Tray column

A perspective view of a tray column is shown in Fig. 5.4-1. The gas flows upward and the liquid downward in the column. On the trays itself there exists a eross flow of the two phases. The horizontal trays have openings (holes, eaps, valves) for enabling the gas flow through the plates. 

Three principles for the design of the openings in the plates can be distinguished (Fig. 5.4-2)  

Sieve trays have a large number of round holes in the plate (6-13 mm in diameter) with an open area of about 8-15% of the active area Aac. The weeping of liquid is prevented by the upward flow of gas through the holes. 

In bubble cap trays weeping of liquid is prevented by design. A chimney with a cap covers the openings in the plate. Bubble caps typically have a diameter of 50-80 mm. The relative free area is about 10-20%. The pressure drop of bubble cap trays is rather high. 

Valve trays have movable elements in the tray openings for adjusting the free area to the actual gas load. This widens the operating range and reduces the pressure drop at high gas loads. 

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Fig. 26. Numbering plates and concentrations in a bubble tray column.

In general, plants using SO2 gas derived from metallic sulfides, spent acids, or gypsum anhydrite purify the gas stream before drying it by cold, ie, wet, gas purification. Various equipment combinations including humidification towers, reverse jet scmbbers, packed gas cooling towers, impingement tray columns and electrostatic precipitators are used to clean the gas. 

If a waste contains a mixture of volatile components that have similar vapor pressures, it is more difficult to separate these components and continuous fractional distillation is required. In this type of distillation unit (Fig. 4), a packed tower or tray column is used. Steam is introduced at the bottom of the column while the waste stream is introduced above and flows downward, countercurrent to the steam. As the steam vaporizes the volatile components and rises, it passes through a rectification section above the waste feed. In this section, vapors that have been condensed from the process are refluxed to the column, contacting the rising vapors and enriching them with the more volatile components. The vapors are then collected and condensed. Organics in the condensate may be separated from the aqueous stream after which the aqueous stream can be recycled to the stripper. 

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. 

Here are two quick approximation methods for bubble cap tray column diameter. 

Percentage of Flood for Existing Ballast Tray Columns... 

Packed columns are gaining ground on trayed columns. Lieberman states that based on his design and operating experience, a properly designed packed tower can have 20-40% more capacity than a trayed tower with an equal number of fractionation stages. 

Data from bubble cap and perforated tray columns for the Murphree vapor plate efficiencies are correlated [86] ... 

For tray columns, bubble caps, valves or sieve, the feed liquid usually enters the column either in between functioning trays or at the top (reflux). The liquid or liq-uid/vapor mixture for flashing liquids must be dispersed uniformly across the tray. Such an arrangement often requires a special tray designed for the purpose to allow... 

Turndown can be applied to all types/styles of tray columns however, it is more relevant to sieve and valve trays. The generally accepted explanation of turndown is as follows [199] (also see Figure 8-101) ... 

Kister et al. [213] have concluded from examining reported cases of cross-flow channeling related to poor sieve tray column performance that under specific conditions the cross-flow channeling does occur. See Figure 8-142 [213] for diagram of the postulated vapor flow across a tray. It is known to occur for valve trays and bubble cap trays. This condition has not been studied very much in the open literature however, several investigators including myself have observed in industrial practice the... 

Design a sieve tray column to separate benzene and toluene to produce 35,000 Ib/hr of benzene as overhead product at atmospheric pressure and use a reflux ratio of 5 1 (reflux returned to net overhead). Using the top tray as the first design basis, which can be followed by other points in the column, determine by the material balance Top tray data ... 

Figure 8-153. Simple side-to-side baffle arrangement, with liquid flow cascades, for baffle tray column. Used by permission. Fair, J. R., Hydrocarbon Processing, V. 72, No. 5 (1993) p. 75, Gulf Pub. Co., all rights reserved.

Example 8-42 Mass Transfer Efficiency Calculation for Baffle Tray Column (used by permission [211])... 

Perry et al. [85] point out that packed columns are more dependent on liquid distribution than trayed columns, as can be appreciated by the differences in the way the liquid must flow down the two types of columns. Liquid distribution quality is measured or described as [85] ... 

Establish the location of the feed, bottoms and overhead compositions on the graph. Draw in the operating lines as for a distillation in a tray column. 

The three primary devices used in this service are (a) open spray columns,(b) packed columns, and (c) tray columns, which are perforated plates, baffle trays, etc. ... 

Fair reports that the data for mass transfer in spray, packed, and tray columns can be used for heat-transfer calculations for these columns. The pressure drop in these types of columns is usually quite low. 

M = molecular weight m = exponent in baffie tray columns... 

Figure 10-173. Direct contact tray column for heat transfer. This could be a baffle tray, sieve type tray, bubble or other contact device, or open spray or random packed column. (Symbols only used by permission Smith, J. H. Hydrocarbon Processing, Jan. 1979, p. 147. Gulf Publishing Company. All rights reserved.)...

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