Process and controller configuration of high purity binary distillation columns in the purification stage of petrochemical plant

This paper reports the systematic selection of process and controller configurations of four distillation columns operated at purification stage in an olefin petrochemical plant. These columns were demethanizer, deethanizer, depropanizer #1 and depropanizer #2 columns, which separated ethane (C
₂
H
₆
) and propane (C
₃
H
₈
) from methane (CH
₄
) and other heavier components (i.e. C
₄₊
). The interconnection of these unit processes propagate the noise and cause severe instabilities towards the final unit process. Therefore, the decision about the choice of the process and control configuration is important in order to ensure the process stability. Such a decision includes the pairing of the manipulated and the controlled variables, the control rules and the inclusion of surge tanks to buffer the disturbances that caused the mentioned instabilities. In particular, the assessment selected three out of six control pairings available for distillation columns. A Proportional - Integral (PI) controller was utilized for all control pairs, and tuned by the Ziegler-Nichols method. In addition, two surge tanks were added as buffers between the demethanizer, the deethanizer and the depropanizer columns. This approach yields the process configuration that was capable of absorbing the disturbance, ensuring a 98-99% product purity and keeping a bias under 2%.