Control system design and analysis for refinery stabilizer

摘要

A refinery stabilizer is a unique type of a fractionator with a multicomponent feed of naphtha, total vapor overhead product and air-cooled condenser. The traditionally reduced 2 x 2 quality control problem is shown to be inadequate. The inherent interaction present within the control loops are accounted for by adding a third variable viz, reflux drum accumulation rate. Light key recovery in the bottom product and heavy key recovery in the top product are chosen as the quality variables. The manipulated variables are the reflux rate, reboiler and condenser duties. The steady state analysis tools reveals inconsistencies between the RGA and SVD pairings. MIC are found to favor RGA pairing and thus the energy balance control scheme are selected. Further interaction compensators are used to reduce the interaction by almost 10 folds (in terms of the condition no.) It is also shown that dynamic elements can be accounted for in the compensation matrixes by calculating the effective dead time in the stripping and rectifying sections. These dead times are readily obtained from the columns hydraulic data.