Transfer units liquid Film control

This operation is so highly liquid-film controlled that the gas-film resistance, for practical purposes, can be neglected. Therefore, the number of transfer units required is ... 

In the case where the rate of catalytic or enzymatic reaction is controlled by the mass transfer resistance of the liquid film around the particles containing catalyst or enzyme, the rate of decrease of the reactant A per unit liquid volume (i.e., (kmol m 3 s )) is given by Equation 7.13 ... 

This form is particularly appropriate when the gas is of low solubility in the liquid and "liquid film resistance" controls the rate of transfer. More complex forms which use an overall mass transfer coefficient which includes the effects of gas film resistance must be used otherwise. Also, if chemical reactions are involved, they are not rate limiting. The approach given here, however, illustrates the required calculation steps. The nature of the mixing or agitation primarily affects the interfacial area per unit volume, a. The liquid phase mass transfer coefficient, kL, is primarily a function of the physical properties of the fluid. The interfacial area is determined by the size of the gas bubbles formed and how long they remain in the mixing vessel. The size of the bubbles is normally expressed in terms of their Sauter mean diameter, dSM, which is defined below. How long the bubbles remain is expressed in terms of gas hold-up, H, the fraction of the total fluid volume (gas plus liquid) which is occupied by gas bubbles. 

The fundamental principles of the gas-to-liquid mass transfer were concisely presented. The basic mass transfer mechanisms described in the three major mass transfer models the film theory, the penetration theory, and the surface renewal theory are of help in explaining the mass transport process between the gas phase and the liquid phase. Using these theories, the controlling factors of the mass transfer process can be identified and manipulated to improve the performance of the unit operations utilizing the gas-to-liquid mass transfer process. The relevant unit operations, namely gas absorption column, three-phase fluidized bed reactor, airlift reactor, liquid-gas bubble reactor, and trickled bed reactor were reviewed in this entry. 

Agitation mainly affects the a in kifl, since it represents the total interfacial area per unit volume of gas plus liquid. It therefore has units of length. The units of kifl are usually expressed in s or min. Typical ki a values for agitated vessels lie between 0.05 to 0.4 s. The mass-transfer coefficient is one of the resistances to transport of species (i) from the gas phase inside the bubble to the bulk fluid outside the bubble. The overall resistance 1/A l is the sum of the inside and outside resistances shown in Equation (9.39), where E is the equilibrium constant. In the great majority of cases, l- This implies that kQ is small compared with k and means that the liquid film resisfance outside the bubble is controlling ... 

The overall coefficient K is the smallest of the three. In the limiting case where one of the film coefficients is much smaller than the other, K is essentially equal to the smaller of the two. We then tend to speak of a system as liquid-phase or gas-phase controlled. The same sort of relationship holds among the NTUs calculated under the different bases, and its inverse connects the heights of the various types of transfer unit. The derivations of all these relationships can be found in any textbook on diffusional operations. Suffice it to say here that we have... 

Increase in mass-transfer rate per unit area. As stated above, agitated gas-liquid contactors are used, in general, when it is necessary to deal with sparingly soluble gases. According to the terminology of the film theory, absorption is then controlled by the liquid resistance, and agitation of the liquid phase could increase the mass-transfer rate per unit area. As will be... 

Relative Kga valid for all systems controlled by mass transfer coefficient (Kg) and wetted area (a) per unit volume of column. Some variation should be expected when liquid reaction rate is controlling (not liquid diffusion rate). In these cases liquid hold-up becomes more important. In general a packing having high liquid hold-up which is clearly greater than that in the falling film has poor capacity. 

Since membrane-cell evaporators do not produce solids, forced-circulation evaporators are used less frequently. Rising-film and falling-film types appear in a number of plants. The rising-film evaporator depends on natural circulation of caustic from the bottom to the top of the tubes. Falling-film evaporators, as shown in Section 9.3.S.2, depend on pumps to lift caustic to the distribution system at the top. These units generally have better heat-transfer coefficients and less tendency to foul. Recirculated units in particular allow good control of flow to maintain a proper film on the tubes. This also permits the designer to provide more turndown capability. Liquid velocities are lower... 

Middleton indicates that for his regime 1 (very slow reaction), where Rl is little affected by the chemical reaction, the interface surface area per unit volume, a, is of little importance since the reaction takes place in the bulk liquid phase, so a bubble colunm is the typical reactor of choice. For Middleton s regimes 11, IV, and V—diflfusional control, very fast reaction, and instantaneous reaction, respectively—both high a and k [ are needed, so a stirred tank is the typical reactor recommended. In regime III—reaction in the mass transfer film—the most important variable is the interface area, so a packed column yielding much liquid surface area may be appropriate. 

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