The four product fractions that are formed during HTL process are organic liquid, aqueous phase fraction, gas and solids. The HTL experiments were carried out in the liquefaction system, which comprised of an autoclave reactor and some auxiliary equipment. The whole body of autoclave was made of 316L stainless steel which ensures the maximum pressure of 40 MPa and temperature of 400 °C, also high corrosion resistance. Temperature was precisely controlled by the controller with a thermocouple inside the reactor. In a typical run, autoclave reactor filled with 33 mL slurry containing of 10–11 wt% dry microalgae (with or without pre-treatment) was tightly sealed with six evenly distributed bolts. Then the reactor headspace was purged with pure nitrogen at 50 mL/min for 5 min. As the reactor cooled down, the pressures and temperatures inside the reactor were recorded. The gas was vented and reaction mixture was collected carefully, and the reactor was rinsed with DCM for at least three times. The rinse solution and another 50 ml DCM were added into the reaction mixture. All of the mixtures were filtered with microporous membranes (0.45 ?m) and the filter residue after drying was defined as the solid residue. Then the filtrate with two phases was separated in a separating funnel. The water-soluble portion was defined as the aqueous products. The DCM soluble portion was defined as the biocrude, and was measured after filtration and evaporation of DCM. As per the central composite design parameters fixed the minimum and maximum value was set. The parameters such as temperature (260 – 320?), time (15 – 60 minutes) and catalyst concentration (0.25 – 1.25) were set respectively. After set these values, the RSM software given the 20 runs and each runs were consist of different temperature, time and catalyst concentration. According to the values, the HTL was performed by every run as mentioned in the table ( ). Then, bio-oil yield was calculated as per the equation ( ). Finally, the 2D graphical and contour plot was performed by RSM software.

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