Disturbance Amplification In a Column Train

Feed disturbances. Disturbances in the feed or operation of a column are often transmitted to downstream columns (e.g., Fig. 19.13a). The disturbance can be either amplified or dampened, with or without a significant time lag. If large enough, it can upset downstream columns. Generally, the smaller the feed rate compared to the internal vapor and liquid traffic, the less sensitive the column to feed disturbances. Superfractionators (high reflux ratios) seldom suffer from feed disturbances, while low-reflux-ratio columns suffer most. 

Factors that commonly cause disturbances to be amplified are 

Insufficient surge volume between columns. This usually compels level controller tuning for a fast response (in the bottom sump or reflux drum), causing flow fluctuations. 

A high ratio of the feed flow rate to the flow rate of the product which is passed to the next column. This is especially true for small bottom streams passed on to the next column. Any feed or even reboiler variations often cause significant fluctuations in bottom flow. 

A preferential baffle in the bottom compartment (Sec. 4.5) increases the tendency of a bottom sump level and, therefore, bottoms flow, to fluctuate. 

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Some factors that can mitigate disturbance amplification in a column train are... 

Figure 19.13 Disturbance amplification in a column train, (a) No heat interaction (6) with heat interaction, poor control (c) with heat interaction, good control. (Parts b and c based on Dak E. Lupfer, Proc. 53rd Ann. Convention of the Gas Processors Assoc., March 25-27,1974. Reproduced courtesy of the Gas Processors Association.)...

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