Noncondensible gas

Calcium C rbon te. Calcium carbonate, like R2O2, affects sulfuric and oleum consumption in the HF process. Sulfuric acid loss is approximately 0.98% H2SO4 for each percentage of CaCO. The carbon dioxide evolved by the reaction increases the noncondensable gas flow, and because it carries HF, contributes to yield losses in the vent stream. 

High temperature steam cools and eventually condenses as it propagates through the oil reservoir. To maintain foam strength as the steam cools, a noncondensible gas, usually nitrogen or methane, is often added to the injectant composition (196). Methods of calculating the optimum amount of noncondensible gas to use are available (197). 

The humidity term and such derivatives as relative humidity and molal humid volume were developed for the air—water system. Use is generally restricted to that system. These terms have also been used for other vapor—noncondensable gas phases. 

However, the reaction with water can be made to be extremely slow. Because the alkaline electrolyte is corrosive toward human tissue as well as toward the materials ia devices, it is more important to have a good seal toward preventing electrolyte leakage ia an alkaline battery than ia a carbon—2iac cell. The formation of a good seal is, however, iacompatible with the formation of a noncondensable gas like hydrogen. 

The HCl gas is absorbed in water to produce 30—40% HCl solution. If the HCl must meet a very low organic content specification, a charcoal bed is used ahead of the HCl absorber, or the aqueous HCl solution product is treated with charcoal. Alternatively, the reactor gas can be compressed and passed to a distillation column with anhydrous 100% Hquid HCl as the distillate the organic materials are the bottoms and are recirculated to the process. Any noncondensible gas present in the HCl feed stream is vented from the distillation system and scmbbed with water. 

The other mechanism appears in scrubbers. When water vapor diffuses from a gas stream to a cold surface and condenses, there is a net hydrodynamic flow of the noncondensable gas directed toward the surface. This flow, termed the Stefan flow, carries aerosol particles to the condensing surface (Goldsmith and May, in Davies, Aero.sol Science, Academic, New York, 1966) and can substantially improve the performance of a scrubber. However, there is a corresponding Stefan flow directed away from a surface at which water is evaporating, and this will tend to repel aerosol particles from the surface. 

A temperature profile of vapor condensing in the presence of a noncondensable gas on a tube wall, as shown in Figure 16 indicates the resistance to heat flow. Heat is transferred in two ways from the vapor to the interface. The sensible heat is removed in cooling the vapor from t to t, at the convection gas cooling rate. The latent heat is removed only after the condensable vapor has been able to diffuse through the noncondensable part to reach the tube wall. This means the latent heat transfer is governed by mass transfer laws. 

Figure 16. Temperature profile showing effect of vapor condensation on a tube wall in the presence of a noncondensable gas.

If steam condenses on a surface, there is no boundary layer the resistance to heat flow is due to scale, metal thickness, and the condensed liquid layer, resulting in a high heat transfer factor. A thin layer of air or other noncondensing gas forms at the surface through which the steam diffuses. The heat transfer factor diminishes rapidly but is considerably higher than in dry convection. 

The effect of a noncondensable gas in the system with a condensable vapor is to significandy reduce the condensing side film coefficient. Henderson and Marcello present data to illustrate the effect. Figures 10-85, 10-86, and 10-86A present the effect of AT with a steam-air system and toluene-... 

A computer program developed hy Volta handles the problem of condensing in the presence of a noncondensable gas for down-flow of either a saturated or superheated gas-vapor mixture vertical tubes. The program is based on a modification of Colburn-Hougen and Bras and is certainly more accurate and easier to use than the lengthy manual calculations. Although the program was written for vertical tubes, it can be used to approximate the results in a horizontal unit, and if the correction factor between vertical and horizontal tube condensation is applied, the compari-... 

The presence of even a small amount of noncondensable gas in the condensing mixture can significantly reduce the condensing heat transfer rates and needs to be recognized. See Figure 10-85. 

Mori, Y, K. Hiyikata, and K Utsunomiya, The Effect of Noncondensable Gas on Film Condensation Along a Vertical Plate in an Enclosed Chamber, ASME Journal Heat Trans., V. 99, May (1977) p. 257. 

Oxygen is not the only noncondensable gas found in boiler circuits, Problems occur due to the presence of carbon dioxide (C02). Carbon dioxide is steam-volatile and reacts with condensing steam to produce carbonic acid, which attacks steel condensate return lines. 

Periodically injecting simultaneously or alternately a noncondensable gas and a foaming composition solution containing alkalis, surfactants and polymers to form combined foam or periodically injecting the gas and the foam previously formed from the solution... 

McNaught, J. M. (1983) An assessment of design methods for condensation of vapors from a noncondensing gas, in Heat Exchangers Theory and Practice (McGraw-Hill). 

Particular care needs to be adopted if a vapor to be condensed has noncondensable gases present. Here the vapor diffuses through the gas to the cold surface where it condenses. But as the condensation proceeds, the concentration of the noncondensable gas increases, which increases the diffusional resistance and decreases the condensing coefficient. To take this into account requires complex models, which is outside the scope of this text. 

A method of calculating the optimum amount of noncondensible gas to use has been reported (409). 

One can easily calculate from such equations, however, what size cavity would undergo maximum expansion when subjected to a given acoustic field. Minnaert, for example, derives (34) (from a simplified model which assumed a noncondensable gas and neglected viscosity) this resonant size of a transient cavity as... 

FIG. 23-36 Omega method solution for orifice flow of flashing liquids and for noncondensable gas plus subcooled liquids. 

For the case of a noncondensable gas and a nonflashing liquid (a nonflashing two-phase fluid), co is simply defined by (Leung, 1996)... 

A number of variations of this test exist. An oven temperature is increased linearly. Continuous monitoring of the temperature and pressure outputs from a sample tube in the oven provides qualitative information about the thermal characteristics of the sample. In many cases, the pressure data can also yield valuable information. Any discontinuity in a plot of In P against 1/T indicates noncondensable gas generation. (The plot is often an essentially straight line if the pressure increase is due solely to the vapor pressure.)... 

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