Tower diameter, sieve trays

Case studies were reported (170,174) of large-diameter (> 15-ft) towers with sieve trays not reaching the expected efficiency. Maldistribution was cited as the culprit or at least one of the causes. Improving liquid flow patterns, often among other modifications, was the fix. The only other evidence that channeling adversely affects tray efficiency comes from the above-mentioned theoretical models. 

Erskine, J. B., and W. Waddington, "Investigation into the Vibration Damage to Large Diameter Sieve Tray Absorber Towers, Paper No. 211, International Sym posium on Vibration Problems in Industry, UKAEA and NPL, Keswick, England 1973. 

McMullan et al describe the modification of a large diameter tower from sieve trays to IMTP packing [10]. Due to the very low liquid flow rates, only three to four distribution points were used per sq ft in the liquid distributor in order to avoid plugging, which could occur with smaller orifices. Experience has demonstrated that uniform liquid and vapor distribution is required to obtain the large number of theoretical stages needed for this separation. This column now is providing a total of 63 theoretical stages in four packed beds. 

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. 

Pulsed packed and sieve tray towers may operate at frequencies of 90 cycles/min and amplitudes of 6-25 mm. In large diameter towers, NETS of about 1 m has been observed. Surface tensions as high as 30-40 dyn/cm have no adverse effect. 

Bubble-cap columns or sieve trays, of similar construction to those described in Chapter 11 on distillation, are sometimes used for gas absorption, particularly when the load is more than can be handled in a packed tower of about 1 m diameter and when there is any... 

Figure 13.31. Assembled sieve tray towers, (a) Flowsketch sieve tray tower in small diameters (Pfaudler Co.).

Sieve trays are widely used in industry with column diameters up to 3.66 m (Ref. A6 p.21.74), this limit was imposed upon the testing procedure. Column diameters of less than 1.5 m would not prove economical under these conditions because of the very large tower height and number of trays required. Although a larger column diameter would substantially reduce the required number of trays,... 

An extractor column is generally a tall, vertical packed tower that has two or more bed sections. Each packed bed section is typically limited to no more than 8 ft tall, making the overall tower height about 40 to 80 ft. Tower diameter depends fully upon liquid rates, but is usually in the range of 2 to 6 ft. Liquid-liquid extractors may also have tray-type column internals, usually composed of sieve-type trays without downcomers. These tray-type columns are similar to duoflow-type vapor-liquid separation, but here serve as contact surface area for two separate liquid phases. The packed-type internals are more common by far and are the type of extractor medium considered the standard. Any deviation from packed-type columns is compared to packing. 

Example 1 Determination of distillation-column diameter on basis of allowable vapor velocity. A sieve-tray distillation tower is to be operated under the following conditions ... 

A sieve-tray tower has an ID of 5 ft, and the combined cross-sectional area of the holes on one tray is 10 percent of the total cross-sectional area of the tower. The height of the weir is 1.5 in. The head of liquid over the top of the weir is 1 in. Liquid gradient is negligible. The diameter of the perforations is in., and the superficial vapor velocity (based on the cross-sectional area of the empty tower) is 3.4 ft/s. The pressure drop due to passage of gas through the holes may be assumed to be equivalent to 1.4 kinetic heads (based on gas velocity through holes). (Tray thickness = hole diameter and active area = 90 percent of total area-see Fig. 16-12). If the liquid density is 50 lb/ft3 and the gas density is 0.10 lb/ft3, estimate the pressure drop per tray as pounds force per square inch. 

Siemens-Martin furnace-regenerator, 590 Sieve tray extractors, 483 capacity, 484,487 diameters, 483, 487 efliciency. 483.487 pulsed, 478,483,487 sizing example, 486 Sieve trays, 428 assembly in a tower, 428 comparison with other types, example, 431... 

The tower will be 3.3 ft in diameter with 22 sieve trays, contacting height of 33 ft and two settling zones on ends of column (8 ft each), yielding a total height of 49 ft. 

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