Knitted mesh spray traps have been widely used in sulfuric acid production since the 1980's. To ensure high chemical stability in service under severe conditions, the meshes are made from corrosion-resistant steel, polypropylene, or fluoroplastic.When operating in an ascending stream, the filtration rate is usually 3-4.5 m/sec with hydraulic resistance 200-400 Pa. The collection efficiency for droplets larger than 5 ~tm is -0.9.Complex spray traps are also of great interest today. Judging from the patent literature, authors are working on the structure of the mesh, changing the strand diameter, the shape and dimensions of the mesh, and the porosity of the stacks.In Russia, sulfuric acid installations are used at the outlet of which are set polypropylene mesh spray traps in the form of cylinders of diameter 40 mm and height 200 ram. The throughput of a single element is about 3.8 m3/h, and the total number of elements in an apparatus can be as high as 2500.With the goal of replacing imported material used in mesh spray traps with domestic material, we conducted comparative tests on three samples of polypropylene mesh: two domestic and one from the Thopse company (Denmark).The tests were conducted on a standard experimental apparatus The tests were conducted at a filtration rate of 3.2 m/sec and initial mist droplet concentration 4.5-5.05 mg/m 3, the hydraulic resistance of the samples ranged from 190 to 240 Pa, the droplet collection efficiency ranged from 0.45 to 0.5.We calculated the mist droplet collection efficiency from the formulas given in [2], for d m = 0.82 ~tm; lg~p = 0.72; Ap = 215 Pa, which gave a theoretical value for the droplet collection efficiency equal to 0.45, i.e., comparable with practical data.A recalculation using the same formulas for the values d m = 5 ~tm and lgtSp = 0.3 (parameters for the droplet size distribution typical for spray traps) for Ap = 215 Pa showed that the spray collection efficiency in this case is increased up to 0.97.Thus the tests confirm that it is possible to effectively use domestic propylene mesh materials for spray traps in sulfuric acid production.

Gas-cleaning equipment (wet-dust and mist eliminators, as well as drop and spray eliminators) in which a gas flow contains drops of liquid are classed as wet precipitators.The disperse composition of suspended particles directly at the inlet to, and outlet from gas-cleaning equipment is usually determined experimentally using an impactor [ 1 ]. The most accurate results may be obtained when samples are removed by the internal-filtration method, i.e., when the impactor is introduced directly inside the gas conduit.The operation of wet precipitators is characterized by a series of phenomena that complicate disperse analysis of the suspended particles contained in the gas flow. The simultaneous existence of solid and liquid suspended particles, and also condensation of water vapor as samples are removed from the saturated gas, and spray carryover at the outlet from the apparatus are therefore possible in the gases being cleaned. The NIIOGAZ has developed methods of dispersion analysis, which make it possible to overcome these difficulties.In cases when the gas flow proceeding into the gas-cleaning equipment is a mist (aerosol) containing exclusively suspended liquid particles (drop diameter dp), the dispersivity of the drops is determined using an impactor designed by the NIFKhI (Model 12) [1], the operating principles of which are the same as those for other similar devices. An AFA type of filter is installed at the outlet from the instrument to determine the quantitative fraction of the finest particles. Figure 1 shows results of the disperse analysis of a sulfuric-acid mist, which was conducted with the use of the NIFKhl impactor by the external-filtration method at the inlet to, and outlet from a wet electrostatic precipitator installed for the production of sulfuric acid by the Cherepovets open joint-stock company Ammofos. The impactor was thermostatically controlled (its heating was called for to eliminate condensation of water from the vapor). The acid content on the plates of impactor was determined by a chemical method (titration).During wet dust collection, samples of saturated gas containing solid suspended particles are also taken by electrically heating the elements of the impactor; this ensures that the temperature of the gas in the device will be maintained 5-10 ~ above the dew point. Figure 2 shows a schematic diagram of the device employed to determine the disperse composition of dust during the wet cleaning of gas.When there is no carryover of drops, however, the impactor is introduced into the gas conduit when the moisture content of the gas is high, mounted with the tip of the gas sampler in the path of the gas, and heated for no less than 30 min to the temperature of the gas flow, after which it is turned toward the dust-laden gas flow and gas samples are taken for disperse analysis. During the experiment, possible condensation of water vapor is monitored continuously. For this purpose, a glass tube is usually installed between the impactor and cooler-condenser; the absence of condensate on the ...

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In etching and galvanic coating, chemical and electrochemical treatments are applied to metals in aqueous solutions of acids, alkalis, and salts. The chemical and electrochemical reactions and the stirring of the solutions with air give rise to gas bubbles, which produce mist droplets when they break.It is found [1] that the maximum rise speed for gas bubbles is 30 cm/sec. When they reach the surface, they break with the formation of droplets with sizes not more than 40 µm [2]. These droplets are of suspension or solution, and they are virtually completely carried off by the air current from the bath, whose speed considerably exceeds the suspension velocity of these droplets.To treat the discharges containing suspended droplets in galvanic processes, common use is made of fibrous filters [3], which provide highly effective trapping. However, sometimes when such filters are used with ventilation discharges, problems arise in regenerating the filter surface. Fibrous mist traps are either self-cleaning or regeneratable [3]. In the first case, droplets deposited on the filter surface accumulate and flow off it, i.e., there is ongoing removal of the trapped droplets.In regenerated filters, the filter surface needs ongoing or periodic flushing, which is usually due to the gases containing not only droplets but also suspended solid particles. The latter may be formed by the drying-up of droplets carried off from the bath.As a rule, the bath temperature is quite high (attains 100°C), while the temperature of the air flow taken from the bath is usually the room value (20-25°C). Then as the droplets are carried away from the source, their size decreases by evaporation.The evaporation rate (kg/sec) is given by an equation [4]:in which D v is the vapor diffusion coefficient in m 2 /sec; C d is the equilibrium concentration of the solvent vapor (water vapor) at the droplet temperature in kg/m 3 ; C g is the concentration of the solvent vapor (water vapor) in the gas flow in kg/m 3 ; Re = v g d d /υ g is Reynolds number; Sc = υ g /D v is the Schmidt number; v g is the speed of the gas flow in m/sec; d d is the droplet diameter in m; and υ g is the kinematic viscosity of the gas flow in m 2 /sec. We estimate the evaporation rate for droplets carried off by air from a bath filled with 10% NaOH solution (used for degreasing metals). The solution temperature has been taken as variable and having the values 80, 60, 45, and 25°C, while the temperature of the air taken off by a side pump [3] was constant at 20°C.The following assumptions were made to use (1):• the size of the transported solution droplets is maximal (d d = 40 µm);• the air is saturated with water vapor (at air temperature 20°C, C g = 17.3·10 -3 kg/m 3 [5]); and • the speed of the outgoing air flow is v g = 10 m/sec.

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