Gases noncondensable

The mixture is cooled and noncondensable gases are scmbbed with water. Some of the resultant gas stream, mainly hydrogen, may be recycled to control catalyst fouhng. The Hquids are fractionally distilled, taking acetone overhead and a mixture of isopropyl alcohol and water as bottoms. A caustic treatment maybe used to remove minor aldehyde contaminants prior to this distillation (29). In another fractionating column, the aqueous isopropyl alcohol is concentrated to about 88% for recycle to the reactor. 

Steps. Thermal-swing cycles have at least two steps, adsorption and heating. A cooling step is also normally used after the heating step. A portion of the feed or product stream can be utilized for heating, or an independent fluid can be used. Easily condensable contaminants may be regenerated with noncondensable gases and recovered by condensation. Water-iminiscible solvents are stripped with steam, which may be condensed and separated from the solvent by decantation. Fuel and/or air may be used when the impurities are to be burned or incinerated. 

Compounds having low vapor pressures at room temperature are treated in water-cooled or air-cooled condensers, but more volatile materials often requite two-stage condensation, usually water cooling followed by refrigeration. Minimising noncondensable gases reduces the need to cool to extremely low dew points. Partial condensation may suffice if the carrier gas can be recycled to the process. Condensation can be especially helpful for primary recovery before another method such as adsorption or gas incineration. Both surface condensers, often of the finned coil type, and direct-contact condensers are used. Direct-contact condensers usually atomize a cooled, recirculated, low vapor pressure Hquid such as water into the gas. The recycle hquid is often cooled in an external exchanger. 

Oil can be deterrnined by extracting the residue using carbon tetrachloride and then evaporating the CCl. Sediment such as iron is deterrnined by dissolving in hydrochloric acid followed by colorimetric estimation. Dissolved noncondensable gases are deterrnined by analysing the atmosphere above the liquid ammonia. 

Enerco, Inc. (Yardley, Pennsylvania) has a 600 tine/d demonstration pyrolysis plant located in Indiana, Pennsylvania. The faciUty operated 8 h/d, 5 d/wk for six months. The process involves pyrolysis in a 5.4 t/d batch-operated retort chamber. The heated tines are broken down to cmde oil, noncondensable gases, pyrolytic filter, steel (qv), and fabric waste. In this process, hot gases are fed direcdy to the mbber rather than using indirect heating as in most other pyrolyses. The pyrolysis plant was not operating as of early 1996. 

Favor condensation (a simple and cheap unit operation) for removal of high boilers from noncondensable gases when cooling water can be used as the condensing medium. 

Drying is an operation in which volatile Hquids are separated by vaporization from soHds, slurries, and solutions to yield soHd products. In dehydration, vegetable and animal materials are dried to less than their natural moisture contents, or water of crystallization is removed from hydrates. In freeze drying (lyophilization), wet material is cooled to freeze the Hquid vaporization occurs by sublimation. Gas drying is the separation of condensable vapors from noncondensable gases by cooling, adsorption (qv), or absorption (qv) (see also Adsorption, gas separation). Evaporation (qv) differs from drying in that feed and product are both pumpable fluids. 

Condensation of pure vapors under laminar conditions in the presence of noncondensable gases, interfacial resistance, superheating, variable properties, and diffusion has been analyzed by Minkowycz and Sparrow [Int. ]. Heat Ma.s.s Tran.sfer, 9, 1125 (1966)]. 

In any event, noncondensable gases should be vented well before their concentration reaches 10 percent. Since gas concentrations are difficult to measure, the usual practice is to ovei vent. This means that an appreciable amount of vapor can be lost. 

To nelp consei ve steam economy, venting is usually done from the steam chest of one effecl to the steam chest of the next. In this way, excess vapor in one vent does useful evaporation at a steam economy only about one less than the overall steam economy. Only when there are large amounts of noncondensable gases present, as in beet-sugar evaporation, is it desirable to pass the vents directly to the condenser to avoid serious losses in heat-transfer rates. In such cases, it can be worthwhile to recover heat from the vents in separate heat exchangers, which preheat the entering feed. 

The noncondensable gases eventually reach the condenser (unless vented from an effect above atmospheric pressure to the atmosphere or to auxiliary vent condensers). These gases will be supplemented by air dissolved in the condenser water and by carbon dioxide given off on decomposition of bicarbonates in the water if a barometric condenser is used. These gases may be removed by the use of a water-jet-type condenser but are usually removed by a separate vacuum pump. 

The primary source of noncondensable gases usually is air dissolved in the condenser water. Figure 11-126 shows the dissolved-gas content of fresh water and seawater, calculated as equivalent air. The... 

By far the largest load on the vacuum pump is water vapor carried with the noncondensable gases. Standara power-plant practice assumes that the mixture leaving a surface condenser will have been cooled 4.2°C (7.5°F) below the saturation temperature of the vapor. This usually corresponds to about 2.5 kg of water vapor/kg of air. One advantage of the countercurrent barometric condenser is that it can cool the gases almost to the temperature of the incoming water and thus reduce the amount of water vapor carried with the air. 

Table 29-4 lists several applications of condensers currently in use. For most operations listed, air and noncondensable gases should be kept to a minimum, as they tend to reduce condenser capacity. 

It is sometimes possible to close an odorous system in order to prevent the release of the odor to the atmosphere. For example, a multiple-effect evaporator can be substituted for an open contact condenser on a process emithng odorous, noncondensable gases. 

Air is usually the basic load component to an ejector, and the quantities of water vapor and/or condensable vapor are usually directly proportional to the air load. Unfortunately, no reliable method exists for determining precisely the optimum basic air capacity of ejectors. It is desirable to select a capacity which minimizes the total costs of removing the noncondensable gases which accumulate in a process vacuum system. An oversized ejector costs more and uses unnecessarily large quantities of steam and cooling water. If an ejector is undersized, constant monitoring of air leaks is required to avoid costly upsets. 

Noncondensable gases at condenser temperature blanket the condenser surface and reduce the condenser capacity. 

Sources of air or inerts include dissolved gas in the cooling water in case of jet condensers, entrainment with steam, entrainment with vapor, leaks, and noncondensable gases. 

Leung, J. C. and Epstein, M., Flashing Two-Phase Flow Including the Effects of Noncondensible Gases, Trans. ASME—Journal of Heat Transfer, 113 (2), 269-272, February... 

The relation applies to systems that potentially are condensable as contrasted to those systems containing noncondensable gases such as air, nitrogen, etc. All of the vapor does not have to be condensed in the unit for the correlation to apply. 

For noncondensing gases and vapors in Equation 10-207 use the average of inlet and oudet gas density referenced to water at 62.4 Ib/fP for the value of s. 

For values of specific gravity for noncondensing gases and vapors use the average density at inlet and oudet conditions referenced to water at 62.4 Ib/fF. 

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