First of all , p-Xylene (para-xylene) is an aromatic hydrocarbon. It is one of three isomers of dimethyl benzene that are collectively known as xylenes. The p- stands for para-, indicating that the two methyl groups on p-xylene occupy the diametrically opposite substituent positions 1 and 4. It is at the positions of the two methyl groups, their arene substitution pattern, that differ from the other isomers, o-xylene, and m-xylene. They all have the same chemical formula C 6 H 4 (CH 3 ) 2 . All xylene isomers mentioned are colorless and highly flammable. The odor threshold for p-xylene is 0.62 parts per million (ppm).
Production of para xylene
Production of para xylene is industrially significant. In general ,its annual demand was estimated at 37 million tons in 2014, and it is still increasing.In addition, p-xylene is produced by catalytic reforming of petroleum naphtha as part of the BTX aromatics (benzene, toluene, and xylene isomers) extracted from catalytic reforming. The p-xylene is then separated in a series of distillation, adsorption, or crystallization and reaction processes from m-xylene, o-xylene, and ethylbenzene. Its melting point is the highest of this series of isomers, but simple crystallization does not allow easy purification due to the formation of eutectic mixtures.
Such separation procedures are major cost factors in production, and the search for alternative methods continues. For example, a reverse osmosis technique has been proposed to improve various aspects of the processes.
Industrial applications of para xylene
Para Xylene is an important chemical feedstock. Among other industrial applications, it is a starting material in the large-scale synthesis of various polymers. In particular, it is an important component in the production of terephthalic acid for polyesters such as polyethylene terephthalate (generally known as PET). It can also be directly polymerized to produce paraxylene.
Toxicity and exposure
Xylenes are not extremely toxic, eg the LD 50 (rat, oral) is 4300 mg/kg. Concerns with xylenes center around narcotic effects. Overexposure to para-xylene in humans can cause headache, fatigue, dizziness, apathy, confusion, irritability, gastrointestinal disturbances including nausea and loss of appetite, flushing of the face, and a feeling of increased body heat. Exposure to xylene vapor above the recommended exposure limit of 100 parts per million (ppm) may cause eye, nose, and throat irritation and possible chest tightness, and abnormal gait.
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