2012_习题课2_Chapters6-12.ppt

Mass Transfer in Multicomponent Mixtures (6) Liu Hui State Key Lab of Chemical Resource Engineering, BUCT 2012 driving force friction with otherspecies ‘j’ MS-Equation with Four Driving Forces activity pressure centrifugal for gases friction forces electrical Review: Exercises in chapters 6-12 6.1 In Figure 6.1 we have written the forces per mole of each of the species. How do you modify the driving force terms to obtain forces per unit mass? In Figure 6.2 we have written the forces per mole of mixture. How do you obtain the driving forces per unit mass of the mixture? Using different units is explored systematically in the Appendix 2 on Units. 6.1 6.2 We consider the ternary mixture of Figure 6.1. Although there are only two independent transport relations, we can write out all three relations. We have put the friction coefficients for these equations in a matrix below: The coefficients in the first two rows are the same as those in the figure. Check that you understand those of the last row for component ‘3’. Note that there are no diagonal terms in the matrix: a component does not exert any friction on itself. The Onsager relation tells us that all three coefficients in the lower left triangle are equal to the corresponding values in the upper right triangle. So there are three different friction coefficients, not six. Extend this reasoning to obtain the number of coefficients in a mixture of four components.  6.2 6.9 Air consists of a mole fraction of 0.21 of oxygen; the rest is nitrogen. In many problems the two components do not move with respect to each other and so behave as a single component. The diffusivities of hydrogen (1)-in-oxygen and hydrogen-in-nitrogen are estimated as and m2 s-1. Write down the Maxwell-Stefan equation for hydrogen, and from this determine the effective diffusivity of hydrogen in air. 6.9 8.1 Distillation of Methanol-Water (Mathcad). In this file we have taken the liquid phase to be ideal.