Mist formation, sulfur

Process air in sulfur-burning plants is dried by contacting it with 93—98 wt % sulfuric acid in a countercurrent packed tower. Dry process air is used to minimise sulfuric acid mist formation in downstream equipment, thus reducing corrosion problems and stack mist emissions. 

The acid mist emission problem can arise from any of several factors. Water vapor in the air feed to the sulfur burner may cause mists because, as the water vapor plus sulfur trioxide stream drops below the dewpoint (condensation) temperature, sulfuric acid aerosol formation occurs. If this cooling happens in the absorbers, it may result in mist losses. Mist formation is minimized or eliminated by adequate drying of the air fed to the sulfur burner with concentrated sulfuric acid. 

Angelo [7] has shown that during periods of continual surface renewal, the actual mass transfer coefficient may be fifteen times as large as that predicted by boundary layer theory. Thus, the unsteady state absorption during surface renewal is a more complex situation not covered by these theories. In order to describe a fog or mist formation>it is necessary to study droplet growth by condensation with no Internal turbulence. Bogaevskii [2] reported water droplets growing by water vapor condensation in a mine shaft to absorb about six times more sulfur dioxide than that predicted by steady state absorption. 

SO3 combines with water vapor present in the gases and the sulfuric acid formed is condensed on specially designed condensers (where acid mist formation is minimized). 

Caution is necessary to avoid condensation of an aqueous phase in the system because of the extreme corrosive nature of the liquid phase. Another operating issue concerns mist formation, a phenomenon which occurs very readily when condensing sulfur, and a series of demisters are necessary to prevent this. Residual sulfur is converted to sulfur dioxide by incineration of the tail gas from the process to prevent emission of other sulfur compounds and to dilute the effluent to reduce ground level sulfur dioxide concentrations. 

Mist formation may also occur in gas/liquid reactions reactants may evaporate, react in the gas phase and form a liquid mist that will not coalesce with the other liquid phase. The temperature of the mist may rise, and uncontrolled reactions may take place in the mist particles. Examples of these are found in the processes for the manufacture of nitric and sulfuric acids, and in the preparation of ammonium nitrite from ammonium carbonate solution and nitrous oxides. In the nitric acid production, the product in the mist may be recovered by an effective separation of the mist particles (with demisters, wet scrubbers or electrostatic filters). But in the ammonium nitrite process, the product formed in the mist phase may subsequently decompose into nitrogen and water, thus reducing the process yield. 

When a liquid or solid substance is emitted to the air as particulate matter, its properties and effects may be changed. As a substance is broken up into smaller and smaller particles, more of its surface area is exposed to the air. Under these circumstances, the substance, whatever its chemical composition, tends to combine physically or chemically with other particles or gases in the atmosphere. The resulting combinations are frequently unpredictable. Very small aerosol particles (from 0.001 to 0.1 Im) can act as condensation nuclei to facilitate the condensation of water vapor, thus promoting the formation of fog and ground mist. Particles less than 2 or 3 [Lm in size (about half by weight of the particles suspended in urban air) can penetrate the mucous membrane and attract and convey harmful chemicals such as sulfur dioxide. In order to address the special concerns related to the effects of very fine, iuhalable particulates, EPA replaced its ambient air standards for total suspended particulates (TSP) with standards for particlute matter less than 10 [Lm in size (PM, ). 

Sulfuric acid is added to the assembled batteries and the plates are formed within the batteries by applying electric voltage. The formation process oxidizes the lead oxide in the positive plates to lead peroxide and reduces the lead oxide in the negative plates to metallic lead. The charging process produces an acid mist that contains small amounts of lead particulate, which is released without emission controls. 

The result can be formation of an aerosol mist of droplets containing intensely irritating sulfuric... 

If a wet method for collection is selected, such as a wet electrostatic precipitator, fiber-type self-draining mist eliminator, or wet scrubber, ammonia can be regenerated from the salt solution by reaction with a readily available metal oxide such as lime or zinc oxide with formation of a stable sulfur product for disposal. These metal oxides, however, as well as their reaction products, are insoluble and could cause deposition on heat transfer surfaces and/or clogging in the regenerating equipment. Therefore, as indicated in Figure 2, to ensure continuity and reliability of the process, a soluble metal oxide was utilized (in the form of sodium hydroxide solution) to regenerate the ammonia in the experimental work described. This procedure also allows more eflFective utilization of the metal oxide the soluble oxide (NaOH) can be regenerated in batch equipment outside the continuous portion of the process by reaction with either the aforestated insoluble reactants, lime, or zinc oxide. Better control is aflForded in a batch reactor with more eflBcient use of reactants. However, in full-scale equipment undersirable deposition of reactant and product may be controllable so that batch operation may not be necessary. 

Purging. In addition to oxidation, sulfate is present from any sulfuric mist removed from the inlet gas and thiosulfate decomposition during the sulfur melting step. While the rate of sulfate formation from all these sources is small, the effect is cumulative and sulfate must be purged from the system. 

Direct contact of concentrated sulfuric acid with plants will result in perforation of the plant tissue and the plant may subsequently die. The most common response of plants to acidic precipitation is low growth and the formation of foliar lesions or areas of dead tissue on the upper surface of the leaves. Necrotic spotting of the epidermis of the leaves after exposure to sulfuric acid mist has been reported in previous investigations. The effects of acid precipitation on plants are shown in Table 35.5. 

Although sulfuric acid results from the reaction of S03(g) with H2O, a for or mist of tiny H2SO4 droplets forms in the reaction chamber. The formation of this mist can be avoided if the SO3 is dissolved in pure H2SO4 to form pyrosulfuric acid, H2S2O4, which is then allowed to react with water to yield H2SO4. 

A common process for the formation of aerosol mists involves the oxidation of atmospheric sulfur dioxide to sulfuric acid, a hygroscopic substance that accumulates atmospheric water to form small liquid droplets ... 

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