Causes of Tray Inefficiency : Out-of-Level Trays

When trays weep, efficiency may not be significantly reduced. After all, the dripping liquid will still come into good contact with the upflowing vapor. But this statement would be valid only if the tray decks were absolutely level. And in the real world, especially in large (>6-ft)-diameter columns, there is no such thing as a “level” tray. Figure 4.3 shows the edge view of a tray that is 2 in out-of-level.

As illustrated, liquid accumulates on the low side of this tray. Vapor, taking the path of least resistance, preferentially bubbles up through the high side of the tray deck. To prevent liquid from leaking through the low side of the tray, the dry tray pressure drop must equal or exceed the sum of the weight of the aerated liquid retained on the tray by the weir plus the crest height of liquid over the weir plus the 2-in out-of-levelness of the tray deck.

Once the weight of liquid on one portion—the lowest area—of a tray deck exceeds the dry tray pressure drop, the hydraulic balance of the entire tray is ruined. Vapor flow through the low area of the tray deck ceases. The aeration of the liquid retained by the weir on the low area of the tray deck stops, and hence the hydraulic tray pressure drop increases even more. As shown in Fig. 4.3, the liquid now drains largely through the low area of the tray. The vapor flow bubbles mainly through the higher area of the tray deck. This phenomenon is termed vapor-liquid channeling. Channeling is the primary reason for reduced distillation tray efficiency, because the vapor and liquid no longer come into good, intimate contact.

The common reason for out-of-levelness of trays is sagging of the tray decks. Sags are caused by pressure surges and sloppy installation. Sometimes the tray support rings might not be installed level, or the tower itself might be out of plumb (meaning the tower itself may not be truly vertical).

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