WO2003037790A2

WO2003037790A2 - Process and apparatus for producing sulfuric acid

Info

Publication number: WO2003037790A2
Application number: PCT/EP2002/011231
Authority: WO
Inventors: Karl-Heinz Daum; Wolfram Schalk
Original assignee: Outokumpu Oyj
Current assignee: Outokumpu Oyj
Priority date: 2001-10-31
Filing date: 2002-10-08
Publication date: 2003-05-08
Anticipated expiration: 2004-04-30
Also published as: DE10153692A1; AR036964A1; AU2002346966A1; PE20030485A1; WO2003037790A3

Abstract

This invention relates to a process and an apparatus for producing concentrated sulfuric acid by means of an absorption tower. To absorb large amounts of S03 (or H2O) in sulfuric acid of different concentration, it is proposed that an empty tube is integrated inside a packed absorber, wherein the gas and the acid are introduced at the head of the empty tube and are cocurrently passed into the lower portion of the succeeding packed absorber, subsequently the gas is passed upwards through the packing, further acid is introduced into the packed absorber above the packings and is countercurrently mixed with the gas in the packing region, the gas from the head of the packed absorber being withdrawn and the acid being discharged from the bottom of the packed absorber.

Description

Process and Apparatus for Producing Sulfuric Acid

Description

This invention relates to a process and an apparatus for producing concentrated sulfuric acid by means of an absorption tower.

Drying and absorption towers in plants for producing sulfuric acid are traditionally designed as countercurrent packed columns, wherein the gas is generally introduced into the packed bed from below and the sulfuric acid is guided countercurrently, i.e. is charged onto the bed from above. There are used various commercially available packings, mostly as random packing, but also as structured packing. In general, there are used ceramic materials. The absorption towers are made both of steel with an inner lining, such as e.g. a plastic film plus acid-resistant brick lining, and also of pure stainless steel. In the case of sulfuric acid plants which are operated with gases of a higher concentration, e.g. > 10-15 vol-% S0₃ at the inlet, the absorbers must be operated with very high specific acid flow rates (spraying density), so that suitable concentrations are maintained for a rather complete absorption. This leads to the fact that pure packed towers are operated on the brink of their load- bearing capacity or would be loaded beyond the flooding point, so that the apparatuses must be designed larger in diameter, although this is not necessary in terms of absorption capacity. A preceding Venturi or empty tube alleviates this situation due to the possibility of reducing the spraying density in the packing portion. Absorption columns with or without preceding Venturi/empty tubes are provided with one or more lateral gas inlet ports, which introduce the gas into the packed column below the packing. With increasing plant capacities and thus increasing tower diameters, the uniform distribution of the gas over the entire absorber cross-section becomes more and more problematic and hence the uniform liquid distribution of the acid, which potentially reduces the absorption capacity of the absorber. In practice, this problem is compensated by overdimensioning the packed bed. A theoretically required filling height of e.g. 1.5 to 1.7 m actually becomes 3 to 4 m. This increases the investment and operating costs.

Another problem in these plants is the adjustment of the acid temperature to a higher level for utilizing the waste heat. The absorption of S0₃ in sulfuric acid and the formation thereof by adding H₂0 is strongly exothermal, the reaction heat generally being dissipated to cooling water by means of an indirect heat exchange. There are used acid temperatures of 70-120°C. If the temperature of the circulating sulfuric acid is increased to 160-230°C, the corresponding heat can be converted into an equivalent amount of steam in a suitable heat exchanger and be utilized. Such processes are commonly used in the industry, but due to the extremely corrosive properties of the sulfuric acid at these temperatures are subject to certain restrictions. A conversion of these plants is very expensive, as the materials used are designed for a certain acid concentration and a defined temperature range. Each deviation from this defined working window makes the corrosion rate increase inadmissibly.

What is known is the so-called Heat Recovery System (HRS), in which a two- stage absorption takes place in two successive packed columns. The wall of the absorption tower is made of stainless steel, which leads to a very narrow restriction of the admissible H₂S0₄ concentration to e.g. 99.0 % to 99.6 % H₂S0₄. The water for forming the sulfuric acid corresponding to the amount of S0₃ absorbed must be added in a suitable apparatus outside the tower under controlled conditions. The required sulfuric acid concentration is adjusted thereby. The high acid concentration leads to an increased formation of acid fumes and an additional effort for the filtration and separation thereof. Deviations from the admissible acid concentration lead to extremely high corrosion rates and an immediate destruction of the apparatus involved.

DE-C-19800800 describes a two-stage process, in which the first stage is a Venturi absorber and the second stage is a packed tower. The process employs separate circuits and conventional acid concentrations of e.g. 95 % to 100 % H₂S0₄. Due to the higher corrosion rate at this acid concentration, the apparatus must, however, partly be made of stainless steels of a very high quality, e.g. 1.4575. These materials can be used in a larger application range of the acid concentration, but they have the disadvantage of the extremely high costs and the restricted processability, such as e.g. welding. In plants of this type, the addition of the water for forming sulfuric acid is usually effected inside the Venturi absorber, either at the bottom or directly in the gas phase. The latter can lead to the formation of extreme amounts of acid fumes and an additional effort for the filtration and separation thereof. The process is suitable for recovering the absorption heat in the form of steam for about 90 %. Both processes are very sensitive as regards malfunctions and have very high investment costs due to the materials used.

Proceeding from this prior art it is the object underlying the invention to develop a process and an apparatus in which large amounts or high concentrations of S0₃ or H₂0 can be absorbed by means of sulfuric acid of different concentration, and in which the process heat can be reused.

In accordance with the invention, this object is solved in that inside a packed absorber an empty tube is integrated, wherein the gas and the acid are introduced at the head of the empty tube and are cocurrently passed into the lower portion of the succeeding packed absorber, subsequently the gas is passed upwards through the packings, further acid is introduced into the packed absorber above the packings and countercurrently mixes with the gas in the packing region, the gas being withdrawn from the head of the packed absorber and the acid being discharged from the bottom of the packed absorber.

Due to the arrangement of the internally disposed cocurrent absorber, the gas distribution to the succeeding packed column is effected radially symmetrically. Therefore, this arrangement is no longer sensitive to large gas quantities, and there is achieved a very uniform gas distribution, which promotes a complete absorption. As a result, both the height of the packing can be reduced and the gas velocity produced in the empty cross-section can be increased, which leads to a smaller and thus less expensive apparatus on the whole.

The absorption means may be made of a steel jacket with inner brick lining, completely of stainless steel or as a combination of a brick-lined design with stainless steel and can be operated for a very wide temperature range of 50 to 250°C. Therefore, an application in a conventional design can be used unchanged both for acid temperatures around 70-120°C (with heat dissipation via cooling water) and for heat recovery with temperatures of 160-230°C. A retrofitting or conversion of a plant of conventional technology to heat recovery is thus easily possible, namely exclusively by changing external equipment parts.

In the case of heat recovery, the apparatus can be operated with conventional acid concentrations, which considerably reduces the formation of acid fumes. A special water mixing means is not required, as industrially successful means are employed. The reduced formation of acid fumes provides for the use of coalescers/demisters for the separation thereof, and the expensive candle-type filters can be omitted.

Embodiments of the process will be explained by way of example with reference to the drawing. The absorber (1 ) comprises a packed absorber (2) with an integrated empty tube (3). The empty tube usually is made of high-alloy stainless steel and, to improve the mixing of the gas with the sulfuric acid, can have a Venturi nozzle (4) in the upper portion. In the middle portion, the empty tube (3) is centrically held in the packed absorber (2) by means of two gas-permeable reinforcements (5). The upper portion of the packed absorber is made of steel or stainless steel, whereas the bottom wall (6) of the packed absorber (2) is made of steel in combination with a brick lining or of stainless steel with or without brick lining. The packing (7) may consist of loose material or of a structured packing. As feed materials, in particular ceramic materials can be used. Through the opening (8) at the head of the empty tube (3), gas is introduced into the absorber.

In the case of a conventional application without recovery of heat, this gas consists of the S0₃ to be absorbed in a concentration of 0.1 to 20 vol-% (the rest are inert gases) and has a temperature of 120-250°C. Via lateral inlet holes (9), sulfuric acid is introduced into the empty tube (3) with a concentration of 98- 99 % and a temperature of 60-130°C. Due to the Venturi nozzle (4), an acceleration of the flow takes place and thus a cocurrent mixing between gas and acid. At the bottom end of the empty tube (3), the acid has a concentration which is increased corresponding to the absorption and a corresponding temperature of 80-150°C. The gas consists of an inert component and the residual non-absorbed S0₃ and has a temperature which at this point is approximately equal to the acid temperature. The sulfuric acid flows to the bottom of the absorber and is discharged through the outlet (10) for further processing. Upon discharge from the empty tube (3), the gas rises and flows through the packing (7) from the bottom to the top. At the same time, further sulfuric acid with 98-99 % H₂S0₄ and a temperature of 60-130°C is introduced into the absorber through the connections (11 ) and distributed rather uniformly over the diameter of the packing (7) by means of a system of distributors (12). In the packing (7), gas and acid are mixed countercurrently. Upon flowing through the packing (7), the acid has a temperature of 60-130°C and a concentration which is increased corresponding to the absorption. Upon flowing through the packing (7), the gas exclusively consists of the inert component and has a temperature which is approximately equal to that of the acid added. The gas flows through the reinforcements (5) in the middle portion of the absorber and leaves the absorber (1) through the outlet (13) for further processing. To the acid having a concentration of 98.5-99.5 % H₂S0 ₎ which has been collected in the lower portion, the required process water is supplied in the interior of the means, and the concentration is thereby reduced to 98-99 % H₂S0 .

Similar conditions exist when H₂0 is absorbed in sulfuric acid. The concentrations must be reversed, i.e. the outflowing sulfuric acid will generally have a lower concentration than the acid added, the acid to be added having a concentration of 94-98.5 %.

When using the absorption means with heat recovery for the absorption of S0₃, approximately the same concentrations should exist as described above. However, the temperature level has been raised to 160-230°C for adding the acid to the Venturi head and 60-130°C at the packing head. To the acid with a concentration of 98.5-99.5 % H₂S0₄, which has been collected in the lower portion, the required process water is supplied in the interior of the means, and the concentration is thereby reduced to 98-99 % H₂S0 . The outflowing acid has a temperature of 160-250°C. This temperature level provides for the conversion of the heat to be dissipated into e.g. low-pressure steam in a suitable apparatus outside the absorption means. Example 1 : Conventional SQ₃ absorber (without heat recovery)

A gas from a sulfuric acid plant on the basis of the combustion of elementary sulfur is supplied to the absorption means as follows:

Example 2: S0₃ absorber with heat recovery

A gas from a metallurgical sulfuric acid plant on the basis of the use of oxygen- enriched melting is supplied to the absorption means as follows:

Claims

Claims:

1. A process of producing sulfuric acid, comprising a packed absorber (2) in which an empty tube (3) is integrated, characterized in that the gas and the acid are introduced at the head of the empty tube (3) and are cocurrently passed into the lower portion of the packed absorber (3), subsequently the gas is passed upwards through the packings (7), further acid is introduced into the packed absorber (2) above the packing (7) and countercurrently mixes with the gas in the packing (7), the gas being withdrawn from the head of the packed absorber (2) and the acid being discharged from the bottom of the packed absorber (2).

2. The process as claimed in claim 1 , characterized in that the sulfuric acid introduced into the absorber (1 ) has a temperature of 50 to 250°C.

3. The process as claimed in claim 1 , characterized in that the sulfuric acid introduced into the absorber (1 ) has a concentration of 98 to 99.5 vol-%.

4. The process as claimed in claim 1 , characterized in that when using the absorption tower with heat recovery, the temperature of the sulfuric acid is 160- 220°C when it is introduced into the empty tube (8), and is 60-110°C when it is introduced above the packing (7).

5. An apparatus for producing sulfuric acid, comprising a packed absorber (2) in which an empty tube (3) is integrated, characterized in that the empty tube (3) in the head of the packed absorber (2) has an opening for introducing gas, that the empty tube (3) in the head of the packed absorber (2) has inlet holes (9) for introducing acid, that in the lower portion of the packed absorber (2) a packing (7) is disposed, through which extends the empty tube (3), that above the packing (7) connections (11 ) are provided for introducing acid, a system of distributors (12) distributing the acid over the surface of the packing, that in the bottom of the packed absorber (2) an outlet (10) is disposed for discharging acid, and that the packed absorber (2) has an outlet (13) for discharging the gas.

6. The apparatus as claimed in claim 5, characterized in that the empty tube has a Venturi nozzle (4).

7. The apparatus as claimed in claim 5, characterized in that the bottom wall (6) of the packed absorber (2) consists of a steel jacket with inner brick lining.

8. The apparatus as claimed in claim 5, characterized in that the bottom wall (6) of the packed absorber (2) consists of stainless steel with inner brick lining.

9. The apparatus as claimed in claim 5, characterized in that the absorber (2) is made of stainless steel.