Decomposition

Decomposition units are used to process and chemically alter the heaviest molecules that emerge from the initial distillation column, producing more valuable products. Depending on its composition, the decomposition product can either be mixed directly with the refinery's bulk gasoline product, or be processed further in other refinery units and later mixed with the gasoline product.

Coking and catalytic cracking units are commonly used for decomposition reactions with the lowest quality petroleum feed. Using a combination of heat and pressure, coking units "crack" or break the bonds of larger hydrocarbon molecules, converting them into smaller hydrocarbon molecules and generating coke, a carbon residue. The cracked hydrocarbons are passed to a fractionator where they are further purified based on molecular weight.

Catalytic cracking units upgrade the quality of their liquid feed, producing a mixture that contains performance-enhancing branched and aromatic hydrocarbons. In addition to feed coming directly from the primary distillation column, catalytic cracking units will often process the heavier molecules emerging from a coking unit.

Production

There are two types of coking units: Delayed and continuous. Delayed coking is a semi-continuous process in which multiple reactor drums are operated in batch mode at high temperatures and pressures, producing the cracked fuel products as well as coke, which is deposited on the reactor walls. The product is sent to a fractionator to be further purified, and the coke is removed from the drums using hydraulic cutters.

Continuous coking units require two separate units. In the first unit, steam is pumped through the base of the reactor, fluidizing a bed of fine coke particles. The hot fluidized particles mix with the hydrocarbon feed, causing the hydrocarbons to decompose, creating lighter hydrocarbons and coke as a by-product. The cracked products leave through the top of the reactor and are sent to a fractionator. Excess coke from the reaction becomes mixed with the fluidized coke. To prevent an accumulation of material, the coke is continually circulated to a second unit, where it is separated by particle size into fractions. The smaller particle fraction is re-heated and sent back to the reactor unit, while the larger particle fraction is removed from the unit to maintain steady state operation.

Three types of catalytic cracking processes exist: fixed-bed , moving-bed , and fluid-bed . Today, fluid-bed, otherwise known as fluidized catalytic cracking (FCC), is the most commonly used catalytic cracking method. FCC units contain zeolite, a clay-like alumina-silica catalyst that provides convenient cracking sites for the hydrocarbon molecules. Furthermore, because of the small size of individual zeolite particles, it behaves like a fluid when it is pumped through pipes or a reactor. Coke buildup results in decreased catalytic activity over time.

To eliminate the layer of coke and regenerate the catalyst, the coke-covered zeolite is burned in the presence of air, forming carbon monoxide and carbon dioxide, which are easily removed from the catalyst-regenerating unit. After being burned and regenerated, the zeolite particles are mixed with the hydrocarbon feed entering the reactor unit. An added bonus to this mixing step is that the zeolite particles effectively transfer heat from the regenerator unit to the hydrocarbon feed, helping vaporize the hydrocarbon feed and eliminating costs for pre-heating.

Purification

Coking and FCC reactor units use similar processes to purify their products. After decomposition, the solid material and vapor hydrocarbons are separated using a cyclcone . In continuous coking units, the solid coke is sent to a solids separator to either be removed from the reaction unit or re-heated and sent back into the reaction. In FCC units, the catalyst is sent to the regeneration unit. Cracked products from delayed coking units are passed straight to a distillation column because the hydrocarbon vapor does not contain any solid material.

(Copyright Mac Process Inc., Kansas City, MO)

After passing through the cyclone separator, the hydrocarbon vapors enter a , where the hydrocarbon vapors separate into heavier and lighter fractions. The heaviest fractions are recycled and sent back to the reactor feed to go through the unit for a second pass and the lightest factions are either blended with the refinery's final gasoline product or processed further in other refinery units.

Acknowledgements

Sierra Monitor Corporation , Milpitas, CA

Monroe Environmental Corporation , Monroe, MI

ROBATEL, Inc. , Pittsfield, MA

Mac Process Inc. , Kansas City, MO

References

"Petroleum Refining Process." OSHA Technical Manual. Section IV: CHapter 2. United States       Department of Labor, Occupational Safety & Health Administration, n.d. Web. 13 Mar.       2016. https://www.osha.gov/dts/osta/otm/otm_iv/otm_iv_2.html

Speight, James G. The Chemistry and Technology of Petroleum. 4th ed. Boca Raton: CRC       Press, 2007. Print.

Developers

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