Efficiency, tray hole diameter

Hole Sizes Small holes slightly enhance tray capacity when limited by entrainment flood. Reducing sieve hole diameters from 13 to 5 mm ( to in) at a fixed hole area typically enhances capacity by 3 to 8 percent, more at low liquid loads. Small holes are effective for reducing entrainment and enhancing capacity in the spray regime (Ql < 20 m3/hm of weir). Hole diameter has only a small effect on pressure drop, tray efficiency, and turndown. 

Hole Diameter The jury is out on the effect of hole diameter on tray efficiency. There is, however, a consensus that the effect of hole diameter on efficiency is small, often negligible. 

Scaleup from Oldershaw column. One laboratory-scale device that found wide application in supplying efficiency data is the Oldershaw column (Fig. 8.13 Ref. 207). This column is available from a number of laboratory supply houses and can be constructed from glass for atmospheric operation or from metal for superatmospheric separations. Small hole diameters and small tray spacings are used. Typical column diameters are 1 to 3 in. 

When vapor and liquid flow rates change appreciably from tray to tray, column diameter, tray spacing, or hole area can be varied to reduce column cost and insure stable operation at high efficiency. Variation of tray spacing is particularly applicable to columns with sieve trays because of their low turndown ratio. 

The static packed column, shown in Fig. 6.3-1, is more efficient with respect to mass transfer. Its design is similar to that of gas/hquid systems (e.g., absorption and distillation). In contrast, the design of a static tray column, used for solvent extraction, is completely different from the corresponding column for gas/liquid service. The sieve trays have very small hole diameters (2-4 mm) and a very small free area 2-4%) (Fig. 6.3-1). 

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. 

Tray System Column diameter, ft Tray spacing, in Pressure, psia Efficiency, % % hole (slot) area Ref. 

Solution Table 14-12 presents measurements by Billet (loc. cit.) for ethyl-benzene-styrene under similar pressure with sieve and valve trays. The column diameter and tray spacing in Billets tests were close to those in Example 9. Since both have single-pass trays, the flow path lengths are similar. The fractional hole area (14 percent in Example 9) is close to that in Table 14-12 (12.3 percent for the tested sieve trays, 14 to 15 percent for standard valve trays). So the values in Table 14-12 should be directly applicable, that is, 70 to 85 percent. So a conservative estimate would be 70 percent. The actual efficiency should be about 5 to 10 percent higher. 

In this type of tray, each opening assembly consists of a cap, or inverted cup, with slots at its base, fixed above an opening. The opening consists of a hole and a riser through which vapor rises from the tray below. Its flow is reversed downward by the cap, after which the vapor flows down around the riser and bubbles out through the slots and into the liquid. Bubble cap diameters are usually about three to four inches. Because of the liquid seal created by the riser, bubble caps can operate at wide ranges of vapor and liquid flows with little loss in tray efficiency. 

Pulsed sieve plate column HETS increases exponentially with diameter 0.4 m at 1 m dia. max. diameter 3 m superficial velocity about 5.5 L/s m sieve holes 3 to 8 mm velocities through the holes <0.2 m/s to minimize the formation of small drops. Tray efficiencies about 20 to 30%. 

Sieve plates numerous small holes of to -fe-in. diameter spaced on uniform triangular pitch to cover about 15 per cent of the plate area require liquid downcomers on each plate sieve trays are cheaper to construct and give lower pressure drop for the same plate efficiencies. 

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