3.1 FAME properties
As-received palm FAME showed a yield of 97.9%. It consisted of saturated and monounsaturated FAMEs as the main composition: 43.6 ± 0.6% methyl palmitate (C16:0), 4.4 ± 0.2% methyl stearate (C18:0) and 41.2 ± 0.1% methyl oleate (18:1). The polyunsaturated FAMEs were composed of methyl linoleate (C18:2) and methyl linolenate (C18:3) of about 9.7 ± 0.3 and 0.2 ± 0.1%, respectively. This composition resulted in the oxidation stability of 12.8 h without adding any additive, while its cloud point was 13.5 °C. The iodine value and acid number were 0.5% g I2/g and 0.2 mg KOH/g, respectively.
3.2 Effect of input power
The studied input power was 50, 75 and 100 W. The gas-filled gap of 1 mm was configured. The reaction was conducted using 25%H2 in the gas mixture at room temperature (starting at 25°C and rising to about 38°C due to the heat from the DBD plasma36). Fig. 2 presents the composition changes at the reaction time of 1 h. The consumption percentage referred to the conversion of the C=C bond into a single bond compared to the feed. The results revealed that applying 100 W of input power provided the highest conversion of polyunsaturated FAME followed by 75 and 50 W, respectively. The reduction of C18:2 and C18:3 from using 100 W was 9.3 (from 9.7 ± 0.3 to 8.8 ± 0.2%) and 27.6% (from 0.2 ± 0.1 to 0.1 ± 0.0%), respectively. In addition, saturated FAME increased as follows: 1.4% for C16:0 (from 43.6 ± 0.6 to 44.2 ± 0.3%) and 11.4% for C18:0 (from 4.4 ± 0.2 to 4.9 ± 0.1%). The response of FAME with input power was according to the expectation – high input power provided higher energy to produce more plasma density37. High plasma density meant a large amount of atomic hydrogen was present to react with C=C bonds.
3.3 Effect of gas-filled gap size
The gap size was configured at 1, 3 and 5 mm. The conditions were 100 W, 25% H2 and room temperature. The reaction took place for 1 h. When considering the percent consumption of the C=C bonds including C18:2 and C18:3, it was found that the smallest gap of 1 mm offered the best overall result with a significant decrease in C18:2 and C18:3 followed by 3 and 5 mm, respectively as presented in Fig. 3. The reduction of C18:2 and C18:3 obtained from the 3 mm gap was 6.2 (from 9.7 ± 0.3 to 9.1 ± 0.1%) and 28.5% (from 0.2 ± 0.1 to 0.1 ± 0.0%), respectively. For 5 mm, C18:2 and C18:3 decreased by 5.2 (from 9.7 ± 0.3 to 9.2 ± 0.1%) and 19.7% (from 0.2 ± 0.1 to 0.1 ± 0.0%), respectively. The gas-filled gap influenced the performance of the plasma catalyzed reaction. Being characteristic of the DBD plasma, the smaller the gap between the two electrodes, the denser the generated microfilament discharges and the resulting higher plasma intensity, and vice versa. On the contrary, a smaller gap might impede gas flow and might result in a diminished quantity of atomic hydrogen. With the result showing the smallest gap of 1 mm performing the best, the effect of higher plasma intensity must have outweighed the effect of gas flow impediment if any.
3.4 Effect of H2 concentration
Firstly, the H2 percentage was increased until plasma could not be sustained. The highest value was slightly over 80% when the plasma visually ceased to exist, confirmed by a sudden drop in the transformer power input (no plasma generation meant no power drawn by the transformer, a phenomenon similar to an AC transformer with an open circuit on the secondary winding which would draw no current on the primary side). The clearly audible high-frequency sound characteristic of a DBD plasma also went silent. Thus, the H2 concentration was examined at 25, 52.5 and 80%. The most appropriate gas-filled gap of 1 mm and the input power of 100 W at room temperature were used. As the solubility of H2 in biodiesel was very low, H2 uptake by the biodiesel in the reaction chamber was negligible. As presented in the study of Tomoya et al.38, H2 can be fairly dissolved in bio-oil. For example, H2 was dissolved in triolein (triglycerides with one unit of glycerol and three units of oleic acid) at a mole fraction of 0.1323 at about 80°C and 7.5 MPa. It was also reported that H2 solubility increased with pressure. This demonstrates that there was a very small amount of H2 incorporated into the liquid phase in this low-pressure and low-temperature treatment regime. Thus, the reaction was two-phase (gas/liquid) that occurred at the plasma-FAME interface. Since the interfacial area remained unaffected with different H2 concentrations, any observed effect on FAME composition changes reflected the effect of H2 concentration.
As shown in Fig. 4, the highest H2 concentration of 80% appeared to show the highest conversion of polyunsaturated FAMEs, followed by 52.5 and 25%, respectively. For the case of 80%H2, C18:2 and C18:3 were decreased by about 13.4 (from 9.7 ± 0.3 to 8.4 ± 0.4%) and 38.0% (from 0.2 ± 0.1 to 0.1 ± 0.0%), respectively, whereas saturated FAME increased as follows: C16:0 by 1.4% (from 43.6 ± 0.1 to 44.2 ± 0.2%) and C18:0 by 20.4% (from 4.4 ± 0.2 to 5.3 ± 0.2%).
3.5 Effect of reaction temperature
The temperatures of 20 ± 2°C, 38 ± 2°C (due to plasma heating only), and 60 ± 2°C were investigated. The reaction conditions were 100 W input power, 1 mm gas-filled gap and 80%H2. Fig. 5 demonstrates the effect of temperature on H-FAME composition, which revealed that temperature played no significant role. High temperature could not enhance the reaction speed, while low temperature could not amplify the benefit of the exothermic hydrogenation. This implied that reduced or elevated temperature was not required for plasma hydrogenation as also revealed in the previous work30. Performing the plasma treatment at ambient temperature is highly energy-efficient as well as cost-effective since no heating or cooling system is required.
3.6 Effect of reaction time
Catalyst-free hydrogenation was performed up to 6 h under the optimal parameters (100 W 1 mm gap and 80%H2 at room temperature), with Fig. 6 displaying the changes of FAME composition. The unsaturated FAMEs were hydrogenated resulting in the saturated ones accumulating over time. H-FAME at 6 h of reaction time composed of 48.2 ± 0.1% C16:0, 9.1 ± 0.1% C18:0, 37.0 ± 0.1% C18:1 and 4.6 ± 0.1% C18:2, while C18:3 completely reacted with hydrogen atoms. When considering the bond dissociation energy (BDE) in a normal alkane, the CH3–nCiH2i+1 bonds are the strongest with BDE of about 364.0 kJ/mol, while the C2H5–nCiH2i+1 bonds are the weakest of about 359.8 kJ/mol, with the bold letters referring to the dissociated atoms39. The ethyl group (–C2H5) in C18:0 could possibly cleavage, causing an increase in C16:0.
The oxidation stability increased with increasing hydrogenation duration due to FAME becoming more saturated. However, FAME at 6 h of plasma treatment was slightly over hydrogenated for its cloud point was 16.5°C, which was marginally above the biodiesel standard of Thailand (16°C maximum). It was found that the optimal hydrogenation time to satisfy the Thai cloud point standard was 5 h and that the compositions of the final product contained saturated FAMEs as the largest component of about 56.0 ± 0.3%. Besides, there were mono- and polyunsaturated FAME of 37.8 ± 0.1 and 5.1 ± 0.1%, respectively as shown in Table 1.
When considering the results obtained in catalytic hydrogenation of palm-based FAME using Pd/SBA-15 in the study of Chen et al.20, the conversion of C18:2 and C18:3 was 37.1 and 63.2%, respectively, after 2 h of reaction time. To achieve a similar level of conversion, the DBD plasma system needed to be conducted for 4 h. At this time, the conversion of C18:2 and C18:3 was 38.7 and 100%, respectively. However, direct comparison of reaction efficacy between a physical catalyst and plasma catalysis cannot be readily made because of different FAME volumes used, as well as different types and amount of energy supplied into each system. Chen et al. 20 studied a continuous process of 0.37 g/min, so for 2 h, the treated volume was 53.5 mL. The total volume used in the present study was 300 mL, which was about 5.6 times higher. The DBD plasma could also take place at ambient conditions and did not require a catalyst, which eliminated the problems of catalyst deactivation and material degradation due to high pressure and high temperature operation.
Table 1 Compositions of FAME and H-FAME (100 W, 1 mm gap, 80%H2, room temperature and 5 h) compared to catalytic hydrogenation
Composition
(%)
|
Present work
|
Ref. Chen et al. 20
|
FAME
|
H-FAME
|
FAME
|
H-FAME
|
Saturated FAME
|
48.0 ± 0.2
|
56.0 ± 0.3
|
49.07
|
51.35
|
Methyl palmitate C16:0
|
43.6 ± 0.1
|
47.5 ± 0.2
|
-
|
-
|
Methyl stearate C18:0
|
4.4 ± 0.1
|
8.5 ± 0.1
|
-
|
-
|
Monounsaturated FAME
|
41.2 ± 0.1
|
37.8 ± 0.1
|
41.01
|
42.27
|
Methyl oleate C18:1
|
|
|
|
|
Cis-
|
41.2 ± 0.1
|
37.8 ± 0.1
|
40.90
|
34.47
|
Trans-
|
0.0 ± 0.0
|
0.0 ± 0.0
|
0.11
|
7.73
|
Polyunsaturated FAME
|
9.8 ± 0.3
|
5.1 ± 0.1
|
9.73
|
6.07
|
Methyl linoleate C18:2
|
9.7 ± 0.2
|
5.1 ± 0.1
|
9.54
|
6.00
|
Methyl linolenate C18:3
|
0.2 ± 0.1
|
0.0 ± 0.0
|
0.19
|
0.07
|
3.7 Reactive species generated during reaction
Reactive species were monitored by optical emission spectroscopy (OES) as presented in Fig. 7. The reaction conditions were: input power of 100W and gas-filled gap of 1 mm at room temperature. The difference in H2 percentage in the mixed carrier gas resulted in dissimilarly observed peaks. At 90%He and 10%H2, all peaks of He species were clearly detected at 336, 356, 388, 501, 587, 667, 706 and 727.5 nm while only one peak of H2 species appeared which was Hα at 656.3 nm. For the case of 20%He and 80%H2, the plasma color was visually observed to became brighter/lighter, having a more whitish tone as shown in Fig. 8. The acquired plasma spectrum showed only the presence of Hα species. These results verified that He and H2 reactive species were indeed generated in the system.
3.8 FTIR analysis
The chemical functional groups were examined by FTIR as shown in Fig. 9. Both FAME and H-FAME consisted of a peak at 3008 cm-1 representing the unsaturated fatty acid methyl esters, C=CH stretching, and the peak decreased due to hydrogenation. The peaks at wave numbers 2922 and 2853 cm-1 were the asymmetric and symmetric stretching vibration of the alkane group, C-H, respectively. The strong peak at 1741 cm-1 corresponded to the ester group, C=O stretching. In addition, the asymmetric stretching of CH3 was detected at wave numbers 1435 and 1460 cm-1 while CH2 was represented at 1361 cm-1. The peak at 1195 cm-1 indicated O–CH3 stretching which was methyl esters. Besides, C-O anti-symmetric and C-O symmetric vibration were present at 1016 and 1169 cm-1, respectively. The peaks at 1244 and 1120 cm-1 corresponded to C-O and C-O-C stretching. The characteristic peaks of cis- and trans- configurations appeared at 722 (cis) and 966 (trans) cm-1. Cis- can be normally detected in FAME and H-FAME, while trans- should not be present in FAME, for it was synthesized from edible oil40. Most importantly, no peak at 911 cm-1 was found — no trans fatty acid methyl ester formation from the hydrogenation reaction using low-temperature DBD plasma.
3.9 H-FAME properties
FAME and H-FAME properties compared to biodiesel standards were demonstrated in Table 2. Feed FAME has high oxidation stability of 12.8 h with the cloud point somehow exceeding the ASTM D6751 requirement. After 5 h of plasma treatment (100 W, 1 mm gap, 80%H2, room temperature), it achieved 20 h of oxidation resistance with an increase of the cloud point from 13.5 to 16°C. If 16.5°C of the cloud point was allowed following 6 h of hydrogenation, the oxidation stability would be higher than 20 h. The more saturation level caused the reduction of iodine value from 50.2 to 43.5, verifying that the DBD plasma system could be practiced for effective H-FAME production. It offered superior performances to a catalytic reaction as presented in the study of Chen et al. 20. In the case of using 0.5 wt.% of the Pd/SBA-15 catalyst at 100°C, 0.3 MPa for palm H-FAME production, the oxidation stability increased by 8.5 h (from 19.4 to 27.9 h) with a small change in the cloud point from 12 to 13°C after 2 h of hydrogenation. A conversion of C18:2 and C18:3 was about 37.11 and 63.16%, respectively, while the saturated- and monounsaturated compositions rose by 4.65 and 3.07%, respectively. There was also trans- formation of about 7.73% (per 100% H-FAME content) after the reaction. The composition still consisted mostly of C18:1, and this was perhaps the reason why the cloud point increased by only 1°C. For the results obtained in the present study, every FAME composition with the C=C bond was hydrogenated into a single bond to a varying degree. The conversion of C18:1, C18:2 and C18:3 was 8.1, 47.4 and 100%, respectively, increasing the saturated FAME by 16.6%. The product could resist oxidation by an additional 7.2 h. However, due to the higher amount of saturated FAME, the increase in the cloud point was greater than that of the catalytic reaction.
The basic parameters directly related to oxidation products including an acid number and a peroxide value were measured. There were several parameters related to oxidation products, but the two were able to be determined with minimum effort by titration. The acid number remained unaffected whereas the peroxide value became lower, signifying that H-FAME exhibited a lower oxidation rate corresponding to the increase in oxidation stability.
Table 2 Properties FAME and H-FAME compared to biodiesel standards (100 W, 1 mm gap, 80%H2, room temperature and 5 h)
Properties
|
FAME
|
H-FAME
|
Standard
|
Oxidation stability (h)
|
12.8
|
20.0
|
> 6.0a, 3.0b, 10.0c
|
Cloud point (°C)
|
13.5
|
16.0
|
< 12.0b,16.0c
|
Iodine value (gI2/100)
|
50.2
|
43.5
|
< 120.0a,b,c
|
Acid number (mgKOH/g
|
0.3
|
0.3
|
< 0.5a,b,c
|
Peroxide value (mequi/kg oil)
|
11.7
|
10.0
|
–
|
a EN14214 (European biodiesel standard), b ASTM D6751 (United States), c Thailand 41-43
3.10 Preliminary design for large-scale H-FAME production
With 100 W of input power to the DBD power supply and with the production rate of 300 mL for 5 h, the energy efficiency in the present experiment was only 66.7 W/L-h. No power was required for FAME heating or catalyst preparation/removal/regeneration. There was no cost for initial catalyst procurement either. For a large-scale production using this novel green and non-thermal DBD plasma hydrogenation, one could employ a large set of electrodes to treat a large FAME surface with a powerful DBD power supply. He and H2 gases could also be completely recycled using a simple recirculating pump operating at ambient conditions, with only occasional replenishment of hydrogen gas to account for the consumed hydrogen by the C=C bonds. Thus, the only major expenditures to produce H-FAME using this novel green technique are electricity, hydrogen gas and FAME. Also, this technique is safe. Even though 80% hydrogen gas concentration was used and would, at first thought, be prone to explosion, in the closed system with no oxidizer present in the reaction chamber, e.g., O2 gas or oxygen atoms in FAME or even in the electrode, the hydrogen gas cannot explode even with the presence of the microfilament discharges. Operating at slightly above one atmosphere would ensure no atmospheric oxygen gas seeping into the system. The reaction chamber and the gas system must also be sealed properly to prevent gas leakage as the hydrogen gas is flammable. Its lower and upper explosive limits in the air at room temperature and atmospheric pressure are 4.3 and 76.5 mol%, respectively. 44 A similar safety infrastructure to that of conventional catalysis will ensure a safe operation of the DBD plama hydrogenation system.