Production of dl-limonene by vacuum pyrolysis of used tires

Various samples of used car and truck tires were pyrolyzed in a batch mode under vacuum and in a continuous feed reactor. The pyrolysis temperature varied in the range of 440-570 degreesC. dl-limonene is a major product formed during the thermal decomposition of rubber under reduced pressure conditions. The pyrolysis oils were distilled to obtain a dl-limonene-rich fraction. The difficulty of obtaining a pure dl-limonene fraction is discussed. A high pyrolysis temperature decreases the dl-limonene yield due to the cracking of the pyrolysis oil. Several secondary organic compounds produced by cracking were identified by gas chromatography/mass spectrometry (GC/MS) analysis. These compounds had a boiling point similar to dl-limonene. The dl-limonene yield decreases with an increase of the pyrolysis reactor pressure. The mechanism of the thermal degradation of tires leading to the formation of dl-limonene is discussed. A dl-limonene-rich fraction was obtained following a series of distillation. Sulfur-containing compounds in the dl-limonene-rich fractions were analyzed by GC using a sulfur specific detector. Several thiophene-derivatives were identified. Quantitative analysis of the sulfur compounds in the dl-limonene rich fractions was made. An olfactometry test was performed on a standard thiophene sample in d- and dl-limonene solutions to establish an approximate threshold value to detect the thiophene odor. (C) 2001 Elsevier Science B.V. All rights reserved.