Catalyst deactivation
Activity of catalysts normally decreases with time. The life of any catalyst generally depends on type of reactions as well as reaction conditions. For example, catalysts for catalytic cracking lose much of their activity within seconds due to carbon deposition on the surface while promoted iron catalysts used in ammonia synthesis have a lifetime of years. For any catalytic process, the life of catalyst is a major economic factor. To regenerate or replace deactivated catalysts, the process needs to be shutdown and consequently production is disrupted. Subsequent separation and regeneration of catalysts also involve time and cost. Therefore, deactivation of catalysts increases the cost of production significantly. Hence, any catalytic process will be economically viable only if regenerations are required infrequently and can be done inexpensively. A catalyst can be deactivated in three ways
- I. Poisoning
II. Fouling
III. Sintering or phase transformation
I. Poisoning
Poisoning basically involve chemisorption of reactants or products or feed impurities on the active sites of the catalyst surface, thereby decreasing the number of active sites available for catalytic reactions. Since poisoning involves chemisorptions, it is known as chemical deactivation. This process can be reversible or irreversible. Compound of sulphur and other materials are frequently chemisorbed on nickel, copper and Pt catalysts. In reversible poisoning, the strength of adsorption bond is not great and activity is regained when the poison is removed from the feed. When the adsorbed material is tightly held on the active sites, poisoning is irreversible and permanent.
II. Fouling
Rapid deactivation can be caused by physical deposition of substance on the active sites of catalysts. Carbon deposition on catalysts used in petroleum industry falls in this category. Carbon covers the active site of the catalysts and may also partially plug the pore entrance. This type of deactivation is partially reversible and regeneration can be done by burning in air.
III. Sintering or phase transformation
Because of local high temperature, support of catalysts or catalyst itself may undergo structural modification or sintering causing a reduction in specific surface area or change in chemical nature of catalytic agent so that it becomes catalytically inactive. Hence, poisoning and fouling are dependent on concentration of reactant or product or impurities. On the other hand, sintering and phase transformation may be assumed to be independent of fluid phase composition. This is also therefore known as independent deactivation.
Steps to reduce deactivation: Following steps can be taken to reduce the possibility of deactivation of catalysts:
a. Removal of poison material form feed
b. Use of hydrogen which reduces coking
c. Removal of hot spot by proper design of reactor /process control to prevent any thermal deactivation.