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The Chemistry and Technology of Petroleum
space velocity (weight or volume of the feedstock charged per hour per weight or volume of catalyst
in the reaction zone). Wide flexibility in product distribution and quality is possible through control
of these variables along with the extent of internal recycling is necessary. Increased conversion can
be obtained by applying higher temperature or higher pressure. Alternatively, lower space velocity
and higher catalyst/feedstock ratio will also contribute to an increased conversion.
When cracking is conducted in a single stage, the more reactive hydrocarbons may be cracked,
with a high conversion to gas and coke, in the reaction time necessary for reasonable conversion of
the more refractory hydrocarbons. However, in a two-stage process, gas and gasoline from a shortreaction-time, high-temperature cracking operation is separated before the main cracking reactions
take place in a second-stage reactor.
19.3.1 FIxed-Bed ProCesses
Historically, the Houdry fixed-bed process, which went on-stream in June 1936, was the first of the
modern catalytic cracking processes. It was preceded only by the McAfee batch process, which
employed a metal halide (aluminum chloride) catalyst but has long since lost any commercial
significance.
In the fixed-bed process, the catalyst in the form of small lumps or pellets was made up in layers
or beds in several (four or more) catalyst-containing drums called converters. Feedstock vaporized
at about 450°C (840°F) and less than 7–15 psi pressure passed through one of the converters where
the cracking reactions took place. After a short time, deposition of coke on the catalyst rendered it
ineffective, and using a synchronized valve system, the feed stream was turned into a neighboring
converter while the catalyst in the first converter was regenerated by carefully burning the coke
deposits with air. After about 10 min, the catalyst was ready to go on-stream again.
Fixed-bed processes have now generally been replaced by moving-bed or fluid-bed processes.
19.3.2 FluId-Bed ProCesses
19.3.2.1 Fluid-Bed Catalytic Cracking
The fluid catalytic cracking (Figure 19.1; Bartholic and Haseltine, 1981) is the most widely used
process and characterized by the use of a finely powdered catalyst that is moved through the reactor
(Figure 19.2) and flow patterns may vary depending upon the precise configuration of the reactor.
The catalyst particles are of such a size that when aerated with air or hydrocarbon vapor, the catalyst behaves like a liquid and can be moved through pipes. Thus, vaporized feedstock and fluidized
catalyst flow together into a reaction chamber where the catalyst, still dispersed in the hydrocarbon
vapors, forms beds in the reaction chamber and the cracking reactions take place. The cracked vapors
pass through cyclones located in the top of the reaction chamber, and the catalyst powder is thrown
out of the vapors by centrifugal force. The cracked vapors then enter the bubble towers where fractionation into cracked light and heavy gas oils, cracked gasoline, and cracked gases takes place.
Since the catalyst in the reactor becomes contaminated with coke, the catalyst is continuously
withdrawn from the bottom of the reactor and lifted by means of a stream of air into a regenerator
where the coke is removed by controlled burning. The regenerated catalyst then flows to the fresh
feed line, where the heat in the catalyst is sufficient to vaporize the fresh feed before it reaches the
reactor, where the temperature is about 510°C (950°F).
19.3.2.2 Model IV Fluid-Bed Catalytic Cracking Unit
This unit involves a procedure in which the catalyst is transferred between the reactor and regenerator
by means of U bends, and the catalyst flow rate can be varied in relation to the amount of air injected
into the spent-catalyst U bend. Regeneration air, other than that used to control circulation, enters the
regenerator through a grid, and the reactor and regenerator are mounted side by side.
The Chemistry and Technology of Petroleum
space velocity (weight or volume of the feedstock charged per hour per weight or volume of catalyst
in the reaction zone). Wide flexibility in product distribution and quality is possible through control
of these variables along with the extent of internal recycling is necessary. Increased conversion can
be obtained by applying higher temperature or higher pressure. Alternatively, lower space velocity
and higher catalyst/feedstock ratio will also contribute to an increased conversion.
When cracking is conducted in a single stage, the more reactive hydrocarbons may be cracked,
with a high conversion to gas and coke, in the reaction time necessary for reasonable conversion of
the more refractory hydrocarbons. However, in a two-stage process, gas and gasoline from a shortreaction-time, high-temperature cracking operation is separated before the main cracking reactions
take place in a second-stage reactor.
19.3.1 FIxed-Bed ProCesses
Historically, the Houdry fixed-bed process, which went on-stream in June 1936, was the first of the
modern catalytic cracking processes. It was preceded only by the McAfee batch process, which
employed a metal halide (aluminum chloride) catalyst but has long since lost any commercial
significance.
In the fixed-bed process, the catalyst in the form of small lumps or pellets was made up in layers
or beds in several (four or more) catalyst-containing drums called converters. Feedstock vaporized
at about 450°C (840°F) and less than 7–15 psi pressure passed through one of the converters where
the cracking reactions took place. After a short time, deposition of coke on the catalyst rendered it
ineffective, and using a synchronized valve system, the feed stream was turned into a neighboring
converter while the catalyst in the first converter was regenerated by carefully burning the coke
deposits with air. After about 10 min, the catalyst was ready to go on-stream again.
Fixed-bed processes have now generally been replaced by moving-bed or fluid-bed processes.
19.3.2 FluId-Bed ProCesses
19.3.2.1 Fluid-Bed Catalytic Cracking
The fluid catalytic cracking (Figure 19.1; Bartholic and Haseltine, 1981) is the most widely used
process and characterized by the use of a finely powdered catalyst that is moved through the reactor
(Figure 19.2) and flow patterns may vary depending upon the precise configuration of the reactor.
The catalyst particles are of such a size that when aerated with air or hydrocarbon vapor, the catalyst behaves like a liquid and can be moved through pipes. Thus, vaporized feedstock and fluidized
catalyst flow together into a reaction chamber where the catalyst, still dispersed in the hydrocarbon
vapors, forms beds in the reaction chamber and the cracking reactions take place. The cracked vapors
pass through cyclones located in the top of the reaction chamber, and the catalyst powder is thrown
out of the vapors by centrifugal force. The cracked vapors then enter the bubble towers where fractionation into cracked light and heavy gas oils, cracked gasoline, and cracked gases takes place.
Since the catalyst in the reactor becomes contaminated with coke, the catalyst is continuously
withdrawn from the bottom of the reactor and lifted by means of a stream of air into a regenerator
where the coke is removed by controlled burning. The regenerated catalyst then flows to the fresh
feed line, where the heat in the catalyst is sufficient to vaporize the fresh feed before it reaches the
reactor, where the temperature is about 510°C (950°F).
19.3.2.2 Model IV Fluid-Bed Catalytic Cracking Unit
This unit involves a procedure in which the catalyst is transferred between the reactor and regenerator
by means of U bends, and the catalyst flow rate can be varied in relation to the amount of air injected
into the spent-catalyst U bend. Regeneration air, other than that used to control circulation, enters the
regenerator through a grid, and the reactor and regenerator are mounted side by side.