Response surface methodology (RSM) based on central composite design (CCD) was used to optimize biodiesel production process from corn oil. The process variables, temperature and catalyst concentration were found to have significant influence on biodiesel yield. The optimum combination derived via RSM for high corn oil methyl ester yield (99.48%) was found to be 1.18% wt catalyst concentration at a reaction temperature of
In the last few years, the world’s energy demand is increasing due to the needs from the global economic development and population growth. However, the most important part of this energy currently used is the fossil energy sources. The problem is fossil fuels are nonrenewable. They are limited in supply and will one day be depleted. There is an increased interest in alternative renewable fuels. As biodiesel is an environmentally friendly fuel, it is the best candidate to replace fossil-diesel, which has lower emissions than that of fossil-diesel, it is biodegradable, nontoxic, and essentially free of sulphur and aromatics [
Renewable feedstocks such as vegetable oils and animal fats have been used as raw materials for biodiesel production [
Vegetable oils are promising feedstocks for biodiesel production since they are renewable origin and can be produced on a large scale. More than 95% of biodiesel production feedstocks come from edible oils since they are varying considerably with location according to climate and availability. In the United States, soybean oil is the most common biodiesel feedstock, whereas in Europe and in tropical countries, rapeseed oil and palm oil are the most common source for biodiesel, respectively, [
Another alternative comes into play when looking to other industries. That is the case of ethanol, whose primary feedstock is corn. Plants ethanol from corn gives the integrated biorefineries a hydrocarbon-based source of renewable carbon for the production of fuels and chemicals. Ethanol is formed when starch is subjected to hydrolysis, followed by glucose fermentation. During this process, also some by-products including corn gluten meal, gluten feed and corn oil are formed. Therefore, corn oil can be extracted as a by-product by using new technology that will make ethanol production more efficient. This corn oil can be converted into a biofuel, such as biodiesel [
Biodiesel is being commercialized as a substitute or blending stock of fossil-diesel. However, biodiesel is less resistant to oxidation than typical fossil fuel and therefore doping of biodiesel in fossil-diesel will affect the stability of fuel significantly [
As corn biodiesel chemically is a mixture of long-chain fatty acid methyl esters (FAMEs), it is more susceptible to autoxidation and thus has a higher level of chemical reactivity than fossil-diesel. This oxidation instability is dependant on the number and location of methylene-interrupted double bounds in the FAMEs. Thus carbons that are simultaneously allylic to two olefinic groups will be extremely susceptible to the initiation of peroxidation. An early study [
A number of reports have appeared on the storage and oxidative stability of biodiesel synthesized from different vegetable and frying oils (including sunflower, soybean and rapeseed oil) [
In a previous work [
In this paper, the storage stability of biodiesel made from corn oil was investigated over a storage time of 30 months under argon atmosphere conditions, whose properties were compared to a corn biodiesel sample which stored under normal oxygen atmosphere. The focus of the study was on the influence of storage time on the biodiesel properties, such as PV, AV, IV and
Factorial design of experiments gives more information per experiment than unplanned approaches; it allows to see interactions among experimental variables within the range studied, leading to better knowledge of the process and therefore reducing research time and costs [
Reactions were carried out in a batch stirred reactor of 500 cm3 volume, equipped with a reflux condenser and a mechanical stirrer. The impeller speed was set at 600 rpm to avoid external mass transfer limitation [
Corn oil was supplied by Koipe Spain. The quality control of the corn oil used in this study is presented in Table
Composition analysis results of corn oil.
PV | AV | IV | Main fatty acid composition (%) | ||||||
(meq/Kg) | (mg KOH/g) | (cst at 40°C) | (mg I2/g) | Palmitic (C16 : 0) | Stearic (C18 : 0) | Oleic (C18 : 1) | Linoleic (C18 : 2) | Linolenic (C18 : 3) | other |
2.26 | 0.23 | 39.28 | 125.4 | 12 | 2.4 | 27.3 | 55.8 | 1 | 1.5 |
PV: peroxide value; AV: acid value;
The corn oil FAMEs were produced via a transesterification process in which a strong alkali catalyst was used. This latter is frequently used in the transesterification reaction, primarily due to its significant advantages in terms of the smaller quantity of catalyst used and the shorter reaction time required [
Corn oil was used as the raw oil to be transesterified with methanol in a reacting tank. The temperature values are below the boiling point of methanol (63°C), to prevent the methanol in the reactant mixture from evaporating. The molar ratio of the methanol and corn oil was set at 6 : 1. The potassium hydroxide was stirred with methanol for 10 minutes using an electric-magnetic stirrer to form potassium methoxide, which was then poured into the reacting tank and mixed with the corn oil. The total reaction time was 60 minutes. Almost total conversion to corn oil FAMEs was achieved quickly after a few minutes from the start of the reaction, depending on the reaction conditions.
At the end of a run the reaction mixture was allowed to cool down. The upper phase consisted of FAME, and the lower phase was glycerol.
Once the glycerol and FAME phases have been separated, the last one was purified by gentle washing with distilled water to remove residual catalyst, glycerol, and soaps. The pH of washing water was initially very high 10.22 due to the dissolved KOH. After 3 successive rinses with water, the washing water became clear and its pH was 7.9. The washing process was continued (twice more) until a pH of about 7 was achieved. Finally, the methyl ester phase was distilled to remove the residual water.
The final water content of the corn oil FAME was less than 0.01%. Water in the sample can promote microbial growth, lead to tank corrosion, participate in the formation of emulsions, as well as cause hydrolysis or hydrolytic oxidation [
Reaction products were monitored by capillary column gas chromatography, using a Hewlett-Packard 5890 series II equipped with a flame ionization detector (FID). The injection system was split-splitless. The carrier gas was helium at a flow rate of 1 mL/minutes. The analytical procedures and operating conditions have been described in detail in a previous work [
Two biodiesel samples, three litres each of were stored at ambient temperature for 30 months at two different storage conditions: the samples were stored in closed glass exposed to daylight, one under normal oxygen conditions and the other under atmosphere argon. During storage, samples were taken out periodically and different quality parameters (PV, AV, IV, and
The synthesis of biodiesel was studied using factorial design of experiments. The experimental design applied to this study was a full two-level factorial design 22 (two factors each, at two-levels) and extended to response surface methodology (RSM).
The response selected,
Selection of the levels was carried out based on the results obtained in preliminary studies [
The statistical analysis was thereafter applied. The experimental matrix for the factorial design is shown in Table
Experimental matrix and experimental results.
Run | |||||
---|---|---|---|---|---|
The use of analysis and factorial design of experiments allowed us to express the amount of ester produced as a polynomial model (if the levels of the factors are equally spaced, then orthogonal polynomials may be used). The response, yield of ester, may be thus expressed as a function of the significant factors.
A linear stage was considered in the first step. Table
22 factorial design: statistical analysis.
Response: | |
---|---|
Number of experiments: | 4 |
Degree of freedom: | 3 |
Results of statistical analysis | |
| 97.16 |
Main effects and interactions | |
Statistical Significance of | |
Confidence level: 95%, | Standard deviation, |
Student’s | Confidence interval: |
Significant effects and Interactions: | |
Statistical significance of curvature | |
Curvature: | |
Confidence curvature interval: | |
Curvature: significant | |
Response equation: | |
Temperature (
The statistical analysis of experimental results revealed that the most significant factor is the catalyst concentration, while it also shows a significant value for curvature for the chosen responses. These data indicate the nonlinearity of the model and thus justifies planning a more complex design to fit the data to a second-order model.
To better predict the effect of variables, a quadratic model was investigated. Here, the 22 experiment design was expanded to a circumscribed central composite design by the addition of 4 new experiments (run 9–12 in Table
The coefficients of (
The statistical model was obtained from coded levels. Equation (
Experimental yield versus temperature and catalyst concentration.
For both linear and nonlinear models, the temperature influence is statistically significant in the range studied. This effect has a positive influence on the response. As the temperature increases, the solubility of methanol in the oil increases and so does the speed of reaction. As a matter of fact, at low temperatures, methanol is not soluble at all in the oil; when the stirring is started an emulsion appears. The reaction takes place at the interface of the droplets of alcohol in the oil and then as soon as the first FAMEs are formed, the alcohol solubilizes progressively because the esters are mutual solvents for the alcohol and the oil.
From the statistical analysis it can be concluded that, within the experimental range, initial catalyst concentration is the most important factor on the transesterification process. It has a positive influence on the response; that is, ester yield increases with increasing catalyst concentration.
The nonlinear model (Central Composite Design) gives the binary influences of all the factors used in the design. Interaction of significant main effects temperature and catalyst concentration (
The ester yield generally increases with increasing catalyst concentration and temperature, but progressively decreases at higher level of temperature and lower level of catalyst concentration. This finding may be explained by the formation of by-products, possibly due to triglycerides saponification processes, a side reaction which is favoured at high temperatures. This side reaction produces potassium soaps and, thus, decreases the ester yield.
The surface plot and contours of ester yield versus temperature and catalyst concentration obtained when individual experimental data are plotted are shown in Figure
Normal probability plot for the methyl esters yield.
Residual plot of methyl esters yield for the second-order model.
Figure
Experimental versus predicted values.
The oxidation reactions affect the fuel quality of biodiesel, primarily during extended storage. The oxidation stability study was conducted for a period of 30 months. At regular intervals, samples were taken to measure the following physico-chemical quality parameters: PV, AV, IV, and
Some of the most important qualities of biodiesel are shown before and after storage in Table
Quality control of corn oil biodiesel fuels used in this study before and after storage time of 30 months compared to EN 14214
Properties | Unit | Corn biodiesel before storage | Corn biodiesel after storage | Specification EN 14214 | |
Under argon atmosphere | Under normal oxygen atmosphere | ||||
Viscosity at 40°C | mm2/s | 3.5–5.0 | |||
Density at 15°C | g/cm3 | 0.86–0.90 | |||
Water content | wt% | ||||
Biodiesel yield | wt% | ||||
Monoglyceride content | wt% | ||||
Diglyceride content | wt% | ||||
Triglyceride content | wt% | ||||
Free glycerol | wt% | ||||
Acid value | mg KOH/g | ||||
Iodine value | mg I2/g | ||||
Peroxide value | meq/Kg | — | |||
Cloud point | °C | — | |||
Pour point | °C | — | |||
Cold filter plugging point | °C |
Although PV is less suitable for monitoring oxidation [
Variation in the peroxide value of corn oil biodiesel stored ■ under normal oxygen atmosphere and ▲ under argon atmosphere.
Once the peroxides have formed, they decompose and interreact to form numerous secondary oxidation products including aldehydes, which further oxidized into acids. Acids can also be formed when traces of water cause hydrolysis of the esters into alcohol and acids. Acid value measured in mg KOH/g is one of the significant indicators of oxidative degradation in lipids. The change in AV during storage for the two corn oil FAME samples is shown in Figure
Variation in the acid value of corn oil biodiesel stored ■ under normal oxygen atmosphere and ▲ under argon atmosphere.
Due to the hydroperoxides decomposition, the oxidative linking of fatty acid methyl ester chains can occur, giving as a result higher molecular weights species. During storage one of the obvious results of polymer formation is an increase in biodiesel viscosity [
Variation in the iodine value of corn oil biodiesel stored ■ under normal oxygen atmosphere and ▲ under argon atmosphere.
One of the oldest and most common methods of determining the level of unsaturation in a fatty oil or ester is the iodine value [
Variation in the kinematic viscosity of corn oil biodiesel stored ■ under normal oxygen atmosphere and ▲ under argon atmosphere.
For biodiesel samples, the peroxide value, acid values, and viscosity tended to increase, and iodine value to decrease over time. The sample stored under argon atmosphere never exceeded the specification limit of the studied parameters along the 30 months of storage. Biodiesel from corn oil stored under argon atmosphere did not demonstrate rapid increase in peroxide value, acid value and viscosity, compared to the one stored under normal oxygen atmosphere.
When storage is done in argon atmosphere condition, at ambient temperature without oxygen availability, PV, AV, and
Biodiesel kept in an argon atmosphere could increase its stability and also show positive effects retarding oxidative degradation of the biodiesel produced from corn oil.
In this work, a fully central composite design has been applied to optimize the synthesis process of FAME from corn oil using potassium hydroxide as catalyst. The study of the factors (temperature and catalyst concentration) affecting the response shows that, within, the experimental range considered, the most important factor is the initial catalyst concentration. For the yield of ester, this factor has a positive influence. The temperature has a positive influence in both responses. The
Biodiesel synthesised, which consists of long-chain FAME, generally suffers from lower oxidation stability. Results from this study suggest that for a remarkably stable biodiesel and in order to avoid oxidation, special precautions must be taken during long storage, such as storage under argon atmosphere. Nevertheless this action delays oxidation but it doesnot prevent it. The results of this work have allowed develop a methodology to overcome the obvious problems of long storage stability of biodiesel. Long-term storage study gives us a better understanding of the effect of the normal oxygen atmosphere on the stability of biodiesel. The use of corn oil as an alternative raw material and renewable feedstock to produce biodiesel which fulfills the specification of EU standards for biodiesel (EN 14214) is of great interest to build an integrated and self-sustained biorefinery.
Financial support from the (CICYT), Spanish Project CTQ 2006-10467/PPQ is gratefully acknowledged.