Gain 5-10% efficiency with this simple 4-pass tray revamp

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%.


View More

  • Safeguard packed column efficiency with proper liquid distribution

    Initial liquid distribution directly affects the efficiency of structured and random packing beds. As such, some form of flow test should be conducted to validate distributor performance prior to column startup. Steady state water flow testing is the most common method of testing distributors. It can ...

  • Separator technology reduces refinery operating costs and downtime

    A US refining company contacted us to discuss a separation problem in the feed to an alkylation process. Caustic entrainment from their selective hydrogenation unit was poisoning the hydrofluoric acid catalyst in the alkylation unit. The continuous phase feed to the alkylation unit consisted of ~180m3/hr ...

  • Tray designs for extreme fouling applications

    Today refiners experience a lot of problems with processing of opportunity or heavy crudes. Such crudes have very high sulfur content and require the addition of amine scavengers before desalting. These amines decompose in the heater and create ammonium chlorides in the presence of water in the top of ...

  • Anti-fouling trays maximize coker main fractionator profitability

    The Coker Main Fractionator is systematically subjected to harsh operating conditions that can lead to deteriorating efficiency and performance due to coking and fouling. Poor reliability results in loss of profitable coking margins for the refinery. The main fractionator vapor feed from the coke drum ...

  • Improve separation in your column by increasing the number of trays

    Refiners often face revamp challenges when trying to improve separation within an existing column. Improving diesel recovery from gas oil, splitting benzene precursors from naphtha reformer charge, or simply minimizing product overlaps after capacity creep can all be difficult when limited by a fi xed ...

  • Gain 5-10% efficiency with this simple 4-pass tray revamp

    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 ...

  • Proper design of mass transfer internals in the FCC flue gas scrubber can help reduce PM emissions

    The EPA’s New Source Performance Standards (40 C.F.R. §60.100-1-0, subpart Ja) regulates refinery particulate emissions, including the discharge of catalyst fines from the FCCU flue gas scrubber stack. Because refiners have traditionally correlated particulate matter (PM) emissions with FCCU ...

  • Improve fouling resistance in your wastewater benzene stripper

    Benzene stripper columns, built so that refineries can meet the National Emissions Standards for Hazardous Air Pollutants (NESHAP), operate with several unique conditions – low vapor rates, high liquid loads, and a high tendency toward fouling make designing well-balanced, effective internals difficult. The ...

  • Important tray design features that improve column operating reliability

    How often does Maintenance personnel open a column during a shutdown and find tray panels fallen without any obvious damage? How about tray valves stuck in the bottoms pump suction? While the initial reaction may be to blame faulty installation where the hardware was not properly tightened, the answer ...

  • Green design practices: focus on efficiency

    Green design is more than recycling scrap materials or calculating carbon footprints. It minimizes negative environmental impact through skillful design and operating practices to produce efficient, better-functioning processes. Because green practices reduce resource requirements, in many cases, they ...

  • Responsive image Digital Refinery
  • Responsive image ITW technologies online cleaning
  • Responsive image Level and density in resid hydrocracking processes
  • Responsive image Monthly technical bulletin
  • Responsive image OpX® Digital
  • Responsive image PROVALVE® Fixed Valve
  • Responsive image Process catalysts
  • Responsive image Tri block, double block and bleed valve from ZWICK
  • Responsive image Shut-off valves
  • Responsive image Trap more foulants with ActiPhase®