Revamping distillation columns
Published by Callum O'Reilly,
Senior Editor
Hydrocarbon Engineering,
Although distillation is energy intensive, it is still the most widely used separation method, and will remain so for many years. To reduce our carbon footprint, it is incumbent upon all engineers involved in the refining and chemical process industries to optimise the design of such columns. The performance of any distillation column can be improved with respect to capacity, separation efficiency, pressure drop, energy efficiency, fouling tendency, and mechanical integrity.
Ultimately the key consideration is cost vs benefits of the improvement.
The mass transfer equipment market divides into three segments approximately equally: new columns, revamps, and maintenance.
Revamping an existing column offers opportunities to improve the performance with respect to all of the aforementioned aspects.
Process engineers have a choice of using trays or packing (random or structured) within a distillation column. The appropriate mass transfer device to use will depend on the service and is a compromise between:
- Capacity (through-put).
- Efficiency (separating power).
- Pressure drop.
- Fouling resistance.
The key difference between trays and packing is pressure drop (ΔP). The maximum recommended ΔP for a trayed column is about 20% of the tray spacing. For a 24 in. tray spacing, this is 4.8 in. of hot liquid and for an air-water system 4.8 in WG (12.0 mbar) per tray, i.e. 2.4 in. WG per ft (6.0 mbar/m) of column. Random packing (for non-foaming systems) is usually designed for a maximum ΔP of 0.5 in. WG per ft (4.0 mbar/m) whereas structured packing maximum ΔP is usually 0.25 in. WG per ft (2.0 mbar/m). Thus random packing and structured packing ΔP is approximately 20% and 10% respectively of a trayed column. Hence, when minimising ΔP is significant, packing is the preferred choice of mass transfer device.
Fouling is a perennial problem in the refining and chemical process industries. In the Kister1 2003 survey it was the top cause of column malfunctions. Packing is rarely a good choice in a fouling service except in some rare cases where ΔP is very critical (e.g. the wash zone bed of a crude vacuum tower). Here, packing is used even with the expectation that the bed will foul up. A well designed wash zone bed may achieve a run length of approximately 5 years before replacement is required, whilst a poorly designed arrangement may fail after just a couple of months.....
Written by Aadam F. Aryan, Distillation Equipment Company Ltd.
This article was originally published in the December 2025 issue of Hydrocarbon Engineering magazine. To read the full article, click here.
Reference
- KISTER, H. Z., ‘What Caused Tower Malfunctions in the Last 50 Years?’, Trans IChemE, Vol. 81, (January 2003).
Read the article online at: https://www.hydrocarbonengineering.com/special-reports/18122025/revamping-distillation-columns/
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