Phase equilibrium chemical absorption

Using Eqs. 13.4-7,13.4-8, and 13.4-9 in the equilibrium relation ofEq. 13.4-6 yields Eq. 13.4-5. Thus, precisely the same equilibrium relation is found between the ammonia partial pressure in the gas phase and the ammonium hydroxide concentration in the liquid phase, independent of the manner in which we presume the absorption-reaction process to take place. Consequently, it is a matter of convenience whether we consider multiphase reactions such as the one here to be chemical equilibrium problems or problems of combined chemical and phase equilibrium. ... 

In processing, it is frequently necessary to separate a mixture into its components and, in a physical process, differences in a particular property are exploited as the basis for the separation process. Thus, fractional distillation depends on differences in volatility. gas absorption on differences in solubility of the gases in a selective absorbent and, similarly, liquid-liquid extraction is based on on the selectivity of an immiscible liquid solvent for one of the constituents. The rate at which the process takes place is dependent both on the driving force (concentration difference) and on the mass transfer resistance. In most of these applications, mass transfer takes place across a phase boundary where the concentrations on either side of the interface are related by the phase equilibrium relationship. Where a chemical reaction takes place during the course of the mass transfer process, the overall transfer rate depends on both the chemical kinetics of the reaction and on the mass transfer resistance, and it is important to understand the relative significance of these two factors in any practical application. 

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). 

The methods based on the equilibrium stage model have existed for over 30 years and refinements continue, but serious development of nonequilibrium models has begun only recently. These methods are an alternative means to the stage model for predicting column performance. They are expected to make inroads, especially for systems for which stage efficiency prediction is very difficult, such as reactive distillation, chemical absorption, and three-phase distillation. However, their progress into systems where efficiency prediction is well-established is likely to be slower. Their complexity due to the restriction to... 

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. 

The emergence of chemical engineering as a professional field of specialized knowledge was catalyzed to a major extent by the systematic classification of apparatus in terms of the Unit Operations. With further progress, the design methods evolved for particular apparatus types have proved equally applicable to other unit operations similar in physical arrangement, material and energy balances, rate behavior, and phase equilibrium. Thus there has been a very extensive development of parallel calculation methods for the separation operations conducted under countercurrent flow conditions—the fluid-fluid operations of distillation, absorption, and extraction. 

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

In the design and dimensioning of absorption equipment the reaction kinetics is of great importance besides the phase equilibrium. If the reaction is very slow, the physical absorption is the dominant mechanism since the residence time of the liquid in the absorber is too low. In fast reactions, the equilibrium line of the chemical system is the relevant quantity. In very fast (instantaneous) reactions, the kinetics of the mass transfer is enhanced, too (see Chap. 4). In industrial practice, only fast reactions are applied to chemical absorption (Table 5.3-3). 

In the vinyl-chloride process, because of the significant differences in the volatilities of the three principal chemical species, distillation, absorption, and stripping are prime candidates for the separators, especially at the high production rates specified. For other processes, liquid-liquid extraction, enhanced distillation, adsorption, and membrane separators might become more attractive, in which case the design team would need to assemble data that describe the effect of solvents on species phase equilibrium, species adsorption isotherms, and the permeabilities of the species through various membranes. 

A chemical reaction between the solvent and the gas component / to be separated is called activated absorption or chemisorption. Chemical attractive forces act between the solvent and component /, resulting in high selectivity and solubility. The phase equilibrium becomes a chemical equilibrium. 

There are some items of interest here. A few basic properties are the basis for separation in two different types of basic separation processes. For example, condensability of a vapor/gas species is useful for vapor absorption as well as for membrane gas separation geometrical partitioning (or partitioning by other means between a pore and an external solution) is useful both in adsorp-tion/chromatography as well as in the membrane processes of dialysis and ultrafiltration, etc. Further, there are many cases where chemical reactions are extraordinarily useful for separation these are not identified here since chemical reactions can enhance separation only if the basic mechanism for separation exists, especially in phase equilibrium based separations. However, there are a few cases where chemical reactions, especially complexations, provide the fundamental basis for separation, as in affinity chromatography, metal extractions and isotope exchange reactions. 

Absorption, Dissociation, and Aqueous-Phase Chemical Reactions The diffusive penetration of gases or gas mixtures into a condensed phase (e g., droplet) is called absorption. In equilibrium, the absorbed gas is dissolved at a certain concentration inside the droplet and the equilibrium vapor pressure over the droplet surface is proportional to the concentration at the droplet surface (Henry s law). The concentration inside the droplet itself can be influenced by dissociation or chemical reactions (sulfur production by oxidation of dissolved SO2 to SOt ). If these processes represent a sink for the solute, the concentration inside the droplet and, consequently, the vapor pressure at the droplet surface is decreased (i.e., mass transfer is enhanced). Typical gases that dissolve into atmospheric water droplets are CO2, SO2, NH3, H2O2, and O3. 

Influence of Chemical Reactions on Uq and When a chemical reaction occurs, the transfer rate may be influenced by the chemical reac tion as well as by the purely physical processes of diffusion and convection within the two phases. Since this situation is common in gas absorption, gas absorption will be the focus of this discussion. One must consider the impacts of chemical equilibrium and reac tion kinetics on the absorption rate in addition to accounting for the effec ts of gas solubility, diffusivity, and system hydrodynamics. 

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 ... 

Most of these aspects of water-sorption equilibrium correspond to the equality of chemical potentials of water in the medium and in the polymer. The consequences of this principle are illustrated by the experiment of Fig. 14.2, where an interface is created between water and a nonmiscible liquid (oil, hydrocarbon, etc.), and a polymer sample is immersed into the organic liquid. It can be observed that, despite the hydrophobic character of the surrounding medium, the sample reaches the same level of water saturation as in direct water immersion or in a saturated atmosphere. What controls the water concentration in the polymer is the ratio C/Cs of water concentrations in the organic phase, where Cs is the equilibrium concentration, which can be very low but not zero. In other words, hydrophobic surface treatments can delay the time to reach sorption equilibrium but they cannot avoid the water absorption by the substrate. 

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