Metallurgical Kinetics_introduction presentation.ppt
- 1. Metallurgical Kinetics Reference books: 1.Metallurgical kinetics: H. S. Ray, PHI India 2.Chemical Reaction Engineering,: Octave Levenspiel, John Willey & Sons, NY, 1995 3.Engineering in Process Metallurgy: R. I. L. Guthrie, Oxford University Press, NY, 1989 4.Process Metallurgy: Jullian Szekely & Nickolas J. Themelis, Willey Interscience, NY, 1971
- 2. Introduction to Kinetics • Thermodynamics Vs Kinetics – Thermodynamics is concerned with initial and final /equilibrium state – Kinetics is concerned with mechanism, time of reaction, productivity – Thermodynamics defines the feasibility of a reaction and kinetics judges the practical feasibility of a process – Thermodynamics quantifies phase composition at equilibrium while kinetics determines the speed at which a reaction proceeds towards equilibrium – Kinetics study is essential for design & process development
- 3. Classification of Kinetics • Homogeneous kinetics – Reaction in single phase – burning of fuel gas in presence of oxygen, or reaction between aqueous solution of acid and base. – Rate depends only on the interactions of reactants. • Heterogeneous kinetics – Reaction takes place at the interface of two distinct phases – gas-solid, gas-liquid, two immisible liquids etc. – Here in addition to reactant interaction, rate can also depend on the transport of the heat and mass across the boundary layer of the interface.
- 4. Importance of Heat & Mass Transfer in Metallurgical system • Metallurgical systems are mostly heterogeneous • Kinetic steps involved in heterogeneous reactions are: – i) Transport of heat/reactant species from bulk of phase A/B to the interface – ii) Chemical reaction at the interface of phase A & B – iii) Transport of the heat/product species from the interface to the bulk of phase A/B.
- 5. Importance of Heat & Mass Transfer in Metallurgical system • Rate Controlling step: – When various steps are connected in series: Slowest step is rate controlling – If step ii) is slowest step> Chemical reaction controlled (likely mechanism at comparatively low temperature) – If step i) or iii) is slowest step > heat /mass transfer controlled (most likely at comparatively high temperature) – When various steps are connected in parallel The fastest step controls the overall rate
- 6. Rate Expression for chemical reaction controlled process n kC R RT E e k k 0 k = chemical rate constant ΔE = Activation Energy
- 7. Rate Expression for Mass Transfer Controlled Process S A B B S A A B A J a b dt dN n A J n n b a g dD s cC s bB g aA ) ( ) ( ) ( ) ( B n = rate of consumption of B per unit time A J = mass flux of reactant A at the gas/solid interface S A = Surface area of the gas/solid interface
- 8. Determination of mass flux at heterogeneous interface • Method 1: If concentration profile is known 0 x A dx dC D J • Method 2: Based on concept of mass transfer coefficient e A b A m A C C k J
- 9. Rate Expression for Heat Transfer controlled process r S B r S B H A q n H A q n R g dD s cC s bB g aA / / ) ( ) ( ) ( ) ( r H =heat of reaction per mole of B consumed q =Heat flux at the gas/solid interface
- 10. Problem-1 Consider a slag (CaO-Al2O3-CaF2) is in contact with a stirred iron- sulphur liquid alloy in a crucible of 0.07 m inner diameter. The metal weighs 4kg and contains 0.1 wt% sulphur. The density of the liquid metal is 7000 kgm-3 . The equilibrium concentration of sulphur in metal in contact with the above slag is 0.005 wt%. Calculate the time when the sulphur concentration in the metal will be 0.06 wt%. Mass transfer coefficient = 5.3x10-4 m/s Ans: 156 seconds
- 11. Problem-2 Reduction of an iron ore carbon composite pellet is controlled by endothermic carbon gasification reaction (CO2+C=2CO). If a 10mm composite pellet is kept in a furnace at 1500K, find out the rate of reduction. Given: The overall reaction Fe2O3+3C=2Fe+3CO. Heat of reaction for carbon gasification is 41 kcal/mole, average temperature of the pellet=1000K, Stefan Boltzmann constant= 5.6x10-8 W/m2 K4, Emissivity of pellet surface=0.5.