Development of Biodiesel Production Processes from Various Vegetable Oils
Abstract
Biodiesel is an alternative fuel to petroleum diesel that is renewable and creates less harmful emissions than conventional diesel thus the use of this fuel is a shift toward “sustainable energy”. Biodiesel can be produced from vegetable oil, animal fat, and organisms such as algae or cyanobacteria. Since vegetable oils are the major source for current commercial biodiesel, they are the focus of this thesis.
The main objective of this Ph.D. research is to develop processes suitable to produce biodiesel from various vegetable oils especially for those of non-edible oils such as used cooking oil, canola oil from greenseed, and mustard oil. An additional objective is to understand the relationship between the parent vegetable oils and the corresponding biodiesel properties.
Used cooking oil was the first vegetable oil investigated in this research. Initially, oil degradation behavior was monitored closely during frying. During 72 hours of frying, acid value and viscosity of the oil increased from 0.2 to 1.5 mgKOH•g-1 and from 38.2 to 50.6 cP, respectively. It was found that ester yield was improved by addition of canola oil to used cooking oil, i.e. addition of 20% canola oil to used cooking oil increased methyl ester yield and ethyl ester yield by 0.5% and 12.2%, respectively. At least 60% canola oil addition is needed to produce ASTM grade ethyl ester biodiesel. The optimum reaction conditions to produce biodiesel are 1% KOH loading, 6:1 alcohol to oil ratio, 600 rpm stirring speed, and either 50°C reaction temperature for 2 hr or 60°C reaction temperature for 1.5 hr for methanolysis and 60°C reaction temperature for 2 hr for ethanolysis.
Among non-edible vegetable oils, greenseed canola oil can be used in the most simple biodiesel production process. In this case, an addition of fresh vegetable oil is not required, because chlorophyll contained in this oil did not play a crucial role in the reaction activity. Methyl ester yields derived from greenseed canola oil without and with 94.1 ppm chlorophyll content are 95.7% and 94.8%, respectively. In contrast, erucic acid contained in mustard oil created difficulties in the production process. Ester yield derived from mustard oil using the conditions mentioned above was only 66% due to the present of unconverted monoglyceride. To obtain a deeper understanding on mustard oil transesterification, its reaction kinetics was studied. In the kinetic study, transesterification kinetics of palm oil was also investigated to study the effect of fatty acid chain length and degree of saturation on the rates of the reactions. It is shown in this research that the rates of mustard monoglyceride transesterification (rate constant = 0.2-0.6 L•mol-1•min-1) were slower that those of palm monoglyceride transesterification (rate constant = 1.2-4.2 L•mol-1•min-1) due to its lower molecular polarity resulting from the longer chain of erucic acid. The activation energy of the rate determining step (in this case, conversion of triglyceride to diglyceride reaction step) of mustard transesterification was, however, 26.8 kJ•mol-1, which is similar to those of other vegetable oils as reported in literature. Despite the presence of unconverted monoglyceride, distillation can be used to obtain a high purity ester.
Several ester properties are determined by characteristics of the parent oil and choice of alcohol used in transesterification. Chlorophyll contained in greenseed canola oil, for example, has an adverse effect on biodiesel oxidative stability. The induction time for methyl ester derived from treated greenseed canola oil (pigment content = 1 ppm) was enhanced by 12 minutes compared to that derived from crude greenseed canola oil (pigment content = 34 ppm). The optimum bleaching process involves the use of 7.5 wt.% montmorillonite K10 at 60°C and stirring speed of 600 rpm for 30 minutes. In addition, it was found that induction time of treated greenseed canola ethyl ester (1.8 hr) was higher than that of methyl ester (0.7 hr), which suggests a better oxidative stability of esters of higher alcohols. Furthermore, the use of higher alcohols instead of methanol produced materials with improved low temperature properties. For example, the crystallization temperatures of monounsaturated methyl, ethyl, propyl, and butyl esters prepared from mustard oil were -42.5°C, -51.0°C, -51.9°C, and -58.2°C, respectively. In contrast, the lubricity of biodiesel is mainly provided by its functional group which is COOCH3 for methyl ester. The use of higher alcohols in transesterification results in a less polar functional group in the corresponding ester molecule, which leads to reduction in ester lubricity. Methyl ester provided the highest lubricity among all esters produced, i.e. wear reduction at 1% treat rate of methyl ester, ethyl ester, propyl ester, and butyl ester are 43.7%, 23.2%, 30.7% and 30.2%, respectively.
The outcomes of this research have been published in several scientific journals and presented at national and international conferences. The published articles and conference presentations are listed at the beginning of each chapter in this thesis.