Condensation Vapors with noncondensables

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. 

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-... 

These high-free-volume polymers also have unusual permeability characteristics with mixtures of condensable and noncondensable gases. For example, in the presence of as little as 1200 ppm of a condensable vapor such as the per-fluorocarbon FC-77 (a perfluoro octane-perfluoro decane mixture), the nitrogen permeability of PTMSP is 20 times lower than the pure nitrogen permeability [71], as shown in Figure 2.41. When the condensable vapor is removed from the feed gas the nitrogen permeability rapidly returns to its original value. The best... 

Specifying overhead vapor product in a system with noncondensables Since the split in the condenser can be very sharp, there will be little freedom of movement. It may be better to specify a variable such as reflux rate, condenser temperature, or any specification on the liquid overhead product (if it exists). 

In practice, the vapor that is to be condensed sometimes contains noncondensible gases such as air. The presence of these noncondensible gases can significantly lower the heat transfer rate from that which would exist under the same circumstances with a pure vapor. A common example is the build-up of air in power plant condensers. These condensers usually operate at a substantial vacuum and some air entrainment is unavoidable. The continuous removal of air by specially designed ejector systems is essential to maintain the condenser vacuum and to maintain acceptable condensation rates. In some chemical plants, the separation of constituents is sometimes produced by condensing one gas from a mixture of gases and in such cases the presence of a noncondensible gas is unavoidable. 

For horizontal shell-side condensers, the condensate falls to the bottom of the shell, and vapor and liquid do not coexist, as assumed by the preceding method. The effect this has on the heat transfer must be considered. It is recommended that shell-side condensers with noncondensable gases present be somewhat overdesigned perhaps 20 percent excess surface should be provided. 

How can you predict the properties of a mixture of a pure vapor (which can condense) and a noncondensable gas at equilibrium A mixture containing a vapor behaves somewhat differently than does a pure component by itself. A typical example with which you are quite familiar is that of water vapor in air. 

Its principle of operation is based on Boyle s Law, in that a fixed large volume of the gas at the unknown pressure is compressed to a fixed small volume. The pressure of the gas following compression is measured by a mercury manometer, and the initial pressure is calculated from this pressure and the volume ratio. Pressures as low as 10-6 mm. Hg can be accurately measured by this gage. A major cause for error is the presence of condensable vapors in the gas being measured. Under such conditions the pressure obtained with this gage is closer to the partial pressure of noncondensables than the total pressure. It is not exactly equal to the partial pressure of noncondensables, however, because the condensable vapors may exert a significant vapor pressure after condensation. 

The condensing vapor may consist of a single substance, a mixture of condensable and noncondensable substances, or a mixture of two or more condensable vapors. Friction losses in a condenser are normally small, so that condensation is essentially a constant-pressure process. The condensing temperature of a single pure substance depends only on the pressure, and therefore the process of condensation of a pure substance is isothermal. Also, the condensate is a pure liquid. Mixed vapors, condensing at constant pressure, condense over a temperature range and yield a condensate of variable composition until the entire vapor stream is condensed, when the composition of the condensate equals that of the original uncondensed vapor.f A common example of the condensation of one constituent from its mixture with a second noncondensable substance is the condensation of water from a mixture of steam and air. 

Condensation of a vapor in the presence of a noncondensable gas is treated elsewhere in this chapter. Figure 14.3 describes the added thermal resistance that occurs due to mass diffusion of the vapor through a noncondensable, gas-rich layer next to the condensate. The case of two condensing vapors is similar to that depicted in Fig. 14.3. Both vapor components condense, but the more volatile one accumulates at the interface and provides a barrier for the less volatile one, similar to a noncondensable gas. Similar effects are also found with multi-component mixtures. 

All vapors to be condensed contain noncondensables. A vapor containing as little as 100 ppm noncondensables can fill a reboiler up with noncondensable gas within less than 10 h if there is no venting (28). 

The largest portion of condensable vapor load is generally product since noncondensable gases are generally saturated with product vapors at process vent conditions. The amount of condensable that enters the vacuum system should be minimized for several reasons ... 

As mentioned above, the crystallization process is not yet complete with the crystallization itself. The suspension produced first has to be separated, while the crystals still have to be dried and packaged. The vapors released have to be condensed and the noncondensable gases extracted from the system by means of a vacuum pump. Figure 11.22 shows a simplified flowchart of the principle, using vacuum evaporation crystallization as an example. 

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