Mists, particle sizes

Figure 14-120 presents an early calculated estimate of mesh efficiency as a fraction of mist-particle size. Experiments by Calvert (R-12) confirm the accuracy of the equation of Bradie and Dickson (Joint Symp. Pioc. Inst. Mech. Eng./Yoikshiie Bi Inst. Chem. Eng., 1969, pp. 24-25) for primary efficiency in mesh separators ... 

Surface in ter/after condenser, 349 System diagrams, 383 Mists, particle sizes, 225 Mixers, jet, 325, 326 Mixers, mechanical components, 289 Baffles, 311 Coils, 312... 

Fog Condensation This is an entirely different way of forming dispersions. Here, the dispersion results from condensation of a vapor rather than mechanical breakup. The particle sizes are usually much finer (0.1 to 30 Im) and are designated as mist or fog. 

FIG. 14-91 Particle -size distribution and mist loading from absorption tower in a contact HC1SO4 plant. [Gillespie and Johnstone, Cbem. Eng. Prog., 51(2), 74 (1955).]... 

Jackson and Calvert [Am. Inst. Chem. Eng. J., 12, 1075 (1966)] studied the collection of fine fuel-oil-mist particles in beds of V2-iu glass spheres, Raschig rings, and Berl and Intalox saddles. The mist had a mass median particle diameter of 6 Im and a standard deviation of 2.0. The collection efficiency as a function of particle size and gas... 

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, ). 

Figure 4-17B. Capture efficiency vs particle size for four standard York-Vane mist eliminators. By permission, Otto H. York Co. Inc.

When a flammable liquid is sprayed as fine droplets into the air, a flammable mixture can result, which may burn or explode. The mist or spray may be formed by condensation of saturated vapors or by mechanical means [40]. As the particle sizes of the liquid become greater than 0.01 mm diameter, the lower flammability limit of the material becomes lower while above 0.01 mm, the LEL is about the same as the vapor. Mechanical engine crankcase explosions of oil mist in air are hazardous, and current practice is to apply explosion relief valves to the crankcase. 

In all these reactors gravity plays an important role. To obtain good contact between phases, we need to overcome the separations that will be driven by gravity whenever the phases have different densities. This requires that the flow conditions and drop, bubble, or particle sizes are properly chosen. It is also important that the phases be separated after the reactor, and mists, emulsions, and dust in separation units can cause major problems in design of these multiphase reactors. Reactor orientation plays an obvious role in any multiphase reaction processes. 

Particle size is shown to have an effect on the distribution and deposit of the insecticide and on foliage injury. A brief description is given of methods of determining particle size. Examples are given of using aerosols and mist blowers in buildings and in the field, and airplanes in the field to illustrate the importance of particle size. 

Tests with Mist Blowers. Work carried oiit in large open warehouses, such as those used for storing tobacco hogsheads U), showed that the particle size must be about 50 microns mass median diameter to prevent too much loss through the large wall openings. The spray was blown over the tops of the hogsheads with a specially built mist blower in order to get proper distribution. 

In treating tall trees with the mist blowers, selection of the proper particle size is of great importance. It has been difficult to secure sufficient deposit on the tops of such trees without overdosing the lower branches and wasting material. This was especially true in certain prefoliage treatments where a heavy deposit was required. It was found that the most efficient particle size was that predicted by Sell s data (3). The size depends on the diameter of the branches and the air velocity available at their elevation, in addition to the velocity required to overcome the force of gravity on the particles. For mist blowers having an air volume of about 10,000 cubic feet per minute, the optimum particle size for deposit at the top of a tree was found to be about 100 microns mass median diameter. 

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