Importance of VLE

Importance of VLE | Thermo. of VLE | Lever Rule & VLE | Molecular Interactions | Molecular Simulation

Vapor-liquid equilibrium (VLE) undergrids the bulk of industrial separation processes, particularly in distillation processes.  In this module, we will consider only binary (two-component) mixtures.  The composition of the liquid phase will be designated by the mole fraction of the more volatile component (lower boiling point), represented by x. The mole fraction of the other component is of course 1 - x. Likewise, the mole fraction of the vapor will be represented by y, with 1 - y being the composition of the other component in the vapor phase. When the liquid mixture is heated to its boiling point, the vapor produced will, in general, have a different composition than the liquid; i.e., x <> y. Thus, boiling the mixture produces a partial separation of the components occurs. If the vapor is collected, it can be condensed and boiled again to further change the composition.

Distillation columns use this principle effectively repeating the separation process at individual trays within a column.  Such columns may have over a hundred trays on which VLE is occurring.  Depicted at the right is a cartoon cut-away of a small section of a tray-type distillation column. Two sieve trays are shown. Hot vapor (shown in white) flows up through the holes in the sieve tray and vigorously bubbles through the liquid (shown in blue). The vapor and liquid mix on this tray and the new mixture boils. The composition of the liquid, x, leaving the will be enriched in the less volatile component while the composition of the vapor, y, will be enriched in the more volatile component. The vapor rises up to the next tray where it will contact a liquid of larger x and mixing and re-equilibration will again occur.  The liquid leaving the tray will flow over the weir and down the downcomer to the next tray where it will mix with the vapor on that tray. In this way, x tends to become smaller at each lower tray, while y tends to increase at each tray higher in the column. VLE refers to the relationship between the x and y values on each tray. 

At equilibrium, the temperature and pressure of the liquid and vapor phases will be equal, but the compositions will not. Why is this?  There are two main factors that make the vapor and liquid compositions different at equilibrium: the pure component vapor pressures and the nonidealities in the liquid phase.  These two factors are discussed more fully in the next section on the Thermodynamics of VLE.  However, to understand fully these factors, one needs to understand the molecular nature of the liquid and vapor phases and how the properties of the fluids arise from the molecular interactions. The section on Molecular Interations will help understand this relationship of molecular interactions and the resultant phase behavior.  Both of these sections should be studied before running the simulator.

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More Background: Importance of VLE | Thermo. of VLE | Lever Rule & VLE | Molecular Interactions | Molecular Simulation