GB1602108A - Catalytic process - Google Patents

Description

(54) CATALYTIC PROCESS (71) We, BOC LIMITED, of Hammersmith House, London, W6 9DX, England, an English company, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to a catalytic process. The process concerned is the catalytic oxidation of sulphur dioxide to sulphur trioxide. If desired, sulphuric acid may then be formed by absorbing the sulphur trioxide in concentrated sulphuric acid. The invention also relates to the manufacture of sulphuric acid.

A typical existing commercial - plant for the manufacture of sulphuric acid involves the steps of forming gas mixture including sulphur dioxide and oxygen, causing the oxygen and sulphur dioxide to react together by passing the gas mixture at elevated temperatures over successive trays of catalyst until most of the sulphur dioxide has been converted to sulphur trioxide, and then absorbing the sulphur dioxide in concentrated sulphuric acid.

Typically, the sulphur dioxide-oxygen mixture may be formed by passing air through a drier, preheating the dried air to approximately 1001500 C, and using the preheated air to support the combustion of sulphur in a sulphur burner or a sulphur-containing material e.g. pyrites in a roasting furnace.

The resulting gas mixture typically contains from 5 to 13% by volume of sulphur dioxide, about 80% by volume of nitrogen and a balance of oxygen. Because the combustion of sulphur proceeds exothermically, the resulting sulphur dioxide-containing gas mixture leaves the burner at a temperature well in excess of the temperature to which the dried air is preheated. Typically, therefore, it is possible to produce the sulphur dioxide containing gas mixture at a temperature of approximately 1000 to 14000 C. The gas is then cooled, for example in a fire tube or water tube waste heat boiler, to a temperature typically in the range 410 to 4500C, which having regard to the kinetics and equilibrium constant of the reaction is the preferred temperature for forming the sulphur trioxide if a conventional vanadium pentoxide catalyst is used.

As in practice it is not possible to convert all the sulphur dioxide to sulphur dioxide in a single stage, more than one catalytic stage is used. Typically, four or five catalytic stages are used. Typically, in a four stage converter, in the first stage from 50 to 70% by weight of the incoming sulphur dioxide is converted to sulphur trioxide, in the second a further 20 to 30% is converted; in the third another 5 to 10% is converted; and in the fourth and final stage 4 to 7% is converted. If desired, the fourth stage may be split into two parts (i.e. there are two spaced apart catalyst beds in it).

As the oxidation of sulphur dioxide to sulphur trioxide is exothermic the gas leaving each catalytic stage is cooled in a heat exchanger or by dilution with air before it enters the next stage.

The gas leaving the final catalytic stage is passed into a column in which it is absorbed in concentrated sulphuric acid. Unconverted sulphur dioxide and residual oxygen and nitrogen are vented from the absorber.

In order to ensure that higher conversion efficiency is obtained, e.g. to ensure that less than 1% by weight (typically less than 0.5%) of the sulphur dioxide remains unconverted, and thereby to keep down the amount of toxic sulphur dioxide in the vented gas, a part of the gas leaving the second or third stage may, if desired, be passed into a second absorber via the air preheater (thereby providing the source of heat for the preheater). In the second absorber the sulphur trioxide is absorbed in sulphuric acid and the unabsorbed sulphur dioxide containing gas warmed by being passed through the interstage heat exchangers (thereby providing the necessary cooling in these heat exchangers) before being introduced into the third or fourth stage. Such a plant is known as a 'double-absorption' plant.

According to the present invention there is provided a process for the production of sulphuric acid, in which a gas mixture which includes sulphur dioxide and oxygen is formed by the reaction of sulphur Or sulphur-containing materials with air unenriched in oxygen; the gas mixture is at elevated temperature passed successively through at least four catalytic stages so as to form sulphur trioxide by reaction between sulphur dioxide and oxygen; commercial oxygen (as herein defined) or oxygen-enriched air is introduced into the gas mixture at one or more regions downstream of the catalyst in the first catalytic stage and upstream of the catalyst in the final stage; the gas mixture is cooled (a) downstream of the catalyst in each stage and upstream of the catalyst in the next downstream stage by a cooling medium with a gaseous cooling medium into which the commercial oxygen or oxygen-enriched air is introduced, and (b) downstream of the final catalytic stage by a cooling medium separate from the commercial oxygen or oxygenenriched air; and the gas mixture after being cooled downstream of the final stage is passed through a sulphur trioxide absorber so as to form sulphuric acid.

By the term 'commercial oxygen' as used herein is meant a gas containing at least 90% by volume of molecular oxygen. Preferably, commercial oxygen containing less than 1% by volume of impurities is used.

The gas mixture passing out of the final catalytic stage can be absorbed in sulphuric acid so as to form sulphuric acid of chosen concentration. Typically, concentrated sulphuric acid may be produced.

The process according to the present invention is particularly suitable for operation on an existing plant for producing sulphuric acid. The existing plant may be of the singleabsorption or double-absorption type. By 'existing plant' as used herein is meant a plant which was originally designed to be operated conventionally and not for operation of the process according to the invention.

The introduction of the commercial oxygen or oxygen-enriched air increases the concentration of oxygen in the gas mixture entering the or each selected stage. This has the consequence of increasing the concentration of sulphur trioxide in the gas mixture over the catalyst immediately downstream of where the commercial oxvgen or oxygen-enriched air is introduced. We believe that it is therefore possible to decrease the proportion of unconverted sulphur dioxide in the gas mixture leaving the final catalytic stage. Thus the gas vented from the process will contain less sulphur dioxide.

The cooling of the gas mixture after each stage may be effected in a heat exchanger.

Alternatively, after the second and subsequent stages the cooling may be effected by diluting the gas mixture with air.

Typically, the incoming gas mixture will contain from, for example, 5 to 13% by volume of sulphur dioxide, a volume of oxygen in excess of the stoichiometric quantity required and a balance of diluent gas (typically nitrogen).

Preferably, at least some of the commercial oxygen or oxygen-enriched air is added to the gas mixture downstream of the catalyst in the first catalytic stage and upstream of the catalyst in the second catalytic stage. We believe that addition of such oxygen at this location rather than between later stages achieves a better utilisation of the oxygen owing to the longer residence time in the catalytic converter and the higher local partial pressure of oxygen that can thereby be achieved. However, on occasions, addition of such oxvgen between the first two stages may give rise to an intolerable temperature rise. Thus, in some instances, at least some of the commercial oxygen or oxygen-enriched air may be added to the gas mixture downstream of the catalyst in the second stage and upstream of that in the third.In addition, or alternatively, it is possible to add the commercial oxygen downstream of the catalyst in the third stage and upstream of the catalyst in the fourth stage. However, adding the oxygen to the gas mixture between the third and fourth stages is, we believe, a less efficient way of utilising it than is addition of such oxygen between earlier stages, particularly if the sulphur dioxide concentration in the incoming gas mixture is above 8% by volume.

If the source of the incoming gas is a sulphur burner (as distinct from an ore roaster) the advantage of employing the commercial oxygen between the first and second catalytic stages or between the second and third catalytic stages, in comparison to between the third and fourth stages becomes progressively greater with increasing sulphur dioxide concentration in the incoming gas mixture.

Sulphur burners typically produce a gas mixture in which the total concentration nf sulphur dioxide shall produce a corresponding decrease in the concentration of oxygen, and this decrease in oxygen concentration is we believe a significant factor lying behind the attainment of the advantage mentioned in the preceding sentence.

If the commercial oxygen or oxygenenriched air is added between the catalyst in the first and the catalyst in the second stage and/or between the catalyst in the second and the catalyst in the third stage, the ratio of weight of molecular oxygen so added per unit time to the weight of sulphuric acid produced in the same way may typically be in the range 10:100 to 25:100.

As aforementioned, we believe that the benefits of adding the commercial oxygen or oxygen-enriched air to the gas mixture tend to become proportionately greater with increasing proportion of sulphur dioxide in the gas mixture entering the first catalytic stage.

Preferably, the gas mixture entering the first catalytic stage contains at least 8 or 9% by volume of sulphur dioxide. Often, it will be preferred that the gas mixture entering the first catalytic stage contains from 8 or 9 to 13% by volume of sulphur dioxide.

If oxygen-enriched air is used its concentration of oxygen is preferably as large as possible as the addition of nitrogen admixed with the oxygen is not advantageous.

The process according to the present invention may be used to increase the proportion of sulphur dioxide converted to sulphur trioxide in an existing plant for producing sulphuric acid, including a reactor in which a gas mixture including sulphur dioxide and oxygen is formed by the reaction of sulphur or sulphur-containing materials with air un enriched in oxygen; at least four catalytic conversion stages for forming sulphur trioxide by reaction between sulphur dioxide and oxygen; means for cooling the gas mixture at a region downstream of the catalyst in each stage and upstream of the catalyst in the next downstream stage by a cooling medium; means for cooling the gas mixture downstream of the final stage, and an absorber for absorbing sulphur trioxide and thereby forming sulphuric acid from the gas mixture after it has been cooled downstream of the final stage.

Depending on the ratio of sulphur dioxide to oxygen in the incoming gas mixture, the conversion that can be achieved is typically at least 98% by volume. Indeed, the total efficiency of conversion of sulphur dioxide to sulphur trioxide is preferably maintained at or increased to 98% by weight or above.

In general, the greater the ratio of sulphur dioxide to oxygen in the incoming gas mix more, the more commercial oxygen or oxygenenriched air will need to be added to the gas mixture to enable a given percentage conversion of the incoming sulphur dioxide to take place. Moreover, as aforementioned, the greater the ratio the more desirable it becomes to add the commercial oxygen or oxygen-entiched air between the respective catalvst lavers of the first and second stage (or somewhat less preferably between the respective catalyst layers of the second and third stages) rather than downstream of the catalyst in the third stage and upstream of the catalyst in the fourth.

The rate of production of sulphuric acid on an existing single absorption plant for producing sulphuric acid may be increased bv adding commercial oxvgen (as herein defined) or oxvgen-enriched air to the gas mixture downstream of the catalyst in the first stage and upstream of the catalyst ill the final stage, the plant thereby performing the process according to the invention, increasing the proportion of sulphur dioxide in the gas mixture entering the first catalyst stage from what it is when the plant is operated conventionally (i.e. without addition of commercial oxygen or oxygenenriched air), and employing less catalyst in the first catalytic stage than is employed when the plant is operated conventionally.

Typically, the oxygen enriched air or commercial oxygen is added between the first and second stages. The increase in the concentration of the sulphur dioxide and the use of less catalyst in the first stage are preferably balanced so as to keep the first stage temperature the same as it is when the plant is operated conventionally. Typically, the sulphur dioxide concentration may be increased to a maximum of 16% by volume; and typically the amount of catalyst used in the first stage may be up to 50% by weight less. The amount of catalyst used in the first stage may be chosen so as to keep the temperature of the gas mixture immediately downstream of the catalyst to below 6000 C.

Desirably the total amount of catalyst and catalyst support material in the first stage remains the same, so as not to alter substantially the heat distribution across the catalyst and supporting material in the first stage.

In most conventional single absorption plant the catalyst tends to deteriorate leading to an increase in the pressure drop in the converter and a reduction in the active surface area of the catalyst. In practice, most of this deterioration (in single absorption plants) takes place in the first stage. The commercial oxygen or oxygen-enriched air may thus be added to an existing single absorption plant so as to counteract this deterioration which leads to progressively lower production of sulphuric acid, or to increased emission of sulphur dioxide. Thus the commercial oxygen or oxygen-enriched air may be added so as to reduce or eliminate any drop in production and/or increase the period of time that the plant is operated before it is shut down to enable the catalyst in the first layer to be changed.Since the deterioration of the catalyst in the first layer is progressive it is desirable to increase pro gressively the proportion of commercial oxygen or oxygen-enriched air that is added.

In starting up a conventional plant from cold a direct or indirect heater is generally used to supply hot gas to raise the temperature of the catalyst layers to above their ignition temperature. The hot gas may contain water vapour which needs to be removed from the process before entering the converter. As a consequence, the sulphuric acid in the 'start up' storage tanks becomes increasingly diluted. However, by adding oxygen or oxygen-enriched air to the gas mixture in accordance with the invention the ignition temperature in the second to fourth stages may be reduced. Therefore, the start-up time may correspondingly be reduced. Hence there mav be less dilution of the start-up sulphuric acid than in a conventionally-operated process.

In addition, we believe that in start-up of conventional plants there is a risk of increased emission of sulphur dioxide.

rn the process according to the invention the catalytic stages may be operated slightly above atmospheric pressures as in a conventional process, or at more elevated pressures for example, 6 bars. Operation of known processes at elevated pressures offers possible advantages of an increase in the efficiency of sulphur dioxide conversion and a reduction in the converter size. However, it is to be appreciated that the process according to the invention offers these advantages when the catalytic stages are operated at pressures only slightly above atmospheric.

The process according to the invention may be operated on a double absorption plant for producing sulphuric acid. Such a plant has an interstage absorber in addition to that in communication with the outlet of the final stage. Typically the inlet to the interstage absorber communicates with an outlet for the penultimate stage and the outlet of the inter-stage absorber communicates with the final stage upstream of the catalyst therein.With such a plant, it is desirable to add the commercial oxygen or oxygenenriched air to the gas mixture downstream of the catalyst in the aforesaid penultimate stage and upstream of the catalyst in the final stage in order to achieve or more of the goals of increasing conversion efficiency or production, or compensating for deterioration of the catalyst in the last stage (such deterioration may be relatively marked owing to a mist of acid being entrained in the gas mixture leaving the interstage absorber).

The process according to the invention will now be described by way of example with reference to the accompanying drawings, of which: Figure 1 is a schematic flow diagram illustrating a single absorption plant with interstage heat exchangers for the manufacture of sulphuric acid, and Figure 2 is a schematic flow diagram illustrating a single absorption plant with interstage air cooling for the manufacture of sulphuric acid.

Referring to Figure 1 of the drawings, a mixture of sulphur dioxide and air unenriched in oxygen is passed into a catalyst reactor 2 via an inlet line 1. The mixture of sulphur dioxide and air may be formed by passing air into a roasting furnace (not shown) in which suiphur-containing materials are burnt.

In the reactor 2 are provided four catalytic stages for reacting the sulphur dioxide with the oxygen in the air to form sulphur trioxide. These stages are indicated by references 4, 6, 8 and 10. In each stage are trays 12 of suitable heterogeneous catalyst. The catalyst will typically be vanadium pentoxide supported on a conventional support material.

The trays of catalyst in each successive stage are separated from one another by means of partitions 14. The partitions prevent gas that has passed over the catalyst in one stage from entering the next stage without passing through a heat exchanger.

The first stage has an outlet 16 downstream of the catalyst in that stage. The outlet 16 terminates in a heat exchanger 18.

The heat exchanger 18 has passages in communication with an inlet 20 to the second stage, the inlet 20 being situated upstream of the catalyst in that stage. The second stage also has an outlet 22 downstream of the catalyst in that stage. The outlet 22 terminates in a heat exchanger 24 which has heat exchange passages communicating with an inlet 26 to the third stage 8, the inlet 26 being situated upstream of the catalyst in that stage. The third stage 6 also has an outlet 28 downstream of the catalyst in that stage. The outlet 28 communicates with the heat exchanger 30. Also in communication with the heat exchanger 30 is an inlet 32 situated upstream of the catalyst in the fourth stage 10. Downstream of the catalyst in the fourth stage 10 is an outlet line 34 which communicates with a heat economiser 36. The economiser 36 has an inlet 37 and an outlet 38 typically for steam.The economiser 36 is constructed so that the gas passing through the outer line 34 does not come into contact with the water but instead heats it by indirect heat exchange. The economiser 36 has an outlet in communication with the bottom region of an absorption column 44. The absorption column 44 has an inlet 46 for recirculated concentrated sulphuric acid. The inlet 46 terminates in a sprayer 48 situated near the top of the column 44. In communication with the bottom of the column 44 is an outlet product 50 for sulphuric acid. At the top of the column 44 is an outlet 52 for unabsorbed gas.

In order to provide cooling for gas passing through the heat exchangers 18, 24, and 30 between respective stages of the reactor 2, passages 53, 54 and 56 are provided. The passages 53 and 54 have a common outlet 58. Preferablv, the cooling fluid in the heat exchangers 18 and 24 is steam from the economiser 36. This steam is superheated as it passes through the exchangers 18 and 24 and the resulting superheated steam may be used for example, to drive a blower used to supply air to a burner (not shown) in which the gas mixture entering the line 1 is formed.

The coolant for the heat exchanger 30 may either be steam from the economiser 36 or air to be preheated before being passed into the roasting furnace.

Means are provided for adding commercial oxvgen or oxygen-enriched air to the gas mixture between the first stage 4 and the second stage 6 of the reactor 2 and between the second stage 6 and the third stage 8 of the converter. Thus, oxygen supply pipelines 60 and 62 extend from a source (not shown) of commercial oxygen and terminate in the inlets 20 and 26 to the second and third stages respectively. Adding the commercial oxygen at such locations ensures good mixing of the added oxygen with the sulphur dioxide containing gas mixture. It is possible, however, to add the commercial oxygen or oxygen-enriched air to the outlets 16 and 22 respectively or to the gas mixture immediately upstream of the respective outlets 16 and 22.

It is also possible to add commercial oxygen to the gas mixture through the pipeline 64.

In typical operation, a gas mixture consisting essentially of sulphur dioxide, oxygen and nitrogen is passed into the line 1 and enters the first stage 4 of the reactor 2. This gas is cooled to a temperature of 420"C before it enters the stage 4. As the gas passes over the catalvst 12 in the stage 4 so the sulphur dioxide reacts with the oxygen to form sulphur trioxide. This is an exothermic reaction and the temperature of the gas leav ing the first stage 4 via the outlet 16 may typically be 6000 C. Tvpically, 55% of the incoming sulphur dioxide may be converted to sulphur trioxide in the first stage 4.

The heat of reaction is removed from the gas mixture in the heat exchanger 18. The gas mixture may be returned to the second stage 6 of the reactor 2 at a temperature of 4200C. Commercial oxygen (substantially pure) is added to the gas mixture as it leaves the heat exchanger 18. Further conversion of the remaining sulphur dioxide takes place as the gas flows over the catalyst 12 in the second stage 6 of the reactor 2. Thus, a further 21.5% of the original sulphur dioxide may typically be converted to sulphur trioxide in the second stage 6. The gas leaving the second stage may typically have a temperature of 555 to 560"C, if the commercial oxygen is not added the temperature may typically rise to 550"C only, and only 20% of the in coming sulphur dioxide may typically be converted in the second stage.It will be noted that this temperature is lower than the exit temperature of the gas leaving the first stage.

This is because less sulphur trioxide is formed in the second stage therefore less heat is evolved.

In the heat exchanger 24 the heat of reaction is removed from the gas leaving the second stage 6. The gas is returned to the third stage 8 at a temperature of 4500 C. A further 15% of the original sulphur dioxide may typically be converted to sulphur trioxide as it flows over the catalyst in the third stage 8.

Typically, the gas leaving the third stage 8 has a temperature in the order of 465"C.

In the heat exchanger 30 the heat of reaction is removed from the gas leaving the third stage. This gas is then returned to the fourth stage upstream of the catalyst 12 in that stage, typically at a temperature of 450"C. The gas mixture then passes over the catalyst 12 in the fourth stage 10. In the fourth stage almost all the residual sulphur dioxide is converted to sulphur trioxide. The thus formed sulphur trioxide containing gas mixture leaves the reactor 2 typically at a temperature of 4550C through the line 34 and enters the economiser 36 in which it causes water to boil, the resultant steam/ water mixture being used to provide cooling in the heat exchanger 24 and 18. The sulphur trioxide containing gas is then passed into the absorption column 44 through the inlet 42.

Concentrated sulphuric acid consisting of approximately 98.5% by volume of sulphuric acid is sprayed into the column 44 from the sprayer 48. The sulphur trioxide is absorbed by this concentrated sulphuric acid and the product of sulphuric acid is formed. The sulphuric acid is withdrawn from the absorption column through the outlet 50. Unabsorbed gas which will consist mainly of nitrogen with some oxygen and traces of unreacted sulphur dioxide is vented through the outlet 52.

Referring now to Figure 2 of the drawings, air unenriched in oxygen is drawn into a compressor 72 through a pipeline 70. It is compressed in the compressor 72 to a pressure typically of 4 psig (1.3 atmospheres). The compressed air passes from the compressor 72 into a sulphur burner via a pipeline 74.

Sulphur is passed into the burner 78 via a line 76. The burner operates at a temperature of 11000C and typically produces a gas mixture containing 10% by volume of sulphur dioxide and 10.9% by volume of oxygen, substantially all the remainder of the gas being nitrogen. This gas mixture passes out of the burner 78 into a waste heat boiler 82 via a pipe 80. In the waste heat boiler 82 the gas is cooled by heat exchange of water supplied through a pipe 84, the water being boiled and the resultant steam leaving the boiler via the pipe 86. Typically, the gas mixture is cooled to a temperature of 4200 C.

The cooled gas leaves the waste heat boiler 82 and passes through a pipeline 88 into a catalytic reactor or converter 90 which has four successive catalytic layers 92, 94, 96 and 98. A vanadium pentoxide catalyst may typically be used. As the gas mixture flows over the catalyst 92 so some of the sulphur dioxide reacts with some of the oxygen to form sulphuric trioxide. This is an exothermic reaction so that the temperature of the gas mixture after it has passed over the first catalyst is greater than it is before it passes over the first catalyst 92.

Typically, the temperature may be raised to a value in the range 590 to 6000C depending on, for example, the composition of the gas mixture. The gas mixture after passing over the first catalyst layer 9 leaves the converter via an outlet 100 and enters a heat exchanger 102 in which at least part of the heat of reaction is removed from the gas mixture by indirect heat exchange with a fluid such as water or steam, supplied through the pipeline 106 and leaving the heat exchanger 102 as superheated steam through pipeline 108. The cooled gas mixture is reutmed to a region just upstream of the second catalyst layer 94 via an inlet 104 to the converter. The pipeline 134 in commune cation with a source (not shown) of commercial oxygen or oxygen-enriched air terminates in the - inlet 104.Thus, the commercial oxygen or oxygen-enriched air may be added to the gas mixture as it passes through the inlet 104. The thus, oxygenenriched gas mixture then passes over the second catalyst layer 94, in which a further portion of the sulphur dioxide in the original gas mixture reacts with oxygen to form more sulphur trioxide. Typically, the temperature of the gas mixture is increased to, say, 520 to 530 C by passing over the second catalyst 94. After it has passed over the second catalyst layer 94 the gas mixture is cooled by having air mixed with it. The air is taken from the pipeline 74 by means of a pipe 110 which terminates in the converter 90 at a region downstream of the second catalyst layer 94 and upstream of the third catalyst 96.A pipeline 136 which communicates with a source (not shown) of commercial oxygen or oxygen-enriched air terminates in the pipeline 110. This enables the air passing through the line 110 to be enriched in oxygen, and, therefore, more importantly, to enrich the gas mixture in oxygen. The air added via the pipeline 110 cools the gas mixture after it has passed over the second catalyst layer 94.

Typically, the gas mixture is cooled to a value in the range 420 to 435"C.

As the cooled gas mixture passes over the third catalytic layer 96, so more of the original sulphur dioxide is converted to sulphur trioxide by reaction with oxygen.

Since the gas mixture is diluted by addition of the air via the pipeline 110, the temperature rise in this stage is much less marked than it is in the previous 2 stages. Typically, the temperature may rise about 30 to 400 C.

Typically, the heat of reaction may be removed from the gas downstream of the third catalyst layer 96 and upstream of the fourth catalyst layer 98 by passing air into this mixture from the pipeline 112, which pipeline communicates with the pipeline 75. Typically, the gas mixture may be cooled to a temperature in the range 435 to 4450 C. Commercial oxygen is added via a pipeline 138 to the air flowing through the pipeline 112, and hence to the gas mixture. The gas mixture then flows over the fourth catalytic layer 98 where a further portion of the original sulphur dioxide is converted to sulphur trioxide by reaction with oxygen.

Typically, about 98% by volume of the original sulphur dioxide is converted to sulphur trioxide as a result of the reaction over the 4 catalytic layers.

The gas leaves the converter typically at a temperature in the order of 440 to 4500 C, the temperature rise over the fourth catalytic layer 98 typically being only about SOC. From the converter 90 the gas mixture containing sulphur trioxide oxygen nitrogen and a small proportion of sulphur dioxide is conducted along a pipeline 114 into an economiser 116 in which the gas mixture is cooled typically to a temperature in the range 80 to 1200C by indirect heat exchange with steam which enters the economiser 116 through a pipe 118 and leaves through a pipe 120.

The cooled sulphur trioxide containing gas mixture then passes via pipeline 122 into an absorber 124 in which it is contacted with a downflowing stream of concentrated sulphuric acid sprayed into the absorber 124 via a sprayer 126 which communicates via a pipeline 128 with a source (not shown) of concentrated sulphuric acid. In consequence, all the sulphur trioxide is absorbed in the sulphuric acid, thereby increasing the concentration of the sulphuric acid. The concentrated sulphuric acid passes out of the absorber 124 through an outlet 132 which communicates with dilution and storage plants. The unabsorbed gas consisting of oxygen and nitrogen together with the small proportion of unconverted sulphur dioxide is vented to the atmosphere via the pipeline 130.

The invention is further illustrated by the following examples.

Examples 1A-1H.

These examples (see Tables 1 and 2) relate to operation of the plant shown in Figure 1.

The Examples show a number of things.

First, an addition of commercial oxygen (approx. 100% pure) to the incoming gas mixture makes possible higher conversion efficiencies than can be achieved when it is not added.

Second, the quantity of commercial oxygen required to obtain the same efficiency of conversion is greater if the oxygen is added downstream of the third stage catalyst and upstream of the fourth stage catalyst than if it is added downstream of the second stage catalyst and upstream of the third stage catalyst Similarly, the quantity of commercial oxygen required to obtain the same efficiency of conversion is greater if the oxygen is added downstream of the third stage catalyst than if the oxygen is added downstream of the first stage catalyst and upstream of the second stage catalyst. Thus, comparatively more oxygen needs to be added with increasing SO2/SO2 ratio inthe incoming gas mixture to achieve or approach a given conversion efficiency.

Fourth, addition of the oxygen between the catalyst of third and the catalyst of the fourth stage becomes even less favourable with decreasing SO202 ratio.

Examples 2A to 2B.

These examples relate to operation of the plant shown in Figure 2.

Example 2A.

The operation of the plant without addition of commercial oxygen or oxygen-enriched air is carried out in the manner illustrated by Table 3.

Example 2B.

The plant is uprated by adding commercial oxygen (c 100% pure) to the gas mixture, at a region downstream of the first catalyst and upstream of the second catalyst and at a region downstream of the second catalyst and upstream of the third catalyst.

In addition to adding the commercial oxygen, some of the first catalyst was removed and replaced with an equivalent volume of 'dummy' catalyst consisting of catalyst support material unimpregnated with catalyst.

Moreover, the proportion of sulphur dioxide in the incoming gas mixture is increased and the proportion of oxygen decreased.

The operation of the plant is illustrated by Table 4.

Example 2C.

The plant is uprated in substantially the same manner as described in respect of Example 2B. However, the commercial oxygen is all added between the third and fourth stages. To achieve the same uprating as in Example 2B a further 3100 tonnes per annum would be required. The operation of the plant is illustrated by Table 5.

Examples 3A and 3B.

Example 3A.

Referring to Example 2A, there is a tendency for the maximum possible production rate to fall. After the catalyst has aged two years, the plant being operated continuously during this period, the maximum possible production may typically have fallen to 1,500 tonnes per week, and the temperatures before and after each catalytic stage may have undergone a corresponding fall as shown in Table 6.

Example 3B.

The production of the plant referred to in Example 3A is restored to 2000 tonnes per week by adding 195 tonnes per week of oxygen to the gas mixture downstream of the first catalyst and upstream of the second catalyst. This corresponds to an increase of 50% by volume in oxygen concentration in the inlet to the second stage. The addition of the oxygen has the effect on the temperatures of the gas mixture as shown in Table 7. TABLE 1

Example IA IB IC ID Commercial oxygen Commercial oxygen Commercial oxygen (c. 100% pure) (c. 100% pure) (c. 100% pure) added between added between added between No commercial 1st and 2nd 2nd and 3rd 3rd and 4th oxygen added stage catalysts stage catalysts stage catalysts % by volume of SO2 in incoming mixture 8.0 8.0 8.0 8.0 % by volume of O2 in incoming mixture 13.0 13.0 13.0 13.0 Temperature of gas mixture in C:: just before 1st stage catalyst 420 420 420 420 just after 1st stage catalyst 595 595 595 595 just before 2nd stage catalyst 425 425 425 425 just after 2nd stage catalyst 500 500 500 500 just before 3rd stage catalyst 435 435 435 435 just after 3rd stage catalyst 450 452 453 450 just before 4th stage catalyst 425 425 425 425 just after 4th stage catalyst 430 430 431 433 % by weight of incoming SO2 converted to SO3 98.2 98.6 98.6 98.6 Commercial oxygen required to achieve conversion expressed as tonne of commercial O2 per tonne of H2SO4 0.12 0.14 0.15 TABLE 2

Example IE IF IG IH Commercial oxygen Commercial oxygen Commercial oxygen (c. 100% pure) (c. 100% pure) (c. 100% pure) added between added between added between No commercial 1st and 2nd 2nd and 3rd 3rd and 4th oxygen added stage catalysts stage catalysts stage catalysts % by volume of SO2 in incoming gas mixture 12.0 12.0 12.0 12.0 % by volume of O2 in incoming gas mixture 9.0 9.0 9.0 9.0 Temperature of gas mixture in C:: just before 1st stage catalyst 420 420 420 420 just after 1st stage catalyst 600 600 600 600 just before 2nd stage catalyst 435 435 435 435 just after 2nd stage catalyst 520 520 520 520 just before 3rd stage catalyst 435 435 435 435 just after 3rd stage caralyst 460 464 470 460 just before 4th stage caralyst 425 425 425 425 just after 4th stage catalyst 432 433 435 437 % by weight of incoming SO2 converted to SO3 97.8 98.3 98.3 98.3 Commercial oxygen required to achieve conversion (expressed as tonne of commercial O2/tonne of H2SO4) 0.20 0.22 0.32 TABLE 3

Rate of production of H2 SO4 at full capacity 100 000 tonnes /annum Proportion of SO2 in incoming gas mixture 10 % by volume Proportion of O2 in incoming gas mixture 10.9 NO by volume Effective SO2 concentration 10 : 7.5 Gas Temperature in OC: just before 1st catalyst 420 just after 1st catalyst 595 just before 2nd catalyst 425 just after 2nd catalyst 520 just before 3rd catalyst 435 * just after 3rd catalyst * just before 4th catalyst just after 4th catalyst 435 Relative quantities of catalyst (by weight) 1st catalyst 1.00 2nd catalyst 1.29 3rd catalyst 1.57 4th catalyst 2.27 % age conversion of SO2 98.3 % by weight * Temperature not normally measured.

TABLE 4

Rate of production of H2 SO4 at updated capacity 117 000 tonnes /annum Proportion of SO2 in incoming gas mixture 11.7 % by volume Proportion of O2 in incoming gas mixture 9.3 % by volume Effective SO2 concentration 11.7 : 8.8 Gas Temperature in OC:: Just before ist catalyst 420 Just after 1st catalyst 600 Just before 2nd catalyst 418 Just after 2nd catalyst 526 Just before 3rd catalyst 420 Just after 3rd catalyst 452 Just before 4th catalyst 445 Just after 4th catalyst 449 Relative quantities of catalyst (by weight) 1st catalyst 0.79 (+ 0.21 dummy) 2nd catalyst 1.29 3rd catalyst 1.57 4th catalyst 2.29 Rate of addition of commercial oxygen (c. 100% pure) between 1st and 2nd catalysts) 3570 tonnes per annum Rate of addition of commercial oxygen (c. 100% pure) between 2nd and 3rd catalysts 5780 tonnes per annum Total rate of addition of commercial oxygen --9350 tonnes per annum % age conversion of SO2 98.3 % by weight TABLE 5

Rate of production of H2 SO4 at uprated capacity 117,000 tonnes/annum Proportion of SO2 in incoming gas mixture 11.7 % vol Proportion of O2 in incoming gas mixture 9.3 % vol Effective SO2 concentration (air dilution) 11.7 : 8.8 Gas temperature in C Just before 1st catalyst 420 Just after 1st catalyst 600 Just before 2nd catalyst 418 Just after 2nd catalyst 520 Just before 3rd catalyst 420 Just after 3rd catalyst 445 Just before 4th catalyst 435 Just after 4th catalyst 453 Relative quantities of catalyst (by weight) 1st catalyst 0.79 (+0.21 dummy) 2nd catalyst 1.29 3rd catalyst 1.57 4th catalyst 2.29 Total rate of addition of commercial oxygen 12,450 tonnes/annum (c. 100% pure) between 3rd and 4th catalyst % age conversion of SO2 98.3 % TABLE 6

Originally After two years Temperature of gas mixture in C:: Just before first catalyst 420 420 Just after first catalyst 595 1 590 Just before second catalyst 425 425 Just after second catalyst 520 518 Just before third catalyst 435 435 * Just after third catalyst * Just before fourth catalyst Just after fourth catalyst 435 435 * Temperature not normally measured.

TABLE 7

Temperature (in C) of gas mixture: Just before first catalyst 420 Just after first catalyst 590 Just before second catalyst 425 Just after second catalyst 522 Just before third catalyst 435 * Just after third catalyst * Just before fourth catalyst Just after fourth catalyst 436 * Temperature not normally measured.

The term 'effective SO2 concentration' as used herein is defined as the ratio xy x+z where x = the (volumetric) flow rate of gas mixture passing from the sulphur burner into the catalytic converter.

y = the percentage (by volume) of sulphur dioxide in the gas mixture passing from the sulphur burner into the catalytic converter.

z = the (volumetric) flow rate of dilation air into the catalytic converter.

WHAT WE CLAIM IS: 1. A process for the production of sulphuric acid, in which a gas mixture which includes sulphur dioxide and oxygen is formed by the reaction of sulphur or sulphur-containing materials with air unenriched in oxygen; the gas mixture is at elevated temperature passed successively through at least four catalytic stages so as to form sulphur trioxide by reaction between sulphur dioxide and oxygen; commercial oxygen (as hereinbefore defined) or oxygen-enriched air is introduced into the gas mixture at one or more regions downstream of the catalyst in the first catalytic stage and upstream of the catalyst in the final stage; the gas mixture is cooled (a) downstream of the catalyst in each stage and upstream of the catalyst in the next downstream stage by a cooling medium separate from the commercial oxygen or oxygenenriched air or by being mixed with a gaseous cooling medium into which the commercial oxygen or oxygen-enriched air is introduced, and (b) downstream of the final catalytic stage by a cooling medium separate from the commercial oxygen or oxygen-enriched air; and the gas mixture after being cooled downstream of the final stage is passed through a sulphur trioxide absorber so as to form sulphuric acid.

2. A process as claimed in claim 1, in which the said gaseous cooling medium is air.

3. A process as claimed in claim 1 or claim 2, in which at least some of the commercial oxygen or oxygen-enriched air is added to the gas mixture downstream of the catalyst in the first catalytic stage and upstream of the catalyst in the second catalytic stage.

4. A process as claimed in claim 3, in which the commercial oxygen or oxygenenriched air is added to the gas mixture at a rate such that the ratio of the weight of molecular oxygen thereby added per unit time to the weight of sulphuric acid produced in the same unit time is in the range 10:100 to 25:100.

5. A process as claimed in any one of the preceding claims, in which at least some of the commercial oxygen or oxygen-enriched air is added to the gas mixture downstream of the catalyst in the second catalytic stage and upstream of the catalyst in the third catalytic stage.

6. A process as claimed in any one of the preceding claims, in which the source of the gas mixture is a sulphur burner.

7. A process as claimed in any one of the preceding claims in which the gas mixture entering the first stage contains at least 8% by volume of sulphur dioxide.

8. A process as claimed in claim 7, in which the gas mixture entering the first stage contains from 8 to 13% by volume of sulphur dioxide.

9. A process as claimed in any one of the preceding claims, in which the total efficiency of conversion of sulphur dioxide to sulphur trioxide is 98% by weight or above.

10. A method of -increasing the proportion of sulphur dioxide converted to sulphur trioxide in an existing plant for producing sulphuric acid including a reactor in which a gas mixture including sulphur dioxide and oxygen is formed by the reaction of sulphur or sulphur-containing materials with air unenriched in oxygen; at least four catalytic conversion stages for forming sulphur trioxide by reaction between sulphur dioxide and oxygen; means for cooling the gas mixture at a region downstream of the catalyst in each stage and upstream of the catalyst in the next downstream stage by a cooling medium; means for cooling the gas mixture downstream of the final stage, and an absorber for absorbing sulphur trioxide and thereby forming sulphuric acid from the gas mixture after it has been cooled downstream of the final stage, which method comprises adding commercial oxygen or oxygen-enriched air to the gas mixture downstream of the catalyst in the first stage but upstream of the catalyst in the final stage so as to perform the process claimed in any one of the preceding claims.

11. A method as claimed in claim 10, in which the proportion of sulphur dioxide converted to sulphur trioxide is increased from below 98% by weight to 98% by weight or more.

12. A method of increasing the rate of production of sulphuric acid on an existing single absorption plant for producing sulphuric acid,

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (21)

**WARNING** start of CLMS field may overlap end of DESC **. The term 'effective SO2 concentration' as used herein is defined as the ratio xy x+z where x = the (volumetric) flow rate of gas mixture passing from the sulphur burner into the catalytic converter. y = the percentage (by volume) of sulphur dioxide in the gas mixture passing from the sulphur burner into the catalytic converter. z = the (volumetric) flow rate of dilation air into the catalytic converter. WHAT WE CLAIM IS:

1. A process for the production of sulphuric acid, in which a gas mixture which includes sulphur dioxide and oxygen is formed by the reaction of sulphur or sulphur-containing materials with air unenriched in oxygen; the gas mixture is at elevated temperature passed successively through at least four catalytic stages so as to form sulphur trioxide by reaction between sulphur dioxide and oxygen; commercial oxygen (as hereinbefore defined) or oxygen-enriched air is introduced into the gas mixture at one or more regions downstream of the catalyst in the first catalytic stage and upstream of the catalyst in the final stage; the gas mixture is cooled (a) downstream of the catalyst in each stage and upstream of the catalyst in the next downstream stage by a cooling medium separate from the commercial oxygen or oxygenenriched air or by being mixed with a gaseous cooling medium into which the commercial oxygen or oxygen-enriched air is introduced, and (b) downstream of the final catalytic stage by a cooling medium separate from the commercial oxygen or oxygen-enriched air; and the gas mixture after being cooled downstream of the final stage is passed through a sulphur trioxide absorber so as to form sulphuric acid.

2. A process as claimed in claim 1, in which the said gaseous cooling medium is air.

3. A process as claimed in claim 1 or claim 2, in which at least some of the commercial oxygen or oxygen-enriched air is added to the gas mixture downstream of the catalyst in the first catalytic stage and upstream of the catalyst in the second catalytic stage.

4. A process as claimed in claim 3, in which the commercial oxygen or oxygenenriched air is added to the gas mixture at a rate such that the ratio of the weight of molecular oxygen thereby added per unit time to the weight of sulphuric acid produced in the same unit time is in the range 10:100 to 25:100.

5. A process as claimed in any one of the preceding claims, in which at least some of the commercial oxygen or oxygen-enriched air is added to the gas mixture downstream of the catalyst in the second catalytic stage and upstream of the catalyst in the third catalytic stage.

6. A process as claimed in any one of the preceding claims, in which the source of the gas mixture is a sulphur burner.

7. A process as claimed in any one of the preceding claims in which the gas mixture entering the first stage contains at least 8% by volume of sulphur dioxide.

8. A process as claimed in claim 7, in which the gas mixture entering the first stage contains from 8 to 13% by volume of sulphur dioxide.

9. A process as claimed in any one of the preceding claims, in which the total efficiency of conversion of sulphur dioxide to sulphur trioxide is 98% by weight or above.

10. A method of -increasing the proportion of sulphur dioxide converted to sulphur trioxide in an existing plant for producing sulphuric acid including a reactor in which a gas mixture including sulphur dioxide and oxygen is formed by the reaction of sulphur or sulphur-containing materials with air unenriched in oxygen; at least four catalytic conversion stages for forming sulphur trioxide by reaction between sulphur dioxide and oxygen; means for cooling the gas mixture at a region downstream of the catalyst in each stage and upstream of the catalyst in the next downstream stage by a cooling medium; means for cooling the gas mixture downstream of the final stage, and an absorber for absorbing sulphur trioxide and thereby forming sulphuric acid from the gas mixture after it has been cooled downstream of the final stage, which method comprises adding commercial oxygen or oxygen-enriched air to the gas mixture downstream of the catalyst in the first stage but upstream of the catalyst in the final stage so as to perform the process claimed in any one of the preceding claims.

11. A method as claimed in claim 10, in which the proportion of sulphur dioxide converted to sulphur trioxide is increased from below 98% by weight to 98% by weight or more.

12. A method of increasing the rate of production of sulphuric acid on an existing single absorption plant for producing sulphuric acid,

including a reaction in which a gas mixture including sulphur dioxide and oxygen is formed by the reaction of sulphur or sulphurcontaining materials with air unenriched in oxygen; at least four catalytic conversion stages for forming sulphur trioxide by reaction between sulphur dioxide and oxygen; means for cooling the gas mixture at a region downstream of the catalyst in each stage and upstream of the catalyst in the next downstream stage by a cooling medium; means for cooling the gas mixture downstream of the final stage, and an absorber for cooling sulphur trioxide and thereby forming sulphuric acid from the gas mixture after it has been cooled downstream of the final stage, which method comprises adding commercial oxygen (as hereinbefore defined) or oxygen-enriched air to the gas mixture downstream of the catalyst in the first stage and upstream of the catalyst in the final stage so as to perform the process claimed in any one of claims 1 to 9, increasing the proportion of sulphur dioxide in the gas mixture entering the first catalytic stage from what it is when the plant is conventionally, and employing less catalyst in the first catalytic stage than is employed when the plant is operated conventionally.

13. A method as claimed in claim 12, in which the proportion of sulphur dioxide in the gas mixture entering the first catalytic stage is increased to a maximum of 16% by volume.

14. A method as claimed in claim 12, in which up to 50% by weight less catalyst is employed in the first stage than is employed when the plant is operated conventionally.

15. A method as claimed in any one of claims 12 to 14, in which the total amount of catalyst and catalyst support material in the first catalytic stage is the same as when the plant is operated conventionally.

16. A method of compensating (so far as rate of production of sulphuric acid is concerned) for deterioration of catalyst in the first catalytic stage of an existing single absorption plant for producing sulphuric acid, including a reactor in which a gas mixture including sulphur dioxide and oxygen is formed by the reaction of sulphur or sulphur-containing materials with air unenriched in oxygen; at least four catalytic conversion stages for forming sulphur trioxide by reaction between sulphur dioxide and oxygen; means for cooling the gas mixture at a region downstream of the catalyst in each stage and upstream of the catalyst in the next downstream stage by a cooling medium; means for cooling the gas mixture downstream of the final stage, and an absorber for absorbing sulphur trioxide and thereby forming sulphuric acid from the gas mixture after it has been cooled downstream of the final stage, which method comprises adding commercial oxygen (as hereinbefore defined) or oxygenenriched air to the gas mixture downstream of the catalyst in the first stage and upstream of the catalyst in the final stage so as to perform the process claimed in any one of claims 1 to 9, the rate of adding the commercial oxygen or oxygen-enriched air being increased as the catalyst in the first stage deteriorates.

17. A process as claimed in claim 1, in which the gas mixture after passing over the catalyst in the penultimate catalytic stage is passed through sulphuric acid, the sulphuric trioxide in the gas mixture being absorbed in the sulphuric acid, and is then returned to the final catalytic stage, the commercial oxygen or oxygen-enriched air being added to the gas mixture downstream of the catalyst in the penultimate stage and upstream of the catalyst in the final stage.

18. A process for the production of sulphuric acid substantially as herein described with reference to Figure 1 or Figure 2 of the accompanying drawings.

19. A method of increasing the proportion of sulphur dioxide converted to sulphur trioxide in an existing plant for producing sulphuric acid, substantially as described in any one of Examples 1B to 1D and IF to IH.

20. A method of increasing the rate of production of sulphuric acid on an existing single absorption plant for producing sulphuric acid substantially as described in Example 2B or 2C.

21. A method of compensating (so far as rate of production of sulphuric acid is concerned) for deterioration of the catalyst in the first catalytic stage of an existing single absorption plant for producing sulphuric acid substantially as described in Example 3B.