Downcomer, distillation pressure drop

A common type of distillation contacting device used in refinery applications is the sieve tray. In the early 50 s and for many years before, the bubble cap tray was the mainstay of the distillation field. A sieve tray consists of a flat plate with regularly spaced holes, normally 1/2 to 1 inch in diameter. Liquid flows horizontally across the tray and into a channel, called a downcomer, which leads to the tray below. The sieve tray exhibits good capacity, excellent efficiency, low pressure drop, and good flexibility i.e., it will operate quite efficiently at tower loadings which are 1/2 to 1/3 of design values. 

The flange leak was taped over, and the exhaust-steam pressure dropped back to 100 mm Hg. The steam required to drive the turbine fell by 18 percent. This incident is technically quite similar to losing the downcomer seal on a distillation tower tray. Again, it illustrates the sort of field observations one needs to combine with basic technical calculations. This is the optimum way to attack, and solve, process problems. 

Example 18.6. A sieve-plate column operating at atmospheric pressure is to produce nearly pure methanol from an aqueous feed containing 40 mole percent methanol. The distillate product rate is 5800 kg/h. (a) For a reflux ratio of 3.5 and a plate spacing of 18 in., calculate the allowable vapor velocity and the column diameter. b) Calculate the pressure drop per plate if each sieve tray is in, thick with j-in, holes on a -in. triangular spacing and a weir height of 2 in. (c) What is the froth height in the downcomer ... 

An important example of a physical process that shows inverse response is the base of a distillation column with the reaction of bottoms composition and base level to a change in vapor boilup. In a binary distillation column, we know that an increase in vapor boilup V must drive more low-boiling material up the column and therefore decrease the mole If action of light component in the bottoms xg. However, the tray hydraulics can produce some unexpected results. When the vapor rate through a tray is increased, it tends to (1) back up more liquid in the downcomer to overcome the increase in pressure drop through the tray and (2) reduce the density of the liquid and vapor Ifoth on the active part of the tray. The first effect momentarily reduces the liquid flow rates through the column while the liquid holdup in the downcomer is... 

Downcomer pressure drop. If the clearance under the downcomer is too low, it may add substantially to the downcomer backup and consequently reduce downcomer capacity. Cases have been reported (61) where column capacity was increased by simply cutting 1 in off the bottom of the downcomer. Methods of estimating the backup caused by hydraulic losses through the opening under the downcomer are available in most distillation texts (48, 319, 371, 409). 

Determine the tray layout and pressure drops for the distillation column in Examples 10-1 and 10-2. Determine if entrainment or weeping is a problem. Determine if the downcomers will work properly. Do these calculations only at the top of the column. 

In both downcomer back up and choke cases, downcomer liquid inventory increases and downcomer liquid backs up until the downcomer froth level reaches the tray above H > Hs). This phenomenon is called downcomer flood. When downcomer flood occurs to any tray, the whole tower will be flooded very quickly. A tower under downcomer flood provides virtually no distillation. In contrast, under tray flood, liquid can still leave the tower and the tower could stiU operate if the control system allows it although distillation efficiency suffers. Downcomer flood can be prevented in design by providing adequate downcomer area and clearance underneath the downcomer and minimizing tray pressure drop. Reducing reflux rate in operation could be effective in avoiding downcomer flood in operation. 

Equation 1 above stales that a tray will become less efficient due to incipient jet flood when the pressure drop per tray, expressed in inches of liquid, equals 22% to 25% of the tray spacing. The inches of liquid term assumes the liquid is deaerated. Of course, the liquid in the downcomers and on the tray decks is closer to a froth than to a flat liquid. The more highly aerated the liquid is (i.e. the more foamlike it becomes), the greater will be the depth of liquid corresponding to a measured external pressure drop. Hence, liquids which foam in distillation columns (such as dirty amine and ethane-rich fractionators) reach their incipient jet flood point at pressure drops below the 20% indicated in Equation 1. 

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