Optimization of Grade Transitions in Polyethylene Solution Polymerization Processes Jun Shi and Lorenz T. Biegler Chemical Engineering Department, Carnegie Mellon University, Pittsburgh, PA 15213, USA Intan Hamdan The Dow Chemical Company, Freeport, TX 77541, USA DOI 10.1002/aic.15113 Published online December 28, 2015 in Wiley Online Library (wileyonlinelibrary.com) This study considers the development of optimization models for grade transition of polyethylene solution polymerization processes. A detailed mathematical model is developed to capture the dynamics of the solution polymerization process. This includes time delay models for vapor and liquid recycle streams as well as a reduced, yet accurate, vapor-liquid equilibrium (VLE) model derived from rigorous VLE calculations. Simultaneous dynamic optimization approach is applied to solve the optimization problem to reduce off-spec production time and transition time. Two optimization for- mulations, single stage and multistage, are developed to deal with single-value target and specification bands of product properties, respectively. The results show significant reductions in grade transition time and off-spec production time. In addition, the multistage formulation designed for problems with specification bands outperforms its single stage counter- part. It minimizes transition time and off-spec production directly, and leads to higher performance control profiles. VC 2015 American Institute of Chemical Engineers AIChE J, 62: 1126–1142, 2016 Keywords: optimization, polymerization, control, mathematical modeling Introduction Linear low-density polyethylene and loop reactors Polyethylene is the most widely used thermoplastic polymer today. Among the large family of polyethylene products, lin- ear low-density polyethylene (LLDPE) has penetrated almost all traditional polyethylene markets. LLDPE is made by copolymerization of ethylene with longer-chain olefins; single-site catalysts are preferred, as they tend to provide a narrower distribution of molecular weight. Solution polymerization in a continuous plant is a typical process for LLDPE production. Two common reactor configu- rations for solution polymerization are stirred-tank reactors and loop reactors. In particular, the loop reactor mainly con- sists of a nonadiabatic tubular system in a closed loop, with at least one heat exchanger for removing the heat generated by polymerization reactions and a pump to circulate the reaction mixture through the pipe. Multiple feed positions are located along the loop for inlet monomer, comonomer and catalyst along with a product outlet. 1 The reactor is operated liquid filled, with temperature and pressure controlled to maintain the reaction mixture in liquid phase. The loop reactor is more effective than a stirred tank as it directly influences heat trans- fer conditions and the mixing of various components in the reaction stream. Moreover, its effectiveness, described in Zacca and Ray, 2 is due to operation at high circulation rates, which allows operations under high polymer concentrations and a high length/diameter ratio, which improves heat transfer conditions. Modeling and simulation studies of loop reactors include Zacca and Ray, 2 where the reactor is modeled as two intercon- nected tubular reactors, and Reginato et al., 3 which develops a nonideal continuous stirred tank reactor (CSTR) model. In Touloupides et al., 4 loop reactors are modeled as an ideal CSTR followed by a semicontinuous product removal unit. As discussed in Zacca and Ray, 2 the dynamic behavior of loop reactors can be characterized by its volumetric recycle ratio. At high recycle ratios, the loop reactor behaves like an ideal CSTR. This reactor type greatly simplifies construction of a detailed dynamic model without incorporating additional information on reactor geometry, which is often proprietary. Grade transitions Various grades of LLDPE tailored to different applications are defined by the specifications of product properties such as melt index (MI) and density. Typically, several grades are pro- duced in the same production line. Due to high inventory cost and volatile market demand, frequent grade transitions are needed. Those grade transitions are considered to be challeng- ing from an operational point of view because both process economics and safety issues should be taken into considera- tion. In certain instances, complex transitions rely heavily on operator/expert experience. Given the large market of LLDPE and the current experience-based transitions, there is a need, and also room to improve transitions and change operating Correspondence concerning this article should be addressed to Lorenz T. Biegler at biegler@cmu.edu VC 2015 American Institute of Chemical Engineers 1126 AIChE Journal April 2016 Vol. 62, No. 4