The production of sulfuric acid from exhaust gases in common industrial operations is normally conducted by a catalytic oxidation stage, where SO2 is converted to SO3 followed by a hydration step whereby SO3 is converted to sulfuric acid vapor. The sulfuric acid is then condensed in a cooling step by indirect heat exchange with a cooling medium, normally air.
U.S. Pat. No. 5,198,206 discloses such a process, in which a gas containing SO3 and an excess of water is subjected to a single wet condensation stage in order to produce condensed sulfuric acid. In this process, sulfuric acid vapors are condensed in a manner where formation of fine acid mist droplets is substantially avoided thereby facilitating the filtration of the acid mist during contact of the gas with a high-velocity filter. Small solid particles acting as nuclei on which the sulfuric acid vapor condenses are provided before the condensation begins in order to restrict the amount of acid mist emitted. This patent further discloses the use of a synthetic feed gas in an experimental setup containing a glass tube for condensing sulfuric acid, where said synthetic gas has been tailored so as to have an acid dew point of 185° C. The synthetic gas entering the single condensing stage corresponds to what is normally obtained after SO2 conversion when lean feed gases, i.e. gases containing well below 6 vol % SO2, for example 0.1 vol % SO2 are treated.
In many practical situations, however, also strong feed gases, i.e. gases containing 6-30%, particularly 10-30 vol % SO2 need to be treated. In plants having a single wet condensing stage, such as in U.S. Pat. No. 5,198,206 the strong feed gas to the plant need to be substantially diluted in air so that the acid dew point of the gas prior to the single wet condensation stage is kept at about 260° C., since the higher the content of SO2 and SO3 in the gas the higher its acid dew point. In this manner, acid mist emissions can be kept at about 10-20 ppmv.
A substantial air dilution, for instance from a strong gas containing 14 vol % SO2 to a gas containing 5-6 vol % SO2 so as to reduce its dew point to 260° C. prior to entering the wet condensing stage has the disadvantage that the process operates with a larger gas flow, and accordingly larger and more expensive plants are required. This problem becomes more pronounced with increasing SO2 in the feed gas. Another disadvantage is that depending on the condenser design and process conditions some acid mist is still emitted from the condenser. Although current acid mist emissions of about 20 ppmv may be permissible, more stringent environmental legislations that will require yet even lower emissions of 5-10 ppmv or even below 5 ppmv are expected in the near future. In particular, acid mist emissions from single wet condensing plants treating gases containing 6 vol % to 30 vol %, particularly 10 vol % to 30 vol % SO2 cannot be kept below 10-20 ppmv H2SO4 by air diluting the gas to a 260° C. acid dew point.
It would therefore be desirable to be able to provide a feed gas to the wet condensation stage with an acid dew point of below 260° C. without requiring a substantial air dilution of the feed gas containing SO2 whilst at the same time achieving acid mist emissions of about 20 ppmv according to current acid mist emission requirements.
It would also be desirable to be able to provide a process of producing sulfuric acid from a feed gas containing SO2 without requiring substantial air dilution of said feed gas whilst at the same time achieving acid mist emissions of 5-10 ppmv or below 5 ppmv.
Another disadvantage of a process with a single wet condensation stage is the equilibrium limitation of the SO2 conversion. Even with the most active commercial SO2 oxidation catalysts, the maximum total SO2 conversion is limited to 99.5-99.7% depending on the O2 concentration in the gas. For higher conversions, expensive tail gas scrubbing with e.g. H2O2 or NaOH must be included. For instance, by air diluting a feed gas containing 8-9 vol % SO2 to a gas containing 5-6 vol % SO2, the equilibrium conversion at 380° C. of SO2 to SO3 in the catalytic converter upstream the condensation stage may only be increased from 99.5 to 99.7%, which falls short of today's SO2 conversion requirements of above 99.7%.
It has been known for years that the conversion of SO2 to SO3 may be increased by introducing a second absorber before the final conversion stage, yet the final absorption stage has normally been conducted under dry conditions, where the molar ratio of SO2+SO3 to water in the feed gas to the SO2 converter is above 1. For instance U.S. Pat. No. 4,368,183 discloses a wet/dry-process of producing concentrated sulfuric acid utilizing an intermediate absorption stage. SO2-containing gases are catalytically converted to SO3 in a first contacting stage in a SO2 converter. The gas containing SO3 and water vapor, where the molar ratio of water to SO3 is below one is then passed to an intermediate absorption stage including a Venturi where sulfuric acid is produced. Dry exit gas from this intermediate stage is returned to the SO2 converter so that remaining SO2 is catalytically converted to SO3 in a second contacting stage and finally passed to an end absorber, in which the final production of sulfuric acid is conducted under dry conditions in the substantially SO2-free gas. Hence, this process can handle feed gases with a molar ratio of SO2+SO3 to water above about 1, whereby the gas emerging from the intermediate absorption tower is dried by the sulfuric acid. For feed gases with more water than SO2+SO3 on a molar basis, the gas emerging from the intermediate absorption stage contains water vapor that condenses in the final absorption tower as an acid mist, which is difficult to remove in an economical way.
It would be desirable to be able to treat feed gases with a molar excess of water vapor compared to SO2+SO3 and still be able to produce concentrated sulfuric acid of above 98 wt % since such wet feed gases are common in industrial operations, for example from sulfur burners, metallurgical operations such as ore roasting and from combustion of hydrogen-containing fuels, e.g. spent acid from petrochemical alkylation, hydrogen sulfide, ammonium sulfate waste and fossil fuels including heavy oil residues and petroleum coke. In particular, it would be desirable to treat these gases whilst at the same time being able to cope with more stringent requirements for SO2 conversion of above 99.7% and acid mist emissions of 5-10 ppmv or below 5 ppmv.
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