Characteristics of Tray Operation

Typical tray layout is shown in Figure 2, and tray operation is shown in Figure 3. High speed photography of a large operating tray indicates that the vapour erupts through the liquid sporadically. The holes that are not erupting do not weep appreciably at a vapour rate above the weep point, although the supporting of the liquid by the vapour is not absolutely complete. The interaction of vapour and liquid on a properly designed tray results in a highly turbulent two-phase mixture of a high specific interfacial area with net

Two-pass tray One-pass tray

Two-pass tray One-pass tray

Figure 2 Tray layouts.

liquid movement in a crossflow direction to the rising vapour stream. The aerated liquid may be either liquid-continuous (froth) at relatively low vapour velocities or vapour-continuous (spray) at high vapour velocities.

The maximum capacity of a sieve tray is reached when the tray is flooded. This may be due to excessive spraying (entrainment) taking place in the intertray space or the froth in the downcomer backing-up to reach the top of the outlet weir. The onset of flooding is accompanied by a sharp increase in tray pressure and a sharp decrease in tray efficiency.

As vapour rates decrease to the point that the vapour flow cannot totally support the liquid on the tray, some liquid will weep through the holes. If the weepage is so severe that no liquid flows over the outlet weir, the tray cannot operate stably under these dumping conditions. The minimum capacity of the tray is normally reached when moderate weepage is encountered. Ideally, a sieve tray should operate in the shaded area shown in Figure 4 to ensure proper operation.

Tray efficiency can be divided into two components:

1. point efficiency as determined by the vertical flow of vapour through the froth;

2. tray efficiency enhancement by the crossflow of liquid.

The physical properties of the vapour-liquid mixture determine the point efficiency, although froth height, which influences the gas residence time, also has a significant effect, especially for low efficiency systems. Liquid flow pathlength determines the liquid residence time and the extent of crossflow tray efficiency enhancement. Entrainment and weeping depress tray efficiency by disrupting the concentration profile in the column. The froth height and the liquid flow path are two parameters that are optimized to give maximum tray efficiency. Other geometric variables, such as open hole area, hole diameter and downcomer arrangement, also affect tray hydraulics and efficiency. The goal for a tray design is to reach maximum tray efficiency without compromising hydraulic stability.

The steps required for tray column design are shown in Figure 5; a detailed discussion of each step is given below.

Solar Panel Basics

Solar Panel Basics

Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.

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