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VLE FROM K-VALUE CORRELATTIONS The partition between liquid and vapor phases of a chemical species is equilibrium ratio, Ki. A boiling L of this composition produce a vapor exactly the same composition L does not change in composition as it evaporates.When x1=y1, the dew point and bubble point curves are tangent to the same horizontal line.A mixture that has a constant composition of liquid and vapor phase.Thus, activity coefficient is introduced in Raoult’s Law 7Īctivity coefficients are function of T & liquid phase composition, x Since For bubble point 8 For dew point 9 VLE BY MODIFIED RAOULT’S LAW The 2nd assumption of Raoult’s Law is abandoned, taking into account the deviation from solution ideality in L phase. 1 written for species 2 Whence y1=1-y2=0.9988, and the vapor phase is nearly pure CO2, as expected. Henry’s law for species 1 & Raoult’s law for species 2 are written With H1=990 bar & P2sat = 0.01227 bar (from steam tables at 100C) Henry’s constant for CO2 in water at 100C is about 990 bar and x1=0.01. For species present as a very dilute solution in liquid phaseĮxample 2 Assuming that carbonated water contains only CO2(1) and H2O(2), determine the compositions of the V & L phases in a sealed can of ‘soda’ & the P exerted on the can at 100C.For pressure low It is so low that it can be assume as ideal gas.Substituting T= 78˚C into (i) and (ii) P1sat = 91.76 kPa P2sat = 46.84 kPa e.g select T= 78˚C Evaluate x1 by Eq. Select T between these two temperatures and calculate P1sat & P2sat for the two temperatures. T1sat & T2sat are calculated from Antoine equation įor P=70kPa, T1sat=69.840C, T2sat=89.580C. 1, This is the liquid-phase composition at point c’ b) When P is fixed, the T varies along T1sat and T2sat, with x1 & y1. 1,Īt point c, the vapor composition is y1=0.6, but the composition of liquid at c’ and the pressure must read from graph or calculated. The corresponding value of y1 is found from Eq. At 750C, the saturated pressure is given by Antoine equation Substitute both values in (A) to find P y1 for a pressure of 70 kPaĪ) BUBL P calculations are required. Vapor pressure for the pure species are given by the following Antoine equations: i ii Raoult’s law equation can be solved for xi to solve for dewpoint calculation (T is given) 4 5Įxample 1 Binary system acetonitrile(1)/nitromethane(2) conforms closely to Raoult’s law. isomers such as ortho-, meta- & para-xylene) Assumptions ĭewpoint & Bubblepoint Calculations with Raoult’s Law FIND GIVEN BUBL P: Calculate and Pįor binary systems to solve for bubblepoint calculation (T is given) 2 3 Valid only if the species are chemically similar (size, same chemical nature e.g.Applicable for low to moderate pressure.10.1 – Shows the P-T-composition surfaces of equilibrium states of saturated V & saturated L of a binary system As pressure reduces, more & more L vaporizes until completed at W point where last drop of L (dew) disappear – dew point: a point when a vapor forms the first droplet of liquid and begins to condense Fig.Liquid at F, reduces pressure at constant T & composition along FG, the first bubble appear at L – bubble point: a point when a liquid forms the first bubble of vapor and begins to evaporate.contact to each other in a closed location.takes place when liquid and vapor are allowed to.e.g: mixture of liquid and vapor at an equilibrium.Binary mixture: Mixture that contains two constituents.rate of evaporation (liquid → vapor) =.A condition where a liquid phase and vapor phase.VLE: State of coexistence of L & V phases.
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The T, P, composition reaches final value which will remain fixed: equilibrium THE NATURE OF EQUILIBRIUM Equilibrium : A static condition in which no changes occur in the macroscopic properties of a system with time. Measures of composition VLE : Qualitative behavior Simple Models for VLE - Raoult’s Law - Dewpoint& Bubblepoint Calculations with Raoult’s Law - Henry’s Law VLE by modified Raoult’s law VLE from K-value correlations.PTT 201/4THERMODYNAMICS SEM 1 (2013/2014) Vapor/Liquid Equilibrium (VLE) Solution Thermodynamic:Ĭhapter Outline (Smith) Nature of Equilibrium