Diffusion and reaction in porous heterogeneous catalysts has been a subject of intensive investigation for almost half a century. Major summary works were published by Wheeler in 1951 and 1955 [1, 2], by Satterfield and Sherwood in 1963 [3], by Satterfield again in 1970 [4], and by Aris in 1975 [5, 6]. Recent years have witnessed no reduction in interest, and literature on the subject continues to proliferate.