The vast majority of industrial distillation columns are equipped with trays or plates (sometimes called "decks" in the petroleum industry) located every 1—3 feet up the column. These trays promote mass transfer of light components into the vapor flowing up the column and of heavy components into the liquid flowing down the column. Vapor-liquid contacting is achieved by a variety of devices. The most widely used trays in recent years have been sieve trays and valve trays because of their simplicity and low cost.
Sieve trays are simple flat plates with a large number of small holes. Vapor flows up through the holes, preventing the liquid from falling through. Liquid flows across each tray, passes over a weir, and drops into a "downcomer," which provides liquid for the tray below through an opening at the base of the downcomer. See Figure 2.3. Valve trays are built with a cap that fits over the hole in the tray and that can move up and down, providing more or less effective hole areas as vapor flow rate changes.
This fairly complex process of flow of vapor up the column and of liquid across each tray and down the column is called tray "hydraulics." It is important in control system design because it imposes very important constraints on the range of permissible liquid and vapor flow rates. If liquid cannot flow down the column, or if vapor-liquid contacting is poor, the separating ability of the column drops drastically.
Vapor flows from one tray up through the tray above it because the pressure is lower on the upper tray. Thus there is an increase in pressure from the top of the column to its base. Liquid must flow against this positive pressure gradient. It is able to do so because the liquid phase is denser than the vapor phase. A liquid level is built up in the downcomer to a height sufficient to overcome the difference in static pressure between the tray onto which the liquid is flowing and the tray from which it is coming.
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