Tray Column Design Department Editor: Kate Torzewski
ere, we present criteria needed or the intelligent, eective specication o trays or a distillation or stripping column. Among the key parameters and accessories during specication are column diameter, tray spacing, number o trays, number o passes, type o downcomers, weir heights, provision o downcomer backups, tray pressure drops, design o bottom seal pan, column bottom arrangement, and nozzle location and orientation.
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Tray Spacing
The vertical distance between adjacent trays varies rom 450 to 900 mm in the columns generally employed in the chemical process industries (CPI), as seen in Table 1. Spacing involves a tradeo between column height and column diameter; with greater tray spacing, the column height increases, while the required diameter decreases. Height/diameter ratios greater than 25–30 are generally not recommended. Tray paSSeS
Set the number o passes such that liquid loads do not exceed 70 m3/h per meter weir length. Ater the number o passes has been selected, i necessary, adjust the column diameter in order to arrive at a minimum path length o 400 mm. •Foracolumndiameterof1,200to Foracolumndiameterof1,200to 2,100 mm: use two passes at maximum •Foradiameterof2,100to3,000mm: Foradiameterof2,100to3,000mm: use three passes at maximum •Foradiameterabove3,000mm:use Foradiameterabove3,000mm:use our passes at maximum DowncomerS
Downcomers are conduits having circular, segmental or rectangular cross sections that convey liquid rom a tray to the one immediately below.
Liquid velocities in downcomers : •Minimallyfoamingliquids: Minimallyfoamingliquids: 0.12–0.21 m/s •Mediumfoamingliquids: Mediumfoamingliquids: 0.09–0.18 m/s •Highlyfoamingliquids: Highlyfoamingliquids: 0.06–0.09 m/s :For Downcomer areas and clearances :For the cross-sectional area o the downcomers, values less than 5–8% o the column
Table 1.
Des Descrip cripti tio on
Guidelines for Selection of Tray Spacing
Tray Spac Spacin ing, g, mm
Comm Co mmen ents ts
Column diameters larger than 3,000 mm
> 600
The tray support beams restrict crawling space available; hence the large tray spacing
Column diameters between 1,200 and 3,000 mm
600
This spacing is sufficiently wide to allow a worker to freely crawl between trays
450
Crawling between the trays is seldom required, because the worker can reach the column wall from the tray manways
> 600
Frequent maintenance is expected
Column diameters between 750 and 1,200 mm Fouling and corrosive service Systems with a high foaming tendency
At least 450 mm, but preferably 600 mm or higher
Required to avoid premature flooding
Columns operating in spray regime
At least 450 mm, but preferably 600 mm or higher
Required to avoid excessive entrainment
Columns operating in froth regime
< 450
cross-sectional area should not be specied. Also, the downcomer width should not be less than 10% o the column diameter. Speciy the downcomer clearance to be less than the outlet weir height; otherwise, vapor will fow up the downcomer rather than through the tray deck above. To achieve a proper Downcomer sealing : To downcomer seal, the bottom edge o the downcomer should be about 10 mm below the top edge o the outlet weir. The downcomer clearance should be selected such that the liquid velocity under the downcomer does not exceed 0.45–0.50 m/s. ouTleT weirS
•Weirheightsinthefrothregimeare Weirheightsinthefrothregimeare restricted to 50–80 mm •Weirheightsforcolumnsoperatingin Weirheightsforcolumnsoperatingin the spray regime should be 20–25 mm Weirloadingsshouldfallwithinthe range o 15 to 70 m 3/h per meter weir length. preSSure Drop
Fortraystofunctionreasonablycloseto their best eciency point, the dry-tray pressure drop must be roughly equal to the hydraulic-tray pressure drop.
Lower tray spacing restricts allowable vapor velocity, thereby promoting froth-regime operation
exceed the clearance normally employed under the tray downcomers. It should be at least 50 mm. The distance that the downcomer extends downward within the seal pan should be about the same as the clearance between downcomer bottom and pan foor. The distance between bottom tray foor and seal pan foor should be 150 mm greater than the normal tray spacing. nozzle orienTaTion
Fornozzlesthatfeedliquidintothetop tray, tray, the nozzle should be perpendicular to the downcomer o the top tray. Nozzles eeding liquid at intermediate trays can be placed anywhere except in the downcomer segment. The space between the two trays should be at least 800 mm. column boTTom
Inlets or the bottom eed and reboiler return lines should be at least 300 mm above the high liquid level. The bottom eed and reboiler return should not impinge on the bottom seal pan, seal pan overfow, or the bottom downcomer. The tops o both pipes should be at least 400–450 mm below the bottom tray.
Seal panS
The clearance between the seal pan foor and the bottom downcomer should
References 1.Mukherjee,S.,TrayColumnDesign:KeepControlofthe Mukherjee,S.,TrayColumnDesign:KeepControlofthe Details, Chem. Eng., September 2005, pp. 52–58.
