The downcomer from the bottom tray is submerged in a seal pan (see Fig. 8.3), to preserve its downcomer seal. I always set the horizontal dimension between the over-flow lip of the seal pan, (dimension y) the downcomer at four inches, so I never have to worry about restricting liquid flow from the bottom tray.… Read More »
Most trays have outlet weirs devoted to maintaining the downcomer seal. But some trays have inlet weirs too, or inlet weirs, but no outlet weirs. A sketch of an inlet weir is shown in Fig. 8.2. Note the horizontal distance between the downcomer and the inlet weir (dimension x). This distance ought to be equal… Read More »
The bottom edge of the downcomer from the tray above should be 0.25 to 0.5 in below the top edge of the weir of the tray below. This is called a positive downcomer seal. Without a positive downcomer seal, vapor will flow up the downcomer and displace the downflowing liquid. This will cause flooding due… Read More »
The vertical edges to the downcomers are bolted to bars welded to the vessel wall. These are called, “downcomer bolting bars.” Gaskets are often used to tightly seal the edge of the tray downcomer to these bars. If the bolts are loose or if the gaskets are missing, vapor will blow into the downcomer and… Read More »
For smaller diameter towers a visual check of tray deck levelness is sufficient. For two-pass trays, a small diameter tower is less than 8 ft. For single-pass trays, a diameter of less than 6 ft is small.For towers of 10 ft or more in diameter, check for out-of-levelness of a tray check using a carpenter’s… Read More »
In a once-through reboiler, the liquid flow coming out of the reboiler is limited to the bottoms product. In a circulating reboiler, the liquid flow coming out of the reboiler can be extremely high. If the reboiler return nozzle is located too close to the bottom tray of the tower, the greater volume of liquid… Read More »
Four types of reboilers are : • Once-through thermosyphon reboilers • Circulating thermosyphon reboilers • Forced-circulation reboilers • Kettle or gravity-fed reboilers There are dozens of other types of reboilers, but these four represent the majority of applications. Regardless of the type of reboiler used, the following statement is correct: Almost as many towers flood… Read More »
The tray temperatures in our preflash tower, shown in Fig. 6.4, drop as the gas flows up the tower. Most of the reduced sensible-heat content of the flowing gas is converted to latent heat of evaporation of the downflowing reflux. This means that the liquid flow, or internal reflux rate, decreases as the liquid flows… Read More »
When we raise the top reflux rate to our preflash tower, the tower-top temperature goes down. This is a sign that we are washing out from the upflowing vapors more of the heavier or higher-molecular-weight components in the overhead product. Of course, that is why we raised the reflux rate. So the reduction in tower-top… Read More »
Up to this point, we have suggested that the weight flow of vapor up the tower is a function of the reboiler duty only. Certainly, this cannot be completely true. If we look at Fig. 6.2, it certainly seems that increasing the heat duty on the feed preheater will reduce the reboiler duty. Let us… Read More »