Columns valve trays

Crude residuum vacuum distillatian column valve trays... 

Cast-iron column shells are satisfactory, but stainless-steel bubble or valve trays are preferred. A flow sheet of a typical tar acid extraction and refining plant is shown in Figure 3. 

Nye Tray, 10-20% increased tray (over sieve or valve) capacity and good efficiency. More capacity from existing column. Improved inlet area for sieve or valve tray with greater area for vapor-liquid disengagement. 

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) ... 

Fair [183] relates sieve trays and includes valve tray remarks to the extensive work done for bubble cap trays. Figure 8-137 and 8-139 show flooding data for 24-in. spacing of bubble cap trays from [81] and represents data well for 36-in. diameter columns, and is conservative for smaller columns. Fair s work has been corrected to 20 dynes/ cm surface tension by ... 

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... 

The chemical engineering undergraduate spends most of his time sizing equipment. Usually in the problems assigned the type of equipment to be used is specified. For a distillation column the student would be told whether it is a bubble cap, a sieve plate, a valve tray, a packed column, or something else, and then asked to size it for a desired separation. In other cases he would be given the size of the specific equipment and asked to determine what the output would be for a given input. 

There are several ways to account for variable pressures. If the total pressure of the column changes but not the pressure drop through the trays (the normal situation in heat-integrated columns, particularly with valve trays whose pressure drops are fairly constant), an approximate variable-pressure model can be used. 

Which of these two opposing effects dominates depends on the tray design and operating level. The pressure drops through valve trays change little with vapor rates unless the valves are completely lifted. Therefore the second effect is sometimes larger than the first. If this occurs, an increase in vapor boiiup produces a transient increase in liquid rates down the column. This increase in liquid... 

Figure 13.41. Efficiencies of some fractionations with several types of trays as a function of vapor factor F = u fp or linear velocity, (a) Data of methanol/water in a column 3.2 ft dia [data of Kastanek, Huml, and Braun, Inst. Chem. Eng. Symp. Ser. 32(5), 100 (1969)]. (b) System cyclohexane/w-heptane in a 4 ft dia sieve column [Sakata and Yanagi, Inst. Chem. Eng. Symp. Ser. 56, 3.2/21 (1979)] valve tray data (Bulletin 160, Glitsch Inc., 1967). (c) Methanol/water [Standart et al., Br. Chem. Eng. 11, 1370 (1966) Sep. Sci. 2, 439 (1967). (d) Styrene/ethylbenzene at lOOTorr [Billet and Raichle, Chem. Ing. Tech. 38, 825 (1966) 40, 377 (1968)]. (e) Ethanol/water (Kirschbaum, Destillier und Rektifiziertechnik, Springer, Berlin, 1969). (f) Methanol/water [Kastanek, Huml, and Braun, Inst. Chem. Eng. Symp. Ser. 32, 5.100, (1969)].

Valve Trays The amount of work reported thus far on valve tray regime transition is small and entirely based on air-water tests. Correlations proposed to date require the knowledge of liquid holdup at transition, which is generally not available, and are therefore of limited application for commercial columns. 

Solution Table 14-12 presents measurements by Billet (loc. cit.) for ethyl-benzene-styrene under similar pressure with sieve and valve trays. The column diameter and tray spacing in Billets tests were close to those in Example 9. Since both have single-pass trays, the flow path lengths are similar. The fractional hole area (14 percent in Example 9) is close to that in Table 14-12 (12.3 percent for the tested sieve trays, 14 to 15 percent for standard valve trays). So the values in Table 14-12 should be directly applicable, that is, 70 to 85 percent. So a conservative estimate would be 70 percent. The actual efficiency should be about 5 to 10 percent higher. 

Only tray-type columns were considered because of the difficulty of incorporating an effective cooling circuit into a packed column. Sieve trays (as opposed to bubble or valve-type trays) were preferred because of the ease of installing cooling coils and also their low unit cost. Details of tray selection are included in Appendix G.l. 

Using low fractional hole areas A fractional hole area reduction to about 5 percent of the bubbling area typically boosts sieve tray turndown to about 3 to 4 1 at the expense of a lower maximum capacity, i.e., of a larger column diameter. This technique is not recommended because traying the column with valve trays is normally a cheaper alternative. 

This means both vapor and liquid loadB are raised and lowered simultaneously. Increasing vapor rate reduces efficiency, while increasing liquid rates raises efficiency. The two effects normally cancel each other, and efficiency is practically independent of load changes (assuming no excessive entrainment or weeping). Figure 7.106 shows a typical dependence of tray efficiency on vapor and liquid loads for a commercial-scale distillation column. Anderson et al. (97) show a similar dependence for several different valve trays. 

For a typical bubble-cap column, the following pressure drops per tray are considered reasonable, and they also would be order-of-magnitude values for sieve trays or valve trays ... 

Design the trays for a distillation column separating dichlorobenzene (DCB) from a high-boiling reaction product. Include designs for sieve trays and valve trays, and discuss the applications of each. The product is temperature-sensitive, so sump pressure should be held at about 100 mmHg (3.9 inHg or 0.13 bar). The separation requires 20 actual trays. 

Because of their proprietary nature, valve trays are usually designed by their respective vendors based on process specifications supplied by the customer. However, most fabricators publish technical manuals that make it possible to estimate some of the design parameters. The procedure for calculating valve-tray pressure drop outlined here has been adapted from the Koch Design Manual. As for the other column specifications required, they can be obtained via the same calculation procedures outlined above for the sieve-tray design. 

The column has 74 valve trays, the design details of which can be found in Example 11. The feed was introduced onto tray 37. 

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