Tube-Side Equation of Continuity for Species A

Following Bird et al. [4], the equation of continuity for species A in terms of M A for constant p and Dab in cylindrical coordinates is 

there is no chemical reaction indicated in this process therefore, there is no generation or consumption of species A (va = 0). Based on the experience, we can anticipate that the convection contribution in the z-direction will be much larger than the diffusion contribution, that is. 

Therefore, the equation of continuity for species A in this system is 

Finally, if we move p inside the derivatives and divide each term by the molecular weight of species A and observe that 

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