DEVELOPMENT OF THERMODYNAMICS TRAINING MATERIAL (DTI PROJECT MPP 9.6)

Transcription

1 DEVELOPMENT OF THERMODYNAMICS TRAINING MATERIAL (DTI PROJECT MPP 9.6) Hugh Davies, Alan Dinsdale, John Gisby, Jim Robinson (NPL) and Fred Hayes (Consultant to NPL) BACKGROUND The teaching of thermodynamics and phase equilibria in Materials Science and Engineering degree courses is becoming less popular than was previously the case 1 despite remarkable advances over the last thirty years in computer software and databases which apply thermodynamic principles to calculate phase equilibria and provide real insight into complex industrial processes involving many different elements and phases. The problem is that students quickly become disillusioned as a result of the large number of equations and proofs without obvious practical application involved in traditional thermodynamics teaching. It has been the aim of this project to develop course materials to help people teaching thermodynamics make use of phase equilibrium calculation software and databases in stimulating interest and an appreciation of the relevance of thermodynamics in their students. Use as part of continuing professional development schemes in industry is also envisaged. Where possible principles are illustrated by means of calculations for real rather than idealised systems undertaken using MTDATA 2 developed at the UK National Physical Laboratory. SCOPE Materials have been developed for courses at three different levels, introductory (covering basic thermodynamic principles), intermediate (covering the relationship between thermodynamics and phase equilibria) and advanced (illustrating how phase equilibrium calculations can be applied to the solution of industrial problems). Teachers would probably want to pick and choose elements from each to suit their own needs. The coverage of each course is outlined below: (1) INTRODUCTION TO THERMODYNAMICS First, second and third laws of thermodynamics State functions (definitions and relationships between them) Isolated systems, closed systems at constant temperature and pressure and closed systems at constant temperature and volume Equilibria between ideal gases, between ideal gases and immiscible solids and between non-ideal phases

2 (2) THERMODYNAMICS AND PHASE EQUILIBRIA Single component phase diagrams The relationship between Gibbs energy and binary phase equilibria Different types of binary phase diagrams Polymorphism, intermediate phases, metastable phases, immiscibility Graphical representation of ternary phase equilibria Monovariant curves, fields of primary crystallisation Isopleths, phase fraction diagrams, Scheil calculations (3) PRACTICAL APPLICATION OF THERMODYNAMICS Storage, retrieval and examination of thermodynamic data Principles of phase equilibrium calculations and models Unary, binary and ternary phase diagrams, Graphical representation of multicomponent equilibria Liquidus contours, primary phase maps, isopleths Equilibrium constraints Non equilibrium calculations - Scheil and para equilibrium Examples of practical applications of each type of calculation Case studies with reference to published sceintific papers MATERIALS (1) INTRODUCTION TO THERMODYNAMICS Notes for twenty lectures in the form of a Word document (currently in draft form) (2) THERMODYNAMICS AND PHASE EQUILIBRIA Notes for twenty lectures in the form of a Word document (currently in preparation) A database (MTTHERM) which allows all of the calculations referred to in the lectures of (1) and (2) to be performed by the student using MTDATA. It also allows the effects of changing temperature, pressure and composition to be explored where appropriate. Instructions on how to undertake the calculations referred to in the lectures of (1) and (2). Students can perform individual calculations interactively or run through the the whole series automatically. (3) PRACTICAL APPLICATION OF THERMODYNAMICS Powerpoint presentation (83 slides) illustrating the variety of diagrams phase equilibrium calculations can produce and their practical uses. Powerpoint presentation (59 slides) of 13 case studies, illustrating selected applications of phase equilibrium calculations in more depth (both presentations completed)

3 A database (MTAPPLIC) which allows the calculations outlined in both Powerpoint presentations to be performed by the student using MTDATA. It also allows the effects of changing temperature, pressure and composition to be explored where appropriate. Instructions on how to undertake the calculations referred to in the Powerpoint presentations. Students can perform individual calculations interactively or run through the the whole series automatically. AVAILABILITY The final form and mechanism for the distribution of the materials for courses (1) and (2) has not yet been finalised but they may be provided as part of a special teaching licence of the MTDATA software. A final decision will be made in consultation with thermodynamics teaching professionals. Materials developed for course (3) have already been trialled successfully in MTDATA training courses for industrial and academic customers and as part of an postgraduate course at Loughborough University. REFERENCES 1. Editorial by Fred Hayes in Journal of Phase Equilibria and Diffusion, 2004, 25(3), MTDATA - Thermodynamics and Phase Equilibrium Software from the National Physical Laboratory, Davies R H, Dinsdale A T, Gisby J A, Robinson J A J, Martin S M, CALPHAD, 2002, 26(2), pp

4 SAMPLE PAGES FROM INTRODUCTION TO THERMODYNAMICS COURSE Numerical Example of use of Kirchhoff Equation The main reduction reaction in the industrial extraction of zinc occurs between zinc oxide and carbon to give metallic zinc and carbon monoxide gas. Under standard state conditions this reaction only becomes feasible at temperatures above 905 o C, which is close to the boiling point of zinc. The reaction of interest can be written as follows: ZnO(s) + C(s) Zn(g) + CO(g) This reaction is endothermic and zinc is produced as a gas within a mixture of CO(g) and CO 2 (g). The mixture is shock cooled by liquid Pb droplets into which the Zn dissolves. It is vital that, prior to shock cooling, the temperature of the Zn(g)/CO(g)/CO 2 (g) mixture is maintained sufficiently high to prevent reoxidation of Zn(g) to ZnO(s) from occurring. This is achieved by the controlled injection of pre-heated air which causes some CO(g) to be oxidised to CO 2 (g), a highly exothermic reaction. This requires an accurate knowledge of r H o for the reduction reaction and its variation with temperature. To calculate this by hand from reference tables involves the following steps: 1) Calculation of r H o (298K) for the following reaction from standard enthalpies of formation of ZnO(s) and CO(g) at 298K: ZnO(s) + C(s) Zn(s) + CO(g) 2) Use C P data and Kirchhoff s equation to calculate r H o at the melting point of Zn 3) Use the enthalpy of fusion (melting) of Zn, fus H o (Zn) to give r H o at the melting point of Zn for the reaction to give liquid Zn: ZnO(s) + C(s) Zn(l) + CO(g) 4) Use C P data and Kirchhoff s equation to calculate r H o at the boiling point of Zn 5) Use the enthalpy of vaporisation (boiling) of Zn, vap H o (Zn) to give r H o at the boiling point of Zn for the reaction to give Zn gas: ZnO(s) + C(s) Zn(g) + CO(g) 6) Use C P data and Kirchhoff s equation to calculate r H o at the temperature of interest. Details of this calculation are set out below.

5 (Three pages of hand calculation omitted for brevity in these sample pages) MTDATA produces the following table: ZnO<c>+C<c>=Zn<g>+CO<g> T Delta Cp Delta H Delta S Delta G Beta K J/(mol.K) J/mol J/(mol.K) J/mol -G/RTln E E E E E E E or a plot of r H o versus T for ZnO<c>+C<c>=Zn<s,l,g>+CO<g>: Conclusion It is clear that the hand calculation is not difficult to perform but is tedious and prone to mistakes due to the number of steps involved. The MTDATA computation is easy, quick, reliable and gives results either as exportable tables, graphs or both.

6 SAMPLE PAGES FROM PRACTICAL APPLICATION OF THERMODYNAMICS COURSE Two of 83 slides showing the variety of diagrams that phase equilibrium calculations can produce and their practical uses.

7 One of thirteen case studies illustrating selected applications of phase equilibrium calculations in more depth.

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