# Purification of Indole Contained in Wash Oil by Combination of Extraction and Crystallization (Part 2: Highly Efficient Purification of Indole in Indole-Concentrated Oil via Solute Crystallization)

## TL;DR

This study presents a highly efficient method to purify indole from wash oil using a combination of extraction and crystallization techniques.

## Contribution

A novel purification process for indole using solute crystallization with optimized conditions is proposed.

## Key findings

- Indole purity of 99.5 wt% was achieved under optimal crystallization conditions.
- A yield of 57.5% was obtained for indole in the generated crystals.
- The proposed method integrates previous extraction techniques with solute crystallization for efficient purification.

## Abstract

To develop a novel process to purify indole in wash oil with high efficiency, a previous study investigated the indole concentration in wash oil using a process combining methanol extraction and n-hexane re-extraction, which allowed the recovery of a raffinate phase containing 73.3 wt% indole-concentrated oil (ICO). In this study, model ICO, prepared based on the components and compositions present in ICO, and n-hexane were used as the raw material and solvent for the solute crystallization (SC) process, respectively, to examine the effect of SC experimental factors and conditions on the purification of indole. A Pyrex batch crystallizer with a jacket was used as the experimental apparatus. The purity and yield of indole in the generated crystals under optimal SC conditions (crystal washing, a crystallization temperature of 283 K, a crystallization time of 10 min, an impeller speed of 0 s−1, and an initial volume ratio of the solvent to model ICO of 15.5) were 99.5 wt% and 57.5%, respectively. Furthermore, this study proposed a novel process for the highly efficient purification of indole contained in wash oil by utilizing the experimental data accumulated from processes in previous research and this study.