When 
, the concentration within pellets drops from Cs to nearly zero in a narrow region near rs. At this condition, only the surface near the outer periphery of the pellet is effective and the catalyst surface in the central portion of the pellet is not utilized. This situation can be caused by large particles, low De or high k1 ; that is very active catalysts. Hence, low effectiveness factors are more likely with very active catalysts. More active the catalyst, the more is the possibility that intrapellet diffusion resistance will reduce global rate. For flat plate or slab of catalysts, opened to reactant only from one side with all other surface being sealed to reactant, the effectiveness factor is given as
where, 
and L = thickness of plate
If the slab is open to reactant on both side of the plate, the expression remains similar with L becoming the half of the plate thickness.
Book Reference :
• J. M. Smith, Chemical Engineering Kinetics , McGrawHill Book Company, 1981
• H. S. Fogler, Elements of Chemical reaction engineering , Prentice Hall of India., 1999
• J.J. Carberry , Chemical and catalytic reaction Engineering, Dover Publications, 2001
• O. Levenspiel , Chemical reaction engineering, John Wiley & sons, 1995