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In the “energy transition era” we are experiencing today, natural gas grows strongly and much faster than either oil or coal, being an environmentally friendly fuel supported by a broad-based demand and the continuing expansion of liquefied natural gas (LNG). Natural gas mainly consists of methane, but also some contaminants are present in it. Among them, nitrogen is an inert gas whose content, if too high, must be reduced to levels acceptable for producing a pipeline-quality gas or LNG. When dilution of the high-nitrogen gas stream with a low-nitrogen gas is not practical, a nitrogen removal unit (known as nitrogen rejection unit or NRU) must be installed. This is becoming more and more important as we shift to lower-quality gas feedstocks. Cryogenic distillation is the only viable option for the removal of N2 on a large scale and in the case of stringent specifications both in the product stream and in the rejected stream. Typically, the feed gas to the NRU has to contain methane and N2 and only very low quantities of compounds (such as CO2) that might freeze at the NRU operating temperatures and cause equipment blockage. Focusing on the production of pipeline-quality natural gas, the aim of this work is to analyze the cryogenic removal of N2 from natural gas streams that also contain CO2 because a CO2-tolerant NRU may lower capital and operating costs reducing the upstream removal of CO2. Different process configurations (i.e., the single-column, the double-column, and the three-column systems) are investigated to determine the maximum allowable CO2 content in the feed gas that avoids solidification within the process and permits reaching the desired value of the Wobbe index. Therefore, a range of applicability for each configuration is determined depending on the N2 and CO2 contents in the feed gas.

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Polythionic acids, whose general formula is H 2 S n O 6 , with n greater than 2, were discovered in the aqueous solution of SO 2 and H 2 S, known as the Wackenroder liquid. Their reactions with each other and with other reagents are, mostly, difficult to characterize, since such compounds readily decompose and interconvert, especially in solution. Nevertheless, they play an important role in technical applications (e.g., gold leaching, magnesium milling, cooling in metal processing) and in reactions of inorganic chemistry of sulfur. A few years ago, Shell−Paques/ Paqell patented the first industrial process for the biological conversion of H 2 S into a colloidal mixture of sulfur and polythionates. Such hydrophilic sulfur can be used as a fertilizer and soil improver in agriculture in all but alkaline soils. Recently, Eni S.p.A. has developed to bench plant scale a new process, the HydroClaus process for the conversion of H 2 S into an acidic hydrophilic slurry of sulfur and polythionate ions. Such a slurry can be used as a soil improver where the very alkaline soil pH hinders the cultivation. The aim of this work is to study the laboratory-scale production of polythionates in view of the novel HydroClaus process scale-up at the industrial level. After the literature related to polythionates and their characterization has been revised, the sulfur-based mixture has been synthesized and the polythionate ions concentration has been determined. Also, the effect of the reaction operating conditions has been investigated to assess how they can influence the nature and the distribution of products in solution.

Acid gas removal from gaseous streams such as flue gas, natural gas and biogas is mainly performed by chemical absorption with amines, but the process is highly energy intensive and can generate emissions of harmful compounds to the atmosphere. Considering the emerging interest in carbon capture, mainly associated with increasing environmental concerns, there is much current effort to develop innovative solvents able to lower the energy and environmental impact of the acid gas removal processes. To be competitive, the new blends must show a CO2 uptake capacity comparable to the one of the traditional MEA benchmark solution. In this work, a review of the state of the art of attractive solvents alternative to the traditional MEA amine blend for acid gas removal is presented. These novel solvents are classified into three main classes: biphasic blends—involving the formation of two liquid phases, water-lean solvents and green solvents. For each solvent, the peculiar features, the level of technological development and the main expected pros and cons are discussed. At the end, a summary on the most promising perspectives and on the major limitations is provided.