Conversion Process: Catalytic cracking Hydrocracking Thermal cracking Coking Solvent De-asphalting

(5) Catalytic cracking Convert heavy hydrocarbon fractions from vacuum distillation into a mixture of more useful products Feedstock undergoes a chemical breakdown, under controlled heat (450 - 500oC) and pressure, in the presence of a catalyst (5) Catalytic cracking Effective catalyst: small pellets of silica, alumina or magnesia and nowadays zeolite Principle reaction: Dehydrogenation of aromatics or naphthenes, which can then add side chains in place of some of the hydrogen attached to the ring of carbons

Catalytic cracking: products Primary goal to make gasoline & diesel Minimize the production of heavy fuel oil Light ends contain large amounts of olefins » Good for chemical feedstock » Can recover chemical grade propylene & ethylene » Propylene, butylene, & C5 olefins can be alkylated for higher yields of high-octane gasoline

Catalytic cracking REGENERATOR REACTOR RISER Catalytic cracking is the most important and widely used refinery process for converting heavy oils into more valuable gasoline and lighter products. The cracking process produces carbon (coke) which remains on the catalyst particle and rapidly lowers its activity. To maintain the catalyst activity at a useful level, it is necessary to regenerate the catalyst by burning off this coke with air. As a result, the catalyst is continuously moved from reactor to regenerator and back to reactor. The cracking reaction is endothermic and the regeneration reaction exothermic.

Catalytic cracking REGENERATOR REACTOR RISER The catalytic-cracking processes can be classified into; Moving-bed unit (like, Thermafor catalytic cracking (TCC)) Fluidized-bed unit (like, fluid catalytic cracker (FCC)) The hot oil feed is contacted with the catalyst in either the feed riser line or the reactor. As the cracking reaction progresses, the catalyst is progressively deactivated by the formation of coke on the surface of the catalyst. The catalyst and hydrocarbon vapors are separated mechanically, and oil remaining on the catalyst is removed by steam stripping before the catalyst enters the regenerator. The oil vapors are taken overhead to a fractionation tower for separation into streams having the desired boiling ranges.

Catalytic cracking REGENERATOR REACTOR RISER The spent catalyst flows into the regenerator and is reactivated by burning off the coke deposits with air. Regenerator temperatures are carefully controlled to prevent catalyst deactivation by overheating. This is done by controlling the air flow to give a desired CO2/CO ratio in the exit flue gases or the desired temperature in the regenerator. The flue gas and catalyst are separated by cyclone separators and electrostatic precipitators. The catalyst in some units is steam-stripped as it leaves the regenerator to remove adsorbed oxygen before the catalyst is contacted with the oil feed.

Catalytic cracking Two basic types of FCC units in use today are the ‘‘side-by-side’’ type, where the reactor and regenerator are separate vessels adjacent to each other, and the stacked type, where the reactor is mounted on top of the regenerator. side-by-side FCC units stacked type FCC units

FCC Process FCCU feed is preheated to 650oF and is fed into the riser The ratio of catalyst: oil = 4:1 to 9:1 by weight. Residence time of the gas < 5 seconds The vapor generated by the cracking process lifts the catalyst up the riser. The vapor velocity at the base of the riser is about 6 m/s and increases to over 20 m/s at the riser exit. FCC Process REACTOR REGENERATOR FCCU feed is preheated to 650oF and is fed into the riser Contains hot catalyst from the regenerator RISER

A spent catalyst is withdrawn from the bottom of reactors and stripped with steam to vaporize the hydrocarbons remaining on the surface Stripping removes most of the hydrocarbon vapors which are entrained between the particles of catalyst FCC Process REACTOR It is desirable to separate the vapor and catalyst as quickly as possible to prevent overcracking of the desired products. REGENERATOR RISER

Catalytic cracking The FCC process employs a catalyst in the form of very fine particles [average particle size about 70 micrometers (microns)] which behave as a fluid when aerated with a vapor. The fluidized catalyst is circulated continuously between the reaction zone and the regeneration zone and acts as a vehicle to transfer heat from the regenerator to the oil feed and reactor. REGENERATOR REACTOR RISER

Catalyst Characteristics of good catalyst: - ability to produce desirable product and not coke - selective to valuable products, eg high octane gasoline is desirable, - stable so it does not deactivate at the high temperature levels in regenerators. - resistant to contamination

Catalyst Three classes: acid-treated natural aluminosilicates amorphous synthetic silica-alumina combinations crystalline synthetic silica-alumina catalyst called zeolite or molecular sieves

Makeup of a catalyst particle

Advantage of using zeolite as catalyst Higher activity Higher gasoline yield Gasoline contains more paraffinic and aromatic HC Lower coke yield Increased isobutane production Higher conversion without overcracking

What deactivates FCC catalyst Conditions in the Regenerator: temperature, water, time Impurities/Poisons in the FCC feed: Va, Ni, Fe, Cu Temporary Poisons: Nitrogen, Carbon on catalyst surface – lowers catalyst activity