where D is the pore diffusion coefficient, C is the gas p ip ] concentration of component i in the macropores, q is the i average amount adsorbed in the crystals, R is the pellet p radius and k is the film mass transfer resistance in the f external surface of the pellet. The sub-index S denotes the geometry of the pellet and has the values of 0, 1 and 2 for infinite slab, infinite cylinder and spheres, respectively.