Phycocyanin to biocrude via the integration of isothermal/fast hydrothermal liquefaction and aqueous phase recirculation: Reaction products and process analyses

Aqueous phase (AP) recirculation is an efficient and economically feasible approach to handle the post waste-water from hydrothermal liquefaction (HTL). In this study, isothermal (300 degrees C, 30 min) and fast HTL (set-point temperature of 500 degrees C, 40 s) of phycocyanin with different mixing ratios (6:0, 4:2, 3:3, 2:4, 0:6) of water and AP products were conducted. The optimum mixing ratios which obtained the highest biocrude yield were further selected for multiple AP recirculation experiments and process analysis. The results show that with the optimum mixing ratio of 2:4, isothermal and fast HTL produced the highest biocrude yields (24.2 wt% and 30.0 wt%) after the 2nd and 1st recycles, respectively. Nitrogen heterocycles from Maillard reaction and amides from acylation reaction were the major compounds in biocrude and AP, which contributed to the biocrude generation and increased the nitrogen content of biocrude. COD and NH3-N contents in HTL wastewater increased while TP contents decreased with the consecutive recirculation of AP. With AP recirculation for three times, nearly half of the wastewater discharges could be avoided. Consequently, AP recirculation is a promising method to enhance the biocrude production and alleviate the environmental impacts of AP discharges.