REACTION KINETICS OF THE ESTERIFICATION OF ETHANOL ...

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Intelligent Column Internals for Reactive Separations Page 2 of 5 Deliverable 22 Issued: 24.08.01 1 SUMMARY AND CONCLUSIONS 2 2 INTRODUCTION 2 3 EXPERIMENTAL 3 4 RESULTS AND DISCUSSION 3 5 LIST <strong>OF</strong> SYMBOLS 5 6 SELECTION <strong>OF</strong> PHYSICAL PROPERTIES 5 1 Summary and conclusions The kinetic parameters have been determined for INTINT reaction system 3: ethanol/acetic acid/ethyl acetate. The reactions considered are the esterification of ethanol (CH3CH2OH, EtOH) and acetic acid (CH3COOH, AcOH) towards ethyl acetate (CH3COOCH2CH3, EtOAc) and water, and the reverse reaction, ester hydrolysis. The catalyst used is the cation exchange resin Purolite CT179. 2 Introduction The work described in this report is a part of the European project “Intelligent Column Internals for Reactive Separations” (INTINT). The report contains the kinetics of reaction system 3. Originally, reaction system 3 was the aldol condensation of acetone towards diacetone alcohol. However, since no stable catalyst for this reaction was available 1 , this process was abandoned. The reaction studied instead is the esterification of ethanol (CH3CH2OH, EtOH) and acetic acid (CH3COOH, AcOH), resulting in the products ethyl acetate (CH3COOCH2CH3, EtOAc) and water, see equation 1: O OH + HO H + H + AcOH EtOH EtOAc This reaction is reversible, and the equilibrium composition is a weak function of temperature. The reaction is acid catalysed, and as for most esterifications usually strong mineral acids (hydrochloric acid, sulfuric acid) are used 2,3 . Cation exchange resins (typically containing sulfonic acid groups, e.g. Amberlyst 15, Purolite CT179) are also widely used, because of their high activity and selectivity (possible side reactions are alcohol dehydration and etherification), and easy separation 2 . Catalyst deactivation appears to be negligible. In practice the equilibrium is often forced towards the 1 INTINT deliverable 21, February 2001 2 Ullmann’s Encyclopedia of Industrial Chemistry, vol. A9 3 Chemical Economics Handbook CD-ROM O O + H 2 O (1)
Intelligent Column Internals for Reactive Separations Page 3 of 5 Deliverable 22 Issued: 24.08.01 ester by azeotropic water removal 2 . In the present investigation the cation exchange resin Purolite CT179 will be used. The intrinsic kinetics of the reaction system are more or less known from literature: the forward reaction rate (ester formation, R1) is a function of EtOH and AcOH concentrations, and the reverse reaction rate (ester hydrolysis, R2) is a function of EtOAc and water. See the kinetic equations below: m EtOH AcOH x x k R1 1 R2 = k2 xEtOAc xH 2O (3) = (2) The equilibrium constant Keq is given by equation 4: K eq k x x 2 = = (4) k x 1 EtOAc H 2O m EtOH x AcOH All concentrations are given as mole fractions. Both k1 and k2 are functions of temperature, according to the Arrhenius equation: k −E A / RT i = ki, oe (5) For porous particles with typical particle diameter 1 mm, intra-particle diffusion and mass transfer from the liquid bulk to the particles are expected to be very important. The kinetics determined in this study therefore only apply to Purolite CT179 with the same particle size distribution. The objective of this study is to find kinetic parameters for the reactions described above (formation and hydrolysis of ethyl acetate). 3 Experimental The catalyst used is the ion exchange resin Purolite CT179, purchased from Purolite. Purolite CT179 is a porous cation exchange resin, containing sulfonic acid groups. The particle diameter was 0.6-1.4 mm. The reagents ethanol (>99.9%), acetic acid (>99%) and ethyl acetate (>99%) were purchased from J.T. Baker, and used as received. Demineralised water was used. Apart from the reactants and products (AcOH, EtOH, EtOAc and water) no other solvents were used. The experimental set-up consisted of a thermostated jacketed glass tube, inner diameter 20 mm. Typically 12 g of catalyst was used, and a constant flow rate of 0.1-25 ml/min was applied. The reactor was kept at 55-65 °C. The effluent was analysed off-line by GC over a CP-wax 58CB column, using a flame ionisation detector. The samples were diluted with acetone prior to injection, and methyl iso-butyl ketone was used as internal standard. 4 Results and discussion The equilibrium constant Keq was determined by leading 0.1 ml/min of a feed solution containing ethanol and acetic acid (in several ratios) through the reactor, containing 12.3 g of Purolite CT179. It was verified that equilibrium was reached by comparison with effluent concentrations reached at higher flow rates: no difference was observed between 0.1 and 0.5 ml/min. The best fit of the re-
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