Ultrasound Intensified Biodiesel Production from Mixed Non–Edible Oil Feedstock
4.2 Material and methods
4.2.3 Experimental protocol for transesterification
The transesterification experiments were performed in two categories, viz. (1) single–step process – optimization of reaction parameters using central composite design (CCD), and (2) Two–step process – to identify role of sonication in enhancement of transesterification reaction. The two–step transesterification process was conducted in two separate stages of esterification followed by transesterification. The esterification reaction was accomplished using H2SO4 catalyst and mechanical agitation, whereas the transesterification reaction was carried out using as–synthesized heterogeneous acid catalyst in presence of ultrasound. Moreover, the role of ultrasound in process intensification was investigated by performing the experiments with mechanical agitation in both the categories under similar optimized conditions. The experimental setup and protocol is described below.
The transesterification experiments with ultrasound (single–step and two–step) were conducted in batch mode in an ultrasound bath (Make: Elma Transonic, Germany, Model: T–460, Total volume: 2 L, frequency: 35 kHz and power input: 35 W). Distilled water was used as medium for ultrasound wave transmission. Transesterification reaction was conducted in a two neck round bottom flask of 25 mL. The total reaction mixture volume for each experiment was 15 mL. A coiled condenser was used for refluxing methanol. The reaction temperature was controlled using circulating water bath (Make: Jeio Tech, Lab Companion, Korea; Model: RW 0525G). The pressure amplitude of ultrasound irradiation differs from point to point in sonication bath, thus the position of round bottom flask was maintained same in all experiments [36].
The experiments with mechanical agitation (400 rpm) were carried out using a magnetic stirrer (Tarson–spinot, Model MC–02) in a simple water bath. Other experimental parameters (i.e. time, reaction volume and conditions) were same as in
ultrasound–assisted experiments. All experiments were carried out in duplicate to check the reproducibility of the results.
Process optimization: Process optimization of single–step transesterification reaction was done using statistical experimental design – CCD design using Minitab 16 software (trial version). The statistical experimental design was used to optimize the yield of transesterification process comprising 3 factors with 3 levels. The operating parameters such as, reaction time, catalyst dosages, oil to alcohol molar ratio, reaction temperature, mode of mixing, etc. have direct impact on the transesterification yield. Among these parameters, catalyst loading (% w/w), alcohol/ oil molar ratio and reaction temperature were chosen as optimization variables. Sonication was applied for mixing of the reaction mixture. The single–step reaction was conducted for 3 h and the levels of process variables were selected on the basis of preliminary experiments. The statistical design comprised of 20 experimental sets with combination of different process variables, details of which are shown in Table 4.1 (A) and (B).
Two–step transesterification process: (1) Esterification experiments: For two–step transesterification process, the esterification step was performed by using homogeneous acid catalyst, aimed at reducing the FFA content of the blended feedstock. The experimental procedure used for esterification was same as described by Choudhury et al. [37]. This involved the use of the alcohol to oil molar ratio of 15:1, with 5% w/w of catalyst concentration and temperature of 338 K. The esterification reaction (total reaction volume 150 mL) was carried out in 250 mL two necked borosilicate round bottom flask fitted with a reflux condenser. Mechanical agitation of reaction mixture was provided at 400 rpm using a magnetic stirrer (Tarson–spinot, model: MC–02).
Reaction was conducted for 1 h followed by separation of reaction mixture into two phases (viz. organic or oil and aqueous or methanol) using separating funnel. Further,
the organic phase was washed 3 times to remove traces of unreacted acid. Later, the organic phase was stored in clean air–tight container after eliminating the moisture by passing it through the activated silica (granules, desiccant ~ 0.2–1 mm, Merck, India) and Whatmann 40 filter paper (with particle retention size of 8 μm). The acid value of final organic phase was determined by using the titration method.
(2) Transesterification experiments: Mixed vegetable oil with reduced FFA content was used as feedstock for transesterification reaction. Operating conditions for transesterification were same as the optimum conditions resulted from CCD statistical design for single–step transesterification. The reaction period was reduced to 1 h (instead of 3 h as in previous case).
Table 4.1 (A): CCD statistical experimental design range and level of independent parameters
Variables Symbol Level of factors coded values (actual values) Catalyst loading (% w/w oil) C –1 (4) 0 (7) +1 (10) Alcohol: oil molar ratio M –1 (7:1) 0 (14:1) +1 (21:1)
Temperature (K) T –1 (328) 0 (333) +1 (338)
Table 4.1 (B): Experimental sets of CCD design with experimental and model predicted triglyceride conversion
Sr.
No.
Catalyst Loading (% w/w)
Molar ratio
Temperature (K)
% Triglyceride conversion (Experimental)
% Triglyceride conversion (Model predicted)
1 4 7 338 42.23 ± 0.79 41.11
2 7 14 328 60.98 ± 1.11 59.51
3 7 7 333 66.87 ± 0.89 69.99
4 4 21 328 18.12 ± 1.37 19.29
5 7 14 333 84.80 ± 1.33 84.43
6 10 14 333 78.89 ± 0.88 76.83
7 10 21 328 31.20 ± 1.08 31.91
8 7 14 333 82.46 ± 0.79 84.43
9 4 7 328 18.34 ± 1.57 16.46
10 7 21 333 64.89 ± 0.69 63.42
11 7 14 338 80.93 ± 1.08 84.05
12 10 7 328 23.76 ± 0.88 25.23
13 7 14 333 85.30 ± 1.42 84.43
14 7 14 333 86.20 ± 0.87 84.43
15 10 7 338 73.90 ± 1.32 72.32
16 7 14 333 86.20 ± 1.57 84.43
17 4 14 333 51.20 ± 0.69 54.91
18 4 21 338 23.18 ± 0.42 21.30
19 7 14 333 84.92 ± 0.67 84.43
Reusability of catalyst: The chloro–sulfonated catalyst was tested for reusability. The catalyst recovered at the end of transesterification reaction (both case, single–step and two–step process) was separated from the reaction mixture by centrifuging it at 6000g for 15 min at 298 K. The separated catalyst was washed 3 times with 5 mL solvent (n–
hexane) to remove impurities, viz. oil, glycerol or methanol, from the catalyst surface.
Further, the catalyst was dried in oven at 393 K for 2 h and used in the next cycle. The procedure was repeated for 3 cycles and the activity of the catalyst was determined on the basis of biodiesel yield. The catalyst reused after 3 cycles was analysed for acid site density calculation and surface morphology was monitored through FE–SEM analysis.