Tray Design and Operation

Once the process design is completed, the equipment design begins. This phase of the design translates the process requirements (i.e., the vapor and liquid loads in each section of the column) into actual hardware.

The hardware design proceeds in two phases: primary (basic) and secondary (detailed layout). The primary phase sets column diameter, type of tray, and split of tray area into bubbling and downcomer areas. This phase also provides a preliminary (and usually close) estimate of tray spacing, number of passes, and other features of tray and down-comer layout such as weir height, fractional hole area, hole diameter, and clearance under the downcomer. These estimates are later firmed up in the secondary phase.

Functionally, the primary phase sets the major equipment requirements, while the secondary phase engineers the finer details. The primary phase has a major impact on column costs, but a relatively small influence on achieving trouble-free operation. These roles are reversed in the secondary phase: it has a relatively small impact on column costs, but a major impact on achieving trouble-free operation.

This chapter deals with the primary phase of hardware design. The secondary phase is outside the scope of this book, and occupies several chapters of a companion book (1).* This chapter describes the physical processes that constrain equipment design, including flooding, entrapment, weeping, pressure drop, clear liquid height, and flow regimes. This chapter then describes how knowledge of these physical processes is harnessed to set hardware design.

The discussions in this chapter emphasize sieve and valve trays, as these trays are most frequently encountered in industrial practice. Several of the considerations also apply to other tray types (e.g., bubble-cap trays). Considerations unique to bubble-cap trays were excluded from this chapter. The infrequent application of this type of tray in modern distillation practice argues against a detailed discus

*References for Chap. 6 appear at the end of Chap. 7.

sion here. A large amount of information on bubble-cap trays is documented in several texts (2—5).

6.1 The Common Tray Types

6.1.1 Description of the common tray types

The bubble-cap tray was the workhorse of distillation before the 1960s. It was superseded by the sieve and valve trays. Presently, bubble-cap trays are specified only for special applications, while sieve and valve trays are the most popular types.

The bubble-cap tray (Fig. 6.1a) is a flat perforated plate with risers (chimneylike pipes) around the holes, and caps in the form of inverted cups over the risers. Several cap designs are shown in Fig. 6.16. The caps are usually (but not always) equipped with slots or holes through which the vapor comes out (Fig. 6.2a). Liquid and froth are trapped on the tray to a depth at least equal to the weir height or riser height, giving the bubble-cap tray a unique ability to operate at low vapor and liquid rates.

The sieve tray (Fig. 6.1c) is a flat perforated plate. Vapor issues from the holes to give a multiorifice effect (Fig. 6.26). The vapor velocity keeps the liquid from flowing down through the holes (weeping). At low velocities, liquid weeps through the holes, bypassing some of the tray and reducing efficiency, giving sieve trays relatively poor turndown. Sieve trays are simple and easy to fabricate, and are therefore relatively inexpensive,

A dual-flow tray (Fig. 6. lei) is a sieve tray with no downcomers. This tray operates with liquid continuously weeping through the holes, hence its low efficiency. Tray froth height diminishes rapidly when vapor velocity is reduced, causing further efficiency deterioration upon turndown. Turndown of a dual-flow tray is even poorer than that of a sieve tray with downcomers. Large-diameter (>8 ft) dual-flow trays are known to sometimes experience instability. Insight into the nature of this instability can be inferred from Yanagi's thorough description of dual flow hydraulics (6). Dual flow trays are prone to channeling, and are therefore sensitive to out of levelness and to liquid distribution.

Due to the absence of downcomers, dual-flow trays give more tray area, and therefore have a greater capacity than any of the common tray types. This makes them an ideal revamp tool if some efficiency can be sacrificed. The absence of downcomers, and the larger open areas, renders dual-flow trays the most suitable to handle highly fouling services, slurries, and corrosive services. Dual-flow trays are also the least expensive to make, and easiest to install and maintain.

Bubble Cap

Figure 6.1 Common tray types, (a) Bubble-cap tray; (6) different types of bubble-cap designs [forts a and b courtesy of Glitsch, Inc.]

Figures 6.3 and 6.4 show different types of valve trays and valve units. Valves can be round or rectangular, with or without a caging structure. A detailed description is available elsewhere {1), and in manufacturers' literature (7-9). The valve disks rise as vapor rate is increased (Fig. 6.2c). The upper limit of opening is controlled by a caging structure or by restrictive legs at the bottom of the valve unit. As vapor rate falls, the disk openings are reduced, or they may settle intermittently over the holes. This stops the liquid from

Gambar Weir Dari Sieve Tray

Figure 6.1 (Continued.) Common tray types. (c> Sieve tray; (d) dual-flow tray. [Pari e courtesy ofGlitsch, Inc.] IPart d courtesy of Fractionation Research Inc. (FRD.)

Figure 6.1 (Continued.) Common tray types. (c> Sieve tray; (d) dual-flow tray. [Pari e courtesy ofGlitsch, Inc.] IPart d courtesy of Fractionation Research Inc. (FRD.)

weeping and gives the valve tray its main advantage—good operation at low flow rates, and therefore, a high turndown.

6.1.2 Comparison of the common tray types

Table 6.1 compares the main tray types. The comparison is general and assumes the trays are properly designed, installed, and operated. Sieve and valve trays have comparable capacity, efficiency, entrain-

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  • vivaldo angelo
    What is flooding and weepingprocess equipment design?
    8 years ago

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