288 Science and Human Welfare
these three processes, but at the present time contact—or
adsorptive—type materials, comprising chiefly silica-alumina
composition, made by synthesis or by the chemical treating
of natural clays, are used almost exclusively for this purpose.
Cracking processes, employing the solid-type catalysts, may
be divided further into two basic types according to the
method of applying the catalyst. One system appears in
which the catalyst mass remains static in the reaction zone
and the vapors pass over the fixed catalyst bed. The Houdry
units exemplify this type. The other is one in which the
catalyst moves continuously through the reaction zone. The
Fluid Flow and TCC belong to this class. The differences be-
tween the TCC, Houdry, and Fluid Flow processes lie chiefly
in the physical methods employed in handling the catalyst,
in its size, in contacting the oil, and in regenerating the
catalyst.
The first Houdry units were placed in operation in 1936.
In this system a number of catalyst units, alternately op-
erated as a catalyst chamber and then being regenerated,
serves to maintain a continuous flow through the complete
unit. In this system the catalyst remains fixed in its case,
serves as a catalyst, is regenerated, and used again.
The TCC process, licensed by Houdry, derives its name
from the Thermofor, spiral-finned, type of clay-burning kiln
which is employed as the catalyst regenerating unit. In the
TCC the catalyst is in the form of granular pellets of from
4 to 60 mesh size. In this form the catalyst mass flows freely
through the equipment. There are separate reactor and re-
generating systems through which the catalyst is con-
tinuously circulated countercurrent to the oil vapors. This
system is recognized as a short, low-cost way to more source
materials for ιoo-octane gasoline, butadiene, and toluene.
Another improvement in catalytic cracking is the Fluid-