Natural gas is the fastest growing fossil fuel in future global energy scenarios. The consequent increase in the global demand for it has led to the need for the exploitation of those reserves that were left undeveloped in the past because of the high CO2 content. The selection of the best CO2 removal process is nowadays a critical concern, since each one has its own advantages and disadvantages when compared to the others. In particular, it is widely acknowledged that, for application to gas reserves characterized by a high CO2 content, conventional technologies based on chemical absorption are too energy-intensive because of the high energy required for solvent regeneration. Thus, new technologies have been recently proposed to treat the raw natural gas extracted from these reserves in a less energy-intensive and cost-effective way. Among these new technologies, low-temperature separation methods and hybrid technologies can be mentioned.

The aim of this work is to compare the performances of a novel technology based on low-temperature distillation with those of a hybrid technology that combines a low-temperature distillation process aimed at performing a bulk CO2 removal with conventional chemical absorption into aqueous alkanolamines solutions as a finishing treatment. Simulations have been performed with Aspen Hysys® and Aspen Plus® and the comparison has been carried out by means of an energy analysis based on the "net equivalent methane" method. Different CO2 contents in the raw natural gas feed stream have been considered in order to establish the breakeven point between the two technologies in terms of CO2 inlet concentration that makes one process less energydemanding than the other one.

1. Introduction

Natural gas is a fossil fuel that contains hydrocarbons and non-hydrocarbon compounds, which are considered as impurities. These impurities must be removed to meet pipeline specifications, enhancing the heating value of the natural gas and avoiding pipelines and equipment corrosion, as well as other problems. In particular, the presence of CO2 and H2S can lead to safety and operational issues. CO2 is a greenhouse gas responsible for global warming, while H2S can cause environmental problems due to the formation of SOx during combustion. Additionally, both can become acidic and corrosive in presence of water, causing potential damages to pipelines and equipment. Moreover, natural gas can be often directed to LNG production [1]. In LNG processing plants, during the cooling process of natural gas to very low temperatures, CO2 can freeze-out, plugging the pipeline systems and causing transportation issues. For all these reasons, the removal of the acid compounds through purification processes is crucial for improving the quality of the product.

In recent years, the interest in natural gas has increased as proved by its growth rate, which is the fastest among fossil fuels (Figure 1): this is due to its inherently environmental benignity, greater efficiency and cost effectiveness [2].

The natural gas extracted from reservoirs has a different composition as a function of the geographical location. Recent studies about its geographical distribution have demonstrated that 40% of the world's natural gas or associated gas reserves currently identified as remaining to be produced are sour and with a high CO2 content [4]. These gas reserves were left undeveloped for many years, but they have recently gained attention due to the continuous increase of the gas demand.

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