Patent Description:
The production of sulphuric acid is well-known for decades. The so-called double absorption process is detailed described in <NPL>. 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 ~<NUM>, the same is inactive at temperatures below ~<NUM>. To avoid damages of the catalyst, typically sulphur dioxide contents are limited to maximally <NUM> vol. -% since otherwise the exothermicity 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 SO<NUM>-concentration by burning sulphur with air, the residual O<NUM>-concentration ex combustion diminishes accordingly and hence the thus reduced O<NUM>/SO<NUM>-ratio would cause a reduced conversion of SO<NUM> and thus an increase of SO<NUM> emissions to the stack, possibly being beyond statutory limits. Therefore, in practice, a typical limit of the SO<NUM>-concentration is around <NUM> vol. -%, -with corresponding residual O<NUM>-concentration of ~<NUM> vol. -% for gas being fed to the catalytic oxidation, i.e. a molar ratio of O<NUM>/SO<NUM> of <NUM>.

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 operating 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 <NUM> vol. -% can be supplied to the catalyst. 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 SO<NUM> converter, typically containing a maximum of <NUM> - <NUM> vol. -% of SO<NUM>. As an oxygen source, ambient dried air is fed into the sulphur combustion furnace. The initial oxygen content of said air amounts to ~ <NUM> vol. The sulphur combustion leads to a consumption of oxygen according to the reaction <MAT>.

The residual O<NUM> content of the gas originating from the combustion with air is correlated to the desired SO<NUM> concentration of said gas, as it can be seen from <FIG>.

Therefore, the resulting ratio of O<NUM>/SO<NUM> shows the course as presented in <FIG>.

In case such gas is fed directly to a catalytic oxidation converter to process the SO<NUM> gas into SO<NUM>, more oxygen is required, according to <MAT>.

Consequently, the theoretical stoichiometric molar ratio of O<NUM>/SO<NUM> of this feed gas must be >= <NUM> to enable a complete conversion. It is obvious that the thermodynamic equilibrium of the technical reaction requires a larger ratio, i.e. stoichiometric 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> for sulphur combustion gas with air and resulting SO<NUM> concentrations of <NUM>-<NUM> vol. -% (with O<NUM> content according to <FIG>.

Obviously, an O<NUM>/SO<NUM> ratio of the gas fed to the catalytic converter below <NUM> does not allow to target full quantitative conversion and current plants are thus designed at a minimum molar O<NUM>/SO<NUM>-ratio of ~ <NUM> to <NUM>, which still provides sufficient excess oxygen to achieve an acceptable overall SO<NUM> conversion of typically <NUM>% with double-absorption technology. The lower the SO<NUM> concentration in the feed gas (and thus the higher the O<NUM>/SO<NUM> ratio), the better is the conversion efficiency, up to and beyond <NUM> %.

It is well-known in the sulphuric acid manufacturing industry that processing high SO<NUM> concentrated gas, such excessively high SO<NUM> 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 <CIT>, higher SO<NUM> concentration fed to the first catalyst bed will result in excessive gas temperatures, based on adiabatic operation. For gas of metallurgical origin, this is presented in <FIG> based on a ratio of O<NUM>/SO<NUM> = ~<NUM>.

A typical process schematic of a conventional sulphuric acid plant based on sulphur combustion with air is presented in <FIG>.

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. <CIT> describes a process applying a gas with ><NUM> vol. -% SO<NUM> with an SO<NUM>/O<NUM>-ratio of ><NUM> and provides examples of SO<NUM>-concentrations up to <NUM> vol. -%, - with and without various recirculation of process gas.

Similarly, <CIT> 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.

From <CIT> a method and plant for generating sulfuric acid from a sulfur-containing material is known. A feed gas comprising more than <NUM> vol. -% SO<NUM> and at least <NUM> vol. -% of CO<NUM> is obtained by burning said sulfur-containing material and a carbon-containing fuel in a first reactor stage. After drying, the gas stream is fed to a multistage converter to form SO<NUM>. Prior to entering the first stage, oxygen is added and the ratio of O<NUM> to SO<NUM> adjusted to below <NUM>, preferably to between <NUM> to <NUM>. In between the first and second stage, said ratio is adjusted to a value in the range <NUM> to <NUM> by oxygen addition. Further, the temperature of the gas stream is adjusted upstream of each stage to optimize the conversion conditions. The obtained SO<NUM> is subsequently absorbed to form sulfuric acid.

<CIT> describes a process and plant for the continuous catalytic oxidation of a feed gas comprising between <NUM> to <NUM> vol. -% SO<NUM> under quasi-isothermal conditions in a tube contactor reactor. Said quasi-thermal conditions are achieved by inducing/dissipating heat in dependency of the SO<NUM> concentration in the feed gas. The O<NUM> to SO<NUM> ratio is adjusted to a range between <NUM> to <NUM> prior to entering the tube reactor by oxygen addition. An only partially converted product gas stream may further be supplied to a standard Double Contact stage, whereby the residual SO<NUM> has to be < <NUM> vol.

<CIT> discloses a process and plant for producing sulfuric acid from a starting gas comprising more than <NUM> vol. -% SO<NUM>. Said gas is supplied to a first contact stage with a O<NUM> to SO<NUM> ratio of less than <NUM>. Thereby, low oxygen content prevents overheating of the catalyst. The O<NUM> to SO<NUM> ratio of the partially converted gas leaving the first stage may be adjusted to hyper- or substoichiometric levels prior to any further contact stage by addition of an oxygen-containing gas.

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<NUM> concentrations of the gas fed to the catalytic conversion to SO<NUM>, 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<NUM> from SO<NUM> containing gas, generated from combustion of elemental sulphur with air, and separate oxygen or oxygen enriched air addition for enhanced SO<NUM> conversion to the catalytic converter, with increased economic efficiency 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 O<NUM>/SO<NUM> molar ratio, sufficient to achieve a reasonable low SO<NUM> stack emission rate and being in line with prevailing statutory requirements.

Said problem is further solved also with a process according to claim <NUM>.

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>. According to said <FIG>. , a typical sulphur combustion gas with <NUM> vol. -% SO<NUM> using solely air as raw material is fed to a first catalytic bed at typically ~<NUM>, thereafter partially oxidized by a catalyst to SO<NUM> to an extend determined by its O<NUM> concentration, adiabatically arriving at a temperature of approximately <NUM>, can achieve an overall conversion of the inlet amount of SO<NUM> to approximately <NUM>%. The related temperature-conversion diagram is presented at <FIG>. Adding an appropriate amount of bulk oxygen to said exit from the first bed, will achieve a virtual overall gas composition of e.g. <NUM> vol. -% SO<NUM> and <NUM> vol. Feeding said gas with a conventional temperature to the second 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 <NUM>% prior to the intermediate absorption of the SO<NUM>. At this <NUM>+<NUM> arrangement, a total desired conversion of ><NUM> % 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 <NUM> and <NUM> vol-%, and said feed gas featuring a molar ratio of oxygen to sulphur dioxide of <NUM> < R < <NUM> of O<NUM>/SO<NUM>. 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 SO<NUM> considering all catalyst beds of the plant. No oxygen addition or oxygen enriched air addition is required when the plant is operated below <NUM> % of name plant capacity and/or below a value of <NUM>,<NUM> for a ratio of the content of not-converted SO<NUM> to the SO<NUM> 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 <NUM> % load and/or said ratio.

The gas leaving the first catalyst bed is cooled and mixed with bulk oxygen or oxygen enriched air prior to entering the subsequent bed, with an amount sufficient to achieve an overall ratio of O<NUM>/SO<NUM> of <NUM> < R < <NUM>, relative to the initial feed gas SO<NUM> concentration to the first catalyst bed.

In addition or alternatively, the gas leaving the second or further subsequent catalyst beds is cooled and mixed with bulk oxygen or oxygen enriched air to achieve an overall ratio of O<NUM>/SO<NUM> of <NUM> < R < <NUM>, relative to the initial feed gas SO<NUM> concentration to the first catalyst bed.

It shall be noted that the above plant load ratio of typically ~<NUM>% is specific to a feed SO<NUM> of <NUM> vol. -% to a "nominal" concentration of <NUM> vol. -% SO<NUM> (= <NUM> / <NUM> = ~<NUM>) is valid for said specific condition. More general, this ratio shall be regarded as being (<NUM> % SO<NUM> / feed SO<NUM> %), thus generalized for all feed concentrations between <NUM> and <NUM> vol.

The process contains the following steps:.

In order to achieve a sustainable operation and activity of the catalyst, it has been found that for a sulphur dioxide content between <NUM> and <NUM> vol-% the range of gas inlet temperature Tin to the first catalytic bed has to be between Tin-min and Tin-max, which are calculated as <MAT> preferably <MAT> and <MAT> preferably <MAT> with c [%-vol. ] defined as the SO<NUM> concentration of the feed gas.

For a sulphur dioxide content above <NUM> vol-% and below <NUM> vol-%, the range of gas inlet temperature Tin is fixed to <NUM> +/- <NUM>, preferably +/- <NUM>. 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 <NUM> and <NUM>:
The method leads to particular good results if the gas feed to the first catalyst bed has a sulphur dioxide content between <NUM> and <NUM> vol. % and/or a molar ratio of O<NUM>/SO<NUM> of <NUM> < R < <NUM>.

Summing up, the combustion gas of the sulphur with air, containing a typical overall ~<NUM> vol. -% SO<NUM> will have a residual oxygen content of O<NUM> of ~ <NUM>,<NUM> vol. -%, thus not suitable to directly feeding a catalyst bed for maximized conversion of SO<NUM> to SO<NUM> (see <FIG>).

Based on sulphur-burning with air, <FIG> illustrates the thermodynamic equilibrium lines vs the operating lines at various SO<NUM> 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. <NUM> vol. -% gas at typically ~<NUM> (starting with <NUM>) and corresponding ~<NUM> % conversion. Obviously, the thermodynamic equilibrium prevents any higher temperature and conversion despite the high SO<NUM> feed concentration. Similarly, an <NUM> %-vol. SO<NUM> gas crosses the equilibrium line at ~<NUM> (starting with <NUM>), at close to <NUM> vol. -% conversion.

While a conventional plant offers a typical turn-down ratio of <NUM>%, this oxygen enhanced process offers a turn-down ratio to <NUM>%. As a result, the exit temperature of said catalyst bed does not exceed the maximum tolerable temperature of say <NUM>. In this context, it is particularly preferred that oxygen is admixed only to a contact stage downstream of a first contact stage, thus with already reduced content of sulphur dioxide and presence of sulphur trioxide, the oxygen content is increased. For a typical arrangement of a <NUM>+<NUM> configuration, i.e. <NUM> catalytic beds prior and <NUM> catalytic bed post the intermediate absorption of SO<NUM>, but can also be applied for any other configuration, e.g. <NUM>+<NUM>, <NUM>+<NUM>, <NUM>+<NUM>, <NUM>+<NUM> 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>. The resulting combustion temperatures are shown therein.

If only air is used as oxygen supplier, a maximum SO<NUM>-concentration of about <NUM> vol. -% can be achieved. Addition of bulk oxygen to the combustion air enables the production of significantly higher SO<NUM> concentrations up to say ~<NUM> vol. -%, whereas this limit is given by the resulting high adiabatic combustion temperature, which should not exceed ~<NUM>, 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 <CIT>, which practically allow the production of virtually <NUM> vol. -% SO<NUM> gas by using pure oxygen.

No O<NUM> addition is needed for cases wherein the plant is operated typically below <NUM>% of nameplate capacity. For this case, it is also preferred to reduce the SO<NUM> concentration of the gas fed to the first catalyst bed down to <NUM> vol. -% SO<NUM>. Preferably, the reduction is done gradually to make no sudden changes.

Moreover, using a catalyst comprising vanadium pentoxide is a preferred embodiment 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 <MAT> and further <MAT> whereas the re-oxidation of the V<NUM>O<NUM> with O<NUM> 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 SO<NUM> containing gas composition characterized by a lack of oxygen well below the stoichiometric ratio of <NUM> will therefore at least diminish the rate of the oxidation reaction remarkably. As described in <CIT>, 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 O<NUM>) to the first contact stage with a temperature of at least <NUM>, and particularly preferably of at least <NUM> and/or with a pressure of <NUM> to <NUM> bar, and particularly preferably of <NUM> to <NUM> bar leads to particularly good conversion rates, -within the limits given by the thermodynamic equilibrium.

A relationship is presented in <FIG>. Because of the inlet gas composition and temperature Tin, the adiabatically achievable exit temperature of the catalytic bed according to the thermodynamic equilibrium, is limited to the values as shown at this <FIG>.

It is preferred that the feed gas to the catalytic converter contains between <NUM> and <NUM> 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. containing ~<NUM> vol. -% SO<NUM> would not lead to any noteworthy overall conversion of SO<NUM> (refer <FIG>).

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

As also explained, the SO<NUM> converter features at least two contact stages or catalyst 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.

Moreover, the invention covers also a plant with the features of claim <NUM> to produce sulphuric acid according to the method known from claims <NUM> to <NUM>. As the essential feature, the plant features a control unit for a sulphur dioxide content between <NUM> and <NUM> 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 <MAT> <MAT> with c [%-vol. ] defined as the SO<NUM> concentration of the feed gas.

For a sulphur dioxide content above <NUM> vol-% and below <NUM> vol-%, the range of gas inlet temperature Tin is controlled or regulated to <NUM> +/- <NUM>.

In particular, the plant for producing sulphuric acid features a control unit wherein the inlet temperature Tin to the first catalyst bed is controlled or regulated depending on the SO<NUM> content of the feed gas within the range of <NUM> and <NUM> vol. -% according to <MAT>.

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.

<FIG> has already been discussed with regard to the understanding underlying the invention.

According to <FIG>, liquid sulphur is fed via line <NUM> to the plant, together with combustion air line <NUM> and oxygen line <NUM>.

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 <NUM>.

Via line <NUM>, the resulting SO<NUM> containing gas is withdrawn and optionally admixed with oxygen passed via line <NUM>. This gas mixture is fed via line <NUM> 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 <NUM> and <NUM>. After the first stage, the respective product gas is passed via line <NUM> into heat exchanger H1 and is returned to the converter CO via line <NUM> and <NUM>, such that it then also contains the added oxygen from line <NUM>.

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

Exemplary at bed <NUM>, but also possible at any other bed upstream, leaving B4, the gas is cooled at heat exchangers H4-<NUM> and H4-<NUM> and then fed to the intermediate absorber for the removal of the SO<NUM> via lines <NUM>,<NUM> and <NUM>. 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-<NUM> via lines <NUM> and <NUM>.

Eventually the gas leaving B5 is fed to the final absorption and cooled at the heat exchanger H5 via lines <NUM> and <NUM>.

According to said <FIG>. , a typical sulphur combustion gas with <NUM> vol. -% SO<NUM> using solely air as raw material is fed to a first catalytic bed at typically ~<NUM>, thereafter partially oxidized by a catalyst to SO<NUM> to an extend determined by its O<NUM> concentration, adiabatically arriving at a temperature of approximately <NUM>, can achieve an overall conversion of the inlet amount of SO<NUM> to approximately <NUM>%. The related temperature-conversion diagram is presented at <FIG>. Adding an appropriate amount of bulk oxygen to said exit from the first bed, will achieve a virtual overall gas composition of e.g. <NUM> vol. -% SO<NUM> and <NUM> vol. Feeding said gas with a conventional temperature to the second 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 <NUM>% prior to the intermediate absorption of the SO<NUM>. At this <NUM>+<NUM> arrangement, a total desired conversion of ><NUM> % can be achieved.

As an example, a current state of the art <NUM>,<NUM> mtpd (metric ton per day) sulphuric acid plant is fed with <NUM>,<NUM> vol. SO<NUM> feed gas originating from sulphur combustion with air. While the gas flow at <NUM>% load (nameplate capacity) amounts to ~<NUM>,<NUM><NUM>/h, such plant offers a typical turndown ratio of <NUM>% 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 ~<NUM>,<NUM><NUM>/h but operated at say <NUM> vol. -% SO<NUM> instead of traditional <NUM> vol. -% SO<NUM>, would lead to an acid capacity of <NUM>,<NUM> mtpd, instead of the original <NUM>,<NUM> mtpd. Accordingly, an inventive acid plant offers a turndown ratio of ~<NUM>% 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>.

While such specific acid plant size results in only <NUM>/<NUM> ~<NUM> % of the conventional size, the resulting capital cost per installed ton of acid production amounts to only ~<NUM>% of a conventional plant. Thus, a potential <NUM>% cost saving, -compared to conventional technology, makes such plant according to this invention very attractive.

Claim 1:
A process for producing sulphuric acid, in which a feed gas containing sulphur dioxide and oxygen at least partly reacts in a converter featuring at least two catalyst beds to form sulphur trioxide at a given temperature, and in which the produced sulphur trioxide containing gas introduced into an absorber wherein it is further converted to sulphuric acid, characterized in that
- the feed gas to the first catalyst bed is generated by combustion of elemental sulphur with air or oxygen enriched air, and
- said feed gas featuring a sulphur dioxide content between <NUM> and <NUM> vol-%, and
- said feed gas featuring a molar ratio of oxygen to sulphur dioxide of <NUM> < R < <NUM> of O<NUM>/SO<NUM>, and
- the gas leaving the first catalyst bed is cooled and mixed with bulk oxygen or oxygen enriched air prior to entering the subsequent bed, with an amount sufficient to achieve an overall ratio of O<NUM>/SO<NUM> of <NUM> < R < <NUM>, relative to the initial feed gas SO<NUM> concentration to the first catalyst bed and
- no oxygen addition or oxygen enriched air addition is required when below a for a load of a ratio of a nominal concentration of <NUM> vol.-% SO<NUM> to the SO<NUM> 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 ratio.