Abstract
In this paper, processes for liquefied natural gas (LNG) production with upstream or integrated natural gas liquids (NGL) removal have been optimized and compared. Since the NGL and LNG production systems use both work and heat to deliver products with different energy quality, it is challenging to measure accurately the thermodynamic efficiency by using conventional energy performance indicators. Thus, two different objective functions, specific energy consumption and exergy efficiency, have been applied in the optimization of these complex systems in order to evaluate the effectiveness of the two performance indicators. The results indicate that use of the exergy-based objective function results in a richer NGL and a larger amount of LNG production with a marginal increase in energy consumption, showing a higher thermodynamic efficiency than the result with the energy-based objective function. Besides, integrated NGL extraction shows a lower thermodynamic performance than upstream removal, indicating that further advanced schemes are required for effective integration of the NGL extraction part in the LNG process.
Highlights
The main objective of this paper is to introduce and evaluate exergy as a key performance indicator that can be used to optimize complex industrial processes
The optimization results with the energy performance indicator (Obj1 in Table 6) shows that the refluxed natural gas liquids (NGL)-liquefied natural gas (LNG) process has the smallest specific energy consumption
The specific energy consumption focuses on the production of NGL and LNG, this performance indicator guided the system to produce a larger amount of end-flash gas, which is less valuable than the LNG product
Summary
The main objective of this paper is to introduce and evaluate exergy as a key performance indicator that can be used to optimize complex industrial processes. Exergy is defined as the maximum amount of work that can be obtained when a system is brought to equilibrium with its surroundings (i.e. temperature, pressure and chemical composition) Exergy measures both the amount (1st Law of Thermodynamics) and quality (2nd Law of Thermodynamics) of energy, and it will be compared with specific energy consumption that is often used in the process industries. Processes for liquefaction of natural gas are prime candidates for use of exergy efficiency as performance indicator These processes operate below ambient, they have two main products (LNG e Liquefied Natural Gas and NGL e Natural Gas Liquids), and they use both work for compression and heat for fractionation. The LNG and NGL products vary in volume and composition depending on the plant configuration and operating parameters Most of these processes have complex flowsheet structures
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