3.1.1 Reaction Models and Mechanisms
Short-chained tertiary olefins can be reacted with short-chained alcohols to form a sub-class of tertiary ethers that have been conveniently designated as fuel ethers. These ethers have useful properties for inclusion in gasoline blends: a high specific energy, a high octane number, a relatively low volatility and oxygen to encourage cleaner burning (Piel and Thomas, 1990). The simplest fuel ether is methyl fer/-butyl ether, MTBE, which is derived from isobutene and methanol:
The reaction requires an acid catalyst to proceed usefully. Protonated ion exchange resins (e.g. Amberlyst 15™) are the most widely used (Jensen, 1995) although experiments have also been conducted with zeolites (e.g. HZSM-5) and sulphur-roasted zircon oxides (Collignon et al., 1997; Quiroga et al., 1997). The reaction is reversible and equilibrium limited in the industrially significant range of temperatures. The reaction details are well understood: the equilibrium constant has been determined as a function of temperature (e.g. Rehfinger and Hoffman, 1995; Zhang and Datta, 1996) and the reaction kinetics have been fitted to a Langmuir-Hinshelwood-Hougen-Watson (LHHW) model (Zhang and Datta, 1996).
The chemistry of ethyl ierf-butyl ether, ETBE, is inherently similar to MTBE. The analogous reaction is:
The reaction is more strongly limited by equilibrium so that the equilibrium conversion from a stoichiometric mixture of reactants at 70°C is only 84.7%. The reaction equilibrium constant (Jensen and Datta, 1995) is given by:
Jensen (1996) also developed a kinetic model for the reaction, utilising another LHHW model. The model pertains to activities (not concentrations) in order to account for the considerable liquid phase non-ideality and uses the UNIFAC method to estimate activity coefficients. The proposed reaction mechanism involves two adsorbed ethanol sites reacting with one adsorbed isobutene site in the rate-determining step, giving a total of three active sites. The rate equation derived from this model (equations 3.4-3.6) produced a better fit with the experimental data than other models. However, a simplified model (equations 3.7-3.8) is applicable for ethanol concentrations less than 4.0 mol%.
= 10.387 + 406059 _2.89055InT-0.0191544T T
EiOH aiBui \
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