دانلود رایگان مقاله ارزیابی مدل ترمودینامیکی برای طراحی و بهینه سازی سیستم گاز مایع

Abstract

Natural gas liquefaction systems are based on refrigeration cycles – they consist of the same operations such as heat exchange, compression and expansion, but they have different layouts, components and working fluids. The design of these systems requires a preliminary simulation and evaluation of their performance. However, the thermodynamic models used for this purpose are characterised by different mathematical formulations, ranges of application and levels of accuracy. This may lead to inconsistent results when estimating hydrocarbon properties and assessing the efficiency of a given process. This paper presents a thorough comparison of six equations of state widely used in the academia and industry, including the GERG-2008 model, which has recently been adopted as an ISO standard for natural gases. These models are used to (i) estimate the thermophysical properties of a Danish natural gas, (ii) simulate, and (iii) optimise liquefaction systems. Three case studies are considered: a cascade layout with three pure refrigerants, a single mixed-refrigerant unit, and an expander-based configuration. Significant deviations are found between all property models, and in all case studies. The main discrepancies are related to the prediction of the energy flows (up to 7%) and to the heat exchanger conductances (up to 11%), and they are not systematic errors. The results illustrate the superiority of using the GERG-2008 model for designing gas processes in real applications, with the aim of reducing their energy use. They demonstrate as well that particular caution should be exercised when extrapolating the results of the conventional thermodynamic models to the actual conception of the gas liquefaction chain.

5. Conclusion

The design of gas liquefaction facilities requires an ad-hoc evaluation of their performance based on the selection of a thermodynamic property model. The GERG-2008 model is currently the most accurate one: it represents all the thermophysical properties of natural gas mixtures under the range of experimental uncertainties. However, equations of state of the cubic, virial and ‘molecularbased’ families are well-known and have been used widely in the oil and gas industry, as well as in the academia. The present work compares these models systematically in three steps, by applying them to the (i) calculation of natural gas properties, (ii) design and (iii) optimisation of three liquefaction plants (cascade, mixedrefrigerant and expander-based). Large deviations are observed for the prediction of the vapour-liquid equilibrium, liquid densities and heat capacities of the natural gas mixtures. These discrepancies have a significant impact on the simulation of liquefaction processes in terms of predicted energy flows and temperature approaches. These inconsistencies may mislead process engineers and decision-makers when building and evaluating these cryogenic systems. The present research supports therefore the recent works in the field of natural gas modelling, which state that cubic, virial, and molecular-based equations of state are not accurate enough. Future work in the field of gas liquefaction optimisation should preferably build on the use of the GERG model, despite its higher mathematical complexity and greater computational requirements.