Synergistic Effects of Skeletal Isomerization on Oleic and Palmitic Acid

Jun 20, 2012 - A 7.5 °C reduction in cloud point is achieved on the esters of a 55:45 (%) oleic acid/palmitic acid ratio, showing that isomerization ...
0 downloads 0 Views 959KB Size
Download PDF
Article pubs.acs.org/EF

Synergistic Effects of Skeletal Isomerization on Oleic and Palmitic Acid Mixtures for the Reduction in Cloud Point of Their Methyl Esters Stephen J. Reaume and Naoko Ellis* Chemical and Biological Engineering, University of British Columbia, 2360 East Mall, Vancouver, British Columbia V6T1Z3, Canada ABSTRACT: Biodiesel is composed of saturated and unsaturated fatty acid esters, both of which have significant effects on the cloud point of biodiesel. These two classes of fatty acids also react differently to isomerization reactions for cloud point improvement. This study examines the effects of isomerization and hydroisomerization on the two most common saturated and unsaturated fatty acids that make up vegetable oils, i.e., palmitic and oleic acids, respectively. The two fatty acids are placed under two different reactions and analyzed both separately and mixed for the cloud point improvement of their esters. The two reactions, isomerization and hydroisomerization, are run at 250 °C and 1 MPa H2 and at 300 °C and 4 MPa H2, respectively. Negative and positive results were shown, i.e., cis to trans isomerization, causing an increase in cloud point, and branching, causing a reduction in cloud point. A 7.5 °C reduction in cloud point is achieved on the esters of a 55:45 (%) oleic acid/palmitic acid ratio, showing that isomerization and hydroisomerization reactions can successfully lower the cloud point of a mixture of fatty acid methyl esters.

■

INTRODUCTION Biodiesel is known as a partial replacement for petroleum-based diesel fuel.1 The major benefits of using biodiesel are reduced CO2 emissions (on the basis of life cycle analysis), in addition to the fuel being produced domestically, adding jobs and revenue to local economies. Other benefits include reduced emissions of CO, SO2, and hydrocarbons.2 With the price of crude oil climbing and world supplies diminishing, along with increased demand, it is clear that a partial replacement of petroleum fuel is necessary.3,4 However, the high cloud point of most biodiesels causes engine problems during the winter months of colder northern climates, particularly in Canada and the northern United States. The cloud point of a substance is the temperature at which solid crystals first start to appear. Below such temperatures, solid crystals can clog fuel lines and injectors, causing engine problems. However, the cloud point can be lowered by creating branched compounds through the processes of hydroisomerization/isomerization.5 Two major classes of fatty acids in oils, unsaturated fatty acids (UFAs) and saturated fatty acids (SFAs), have significant effects on the cloud point of the subsequent esters (biodiesel). Melting points are a good indicator of the cloud point and, thus, used for pure compounds where cloud point data are lacking. SFA esters have high cloud points, while UFA esters have lower cloud points. UFAs yield esters with lower cloud points; however, SFAs provide much more desirable esters in terms of fuel stability. For example, SFA esters, such as methyl palmitate, have oxidative stability index (OSI) values of >40 h, whereas UFA esters, such as methyl oleate (one double bond) and methyl linoleate (two double bonds), have values of 15.1 and 3.4 h, respectively.6 Methyl palmitate and methyl stearate methyl esters have melting points of approximately 30 and 39 °C, respectively, while methyl oleate and methyl linoleate methyl esters have melting points of −20 and −35 °C, respectively.7 Branching has a positive effect on melting points; i.e., palmitic acid methyl ester has a melting point of 30 °C, © 2012 American Chemical Society

while the branched isopalmitic acid methyl ester has a melting point of 16 °C.7 Furthermore, branching a UFA and subsequently hydrogenating the double bond can lower the cloud point of the ester and significantly increase the fuel stability.8 A similar effect to branching the hydrocarbon chain of the biodiesel molecule is to use a higher alcohol or branched alcohol (isopropanol or tert-butanol) to create the ester. These alcohols have been shown to lower the cloud point of the biodiesel by 10 °C over the use of methanol.9,10 The drawback is the increased cost and decreased availability of the alcohols compared to methanol.11 A second drawback is that higher alcohols require a stronger catalysts and very high alcohol/oil ratios to produce biodiesel from vegetable oils.9,11 The decreased availability and the requirement of large amounts of alcohol could lead to feedstock problems as production increases. Two reactions that can branch a hydrocarbon are isomerization and hydroisomerization. Fatty acids can be branched in a similar process as a hydrocarbon. The two reactions branch a hydrocarbon chain at two different sites.12,13 In isomerization, an olefinic hydrocarbon is required for its double-bond site. The methyl or ethyl branch is formed at the double-bond site through the formation and destruction of a carbocation ring.14 For the hydroisomerization reaction, no double bond is required because the reaction creates and destroys one through an extra dehydrogenation/hydrogenation step.15 Methyl branching of oleic acid has been previously accomplished through isomerization with zeolites,8,16 with and without a reduction in cloud point.5 As well, hydroisomerization with platinum-impregnated zeolites has shown great success on hydrocarbon compounds.17 However, hydroisomerization using Received: November 28, 2011 Revised: June 18, 2012 Published: June 20, 2012 4514

dx.doi.org/10.1021/ef3006564 | Energy Fuels 2012, 26, 4514−4520

Energy & Fuels

Article

Figure 1. Desired and undesired products from isomerization/hydroisomerization of OA based on cloud point.

fatty acids has produced low yields caused by the presence of the carboxylic acid site.16 For the two different reactions of isomerization and hydroisomerization, two catalysts are required. For the isomerization reaction, zeolites can provide the required acid sites and are commercially available. The isomerization reaction needs a π bond to create a methyl branch on the hydrocarbon chain. Because there is no π bond for the SFA, these compounds cannot be directly isomerized using zeolite catalysts. Instead, a hydrogenation/dehydrogenation catalyst is required to temporarily dehydrogenate the chain and create the necessary π bond for the SFAs.15 Platinum works well as a hydrogenation/dehydrogenation catalyst and can be easily added to the support structure of the zeolite framework. The addition of Pt to the zeolite creates a bifunctional catalyst capable of dehydrogenating and isomerizing the SFA, thus, carrying out the second reaction: hydroisomerization. This study uses the UFA oleic acid and the SFA palmitic acid, because they are the most common fatty acids in vegetable oils suited for biodiesel production.18 The fatty acids oleic acid (OA) and palmitic acid (PA) and not the fatty acid methyl esters are used for the reason that fatty acids have greater reaction rates in isomerization and hydroisomerization reactions over the respective fatty acid methyl esters.5 The two different reactions branch the compounds at two different points. In the isomerization reaction, OA is branched by creating a methyl side chain at the point of the double bond.5 However, this branching does not always lower the cloud point of the esters, because of the shift from cis to trans configuration of the double bond. The cis/trans isomerization occurs at a much faster rate than the branching because of the thermodynamic stability of the trans double bond.19,20 The cloud point increase is due to the cis configuration, which is the configuration in vegetable oils, having a low melting point of −20 °C, while the trans configuration has a high melting point of 10 °C.18 Additionally, hydroisomerization has a negative side reaction, the hydrogenation of the double bond that turns the UFA into a SFA. This has detrimental effects on the melting point of the esters given that the UFA oleic acid methyl ester has a melting point of −20 °C and the subsequent SFA stearic acid methyl ester has a melting point of 37 °C.18 The desired and undesired reactions are shown in Figure 1. These two undesired reactions, hydrogenation and cis/trans isomerization, are not present in the reactions with the SFA palmitic acid because no double bond is present, as shown in Figure 2. The branching can have a synergistic effect on the cloud point of a sample. The two different branching positions (middle and end of chain) can lower the cloud point of the sample more than the effect that they have separately. Thus, the

Figure 2. Hydroisomerization of PA.

objective of this study is to test the effects of the isomerization and hydroisomerization reactions on the cloud point of saturated and unsaturated fatty acid methyl esters. This is accomplished with the use of palmitic (saturated) and oleic (unsaturated) acids. The two acids are reacted separately using hydroisomerization and isomerization reactions individually, as well as together in a mixture.

■

EXPERIMENTAL SECTION

Materials. Reactants used in the isomerization and hydroisomerization are palmitic acid (98% Acros Organics, Fair Lawn, NJ) and oleic acid (NF/FCC, Fisher Scientific, Fair Lawn, NJ). The catalyst used in the two experiments was beta zeolite (CP814E, Zeolyst International, Kansas City, MO) and was impregnated with a solution of tetraammonium platinum chloride [(NH3)4PtCl2, Aldrich Chem. Co., Milwaukee, WI] using the incipient wetness method. The reactant gas used to pressurize the reaction vessel was hydrogen gas (H2) (Praxair, Mississauga, Ontario, Canada). In the esterification reactions, methanol (Fisher Scientific, Fair Lawn, NJ) was used with a catalyst of sulfuric acid (BDH, 95−98%). Catalyst Preparation. Two different zeolite catalysts are prepared for this study: with or without platinum. The beta zeolite used for both reactions (isomerization and hydroisomerization) was CP814E (Zeolyst International), with the specifications of Si/Al molar ratio of 25 and surface area of 680 m2/g. The catalyst used for the isomerization reaction was calcined at 500 °C for 3 h, while the catalyst for the hydroisomerization reactions was impregnated with platinum. The compound used for impregnation was (NH3)4PtCl2. A 10 mL solution of 0.0205 M (NH3)4PtCl2 is added to 8 g of beta zeolite by incipient wetness to give a platinum loading of 0.5 wt %. The sample is then dried in an oven at 110 °C for 24 h. To achieve platinum in the ground state, it was reduced in a tube furnace at 350 °C in a H2 environment for 3 h. Isomerization. The beta zeolite catalyst without Pt is used for isomerization. A Parr 4848 autoclave is charged with 30 g of fatty acid and 1.2 g of catalyst to give 4 wt % catalyst. The reactor is purged with H2 gas for 5 min and heated to 260 °C while pressurized with H2 to 1.5 MPa and stirred at a rate of 600 rpm. The reaction is allowed to proceed for 6 h. When the reaction is complete, the reactants are cooled to 80 °C. The reactor is then depressurized, and contents are removed. The products are centrifuged to remove all traces of catalyst from the reacted fatty acid. Lastly, the reaction product is stored in a 4515

dx.doi.org/10.1021/ef3006564 | Energy Fuels 2012, 26, 4514−4520

Energy & Fuels

Article

Table 1. GC/MS Analysis of the Methyl Esters of the Isomerization/Hydroisomerization Reactions on OA and PAa branched compounds (%) entry

sample

CP (°C)

PAME (%)

OAME (%)

SAME (%)

C16

C18

other CC species (%)

di-CC species (%)

cracking products (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

100PA 100HP 100IP 100PAT 100OA 100IO 100HO 100OAT 55OA/45PA 55IO/45PA 55OA/45HP 55IO/45HP 55HO/45PA 55OA/45IP 55HO/45IP 55HO/45HP 55IO/45IP 55OA/45PAT

29.0 19.1 26.9 18.6 −16.1 −3.7 28.2 5.6 17.2 13.5 12.2 10.7 25.2 16.8 24.8 23.6 13.8 9.7

92 44 88 42 4.5 0 0 2 43 43 21 22 42 39 40 20 40 21

0 3 0 2 88 41 6 5 46 15 49 21 8 48 5 7 25 22

0 0 0 0 0 0 52 38 0 0 0 0 25 0 30 28 0 0

0 48 3 47 0 0 0 0 0 0 18 20 0 3 4 21 3 23

1 0 0 0 0 34 21 44 0 21 0 20 17 0 17 16 16 27

0 0 2 0 2 15 0 0 2 7 2 6 0 0 0 2 8 0

0 0 0 0 0 3 0 0 2 0 2 2 0 0 0 0 0 0

0 3 1 5 0 2 4 4 0 1 3 3 2 1 3 5 0 5

a

OA, oleic acid; CC, unsaturated fatty acid other than oleic acid; IO, isomerized oleic acid; PAME, palmitic acid methyl ester; HO, hydroisomerized oleic acid; OAME, oleic acid methyl ester; PA, palmitic acid; SAME, stearic acid methyl ester; IP, isomerized palmitic acid; C16, mixture of 16 carbon length molecules containing a methyl branch; HP, hydroisomerized palmitic acid; C18, mixture of 18 carbon length molecules containing a methyl branch; T, two reactions run together; 55/45, 55:45 (wt %); and CP, cloud point.

cool, dry, dark place until it is analyzed. Because of the small size of the reactor and large volume of material needed for the whole study, multiple reactions are carried out and all reaction products are thoroughly mixed before analysis. Hydroisomerization. The beta zeolite containing 0.5% Pt by weight is used for the hydroisomerization reaction. A Parr 4848 autoclave is charged with 30 g of fatty acid and 1.2 g of catalyst to give 4 wt % catalyst. The reactor is then purged with H2 gas for 5 min, heated to 300 °C while pressurized to 4.0 MPa, and stirred at 600 rpm. After 16 h, the reactor is cooled to 80 °C stopping the reaction and depressurized. The reaction products are removed from the vessel and centrifuged. The supernatant is cooled and stored for further analysis. Just as in the isomerization steps, a large volume of material is collected and thoroughly mixed before analysis. Centrifugation. The products of the isomerization and hydroisomerization are too viscous to be filtered; thus, centrifugation is used to remove the catalyst particles. Because of the relatively high melting point of the fatty acids, the mixture was heated to 70 °C before centrifugation. The centrifuge process was 12 000 rpm for 5 min. This process was completed after every reaction to ensure that there is no further reaction once the sample was removed from the reactor. Esterification. For the fatty acids to be tested for cloud point and composition, they are esterified with methanol to create a fatty acid methyl ester (FAME). This is due to the high cloud point of saturated fatty acids, which is above the limit of the cloud point analyzer (from −40 to 50 °C). Moreover, fatty acids have much higher melting points, leading to very high residence times in the gas chromatography (GC) column. Esterification was carried out in an Omni-Reacto Station (Thermo Scientific) using sulfuric acid as a catalyst. Approximately 20 g of reacted fatty acid is charged into the reactor with 25 g of methanol and 0.4 g of sulfuric acid. This gives a methanol/fatty acid molar ratio of 10:1 and 2 wt % catalyst. Sulfuric acid and methanol are first added and allowed to mix thoroughly at 350 rpm. Next, the fatty acid is added to the mixture, and the temperature is set to 65 °C. The reaction is allowed to proceed for 2 h under reflux conditions. Once the reaction is complete, the ester is cooled and water-washed 3 times to remove all traces of methanol, acid, and other impurities. The

washed ester is then dried using anhydrous CaCl2, and an acid number is taken to ensure >99% conversion. Simulation Calculations. Simulations of the molecules were performed with density functional theory (DFT) calculations using Materials Studio 4.4.0.0 (Accelrys, Inc.) software. The software predicts the most stable configuration of a system based on geometric and electrical properties of the molecules. Two systems were chosen to study the effects of adding a methyl branch to an ester: first, a control of two methyl palmitate molecules, and second, one methyl palmitate and one methyl isopalmitate. The simulations were run at 0 K, to study the molecules at their lowest energy state. At temperatures higher than 0 K, the increased energy can force the molecules apart, not based on electrical or spacial properties. Studying at 0 K gives the orientation of the molecules purely based on these properties. Experimental Error. To reduce the risk of experimental error because of the large number of samples and relatively small sample size, a react and mix approach was adopted. This method reacts the two fatty acids, OA and PA, separately in large batches, creating four reacted batches of isomerized oleic acid (IO), hydroisomerized oleic acid (HO), isomerized palmitic acid (IP), and hydroisomerized palmitic acid (HP). These batches are combined to create the proper mixtures and prepared and reacted in triplicates, where the standard deviations are shown in the graphs. Product Analysis. Fourier Transform Infrared (FTIR) Spectroscopy. For infrared spectroscopic analysis, a Varian 3100 Excalibur FTIR spectrometer was used for the detection in bond frequencies, which makes it useful in studying whether a certain bond is created or removed from a compound. Two bonds were of importance in this study: the methyl group (CH3) to determine branching and the methylene (CH2) group to see whether the hydrocarbon chain was shortened. A Varian 3100 FTIR spectrometer was equipped with an attenuated total reflectance (ATR) crystal. Before a sample is run, a background scan of the crystal is taken, so that the baseline can be subtracted to obtain the absorbance for the sample. The absorbance of each sample was taken at the wavelength ranges of 4500−650 cm−1. A small amount of sample, 50 μL, was placed on a horizontal ZnSe crystal, scanned 64 times, and co-added together at a resolution of 4 4516

dx.doi.org/10.1021/ef3006564 | Energy Fuels 2012, 26, 4514−4520

Energy & Fuels

Article

cm−1. The signal was analyzed using Varian Resolutions Pro software, which employed baseline and ATR corrections to the chromatographs. Gas Chromatography/Mass Spectrometry (GC/MS). Analysis of the reaction products was carried out with the use of a Varian CP 3800 gas chromatograph and Varian 4000-8 mass spectrometer. The column used was a 60 m × 0.25 mm inner diameter CP 50 wax column. The injector temperature was set constant at 230 °C. The column oven temperature started at 100 °C and was held for 0.5 min, increased to 150 °C at 10 °C/min and held for 5 min, and then increased to 220 °C at 5 °C/min and held for 5.5 min. The carrier gas (He) began at a flow rate of 0.5 mL/min for 15 min, then increased to 2.0 mL/min at 0.3 mL/min, and held for 10 min for a total run time of 30 min. Cloud Point. The cloud point analysis was carried out on the cloud, pour, and freeze point analyzer, model PSA-70X (Phase Technologies, Richmond, British Columbia, Canada) with an accuracy of ±1 °C. At the start of an analysis, a standard sample of known cloud point was tested to be within 1 °C of its cloud point to ensure proper operation of the instrument. The sample cup was then cleaned with heptane and flushed with 150 μL of sample twice. The sample of unknown cloud point was tested by placing 150 μL inside the analyzer and lowering the temperature at a rate of 1.5 °C/min until crystals first appeared. The cloud point analyzer meets and complies with the American Society for Testing and Materials (ASTM) D5773 method for testing of the cloud point.

■

RESULTS AND DISCUSSION The objective of the study is to find the optimal conditions that give the greatest cloud point reduction. The results of the various isomerization and hydroisomerization reactions on OA and PA are shown in Table 1. The optimal conditions for cloud point reduction were obtained when a mixture of the fatty acids were reacted together by both isomerization and hydroisomerization. This condition gave an ester cloud point reduction of 7.5 °C and 50% branched material, which was the highest amount of any sample. The isomerization conversions listed in Table 1 are in agreement with previous studies yielding conversions to iso species of 30−40%.8,16 The hydroisomerization conversions, on the other hand, are much higher than a patented study of 6% conversion to branch chain products.21 For example, Table 1 shows conversions of up to 48% using palmitic acid as the starting material and 20% using oleic acid as the starting material. The mixtures achieved a total combined conversion of approximately 50% branch chain products. Additionally, the cloud point of mixtures 10, 12, and 18 from Table 1 was lowered in all cases from the starting material. The reductions of 3.7−6.5 °C are consistent with a study by Yori et al., who achieved cloud point reductions of 4−6.5 °C using solid acid catalysts and a biodiesel mixture.22 Figure 3 shows the major effects of the individual reactions on the mixture of OA and PA based on cloud point. This allows for the examination of the effect of either isomerization or hydroisomerization on a single fatty acid, while all other conditions remain constant. For example, Figure 3A shows the effects of isomerization of OA on the cloud point, where a reduction is shown in all but one of the cases. The reason for the increase in cloud point for the 100OA sample is the cis/ trans isomerization changing the OA to a much higher cloud point trans configuration molecule.7 Figure 3B, on the other hand, shows the effects of hydroisomerization of OA on the cloud point, which increased for all cases. This is caused by the hydrogenation of the double bond creating a saturated compound with a much higher cloud point than that of OA.7 The hydrogenation reaction has a higher reaction rate than the hydroisomerization reaction; therefore, the hydrogenation is

Figure 3. (A) Effect of isomerization of OA on the cloud point of the ester (X = OA for the unreacted sample and IO for the reacted sample). (B) Effect of hydroisomerization of OA on the cloud point of the ester (X = OA for the unreacted sample and HO for the reacted sample). (C) Effect of hydroisomerization of PA on the cloud point of the ester (X = PA for the unreacted sample and HP for the reacted sample). OA, oleic acid; PA, palmitic acid; IO, isomerized oleic acid; HO, hydroisomerized oleic acid; and HP, hydroisomerized palmitic acid.

complete well before the isomerization.23 The melting point data from the literature illustrate a greater increase in the melting point of methyl oleate to methyl stearate than to methyl isostearate, where the melting points are −20, 38, and 26 °C, respectively.7 Therefore, this gives a 58 °C increase with only a maximum potential decrease of 12 °C. Hence, the hydrogenation of unsaturated fatty acids must be avoided if any reduction in cloud point or melting point is to be achieved. Figure 3C shows the reduction of cloud point through hydroisomerization of PA. The only reaction present was the branching of the PA, where there was no cis/trans isomerization or hydrogenation, which could cause an increase in cloud point. Note that isomerization of PA is left out of the figure because the isomerization has shown a negligible effect on the saturated fatty acid. Table 1 shows that, when OA is reacted under isomerization conditions, the ester cloud point increases from −16.1 to −3.7 °C, in samples 5 and 6, respectively. However, when mixed with PA, the isomerized OA reduces the ester cloud point of the mixture from 17.2 to 13.5 °C, in samples 9 and 10, 4517

dx.doi.org/10.1021/ef3006564 | Energy Fuels 2012, 26, 4514−4520

Energy & Fuels

Article

respectively. The cloud point is also reduced in samples 11 to 12 from 12.2 to 10.7 °C, respectively. In both reduction cases, the only reaction was isomerization on the OA fraction. This effect is caused by mixing the fatty acids OA and PA, as opposed to using a single unsaturated fatty acid. This leaves a lower initial concentration of unsaturated fatty acid, greatly affecting the cloud point of the ester. The effect of branched and unsaturated compounds on the cloud point is shown in Figures 4 and 5, respectively. Figure 4 shows that increased

Figure 6. Synergistic effects of methyl branching on improving the cloud point of a mixture of fatty acid methyl esters (arrows indicate reference axis: area % for bar graph and cloud point for line graph): 55OA/45PA, 55% oleic acid/45% palmitic acid; 55IO/45PA, 55% isomerized oleic acid/45% palmitic acid; 55OA/45HP, 55% oleic acid/ 45% hydroisomerized palmitic acid; 55IO/45HP, 55% isomerized oleic acid/45% hydroisomerized palmitic acid; and 55OA/45PAT, 55% oleic acid/45% palmitic acid run under both isomerization and hydroisomerization.

Figure 4. Effect of iso-fatty acid methyl esters (branched compounds) on the cloud point of fatty acid methyl ester mixtures.

55OA/45HP samples have one branching reaction each, i.e., either isomerization or hydroisomerization. The 55IO/45HP sample undergoes two branching reactions. Similarly, in the 55OA/45PAT sample, there are now two branching reactions; however, double the effect is produced because of both isomerization and hydroisomerization occurring on each component, yielding in effect four branching reactions. The lower cloud point from increased branching may be attributed to the reduction in London dispersion forces that occur between the hydrocarbon tails. The increased distance caused by the methyl group reduces the intermolecular dispersion forces, causing the molecules not to stack as well as the straightchain molecules, which in effect lowers the cloud point of the mixture. To further confirm the trend, molecular simulation has been conducted to show the increased space between the molecules caused by the branching, as shown in Figure 7. The simulations indicate that the average distance between the carbon backbone of the esters increases from 2.85 to 4.35 Å, specifically because of the addition of the methyl branch. This increase in distance causes a reduction in molecular dispersion attractive forces, resulting in a more fluid structural arrangement, therefore lowering the cloud point of the mixture. In addition to the increased spacing between the carbon chains, the methyl branch forces the oxygen molecules in the ester bond closer, causing the bond to rotate. This rotation forces the methyl group from the ester bond into another spacial plane, which would force molecules in that plane further away from the esters. The isomerization and hydroisomerization reactions create several byproducts in very low concentrations. These compounds include cracking products, i.e., smaller chain fatty acids, alkanes, and olefinic hydrocarbons. The cracking products are grouped together and shown in Table 1. One product not present in any sizable quantity is the dimer compounds. These did not show up in the GC/MS analysis. Additionally, evidence of their structure was shown in the FTIR

Figure 5. Effect of unsaturated fatty acid methyl esters on the cloud point of fatty acid methyl ester mixtures.

branching and a reduction in cloud point follow a linear trend. On the other hand, Figure 5 shows that increasing the unsaturated portion only largely affects the cloud point at concentrations of 60% unsaturated compounds. Therefore, with a maximum concentration of 55% unsaturated compounds, the effect on the cloud point is lower than increasing from 60 to 100% unsaturated compounds. However, the addition of branched compounds is significant in terms of cloud point reduction. This general trend is shown with hydrocarbons, where the effects of mixing branched alkanes with straight-chain alkanes resulted in the reduction of the melting point of the mixture in a linear trend. Similarly, the addition of double-bond alkenes does not significantly affect the melting point until concentrations reach near 75%.24 Figure 6 shows the synergistic effect of combining both isomerization and hydroisomerization to lower the cloud point of a fatty acid mixture. The highest cloud point is the control sample of 55OA/45PA ester. The cloud point is decreased as the reactions proceed from isomerization only (55IO/45P), to hydroisomerization only (55OA/45HP), to isomerization and hydroisomerization (55IO/45HP), and last to the reactions run together (55OA/45PAT). The general trend in cloud point reduction is caused by the increase in branching reactions (isomerization/hydroisomerization). The 55IO/45PA and 4518

dx.doi.org/10.1021/ef3006564 | Energy Fuels 2012, 26, 4514−4520

Energy & Fuels

Article

separately and together. Isomerization had a negative effect on the pure oleic acid sample, increasing the ester cloud point; however, when oleic acid was isomerized and mixed with palmitic acid, the ester cloud point was reduced. Hydroisomerization indicated a negative effect on oleic acid and a positive effect on palmitic acid with respect to ester cloud point reduction. The optimal run combination with respect to ester cloud point reduction was the mixture of oleic acid and palmitic acid run under both reactions together. This optimal condition gave an ester cloud point reduction of 7.5 °C and a total branched component of 50%. This study has successfully proven that, when combined, the two reactions, isomerization and hydroisomerization, can lower the cloud point of a mixture of UFAs and SFAs.

■

AUTHOR INFORMATION

Corresponding Author

*Telephone: 604-822-1243. Fax: 604-822-6003. E-mail: nellis@ chbe.ubc.ca.

Figure 7. Simulated stable configurations of (A) methyl palmitate and methyl palmitate and (B) methyl palmitate and branched methyl isopalmitate.

Notes

The authors declare no competing financial interest.

■

results (Table 2), however, in readings of