Spray towers, mass-transfer coefficients

Correlations for estimating flooding rates are available, and data for mass-transfer coefficients and axial mixing have been summarized (23, 43, 72, 76]. Although axial mixing is less severe than in spray towers, mass-transfer rates are poor. It is recommended instead that sieve-tray towers be used for systems of low interfacial tension and mechanically agitated extractors for those of high interfacial tension. 

Mass transfer. It is not yet possible to predict the mass transfer coefficient with a high degree of accuracy because the mechanisms of solute transfer are but imperfectly understood as discussed Light and Conway(14), Coulson and Skinner(15) and Garner and Hale 16 1. In addition, the flow in spray towers is not strictly countercurrent due to recirculation of the continuous phase, and consequently the effective overall driving force for mass transfer is not the same as that for true countercurrent flow. 

The mass-transfer coefficients are taken from [14], assuming that they are the same order of magnitude in a packed bed and spray tower ... 

Gravity spray towers superficial velocity about 5.5 L/s m mass transfer coefficients liquid phase ... 

Spray Towers. The performance of a spray tower will be a function of the variation of the individual mass-transfer coefficients k of the two phases and the interfacial area a with operating conditions. The value of k for the dispersed phase can be expected to depend on drop size, diffusivity, and... 

Fio. 10.25. Dispersed-phase mass-transfer coefficients spray tower, 2-in. diam. (51). 

An interesting situation arises in processes where the reaction product P evaporates and is taken out of the reactor with the gas phase (the supply phase). Let us assume that there are no chemical reactions in the gas phase, e.g., l ause the liquid phase reaction is catalysed. We consider the case of rapid reactions, so that all the desired product P is formed in the diffusion layer in the liquid phase, close to the interface. When P can undergo undesired reactions in the liquid phase it is essential to remove P as effectively as we can, e.g., by creating a large surface area and very high gas-phase mass transfer coefficients. At the same time it is essential that the volume of the liquid phase is minimized, since decomposition of P will occur just there. The obvious choice would then be a configuration where the liquid is the dispersed phase, such as in a spray tower or a spray cyclone, provided the heat removal rate is sufficient. Another suitable arrangement could be a gas/liquid packed bed or a wetted wall column. The latter reactor type is very suitable for heat removal (section 4.6.3.1)... 

Equation 7-22 can be used to predict the efficiency when the overall mass transfer coefficient for a given absorber is known. The overall mass transfer coefficient is experimentally determined from pilot plant and full-sized units. For the GEESI open spray tower, the overall mass transfer coefficient, K a, has been correlated to three variables gas velocity, liquid density, and inlet SO2 concentration ... 

In most types of separation equipment such as packed or spray towers, the interfacial area that is effective for mass transfer cannot be accurately determined. For this reason it is customary to report experimentally observed rates of transfer in terms of transfer coefficients based on a unit volume of the apparatus rather than on a unit of interfacial area. Such volumetric coefficients are designated as Kca, kLa, etc., where a represents the interfacial area per unit volume of the apparatus. Experimentally observed variations in the values of these volumetric coefficients with variations in flow rates, type of packing, etc., may be due as much to changes in the effective value of a as to changes in k. Calculation of the overall coefficients from the individual volumetric coefficients is made by means of the equations... 

Spray tower volumetric heat transfer coefficient, Uy = 1.8-5 kW/m °C superficial gas velocity = 1 m/s mass loading liquid to gas = 1-50/1. 

We begin this chapter with a comparison of the mechanisms responsible for mass and heat transfer. The mathematical similarities suggested by these mechanisms are discussed in Section 21.1, and the physical parallels are explored in Section 21.2. The similar mechanisms of mass and heat transfer are the basis for the analysis of drying, both of solids and of sprayed suspensions. However, the detailed models differ, as shown by the examples in Section 21.3. In Section 21.4, we outline cooling-tower design as an example based on mass and heat transfer coefficients. Finally, in Section 21.5, we describe thermal diffusion and effusion. 

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