Gas film control

In situations where the gas film resistance is predominant (gas film-controlled situation), k Pis much smaller than and the tie line is very steep. approachesjy so that the overall gas-phase driving force and the gas-film driving force become approximately equal, whereas the Hquid-film driving force becomes negligible. From equation 7 it also follows that in such cases. The reverse is tme if the Hquid film resistance is controlling. Since the... 

Mass-transfer theory indicates that for trays of a given design the factors most hkely to inflnence E in absorption and stripping towers are the physical properties of the flnids and the dimensionless ratio Systems in which the mass transfer is gas-film-controlled may be expected to have plate efficiencies as high as 50 to 100 percent, whereas plate efficiencies as low as 1 percent have been reported for the absorption of gases of low sohibility (large m) into solvents of relatively high viscosity. 

Assuming that the absorption process is gas-film controlled, and that the concentration of the solute is small (i.e., 1 - y = 1), then ... 

The transfer of mass between phases in a packed tower occurs either as essentially all gas film controlling, all liquid film controlling, or some combination of these mechanisms (see Figure 9-69). To express the ease (low number... 

The transfer process is termed gas film controlling if essentieilly all of the resistance to mass transfer is in the gas film. This means that the gas is usually quite soluble in, or reactive with, the liquid of the system. If the system is liq-... 

For predominately liquid film controlling system, A Hg is almost negligible and Hql = Hl liketvise for gas film controlling, HiyA is negligible and Hqg = Hg-... 

Diffusion coefficients are used to estimate K a values for gas film controlling systems ... 

Special cases arise from each of equations 9.2-13 and -14, depending on the relative magnitudes of kAg and kA(. For example, from equation 9.2-13, if kAg is relatively large so that VkAg HAlkA(, then KAg (and hence NA) is determined entirely by kA(, and we have the situation of liquid-film control. An important example of this is the situation in which the gas phase is pure A, in which case there is no gas film for A to diffuse through, and kAg -> >. Conversely, we may have gas-film control. Similar conclusions may be reached from consideration of equation 9.2-14 for KA(. In either case, we obtain the following results ... 

Two extreme cases of equation 9.2-22 or -22a or -22b arise, corresponding to gas-film control and liquid-film control, similar to those for mass transfer without chemical reaction (Section 9.2.2). The former has implications for the location of the reaction plane (at distance 8 from the interface in Figure 9.6) and the corresponding value of CB. These points are developed further in the following two examples. 

That is, S decreases as cB increases. 8 can only decrease to zero, since the reaction plane cannot occur in the gas film (species B is nonvolatile). At this condition, the reaction plane coincides with the gas-liquid interface, and pAi, cAi, and cB (at the interface) are all zero. This corresponds to gas-film control, since species A does not penetrate the liquid film. 

To obtain (—rA) for gas-film control, we may substitute cBmax from equation 9.2-23 into equation 9.2-22, and again use kM HAkAg, together with equation 9.2-21 ... 

Note that the enhancement factor E is relevant only for reaction occurring in the liquid film. For an instantaneous reaction, the expressions may or may not involve E, except that for liquid-film control, it is convenient, and for gas-film control, its use is not practicable (see problem 9-12(a)). The Hatta number Ha, on the other hand, is not relevant for the extremes of slow reaction (occurring in bulk liquid only) and instantaneous reaction. The two quantities are both involved in rate expressions for fast reactions (occurring in the liquid film only). 

The values of both t and /, are the same as in part (a), 20 and 6.67 min, respectively. The latter is because, for reaction and gas-film control, the forms of the expressions for t, are the same with respect to the parameters that change. 

B = 0.80, t, which is a measure of the size of reactor, is about 1.7 min for ash-layer control, 9.5 min for reaction control, and 14.5 min for gas-film control. The relatively favorable behavior for ash-layer diffusion control in this example reflects primarily the low value of (1.67 min versus 6.67 min for the other two cases) imposed. 

If the results cited above are put another way, from the point of view of determining fB for a given f (or reactor size), a mean residence time of 1.7 min gives fB = 0.80 for ash-layer control, as noted, but only 0.37 for reaction control, and only 0.23 for gas-film control. 

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. 

For very soluble gases, is small and K q —k g and the system is gas-film controlled for rather insoluble gases, is large and Ki —k[ and liquid-film control operates. 

We have to guess the value of. It can be anywhere between 0 Pa (gas film controls) up to 20 Pa (liquid film controls). Let us guess no gas-phase resistance. Then = Pa in which case... 

Experiments are carried out at atmospheric pressure on the absorption into water of ammonia from a mixture of hydrogen and nitrogen, both of which may be taken as insoluble in the water. For a constant mole fraction of 0.05 of ammonia, it is found that the absorption rate is 25 per cent higher when the molar ratio of hydrogen to nitrogen is changed from 1 1 to 4 1. Is this result consistent with the assumption of a steady-state gas-film controlled process and, if not, what suggestions have you to make to account for the discrepancy ... 

The assumption of a gas-film controlled process may not be valid. If there is a liquid-film resistance, the effect of increasing the gas-film diffusivity will be less than predicted for a gas-film controlled process. 

Figure 2-4 Typical concentration profiles of instantaneous reaction between the gas A and the reactant C, based on film theory, ids Diffusion controlled - slow reaction, (fcl kinetically controlled-slow reaction, (c) gas-film-controlled desorption - fast reaction, 0 liquid-film-controlled desorption-fast reaction, (e) liquid-film-controlled absorption -instantaneous reaction between A and C, (/) gas-film-controlled absorption-instantaneous reaction between A and C, (g) concentration profiles for A, B, and C for instantaneous reaction between A and C-both gas- and liquid-phase resistances are comparable.1 2...

When H is less than about 5, we have water film control when H is larger than 500, gas film control prevails. For compounds with intermediate values for H, both films contribute to gas transfer resistance. Large molecules or polar compounds like phenols are air-film controlled. Small molecules and nonpolar compounds are water-film controlled. 

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