Article pubs.acs.org/EF
Kinetics of Triazabicyclodecene-Catalyzed Canola Oil Conversion to Glycerol-free Biofuel Using Dimethyl Carbonate Mohammad R. Islam, Yogesh M. Kurle, John L. Gossage, and Tracy J. Benson* Center for Chemical Energy Engineering, Dan F. Smith Department of Chemical Engineering, Lamar University, Post Office Box 10053, Beaumont, Texas 77710, United States ABSTRACT: In this study, transesterification kinetics of canola oil with dimethyl carbonate (DMC) was investigated, intending to produce glycerol-free biofuel using triazabicyclodecene (TBD) as a catalyst. The triglyceride/DMC reaction in a stirred batch reactor produces a mixture of fatty acid methyl esters (FAMEs) and fatty acid glycerol carbonates (FAGCs), which are soluble in FAMEs and can then be used as a motor fuel. The main factors affecting the yield of biofuel, such as the molar ratio of oil/DMC, catalyst loading, reaction temperature, and reaction time, are discussed. A reaction mechanism has been proposed for the TBDcatalyzed transesterification. For the conversion of triglycerides, a second-order reaction rate with rate constants of 1.09 × 10−2, 1.65 × 10−2, and 2.30 × 10−2 L mol−1 min−1 at 50, 60, and 70 °C, respectively, was determined from the regression of the experimental data. The corresponding value of activation energy was 36.36 kJ mol−1. The maximum yield of 97.99% from transesterification of canola oil was obtained within 120 min.
1. INTRODUCTION Biodiesel, primarily produced from domestic and renewable biological sources, has been recognized as an environmentally friendly alternative fuel that has potential to displace petroleum-based diesel fuel.1,2 Technically, biodiesel is defined as a fuel comprised of monoalkyl esters of long-chain fatty acids derived from vegetable oils or animal fats, designated B100, and meeting the requirements of ASTM D6751.3 Currently, the base-catalyzed transesterification method is the most common and commercially available process for biodiesel production. The reaction of lipid feedstock, such as soybean, canola, rapeseed, sunflower, palm, and coconut oils, which are composed of C14−C20 fatty acid triglycerides (TG),4 with methanol is conducted using homogeneous base catalysts, such as NaOH (forming NaOCH3) and KOH (forming KOCH3), to produce long-chain fatty acid methyl esters (FAMEs) and glycerol as a byproduct.5−7 As the production of biodiesel is growing worldwide, the burden of excess low-value glycerol that is produced in traditional transesterification continues to form levels of uncertainty that may thwart the growth of the biodiesel industry. Indeed, the glycerol that is coming from the biodiesel industry is “greener”; i.e., it is not produced petrochemically from propane. However, to avoid the co-production of glycerol, methylation reactants, alternative to alcohols, have been studied in recent years8−11 to produce other economically valuable coproducts using the glycerol moiety. The chemical process model developed by Kurle et al. using a methanol substitute illustrates the viability of the glycerol-free biofuel.12 In the present study, triazabicyclodecene, a strong basic, cyclic guanidine organic catalyst, was used for the transesterification of canola oil by producing needed methoxy ions from dimethyl carbonate (DMC). In a preliminary study, DMC gave higher yields and insignificant amounts of byproducts, required no post-reaction water washing, and provided easier separation of excess DMC by flash distillation.12 The reaction between TG and DMC avoids the production of glycerol and, instead, produces a mixture of FAMEs and cyclic glycerol © 2013 American Chemical Society
carbonate esters of fatty acids, known as fatty acid glycerol carbonates (FAGCs) (Figure 1A). Because the reaction products contain mostly FAMEs and FAGCs, the theoretical mass yield of fuel in the DMC biofuel process is 9.70% higher than that of the conventional methanol biodiesel process. The presence of more oxygenated FAGCs in DMC biofuel may influence both fuel and flow properties,10 while the distribution of main polycyclic aromatic hydrocarbons (PAHs) in emissions is less (despite having a molar mass of 86 Da larger) than those of the corresponding FAMEs.10 It was also reported on the fuel performance that the net heating value of DMC biofuel is higher than that of traditional B100.10 Therefore, FAGCs, which are miscible with FAMEs, can be considered as a biofuel component along with FAMEs. A minor fraction of FAGCs can react with DMC and form glycerol dicarbonate (GDC) (Figure 1B). For a system that is not completely water-free, GDC can produce glycerol carbonate and methanol (Figure 1C). Although there are a number of kinetic studies in the literature on the transesterification of vegetable oils with alcohol, transesterification of vegetable oils with alternative methylating agents, such as DMC, has not been widely studied. Therefore, in this present work, a kinetic model was developed that describes a homogeneous reaction system consisting of canola oil and DMC for the production of biofuel using the TBD catalyst. The study was mainly focused on the influences of the temperature, catalyst loading, and oil/DMC molar ratio on the progress of the transesterification reaction. The nonlinear regression analysis of the experimental data, processed by POLYMATH 6.0, provided values for the rate constants. Corresponding activation energies were calculated from an Arrhenius relationship. An attempt has been made to Received: November 9, 2012 Revised: February 16, 2013 Published: February 18, 2013 1564
dx.doi.org/10.1021/ef400048v | Energy Fuels 2013, 27, 1564−1569
Energy & Fuels
Article
Figure 1. Transesterification of oil leading to (A) TBD-catalyzed DMC biofuel, (B) series reaction for FAGC conversion, and (C) side reaction of GDC. process. The sample preparation described here (including H2SO4, NaCl, and BHT) was used solely for analytical purposes and would not be used for the manufacturing of biofuel. The samples for GC were taken directly from the oil phase and passed through anhydrous sodium sulfate to remove trace amounts of water and diluted again with a toluene/BHT mixture. Each reaction was conducted twice at the same condition, and each sample was analyzed twice using GC− FID. The averaged values were used in the kinetic model. 2.4. Sample Analysis with GC−FID. A Varian CP-3800 equipped with a FID was used for analysis of kinetic results, including intermediate compounds. The GC column was a MXT-65TG (15 m × 0.25 mm inner diameter, with a 0.10 μm film thickness) manufactured by Restek, Inc. (Bellefonte, PA), specially tested for TG and stable to 370 °C. Helium was used as the carrier gas. The GC oven temperature was programmed at an initial temperature of 50 °C, held for 1 min, ramped to 180 °C at 15 °C min−1, ramped to 230 °C at 7 °C min−1, then ramped again to 370 °C at 15 °C min−1, and finally, held for 11.2 min. The sample injection volume was 1 μL, and the peak identification was made through comparing the retention times between the samples and standard compounds. A typical GC−FID trace is shown in Figure 2. To that end, samples were diluted appropriately to establish well-defined peaks throughout an experimental run. 2.5. Product Characterization. The percent yield of biofuel was calculated using the formula: yield (%) = (total actual weight of biofuel/total theoretical weight of biofuel) × 100, where biofuel includes both FAMEs and FAGCs. The concentration of TG (CTG) and FAME (CFAME) samples was determined by GC−FID as described. The FAME concentration obtained from GC−FID includes FAMEs formed in the first reaction (CFAME1) in Figure 1A and FAMEs formed in the second reaction (CFAME2) in Figure 1B. The synthesis techniques of FAGCs and GDC have been relatively little reported in the literature, and these two chemicals are not available to purchase from the market; therefore, the concentrations of FAGCs and GDC at any particular time were calculated from stoichiometric relations with the known substances, as shown in the following equations:
elucidate the transesterification reaction mechanism for the TBD-catalyzed process with DMC as an acyl acceptor.
2. MATERIALS AND METHODS 2.1. Chemicals. Refined canola oil (ACH Food Companies, Inc., Memphis, TN) was purchased from a local grocery store. Anhydrous DMC (≥99.0%), 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD, ≥98.0%), sulfuric acid (95.0−98.0%), sodium chloride (≥99.5%), anhydrous toluene (≥99.9%), butylated hydroxytoluene (BHT, ≥99.0%), and anhydrous sodium sulfate (≥99.0%) were purchased from Sigma− Aldrich. Canola oil and DMC were dried using sodium sulfate to remove trace amounts of water because water would lead to an unwanted side reaction (shown in Figure 1C). The standards for gas chromatography−flame ionization detector (GC−FID) analysis, including TG (triolein and tripalmitin) and FAMEs (methyl oleate and methyl palmitate), were also purchased from Sigma−Aldrich. Canola oil, although refined, had a tested acid value of 0.08, determined by KOH titration. A low free fatty acid (FFA) content (i.e.,
