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… Read More »
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,… Read More »
One of the most frequent causes of flooding is the use of carbon steel trays. Especially when the valve caps are also carbon steel, the valves have a tendency to stick in a partially closed position. This raises the pressure drop of the vapor flowing through the valves, which, in turn, pushes up the liquid… Read More »
Figure 3.8 is a realistic picture of what we would see if our towers were made of glass. In addition to the downcomers and tray decks containing froth or foam, there is a quantity of spray, or entrained liquid, lifted above the froth level on the tray deck. The force that generates this entrainment is… Read More »
To summarize, the total height of clear liquid in the downcomer is the sum of four factors: • Liquid escape velocity from the downcomer onto the tray below. • Weir height. • Crest height of liquid overflowing the outlet weir. • The pressure drop of the vapor flowing through the tray above the downcomer. Unfortunately,… Read More »
We have yet to discuss the most important factor in determining the height of liquid in the downcomer. This is the pressure drop of the vapor flowing through the tray deck. Typically, 50 percent of the level in the downcomer is due to the flow of vapor through the trays. When vapor flows through a… Read More »
Referring to Fig. 3.5, note that the downcomer B is flooding. The cause is loss of the downcomer seal. The height of the outlet weir is below the bottom edge of the downcomer from the tray above. This permits vapor to flow up downcomer B. The upflowing vapor displaces the downflowing liquid. That is, the… Read More »
Liquid flows across a tray deck toward the outlet weir. The liquid overflows the weir, and drains through the downcomer, to the tray below. Vapor bubbles up through the sieve holes, or valve caps, on the tray deck, where the vapor comes into intimate contact with the liquid. More precisely, the fluid on the tray… Read More »
Distillation trays in a fractionator operate between 10 and 90 percent efficiency. It is the process person’s job to make them operate as close to 90 percent efficiency as possible. Calculating tray efficiency is sometimes simple. Compare the vapor temperature leaving a tray to the liquid temperature leaving the trays. For example, the efficiency of… Read More »
Referring back to Figure 10—1, remember that we have compared the actual gas compressor speed to the speed indicated by the curve that passes through point “A”. We calculated point “A” from the natural gas flow, and the observed suction and discharge pressure. We said that if the measured gas compressor speed exceeded the speed… Read More »