Tray Devices

While there are perhaps five basic distillation trays suitable for industrial use, there are many design variations of differing degrees of importance and a confusing array of trade names applied to their products by tray manufacturers. The most modern and commonly used devices include sieve, valve, bubble cap, dual flow, and baffle trays - each with its advantages and preferred usage. Of these, the sieve and valve type trays currently are most often specified.

For a better understanding of tray design, Figure 4 defines and locates typical tray components. The material of construction usually is 14 gauge with modern trays adopting the integral truss design which simplifies fabrication. A typical truss tray is shown in Figure 5. For columns less than 3 ft (0.9m) in diameter, it is not possible to assemble the truss trays in the column; therefore, trays must be preassembled on rods into a cartridge section for loading into the column. Figure 6 shows this arrangement in scale model size.

The hydraulic design of a tray is a very important factor. The upper operating limit generally is governed by the flood point, although in some cases, entrainment also can restrict performance before the onset of flooding. Flooding is usually caused by either massive entrainment, termed jet flooding, or by downcomer back-up. Downcomer back-up occurs when a tray design provides insufficient downcomer area to allow for the liquid flow or when the pressure drop across the

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Downcomer Distillation
Figure 4. Tray component terminology. Figure 6. Cartridge tray assembly.

tray is high, which forces liquid to back up in the downcomer. When the downcomer is unable to handle all the liquid involved, the trays start to fill and pressure drop across the column increases. This also can occur when a highly foaming liquid is involved. Flooding associated with high tray pressure drops and small tray spacing takes place when the required liquid seal is higher than the tray spacing. Downcomer design also is particularly important at high operating pressure due to a reduction in the difference between vapor and liquid densities.

The lower limit of tray operation, meanwhile, is influenced by the amount of liquid weeping from one tray to the next. Unlike the upward force of entrainment, weeping liquid flows in the normal direction and considerable amounts can be tolerated before column efficiency is significantly affected. As the vapor rate decreases, however, a point eventually is reached when all the liquid is weeping and there is no liquid seal on the tray. This is known as the dump point, below which there is a severe drop in efficiency.

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