Since the control volume in Figure 15-8 was set to move at the average molar velocity v_{z,ref,mol}, the sum of J_{i,z} is equal to zero for any system. Thus, for a binary system, only one J_{i,z} is independent. However, there are two independent fluxes N_{i,z}. In order to use the Stefan-Maxwell formulation in practical problems, we need another relationship (called a bootstrap equation) that allows us to determine the additional independent flux N_{i,z} In other words, we need to tie the moving J_{i,z} to the stationary N_{i,z} . The form of this additional equation depends on the situation. In general, N_{i,z} = C_{i}v_{i} = C_{m}y_{i}v_{i}, and if one of the v_{i} is known, we can calculate the required unknown flux N_{i,z}.

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