COKING OF CRACKING CATALYSTS.

Fouling or ″coking″ is an integral feature of catalytic cracking, and can rapidly reduce the activity of a catalyst and so necessitate frequent regeneration and replacement. The composition of coke, essentially polynuclear aromatic compounds, changes with time; usually the molecular mass increases by the addition of olefins to the coke, and the H/C ratio decreases by H loss in polymerization and cyclization reactions. Despite these two processes occurring together, the rate of coke build-up can be expressed by the Voorhies equation. Non-kinetic investigations tend to favor a Langmuir-Hinshelwood mechanism. On the basis of this conclusion the author is preparing amorphous silica-alumina catalysts with controlled site spacings. If there is a decrease in coking with increased spacing then the Langmuir-Hinshelwood mechanism prevails.