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Effect of Component Numbering on the Fick Matrix

Example 4.2.4 Effect of Component Numbering on the Fick Matrix [Pg.84]

The preceding examples illustrated some of the ways in which the Fick matrix [D] depends on the composition of the mixture and on the binary (Maxwell-Stefan) diffusion coefficients. One further property of [D] is that both the sign and magnitude of the elements of [ ] depends on the order in which the components are numbered. Wesselingh (1985) provided a dramatic and elegant illustration of this fact for the system H2-N2-CCI2F2. At a temperature of 298 K and a pressure of 101.3 kPa the diffusion coefficients of the three binary pairs that make up the mixture are [Pg.84]

The ratio of the largest to the smallest of these coefficients is nearly an order of magnitude thus we may expect the Fick matrix [D] to show a strong composition dependency. [Pg.84]

SOLUTION For each of the three possible choices for component 3 there are two ways to order the components. We will calculate the elements of [D] for three ways of ordering the components obtained by rotating the component numbers in order. With the compo- [Pg.84]

Another aspect of the choice of component numbering relates to the accuracy of the calculations it is not a good idea to calculate small fluxes by subtracting two Targe numbers. Put another way, if the flux of component i is expected to be small, do not label this as solvent species n Include i as one of the first n - 1 species. Greater accuracy is obtained by choosing n to be the species with the highest concentration.  [Pg.86]




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