Plate, bubble-cap

Nonisothermal Gas Absorption. The computation of nonisothermal gas absorption processes is difficult because of all the interactions involved as described for packed columns. A computer is normally required for the enormous number of plate calculations necessary to estabUsh the correct concentration and temperature profiles through the tower. Suitable algorithms have been developed (46,105) and nonisothermal gas absorption in plate columns has been studied experimentally and the measured profiles compared to the calculated results (47,106). Figure 27 shows a typical Hquid temperature profile observed in an adiabatic bubble plate absorber (107). The close agreement between the calculated and observed profiles was obtained without adjusting parameters. The plate efficiencies required for the calculations were measured independendy on a single exact copy of the bubble cap plates installed in the five-tray absorber. 

Steam is introduced at the base of the whiskey column through a sparger. Where economy is an important factor, as in a fuel alcohol plant, a calandtia is employed as the source of indirect heat. The diameter of the stiU, number of perforated and bubble cap plates, capacity of the doubler, and proof of distiUation are the critical factors that largely determine the characteristics of a whiskey. 

Three principal vapor—Hquid contacting devices are used in current crossflow plate design the sieve plate, the valve plate, and the bubble cap plate. These devices provide the needed intimate contacting of vapor and Hquid, requisite to maximizing transfer of mass across the interfacial boundary. 

Research. Much of the research on commercial-size distiUation equipment is being done by Fractionation Research, Inc. (FRI), a nonprofit, industry-sponsored, research corporation. The industrial sponsors are fabricators, designers, and constmctors, or users of distiUation equipment. PubHcations include Hquid mixing on sieve plates (91), bubble cap plate efficiency (92), and sieve plate efficiency (93,94). A motion picture of downcomer performance is also avaUable (95). References 96 and 97 cover the Hterature from 1967 to 1990. 

Figure 14-25 or Eq. (14-92) may be used for sieve plates, valve plates, or bubble-cap plates. The value of the flooding vapor velocity must be considered as approximate, and prudent designs call for approaches to flooding of 75 to 85 percent. The value of the capacity parameter (ordinate term in Fig. 14-25) may be used to calculate the maximum allowable vapor velocity through the net area of the plate ... 

The open area for these plates ranges from 15 to 30 percent of the total cross section compared with 5 to 15 percent for sieve plates and 8 to 15 percent for bubble-cap plates. Hole sizes range from 6 to 25 mm (1/4 to 1 in), and slot widths from 6 to 12 mm (14 to V2 in). The Turbogrid and Ripple plates are proprietary devices. 

Sieve plates usually have negligible hydraulic gradient. Bubble-cap plates can have significant gradient because of the blockage by the caps. Valve plates presumably are intermediate, with hydraulic-gradient characteristics approaching those of sieve plates. 

For bubble-cap plates, hydraulic-gradient must be given serious consideration. It is a function of cap size, shape, and density on the plate. Methods for analyzing bubble-cap gradient may be found in the chapter by BoUes (Smith, De.sign of Equilibrium Stage Proce.s.se.s, Chap. 14, McGraw-Hill, New York, 1963) or in previous edition of this handbook. 

For sieve or valve plates, h = h , outlet weir height. For bubble-cap plates, h = height of static seal. Tbe original references present vaH-dations against laboratoiy and small-commercial-column data. Modifications of tbe efficiency equation for absorption-stripping are also included. 

Sieve plates have less back mixing than bubble-cap plates because of less obstruction to flow. 

For bubble-cap plates, the eddy-diffusion correlation in the AlChE Bubble-Ti ay Design Manual should be used. 

Bubble-cap plate—90-mm (3.5-in) round caps, 30-mm (1.2-in) static submergence... 

The vapors from a fluid catalyst unit carry a small amount of fine catalyst particles which might clog the narrow clearances of a conventional bubble cap plate. 

Sutherland [69] reports for air-water entrainment of 0.0001 to 0.1 lb liquid/lb vapor, averaging 0.01 for 15-in. tray spacing at hole velocity Fj values of 3 to 15. Fh = Vq These values are 1-10% of bubble cap plates. Simkin et al., [64] reports a comparison with the Turbogrid tray giving only 3-60% of the entrainment of bubble caps over a wide range of operation. 

This method of predicting plate efficiency, published in 1958, was the result of a five-year study of bubble-cap plate efficiency directed by the Research Committee of the American Institute of Chemical Engineers. 

Methods for predicting the entrainment from sieve plates are given in Section 11.13.5, Figure 11.27 a similar method for bubble-cap plates is given by Bolles (1963). 

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. 

The most significant feature of the bubble-cap plate is that the use of risers ensures that a level of liquid is maintained on the tray at all vapour flow-rates. 

Cost. Bubble-cap plates are appreciably more expensive than sieve or valve plates. The relative cost will depend on the material of construction used for mild steel the ratios, bubble-cap valve sieve, are approximately 3.0 1.5 1.0. 

For a given tray layout there are certain limits for the flows of vapour and liquid within which stable operation is obtained. The range is shown in Figure 11.54, which relates to a bubble-cap plate. The region of satisfactory operation is bounded by areas where... 

Figure 4. Industrial gas distributors (A) perforated plate (B) nozzle plate (C) bubble-cap plate.

In eq 10, ho is the height above the grid where the bubbles form (for a porous plate, ho = 0 for a perforated plate, ho = L for a nozzle plate, ho is the height of the outlet opening above the plate and for a bubble-cap plate, ho is the height of the lower edge of the cap above the plate). 

Example 3 Determination of pressure drop and liquid height in downcomer for bubble-cap plate. The following specifications apply to a bubble-cap plate ... 

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