A Novel Approach to Design and Implementation of Multivariable Process Controllers for C3 Fractionation Columns

Most of multivariable process controllers (MPC) for C3 fractionation columns in olefin plants require good column pressure control and include the PIC as a manipulated variable. This is due to the fact that big pressure swings can cause significant propylene product swings. In this application, two C3 fractionators share one reflux drum, resulting in significant pressure interactions. To deal with this problem, a new DMC control design was developed and implemented which uses a reflux ratio controller to reject pressure-related disturbances and floats column pressures. To make sure the controller design is feasible for the low-feed propane and propylene ratio (which results in low bottom draw flow), a dynamic model was developed to evaluate dynamic behavior and to compare different control schemes. The new control design scheme provides a good alternative for C3 fractionator MPC implementation.