Wei-Cheng Wang Department of Mechanical and Aerospace Engineering, North Carolina State University, Raleigh, NC 27695 William L. Roberts Department of Mechanical and Aerospace Engineering, North Carolina State University, Raleigh, NC 27695; Clean Combustion Research Center, King Abdullah University of Science and Technology, Thuwal, Saudi Arabia Larry F. Stikeleather Department of Biological and Agricultural Engineering, North Carolina State University, Raleigh, NC, 27695 Hydrocarbon Fuels From Gas Phase Decarboxylation of Hydrolyzed Free Fatty Acid Gas phase decarboxylation of hydrolyzed free fatty acid (FFA) from canola oil has been investigated in two fix-bed reactors by changing reaction parameters such as tempera- tures, FFA feed rates, and H 2 -to-FFA molar ratios. FFA, which contains mostly C 18 as well as a few C 16 ,C 20 ,C 22 , and C 24 FFA, was fed into the boiling zone, evaporated, car- ried by hydrogen flow at the rate of 0.5–20 ml/min, and reacted with the 5% Pd/C catalyst in the reactor. Reactions were conducted atmospherically at 380–450  C and the prod- ucts, qualified and quantified through gas chromatography-flame ionization detector (GC-FID), showed mostly n-heptadecane and a few portion of n-C 15 , n-C 19 , n-C 21 , n-C 23 as well as some cracking species. Results showed that FFA conversion increased with increasing reaction temperatures but decreased with increasing FFA feed rates and H 2 - to-FFA molar ratios. The reaction rates were found to decrease with higher temperature and increase with higher H 2 flow rates. Highly selective heptadecane was achieved by applying higher temperatures and higher H 2 -to-FFA molar ratios. From the results, as catalyst loading and FFA feed rate were fixed, an optimal reaction temperature of 415  C as well as H 2 -to-FFA molar ratio of 4.16 were presented. These results provided good basis for studying the kinetics of decarboxylation process. [DOI: 10.1115/1.4006867] Keywords: Gas phase decarboxylation, free fatty acid, Pd/C, biofuel 1 Introduction Biofuel is a renewable fuel chemically equivalent to current pe- troleum based fuels, and it can be produced from plant biomass or lipids which contains mostly triglycerides, such as vegetable oils or animal fats [1–4]. The term biofuel mainly comes from mono- alkyl esters of long chain fatty acids. Several methods have been developed to transform free fatty acid (FFA) to the hydrocarbon fractions suitable for fuel blending. Deoxygenation, which breaks the hydrocarbon chains and removes oxygen, has been reported recently [5]. The decarboxylation of FFA is one pathway for deoxygenation that results in the removal of the carboxyl group and has been studied extensively in liquid phase [6–14]. Decar- boxylation of FFA derived from various crude lipids has been applied as the second stage of the patent process [15,16], which converts hydrolyzed FFA into hydrocarbon fuels. Liquid phase deoxygenation was first conducted with stearic acid over the carbon-supported metal catalyst [7]. The reaction temperatures were 300–360  C along with the pressure of 17–40 bars in order to maintain the reactant as liquid phase. N-Heptadecane, the main liquid phase product, was produced with high selectivity. In addition, n-pentadecane was also formed by this reaction [8]. In this process, carbon dioxide, carbon monox- ide, methane, and propane were observed in the exhaust gas [8]. Catalyst plays an important role in the deoxygenation reaction. After trying various metals, supports and loadings of the catalyst, 5 wt. % Pd/C has been chosen as the preferred catalyst for this reaction [9]. The analysis of exhaust gas, CO 2 and CO, indicates two pathways of deoxygenation, decarboxylation, and decarbonyla- tion [10]. These two pathways were determined by the hydrogen partial pressure of the carrier gas and affected the reaction rates as well as catalyst turn-over frequencies (TOFs) [11]. Moreover, un- saturated FFAs such as oleic acid and linoleic acid, were converted into saturated diesel fuel range hydrocarbons via decarboxylation reactions under similar conditions and catalyst [12]. Temperature strongly enhances or suppresses the reaction. Increasing the temperature leads to the reduced probability of the colliding molecules capturing one another and causes a decrease of the activation energy [8]. According to Lestari et al. [8], the time required for completing FFA conversion decreased with increasing reaction temperature. Higher temperature results in an improved unsaturated FFA conversion and selectivity for n-heptadecane [12]. Sna ˚re et al. shows that complete conversion of palmitic acid was achieved at 300  C in 110 min, whereas at 260  C only 50% was converted [12]. Hydrogen and helium were used as the carrier gases in the de- carboxylation process [5]. Hydrogen is required in activating Pd and helps organic species desorb from metal surface [5]. Also, hydrogen helps to saturate the double bonds on the unsaturated FFAs [10]. It appeared that over Pd/C in the presence of hydrogen, the conversion of unsaturated FFA increased compared with the reactions with inert gas [12]. After testing different H 2 /He ratios, Ma ¨ki-Arvela et al. [9] confirmed the fact that hydrogen not only benefits the conversion of FFAs after the prolonged reaction time but retards the catalyst deactivation from coke formation. How- ever, the increasing of H 2 partial pressure decreases the rate of de- carboxylation and ends up with lower CO 2 selectivity [11]. For liquid phase decarboxylation, high H 2 partial pressure moved the reaction pathway toward decarbonylation instead of decarboxyl- ation and the concentration of CO in the exhaust gas was increased [5]. The catalyst was poisoned by CO and the decarbox- ylation pathway was strongly inhibited. Evidently, low partial pressure of hydrogen gives better TOFs and maintains the catalyst activity [7]. Five percent H 2 in carrier gas was found to give the best results among all the investigations [7,11]. On the other hand, decarboxylation was also performed without hydrogen via using hydrotalcites with MgO contents [13]. Although 98% conversion of FFA(s) was achieved at 400  C, the selectivity of heptadecane is not high due to the accompanying of cracking. Besides, different feedstocks, the use of solvent as well as the catalyst loading affect decarboxylation. Pure fatty acids have con- sistent rates of decarboxylation, whereas significant catalyst poi- soning occurred with FFAs with impurities, such as phosphorus content [17]. Heptadecane as solvent, due the lower vapor Contributed by the Petroleum Division of ASME for publication in the JOURNAL OF ENERGY RESOURCES TECHNOLOGY. Manuscript received October 15, 2011; final manuscript received May 8, 2012; published online June 21, 2012. Assoc. Editor: Andrew K. Wojtanowicz. Journal of Energy Resources Technology SEPTEMBER 2012, Vol. 134 / 032203-1 Copyright V C 2012 by ASME Downloaded 28 Aug 2012 to 152.1.237.89. Redistribution subject to ASME license or copyright; see http://www.asme.org/terms/Terms_Use.cfm