Absorption with chemical reaction equilibrium

Group methods are limited to dilute systems, where A -values could be considered as constant. In the case of more concentrated mixtures we recommend the use of computer methods. Attention has to be paid care to correct description of gas-liquid equilibrium. The use of asymmetric convention for /C-values could overcome the drawback of using the Henry-law for concentrated solution (see Chapter 6). In the case of absorption with chemical reaction the methods based on the integration of mass transfer equations are recommended. Some specialised simulation packages have capabilities in this area. 

Reactive absorption represents a process in which a selective solution of gaseous species by a liquid solvent phase is combined with chemical reactions. As compared to purely physical absorption, RA does not necessarily require elevated pressure and high solubility of absorbed components because of the chemical reaction, the equilibrium state can be shifted favorably, resulting in enhanced solution capacity (17). Most RA processes involve reactions in the liquid phase only in some of them, both liquid and gas reactions occur (18,19). 

Many important chemical reactions take place in the aqueous component of the atmospheric aerosol or in fog droplets. An example is the solution-phase oxidation of SO2 to SO. Such reactions may drive the dilTusioiial transport of reactants from the gas to the particles followed by absorption and chemical reaction. If the chemical reactions are slow compared with the gas- and aero.sol-phase transport rates, the dissolved reactive species will be nearly in equilibrium between the gas and particles. 

A liquid is in contact with a well-mixed gas containing substance A to be absorbed. Near the surface of the liquid there is a film of thickness 8 across which A diffuses steadily while being consumed by a first-order homogeneous chemical reaction with a rate constant ky At the gas-liquid interface, the liquid solution is in equilibrium with the gas and its concentration is cAl at the other side of the film, its concentration is virtually zero. Assuming dilute solutions, derive an expression for the ratio of the absorption flux with chemical reaction to the corresponding flux without a chemical reaction. 

Many commercial absorption processes involve a chemical reaction between the solute and the solvent. The occurrence of a reaction affects not only gas-liquid equilibrium relationships but also the rate of mass transfer. Since the reaction occurs in the solvent, only the liquid mass transfer rate is aflected. Normally, the effect is an increase in the liquid mass transfer coefficient k. The develtqiment of correlations to-predicting the degree of enhancement for various types of chemical reaction and system configuration has been the subject of numerous studies. Comprehensive discussions of the theory of mass transfer with chemical reaction are presented in recent books by Astarita, Danckweits, and Astarita et al. ... 

With a reactive solvent, the mass-transfer coefficient may be enhanced by a factor E so that, for instance. Kg is replaced by EKg. Like specific rates of ordinary chemical reactions, such enhancements must be found experimentally. There are no generalized correlations. Some calculations have been made for idealized situations, such as complete reaction in the liquid film. Tables 23-6 and 23-7 show a few spot data. On that basis, a tower for absorption of SO9 with NaOH is smaller than that with pure water by a factor of roughly 0.317/7.0 = 0.045. Table 23-8 lists the main factors that are needed for mathematical representation of KgO in a typical case of the absorption of CO9 by aqueous mouethauolamiue. Figure 23-27 shows some of the complex behaviors of equilibria and mass-transfer coefficients for the absorption of CO9 in solutions of potassium carbonate. Other than Henry s law, p = HC, which holds for some fairly dilute solutions, there is no general form of equilibrium relation. A typically complex equation is that for CO9 in contact with sodium carbonate solutions (Harte, Baker, and Purcell, Ind. Eng. Chem., 25, 528 [1933]), which is... 

In some absorption processes, especially where a chemical reaction occurs, there is a liberation of heat. This generally gives rise to an increase in the temperature of the liquid, with the result that the position of the equilibrium curve is adversely affected. 

In complete equilibrium, the ratio of the population of an atomic or molecular species in an excited electronic state to the population in the groun d state is given by Boltzmann factor e — and the statistical weight term. Under these equilibrium conditions the process of electronic excitation by absorption of radiation will be in balance with electronic deactivation by emission of radiation, and collision activation will be balanced by collision deactivation excitation by chemical reaction will be balanced by the reverse reaction in which the electronically excited species supplies the excitation energy. However, this perfect equilibrium is attained only in a constant-temperature inclosure such as the ideal black-body furnace, and the radiation must then give -a continuous spectrum with unit emissivity. In practice we are more familiar with hot gases emitting dis-... 

A more comprehensive analysis of the influences on the ozone solubility was made by Sotelo et al., (1989). The Henry s Law constant H was measured in the presence of several salts, i. e. buffer solutions frequently used in ozonation experiments. Based on an ozone mass balance in a stirred tank reactor and employing the two film theory of gas absorption followed by an irreversible chemical reaction (Charpentier, 1981), equations for the Henry s Law constant as a function of temperature, pH and ionic strength, which agreed with the experimental values within 15 % were developed (Table 3-2). In this study, much care was taken to correctly analyse the ozone decomposition due to changes in the pH as well as to achieve the steady state experimental concentration at every temperature in the range considered (0°C [Pg.86]

A convention used in most literature on ozone mass transfer and in the rest of this book is to define the mass transfer coefficient as the one that describes the mass transfer rate without reaction, and to use the enhancement factor E to describe the increase due to the chemical reaction. Furthermore, the simplification that the major resistance lies in the liquid phase is used throughout the rest of the book. This is also based on the assumption that the mass transfer rate describes physical absorption of ozone or oxygen, since the presence of a chemical reaction can change this. This means that KLa - kLa and the concentration gradient can be described by the difference between the concentration in equilibrium with the bulk gas phase cL and the bulk liquid concentration cL. So the mass transfer rate is defined as ... 

The circumstellar chemistry is often subdivided into three main zones, which are determined by a comparison of the characteristic dynamic flow time, R/vx, with the chemical reaction times (Lafont et al. 1982 Omont 1987 Millar 1988). (i) In the region closest to the star (perhaps R 1014 cm), the density is sufficiently high that three-body chemical reactions occur in a time short compared to the dynamic time. In this regime, we expect the chemical abundances to approach thermodynamic equilibrium, (ii) Somewhat further away from the star (1014 cm < R < 1016 cm), there is a freeze-out of the products of the three-body reactions (McCabe et al. 1979). In this region, two-body reactions dominate the active chemistry, (iii) Finally, far from the star (R > 1016 cm), the density becomes sufficiently low that the only significant chemical processing is the photodestruction that results from absorption of ambient interstellar ultraviolet photons by the resulting molecules that flow from the central star. 

From Eqn. (14) it follows that with an exothermic reaction - and this is the case for most reactions in reactive absorption processes - decreases with increasing temperature. The electrolyte solution chemistry involves a variety of chemical reactions in the liquid phase, for example, complete dissociation of strong electrolytes, partial dissociation of weak electrolytes, reactions among ionic species, and complex ion formation. These reactions occur very rapidly, and hence, chemical equilibrium conditions are often assumed. Therefore, for electrolyte systems, chemical equilibrium calculations are of special importance. Concentration or activity-based reaction equilibrium constants as functions of temperature can be found in the literature [50]. 

Preceding chapters have dealt largely with pure substances or with constant-composition mixtures. e.g., air. However, composition changes are the desired outcome, not only of chemical reactions, but of a number of industrially important mass-transfer operations. Thus composition becomes a primary variable in the remaining chapters of tliis text. Processes such as distillation, absorption, and extraction bring phases of different composition into contact, and when tlie phases are not in equilibriimi, mass transfer between the phases alters their compositions. Botli tlie extent of change and tlie rate of transfer depend on the departure of the system from equilibrium. Thus, for quantitative treatment of mass transfer the equilibrium T, P, and phase compositions must be known. 

In one way or another, picoseconds after the initial excitation, the molecule will typically find itself thermally equilibrated with the surrounding medium in a local minimum on the S, T, or So surfaces in S if the initial excitation was by light absorption, in T, if it was by sensitization or if special structural features such as heavy atoms were present, and in Sq if the reaction was direct. Frequently, the initially reached minimum in S (or T,) is a spectroscopic minimum located at a geometry that is close to the equilibrium geometry of the original ground-state species, so no net chemical reaction can be said to have taken place so far. 

Excited electronic states thus give rise to photon emission with a yield smaller than unity on the other hand, absorption of these photons produces, in turn, excited electronic states, also with a yield smaller than unity. Consequently, if one neglects the possibility for the photons to escape from the solid, a quasi-equilibrium is established between these two forms of energy between which the near totality of the incident energy is recovered. However, every conversion from one form to the other is accompanied by a release of thermal energy. If the irradiated system does not use the energy in either of these forms for certain definite purposes, such as chemical reaction for instance, the totality of this energy will be finally converted into heat. 

Let us consider the energy change associated with a chemical reaction (Figure 2.1). Transformation from a reactant to a product often gives rise to the release or absorption of energy. If equilibrium exists between the reactant and the product, the amount of the reactant and that of the product in the system are determined by the energy difference between them as well as temperature. If two products are formed in the equilibrium. 

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