Mist flow, condensation

Natural ventilation is the controlled flow of air through doors, windows, vents, and other purposely provided openings caused by stack effect and wind pressure. Natural ventilation is used in spaces with a significant heat release, when process and hygienic requirements for indoor air quality allow outdoor air supply without filtration and treatment. Natural ventilation cannot be used when incoming outdoor air causes mist or condensation. Natural ventilation allows significant air change rates (20 to 50 ach) for heat removal with ntinimal operation costs. 

The basic assumptions implied in the homogeneous model, which is most frequently applied to single-component two-phase flow at high velocities (with annular and mist flow-patterns) are that (a) the velocities of the two phases are equal (b) if vaporization or condensation occurs, physical equilibrium is approached at all points and (c) a single-phase friction factor can be applied to the mixture if the Reynolds number is properly defined. The first assumption is true only if the bulk of the liquid is present as a dispersed spray. The second assumption (which is also implied in the Lockhart-Martinelli and Chenoweth-Martin models) seems to be reasonably justified from the very limited evidence available. 

The sample solution is drawn first into a nebulizer by the flow of support gas where it forms a mist or aerosol. Fuel gas is introduced and the mixture passed to a spray chamber where large droplets condense and run to waste. Alternatively a small volume of solution (10-100 pi) may be injected directly into the orifice of the nebulizer using a micropipette. 

For condensation inside long tubes, when the condensation rate or the length are large, the flow regime becomes annular with a vapour core. At even higher vapour velocities, the vapour core may contain a mist of liquid droplets. 

Kraft pulp mills Digesters batch and continuous Multiple-effect evaporators Recovery furnace Weak and strong black-liquor oxidation Smelt tanks Lime kiln Mercaptans, methanol (odors) H2S, other odors H2S, mercaptans, organic sulfides, and disulfides h2s Particulates (mist or dust) Particulates (dust), H2S Condensers and use of lime kiln, boiler, or furnaces as afterburners Caustic scrubbing and thermal oxidation of noncondensables Proper combustion controls for fluctuating load and unrestricted primary and secondary air flow to furnace and dry-bottom electrostatic precipitator noncontact evaporator Packed tower and cyclone Demisters, venturi, packed tower, or impingement-type scrubbers Venturi scrubbers... 

An extra feature of the equipment shown in Figure 21.3 is the design of the condensation train. It is known that under certain conditions, the pyrolytic products when condensed, form a mist or aerosol, similar to the smoke produced by cigarettes or a barbecue. These aerosols, may accumulate as a wax on the walls of condensers, but often they do not settle inside collection vessels. This problem was solved by making the gases flow upwards through a vertical condenser, effectively creating a reflux-like effect, similar to a distillation column. Furthermore, this condenser was operated at a temperature that cooled the products to a point at which they condensed, but did not solidify, and allowed the liquid droplets to coalesce and hence be collected. The optimal operation of the condenser was with water at temperatures between 50 and 60°C [86]. 

In the new process, other separation zones are added after the normal first separation zone. The heavy phase from the first zone is sent to a suipper and the vaporized solvent, steam, and fine mist particles are withdrawn from the top and are conveyed to the second separation zone. In the second separation zone, the stream consisting of solvent, steam, and mist particles flow through a circuitous path where it is contacted couniercurrently with a sU eam of light oil which scrubs the asphaltene and resin mist from the solvent. A portion of the light phase oil containing asphaltenes and resins is recycled to the second separation zone or it can be recycled to the first separation zone. The solvent and steam mixture leaving the second zone is conveyed to a solvent condenser and water separation zone. No emulsification problems are exp>erienced because the mist particles have been removed in the first separation zone. 

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