(747d) Reactor Network Development for Rigid Polyols Production

Rigid polyols are defined as polyether polyols formed by alkoxylation of initiators with 3 to 8 active sites to form polyethers used with isocyanates to manufacture closed cell foams.Closed cell foams are used in the production of refrigerators, building and construction insulation. Demand for these products have increased significantly in the last 10 years and are expected to continue that growth in the next 10 years, and tight product specifications are imposed based on properties related to the molecular weight distribution (MWD).

In most cases, semi-batch reactors are used to produce rigid polyols, with only a few examples of continuous reactors in the patent literature. As the product demand of a polymer increases, larger or more semi-batch reactors are built to meet the demand, and the capital cost per production capacity is predictably lowered. However, this capital cost is still too high for re-investment, and alternative lower capital cost solutions, such as switching the production from semi-batch to continuous processes, could help sustain growth. Hence, continuous processes can reduce the capital cost and the cost required for heat transfer equipment, since the heat load is more evenly distributed over time. In addition, continuous reactors are easier to optimize at a steady state to maximize the overall production rate. However, continuous reactors are not as flexible regarding multiple products, product transition can be difficult and scale-up of the products need extra investment. The motivation of this study is to transform the production from the existing semi-batch reactors to continuous reactors, such as plug flow and continuous stir tank reactors, to lower the capital cost and to maintain product specifications based on properties of the MWD. Prior to developing an optimization superstructure, which predicts an optimal combination of CSTRs and/or PFRs in series/parallel, the reaction mechanism and the kinetic parameters also needs to be determined.

Here, the kinetic parameter estimation problem, which includes MWD information, can be written in a general form as a dynamic optimization problem. Moreover, the simultaneous collocation method can be adopted to deal with the dynamic optimization problem for the reactor network. This method discretizes the continuous time or length horizon into a finite element mesh, and then the differential-algebraic equation optimization problems are converted into nonlinear programming problems. The differential and algebraic state variables z and y, respectively can be denoted by the Runge-Kutta basis representation. Furthermore, a three-point Radau collocation is applied to parameter estimation model. The model is constructed in AMPL and solved by NLP solver IPOPT. This optimization is applied to synthesis an optimal reactor network for rigid polyol production, where we demonstrate significant performance improvements over previous studies.