Plates cross-flow

Plate Columns. There are two general types of plates in use crossflow and counterflow. These names refer to the direction of the Hquid flow relative to the rising vapor flow. On the cross-flow plate the Hquid flows across the plate and from plate to plate via downcomers. On the counterflow plate Hquid flows downward through the same orifices used by the rising vapor. 

The cross-flow plate (Fig. 14-16 ) utilizes a hquid downcomer and is more generally used than the counterflow plate (Fig. 14-16Z ) because of transfer-efficiency advantages and greater operating range. The liquid-flow pattern on a cross-flow plate can be controlled by placement... 

FIG. 14-16 (a) Cross-flow plate (side view), (h) Countercurrent plate (side... 

Most new designs of cross-flow plates employ perforations for dispersing gas into hquid on the plate. These perforations may be simple... 

TABLE 14-5 Selection of Cross-Flow-Plate Flow Pattern ... 

FIG. 14-17 Common liquid-flow patterns, cross-flow plates. (Smith, Design of Eqiiihhriiim Stage Processes, McGraw-Hill, New York, 1963. )... 

Historically the most common gas disperser for cross-flow plates has been the bubble cap. This device has a built-in seal which prevents liquid drainage at low gas-flow rates. Typical bubble caps are shown in Fig. 14-20. Gas flows up through a center riser, reverses flow under the cap, passes downward through the annulus between riser and cap, and finally passes into the liquid through a series of openings, or slots, in the lower side of the cap. 

Bubble caps were used almost exclusively as cross-flow-plate dispersers until about 1950, when they were largely displaced by simple or valve-type perforations. Many varieties of bubble-cap design were used (and therefore are extant in many operating columns), but in most cases bell caps of 75- to 150-mm (3- to 6-in) diameter were utilized. 

For cross-flow plates, net area is the column cross section less that area blocked by the downcomer or downcomers (Fig. 14-22). The vapor velocity in the net area represents an approach velocity and thus controls the level of liquid entrainment. For counterflow plates, net area is the same as the column cross section, since no downcomers are involved. 

Plate Layouts Cross-flow plates, whether bubble-cap, sieve, or valve, are similar in layout (Fig. 14-28). Possible zones on each plate are Active vapor-dispersion Peripheral stiffening and support Disengaging Distributing Downcomer... 

The peripheral stiffening zone (tray ring) is generally 25 to 50 mm (1 to 2 in) wide and occupies 2 to 5 percent of the cross section, the fraction decreasing with increase in plate diameter. Peripheiy waste (Fig. 14-28) occurs primarily with bubble-cap trays and results from the inabihty to fit the cap layout to the circular form of the plate. Valves and perforations can be located close to the wall and little dead area results. Typical values of the fraction of the total cross-sectional area available for vapor dispersion and contact with the liquid for cross-flow plates with a chord weir equal to 75 percent of the column diameter are given in Table 14-6. 

FIG. 14-29 Pressure-drop contributions for cross-flow plates, = pressure drop through cap or sieve, equivalent height of plate liquid = height of weir ... 

Cross-flow plates are the most common type of plate contactor used in distillation and absorption columns. In a cross-flow plate the liquid flows across the plate and the vapour... 

Figure 11.17. Typical cross-flow plate (sieve)...

Other types of plate are used which have no downcomers (non-cross-flow plates), the liquid showering down the column through large openings in the plates (sometimes called shower plates). These, and, other proprietary non-cross-flow plates, are used for special purposes, particularly when a low-pressure drop is required. 

This is the simplest type of cross-flow plate. The vapour passes up through perforations in the plate and the liquid is retained on the plate by the vapour flow. There is no positive vapour liquid seal, and at low flow-rates liquid will weep through the holes, reducing the plate efficiency. The perforations are usually small holes, but larger holes and slots are used. 

In which the vapour passes up through short pipes, called risers, covered by a cap with a serrated edge, or slots. The bubble-cap plate is the traditional, oldest, type of cross-flow plate, and many different designs have been developed. Standard cap designs would now be specified for most applications. 

Cross flow plates are the most common type of plate contactors used in tray distillation column. In a cross-flow plate, the liquid flows across and down from the plate and the vapour flows up through the plate. The flowing liquid is transferred from plate to plate through vertical channels called downcomers. A pool of liquid is retained on the plate by an outlet... 

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