Non-Ideal Mixtures

After the above discussion on RD of ideal ternary mixtures, in this section two nonideal ternary systems are considered. These are the heterogeneously catalyzed syntheses of the fuel ethers MTBE (methyl tert-butyl ether) and TAME (tert-amyl methyl ether) by etherification of methanol with isobutene or isoamlyenes respectively. Both reaction systems have enormous industrial importance because of the outstanding antiknock properties of MTBE and TAME as gasoline components. Synthesis of MTBE

The fuel ether MTBE is synthesized by the liquid-phase reaction of isobutene (IB) and methanol (MeOH) using macroreticular sulfonic acid ion-exchange resins as catalysts. The stoichiometric equation is given by

The microkinetics of this reaction were investigated in detail by Rehfrnger and Hoffmann [7]. These authors proposed the following rate expression in terms of liquid-phase activities: the derivation is described in Section 5.4.3

In (5.37), r stands for the volumetric reaction rate, cL represents the concentration of acid groups per unit volume of catalyst, and ai is the liquid-phase activity of component i. The temperature dependence of the reaction rate constant k can be expressed by the Arrhenius equation. All kinetic and thermodynamic parameters can be found elsewhere [7].

Fig. 5.16 shows residue curve maps for four selected values of the Damkohler number Da at an operating pressure of p = 0.8 MPa. The residue curve map for distillation without reaction (Da = 0, Fig. 5.16a) shows one saddle point, which is the binary azeotropic point between MeOH and MTBE. The second binary azeo-tropic mixture of MeOH and IB represents an unstable node. These two points are linked by a separatrix, which acts as a distillation boundary. Consequently, two stable nodes exist in the system: one stable node at pure MTBE for initial compositions below the distillation boundary and another stable node at pure MeOH fo mixtures having an initial composition above the distillation boundary.

By increasing the Damkohler number, the influence of the chemical reaction is fortified. For Da = 10~4 (Fig. 5.16b) the shape of trajectories starting with a relatively high mole fraction of MeOH is still similar to those of distillation without reaction, so that pure MeOH remains a stable node in the system. Near the MTBE vertex, the conditions change remarkably. The reaction vector is pointed towards the chemical equilibrium line (dashed curve). As a consequence, the stable node moves from

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