Reactive Ideal Binary Mixtures

the role of reaction kinetics is analyzed considering RD processes for the simple reversible reaction Aj o Aj in an ideal binary mixture. The educt Aj is assumed to be the reaction component with the higher boiling point, so the product A2 is obtained in the distillate. The reaction can be carried out in an RD column sequence with an external recycling loop (Fig. 5.1), a non-RD column on top of a reactive reboiler (Fig. 5.2), or a full RD column (Fig. 5.3). More possible configurations are analyzed elsewhere [1]. 

Distillation column with reactive total reboiler 

In the very early stage of conceptual process design, very little is known about the reaction kinetics, so a macrokinetic power-law expression in terms of liquid-phase mole fractions should be used 

For the sake of a simplified nomenclature, the component indices are dropped by setting Xi = x and x = 1 - x. In addition, a dimensionless reaction rate r is introduced 

As a special case of (5.2), the rate of a first-order reaction (n = 1) is given by 

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Binary copolymerization resembles distillation of a bicomponent liquid mixture, with a reactivity ratio corresponding to the ratio of vapor pressures of the pure components in the latter case. The vapor-liquid composition curves of ideal binary mixtures have no inflection points and neither do the polymer-composition curves for random copolymerizations, in which/ r2 — 1 (Fig. 7-1). For this reason, such comonomer systems are sometimes called ideal. 

As has been outlined in the first sections of this chapter, RD processes can efficiently replace reactor-separator flow-sheets by internalization of external recycling loops. This is demonstrated for a simple isomerization reaction in an ideal binary mixture. It is clearly shown that hybrid columns combining non-reactive and reactive sections overcome the restrictions of fully RD columns. The most simple and effective solution for isomerization reactions is a reactive total reboUer with a non-reactive column on top. 

Sundmacher and Qi (Chapter 5) discuss the role of chemical reaction kinetics on steady-state process behavior. First, they illustrate the importance of reaction kinetics for RD design considering ideal binary reactive mixtures. Then the feasible products of kinetically controlled catalytic distillation processes are analyzed based on residue curve maps. Ideal ternary as well as non-ideal systems are investigated including recent results on reaction systems that exhibit liquid-phase splitting. Recent results on the role of interfadal mass-transfer resistances on the attainable top and bottom products of RD processes are discussed. The third section of this contribution is dedicated to the determination and analysis of chemical reaction rates obtained with heterogeneous catalysts used in RD processes. The use of activity-based rate expressions is recommended for adequate and consistent description of reaction microkinetics. Since particles on the millimeter scale are used as catalysts, internal mass-transport resistances can play an important role in catalytic distillation processes. This is illustrated using the syntheses of the fuel ethers MTBE, TAME, and ETBE as important industrial examples. 

Note 2 In the special case of an ideal binary copolymerization in which r 2 = Z2 = 1, the two monomers show equal reactivities toward both types of propagating species. Thus, k = k2 and ky2 = k22- Hence, copolymerization of a mixture of two monomers with any ratio of monomer concentrations in the monomer feed gives rise to a copolymer in which the molar ratio of monomer units is identical to that in the monomer feed. (See also azeotropic copolymerization.)... 

Reactive absorption processes occur mostly in aqueous systems, with both molecular and electrolyte species. These systems demonstrate substantially non-ideal behavior. The electrolyte components represent reaction products of absorbed gases or dissociation products of dissolved salts. There are two basic models applied for the description of electrolyte-containing mixtures, namely the Electrolyte NRTL model and the Pitzer model. The Electrolyte NRTL model [37-39] is able to estimate the activity coefficients for both ionic and molecular species in aqueous and mixed solvent electrolyte systems based on the binary pair parameters. The model reduces to the well-known NRTL model when electrolyte concentrations in the liquid phase approach zero [40]. 

The parameter a in Eq. 6 is the ratio of the effective binary coefficient of ordinary diffusion for the reactive species and the mixture of other void gases to the coefficient of ordinary self diffusion for the reactive species. As shown in the Appendix, this parameter depends only on the composition of the gas mixture and is independent of both the pressure and temperature for ideal gases. Thus for fixed gas composition, the parameter a is constant. 

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