Efficiency, tray loads

Reducing the pumparound duty increases the tray loadings on trays 1 through 7. But in so doing, the trays operate closer to their incipient flood point. This is fine. The incipient flood point corresponds to the optimum tray performance. But if we cross over the incipient flood point, and trays 5, 6, and 7 actually start to flood, their fractionation efficiency will be adversely affected. Then, as we decrease the pumparound heat-removal duty, the mutual contamination of diesel and gas oil will increase. 

Carrots, potatoes, apples, and green beans dried in this modified CFB at an air velocity of 2400 ft/min and 240°F showed that a weight reduction of 50% could be achieved in less than 6 min for all items. In comparison with a tunnel dryer with a cross-flow air velocity of 780 ft/min, 160°F temperature, and 2 Ib/fF tray loading, it was shown that average drying rate in a modified CFB (air velocity 2400 ft/min) was 5.3 times the cross-flow value. This increase in drying rate (three times the theoretical value) was due to high efficiency of the air-to-particle contact achieved in the CFB. 

At low vapor rates, tray pressure drop decreases hence, tray leakage is increased. This reduces tray fractionation efficiency. Then, to achieve the desired product split, a higher reflux ratio, which wastes reboiler energy, is needed. Figure 9-3 shows a typical relationship of tray efficiency vs load. When assembly of the tray sections is less than perfect, the turndown efficiency of the tray is further degraded. 

Performance data on some typical tray and compartment diyers are tabulated in Table 12-10. These indicate that an overall rate of evaporation of 0.0025 to 0.025 kg water/(s m") of tray area may be expected from tray and tray-truck diyers. The thermal efficiency of this type of diyer will vary from 20 to 50 percent, depending on the diying temperature used and the humidity of the exhaust air. In diying to very low moisture contents under temperature restrictions, the thermal efficiency may be in the order of 10 percent. The major operating cost for a tray diyer is the labor involved in loading and unloading the trays. About two labor-hours are required to load and unload a standard two-truck tray diyer. In addition, about one-third to one-fifth of a... 

Refitting of a tray-type column is desired to increase loading, increase efficiency, and/or decrease pressure drop. Structured packing is pariicularly applicable in this case. 

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. 

A tray is flexible when it operates with acceptable efficiency under conditions which deviate significandy from those established for design. The usual changes affecting flexibility are vapor and/or liquid loading. A tray may operate down to 50% and up to 120% of vapor load, and down to 15% and up to 130% of liquid load and still be efficient. Beyond these points its efficiency may fall off, and the flexible limits of the tray would be established. 

Figure 8-147 indicates minimum values of Fh to initiate acceptable bubbling tray action. Efficiency at this activation or load point might be expected to be low however Myers results indicate good values at this rate. 

It is a characteristic of process equipment, that the best operation is reached, at neither a very high nor a very low loading. The intermediate equipment load that results in the most efficient operation is called the the best efficiency point. For distillation trays, the incipient flood point corresponds to the best efficiency point. We have correlated this best efficiency point, for valve and sieve trays, as compared to the measured pressure drops in many chemical plant and refinery distillation towers. We have derived the following formula ... 

Hole Sizes Small holes slightly enhance tray capacity when limited by entrainment flood. Reducing sieve hole diameters from 13 to 5 mm ( to in) at a fixed hole area typically enhances capacity by 3 to 8 percent, more at low liquid loads. Small holes are effective for reducing entrainment and enhancing capacity in the spray regime (Ql < 20 m3/hm of weir). Hole diameter has only a small effect on pressure drop, tray efficiency, and turndown. 

Dual-Flow Trays These are sieve trays with no downcomers (Fig. 14-27b). Liquid continuously weeps through the holes, hence their low efficiency. At peak loads they are typically 5 to 10 percent less efficient than sieve or valve trays, but as the gas rate is reduced, the efficiency gap rapidly widens, giving poor turndown. The absence of downcomers gives dual-flow trays more area, and therefore greater capacity, less entrainment, and less pressure drop, than conventional trays. Their pressure drop is further reduced by their large fractional hole area (typically 18 to 30 percent of the tower area). However, this low pressure drop also renders dual-flow trays prone to gas and liquid maldistribution. 

Vapor-Liquid Loads and Reflux Ratio Vapor and liquid loads, as well as the reflux ratio, have a small effect on tray efficiency (Fig. 14-43) as long as no capacity or hydraulic limits (flood, weep, channeling, etc.) are violated. 

Adjust the downcomer flood approximately equal to the tray active area flood. With these two flood values equal or nearly equal, the tray is considered to have a balance of loading between the downcomer loading and the active tray area loading. This balance ensures that the tray will operate efficiently even if it has less than 50% flood loadings. Review these two flood values (downcomer and active area flood values) carefully and make adjustments, especially in new tray design, ensuring that these flood values are close to equal. 

The number of trays is determined by dividing the theoretical number of stages, which is obtained from the relationships in Section III, by the appropriate tray efficiency. It is best to use experimental efficiency data for the system when available, but caution is required when extending such data to column design, because tray efficiency depends on tray geometry, liquid and gas loads, and physical properties, and these may vary from one contactor to another. In the absence of data, absorption efficiency can be estimated using O Connell s empirical correlation. This correlation should not be used outside its intended range of application. 

Limitations. The Chan and Fair correlation generally gave good predictions when tested against a wide data bank, but ita authors also observed some deviations. Chan and Fair (184,135) describe it as tentative until more data becomes available. Lockett (12) notes that the Chan and Fair correlation inherited the tendency to predict high point efficiencies from (be AIChE correlation. Lockett also points out that the presence of the FF term in Eq. (7.19) implies that efficiency depends on tray spacing for fixed vapor and liquid loads. This implication is supported neither by theoretical nor by experimental evidence, and is considered by Lockett as hardly reasonable."... 

Vapor-liquid loads. A higher vapor load reduces the vapor contact time but also increases the interfacial area (136,137). These two factors have counteracting effects on tray efficiency. Usually, the contact time dominates, and efficiency decreases with higher vapor rates (185). A higher liquid load increases tray efficiency (185) because it increases tray liquid holdup, and therefore vapor contact time. 

Figure T.10 Some factors affecting sieve tray efficiency. FRI data, total reflux, DT = 4 It, S = 24 in, hu, = 2 in, dH = 0.5 in. Both parts show a small efficiency rise with pressure. Both parts show little effect of vapor and liquid loads above about 40 percent of flood, (a) Showing efficiency reduction when fractional hole area is increased from 8 to 14 per-cent of the bubbling area (6) emphasizing little effect of vapor and liquid loads, and an efficiency increase with pressure. Af 0.14 (Both parts repeated with permission from T. Yanagi and If. Sakata, lad. Eng. Chan. Proc. Use. Dev. 21, p. 712, copyright 19S2, American Chemical Society.)...

This means both vapor and liquid loadB are raised and lowered simultaneously. Increasing vapor rate reduces efficiency, while increasing liquid rates raises efficiency. The two effects normally cancel each other, and efficiency is practically independent of load changes (assuming no excessive entrainment or weeping). Figure 7.106 shows a typical dependence of tray efficiency on vapor and liquid loads for a commercial-scale distillation column. Anderson et al. (97) show a similar dependence for several different valve trays. 

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