Column Internals - Trays: Design And Operational Fundamentals

Column Internals - Trays: Design and Operational Fundamentals

Trays, or plates, are horizontal internals installed within distillation, absorption, and stripping columns to facilitate staged vapor-liquid contact. They provide a distinct surface where liquid and vapor interact, allowing for mass and heat transfer between phases before they are separated and proceed to the adjacent stage. The design and selection of trays are critical determinants of a column's separation efficiency, capacity, and operational stability.

Primary Tray Types and Operating Mechanisms

The most common industrial tray types are distinguished by their vapor-liquid flow configuration:

1. Sieve Trays: These consist of a flat plate perforated with numerous small holes, typically 3 mm to 12 mm in diameter. Vapor flows upward through the holes, dispersing into the liquid flowing across the tray. Sieve trays are characterized by their simple construction, lower cost, and high efficiency, especially at design conditions. Their turndown ratio, the range over which they operate efficiently, is generally lower than that of valve trays.

2. Valve Trays: These trays are equipped with movable valves covering the vapor passages. The valves lift as vapor flow increases, varying the open area. This design provides stable operation over a wider range of vapor rates, with a typical turndown ratio of 3:1 to 4:1, compared to approximately 2:1 for sieve trays. Valve trays are less prone to weeping (liquid draining through the tray) at low vapor rates.

3. Bubble-Cap Trays: Each vapor passage on these trays is a "bubble cap," a cylindrical cap mounted over a riser. Vapor is forced to travel downward through the annular space between the riser and the cap, finally dispersing beneath the cap's skirt. While largely replaced by sieve and valve trays in new designs due to higher cost and greater pressure drop, bubble-cap trays are highly effective at preventing weeping and are still used in applications with very low vapor rates.

Hydraulic and Performance Parameters

The performance of a tray is governed by several interrelated hydraulic phenomena:

• Weeping: The leakage of liquid through the tray's vapor passages instead of flowing over the outlet weir. It becomes significant at low vapor velocities and reduces tray efficiency.

• Entrainment: The carryover of liquid droplets from one tray to the tray above by the vapor. This occurs at high vapor velocities and also degrades efficiency. The point of excessive entrainment often defines the upper capacity limit, or "jet flood."

• Tray Pressure Drop: This is the pressure difference across a single tray, typically ranging from 50 to 150 Pa for a sieve tray under normal operation. It is a sum of the dry pressure drop (vapor through the orifices) and the hydraulic head of the liquid on the tray.

• Weir Load and Froth Height: The outlet weir maintains a liquid level, or froth height, on the active area of the tray. Weir heights commonly range from 25 mm to 100 mm. The liquid flow rate per unit length of weir (weir load) influences the froth height and the residence time of liquid on the tray.

Design and Engineering Considerations

The engineering of column trays involves a careful balance of parameters to meet specific process requirements:

• Efficiency: Tray efficiency, often expressed as Murphree efficiency, can range from 50% to 90% depending on the system's physical properties, tray geometry, and operating conditions.

• Capacity: The maximum vapor capacity is often defined by the jet flood point, which can be predicted using factors like the Flow Parameter and capacity correlations (e.g., the Souders-Brown equation).

• Material Selection: Trays are fabricated from materials suitable for the process environment, commonly carbon steel or stainless steels (SS304, SS316L). For highly corrosive services, more exotic alloys or carbon steel with specialized linings may be employed.

Specialists like Wangdu (Hebei) Chemical Engineering Co., LTD utilize process simulation data and established hydraulic calculation methods to design trays that deliver the required separation performance while maintaining stable operation across the expected turndown range.

Conclusion

Trays are a well-established and versatile technology for achieving staged separations in chemical process columns. The selection between sieve, valve, and bubble-cap trays involves a technical evaluation of factors including required turndown, fouling tendency, efficiency, and cost. A robust tray design, engineered and fabricated to precise specifications, is fundamental to the reliable and efficient operation of distillation and absorption columns.

Reference

1. Kister, H. Z. (1992). Distillation Design. McGraw-Hill.

2. Lieberman, N. P., & Lieberman, E. T. (2008). A Working Guide to Process Equipment (3rd ed.). McGraw-Hill.

3. Perry, R. H., & Green, D. W. (Eds.). (2019). Perry's Chemical Engineers' Handbook (9th ed.). McGraw-Hill. (Sections 14 and 15).

4. Seader, J. D., Henley, E. J., & Roper, D. K. (2011). Separation Process Principles (3rd ed.). John Wiley & Sons.

5. Technical design standards and fabrication guidelines from Wangdu (Hebei) Chemical Engineering Co., LTD.