Kinetics Of Esterification Of Lactic Acid With Ethanol Catalyzed By Cation-Exchange Resins

Reactive & Functional Polymers 61 (2004) 101–114

REACTIVE & FUNCTIONAL POLYMERS
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Kinetics of esterification of lactic acid with ethanol catalyzed by cation-exchange resins
Yang Zhang, Li Ma, Jichu Yang *
Department of Chemical Engineering, Tsinghua University, Beijing 100084, People’s Republic of China Received 17 March 2004; accepted 26 April 2004 Availableonline 25 June 2004

Abstract The esterification of lactic acid with ethanol was carried out in the presence of five different cation-exchange resins. The effect of catalyst type, catalyst loading, and temperature on reaction kinetics was evaluated. In order to study which components had the strongest adsorption strength on the resin surface, two simplified mechanisms based on Langmuir– Hinselwoodmodel were compared by correlating the experimental data. FTIR method was used to verify the rationality of the mechanism. Nonideality of the liquid phase was taken into account by using activities, which were predicted by UNIFAC method instead of concentrations. The thermal stability and mechanical strength of the resin catalysts were tested by SEM. Ó 2004 Elsevier B.V. All rights reserved.Keywords: Lactic acid; Esterification; Ethanol; Kinetics; Cation-exchange resin

1. Introduction Ethyl lactate is an important organic ester, which is biodegradable and can be used as food additive, perfumery, flavor chemicals and solvent, which can dissolve acetic acid cellulose and many resins [1]. Furthermore, the esterification of lactic acid with ethanol is a step in the purification of lactic acid byreactive distillation [2,3]. The conventional way to produce ethyl lactate is the esterification of lactic acid with ethanol
Corresponding author. Tel.: +86-10-62-788568/785514; fax: +86-10-62-770304. E-mail address: yjc-dce@mail.tsinghua.edu.cn (J. Yang).
*

catalyzed by sulphuric acid. Since this kind of homogeneous catalyst may cause a lot of problems, many heterogeneous solid catalystswere used in the reaction such as ion-exchange resin, clays and clay supported heteropoly acids [4]. Among these catalysts, cation-exchange resin is a perfect substitute which has many advantages such as: (a) corrosion problems could be avoided and it is easier to dispose of the waste liquor from the reaction mixture; (b) continuous operations in columns are possible; (c) the catalyst can be easilyremoved from the reaction products by decantation or filtration; (d) the purity of the products is higher since side reactions can be eliminated or are less significant [5,6].

1381-5148/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.reactfunctpolym.2004.04.003

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Y. Zhang et al. / Reactive & Functional Polymers 61 (2004) 101–114

Nomenclature ai ci DA De EADH Keq k k0 ki0 ki M MRD N nLA;0 R r r0 S SRS activity of i (mol/l) concentration of i at the surface of the catalyst (mol/l) molecular diffusion coefficient effective diffusion coefficient apparent activation energy (kJ molÀ1 ) enthalpy change (kJ molÀ1 ) reaction equilibrium constant reaction rate constant (mol gÀ1 minÀ1 ) pre-exponential factor (mol gÀ1 minÀ1 ) adsorption coefficient at initialtemperature adsorption coefficient molar mass of the solvent mean relative deviation number of data points initial molar amount of lactic acid (mol) gas constant (J molÀ1 KÀ1 ) reaction rate (mol gÀ1 minÀ1 ) radius of catalyst particle vacant site on catalyst surface sum of residual squares T t W X VA temperature (K) time (min) dry catalyst weight (g) conversion molar volume at boiling point

Greeksymbols c activity coefficient e porosity s tortuosity / thiele modulus U0 association factor l viscosity (Pa s) Subscripts calc calculated values exp experimental values LA lactic acid E ethanol EL ethyl lactate W water

The industrial production of esters by esterification of acid and alcohol was first carried out in a continuous stirred tank reactor (CSTR) and later in a catalytic distillation column...