NZ203892A - A process and apparatus for the production of sulphuric acid - Google Patents

Description

2 038 Priority Daie(s): .v Complete Specification Filed: Class: C&.) B. 177.f SP... <S3 Publication Date: .. Z1.F.EB.1986" P.O. Journal, No: . lQ NEW ZEALAND PATENTS ACT, 1953 No: Date: COMPLETE SPECIFICATION IMPROVED SULFURIC ACID PLANT . £ 14APR1983 feci Ve, MONSANTO COMPANY, a corporation organized and existing under the laws of the State of Delaware, United States of America, of 800 North Lindbergh Boulevard, St. Louis, Missouri 63167, United States of Ameri ca, 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: (followed by page -la-) 1> A CT - - 1Q - IMPROVED SULFURIC ACID PLANT BACKGROUND OF THE INVENTION This invention relates to a process for the production of sulfuric acid and more particularly, relates to an improved multistage oxidation and absorption process wherein the gas fed into the sulfuric acid plant has a higher concentration of sulfur dioxide than has been attempted before.

Sulfur d i ox i d e - c on t a i n i n g gas is currently produced in a wide variety of industrial processes. Considerable concern has arisen over the problem of lowering the content of sulfur oxides in waste gas released to the atmosphere in view of the obvious pollution problems. At the present, sulfur dioxide-containing gas is treated both to lower the content of sulfur oxides in the stack gases and to provide usable products, such as sulfuric acid, oleum or liquid sulfur trioxide.

Sulfuric acid and other important products originate by reacting sulfur dioxide-containing gases with excess air while passing the gases at a suitable temperature over oxidation catalysts. At a predetermined minimum temperature, referred to as the initiation temperature, the following reaction takes place: S02 + 1/2 02 S03 + heat.

As is widely known, a final more nearly complete conversion of sulfur dioxide to sulfur trioxide 2 038 92 may be obtained if the heat energy generated by the conversion is removed from the process to maintain the operating temperature at a level favoring the formation of sulfur trioxide and if the sulfur trioxide is 5 removed from the sulfur dioxide-containing gas stream during the above oxidation reaction. Thus, a series of oxidation stages are utilized with cooling of the gas between each oxidation stage and with an intermediate absorption stage which removes the sulfur 10 trioxide from the gas stream. Sulfur trioxide is typically removed from the gas in the absorption stages by washing with sulfuric acid before the gas is passed to a succeeding oxidation stage.

A conventional multiple stage oxidation 15 and absorption process, as disclosed in U.S. Patent 3,525,587, utilizes a sulfur dioxide-containing feed gas which is cleaned, diluted, and dried prior to its introduction to the first oxidation stage. This sulfur dioxide-containing gas has a sulfur dioxide content 20 of less than about 9%. It is also known that conventional sulfuric acid plants have successfully operated with a feed gas having a concentration of sulfur dioxide of approximately 10-1/2%.

The oxidation of sulfur dioxide to sulfur 25 trioxide is an exothermic reaction. In the past, the design of sulfuric acid plants has concentrated on using the heat that is created within the process where required to heat the gas streams entering oxidation stages to the initiation temperature required for the 30 conversion of sulfur dioxide to sulfur trioxide. The gas stream entering the first oxidation stage may require heating and the gas stream entering an oxidation stage after passage through the intermediate absorber stage does require heating to initiate the oxidation reaction. 35 In general, heat is generated in excess of that 203892. required for this function. Therefore, economizers,that is heat exchangers with one side connected within the sulfuric acid process and the other side connected externally of the sulfuric acid process, are used to 5 provide hot water, low pressure steam or a heated process stream for use external to the boundaries of the sulfuric acid plant. The requirements of the sulfuric acid plant have restricted this external use of the generated heat.

STATEMENT OF THE INVENTION It is an object of this invention to provide a sulfuric acid plant which utilizes a feed gas stream which contains at least approximately 12% sulfur dioxide. It is a further object of this inven-15 tion to provide a sulfuric acid plant with a greatly reduced consumption of energy when compared to a typical plant of the prior art. It is yet another object of this invention to produce a sulfuric acid plant with a greatly increased steam production.

Those and other objects are obtained by a sulfuric acid plant which utilizes a feed gas stream which contains at least approximately 12% by volume sulfur dioxide.

Please note all percentages in relation to sulfur dioxide contained in this specification and the appended claims are by volume. The greater concentration of sulfur dioxide within the feed gas stream, when compared to the prior art, allows the use of smaller equipment within the sulfuric acid plant because smaller volumes of gas must be handled to produce a specified daily capacity of sulfuric acid. The reduced volumes of gas create another advantage of the sulfuric acid plant of this invention by reducing the energy consumption of the plant. The electrical or steam turbine power required for the main compressor of a sulfuric acid plant is typically 80 to 90% of the energy consumption. within the plant. A reduction in the quantity of gas that must be moved through the plant by the main compressor allows a reduction in size of the compressor and a reduction in the energy consumed by that 5 compressor. Since smaller quantities of gas must be heated within the plant, more energy may be transferred to generate steam within economizers in the sulfuric acid plant. These advantages are created while maintaining the conversion of sulfur dioxide 10 to sulfur trioxide at the levels demonstrated by conventional sulfuric acid plants, that is at conversion rates of up to 99.7% or more.

The equipment used in the sulfuric acid plant of this invention will be the same as the equip-15 ment used in a conventional sulfuric acid plant with a few distinctions. Since the volume of gas that must be handled by the equipment will be reduced for a given size of sulfuric acid plant, the equipment will be smaller than that used in the conventional plant. 20 In addition, the sulfuric acid plant of this invention will have a converter, a vessel for the catalytic conversion of sulfur dioxide to sulfur trioxide, that has at least five catalyst beds rather than the four catalyst beds which are typically used in conventional 25 sulfuric acid plants. In the converter, a first oxidation stage will be comprised of at least three catalyst beds. The gas will be cooled as it passes from one catalyst bed to another catalyst bed in the first oxidation stage and after passing entirely through the first 30 oxidation stage, the gas will flow through an intermediate absorption tower to remove the sulfur trioxide which has been produced. Following the passage through the intermediate absorption tower, the gas stream will pass through the second oxidation stage which 35 comprises at least two catalyst beds. The gas will be 2 03892 cooled as it passes between successive catalyst beds of the second oxidation stage and will then flow to a final absorption tower. Before the final catalyst bed of the second oxidation stage, a small amount of 5 dry air or oxygen may be added to the gas stream to cool the gas entering the second catalyst bed and to provide an overabundance of oxygen for completion of the catalytic reaction of sulfur dioxide to sulfur trioxide.

In a conventional sulfuric acid plant the excess heat generated is used in economizers to heat boiler feed water for other processes. Steam generation is typically a very minor function and any steam generated is usually low pressure steam. In the 15 sulfuric acid plant of this invention, the higher concentration of sulfur dioxide in the gas stream creates higher temperatures during the conversion process.

Since a smaller quantity of gas must be heated in this sulfuric acid plant, the much greater quantities of 20 heat that are available are used to generate steam, especially high pressure superheated steam which may be used to drive a turbogenerator for the generation of electricity and low pressure steam which may be used elsewhere in the process. 2 5 DESCRIPTION OF THE DRAWINGS The drawing is a schematic illustrating the flow of the process gas through the sulfuric acid plant of this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT 30 The drawing illustrates a process flow diagram for a sulfuric acid plant which burns sulfur to supply the sulfur dioxide feed stream to the sulfuric acid plant. For a metallurgical plant or a feed gas *>■ f) Jf Q ^ M ^ O supplied from a roaster oven, the sulfuric acid plant would remain the same as will be described for the sulfur burner-fed plant. However, in those instances the feed gas conditioning equipment will differ from that discussed below. In the sulfur burner-fed plant the process gases which are created must be cooled prior to introduction to the converter. In a metallurgical plant the process gases must be heated prior to their introduction to the converter.

Looking now at the drawing, sulfur is fed to a sulfur burner 10. Blower 12 supplies air through duct 16 to drying tower 14. Alternately, air can be drawn through the drying tower on the suction side of the blower. The dried air is carried to sulfur burner 10 by duct 18. The sulfur is burned, or oxidized within the sulfur burner to form sulfur dioxide.

The sulfur dioxide leaves the sulfur burner 10 by duct 20. This sulfur dioxide-rich gas stream has a temperature higher than the initiation temperature for the catalytic conversion of sulfur dioxide to sulfur trioxide. The higher temperature favors the dissociation of sulfur trioxide to sulfur dioxide; therefore, the excess heat must be removed from this stream. The oxidation of sulfur dioxide to sulfur trioxide is an exothermic reaction, thus the removal of heat from the sulfuric acid plant becomes a major concern. Therefore, the feed streams to the catalyst beds are maintained at the minimum temperature necessary to initiate the oxidation reaction. As shown in the drawing, duct 20 conveys the sulfur dioxide-1 aden feed gas to a first boiler 22, or equivalent heat exchanger used to transfer heat from the sulfuric acid plant to an external process such as the heating of boiler feed water or the generation of steam.

The feed gas is cooled in first boiler 22 to a temperature near the desired input temperature for the converter and is then conveyed by duct 24 to the converter 30, Converter 30 comprises a vessel for the catalytic conversion of sulfur dioxide to sulfur trioxide 5 having a plurality of catalyst beds which are divided into a first oxidation stage 32 and a second oxidation, stage 34. In the preferred configuration shown in the Figure, converter 30 has a first oxidation stage 32 comprised of three catalyst beds and a second oxidation stage 10 34 comprised of two catalyst beds. Between each catalyst bed of the first oxidation stage 32 and the second oxidation stage 34, the process gas is cooled to remove the heat generated during the oxidation of the sulfur dioxide. Between the first oxidation stage 32 and the second 15 oxidation stage 34, the gas stream is carried through an inte rpass absorber 50 to remove the sulfur trioxide which has been produced from the gas stream so as to provide a feed to the second oxidation stage 34 that is again rich in sulfur dioxide.

Ret urning now to the sulfur dioxide-laden feed gas stream which is conveyed from first boiler 22 to the converter by duct 24, the feed gas stream enters the first conversion stage 32 of converter 30 and passes through a catalyst bed 35. Passage through 25 the catalyst bed 35 causes a reaction which oxidizes a portion of the sulfur dioxide to sulfur trioxide and creates heat. This heated gas stream is carried to second boiler 26, or equivalent heat exchanger device. The hot gas stream is used in second boiler 26 to 30 generate additional steam or heated boiler feed water for use externally of the sulfuric acid plant. The cooled gas stream is returned to the converter 30 and enters catalyst bed 36, the second catalyst bed.

After passing through the catalyst bed 36, in which 2 03892 ; additional sulfur dioxide is oxidized, the again heated [ gas stream is carried to a heat exchanger 52, the hot { i interpass heat exchanger. This hot gas stream is I used in heat exchanger 52 as the final heating of a j chilled gas stream returning from the interpass j absorption tower 50. From heat exchanger 52 the gas ! stream is returned to the converter 30 where it enters | ! the third catalyst bed 37 of the first oxidation stage 32 .

After the gas stream passes through the third catalyst bed 37, the gas has completed its passage through the first oxidation stage 32 of converter 30.

Greater than 90'/. of the sulfur dioxide in the feed j i stream entering the first catalyst bed has been con- j verted or oxidized to sulfur trioxide when the gas j stream exits from the catalyst bed 37. The oxidation ! j reaction approaches an equilibrium thus some of the sulfur trioxide must be removed from the gas stream to enable the remaining sulfur dioxide to be oxidized 20 easily. Therefore, the gas stream exiting from the first oxidation stage 32 passes through the interpass absorption tower 50. The interpass absorption tower 50 operates at a cool temperature so that the sulfur trioxide may be absorbed within a liquid stream of 25 sulfuric acid or oleum. To cool the gas stream exiting from the first oxidation stage 32 the gas stream passes through heat exchanger 54. In this heat exchang- | er the hot gas stream from the first oxidation stage j 32 is cooled while the cold gas stream exiting from 30 the interpass absorption tower 50 is heated. Only a portion of the heat within the gas stream reaching heat exchanger 54 is necessary to heat the cold gas returning from the interpass absorption tower 50. Therefore, j rather than wasting the remaining excess heat within j i this gas stream, the gas stream passes through 2 036 9 ? economizer 56 before it enters the interpass absorption tower 50 to further reduce the gas temperature by again providing heat for boiler feed water, steam or process heating externally from the sulfuric acid 5 plant- From economizer 56 the gas stream enters the interpass absorption tower 50 and the sulfur trioxide is removed.

As noted above, when the cool gas stream now depleted in sulfur trioxide leaves the interpass 10 absorption tower 50 it passes through heat exchanger 54 which partially reheats the gas stream to the minimum temperature necessary for the oxidation reaction within the next catalyst bed 38. Heat exchanger 54 is sized such that the gas stream returning from the 15 interpass absorption tower 50 to the second oxidation stage 34 of converter 30 must have additional heating. For this reason the gas stream, after passing through heat exchanger 54, also passes through heat exchanger 58 and heat exchanger 52. As noted above, heat 20 exchanger 52 receives its heat from the hot gas stream flowing from catalyst bed 36. The gas flowing from catalyst bed 36 is cooled in heat exchanger 52 so that the gas stream returning to catalyst bed 37 is at the initiation temperature for the oxidation 25 reaction. After passage through the heat exchanger 52, the gas stream from the interpass absorption tower 50 has been heated to the initiation temperature for the catalytic oxidation of sulfur dioxide to sulfur trioxide. This heated gas stream enters the second 30 oxidation stage 34 of converter 30 by entering catalyst bed 33. Passage of the gas stream through catalyst bed 38 oxidizes some of the remaining sulfur dioxide and again heats the gas stream. The heated gas stream exits from the converter 30, passes through heat exchanger 58, and after cooling returns to the converter 30 for passage through catalyst bed 39, the final catalyst bed. After passage through catalyst bed 39 the conversion process is completed and up to 99.7% or more of the sulfur dioxide in the original feed gas stream that entered the converter will have been converted to sulfur trioxide. The hot gas exiting from the catalyst bed 39 is conveyed to economizer 60 which, if desired, may be used to superheat the steam generated in boiler 22 and boiler 26. From the economizer 60 the cooled gas stream enters the final absorption tower 70 in which the sulfur trioxide is removed from the gas stream. After passage through the final absorption tower 70 the gas stream, now depleted of sulfur trioxide and sulfur dioxide, passes through a stack to be dispersed within the atmosphere.

Though it is not shown on the drawing, a small amount of dry air or oxygen may be added to the gas stream between catalyst bed 38 and catalyst bed 39, the catalyst beds of the second oxidation stage 34.

This addition of air or oxygen between these catalyst beds serves the dual purpose of providing additional cooling to cool the gas to an optimum temperature for high overall conversion of sulfur dioxide to sulfur trioxide and improving the catalyst performance by providing an excess of oxygen at the final catalyst bed. By this means the operable range of sulfur dioxide and oxygen concentrations can be extended beyond the ranges necessary in the gas stream entering the first oxidation stage 32. Also, by adding oxygen at this point of the process the amount of catalyst required can be reduced. This result is reached because the addition of oxygen increases the oxygen to sulfur dioxide ratio as the gas stream enters the catalyst bed 39 which favors the conversion of sulfur dioxide to sulfur trioxide. 2 038 92 -ii- Operation of the sulfuric acid plant of this invention with a sulfur dioxide concentration I in the feed gas to the first oxidation stage of the j converter of at least approximately 12% creates 5 several benefits. In conventional sulfuric acid i plants the sulfur dioxide~ 1 aden feed gas is burned with or diluted by excess air until the sulfur dioxide reaches a concentration of approximately 9 to 10%.

In the plant of this invention, with a feed gas stream i having a sulfur dioxide concentration of at least j approximately 12%, the volume of air that must be conveyed through the plant is reduced from that volume •! conveyed in a conventional plant. The smaller volume j of gas circulating through the plant enables the usage 15 of smaller equipment within the plant, including a smaller main compressor. The size of the main compressor, and its energy requirements, and the size of the ducts and columns within the plant may be reduced.

Since typically 30 to 90% of the energy consumed 20 within the sulfuric acid plant is required to power the electrical or steam turbine for the main compressor, reducing the compressor's size will reduce the energy consumed within the sulfuric acid plant. The reduction of gas volume also increases the energy available for 25 heat recovery as a smaller volume of gas requires | heating, for example after passage through the interpass f ■ absorption tower. In addition, the oxidation of a higher concentration of sulfur dioxide in a smaller total volume of gas produces higher exothermic tempera-3Q tures which may be used to generate high pressure, super- j heated steam for use outside the sulfuric acid plant. The j. quantity, pressure and temperature of the steam generated within the sulfuric acid plant of this invention is unavailable in conventional sulfuric acid plants. This steam may be used for the generation of electricity to further reduce the energy costs to the plant owner. In a conventional sulfuric acid plant typically steam is generated having a pressure of approximately 600 pounds per square inch and this steam may be utilized to generate approximately 10,000 kilowatts of electrical power. The sulfuric acid plant of this invention generates a greater quantity of steam than the conventional plant. The steam, which has a pressure of approximately 800 pounds per square inch, may be used to generate approximately 14000 kilowatts of electrical power. If electrical power has a value of five cents C$0.05) per kilowatt hour, the increased electrical power generated by the sulfuric acid plant of this invention has an approximate value of 51,600,000 per year.

The foregoing description of this invention is not intended as limiting the invention. Many variations and modifications may be made without departing from the spirit and scope of this invention. 203893

Claims (13)

WHAT WE CLAIM IS:

1. A process for the production of sulfuric acid comprising: creating a gas stream containing a concentration of at least substantially 12% by volume sulfur dioxide and sufficient oxygen to convert said sulfur dioxide to sulfur trioxide; feeding said gas stream to a converter; converting said sulfur dioxide to sulfur trioxide in said converter - said converter having first and second oxidation stages - said first oxidation stage having at least three catalyst beds and said second oxidation stage having at least two catalyst beds; and absorbing said sulfur trioxide to provide sulfuric acid in an interpass absorption tower following said first oxidation stage and ffi OZl d-* further absorbing said sulfur -dioxide in a final absorption tower following said second oxidation stage.

2. The process of Claim 1 wherein said first oxidation stage has three catalyst beds and said second oxidation stage has two catalyst beds.

3. The process of Claims 1 or 2 further including passing said gas stream through at least two economizers to remove the heat created by converting said sulfur dioxide to sulfur trioxide.

4. The process of Claim 3 wherein said gas stream passes through a first economizer following said first oxidation stage and a second economizer following said second oxidation stage.

5. The process of Claim 3 further including converting greater than 99.0% of said sulfur dioxide to sulfur trioxide. 2G3892 - 14 -

6. The process of Claim 5 further including adding dry air to said gas stream between two of said catalyst beds of said second oxidation stage.

7. An apparatus for the oxidation of sulfur dioxide to sulfur trioxide comprising a vessel having first and second oxidation stages, said first oxidation stage having at least three catalyst beds and said second oxidation stage having at least two catalyst beds.

8. The apparatus of Claim 7 wherein said first oxidation stage has three catalyst beds and said second oxidation stage has two catalyst beds.

9. The apparatus of Claims 7 or 8 further including means to introduce dry air between two of said catalyst beds of said second oxidation stage.

10. A process for the production of sulfuric acid comprising: (1) creating a gas stream containing a concentration of at least substantially 12% by volume sulphur dioxide and sufficient oxygen to convert said sulfur dioxide to sulphur trioxide; (2) cooling said gas stream to a minimum temperature for initiation of the catalytic oxidation of sulfur dioxide to sulfur trioxi de; (3) feeding said gas stream to a converter for oxidizing said sulfur dioxide to sulfur trioxide; (4) oxidizing greater than 99.0% by volume of said sulfur dioxide to sulfur trioxide in said converter - said converter having first and second oxidation stages - said first oxidation stage having 20389 - 15 - at least three catalyst beds and said second oxidation stage having at least two catalyst beds; (5) passing said gas stream sequentially through said catalyst beds of said first oxidation stage while cooling said gas stream to a minimum initiation temperature between said catalyst beds to oxidize a portion of said sulfur dioxide to sulfur trioxide; (6) passing said gas stream through an economizer following said first oxidation stage to create a cool gas stream by removing the heat created during said oxidation of sulfur dioxide to sulfur trioxide; (7) absorbing said trioxide in said cooled gas stream in an interpass absorption tower to produce sulfuric acid and to provide a gas stream depleted in sulfur trioxide; (8) reheating said gas stream depleted in sulfur trioxide to a minimum initiation temperature and passing said gas stream sequentially through said catalyst beds of said second oxidation stage while cooling said gas stream to a minimum initiation temperature between said catalyst beds to complete the oxidation of said sulfur dioxide to sulfur trioxide; and (9) passing said gas stream through an economizer following said second oxidation stage to remove the heat created during said oxidation of sulfur dioxide to sulfur trioxide; and (10) absorbing said sulfur trioxide in said cooled gas stream in a final absorption tower to produce sulfuric acid and to provide a gas stream depleted in sulfur trioxide and sulfur dioxide before releasing said gas stream to the atmosphere. - 16 -

11. A process as claimed in any one of claims 1 to 6 when performed substantially as hereinbefore described with reference to the accompanying drawing.

12. Apparatus as claimed in any one of claims 7 to 9 substantially as herein described.

13. A process as claimed in claim 10 substantially as hereinbefore described with reference to the accompanying drawing. DATED THIS IU ^ DAY OF 1 19 ^ £ A. U. PAR K & SON PER AGENTS FOR THE APPLICANTS