Absorption phase equilibria

Thus, sensors based on absorption (phase equilibrium) measure activity and, if only one type of sorption mechanism is involved, their response is logarithmic. 

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. 

Benzene is to be absorbed from coal gas by means of a wash-oil. The inlet gas contains 3 per cent by volume of benzene, and the exit gas should not contain more than 0.02 per cent benzene by volume. The suggested oil circulation rate is 480 kg oil/100 m3 of inlet gas measured at 273 K and 101.3 kN/m2. The wash oil enters the tower solute-free. If the overall height of a transfer unit based on the gas phase is 1.4 m, determine the minimum height of the tower which is required to carry out the absorption. The equilibrium data are ... 

The mechanism of transfer of solute from one phase to the second is one of molecular and eddy diffusion and the concepts of phase equilibrium, interfacial area, and surface renewal are all similar in principle to those met in distillation and absorption, even though, in liquid-liquid extraction, dispersion is effected by mechanical means including pumping and agitation, except in standard packed columns. 

These distinctions between the two operations are partly traditional. The equipment is similar, and the mathematical treatment, which consists of material and energy balances and phase equilibrium relations, also is the same for both. The fact, however, that the bulk of the liquid phase in absorption-stripping plants is nonvolatile permits some simplifications in design and operation. 

Although we have assumed that there is no mucosal diffusional barrier for the CYP3A substrate, the unbound inhibitor concentration in the intestinal epithelia (7gm) may or may not be equivalent to that in plasma and the liver (7U). 7gm may exceed the unbound portal plasma concentration during the inhibitor absorption phase or be less than the unbound portal concentration postabsorption if there is not rapid equilibrium between the intracellular and portal plasma compartments (i.e., a basolateral membrane diffusional barrier exists). This obviously makes it challenging to anticipate the quantitative effect of an inhibitor on intestinal first-pass metabolism. 

J. Krissmann, M. A. Siddiqi, K. Lucas, Absorption of sulfur dioxide in dilute aqueous solutions of sulfuric and hydrochloric acid, Fluid Phase Equilibriums, 1997, 141, S221-S233. 

For the right choice of the selective solvent and for the development and design of separation processes, a reliable knowledge of the phase equilibrium behavior (extractive distillation vapor-liquid equilibrium (VLE) extraction liquid-liquid equilibrium (LLE), absorption gas-liquid equilibrium (GLE)) is required. This information is available from phase equilibrium thermodynamics. 

The resistance to mass transfer according to (1.221) and (1.223) is made up of the individual resistances of the gas and liquid phases. Both equations show how the resistance is distributed among the phases. This can be used to decide whether one of the resistances in comparison to the others can be neglected, so that it is only necessary to investigate mass transfer in one of the phases. Overall mass transfer coefficients can only be developed from the mass transfer coefficients if the phase equilibrium can be described by a linear function of the type shown in eq. (1.217). This is normally only relevant to processes of absorption of gases by liquids, because the solubility of gases in liquids is generally low and can be described by Henry s law (1.217). So called ideal liquid mixtures can also be described by the linear expression, known as Raoult s law. However these seldom appear in practice. As a result of all this, the calculation of overall mass transfer coefficients in mass transfer play a far smaller role than their equivalent overall heat transfer coefficients in the study of heat transfer. 

In phase equilibrium, mole fraction in each of the phases is the most common way to express composition that goes along with mole balances around the system being studied. There is also the possibility to use mass fraction for defining composition. Mass balances around the system can then be made. For some situations, absorption column design, for example, the composition may be expressed as mass of solute per mass of solvent. 

Propane (the key component) is recovered in one case from a nitrogen-propane gas mixture by absorption with a liquid consisting mostly of decane. In another case propane is recovered from a decane-propane solution by stripping it with a gas that is mostly nitrogen. All the feed streams are maintained at a temperature of 24°C and a pressure of 1725 kPa. The absorption or stripping takes place in an insulated vessel maintained at 1725 kPa, where the vapor and liquid feeds mix, reach phase equilibrium, then separate into vapor and liquid products. The feed and product compositions and thermal conditions are given in Table 8.1. 

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. 

There are a number of sources for phase equilibrium data and computational methods (see E4.1, below). Most of the material focuses on vapor-liquid equilibrium (VLB) since this information is used extensively for distillation, absorption, and stripping. The most complete VLB literature is a series of books by Hala et al. (1967, 1968). Additional information can be found in Hirata et al. (1975) and Gmehling et al. (1979). For light hydrocarbon systems, the Natural Gas Processors Association has published a data book (1972). A very useful and extensive source, including solid-liquid and liquid-liquid as well as VLB information, has been written by Walas (1985). This book contains both source data and methodology and contains sample calculations. 

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. 

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. ... 

The McCabe-Thiele constructions described in Chapter 8 embody rather restrictive tenets. The assumptions of constant molal overflow in distillation and of interphase transfer of solute only in extraction seriously curtail the general utility of the method. Continued use of McCabe-Thiele procedures can be ascribed to the fact that (a) they often represent a fairly good engineering approximation and (b) sufficient thermodynamic data to justify a more accurate approach is often lacking. In the case of distillation, enthalpy-concentration data needed for making stage-to-stage enthalpy balances are often unavailable, while, in the Case of absorption or extraction, complete phase equilibrium data may not be at hand. 

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