Experimental Study on Enrichment of Indole in Wash Oil by a Combined Process of Extraction and Re-Extraction

Abstract

Wash oil, one of the by-products of coal tar distillation, contains useful nitrogen heterocyclic compounds (NHC) such as indole (IN), which is used as a raw material for perfume, essential amino acid, and pharmaceuticals. In this study, as a pre-treatment step for purification of IN, which is about 5.75% contained in wash oil, the enrichment of IN in wash oil was examined by combining formamide extraction (FE) and n-hexane re-extraction (HRE). The raw material for the re-extraction was a mixture of the preceding formamide extract phase acquired through batchwise 5-stage co-current extraction trial under fixed FE condition. As the 5-stage FE and HRE progressed, the IN yield based on IN in the wash oil was approximately 73.3%, and the IN concentration in the raffinate oil acquired from the re-extraction run was highly enriched to about 92.4%. In addition, the IN enrichment efficiency of the FE-HRE combined process (FHC) employed in this study was compared with methanol extraction (ME)-hexane re-extraction (HRE) combination (MHC).