Direct n-hexane extraction of wet sewage sludge at thermal and pressurized conditions: A preliminary investigation on its process and product characteristics

- A hydrothermal liquefaction route of wet sewage sludge with a pretreatment process is proposed reasonably.
- Wet sewage sludge can be extracted directly by n-hexane at thermal and pressurized conditions to produce crude sludge oils.
- The extraction process characteristics at thermal and pressurized conditions are illustrated preliminarily.
- The produced crude sludge oil has a high yield of 22.8-24.2% and a low O content of 3.5-6.0%.
- Both aqueous phase and residue are suitably recycled for the hydrothermal liquefaction.

This paper proposed a hydrothermal liquefaction route of wet sewage sludge (WSS) with a pretreatment process, which is the direct solvent extraction of original WSS at thermal and pressurized conditions. Thereby, a preliminary investigation was made on the direct n-hexane extraction process of original WSS and its product characteristics. The extraction process, which has these characteristics with the removal of O/N/S heteroatoms (especially decarboxylation) and the efficient separation of water, is suitable and efficient for directly extracting crude sludge oils from original WSS. The favorable temperature for the extraction process is above 260 °C, where the crude sludge oil possesses a high yield (22.8-24.2%) and a low O content (3.5-6.0%). The produced aqueous phase mainly contains alcohols, carboxylic acids, aldehydes, and ketones, which can take a promotion on the thermal dissolution during thermo-chemical conversion of organics. The produced residue contains a large quantity of solvent-soluble organics such as asphaltenes and preasphaltenes. Therefore, it is inferred that produced residue and aqueous phase are suitably recycled for the hydrothermal liquefaction to further produce crude bio-oils in higher temperatures.