GB2024180A - Catalytic oxidation of SO2 to SO3 - Google Patents

Abstract

In a process for the production of sulphuric acid by burning sulphur in air and catalytically oxidising the SO2 in the resulting gases to SO3 by passing a mixture of SO2 and oxygen through a plurality of catalytic reaction layers at elevated temperature to effect a two- stage conversion whereof the first stage includes a plurality of reaction layers, separating by interstage absorption the SO3 in the gaseous product of the first stage, recycling the gaseous residue of the interstage absorption to the second stage of the conversion, and separating by final absorption the SO3 in the gaseous product of the second stage, the gaseous residue of the interstage absorption is preheated in passing to the second conversion stage by a heat- exchanging relationship with, successively, the gaseous product of the last layer of the first conversion stage and the gaseous product of the penultimate layer of the first conversion stage, and the gaseous product of the first stage is cooled in passing to the interstage absorption by a heat-exchanging relationship with successively the gaseous residue passing to the second conversion stage and air to be used for sulphur combustion.

Description

SPECIFICATION Improvements in or relating to the production of sulphuric acid This invention relates to a process for the production of sulphuric acid by burning sulphur and catalytically oxidizing the SO2 to S03, and is particularly concerned with such a process of the kind comprising passing a mixture of SO2 and oxygen through a plurality of catalytic reaction zones at elevated temperature to effect a two-stage conversion whereof the first stage includes at least three reaction zones, separating by absorption the S03 in the gaeous product of the first stage, recycling the gaseous residue of the absorption to the second stage of the conversion, and separating by absorption the SO3 in the gaseous product of the second stage.

To obtain a high conversion degree of SO2 to S03 at least two oxidation stages with interstage absorption of S03 are used in to-day's techniques.

There are known a number of methods for transferring the heat from gases during the oxidation of 502 to S03 and for heating gases which pass to the second stage of the conversion. Most frequently used heat exchange systems are surface heatexchangers. Such systems are described in the book "Sulphuric acid" Handbook for engineer and technician, chapter IX; in U.S. patent No. 3,615,197 and in German patent No. 1,567,672. In such a system of gases, after passing through the successive catalytic beds; are cooled in heat-exchangers. Usually, the gases which flow to the second stage of the conversion are heated successively in heatexchangers disposed downstream of respectively the last bed of the first stage of the conversion, the last but one bed, and the first bed.

On the other hand, the gases passing to the interstage absorption location are cooled successively in a heat-exchanger at the location where the air for sulphur combustion is heated, and in a heat-exchanger at the location where the gases which flow to the second stage of the conversion are heated. The disadvantage of the above-described system consists in that there is a great number of connections between exchangers; moreover, the unfavourable temperature difference results in larger heat-exchange surfaces, and some of the exchangers are exposed to increased corrosion. The considerable number of heat-exchange apparatuses used results in increased resistance to flow and consequently greater energy consumption. Evidently, such a system requires a considerable area for installation.

Another solution concerns a system where the gases leaving the last but one bed of the first conversion stage are cooled by means of saturated vapour, and the gases passing to the interstage absorption are cooled in a water heater which must be made of pipes covered with cast-iron. The gases entering the second conversion stage are heated in firstly a heat exchanger by exchanging the heat with gases which pass to the final absorption column, and secondly in a heat-exchanger downstream of the first catalytic bed. The disadvantage of this system consists in the high cost of the economizer made of pipes covered with cast-iron, and its great weight. Similar systems are described in the U.S.

patents Nos. 3,443,896,3,536,446 and 3,620,673.

The object bf the present invention is to conduct the process of converting SO2 to SO3 in such a manner as to intensify the heat-exchange while simultaneously reducing the dimensions and weights of the heat-exchangers and decreasing the consumption of electrical energy.

According to the present invention we provide a process for the production of sulphuric acid by burning sulphur and catalytically oxidizing the SO2 to SO3, comprising passing a mixture of SO2 and oxygen through a plurality of catalytic reaction zones at elevated temperature to effect a two-stage conversion whereof the first stage includes at least three reaction zones, separating by absorption the SO3 in the gaseous product of the first stage, recycling the gaseous residue of the absorption to the second stage of the conversion, and separating by absorption the SO3 in the gaseous product of the second stage, wherein the gaseous residue of the absorption is preheated in passing to the second conversion stage by a heat-exchanging relationship with, successively, the gaseous product of the last zone of the first conversion stage and the gaseous product of the penultimate zone of the first conversion stage, and the gaseous product of the second stage is cooled in passing to the interstage absorption by a heat-exchanging relationship with successively the gaseous residue passing to the second conversion stage and airto be used for sulphur combustion.

Preferably the reaction zones are in a multi-plate catalytic reactor, the catalyst in vanadium, and the heat-exchange is effected in surface heatexchangers.

Preferably also the gases leaving the first plate of the catalytic reactor of the first oxidation stage are cooled by surface cooling by means of saturated water vapour generated in a boiler, and the gases leaving the second oxidation stage are cooled in a surface water-heater.

A modification of the process involving the catalytic oxidation of SO2 to SO3 consists in that the agent cooling the gases from the first plate of a multi-plate second oxidation stage is water vapour.

By virtue of the present invention there is provided improved efficiency in heat exchange, reduced weight of construction materials, and partial elimination of more expensive materials. Simultaneously, the consumption of electrical energy needed to propel an air blower has been reduced. The proposed solution of contact node requires a smaller area for installation and enables the process to be conducted under variable load, even up to 50% of the nominal load.

Examples embodying the invention will now be described with reference to the accompanying drawings in which Figures 1 and 2 show respectively four - and five - plate apparatuses for use in SO2 conversion: Example 1. The gases from sulphur combustion in an amount of 84000 Nm3/h containing 10% of SO2 and having a temperature of 430"C are introduced into a four - plate catalytic reactor 1. After passing through the first plate of the catalytic reactor, the temperature of the gases increased to 602 C; the gases were cooled in a steam superheater 2 to a temperature of 440"C and then conducted to the second plate.The gases from the second plate were cooled in a surface heat-exchanger 3 to a temperature of 440"C by means of gases recycled from the inter-stage absorption.

The gases leaving the third plate of the first conversion stage were firstly cooled in a heatexchanger 4 in which the gases from the inter-stage absorption were heated, and secondly in a heat exchangers in which air 12 used for sulphur combustion was heated. The stream of gases 11 cooled to a temperature of 1900C was conducted to the inter-stage absorption. The stream of gases 10 leaving the inter-stage absorption and having a temperature of 65"C was passed to the heatexchangers 4 and 3 where it was heated to a temperature of 420"C. The heated gases containing unreacted S02 in an amount of 0.6% by volume were conducted to the fourth plate, that is to the second conversion stage, where the further oxidation of SO2 to SO3 took place.The degree of conversion attained was 99.75%. The gases leaving the fourth plate were cooled to a temperature of 1800C in a water heater 6 whence a stream 14 was directed to the final absorption.

Example 2. Figure 2 shows a five-plate catalytic reactor in which three plates constitute the first conversion stage and two plates form the second stage. The gases leaving the first plate of the second conversion stage are cooled by surface cooling with water vapour in a superheater 16. The vapour from this superheater is then conducted to cool the flow of gases from the first plate of the first conversion stage in a superheater 2.

As a result of using the method according to the invention the surface needed for heat exchange was reduced, and consequently the weight of the installation in the contract node was decreased by about 15%. The area necessary for the contact node was also reduced. Due to the reduced quantity of apparatuses, the resistance to gas flow was reduced on average by 150 mm H20, and consequently the consumption of electrical energy was reduced by 100 kW/hour.

Claims (6)

CLAIMS 1. A process for the production of sulphuric acid by burning sulphur and catalytically oxidizing the SO2 to S03, comprising passing a mixture of SO2 and oxygen through a plurality of catalytic reaction zones at elevated temperature to effect a two-stage conversion whereof the first stage includes at least three reaction zones, separating by absorption the S03 in the gaseous product of the first stage, recycling the gaseous residue of the absorption to the second stage of the conversion, and separating by absorption the SO3 in the gaseous product of the second stage, wherein the gaseous residue of the absorption is preheated in passing to the second conversion stage by a heat-exchanging relationshep with, successively, the gaseous product of the last zone of the first conversion stage and the gaseous product of the penultimate zone of the first conversion stage, and the gaseous product of the second' stage is cooled in passing to the interstage absorption by a heat-exchanging relationship with successively the gaseous residue passing to the second conversion stage and air to be used for sulphur combustion. 2. A process according to claim 1, wherein the gaseous product of the first reaction zone of the first oxidation stage is cooled by heat-exchange with saturated water vapour, and a gaseous product of the second oxidation stage is cooled by heatexchange in awatersuperheater. 3. A process according to claim 2, wherein the second stage includes two reaction zones, and the gaseous product of the first reaction zone is cooled by heat-exchange with water vapour. 4. A process according to any one of the preceding claims, wherein the reaction zones are in a multi-plate catalytic reactor, the catalyst is vanadium, and the heat exchange is effected in surface heat-exchangers. 5. A process for the production of sulphuric acid by burning sulphur in air and catalytically oxidizing S02 to S03, substantially as hereinbefore described with reference to Figure 1 or Figure 2 of the accompanying diagrams. CLAIMS -

1. A process for the production of sulphuric acid by catalytically oxidising in two stages with interstage absorption gases produced by the combustion of sulphur in air and containing SO2 and oxygen, comprising contacting the gases in a first oxidising stage at elevated temperature with a plurality of layers in succession of a multi-layer catalytic reactor to produce gases containing SO3, passing the first stage gaseous product to an inter-stage absorber to remove the SO3 and produce an interstage gaseous residue, pre-heating the interstage gaseous residue by passing same through, successively, first and second heat exchangers, contacting the pre-heated interstage gaseous residue in a second oxidising stage with a layer of catalyst to produce gases containing S03, and passing the second-stage gaseous product to a final absorber to remove the S03, wherein said first and second heat exchangers are heated by gas streams passing respectively from the final catalyst layer of the first stage to the inter-stage absorber and from the penultimate catalyst layer of the first stage to the final catalyst layer thereof, and the combustion air is pre-heated by passing same through a third heat exchangers heated by a gas stream passing from the first heat exchanger to the inter-stage absorber.

2. A process according to claim 1, wherein the first oxidising stage of the catalytic reactor has at least three layers of catalyst.

3. A process according to claim 2, wherein the gases passing from the first layer to the second layer of the first oxidising stage are cooled in a heat exchanger by saturated water vapour, and the gases passing from the second oxidising stage to the final absorber are cooled in a water superheater.

4. A process according to any one of claims 1 to 3, wherein the second oxidising stage of the catalytic reactor has two layers of catalyst for successive throughglow of the gases, and the gases between the two layers are cooled by water vapour

5. A process according to any one of the preceding claims, wherein the heat exchangers are of the plate type.

6. A process for the production of sulphuric acid, substantially as hereinbefore described with reference to Example 1 of Example 2.