Optimization of process parameters for bio-crude and value-added chemical recovery through hydrothermal liquefaction of microalgae

Hydrothermal liquefaction (HTL) is a promising conversion technique utilized in converting microalgal biomass into value-added products like bio-crude and bio-char. On processing this bio-crude based on the applications leads to biofuel, aqueous phase, and value-added chemicals which have many applications. For this study, Nannochloropsis oculata (NO), Chlorella vulgaris (CV), and Arthrospira platensis (AP) were selected for bio-crude production using the HTL process. The process parameters like temperature (250-300 degrees C), residence time (30-60 min), and microalgae to water loading ratio (4-10) were chosen using Box-Behnken design using response surface methodology to obtain maximum bio-crude yield. A total of 17 experiments for each microalga were conducted for optimization of process parameters. The study suggested it is a quadratic model that fits all three process parameters interacting with each other and having R-squared values of 0.9704 (NO), 0.9827 (CV), and 0.9884 (AP). For NO, the highest yield of 53.78% was obtained (280 degrees C, 60 min, 9.5); similarly, the maximum yield of 48.04% (290 degrees C, 60 min, 10) and 44.65% (278 degrees C, 57min, 7.5) was obtained for CV and AP. GC-MS analysis of bio-crude showed the presence of major compounds like phenol, alcohols, acids, and esters. Elemental analysis for the bio-crude was performed and NO had a higher HHV value of 39.85 MJ/kg followed by CV (33.88 MJ/kg) and AP (32.47 MJ/kg).