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284 Advances in Eco-Fuels for a Sustainable Environment

Table 10.3 Coded and actual levels utilized in the Box-Behnken experimental method

Coded and actual values for the levels in the experimental
design
Low Center High
Parameters
Levels 1 0 +1
Temperature (°C) 250 300 350
Holding time (min) 0 30 60
Initial pressure (MPa) 0.10 2.55 5

250–350°C was selected in agreement with a previous study undertaken by Jazrawi
et al. [45]. The effect of reaction time on the biocrude yield was assessed from
0min (the time when the target temperature is attained) to 60min in agreement with
the previous work undertaken by Audo et al. [46]. The effect of the initial reactor pres-
sure on the biocrude yield was investigated ranging from 0.1MPa (ambient pressure)
to 5MPabar, also in agreement with the previous work undertaken by Audo et al. [46].
According to the previous experimental studies in the area of biomass HTL
processing, the reaction time after the target temperature is attained has been specified
as the holding time in the present study [47–49]. The temperature has been specified as
the target temperature imposed during the hydrothermal liquefaction process.
Table 10.3 summarizes the levels of the Box-Behnken design method and the asso-
ciated values of the operating variables that were investigated in this study. The sta-
tistical analysis of the experimental results was performed using Minitab 17.1.0
(Minitab, United States), as a suitable statistical tool.
Based on the number of operating variables investigated in this study, the exper-
imental results obtained will be fitted to the second-order polynomial regression
model expressed as follows,

3 3 3
X X 2 XX
Y o ¼ X o + a i X i + a ii X i + a ij X i X j (10.1)
i¼1 i¼1 i¼1 j¼1

where Y o represents the biocrude yield (wt%), X o represents the model intercept, X i
(X j ) represents the ith (jth) operating variable (i.e., target temperature T, holding time
t, or initial pressure p), and a i , a ii , and a ij represent the model regression coefficients.

10.2.3 Experimental method

Hundred grams of a wet digestate mixture containing 3.02g of solids (composition
reported in Table 10.2) was initially mixed to ensure uniformity and then introduced to
a batch reactor and sealed. Leak tests under high operating pressures were initially per-
formed prior to reactor purging using nitrogen gas for air displacement. The conditions
oftemperature,holdingtime,andinitialpressure wereimposed accordingtothe specified
Box-Behnken experimental design runs shown in Table 10.4. The batch reactor is a