Gas absorption with chemical reaction

F. Gas Absorption with Chemical Reaction—Time-Dependent Bulk Concentration. 346... 

Figure 6.3 Gas absorption with chemical reaction (a) rapid second-order reaction and (b) slow pseudo first-order reaction.

Many processes of gas absorption with chemical reaction are set up at high pressures, result of technical and/or economical requirements. That is, for example, processes of hydrocracking and hydrorefining of heavy oils and processes of oxydation of liquid effluents. However, if many chemical systems are found to determine the mass transfer parameters in an industrial reactor at atmospheric pressure by using the chemical method, they become scarce at elevated pressures. Several physical and chemical methods have been proposed chemical methods present some severe drawbacks, since one has to replace the gas-liquid system of interest by another one, presenting different physical properties (specially a different coalescence behaviour). 

In the overall picture, different expressions are proposed for the rate of gas absorption with chemical reaction, depending on the forms of the enhancement factor E corresponding to different kinetic regimes, going from reaction-controlling to mass transfer-controlling. Typical cases are ... 

Reaction between a gas and a liquid normally involves absorption and physical solution of the gas followed by homogeneous reaction between the dissolved species. The problem of gas absorption with chemical reaction has been extensively studied and in such systems the observed rate of gas absorption will be a function of the chemical reaction rate, the diffusion of the dissolved gas in the liquid, and, possibly, the fluid dynamics of the system (the rate of surface renewal) if surface tension driven or other circulation effects occur. There is no evidence of these so far in the thin films employed in practical catalysts. Danckwerts and Astarita give comprehensive treatments of the subject of gas absorption accompanied by reaction. 

Consider gas absorption with chemical reaction in an agitated tank. [13] The governing equation and boundary conditions for dimensionless concentration are given by ... 

Design procedures of contactors for simultaneous gas absorption with chemical reaction require all the data -such as flooding, hold-up, ki a and k a and axial dispersion coefficients whenever they are relevant-which are normally required for the design of physical gas absorbers too. Further to these data, separate values of kL and a are also required in order to estimate the enhancement factor using one of the absorption-reaction models. 

Porter,K. "The effect of contact time distribution on gas absorption with chemical reaction".Trans.Instn.Chem.Enqrs. 44 (1966) T25. 

The subject of carrier-mediated or facilitated gas transport in liquid film membranes has been an active field of research the past 15-20 years. It refers to the enhancement of gas transport attributable to reversible reaction of physically dissolved gas with non-volatile, diffusible solutes. The underlying physics are analogous to those governing gas absorption with chemical reaction. However, the two processes differ insofar as the membrane process has been primarily operated in the steady-state, and without the occurrence of irreversible reactions. 

Figure 5A3. The enhancement factor E for gas absorption with chemical reaction, as a function of the Hatta number (9), for various values of the hinterland ratio ( 6), according to eq. (5A.10).

Mann, R., and M. Moyes (1977). Exothermic gas absorption with chemical reaction, AIChE J., 23, 17-23. 

A necessary prerequisite to understanding the subject of absorption with chemical reaction is the development of a thorough understanding of the principles involved in physical absorption, as discussed earlier in this section and in Section 5. There are a number of excellent references the subject, such as the book by Danckwerts Gas-Liquid Reactions, McGraw-Hill, New York, 1970) and Astarita et al. Gas Treating with Chemical Solvents, Wiley, New York, 1983). 

While the carbon dioxide/caiistic test method has become accepted, one should use the results with caution. The chemical reaction masks the effect of physical absorption, and the relative values in the table may not hold for other cases, especially distillation applications where much of the resistance to mass transfer is in the gas phase. Background on this combination of physical and chemical absorption may Be found earher in the present section, under Absorption with Chemical Reaction. ... 

Only physical absorption from dilute gases has been considered in this section. For a discussion of absorption from concentrated gases and absorption with chemical reaction, the reader should refer to Volume 2, or to the book by Treybal (1980). If the inlet gas concentration is not too high, the equations for dilute systems can be used by dividing the operating line up into two or three straight sections. 

In this paper a transfer model will be presented, which can predict mass and energy transport through a gas/vapour-liquid interface where a chemical reaction occurs simultaneously in the liquid phase. In this model the Maxwell-Stefan theory has been used to describe the transport of mass and heat. On the basis of this model a numerical study will be made to investigate the consequences of using the Maxwell-Stefan equation for describing mass transfer in case of physical absorption and in case of absorption with chemical reaction. Despite the fact that the Maxwell-Stefan theory has received significant attention, the incorporation of chemical reactions with associated... 

The influence of pressure on the mass transfer in a countercurrent packed column has been scarcely investigated to date. The only systematic experimental work has been made by the Research Group of the INSA Lyon (F) with Professor M. Otterbein el al. These authors [8, 9] studied the influence of the total pressure (up to 15 bar) on the gas-liquid interfacial area, a, and on the volumetric mass-transfer coefficient in the liquid phase, kia, in a countercurrent packed column. The method of gas-liquid absorption with chemical reaction was applied with different chemical systems. The results showed the increase of the interfacial area with increasing pressure, at constant gas-and liquid velocities. The same trend was observed for the variation of the volumetric liquid mass-transfer coefficient. The effect of pressure on kia was probably due to the influence of pressure on the interfacial area, a. In fact, by observing the ratio, kia/a, it can be seen that the liquid-side mass-transfer coefficient, kL, is independent of pressure. 

In a number of important industrial processes, it is necessary to carry out a reaction between a gas and a liquid. Usually the object is to make a particular product, for example, a chlorinated hydrocarbon such as chlorobenzene by the re action of gaseous chlorine with liquid benzene. Sometimes the liquid is simply the reaction medium, perhaps containing a catalyst, and all the reactants and products are gaseous. In other cases the main aim is to separate a constituent such as C02 from a gas mixture although pure water could be used to remove CO2, a solution of caustic soda, potassium carbonate or ethanolamine has the advantages of increasing both the absorption capacity of the liquid and the rate of absorption. The subject of gas-liquid reactor design thus really includes absorption with chemical reaction which is discussed in Volume 2, Chapter 12. 

There continue to be many absorbers for the removal of water-soluble gases. Acid gases and some volatile organic compounds can be absorbed readily in water by the types of equipment previously discussed. These processes are essentially absorption with chemical reaction. For a discussion of absorption in air pollution control and a description of several absorption systems for sulfur dioxide and nitrogen oxide removal, see Schnelle and Brown. A more detailed discussion of many more processes for flue gas desulfurization employing absorption is given by Lunt and Cunic. ... 

Advanced recovery can be realised by means of a gas-absorption device. The absorbent can be a process stream or a recycled solvent (Fig. 7.16c). When reactive components are present in the gas phase the separation can involve absorption with chemical reaction. If the pressure of the original stream is not sufficient, vapour recompression could be used to improve the separation after a first flash (Fig. 7.16d). 

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. 

Danckwerts,P.V. and A.J.Gillham. "The design of gas absorbers. I.Methods for predicting rates of absorption with chemical reaction in packed columns and tests with 1 1/2 in Raschig rings". Trans.Instn.Chem.Engrs. 44 (1966) T42. 

The partial pressure of the transported species is maintained constant in the two gas phases separated by the membrane, by virtue of either the velocity of a sweep gas or the volume of closed gas phases. The membranes are generally in the form of a soaked filter oaoer, or of homogeneous solution supported by highly permeable polymer membranes. At one membrane face there is absorption with chemical reaction at the other there is stripping. Steady-state operation is ensured by zero net conversion within the membrane as a whole. 

Hqq is related to the individual coefficients by Eq. (16-27a). Unfortunately, it is dangerous to use Eqs. (16-37) and (16-38) to determine the values for Hl and Hq for co-current columns because the correlations are based on data in countercurrent columns at lower gas rates than those used in co-current columns. Reiss (1967) reviews co-current contactor data and notes that the mass transfer coefficients can be considerably higher than in countercurrent systems. Gianetto et al. (1973) operated with a 15-fold velocity increase and observed a 40-fold increase in Icl when liquid-phase resistance controlled. They recommended co-current operation for absorption with chemical reaction. Harmen and Perona (1972) did an economic conparison of co-current and countercurrent columns. For the absorption of CO2 in carbonate solutions where the reaction is slow they concluded that countercurrent operation is more economical. For CO2 absorption in monoethanolamine (MEA), where the reaction is fast, they concluded that countercurrent is better at low liquid fluxes whereas co-current was preferable at high liquid fluxes. 

The technique involving simultaneous absorption with chemical reaction and physical desorption was employed to determine mass transfer coefficients with and without chemical reaction under identical hydrodynamic conditions. The gas phase consisted of CO2 and N2, and the liquid phase consisted of 0.2M NaOH solution containing dissolved oxygen. 3mm and 4mm glass beads were used as the solid phase. 

Gas and liquid samples were analyzed by gas chromatography and using an oxygen meter, respectively. The rate of CO2 absorption with chemical reaction was obtained from the gas phase anlaysis. 

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