A simultaneous Dynamic optimization approach for natural gas processing plants

JUAN I. LAIGLECIA; RODRIGO LOPEZ-NEGRETE; MARIA SOLEDAD DIAZ; LORENZ T. BIEGLER

Savannah

Congreso; Foundations on Computer Aided Process Operations; 2012

Carnegie Mellon University

In this work we address dynamic optimization of natural gas processing plants through the use first principle models and full discretization of both control and state variables. The optimization problem includes rigorous models for cryogenic countercurrent heat exchangers with partial phase change, separation tanks, distillation columns and turboexpanders. Thermodynamic predictions are made with a cubic equation of state. The partial differential algebraic equation system is transformed into ordinary differential-algebraic equations (DAEs) by applying the method of Lines for the spatial coordinate in cryogenic heat exchangers. The resulting optimization problem is formulated and solved by applying orthogonal collocation on finite elements, and the large-scale Nonlinear Programming (NLP) problem is solved with a Newton-based Interior Point method. The objective is to switch between operating modes to minimize the offset between current ethane recovery and a set point value. Numerical results provide temporal and spatial profiles of controlled and manipulated variables, while fulfilling specific path constraints associated to ethane extraction processes. In particular, the tight integration between process units as well as path constraints has been efficiently handled with low computational time