Tray Column Design Department Editor: Kate Torzewski
Chemical Engineering©
Here, we present criteria needed for the intelligent, effective specification of trays for a distillation or stripping column.
Among the key parameters and accessories during specification are column diameter, tray spacing, number of trays, number of passes, type of downcomers, weir heights, provision of downcomer backups, tray pressure drops, design of bottom seal pan, column bottom arrangement, and nozzle location and orientation.
Tray Spacing
The vertical distance between adjacent trays varies from 450 to 900 mm in the columns generally employed in the chemical process industries (CPI), as seen in Table 1.
Spacing involves a tradeoff 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 of passes such that liquid loads do not exceed 70 m3/h per meter weir length.
After the number of passes has been selected, if necessary, adjust the column diameter in order to arrive at a minimum path length of 400 mm.
Downcomers are conduits having circular, segmental or rectangular cross sections that convey liquid from a tray to the one immediately below.
Liquid velocities in downcomers:
Also, the downcomer width should not be less than 10% of the column diameter.
Specify the downcomer clearance to be less than the outlet weir height; otherwise, vapor will flow up the downcomer rather than through the tray deck above.
Downcomer sealing: 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.
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
Pressure Drop
For trays to function reasonably close to their best efficiency point, the dry-tray pressure drop must be roughly equal to the hydraulic-tray pressure drop.
Seal Pans
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.
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.
Nozzle orientation
For nozzles that feed liquid into the top tray, the nozzle should be perpendicular to the downcomer of the top tray.
Nozzles feeding 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 for the bottom feed and reboiler return lines should be at least 300 mm above the high liquid level.
The bottom feed and reboiler return should not impinge on the bottom seal pan, seal pan overflow, or the bottom downcomer.
The tops of both pipes should be at least 400–450 mm below the bottom tray.
References
1. Mukherjee, S., Tray Column Design: Keep Control of the Details, Chem. Eng., September 2005, pp. 52–58.
The vertical distance between adjacent trays varies from 450 to 900 mm in the columns generally employed in the chemical process industries (CPI), as seen in Table 1.
Spacing involves a tradeoff 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 of passes such that liquid loads do not exceed 70 m3/h per meter weir length.
After the number of passes has been selected, if necessary, adjust the column diameter in order to arrive at a minimum path length of 400 mm.
- For a column diameter of 1,200 to 2,100 mm: use two passes at maximum
- For a diameter of 2,100 to 3,000 mm: use three passes at maximum
- For a diameter above 3,000 mm: use four passes at maximum
Downcomers are conduits having circular, segmental or rectangular cross sections that convey liquid from a tray to the one immediately below.
Liquid velocities in downcomers:
- Minimally foaming liquids: 0.12–0.21 m/s
- Medium foaming liquids: 0.09–0.18 m/s
- Highly foaming liquids: 0.06–0.09 m/s
Also, the downcomer width should not be less than 10% of the column diameter.
Specify the downcomer clearance to be less than the outlet weir height; otherwise, vapor will flow up the downcomer rather than through the tray deck above.
Downcomer sealing: 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.
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
- Weir heights in the froth regime are restricted to 50–80 mm
- Weir heights for columns operating in the spray regime should be 20–25 mm
Pressure Drop
For trays to function reasonably close to their best efficiency point, the dry-tray pressure drop must be roughly equal to the hydraulic-tray pressure drop.
Seal Pans
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.
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.
Nozzle orientation
For nozzles that feed liquid into the top tray, the nozzle should be perpendicular to the downcomer of the top tray.
Nozzles feeding 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 for the bottom feed and reboiler return lines should be at least 300 mm above the high liquid level.
The bottom feed and reboiler return should not impinge on the bottom seal pan, seal pan overflow, or the bottom downcomer.
The tops of both pipes should be at least 400–450 mm below the bottom tray.
References
1. Mukherjee, S., Tray Column Design: Keep Control of the Details, Chem. Eng., September 2005, pp. 52–58.
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