Increasing demand for gasoline, along with the need to produce high-octane gasoline for increasingly more powerful spark ignition engines, led to the development and maturation of catalytic cracking processes just before and during World War II. Following the development of a fixed-bed (Houdry process, 1936) and a moving-bed (Thermafor Catalytic Cracking, 1941) catalytic cracking process, fluid-bed catalytic cracking (FCC, 1942) became the most widely used process worldwide because of the improved thermal efficiency of the process and the high product selectivity achieved, particularly after the introduction of crystalline zeolites as catalysts in the 1960s.
The list below shows a timeline for the development of the catalytic cracking processes. The evolution of catalytic cracking processes is an exemplary showcase in chemical engineering for discussing the advancement of reactor configuration, driven by energy conservation and process kinetics. The evolution of these processes is discussed in the following subsections.
McAfee (1915)
Batch reactor catalytic cracking to produce light distillates
Catalyst: A1Cl3 – A Lewis acid, electron acceptor
Alkane – electron(abstracted by A1Cl3)→ a carbocation(+)→ ionic chain reactions to crack long chains
Houdry (1936) - a commercial process
Continuous feedstock flow with multiple fixed-bed reactors
Cracking/catalyst regeneration cycles
Catalyst: clays, natural alumina/silica particles
Thermafor Catalytic Cracking (TCC) (1942)
Continues feedstock flow with moving-bed catalysts
Catalyst: synthetic alumina/silica particles
Higher thermal efficiency by process integration
Fluid Catalytic Cracking (FCC) (1942)
Continuous feedstock flow with fluidized-bed catalysts
Catalyst: synthetic alumina/silica+zeolites (1965)
The first catalytic cracking process was developed as a batch process (McAfee, 1915) shortly after the development of a thermal cracking process. The process used Lewis acid catalysts (e.g., AlCl3) for cracking. These catalysts were expensive and corrosive. In addition to these impediments, use of a batch reactor in the McAfee process did not allow large-scale commercialization of this process. The first full-scale commercial process, the Houdry Catalytic Cracking, used much less expensive catalysts, such as clays, and natural alumina and silica particles. Figure 7.5 shows the configuration of the Houdry Catalytic Cracking process. For cracking, gas oil feed was heated to 800°F and fed to a fixed-bed reactor packed with the catalyst particles. Cracking products are sent to a fractionator to be separated into gas, gasoline, light cycle oil (LCO) and heavy cycle oil (HCO) products.
A series of swing reactors were needed to switch the feed flow from one reactor to another after approximately 10 minutes of operation. The switch to a swing reactor was necessary because of rapid coking on catalysts which, being natural materials, had a wide range of activity. Rapid coking on silica/alumina particles deactivated these catalysts and led to plugging of the reactors. After the flow was switched to another reactor, the isolated reactor was stripped with steam for five minutes to remove the liquid products adsorbed on catalyst particles. After stripping with steam, the deactivated catalysts were regenerated by burning off the coke on catalysts with hot air introduced to the reactor. Catalyst regeneration also takes approximately 5 minutes before the reactor with regenerated catalyst is ready to accept the feed again. By this time, the second reactor would be ready for the 10-minute cycle of steam stripping and catalyst regeneration. Having a third reactor in the plant would help deal with any delays/problems in reactor preparation. Considering that catalytic cracking is an endothermic process, the heat generated from burning the coke off the catalyst could be used partially to heat the catalyst particles for the endothermic reaction. A large portion of the heat in the flue gases from coke combustion was not available for the process. Therefore, the thermal efficiency of the Houdry Process was low.
Thermafor (also referred to as “thermofor” in some sources) Cracking Process was introduced for better integration of thermochemistry (endothermic cracking and exothermic catalyst regeneration) by introducing a moving-bed configuration, rather than a fixed-bed, as shown in Figure 7.6. Catalysts used in this process were synthetic alumina/silica beads that have more homogeneous and consistent properties (e.g., activity) than the natural minerals. Catalysts particles and the feed are introduced from the top of the reactor, and the catalyst particles move downward with gravity as the cracking reactions take place on the catalyst surfaces. Steam is injected from the bottom of the reactor to carry the cracking products to the fractionator for recovery. As the particles move down the reactor, they are deactivated by coke build-up on active sites. The deactivated catalysts removed from the bottom of the reactor are sent to a regenerator unit where the coke on catalysts surfaces are burned off and the heated catalysts particles are recycled to the top of the reactors by bucket elevators. Hot catalyst particles provide most of the heat necessary for the cracking reactions in the reactor. Although the thermal efficiency of TCC is higher than that of the Houdry process, there was still a significant amount of heat loss during the transport of heated catalyst particles by bucket elevators.
Fluid Catalytic Process, also introduced in 1942, offered an excellent integration of the cracking reactor and the catalyst regenerator that provides the highest thermal efficiency, as shown in Figure 7.7. In FCC, a fluidized-bed (or fluid-bed) of catalyst particles is brought into contact with the gas oil feed along with injected steam at the entrance (called the riser) of the reactor. The hot catalyst particles coming from the regenerator unit evaporate the feed gas oil upon contact in the riser, and the cracking starts as the gas oil vapors and the catalyst particles move upward in the reactor. The temperature of the catalyst particles drops as the evaporation of gas oil and endothermic cracking reactions proceed during the upward movement. Cracking reactions also deposit a significant amount of coke on the catalysts, leading to the deactivation of the catalyst. After removing the adsorbed hydrocarbons by steam stripping, the coked catalyst is sent to the regeneration unit to burn off the coke with air. Heat released from burning the coke deposit increases the temperature of the catalyst particles that are returned to the riser to complete the cycle. Burning off the rejected carbon (coke) in the regenerator provides the energy necessary for cracking without much loss, thus increasing the thermal efficiency of the process. The cracking products are sent to the fractionator for recovery after they are separated from the catalyst particles in the upper section of the reactor [3].
In the reactor, the cracking reactions initiate on the active sites of the catalysts with the formation of carbocations and the subsequent ionic chain reactions produce branched alkanes and aromatic compounds to constitute the crackate (cracked gasoline with high octane number), light olefins, cycle oils, and slurry oil that are sent to the fractionator. A carbon-rich byproduct of catalytic cracking, termed “coke,” deposits on catalyst surfaces and blocks the active sites. FCC is considered a carbon rejection process because the coke deposited on the catalyst surface and eventually burned off for heat is rich in carbon and thus enables the production of large quantities of a light distillate (crackate) in the process without the addition of hydrogen.
Two different configurations of the commercial FCC processes exist depending on the positions of the reactor and the regenerator: they can be side by side or stacked, where the reactor is mounted on top of the regenerator. Major licensor companies that offer FCC processes with different configurations include Kellogg Brown & Root, CB&I Lummus, ExxonMobil Research and Engineering, Shell Global Solutions International, Stone & Webster Engineering Corporation, Institut Francais du Petrole (IFP), and UOP. Figure 7.8 shows examples of Exxon and UOP designs [1,4]. The UOP design of high-efficiency two-stage regenerator units offer advantages of uniform coke burn, higher conversion of CO to CO2 and lower NOx emissions among others. Another modification to FCC plants could be the installation of a catalyst cooler, which may provide better control of the catalyst/oil ratio; the ability to optimize the FCC operating conditions, increase conversions, and process heavier residual feedstocks; and better catalyst activity and catalyst maintenance [3].
In the first video below, the animation of an explosion in an FCC unit in 2015 (7:12 minute long) provides a good review of the FCC process, and points out the potential hazards of working with hydrocarbons exposed to high temperatures in refinery units:
One of the significant developments in FCC practice was the introduction of zeolite catalysts in 1965. Catalysts and additives play a major role in the selectivity and flexibility of FCC processes. FCC catalyst consists of a fine powder with an average particle size of 60–75 μm and a size distribution ranging from 20 to 120 μm. Four major components make up the catalysts: zeolite, active matrix, filler, and binder. Each of these constituents has a unique role to play, but zeolite is the key component that is more active and selective for high-octane number gasoline production [4]. Table 7.4 compares the octane numbers of some refinery products and FCC gasoline.
Product |
RON (600 rpm) |
MON (900 rpm) |
---|---|---|
Regular - Premium Gasoline | 90-100 | 80-90 |
Straight Run Gasoline | 60-68 | 60-68 |
FCC Gasoline (light) | 93 | 82 |
FCC Gasoline (heavy) | 95 | 85 |
Solve a problem on the material balance for the regenerator in Fluid Catalytic Cracking Process.
Burning the coke deposited on the catalyst particles generates all the heat necessary for catalytic cracking. Therefore, the coke burning rate is a critical parameter to control the rate of cracking. The composition of dry flue gas from the regenerator of an FCC unit is given in vol% as follows:
N2: 81.6
CO2:15.7
CO: 1.5
O2: 1.2
The dry air flow rate to the regenerator is given as 593 SCMM (standard cubic meters per minute). Considering that a significant portion of coke is carbon, calculate the carbon burning rate in the regenerator in kg/min. Remember: 1 kgmole at STP = 22.4 m3)