Loading tray

Trays may be square or rectangular, with 0.5 to I m" per tray, and may be fabricated from any material compatible with corrosion and temperature conditions. When the trays are stacked in the truck, there should be a clearance of not less than 4 cm between the material in one tray and the bottom of the tray immediately above. When material characteristics and handling permit, the trays should have screen bottoms for additional diying area. Metal trays are preferable to nonmetalhc trays, since they conduct heat more readily. Tray loadings range usually from I to 10 cm deep. 

A further benefit in everyday operation is that samples and standards can be separated on the sample tray, thus simphfying sample preparation and tray loading. The instructions for cup positions are simply entered with the other run parameters when the operator specifies the analysis conditions. 

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. 

The downcomer percent flood is a critical tray-loading factor. If it is over 90%, then tray flooding failure is likely. If it is below 20%, tray vapor blowthrough (downcomer side) is likely to happen. It is therefore important to keep the downcomer flood below 90% and above 20%. One more limiting downcomer flood value, the active area flood, is also important. The active area flood calculation is covered in the following section. 

A typical bubble cap design is shown in Fig. 3.8. Similar to valve-type trays, the bubble cap tray has a tray loading factor XSB, which is dimensionless. The following three equations reveal the derivation of XSB ... 

Mass ratio tray loading factor used in Eqs. (3.95) and (3.96) to calculate KSB ... 

The following curve-fitted equations for sieve tray loading refer to Fig. 18-10 in the Chemical Engineer s Handbook [10,11]. 

A large floor space is required for the oven and tray loading facilities. 

It is relatively easy to set and control the optimum drying conditions in cabinet dryers. For this reason, various heat-sensitive food materials can be dried in small batches. The heat source is usually steam batteries or steam coils. The air from a centrifugal fan is passed through the coils and then baffled across the trays loaded with the product. The trays may either be loaded onto trolleys in stacks of 10-12 or may be stacked individually into the slots of the cabinet. The movement of trays may be manual or mechanically assisted depending on the size and capacity of the dryer. The hot air in almost all cabinet dryers is introduced at the top, and provision is made to recycle the air so that its total drying potential is ntilized by the time it discharges to the atmosphere. A schematic illnstration of a cabinet dryer and its principle of operation are shown in Fignre 24.7. 

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. 

In tray dryers, which are also called shelf, cabinet, or compartment dryers, the material, which may be a lumpy solid or a pasty solid, is spread uniformly on a metal tray to a depth of 10 to 100 mm. Such a typical tray dryer, shown in Fig. 9.2-1, contains removable trays loaded in a cabinet. 

The preceding calculation assumes an infinite number of contacting stages in the slurry oil pumparound. Detailed tray-to-tray computer simulations show that for three theoretical stages in the slurry oil pumparound, the maximum tray loading is about 65% above that calculated using 700,000 Ib/hr vapor and 980"F. The computer simulation takes into account reduced temperature, liquid flows, and vapor density. 

I have found, for most refinery services, when the above percentage exceeds 20% to 25%, a further increment in tray loading will promote tray flooding and entrainment. In the case of No. 2 crude unit, the 28% value calculated in the equation above explained why the FCCU feed was black The wash oil trays were flooding and severe entrainment of black resid was contaminating the gas oil product. 

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