Equation-based Model for Simultaneous Working Fluid - Process - Heat Integration Optimization of Organic Rankine Cycle for Liquefied Natural Gas regasification

orc2019 Tracking Number 137

The organic Rankine cycle (ORC) is a promising energy recovery technology that finds application in a variety of energy sources due to its exceptional flexibility to operate for a wide temperature range [1]. Among different energy sources, liquefied natural gas (LNG) is a great candidate because of its low temperature (110 K), which can significantly increase the efficiency of low grade heat recovery (<373 K). Its massive heat requirement (tens of MW) during regasification also guarantees a substantial energy output [2]. Despite its great potential, design of the Organic Rankine Cycle for LNG regasification is extremely challenging due to the complexity of the problem, which must consider process design, heat integration, and composition of its working fluid at the same time. Usually, optimization of the aforementioned problem is formulated as a mixed-integer nonlinear programming (MINLP) problem. However, the optimization problem is computationally expensive due to the large number of binary variables used to model heat integration and heat capacity variation of process streams during temperature exchange [3]. Exacerbating the problem is the use of oversimplified thermodynamics embedded in the model, which can make the results inaccurate since the thermodynamics used is not rigorous. In this paper, an equation-oriented model is developed for simultaneous process-heat integration-working fluid optimization of LNG regasification process using ORC for cold energy recovery. To address the computational difficulties, we present a novel formulation approach for heat integration that significantly reduces binary variables compared with conventional formulation. Additionally, an efficient modeling method is proposed to accommodate the heat capacity change of streams as a result of phase change during heat exchange. We also integrate a rigorous thermodynamics module based on SRK equation of state in our model to ensure accurate computation of thermodynamics properties (e.g. vapor-liquid equilibrium and enthalpy) so that the composition of the work fluid can be optimized. Thanks to the efficient formulation and rigorous thermodynamics calculation, our model is capable of solving the solving the simultaneous optimization problem effectively. Simple examples are presented to illustrate the proposed model, then a LNG regasification process design problem is used to demonstrate the efficiency the proposed model. References [1] Tchanche BF, Lambrinos G, Frangoudakis A, Papadakis G. Low-grade heat conversion into power using organic Rankine cycles–A review of various applications. Renewable and Sustainable Energy Reviews 2011;15:3963-79. [2] Gómez MR, Garcia RF, Gómez JR, Carril JC. Review of thermal cycles exploiting the exergy of liquefied natural gas in the regasification process. Renewable and Sustainable Energy Reviews 2014;38:781-95. [3] Kamath RS, Biegler LT, Grossmann IE. Modeling multistream heat exchangers with and without phase changes for simultaneous optimization and heat integration. AIChE J 2012;58:190-204.