3.1 Effect of MeOK concentration on product composition of the liquid fraction
The advantage of using a feeding pump in a semi-continuous reactor is that it allows fractional analyses for different operation durations or volumes. Such data could be used to design suitable conditions for optimization of in-situ TE. The effect of MeOK concentration (1.0 – 2.5 g KOH in 100 mL methanol) on LF composition was investigated to determine the boundary between the pretreatment and biodiesel fractions. The total mass of SCGs was 60 g and MeOK loading volume was 240 mL. The presence of MeOK, soap, FAME, and TG in 25 mL aliquots of the LF were examined via HPLC-ELSD as shown in Fig.2. The composition of the first and second aliquots of the LF could not be analyzed because the soap signal was very broad and overlapped with the biodiesel signal. Further, only 175 mL of the LF could be collected because some part of the LF was absorbed by the SCGs, and therefore could not be drawn out under vacuum pressure (6.0–6.2 kPa).
As demonstrated by the results obtained, most of the MeOK catalyst was consumed during saponification with FFA in the early stage of in-situ TE (Fig.2A), resulting in the formation of soap (Fig.2B). Moreover, the higher the MeOK concentration, the higher amount of soap formed, with highest soap formation recorded at 2.50 g KOH in 100 mL methanol. This could reduce biodiesel yield remarkably (Fig.2C) because at high MeOK concentrations, saponification is enhanced and TG is converted to soap instead of FAME. Further, high soap content in the final biodiesel product could lead to difficulties during biodiesel purification by water washing, which in turn can result in a significant loss of the biodiesel product.
Low concentrations of MeOK were beneficial in reducing soap formation. However, it was not sufficient for FAME synthesis, thereby leading to low FAME yield and high TG content in the LF (Fig.2C and 2D). If there are residual TG in the LF, the final biodiesel product does not satisfy the biodiesel standard. Therefore, it is essential to select a suitable boundary to separate the pretreatment fraction (i.e., LF with high soap content) from the FAME fraction. Further, a MeOK concentration at which the highest FAME yield can be achieved with the lowest soap and TG contents in the LF needs to be identified.
Based on these results, the first 50 mL of the LF (i.e., the first and second 25 mL aliquots) was classified as the pretreatment fraction. MeOK concentration between 1.0–1.5 g KOH in 100 mL methanol was set as the experimental boundary, which is to be optimized in the subsequent step.
3.2 Optimizing MeOK concentration and SRT
In a previous study [17], we found that in-situ TE could have two reactive extraction rates based on the location of the oil. Oil located on the surface was rapidly converted to FAME and simultaneously extracted to the LF at a high rate. However, it took much more time for alkaline methanol to diffuse through the SCGs matrix via small porous channels and reach oil trapped within the matrix. This could severely reduce the performance of the type of semi-continuous reactor used in this study compared to the performance achieved in our previous study.
Insufficient mixing is one of disadvantages of using a semi-continuous reactor because this reactor does not have a mixing instrument. In this study, gravity and the pressure exerted by the peristaltic pump were sufficient for the MeOK solution to diffuse through the SCGs in the column. This implies that solid retention time (SRT), which is the time that SCGs is submerged in the solution, is a crucial parameter and therefore this parameter was optimized in addition to MeOK concentration. The SRT of this study was adjusted by the feeding rate of the peristaltic pump.
The CCRD was applied to optimize the conditions for in-situ TE in a semi-continuous column reactor in terms of MeOK concentration and SRT. The design of experimental conditions based on CCRD, and the corresponding biodiesel yields and residual TG contents of defatted SCGs are presented in Table 1. Results of ANOVA indicated that both MeOK concentration and SRT significantly affected FAME yield and residual TG level in defatted SCGs (p-value <0.01). However, at the 95% confidence level, no effect of MeOK concentration and SRT on FAME yield and residual TG level in defatted SCGs was observed. Moreover, as revealed by HPLC-ELSD results, TG was not detected in the product fraction at a detection limit of 100 ppm; only FAME was detected.
Table 1 Experimental design based on CCRD to identify optimal MeOK concentration and SRT for in-situ TE, as determined based on FAME yields and residual TG levels in defatted SCGs.
Treatment
|
[MeOK]
(g KOH in 100 mL methanol)
|
SRT (min)
|
FAME Yield (%)
|
Residual TG Level in Defatted SCGs (%mass)
|
|
|
Experimental
|
Predicted
|
Experimental
|
Predicted
|
|
1
|
1.00
|
30.48
|
57.16±0.71
|
57.37
|
5.64±0.03
|
5.25
|
|
2
|
1.00
|
60.95
|
63.81±0.89
|
68.46
|
4.95±0.16
|
4.20
|
|
3
|
1.50
|
30.48
|
77.10±1.42
|
75.68
|
3.00±0.54
|
3.18
|
|
4
|
1.50
|
60.95
|
84.46±0.09
|
87.47
|
1.64±0.18
|
1.46
|
|
5
|
0.90
|
45.71
|
64.46±1.12
|
61.70
|
4.25±0.03
|
4.94
|
|
6
|
1.60
|
45.71
|
88.55±1.59
|
88.09
|
1.66±0.19
|
1.54
|
|
7
|
1.25
|
24.16
|
59.98±2.34
|
61.51
|
4.75±0.46
|
4.78
|
|
8
|
1.25
|
67.26
|
82.43±0.70
|
77.68
|
2.29±0.03
|
2.83
|
|
9
|
1.25
|
45.71
|
83.27±0.92
|
83.84
|
3.34±0.02
|
3.13
|
|
10
|
1.25
|
45.71
|
82.72±0.74
|
83.84
|
2.91±0.04
|
3.13
|
|
In terms of FAME yield, optimal MeOK concentration and SRT can be determined based on a quadratic model with two levels, as shown in Eq.2 (adjusted R2 0.92). The predicted value of residual TG level in the defatted SCGs was calculated based on Eq.3 (adjusted R2 0.88). Both equations for predicting FAME yield and residual TG level in defatted SCGs were passed through a goodness-of-fit test (p-value <0.05). The predicted values (Table 1) were used to generate contour graphs for FAME yields and residual TG in the defatted SCGs, which are shown in Fig.3A and Fig.3B, respectively. It should be noted that the predicted values are valid only the experimental range used in this study.
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)
where X1 is the MeOK concentration (g KOH in 100 mL methanol) and X2 is the SRT (min).
According to model prediction, the optimal conditions were MOK concentration of 1.5 g KOH in 100 mL methanol and SRT of 52.15 min, with MeOK loading of 4 mL g-1 SCGs and feeding rate of 5.25 mL min-1 at 30 ˚C. Under these optimal conditions, biodiesel yield of 88.1% (11.4 g FAME per 100 g SCGs) was achieved. Further, analysis of the product fraction by HPLC-ELSD at a detection limit of 100 ppm did not reveal the presence of TG under these conditions.
The average residual TG content in the defatted SCGs from all four column reactors was 1.5 %mass. The defatted SCGs from the first column reactor had the lowest residual TG content (0.7 %mass), followed by those from the second (0.8 %mass), third (1.4 %mass), and fourth (3.1 %mass) column reactors.
A comparison of the advantages of using MeOK solution in a semi-continuous column reactor and those of using other types of reactor and operating conditions for in-situ TE is presented in Table 2. A semi-continuous column reactor has a superior in lowering the methanol/ethanol usage, compared to batch reactor type. This is due to the multistage countercurrent extraction found in continuous and semi-continuous reactor types. The fresh solvent contacts the first oil-bearing material in first column, and the extracted liquid is fed to the subsequence columns. Such operation conditions provide the most dilute extract contacts the oil-bearing material with the least amount of residual oil which enhances the diffusive flux by the difference in concentration gradient of oil between solvent and oil-bearing material [18].
Table 2 A comparison of in-situ TE processes performed with different operating systems and under different conditions.
Feedstock
|
Reactor
|
Alcohol, Catalyst, and Co-solvent
|
Temperature
|
Time
|
Yield
|
(mass per batch)
|
(°C)
|
(h)
|
(%)
|
Rice Bran (10 g)
|
Batch
|
Step 1: Methanol (15 mL g-1), H2SO4 (18 %mass of biomass)
|
60
|
0.2
|
96.8 – 97.4
(FAME Yield)
|
[3]
|
Step 2: Methanol (0.8 mL g-1), Sodium hydroxide (5.0 M)
|
60
|
0.8
|
Jatropha (20 g)
|
Batch
|
Step 1: Ethanol (10.5 mL g-1), H2SO4 (7.5 %mass of biomass)
|
Microwave
|
0.6
|
97.2
(Fatty acid ethyl ester Yield)
|
[19]
|
Step 2: Ethanol (4.1 mL g-1), KOH (5.0 M)
|
Microwave
|
0.2
|
SCGs (4 kg)
|
Batch
|
Step 1: Methanol (3.3 mL g-1)
|
45
|
1.0
|
81.8
(Biodiesel Yield)
|
[5]
|
Step 2: Methanol (2.9 mL g-1), KOH (0.225 M)
|
60
|
3.0
|
SCGs (1 g)
|
Semi-continuous (single state)
|
Supercritical Methanol (5 mL g-1), water (60 %mass of biomass) under 9 MPa
|
270
|
0.3
|
86.3
(FAME Yield)
|
[4]
|
SCGs (15 g)
|
Semi-continuous (single state)
|
Step 1: Methanol (3.7 mL g-1)
|
30
|
1.0
|
88.8
(FAME Yield)
|
[6]
|
Step 2: Methanol (2.0 mL g-1), KOH (0.446 M), Isopropanol (0.7 mL g-1) as co-solvent
|
30
|
2.0
|
SCGs (60 g)
|
Semi-continuous (multistate)
|
Methanol (3.5 mL g-1), KOH (0.268 M) under 0.1 MPa
|
30
|
0.9
|
88.1
(FAME Yield)
|
This study
|
Tuntiwiwattanapun et al. [20] compared the canola oil extraction performance between single stage extraction and triple stage extraction using aqueous surfactant solution. The results showed that the triple stage extraction had higher oil extraction yield than single stage by 50%. Co-solvent such as isopropanol could compensate the lower FAME yield from single stage extraction. Our previous study [6] showed that using a single column in semi-continuous column reactor achieved the FAME yield only 60.9%. Additional of isopropanol at 25% volume in methanol could improve FAME yield to 88.8%, in which FAME yield was similar to multistage semi-continuous column reactors applied in this work (Table 2).
As MeOK solution was reused in this study, the ratio of alcohol to oil-bearing biomass is obviously lower than that reported in other studies. Compared to our previous study with a batch reactor [5], the semi-continuous column reactor used in this study could reduce the amount of MeOK solution required by half. In a case of SCGs, the pH of MeOK solution after in-situ TE was reduced, especially in low concentration of MeOK [6]. This might be due to the high lignin content in SCGs. KOH catalyst reacts with lignin, which similar to kraft reaction in paper pulp process. The insufficient KOH concentration led to low FAME recovery in in-situ TE, especially in batch reactor type. For semicontinuous reactor, this problem could be minimized due to the multistage counter current extraction which provides the fresh MeOK solution into the system. In addition, the dissolved lignin by KOH could be separated as the pretreatment fraction with the soap formation.
Therefore, applying semi-continuous column reactor could markedly reduce the energy requirement for methanol recovery, which has been identified as the most energy intensive step of in-situ TE based on a life cycle assessment [5]. Beside catalyzed alcohol, the other alternative reagent such as ionic liquid and deep eutectic solvent could be used in this reactor [21-23].