Fatty acid methyl esters (FAME) are the main components of biodiesel. They are the result of the transesterification of vegetable oils (triglyceride or TG) with methanol using an alkali or acid as a catalyst. The overall reaction is:
TG+3ROH → 3R'Co2R+GL (1)
Triglyceride + Alcohol → Methyl Esters + Glycerol
This reaction can be broken down into three steps:
TG + ROH ? DG + R'Co2R (2)
DG+ ROH ? MG + R'Co2R (3)
MG + ROH? GL + R'Co2R (4)
Where DG and MG are diglycerides and monoglycerides, respectively. It can be observed from (1) that the required stoichiometric ratio of alcohol to TG is 3:1. However, since the set of reactions are reversible, excess alcohol is used to maximize ester yield.
The data shown below was obtained in a batch reactor using soybean oil and methanol (6:1 ratio of alcohol to TG) in attempt to model the hydrolysis of TG, presented by equation (2). Concentrations were measured at different points in time for three temperatures: 25, 40 and 60oC.
a) Evaluate the data and formulate a rate law. Calculate the specific rate constants and order of reaction.
b) Calculate the activation energy and preTexponential factor.
c) The data presented made use of ultrasonic mixing. Literature suggests that the apparent specific rate constant of the pseudoTreaction (kTG') at 60°C is 0.036 (wt% minT1) when traditional mixing is used. Did the ultrasonic mixing affect the apparent specific rate? If so, what may this be attributed to?
Table 1. Experimental Data for triglyceride transesterfication.
|
25°C
|
40°C
|
60°C
|
|
Concentration TG (wt%)
|
Time (min)
|
Concentration TG (wt%)
|
Time (min)
|
Concentration TG (wt%)
|
Time (min)
|
|
66.7
|
0
|
42.5
|
0
|
73.7
|
0
|
|
49.4
|
1
|
11.0
|
2
|
5.8
|
2
|
|
25.8
|
3
|
7.0
|
3
|
2.7
|
3
|
|
13.5
|
5
|
4.3
|
5
|
2.5
|
5
|
|
11.0
|
8
|
3.0
|
7
|
1.2
|
8
|
|
5.5
|
17
|
2.7
|
8
|
0.9
|
10.5
|
|
3.3
|
28
|
1.5
|
15
|
|
|
|
2.6
|
35
|
|
|
|
|