1. Introduction
It is clearly established that the use of chemical absorption processes for carbon dioxide separation is considered as a mature technique with a relatively low cost that allows the treatment of gas streams with high flow rates.1 This type of operation for carbon dioxide separation implies the use of chemical solvents, generally based on amines. In general, the use of primary and secondary amines is highly recommended, taking advantage of the high reaction rate with carbon dioxide in aqueous solution and the low cost.2 One of the most important limitations of this type of solvents is related to the low carbon dioxide loading that can be reached, limited by the reaction mechanism based on the production of carbamate.3,4 Another negative characteristic of this type of solvents that includes primary and secondary amines is the high cost associated to solvent regeneration procedures5,6that generally accounts for 70% of the costs of the overall absorption/desorption process.7 In addition other researchers8 consider that a higher stability of solvents can be reached avoiding the use secondary centres due to the formation of nitrosamines.
Taking into account these negative characteristics the development of new solvents tries to reach: (i) high absorption rate and carbon dioxide loading and (ii) negligible solvent degradation, corrosion and energy consumption. To reach these aims one of the proposed alternatives is the use of solvents that include a mixture of amines.9 The aim of using this type of mixtures is centred on taking advantage of the positive characteristics of tertiary or sterically hindered amines that can overcome the negative aspects of primary and secondary amines, previously commented. It must be taken into account that solvents based on tertiary amines have shown a low reaction rate in comparison to primary and secondary amines.10 Moreover this type of solvents can reach higher values of carbon dioxide loading, thus leading to a better regeneration process that implies lower costs, degradation and corrosion.11 Several studies have been carried out in order to analyse the behaviour of mixtures of amines in the carbon dioxide absorption process comparing their performance to those obtained for the individual amines reaching in certain cases better results than the conventional solvent.12-14 Also a research line with very interesting results is centered on the use of di- or polyamines. One of the most important molecules in this research line is piperazine that has been used in an important number of research studies and papers, from basic studies to industrial applications.15,16
Present research work evaluates other type of chemical solvents different than conventional ones, trying to take advantage of combining different amine centres in the same solvent, inspired by promising results obtained while working with mixtures of different amines. A previous study analysed the use of diamines in the chemical solvents for carbon dioxide absorption using mineralisation for solvent regeneration.17 This work shows fast kinetics and low energy requirements that indicate this type of solvents as promising ones. A previous work18 of our team showed that a diamine allows to reach similar values that amines blends for absorption rate and carbon dioxide loading. Taking into account these previous results, present work is focussed on solvents with N,N-dimethylethylenediamine (a diamine with different amino centres - primary and tertiary) that has been considered as a promising diamine for chemical solvent formulation.17 Our work is centred on a comparison with the solvent based on amines blends (maintaining type and concentration) in order to evaluate the strength and weakness of this type of solvents. The influence of amine ratio, concentration and gas flow rate over the absorption process has been analysed.