(78e) Practical Tray Capacity: A Shortcut Analysis

A conventional tray attains maximum capacity by balancing the downcomer volume for degassing[1] versus the volume above the tray for de-entrainment[2]. This balance shifts with a complex interplay of tray spacing, tray type, special tray features, relative and absolute liquid and vapor loads, and physical properties. Detailed design methods and extensive experience are needed to balance these loads.

Nevertheless, need for shortcut methods remains. Shortcut methods can be used for rough sizing with limited information. They are also useful for less experienced engineers as a reasonableness check for existing equipment and new designs. Additionally, shortcut tray evaluation methods can be used as part of quick conceptual studies to rapidly evaluate process changes.

The first soundly based shortcut tray hydraulic method was developed by Souders and Brown in 1934 [1]. Latter efforts included work by Fair [2], Smith et. al. [3], Andersen and Phillips [4], Kister and Doig [5]and many others.

Recent work needed included an evaluation of many towers in a major facility. The question that needed answering is what would be the maximum expected shell capacity for a conventional tray on existing spacings. A shortcut method would save considerable time compared to evaluating 152 tower sections by rigorous methods.

However, it quickly became apparent that the shortcut methods gave widely different answers for the same systems. This paper looks at a variety of shortcut methods and compares them to the ‘optimized’ conventional designs for the towers.

[1] M. Souders and G. G. Brown, “Design of Fractionating Columns, Entrainment, and Capacity,” Ind Eng Chem, vol. 38, no. 1, pp. 98–103, Jan. 1934.

[2] J. R. Fair, “How to Predict Sieve Tray Entrainment and Flooding,” PetChem Eng, vol. 33, no. 10, pp. 45–52, Sep. 1961.

[3] R. B. Smith, T. Dresser, and S. Ohlswager, “Tower Capacity Rating Ignores Trays,” Hyd Eng and Pet Ref, vol. 42, pp. 183–184, May 1963.

[4] A. E. Andersen and E. M. Phillips, “Now Figure Tray Flooding Closer,” Hyd Proc Petr Ref, vol. 43, no. 8, pp. 159–166, Aug. 1964.

[5] H. Z. Kister and I. D. Doig, “Entrainment flooding prediction,” Hyd Proc, vol. 56, no. 9, pp. 149–151, Sep. 1977.

[1] Removing vapor from the descending liquid.

[2] Removing liquid from the ascending vapor.