Perforated sieve plates/trays

Entrainment about three times that of perforated type plate or sieve tray. Jet-action accompanies bubbling. 

In plate columns the two phases are intensively mixed on each plate and separated between each plate (Fig. 6.7-5). For the distribution of the light phase through the liquid a lot of devices were developed. The simplest one is a perforated sieve tray, where the supercritical phase can pass through. To avoid weeping of the liquid through the holes different devices like bubble caps or valves (Fig. 6.7-6) were developed. 

The column shown in Fig. 17.1 often contains a number of perforated plates, or trays, stacked one above the other. A cascade of such trays is called a sieve-plate column. A single sieve plate is shown in Fig. 17.2. It consists of a horizontal tray A carrying a down pipe, or downcomer, C, the top of which acts as a weir, and a number of holes B. The holes are all of the same size, usually 5 to in. in... 

Sieve plate A, tray or plate B, perforations C, downcomer to plate below D downcomer from plate above. 

Perforated-plate Towers. In the perforated-plate, or sieve-plate, column, the dispersed phase is repeatedly coalesced and redispersed by causing it to flow through a series of trays in which a large number of small holes have been punched or drilled. In the simplest type, the plates are similar to the side-to-side baffles described above, except that they are perforate. Hunter and Nash (42) describe a successful installation of this type for dephenolating gas liquor consisting of a 46-ft,-high shell, 5 ft. in diameter, in which the baffles each contain two hundred holes. 

Whenever three to nine theoretical stages are necessary, packed columns are the preferred devices. The packing provides a tortuous flow path for the dispersed phase, thereby increasing contact time between phases. The packing also restricts axial mixing of the continuous phase. Sieve-plate columns also have been employed for these same applications. The perforations in the trays serve to produce the dispersion, while the downcomers (or upcomers) conduct the continuous phase from tray to tray. Sieve plates create a series of short spray columns with the continuous phase being partially mixed in the downcomers. Because packed and sieve-trayed columns impart only a minimum of energy to the system, they are preferred for low interfacial tension systems (less than 12 dyne/cm) to avoid emulsification. 

Holes are sometimes drilled in side-to-side baffle trays, and the extreme in such a trend results in perforated trays, sieve plates, Turbogrid trays, and Ripple trays. In these, both the down-flowing liquid and the rising vapor pass through the same openings. 

Column diameter for a particular service is a function of the physical properties of the vapor and liquid at the tray conditions, efficiency and capacity characteristics of the contacting mechanism (bubble trays, sieve trays, etc.) as represented by velocity effects including entrainment, and the pressure of the operation. Unfortunately the interrelationship of these is not clearly understood. Therefore, diameters are determined by relations correlated by empirical factors. The factors influencing bubble cap and similar devices, sieve tray and perforated plate columns are somewhat different. 

Perforated plates without downcomers have only recently been included in commercial equipment. The data for rating the performance is not adequately covered in the literature, since the present developments in industrial equipment have not been released. The information included here is based only on available data and experience, yet it may serve as a basis for rating, because the basic nature of the contact is quite analogous to the sieve tray. The limits of performance are not well defined therefore the methods oudined cannot be considered firm. However, they are adequate for many applications and as the basis for further study. 

This is the case with diameter determination. The relation of Equation 8-250 for the perforated tray or sieve tray with downcomers can be used for the plate without downcomers. Generally, the liquid level and foam-froth height will be higher on this tray, hence the ralue of h., clear liquid on the tray, may range from 1-in. to 6-in. depending on the service. 

Hole size is as important in perforated plates without downcomers as far the sieve tray. Published data limits a full analysis of the relationships however, the smaller holes, Ys-in., Me-in., 4-in. appear to give slightly higher efficiencies for the same tray spacing [47]. Unfortunately the data [69] for the larger %-in. holes was not evaluated for efficiencies. Experience has indicated efficiencies equal to or only slightly, 10-15%, less for M-in. holes w hen compared to Me-in. holes for some systems. Holes as small as Mfrin., %2-in. and Me-in. were considered unsatisfactory for high surface tension materials such as water [47]. 

The extent of entrainment of the liquid by the vapour rising over a plate has been studied by many workers. The entrainment has been found to vary with the vapour velocity in the slot or perforation, and the spacing used. Strang 60-1, using an air-water system, found that entrainment was small until a critical vapour velocity was reached, above which it increased rapidly. Similar results from Peavy and Baker 6 11 and Colburn 62 have shown the effect on tray efficiency, which is not seriously affected until the entrainment exceeds 0.1 kmol of liquid per kmol of vapour. The entrainment on sieve trays is discussed in Section 11.10.4. 

Sieve trays (Fig. 14-18a) are perforated plates. The velocity of upflowing gas keeps the liquid from descending through the perforations (weeping). At low gas velocities, liquid weeps through the perforations, bypassing part of the tray and reducing tray efficiency. Because of this, sieve trays have relatively poor turndown. 

Two types of trays are most common sieve trays and valve trays. A sieve tray is a simple perforated plate. Gas issues from the perforations to give a multiorifice effect liquid is prevented from descending the perforations or weeping by the upward motion of the gas. At low gas flow rates, the upward gas motion may be insufficient to prevent weeping. 

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