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Picking the right workhorse

Published by , Senior Editor
Hydrocarbon Engineering,

In pressure distillation towers, the distillation tray is the workhorse of the mass transfer industry. Particularly for light hydrocarbon fractionation trains in olefins plants and NGL fractionating plants, the distillation tray is the device of choice to reliably and economically produce high purity products.

The available tray technologies fall into three major categories:

  • Crossflow trays.
  • Counter-flow trays.
  • Cocurrent-flow or ultra-high capacity trays.

Crossflow trays

The first category, crossflow trays, are the most widely applied tray type. These include perforated or sieve trays, bubble-cap trays and floating- and fixed-valve trays. These devices demonstrate a good balance between investment cost, capacity and efficiency, and are typically used for newly constructed process plants, particularly in pressure distillation of light hydrocarbons.

Later in the life of process units, there is often a desire for an incremental capacity increase. If it is possible to debottleneck the fractionating trains by only revising the column internals, the economic payback for such an incremental capacity increase is often very attractive. Random packing was, for a time, employed to increase the capacity of tray towers, but this is an expensive and time-consuming revamp strategy. Since the late 1980s, the mass transfer equipment companies have developed an array of improved higher capacity trays. Some of these are enhanced crossflow trays that maintain the crossflow configuration of standard trays, but use various enhancements to improve both the tray capacity and efficiency. Special downcomer shapes are employed to enlarge the bubbling area of the tray. Improved valve shapes are also employed to reduce entrainment and further increase capacity. Longer flow paths and/or more uniform residence time results in enhanced separation efficiency.

Counter-flow trays

The other main category of higher capacity trays are counter-flow trays which use a multitude of downcomers to reduce the liquid froth heights on the tray deck. With counter-flow trays, the liquid flow path across the vapour/liquid contacting zone of the tray is relatively short, and less uniform than that which occurs on crossflow trays. This results in operation close to the point efficiency with little-to-no crossflow enhancement, so that this category of trays often exhibits reduced tray efficiencies compared to standard or enhanced crossflow trays…

Written by Alessandro Ferrari, Izak Niewoudt, and Neil Sandford, Koch-Glitsch.

This article was originally published in the April 2020 issue of Hydrocarbon Engineering. To read the full article, and other great technical articles in this issue, view the full issue here. You can also register to receive a free regular copy of the magazine here.

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