DISTILLATION
� TRAY SPACING The vertical distance between adjacent trays varies between 450 and 90 0 mm in the columns generally employed in the chemical process industries (CPI). Spacing involves a tradeoff between column height and column diameter: with greater tray spacing, the column height increases while the required diameter decreases. In most cases, the two effects cancel each other from an economic standpoint. There are, however, additional constraints that govern tray spacing; the relevant guidelines appear in the Table. Multiple passes are used when the liquid load is relatively high for a single pass. The capacities of the tray and the downcomer rise, and the pressure drop across the tray becomes smaller. However, the tray efficiency declines because th e liquid path becomes shorter; the latter effect may also complicate the provision of tray manways (discussed in more detail below). Trays containing more than two liquid passes are prone to liquid and vapor maldistribution because of nonsymmetrical panels. If possible, trays having odd numbers of passes should not be used.
GUIDELINES FOR SELECTION OF TRAY PASSES • Set the number of passes such that liquid loads do not exceed 70 m 3/h per meter of weir length • After the number of passes has been selected, check the path length. If that length works out to be less than 400 mm, the installation of internal manways is not feasible. If employment of multiple passes becomes mandatory, it may be necessary to adjust the column diameter in order to arrive at the minimum path length of 400 mm • To avoid short liquid paths , adhere to the following guidelines: for column diameters of 1,200 to 2,100 mm, a maximum of two passes; for diameters of 2,100 to 3,000 mm, a maximum of three passes; for diameters above 3,000 mm, use four passes at most
DOWNCOMERS Downcomers are conduits having circular, segmental or rectangular cross sections that convey liquid from a tray to the one immediately below. Various types appear in Figure 1. The version most often found in process plants is the straight, segmental vertical downcomer. It is simpler and less expensive than sloped downcomers. On the other hand, the sloped versions make the best use of the column cross-sectional area for downflow — they maximize the active area on the lower of the two trays, and at the top of the downcomer,
there is adequate space for the liquid and vapor to separate from each other.
RECOMMENDED DOWNCOMER VELOCITIES • Minimally foaming liquids: 0.12–0.21 m/s • Liquids with medium foaming tendency: 0.09– 0.18 m/s • Highly foaming liquids: 0.06–0.09 m/s
DOWNCOMER AREAS, CLEARANCES • For the cross-sectional area of the downcomers,values less than 5 to 8% of the column cross-sectional area should not be specified • For single pass trays, the downcomer width should not be less than 10% of the column diameter • The downcomer clearance should be less than the outlet weir height
DOWNCOMER SEALING In general,to achieve a proper downcomer seal, the bottom edge of the downcomer should be about 10 mm below the top edge of the outlet weir. (This statement is more of a guideline than a rule). According to another guideline, the downcomer clearance should be selected such that the liquid velocity under the downcomer does not exceed 0.45–0.50 m/s. The best downcomer clearance can only be specified by the tray vendor.
Tray Spacing, mm > 600
L W Q L Q D u S ρ
Downcomer entrance area Vapor load Liquid volumetric volumetric flowrate, or moles moles of liquid in kettle Length of outlet weir Liquid load Downcomer load Superficial vapor velocity Density GENERAL SUBSCRIPTS
V
L
Vapor
Liquid
• The distance that the downcomer extends downward within the seal pan should be about the same as the clearance between downcomer bottom and pan floor • The distance between bottom tray floor and seal pan floor should be 150 mm greater than the normal tray spacing
TRAYS Vapor load: Liquid load:
CS
=
Q L
Downcomer load: Q D
u S
=
=
ρ V ρL
−
ρ V
L L W L A D
OUTLET WEIRS The purpose of an outlet weir is to keep the liquid level on the tray at the intended value. Proper functioning of a weir is especially important if the tray operates in the froth regime, the most common flow regime for distillation in the CPI. • Weir heights in the froth regime are therefore restricted to 50–80 mm. • The preferred weir height for columns operating in the spray regime is 20–25 mm. • Weir loadings should fall within the range of 15 to 70 m3/h per meter of weir length.
PRESSURE DROP For trays to function reasonably close to th eir best efficiency point, the dry-tray pressure drop must be roughly equal to the hydraulic-tray pressure drop. Guidelines for seal pans (Figure 2): • To forestall problems due to solids accumulation, the clearance between the seal pan floor and the bottom downcomer should exceed the clearance normally employed under the tray downcomers. It should be at least 50 mm
GUIDELI NES FOR SELECTIO N OF TRAY TRAY SPACING SPACING Description Column diameters larger than 3,000 mm
NOMENCLATURE
A D C S L
Comments The tray support beams restrict crawling space available; hence the large tray spacing Column diameters between 600 This spacing is suf ficiently wide to allow a 1,200 mm and 3,000 mm worker to freely crawl between trays Column diameters between 450 Crawling between the trays is seldom re750 mm and 1,200 mm quired, because the worker can reach the column wall from the tray manways Fouling and corrosive service > 600 Frequent maintenance is expected Systems with a high foaming At least 450 mm, but pre- Required to avoid premature flooding tendency ferably 600 mm or higher Columns operating in spray At least 450 mm, but pre- Required to avoid excessive entrainment regime ferably 600 mm or higher Columns operating in froth < 450 Lower tray spacing restricts allowable vapor regime velocity, thereby promoting froth-regime operation
Of these three types of downcomers, downcomers, the segmental version is the one in widest use at chemical process plants FIGURE 1.
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Note the adverse effects of improper improper clearance (too large or too small) for the downcomer feeding Tray #11 FIGURE 2.
Reference Mukherjee, S., “Tray Column Design: Keep Control of the Details,” Chem. Eng., Eng., September 2005, pp. 52–58.
