A BBREVIATIONS
Scheme 2.5: Scheme 2.5: Simultaneous esterification and transesterification by heterogeneous acid catalyst
3.2 Experimental and Mathematical Modelling .1 Approach
3.2.3 Experimental setup and protocol
The experiments were conducted in two categories, viz. control and test. The control experiment was performed with mechanical stirring in a 50 mL two-neck round bottom flask.
A magnetic stirrer was used to agitate reaction mixture (with total volume of 25 mL) at 600 rpm for 5 h. The catalyst concentration was 8% w/v oil (1.3 g), with alcohol to oil molar ratio of 12:1, and the temperature of the reaction being 65oC. Samples of the reaction (500 µL aliquots) were withdrawn at 1 h interval to assess the conversion of the triglycerides (analysis of the procedure is described subsequently). The time data of conversion is depicted in Fig.
3.1. The total conversion obtained in the control experiment (at the end of 5 h) was approx.
56%. This value is significantly lesser than the conversion reported by Liu et al. [3]. We conjectured that this is an effect of presence of inert carbonate phase in the catalyst. To affirm this, we have conducted XRD analysis of the calcined calcium oxide, results of which have been described subsequently. Another probable cause behind this effect is improper mixing and distribution of the catalyst particles in the reaction mixture.
A schematic of the experimental setup is given in Fig. 3.2. An ultrasound bath (Elma Trans–sonic T–460 type, Germany, capacity: 2 L, frequency: 35 kHz, power: 35 W) was used for sonication of the transesterification reaction mixture. 2/3rd of bath volume was filled with water, which acted as medium for transmission of ultrasound. All experiments were carried out in a 50 mL two neck round bottom flask made of borosilicate glass. Since the temperatures of the transesterification reaction system were near or even at the boiling point of methanol, a reflux condenser was employed so as to condense the methanol vapors and return them to reaction mixture.
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0 10 20 30 40 50 60
0 1 2 3 4 5 6
% FAME
Time (h)
Figure 3.1: Results of the control experiments with mechanical stirring (reaction parameters:
alcohol to oil molar ratio = 12:1, temperature = 65oC, catalyst concentration = 8 wt% oil, agitation speed = 600 rpm)
The temperature of water in the bath was maintained at desired levels by means of a variable energy input heating element. The bath was sealed to minimize as much heat loss as possible from the medium. The intensity (or pressure amplitude) of the ultrasound wave field shows significant spatial variation [19], and hence, the position of the reaction flask was maintained carefully the same in all experiments. The actual energy dissipation and pressure amplitude of the ultrasound wave in the bath was characterized using calorimetry [20].
As per the Box–Behnken statistical experimental design for 3 factors and 3 levels, 15 sets of experiments have been conducted for different combinations of the parameters. The experimental conditions in these sets are given in Table 3.2A, while the exact composition of reaction mixture in each set is given in Table 3.2B. Prior to performing the Box–Behnken experimental design, preliminary studies on the effect of time in the transesterification reaction was investigated and based on these results the time of sonication in all experiments was fixed as 60 min. The reaction mixture was intermittently stirred to avoid settling of large catalyst particles. In each set, experiments have been done in triplicate (3 runs) so as to assess
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the reproducibility of results. The progress of the transesterification reaction was monitored by withdrawing 500 mL aliquots of the reaction mixtures after every 12 min. These aliquots were immediately mixed with hot aqueous suspension of Na2CO3 and centrifuged at 9000 rpm for 10 min to separate the solid catalyst as well as the aqueous (or glycerol and alcohol) phase. The organic or biodiesel layer was then analyzed to determine the yield of FAME or biodiesel.
Figure 3.2: Schematic of the experimental setup
Determination of reaction kinetics and %FAME yield: The gross conversion of triglycerides in soybean oil was determined using 1H NMR (Varian 400 MHz FT–NMR) spectroscopic analysis with CDCl3 as solvent and TMS (tetramethyl silane) as internal standard [21, 22].
The conversion (X) of triglycerides in soybean oil to fatty acid methyl esters is determined using following equation: X =
(
2×AME)
×100(
3×Aα −CH2)
, where, AME = integration value of the protons of the methyl esters (the strong singlet peak at 3.6 ppm), Aα–CH2 = integration value of the methylene protons at 2.3 ppm.TH-1254_08615106
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(A) Box–Behnken experimental design matrix Sl.
No.
Temperature (K)
Molar ratio
Catalyst (wt%
oil)
% FAME yield with standard
deviation
Fitted
% FAME yield
1 313.15 6 4 15.8 ± 0.26 15.8
2 338.15 6 4 71 ± 0.83 71.3
3 313.15 18 4 9.2 ± 0.11 8.9
4 338.15 18 4 40.1 ± 0.65 40.1
5 313.15 12 1 1.5 ± 0.47 1.3
5 338.15 12 1 29.9 ± 0.8 29.5
7 313.15 12 7 26.3 ± 0.62 26.8
8 338.15 12 7 85.2 ± 0.4 85.4
9 325.65 6 1 13.8 ± 0.82 13.9
10 325.65 18 1 2.1 ± 0.25 2.5
11 325.65 6 7 62.7 ± 0.75 62.3
12 325.65 18 7 35.8 ± 0.45 35.6
13 325.65 12 4 75.1 ± 0.95 75.0
14 325.65 12 4 74.9 ± 1.07 75.0
15 325.65 12 4 75.1 ± 0.95 75.0
(B) Composition of reaction mixture in Box–Behnken experimental design matrix
Sl No.
Temperature (oC)
Molar ratio
Catalyst (wt% oil)
Oil Volume (ml)
Methanol Volume (ml)
Methanol / Oil volume ratio
Catalyst added (g)
1 40.0 6 4 20 5 0.25 0.73
2 65.0 6 4 20 5 0.25 0.73
3 40.0 18 4 14.2 10.8 0.76 0.52
4 65.0 18 4 14.2 10.8 0.76 0.52
5 40.0 12 1 16.6 8.4 0.51 0.15
5 65.0 12 1 16.6 8.4 0.51 0.15
7 40.0 12 7 16.6 8.4 0.51 1.06
8 65.0 12 7 16.6 8.4 0.51 1.06
9 52.5 6 1 20 5 0.25 0.18
10 52.5 18 1 14.2 10.8 0.76 0.13
11 52.5 6 7 20 5 0.25 1.27
12 52.5 18 7 14.2 10.8 0.76 0.91
13 52.5 12 4 16.6 8.4 0.51 0.6
14 52.5 12 4 16.6 8.4 0.51 0.6
15 52.5 12 4 16.6 8.4 0.51 0.6
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Determination of transesterification kinetics: The time data of conversion of triglycerides obtained with help of 1H NMR spectroscopy, as described above was used to determine the kinetic parameters of the process. The transesterification reaction progresses in three consecutive and reversible reactions steps as follows: (1) reaction of triglyceride (TG) and methanol to produce diglycerides (DG) and 1 molecule of methyl ester, (2) further reaction of diglycerides to yield monoglycerides (MG) and another molecule of methyl ester, and finally (3) reaction of monoglyceride to yield one mole of methyl ester and one mole of glycerol. In our analysis, we have determined the kinetic constant of the overall reaction, without accounting for the intermediate steps. The overall reaction is assumed to be governed by pseudo 1st order kinetics [23], and the following equation has been fitted to the conversion–
time (X vs. t) data so as to obtain the kinetic constant:
( )
ln 1−X = −kt (3.2)
where X = conversion of triglyceride at any time t. Plot of−ln(1−X) vs t gives the kinetic constant k as the slope.
Determination of activation energy (Arrhenius plot): The Arrhenius equation gives a relationship between the specific reaction rate constant (k), absolute temperature (T) and the energy of activation (Ea) as: k =A×exp
(
−Ea RT)
where A is the frequency factor and R is universal gas constant (J mol−1 K−1). This equation is rewritten as:ln( ) Ea ln( )
k A
RT
= − + (3.3)
We have used three reaction temperatures (as mentioned in Table 3.1) in our statistical experimental design. Experiments have been conducted using optimum values of catalyst loading and alcohol to oil molar ratio (as determined from the statistical experimental design) at these three reaction temperatures. Plot of ln (k) vs 1/T gives slope equal to (−Ea /R) from which activation energy can be determined.
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