Trays spacing

A tray spacing of 18 in is recommended for columns 2 V2 to 4 ft in diameter (48). Here crawling between trays is seldom required, because a worker can reach the column wall from the tray manway. For such columns, the recommended tray spacing is lowered to reduce the support problems associated with tall and thin columns (see item 8 below). 

A tray spacing smaller than 18 in makes access for maintenance difficult. It is therefore generally not recommended, but is often used (257, 272, 409). 

When the economic trade-off between column height and column diameter is constrained, optimum tray spacing is dictated by cost considerations, while access considerations assume a secondary role. Some situations where the trade-off is constrained are 

When column height is restricted Examples include a column that needs to be enclosed inside a building, a cold box, or another expensive shield (and perforating the roof is impractical or costly) a tall column that is to be erected in the vicinity of an airport (and must comply with a maximum height regulation) and a column 250 to 

When column diameter is restricted Examples include small columns, where the diameter needs to be at least 2 V2 to 3 ft to permit personnel entry an existing column used for a new service or a hydraulically underloaded section of a constant-diameter column. When the diameter in such columns is hydraulically oversized, it becomes attractive to minimize tray spacing, thus utilizing excess capacity to reduce column height or increase the number of trays. 

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The tray-free area can be decreased at intervals from top to bottom as the density differential between the aqueous phase and the working solution widens. This adjustment maintains a nearly constant depth of coalesced working solution beneath each tray. For this type extractor the distance between trays (spacings) is constant from top to bottom. Alternatively, the tray area can be held constant and the height of the coalesced layer beneath the tray permitted to vary, thus providing the needed pressure drop for flow. 

Fig. 18. Flooding correlation for crossflow trays (sieve, valve, bubble-cap) where the numbers represent tray spacing in mm. Also shown are approximate...

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. 

Disperser System Column diameter, ft Tray spacing, in Pressure, psia Static submergence, in Efficiency, % Remarks Ref. 

Testing of plates and other devices is carried out by Fractionation Research, Inc. for industrial sponsors. Some of the test data for sieve plates have been published for the cyclohexane//i-heptane and isobu-tane//i-butane systems. Representative data are shown in Fig. 14-43. These are taken from Sakata and Yanagi Jn.stn. Chem. Engis. Symp. See. No. 56, 3.2/21 (1979)] and Yanagi and Sakata [Jnd. Eng. Chem. Proc. Des. Devel, 21, 712 (1982)]. The column diameter was 1.2 m, tray spacing was 600 mm, and weir height was 50 mm. 

Fair s empirical correlation for sieve and bubble-cap trays shown in Fig. 14-26 is similar. Note that Fig. 14-26 incorporates a velocity dependence (velocity) above 90 percent of flood for high-density systems. The correlation implicitly considers the tray design factors such as the open area, tray spacing, and hole diameter through the impact of these factors on percent of flood. 

Figure 1. F Factor as a function of column pressure drop and tray spacing.

F = Factor for fractionation allowable velocity P = Column pressure, psia T = Tray spacing, in. 

Smith recommends obtaining the settling height (tray spacing minus clear liquid depth) by applying the familial Francis Weir formula. For our purposes of rapidly checking column diameter, a faster approach is needed. 

P = Pressure drop in inches of water SG = Specific gravity of the liquid on the tray at the appropriate temperature T = Number of trays T, = Tray spacing, in. 

For a 15-inch spray height, a tray spacing of at least 21 inches is recommended. 

The capacity factor CAF<, increases with increasing tray spacing up to a limiting value. Limits reached... 

GPM = Column liquid loading, gal/min NP = Tray number of flow paths or passes TS = Tray spacing, in. 

Avoid vapor entry close to a liquid level. Reboiler vapor should enter the bottom of a fractionator a distance of at least tray spacing above high liquid level. Tray damage can result if the liquid is disturbed. 

For estimating downcomer area. Reference 3 gives a correlation of design liquid traffic versus tray spacing. The developed equation is ... 

Improper tray spacing at feed location. Premature flooding. Design error. 

Downcomer Backup Flood. For downcomer backup. Equation 4 can be used. Reference 15 states that if the downcomer backup for valve trays exceeds 40% of tray spacing for high vapor density systems I3.01bs/ft-), 50% for medium vapor densities, and 60% for vapor densities... 

Sieve plates are used with 0.41-m (16-inch) tray spacing. Height of column shell ... 

The maximum allowable superficial velocity of air in the stripper can be estimated using the following expression (based on the data provided by Peters and Timmerhaus (1991) for 0.41-m tray spacing) ... 

A tray spacing of 0.41 m may be employed. All columns are assumed to have an overall efficiency of 25%. The fixed cost of the column is obtained by multiplying the equipment cost by a factor of 5 to account for installation, instrumentation and other ancillary devices. 

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