Optimizing the Process Conditions of Corn Oil Methyl Ester for Bioresin Production

doi:10.21203/rs.3.rs-3136748/v1

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Optimizing the Process Conditions of Corn Oil Methyl Ester for Bioresin Production

https://doi.org/10.21203/rs.3.rs-3136748/v1

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The conversion of corn oil into quality methyl ester through the transesterification process is an age-old concept. Because of its higher degree of unsaturation, similar to soybean oil, transesterified corn oil has promising properties to produce bio-based resin. Replicating published methods did not result in desired quantity and quality of corn methyl ester needed for bio-resin production, which requires further investigation. This research investigated the effect of different reaction conditions on the yield and quality of methyl ester produced from corn oil. The reaction was conducted at 60°C with a methanol-oil ratio of 6:1 and NaOH as a catalyst. Hypothesized recovered yield (> 70%) of corn methyl ester was achieved after stopping the reaction with HCl. The process was conducted at different reaction times (0.5, 1, and 1.5 h) and acid amounts (0, 1.3, 2.6, 3.9, and 5.2 mL). The methyl ester yield ranged from 45–79%. A statistical model was obtained with linear and quadratic terms, and the recovered yield varied significantly with the acid amount, reaction time, and their interactions. The addition of 2.6 mL acid after the transesterification process resulted in a 15–25% increased yield compared to no acid treatment. On the other hand, the yield was reduced 18–24% with increasing time when no acid was added. All the measured characteristics of the produced corn methyl ester sample were found within the limits of ASTM D6751 pure methyl ester. Overall, the optimization of the transesterification process showed promise in increasing the yield of quality methyl ester from corn oil.

Biodiesel

Transesterification

Reaction time

Neutralization

Epoxy resins can be synthesized from plant-based oils. Most of the commercially available epoxy resins are derived from petroleum sources and have many environmental and sustainability concerns. Vegetable oils can compete with petroleum sources because they are readily biodegradable and grown in a sustainable manner [1]. Vegetable oils with higher amount of unsaturated fatty acids can be converted into epoxidized oils. The best compositions of unsaturated fatty acids for use in the production of epoxy fatty acid are oleic, linoleic and linolenic groups [2]. These fatty acids are plentiful in linseed oil, rapeseed oil, soybean oil, corn oil, and sunflower oil [3].

Epoxidized sucrose esters of fatty acid (ESEFA) are novel bio-based resins pioneered at North Dakota State University from different vegetable oils, including soybean oil, for over a decade [4], and the produced ESEFA and their derivatives have been used in a number of coatings and thermoset materials applications. Corn oil has a similar degree of unsaturation as soybean oil; hence, it can be another potential source for producing ESEFA resins. Corn oil is usually produced from corn germ by combining expelling in continuous screw presses with solvent extraction of the press cake. The corn germ extraction methods can be classified as dry and wet methods. The ethanol industry mainly uses the dry method, and the oil from the wet process is primarily intended for human consumption. In 2021, 3.52 MT (million tons) of corn oil was produced worldwide, representing about 2% of the total worldwide vegetable oil production [5]. The United States is the largest corn oil producer in the world with 1.9 MT (million tons) of production per year which is half of the world's corn oil production per year. Compared with the worldwide production of other vegetable oils, corn oil ranks tenth [6] and corn oil is less expensive than most other types of vegetable oils. According to National Weekly Ag Energy Round-Up-2022 (on second week of June, 2022), the average US wholesale price for crude corn oil was $0.80 per pound whereas crude soybean oil is priced at $0.92 per pound [7].

The production of ESEFA from corn oil requires highly purified corn methyl ester. Transesterification of vegetable oils is the most effective process for transforming the triglyceride molecules into molecules of fatty acid methyl or ethyl esters. As a renewable resource, corn oil (usually rich in oleic and linoleic acids) can be epoxidized using hydrogen peroxide in the presence of either acetic acid or formic acid. The same reaction principle can be applied to corn oil methyl ester after it has been esterified with sucrose to produce a bio-based resin. Some specific quality of the methyl ester influences the production of ESEFA. Glycerol and water content of the methyl ester are important, because transesterification separates the glycerol from the fatty acid, and moisture content can impact the efficiency of the catalyst used in the esterification reaction to produce sucrose ester.

Several published reports showed corn oil methyl ester yield in the range of 85–95% [8–10]. However, most of these yields were based on the conversion rate of triglycerides, which does not provide the actual amount of ester produced during transesterification. KOH and NaOH are commonly used catalysts in the transesterification of vegetable oils, due to their low cost, high efficiency, and ease of availability [11]. The homogeneously catalyzed reaction is commonly known as a one-step process in the presence of base catalysts. According to previous studies, base catalysts initiate the saponification reaction, which reduces the yield of methyl esters and makes separation of methyl esters from the glycerol phase difficult [12, 13]. Vávra et al. [14], stated that the transesterification reaction is reversible and has to be stopped before the methanol is removed. During the methanol removal process from the entire reaction mixture in the presence of the base catalyst, reverse transesterification to raw materials occurred [15], leading to the low yield of methyl ester.

To increase methyl ester yield by reducing saponification and reverse transesterification, some acid can be added to neutralize the catalytically active methoxide ions and to stop the transesterification process [16, 17]. In some previous studies, the problem associated with phase separation between glycerol and methyl ester was reported [18, 19], and the neutralization step after the reaction by adding acid can minimize this problem [20, 21]. However, this catalyst neutralization step is usually used in kinetics studies during methyl ester production and to recover quality glycerol as a valuable by-product of transesterification. Some critical parameters of transesterification reaction, such as temperature, alcohol to oil molar ratio, and catalyst concentration, have been studied for methyl ester yield from corn oil. Veljković et al. [22], reviewed these conditions and concluded that higher corn oil triacylglycerol conversion (> 90%) was achieved using NaOH catalyst at a concentration of 1.25% at 60 ºC and 6:1 methanol to oil molar ratio. However, there is little available information about stopping the transesterification reaction after corn oil was transesterified. El Boulifi et al. [8], reduced the pH of the system by washing with distilled water during separation. Moreover, various reports mentioned different reaction times for corn oil transesterification, ranging from 0.5–1.5 h [22]. In this context, it is necessary to optimize the corn oil methyl ester yield in terms of stopping the reaction with different amounts of acid added after different reaction times. It is also essential to compare if the neutralization affects the yield and quality of corn oil methyl ester compared to other commonly used vegetable oils such as soybean and canola oils.

In this study, the objective was to optimize the yield and quality of corn methyl ester for the production of epoxidized sucrose esters of fatty acid and also to illustrate the effect of using acid at the end of the base-catalyzed transesterification of different vegetable oil in terms of methyl ester yield.

Materials

Commercial edible grade (brand name: Our Family) corn oil, soybean oil and canola oil were bought from the local supermarket. Methanol of 99.8% purity was supplied from VWR (PA, USA). Sodium hydroxide (NaOH) pellets (reagent grade, ≥ 98%, anhydrous) purchased from Sigma-Aldrich. Magnesium sulfate (MgSO₄), anhydrous ≥ 99%, powder was purchased from J.T. Baker (PA, USA). A working solution of 6M HCl was prepared from ACS grade HCl solution (supplied from EMD Millipore, Germany). All the chemicals were used without any further purification.

Methyl Ester Production

The methyl esters were produced via a transesterification process in which NaOH was used as a catalyst. 100 g of different oil samples (corn, canola and soybean) were heated to 50°C in a 250-mL Erlenmeyer flask. The heating helped to decrease the viscosity of the oil and thus facilitated the activity for methanol. Sodium methoxide solution was prepared separately by dissolving 1.25 g of NaOH in 22-mL of methanol using a magnetic hotplate stirrer. The methanol amount for the sodium methoxide solution was based on the molar ratio of the methanol and corn oil at 6:1. Prepared sodium methoxide solution was poured into the reaction flask when the corn oil reached 50°C. Heating continued and was maintained at 60 ± 2°C, below the boiling point of methanol (63°C). The temperature of the reactant was monitored using Graphtec mini data logger (Model GL220). Stirring on the hot plate was done using a magnetic stirrer at 600 rpm. The reaction was timed as soon as the methanol/catalyst mixture was added. The total reaction time was 1 h for the neutralization experiment when different vegetable oils were used for methyl ester production. In the methyl ester yield optimization study with corn oil the reaction time was varied from 0.5–1.5 h (Table 1). After completion of the reaction time mentioned, a fixed amount of acid (5.2 mL) was added in the neutralization experiment and different acid amounts (0-5.2 mL) were used for the optimization study with corn oil (Table 1) to produce methyl ester. Neutralization was done by using 6M HCl and the amounts were calculated based on the stoichiometry between HCl and NaOH used. The neutralized reaction mixture was transferred into a 500-mL separatory funnel and left overnight for cooling and separation of the phases of glycerol and corn oil methyl ester. Dark glycerol phase was discarded followed by the phase separation and the methyl esters phase was washed and purified with distilled water.

Table 1

Experimental matrix with corn oil and their results
Run	Factors			Yield of corn oil methyl ester (%)
	Reaction time (h)	Acid amount (mL)	Neutralization (%)	Experimental*	Predicted	Relative error (%)**
01	0.5	0		69.6 ± 1.5 _ab	65.1	6.5
02	1	0	0	52.3 ± 4.6 _c	56.7	8.3
03	1.5	0		45.9 ± 5.1 _c	48.3	5.2
04	0.5	1.3		70.1 ± 3.2 _ab	73.5	4.8
05	1	1.3	25	71.4 ± 6.1 _ab	67.8	5.0
06	1.5	1.3		66.7 ± 2.9 _b	62.2	6.8
07	0.5	2.6		75.9 ± 6.1 _ab	77.8	2.4
08	1	2.6	50	75.9 ± 4.9 _ab	74.8	1.4
09	1.5	2.6		71.9 ± 3.2 _ab	71.9	0.1
10	0.5	3.9		75.9 ± 2.5 _ab	77.8	2.5
11	1	3.9	75	77.7 ± 2.8 _ab	77.6	0.1
12	1.5	3.9		75.7 ± 0.8 _ab	77.5	2.4
13	0.5	5.2		74.9 ± 1.7 _ab	73.8	1.5
14	1	5.2	100	79.1 ± 4.8 _a	76.4	3.4
15	1.5	5.2		77.1 ± 2.9 _ab	78.9	2.4
different letters indicate significant differences (at α = 0.05); *Relative error (%) = [abs value of (Experimental-Predicted)/Experimental]×100

Firstly, the excess methanol from the methyl esters phase was removed by evaporation under vacuum. This was done by heating the washed methyl ester under 85–100 kPa vacuum pressure at 65°C for 30 min. The methyl esters were further purified by gentle washing with distilled water to remove residual catalyst, glycerol and soaps (aqueous phase). The mixture was allowed to settle for 20 min followed by each washing for complete removal of the aqueous phase. After three successive washes with distilled water (25 mL in each wash), the aqueous phase became clear. Afterwards, the resulting solution was dried overnight using anhydrous MgSO₄. Finally, the produced methyl ester was filtered under vacuum to remove the hydrated MgSO₄.

Neutralization Experiment using Three Different Vegetable Oils

To investigate the effect of neutralization on yield, methyl ester was produced from corn oil, soybean oil, and canola oil. Two different reaction conditions, each with three replicates were used in this experiment. The first reaction condition was to produce methyl ester without adding any acid, and the other was to add 5.2 mL of 6M HCl to stop the reaction before separation. The reaction ran for 60 min, and other parameters were the same as described in the previous section. The yield of the produced methyl ester was determined using Eq. (1).

$$\text{Yield (%)=}\frac{\text{Weight of refined methyl ester}}{\text{Theoretical weight of methyl ester}}\text{× 100}$$

where the theoretical weight of the methyl ester was calculated by multiplying the weight of oil used with a conversion factor of 0.96.

This conversion factor was based on the average molecular weight of the corn oil triglyceride and its fatty acid methyl esters, and the stoichiometry of the transesterification reaction. The percent yield in Eq. (1) is the recovered yield indicating how much methyl ester can be isolated from the reaction mixture, and the theoretical weight of the methyl ester is the reaction yield which means how much oil is converted to methyl ester. The fatty acid profile of corn oil (Table 2) was used to calculate the average molecular weight of the triglyceride in corn oil (869 g/mol) and its fatty acid methyl ester (277.5 g/mol). For soybean oil, the average molecular weight of the triglyceride and its fatty acid methyl ester was 870.5 g/mol and 278 g/mol, respectively. For canola oil it was 900.23 g/mol and 287.4 g/mol, respectively.

Experimental Design to Optimize Corn Oil Methyl Ester

A full factorial design with two factors was designed to optimize the methyl ester production from corn oil. The two selected factors were the reaction time (A) and the acid amounts used to stop the reaction (B). Factor A had three levels (0.5, 1 and 1.5 h) and factor B had five levels (0, 1.3, 2.6, 3.9 and 5.2 mL). The experimental matrix was tabulated in Table 1. A total of 15 treatment combinations were obtained and each treatment was conducted in triplicate. The response variable was the yield of methyl ester. Following the completed experiments, the response variable (methyl ester yield) was fitted into a second order polynomial model in order to correlate the response variable to the independent variable. The general form of the model equation was as follows in Eq. (2):

$$\text{Y}= {\text{b}}_{0}+ \sum {\text{b}}_{\text{i }}{\text{X}}_{\text{i}}+\sum {\text{b}}_{\text{ii }}{{\text{X}}_{\text{i}}}^{2}+\sum {\text{b}}_{\text{ij }}{\text{X}}_{\text{i}}{\text{ X}}_{\text{j}}$$

where Y is the yield of methyl ester, X_i and X_ij are the factors/independent variables, b₀ is the constant regression coefficient, b_i, b_ii and b_ij (i,j = 1,2) are linear, quadratic and two-way regression coefficients, respectively.

Statistical Analysis

Statistical analysis was performed using Minitab 21 (State College, PA) at a 0.05 significance level. In addition, One-way analysis of variance (ANOVA) was also computed to evaluate the statistical significance and validity of the model. The influence of the process factors on methyl ester yield was analyzed using main effect and interaction effect. The optimal condition providing the maximum methyl ester content was determined by solving the model equation.

Analytical Methods

The Official American Oil Chemists' Society [23] methods were used to determine the fatty acid composition (Ce 1e-62), acid value (Cd 3a-63) and saponification value (Cd 3a-94) of the oils. The quality of the produced methyl ester was analyzed in terms of viscosity (ASTM D445), acid value (ASTM D664), cloud and pour point (ASTM D2500) using the methods according to ASTM standards. The water content of the methyl ester was determined using Karl Fischer titration. The total glycerin was calculated using the SafTest™ total glycerin kit [24]. This glycerin kit was brought from MP biomedical; Solon, OH, USA. A total glycerin reagent which is a lipase enzyme was used to convert bound glycerin into free glycerin of the sample. Then a SafTest™ analyzer which is a spectrophotometer at 570 nm was used to quantify the total free glycerin content. The obtained characteristics of methyl ester were then compared with the standards such as American Society for Testing and Materials (ASTM D6751) and previously published literature.

Effect of Neutralization on Yield of Corn Oil Methyl Ester Compared to Other Vegetable Oils

This research started by producing corn oil methyl ester using homogeneous base catalyst (NaOH pellets) in lab-scale. However, the recovered yield of the corn methyl ester was very low (< 50%) because it was difficult to separate the methyl ester from the glycerol phase (Fig. 1). There was no distinct separation phase for the corn methyl ester compared to that of vegetable (soybean) oil methyl ester as shown in Fig. 1. The corn methyl ester had significant soap formation and was mixed in the aqueous phase. It was important to investigate the best way to address the poor separation of corn methyl ester.

Poor separation of corn oil methyl ester can be explained by looking at some properties of the different oils tabulated in Table 2. Corn oil has the highest saponification value and ester value among three oils (Table 2). A low saponification value usually indicates easy soap formation at a higher temperature. Canola oil had the lowest saponification value (Table 2) causing more soap formation during transesterification reaction. The soap increases the methyl ester solubility in glycerol and results in emulsification of the ester and glycerol, which causes difficulties in the separation of the esters, thus reduces the recovered yield.

Table 2

The iodine, acid and saponification value of corn, canola and soybean oils including distribution of fatty acids
Oils	Saponification value (mg KOH g^− 1)	Ester value (mg KOH g^− 1)	Distribution of Fatty acid, %
Oils	Saponification value (mg KOH g^− 1)	Ester value (mg KOH g^− 1)	Palmitic (16:0)	Stearic (18:0)	Oleic (18:1)	Linoleic (18:2)	Linolenic (18:3)	Others
Corn	193.3	182.4	13.2	1.6	28.2	54.7	N/A	2.2
Canola	186.9	179.5	4.5	0.2	63.1	1.9	19.5	10.8
Soybean	192.9	179.6	11.5	3.7	21.0	55.8	7.3	0.8

Saponification values include the neutral fatty acids and free fatty acid content present in the oil. On the other hand, the ester value represents the amount of neutral fat in the oil, which is directly associated with acid and saponification values. A high ester value indicates a high amount of ester with a low molecular weight fatty acid indicates the availability of short-chain fatty acid. Table 2 shows that the ester value is higher in corn oil than in other oils, and it may hinder the separation of glycerol and ester. Saponification is a side reaction happening along with transesterification. Saponification occurs when the free hydroxide of a catalyst breaks the ester bonds between fatty acids and glycerol in a triglyceride, resulting in more free fatty acids and glycerol. The ester bond is more prone to break down with short chain fatty acid. The sodium used for a catalyst is then bound with the fatty acid and unusable, thus complicating the separation and recovery of esters.

The soybean oil had a similar saponification value to the corn oil and a similar ester value to the canola oil (Table 2). Nevertheless, it is already evident from Fig. 1 that the soybean oil had less soap formation during reaction, when compared to the corn oil resulting in better methyl ester yield. Soybean oil had the lowest amount of oleic acid among others, and free fatty acid is often expressed as percent oleic acid. The presence of low oleic acid could be the reason for less soap formation during saponification towards transesterification of soybean oil.

To improve the recovered yield, the effect of stopping or neutralizing the reaction of the different corn, canola, and soybean methyl esters was studied. These are commonly used oils in the US and the expectation is that the yield of methyl ester production will be similar since the refining steps are also similar. The results illustrated in Fig. 2 revealed that neutralization has a positive effect on the yield of methyl ester. The recovered yield increased with the use of acid to stop the reaction. The methyl ester yield was 52–55% for corn and canola oil samples that were not neutralized. This yield significantly increased to 75% when the reaction was neutralized using acid. However, it was also visible that soybean methyl ester yield in reaction with no neutralization was around 72%, which was not significantly different with corn and canola methyl ester yield neutralized with acid. Neutralizing the reaction of soybean methyl ester significantly increased the yield to 88%. and was comparable to the maximum expected theoretical yield of 90%. The small variation might be an indication of some esters were washed during washing.

This finding is interesting because the recovered yield is dependent on how well the organic phase separated from the aqueous phase and also the formation of soap that usually dissolves in the aqueous phase. The higher recovered yield seen with soybean oil is likely due to less soap formation. Moreover, the added HCl is more strongly attracted to the metal ion on the sodium soap than the fatty acid chain. So, the metal ion combines with the Cl from the HCl to produce NaCl, and the hydrogen freed from the HCl converts the fatty acid chain to free fatty acid. In this way, the soap formation was reduced, and an increasing recovered yield was observed when acid was added after the transesterification reaction.

Most previous studies on the transesterification of these three mentioned oils were mainly focused on the property of methyl ester from different oils and their combustion performance. The findings of this present study is quite similar to Karademir and Karademir [25] who measured the efficiency of biodiesel production from soybean, corn and canola oil. Their result also showed a better ester conversion rate in soybean compared to corn or canola. But their yield measurement was based on total ester rate and the percent of linoleic acid conversion through gas chromatography instrument rather than the recovered yield. Moreover, the previous studies did not use any neutralization step during their experiment. The neutralization step of stopping transesterification reaction and the recovered yield on the basis of weight differentiates the present study from previous studies. Overall, the differences observed with the recovered yield of methyl ester justified why optimizing the process steps for each type of vegetable oil used is important. Hence, it was essential to optimize the process for corn oil which was the focus of this research.

Modelling the Factors Influence the Yield of Corn Oil Methyl Ester

With the finding of the importance of neutralization towards increasing the yield during transesterification, it was important to identify the amount of acid needed and its interaction with the total reaction time. The experimental runs corresponding to the factorial design along with the values of the yield for each run are presented in Table 1, where the predicted value is based on the statistical model. The model to predict the yield of methyl ester was first developed by considering both the linear and quadratic terms in Eq. 2. For developing a quadratic model, the present experimental design was expanded to a central composite design by the addition of three new runs (at time 1 h and stopping the reaction with 2.6 mL HCl). A statistical analysis was carried out on these experimental values, and the main effects and interaction effects of the factors were determined. Initially, both time (A) and square of time (A²) in the model were not statistically significant at p-value < 0.05. Therefore, a reduced model was developed using forward selection method as seen in Eq. (3) and the ANOVA results of the developed model are shown in Table 3.

$$\text{Yield (%) = 73.45-16.77A+0.31B-0.003}{\text{B}}^{\text{2}}\text{+0.22 AB}$$

Table 3

ANOVA results for the reduced quadratic model
Source of variation	Degree of freedom	Adjusted Sum of squares	Adjusted Mean Square	F-Value	P-Value
Model	4	3421.9	855.5	43.2	< 0.0001
Linear	2	2432.9	1216.4	61.4	< 0.0001
Time (A)	1	253.7 (6.0%)	253.7	12.8	0.001
Acid amount (B)	1	2179.2 (51.7%)	2179.2	110.0	< 0.0001
Acid Amount× Acid Amount (B²)	1	539.2 (12.8%)	539.2	27.2	< 0.0001
Interactions (A×B)	1	449.8 (10.7%)	449.8	22.7	< 0.0001
Error	40	792.2 (18.8%)	19.8
Lack-of-Fit	10	339.6 (8.1%)	34.0	2.3	0.042
Pure Error	30	452.6 (10.7%)	15.1
Total	44	4214.1

According to the ANOVA result (Table 3), the reaction time (A), the acid amounts used to stop the reaction (B), square term of acid amount (B²) and interaction term (A×B) significantly affected the yield at 95% confident level. The p-values (< 0.0001) indicate that all of the factors were important in modeling the transesterification reaction. The ANOVA table shows that almost 82% of the source of variation (SOV) in the process was due to variation in the levels of the selected factors. Acid amount (B) contributed more than 50% to the variation in the model. Both the squared term of acid amount (B²) and interaction term (A×B) had similar contribution to predict the yield of methyl ester (12.8% and 10.7%, respectively). Time (A) had contributed only 6% in the variation of the developed model. The source of variation for each factor is represented in the Pareto chart in Fig. 3. to present the magnitude and the importance of the effects. On the Pareto chart, bars that cross the reference line are statistically significant.

However, the lack of fit was slightly significant at p-value = 0.042 in Table 3, which was very close to being insignificant at p-value > 0.05. The significant error term was probably due to the elimination of square term of time (A²) from the initial model and higher relative deviation (greater than 5%) occurred in experimental run 1 to 6 (Table 1). The developed model indicates that for the reactions with no acid addition, the separation between methyl ester and glycerol becomes more difficult, as the reaction time increases.

The significance of each coefficient in Eq. (3) was evaluated by the p-value shown in Table 4. The smaller the magnitude of the P-value, the more significant is the corresponding coefficient. From Table 4, it can be seen that all terms in the model were found to be statistically significant. The acid amount (B) term had the most significant effect, followed by the square of acid amount (B²) and interaction (AB) term. The reaction time (A) had the least impact and a similar trend is also seen at 5% significance level with the Pareto chart at Fig. 3.

Table 4

Regression coefficients of the predicted model
Term	Coefficient	P-Value
Constant	74.80	0.000
Time (A)	-2.91	0.001
Acid Amount (B)	9.84	0.000
Acid Amount × Acid Amount (B²)	-8.27	0.000
Time × Acid Amount (AB)	5.48	0.000

Reaction time (A), amount of acid (B), and time-acid amount interaction effects (AB) were fitted by multiple regression analysis to develop a linear model. This linear model was developed to improve on the quadratic models. The adequacy of all the statistical models (linear, quadratic, quadratic with forward selection) was compared by model summary statistics (Table 5). The linear model maximizes the value of R² and R²_adj and minimizes the standard deviation compared to the other quadratic models. The high R² value refers to the acceptable goodness of fit of the linear model. The R² value 0.893 of the linear model revealed that 89.3% of the variation in the response was due to the difference observed in the factors. The linear model to predict methyl ester yield (response function) for the significant main effects and interactions is in Eq. (4).

$$\text{Y}\text{i}\text{e}\text{l}\text{d} \left(\text{%}\right) = 70.66-2.91\text{A}+9.84\text{B}+5.48 \text{A}\text{B}$$

Table 5

Model summary statistics of different model
Model type	Standard Deviation	R²	R²_adj	R²_pred
Linear	3.885	0.893	0.843	0.758
Quadratic	4.484	0.814	0.793	0.751
Quadratic with forward selection	4.450	0.812	0.793	0.763

Main Effect and Interaction Effect on Corn Oil Methyl Ester Yield

Figure 4 and 5 illustrate the main effects and interaction effects of the treatment combinations on the yield of corn oil methyl ester. The model results indicate that both the main effects’ such as reaction time and acid amounts were statistically significant. Moreover, the interaction between the reaction time and amount of acid added was also significant. In the main effects plot (Fig. 4), reaction time at 0.5 h and 1 h were associated with the highest mean yield. The methyl ester yield at 1.5 h was significantly different than the yield at 0.5 h and 1 h. The decreasing trend of the yields with increasing time was also evident in Table 4. The negative coefficient of factor time (A) in Table 4 indicates a reduction in methyl ester yield with the increasing reaction time.

Highest yield was achieved by stopping the transesterification reaction with 5.2 mL of 6 M HCl. However, the highest yield was not statistically different for 2.6 mL and 3.9 mL of acid addition to the reaction. Lowest yield of methyl ester was found when no acid was added. Even with a little acid added to stop the reaction, the yield increases significantly (from 55.9% with no acid to 69.4% with 1.3 mL of acid). The positive coefficient of factor acid amount (B) in Table 4 indicates an increasing trend of methyl ester yield with the increasing acid amount. The negative coefficient of the square of acid amount (B²) indicates a concave curvature relationship with yield (Table 4; Fig. 4).

In the interaction plot (Fig. 5), the highest yield (79.1%) of methyl ester was achieved with the treatment condition of 1 h reaction time and 5.2 mL acid amounts used to stop the reaction. The significant differences of means were not illustrated here in Fig. 5, but it was already presented in Table 1 and Fig. 4. From Fig. 5, it was evident that the methyl ester yield of corn oil depends both on reaction time and the amount of acid added to stop the reaction. Figure 5 exhibited a trend of increasing yield with the increasing time and acid amount. The only exception happened when no acid was added to stop the reaction. When no acid was used, the yield of methyl ester reduced significantly from 69.6% at 0.5 h to 52.3% and 45.9% at 1.0 h and 1.5 h, respectively. From practical observation, the separation between glycerol and methyl ester became very difficult when no acid was added. Moreover, the interaction effect only occurs between 0.5-1 h. As the reaction time extended beyond 1 h, the interaction was not seen, and methyl ester yield was also slightly reduced.

Quality of Methyl Ester

Characteristics of the produced methyl ester from the experimental runs were tabulated and compared with standard methyl ester values and other previous studies as shown in Table 6. The selected characteristics were water content, viscosity, total glycerin content, acid value, cloud point, and pour point. For sucrose esters of fatty acid (bioresin) production using methyl esters, water content of the methyl ester is considered to be the most important parameter. This is because high water content slows down the catalysts by not only participating in the formation of emulsions but also causing hydrolysis or hydrolytic oxidation during the esterification reaction [26]. The water content of the produced corn oil methyl ester was well below the limit of ASTM D6751 (Table 6). Some of the previous studies have reported a very high water content of the methyl ester and might be an indication of using a different drying agent (diatomaceous earth) than that used in present study [27]. Both diatomaceous earth and MgSO₄ have been reported to be good drying agents for oil but maybe the vacuum drying may have helped lower the moisture content. The amount of drying agent can also impact the water content in the final product. The kinematic viscosity and total glycerin content were also found to be within the limits of ASTM standards and comparable to the previous studies as well (Table 6). The low total glycerin content indicated that the conversion was good and very little impurity was present. The acid value found in this study was very similar to [28]. The low acid value may be due to the difference of acid value in the corn oil used.

Table 6

Characterization of corn oil methyl ester
Characteristics	ASTM D6751 (limit)	Corn oil methyl ester (present study)^*	El Boulifi et al. [8]
Water content, ppm	< 500	242.05 ± 42.01	323	400	892
Viscosity, mm² s^− 1	1.9–6.0	4.55 ± 0.18	4.33	4.14	4.55
Total glycerin, %	< 0.24	0.05 ± 0.01	0.18	nd	nd
Acid value, mg KOH g^− 1	< 0.5	0.49 ± 0.08	0.04	0.49	0.25
Cloud point, °C	Report	-1.96 ± 0.51	-3.6	-5	nd
Pour point, °C	Report	-4.41 ± 2.06	-6	-6	nd
^*n = 30 (15 sample with two replicates) displayed as average ± standard deviation, nd: not determined

Cloud point and pour point was a main indicator of cold flow properties of methyl esters. However, ASTM D6751 requires that cloud point and pour point be reported. The observed cloud point and pour point values for the present study were − 1.96°C and − 4.41°C, respectively. Other studies have reported lower cloud point and pour point for their methyl esters, indicating good cold flow properties (Table 6). In our study, this characteristic has less attention as it was mainly related with fuel properties. Overall, it was evident that despite having a low yield in some of the experimental runs, the quality was still within acceptable limits.

The production of methyl ester from commercial corn oil via transesterification was optimized using a full factorial design. Results indicated that the acid amount used to stop the reaction (both linear and quadratic terms), the reaction time (only linear term), and their two-way interaction were statistically significant at 0.05 significance level. The developed quadratic model adequately described the transesterification process. After 1.5 hours of reaction time and complete neutralization of the NaOH catalyst included in the experiment, a maximum corn oil methyl ester yield of 78.9% was predicted. The acid amount factor and its quadratic terms had the most influence on the transesterification reaction. Compared to the treatment with the least amount of acid, the yield was much lower when no acid was added. The characteristics of the corn oil methyl ester generated in this research complemented those of existing standards and prior reports. Present findings of this study will pave the way to synthesize quality bioresin (epoxidized sucrose esters) from corn oil. Future studies will be looking at how the properties of corn oil methyl ester influence the production of sucrose ester.

Funding

The authors thank the North Dakota Corn Utilization Council (FAR0032322) for funding this research study. Additional support comes from North Dakota Agricultural Experiment Station and USDA-NIFA Hatch multistate ND01491.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Data Availability

Data will be made available on request.

Author Contribution

All authors contributed to the study conception and design. Material preparation, data collection, and analysis were performed by Md. Sanaul Huda, Michael Odegaard , Niloy Chandra Sarker, Dean Webster and Ewumbua Monono. The first draft of the manuscript was written by Md. Sanaul Huda, and all authors commented on previous versions of the manuscript. All authors read and approved the final manuscript.

Acknowledgements

The authors are very grateful to Dr. Brent Hulke from USDA, Sunflower and Plant Biology Research in Fargo, ND for providing the fatty acid analysis of different oils used in the experiment. The authors also would like to acknowledge Andrew Taylor of the North Dakota State University Graduate Center for Writers for his writing consultation to prepare the manuscript.

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Optimizing the Process Conditions of Corn Oil Methyl Ester for Bioresin Production

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