Baffle tray tower

Fig. 7 Baffle tray tower. (View this art in color at www. dekker.com.)...

Figure 17.11. Types of contactors for reacting gases with liquids many of these also are suitable for reacting immiscible liquids. Tanks (a) with a gas entraining impeller (b) with baffled impellers (c) with a draft tube (d) with gas input through a rotating hollow shaft, (e) Venturi mixer for rapid reactions, (f) Self-priming turbine pump as a mixer-reactor, (g) Multispray chamber. Towers (h) parallel flow falling film (i) spray tower with gas as continuous phase (j) parallel flow packed tower (k) counter flow tray tower. (1) A doublepipe heat exchanger used as a tubular reactor.

Baffle Trays Baffle trays ( shed decks, shower decks ) (Fig. 14-28 ) are solid half-circle plates, sloped slightly in the direction of outlet flow, with weirs at the end. Gas contacts the liquid as it showers from the plate. This contact is inefficient, typically giving 30 to 40 percent of the efficiency of conventional trays. This limits their application mainly to heat-transfer and scrubbing services. The capacity is high and pressure drop is low due to the high open area (typically 50 percent of the tower cross-sectional area). Since there is not much... 

Superheated feeds from chemical reactors or petroleum cracking units are a special case, often requiring a baffle-tray desuperheating section in the tower. 

In a typical design, this tower will contain at least four open-type trays in the lower section and a minimum of seven fractionating trays in the upper section. The function of the lower section is to cool the incoming gas by sensible heat transfer with a pumparound of cooled quench oil (bottoms). This quench oil will leave the column at 350° to 400°F, and the pumparound will be cooled to between 270° and 330°F before being returned to irrigate the lower section trays. At least 90% of the components in the cracked gas feed that are heavier than C-lOs will be condensed by these angle trays, baffle trays, or splash decks. A small slip-stream of bottoms is sent to a stripper to remove the C-8 and lighter components, because the stripped bottoms have only fuel value. 

The lower sections of water quench towers typically employ baffle trays, splash decks, or angle trays. These devices have a very low efficiency, so that 6 to 12 actual trays are installed, which develop a pressure drop of only 0.02 psi per actual tray. If the effluent liquid from this lower section of the tower could be heated an additional 5°F, the useful heat available for cold section reboilers would increase from 77% to 82% of the total heat load on the water quench column. When this section is equipped with high capacity packing, the exit liquid temperature can be raised to within about 5°F of the inlet vapor adiabatic saturation temperature. This can be done without increasing the pressure drop compared to the trays. Normally a 10-ft to 14-ft depth of 70IMTP pacldng is specified for the lower section. Such a packed depth provides a 70% to 100% increase in theoretical stages compared to the trays that are replaced. 

The absorber may be any efficient liquid-gas contacting device such as a packed tower or a column provided with multiple spray nozzles. Since it is claimed that no solids precipitate during absorption, no special provisions are taken to prevent plugging. The digester may be located in the bottom of the absorber or may be a separate vessel of sufficient capacity to allow the reaction to go to completion. The acidificr, if required, may be a stirred ves.sel or a column provided with baffle trays. 

Total number of actual trays in tower Number of caps per tray Number of slots per bubble cap Valve density, number of valves per ft or Number of valve units on a valve tray Depth of notches in weir, in or Exponent defined by Equations 8-288 and 327 Dry tray pressure drop for 50% cut baffles, in. liquid per baffle or... 

Deviations from optimal design of trays, packings and other tower internals, e.g., distributors and baffles. 

Transfer of heat by direct contact is accomplished in spray towers, in towers with a multiplicity of segmented baffles or plates (called shower decks), and in a variety of packed towers. In some processes heat and mass transfer occur simultaneously between phases for example, in water cooling towers, in gas quenching with water, and in spray or rotary dryers. Quenching of pyrolysis gases in transfer lines or towers and contacting on some trays in fractionators may involve primarily heat transfer. One or the other, heat or mass transfer, may be the dominant process in particular cases. 

FIG. 15-41 Baffle towers, (a) Side-to-side flow at each tray, b) Center-to-center flow fdisk-and-doughnut style), fc) Center-to-side flow. [Reprinted frcm Treybai, Liquid Extraction (McGraw-HtU, 1963), with permisswn. Cepyright 1963 McGraw-Hill, Inc.]... 

Baffle columns and shower tray columns, shown in Fig. 2.27, are characterized by relatively low liquid dispersion and very low pressure drops. The major application of this type of flow regime is in cooling towers, where the water flows across wooden slats and very large volumes of gas are handled. Here economics dictate that fans rather than compressors be used. Some gas absorption and vacuum distillation columns employ baffle or shower trays. 

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