please can anyone help with this article?Notes on the diffusion-type model for the longitudinal mixing of fluids in flow: O. Levenspiel and W. K. Smith, Chem. Engng Sci.6: 227–233, 1957 and Levenspiel, O. (1972) Chemical Reaction Engineering. 2nd Ed., Wiley,
New York, N.Y. Many thanks.