Using the analogies from Section 3.1 and the results of Section 4 (see also Bertoli et al. ), it can be stated that for co-current flow and finite but not null model parameter values, the roots 𝜆 ́𝑗 (𝑗 = 0,1,2) are positive and distinct real numbers. For counter-current flow in the inner annular section, we can have a positive real root and the other two, both negative real roots, with the possibility (unconfirmed) of a double real root or a complex conjugate. For the parametric region 10 -10 < 𝜉 ́𝑞 ≤ 100 (𝑞 = 1 -4), the roots are distinct, one positive real and two negative real ones.