2012
Ind. Eng. Chem. Res. 1997, 36, 2012-2018
A Comparative Thermodynamic and Kinetic Study of the Reaction between Olefins and Light Alcohols Leading to Branched Ethers. Reaction Calorimetry Study of the Formation of tert-Amyl Methyl Ether (TAME) and tert-Butyl Isopropyl Ether (IPTBE) Lluı´s Sola` and Miquel A. Perica` s* Servei de Calorimetria de Reaccio´ , Departament de Quı´mica Orga` nica, Facultat de Quı´mica, Universitat de Barcelona, Martı´ i Franque` s 1, 08028 Barcelona, Spain
Fidel Cunill† and J. Felipe Izquierdo Departament d’Enginyeria Quı´mica, Facultat de Quı´mica, Universitat de Barcelona, Martı´ i Franque` s 1, 08028 Barcelona, Spain
The liquid-phase additions of isopropyl alcohol to isobutene to give tert-butyl isopropyl ether (IPTBE) and of methanol to a mixture of 2-methyl-2-butene (2M2B) and 2-methyl-1-butene (2M1B) to give tert-amyl methyl ether (TAME) have been studied in a calorimetric reactor. The heat capacity of IPTBE and TAME and the enthalpy change associated with the IPTBE and TAME synthesis reactions have been determined. At 343 K, the standard molar reaction enthalpy for TAME formation from 2M1B is -34.9 ( 2.5 kJ‚mol-1 and from 2M2B is -27.1 ( 2.5 kJ‚mol-1. At 298 K, the standard molar enthalpy for IPTBE formation is -21.7 ( 1.6 kJ‚mol-1. A determination of the apparent activation energy of 70.3-78.4 kJ‚mol-1 for the IPTBE synthesis has been performed graphically from the plots of heat flow rate vs time. TAME heat capacity in the liquid phase has been found to follow the equation Cp(J‚mol-1‚K-1) ) 1059.56.1271(T/K) + 1.1093 × 10-2(T/K)2 and that of IPTBE the equation Cp(J‚mol-1‚K-1) ) 1181.56.7818(T/K) + 1.2186 × 10-2(T/K)2. Comparison of thermodynamic and kinetic data obtained for the IPTBE system with that previously reported for tert-butyl methyl ether (MTBE) and tert-butyl ethyl ether (ETBE) is also performed. Introduction The lowest tertiary aliphatic ether, methyl tert-butyl ether (MTBE), is the oxygenate preferred by refiners, and it is being extensively used in gasolines to reduce evaporative and tail-pipe emissions and as an octane number improver. However, its growing demand, its relatively high blending vapor pressure, and its dependence on methanol and isobutene have promoted the evaluation and use of other tertiary ethers. Nowadays, higher molecular weight ethers such as ethyl tert-butyl ether (ETBE) and tert-amyl methyl ether (TAME) are already being produced in refineries and blended in gasoline (Unzelman, 1995; Haigwood and Stepan, 1995). Other ethers, such as diisopropyl ether (DIPE) and tert-butyl isopropyl ether (IPTBE), are also being evaluated for these purposes. The main advantage of both DIPE and IPTBE lies in the fact that they are completely refinery-based oxygenates, the sole source of the oxygen atoms in their molecules being water. TAME is currently made by the liquid-phase reaction of the reactive isoamylenes (2-methyl-1-butene and 2-methyl-2-butene) with methanol under catalysis by a sulfonic acid resin, working under pressure (700-900 kPa) at temperatures in the range 313-373 K. Under standard operation conditions, isoamylene equilibrium conversion is about 70% (Rock, 1992). Up to now, few studies have been devoted to the thermodynamic properties of this reacting system (Rihko et al., 1994; Oost
and Hoffman, 1995; Piccoli and Lovisi, 1995; Serda` et al., 1995). All of them use equilibrium constants based on the activities of the compounds to calculate the standard enthalpy change of the reactions. IPTBE, in turn, can be prepared by adding isopropyl alcohol to isobutene over an acidic resin such as in the case of TAME. However, no detailed chemical and technical information on the reaction has been disclosed up to now. Bench-scale calorimetry represents a powerful tool to study chemical processes and has been extensively used in recent years (Regenass, 1985, 1987; Shatynski and Hamesian, 1993; Stoessel, 1995). Process monitoring, as well as the determination of the thermodynamic and kinetic parameters of a chemical reaction, can be easily performed without interfering with the process, and this represents a significant advantage to other methodologies (Regenass, 1983). We report in the present paper a thermodynamic study of the liquid-phase synthesis of TAME using a reaction calorimeter, as well as a thermodynamic and kinetic study for the liquid-phase synthesis of IPTBE. A comparison of the values obtained in this work with values reported in previous communications for the MTBE and ETBE systems is also performed. The heat capacities of TAME and IPTBE, determined by both reaction calorimetry and differential scanning calorimetry (DSC) techniques, are also reported Experimental Section
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[email protected]. † Telephone: 34-3-4021304. Fax: 34-3-4021291. E-mail:
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(i) Materials. Methanol and isopropyl alcohol (99.9%) were obtained from Romil Chemicals Ltd. (Shepshead, U.K.) and were dried and stored over molecular sieves © 1997 American Chemical Society
Ind. Eng. Chem. Res., Vol. 36, No. 6, 1997 2013
(Fluka, Buchs, Switzerland). Isobutene (99% pure) was supplied by Carburos Meta´licos (Barcelona, Spain) and used without further purification. 2-Methyl-2-butene (2M2B) (85%) and tert-amyl methyl ether (TAME) (>97%) were both obtained from Fluka (Buchs, Switzerland). 2-Methyl-2-butene was found to contain 14% of the also reacting isomer 2-metyl-1-butene (2M1B). Pure 2M1B and 2M2B were used for analytical calibrations. tert-Butylisopropyl ether (IPTBE) was prepared by reaction of isopropyl alcohol with isobutene and was purified by washing with water (to extract isopropyl alcohol) followed by distillation of isobutene traces and finally dried over MgSO4. IPTBE (>99.5%, GC) was obtained in this way. The catalyst used was the ion-exchange resin Lewatit K2631 from Bayer AG (Leverkusen, FGR). The commercial resin was ground and sieved, and the fraction with diameters between 0.063 and 0.16 mm was used. Prior to use, the catalyst was dried at 110 °C under vacuum (1 mmHg) for 3 h. The amount of water left is negligible, and the corresponding tert-butyl alcohol could not be detected in any experiment. (ii) Apparatus. A commercially available (Mettler RC1) computer-controlled reaction calorimeter was used throughout the study. The calorimeter and its working principle have been thoroughly described in previous communications (Sola` et al., 1994, 1995). The reaction vessel is a 1-L jacket-cooled stirred tank (Mettler MP10 reactor) suitable for work under pressure ( EtOH > IPOH. This ultimately reflects the variation in the nucleophilic character of the studied alcohols. Nomenclature A ) reactor area for heat transfer (m2) dB/dt ) rate of addition (kg/s) cj ) molar concentration of compound j (mol‚L-1) Cpdos ) specific heat capacity of the dosed material (J‚kg-1‚K-1) Cpr ) specific heat capacity of the reacting mixture (J‚kg-1‚K-1) dTr/dt ) reaction temperature variation rate (K‚s-1) Ea ) apparent activation energy (kJ‚mol-1) f ) general function GC ) gas chromatography I ) isobutene k ) rate coefficient (units depending on reaction order) k0 ) Arrhenius preexponential factor (units depending on reaction order) nI0 ) initial number of moles of isobutene
nIf ) final number of moles of isobutene nI ) number of moles of isobutene nj ) number of moles of compound j mr ) mass of reacting mixture (kg) Q ) total heat of reaction (kJ) qr ) heat flow rate (W) qacc ) accumulated heat flow rate (W) qcal ) calibration heat flow rate (W) qdos ) heat flow rate due to dispensing (W) qflow ) heat flow rate through the reactor jacket (W) qloss ) heat flow rate through the reactor’s lid (W) r ) intensive reaction rate (mol‚(L‚s)-1) R ) ideal gas constant (J/(mol‚K)) t ) time (s) T ) temperature (K) Tj ) jacket reactor temperature (K) Tr ) reacting mixture temperature (K) U ) overall heat-transfer coefficient (W‚m-2‚°C-1) V ) reaction mass volume (I) V0 ) initial reaction mass volume (I) Vf ) final reaction mass volume (I) Xj ) conversion of reactant j (dimensionless) ∆H° ) standard enthalpy change of reaction (kJ‚mol-1) w/w ) percentage in weight Subscripts a ) apparent e ) equilibrium I ) isobutene i ) initial f ) final MTBE ) tert-butyl methyl ether ETBE ) tert-butyl ethyl ether IPTBE ) tert-butyl isopropyl ether TAME ) tert-amyl metyl ether 0 ) initial
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Received for review December 3, 1996 Revised manuscript received March 3, 1997 Accepted March 4, 1997X IE960753N
X Abstract published in Advance ACS Abstracts, April 15, 1997.
