Controlling resistances

Fig. 6. Concentration profiles through an idealized biporous adsorbent particle showing some of the possible regimes. (1) + (a) rapid mass transfer, equihbrium throughout particle (1) + (b) micropore diffusion control with no significant macropore or external resistance (1) + (c) controlling resistance at the surface of the microparticles (2) + (a) macropore diffusion control with some external resistance and no resistance within the microparticle (2) + (b) all three resistances (micropore, macropore, and film) significant (2) + (c) diffusional resistance within the macroparticle and resistance at the surface of the...

This is the important rule of additivity of resistances. In practice, and are often of the same order of magnitude, but the distribution coefficient m can vary considerably. For solutes which preferentially distribute toward solvent B, m is large and the controlling resistance Hes in phased. Conversely, if the distribution favors solvent A the controlling mass-transfer resistance Hes in phase B. 

With good diy scrubbing sorbents, the controlling resistance for gas cleaning is external turbulent diffusion, which also depends on energy dissipated by viscous and by inertial mechanisms. It turns out to Be possible to correlate mass-transfer rate as a fimctiou of the fric tiou Factor. 

Electrostatic precipitators are satisfactory devices for removing small particles from moving gas streams at high collection efficiencies. They have been used almost universally in power plants for removing fly ash from the gases prior to discharge. Electrostatic precipitators have the capability of fine particulate control. Resistivity plays an important role in determining whether a particle can be readily collected in this device. 

Equipment Material Treated Agitation Heating Smface Overall Coeff. Btu/hr (Ft2)(°F) Controlling Resistance... 

Where the use of zinc anodes is practicable, the low driving potential is a great advantage since the resistance of the steel to be cathodically protected is the controlling resistance, and the current output of the anode varies with the requirements of the cathode. Thus it can be said that zinc anodes are largely self-governing. 

Kl = 2xl0 3(323cm/min)°-7(0.051)a2 = 7.9X10 4S Liquid film was the controlling resistance. 

In evaluating their results they assumed the film theory, and, because the oxygen is sparingly soluble and the chemical reaction rate high, they also assumed that the liquid film is the controlling resistance. The results were calculated as a volumetric mass-transfer coefficient based, however, on the gas film. They found that the volumetric mass-transfer coefficient increased with power input and superficial gas velocity. Their results can be expressed as follows ... 

Assume that this is the controlling resistance so that U=h. A kinetic model is needed for Rp and for the instantaneous values of and Iw The computer program in Appendix 13 includes values for physical properties and an expression for the polymerization kinetics. Cumulative values for the chain lengths are calculated as a function of position down the tube using... 

The exact pathologic cause of ADHD has not been identified. ADHD is generally thought of as a disorder of self-regulation or response inhibition. Patients who meet the criteria for ADHD have difficulty maintaining self-control, resisting distractions,... 

Burgmayer and Murray [40] reported electrically controlled resistance to the transport of ions across polypyrrole membrane. The membrane was formed around a folded minigrid sheet by the anodic polymerization of pyrrole. The ionic resistance, measured by impedance, in 1.0 M aqueous KC1 solution was much higher under the neutral (reduced) state of the polymers than under the positively charged (oxidized) state. The redox state of polypyrrole was electrochemically controlled this phenomenon was termed an ion gate, since the resistance was varied from low to high and vice versa by stepwise voltage application. 

Voehringer DW et al. Gene microarray identification of redox and mitochondrial elements that control resistance or sensitivity to apoptosis. Proc Natl Acad Sci USA 1999 97 2680-2685. 

Determine the controlling resistance. From the isotherm, nref = (500 x... 

From Eq. 3.9, the greater the thickness (5) of each phase the larger the resistance to solute transfer. Although the use of 5 is convenient for modeling and conceptualizing WBL resistance, it is largely fictitious, as complex hydrodynamics control resistance to mass transfer across the WBL in environmental exposures (see Section 3.6.5. for a more in- depth discussion on this phenomenon). 

The rate of mass transfer of a snbstance across a water-gas bonndary is controlled by the diffnsion film model as well. Gas transfer from a water sonrce is faster than from a solid sonrce, and the chemical does not nndergo a chemical reaction during the transfer process. Under these conditions, the interface concentration may be interpreted in terms of the Henry constant (K ), which indicates whether the controlling resistance is in the liqnid or the gas film. When 5, a water film is the controlling factor, while a gas film controls the behavior when K >500. 

What is the most reasonable interpretation, in terms of controling resistances, of the kinetic data of Table E18.7 obtained in a basket type mixed flow reactor if we know that the catalyst is porous Assume isothermal behavior. 

Find the rate of absorption, the controlling resistance, and what is happening in the liquid film, at a point in the column where Cb = 500 mol/m. ... 

Figure 25.5 Representation of a reacting particle when diffusion through the gas film is the controlling resistance.

This expression may be integrated for the various controlling resistances. ox film resistance controlling, Eq. 25.11 with Eq. 6 yields... 

Since a hard product material is formed during reaction, film diffusion can be ruled out as the controlling resistance. For chemical reaction controlling Eq. 25.38 shows that... 

As the experimentally found diameter dependency lies between these two values, it is reasonable to expect that both these mechanisms offer resistance to conversion. Using in turn ash diffusion and chemical reaction as the controlling resistance should then give the upper and lower bound to the conversion expected. 

Mass transfer rates are increased in the presence of eruptions because the interfacial fluid is transported away from the interface by the jets. For mass transfer from drops with the controlling resistance in the continuous phase, the maximum increase in the transfer rate is of the order of three to four times (S8), not greatly different from the estimate of Eq. (10-4) for cellular convection. This may indicate that equilibrium is attained in thin layers adjacent to the interface during the spreading and contraction. When the dispersed-phase resistance controls, on the other hand, interfacial turbulence may increase the mass transfer rate by more than an order of magnitude above the expected value. This is almost certainly due to vigorous mixing caused by eruptions within the drop. 

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