Sieve trays tray spacing

Fig. 18. Flooding correlation for crossflow trays (sieve, valve, bubble-cap) where the numbers represent tray spacing in mm. Also shown are approximate...

Example 8 Calculation of Rate-Based Distillation The separation of 655 lb mol/h of a bubble-point mixture of 16 mol % toluene, 9.5 mol % methanol, 53.3 mol % styrene, and 21.2 mol % ethylbenzene is to be earned out in a 9.84-ft diameter sieve-tray column having 40 sieve trays with 2-inch high weirs and on 24-inch tray spacing. The column is equipped with a total condenser and a partial reboiler. The feed wiU enter the column on the 21st tray from the top, where the column pressure will be 93 kPa, The bottom-tray pressure is 101 kPa and the top-tray pressure is 86 kPa. The distillate rate wiU be set at 167 lb mol/h in an attempt to obtain a sharp separation between toluene-methanol, which will tend to accumulate in the distillate, and styrene and ethylbenzene. A reflux ratio of 4.8 wiU be used. Plug flow of vapor and complete mixing of liquid wiU be assumed on each tray. K values will be computed from the UNIFAC activity-coefficient method and the Chan-Fair correlation will be used to estimate mass-transfer coefficients. Predict, with a rate-based model, the separation that will be achieved and back-calciilate from the computed tray compositions, the component vapor-phase Miirphree-tray efficiencies. 

Testing of plates and other devices is carried out by Fractionation Research, Inc. for industrial sponsors. Some of the test data for sieve plates have been published for the cyclohexane//i-heptane and isobu-tane//i-butane systems. Representative data are shown in Fig. 14-43. These are taken from Sakata and Yanagi Jn.stn. Chem. Engis. Symp. See. No. 56, 3.2/21 (1979)] and Yanagi and Sakata [Jnd. Eng. Chem. Proc. Des. Devel, 21, 712 (1982)]. The column diameter was 1.2 m, tray spacing was 600 mm, and weir height was 50 mm. 

Fair s empirical correlation for sieve and bubble-cap trays shown in Fig. 14-26 is similar. Note that Fig. 14-26 incorporates a velocity dependence (velocity) above 90 percent of flood for high-density systems. The correlation implicitly considers the tray design factors such as the open area, tray spacing, and hole diameter through the impact of these factors on percent of flood. 

Devices that are stagelike in characder (sieve trays, compartmented extractors, etc.) are perhaps better treated by a somewhat different procedure which space does not permit outlining here. See Sleicher [Am. Inst. Chem. Eng. J., 6,. 529 (I960)], Miyauchi and Vermeulen [Ind. Eng. Chem. Fundam., 2, 304 (1963)], and Van der Laan [Chem. Eng. ScL, 7, 187(1958)]. 

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. 

Sieve plates are used with 0.41-m (16-inch) tray spacing. Height of column shell ... 

This specialized Sieve Tray design is of high efficiency and operates with exceptional short tray spacings, sometimes as low as 6 in. between trays. 

Tray spacing can usually be about 6 in. less than for a corresponding bubble tray. Sieve trays are operating on spac-ings of 9 in. and up to 30 in., the latter being necessary for high vacuum service. Spacing of 12-16 in. is common. 

As tray spacing increases, entrainment reduces in quantity, but does increase with the sieve tray hole diameter [183, 184], but generally increases with reduction in hole... 

Figure 8-137. Flooding capacity, sieve trays weir height is less than 15% of tray spacing low- to non-foaming system hole area at least 10% hole sizes Ms-in. to M-in. dia. surface tension = 20 dynes/cm. Used by permission, Fair, J. R., Petro/Chem. Engineer, Sept (1961), p. 46, reproduced courtesy of Petroleum Engineer International, Dallas, Texas.

Fair [183] relates sieve trays and includes valve tray remarks to the extensive work done for bubble cap trays. Figure 8-137 and 8-139 show flooding data for 24-in. spacing of bubble cap trays from [81] and represents data well for 36-in. diameter columns, and is conservative for smaller columns. Fair s work has been corrected to 20 dynes/ cm surface tension by ... 

Figure 8-140B. Sieve tray flooding, 6-in. tray spacing.

Figure 8-140. Studies of sieve tray and bubble cap tray flooding (24-in. tray spacing). (Note that the references listed on the illustrations in Figure 8-140 are from the original source, while Ref. 185 Is from this text.) Used by pennission. Fair, J. R., Petro/Chem Engineer, Sept. (1961) p. 45, reproduced courtesy Petroleum Engineer International Dallas, Texas.

Large fractional hole area, long flow path relative to tray spacing and high liquid flow rate are the key factors leading to the formation or intensification of vapor cross-flow channeling on sieve and valve trays. 

Hole size is as important in perforated plates without downcomers as far the sieve tray. Published data limits a full analysis of the relationships however, the smaller holes, Ys-in., Me-in., 4-in. appear to give slightly higher efficiencies for the same tray spacing [47]. Unfortunately the data [69] for the larger %-in. holes was not evaluated for efficiencies. Experience has indicated efficiencies equal to or only slightly, 10-15%, less for M-in. holes w hen compared to Me-in. holes for some systems. Holes as small as Mfrin., %2-in. and Me-in. were considered unsatisfactory for high surface tension materials such as water [47]. 

Tray efficiency is as high as for bubble caps and almost as high as sieve trays. It is higher than bubble caps in some systems. Performance indicates a close similarity to sieve trays, since the mechanism of bubble formation is almost identical. The real point of concern is that the efficiency falls off quickly as the flow rate of vapor through the holes is reduced close to the minimum values represented by the dump point, or point of plate initial activation. Efficiency increases as the tray spacing increases for a given throughput. 

Thus, a 20-foot baffle tray section, with 50% cut baffles on 24-in. spacing can contain 10 elements and produce 5.2 theoretical stages of separation. A corresponding crossflow sieve tray section, with 10 trays at 90% efficiency (16), can produce 9 theoretical stages. This ratio is about as expected. 

Trays are usually designed with F-factor from 0.25 to 2.0 for a turndown of 8 1. Pressure drop per theoretical stage falls between 3 and 8 mm Hg. Note that bubble cap trays are on the high side and sieve trays are on the lower end of the range. Varying tray spacing and system efficiency, the HETP for trays are usually between 24 in. and 48 in. [133]. The C-factor is the familiar Souders and Brown capacity equation. 

Sieve tray towers have holes of only 3-8 mm dia. Velocities through the holes are kept below 0.8 ft/sec to avoid formation of small drops. Redispersion of either phase at each tray can be designed for. Tray spacings are 6-24 in. Tray efficiencies are in the range of 20-30%. 

The 23-cm-diameter distillation column under study is used to separate ethanol and water. It contains 12 sieve trays with a 30-cm spacing (Fig. 11) as well as three possible feed locations, an external reboiler, and two condensers, which are used at the bottom and the top of the column. The second condenser is also used as a reflux drum a pump sends the reflux back to the column (tray 1) and the product to the product tank. 

The extent of entrainment of the liquid by the vapour rising over a plate has been studied by many workers. The entrainment has been found to vary with the vapour velocity in the slot or perforation, and the spacing used. Strang 60-1, using an air-water system, found that entrainment was small until a critical vapour velocity was reached, above which it increased rapidly. Similar results from Peavy and Baker 6 11 and Colburn 62 have shown the effect on tray efficiency, which is not seriously affected until the entrainment exceeds 0.1 kmol of liquid per kmol of vapour. The entrainment on sieve trays is discussed in Section 11.10.4. 

Sieve tray tower Take 1.5 ft tray spacing, 0.25 in. holes on 0.75 in. triangular spacing. The downcomer area is found with Eq. (14.36) ... 

The conditions of Example 13.15 will be used. This is the case of a standard sieve tray with 24 in. spacing and to operate at 80% of flooding. The entrainment correlation is Figure 18.4 for which the value of the abscissa was found to be... 

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. 

An initial estimate of 0.5 m for the tray spacing is selected based upon typical values suggested in Ref. A6 (p.21.75). The accuracy of this estimate can be tested in later calculations after complete specification of the sieve plate. 

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