Plate towers sieve trays

Figure 12.6-2. Extraction towers (a) perforated-plate or sieve-tray tower, b) agitated extraction tower.

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. 

Adsorbers, distillation colunuis, and packed lowers are more complicated vessels and as a result, the potential exists for more serious hazards. These vessels are subject to tlie same potential haz. uds discussed previously in relation to leaks, corrosion, and stress. However, llicse separation columns contain a wide variety of internals or separation devices. Adsorbers or strippers usually contain packing, packing supports, liquid distributors, hold-down plates, and weirs. Depending on tlie physical and chemical properties of the fluids being passed tlirough tlie tower, potential liazards may result if incompatible materials are used for llie internals. Reactivity with llie metals used may cause undesirable reactions, which may lead to elevated temperatures and pressures and, ullinialely, to vessel rupture. Distillation columns may contain internals such as sieve trays, bubble caps, and valve plates, wliicli are also in conlacl with tlie... 

Perforated-plate towers, 651-652, 662, 681 (See also Sieve trays)... 

In pulsed sieve-plate towers, the entire column cross-section is occupied with trays, and thus the lighter phase passes through the holes in the upward stroke and the heavy phase in the downward stroke. This will continuously create new interfaces, which improves the mass transfer. By low pulsation intensities the dispersed phase is discon-tinuously moving through the holes (mixer-settler mode). The appropriate relation... 

Gas absorption can be carried out in a column equipped with sieve trays or other types of plates normally used for distillation. A column with trays is sometimes chosen instead of a packed column to avoid the problem of liquid distribution in a large diameter tower and to decrease the uncertainty in scaleup. The number of theoretical stages is determined by stepping off plates on a y-x diagram, and the number of actual stages is then calculated using an average plate efficiency. The plate and local efficiencies are defined in the same way as for distillation [Eqs. 

Sieve-tray towers are very effective, both with respect to liquid-handling capacity and extraction efficiency, particularly for systems of low interfacial tension which do not require mechanical agitation for good dispersion. The general assembly of plates and downspouts is much the same as for gas-liquid contact except that a weir is not required. Towers packed with the same random packing used for gas-liquid contact have also been used for liquid extractors however, mass-transfer rates are poor. It is recommended instead that sieve-tray towers be used for systems of low interfacial tension and mechanically agitated extractors for those of high interfacial tension (Treybal, 1980). 

Plate Columns. The much preferred plate is the sieve tray. Columns have been built successfully in diameters larger than 4.5 m. Holes from 0.64 to 0.32 cm in diameter and 1.25 to 1.91 cm apart are commonly used. Tray spacings are much closer than in distillation—10 to 15 cm in most applications involving low-interfacial-tension liquids. Plates are usually built without outlet weirs on the downspouts. A variation of the simple sieve column is the Koch Kascade Tower , where perforated plates are set in vertical arrays of moderately complex designs. 

A distillation tower with sieve trays is to separate benzene from monochlorobenzene. Conditions at a plate near the bottom of the column are... 

Perforated-plate Towers. In the perforated-plate, or sieve-plate, column, the dispersed phase is repeatedly coalesced and redispersed by causing it to flow through a series of trays in which a large number of small holes have been punched or drilled. In the simplest type, the plates are similar to the side-to-side baffles described above, except that they are perforate. Hunter and Nash (42) describe a successful installation of this type for dephenolating gas liquor consisting of a 46-ft,-high shell, 5 ft. in diameter, in which the baffles each contain two hundred holes. 

Various types of tray (plate) towers for absorption and distillation. In order to efficiently contact the vapor and liquid in absorption and distillation, tray (plate) towers are often used. A very common type of tray contacting device is the sieve tray, which is shown schematically in Fig. 10.6-la and in Section 11.4A for distillation. 

Continuous reactors are at work all the time. This means newly introduced reactants mix to some extent with products. This extent is termed backmixing. A tower has many plates or baffles in it and experiences less backmixing as, for instance, a tank with no plates. Continuous reactors can then be found within towers and columns. Towers may be packed or plate (bubble cap or sieve tray) type. Optimum reactor design attempts to curtail the amount of dead space or areas where no reaction is taking place. It is also possible to have reactants take a shorter path than is necessary for optimum reaction. This is called shortcircuiting. 

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