Liquid-film controlling

Film Control. The designer needs to know whether his system is gas or liquid film controlling. For commercial processes this is known. 

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... 

From physical properties of system, determine Iqj and kL- If system is known or can be assumed to be essentially all gas or all liquid film controlling, then only the controlling k need be calculated. For greater accuracy, both values are recommended, because very few systems are more than 80% controlled by only one k. 

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-... 

Overall mass transfer coefficient based on liquid phase, lb mol/ (hr) (ft ) (lb mol/ft ) Overall mass transfer coefficient based on liquid film controlling, lb mol (hr) (ft ) (lb mol/ft3)... 

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. 

What is the form of equation 9.2-22 for liquid-film control ... 

For liquid-film control, there is no gas-phase resistance to mass transfer of A. Thus, in equation 9.2-14, l/kA( l/HAkAg, and KA( = kM, so that equation 9.2-22b may be... 

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). 

Consider a packed tower for the absorption of A from a gas containing inert material (in addition to A) by a liquid containing B (nonvolatile) under continuous, steady-state conditions. Absorption is accompanied by the instantaneous reaction A(g) + bB(() - products. Assume the overall process is liquid-film controlled. 

Flow through the liquid film controls the mass transfer if HA > 250 atm (mole fraction)-1. 

Figure 12.19. Driving force in gas and liquid-film controlled processes. The Figure shows the operating line...

If the liquid film controls the process, X, equals Xe and the driving force Xt — X is given in Figure 12.19 by the line DR. The evaluation of the integral ... 

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... 

From Figure 4.24, it is clear that in the case of liquid-film control, the curve has a tail in the beginning, while in the case of sohd diffusion control, the tail is at the end. 

Air-water exchange of PCE is liquid-film controlled (Table 20.56). For BC, the size of die Henry s law coefficient (K = 0.015) suggests a slight influence of the air-film which you may disregard. To relate v vw of PCE and BC, combine Eqs. 18-55 and 20-29 into the simple expression ... 

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...

The primary variable that determines whether the controlling resistance is in the liquid or gas film is the H or Henry constant. As shown in Figure 5.15, and as is apparent from equation 39, for small values of H the water phase film controls the transfer, and for high values of H the transfer is controlled by the air phase film. Gas transfer conditions that are liquid film controlled sometimes are expressed in terms of thickness, Zw, of the water film. As indicated by equation 38, this can be done from a measured value of (or K,o,) and the diffusion coefficient of the substance Zw decreases with the extent of turbulence (current velocity, wind speed, etc.). Typical values for are in the range of micrometers for seawater, a few hundred micrometers in lakes and up to 1 nun in small wind-sheltered water bodies (Brezonik, 1994). 

CALCULATION OF TOWER HEIGHT. An absorber can be designed using any of the four basic rate equations, but the gas-film coefficients are most common, and the use of K a will be emphasized here. Choosing a gas-film coefficient does not require any assumption about the controlling resistance. Even if the liquid film controls, a design based on K/i is as simple and accurate as one based on K a or k a. 

When the gas being absorbed is only slightly soluble, the liquid film usually has the controlling resistance because of the high value of m [see Eq. (22.12)], and the corresponding low value of K a leads to a low plate efl5dency [Eq. (21.77)]. The effect of liquid viscosity is also more important when the liquid film controls, because the lower diffusivity in the liquid phase has a more direct influence on... 

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