KR100310109B1 - Absorption of sulfur oxides and simultaneous production of ammonium sulfate - Google Patents

Abstract

The present invention relates to a process for removing sulfur oxides from a sulfur dioxide containing gas and simultaneously producing ammonium sulfate. The process involves first passing a hot sulfur oxide containing gas through a precleaner to bring the gas into contact with a saturated aqueous ammonium sulphate solution that is recycled to the precleaner and passing the precleaned gas through an absorber to contact the precleaned gas with a diluted aqueous ammonium sulphate solution. Is performed. Sulfur oxide in the sulfur oxide containing gas is absorbed with a dilute aqueous solution of ammonium sulfate in the absorber and the washed gas is removed from the absorber. The diluted aqueous ammonium sulfate solution is treated with ammonia and air and the absorbed sulfur dioxide is converted to ammonium sulfate in the liquid. The diluted aqueous ammonium sulphate solution is recycled to contact the precleaned gas in the absorber. The diluted aqueous ammonium sulphate solution is removed from the absorber and added to the saturated aqueous ammonium sulphate solution in the precleaner to saturate due to the evaporation generated by the hot gas. The ammonium sulfate crystal form in the saturated aqueous ammonium sulfate solution in the precleaner is recovered as a product from the saturated aqueous ammonium sulfate solution extracted from the precleaner.

Description

Absorption of sulfur oxides and simultaneous production of ammonium sulfate

The present invention relates to an improved method of removing sulfur oxides from a sulfur oxide-containing gas, more particularly an improved method of absorbing sulfur oxides from a gas containing sulfur oxides and simultaneously producing ammonium sulfate.

To alleviate the severe environmental and health damage associated with sulfur dioxide, many countries require legislation to control the release of sulfur dioxide from sources such as fossil fuel fired boilers, smelters, sulfuric acid plants, pulp and paper milling operations. The most widely used method of controlling sulfur dioxide is based on contacting the flue gas in the form of an aqueous slurry with limestone or lime. In most cases, the by-products are used as road fillers to be disposed of or converted to gypsum, which is used to make wallboard and cement. In some cases, useful alkaline by-products such as pure sulfur dioxide, sulfuric acid and sulfur have been recovered using other alkaline reagents such as sodium compounds, magnesium compounds and ammonia.

Ammonia or ammonia cleaning solutions are also well known as desulfurization agents of flue gases. In US Pat. No. 4,690,807, incorporated herein by reference, gas containing one or more sulfur oxides is treated with ammonia water to produce ammonium sulfate in a single vessel comprising an absorption tower and a liquid reservoir. The sulfur oxide gas (es) are removed from the gas containing sulfur oxides by contact with aqueous ammonium sulfate solution. Sulfur oxide gases such as sulfur dioxide become acidic as they are absorbed into the ammonium sulfate solution. In US Pat. No. 4,690,807, ammonia is injected into ammonium sulfate solution to maintain the desired pH level sufficient to neutralize the acidic solution to prevent excessive ammonia loss. In the neutralized ammonium sulfate solution in which sulfur oxide gas is absorbed, an oxidizing medium such as air is injected, thereby forming ammonium sulfate. The ammonium sulfate product can be recovered as ammonium sulfate crystal suspension, as saturated solution and / or as unsaturated solution. In US Pat. No. 4,690,807, the recovered ammonium sulfate can be further crystallized and / or dehydrated and dried for convenience in handling and storage.

In general, it is known that the faster the oxidation rate, the more economical the method depends on the oxidation rate. It has been found that the rate of oxidation of ammonium sulfite / ammonium sulfite formed by absorbing sulfur dioxide in ammonium sulfate solution and reacting with ammonia depends on the concentration of ammonium sulfate in the ammonium sulfate solution and the rate of oxidation decreases with increasing concentration. Although it is efficient to remove sulfur dioxide from flue gas and simultaneously produce ammonium sulfate in the system and method of US Pat. No. 4,690,807, it is always desirable and advantageous to improve the cleaning efficiency of the gas. It would therefore be advantageous to provide a method and apparatus for removing sulfur oxides from a sulfur oxide containing gas simultaneously with the production of ammonium sulfate, which increases the rate of oxidation by reducing the concentration of ammonium sulfate used to absorb the sulfur oxide gas. .

In order to achieve these and other advantages, and in accordance with the purpose of the present invention, an improved method and system for removing sulfur oxides from gases containing sulfur oxides, as embodied and explicitly described herein, is provided. The efficiency of removing sulfur oxides from the flue gas was improved by absorbing sulfur dioxide in dilute ammonium sulfate solution or dilute ammonium sulfate and oxidizing ammonium sulfite. The resulting ammonium sulfate dilution is evaporated in a preclean vessel to form a saturated aqueous ammonium sulfate solution containing a suspension of ammonium sulfate crystals.

The hot flue gas containing sulfur dioxide and optionally HCl and HF is contacted with saturated aqueous ammonium sulfate solution in a preclean vessel providing close contact between the hot flue gas and saturated aqueous ammonium sulfate solution. The preliminary washing vessel is supplied with a dilute aqueous ammonium sulfate solution from a sulfur dioxide absorption vessel. The hot flue gas is cooled in a preclean vessel and saturated with water vapor. When HCl or HF are present, they are also removed in the precleaner. The pH of the preclean solution is low (eg about pH 0.5 to 3.0) and therefore sulfur dioxide in the precleaner is rarely removed.

The droplets entrained in the demister are then removed, and the cooled gas is then introduced into a sulfur dioxide absorption vessel to remove sulfur dioxide from the gas using a dilute aqueous ammonium sulfate solution. The absorbed sulfur dioxide is reacted with ammonia in the liquid to form ammonium sulfite and ammonium sulfate is formed by oxidizing by injecting an oxygen-containing gas, preferably air. Ammonia may be introduced into the liquid together with the air for oxidation, or ammonia and air may be injected separately. The sulfur dioxide absorption vessel provides intimate contact of the sulfur dioxide containing gas with a dilute aqueous ammonium sulfate solution.

According to the present invention, further ammonium sulfate crystals and sulfur oxides are contacted by contacting a hot gas containing sulfur oxides in a preclean vessel with a saturated aqueous ammonium sulfate solution in which ammonium sulfate crystals are suspended, and by adiabatic cooling the gas stream to evaporate water. And a precleaned gas containing water vapor. Any HCl and / or HF in the sulfur oxide-containing gas stream is removed in the precleaner. Saturated aqueous ammonium sulphate solution, through which the sulfur oxide-containing gas has passed, is collected in a preclean reservoir. The saturated aqueous ammonium sulfate solution through which hot gas containing sulfur oxides passes in the preclean container is the saturated aqueous ammonium sulfate solution recycled from the pre-cleaner reservoir, and the saturated aqueous ammonium sulfate solution in which the ammonium sulfate crystals are suspended is It is removed from the precleaner reservoir for recovery.

The precleaned gas containing sulfur oxides and water vapor passes through a demister to remove entrained saturated aqueous ammonium sulfate solution and the ammonium sulfate crystals suspended therein. The precleaned gas containing sulfur oxides and water vapor is passed through a demister and then contacted with a dilute aqueous solution of ammonium sulfate in the sulfur oxide absorber to form a dilute aqueous solution of ammonium sulfate containing the absorbed sulfur oxides and the cleaned gas. Dilute ammonium sulphate aqueous solution in which sulfur oxide gas is absorbed is collected in the absorber reservoir. By introducing ammonia into the diluted aqueous ammonium sulfate solution absorbed by the sulfur oxide gas and introducing the oxygen-containing gas into the diluted aqueous ammonium sulfate solution absorbed by the sulfur oxide in the absorber reservoir, the absorbed sulfur oxide gas was absorbed into the ammonia and oxygen-containing gas. Reaction with oxygen forms ammonium sulfate in a dilute aqueous solution of ammonium sulfate contained in the absorber reservoir. The dilute ammonium sulfate aqueous solution in which the precleaned gas containing sulfur oxides is contacted in the sulfur oxide absorber is formed from the dilute ammonium sulfate aqueous solution recycled from the absorber reservoir, and the washed gas is recovered from the sulfur oxide absorber.

Meanwhile, according to a specific aspect of the present invention, after removing the saturated ammonium sulfate solution from the pre-cleaner reservoir, the ammonium sulfate crystals are separated from the saturated ammonium sulfate solution in which the ammonium sulfate crystals are suspended, and the saturated ammonium sulfate aqueous solution optionally removed from the ammonium sulfate crystals. Recycle to the precleaner and / or prewasher reservoir. According to another aspect of the present invention, the extraction stream of saturated aqueous ammonium sulfate solution is optionally removed from the precleaner, such as impurities, chlorides and chlorides present as reaction products from other components trapped in or contained in the gas. (Or) treated to remove fluoride. According to another aspect of the present invention, dilute aqueous ammonium sulphate solution recovered from the absorber reservoir is added to the saturated aqueous ammonium sulfate solution in the precleaner as supplemental water, and optionally to the saturated aqueous ammonium sulphate solution recovered from the absorber reservoir. Clean the demister between the precleaner and the absorber before application. According to another aspect of the invention, make-up water is added to the dilute aqueous ammonium sulfate solution in the absorber.

As used herein, sulfur oxides generally refer to sulfur dioxide for convenience. The sulfur oxide gas may also contain other components, including gases that react with ammonium sulfate, such as hydrogen chloride, hydrogen fluoride, and the like, and mixtures thereof.

The ammonium sulfate product can be used directly as a fertilizer or in admixture with other fertilizers.

It is to be understood that the foregoing general description and the following detailed description are exemplary and explanatory and are intended to further explain the invention as claimed.

The accompanying drawings, which are incorporated in and constitute a part of the specification, describe the present invention in detail, and together with the description of the present invention, serve to explain the advantages and features of the present invention.

In the present invention, a gas containing one or more sulfur oxides is contacted with a spray solution, mist (in droplet form), or a dilute aqueous ammonium sulfate solution in a suitable form in a sulfur oxide absorber (absorption vessel) for removing sulfur oxides. The treatment forms an aqueous ammonium sulphate solution with absorbed sulfur oxides and a cleaned gas. Dilute ammonium sulfate aqueous solution in which sulfur oxides are absorbed is collected in a reservoir, commonly referred to herein as an absorber reservoir. Ammonia is introduced into a dilute aqueous ammonium sulfate solution in which sulfur oxide gas is absorbed. Ammonia may be introduced into the aqueous ammonium sulphate solution at several points in the absorber, preferably at several points within the system where the absorber reservoir and / or the dilute aqueous ammonium sulphate solution is circulated or recycled from the absorber reservoir to the dilute ammonium sulphate aqueous solution cleaning zone in the absorption vessel. . Ammonia reduces the acidity of dilute aqueous ammonium sulfate solution with sulfur dioxide absorbed. Dilute ammonium sulfate aqueous solution treated with ammonia and with reduced acidity is referred to herein as neutralized dilute aqueous ammonium sulfate solution.

Oxygen-containing gases, such as air, generally air, are introduced into a dilute aqueous solution of ammonium sulfate absorbed by sulfur dioxide in the absorber reservoir to react the absorbed sulfur oxide gas with oxygen in the ammonia and oxygen-containing gas to form ammonium sulfate in the absorber reservoir. .

The dilute aqueous ammonium sulfate solution in contact with the gas containing sulfur oxides in the sulfur oxide absorber is removed from the absorber reservoir and formed from the dilute aqueous ammonium sulfate solution and circulated by suitable means such as a pump through a suitable conduit such as a pipe. do. The washed gas is recovered from the sulfur oxide absorber, and the diluted aqueous ammonium sulfate solution recovered from the sulfur oxide absorber is collected in the absorber reservoir and recovered therefrom and supplied to the precleaner as an ammonium sulfate supplement solution. Saturated by evaporation.

Before introducing the gas into the sulfur oxide absorber to remove the sulfur oxides, the gas is prewashed in a preclean vessel, which adiabatically cools the gas stream and consequently evaporates water in a saturated aqueous ammonium sulfate solution. A concentrated or saturated aqueous solution of ammonium sulfate is formed in which the crystals are suspended. Hot gases containing sulfur oxides are contacted with a spray, mist (droplet form) or other suitable form of saturated aqueous ammonium sulfate solution in a preclean vessel, and the cooled preclean gas and water vapor containing sulfur oxides are precleaned. By recovering from and passing through a precleaner demister, the entrained saturated aqueous ammonium sulfate solution containing crystals is removed therefrom before the precleaned gas containing sulfur oxides and water vapor is passed through the sulfur oxide absorber. Thus, since water vapor is removed from the precleaned gas stream, the saturated aqueous ammonium sulphate solution in the precleaner is concentrated to crystallize the ammonium sulphate and a saturated aqueous ammonium sulphate solution in which the ammonium sulphate crystals are suspended in the precleaner is formed.

Saturated ammonium sulphate aqueous solution in which the ammonium sulphate crystals are suspended is collected in a reservoir commonly referred to as a preclean reservoir.

A saturated ammonium sulfate aqueous solution in contact with a hot flue gas containing sulfur oxides in a preclean vessel is formed from a saturated aqueous ammonium sulfate solution in which suspended (recirculated) ammonium sulfate crystals removed from the prewasher reservoir are suspended, It is circulated through a conduit by suitable means, such as a pump. The saturated aqueous ammonium sulphate solution, in which water vapor has been removed by contact with a hot gas, is collected in a prewasher reservoir, from which the ammonium sulphate crystals can be recovered as a saturated aqueous ammonium sulphate solution to recover the ammonium sulphate crystals as a product.

The saturated aqueous ammonium sulfate solution may be stirred in the region of the recovery zone by any suitable stirring means such as one or several stirrers. By stirring the saturated aqueous ammonium sulfate solution in the recovery zone, a slurry of ammonium sulfate crystals is maintained in saturated aqueous ammonium sulfate solution and prevents the deposition of ammonium sulfate crystals at the bottom of the prewasher reservoir. The saturated aqueous ammonium sulfate solution in which the ammonium sulfate crystals are suspended may be removed from the recovery zone by any suitable method, for example by gravity removal means present at or near the bottom of the pump or prewash reservoir. Any crystals contained in a saturated aqueous ammonium sulfate solution can be easily removed, for example, by filtration. The product may also be classified, recrystallized and / or dehydrated and dried for convenience in handling and storage.

According to one aspect of the present invention, a saturated ammonium sulfate solution in which ammonium sulfate crystals are suspended is passed from a precleaner to a classifier; Coarse crystals of ammonium sulfate are separated from the liquid in a classifier; Liquid containing crystals of ammonium sulfate having a particle size smaller than the coarse crystals is recycled to a preclean vessel or prewasher reservoir.

There are no specific restrictions on the particle size of the particles that are separated in the classifier, for example the hydroclone, but those skilled in the art can easily select the desired size of the ammonium sulfate crystals from which the ammonium sulfate crystals are separated from the saturated aqueous ammonium sulfate solution. have. In the present specification, there is no intention to limit the specific size of the ammonium sulfate crystals separated from the liquid, but the coarse crystals may generally be defined as having a size of about 300 microns or more, for example. Typically, liquids containing ammonium sulfate crystals having a particle size smaller than coarse crystals as used herein, i.e., containing crystals of about 300 microns or less, can be recycled to a preclean vessel or prewasher reservoir. have.

The process of the invention is easily carried out in a continuous manner. Any other sulfur oxide-containing gas obtained from flue gas or preferably any other source having dust or other particles removed therefrom is introduced into the precleaner and co-flowed with a saturated ammonium sulfate aqueous solution flow. The cooled and precleaned gas exits the preclean vessel through a precleaner demister and is introduced into the sulfur oxide absorption vessel in a continuous manner to countercurrent to a dilute aqueous ammonium sulphate flow. The cleaned gas flows continuously from the absorption vessel and is removed through suitable means, for example a stack, by passing the cleaned gas through a demister to remove the dilute aqueous ammonium sulfate solution therefrom. Generally, the gas is continuously moved through the system using one or more fans, for example an induction gas fan.

Gases pass continuously through the precleaner and absorber, while each liquid is continuously recycled to a spray, mist or other suitable form, providing intimate contact between the gas and the liquid in each reaction vessel and saturating with ammonium sulfate crystals. The aqueous ammonium sulphate solution is continuously collected from the precleaner, and the diluted aqueous ammonium sulfate solution is continuously collected from the absorber and fed to the precleaner.

The liquid amount is maintained by any suitable means in the preclean vessel and prewash reservoir and in the sulfur oxide absorption vessel and reservoir. On the other hand, in the present invention, the amount of liquid in the preclean vessel and / or prewasher reservoir tends to decrease due to the evaporation of water and the recovery of the liquid product containing ammonium sulfate crystals. To reverse this tendency and maintain a constant amount of liquid in the preclean vessel, an appropriate amount of dilute aqueous ammonium sulfate solution is withdrawn from the absorber reservoir and added to the prewasher reservoir. This recovery tends to reduce the amount of liquid in the absorber reservoir, so that an appropriate amount of make-up water is added to the absorber vessel or absorber reservoir to maintain a constant amount of liquid. SO exact concentration of ammonium sulfate that can be stored in the _second absorption vessel is a function of the amount of water absorbed and the amount applied to the container and absorption of SO ₂ oxidation with ammonium sulfate. The amount of water added to the absorption vessel is equal to the amount of liquid recovered and the factor controlling the amount of water acceptable is the evaporation rate since this water is added to the preclean vessel to compensate for the loss due to evaporation. The evaporation rate is a function of the temperature of the hot gas introduced into the preclean vessel. 3 shows the effect on SO ₂ concentration and gas temperature on the concentration of ammonium sulfate that can be maintained in the liquid of the absorption vessel.

Replenishment water is preferably added to the absorption vessel through the absorber demister in the absorption vessel to clean any deposits on the absorber demister blade and collect it in the absorption vessel. Similarly, dilute aqueous ammonium sulfate solution recovered from the absorption vessel is first used to clean the precleaner demister before adding it to the preclean vessel as supplemental water.

In the method of the present invention, it is important to maintain the aqueous ammonium sulfate solution in the absorption vessel and the absorber reservoir with the diluted aqueous solution of ammonium sulfate to improve the oxidation efficiency of sulfur dioxide absorbed in the fluid. This is accomplished by maintaining the dilute concentration of ammonium sulfate in the absorber liquid by removing the dilute aqueous ammonium sulfate solution from the absorber and adding supplemental water so that sulfur dioxide is oxidized to the maximum or near maximum as shown in the graph of FIG. do. As shown in FIG. 1, an aqueous ammonium sulphate solution of ammonium sulfite / anmonium bisulfite was tested to demonstrate that the rate of oxidation depends on the concentration of ammonium sulphate in the liquid. As shown in Figure 1, as the ammonium sulfate is saturated, the oxidation rate of mM / l / min decreases substantially, for example as the concentration of ammonium sulfate in the aqueous solution approaches at least about 40% by weight. The present invention provides a process for the absorption of sulfur dioxide and a rapid oxidation of the aqueous solution of dilute ammonium sulphate in the absorption unit, which occurs with the concentration of the diluted aqueous solution of ammonium sulphate in the preclean unit.

In the process of the present invention, a saturated aqueous ammonium sulphate solution generally contains from about 45 to about 50% by weight of ammonium sulfate in water and the dilute aqueous ammonium sulphate solution will generally contain ammonium sulfate in an amount less than that required for saturation. will be. However, in a preferred embodiment of the present invention, the dilute ammonium sulfate aqueous solution or diluent contains up to about 35% by weight of ammonium sulfate in water. There is no lower limit to the concentration of dilute aqueous ammonium sulphate solution or diluent, but it is generally or almost advantageous to use dilute aqueous ammonium sulphate solution containing less than about 5 wt%, more preferably less than about 15 wt% ammonium sulphate in water. Not. In a most preferred embodiment of the invention, the concentration of dilute aqueous ammonium sulfate solution is maintained at a concentration of about 10 to 35% by weight of ammonium sulfate in water.

In the prewasher reservoir, the saturated aqueous ammonium sulfate solution tends to be supersaturated to allow the growth of the ammonium sulfate crystals as mentioned above and such crystals are removed continuously or intermittently as desired from the saturated aqueous ammonium sulfate solution. Thus, the prewasher reservoir is for the growth of ammonium sulfate crystals and serves as a container to prevent supersaturation of the aqueous ammonium sulfate solution.

As mentioned above, the hot flue gas may also contain hydrogen chloride and / or hydrogen fluoride gas. If such gas is present in the hot sulfur oxide-containing flue gas, it is removed from the preclean vessel to form an ammonium salt. The salts can be removed together with the ammonium sulphate product or separately by taking the extract stream of the aqueous ammonium sulphate solution from the precleaner.

In general, the apparatus of the present invention for the removal of sulfur oxide gas from high temperature sulfur oxide-containing gases and for the simultaneous production of ammonium sulphate is characterized in that a precleaner unit or tower associated with a suitable precleaner reservoir, and sulfuric acid associated with a suitable absorber reservoir A cargo absorber unit or tower. The prewasher reservoir has an ammonium sulfate product recovery zone. Dust collectors or other particle removal means are typically located in an upstream stream of the preclean vessel. Means are provided for delivering gas from a dust collector to a preclean vessel and to a sulfur oxide absorption vessel or tower having means for removing the cleaned gas from the preclean vessel. Means are provided for transferring the dilute aqueous ammonium sulphate solution from the absorber reservoir to the precleaner reservoir and adding supplemental water to the absorber unit.

The precleaner unit is for washing hot sulfur oxide gas with a saturated aqueous ammonium sulfate solution and for extracting or removing a saturated aqueous ammonium sulfate solution in which ammonium sulfate crystals are suspended from a preclean vessel, a preclean reservoir, a prewash demister, and a prewash reservoir. Means, means for introducing a dilute aqueous ammonium sulphate solution, means for contacting a gas with a saturated aqueous solution of ammonium sulphate in the precleaner, means for recovering a saturated ammonium sulphate solution from the precleaner reservoir and contacting the gas in the precleaner, Any suitable design may be included, including means for flowing the sulfur oxide-containing gas in parallel with a flow of saturated aqueous ammonium sulphate solution in the precleaner and means for stirring the saturated aqueous ammonium sulphate solution in the preclean reservoir. The preclean vessel and prewash reservoir may be a single unit with a preclean reservoir located at the bottom of the preclean vessel, or the prewash reservoir may be connected by suitable means as a vessel or tank separate from the preclean vessel and saturated aqueous ammonium sulphate solution. Can be brought from the preclean vessel to the prewasher reservoir, preferably by gravity.

The absorber unit allows the sulfur oxide gas to be absorbed in intimate contact in dilute aqueous ammonium sulfate solution, for example in the form of a spray solution or mist and flows through the absorber reservoir to flow the dilute ammonium sulfate aqueous solution from the absorption vessel to the absorber reservoir. Means for contacting the gas with a dilute aqueous solution of ammonium sulphate in the absorption vessel, means for recovering the dilute aqueous ammonium sulphate solution from the absorber reservoir and contacting the gas in the absorber, and refluxing the gas entering the absorption vessel into the dilute aqueous solution of ammonium sulphate. Means for passing in a direction, means for removing the dilute aqueous ammonium sulfate solution from the absorber, absorber demister, ammonia and / or aqueous ammonia and air for removing the diluted aqueous ammonium sulfate solution from the cleaned gas. To inject It can be of any design, including means, means for removing the cleaned gas from the absorbent vessel, and means for applying replenishment water to the absorber unit. The absorbent vessel and absorber reservoir may be a single unit or may be separate units that flow through one another to allow dilute aqueous ammonium sulfate solution to flow from the absorber vessel to the absorber reservoir, preferably by gravity.

In FIG. 2, a hot gas stream containing sulfur dioxide gas is generated by boiler 2 from source 3 and gas 4. The hot sulfur dioxide passes from the boiler (2) through a suitable flue gas conduit (5) to the dust collector (4) so that the particulate material is known in the art, for example cyclone separators and / or bags. The ash is removed by a bag house and the ash is removed via a suitable hopper in the recovery unit 6. The hot sulfur dioxide containing gas from which the dust particles have been removed is delivered to the induction gas fan 8 which propels the gas through the gas conduit 7. The hot sulfur dioxide containing gas is delivered from the fan 8 through the gas conduit 9 and through the gas conduit 32 into the precleaner 10.

The precleaner 10 has a top 12 defined herein as a preclean vessel 12 and a bottom 14 defined herein as a preclean reservoir 14. The precleaner reservoir 14 has a saturated aqueous ammonium sulphate solution, which is removed from the precleaner reservoir 14 by means of a pump 22 via a suitable conduit 24 and located in the precleaning vessel 12. 16) is recovered as a spray liquid or mist. Conduit 26 distributes the recovered saturated aqueous ammonium sulphate solution into a distributor (not shown) to clean the inner wall of the preclean vessel 12 with a preclean wall of the liquid down the stream to deposit solid material or To prevent deposition on walls; The injection means 16 introduces a saturated aqueous ammonium sulphate solution 34 into the preclean vessel 12 in a co-current direction with a flow of hot sulfur dioxide gas entering the precleaner 10 from the gas conduit 32. Conduit 36 has the outlet of precleaner 10.

The hot sulfur dioxide containing flue gas is generally adiabatic cooled in the preclean vessel 12 at a temperature of about 93 ° C. (200 ° F.) to 204 ° C. (400 ° F.). Cooling takes place at a constant temperature and absorbs heat as evaporative heat from hot sulfur dioxide containing gas, and the temperature is kept constant. Water from saturated aqueous ammonium sulphate solution is water vapor in the hot sulfur dioxide containing gas and the sulfur dioxide containing gas flows out of the precleaner 10 among other components as a cooled precleaning gas containing sulfur dioxide and water vapor. Evaporation of water from saturated aqueous ammonium sulphate solution concentrates the aqueous ammonium sulphate solution to form ammonium sulfate crystals in the liquid. Saturated ammonium sulphate aqueous solution in which the ammonium sulphate crystals are suspended is collected in the precleaner reservoir 14 and sprayed in the upper portion 12 of the precleaner 10 as mentioned above via a conduit 24 by means of a pump 22. (16) Recover into. The temperature of the precleaned sulfur dioxide containing gas upon exiting precleaner 10 is from about 43 ° C. (110 ° F.) to about 66 ° C. (150 ° F.). Precleaned flue gas in gas conduit (36) is passed through precleaner demister (20), where the entrained saturated aqueous ammonium sulfate solution in which ammonium sulfate crystals are suspended is removed from the precleaned gas and the liquid and suspended crystals are suitable. The conduit 38 is withdrawn to the prewasher reservoir 14.

The precleaned gas passing through precleaner demister 20 passes through gas conduit 75 to reach absorber 40, where sulfur dioxide is removed from the gas by absorption.

Absorber 40 has a top defined as absorbent vessel 42 and a bottom defined as absorber reservoir 44. Absorber reservoir 44 contains dilute aqueous ammonium sulfate solution 48. Flue gas enters absorber 40 through gas conduit 75 at the bottom of absorption vessel 42 but above the height of the dilute aqueous ammonium sulfate solution 48.

Dilute aqueous ammonium sulfate solution 48 is recycled from absorber reservoir 44 through conduit 72 by means of pump 54 and through conduits 52 and 50 to reach absorber injection vessel 74. Where the dilute aqueous ammonium sulphate solution is absorbed in the direction of flow and countercurrent of the pre-cleaned gas stream, which is a washed gas entering the absorber 40 in the gas conduit 75 and exiting the absorber 40 in the gas conduit 77. Form mist or spray liquid that passes into the vessel 42.

The diluted aqueous ammonium sulfate solution absorbs sulfur dioxide, and the diluted aqueous ammonium sulfate solution absorbed sulfur dioxide passes through the absorber reservoir 44, preferably by gravity, and accumulates in the reservoir and absorber spray means 74 from the absorber reservoir 44. Is recovered.

In absorber 40, ammonia gas from source 60 is delivered to absorber reservoir 44 via conduits 63 and 58 by suitable pump 62. At the same time, an oxygen source, such as air from source 80, is delivered to absorber reservoir 44 through conduit 82. The reaction of sulfur dioxide absorbed in dilute aqueous ammonium sulfate solution with ammonia and air produces ammonium sulfate.

The diluted aqueous ammonium sulphate solution 48 is withdrawn from the absorber reservoir 44 through conduit 68 and delivered by pump 64 through conduit 46 to precleaner demister 20 and then conduit 38. Delivered to prewasher reservoir 14 to provide supplemental liquid lost through evaporation and withdrawal of product in prewasher 10. As the dilute aqueous ammonium sulfate solution is removed from the absorber reservoir 44, supplemental water from source 78 is added to the absorber 40 via conduits 88 and 89, which are respectively sprayed means 92 and 76. Is injected into the absorption vessel 42. The water entering the absorption vessel 42 through the conduit is preferably used in conjunction with the spray remover 95, ie the absorber demister 95. The cleaned gas may exit the absorber 40 in the gas conduit 77, pass through any gas reheater 90 and then pass through the gas conduit 86 to the chimney 84 and the cleaned gas may be Emitted into the atmosphere.

The prewasher reservoir 14 is equipped with one or more agitators 18, and the absorber reservoir 44 is equipped with one or more agitators 70 to continue stirring the liquid in each reservoir.

Although not shown in the figure, the prewasher reservoir 14 can be a tank or unit separate from the preclean vessel 12 and the absorber reservoir 44 is adapted to allow liquid to flow from the vessel to each reservoir. It may be a unit or tank separate from the absorbent vessel 42 as provided.

Saturated ammonium sulphate aqueous solution in which the ammonium sulphate crystals are suspended can be used in various ways to separate the ammonium sulphate crystals from the saturated aqueous ammonium sulphate solution when removed from the precleaner reservoir 14 through the conduit 28. As shown in FIG. 2, the liquid containing ammonium sulfate crystals is delivered to the product recovery unit 100 via conduit 138 using pump 30. The liquid containing ammonium sulfate crystals is delivered to a primary dehydration device, for example hydroclone 120. The backflow 122 containing the hydroclone 120 and the ammonium sulfate crystal mass is further dehydrated in the centrifuge 123 to form an ammonium sulfate crystal coke containing about 2% water. The caking binder is then fed to a mill / mill (not shown) and fed by the dryer 124 or directly into the dryer 124 to provide a dry ammonium sulfate product. Dry ammonium sulphate product 126 is stored, while saturated aqueous ammonium sulphate solution containing particulates and crystals is passed through conduit 136 and recovered to recycle tank 128. The liquid removed in the centrifuge 123 is recovered through the conduit 140 to the recirculation tank 128 by the action of the pump 142. Optionally, a small amount of extraction stream 143 can be removed to remove any impurities from the system. As desired, the saturated ammonium sulphate aqueous solution maintained by the stirrer 132 in the recirculation tank 128 is kept in the agitated state by the pump 130 to remove the pre-cleaner reservoir through the conduits 134 and 15. Transfer to (14) and add to the saturated aqueous ammonium sulfate solution (34) in which the ammonium sulfate crystals are suspended.

The air is absorbed into the absorber reservoir 44 by any suitable air injection system, for example an air spreader arranged in a network structure of perforated pipes (not shown) known in the art and disclosed in US Pat. No. 4,690,807. Pour into a dilute aqueous ammonium sulfate solution.

As described above, the pH is maintained such that the acidity of the absorbed sulfur dioxide in the dilute aqueous ammonium sulfate solution in the absorber reservoir 44 is reduced. The pH of the dilute aqueous ammonium sulfate solution in the absorber is generally maintained at about 5-6, more preferably at about 5.2 to about 5.8. The ammonia, preferably injected in the form of gaseous ammonia, is applied to a spreader arranged in the network structure of any suitable means known in the art, for example the perforated pipe (not shown) disclosed in US Pat. No. 4,690,807. By injection into the absorber reservoir 44.

The following specific examples illustrate the method of the present invention. These examples are for illustrative purposes only and are not intended to limit the invention.

Example 1

This example does not describe the method of the present invention but merely describes a method of obtaining data for the graph of FIG. 1 described in detail above.

This example is a study to determine the cause of the variable oxidation rate of the prior art as a bench scale, in a specific case, was carried out below the generally expected oxidation rate.

An aqueous solution containing 20 ppm iron as a ferrous catalyst with varying concentrations of ammonium sulfate and no catalyst was added to several beakers. Various concentrations of sodium sulfite were added to the aqueous ammonium sulfate solution (starting at 10,000 ppm sulfite) and the pH was adjusted to pH 5 with sulfuric acid. The temperature was maintained at 54 ° C. (130 ° F.). Air was passed through a beaker and the rate at which sodium sulfite disappeared in each sample was measured. Data from these experiments were obtained and shown in FIG. 1 and discussed below.

Example 2-7

The examples described below were carried out in an intermediate test plant scale absorber shown in FIG. The source of flue gas is the rupture port of the boiler conduit, and the concentration of sulfur dioxide was increased by the use of a suitable meter and by the use of sulfur dioxide tanks before the flue gas conduit operation.

Using aqueous ammonium sulfate solution in the apparatus of FIG. 2, sulfur dioxide removal efficiencies were compared at various concentrations of sulfur dioxide, expressed in parts per million (ppm) in the table below. In Case I, six working injection banks 74 were used for absorption vessel 42 and in Case II four working injection banks 74 were used for absorption vessel 42. The pH of the dilute ammonium sulfate aqueous solution 48 used in the absorber 40 is shown in Example II of the table below. The amount of ammonium sulfite present in the ammonium sulfate aqueous solution 48 is also shown in the table below.

From the data in the table, it can be seen that the sulfur dioxide removal efficiency is very high when using the method of the present invention using ammonia as the absorber. As shown in the table above, sulfur dioxide removal efficiency varies with sulfur dioxide concentration. Moreover, sulfites in the liquid (in the form of ammonium sulfite) significantly increase the removal efficiency. In the pH range 5.2-5.8, the ammonia concentration gradient is low, 0-3 ppm. Turbidity did not increase during operation. The ammonium sulphate product isolated as (crystal) from the above process had a purity of at least 99.5% with a moisture content of less than 1.2% and 90% of the grain crystals showed a particle size of at least 300 microns.

It will be apparent to those skilled in the art that various modifications and variations can be made to the methods of the invention without departing from the spirit and scope of the invention. Thus, it is intended that the present invention cover the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.

1 is a graph showing the correlation between the rate of oxidation of ammonium sulfite and the concentration of ammonium sulfate in solution.

2 shows a sulfur dioxide source, a sulfur dioxide pre-cleaner, an ammonium sulfate crystal separator, a sulfur dioxide absorption unit, a cleaning gas processor, and the like.

3 is a graph showing the correlation between the concentration of sulfur dioxide, the input gas, and the ammonium sulfate concentration in the sulfur dioxide absorber at two different input gas temperatures.

Explanation of symbols for the main parts of the drawings

2: boiler 3: source

4: Dust Collector 6: Recirculation Unit

8: Induction gas fan 10: Precleaner

12: prewash container 14: prewash reservoir

16: injection means 18: stirrer

20: prewasher demister 34: saturated aqueous ammonium sulfate solution

40: absorber 42: absorption vessel

44 absorber reservoir 48 dilute aqueous ammonium sulfate solution

60: ammonia source 70: stirrer

74 absorber injection container 76 injection means

78: supplemental water source 80: oxygen source

84: chimney 90: gas reheater

92: injection means 95: demister

100: product recovery unit 120: primary dehydrator (hydroclone)

122: reflux 123: centrifuge

124: dryer 126: dry ammonium sulfate product

132: stirrer 143: extraction stream

5,15,24,26,28,38,46,50,52,53, 58, 63,68,72,82,88,89,134,136,138,140: conduit

7,9,32,33,36,75,77,86: gas conduits 22,30,62,64,128,130,142: pumps

Claims (14)

(a) The hot gas containing sulfur oxides is contacted with a saturated aqueous ammonium sulfate solution in a pre-cleaning vessel and the gas is evaporated by adiabatic cooling of the gas to form a saturated aqueous ammonium sulfate solution in which ammonium sulfate crystals are suspended. And forming a precleaned gas containing water vapor; (b) collecting a saturated aqueous ammonium sulfate solution in which the gas containing sulfur oxides has passed in the pre-cleaner reservoir in the pre-cleaning container, and storing the saturated ammonium sulfate solution in contact with a hot gas containing sulfur oxides. Recirculating from the pre-clean container; (c) removing the saturated aqueous ammonium sulfate solution from which the ammonium sulfate crystals are suspended from the precleaner reservoir; (d) passing said precleaned gas containing sulfur oxides and water vapor through a demister to remove entrained saturated aqueous ammonium sulfate solution containing ammonium sulfate crystals therefrom; (e) contacting the precleaned gas containing sulfur oxide and water vapor passed through the demister with a diluted aqueous ammonium sulfate solution in a sulfur oxide absorber to form a diluted aqueous ammonium sulfate solution and a cleaned gas; (f) collecting the diluted aqueous ammonium sulfate solution in which the sulfur oxide gas is absorbed into an absorber reservoir; (g) introducing ammonia into the diluted aqueous ammonium sulfate solution in which sulfur oxide gas is absorbed; (h) introducing an oxygen-containing gas into the dilute aqueous ammonium sulfate solution in which the sulfur oxide in the absorber reservoir is absorbed, thereby reacting the absorbed sulfur oxide gas with oxygen in the ammonia and oxygen-containing gas to form an ammonium sulfate product in the absorber reservoir. Doing; (i) contacting the diluted aqueous ammonium sulfate solution recycled from the absorber reservoir with the precleaned gas containing sulfur oxides in a sulfur oxide absorber; And (j) A method for treating hot gas containing sulfur oxides, comprising recovering the cleaned gas. The method of claim 1, further comprising recovering the diluted aqueous ammonium sulfate solution from the absorber reservoir and adding the diluted aqueous ammonium sulfate solution to the preclean vessel. The method of claim 2, wherein the diluted aqueous ammonium sulfate solution recovered from the absorber reservoir is fed from the absorber reservoir to the demister before adding to the preclean vessel. The method of claim 1 further comprising adding supplemental water to the absorber. The method of claim 1 further comprising removing the crystal of ammonium sulfate from a saturated aqueous ammonium sulfate solution in which the ammonium sulfate crystals are suspended. The method of claim 5 wherein the saturated aqueous ammonium sulfate solution is recycled to a preclean vessel or prewasher reservoir. 2. The saturated ammonium sulfate solution in which ammonium sulfate crystals are suspended is transferred from a precleaner to a classifier, separating coarse crystals of ammonium sulfate from the aqueous solution, and containing ammonium sulfate crystals having a smaller particle size than the coarse crystals. Recycling the aqueous solution to a preclean vessel or prewasher reservoir. The method of claim 1 wherein the sulfur oxide is sulfur dioxide. The method of claim 1 wherein the oxygen containing gas is air. The method of claim 1 wherein the amount of ammonia introduced into the dilute aqueous ammonium sulphate solution is an amount sufficient to reduce the acidity of the ammonium sulphate solution absorbed sulfur oxides. The method of claim 1 wherein the amount of ammonia introduced into the dilute aqueous ammonium sulphate solution is an amount sufficient to prevent loss of ammonia from the dilute aqueous ammonium sulphate solution. The method of claim 1, further comprising passing the cleaned gas through a demister to remove droplets of the diluted aqueous ammonium sulfate solution. The method of claim 1, wherein the hot gas introduced into the precleaner contains HCl, HF, or mixtures thereof, which are removed in the prewasher. The method of claim 1, wherein the concentration of ammonium sulfate in the dilute aqueous ammonium sulfate solution is maintained at a concentration of 5 to 35 weight percent based on the total weight of the liquid.