Expansion turbines condensing streams

Figure 6.2 shows the process flow diagram for such a plant. Air is compressed to 7-8 bar, precooled in countercurrent heat exchangers and then cooled in a chiller to near the water freezing point before the condensate is drained. Thereafter, the air is further cooled by heat exchange to something like 180°K, and the carbon dioxide, acetylene and other hydrocarbons are removed in other adsorbers. The air, which is now clean, is further cooled down in countercurrent heat exchangers and flashed in an expansion turbine. A slip stream of air... 

Valves are often used to reduce the pressure of a gas or liquid process stream. By replacing the valve with a turbine, called an expander, turboexpander, or expansion turbine in the case of a gas and a liquid expander or radial-infiow, power-recovery turbine in the case of a liquid, power can be recovered for use elsewhere. Power recovery from gases is far more common than from liquids because for a given change in pressure and mass flow rate, far more power can be recovered from a gas than from a liquid because of the lower density of the gas. Equations for f.o.b. purchase costs of power recovery devices are included in Table 16.32 in terms of horsepower that can be extracted. Typical efficiencies are 75-85% for gases and 50-60% for liquids. Condensation of gases in expanders up to 20% can be tolerated, but vapor evolution from liquid expansion requires a special design. Whenever more than 100 Hp for a gas and more than 150 Hp for a liquid can be extracted, a power recovery device should be considered. 

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