Dual flow tray

Fig. 9.4 Sieve trays with downcomers for liquid systems with (a) high interfacial tension and dual-flow trays for liquid systems and (b) with low interfacial tension. In the case of downcomers, only the phase to be dispersed flows through the holes. The droplets are formed by jet disintegration. In dual-flow trays, both hquids flow through the same holes alternately. The larger drops spUt because of colhsion with the tray.

Dual-Flow Trays These are sieve trays with no downcomers (Fig. 14-27b). Liquid continuously weeps through the holes, hence their low efficiency. At peak loads they are typically 5 to 10 percent less efficient than sieve or valve trays, but as the gas rate is reduced, the efficiency gap rapidly widens, giving poor turndown. The absence of downcomers gives dual-flow trays more area, and therefore greater capacity, less entrainment, and less pressure drop, than conventional trays. Their pressure drop is further reduced by their large fractional hole area (typically 18 to 30 percent of the tower area). However, this low pressure drop also renders dual-flow trays prone to gas and liquid maldistribution. 

In general, gas and liquid flows pulsate, with a particular perforation passing both gas and liquid intermittently, but seldom simultaneously. In large-diameter (>2.5-m, or 8-ft) dual-flow trays, the pulsations sometimes develop into sloshing, instability, and vibrations. The Ripple Tray is a proprietary variation in which the tray floor is corrugated to minimize this instability. 

With large holes (16 to 25 mm), these trays are some of the most fouling-resistant and corrosion-resistant devices in the industry. This defines their main application highly fouling services, slurries, and corrosive services. Dual-flow trays are also the least expensive and easiest to install and maintain. 

A wealth of information for the design and rating of dual-flow trays, much of it originating from FRI data, was published by Garcia and Fair [Ind. Eng. Chem. Res. 41 1632 (2002)]. 

Figure 6.1 Continued) Common tray types, (c) Sieve tray id) dual flow tray. [Pari e courtesy ofGlitsch, Inc,] [Peal d courtesy of Fractionation Research Inc. (FRI)-l...

Type Sieve tray Valve tray Bubble-cap tray Dual-flow tray... 

FIG. 14-27 Other trays, (a) Bubble-cap tray, (b) Dual-flow tray. [Fart a, courtesy of Koch-Glitsch LF pari h, courtesy of Fractionation Research Inc. (FBI). ]... 

In systems with a low surface tension, the hole size of the sieve tray with downcomer should be smaller than 2 mm to prevent the continuous phase percolating through. For the treatment of such systems, dual flow trays without downcomers are more suitable. On a dual flow tray, the disperse and continuous phases flow in turn through the base plate holes and between the trays, producing strongly circulating convection cells. 

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