Sieve trays efficiency

Figure T.10 Some factors affecting sieve tray efficiency. FRI data, total reflux, DT = 4 It, S = 24 in, hu, = 2 in, dH = 0.5 in. Both parts show a small efficiency rise with pressure. Both parts show little effect of vapor and liquid loads above about 40 percent of flood, (a) Showing efficiency reduction when fractional hole area is increased from 8 to 14 per-cent of the bubbling area (6) emphasizing little effect of vapor and liquid loads, and an efficiency increase with pressure. Af 0.14 (Both parts repeated with permission from T. Yanagi and If. Sakata, lad. Eng. Chan. Proc. Use. Dev. 21, p. 712, copyright 19S2, American Chemical Society.)...

O Connell derived his correlation from binary systems in distillation service with bubble-cap trays. Calculated values are slightly conservative for sieve and valve trays. Credit for the slight improvement in valve and sieve tray efficiency should be ignored and counted as a design margin. A separate correlation was developed for absorption services. 

Garcia, J. A. 1999. Fundamental Model for the Prediction of Distillation Sieve Tray Efficiency Hydrocarbon and Aqueous Systems. Ph.D. dissertation, Univ. of Texas at Austin. 

The best way to determine efficiency is to have data for the chemical system in the same type of column of the same size at the same vapor velocity. If velocity varies, then the efficiency will follow Figure 10-13. The Fractionation Research Institute (FRl) has reams of efficiency data, but until recently, most of the data were available to members only. Most large chemical and oil conpanies belong to FRI. The second best approach is to have efficiency data for the same chemical system but with a different type of tray. Much of the data available in the literature are for bubble-cap or sieve trays. Usually, the efficiency of valve trays is equal to or better than sieve tray efficiency, which is equal to or better than bubble-cap tray efficiency. Thus, if bubble-cap efficiencies are used for a valve tray column, the design will be conservative. The third best approach is to use efficiency data for a similar chemical system... 

The efficiency of valve tray depends upon the vapor velocity, the valve design, and the chemical system being distilled. Except at vapor flow rates near flooding, the efficiencies of valve trays are equal to or higher than sieve tray efficiencies, which are equal to or higher than bubble-cap tray efficiencies. Thus, the use of the efficiency correlations discussed earlier will result in a conservative design. 

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. 

For sieve trays, Chan and Fair [Ind. Eng. Chem. Pioc. Des. Dev., 23, 814 (1983)] used a data bank of larger-scale distillation column efficiencies to deduce the following expression for the product kcCi ... 

FIG. 23-38 Efficiency and capacity range of small-diameter extractors, 50 to 150 mm diameter. Acetone extracted from water with toluene as the disperse phase, V /V = 1.5. Code AC = agitated cell PPC = pulsed packed column PST = pulsed sieve tray RDC = rotating disk contactor PC = packed column MS = mixer-settler ST = sieve tray. (Stichlmair, Chem. Ing. Tech. 52(3), 253-255 [1980]). 

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. 

Trays operate within a hydraulic envelope. At excessively high vapor rates, liquid is carried upward from one tray to the next (essentially back mixing the liquid phase in the tower). For valve trays and sieve trays,. i capacity limit can be reached at low vapor rates when liquid falls through the rray floor rather than being forced across the active area into tlic downcomers. Because the liquid does not flow across the trays, it rass.scs contact with the vapor, and the separation efficiency drops dramatically. ... 

Trays are generally divided into four categories (1) sieve trays, (2 ) val ve tray s. (3) bubble cap trays, and (4) high capacity/high efficiency trays. 

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

Column diameter for a particular service is a function of the physical properties of the vapor and liquid at the tray conditions, efficiency and capacity characteristics of the contacting mechanism (bubble trays, sieve trays, etc.) as represented by velocity effects including entrainment, and the pressure of the operation. Unfortunately the interrelationship of these is not clearly understood. Therefore, diameters are determined by relations correlated by empirical factors. The factors influencing bubble cap and similar devices, sieve tray and perforated plate columns are somewhat different. 

The pressure drop of these trays is usually quite low. They can be operated at an effective bubbling condition wnth acceptable efficiencies and low pressure drops. For more efficient operation the clear liquid height on the tray appears to be. similar to the sieve tray, i.e., 1.5-2-in. minimum. This is peculiar to each system, and some operate at 1 in. with as good an efficiency as when a 2-in. is used. When data is not available, 2 in. is recommended as a median design point. 

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%. 

Chan, H. and Fair, J. R. (1984a) Ind. Eng. Chem. Proc. Des. Dev. 23, 814. Prediction of point efficiencies on sieve trays. 1. Binary systems. 

The effects of liquid viscosity on tray efficiency have been studied by Drickamer and Bradford(58) and () Co nt ij. 59- and these are discussed in Section 11.10.5. Surface tension influences operation with sieve trays, in relation both to foaming and to the stability of bubbles. 

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. 

---