Ind. Eng. Chem. Res. 2009, 48, 4757–4767
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Hydrolysis of Triglycerides Using Solid Acid Catalysts Kanokwan Ngaosuwan,†,‡ Edgar Lotero,† Kaewta Suwannakarn,† James G. Goodwin, Jr.,*,† and Piyasan Praserthdam‡ Department of Chemical and Biomolecular Engineering, Clemson UniVersity, Clemson, South Carolina 29634, and Center of Excellence on Catalysis and Catalytic Reaction Engineering, Department of Chemical Engineering, Faculty of Engineering, Chulalongkorn UniVersity, Bangkok 10330, Thailand
The commercial production of free fatty acids (FFAs) is carried out through the noncatalytic hydrolysis of triglycerides (TGs) using great amounts of superheated steam in large reactors made of expensive corrosionresistant materials, making the process energy intensive and costly. In this study, the feasibility of a continuous reaction system using a solid acid for the hydrolysis of TGs at atmospheric pressure has been investigated. This research was to explore the viability of heterogeneous catalyzed hydrolysis of oils and fats for the synthesis of FFAs, a platform reaction of the oleochemical industry and a possible reaction in a novel two-step (hydrolysis-esterification) biodiesel synthesis process using low-cost feedstocks containing >5-15% FFAs. Tricaprylin (TCp) was used as a model compound representing TGs in order to obtain reliable intrinsic kinetics. Using tungstated zirconia (WZ) and the solid acid composite SAC-13 (Nafion resin nanoparticles supported on mesoporous silica) as catalysts, the hydrolysis of TCp was carried out at 110-150 °C in a semibatch reactor with continuous addition of water at low flow rates capable of achieving 100% selectivity of the carboxylic acid side chains on the triglyceride to HCp. The characteristics of the catalysts played an important role in reaction selectivity, apparent activation energy, and deactivation. Catalyst recycling experiments showed continuous activity loss for both catalysts. Characterization of the used catalysts indicated that deactivation was likely caused by the strong adsorption of bulky reaction intermediates on the catalytic acid sites, blocking reactant accessibility. For WZ, recalcination in air was an effective regeneration method resulting in the recovery of 100% of its original activity. Regeneration by calcination was not possible for SAC-13 due to its temperature sensitivity. Methanol washing of used WZ and SAC-13 catalysts only partially regenerated catalyst activity. 1. Introduction Free fatty acids (FFAs) are major components and precursors for a great variety of products such as soaps, detergents, fatty alcohols, cosmetics, pharmaceuticals, and food.1 Lately, in mixtures with triglycerides (fats and oils), they have also become a low-cost raw material for biodiesel synthesis to make biodiesel more cost-competitive with petroleum diesel. At moderate temperatures, triglycerides (TGs) (main components of vegetable oils and fats) can be hydrolyzed with water/steam to produce 3 mol of FFAs and 1 mol of glycerol by the following consecutive reactions: TCp + H2O a DCp + HCp
(I)
DCp + H2O a MCp + HCp
(II)
MCp + H2O a GL + HCp
(III)
where TCp ) tricaprylin, DCp ) dicaprylin, MCp ) monocaprylin, GL ) glycerol, and HCp ) caprylic acid. As oils and water are immiscible, the hydrolysis of oils can be hindered by solubility issues. According to the literature, however, conducting the reaction at high temperatures increases the solubility of water in the oil phase, minimizing this limitation.2 The use of high temperature not only affects the solubility of water in the oil but also improves the kinetics of the process. Currently, fatty acids are obtained commercially from the reaction of vegetable oils and/or animal fats with superheated * Corresponding author. Phone: (864) 656-6614. Fax: (864) 6560784. E-mail:
[email protected]. † Clemson University. ‡ Chulalongkorn University.
steam. Conventionally, the reaction is carried out at 100-260 °C and 100-7000 kPa using a 0.4-1.5 wt % water-to-oil ratio.3 Different variations of this technology have been used by industry (e.g., the Twitchell process, the Colgate-Emery synthesis, and the Eisenlohr process). These processes are normally referred to as fat splitting.4-6 One of the earliest industrial processes used for the preparation of FFAs by TG hydrolysis was the Twitchell process. In it, the Twitchell reagent was used to catalyze the hydrolysis of vegetable oils at the boiling point of water at atmospheric pressure using open steam for 36-48 h. Because of the time required, the high amount of steam, and the use of strong homogeneous acids, the Twitchell process is no longer of commercial importance.7 The batch autoclave process is the oldest commercial method currently used for fat splitting. This technology produces lightcolored free fatty acid mixtures and takes only about 6-10 h for the whole process. The batch autoclave process is faster than the Twitchell process due to operation at high pressure (1135 kPa), the whole process taking only about 6-10 h. Soap formation during reaction (due to the presence of the metal species added), high consumption of steam, and the requirement of a distillation unit to further clean the FFAs are among the main drawbacks of this process.8 The continuous countercurrent, high-pressure fat-splitting process, more popularly known as the Colgate-Emery process, is the method most currently used for TG hydrolysis. The high temperature and pressure used permit short reaction times. Even though the noncatalytic technology used in this process for TG hydrolysis is effective, it requires a high capital investment and high operating cost associated with the great amount of steam
10.1021/ie8013988 CCC: $40.75 2009 American Chemical Society Published on Web 04/27/2009
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Ind. Eng. Chem. Res., Vol. 48, No. 10, 2009
needed for optimum processing.9 Another important factor that adds to the operational cost is that polyunsaturated fats can experience substantial degradation and must be extensively purified by distillation for most uses. The oil-water ratio is an important factor for this reaction. For instance, King et al.4 obtained high TG conversions with reaction times shorter than 10 min by conducting the hydrolysis of soybean oil in subcritical water at 270-340 °C and 13 000 kPa with a low molar oil-to-water ratio (1:5). Pinto and Lancas10 reported the effect of using subcritical water on the reaction rate of corn oil hydrolysis using an oil-to-water mass ratio of 85:15. No oil conversion was found at 150 and 200 °C, but at 250 and 280 °C conversions up to 80% and 100% were achieved, respectively. Recently, supercritical carbon dioxide (SC-CO2) has been used as a medium for the hydrolysis of triolein in order to improve the solubility of water in the nonpolar TG-rich phase.11 A 100% triolein conversion was obtained within 3 h at 250 °C and 8000 kPa. However, as already mentioned, operation at high temperatures and high pressures, as required for supercritical and near critical water processing, involves high capital and operating costs, which undermines this technology. The study reported here focused on the applicability of solid acid catalysts for the hydrolysis of oils and fats as a means to ultimately lower the capital and operating costs for TG hydrolysis by conducting the reaction under moderately low reaction conditions (110-150 °C and atmospheric pressure). Such a process could be applied in a novel two-step (hydrolysis-esterification) biodiesel synthesis process using low-cost feedstocks containing >5-15% FFAs. Since the esterification of FFAs is faster than transesterification of triglycerides on acid catalysts, there could be a possibility to construct a more efficient biodiesel synthesis process around the use of two-step hydrolysis-esterification on solid acids rather than what is now done with three-step pre-esterification (homogeneous acid catalyzed)sseparation (removal of acid and water)stransesterification (homogeneous base catalyzed).12 The research involved an investigation of the kinetics of solid acid catalyzed hydrolysis of tricaprylin (TCp) at atmospheric pressure. TCp was used as a model compound for larger TGs and for mixtures (as are typical in fats and oils) in order to facilitate the kinetic study. The catalysts chosen for this study were SAC-13 and tungstated zirconia (WZ). SAC-13 is a Nafion/silica nanocomposite catalyst, containing only Brønsted acid sites with an acid strength similar to concentrated H2SO4, as estimated by Hammett H0 values (-H0 ∼ 12).13 As a result, SAC-13 should catalyze TG hydrolysis, on a site basis, as effectively as H2SO4.14,15 The other catalyst chosen for this study, WZ, is a strong inorganic solid acid catalyst, which has been used successfully for a wide range of acid-catalyzed reactions, such as dehydration, esterification, hydrocarbon isomerization, and cracking.16-19 A threephase reaction system configuration operating at atmospheric pressure was used, with water being continuously pumped at a low flow rate into a slurry of the TG and solid catalyst at >100 °C. This three-phase configuration limited the amount of water in the reactor at any time. This was done as it is known that water has a deleterious poisoning effect on Brønsted acid catalysts.14,20 Moreover, a well-stirred semibatch reactor was used to measure initial reaction kinetics and longer term conversion data since use of a fixed bed reactor for three-phase reaction studies is more problematic for measuring accurate kinetics. The well-stirred semibatch reactor functions more or less like a CSTR during the initial conversion (200 mesh (
