Petrochemicals: Aromatic Compounds

James Richard Fromm


The petrochemicals industry is broadly defined as that industrial activity which uses petroleum or natural gas as a source of raw materials and whose products are neither fuels nor fertilizer. The petrochemical industry begins with oil refineries or extracting plants built to remove ethane and higher hydrocarbons from natural gas streams; sometimes methane itself is used as a source material or feedstock. The industry is so varied that analysis by specific compound or class of compound is the most effective method of presentation.

Present world data on production of organic industrial compounds is not easily obtainable. The Table below gives production figures for the United States for the fifteen organic chemicals produced in greatest quantity. This list excludes fuels, such as methane, ethane, propane, and butane, and also gasoline which includes aromatic compounds (toluene, xylene, etc.) to raise its octane rating. Only the major sources are given; reference to another compound included in the Table is indicated by an asterisk.


Table: Production of Industrial Organic Compounds, U.S.A.
Compound Production (Tg, 1985) Production (Tg, 1975) Source
       
Ethene 13.54 9.99 Ethene
Propene 6.75 4.43 Propane
Dichloroethane 5.49 3.59 Ethene*, Chlorine
Vinyl Chloride 4.29 2.60 Dichloroethane*
Benzene 4.26 4.81 Refinery
Styrene 3.46 2.86 Ethylbenzene*
Ethylbenzene 3.35 2.78 Benzene*, Ethene*
Terephthalic Acid 2.94 2.29 Xylene*
Formaldehyde 2.54 2.55 Methanol*
Ethylene Oxide 2.46 1.90 Ethene*, Oxygen
Xylene 2.41 3.30 Refinery
Toluene 2.30 3.73 Refinery
Methanol 2.27 2.83 Methane
Ethylene Glycol 1.90 1.52 Ethylene Oxide*
Butadiene 1.06 1.47 Butanes
       
The unit of one Tg used in this Table is also one million Metric Tons (MMT).      

An overview of the petrochemicals industry and a discussion of the aliphatic compounds of the petrochemical industry are given in another section. We now turn our attention to the aromatic compounds of the petrochemical industry.

Aromatic compounds are those containing one or more benzene rings or similar ring structures. The majority are taken from refinery streams which contain them and separated into fractions, of which the most significant fractions are benzene, C6H6, methylbenzene or toluene, C6H5CH3, and the dimethylbenzenes or xylenes, C6H4(CH3)2. Napthalene, containing two linked benzene rings, was in 1960 virtually all produced as a by-product of coal coking and coal tar distillation. Now more than half is produced from petroleum sources. Most of the aromatic compounds are used by the oil refineries to raise the octane rating of gasoline, and that used in the chemical industry is of lesser significance in terms of tonnage.

Benzene is used for many purposes, of which the largest use by far is for manufacture of styrene,

C6H5CH=CH2.

The process proceeds through ethylbenzene which is produced by reaction of benzene and ethene at 95oC in the presence of a catalyst:

C6H6 + CH2=CH2 rarrow.gif (63 bytes) C6H5CH2CH3.

In the presence of a catalyst and superheated steam ethylbenzene dehydrogenates to styrene:

C6H5CH2CH3 rarrow.gif (63 bytes) C6H5CH2=CH2 + H2.

Toluene is usually added to the gasoline pool or used as a solvent, but it can be dealkylated to benzene by catalytic treatment with hydrogen:

C6H5CH3 + H2 rarrow.gif (63 bytes) C6H6 + CH4.

Similar processes are used for dealkylation of methyl-substituted napthalenes.

Of the xylenes, ortho-xylene is used to produce phthalic anhydride and other compounds. Para-xylene is used in the production of polyesters in the form of terephthalic acid or its methyl ester. Terephthalic acid is produced from para-xylene by two reactions in four steps. The first of these is oxidation with oxygen at 190oC:

CH3-C6H4-CH3 + O2 rarrow.gif (63 bytes) HOOCC6H4CH3.

This is followed by formation of the methyl ester at 150oC:

HOOCC6H4CH3 + CH3OH rarrow.gif (63 bytes) CH3OOCC6H4CH3.

Repetition of these steps gives the methyl diester of terephthalic acid: This diester, CH3OOCC6H4CCOOCH3, when polymerized with ethylene glycol at 200oC, yields the polymer after loss of methanol to give a monomer. The polymerization step requires a catalyst.


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Copyright 1997 James R. Fromm