Transfer effective

Discussion of the concepts and procedures involved in designing packed gas absorption systems shall first be confined to simple gas absorption processes without compHcations isothermal absorption of a solute from a mixture containing an inert gas into a nonvolatile solvent without chemical reaction. Gas and Hquid are assumed to move through the packing in a plug-flow fashion. Deviations such as nonisotherma1 operation, multicomponent mass transfer effects, and departure from plug flow are treated in later sections. 

Reactor Configuration. The horizontal cross-sectional area of a reactor is a critical parameter with respect to oxygen mass-transfer effects in LPO since it influences the degree of interaction of the two types of zones. Reactions with high intrinsic rates, such as aldehyde oxidations, are largely mass-transfer rate-limited under common operating conditions. Such reactions can be conducted effectively in reactors with small horizontal cross sections. Slower reactions, however, may require larger horizontal cross sections for stable operation. 

Relations for transport properties such as viscosity and thermal conductivity are also required if wall friction and heat-transfer effects are considered. 

Heat transfer and mass transfer occur simultaneously whenever a transfer operation involves a change in phase or a chemical reaction. Of these two situations, only the first is considered herein because in reacting systems the complications of chemical reaction mechanisms and pathways are usually primary (see HeaT-EXCHANGETECHNOLOGy). Even in processes involving phase changes, design is frequendy based on the heat-transfer process alone mass transfer is presumed to add no compHcations. But in fact mass transfer effects do influence and can even limit the process rate. 

One goal of catalyst designers is to constmct bench-scale reactors that allow determination of performance data truly indicative of performance in a full-scale commercial reactor. This has been accompHshed in a number of areas, but in general, larger pilot-scale reactors are preferred because they can be more fully instmmented and can provide better engineering data for ultimate scale-up. In reactor selection thought must be given to parameters such as space velocity, linear velocity, and the number of catalyst bodies per reactor diameter in order to properly model heat- and mass-transfer effects. 

In rotary devices, reradiation from the exposed shelf surface to the solids bed is a major design consideration. A treatise on furnaces, including radiative heat-transfer effects, is given by Ellwood and Danatos [Chem. Eng., 73(8), 174 (1966)]. For discussion of radiation heat-transfer computational methods, heat fliixes obtainable, and emissivity values, see Schornshort and Viskanta (ASME Paper 68-H 7-32), Sherman (ASME Paper 56-A-III), and the fohowing subsection. 

When two-phase mass transfer is required to supply reactants by mixing for a chemical reaction, the most important factor to consider is whether the mass transfer controls the operation or whether the chemical reaction controls it. This can be done by increasing the mixer speed to a point w here mass transfer effects become very high and the operation is limited by the chemical reaction. 

The mass transfer effect is relevant when the chemical reaction is far faster than the molecular diffusion, i.e. Ha > 1. The rapid formation of precipitate particles should then occur spatially distributed. The relative rate of particle formation to chemical reaction and/or diffusion can as yet be evaluated only via lengthy calculations. 

Second, using the fully relativistic version of the TB-LMTO-CPA method within the atomic sphere approximation (ASA) we have calculated the total energies for random alloys AiBi i at five concentrations, x — 0,0.25,0.5,0.75 and 1, and using the CW method modified for disordered alloys we have determined five interaction parameters Eq, D,V,T, and Q as before (superscript RA). Finally, the electronic structure of random alloys calculated by the TB-LMTO-CPA method served as an input of the GPM from which the pair interactions v(c) (superscript GPM) were determined. In order to eliminate the charge transfer effects in these calculations, the atomic radii were adjusted in such a way that atoms were charge neutral while preserving the total volume of the alloy. The quantity (c) used for comparisons is a sum of properly... 

We conclude that more work is need<. In particular it would be useful to repeat the TB-LMTO-CPA calculations using also other methods for description of charge transfer effects, e.g., the so-called correlated CPA, or the screened-impurity modeP. One may also cisk if a full treatment of relativistic effects is necessary. The answer is positive , at least for some alloys (Ni-Pt) that contain heavy elements. 

Most of the present implementations of the CPA on the ab-initio level, both for bulk and surface cases, assume a lattice occupied by atoms with equal radii of Wigner-Seitz (or muffin-tin) spheres. The effect of charge transfer which can seriously influence the alloy energetics is often neglected. Several methods were proposed to account for charge transfer effects in bulk alloys, e.g., the so-called correlated CPA , or the screened-impurity model . The application of these methods to alloy surfaces seems to be rather complicated. 

Figure 1 A dilute alloy system, showing a substitutional impurity, an interstitial impurity and an electromigration defect, and its reference system, the unperturbed host system. Some charge transfer effects are shown. Lattice distortion effects are omitted.

When fluid swirling is prominent, it is difficult or impossible to reproduce the same mass transfer effect in any other size vessel, and hence cannot be reproduced by geometric similarity [21]. 

E = superficial liquid entrainment, Ib/hr/fL or, heat transfer effectiveness. 

In addition to impurities, other factors such as fluid flow and heat transfer often exert an important influence in practice. Fluid flow accentuates the effects of impurities by increasing their rate of transport to the corroding surface and may in some cases hinder the formation of (or even remove) protective films, e.g. nickel in HF. In conditions of heat transfer the rate of corrosion is more likely to be governed by the effective temperature of the metal surface than by that of the solution. When the metal is hotter than the acidic solution corrosion is likely to be greater than that experienced by a similar combination under isothermal conditions. The increase in corrosion that may arise through the heat transfer effect can be particularly serious with any metal or alloy that owes its corrosion resistance to passivity, since it appears that passivity breaks down rather suddenly above a critical temperature, which, however, in turn depends on the composition and concentration of the acid. If the breakdown of passivity is only partial, pitting may develop or corrosion may become localised at hot spots if, however, passivity fails completely, more or less uniform corrosion is likely to occur. 

Very recently, Luckner et al. (116) obtained initial rate data for the metathesis of propene using the W0r-Si02 catalyst at flow rates where mass transfer effects were found to be negligible. Their experimental data referring to measurements at 0.1 to 0.9 MNm-2 and 672 to 727 K could be correlated by Eq. (53). 

Among the earlier studies of reaction kinetics in mechanically stirred slurry reactors may be noted the papers of Davis et al. (D3), Price and Schiewitz (P5), and Littman and Bliss (L6). The latter investigated the hydrogenation of toluene catalyzed by Raney-nickel with a view to establishing the mechanism of the reaction and reaction orders, the study being a typical example of the application of mechanically stirred reactors for investigations of chemical kinetics in the absence of mass-transfer effects. 

In this region, the mass transfer effects are small and the rate is determined almost entirely by the reaction kinetics. 

Thus t]X s 1, corresponding to the region where mass transfer effects dominate. The concentration profile is given by equation 10.198 as ... 

Boguslavsky, L. I. Electron Transfer Effects and the Mechanism of the Membrane Potential 18... 

For the lower heat transfer surfaces in Fig. 2.60 to contribute to the energy transport, the solid should be an effective conductor of heat through its thickness. In other words, conjugate heat transfer effects should not create a more significant resistance to heat flow than that of the fluid in the channel. Since the heat transfer coefficient is generally a maximum at CHF, this leads to... 

Most of heat transfer correlations are based on data obtained in flow boiling from relatively large diameter conduits and the predictions from these correlations show considerable variability. Effects of superficial liquid and gas velocity on heat transfer in gas-liquid flow and its connection to flow characteristics were studied by Hetsroni et al. (1998a,b, 2003b), Zimmerman et al. (2006), Kim et al. (1999), and Ghajaret al. (2004). However these investigation were carried out for tubes of D = 25—42 mm. These data, as well as results presented by Bao et al. (2000) in tubes of L> = 1.95 mm and results obtained by Hetsroni et al. (2001), Mosyak and Hetsroni (1999) are discussed in the next sections to clarify how gas and liquid velocities affect heat transfer. Effects of the channel size and inclination are considered. 

Fig. 1 shows the thermal decomposition curves of HDPE mixed with Al-MCM-41, with respect to time, at isothermal operating temperatures. Lag periods were formed at the initial stage of decomposition, possibly due to the heat transfer effect, which could delay the decomposition of a sample until the latter reaches the operating temperatures. As the reaction ten erature increased, the reaction time became noticeably shorter. The shortening of the reaction time was clearly observed when the reaction occurred at the reaction teirperatures between 420 and 460 °C. The HDPE on Al-MCM-41-P decomposed faster than that on blank and that on A1-MCM-41-D, as shown in Fig. 1(b). 

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