Tray details

Tray details will be considered in a later example. 

Figure 8-86. Typical bubble cap tray details 3-ft-O-in. diameter column, 4-in. caps.

Preliminary Process Flowsheet. This will show major equipment and lines, preliminary equipment details (vessel diameter, number of trays, pump flow and driver horsepower, etc.), major instrumentation, and, it is hoped, have a material balance at the bottom of each drawing with flows keyed to a numbering system on the diagram. The process flowsheets should cover both the process and utility sides of the plant. 

Burning Rate - Defined as the rate (in millimeters per minute) at which a pool of liquid decreases as the liquid bums. Details of measurements are provided by D. S. Burgess, A. Strasser, and J. Grumer, Diffusive Burning of Liquid Fuels in Open Trays, Fire Research Abstracts and Reviews, 3, 177 (1961). 

Once the operating conditions are established for a tower, its diameter and height can be chosen using data available from tray and packing manufacturers. The details of tower diameter selection, tray spacing, and internal design are beyond the scope of this text. 

It is essential to realistically establish the condensing conditions of the distillation overhead vapors, and any limitations on bottoms temperature at an estimated pressure drop through the system. Preliminary calculations for the number of trays or amoimt of packing must be performed to develop a fairly reasonable system pressure drop. With this accomplished, the top and bottom column conditions can be established, and more detailed calculations performed. For trays this can be 0.1 psi/actu-al tray to be installed [149] whether atmospheric or above, and use 0.05 psi/tray equivalent for low vacuum (not low absolute pressure). 

The American Institute of Chemical Engineer s Distillation Tray Efficiency Research [2] program has produced a method more detailed than the short-cut methods, and correspondingly is believed to produce reliable results. This method produces information on tray efficiencies of new systems without experimental data. At present there is not enough experience rvith the method and its results to evaluate its complete range of application. 

Murphree [85] developed point and overall distillation tray efficiencies, which are examined in detail in Reference 2. The expressions are [59] ... 

In Table 8-2 Proctor [178] compares efficiencies of sieve and bubble cap trays (plates). He concludes that the sieve design provides a 15% improvement in plate efficiencies. To fully evaluate the actual efficiencies in any particular system, the physical properties, mechanical details of the trays, and flow rates must be considered. See Reference 2 also. 

When the feed point is located by a match from tray-bytray calculation, the correct point can be established with greater confidence, but still alternate nozzles are suggested since even these detailed calculations can be off to a certain extent. 

Rigorous tray-by-tray computations for multicomponent mixtures of more than three components can be very tedious, even when made omitting a heat balance. Computers are quite adaptable to this detail and several computation methods are in use. 

The simplest tray arrangement considering fluid flow and mechanical details is the cross-flow shown in Figure 8-84 (page 137). It fits the majority of designs. WTien liquid flows become small with respect to vapor flows the... 

Figure 8-86 (page 139) is a 3-ft 0-in. diameter tray, and is representative of details associated with tray design.A typical 4-in. pressed cap is shown in Figures 8-79 and 81. 

WEIR DETAIL B" Scale lV2 =1 -0 Fabricator shall furnish S-3/8"0 Machins bolts with Hex Nuts Washers for each tray. 

For stepped caps (tray level, risers of different heights) or cascade tray (tray consisting of two or more levels) care must be taken to analyze all the conditions associated with level changes on the tray. Reference 5 discusses this in some detail. 

From a consideration of contacting requirements, a tower 9.5 ft in diameter is selected. Other pertinent details are 24-in. tray spacing, 1-in. weir height, /e-in. dia. holes, 10% hole area (referred to active area) and 8.3 ft downcomer area. 

Some of the details of tray layout are given in Figure 8-67A. The working details can be set by the required performance. 

Since standard details for fabrication are already available for a tray with a 19.5-in. w eir in a 30-in. tower (65% of dia.), try this as first tray examined. This is 6.8% of tower cross-sectional area. Dowmeomer area = 0.068 (4.91) = 0.334 ft. ... 

Mechanical tray layout details. Allow a total of 3H-in. on diameter for extension of tray ring-type support into the tower. This reduces available tray area. Other support details might make more area arail-able. Each must be examined. 

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