Disc-and-Donut Tray Design for Severe Fouling and Solids-Containing Service

A disc-and-donut tray is selected when keeping a tower open is more important than achieving the efficiency of a conventional crossflow tray. Instead of using a full tray deck with many small vapor openings and downcomers, the tower contains alternating circular discs and annular “donut” baffles. Liquid cascades from one element to the next, while ascending vapor passes through the changing central and peripheral flow windows.

The geometry is simple, but successful design still requires control of open area, liquid trajectory, vapor velocity, spacing, support strength and deposit removal. Calling the device “non-plugging” without examining these details can produce erosion, entrainment or solids accumulation in new locations.

How the Alternating Geometry Works

A disc is a circular plate installed near the center of the tower, leaving an annular opening between the plate edge and the shell. The next element below is a donut-shaped annular plate attached near the shell, leaving a central opening.

Liquid falling from a disc passes through the outer annulus and lands on the donut below. It then travels inward and falls through the donut’s central opening onto the next disc. This alternating pattern creates a repeated outward-and-inward liquid path.

Vapor flows upward through the same large openings in the opposite overall direction. Contact occurs mainly where vapor crosses the falling liquid curtains, droplets and splashing liquid on the baffle surfaces. There is no conventional outlet downcomer maintaining a uniform froth depth across a full active deck.

Why the Design Resists Fouling

Conventional sieve and valve trays contain numerous relatively small openings, narrow clearances and areas where liquid velocity may be low. Sticky solids, coke, salts or polymers can restrict these passages.

A disc-and-donut arrangement replaces those features with large central and annular openings. It has no movable valves and normally avoids narrow downcomer entrances. Solids have a clearer route downward with the liquid, and the exposed baffle surfaces are easier to wash than the underside of a densely perforated tray.

This does not mean deposits cannot form. Material can still settle on horizontal surfaces, collect near supports or bake onto hot metal. The advantage is that moderate accumulation is less likely to block the tower’s principal vapor path.

Open Area Must Be Evaluated at Both Elements

The disc and donut do not automatically provide equal vapor-flow areas. The annular opening around a disc and the central opening inside a donut must each be calculated at actual tower dimensions.

If the two areas differ greatly, the smaller opening becomes the repeating hydraulic restriction. Vapor velocity rises there, increasing pressure drop and the potential for liquid entrainment. The alternating restriction can also produce different spray patterns at successive levels.

Shell diameter, shell ovality and installation clearance affect the real annular area around a disc. Similarly, support members crossing a donut’s central opening reduce its net area. Hydraulic sizing must use the unobstructed free area after deducting beams, clips and other hardware.

Liquid Trajectory Determines Contact and Wall Wetting

The liquid must reach the next baffle rather than fall directly through several levels. Disc diameter, donut opening diameter and vertical spacing control this trajectory.

A disc that is too small may send most liquid through the surrounding annulus without spreading it across the donut below. A disc that is too large restricts vapor flow near the shell and may create a strong upward jet that throws liquid onto the wall.

The donut must receive the outer liquid curtain and redirect it toward the center. Its surface slope, edge detail and wetting behavior influence whether liquid forms a distributed curtain or several concentrated streams.

Poorly distributed feed entering the top of the section will not necessarily correct itself immediately. The first disc or donut should be positioned and sized to intercept the incoming liquid pattern.

Pressure Drop and Efficiency Are Deliberate Tradeoffs

Disc-and-donut trays generally provide less controlled vapor-liquid contacting than a properly loaded valve or sieve tray. They should not be selected merely because they appear mechanically simple.

Their value is greatest in washing, quenching, heat-transfer or rough fractionation zones where run length and solids passage dominate. If tight product purity requires a high number of theoretical stages, the lower efficiency per installed level may demand more height than the tower has available.

Pressure drop is often moderate because the principal openings are large, but it can increase sharply if the openings are undersized or liquid curtains become excessively dense. A design should evaluate the expected vapor velocity through both the central and annular windows at every operating case.

Solids Behavior Must Be Defined

The term “solids-containing service” covers very different materials. Hard mineral particles may erode landing zones and baffle edges. Sticky polymer may adhere to the plate surface. Coke fines may settle behind structural members, while crystalline salts may grow where local evaporation occurs.

For erosive service, replaceable wear plates or increased thickness may be needed where liquid repeatedly strikes the baffle. For sticky service, sloped surfaces and smooth transitions can reduce deposit retention. Cleaning nozzles may be useful, but their spray must reach both the top and underside of the baffles.

The bottom of the section must also provide a route for removed solids. An open tray section is of limited value if the particles later collect in a restricted sump outlet.

Mechanical Design Is Not Automatically Simple

Each disc is supported near its center or by beams spanning the tower. Each donut transfers load toward the shell or a support ring. These two support arrangements produce different stress and vibration behavior.

The design must consider dead weight, liquid accumulation, deposit load, maintenance personnel, pressure-surging loads and thermal expansion. Slender supports located in high-velocity vapor openings can vibrate. Wide unsupported plates can deflect and change the designed clearance.

Installation tolerances are also important. An off-center disc creates a narrow annulus on one side and a wide opening on the other. The resulting vapor imbalance can push liquid toward one wall and accelerate local fouling.

When to Select—and When Not to Select—It

Disc-and-donut trays are strong candidates when the service has severe fouling, large solids, heavy wash liquid or a need for simple cleanable internals. They can be useful in refinery wash zones, slurry strippers and other dirty services where conventional tray openings would lose capacity during the run.

They are a poor default choice when high separation efficiency, precise stage performance or very wide turndown is the governing objective. The selection should compare expected run length, pressure drop, required contacting duty, available tower height and cleaning strategy.