Chemistry of inorganic compounds – Modifying or removing component of normally gaseous mixture – Sulfur or sulfur containing component
Patent
1995-06-09
1997-10-07
Straub, Gary P.
Chemistry of inorganic compounds
Modifying or removing component of normally gaseous mixture
Sulfur or sulfur containing component
423241, 423486, 423507, 423522, C01B 1720, C01B 701, C01B 1769
Patent
active
056744641
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a process for the removal of sulphur dioxide from waste gases which includes the catalytic oxidation of hydrogen bromide as an intermediate step and the formation of sulphuric acid as a useful end product.
The invention is based in the well-known reaction of bromine with sulphur dioxide to form sulphuric acid and hydrobromic acid:
The desulphurisation is carried out by contacting the waste gases with an aqueous solution containing both acids and a small quantity of bromine. During the reaction, the two acids are produced simultaneously in the solution.
Subsequently, the acids are separated from the solution in such a way that hydrogen bromide is evaporated, brought in contact with an excess of oxygen (air) and catalytically oxidized to bromine according to the following reaction:
In this way, the consumed bromine is completely recycled and the overall reaction of the process is the formation of sulphuric acid from sulphur dioxide according to the following reaction:
The single steps of the process are all well-known reactions, but have never before been combined in a single process.
Known wet regenerative flue gas desulphurisation processes include the process which is known as the Ispra Mark 13A process which is based on the above reaction (1).
In this process, the regeneration of the consumed bromine is carried out by electrolysis of the hydrogen bromide. Aspects of this process are discussed in EP-A-0016290 and EP-A-0171570. The process has been successfully developed to a large pilot plant scale and extensive market studies and comparative cost estimations have been carried out.
During these studies it appeared that the investment and operation costs of the electrolyzer would be very high. As an example, it was estimated that in case of a 500 MW power station operating with coal containing 2 wt % of sulphur, the electrolyzer represents about 25% of the total investment costs and about 30% of the viable operating costs.
Alternatives to the electrolysis step have been investigated, but, until now could not be realized. For example, catalytic oxidation of hydrogen bromide in the gas phase with oxygen present in the flue gas is not feasible due to the presence of sulphur dioxide. The eventually formed bromine will react directly with SO.sub.2 to SO.sub.3 and subsequently to H.sub.2 SO.sub.4 in the vapour phase. This will give rise to the formation of an excessive amount of very fine acid droplets (areosol formation), which are extremely difficult to abate.
The arrangements of the present invention, however, allow for the separation of the hydrogen bromide to be oxidized in a sufficiently pure form to avoid the mist/aerosol formation. This result is obtained by a modification of the procedure of acid concentration of the original so called Ispra mark 13A process.
The vapour phase oxidation of hydrogen bromide (reaction (2)) is also a known process. There are a considerable number of disclosures for the catalyic gas phase oxidation of hydrogen bromide. Nearly all apply to the treatment of gas mixtures resulting from bromination reactions in organic chemicals manufacturing processes.
The surprising innovation of the present invention is that by making use of the particular properties of the vapour/liquid equilibrium of the reactor liquid, it has been possible to design a system to regenerate the consumed bromine by catalyic vapour phase oxidation, and to avoid the electrolysis step, which is cumbersome and expensive. This represents a considerable advantage with respect to the electrolytic decomposition of hydrogen bromide.
Accordingly, the present invention provides a process for the removal of sulphur dioxide from waste gas which process comprises contacting a waste gas with an aqueous solution containing sulphuric acid, hydrogen bromide and bromine to form sulphuric acid and hydrogen bromide; catalytically oxidizing in the vapour phase the hydrogen bromide formed to bromine and thereafter recycling the bromine to the first step of the process.
The SO.sub.2 -containing flue
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patent: 3353916 (1967-11-01), Lester
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patent: 4668490 (1987-05-01), van Velzen et al.
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Langenkamp Heinrich W.
Papameletiou Dimosthenis
van Velzen Daniel
European Economic Community
Straub Gary P.
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