Gas-phase response

Figure 4.2 shows the computer simulation results of such a dynamic aeration experiment. The y-axis shows the response fraction with respect to time. The gas phase response is typically first-order, and the liquid phase shows some lag or delay on the signal. The electrode response is much more delayed for a slow-acting electrode.4... 

Figure 2. Mixing behavior of the reactor cell gas phase response to a step increase in inlet CO concentration.

Figure 9. The gas phase responses for ir,Ns, 14Nt and UN1SN obtained after substitution of a feedstream containing I5NO/Ar by a stream containing 1ANO/HJAr Pso = 8 X 10 s atm, PH% — 7yfl0 2 atm, and T = 438 K.

From Figure 8.8, Vf > vp, so cbcf/dxp > 1. Hence, transient effects will cause a lower speed for wind-aided spread. Again, since the gas phase response time is much faster than the solid, we can use steady gas phase results for qf and <5f in these formulas for flame spread on surfaces. 

COMMENTS This calculation assumes aU water generated goes directly to liquid and none to vapor, which is unlikely to be true, so that the time scale required for liquid water accumulation will be even greater than calculated. In a PEFC, this slow time scale, the slow time scales of evaporation or drainage from the porous media and electrolyte, coupled with the nearly instantaneous electrochemical and gas-phase response times lead to hysteresis in a polarization curve and the fuel cell memory effect discussed. 

As also noted in the preceding chapter, it is customary to divide adsorption into two broad classes, namely, physical adsorption and chemisorption. Physical adsorption equilibrium is very rapid in attainment (except when limited by mass transport rates in the gas phase or within a porous adsorbent) and is reversible, the adsorbate being removable without change by lowering the pressure (there may be hysteresis in the case of a porous solid). It is supposed that this type of adsorption occurs as a result of the same type of relatively nonspecific intermolecular forces that are responsible for the condensation of a vapor to a liquid, and in physical adsorption the heat of adsorption should be in the range of heats of condensation. Physical adsorption is usually important only for gases below their critical temperature, that is, for vapors. 

Solvent Recovery. Most of the activated carbon used in gas-phase applications is employed to prevent the release of volatile organic compounds into the atmosphere. Much of this use has been in response to environmental regulations, but recovery and recycling of solvents from a range of industrial processes such as printing, coating, and extmsion of fibers also provides substantial economic benefits. 

When liquid content of the feed is high, a condenser and a separator are needed. The liquid-to-gas ratio can be as high, so that even at reaction temperatures a liquid phase is present. The reactor still performs as a CSTR, however the response time for changes will be much longer than for vapor phase alone. Much lower RPM will be needed for liquid-phase studies (or liquid and gas phase experiments) since the density of the pumped fluid is an order-of-magnitude greater than for vapor phase alone. In this case a foamy mixture or a liquid saturated with gas is recirculated. 

The surface potential of a liquid solvent s, %, is defined as the difference in electrical potentials across the interface between this solvent and the gas phase, with the assumption that the outer potential of the solvent is zero. The potential arises from a preferred orientation of the solvent dipoles in the free surface zone. At the surface of the solution, the electric field responsible for the surface potential may arise from a preferred orientation of the solvent and solute dipoles, and from the ionic double layer. The potential as the difference in electrical potential across the interface between the phase and gas, is not measurable. However, the relative changes caused by the change in the solution s composition can be determined using the proper voltaic cells (see Sections XII-XV). 

---