Sieve trays with downcomers design

A splash baffle is recommended when liquid flow rate is less than 0.1 gpm per inch of outlet weir (144). One successful application of tailor-designed sieve trays with splash baffles at liquid rates ranging from 0.01 to 0.07 gpm per inch of weir has been described (374). Splash baffles should only be used at low liquid loads, because they restrict the downcomer inlet area and can lead to premature downcomer choke. A calming zone upstream of the baffle is sometimes used to minimize this problem (374). Splash baffles also increase tray pressure drop and froth regime entrainment (31). 

A common type of distillation contacting device used in refinery applications is the sieve tray. In the early 50 s and for many years before, the bubble cap tray was the mainstay of the distillation field. A sieve tray consists of a flat plate with regularly spaced holes, normally 1/2 to 1 inch in diameter. Liquid flows horizontally across the tray and into a channel, called a downcomer, which leads to the tray below. The sieve tray exhibits good capacity, excellent efficiency, low pressure drop, and good flexibility i.e., it will operate quite efficiently at tower loadings which are 1/2 to 1/3 of design values. 

Example 8-38 Sieve Tray Design (Perforated) with Downcomer... 

Fig. 3.1. Also note that this nonconventional design has the downcomer outlet area as additional active tray area. This additional active area is the tray deck area under the downcomer having valves, bubble caps, or sieve holes that allow the gas to pass through under the liquid downcomer area of the next tray up. ICPD tray programs dealing with the design and rating of sieve, bubble cap, and valve-type trays allow this active area input. This is an option shown in Table 3.1, which is offered in the three tray design/rating computer programs given in this book.

Sieve tray tower Design is based on a coalesced layer of 1 inch h = 2.54 cm) with 50% of pressure drop through perforations and 50% through downcomer. An 18-inch tray spacing is used. 

Assume 99% absorption use the Kremser equation for the design of dilute units with molar absorption factor for economic recovery of the solvent F/Fgm =1.4 with general range 1.2 to 2. Tray spacing = 0.6 m with valve or sieve trays. See also Section 16.11.4.2 for such details as downcomer sizing. 

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