only air is used as oxygen supplier, a maximum SC -concentration of about 21 vol.-% can be achieved. Addition of bulk oxygen to the combustion air enables the production of significantly higher SO2 concentrations up to say ~25 vol.-%, whereas this limit is given by the resulting high adiabatic combustion tempera ture, which should not exceed ~2000°C, considering the mechanical design of the downstream equipment, e.g. waste heat boiler. To overcome this restriction, recirculation techniques have been developed earlier as it is e.g. shown in WO 2005/105666, which practically allow the production of virtually 100 vol.-% SO2 gas by using pure oxygen.

No O2 addition is needed for cases wherein the plant is operated typically below 65% of nameplate capacity. For this case, it is also preferred to reduce the SO2 concentration of the gas fed to the first catalyst bed down to 11.5 vol.-% SO2. Preferably, the reduction is done gradually to make no sudden changes. Moreover, using a catalyst comprising vanadium pentoxide is a preferred embod iment of this invention, as this catalyst is well established in the market.

Simplified, the mechanism of the oxidation of sulphur dioxide with oxygen to form sulphur trioxide using conventional vanadium-based catalyst is characterized by

V2O5 + SO2 - V2O4 + SO3 and further

V2O4 + ½ O2 - V2O5 whereas the re-oxidation of the V2O4 with O2 is regarded to be the limiting factor of the heterogeneous catalytic reaction with a vanadium-based catalyst.

Coming back to the basic idea underlying the invention, it can also bee seen from the equations that SO2 containing gas composition characterized by a lack of ox ygen well below the stoichiometric ratio of 0.5 will therefore at least diminish the rate of the oxidation reaction remarkably. As described in WO 2005/105666 A2, this diminishing effect can be offset by an increased temperature of reaction, thus compensating the said reduced rate by applying Arrhenius’ law.

In this context it has been found that supplying a sub-stoichiometric contact gas (with regard to O2) to the first contact stage with a temperature of at least 450 °C, and particularly preferably of at least 470 °C and/or with a pressure of 1 to 30 bar, and particularly preferably of 1.5 to 10 bar leads to particularly good conversion rates, -within the limits given by the thermodynamic equilibrium.

A relationship is presented in fig. 8. Because of the inlet gas composition and temperature Ti _n , the adiabatically achievable exit temperature of the catalytic bed according to the thermodynamic equilibrium, is limited to the values as shown at this fig. 8. It is preferred that the feed gas to the catalytic converter contains between 1 and 5 vol.-% of residual oxygen. These are oxygen contents typically found in off gases from sulphur burning, which means that such off gases can be directly used in a conversion according to this invention. However, such a gas, e.g. con taining ~18 vol.-% SO2 would not lead to any noteworthy overall conversion of SO2 (refer fig. 6).

As already touched above, it is particularly preferred that the feed gas to the SO2 oxidation catalyst originates from a combustion of elemental sulphur with air only.

As also explained, the SO2 converter features at least two contact stages or cat alyst beds, whereby oxygen is admixed to at least one contact stage such that the equilibrium at the given temperature in the stage depending on the sulphur dioxide content is reached. Such a design offers the possibility to cool between the stages.

It is the basic idea of this invention that via a line an amount A of the admixed oxygen is added into one catalyst bed whereby the amount A is calculated such that the equilibrium at the given temperature in the relative catalyst bed depending on the sulphur dioxide content is reached.

Moreover, the invention covers also a plant with the features of claim 20 to pro duce sulphuric acid according to the method known from claims 1 to 10. As the essential feature, the plant features a control unit for a sulphur dioxide content between 14.0 and 19.0 vol-% the range of the gas inlet temperature Tin into to the first catalyst bed is controlled or regulated to a range between Tin-min and Tin- max, which are calculated as

Tin-min = - 0.066 x c ³ + 3.3 x c ² - 44.2 x c + 585 and Tin-max = - 0.066 x c ³ + 3.3 x c ² - 44.2 x c + 615 with c [%-vol.] defined as the SO2 concentration of the feed gas.

For a sulphur dioxide content above 19.0 vol-% and below 20.0 vol-%, the range of gas inlet temperature T, _n is controlled or regulated to 490 °C +/- 15 °C.

In particular, the plant for producing sulphuric acid features a control unit wherein the inlet temperature T, _n to the first catalyst bed is controlled or regulated depend ing on the SO2 content of the feed gas within the range of 11 and 19 vol.-% accord ing to

Tin = - 0.066 x ^c3 + 3.3 x c ² - 44.2 x c + 600 with a tolerance of +/- 10°C, whereas c = SO2 concentration of the feed gas to the first catalyst bed [vol.-%], or when the sulphur dioxide content c is more than 19 vol-%, then T, _n is fixed to 490 °C +/- 10°C.

Additional features, advantages and possible applications of the invention are found in the following description of exemplary embodiments and the drawings. All the features described and/or illustrated graphically form the subject matter of the invention, either alone or in any desired combination, regardless of how they are combined in the claims or in their references back to preceding claims.

The drawings show schematically: Fig. 1 : Sulphur burning with air - residual O2

Fig. 2: O2/SO2 ratio of gas based on S-combustion with air Fig. 3: Maximum achievable conversion from SO2 to SO3 of S-burner gas Fig. 4: Combustion temperature of sulphur Fig. 5: Gas exit temperature vs. feed SO2 concentration

Fig. 6: Oxidation of SO2 gas (18%S0 ₂ ) with O2 (2.6%-vol.)

Fig. 7: Conversion of concentrated S-burner gas vs. SO2 concentration to bed#1 Fig. 8: Gas inlet and exit temperature bed#1 vs. S-burner feed gas

Fig. 9: Acid plant concept contact section using sub-stoichiometric feed gas to bed#1 Fig. 10: Part load operation 7,800 mtpd acid plant

Fig. 11 : Conventional acid plant with 3+2 arrangement

Fig. 12: Sub-stoichiometric operation of bed#1

Figures 1 to 11 has already been discussed with regard to the understanding un derlying the invention. According to fig. 12, liquid sulphur is fed via line 1 to the plant, together with com bustion air line 2 and oxygen line 3.

At least one burner S is foreseen in a combustion chamber CC. Said combustion chamber CC is preferably situated in the same housing HO, wherein also at least one heat exchanger E is positioned.

Optionally, oxygen can be added to the combustion chamber CC via line 5.

Via line 7, the resulting SO2 containing gas is withdrawn and optionally admixed with oxygen passed via line 6. This gas mixture is fed via line 8 into converter CO.

The converter CO features five catalyst stages, also called catalyst beds B1 to B5, whereby the number can be freely chosen, preferably between 2 and 12. After the first stage, the respective product gas is passed via line 9 into heat exchanger H1 and is returned to the converter CO via line 10 and 11, such that it then also contains the added oxygen from line 4.

This continues then to the second catalytic bed B2 and the gas leaves this said bed towards a heat exchanger H2 via line 12 and returns to the converter bed 3 B3 via line 13. Similarly, and typically this continues accordingly to bed B4.

Exemplary at bed 4, but also possible at any other bed upstream, leaving B4, the gas is cooled at heat exchangers H4-1 and H4-2 and then fed to the intermediate absorber for the removal of the SO3 via lines 16,17 and 18. Returning from the intermediate absorber, the gas is heated up to the desired gas inlet temperature of the last bed B5 at the heat exchanger H4-1 via lines 19 and 20.

Eventually the gas leaving B5 is fed to the final absorption and cooled at the heat exchanger H5 via lines 21 and 22. According to said fig. 12., a typical sulphur combustion gas with 18 vol.-% SO2 using solely air as raw material is fed to a first catalytic bed at typically ~470°C, thereafter partially oxidized by a catalyst to SC to an extend determined by its O2 concentration, adiabatically arriving at a temperature of approximately 580°C, can achieve an overall conversion of the inlet amount of SO2 to approximately 28%. The related temperature-conversion diagram is presented at Fig. 9. Adding an appropriate amount of bulk oxygen to said exit from the first bed, will achieve a virtual overall gas composition of e.g. 18 vol.-% SO2 and 14.7 vol.-% O2. Feeding said gas with a conventional temperature to the second catalytic bed, and in con tinuation to further subsequent catalytic beds with appropriate intermediate cool ing between beds, will in this example arrive at a total degree of conversion of approximately 96% prior to the intermediate absorption of the SO3. At this 4+1 arrangement, a total desired conversion of >99.8 % can be achieved.

Example according to a process shown in fig.12

As an example, a current state of the art 5,000 mtpd (metric ton per day) sulphuric acid plant is fed with 11 ,5 vol.-%. SO2 feed gas originating from sulphur combus tion with air. While the gas flow at 100% load (nameplate capacity) amounts to ~400,000 Nm ³ /h, such plant offers a typical turndown ratio of 40% load. It must be noted that the entire plant is virtually sized on the basis of overall gas flow rates and only to a lower degree on actual acid production.

Thus, an acid plant designed in accordance with the invention for same gas throughput of ~400,000 Nm ³ /h but operated at say 18 vol.-% SO2 instead of tra ditional 11.5 vol.-% SO2, would lead to an acid capacity of 7,800 mtpd, instead of the original 5,000 mtpd. Accordingly, an inventive acid plant offers a turndown ratio of ~25% load. As a result, cost savings per ton of installed acid plant capacity are evident, and the operational flexibility is significantly increased. The gas-flow vs. plant load (%) diagram is presented at fig. 10.

While such specific acid plant size results in only 18.0/11.5 ~64 % of the conven- tional size, the resulting capital cost per installed ton of acid production amounts to only ~75% of a conventional plant. Thus, a potential 25% cost saving, -com pared to conventional technology, makes such plant according to this invention very attractive. The significant cost savings resulting from this invention are partly offset by the cost of provision of the bulk O2, whereas its use can be minimized by this inven tion. Typically, whenever the plant load is below say ~65% of nominal capacity, no oxygen would be required, and the plant would be operated with conventional parameters.

List of Reference Numerals

1 -4, 7-22 line 5, 6 line optional

S burner cc combustion chamber

HO housing

E heat exchanger CO converter

H 1 , H2, H3, H5 heat exchanger H4-1 , H4-2 heat exchanger B1 - B5 catalyst bed