Mist, acid

Lead—copper alloys are also used as tank linings, tubes for acid mist precipitators, steam heating pipes for sulfuric acid or chromate plating baths, and flashing and sheeting (see Tanks AND pressure vessels). 

Another concentration method involves passing an inert gas such as N2 or CO2 through the reaction medium (12). As the gas passes through, it becomes humidified and carries captured water with it. Most of the energy required for the gas humidification comes from the heat of reaction. An advantage is that expensive drying equipment is not needed. Also, the sulfuric acid mist formed in typical concentrators is minimized. Du Pont uses a similar process in its nitrobenzene production faciUty. 

Thermal Process. In the manufacture of phosphoric acid from elemental phosphoms, white (yellow) phosphoms is burned in excess air, the resulting phosphoms pentoxide is hydrated, heats of combustion and hydration are removed, and the phosphoric acid mist collected. Within limits, the concentration of the product acid is controlled by the quantity of water added and the cooling capabiUties. Various process schemes deal with the problems of high combustion-zone temperatures, the reactivity of hot phosphoms pentoxide, the corrosive nature of hot phosphoric acid, and the difficulty of collecting fine phosphoric acid mist. The principal process types (Fig. 3) include the wetted-waH, water-cooled, or air-cooled combustion chamber, depending on the method used to protect the combustion chamber wall. 

Figure 9 shows the sampling train for sulfuric acid mist collection (13). The first impinger contains 80 wt % isopropyl alcohol and the second and third contain 3 wt % H2O2. The first impinger and filter retain the acid mist and SO the next two retain the SO2. After sampling, the filter is added to the contents of the first impinger and the total acid is titrated and reported as sulfuric acid. 

For acid mists, the Brink impactor is often used (Fig. 10) (17). The mist is first drawn through a cyclone to remove particles larger than 3 fim. A five-stage impactor is used to classify mist particles of diameter 0.3—3.0 fim. 

Health and Safety. Halosilane vapors react with moist air to produce the respective hydrohalogen acid mist. Federal standards have not set exposure to halosilanes, but it is generally beheved that there is no serious risk if vapor concentrations are maintained below a level that produces an irritating concentration of acid mist. The exposure threshold limit value (TLV) for HCl is 5 ppm, expressed as a ceiling limit. Because most people experience odor and irritation at or below 5 ppm, HCl is considered to have good warning properties. 

The sulfur trioxide produced by catalytic oxidation is absorbed in a circulating stream of 98—99% H2SO4 that is cooled to approximately 70—80°C. Water or weaker acid is added as needed to maintain acid concentration. Generally, sulfuric acid of approximately 98.5% concentration is used, because it is near the concentration of minimum total vapor pressure, ie, the sum of SO, H2O, and H2SO4 partial pressures. At acid concentrations much below 98.5% H2SO4, relatively intractable aerosols of sulfuric acid mist particles are formed by vapor-phase reaction of SO and H2O. At much higher acid concentrations, the partial pressure of SO becomes significant. 

Small amounts of sulfuric acid mist or aerosol are always formed in sulfuric acid plants whenever gas streams are cooled, or SO and H2O react, below the sulfuric acid dew point. The dew point varies with gas composition and pressure but typically is 80—170°C. Higher and lower dew point temperatures are possible depending on the SO concentration and moisture content of the gas. Such mists are objectionable because of both corrosion in the process and stack emissions. 

More recentiy, sulfuric acid mists have been satisfactorily controlled by passing gas streams through equipment containing beds or mats of small-diameter glass or Teflon fibers. Such units are called mist eliminators (see Airpollution control methods). Use of this type of equipment has been a significant factor in making the double absorption process economical and in reducing stack emissions of acid mist to tolerably low levels. 

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. 

Acid mist eliminators use three aerosol collection mechanisms inertial impaction, interception, and Brownian motion. Inertial impaction works well for aerosols having particle diameters larger than 3 p.m Brownian motion and interception work well with aerosols having smaller particle diameters. 

Packed fiber bed mist eliminators can be designed to operate at almost any desired particle collection efficiencies, depending on the allowable pressure drop and cost. A good discussion of sulfuric acid mist generation, control, and mist eliminator design is available (109,110). 

S. M. Horvath and co-workers. Effects of Sulfuric Acid Mist Exposure on Pulmonary Function, EPA-600/S1-81-044, issued as PB81-208977 by NTIS, Washington, D.C., June 1981. 

Safety. Chlorosulfuric acid is a strong acid and the principal ha2ard is severe chemical bums when the acid comes into contact with body tissue. The vapor is also ha2ardous and extremely irritating to the skin, eyes, nose, and respiratory tract. Exposure limits for chlorosulfuric acid have not been estabhshed by OSHA or ACGIH. However, chlorosulfuric acid fumes react readily with moisture in the air to form hydrochloric and sulfuric acid mists, which do have estabhshed limits. The OSHA 8-h TWA limits and ACGIH TLV—TWA limits are sulfuric acid = 1 mg/m hydrochloric acid = 5 ppm or 7 mg/m (ceiling limit). 

SpiHs can also be diluted with large volumes of water. Care should be taken, however, because chlorosulfuric acid reacts violentiy with water Hberating heat, hydrochloric acid, and sulfuric acid mists and steam. The water should be appHed from a safe distance upwind of the spiH using a fog no22le. Remaining traces of acid should be neutrali2ed with soda ash, caustic soda, or lime before disposal. 

The primary routes of entry for animal exposure to chromium compounds are inhalation, ingestion, and, for hexavalent compounds, skin penetration. This last route is more important in industrial exposures. Most hexavalent chromium compounds are readily absorbed, are more soluble than trivalent chromium in the pH range 5 to 7, and react with cell membranes. Although hexavalent compounds are more toxic than those of Cr(III), an overexposure to compounds of either oxidation state may lead to inflammation and irritation of the eyes, skin, and the mucous membranes associated with the respiratory and gastrointestinal tracts. Skin ulcers and perforations of nasal septa have been observed in some industrial workers after prolonged exposure to certain hexavalent chromium compounds (108—110), ie, to chromic acid mist or sodium and potassium dichromate. 

The oxidation catalyst (OC) operates according to the same principles described for a TWO catalyst except that the catalyst only oxides HC, CO, and H2. It does not reduce NO emissions because it operates in excess O2 environments. One concern regarding oxidation catalysts was the abiUty to oxidize sulfur dioxide to sulfur trioxide, because the latter then reacts with water to form a sulfuric acid mist which is emitted from the tailpipe. The SO2 emitted has the same ultimate fate in that SO2 is oxidized in the atmosphere to SO which then dissolves in water droplets as sulfuric acid. 

Typical applications in the chemical field (Beaver, op. cit.) include detarring of manufactured gas, removal of acid mist and impurities in contact sulfuric acid plants, recovery of phosphoric acid mists, removal of dusts in gases from roasters, sintering machines, calciners, cement and lime Idlns, blast furnaces, carbon-black furnaces, regenerators on fluid-catalyst units, chemical-recoveiy furnaces in soda and sulfate pulp mills, and gypsum kettles. Figure 17-74 shows a vertical-flow steel-plate-type precipitator similar to a type used for catalyst-dust collection in certain fluid-catalyst plants. 

Sulfur Dioxide EPA Method 6 is the reference method for determining emissions of sulfur dioxide (SO9) from stationary sources. As the gas goes through the sampling apparatus (see Fig. 25-33), the sulfuric acid mist and sulfur trioxide are removed, the SO9 is removed by a chemical reaction with a hydrogen peroxide solution, and, finally, the sample gas volume is measured. Upon completion of the rim, the sulfuric acid mist and sulfur trioxide are discarded, and the collected material containing the SO9 is recovered for analysis at the laboratory. The concentration of SO9 in the sample is determined by a titration method. 

Sonic agglomerators, which have been used experimentally for sulfuric acid mists and as mist eliminators. Commercial development is not projected at this time because the energy requirements are considerably greater than those for venturi scrubbers of similar capacity. 

Sulfuric Contact SO2, acid mist Scrubbers with mist eliminators, ESPs... 

Strong-inorganic-acid mists containing sulphuric acid (occupational exposure to) (Vol. 54 1992)... 

Sulphuric acid mist and SO, from stationary sources 8... 

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