Seperation-Distillation notes

Seperation-Distillation notes - McGill University,...

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McGill University, Department of Chemical Engineering - CHEE351: Separation Processes Distillation - 1 Notice: These notes have not been revised and may contain errors. Use your textbook as the primary learning reference. The notes are almost entirely based on your textbook. Vapor-Liquid Separation Processes VAPOR-LIQUID EQUILIBRIUM RELATIONS - Phase Rule and Raoult’s Law One Component As in the gas-liquid systems, the equilibrium in vapor-liquid systems is restricted by the phase rule: F=C-P+2 For an one-component system: C=1, P=2 F=1 degrees of freedom Four working variables are (but the last two do not change, i.e. x=y= 1) ¾ temperature, ¾ pressure, ¾ composition of y A in the vapor phase, and ¾ x A in the liquid phase. So, if we set up the pressure, the temperature is automatically set. The relationship between temperature and pressure for which two phases co-exist at equilibrium is called the vapor pressure curve. This diagram summarizes all the vapor-liquid phase behavior for a one-component system. Two Components Now, for a two-component system, the phase rule tells: F = C - P + 2 C=2, P=2 F=2 degrees of freedom Four working variables are: ¾ temperature, ¾ pressure, ¾ composition of y A in the vapor phase, and ¾ x A in the liquid phase. The composition of water ( B ) is fixed if y A or x A is specified, since: y A + y B = 1.0 and x A + x B = 1.0. Now, considering the four variables and if we fix p , then only one more variable can be set. If we set the liquid composition, the temperature and vapor composition are automatically set. As we can see, the vapor-liquid phase behavior of two-component systems is more complicated then the one- component system, because we have an additional variable composition , as well as T and P. Instead of a single temperature at which both liquid and vapor can co-exist, there is a range of temperatures. © S.Omanovic, 2005
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McGill University, Department of Chemical Engineering - CHEE351: Separation Processes Distillation - 2 BOILING-POINT DIAGRAMS AND XY PLOTS At a given pressure ( P = const), we can summarize the phase behavior by means of a Txy diagram (boiling point diagram): FIGURE 11.1-1. Boiling point diagram for benzene (A)-toluene (B) at 101.325 kPa (1 atm) total pressure. If the temperature is low enough, we will have just one phase: liquid. As we heat this mixture to the temperature corresponding to the lower curve, we would begin to see bubbles of vapor formed in the liquid as more heat is added. This temperature at which the bubbles first form is called the bubble point -when heating a subcooled liquid, the temperature at which the first bubble forms The upper line is the saturated vapor line (the dew-point line ) and the lower line is the saturated liquid line (the bubble-point line ) . The two-phase region is in the region between these two lines.
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Seperation-Distillation notes - McGill University,...

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