Crossflow

S. V. Cabibbo and A. J. Teller, "The Crossflow Scmbber—A Digital Model for Absorption," Paper No. 69-186, APCA 62nd Annual Meeting New York, NY, (June 22—26, 1969). 

J. Murkes and C. G. Cadsson, Crossflow Filtration, John Wiley Sons, Inc., New York, 1988. 

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. 

Fig. 15. Flow pattern in a crossflow plate distillation column.

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. 

Multiple IJquid-Path Plates. As the Hquid flow rate increases in large diameter crossflow plates (ca 4 m or larger), the crest heads on the overflow weirs and the hydrauHc gradient of the Hquid flowing across the plate become excessive. To obtain improved overall plate performance, multiple Hquid-flow-path plates maybe used, with multiple downcomers. These designs are illustrated and discussed in detail in the Hterature (49). 

Fig. 18. Flooding correlation for crossflow trays (sieve, valve, bubble-cap) where the numbers represent tray spacing in mm. Also shown are approximate...

Fraction of crossflow area ovailable for bypass flow, F p... 

FIG. 14-24 Performance of two crossflow plates operating at 0.13 bar pressure and total reflux. Test mixture etbylbenzene/styrene. Spacing between plates is 0.50 m, and outlet weir height is 38 mm. Ut = superficial vapor velocity, pc = vapor density. [Billet, Comad, and Giuhh, I. Chem. E. Symp. Ser. No. 32, 5, 111 (1969).]... 

FIG. 14-25 Flooding correlation for columns with crossflow plates (sieve, valve, bubble-cap). [Fair, Pet/Chem Eng 33(10), 45 (September 1961),]... 

Example 9 Loading/Flooding of a Distillation Plate An available sieve plate column of 2.5-m diameter is being considered for an etbylben-zene/styrene separation. An evaluation of loading at the top plate will be made. Key dimensions of the single-crossflow plate are ... 

Pressure Drop Methods for estimating fluid-dynamic behavior of crossflow plates are analogous, whether the plates be bubble-cap, sieve, or valve. The total pressure drop across a plate is defined by the general equation (see Fig. 14-29)... 

FIG. 14-34 Friction factor for frotF crossflow, sieve plates. To convert incFes to millimeters, multiply by 25.4. (Smith, Design of Equilibrium Stage Processes, McGraw-Hill, New Yoih, 1963. )... 

Ejfects of Gas and Liquid Mixing As noted previously, it is necessary in most instances to convert point efficiency E g to Murphree plate efficiency E, ,. This is true because of incomplete mixing only in small laboratoiy or pilot-plant columns, under special conditions, is the assumption E g = E, , likely to be valid. For a crossflow plate with no hquid mixing there is plug flow of hquid. For this condition of liquid flow, Lewis [Ind. Eng. Chem., 28, 399 (1936)] analyzed effects of gas mixing on efficiency. He considered three cases ... 

FIG. 14-43 Overall (Murphree) efficiencies of sieve plates with hole/active area ratios of 0.08 and 0.14. Efficiency values greater than 1.0 (100%) result from crossflow effects (Figs. 14-38, 14-39). [Yanagi and Sakata, Ind. Eng. Chem., Proc. Des. Devel., 2i, 712 (J.9S2).] Reproduced with permission, copyright 1982 American Chemical Society. 

Crossflow units — Flat sheet membranes - Rotating filter elements... 

The arbor (wicket) heater is a substantially vertical design in which the radiant tubes are inverted Us connecting the inlet and outlet terminal manifolds in parallel. An overhead crossflow convection bank is usually included. This type of design is good for heating large gas flows with low pressure drop. Typical duties are 53 to 106 GJ/h (50 to 100 10 Btu/h). 

Cooling towers are broadly classified on the basis of the type of draft natural draft (natural convection), mechanical draft (forced convection) and mechanical and natural. Further distinction is made based on (1) the type of flow i.e. - crossflow, counterflow, cocurrent flow (2) the type of heat dissipation-wet (evaporative cooling), dry, wet-dry and (3) the type of application-industrial or power plant. Each of the major types of cooling towers has a distinct configuration. The major designs are summarized in Figures 1 through 8 and a brief description of each follows. 

Another type of crossflow cooling tower is the wet-dry tower, which consists of a normal crossflow tower over which a few air coils are placed. The hot water is first cooled by an air cooled heat exchanger and then drops to the wet cooling tower where more cooling is obtained by the evaporative mechanism. Figures 5 and 6 provide examples. In contrast, deck-filled towers contain tiers of splash bars or decks to aid in the breakup of water drops to increase the total water surface and, subsequently, the evaporation rate. 

Hot water basins are used to distribute water in crossflow towers. Here, water is pumped to an open pan over the wet deck fill. The bottom of the pan has holes through which water is distributed. Manufacturers will fit specially shaped plastic drip orifices into the holes to give the water an umbrella shape for more uniform distribution. Different size orifices are used for different flow rates. Ideally, the basin will be almost full at maximum flow. This way, sufficient depth is retained for good water distribution as turn down occurs. The turn down ratio can be extended by the addition of hot water basin weirs- a pattern of baffles perhaps 2... 

Basin The area at the bottom of the tower for collecting cold water. Crossflow towers have a hot water distribution basin at the top and, in some cases, a water basin between the top and bottom basins. 

Crossflow A system in which air encounters the hot water at a 90° angle. Air enters through the entire sidewall and moves horizontally through the fill and water. 

Diffusion (Redistribution) Deck A device below the hot water distribution basin of a crossflow tower to break up the water going through the orifices before it goes through the fill. 

Distribution System Mechanical method of passing hot water over the fill uniformly. Low-pressure spray-through piping and nozzles are usually used in counterflow towers gravity drop is normally utilized in crossflow towers. 

Crossflow Filters - These are usually membrane-type filters used for ultrafiltration. In the field of biotechnology these types of filters are used in ultrafiltration devices used in concentrating solutions, and performing buffer exchanges. 

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