(177b) High-Resolution Process Design: Flowsheet Optimization with Embedded High Fidelity Unit Models

R.C. Pattison and M. Baldea

McKetta Department of Chemical Engineering, The University of Texas at Austin, Austin, TX 78712

Optimal process design is key to maximizing the profitability of chemical plants. Due to the large upfront capital investments required to construct new processes or retrofit existing plants, it is imperative that the design is optimized using modern gradient-based optimization tools coupled with detailed models of the processing units present in the flowsheet.

Equation-based simulation and optimization have witnessed substantial progress, and most current simulators offer equation-oriented simulation options as well. In this presentation, we illustrate a novel equation-oriented simulation tool which allows for high-fidelity models of select unit operations (including reactors, multi-stream heat exchangers) to be easily embedded into the process flowsheet model. These models capture phenomena occuring over multiple length scales, including smaller dimensions (e.g., diffusion and reaction in a catalyst pellet) which are typically not considered in flowsheet simulation or optimal design calculations.

Our framework is based on a pseudo-transient representation of the unit operation models developed in our previous work [1-3]. The pseudo-transient modeling framework relies on a systematic reformulation of the material and energy balances into a differential and algebraic equation model having the same steady state solution to that of the original algebraic equation model. The (typically) partial differential equations associated with detailed unit models lend themselves naturally to a pseudo-transient reformulation which can then be incorporated into the process flowsheet model. This, in turn, enables the detailed steady state unit designs to be optimized simultaneously with the design of the entire process flowsheet. To this end, we use a time relaxation-based optimization algorithm previously developed in our group [1].

The framework will be illustrated with industrial case studies, including the design optimization of an intensified dimethyl ether production process featuring a dividing wall distillation column and the optimal design of a natural gas liquefaction process.

References

[1] Pattison, R.P.; Baldea, M. Equation-oriented flowsheet simulation and optimization using pseudo-transient models. AIChE J., 2014, 60, 4104-4123.

[2] Pattison, R.P.; Baldea, M. Multistream heat exchangers: Equation-oriented modeling and flowsheet optimization. AIChE J., 2015, 61, 1856-1866.

[3] Pattison, R.P.; Gupta, A.M.; Baldea, M. Equation-oriented optimization of process flowsheets with dividing-wall columns. AIChE J., 2016, 62, 704-716.