The Multivariable Control of Carbon Dioxide Absorption System using the Proportional Integral Plus (PIP) Controller

The present article investigates the multivariable control of the carbon dioxide absorption process of the Shiraz petrochemical ammonia unit using the proportional-integral-plus (PIP) controller without limitation on the number of controlled variables. The PIP controller is a rational extension of conventional PI/PID controllers with extra dynamic feedback and input compensators. A multiinput – multi-output (MIMO) square model was extracted from the step response test. In this way, input water flow rate to carbon dioxide absorption system, the heat duty of input absorbent cooler to tray (1) of absorption tower and re-boiler heat duty of stripping tower are chosen as manipulated variables (inputs), while CO2 mole fraction in absorption tower vapor product, H2O mole fraction in absorption tower liquid product and tray temperature No.36 of stripping tower are determined as controlled ones (outputs). The system identification is performed with three input and three output variables using the step response test. As a result, continuous and discrete-time transfer function matrices and suitable NMSS model for PIP controller are reported. Finally, to evaluate the PIP control performance, the feed flow rate increases by 2%. The results display the proper performance of the designed PIP controller for both eliminations of disturbance and tracking of set point.