The design of 4-pass trays can be complex. It not only requires a close evaluation of the mechanical design but also the process response to that design at various flow rates. The balancing of the fluid flows across the tray can have a substantial effect on the tray performance, namely efficiency.

Many existing columns are operating with older 4-pass designs. While some of these are well designed, others could use some improvement. Since 4-pass trays always involve a fairly large column diameter, it can pay substantially to recheck your 4-pass tray designs to make sure that they are properly balanced and giving optimum performance. In some cases, a quick win can be gained simply by rebalancing the vapor-liquid ratio with picket fence weirs to improve efficiency.

4-Pass Trays: Vapor to Liquid Ratio is the Key
The pressure drop across 4-pass trays cannot be determined by simply looking at individual panels alone. Pressure drop is determined by the summation of the pressure drop across two successive trays. By definition, the pressure drop across Panel A plus Panel C must be equal to the pressure drop across Panel B plus Panel D.

There are two recognized standards for designing the complex 4-pass tray geometry: “Equal Bubbling Area” or “Equal Flow Path Length”.

In an Equal Bubbling Area design, the active area of the tray deck is the same for Panels A, B, C, and D. This design method assumes that the vapor will be split into quarters and flow equally to each of the panels. To maintain a constant liquid loading on the panels, the center downcomer weir length must be the same as the length of the side downcomer weir. The V/L ratio on Panels A and B must be near unity at all loading conditions with picketing.

In an Equal Flow Path Length design, the downcomers are arranged such that an equal flow path length is created for all the panels. By definition then, the side panels A&C will have less bubbling area than the central panels B&D. Liquid flow is fed to each panel proportional to its area. The smaller flow to the side downcomer makes the equal flow path length design have a lower weir loading, thereby providing some capacity advantage. Proper management of the V/L ratio for these trays requires some picketing, but typically not as much as for that of an equal bubbling area design.

Regardless of which style design is used, the ratio of the vapor flow rate divided by the liquid flow rate on any tray panel should be the same as the other tray panels. By ensuring that the V/L ratio is the same, the tray efficiency on each tray panel is maximized. Equal Bubbling Area and Equal Flow Path Length designs can both provide similar good efficiency if properly configured.

An Easy Fix to Rebalance Existing 4-Pass Trays
Installing picket fence weirs can shift the liquid flow across the passes into balance. This can be accomplished with a simple bolt-in installation during an outage. While the new picketed weirs design may not quite meet the definition of either Equal Flow Path Length or Equal Bubbling Area, it will represent a significant improvement in V/L balance and improve tray efficiency. In severely unbalanced trays, where V/L may vary from 0.8 to 1.2 across the panels, picketing can improve efficiency by 5-10%.