Searching
for a Solution
 Typical
catalysts in the early days of cracking were acid-treated clays or
chemically made silica-alumina mixtures. Cracking catalysts that worked
well exhibited the same behavior; they were active for a short period
of time and then became covered with a deactivating layer called coke.
This coke layer could be removed by heating and burning, but the regeneration
was slow (minutes) compared to the time the catalyst was active (seconds).
For every 20 molecules of heavy oil put into the cracking reaction,
18 would crack to smaller molecules, but two molecules would combine
to make an even larger molecule. This larger molecule stuck to the
catalyst surface and eventually became the coke that deactivated the
catalyst. To this day, no exceptions to this behavior have been seen,
although the amount of the heavy oil that becomes coke is less as
catalysts have been improved.
The
efficient way to use such catalysts, understood by all the organizations
involved, was to move the catalyst from one reactor (where cracking
was done) to another reactor (where regeneration was done). The
problem was to move the catalyst from the reactor, which contains
the hydrocarbon feed and products, to the regenerator, into which
air is forced to burn the carbon off the catalyst particles, without
the regenerator air contacting the hydrocarbon. Steam was used as
a stripping agent to separate the air and hydrocarbon vessels.
A
Solution: The Fluidized Bed
The
Catalytic Research Associates decided to focus on fine powder catalysts.
In small units, it was easy to circulate the powders through a reactor,
stripper and regenerator using screw-type conveyors, but these devices
plugged up or wore rapidly in larger units. It was well known that
a high-velocity gas flow blows powdered solids up (or down) a pipe,
but Warren K. Lewis and Edwin R. Gilliland of the Massachusetts
Institute of Technology, while working with Standard Oil Company
of New Jersey, suggested that a low velocity gas flow through a
powder might "lift" it enough to cause it to flow in a
manner similar to a liquid.
This
was quickly found to be true, and the M. W. Kellogg Company constructed
a large pilot plant in Standard's Baton Rouge refinery. The pilot
plant began operation in May of 1940. Based on its success, the
construction of the first commercial plant began in September in
the tense months just before World War II. The first Model I Fluid
Catalytic Cracker (FCC) was completed on May 1, 1942 and began operating
on May 25 in Baton Rouge at the Standard Oil Company refinery. Called
PCLA-1 (Powdered Catalyst Louisiana), it was the first commercial
fine powder circulating fluid bed reactor.
Between
the May 1940 decision to build a Model-I design and the May 1941
decision to install additional catalytic crackers at the Baton Rouge
refinery, large pilot plant work demonstrated that a Model II design
used less steel and had a more efficient method of operation. The
first two Model II units, PCLA-2 and PCLA-3 were built right next
to PCLA-1. Constructed in 1942 and 1943, they incorporated changes
reflecting improved understanding of the process and the catalysts.
They still operate today. PCLA-1 was shut down in October 1963 and
subsequently dismantled.
Since
the fluid bed designs needed less steel, less piping, and fewer
valves than a fixed-bed unit for the same amount of heavy oil cracking
capacity, war-time pressures to conserve strategic materials resulted
in the construction of 34 more FCC units. These reactors were built
and operating by 1945 to help supply the large volume of high-octane
aviation gasoline needed for the Allied forces in World War II,
along with the feedstocks needed for the wartime synthetic rubber
industry.
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