Flashing Feed Distributors

Flashing Feed and Vapor Distributors When the feed or reflux is a flashing feed, the vapor must be separated out of the liquid before the liquid enters a liquid distributor. At low velocities (only), this can be achieved by a bare nozzle (Fig. 14-70a). A V baffle (Fig. 14-70h) is sometimes installed as a primitive flashing feed or vapor distributor. 

FIG. 14-70 Flashing feed and vapor distributors, (a) Bare nozzle, (b) Rounded V baffle, (c) Peripheral flash box—the box extends right around the tower wall, with the collected liquid descending via downpipes to a liquid distributor below, (d) Gallery distributor—the feed enters the gallery area (upper plate). (Parts a-c, courtesy of Sulzer Chemtech part d, courtesy of Koch-Glitsch LP.)... 

Feed distributors should be located at least 8 in above the tray floor for liquid feeds, and at least 12 in above the tray floor for flashing feeds (207). A distributor of the type shown in Fig. 2.2, arrangement i, is best located so that its centerline is two-thirds of a tray spacing above the tray floor (207). 

The common types of flashing feed and vapor distributors are the baffle type (Fig. 3.9a), the vapor-liquid separator type (Fig. 3.96), the gallery type (Fig. 3.9c), and the tangential entrance tjrpe (Fig. 2.2j). Some of the vapor distributors discussed in Sec. 3.12 are also sometimes used for flashing feeds, especially when liquid distribution to the section below is not critical (e.g., when it contains trays). 

Figure 3.9 Flashing feed distributors, (a) Baffle-type distributor, (b) vapor-liquid separator-type distributor (c) gallety-type distributor. (Parts a, c Gilbert K. Chen, excerpted by special permission from Chemical Engineering, March 5,1984, copyright by McGraw-HiU, Inc., New York, NY 10020 part b reprinted courtesy of Norton Company.)...

When a flashing feed distributor is installed between packed beds (rather than at the top of the columns), vapor risers should be equipped with hats to prevent liquid from the packed bed above from entering the risers. Guidelines 1, 3, and 7 in Sec. 3.10 are also applicable for flashing feed distributors. 

Ethylene feed and catalyst solution enter the bottom of the cylindrical reactor where the reaction proceeds at 125-130 C (255-265 °F) and 8-9 atm. (100-115 psig). The reactor contains internal distributors to insure good vapor-liquid distribution. Ethylene conversion is 96.7< 7o and selectivity to acetaldehyde is 9B.2 o. Even though there is a high exothermic heat of reaction, the reactor temperature is nearly isothermal because of the large quantity of catalyst solution circulated to the reactor. The reactor effluent is flashed adiabatically. Acetaldehyde product, unreacted ethylene, and flashed steam constitute the overhead vapor from the flash drum, and the catalyst solution is pumped from the bottom. 

If the column feed is vapor, then the foregoing guidelines apply to the location of the feed point with respect to matching the upflowing vapor within the column. If the feed is a vapor-and-liquid mixture or a liquid that flashes on entering the column, special adaptors are added to the feed distributor to separate the vapor and liquid feed portions. In this case, the feed location should be selected based on the major molar flow (vapor or liquid) of the feed in the fully flashed condition. 

The solvent normally is added to the distillation column above the feed point, while above the solvent feed point is a wash bed to which some of the liquid distillate is returned as reflux. This arrangement prevents any solvent vapor being carried overhead from the column, which would represent a solvent loss from the system as well as contamination of the distillate product. The solvent feed distributor may require special design if the mixture of reflux and solvent feed either foams or flashes. 

As we have discussed, uniformity of liquid distribution is essential to the attainment of the maximum packed bed efficiency. In those cases where a mixed vapor/liquid feed is used, or where the liquid feed would flash on entering the column, special designs are necessary. To avoid excessive turbulence in the liquid distributor, it is customary to install a device that separates the vapor and liquid phases ahead of the final distributor. Such a device used for small columns is located immediately above the distributor (Figure 10-15). For large diameter towers where feed... 

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