Process Water Recycle in Hydrothermal Liquefaction of Microalgae To Enhance Bio-oil Yield

In this work, the effect of recycling the process water (PW) of hydrothermal liquefaction (HTL) to the HTL reactor was investigated, with the objective being to recover carbon from the organic content of the PW and to develop a solvent-free process. When recycling twice the PW at 220, 240, and 265 °C, a significant increase in bio-oil production is observed at all temperatures. An extended series of recycle experiments was performed at 240 °C, showing that the bio-oil yield increased up to stationary level, after six recycles. To investigate the role of accumulated compounds as acetic acid, additional experiments were carried out with dilution and spiking of recycled PW. PW recycling not only increases bio-oil yield but also enables to reduce operating temperature (and, hence, costs) of HTL; therefore, it is an essential element in developing a solvent-free process for biocrude production from microalgae.