Biodiesel is a clean and renewable resource that consists of mono-alkyl esters of long chain fatty acid, which could be obtained from the transesterification reaction of vegetable oils and animal fats with alcohols and catalysts. Biodiesel yield has typically been determined using expensive and laborious techniques. The attempt of this study was to examine the potential of quantifying the biodiesel conversion in real time using refractive index in transesterification process of canola oil with methanol and KOH. Biodiesel yields at five different mixing intensities and reaction times were measured using a refractometer. The measured results were then compared with analytical data obtained from thermogravimetric analysis (TGA) technique over a temperature range of 25℃ - 600℃. Experimental results indicated that the FAME conversions at different mixing intensity and reaction time measured from refractometer correlated well to the relative weight losses from TGA method with R2 = 0.93 (p ≤ 0.05); however, the refractometer may over-estimate the biodiesel yield when the reaction rate was too low. Overall, the refractometer technique is cheaper and easier to manage and could provide a reliable prediction of biodiesel yield in real time.
Fossil fuels, including oil, coal, natural gas, are still the leading energy for human activities. However, these resources, which were formed over hundreds of millions of years, are non-renewable and will run out in the near future [
Numbers of biodiesel characterization methods have been proposed and investigated by many researchers, including thermogravimetric analysis (TGA), gas chromatography (GC), near-infrared spectroscopy (NIRS), and high-performance liquid chromatography (HPLC) and Proton nuclear magnetic resonance (1H NMR) spectroscopy [
TGA is a less expensive technique to characterize the thermal stability of materials and assess the conversion of biodiesel yield when compared with the mentioned techniques without requirement of pretreatment [
Although TGA does not require pretreatment of samples, it cannot provide real time monitoring of biodiesel conversion because it requires heating of samples, which may take a few hours. Comparatively, the characterization of biodiesel using refractometer is much cheaper than TGA and has a higher time resolution. The method of using refractive index for biodiesel synthesis monitoring was first examined by Xie and Li [
Therefore, the objective of the present work is to test the agreement of TGA and refractometer for determining FAME content from the transesterification process in canola oil and methanol with KOH, and thereby to evaluate the performance of refractometer for analyzing real time biodiesel production in research and industrial settings.
Canola oil was obtained from Montreal local market. HPLC grades of methanol (99.8%) was purchased from Fisher Scientific. Potassium Hydroxide (86.4%) was purchased from Fisher Scientific as the chemical catalyst. Biodiesel standard for the calibration was purchased from LGC Standards (USA).
The biodiesel production set up consisted of a 1-L glass reactor, an overhead agitator was equipped with a digital display unit and a two flat-blade paddle agitator. The impeller diameter and the blade width were 60 mm and 20 mm, respectively. The speed range of the agitator varied from 80 rpm to 1000 rpm. A Thermo Scientific Sorvall Legend XT centrifugation was used for separating the materials in each sample.
The methanol: oil volume corresponding to a molar ratio of 6 was estimated at 1:3.96. All the experiments were conducted at room temperature (25˚C ± 1.5˚C) and atmospheric pressure. The impeller speeds were 200, 250, 275, 300 and 325 rpm to cover the whole range of mixing from immiscible phase to uniform dispersion. The blade was set at 1/2 H (47.5 mm from the bottom). The total height (H) of the emulsion was 95 mm.
The biodiesel was produced with canola oil and methanol with KOH as catalyst. 2.74 g of potassium hydroxide was dissolved in 100 ml of methanol in a conical flask. The mixture was continuously stirred until all the KOH were dissolved into the methanol. Afterwards, 595 ml of canola oil was poured into the reacting vessel, followed by 50 ml of pure methanol. After the two solutions were separate and stable, the remaining 100 ml of the methanol was added gently with KOH. The agitator is then started at the preset speed to attain a specific rpm (200, 250, 275, 300 or 325).
To investigate the processes of transesterification reaction, 5 ml sample was withdrawn from the mixing reactor by a 2-ml pipette during the agitating process at different times. To ensure sample was taken randomly, each sample set was drawn from bottom, middle and top of stirred vessel for a proper representation of the entire sample. The reaction was stopped at the set time with the immediate addition of few drops of pH 6.2 phosphate buffer and allowed to stay for 5 min. The sample was then centrifugated at 3500 rpm for 5 min after which it was separated into upper phase (biodiesel and residual canola oil) and lower phase (methanol, glycerine and rest buffer). The lower phase was decanted, and the upper phase was kept in the refrigerator for further analysis.
Thermogravimetric analysis (TGA) of the unwashed biodiesel was conducted by 10 µL samples of biodiesel/oil mixture heated at a constant heating rate of 10˚C/min in the nitrogen atmosphere in a titanium pan. The temperature ranged from 25˚C to 600˚C. Based on previous TGA study, the heating rate of 10˚C/min has been reported to be a reliable heating rate and it minimizes experimental error compared with other heating rates [
Refractometer analysis was considered as a simplified alternative method to several analytical methods including the thermogravimetric analysis used in the study [
The performance of the refractometer in determining the biodiesel yield was estimated using the calibration curve. The computed biodiesel yields from refractive indexes were compared with the TGA results using two statistical indices, including the percent bias (PBIAS) and coefficient of determination ( R 2 ). Percent bias (PBIAS) determines the difference of mean values between refractometer and TGA measured values (Equation (1)). Coefficient of determination ( R 2 ) is defined as the proportion of variation of y data that was explainable by variation in x data (Equation (2)) [
P B I A S = x i − y i x i × 100 % (1)
R 2 = 1 − ∑ i = 1 n ( y i − x i ) 2 ∑ i = 1 n ( x i − x a ) 2 (2)
where x i is the biodiesel yield from TGA, y i is the computed yields by refractive index, n is the total number of measurements, x a is the average value of TGA measured yields.
Typical TGA thermographs for biodiesel production over the mixing time are shown in
to the very low reaction rate at the start of the process due to the mass transfer limitations between methanol and oil phase [
After 5 min’s agitation, weight losses as indicated by TGA shows only 0.47% biodiesel was produced at 200 rpm, while up to 66.63% biodiesel was obtained when mixing intensity reached to 325 rpm for the same time duration. Experimental evidence showed that with an increase of mixing intensity, biodiesel yield increased accordingly after 5 min’s agitation. As mixing continued for 10 min marginal increase in biodiesel yield was observed for lower rpm. For instance, 0.98% yield was observed when agitation speed was at 200 rpm, but significantly higher yield of 70.06% and 78.75% were obtained at 250 and 275 rpm, respectively. A comparison of the mean biodiesel yield using Tukey-Kramer HSD shows that there is no significant difference (p > 0.05) in yield beyond 275 rpm. This is evident in the less than 1.5% variation in the biodiesel yield at 300 and 325 rpm. Similar minimum variation was observed at higher mixing time.
The refractometer was calibrated using the standard pure biodiesel (B100) and canola oil from 0 to 100% with an interval of 20%. The linear relationship between the biodiesel conversion percentage and refractive index is shown in
the experiment is shown in Equation (3). The estimated coefficient of determination shows that 97.99% of the variation in the refractive indexes could be explained by the variation in biodiesel yield thus indicating the statistical reliability of the linear model.
As illustrated in
The computed biodiesel yields from measured refractive indexes at five agitation intensities as a function of agitation time is shown in
Thermogravimetric techniques for biodiesel yield assessment have been a validated method for assessment reported by several authors. It is comparatively simple, requiring no sample preparation compared to the mainstream analytical
methods-Spectroscopy techniques. However, the cost of a thermal analyzer is more than $50,000. In a well-established laboratory in developed countries, such equipment may be available but for laboratories and many developing countries thermal analyzer may not exist. On the other hand, a simple refractometer costs less than $100 hence could be an alternative for the much complex and expensive analytical equipment.
Again, the average processing time for analysis using the TGA approach was 50 ± 5 min including prepping the equipment. The refractometer, in addition to requiring no sample preparation, less than a minute is required to determine the refractive index from which the yield is determined. Thus, the refractometer has the potential of providing a real time determination of the biodiesel yield.
Although both methods predicted lowest yields at an agitation intensity of 200 rpm, TGA measured yields were only 0.3% - 2.5% (1.1% on average), while computed values from refractive indexes were 5.5% - 17.3% (11.2% on average).
| RPM | Mean % biodiesel | PBIAS | R2 | |
|---|---|---|---|---|
| TGA | RM | |||
| 200 | 1.1 | 11.2 | −918% | 0.85 |
| 250 | 51.3 | 58.7 | −14% | 0.95 |
| 275 | 62.3 | 64.9 | −4% | 0.92 |
| 300 | 65.3 | 65.9 | −1% | 0.96 |
| 325 | 64.2 | 67.0 | −4% | 0.96 |
It should be noted that the PBIAS of biodiesel yields between two methods were extremely high (−918%) and R2 was 0.85 (
When agitation intensity was increased to 250 rpm, average biodiesel yields measured by TGA and refractometer increased to 51.3% and 58.7%, respectively. The difference between two measurements was within 15% and R2 was 0.95. Lower PBIAS were found when agitation speed continued to increase to 275 - 325 rpm, with percent difference between 1% - 4% and R2 from 0.92 - 0.96.
In general, the biodiesel yields computed from measured refractive indexes were comparable to TGA measure values, and both methods provided similar trend of increasing yields with elevating agitation intensities. As illustrated in
The biodiesel yield data was fitted to an exponential rise model shown in Equation (3) to predict the biodiesel yield as a function of the mixing time.
Y = a ( 1 − e − b t ) (3)
where Y represents the biodiesel yield and a represents a mixing constant reflecting agitator resistance and b represents the rate of biodiesel yield.
The result shows that the model was a good fit for predicting biodiesel yield using mixing time. The model parameters shown in
| RPM | TGA | RM | ||||
|---|---|---|---|---|---|---|
| a | b | R2 | a | b | R2 | |
| 200 | 1539 | 6.82E−05 | 0.85 | 14.23 | 0.0254 | 0.67 |
| 250 | 107.10 | 0.0807 | 0.94 | 98.67 | 0.1194 | 0.97 |
| 275 | 86.59 | 0.2152 | 0.96 | 81.24 | 0.3014 | 0.99 |
| 300 | 84.17 | 0.2728 | 0.99 | 79.08 | 0.3850 | 0.99 |
| 325 | 85.20 | 0.2528 | 0.97 | 82.79 | 0.3319 | 0.99 |
decline in the agitation resistance “b” was observed as speed increased. It is important to note that there was a wide variation in the two methods at lower speed, thus it supports the earlier assertion that the refractometer over predicts at lower speed. The changes in model parameter “a” and “b” were not significantly different (p < 0.05) at higher speeds (rpm > 275). Overall, the rate of biodiesel yield was relatively higher with the refractometer technique compared to the TGA. The rate of biodiesel yield ranged from 0.000068 - 0.2528 and 0.0254 - 0.3850 per minute for thermogravimetric and refractometer techniques, respectively. Some investigators had also observed the higher rates and decreased agitation resistance which have been described as shorter delay in FAME appearance with increasing agitation speed [
This paper examined the potential of using a refractometer to measure the biodiesel yield in real time from the transesterification reaction in canola oil and methanol with the presence of KOH as the catalyst. The computed biodiesel yields under five different agitation intensities and five reaction times were compared with the measured values by TGA. Our results indicated that the values measured from the two methods were comparable within 9% difference on average for all agitation intensities and reaction times. The linear fit between the two data series results in an average R2 value of 0.93. The linear regression between TGA biodiesel yields and refractometer measured yields suggests that the refractometer can accurately quantify the amount of biodiesel present. Finally, compared with TGA method, using the refractometer for monitoring the transesterification of vegetable oils with methanol has comparable accuracy and it is faster, more convenient and cheaper than TGA method with portable devices, which makes it a more appropriate method for real-time process monitor purposes.
The authors declare no conflicts of interest regarding the publication of this paper.
Li, S., Kwofie, E.M. and Ngadi, M. (2020) Comparative Evaluation of Thermogravimetric and Refractive Index Techniques in Determining Biodiesel Yield. Journal of Sustainable Bioenergy Systems, 10, 30-42. https://doi.org/10.4236/jsbs.2020.101003