Column Internals From China

Column Internals: A Technical Overview of Trays

Trays, or plates, are fundamental internal components used in vertical process columns for facilitating mass and heat transfer between liquid and vapor phases. They are employed in unit operations such as distillation, absorption, and stripping. 

Wangdu (Hebei) Chemical Engineering Co., LTD incorporates various tray designs into process columns, focusing on achieving specified separation efficiency, throughput, and operational stability for clients.

The primary function of a tray is to provide a stage for intimate contact between the ascending vapor and descending liquid. The most common designs include sieve trays, valve trays, and bubble-cap trays, each with distinct operational characteristics.

Sieve Trays consist of a plate with numerous perforations, typically 3 to 12 mm in diameter. Vapor passes upward through these holes, dispersing into the liquid flowing across the tray. Sieve trays are noted for their cost-effectiveness and lower pressure drop per tray. Their operational flexibility is somewhat less than other types, with a typical turndown ratio (the ratio of the maximum to the minimum operable vapor load) of around 2:1. They are susceptible to weeping at low vapor velocities.

Valve Trays are similar to sieve trays but feature movable valves that cover the perforations. These valves open and close in response to vapor flow. At low flow rates, valves partially close to reduce liquid weeping, and at high flow rates, they open fully. This design offers improved turndown ratios, often in the range of 4:1, providing greater operational flexibility compared to sieve trays. The pressure drop is generally slightly higher than that of a sieve tray but lower than a bubble-cap tray.

Bubble-Cap Trays, an older design, feature risers covered by caps. Vapor is forced to pass downward and then upward through slots in the cap, bubbling into the liquid. This design minimizes liquid leakage (weeping) at very low vapor rates, making it suitable for services with extremely wide flow variations or low operating pressures. However, bubble-cap trays have a higher pressure drop, greater cost, and are more complex to fabricate than sieve or valve trays.

Key performance parameters for trays include:

• Pressure Drop: Typically ranging from 50 to 150 Pa per tray for sieve and valve trays under normal load conditions. This is a critical factor in vacuum distillation where total column pressure drop must be minimized.

• Efficiency: Expressed as Murphree Vapor Efficiency, which can range from 50% to 90% depending on the physico-chemical properties of the system, tray geometry, and operating rates.

• Weeping and Dumping: Occurs when vapor velocity is insufficient to hold liquid on the tray.

• Flooding: The ultimate capacity limit of a tray, occurring when liquid cannot flow down the column or when excessive entrainment takes place. The F-factor (F_s = u_v √ρ_v, where u_v is vapor velocity based on the active bubbling area and ρ_v is vapor density) is a common parameter for predicting flood; a typical value for a tray spacing of 0.6 m might be 2.5 Pa^0.5.

Wangdu (Hebei) Chemical Engineering Co., LTD selects tray types based on a detailed analysis of process conditions, including flow rates, operating pressure, fouling potential, and required separation efficiency.

Reference

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

Lieberman, N. P., & Lieberman, E. T. (2008). A Working Guide to Process Equipment (4th ed.). McGraw-Hill.

Perry, R. H., & Green, D. W. (Eds.). (2007). Perry's Chemical Engineers' Handbook (8th ed.). McGraw-Hill.