Systems with Liquid-Phase Splitting

Little research has been done on reacting systems with simultaneous distillation and liquid-phase splitting. Ung and Doherty [13] presented a theory of phase equilibrium in multiple reaction systems, which also covers heterogeneous liquid sys- 

For this reaction system, the liquid-phase non-ideality can be described by the UNIFAC activity coefficient model [20]. Panneman and Beenackers [21] studied the reaction kinetics of (5.42) catalyzed by macroporous strongly acidic ion-ex-change resins. Based on their results, Qi et al. [19] proposed an activity-based reaction rate expression for this reaction 

The pseudohomogeneous chemical equilibrium (PCE) for the cyclohexanol reaction system is illustrated in Fig. 5.21a and the non-reactive isobaric L-L phase diagram is plotted in Fig. 5.21b. The rafSnate phase is very dose to the pure water vertex (see enlarged view in the right block). The two L-L envelopes intersect the PCE at two points x = (0.3466, 0.1840) and x = (0.0003, 0.9969). The two parts of the PCE outside the L-L region and the so called unique reactive liquid-liquid tie line [19] comprise the heterogeneous chemical equilibrium line (HCE), which is the bold line in Fig. 5.21b. 

For the heterogeneous liquid system, the reaction was assumed to take place in both liquid phases. When we compare the RCMs at the same Da for the pseudohomogeneous and heterogeneous systems, again we can find that the properties (including the reactive azeotrope) of the RCMs outside the L-L region are identical. But they are distinct inside the L-L region because of their different chemical equilibrium curves (compare Fig. 5.21). 

Recently, Steyer et al. [22] presented an extended analysis of cyclohexanol synthesis and splitting, supposing that the chemical reaction will proceed in the two liquid phases at different rates, that is with different Damkbhler numbers. Based on their results, these authors proposed a flow-sheet of two coupled RD columns for the separation of cyclohexene/cyclohexane mixtures. 

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MESH) equations which are solved for the whole column, decanter included and taking into account the liquid-liquid phase split. Numerical treatment of the Differential Algebraic Equation (DAE) system and discrete events handling is performed with DISCo, a numerical package for hybrid systems with a DAE solver based on Gear s method. The column technical features and operating conditions are shown in Table 4. A sequence of two operational batch steps, namely... 

If the liquid mixture is extremely non-ideal, liquid phase splitting will occur. Here, we first consider the hypothetical ternary system. The physical properties are adopted from Ung and Doherty [17] and Qi et al. [10]. The catalyst is assumed to have equal activity in the two liquid phases. The corresponding PSPS is depicted in Fig. 4.5, together with the liquid-liquid envelope and the chemical equilibrium surface. The PSPS passes through the vertices of pure A, B, C, and the stoichiometric pole Jt. The shape of the PSPS is affected significantly by the liquid phase non-idealities. As a result, there are three binary nonreactive azeotropes located on... 

The question of whether it is possible to predict the concentration dependence of from information on Ky via equation (4.22) is discussed here using the esterification of 1-butanol with acetic acid at 80 °C as an example. The experimental data given in Fig. 4.9 show that is a strong function of the composition for that liquid-phase reaction. The numbers for are in the range 2-8 and systematically vary with composition. In the studied system, there is a liquid-liquid miscibility gap intersecting with the chemical equilibrium surface. No measurements with liquid-liquid phase split were carried out in the present work. 

As shown above, reaction kinetics have a significant influence on RD process performance in binary mixtures and the same is true for multicomponent mixtures. In the following, the attainable products of kinetically controlled RD processes are analyzed, first for ideal ternary mixtures, then for non-ideal ternary mixtures occurring in industrially important fuel ether synthesis, and finally for an extremely non-ideal system with potential liquid-phase splitting. In all cases, reversible reactions of type A + B o C are considered. 

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. 

Figure 3. The pressure composition sections of the binary system carbon dioxide/decane at (a) 260.0 K, with liquid-liquid phase split and (b) 319.3 K. Both figures have been obtained from the PR equation of state. Open ircles are experiments [22].

Adrian et al. (2000) have reported a novel high-pressure liquid-liquid extraction process with reference to processing in biotechnology the example of cardiac glycosides (digitoxin and digoxin) is cited. A completely miscible, binary system of water and a hydrophobic organic solvent like ethanol can split into two liquid phases when a near-critical gas (e.g. CO2) is added. The near-critical C02/water/l-propanol system is reported, for which possibilities for industrial exploitation exist. 

In a binary system more than two fluid phases are possible. For instance a mixture of pentanol and water can split into two liquid phases with a different composition a water-rich liquid phase and a pentanol-rich liquid-phase. If these two liquid phases are in equilibrium with a vapour phase we have a three-phase equilibrium. The existence of two pure solid phases is an often occuring case, but it is also possible that solid solutions or mixed crystals are formed and that solids exists in more than one crystal structure. 

An application has been found in which a system that exhibits an upper, or lower, critical consolute point, UCST or LCST, respectively, is utilized. At a temperature above or below this point, the system is one homogeneous liquid phase and below or above it, at suitable compositions, it splits into two immiscible liquids, between which a solute may distribute. Such a system is, for instance, the propylene carbonate - water one at 25°C the aqueous phase contains a mole fraction of 0.036 propylene carbonate and the organic phase a mole fraction of 0.34 of water. The UCST of the system is 73 °C (Murata, Yokoyama and Ikeda 1972), and above this temperature the system coalesces into a single liquid. Temperature cycling can be used in order to affect the distribution of the solutes e.g. alkaline earth metal salts or transition metal chelates with 2-thenoyl trifluoroacetone (Murata, Yokayama and Ikeda 1972). 

The physical picture that underlies this behavior, as pointed out first by Elgin and Weinstock (1), is the salting out effect by a supercritical fluid on an aqueous solution of an organic compound. As pressure is increased, the tendency of the supercritical fluid to solubilize in the organic liquid results in a phase split in the aqueous phase at a lower critical solution pressure (which varies with temperature). As pressure is further increased, the second liquid phase and the supercritical phase become more and more similar to each other and merge at an upper critical solution pressure. Above this pressure only two phases can coexist at equilibrium. This pattern of behavior was also observed by Elgin and Weinstock for the system ethylene - acetone - water at 288 K. In addition, the same type of... 

The behavior of the Fick diffusion coefficient in nonideal systems may be complicated, while the Maxwell-Stefan diffusion coefficients behave quite well, and are always positive for binary systems. In nonideal binary systems, the Fick diffusivity varies with concentration. As seen in Figure 6.1, water-acetone and water-ethanol systems exhibit a minimum diffusivity at intermediate concentrations. Table 6.1 displays the dependency of binary diffusivity coefficients on concentration for selected alkenes in chloroform at 30°C and 1 atm. As the nonideality increases, mixture may split into two liquid phases at certain composition and temperature. 

An example of heterogeneous-azeotrope formation is shown in Fig. 13-8 for the water-normal butanol system at 101.3 kPa. At liquid compositions between 0 and 3 mol % butanol and between 40 and 100 mol % butanol, the liquid phase is homogeneous. Phase splitting into two separate liquid phases (one with 3 mol % butanol and the other with 40 mol % butanol) occurs for any overall liquid composition between 3 and 40 mol % butanol. A minimum-boihng heterogeneous azeotrope occurs at 92°C (198°F) when the vapor composition and the overall composition of the two liquid phases are 25 mol % butanol. 

A feed composition in the metastable set is stable to infinitesimal composition disturbances but is unstable to finite ones hence, for such a composition phase splitting can only occur by nucleation, and not simply by Brownian motion (which, at most, supposedly results in infinitesimal composition disturbances). Hence, a metastable composition may be observed as a one-phase system in the laboratory Superheated liquids and subcooled vapors are elementary one-component examples. In contrast with this, one-phase spinodal compositions, by virtue of being unstable to infinitesimal perturbations, will never be observed in the laboratory. 

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