Abstract
A fundamental study on uncatalyzed esterification of various biomass-derived aliphatic carboxylic acids with stoichiometric amount of ethanol has been investigated in an isothermal batch reactor, with the objective to convert carboxylic acids to corresponding ethyl esters and to determine both the kinetic and thermodynamic parameters. The effects of temperature on the conversion of carboxylic acid, kinetic and thermodynamic parameters have been investigated. Temperature was found to have significant effect on the rate of reaction and conversion of carboxylic acid. A simple second order reversible kinetic model was developed to determine the kinetic and thermodynamic parameters. The thermodynamic and kinetic parameters varied for uncatalyzed esterification reaction of both short-chain and long-chain carboxylic acids considered. The predicted data from the kinetic model were correlated with experimental data and the two sets of data agreed reasonably well for the uncatalyzed esterification systems. It was observed that the Van't Hoff plot for uncatalyzed esterification of linoleic acid was non-linear curve, whereas for the Arrhenius and Eyring plots, they were linear. Additional experiments to assess the catalytic and corrosion effects of several metallic substances revealed Inconel 625 alloy, nickel wire and stainless steel materials were susceptible to corrosion problem with uncatalyzed esterification reaction at elevated reaction temperatures. However, tantalum and grade-5 titanium materials were corrosion resistance metals, suitable for similar reaction conditions and this can encourage the design of a flow reactor system. Although, uncatalyzed esterification of carboxylic acids at elevated reaction temperature is still at laboratory scale. It is our hope that the estimated kinetic and thermodynamic parameters would be the guiding tools for reactor scale-up, thus providing a new perspective into the conversion of biomass-derived carboxylic acids into value-added ester products under uncatalyzed esterification reaction system.
Highlights
In recent years, chemical processing industry is facing a regulatory challenge to replace hazardous petroleum-derived solvents such as chlorofluorocarbons, ethylene glycol ethers, methylene chloride, chloroform and Methyl Ethyl Ketone (MEK)with environmentally friendly solvents and renewable chemicals (Argonne National Laboratory, 1998; Rathin and Shih-Perng, 1998; Carole et al, 2004)
Chemicals and materials: Chemicals used for reaction were lactic acid (>98%, Fisher Scientific), levulinic acid (>98%, Acros Chemicals), acetic acid (>98%, Fisher Scientific), formic acid (>98%, Acros), linoleic acid (>99%, Acros), anhydrous ethanol (AAPER Chemicals), ethyl levulinate (>99%, Acros Chemicals), ethyl lactate (>97%, Acros Chemicals), ethyl acetate
Equilibrium constant: The temperature and molar ratio of ethanol to carboxylic acid are two variables affecting the conversion of carboxylic acid
Summary
With environmentally friendly solvents and renewable chemicals (Argonne National Laboratory, 1998; Rathin and Shih-Perng, 1998; Carole et al, 2004). In this regard, the increased demand for value-added solvents and renewable chemicals has prompted the chemical industry to initiate the esterification of biomass-derived carboxylic acids to synthesize corresponding non-toxic biodegradable esters, which have significant applications in various areas like pharmaceuticals, plasticizers, solvents, food flavors, coating and fragrance (Bart et al, 1994; Sanz et al, 2002; Ayoub, 2005; Delgado et al, 2007). The homogeneous acid catalyst can erode process equipment; miscibility of acid catalyst with the reaction medium requires expensive downstream separation operations; there are possible side reactions such as dehydration and etherification; and acid
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