Optimization Modeling for Advanced Syngas to Olefins Reactive Systems

Reactor designs with mixed catalysts play an important role transforming a multiple reactor system to single-shot reactors. In addition to savings in capital and ease of implementation, single-shot reactors are useful to break equilibrium limitations, therefore increasing the yield and selectivity of desired product as shown in literature. However, the nonlinear and highly exothermic nature of mixed-catalyst systems makes it difficult for commercial process simulation and optimization tools to optimize these systems. This study describes the development and application of optimization strategies for mixed-catalyst, single-shot reactors for syngas-to-olefin processes. Finding the optimal catalyst distribution is challenging and requires advanced solution strategies for singular optimal control problems, which are poorly conditioned and often lead to flat response surfaces. The graded bed and partial moving finite element approaches are used to find the optimal catalyst distribution that maximizes the olefins yield. 1.3 % increase in the yield is observed from 1 zone to 3 zones. The yield further improves from 3 zone to the exact solution by 0.2%. This improvement can be achieved by changing only the catalyst distribution, without any extra investment. 

To account for the uncertainty in kinetic parameters, a two-stage multi-scenario problem formulation is implemented, and a robust reactor design is achieved. The robust design guarantees the desired olefins to paraffins ratio in the reactor, even for the worst set of kinetic parameters. 

The aim of this study is to come up with an optimization framework that can solve mixed catalyst reactive systems, which have singular characteristics that can cause numerical instabilities. Also, thermal runaway constraints are embedded into the optimization formulation for a safe operation. Finally, the robust design handles the uncertain parameters. This framework is applied to syngas-to-olefins reaction network and can be easily extended to other mixed-catalyst reactive systems.