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Oct-2001

High capacity distillation revamps

Understanding how high-capacity trays work assists in the selection and design process and can also help in the avoidance of failures

Daryl Hanson and Edward Hartman
Process Consulting Services
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Article Summary
In the past 10 years the mass transfer industry has developed a number of tray designs to increase column capacity by 10–30 per cent over well-designed conventional trays. While some of these trays, such as UOP’s and Shell’s have been in service for more than 20 years, several have only been commercialised in the past five. High capacity tray designs employ several different mechanical features to increase capacity.

Maximising tray hydraulic capacity always reduces tray efficiency and/or operating flexibility. Balancing capacity, efficiency, and operating flexibility is the challenge facing the engineer performing a revamp. Some designs have inherently more efficiency due to the longer distance the liquid travels across the tray. Others have extremely high capacity, lower efficiency and little turndown. Which high capacity tray is needed depends on the specific process objectives.

High capacity trays increase column vapour and liquid capacity by increasing active area, increasing downcomer capacity, decreasing weir loading and reducing hydraulic gradient.

Trays flood when the liquid and/or the vapour rate exceed the capacity of the particular design. In some cases an optimised conventional tray will debottleneck the column. In those instances where it is necessary to use high capacity trays, one or all of the four principles to increase vapour and liquid handling may be needed. Often, tray capacity, column efficiency, and operating flexibility will need to be balanced to meet revamp objectives. Two case studies, described later, highlight this balance.

Tray capacity
Trays flood because the active area and/or the downcomers are not capable of handling the operating vapour and liquid rates. Figure 1 shows a conventional two-pass tray where liquid flows down the column through downcomers, while vapour flows up through the active area. Incipient flooding begins when liquid is entrained with the rising vapour and reaches the tray above. If the downcomers have the capacity to handle both the entrained liquid plus the normal liquid flow, then fractionation will suffer but the column will be operable.

Once the downcomer capacity is exceeded, the tray vapour space begins to fill with liquid. At this point the tray will hydraulically flood, common symptoms being periodic loss of level below the flooded trays followed by “dumping” of the accumulated liquid causing high levels. When trays flood the column pressure drop increases.

Trays fractionate by mixing the liquid and vapour phases together on and above the active area. The mixed phases separate by Stoke’s Law with the liquid settling onto the tray deck or into the top of the downcomer, while liquid-free vapour rises up the column through the next tray.

Jet flooding occurs when the localised velocity through the tray deck is high enough to entrain liquid to the tray above (Figure 2). Jet flooding is common in low and moderate pressure services such as atmospheric crude, FCC main fractionator, or naphtha splitters. These low to moderate pressure systems have high vapour rates and relatively low liquid rates in the fractionating sections.

Tray downcomers must have the capacity to handle the liquid flowing from one tray to the next without backing liquid onto the tray deck. A downcomer can flood by two distinct mechanisms. Downcomer choke flood occurs when the rate of liquid entering the top of the downcomer is too high to allow entrained vapour to disengage.

The downcomer chokes with liquid and vapour, and when it does the liquid level on the tray deck builds up and the column floods. On a simple basis, downcomer backup flood occurs when the liquid and froth level in the downcomer exceeds the tray spacing plus the weir height. Liquid fills the normal vapour space above the tray deck, causing flooding.

Downcomer backup is impacted on by several tray design parameters with the pressure loss components shown below:
Dry tray pressure drop – vapour induced loss
Weir height – set by tray designer
Weir crest (height of liquid flowing over the weir) – gpm/inch of weir
Liquid gradient
Pressure loss under the downcomer or through dynamic seal – velocity
Downcomer backup determines the maximum and minimum operable liquid rates through a high capacity tray, assuming choke is not occurring. There must be enough liquid height in the downcomer to overcome the pressure drop of the various components.

Total liquid height in a downcomer results from the dry tray pressure-drop (vapour induced pressure drop through the sieve holes or valves), weir height, height of liquid flowing over the weir (gpm/inch of weir), liquid gradient and pressure loss caused by liquid flowing under the downcomer or through the dynamic seal.

Liquid level on the tray deck is not uniform and this reduces tray capacity. The liquid gradient is the driving force needed to push the liquid across the tray from inlet to outlet. This gradient causes non-uniform vapour flow rate through the active area with the variability depending on the magnitude of the gradient. Liquid gradient causes high vapour velocity near the outlet weirs, which results in localised flooding of the tray active area. High capacity tray revamps need to balance hydraulic capacity, turndown, and column efficiency to meet fractionation objectives. Each factor impacting on tray capacity should be reviewed independently. In practice, the designer concurrently evaluates all the factors involved, otherwise an unwanted limit can be created.
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