Aromatics

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Aromatics generally refer to a class of hydrocarbons containing one or more conjugated cyclic (benzene) rings, cyclic unsaturated hydrocarbons with alternating double bonds. These hydrocarbons are found in almost every petroleum mixture. Aromatics are commonly fragrant compounds and hence the name.

Benzene (C₆H₆) is the simplest aromatic, toluene (C₇H₈) is benzene with one methyl group, and xylenes (C₈H₁₀) are dimethylbenzenes. Xylene refers to three isomers of dimethylbenzene and differs by the positions of the two methyl groups on the benzene ring:

• Ortho-xylene or "o-Xylene" (1,2-dimethylbenzene) has methyl groups on the 1 & 2 position carbon atoms.
• Meta-xylene or "m-Xylene" (1,3-dimethylbenzene) has methyl groups on the 1 & 3 position carbon atoms.
• Para-xylene or "p-Xylene" (1,4-dimethylbenzene) has methyl groups on the 1 & 4 position carbon atoms.

These xylenes are often shipped and sold as mixed xylenes, but are separated for these end uses. Under standard conditions, benzene, toluene, and xylene (BTX) are in liquid form, whereas higher aromatics, such as naphthalene, occur as solids.

Source: Benzene, Toluene, and ortho-, meta-, and para-xylene, Wikipedia Commons

Downstream products made from aromatics

Aromatic compounds are important for the production of polymers, various other chemicals and different consumer products like solvents, paints, polishes and pharmaceuticals. They serve as high-octane gasoline components and feedstock for various polymers, resins and other chemicals. Aromatics are precursors to a wide range of chemicals.

Source: BTX Value Chain, U.S. Department of Energy

Benzene derivatives

The majority of the benzene production is processed to ethylbenzene, a precursor to styrene monomer, which is polymerised to polystyrene and used to make plastics with applications like food packaging, thermal insulation and appliances. Benzene is also used to produce Cumene (isopropylbenzene), used for the production of phenol and acetone, used for resins and adhesives. Cyclohexane, another derivative of benzene, is used to manufacture nylon fibres, which are then processed into textiles and engineering plastics. Benzene also finds application in the production of various types of rubbers, lubricants, dyes, detergents, drugs, explosives, and pesticides.

Toluene derivatives

Toluene is used as a solvent for thinners, paints, lacquers, and adhesives. It is also used as an additive for gasoline and for the manufacture of explosives. If needed, toluene can be converted into benzene or xylene, allowing access to a variety of aromatic downstream products. The process of converting toluene into benzene is known as “hydrodealkylation,” and the process of converting toluene into a mixture of benzene and xylene is known as ”transalkylation.”

Xylene derivatives

Para-xylene is transformed into terephthalic acid and dimethyl terephthalate, both monomers used in the production of Polyethylene Terephthalate (PET) plastic bottles and polyester clothing. Most para-xylenes are consumed for the production of PET and polyester. Ortho-xylene is used for the production of phthalic anhydride and phthalate esters used as plasticizers. Meta-xylene converted to isophthalic acid derivatives is used as a component of alkyd resins. Xylenes are also used as solvents in printing, rubber, and leather industries, for thinning paints and varnishes, as a cleaning agent for steel, silicon wafers, and integrated circuits.

Production of aromatics

Aromatics are produced from various refinery and chemical processes that also produce fuels and olefins:

• Steam cracking: Thermal cracking of naphtha or light hydrocarbons for olefin production also yields pyrolysis gasoline, a by-product which is rich in aromatics. Benzene, toluene, and xylene can be extracted and purified from pyrolysis gasoline using BTX extraction units.
• Fluid catalytic cracking (FCC): FCC units in refineries crack heavy vacuum gas oils into gasoline-range products. In this catalytic process, many cracked fragments dehydrogenate and rearrange into aromatic rings, which are then recovered.
• Catalytic reforming (CR): CR is a process that produces high-octane gasoline and other high-value petrochemical molecules. Naphtha reforming in refineries converts low-octane naphtha into high-octane reformate. Reforming creates significant quantities of aromatics as products. The reformate, which is a blend of high-octane compounds, is further treated by solvent or extractive distillation to recover bulk BTX aromatics.
• Methanol-to-aromatics (MTA): This relatively newer route uses methanol synthesized from natural gas, coal gasification or biomass as feedstock. Methanol is selectively converted to aromatics over zeolite catalysts. When the feed methanol is sourced from green hydrogen and captured CO₂, this process can offer a carbon-neutral pathway to BTX production and is under development as a supplement to conventional routes.
• Coal pyrolysis and gasification: Coal carbonization in coke ovens produces coal tar, a complex mixture rich in benzene compounds. Benzene was first isolated from coal tar in 1845 and four years later, the first industrial-scale production of benzene began using the coal-tar method.

Feedstocks

Major feedstocks for aromatics include:

• Petroleum naphtha: Naphtha derived from crude oil is used in catalytic reformers, FCCs and steam crackers to produce aromatics.
• Pyrolysis gasoline (Pygas): Pygas is a byproduct of ethylene or propylene steam crackers and is rich in high-octane compounds. Hydrogenated pygas is fractioned to recover benzene and toluene.
• Natural gas: Aromatics can be produced from natural gas through a direct conversion process or by first converting it to methanol.
• Coal and coke oven byproducts: Both coal tar from coal carbonization and coke oven gas contain aromatics, which are then separated out.
• Waste materials: Using waste materials such as biomass, agricultural residues, plastic waste, and industrial byproducts is an emerging sustainable alternative to using fossil fuels. Various methods like pyrolysis, lignin depolymerization, cellulose depolymerization, hemicellulose to furfural, CO2 capture and chemocatalytic techniques are used to convert waste to valuable aromatics.