Reactive phase equilibrium

The solution to (P12) gives us the optimal separation profile as a function of age within the reactor. However, except in the case of reactive phase equilibrium, the assumption of a continuous separation profile is not really required. Furthermore, a continuous separation profile may not be implementable in practice. To address this, we take advantage of the structure of a discretization procedure for the differential equation system. In this case, we choose orthogonal collocation on finite elements to discretize the above model. This results... 

Formaldehyde is a low-boiling substance with a normal boiling point of approx. 254 K. It is not stable in its pure form, so it usually occurs in aqueous or methanolic solutions. Mixtures of formaldehyde and water or alcohols are not binary solutions in the usual sense, as formaldehyde reacts with both of them to a wide variety of species which are not stable as pure compounds themselves. Therefore, the standard procedure for building up a thermodynamic model by setting up the pure component properties and the binary interaction parameters fails in this case. The formaldehyde-water-methanol system is a good example f[Pg.567]

These chemically reactive phases are prepared by slow cooling of melts with the appropriate composition under an inert atmosphere or vacuum. Equilibrium is slow to be attained at the low temperatures necessary to prevent dissociation at 6.9°C Na2K dissociates into a (solid solution of K in Na) and liquid (60/40 Na/K). The KjCs and K7CSJ phases are even less stable and result from cooling mixtures of the elements of the desired stoichiometry to — 100°C in a metal beaker under argon. ... 

Intelligent engineering can drastically improve process selectivity (see Sharma, 1988, 1990) as illustrated in Chapter 4 of this book. A combination of reaction with an appropriate separation operation is the first option if the reaction is limited by chemical equilibrium. In such combinations one product is removed from the reaction zone continuously, allowing for a higher conversion of raw materials. Extractive reactions involve the addition of a second liquid phase, in which the product is better soluble than the reactants, to the reaction zone. Thus, the product is withdrawn from the reactive phase shifting the reaction mixture to product(s). The same principle can be realized if an additive is introduced into the reaction zone that causes precipitation of the desired product. A combination of reaction with distillation in a single column allows the removal of volatile products from the reaction zone that is then realized in the (fractional) distillation zone. Finally, reaction can be combined with filtration. A typical example of the latter system is the application of catalytic membranes. In all these cases, withdrawal of the product shifts the equilibrium mixture to the product. 

Property parameters. The physical property parameters include state of matter, phase equilibrium, thermal, mechanical, optical, and electromagnetic properties. The chemical property parameters include preparation, reactivity, reactants and products, kinetics, flash point, and explosion limit. The biological property parameters include toxicity, physiological and pharmaceutical effects, nutrition value, odor, and taste. 

The Me3Ge+ cation can also form adducts with arenes166 and quantitative gas-phase equilibrium measurements show that the stability of the adducts (—AG°) obeys the order 1,3-Me2C6H4 > h2o > MePh. The thermochemistry and reactivity of the [Me3Ge+.arene] adducts suggest that the most likely structure is that of a sigma complex. 

Section 4.2 is focused on phase equilibrium-controlled vapor-liquid systems with kinetically or equihbrium-controlled chemical reactions. The feasible products are kinetic azeotropes or reactive azeotropes, respectively. 

In the reactive case, r is not equal to zero. Then, Eq. (3) represents a nonhmoge-neous system of first-order quasilinear partial differential equations and the theory is becoming more involved. However, the chemical reactions are often rather fast, so that chemical equilibrium in addition to phase equilibrium can be assumed. The chemical equilibrium conditions represent Nr algebraic constraints which reduce the dynamic degrees of freedom of the system in Eq. (3) to N - Nr. In the limit of reaction equilibrium the kinetic rate expressions for the reaction rates become indeterminate and must be eliminated from the balance equations (Eq. (3)). Since the model Eqs. (3) are linear in the reaction rates, this is always possible. Following the ideas in Ref. [41], this is achieved by choosing the first Nr equations of Eq. (3) as reference. The reference equations are solved for the unknown reaction rates and afterwards substituted into the remaining N - Nr equations. 

The wave and pulse patterns of nonreactive separation processes, as well as the integrated reaction separation processes illustrated above, can be easily predicted with some simple graphical procedures derived from Eqs. (4) and (5). The behavior crucially depends on the equilibrium function y(x) in the nonreactive case, and on the transformed equilibrium function Y(X) in the reactive case. In addition to phase equilibrium, the latter also includes chemical equilibrium. An explicit calculation of the transformed equilibrium function and its derivatives is only possible in special cases. However, in Ref. [13] a numerical calculation procedure is given, which applies to any number of components, any number of reactions, and any type of phase and reaction equilibrium. 

As in reactive distillation and reactive chromatography, many sorption-enhanced reaction processes are controlled by phase equilibrium in addition to reaction equilibrium. The situation is different for membrane reactors, where phase equilibrium between the phases adjacent to the membrane is often trivial and the process is... 

The first two items are particularly powerful. The result is that a reactive distillation setup offers the possibility of achieving simultaneously high conversion for both reactants, with stoichiometric consumption of reactants at optimal selectivity. The third item indicates that the reactive distillation is of great interest for equilibrium constrained reactions. Taking advantage of exothermal reactions depends on the temperature level that can be allowed by the phase equilibrium. 

Therefore, adopting the solution of reactive distillation instead of separate reaction and separation units does not lead automatically to a more efficient process. Matching the conditions of separation and reaction in the same device requires careful design. The element with the highest impact is the chemical reaction. The key condition for an efficient and competitive process by reactive distillation is the availability of a superactive catalyst capable to compensate the loss in the driving force by phase equilibrium, but at the same time ensuring a good selectivity pattern. 

A modern alternative is the use of reactive distillation. At first sight appealing, this raises a number of problems. The reaction rate is considerably reduced with respect to a homogeneous liquid process because of the lower propylene concentration due to phase equilibrium. In addition, the countercurrent flow of reactants... 

Inside the reactive zone, chemical and phase equilibrium occur simultaneously. The composition of phases can be found by Gibbs free-energy minimization. The UNIQUAC model is adopted for phase equilibrium, for which interaction parameters are available, except the binary fatty-ester/water handled by UNIFAC-Dortmund. 

Figure A.2 (right) emphasizes a particular position where phase equilibrium and stoichiometric lines are collinear. In other words the liquid composition remains unchanged because the resulting vapor, after condensation, is converted into the original composition. This point is a potential reactive azeotrope, but when the composition satisfies chemical equilibrium too it becomes a true reactive azeotrope. Some examples of residue curve maps are presented below. Ideal mixtures are used to illustrate the basic features, which may be applied to some important industrial applications.

The accuracy of the thermodynamic data has a significant effect on RCM computation. In the case of slow reactions both kinetics and phase equilibrium should be modelled accurately. If the reaction is fast enough the chemical reaction prevails. In many cases chemical equilibrium may be taken as the reference. Consequently, accurate knowledge of the chemical equilibrium constant is needed. When reactive azeotropes and/or phase splitting might occur accurate modelling of phase equilibrium is also needed. 

To perform energy balance calculations on a reactive system, proceed much as you did for nonreactive systems (a) draw and label a flowchart (b) use material balances and phase equilibrium relationships such as Raoult s law to determine as many stream component amounts or flow rates as possible (c) choose reference states for specific enthalpy (or internal energy) calculations and prepare and fill in an inlet-outlet enthalpy (or internal energy) table and (d) calculate AH (or AC/ or A/C), substitute the calculated value in the appropriate form of the energy balance equation, and complete the required calculation. 

Once this interpretation has been established, MODEL.LA. (a) generates all the requisite modeling elements and (b) constructs the modeling relationships, such as material balances, energy balance, heat transfer between jacket and reactive mixture, mass transport between the two liquid phases, equilibrium relationships between the two phases, estimation of chemical reaction rate, estimation of chemical equilibrium conditions, estimation of heat generated (or consumed) by the reaction, and estimation of enthalpies of material convective flows. In order to automate the above tasks, MODEL.LA. must possess the following capabilities ... 

Simultaneous Representation of Chemical Reaction and Phase Equilibrium and the Evaluation of Phase Envelopes of Reactive Mixtures... 

Phase separation is controlled by phase equilibrium relations or rate-based mass and heat transfer mechanisms. Chemical reactions are controlled by chemical equilibrium relations or by reaction kinetics. For reactive distillation to have practical applications, both these operations must have favorable rates at the column conditions of temperature and pressure. If, for instance, the chemical reaction is irreversible, it may be advantageous to carry out the reaction and the separation of products in two distinct operations a reactor followed by a distillation column. Situations in which reactive distillation is feasible can result in savings in energy and equipment cost. Examples of such processes include the separation of close-boilers, shifting of equilibrium reactions toward higher yields, and removal of impurities by reactive absorption or stripping. 

Thus, tempermare. pressure, and the chemical potential of each distributed component are uniform for a closed system in phase equilibrium. If the system contains chemically reactive species, then additional equations ate required to characterize the equilibrium state. 

The COt Acceptor Gasification Process is discussed in light of the required properties of the CaO acceptor. Equilibrium data for reactions involving the CO% and sulfur acceptance and for sulfur rejection jit the process requirements. The kinetics of the reactions are also sufficiently rapid. Phase equilibrium data in the binary systems CaO-Ca(OH)t and Ca(OH)jr-CaCOs show the presence of low melting eutectics, which establish operability limits for the process. Data were obtained in a continuous unit which duplicates process conditions which show adequate acceptor life. Physical strength of many acceptors is adequate, and life is limited by chemical deactivation. Contrary to earlier findings both limestones and dolomites are equally usable in the process. Melts in the Ca(OH)2-CaC03 system are used to reactivate spent acceptors. 

Flow may also result in mechano-chemical degradation processes that generate reactive sites, viz., radicals, peroxides, acids, etc. Furthermore, trans-esterification and ester-amide exchange reactions are well documented. These reactions affect the phase equilibrium as well as the regularity of the chain structure, thus dispersion in the blend and its crystallinity. 

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