Mass Transfer Coefficients for Tower Packings

To this point in our narrative, we confined ourselves to mass transfer in and aroimd simple geometries such as channels of various types and exterior flow about flat plates, cylinders, and spheres. 

In this section we turn our attention to more complex shapes represented by tower packings used in operations such as gas absorption, stripping, and distillation. The operation of such columns is addressed in more detail in Chapter 8. 

Pressure drop. This item, which is often overlooked in preliminary design work, dictates the pumping costs and is the principal contributor to the operating cost of a tower. Even a 10 to 20% reduction in AP is considered a major victory. 

Promotion of lateral distribution and surface renewal of the liquid solvent. Both of these items improve the mass transfer rate and result in a reduction in tower height. 

Resistance to corrosion and high temperatures. Packings are now available in a wide variety of materials, including plastic and high-performance metals and alloys. 

The mass transfer characteristics obtained on packings are reported in several different ways. At the more fundamental level, we extract volumetric mass transfer coefficients from the experimental performance data. These coefficients, which we have encountered before in Illustrations 2.2 and 2.3, consist of the product of a film coefficient and the nominal specific surface area a of the packing, expressed in units of m per m of packing. If we use the molar concentration-based coefficient or K, which has units of m/s, then the 

Mass Transfer Coefficients in Various Commercial Packings 

Quantitative prediction methods for volumetric mass transfer coefficienfs that rely on empirical coefficients for each particular packing and packing size have been developed and can be found in the relevant literature. We do not often resort to complete predictions of this type, and it is more common to use relations that will extend known coefficients, such as those listed in Table 5.6, to a different set of conditions. This can be done in an approximafe fashion using fhe following proportionalities  

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