Structured catalysts for Low temperature cos hydrolysis Process intensification

Abstract

This PhD project has been focused on the process intensification of COS hydrolysis, a reaction which stays in the framework of acid gas cleaning and that nowadays drives increasing attention by the petrochemical industry. The rising economy, the increase of the demand for energy and fuels, and the consequent increase in exploitation of natural resources, have originated growing environmental concerns. Among these, the attention to sulfur contents in fossil fuels has recently received a deep focus. It is well known that most of the sulfur-derived compounds are toxic for human health, and their presence in the atmosphere could be related to the acid rains. The topic plays a key role in modern energy scenario, and it has become an incentive in improving the existent desulfurization technologies. The sulfur compounds are conventionally distinguished in organic and inorganic. Carbonyl sulfide is an organic sulfur compound, whose emissions in the atmosphere have anthropogenic roots in desulfurization processes. In this PhD project, the problematic of carbonyl sulfide abatement has been addressed, and the existent removal technologies have been discussed, highlighting the limitations of the nowadays processes. The core of this three-years project has been the proposal and investigation of some innovative solutions to overcome the most relevant issues. Considering the present state-of-the-art, the research could provide a noteworthy improvement to the existent technologies. Hence, the shared opinion of the scientific committee of this thesis was to address the aims in two main directions: the development of a catalyst to efficiently conduct this reaction, and the development of a process technology to provide a potentially competitive industrial solution. Once reached the milestones of finding a low-temperature active formulation, enumerating the criticalities of the process, it was offered a solution to enhance the reaction performances while demonstrating the potentiality of a coupled configuration. An open architecture configuration constituted by a closed box in with the hydrolysis reactor and the absorber were able to work at the same temperature condition was tested, assessing the feasibility of the process integration. Then, the process intensification of COS hydrolysis was addressed following two different approaches. On one hand, the optimized formulation was transferred on a structured catalyst, and the advantages that this solution provided to the process were highlighted, demonstrating how the micrometric layer of active phase deposited on the carrier reduces the diffusion limitation typical of this system. Then, the activity of the structured catalyst was investigated in a remarkably wide range of operating conditions, considered the extreme variability of a real tail gas composition. Once the complete overview of the behavior of the catalyst was achieved, the collected data were employed to develop a kinetic model able to predict the performances of the process in a furtherly broad condition spectrum. In addition, the stability of the catalyst was evaluated, pointing out its ability of keeping unvaried performances, despite the formation of sulfates species on the surface due to critical reaction conditions. Afterwards, the study of COS removal was conducted in liquid phase in presence of an aqueous solution of a customized tertiary amine. The evaluation of the effect of the presence of different packing materials with different shapes was performed. Then, the aging of the amine solution was evaluated in presence of COS and H2S, and the experimental campaign allowed to observe that the removal in liquid phase goes through an aliquot of physical absorption and a sensible extent of reaction in liquid phase, with the portion of liquid water present in the solution.

Finally, a new experimental setup was designed for evaluating an unexplored technology: a three- phase system for the performance of COS hydrolysis. The system was composed of the structured

catalyst – for a fast and efficient conversion of the COS present in a gaseous stream – and the amine solution, in which the catalyst was completely immersed, to continuously subtract the produced H2S. The feasibility of this application was demonstrated with an outstanding success in the experiments: the three-phase system allowed to completely remove the COS – together with the produced H2S –

obtaining a clean outlet gas in a single room-temperature operating unit, excellently dwarfing the performances of both the constituting processes. [edited by Author]