Use of Optimization in Evaluation and Design of Liquefaction Processes for Natural Gas

Liquefied natural gas (LNG) is a fast growing energy carrier suitable for transport of natural gas when the distance from source to market is long and/or the volumes are moderate. In order to liquefy the natural gas, and thereby reduce its volume, energy demanding low-temperature refrigeration over a wide temperature range is required. Hence, energy efficient designs depend on small temperature differences in heat transfer. Over the years, many process concepts have been proposed for liquefaction of natural gas, with different advantages and disadvantages. While development of LNG processes for a long time was concerned primarily with increased capacity and energy efficiency, the emergence of new applications such as remote gas and floating operations have put focus also on properties such as safety, environmental impact, compactness, operability and flexibility. For all applications, optimization is essential for fair comparison of different process concepts, in order to minimize cost, energy use and environmental impact. Due to characteristics of the liquefaction processes and the rigorous thermodynamic models required for practical feasibility of the design, optimization of LNG processes is a complex problem. In order to accommodate this challenge, the focus of this work has been to improve the optimization problem formulation through use of thermodynamic analysis and insight, with the objective of enabling rigorous and robust optimization of both simple and complex process concepts. In this work, the refrigeration processes have been modelled with a commercial process simulation tool using cubic equations of state. Optimization has been performed using a sequential quadratic programming algorithm. A stochastic search algorithm (simulated annealing) has also been tested. The observed performance was, however, better for the deterministic search method. Processes studied in this project include single mixed-refrigerant process, single and dual nitrogen expander processes, pure-refrigerant cascade processes and a dual mixed-refrigerant process. Optimization studies of nitrogen expander processes have been carried out both with a simplified process model assuming perfect gas behaviour of the refrigerant and a rigorous process model using a cubic equation of state. Comparison of the results indicated agreement between the models only for a limited number of cases. In the majority of the case studies, a better solution was found for the rigorous model accounting for the non-idealities of the refrigerant. The intermediate pressure levels in multi-stage compression with intercooling have been optimized for the case of perfect gas behaviour and constant isentropic efficiency. For the case of different suction temperatures in the different compression stages, the results indicate that the optimal intermediate pressure levels are characterized by uniform discharge temperatures for the compression stages rather than uniform pressure ratios. A heuristic rule for the optimal intermediate pressure in multi-stage real gas compression has also been proposed. For a single mixed-refrigerant cycle this was proven to generally provide high accuracy. The influence of the choice of decision variables and bounds on the optimization search performance has been illustrated for optimization of pure-refrigerant cascade processes. Compared to a fairly obvious choice of variables, a set of decision variables based on process characteristics was found to give significant improvement in the success rate of the optimization search. Exergy analysis of cascade refrigeration processes have been used to illustrate the interaction between the different refrigeration cycles in a cascade. The results demonstrate that the solution that provides the smallest compression power in a single cycle not necessarily coincides with the solution that gives the smallest power consumption for the overall process. Based on these findings, an approach for design and optimization of complex cascade processes has been proposed as an alternative to simultaneous optimization of all variables. In the suggested approach, the load distribution between the different cycles in the cascade is optimized in an outer loop, while the different refrigeration cycles are optimized sequentially in an inner loop starting from the lowest temperature level. The principles of the procedure have been illustrated for a dual mixed-refrigerant process. Studies on the influence of constraint formulations for optimal trade-off between operating and investment costs in LNG process design have proven the inadequacy of the common approach with a minimum temperature difference constraint. A case study presented for a single mixed-refrigerant process illustrated that significant savings in energy use could be realized by optimal distribution of heat transfer driving forces