(106f) Optimization of Natural Gas Liquefaction Process Using Detailed Differential Algebraic Equation Based Multi-Stream Heat Exchanger Design

Natural Gas has become an increasing source of energy around the world. It is seen as a transition fuel from traditional energy sources like coal and oil to renewable energy sources like solar and wind. Worldwide trade of natural gas has also increased with many nations using it as the fuel in their power generating plants. Transporting natural gas overseas requires liquefaction under high pressure and cryogenic temperatures.

The liquefaction of natural gas is an energy intensive process where a mixture of hydrocarbon and nitrogen is used as the mixed refrigerant to cool the gas inside a multi-stream heat exchanger. Most optimization studies considering natural gas liquefaction process do not include the design effects of the multi-stream heat exchanger. This could lead to sub-optimal or infeasible solution to the real world flowsheet optimization problem. To the best of author’s knowledge, only Tsay et al. (2017) implemented detailed design of multi-stream heat exchanger inside natural gas flowsheets.

In this study, we formulate a DAE based model for multi-stream heat exchanger design which incorporates phase change and heat exchanger between the natural gas and mixed refrigerant streams. The design equations of a spiral wound coil heat exchanger are used for the detailed design of the multi-stream heat exchanger. The phase change is modeled using complementarity constraint based flash equations and the heat transfer is modeled based on first principles heat partial differential equation.

We use the trust-region filter based method (by Eason and Biegler, 2018) to embed the detailed DAE model into the natural gas liquefaction process flowsheet. Reduced models are used to represent the DAE model inside the flowsheet models which are updated after each trust region iterations using the solution from the DAE design model. The filter is used to guarantee convergence to both feasible and optimal point of the overall design problem of natural gas liquefaction process with detailed heat exchanger design. We will present the DAE model formulation and the trust-region algorithm along with results from a real world natural gas liquefaction flowsheet optimization problem.

References:-

1. Eason, J.P. and Biegler, L.T. (2018), Advanced trust region optimization strategies for glass box/black box models. AIChE J, 64: 3934-3943
2. Tsay, C., Pattison, R.C. and Baldea, M. (2017), Equation‐oriented simulation and optimization of process flowsheets incorporating detailed spiral‐wound multistream heat exchanger models. AIChE J., 63: 3778-3789.