Study on Kinetics of Transesterification of Biodiesel in Zanthoxylum bungeanum seed Oil Ethyl Ester

Zanthoxylum bungeanum seed oil (ZSO) was the by-product of the zanthoxylum industry, and was a kind of cheap and abundant source in china, which can be great potential of use as a feedstock for biodiesel production in terms of reducing the producing cost. Kinetics of Transesterification of Biodiesel from Zanthoxylum bungeanum seed Oil Ethyl Ester using Sodium ethoxide as an alkaline catalyst was studied in this paper. The results showed that the progression of transesterification was grade 1.5, the reaction activation energy was 17.876kJ•mol-1 and the pre-exponential factor was 0.8521L/(mol•min). The contrast between experimental value and predicted value displayed the dynamics model had a good veracity, and can be applied for predicting reaction rate of progress.

Biodiesel is usually produced by transesterification of vegetable oils or animal fats [1] . The transesterification reaction is a chemical reaction of triglyceride and a short chain alcohol under the action of a catalyst to produce a fatty ester monoester [2][3][4] . Studies on the transesterification kinetics of biodiesel from palm oil, cottonseed oil, soybean oil, rapeseed oil, and tung oil have been reported [5][6][7][8] . Kinetic studies of transesterification of ZSO with methanol to prepare biodiesel are also available on literature [9] . It is generally believed that fatty acid alkyl esters are prepared by transesterification, using different short chain alcohols or selecting different lipids, and the kinetic model and parameters of the transesterification reaction are also different [10,11] . At present, there is no report on the reaction kinetics of the transesterification of ZSO with bioethanol.
In this paper, Kinetics involved in the process of biodiesel production and its stability have been critically studied. The research could provided a substantial and theoretic foundation for biodiesel production of ZSO ethyl ester.

Materials and reagents
ZSO was provided by a local company located in Hancheng, Shaanxi Province, China. This is a high yield area for Z. bungeanum Maxin. Eethanol, Sodium ethoxide, change-color-silica gel and other chemicals were of analytical reagent grade. The pretreated ZSO used in this experiment, with the acid value and the saponification value of 1.56 mg KOH/g and 189.3 KOH/g, was adapted for biodiesel preparation while alkali was as catalyst.

Measurement of biodiesel conversion rate
The conversion rate of biodiesel was calculated by measuring the conversion rate of glycerin. Glyceric spectrophotometer method was improved in the experiment based on the research by Yan jie [12] . The method adopted was proved well at veracity and credibility [13] .

The Kinetics of Biodiesel Production from ZSO by Transesterification
In this paper, the reaction kinetics of preparation of biodiesel from ZSO ethyl ester by alkali-catalyzed transesterification was studied for the first time. A macro-reaction kinetics model was established and relevant kinetic parameters were calculated to lay a theoretical foundation for the production and application of ZSO ethyl ester biodiesel.
In order to simplify the kinetic model for the preparation of biodiesel from ZSO, it is assumed here that the transesterification reaction is completed in one step, and the reaction equation is simplified as follows.
Triglyceride(A) + 3ethanol → 3 Fatty acid ethyl ester(FAEE) + Glycerol When the absolute amount of absolute ethanol in one of the reactants is excessive, the positive reaction rate is much greater than the reverse reaction, and its influence on the reaction can be ignored. Therefore, the transesterification reaction kinetic equation can be expressed as (1): When there is a far excess of ethanol, it can be considered that the concentration remains substantially unchanged during the reaction. Formula (2) can be converted to: ，activation energies E a would be gained by the linear slope，frequency factor(or pre-exponential factor called) can be acquired by the linear intercept.

Results and discussion
For getting partial progression α of transesterification on ZSO, two times, three times, four times, five times and six times of excessive ethanol respectively were used in the reaction, namely the ethanol-to-oil molar ratio of 9:1, 12:1, 15:1, 18:1, and 21:1 adopted. The reaction condition was at catalyst amount of 1.7%, reaction time 120min, reaction temperature of 45℃, 55℃, 65℃, and 75℃ fixed respectively. Spectrophotometric was adopted to determine biodiesel conversion of under different conditions, and the relationship between ln(Δc/Δt) and lnc under different temperature conditions could be obtained, the results showed as Fig. 1. For getting partial progression β of transesterification on ZSO, the ZSO molar ratio added into reaction system was excessive for two times, three times, four times, five times and six times to ethanol when the partial progression of ethanol was calculated, such as the ethanol-to-ZSO molar ratio of 3:3, 3:4, 3:5, 3:6, and 3:7 employed. The other reaction condition were respectively at 1.7% catalyst amount, reaction time of 120min, and reaction temperature fixed separately of 45℃, 55℃, 65℃ and 75℃. The relationship between ln(Δc/Δt) and lnc under different temperature conditions could be obtained, the result of experiment was showed as Fig. 2.

Fig. 2. The relation between ln(Δc/Δt) and lnc under different temperature
According to Fig.1 and Fig.2, the reaction order α and β corresponding to ZSO and ethanol respectively, and the reaction rate constants k 1 and k 2 can be obtained at different temperatures. The results are shown in Table 1 and 2. From the table, we can see that the linear correlation of the fitted linear equations are good, and the reaction rate constants increase with the increase of temperature. The reaction grades under different temperature conditions were averaged to obtain a reaction grade of 1.5 for the preparation of ZSO ethyl ester biodiesel by alkali-catalyzed transesterification.
Take 1/T as abscissa and lnkA as ordinate drawing, a straight line can be obtained, as shown in Fig. 3. Apparent activation energy E a =17.876kJ • mol -1 , would be obtained from the slope of the regression equation; The apparent frequency factor A=e -0.16 =0.8521L/(mol min) could be found by the intercept, which is consistent with the conclusions of literature [14,15] . Therefore, the kinetic equation at the reaction temperature of 75°C can be expressed as r A =1.602×10 -3 c A 1.0916 c B 0.5368 . According to formula (4), the theoretical conversion rate of ZSO ethyl ester biodiesel at different reaction temperatures and reaction times can be predicted and verified by experiments, the results are shown in Fig. 4. As can be seen from the figure, the actual measurement values are in good agreement with the predicted values, indicating that the macro-kinetic model is accurate and reliable.

Conclusion
The kinetic study on the preparation of biodiesel from ZSO by alkali-catalyzed transesterification showed that the transesterification reaction order of ZSO and ethanol was 1.5, and the activation energy of reaction was 17.876 kJmol -1 , and the pre-exponential factor was 0.8521L/(molmin). The kinetic equation in 75℃ is r A =1.602×10 -3 c A 1.0916 c B 0.5368 . The contrast between experimental value and predicted value displayed the dynamics model had a good veracity, and can be applied for predicting reaction rate of progress.