Maximum Operational Capacity MOC Concept and Traps

The maximum operational capacity or throughput is defined (15) as the Maximum vapor rate that provides normal efficiency of a packing (i.e., point F in Fig. 8.16a). 

MOC in Fig. 8.16d is difficult and leaves a lot of room for subjectivity. Further, MOC determination is sensitive to the accuracy of efficiency measurements—factors such aa liquid and vapor distribution, sampling procedure, end effects at the top and bottom of the packings, and the availability of a large number of efficiency points near the MOC, where conditions may not be steady-state. For instance, for one set of published experimental data reported by Eckert and Walter (61), MOC increased by about 10 percent due to reducing packed height from 10 to 5 ft- 

The MOC is a useful upper-capacity criterion, specifically related to packing efficiency. However, the MOC is far from the panacea for resolving the uncertainties inherent in the flood-point concept, as it substitutes those for new uncertainties. 

Pressure drop This is often used to specify packed tower capacity. The application of this criterion, and the interpretation of packing pressure drop data are not trouble-free. Some inherent limitations and traps are 

Dry-packed beds have higher pressure drops than wet-packed beds. Billet (56) and Kister (40) report cases where changing from dry to wet packing increased column capacity by 5 percent and reduced pressure drop by 10 percent. Ludwig (63) reports cases where this pressure drop reduction was 50 to 60 percent. The 

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