Nucleation, the initial process in vapor condensation, crystal nucleation, melting, and boiling, is the localized emergence of a distinct thermodynamic phase at the nanoscale that macroscopically grows in size with the attachment of growth units. These phase changes are the result of atomistic events driven by thermal fluctuations. The occurrence of atomistic level events with the length scales on the order of 10 −10 m and time scales of 10 −13 S equivalent to the vibrational frequency of atoms makes the nucleation a very complicated phenomenon to study. Even though abundant literature is available about fundamental aspects of nucleation, the knowledge on these phenomena is far from complete. The classical pathway to nucleation which was once considered to have general applicability to all nucleating systems is gradually giving way to a nonclassical pathway which is now considered as a dominating mechanism in solution crystallization and other systems. In this review, an attempt is made to compare underlying physical principles involved in various nucleating systems and their theoretical treatment based on classical nucleation theory, and other important theories such as a density functional approach and diffuse interface theory. The limitations of classical theory, the gradual evolution of a nonclassical two-step pathway to nucleation, and the questions that have to be addressed in the future are discussed systematically.
The present study in Rotary Disc Contactors, an attempt has been made to develop a new correlation for the direct estimation of dispersed phase hold-up with the knowledge of known operating system variables and also to estimate the different regions of effective operation in terms of rotor critical speed. New generalized correlation involving Froude Number, phase¯ow rates, modi®ed property group along with those of geometry factor is proposed for both no solute transfer as well as mass transfer conditions. A large bank of published literature data from 13 different sources (1248 data points with 14 liquid systems) with a wide range of variables, along with those of author's data (291 measurements with four liquid systems) were used. The proposed correlations are far more accurate and simple to use, than those of previously reported.List of symbols D r Rotor disc diameter, m D s Stator ring opening, m D t Column diameter, m Fr Froude Number gaN 2 Á D r G f Geometry group, D s aD r À2X1 Á D r aD t À2X5 Á Z c aD r À0X75 Â Ã g Acceleration due to gravity, m/s 2 Mo Morton number, c 3 Á Dq a l 4 c Á g À Á Â Ã N Rotor speed, rps U Super®cial velocity, m/s U 0 Characteristic velocity of droplets, m/sec U s Slip velocity, m/sec Z c Compartment height, m D Difference e Fractional hold-up c Interfacial tension, N/m l Viscosity, cp q Density, kg/m 3 w 1 U Ã d À Á 1X0 1 U c aU d 0X15 w 2 Fr À0X33 Mo À0X07 Dqaq c À0X2
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