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 It was superseded by the sieve and valve trays.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) etround the holes, and caps in the form of inverted cups over the risers. Severed 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. 6.2o). Liquid euid froth are trapped on the tray to a depth at least equal to the weir height or riser hei t, giving the bubble-cap tray a unique ability to operate at low vapor and liquid rates. 

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. 

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TABLE 6.1 Comparison of the Common Tray Types (Conr/mNKQ... 

Various types of filtration equipment are available commercially and can be operated in batch, semicon-tinuous, or continuous modes. Among the commonly used types are the plate and frame filter, rotary drum filter, leaf filter, plate filter, and tray filter. Apart from the plate and tray filters, all other are enclosed and therefore are easy to work with when sterility of the solids is an important issue. Moreover, all these filters are examples of dead-end filters. Cross-flow filtration is mostly used in the purification stage through membranes with very low pore sizes and is discussed later. 

A common type of distillation contacting device used in refinery applications is the sieve tray. In the early 50 s and for many years before, the bubble cap tray was the mainstay of the distillation field. A sieve tray consists of a flat plate with regularly spaced holes, normally 1/2 to 1 inch in diameter. Liquid flows horizontally across the tray and into a channel, called a downcomer, which leads to the tray below. The sieve tray exhibits good capacity, excellent efficiency, low pressure drop, and good flexibility i.e., it will operate quite efficiently at tower loadings which are 1/2 to 1/3 of design values. 

These three types of trays have a common feature in that they all have separate downcomers for the passage of liquid from each tray to the one below. There is another class of tray which has no separate downcomers and yet it still employs a tray type of construction giving a hydrodynamic performance between that of a packed and a plate column. Two examples of this type of device are the Kittel plate and a Turbogrid tray(53). Design data for these trays are sparse in the literature and the manufacturer s recommendations should be sought. 

It has already been noted that trays which are suitable for distillation may be used for absorption duties though in general lower efficiencies will be obtained. In Chapter 11, the design of trays for common contacting devices is considered and the methods presented in that chapter are generally applicable. The most commonly used tray types are shown in Figure 11.50a with the crossflow tray being the most popular. 

Sublimation temperatures are in the range of —10 to —40°C and corresponding vapor pressures of water are 2.6-0.13 mbar. Cabinet tray dryers are the most commonly used type. The trays are lifted out of contact with hot surfaces so the heat transfer is entirely by radiation. Loading of 2.5 lb/sqft is usual for foodstuffs. Drying capacity of shelf-type freeze dryers is 0.1-1.0kg/(hr)(m2 exposed surface). Another estimate is 0.5-1.61b/(hr)(sqft). The ice surface has been found to recede at the rate of 1 mm/hr. Freeze drying also is carried out to a limited extent in vacuum pans, vibrating conveyors, and fluidized beds. Condensers operate as low as —70°C. 

An extractor column is generally a tall, vertical packed tower that has two or more bed sections. Each packed bed section is typically limited to no more than 8 ft tall, making the overall tower height about 40 to 80 ft. Tower diameter depends fully upon liquid rates, but is usually in the range of 2 to 6 ft. Liquid-liquid extractors may also have tray-type column internals, usually composed of sieve-type trays without downcomers. These tray-type columns are similar to duoflow-type vapor-liquid separation, but here serve as contact surface area for two separate liquid phases. The packed-type internals are more common by far and are the type of extractor medium considered the standard. Any deviation from packed-type columns is compared to packing. 

Liquid collectors are installed when liquid must be collected for redistribution or drawoff (e.g., for external cooling). The common device used is a chimney tray, which is similar to an orifice redistributor, but without perforations. Another common device is the Chevron-type collector, which is a series of Chevron blades, with liquid being collected at the bottom of the blades. 

Vertical tray-type deodorizers are probably the most commonly used type of continuous deodorizers. Their design is based on a series of trays or compartments stacked vertically in a cylindrical shell, with each tray designed for a specific task (e.g., De Smet—Figure 23). All operations, heating, deodorization, and heat... 

Sublimation temperatures are in the range of —10 to —40°C and corresponding vapor pressures of water are 2.6-0.13mbar. Cabinet tray dryers are the most commonly used type. The trays are lifted out of contact with hot surfaces so the heat transfer is entirely by radiation. Loading of 2.5 Ib/sqft is usual for foodstuffs. 

There are three types of deodorization operations. The batch process is the least common, due to its low efficiency and inconsistent product quality. Semi-continuous and continuous deodorizers have improved processing efficiency. There are several configurations of the continuous deodorizer, including singleshell cylindrical vessel type, vertically stacked tray type, and the thin-film packed column type. This last provides excellent fatty acid stripping with minimum use of steam, but it achieves neither desired heat bleaching nor effective deodorization due to the relatively short retention time. A retention vessel has to be used after the column distillation (De Greyt and Kellens 2000). 

The common types of flashing feed and vapor distributors are the baffle type (Fig. 3.9a), the vapor-liquid separator type (Fig. 3.96), the gallery type (Fig. 3.9c), and the tangential entrance tjrpe (Fig. 2.2j). Some of the vapor distributors discussed in Sec. 3.12 are also sometimes used for flashing feeds, especially when liquid distribution to the section below is not critical (e.g., when it contains trays). 

Conventional dryers used in the mineral-processing industry are classified as hearth type, shaft type, and grate type. Other types of dryers used less commonly in current practice are the spray type, fluid-bed type, pneumatic or flash type, conveyor type, drum type, stationary- and rotating-tray type, infrared type, and others. Only the more commonly used dryers are discussed in this chapter. 

A number of factors influence the selection of a dryer from the many different types available. These factors are dominated by the nature of the feed, whether it be granular solids, a paste, a slab, a film, a slurry, or a liquid. Other factors include the need for agitation, the type of heat source (convection, radiation, conduction, or microwave heating), and the degree to which the material must be dried. The most commonly employed continuous dryers include tunnel, belt, band, turbo-tray, rotary, steam-tube rotary, screw-conveyor, fluidized-bed, spouted-bed, pneumatic-conveyor, spray, and drum dryers. 

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