Tray Columns Design

K. T. Chuang and K. Nandakumar,

University of Alberta, Edmonton, Alberta, Canada

Copyright © 2000 Academic Press

Distillation has remained an important separation technology for the chemical process industries. In 1997 it was reported in the journal Chemical Engineering that about 95% of all worldwide separation processes use this technology. In the USA alone, some 40 000 distillation columns represent a capital investment of about US $8 billion. They consume the energy equivalent of approximately 1 billion barrels of crude oil per day. Such columns are used in refineries, petrochemical plants, gas processing plants and organic chemical plants to purify natural gas, improve gasoline, produce petrochemicals and organic products, recover pollulant species, etc.

Distillation can be carried out in a tray or a packed column. The major considerations involved in the choice of the column type are operating pressure and design reliability. As pressure increases, tray coulmns become more efficient for mass transfer and can often tolerate the pressure drop across the trays. The design procedure for the large diameter tray column is also more reliable than that for the packed column. Thus, trays are usually selected for large pressurized column applications.

Distillation trays can be classified as:

1. cross-flow trays with downcomers (see Figure 1A);

2. countercurrent trays without downcomers (also known as dual-flow trays) (see Figure 1B).

The-dual flow tray allows the gas and liquid to pass through the same tray openings. This results in a limited operating range because the dispersion height is very sensitive to the gas/liquid flow rates. In general, dual-flow trays are employed only in cases where high capacity or high resistance to fouling are required. However, because of its narrow operating range, the market share is small and such trays will not be discussed further.

The cross-flow tray utilizes a weir on the down-comer to control the spray height on the tray, and thus provides a stable gas-liquid dispersion over a wide range of gas/liquid flows. A tray is the combination of a tray deck, where froth is generated to provide vapour-liquid contact, and a downcomer, where the vapour-liquid mixture is separated. The bulk of the vapour rises from the aerated liquid through the vapour disengagement space to the tray above. However, the passage of the liquid from the top to the bottom of the column occurs mainly via downcomers.

There are three types of cross-flow trays: (1) sieve, (2) valve and (3) bubble cap. Among them, sieve trays offer high capacity and efficiency, low pressure drop, ease of cleaning, and low capital cost, but smaller turndown ratio. Although the design procedure is similar for all three types of trays, only sieve tray performance data are readily available in the public domain. The valve and bubble cap designs are often protected by patents, and thus the performance data are supplied by the vendors. This article describes the procedure for designing an optimum sieve tray. A similar procedure can be applied in principle to the valve and bubble cap trays, provided critical performance data are available.

The cost of a tray column is determined by two factors:

1. column diameter, which determines the throughput;

Figure 1 (A) Sieve ray with downcomer in a 30 cm diameter column. (B) Dual-flow tray in a 30 cm diameter column.

2. column height, which delivers the number of equilibrium stages required for the separation.

The minimum cost is generally achieved when the column volume is minimized. The final selection of the tray design is based on the combined cost of the column shell, internals and installation.

It should be noted that the fraction of the cross-sectional area available for vapour-liquid disengagement decreases when the downcomer area is increased. Thus, optimum design of the tray involves a balance between the tray area and the downcomer area (i.e. the capacity for the tray deck should match the capacity of the downcomer). The correlations for sizing trays are implicit in column diameter, tray spacing and tray geometry, thus requiring trial-and-error calculations to arrive at the final selection.

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