Temporary catalyst poisons

The resistance of new zeolitic catalysts to temporary and permanent catalyst poisons is essential to the economic and commercial success of a zeolitic based cumene process. The following commercial data obtained using beta zeolite as a catalyst illustrates the outstanding ability of beta zeolite to cope with a wide range of feedstock contaminants ... 

Figure 3. Temporary effect of catalyst poisons on silica-based catalysts. (Reproduced from Ref. 10 Copyright 1968, American Chemical Society.)...

Halogens and volatile nitrogen compounds generally act as weak catalyst poisons or inhibitors and lead to a reversible or temporary lowering of the catalyst activity. 

In the case of gaseous catalyst poisons, a distinction can be made between permanent poisons causing an irreversible loss of catalytic activity and temporary poisons which lower the activity only while present in the synthesis gas. This distinction is fully discussed in the book by Nielsen. Permanent poisons such as sulfur accumulate upon the catalyst surface and may be detected by chemical and spectroscopic analysis, while temporary poisons do not interact nearly as strongly with the catalyst. It is very difficult to detect temporary poisons by means of post-analytical methods. The principal temporary poisons are oxygen, carbon oxides, and water. Since the catalyst also contains percent amounts of oxygen... 

Fouling Salt formation can build-up on the catalyst surface effectively limiting accessibility. Ammonium bisulfate can form at low temperatures. This foulant can be removed by increasing temperature and is considered a temporary poison. 

All licensors agree on the necessity of hydrotreating the feed to lower the level of poisons for the platinum-based reforming catalyst. Temporary poisons are sulfur and nitrogen, while As, Pb, and other metals are permanent poisons. Proper conditions of hydrogen, pressure, temperature, and space velocities are able to reduce these poisons to the acceptably low levels of modern catalysts. Numerous process design modifications and catalyst improvements have been made in recent years. 

Sulfur, chlorine, and phosphorus are classed as temporary poisons in reforming reactions, because the activity of the catalyst is recovered when the poison... 

Moreover, the kinetics are further complicated by the high level of water in the reactor inlet, deriving from its high concentration in both fresh and recycled ethanol, that acts as a temporary poison for the catalyst and strongly reduces the reaction rate [6]. 

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