Turndown packed columns

An attractive feature of packed beds for mass and heat transfer operations is that the bed itself offers better turndown characteristics than the equivalent tray column. The mass transfer efficiency of a packed column with a good distributor is about as good at both low and high loadings. This favorable comparison is evident from the data in Fignres 12.26 and 12.49 (same colnmn and test mixture). The liquid distribution is the key, as indicated in Figure 12.50. 

The main consideration for introducing reflux or intermediate feed into a packed tower is adequately distributing the incoming stream to the packing. Unlike most tray columns, packed towers are sensitive to distribution. Maldistribution is detrimental to packing efficiency and turndown. The main devices that set the quality of distribution in a packed column are the top (or reflux) distributor, the intermediate feed distributor, the redistributor, and sometimes the vapor distributor. Adequate hydraulics in the inlet area is also important failure to achieve this can affect distributor performance and can also cause premature flooding. 

Maximum vapour rates on packed and tray columns are similar, being about 1.4-1.9m/s based on the empty column area. As indicated under Pressure drop , the pressiue drops over trays will be a good deal greater so that the absolute pressure at the column top is likely to be lower in vacuum operation and the mass flow will be correspondingly less. Vendors of column internals, most of whom are able to supply both trays and column packing systems, can provide information on the details of their products in respect of their liquid and vapour performance, but a turndown of 3 1 on packed columns and 5 1 on trays should be attainable (Fig. 4.7). 

Packed towers are used increasingly in small fractionating towers and absorbers where accessibility is restricted. Their use is also advantageous where corrosion control is essential. Because of liquid-distribution concerns, packing still is usually not recommended for large-diameter columns or where a high turndown ratio is required. All packings tend to spread liquid as it flows downwards. This characteristic is adequate in... 

For distillation columns Add sufficient trays to account for disturbances and anticipated expansion of production. The expected turndown ratios affect the choice of internals in a distillation column with a large turndown ratio, suggesting the use of bubble caps a low turndown ratio may point to structured packings. At the same time, safety and hazard analysis indicates that we want a minimum of liquid holdup in the system. 

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