Packed columns maldistribution

The packing cannot be higher than a maximum value because the liquid has the tendency to pass to the wall of the column. Therefore at given sections the liquid has to be collected and re-distributed to the next layer of packings. A maldistribution of the liquid results in low separation efficiency. 

Performance prediction. Due to their sensitivity to maldistribution there is greater uncertainty in predicting packed column performance. 

Liquid redistributors are installed at frequent intervals in a packed column to remix the liquid, thus counteracting the propagation of maldistribution effects and the natural tendency of liquid to migrate toward the wall. A common design practice is to redistribute the liquid every 20 ft (6-7 m). 

Low liquid rstea. With the aid of serrated weirs, splash baffles, reverse-flow trays (40), and bubble-cap trays, low liquid rateB can be handled better in tray columns. Packed columns suffer from liquid dewstting and maldistribution at low liquid rates. 

Maldistribution in packed columns may cause a severe reduction in column efficiency. For 1-in packings, HETP may increase fay a factor as high as 2 or 3 due to maldistribution (116,131,132). 

A packed column has reasonable tolerance for a uniform or smooth variation in liquid distribution and for a variation that is totally random small-scale maldistribution). However, the impact... 

Maldistribution Departure from uniform distribution of the phases in a packed column can be caused by ... 

Hoek, P.J. Wesselingh, J.A. Zuiderweg, F.J. Small scale and large-scale liquid maldistribution in packed columns. Chem. Eng. Res. Des. 1986, 64, 431-449. 

Hoak, P. J., Large and Small Scale Liquid Maldistribution in Packed Column, Ph.D. Thesis, Delft University, Netherland, 1983. 

In packed columns a non-uniform distribution (maldistribution) is to be expected both in the vapour and in the liquid phase. In the liquid it occurs for these reasons... 

The efficiency rtm/Ko of a packed column then is a function of maldistribution I and degree of mixing m per separating stage ... 

The main consideration for introducing reflux or intermediate feed into a packed tower is adequately distributing the incoming stream to the packing. Unlike most tray columns, packed towers are sensitive to distribution. Maldistribution is detrimental to packing efficiency and turndown. The main devices that set the quality of distribution in a packed column are the top (or reflux) distributor, the intermediate feed distributor, the redistributor, and sometimes the vapor distributor. Adequate hydraulics in the inlet area is also important failure to achieve this can affect distributor performance and can also cause premature flooding. 

When a sparger pipe enters an intermediate feed, vapor jets must not impinge on redistributor liquid surface or other packed-column internals. Liquid surface agitation in redistributors, or mechanical damage to internals, can cause maldistribution, which can be detrimental to column efficiency. 

In packed columns, liquid overflowing the chimneys may preferentially descend into some risers (often the peripheral), with vapor ascending through the others (often the central risers). This may cause severe maldistribution of vapor to the bed above. 

Maldistribution in packed columns can be detected in the field by the following techniques... 

Meier, W., and M. Huber, "Measurements of the Number of Theoreticed Plates in Packed Columns with Artificial Maldistribution, I. Chem. E. Symp. Ser. 32, 1969, p. 4 31. 

Yuan, H. C., and L. Spiegel, "Theoretical and Experimental Investigation of the Influence of Maldistribution on the Number of Theoretical Stages in Packed Columns at Partial Reflux, Proc. 2nd World Congress of Chemical Engineering, Montreal, Canada, Oct., 1981, p. 274,... 

An issue that is not adequately addressed by most models (EQ and NEQ) is that of vapor and liquid flow patterns on distillation trays or maldistribution in packed columns. Since reaction rates and chemical equilibrium constants are dependent on the local concentrations and temperature, they may vary along the flow path of liquid on a tray, or from side to side of a packed column. For such systems the residence time distribution could be very important, as well as a proper description of mass transfer. On distillation trays, vapor will rise more or less in plug flow through a layer of froth. The liquid will flow along the tray more or less in plug flow, with some axial dispersion caused by the vapor jets and bubbles. In packed sections, maldistribution of internal vapor and liquid flows over the cross-sectional area of the column can lead to loss of interfacial area. 

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... 

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...

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