Sieve and Valve Trays

Tray design encompasses the determination of the column diameter and the tray spacing as well as a number of mechanical considerations. The scope of the material in this chapter is limited primarily to the fundamentals involved in the design of single-pass sieve trays. The fundamentals involved in the design of valve trays are essentially the same as those involved in sieve trays. No attempt is made to treat bubble-cap trays, since valve and sieve trays have been used extensively in new installations since the early 1950s. Up until that time, bubble-cap trays were used almost exclusively. The design of bubble-cap trays has been treated by a number of authors see for example Van Winkle.17 

Pictures of a typical sieve tray and valve tray are shown in Figs. 12-1 and 12-2, respectively. Sieve trays consist of metal plates with small circular perforations. The valve of a valve tray consists of a self-regulating variable orifice (Fig. 12-2) which adjusts its opening in proportion to the total flow rate of the vapor. Most of the equations for sieve trays are also applicable for valve trays. A treatment of sieve trays is presented in Sec. 12-1 and the modifications of these equations as well as additional equations needed to describe valve trays are presented in Sec. 12-2. 

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Figure 8-78. Glitsch Nye Tray action to improve conventional sieve and valve tray performance by 10-20%. Used by permission, Glitsch, Inc., Bui. GLI-5138.

Turndown can be applied to all types/styles of tray columns however, it is more relevant to sieve and valve trays. The generally accepted explanation of turndown is as follows [199] (also see Figure 8-101) ... 

Hsieh and McNulty [210] developed a new correlation for weeping of sieve and valve trays based on experimental research and published data. For sieve trays the estimation of the weeping rate and weep point is recommended using a two-phase countercurrent flow limitation model, CCFL. 

The weeping rate of the sieve tray is strongly influenced by the gas flow rate, that is, the weeping rate will increase as the gas flow rate reduces below the weep point, i.e., where the weeping starts. Note the comparison of sieve and valve trays during weeping, Figure 8-135 [210]. 

Kister and Haas [184] recommend using 25 dynes/cm in Equation 8-286 when the actual surface tension is a 25 dynes/cm. This correlation is reported [94, 184] to give better effects of physical properties, and predicts most sieve and valve tray entrainment flood data to 15 to 20%, respectively. 

Kister and Haas [184] analyzed sieve and tray data as earlier described [94] and then related their results to the application for sieve and valve trays. 

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. 

Kister. H.Z. and J.R. Haas Predict Emrainment Hooding on Sieves and Valve Trays." Chem. Eng. Prtpg.. 63 (September 1990). 

For most sieve and valve trays, the hydraulic gradient is small and can be dropped from Eq. (14-108). Some calculation methods are... 

The O Connell correlation was based on data for bubble-cap trays. For sieve and valve trays, its predictions are likely to be slightly conservative. 

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 valve trays, the perforations are equipped with valve units (Fig. 19). At high gas rates, the gas force opens the valves, thus providing area for gas flow. At low gas rates, there is insufficient force to keep many of the valves open, and these close, preventing the liquid from weeping. Sieve and valve trays show comparable capacity, efficiency, and... 

The discussions in this chapter emphasize sieve and valve trays, as these trays Eire most frequently encountered in industrial practice. Several of the considerations also apply to other tray types (e.g., bubble-cap trays). Considerations unique to bubble-cap trays were excluded from this chapter. The infrequent application of this type of tray in modern distillation practice argnes against a detailed discus-... 

The bubble-cap tray was the workhorse of distillation before the 1960s. It was superseded by the sieve and valve trays. Presently, bubble-cap trays are specified only for special applications, while sieve and valve trays are the moat popular types. 

Table 6.1 compares the main tray types. The comparison is general and assumes the trays are properly designed, installed, and operated. Sieve and valve trays have comparable capacity, efficiency, entrain-... 

Figure IW Weep point prewure balance for sieve and valve trays, (a) Sieve tray (6] well-designed valve tray, (c) valve tray with too many velvet or with valves that are too light (d) valve tray with too many valves, but fewer than in C (a) well-designed valve trey with two valve weights. (From w. h. Haifa, Chem. Eng. Png., 72 (B), p 43 (September 1876), reprinted courtesy of the American Institute of Chemical Engineers.) 305... 

The O Connell correlation was based on data for bubble-cap trays, and it was stated (131) to predict 90 percent of the efficiency data within 10 percent, both for distillation and absorption. For sieve and valve trays, its predictions are likely to be slightly conservative (151). Ludwig (4) warns that O Connell s absorber correlation (Fig. 7.55, sometimes predicts efficiencies that are too high He believes that it can be used for stripping of gases from rich oils and for absorbers provided care is exercised not to accept too high values. 

Capps [188] examines sieve and valve tray capacity performance and Figure 8-151 [188] is offered for preliminary colunm sizing or for determining whether a debottlenecking study is justified. The correlation for flooding, tray rating, and design of a tray are all based on the capacity factor, Cx, equation (Souders and Brown [68] by Capps [188]). 

Kister, H. Z. andj. R., Haas, Predict Entrainment Flooding on Sieve and Valve Trays, American Institute of Chemical Engineers, Chem. Eng. Aog. V. 86, No. 9 (1990), p. 63. 

Due to the need to use case-by-case analysis the Kister studies [136, 137] focused on item 1. The data evaluated came from published reports by Fractionation Research (FRI) and Separation Research Program (SRP) at the University of Texas, taken from commercial size equipment rather than laboratory research columns. The FRI data includes No. 2 and No. 2.5 Nutter random rings packing, aind Norton s Intalox 2T structured packing, each considered currently state-of-the-art or close to it, while the sieve and valve trays were of FRI s latest designs, plus Nutter s proprietary valve trays, all using 24-in. tray spacing. 

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