Tray Diameter

Typically, the column diameter D is based on a specified fractional approach to flooding/ Then, 

Oliver (1966) suggests that Ad/At must be chosen, based on the value of X, as 

Because of the need for internal access to columns with trays, a packed column is generally used if the diameter calculated from equation (4-33) is less than 60 cm. Tray spacing must be specified to compute column diameter, as shown by equation (4-32). On the other hand, recommended values of tray spacing depend on column diameter, as summarized in Table 4.3. Therefore, calculation of the tower diameter involves an iterative procedure (a) an initial value of tray spacing is chosen (usually, t = 0.6 m) (b) the tower diameter is calculated based on this value of tray spacing (c) the value of t is modified as needed according to the recommendations of Table 4.3 and the current estimate of D (d) the procedure is repeated until conver- 

Example 4.6 Design of a Sieve-Tray Column for Ethanol Absorption 

Design a sieve-tray column for the ethanol absorber of Example 4.4. For alcohol absorbers, Kister (1992) recommends a foaming factor Ff = 0.9. The liquid surface tension is estimated as a = 70 dyn/cm. Take do- 5 mm on an equilateral-triangular pitch 15 mm between hole centers, punched in stainless steel sheet metal 2 mm thick. Design for an 80% approach to the flood velocity. 

The column cross-sectional area, and hence the tray diameter, is sized such as to prevent flooding. Tray sizing is initially based on preventing entrainment flooding, but is subsequently checked for downcomer flooding and other considerations. Above a certain vapor velocity, the vapor flood velocity, liquid droplets are entrained with the vapor, causing flooding. The tray should be sized such that the actual vapor velocity is below the vapor flood velocity. 

At the vapor flood velocity Uf, the weight of the droplet, F, is balanced by the 

If the liquid droplet diameter is its density is and the surrounding vapor density is py, the following relations hold  

The drag force is given in terms of the droplet cross-sectional area, the vapor velocity and density, and the drag coefficient Cp. At the vapor flood velocity, the drag force is 

The vapor flood velocity is obtained by substituting the above force expressions in the force balance equation and solving for U/. 

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To determine the column (with trays) diameter, an approach [130] is to (1) assume 0 hours (2) solve for V, Ib/hr vapor up the column at selected, calculated, or assumed temperature and pressure (3) calculate column diameter using an assumed reasonable vapor velocity for the type of column internals (see section in this volume on Mechanical Designs for Tray Performance ). 

Weir heights are 2 in., weir lengths about 75% of tray diameter, iiquid rate a maximum of about 8 gpm/in. of weir muitipass arrangements are used at high liquid rates. 

Tray stack heigrt, ft. Tray diameter, ft. Tray spacing, in. Tray type Material... 

Weir heights of 2 in. are fairly standard and weir lengths about 75% of the tray diameter. For normal conditions downcomers are... 

Tray diameters may be approximated with Figure 13.34 which is for normal systems, 24 in. tray sparing, and 80% of flooding. For other tray sparings, corrections may be approximated with the... 

The allowable vapor velocity and the corresponding tray diameter are represented by the work of Souders and Brown, which is cited in standard textbooks, for example Treybal (1980). Its equivalent is the Jersey Critical formula,... 

Liquid Flow Patterns on Large Trays The most popular theoretical models (below) postulate that liquid crosses the tray in plug flow with superimposed backmixing, and that the vapor is perfectly mixed. Increasing tray diameter promotes liquid plug flow and suppresses backmixing. 

If you are using the program supplied on the companion CD, go to the form 2 screen called the Program Run form and notice that there are three remaining items to input tray diameter, width, and number of liquid passes. You must choose a value for each. Form 2 of the Tray 10 program is shown in Table 3.2. 

Most popular theoretical models (such as the AlChE and the Chan and Fair models, Sec. 7.2.1) postulate that liquid crosses the tray in plug flow (Fig. 7.7a) with superimposed backmixing, and that vapor is perfectly mixed. Increasing tray diameter promotes liquid plug flow and suppresses backmixing. This should enhance efficiency in large-diameter columns, but such enhancement has not been observed (147,148). Liquid maldistribution is the common explanation to the observation. 

Weir heights of 2 in. are fairly standard and weir lengths about 75% of the tray diameter. For normal conditions downcomers are sized so that the depth of liquid in them is less than 50% and the residence time more than 3 sec. For foaming and foam-stable systems, the residence time may be two to three times this value. The topic of tray efficiency is covered in detail in Section 13.6, but here it can be stated that they are 80-90% in the vicinity of F = = 1.0(ft/sec)(lb/cuft) for mixtures similar to water... 

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