Phase equilibrium irreversible chemical reaction

Consider a physical system shown schematically in Figure 1. A fluid stream containing reactant A is moving upwards in plug flow with a constant velocity U. The reactant is adsorbed by a stream of solid catalytic particles falling downwards with a constant velocity V and occupying the void fraction of 1 - e. On the surface of catalyst an irreversible chemical reaction A - B is occurring and the product B is then rapidly desorbed back into the fluid phase. Instantaneous adsorption equilibrium for the species A is assumed. 

Thermodynamics and statistical mechanics deal with systems in equilibrium and are therefore applicable to phenomena involving flow and irreversible chemical reactions only when departures from complete equilibrium are small Fortunately this is often true in combustion problems, but occasionally thermodynamical concepts yield useful results even when their validity is questionable [for example, in the analysis of detonation structure (see Section 6.1.5) and in transition-state theory (see Section B.3.4)]. The presentation is restricted to chemical systems appropriate independent thermodynamic coordinates are pressure, p, volume, V, and the total number of moles of a chemical species in a given phase, N-, Moreover, results related to combustion theory are emphasized. 

Irreversible processes of phase transfer and chemical reaction within a closed system, whether homogeneous (a single phase) or heterogeneous (more than one phase), lead to T djS > 0. At equilibrium, T djS = 0. For fixed S and V constraints, dE = —T djS. A reversible process corresponds to zero internal entropy change and a minimum in dE. 

First of all, three special cases of vapor-hquid equilibrium of binary mixtures are presented qualitatively in Fig. 5.1-2. Considered are ideal mixtures (case A ), mixtures with total miscibiUty gap in the hquid phase (case B), and mixtures with irreversible chemical reaction in the liquid phase (case C). By converrtion, the symbols X and y denote the molar fraction of the low-boiling component a in the hq-ttid and the gas phase, respectively. 

Big. 5.1-2 Vapor-liquid equilibrium of three binary mixtures (A) ideal system, (B) system with a total miscibility gap in the liquid phase, and (C) system with irreversible chemical reaction in the liquid phase... 

Fig. 5.3-11 Phase equilibrium of chemical absorption at irreversible (left) and reversible (right) chemical reactions...

Depending on the nature of the system, the adsorption process can be either reversible or irreversible. In the first case an adsorption equilibrium exists between the particles adsorbed on the adsorbent s surface and the particles in the electrolyte (or in any other phase contacting with the adsorbent). After removing the substance from the electrolyte, adsorbed particles leave the surface and reenter into the electrolyte. In the case of an irreversible adsorption, the adsorbed particles remain at the surface even if their concentration in the bulk phase drops to zero. In this case the adsorbed particles can be removed from the surface only by means of a chemical reaction... 

A homogeneous reaction is one that involves only one phase. A heterogeneous reaction involves more than one phase, and reaction usually occurs at, or very near the interface between the phases. An irreversible reaction is one that proceeds in only one direction and continues in that direction until the reactants Types of reactions are exhausted. A reversible reaction, on the other hand, can proceed in either direction, depending on the concentrations of reactants and products relative to the corresponding equilibrium concentrations. An irreversible reaction behaves as if no equilibrium condition exists. StrictW speaking, no chemical reaction is completely irreversible, but in very many reactions the equilibrium point lies so far to the right that they are treated as irreversible reactions. 

The systems which we shall consider are therefore those which are in partial equilibrium. Equilibrium is already established with respect to certain variables such as temperature and pressure, and no irreversibility is associated with any change in these variables. On the other hand equihbrium is not attained with respect to redistribution of matter among constituents susceptible to chemical reaction, nor with respect to redistribution of matter among the different phases of the system, nor in general with respect to any changes which can be characterized by the parameter (c/. chap. I, 8). 

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. 

A case of practical interest is a chemical reactor coupled with a separation section, from which the unconverted reactants are recovered and recycled. Let s consider the simplest situation, an irreversible reaction A—>B taking place in a CSTR coupled to a distillation column (Fig. 13.5). Here we present results obtained by steady state and dynamic simulation with ASPEN Plus and ASPEN Dynamics. The reader is encouraged to reproduce this example with his/her favourite simulator. The species A and B may be defined as standard components with adapted properties. In this case, we may take as basis the properties of n-propanol and iso-propanol, and assume ideal phase equilibrium. The relative volatility B/A increases at lower pressures, being approximately 1.8 at 0.5 atm. We consider the following data nominal throughput of 100 kmol/hr of pure A, reactor volume 2620 1, and reaction constant =10 s". For stand-alone operation the reaction time and conversion are r= 0.106 hr and = 0.36. 

There is no method for theoretical prediction or correlation of VLE data in reacting systems. It has to be remembered that equilibrium in such systems means not only concentration and thermal equilibrium but also absence of chemical change. In other words, the chemical reaction must also be at equilibrium or, if it is irreversible, it must have reached completion. This poses the question of how to express the interphase concentration driving force in, e.g., a packed column in the presence of, say, a slow reaction. Should it be the difference between the local concentration of a component in the vapour phase and a true equilibrium value corresponding to a given liquid concentration of the corai)onent or should a pseudo equilibrium value, which... 

Chemical reactions in the liquid phase are either reversible or irreversible. Typical reversible reactions are involved in the absorption of H2S into ethanolamines, or the absorption of CO2 into alkali carbonate solutions. These reversible reactions permit the resultant solution to be regenerated so that the solute can be recovered in a concentrated form. Some irreversible reactions are the absorption of NH3 into dilute acids and the absorption of CO2 into alkaline hydroxides. The solute in such absorptions is so tightly bound in the reaction product that there is no appreciable vapor pressure of solute above the liquid phase. Under these conditions, regeneration of the solute is not possible, and the reacting component in the liquid is consumed. The purpose of such a reactant is to increase the solubility of the solute in the liquid phase and/or reduce the liquid-film resistance to mass transfer. Much theoretical work has been conducted since the 1950s to study diffusion and reaction in the liquid phase [19]. To calculate the effect of the rate of chemical reaction on the mass transfer requires the prediction of physical/chemical constants of salt solutions, such as equilibrium constants, reaction velocities, solubilities, and diffusion coefficients. Often, these constants must be available at elevated temperatures and/or pressures. 

When the reaction between solvent B and solute C is irreversible, the value of Xe approaches zero. Such would be the case if H2S were extracted from LPG using an aqueous solution of caustic soda as the solvent. However, if the chemical reaction is reversible, so that the solute can be recovered, then Xe will have a low value that is dependent on the concentrations of the reactants and the temperature. Such a system might be the extraction of H2S from LPG using an aqueous ethanolamine solvent. The extract phase subsequently would be regenerated to permit reuse of the amine solvent. As shown in Figure 11-6, the equilibrium concentration in the extract approaches a constant value limited by the concentration of the reactant in the solvent and the stoichiometry. 

A baseline potential pulse followed each current pulse in order to strip extracted ions from the membrane phase and, therefore, regenerated the membrane, making it ready for the next measurement pulse. This made sure that the potentials are sampled at discrete times within a pulse that correspond to a 6m that is reproducible from pulse to pulse. This made it possible to yield a reproducible sensor on the basis of a chemically irreversible reaction. It was shown that the duration of the stripping period has to be at least ten times longer than the current pulse [53], Moreover the value of the baseline (stripping) potential must be equal to the equilibrium open-circuit potential of the membrane electrode, as demonstrated in [52], This open-circuit potential can be measured prior to the experiment with respect to the reference electrode. 

Several key questions must be answered initially in a study of reaction chemistry. First, is the reaction sufficiently fast and reversible so that it can be regarded as chemical-equilibrium controlled Second, is the reaction homogeneous (occurring wholly within a gas or liquid phase) or heterogeneous (involving reactants or products in a gas and a liquid, or liquid and a solid phase) Slow reversible, irreversible, and heterogeneous (often slow) reactions are those most likely to require interpretation using kinetic models. Third, is there a useful volume of the water-rock system in which chemical equilibrium can be assumed to have been attained for many possible reactions This may be called the local equilibrium assumption. 

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