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Thermal depolymerization of biogas digestate 287

reactor walls. Filtration of the product mixture is then undertaken using Whatman No. 1
filter papers to enable the separation of the insoluble residues from the liquid phase. The
filtration operation was performed twice to ensure optimal insoluble solid residue
removalfromtheliquidproductphase.Therecoveredliquidproductwasthenmixedwith
CH 2 Cl 2 (volume ratioof1to1)andtheresultingtwoimmiscible phases,composedofthe
nonpolar CH 2 Cl 2 phase and the aqueous phase, separated using a funnel to enable
biocrude recovery. Drying of the separated biocrude CH 2 Cl 2 phase mixture, which con-
tains the target biocrudeproduct,wasundertakentovaporizethe CH 2 Cl 2 solvent present.
Each experimental run was undertaken in duplicate and mean values reported.
Having determined the biocrude yield under different experimental conditions, the
significance of each operating variable to the hydrothermal liquefaction was investi-
gated for completeness. The significance of each process variable was assessed via the
comparison of the statistical student F-value of each variable with the critical F-value
based on the experimental data [51, 52]. Typically, the impact of an operating variable
on the hydrothermal liquefaction process is considered significant if the calculated
statistical student F-value of the experimental data of the variable is greater than
its critical F-value. The critical F-value of the operating variable can be determined
by using the critical F-value table [53]. The critical F-value for the present study for a
confidence level of 95% was determined to be 4.8.

10.2.4 Determination of the appropriate conditions for enhanced
biocrude yield
The values of the system variables of initial reactor pressure, reaction temperature,
and holding time required to facilitate maximum biocrude yield were estimated from
the empirical relation developed according to the second-order polynomial expression
in Eq. (10.1) and determined from the experimental data presented in Table 10.4.
These values were estimated using the numerical optimization algorithm
(desirability-function) of Minitab [54]. The details of the desirability-function
approach can be found elsewhere in Ref. [55]. To experimentally test the validity
of the estimated conditions for maximum biocrude yield, duplicate experiments were
undertaken at the estimated conditions specified for maximum biocrude yield. The
average value of the experimentally determined biocrude yields from the duplicate
runs was then compared against the predicted optimum biocrude yield.
Under the conditions for optimized biocrude yield, the associated yields of the
insoluble solids (biochar), the soluble solids present in the post-HTL water, and the
gas phase products were also determined.
To determine the yield of the insoluble solid residue (biochar), the product recov-
ery steps presented in Fig. 10.2 were utilized. The biochar residue was recovered after
the filtration operation was oven dried to constant mass at 105°C. The mass of the
dried biochar was subsequently measured using a weighing balance (Mettler Toledo,
3
New Zealand) equipped with an electronic scale with a resolution of 10 g. The
biochar yield in wt% on a dry basis (Y biochar ) was then determined as follows,

m biochar
Y biochar ¼ 100 (10.3)
m dig: dbð Þ