Optimal Grade Transition for Polyethylene Reactors via NCO Tracking

Abstract

In fluidized-bed gas-phase polymerization reactors, several grades of polyethylene are produced in the same equipment by changing the operating conditions. Transitions between the different grades are rather slow and result in the production of a considerable amount of off-specification polymer. Grade transition improvement is viewed here as a dynamic optimization problem, for which numerous approaches exist. Numerical optimization based on a nominal process model is typically insufficient due to the presence of uncertainty in the form of model mismatch and process disturbances. This paper proposes to implement optimal grade transition using a measurement-based approach instead. It is based on tracking the necessary conditions of optimality (NCO tracking) using a decentralized control scheme. For this, the nominal input profiles are dissected into arcs and switching times that are assigned to the various parts of the NCO. These input elements are then adapted using appropriate measurements. NCO tracking is used to determine optimal grade transition in polyethylene reactors. The problem of minimizing the transition time from a steady state of low melt index to that of high melt index is studied, with the feeds of hydrogen and inert and the output flow rate considered as manipulated variables. In the optimal solution, all arcs are determined by path constraints, and all switching times are determined by path and terminal constraints, which significantly eases the adaptation. The on-line and run-to-run adaptation of these parameters is illustrated in simulation.

Optimal grade transition in industrial polymerization processes via NCO tracking

“…At this point, it is important to mention that there exists no complete and systematic procedure for deriving a solution model, Physical insight is necessary as it helps characterize the optimal nominal solution, choose an appropriate pairing of manipulated and controlled variables, and decide on the feedback implementation type (online or run‐to‐run). Initial experiences have been made with various processes that include semibatch reactors for specialty chemicals (Srinivasan and Bonvin,2006; Bonvin et al,2006), polymerization reactors (François et al,2004; Bonvin et al,2005; Chatzidoukas et al,2005), bioreactors (Kadam et al,2006), and batch‐distillation columns (Welz et al,2006a; Welz et al,2006b).…”

Broadening the polyethylene molecular weight distribution by controlling the hydrogen concentration and catalyst feed rates

“…This ideal condition, however, does not always exist. There are a number of efforts in the literature to deal with this issue [1][2][3][4][5][6][7][8][9][10][11]. However, all these efforts have dealt only with melt index (or alternatively the molecular weight) and density of polymers and not the molecular weight distribution of the formed polymer.…”

Optimal grade transition control for liquid‐propylene polymerization reactor

“…Bonvin et al [6] studied the grade transition problem using the measurement-based approach. This method is based on tracking the necessary conditions of optimality, which are determined using Pontryagin's Minimum Principle, using a decentralized control scheme.…”