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A Control Method for the Cryogenic Distillation Column with a Feedback Stream

Toshihiko Yamanishi, Mikio Enoeda, Kenji Okuno, Robert H. Sherman

Fusion Science and Technology

Volume 29 / Number 2 / March 1996 / Pages 232-243


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A control method was proposed for the cryogenic distillation column with a feedback stream. The top and bottom flow rates of the column are adjusted for the variation of external feed composition to control product purity. The flow rate of the side stream and the power of the reboiler heater are promptly and linearly changed with the corresponding variation of external feed flow rate. Ordinary columns with no feedback stream are first-order lag systems for the case where the top flow rate is chosen as a manipulated variable. On the other hand, the column with a feedback stream is a second-order lag system even in this case. The parameter-setting method of the proportional-integral (PI) controller was proposed to predict the unstable region in the control of the column. The method can also be applied to the case where the measurement of the controlled variable is accompanied by a long time lag. However, the longer time lag requires a larger integral time, and the larger integral time brings a larger overshoot and slower damping for the controlled variable. For this case, the promptness of the control can be improved by introducing the PI derivative controller.

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