Perforated trays with downcomers

Sieve Tray or Perforated Tray With Downcomers... 

Figure 8-67A. Sieve or perforated tray with downcomers.

Figure 8-120. Selection guide, perforated trays with downcomers. Used by permission, Chen-Jung and Hodson, J. R. Petroleum Refiner, V. 37 (1958) p. 104, Gulf Publishing Co., all rights reserved.

More elaborate arrangements are the familiar bubble-plate (Fig. 16-6) and such modifications as tunnel trays (vapor slots and vapor troughs extend across the plate) and Uniflux trays, float-valve bubble trays, perforated trays with downcomers for the liquid, and Kaskade-type (Bentuii and Flexitray) trays. 

This is the case with diameter determination. The relation of Equation 8-250 for the perforated tray or sieve tray with downcomers can be used for the plate without downcomers. Generally, the liquid level and foam-froth height will be higher on this tray, hence the ralue of h., clear liquid on the tray, may range from 1-in. to 6-in. depending on the service. 

At the flood point, liquid continues to flow down the column, but builds up at a greater rate from tray to tray. Sutherland [69] demonstrated that flooding moves up the column from the point of origin. For this reason it is important to design perforated trays without downcomers with extra care, as changing internal rates are quickly reflected in performance if the proper hole requirements are not met. They are a usefiil tray for steady state operations. 

Chlorohydrin solution is pumped from the chlorinator to the saponifier. It is mixed in the feed line with a 10% lime slurry and preheated by injection of live 25 psig steam to a temperature of 200°F. Stripping steam is injected at the bottom of the saponifier, which has six perforated trays without downcomers. Propylene... 

Example 8-38 Sieve Tray Design (Perforated) with Downcomer... 

Sieve or perforated trays. These are much simpler in construction, with small holes in the tray. The liquid flows across the tray and down the segmental downcomer. Figure 11.52 indicates the general form of tray layout. 

There is essentially no published work on specific tests with these trays as relates to entrainment, etc. However, the very close similarity between a perforated plate without downcomers and one with downcomers is sufficient to justify using some data for one in the design of the second. 

The single-pass sieve tray with crossflow appears to be the most widely used type of tray today. In a tray of this type, the vapor passes upward through the perforations (or valves) and then through the liquid on the tray. The liquid flows down through the column and enters each plate by flowing under the downcomer weir. Then the liquid flows across the plate and over the outlet weir and into the downcomer to the plate below as shown in Fig. 12-3. In order for the liquid to flow across each plate in succession as it passes down through the column, a liquid head in each downcomer is required as depicted in Fig. 12-3. 

Determining the number of theoretical and actual trays in a distillation column is only part of the design necessary to ensure system performance. The interpretation of distillation, absorption, or stripping requirements into a mechanical vessel with internal components (trays or packing, see Chapter 9) to carry out the function requires use of theoretical and empirical data. The costs of this equipment are markedly influenced by the column diameter and the intricacies of the trays, such as caps, risers, weirs, downcomers, perforations, etc. Calcvdated tray efficiencies for determination of actual trays can be lost by any unbalanced and improperly designed tray. 

The perforated plate, punched plate, or Dual-Flow plate are terms used to refer to a tray operating without downcomers, with rapor and liquid passing countercurrent through perforations in the tray. The Dual-Flow term has been coined by Fractionation Research, Inc., and its design know-how is restricted to contributing members, and cannot be presented in this book. 

A typical arrangement (Fig. 17) consists of a vertical tower fitted with horizontal plates or trays, on which liquid and gas are contacted. Each tray is equipped with gas passages, which may be perforations in the tray floor or other devices such as valves or bubble caps that disperse the rising gas into the liquid layer. The liquid layer on the tray is maintained by the outlet weir. Liquid descends from each tray to the tray below via a downcomer. 

A variation of the normal sieve tray is the Turbogrid tray developed by the Shell Development Company. The Turbogrid tray uses slots instead of circular holes and operates as a counterflow plate unit with no downcomers. A typical tray consists of a flat grid of parallel slots. The slots can be stamped perforations in a flat metal sheet or the open spaces between parallel bars. 

The gas in the scrubber rises through a series ottrays (metal plates perforated with many small holes), and the solvent flows over the trays and through downcomers to the trays below. Gas bubbles emerge from the holes in each tray and rise through the covering liquid, and SO2 diffuses out of the bubbles and into solution. 

Sieve tray tower Design is based on a coalesced layer of 1 inch h = 2.54 cm) with 50% of pressure drop through perforations and 50% through downcomer. An 18-inch tray spacing is used. 

---