CH2351 Chemical Engineering Thermodynamics II Unit – I, II www.msubbu.in PhasewwwEquilibria .m sub bu .in Dr. M. Subramanian Associate Professor Department of Chemical Engineering Sri Sivasubramaniya Nadar College of Engineering Kalavakkam – 603 110, Kanchipuram (Dist) Tamil Nadu, India msubbu.in[AT]gmail.com Jan-2012 Contents Criteria for vapor-liquid equilibrium between phases in multi component non-reacting systems in terms of chemical potential and fugacity, estimation of fugacity of liquid, Raoult’s law, constant temperature and constantwpressure VLE (Pxy, Txy diagrams), effect ww of temperature and pressure .on ms VLE, P-T diagram, deviations from bb ideality – positive and negative udeviations, models for activity u.in coefficient, azeotropes – minimum and maximum boiling azeotropes Jan-2012 M Subramanian Ideal Solution ww w.m sub bu .in ww w.m sub bu .in ww w.m sub bu .in Deviations from Ideal Solution ww w.m sub bu .in Minimum Boiling Azeotrope ww w.m sub bu .in Maximum Boiling Azeotrope ww w.m sub bu .in x-y Diagrams ww w.m sub bu .in Raoult’s Law ww P = xAPAsat + xBPBsat yA = xAPAsat/ P w.m sub bu .in ww w.m sub bu .in ww w.m sub bu .in ww w.m sub bu .in Effect of Temperature ww w.m sub bu .in Effect of Pressure ww w.m sub bu .in P-T Diagram ww w.m sub bu .in Deviation from Ideal Solution • The activity coefficients express the departure from ideality and thus define the excess Gibbs energy of the solution. • Deviation from ideality is said to be positive when γ > 1 (ln γ is ww γ < 1(ln γ is negative). positive) and negative when w.m sub bu .in Positive Deviation from Raoult’s Law P= γΑxA pA sat+ γBxBpBsat ww γi w.m greater than 1 ln γi positive sub bu .in Negative Deviation From Raoult’s Law P= γΑxA pA sat+ γBxBpBsat ww γi w.m less than 1 ln γi negative sub bu .in Azeotropes • If azeotropes are not present, a fractional distillation can eventually separate the mixture into the pure components, with the component with the higher vapor pressure ending up as the distillate and the less volatile component (known as the residue) left in the distillation pot. ww w .m a fractional distillation can produce a • For a minimum boiling azeotrope, sub distillate with the azeotropic composition and a residue that is one of bu . in composition of the starting the pure components, depending on the mixture. • For a maximum boiling azeotrope a fractional distillation can produce one of the pure components as the distillate, and a residue with the azeotropic composition. ww w.m sub bu .in ww w.m sub bu .in ww w.m sub bu .in ww w.m sub bu .in ww w.m sub bu .in ww w.m sub bu .in Positive homogeneous azeotropes in mixtures of ethanol(1) and benzene(2). The Pxy diagram is at 370 K, and the Txy diagram is at 2.5 bar. Filled circles locate the azeotropes. ww w.m sub bu .in Negative homogeneous azeotropes (dots) in mixtures of acetone(1) and chloroform(2). The Pxy diagram is at 50°C; the Txy diagram is at 0.75 bar. ww w.m sub bu .in Double Azeotrope ww w.m sub bu .in VLE Calculations of Non-ideal Solutions • Obtain γi from azeotropic composition data • Evaluate model parameters of γi-xi models based on the above data ww w.m sub • Make VLE calculation with yiP = γbixu.iPisat in ww w.m sub bu .in The equilibrium flash of a multicomponent liquid is also an isenthalpic process and may be visualized as a simple distillation process using a single equilibrium stage. VLE Problems Problem Name Bubble P Knowns T, xi Unknowns to find P, yi Dew P T, yi www. P, xi ms ub bu Bubble T P, xi Dew T P, yi T, xi Flash T, P, zi xi, yi, nv/n .in T, yi Henry’s Law & Raoult’s Law • As early as in 1803 William Henry showed empirically that the vapor pressure of a solute i is proportional to the concentration of solute i: where xi is the mole fraction solute and kH,i is known as the Henry’s law constant. ww w.m su bb demonstrated that at low • More than 80 years later François Raoult u.in concentrations of a solute, the vapor pressure of the solvent is simply where xi is the mole fraction solvent and pi* is the vapor pressure of the pure solvent. • Raoult’s and Henry’s laws are often termed ‘limiting laws’. This use reflects that real solutions often follow these laws at infinite dilution only. ww w.m sub bu .in Comparison of Raoult’s law and Henry’s law Schematic illustrations of the saturated ww vapor pressures above two different w.m non-ideal solutions. sub bu .in The continuous solid curves represent the behavior of the real solutions, the dashed lines represent Raoult’s law and short solid lines represent Henry’s law. ww w.m sub bu .in ww w.m sub bu .in ww w.m sub bu .in ww w.m sub bu .in
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