Phase Mass Transfer with Chemical Reactions

Liquid Phase Mass Transfer with Chemical Reactions 

So far, we have considered pure physical mass transfer without any reaction. Occasionally, however, gas absorption is accompanied by chemical or biological reactions in the liquid phase. For example, when CO2 gas is absorbed into an aqueous solution of Na2CO3, the following reaction takes place in the liquid phase. 

In an aerobic fermentation, the oxygen absorbed into the culture medium is consumed by microorganisms in the medium. 

In general, the rates of the mass transfer increase when it is accompanied by reactions. For example, if k indicates the liquid phase coefficient, including the effects of the reaction, then the ratio E can be defined as  

1 latta [5] derived a series of theoretical equations for E, on the basis of the film model. Experimental values off agree with the Hatta theory, and also with theoretical values of f derived later by other investigators, on the basis of the penetration model. 

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Liquid Phase Mass Transfer with Chemical Reactions I 83... 

In order to establish the conditions for thermodynamic feasibility of reactive mass exchange, it is necessary to invoke the basic principles of mass transfer with chemical reactions. Consider a lean phase j that contains a set Bj = z —... 

Hofmann, H., 1983. in Mass Transfer with Chemical Reaction in Multiphase Systems, Vol. II Three-Phase Systems , Erdogan, A. (Ed.), Martinus Nijhoff Publishers, the Hague. 

Fig. 4.2. Liquid phase concentration profiles for mass transfer with chemical reaction - film theory...

Estimate the asymptotic Sh for mass transfer with chemical reaction for a circular cross section microchannel with the diameter of 500 pm for Ball equal to 0.1,1, and 100. Also, estimate the mass transfer coefficient for a gas-phase system with molecular diffusivity (B ) of 10" m s . For circular tube Sh = 3.66 and Sh = 4.36. 

Mass transfer with chemical reaction in multiphase systems" covers, indeed, a large area. Table 1 shows a general classification of the systems encountered. From the possible two-phase systems, solid-solid reactions, liquid-solid (reactive or catalytic) and gas-solid (reactive or catalytic) reactions are not discussed here. The first one was reviewed by Tamhankar and Doraiswamy (2) and gas-solid (reactive) systems, such as, coal gasification, calcination of limestone, reduction of ores, etc. have been treated in some detail in recent reviews (3-5). The industrially important fluid-solid catalytic processes were the topic of a previous Advanced Study Institute (6) and have been also discussed authoritatively elsewhere (5,7). Concerning solid (reactive)-liquid two-phase systems, only some interesting examples are presented in Table 2 (1). 

Schumpe, A. and W.-D. Deckwer. Paper presented at Int. Symp. on "Mass Transfer with Chemical Reaction in Two-Phase Systems ACS meeting, March 29 - April 3, 1981, Atlanta, USA... 

A detailed and very fundamental analysis of mass-transfer with chemical reaction in liquid-liquid dispersions has been published recently by Tavlarides and Stamatoudis (10). These authors point out that the design and analysis problem depends on the phase in which the reaction occurs, whether multiple reactions are involved, the relative magnitudes of the rates of mass transfer and reaction, and upon macromixing processes of the dispersed and continuous phases. Hence, they first examine dispersion phenomena such as coalescence and breakage of droplets, and drop size distribution. The topics discussed by the authors (10)... 

Vasudevan,T.V. and M.M.Sharma. Some aspects of process design of liquid-liquid reactor. (Int. Sympo-siuir. on Mass Transfer with Chemical Reaction in Two-Phase Systems", ACS-Meeting, Atlanta, 1981). 

The phenomenon of "mass transfer with chemical reaction" takes place v/henever one phase is brought into contact with one or more other phases not in chemical equilibrium with it. This phenomenon has industrial, biological and physiological importance, in chemical process engineering, it is encountered in both separation processes and reaction engineering. In some cases, a chemical reaction may deliberately be employed for speeding up the rate of mass transfer and/or for increasing the capacity of the solvent in other cases the multiphase reaction system is a part of the process with the specific aim of nroduct formation. 

A local overaU gas-phase coefficient for mass transfer with chemical reaction may be defined by the equation... 

In order to establish the conations for thermodynmnic feasibility of reactive mass exchange, it is necessary to invoke the basic principles of mass transfer with chemical reactions. Consider a lean phase j that contains a set Bj = [B j z — I,..., NZy of reactive species (i.e., the set Bj contains NZj reactive species, each denoted by B j, where the index z assumes values from 1 to NZy). These species react with the transferable key solute. A, or ammig themselves via Qj independent chemical reactions which may be represented by... 

Hofmann, H., in E. Alper (Ed.), Mass transfer with chemical reaction in multi phase systems, vol II, NATO ASI Series No. 73, Martinus Nijhoff Publishers, The Hague (1983) 171-197. 

The mass transfer with chemical reaction in the liquid phase has the... 

Carbon dioxide gas diluted with nitrogen is passed continuously across the surface of an agitated aqueous lime solution. Clouds of crystals first appear just beneath the gas-liquid interface, although soon disperse into the bulk liquid phase. This indicates that crystallization occurs predominantly at the gas-liquid interface due to the localized high supersaturation produced by the mass transfer limited chemical reaction. The transient mean size of crystals obtained as a function of agitation rate is shown in Figure 8.16. 

In general, the concentration of the reactant will decrease from CAo in the bulk of the fluid to CAi at the surface of the particle, to give a concentration driving force of CAo - CAi)-Thus, within the pellet, the concentration will fall progressively from CAi with distance from the surface. This presupposes that no distinct adsorbed phase is formed in the pores. In this section the combined effects of mass transfer and chemical reaction within the particle are considered, and the effects of external mass transfer are discussed in Section J 0.8.4. 

In the case where the blood flow is turbulent, we can use the concept of enhancement factor for the case of liquid-phase mass transfer with a chemical reaction (Section 6.5). Thus,... 

For multiphase reactive systems of types (a) and (b), at least one of the reactants has to reach the reaction zone from a different phase. In such systems, generally mass transfer between these two different phases (and its interaction with chemical reactions) is of primary importance and turbulent mixing is often of secondary importance. For such systems, modeling multiphase flows as discussed in Chapter 4 is directly applicable. The only additional complexity is the possibility of interaction between mass transfer and chemical reactions. The typical interphase mass transfer source for component k between phases p and q can be written (for the complete species conservation equation, refer to Chapter 4) ... 

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