Sieve tray capacity

Figure 8-123 illustrates a typical sieve tray capacity chart. Entrainment by jet flooding or limitation by downcomer flooding are two of the main capacity limiting factors. The liquid backup in the downcomer must balance the pressure drop across the tray, with the process balance [209]. 

Figure 8-123. Sieve tray capacity chart. Used by permission, Biddulph, M. W., et al. The American Institute of Chemical Engineers, Chem. Eng. Prog. V. 89 No. 12 (1993), p. 56, all rights reserved.

The capacity-factor correlation These values of K and zt-b are substituted into the Bode form above to give the sieve-tray capacity factor correlation. 

Sieve tray capacity limited by coalesced layer flood... 

Sieve Tray Capacity at Flooding The capacity of a sieve tray is determined by hydraulic mechanisms involved in flooding and is not... 

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. 

Capacity Quite similar to sieve tray, as high or higher than bubble cap tray from 50% up to 100% design rate (varies with system and design criteria). Performance at specification quality falls off at lower rates. 

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. 

In some sieves the capacity is 1.5 to as much as 3 times that of a bubble cap tray provided careful consideration has been given to all design features. 

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.

Capps [188] compares valve and sieve tray performance as related to capacity and flooding. Also see sieve tray section presented earlier in this chapter. 

Kister et al. [136] prepared one of the few comprehensive distillation studies for the application selection of valve and sieve trays compared to random or structured packing. This reference is based on a more comprehensive evaluation of accumulated data by the same authors [137]. Many separate studies have been conducted for trays [138] including bubble caps as well as various packings, but few, if any attempt to establish similar conditions to make a viable comparison as is attempted in References 136, 137. There are four main differences related to capacity and separation [136] when considering ... 

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. 

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. 

Dual-Flow Trays These are sieve trays with no downcomers (Fig. 14-27b). Liquid continuously weeps through the holes, hence their low efficiency. At peak loads they are typically 5 to 10 percent less efficient than sieve or valve trays, but as the gas rate is reduced, the efficiency gap rapidly widens, giving poor turndown. The absence of downcomers gives dual-flow trays more area, and therefore greater capacity, less entrainment, and less pressure drop, than conventional trays. Their pressure drop is further reduced by their large fractional hole area (typically 18 to 30 percent of the tower area). However, this low pressure drop also renders dual-flow trays prone to gas and liquid maldistribution. 

---