A distillation tray works efficiently when the vapor and liquid come into intimate contact on the tray deck. To this end, the liquid should flow evenly across the tray deck. The vapor should bubble up evenly through the perforations on the tray deck. The purpose of the outlet weir is to accomplish both these objectives, as follows:
1. Uneven liquid flow across the tray deck is particularly detrimental to good vapor-liquid mixing. For example, if half
of the tray deck has stagnant liquid, then the vapor bubbling through the stagnant liquid cannot alter its composition.
Let me explain. A tray deck is a flat plate with holes. Liquid runs across the plate. Vapor bubbles up through the holes. If liquid only runs across part of this plate, vapor will still bubble up through the holes in the whole plate.
The vapor bubbling up through that portion of the tray deck where the liquid flow is active will mix with the flowing liquid. The flowing liquid will wash out the heavier components from the rising vapors.
On the other hand, the vapor bubbling up through that portion of the tray deck where the liquid flow is zero will also mix with the stagnant liquid. But it’s like trying to wash dirty clothes in dirty water. The stagnant liquid cannot wash out the heavier components from the vapors, because it is already saturated with these heavier components.
Uneven liquid flow is promoted by the outlet weir being out of level. Liquid will tend to flow across that portion of
the tray with a lower than average weir height. The portion of the tray upstream of the high part of the outlet weir will contain stagnant liquid. However, if the crest height (i.e., the height of liquid over the weir) is large, compared to the out-of-levelness of the tray, then an even liquid flow across the tray will result. To achieve a reasonable crest height above the outlet weir, a weir loading of at least 2 GPM per inch of weir length is needed. When liquid flows are small, the tray designer employs a picket weir, as shown in Fig. 4.1.
2. Uneven vapor flow bubbling up through the tray deck will promote vapor-liquid channeling. This sort of channeling accounts for many trays that fail to fractionate up to expectations. To understand the cause of this channeling, we will have to quantify total tray pressure drop.