Preliminary Specifications

Tray Spacing

Tray spacing is set by maintenance requirements, and also by support structure design in large-diameter columns. Sufficient crawl space must be provided for tray cleaning and repair. From these considerations, the minimum tray spacing is about 12 in (30 cm) for column diameter less than 5 ft, and (150cm) and 18 in (45 cm) for a column diameter greater than 10 ft (300 cm). In general, it is best to keep tray spacing to a minimum, which is often the most economical.

Downcomer Area

The downcomer area at the top is sized such that the velocity of the ascending vapour bubbles exceeds the downflow velocity of the liquid. The size is related to the stability of the froth in the downcomer and determined by the residence time required for achieving the separation of the two-phase mixture. For non-foaming systems, such as lower alcohols, a residence time of 3 s is sufficient, whereas for extremely high foaming systems such as caustic regenerators, 9 s is required.

To prevent the liquid coming off the bubbling area from splashing against the column wall, the minimum downcomer width is 5 in (12.7 cm). Also, the minimum side chord length should be 60% of the column diameter. This is required to maintain good liquid distribution on the tray.

Since the separation of the vapour-liquid mixture is complete at the bottom of the downcomer, a sloped downcomer can be used to maximize the active tray area. In this case, the downcomer area at the bottom should be about 60% of that at the top.

Jet flooding

/ Design / uncertainty

Excessive entrapment

Downcomer flooding

Jy y

Severe weeping

Figure 4 Sieve tray performance diagram.

Liquid flow rate

Figure 4 Sieve tray performance diagram.

Figure 5 Sieve tray design procedure.

It should be noted that the downcomer area occupies only a small fraction of the cross-sectional area. Thus, a small overdesign does not result in a significant economic penalty.

Column Diameter

The column diameter can be calculated once the tray spacing and downcomer area have been specified. The Fair correlation, based on the Souders and Brown criterion, is recommended by most designers. The vapour flooding velocity can be calculated from eqn [1].

In eqn [1] CSB is the Souders-Brown coefficient, pL and aL (dyne cm"1) are liquid density and surface tension, respectively, and pV is the vapour density in the same units as pL. UN,f is based on the net area, AN(ft2), which is the active area plus one downcomer area. The unit for UN,f is fts"1. The most popular empirical formula for calculating CSB is given in eqn [2].

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