Sieve plates operating limits

The following discussion is limited to the usual type of column, equipped with sieve plates, operating at pressures not far from atmospheric, and treating mixtures having ordinary properties. 

Satisfactory operation will only be achieved over a limited range of vapour and liquid flow rates. A typical performance diagram for a sieve plate is shown in Figure 11.26. 

Pulsed sieve-plate columns have shown good efficiencies for frequencies between 60 and 150 strokes/min and amplitudes under 1 cm. In general, a plate spacing of 10 cm is considered most favorable. However, these columns operate well in only a limited load range and are also susceptible to fouling, particularly with sticky products. To some extent, the reduction in efficiency at underload can be compensated by changing the pulsation characteristics the load can also be artificially increased by recycling the raffinate. 

A sieve-tray column with 15 plates is used to prepare 99 percent methanol from a feed containing 40 percent methanol and 60 percent water (mole percent). The plates have 8 percent open area, in. holes, and 2-in. weirs with segmental downcomers, (a) If the column is operated at atmospheric pressure, estimate the flooding limit based on conditions at the top of the column. What is the F factor and the pressure drop per plate at this limit (Z>) For the flow rate calculated in part (a) determine the F factor and the pressure drop per plate near the bottom of the column. Which section of the column will flood first as the vapor rate is increased ... 

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