Sieve Tray Design

Capacity

High

High to very high

Moderately high

Very high

Efficiency

High

High

Moderately high

Lower than other types

Turndown

About 2:1. Not generally suitable for operation under variable loads

About 4-5:1. Some special designs achieve (or claim) 10:1 or more

Excellent, better than valve trays. Good at extremely low liquid rates

Low, even lower than sieve trays (10). Unsuitable for variable load operation

Entrainment

Moderate

Moderate

High, about 3 times higher than sieve trays (4)

Low to moderate

Pressure drop

Moderate

Moderate. Early designs somewhat higher. Recent designs same as sieve trays

High

Low to moderate

Cost

Low

About 20 percent higher than sieve trays (11)

High. About 2-3 times the cost of sieve trays (10,11)

Low

Maintenance, fouling tendency, and effects of corrosion are least in sieve trays, although these are not much greater for valve trays.

In general, bubble-cap trays and dual-flow trays are mainly used in special applications. Bubble caps are preferred either when extremely high turndown is required or when leakage must be eliminated. Dual-flow trays are preferred for handling slurries, highly corrosive and highly fouling services, or when a column is revamped for high capacity.

For most other services, either sieve or valve trays are the best choice. Sieve trays are at an advantage when the service is fouling, or corrosive, or when turndown is unimportant, while valve trays are preferred when turndown is important. With high energy costs, the energy saved during even short turndown periods usually justifies the relatively low cost difference between valve and sieve trays. This has made valve trays most popular.

table 6.1 Comparison of the Common Tray Types {Continued)

Type

Sieve tray

Valve tray Bubble-cap tray Dual-flow tray

Mainte-

Low to moderate Relatively high Low

Fouling tendency

Effects of corrosion

Availability Well known of design information

Other

Low to moderate High. Tends to collect solids

Low to moderate High

Proprietary, but Well known information readily available

Extremely low. Suitable where fouling is extensive and for slurry handling.

Very low

Some information available

Instability sometimes occurs in large diameter (>8 feet) col-

Main applications

Most columns when turndown is not critical

Share of the market (11)

1. Most columns,

2. Services where turndown is important

1. Extremely low flow conditions

2, Where leakage must be minimized

1. Capacity revamps where efficiency and turndown can be sacrificed

2. Highly fouling and corrosive services

No information

6.2 Tray Capacity Limits 6.2.1 Hie classical hydraulic model

Figure 6,5 illustrates the classical hydraulic model of a fractionation tray. Liquid enters the tray from the downcomer of the tray above. The liquid entering the tray is aerated with vapor rising from the tray below to form froth on the tray. The froth flows across the tray until it reaches the outlet weir. The froth then flows over the weir into the downcomer, where the vapor is disengaged from the liquid.

Recent work (Sec. 6.4.1) has shown that this model is an oversimplification of the processes occurring on a distillation plate. Nevertheless, many of the modern design procedures are based on this model and are expressed in terms of this model.

Sieve Tray Stability Diagram

Figure 6.6 is a typical tray stability diagram. The area of satisfactory operation (shaded) is bound by the tray stability limits. These limits are discussed in the following sections. The upper capacity limit is the onset of flooding. At moderate and high liquid flow rates, the entrapment (jet) flooding limit is normally reached when vapor flow is raised, while the down comer flooding limit is normally reached when liquid flow is raised. When flows are raised while the column operates at constant LIV (i.e., constant reflux ratio), either limit can be reached. At very low liquid rates, as vapor rate is raised, the limit of excessive entrainment is often reached.

As vapor rate is lowered, either at constant liquid rate or at a constant LfV ratio, the limit of excessive weeping is reached. This limit is not identical with the weep point, as some weeping can usually be tolerated.

6.2.2 Tray stability diagram

Figure 6.6 is a typical tray stability diagram. The area of satisfactory operation (shaded) is bound by the tray stability limits. These limits are discussed in the following sections. The upper capacity limit is the onset of flooding. At moderate and high liquid flow rates, the entrapment (jet) flooding limit is normally reached when vapor flow is raised, while the down comer flooding limit is normally reached when liquid flow is raised. When flows are raised while the column operates at constant LIV (i.e., constant reflux ratio), either limit can be reached. At very low liquid rates, as vapor rate is raised, the limit of excessive entrainment is often reached.

As vapor rate is lowered, either at constant liquid rate or at a constant LfV ratio, the limit of excessive weeping is reached. This limit is not identical with the weep point, as some weeping can usually be tolerated.

6.2.3 Definitions of tray area, vapor load, and liquid load

Area definitions. Areas used for defining tray velocity are:

Entrainment Sieve Tray
Figure 6.6 Sieve tray performance diagram.

Total tower cross-section area, AT; the inside cross-section area of the empty tower (without downcomers or trays).

Net area, AN: the total tower cross-section area Aj> less the area at the top of the downcomer (sometimes referred to as free area; the term free area has been used inconsistently in the literature). The net area represents the smallest area available for vapor flow in the intertray spacing.

Bubbling area, AB: the total tower cross-section area less the total of downcomer area, downcomer seal area, and any other nonper-forated regions (often referred to as active area, Aa). In practice, nonperforated regions less than 4 in wide are counted as perforated areas, while regions wider than 4 in are counted as nonperforated areas. The bubbling area represents the area available to vapor flow near the tray floor.

Hole area, Ah: the total area of perforations on the tray. The hole area is the smallest area available for vapor passage.

Slot area, As: the total (i.e., of all the valves) vertical curtain area through which vapor passes in a horizontal direction as it leaves the valves. It is based on the narrowest opening of each valve. The slot area is a function of the number of valves that are open. The slot area is usually the smallest area available for vapor flow on a valve tray.

Open slot area, ASo (valve trays): the slot area when all the valve units are fully open.

Fractional hole area, Af: the ratio of hole area to bubbling area (sieve trays) or slot area to bubbling area (valve trays).

Vapor load definitions. One vapor load term is VLOAD

This term is inconvenient because it is based on CFS and not on a velocity. A more convenient vapor load term is the F-factor,

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  • Hagosa
    How valve trays work diagram?
    6 years ago
  • abrha
    What is the difference between sieve trays and valve trays?
    4 years ago
  • Aatifa
    What is the differenec between sieve tray and valve tray?
    2 years ago
  • Fred
    How sieve trays works?
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