Transfer with and without chemical reaction

I. Residence-Time Model for Total Mass Transfer with and without Chemical Reaction... 

Gal-Or and Hoelscher (G5) have recently proposed a mathematical model that takes into account interaction between bubbles (or drops) in a swarm as well as the effect of bubble-size distribution. The analysis is presented for unsteady-state mass transfer with and without chemical reaction, and for steady-state diffusion to a family of moving bubbles. 

Two books deal almost exclusively with the subject of mass transfer with chemical reaction, the admirably clear expositions of Astarita (A6) and Danckwerts (D2). Since then a flood of theoretical and experimental work has been reported on gas absorption and related separations. The principal object of this chapter is to present techniques, results, and opinions published mainly during the last 6 or 7 years on mass-transfer coefficients and interfacial areas in most types of absorbers and reactors. This necessitates some review of mass transfer with and without chemical reaction in the first section, and comments about the simulation of industrial reactors by laboratory-scale apparatus in the concluding section. Although many gas-liquid reactions are accompanied by a rise in temperature that may be great enough to affect the rate of gas absorption, our attention here is confined to cases where the rise in temperature does not affect the absorption rate. This latter topic (treated by references B20, TIO, S3, T3, V5) could justify another complete chapter. 

The sections which follow outline the general multidimensional distribution theory. Applications of the theory are discussed to describe droplet size distributions and mixing frequencies in chemically equilibrated systems, the effects of droplet mixing on the extent of reaction, the analysis of mass transfer with and without chemical reaction, hydrocarbon fermentation, and emulsion polymerization. 

The main advantage of the Eulerian-Lagrangian formulation comes from the fact that each individual bubble is modeled, allowing consideration of additional effects related to bubble-bubble and bubble-liquid interactions. Mass transfer with and without chemical reaction, bubble coalescence, and redispersion, in principle, can be added directly to an Eulerian-Lagrangian hydrodynamic model. The main disadvantage of the Eulerian-Lagrangian approach is that only a limited number of particles (bubbles) can be tracked, such as when the superficial gas velocity is low (Chen et al., 2005), due to computer limitations. 

Kashid, M. N., Agar, D. W., Turek, S. (2007b). CFD modelling of mass transfer with and without chemical reaction in the liquid-liquid slug flow microreactor. Chemical Engineering Science, 62, 5102-5109. 

Kashid et al. [46] investigated mass transfer with and without chemical reaction in a liquid-liquid microreactor with slug flow (Figure 12.6). The authors modified Equations 12.21 and considered the computational domain to be two dimensional ... 

Solutions for diffusion with and without chemical reaction in continuous systems have been reported elsewhere (G2, G6). In general, all the parameters in this model can be determined or estimated, and the theoretical expressions may assist in the interpretation of mass-transfer data and the prediction of equipment performance. 

In this chapter, consideration will be given to the basic principles underlying mass transfer both with and without chemical reaction, and to the models which have been proposed to enable the rates of transfer to be calculated. The applications of mass transfer to the design and operation of separation processes are discussed in Volume 2, and ihe design of reactors is dealt with in Volume 3. 

Ba1di,G. and S.Sicardi. "A model for mass transfer with or without chemical reaction in packed columns". Chem.Engng.Sci. 30 (1975) 617. 

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-to-liquid mass transfer can take place from a gas phase to a liquid phase (and vice versa) with or without chemical reactions. The concentration gradient of a transferred component in the bulk fluid and in the fluid at the interface is the driving force for mass transfer. When the mass transport occurs in a phase that is moving, the transport of the component is known as convective mass transfer. Convective mass transfer... 

Three general classes of kinetic models that may apply to systems with rate control by mass transfer in the liquid or by interdiffusion in the particle with or without chemical reaction will be briefly reviewed here (for more detail, see [Helfferich, 1962a Helfferich and Hwang, 1988]). In particular I he following models will be examined liquid-phase mass transfer with linear driving force, Nernst-Planck models for intraparticle diffusion without reac-lion, and, Nernst-Planck models for intraparticle diffusion with accompanying reaction. 

For reactor scale-up for a given system, one may proceed as follows. First, a stirred bench scale reactor should be chosen with standardized dimensions. The net superficial gas flow rate should be kept within the indicated limits. Mass transfer measurements (with or without chemical reaction) should be carried out under realistic conditions. The influence of the stirrer speed should be measured accurately over a wide range. The superficial gas flow rate may also be varied, e.g., between 1 and 4 cm/s. With these experiments the constants in eq. (4.62) can be determined. The resulting equation may be used with confidence for the larger scale. The variables e and w, on the larger scale, should be within the limits of the bench scale tests ... 

One can also frequently choose between a purely mass-transfer operation and a chemical reaction or a combination of both. Water can be removed from an ethanol-water solution either by causing it to react with unslaked lime or by special methods of distillation, for example. Hydrogen sulfide can be separated from other gases either by absorption in a liquid solvent with or without simultaneous chemical reaction or by chemical reaction with ferric oxide. Chemical methods ordinarily destroy the substance removed, while mass-transfer methods usually permit its eventual recoveiy in unaltered form without great difficulty. 

There are two very active special fields of phase-transfer appHcations that transcend classes (/) and 2) metal—organic reactions both with and without added bases, and polymer chemistry. Certain chemical modifications of side groups, polycondensations, and radical polymerizations can be influenced favorably by PTC. 

Steady state models of the automobile catalytic converter have been reported in the literature 138), but only a dynamic model can do justice to the demands of an urban car. The central importance of the transient thermal behavior of the reactor was pointed out by Vardi and Biller, who made a model of the pellet bed without chemical reactions as a onedimensional continuum 139). The gas and the solid are assumed to have different temperatures, with heat transfer between the phases. The equations of heat balance are ... 

Equations describing the transfer rate in gas-liquid dispersions have been derived and solved, based on the film-, penetration-, film-penetration-, and more advanced models for the cases of absorption with and without simultaneous chemical reaction. Some of the models reviewed in the following paragraphs were derived specifically for gas-liquid dispersion, whereas others were derived for more general cases of two-phase contact. 

This model is proposed for steady-state mass transfer without chemical reaction from swarms of moving bubbles with clean interfaces and without interaction between adjacent bubbles. 

Conducted in 10% CH2Cl2-90% acetonitrile for compounds [54] and [56] and in acetonitrile [55] upon addition of 2 equiv of the respective cation supporting electrolyte, 0.10 mol dm-3 TBABF4. The potential of the reduction current peak r, reversible q, quasi-reversible s, single reduction peak without corresponding reoxidation peak ec, electron transfer followed by a chemical reaction ec, ad, electron transfer followed by a chemical reaction with insoluble product which adsorbs on to the electrode surface. Prewaves are in parentheses. 

Table 1. Dimensionless mass flux for mass transfer without chemical reaction. [The results obtained from eqs (21), (23) and (24) are compared with the results obtained from the numerical model]...

Photophysics involves the absorption, transfer, movement, and emission of electromagnetic, light, energy without chemical reactions. By comparison, photochemistry involves the interaction of electromagnetic energy that results in chemical reactions. Let us briefly review the two major types of spectroscopy with respect to light. In absorption studies, the detector is placed... 

It is obvious that re-atomization yields decrease the mean diameter of the liquid droplets and thus an increased interface area at the same time, it results in reduced average transfer coefficients, because heat and mass transfer coefficients between gas flow and particle or droplet are in positive correlation with the diameter of the particle or droplet, while coalescence of droplets yields influences opposite to those described above. In their investigation on the absorption of C02 into NaOH solution, Herskowits et al. [59, 60] determined theoretically the total interface areas and the mass transfer coefficients by comparing the absorption rates with and without reaction in liquid, employing the expression for the enhancement factor due to chemical reaction of second-order kinetics presented by Danckwerts [70],... 

This model was applied to the same data for batch and flowthrough systems with and without acid addition as for the previous two models, and some of the xylan conversion predictions calculated from the data and concentration predictions via Eq. 8 are summarized in Figs. 5 and 6 for batch and flowthrough systems, respectively. Tables 4 and 2 present the parameters and the SSE values for the branched pore model, respectively. Overall, although some data are better matched than others, hemicellulose hydrolysis models based on mass transfer alone can predict performance in batch and flow systems as well as, if not better than, reaction-only models. In addition, the changes in mass transfer coefficient with flow are consistent with expectations for a mass transfer model but not for strictly a chemical reaction. 

When this conjugation occurs, the level of active (corrosive) acid is substantially decreased. No simple quantitive correlation has been shown between the acidity (pKa) of acids in hydrocarbon formulation and low polar solvents (Coetzee, 1967). Acid-base interaction with and without proton transfer (PT) (BH+A B...(HA)m) has been related to acid and base enthalpies of reaction (Pawlak and Bates, 1982), the infrared carbonyl stretching band and gradual appearance of the asymmetric COO band (Lindeman and Zundel, 1972 Magonski and Pawlak, 1982), changes in pH (Kuna et al., 1982 Pawlak et al., 1982), NMR proton chemical shifts (Magonski and Pawlak, 1982), and dipole moments (Sobczyk and Pawelka, 1979). These parameters depend upon the acid-base strength of the partners, ApKa(PT) the difference between the pKa(acceptor) and pKa(donor) on the water scale (Sobczyk, 2001). 

INSTABILITIES OF FLOWS WITH AND WITHOUT HEAT TRANSFER AND CHEMICAL REACTION... 

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