Control resistance

E] Based on oxygen transfer from water to air 77 F. Liquid film resistance controls. (Dwnei- 77 F = 2.4 X 10 ). Equation is dimensional. Data was for thin-waUed polyethylene Raschig rings. Correlation also fit data for spheres. Fit 25%. See Reiss for graph. 

E] Ammonia absorption into water from air at 70 F. Gas-film resistance controls. Thin-waUed polyethylene Raschig rings and 1-inch Intalox saddles. Fit 25%. See Reiss for fit. Terms defined as above. 

In addition to secondarv resistance control, other devices such as reactors and thyristors (solid-state controllable rectifiers) are used to control wound-rotor motors. Fixed secondary reactors combined with resistors can provide veiy constant accelerating torque with a minimum number of accelerating steps. The change in slip frequency with speed continually changes the effective reac tance and hence the value of resistance associated with the reactor. The secondaiy reactors, resistors, and contacts can be varied in design to provide the proper accelerating speed-torque curve for the protection of belt conveyors and similar loads. 

Note that H is simply Henry s constant corrected for units. When the solute gas is readily soluble in the liquid solvent, Henry s law constant (H or H ) is small and Kj approximately equals k, and the absorption process is controlled by the gas film resistance. For systems where the solute is relatively insoluble in the liquid, H is large and K( approximately equals k, and the absorption rate is controlled by the liquid phase resistance. In most systems, the solute has a high solubility in the solvent selected, resulting in the system being gas film resistance controlled. 

Resistivity. Control of the resistivity of the mud and mud filtrate while drilling may be desirable to permit better evaluation of formation characteristics from electric logs. The determination of resistivity is essentially the measurement of the resistance to electrical current flow through a known sample configuration. Measured resistance is converted to resistivity by use of a cell constant. The cell constant is fixed by the configuration of the sample in the cell and is determined by calibration with standard solutions of known resistivity. The resistivity is expressed in ohm-meters. 

Similar considerations also apply to the dielectric films formed on the metal surface during anodising, and, for example, in the case of the valve metals (Al, Ti, Ta, Nb, etc.) IR drops of hundreds of volts may be produced by the anodic oxide film formed on the metal surfaces. Paint films applied to a metal surface also exert resistance control see Section 14.3). 

Fig. 1.27 Evans diagrams illustrating (a) cathodic control, (b) anodic control, (c) mixed control, (d) resistance control, (e) how a reaction with a higher thermodynamic tendency ( r, ii) may result in a smaller corrosion rate than one with a lower thermodynamic tendency and (/) how gives no indication of the corrosion rate...

Figures 1.27a to d show how the Evans diagram can be used to illustrate how the rate may be controlled by either the polarisation of one or both of the partial reactions (cathodic, anodic or mixed control) constituting corrosion reaction, or by the resistivity of the solution or films on the metal surface (resistance control). Figures 1. lie and/illustrate how kinetic factors may be more significant than the thermodynamic tendency ( , u) and how provides no information on the corrosion rate.

Denver, Colorado (Refs. 16 and 19) 3 (0 in buildings) A propane release at a polymerization unit in a process plant resulted in a blast that destroyed the process unit. The blast-resistant control house, located only 98 ft (30 m) from the blast center, sustained little damage. 

Mackay, D. Shiu, W.Y., Chau, E. (1983) Calculation of diffusion resistance controlling volatilization rates of organic contaminants from water. Can. J. Fish. Aqua. Sci. 40, 295-303. 

The solvent enters the tower free of H2S and leaves containing 0.013 kmol of H2S/kmol of solvent. If the flow of inert hydrocarbon gas is 0.015 kmol/m2s of tower cross-section and the gas-phase resistance controls the process, calculate ... 

The overall absorption coefficient KGa may be taken as 1.5 x 10 4 kmol/[m3s (kN/m2) partial pressure difference] and the gas film resistance controls the process. 

The signal generated by the complex is governed by several physical phenomena associated with the matrix thickness. As soon as the probe is placed in contact with the analyte, external mass transfer controls the movement of the analyte toward the surface of the optical probe.(S4) The osmotic pressure and Gibbs free energy dictate the permeation of the analyte into the matrix. Once the analyte has penetrated the matrix, internal mass transfer resistance controls the movement of the analyte in the matrix. Eventually, the probe reaches a steady state of equilibrium with molecules continuously moving in and out of the matrix. 

Gas film resistance controls for highly soluble gases. 

Liquid film resistance controls for slightly soluble gases. 

At both ends of the tower fc sPA < therefore, gas-phase resistance controls... 

As with particles of constant size, let us see what rate expressions result when one or the other of the resistances controls. 

Combination of Resistances. The above conversion-time expressions assume that a single resistance controls throughout reaction of the particle. However, the relative importance of the gas film, ash layer, and reaction steps will vary as particle conversion progresses. For example, for a constant size particle the gas film resistance remains unchanged, the resistance to reaction increases as the surface of unreacted core decreases, while the ash layer resistance is nonexistent at the start because no ash is present, but becomes progressively more and more important as the ash layer builds up. In general, then, it may not be reasonable to consider that just one step controls throughout reaction. 

Predictability of Film Resistance. The magnitude of film resistance can be estimated from dimensionless correlations such as Eq. 24. Thus an observed rate approximately equal to the calculated rate suggests that film resistance controls. 

When in plug flow all solids stay in the reactor for the same length of time. From this and the kinetics for whatever resistance controls, the conversion X R) for any size of particle R can be found. Then the mean conversion of the solids leaving the reactor can be obtained by properly summing to find the overall contribution to conversion of all sizes of particles. Thus,... 

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

For ash resistance controlling replacement of Eq. 25.18 in Eq. 6 followed by integration leads to a cumbersome expression which on expansion yields [see Yagi and Kunii (1961)]... 

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