light oil
The oily product, lighter than water, forming the chief fraction of the first distillate of coal tar, and consisting largely of benzene and toluene.
light oil: benzene-rich distillate from coal tar processing
Light oil is the volatile organic fraction recovered in the first stage of coal tar distillation, boiling between roughly 80 and 170 degrees Celsius. It floats on water and comprises mostly aromatic hydrocarbons, primarily benzene (about 45 percent) and toluene (about 25 percent), along with xylene and other C6 to C9 aromatics. This fraction separates naturally as the tar is heated and its vapors condense in stages, with light oil coming off first because of its lower boiling point.
The recovery process depends on cooling and condensation. As coal tar vapor rises through distillation columns or passes over cooling surfaces, heavy tars drop out first. Light oil condenses next and is drawn off; heavier fractions remain. The collected liquid is then refined further by fractional distillation or solvent extraction to isolate pure benzene, toluene, and xylene for chemical synthesis, or blended as a mixed aromatic concentrate for paint thinners, industrial solvents, and fuel blending.
Grades and practical use
Light oil quality varies with coal type and operating temperature. Coke plant distillation typically yields 10 to 15 liters of light oil per ton of coal processed. The product is often sold as tar acids (containing phenols), tar bases (containing pyridines and quinolines), or neutral oil, depending on what impurities remain and what the buyer needs. Benzene and toluene extracted from light oil feed petrochemical plants, while lower-purity grades become wood preservatives, insecticides, or motor fuel components.
Contamination and separation failures are common problems. If cooling is inadequate, light oil carries over into heavier fractions and is lost. If tar moisture is not removed before distillation, water emulsions form and clog equipment. The aromatics in light oil are volatile and toxic; vapors accumulate in confined spaces, and skin contact causes dermatitis. Historical records from coal tar works document respiratory illness in workers handling freshly distilled material.
The term originated in 19th-century gas and coke manufacturing, where coal tar byproduct was fractionated by simple heating. Chemists labeled the early condensate light oil to distinguish it from the heavy tars and creosote that followed. This vocabulary persists in modern coal chemistry even though synthetic aromatic production now dominates markets. Light oil remains a reference point for understanding coal tar composition and a useful intermediate for small-scale aromatic recovery where dedicated benzene plants are uneconomic.