Redistributor Drawoffs

In packed columns, liquid may sometimes be withdrawn from a redistributor. This drawoff method is suitable only when the liquid withdrawn is a small fraction of the column liquid. Compared to the chimney tray, this method consumes far less vertical height and is therefore less expensive. 

Liquid is withdrawn from the redistributor either via an overflow pipe or using a submerged drawoff nozzle. When an overflow arrangement is used, the drawoff may frequently run dry, and flow to downstream units may be intermittent or unstable. When a submerged drawoff nozzle is used, excessive drawoff rates may lower the liquid level in the redistributor and cause poor distribution to the bed below. 

Redistributors usually provide insufficient residence times for vapor disengagement (especially if an overflow pipe is used), and venting must be completed downstream of the column outlet. Downstream piping must be sized for self-venting flow (Fig. 4.5). 

When this arrangement is used, caution is required to ensure that the drawoff does not generate large-scale maldistribution of liquid to the bed below (see Sec. 3.1) and that unirrigated areas do not form on top of the packing under the drawoff pipe or sump. This is most likely to be a problem when a submerged drawoff nozzle or sump is used. 

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Liquid collectors are installed when liquid must be collected for redistribution or drawoff (e.g., for external cooling). The common device used is a chimney tray, which is similar to an orifice redistributor, but without perforations. Another common device is the Chevron-type collector, which is a series of Chevron blades, with liquid being collected at the bottom of the blades. 

When a liquid product or pumparound stream is drawn from a redistributor, the area under the drawoff box must be properly irrigated. The author is familiar with one large-diameter (> 20-ft) column that had a redistributor containing a central 2-ft-wide liquid collecting draw box extending the full tower diameter with no irrigation underneath. The column performed extremely poorly, but mainly due to other more severe sources of maldistribution. 

Chevron collectors are used in packed columns as liquid collectors for partial drawoff or for feeding to a redistributor which is not selfcollecting (e.g., a notched-trough redistributor). They are sometimes also used as total drawoffs, but are less suitable than chimney trays for this purpose. The chevron collector (Fig. 4.13) consists of evenly spaced chevron blades several inches high. Liquid collects at the bottom of the blades and runs into a draw pan. From there it can be taken out or fed to a redistributor. 

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