Extraction of phenolic compounds from hydrothermal processing of black liquor: Effect of reactor type and pH of recovered liquid phase

Abstract

The goal of this work was to study the effect of pH on the extraction yield of phenolic compounds from liquid phases recovered after the hydrothermal treatment of black liquor in the batch and continuous reactors at 280 and 350 °C. The pH values of the recovered liquid phases under investigation were adjusted to 9, 8, and 7. The liquid-liquid extraction was subsequently performed using ethyl acetate for 4 h at room temperature. The identification and quantification of phenolic products were conducted using GC-Mass analysis. The main monomeric compounds identified in the aqueous phase were phenol, guaiacol, and syringol. The results indicated that reducing the pH slightly improved the phenol and guaiacol extraction from the various recovered liquid phases after hydrothermal treatment. However, reducing the pH of liquid phases greatly increased the extraction yield of syringol. This was very noticeable in the recovered liquid phases after treatment at 280 °C, where the quantity of syringol was initially high. Additionally, a lower pH hindered the ethyl acetate hydrolysis during liquid-liquid extraction. The extraction experiments with various pH also confirmed the superior performance of the continuous reactor over the batch reactor to obtain the phenolic compounds. In addition, the phenolic products degraded as the treatment temperature increased from 280 to 350 °C.