Example 3. Varying the Composition of the Feed to a Flash Tank

Let's now explore what happens to the outlet streams as the feed to an isothermal flash tank changes composition. Set the temperature of the simulation at 320 K and the mole fraction at 0.5. This represents a 50-50 mixture for the feed. What phase(s) is/are present in the outlet streams?  Now shift z to 0.65.  The system is now in the two-phase region.  Note the mole fractions of the vapor and in the liquid phase.  Also note the fraction vaporized.  Now shift z to 0.75.  Note that the mole fractions in the two phases did not change.  Why don't they?  How can the overall mole fraction change from 0.5 to 0.65 without the compositions in the two equilibrium phases changing?  Note that the compositions are fixed in the two phase region by the temperature and pressure of the system.  They can not change as long as there is equilibrium between the two phases at the same temperature and pressure.  Can you see why this is from a molecular level considering the balance between the kinetic energy and the attractive forces that molecules feel that leads to two equilibrium phases?  What does change is the relative amounts in each phase.  Thus, if you change the feed composition to the flash tank, the compositions of the liquid and vapor exiting the flash tank will be the same (fixed by the thermodynamic equilibrium conditions), but the relative amounts obtained in the two phases will vary as you change z.

What is the composition of the vapor and liquid phases for this isotherm at the dew point?  What is the fraction vaporized?  Slide the composition slider over to where you think the dew point is.  Did you get all vapor?  Try to pick off the bubble-point composition at this temperature from the simulator by moving z until the vapor phase vanishes.  Does the Txy plot verify that you are close to the liquidus or liquid saturation line?
 

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More Examples: Example 1 | Example 2 | Example 4 | Example 5