Maldistribution, packings, liquid column diameter

Because a packing support plate usually is located immediately above the gas inlet in an absorber or the reboiler return in a distillation column, this plate could be used to control vapor distribution. Obviously, vapor maldistribution can reduce column efficiency in the same way as liquid maldistribution although due to the turbulence in the vapor phase, its rate of radial cross-mixing is at least three times that of the liquid phase. The potential for vapor maldistribution increases as column diameters or operating pressures increase. Fortunately, the vapor phase tends to maintain a uniform distribution once it has been established. Thus, usually only the packing support plate immediately above the vapor inlet needs to act as a vapor distributor. This support plate should be located at least one vapor-inlet diameter plus 12-in. above the center-line of the vapor inlet nozzle. 

Fig. 5.4-24 Maldistribution in a packed column (metal Pall rings 35 mm). Column diameter D = 0.63 m, packing height H = 6.8 m. Gas load F = 1.1 Pa, liquid load B = 5.2 w /(jn -h). Ley Uniform liquid feed. Rigte Single point liquid feed...

Liquid maldistribution affects mass transfer efficiency of columns. However, in tray columns maldistribution is restricted to a single tray since the hquid is well mixed in the subsequent downcomer and evenly redistributed to the next tray. In this respect, tray columns are better suited for large diameter columns than packed... 

Feed concentration < 8 to 10 %. Multistaging is easy with up to 500 theoretical stages. Scale up based on total liquid flowrate. HETS is a function of the packing diameter, and the local fluid maldistribution. HETS = 0.1 - 0.3 cm. Residence times in a column are in the order of 3 min. 

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