Optimisation of a catalytic hydrothermal liquefaction process using central composite design for yield improvement of bio-oil

Hydrothermal liquefaction (HTL) or hydrous pyrolysis is a thermochemical de-polymerisation process in conversion of wet biomass into bio-crude oil and chemicals at moderate temperatures and high pressures. The notable applications of these bio-oils are in boilers, engines and turbines for generation of heat and power or for extraction of chemicals. In this HTL process, experiments were conducted using rice straw as biomass, glycerol as co-solvent and sodium carbonate as catalyst. The effect of temperature (220°C, 240°C, 260°C, 280°C), reaction time (15 min, 30 min, 45 min, 60 min), catalyst dosage (5wt%, 12.5wt%), biomass to water ratio (1:5, 1:10) and glycerol to water ratio (0:10, 5:5) were systematically studied and analysed for evaluation of process performance in terms of yield and selectivity of bio-oil. Central composite design (CCD) was further used to optimize the process variables like temperature, reaction time, catalyst, solvent and co-solvent. Finally, the optimal operating conditions of 12.5 wt% of Na2CO3 at 260°C for 60 min at equal ratios of water and glycerol are established using CCD and were experimentally verified. The obtained product was analysed using elemental analysis, FTIR, GC-MS for determination of properties and it was found that the main compounds constitute ketones, alcohols, phenolics and aldehydes as main compounds in the HTL process.