Increase capacity in your column with a tray design revamp

When trying to increase column capacity beyond design rates, an operator may notice loss in separation efficiency or high pressure drop, indicative of either vapor phase or liquid phase hydraulic limitations. An optimized tray design can eliminate hydraulic constraints to provide better column capacity and performance.

Common Vapor Constraints to Raising Throughput
In systems limited by higher vapor rates (typically lower pressure systems), jet flooding may constrain the column. Jet flooding, also called entrainment flooding, is caused by massive entrainment of liquid to the tray above. As the tray loading increases, a two-phase mixture of spray or froth begins to occupy more and more of the clear vapor space between the trays. At very high vapor rates, the mixture fills up the entire tray spacing, causing entrainment of significant amounts of liquid to the tray above. Typically, 10% entrainment can be tolerated with only a small influence on the column operation, but anything substantially over 10% will create noticeable problems.

The recirculation of entrained liquid decreases efficiency by moving heavier components up the column, contaminating lighter products. Entrainment also recycles liquid upward, increasing tray weir loads, froth height, and downcomer loads. All these factors decrease the trays hydraulic capacity, eventually leading to column flooding.

Jet flood can be alleviated through a tray revamp by increasing active area, increasing tray spacing, or switching to a high capacity deck design. An increase in active area serves to lower the overall vapor velocity upwards through the column. In simplest terms, a 10% increase in active area should lead approximately to a 10% increase in capacity. An increase in tray spacing just increases the distance that the vapor has to travel before it entrains liquid. This technique does not increase capacity in a linear fashion. Generally speaking, capacity increases with the square root of the tray spacing.

High capacity deck designs usually use smaller orifices. Smaller sieve holes or valves tend to reduce localized momentum of the vapor flowing through the orifices. Essentially, smaller streams have a more difficult time penetrating the froth on the tray deck and subsequently do not carry as much entrainment upward. Somewhat counter-intuitively, small orifices also have a lower overall pressure drop which helps to reduce downcomer backup.

Common Liquid Constraints to Raising Throughput
In systems limited by higher liquid rates (typically higher pressure systems well above atmospheric pressure), downcomer flooding will likely limit column capacity. With conventional cross-flow trays, aerated froth enters the downcomer from the tray above. As the froth flows into the downcomer, much of the vapor disengages from the liquid and returns to the tray space above, leaving a level of clarified liquid in the downcomer that flows to the tray below. High liquid loads can create either downcomer backup flooding or downcomer choking.

Backup flooding occurs when the froth height in the downcomer backs up over the weir of the tray above. It commonly results from a hydraulic restriction at the downcomer exit along with high overall tray pressure drop. Downcomer choking, in contrast, is a function of the downcomer entrance conditions. When the velocity of the froth entering the downcomer is too high, the vapor is not able to properly disengage from the liquid. This creates a low density, high volume froth that simply cannot flow through the downcomer without backing up onto the tray above.

Downcomer floods can be alleviated with a tray revamp that incorporates modifications of the downcomer geometry or type. Increasing downcomer top width is a standard method to handle choking problems. It lowers the top froth velocity, allowing improved vapor disengagement. Downcomer backup can be addressed by increasing tray spacing, decreasing the bottom downcomer frictional resistance, and/or lowering the overall tray pressure drop.

Sulzer’s High Capacity Tray Technology
VGPlusTM designs typically use a combination of high capacity decks and high performance downcomers to increase capacity from 10-30% over previous designs. The key is to use a balanced design to handle both the liquid and vapor loads within the column.


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