Pilot-Scale Continuous Flow Reactor for Hydrothermal Liquefaction of Algae

Algae-based biofuels have attracted much attention due to their advantages of not competing with land for food production, fewer terrestrial and weather limitations, higher growth rates, and strong CO₂-mitigation abilities. In recent years, research has focused on hydrothermal liquefaction (HTL) of algae since HTL does not require that the algae be dried before processing. Hydrothermal liquefaction uses high temperatures (270-350 °C) and high pressures (80-173 bar) to produce relatively high yields of bio-oil that can be upgraded to liquid transportation fuels. Most research to date has been conducted in batch reactors; shifting from batch reactors to continuous flow systems is an important next step towards commercial-scale processing.

This study presents the development and preliminary testing of a pilot-scale continuous flow reactor for hydrothermal liquefaction of fresh water microalgae at low solids loadings. A significant challenge to be overcome is the tendency of the algae feedstock and product slurries to create clogs inside filters and at bends and joints in the tubing even at very low solids loadings (<1 wt.%). To address such issues, a dual high-pressure cylinder filter system was designed to allow one filter to run while the other is being cleaned. Solid particles (char and ash), ranging from 6 to 25 µm, are captured by the cylinder filters and collected in blowdown pots. The filter elements can be cleaned in place by flushing back the product fluid. Tubing size and configuration was carefully considered to accommodate a narrow diameter desirable for the high pressure and to prevent unnecessary clogging opportunities. The goal of the reactor is to be able to perform continuous HTL on slurries with solid algae contents of 5-10 wt.%, and to produce char-free bio-oils.