A perspective of solutions for membrane instabilities in olefin/paraffin separations: a review

Light olefins are mainly produced by naphtha steam cracking, which is among the more energy intensive processes in the petrochemical industry. To save energy, some alternatives have been proposed to partially replace or combine with cryogenic distillation the conventional technology to separate olefins and paraffins. Within this aim, facilitated transport membranes, mainly with Ag+cations as selective carriers, have received great attention owing to the high selectivity and permeance provided. However, to be used industrially, the undesirable instability associated with the Ag+ cation should be considered. Poisonous agents and polymer membrane materials are sources of Ag+ deactivation. In recent years, great achievements on the separation performance have been reported, but the current challenge is to maintain the selectivity in long-term separation processes. This work presents a critical analysis of the potential causes of Ag+ deactivation and points out some alternatives that have been proposed to overcome the hurdle. This review highlights and critically analyses some perspectives of the ongoing development and application of facilitated transport membranes.The authors thank the Brazilian Federal Agency for Support and Evaluation of Graduate Education − CAPES (PDSE Grant 88881.134232/2016-01) and the Spanish Ministry of Economy, Industry and Competitiveness (CTQ2015-66078-R and CTQ2016-75158-R projects, Spain-FEDER 2014-2020) for financial support

Kinetics of reactive absorption of propylene in RTIL-AG+ media

In the present work the kinetics of absorption of propylene in Ag+-BMImBF4 medium have been analysed, discriminating the kinetic regime and determining the expression and parameters of the rate of reactive absorption as a function of the concentration of both reagents. A stirred cell reactor was used to obtain the kinetic information by loading pure absorption gas and recording the total pressure decrease at different silver salt concentrations [AgBF4 = 0–1 M] at 288–308 K. The experimental values of the enhancement factors (EA) due to the presence of the chemical reaction indicate that for concentrations equal to or higher than 0.25 M of Ag+ in BMImBF4 it is possible to assume instantaneous chemical reaction between propylene and silver ions. Therefore, the rate of absorption in this case is governed by the rate at which dissolved C3H6 and Ag+ diffuse to the reaction plane from the G–L interface and liquid bulk, respectively. Physico-chemical parameters necessary to predict the absorption rate have been experimentally determined in this work. The liquid mass transfer coefficient (kL) was obtained in a non-reactive BMImBF4 medium as a function of stirring speed and temperature, showing a significant increase from 1.61 × 10-6 to 17.9 × 10-6 m s-1 when the temperature increased from 288 to 308 K at 500 rpm. Experimental viscosity measurements for the reactive system Ag+-BMImBF4 are reported in order to predict the diffusion coefficient of propylene in the absorptive media. Diffusivity of the silver ions in BMImBF4 has been determined by the chronoamperometry technique, showing a concentration dependency in the range of AgBF4 = [0.05–0.25 M]

Kinetics of reactive absorption of propylene in RTIL-AG+ media

In the present work the kinetics of absorption of propylene in Ag+-BMImBF4 medium have been analysed, discriminating the kinetic regime and determining the expression and parameters of the rate of reactive absorption as a function of the concentration of both reagents. A stirred cell reactor was used to obtain the kinetic information by loading pure absorption gas and recording the total pressure decrease at different silver salt concentrations [AgBF4 = 0–1 M] at 288–308 K. The experimental values of the enhancement factors (EA) due to the presence of the chemical reaction indicate that for concentrations equal to or higher than 0.25 M of Ag+ in BMImBF4 it is possible to assume instantaneous chemical reaction between propylene and silver ions. Therefore, the rate of absorption in this case is governed by the rate at which dissolved C3H6 and Ag+ diffuse to the reaction plane from the G–L interface and liquid bulk, respectively. Physico-chemical parameters necessary to predict the absorption rate have been experimentally determined in this work. The liquid mass transfer coefficient (kL) was obtained in a non-reactive BMImBF4 medium as a function of stirring speed and temperature, showing a significant increase from 1.61 × 10-6 to 17.9 × 10-6 m s-1 when the temperature increased from 288 to 308 K at 500 rpm. Experimental viscosity measurements for the reactive system Ag+-BMImBF4 are reported in order to predict the diffusion coefficient of propylene in the absorptive media. Diffusivity of the silver ions in BMImBF4 has been determined by the chronoamperometry technique, showing a concentration dependency in the range of AgBF4 = [0.05–0.25 M]

Water and methanol permeation through short-side-chain perfluorosulphonic acid ionomeric membranes

The water and methanol transport into a short-side-chain perfluorosulphonic acid ionomeric (PFSI) membrane suitable for application in proton exchange membrane fuel cells (PEMFC), namely Hyflon® Ion, was studied between 35 and 65 C. In particular, the permeabilities of pure water, pure methanol and their mixtures at different temperatures were measured through pervaporation experiments, at various values of feed composition. Due to the presence of mutual interactions between permeants as well as among penetrants and polymeric matrix, the composition of the feed solution affects the membrane permeability in a way which cannot be predicted on the basis of permeability data of the pure liquid components alone. It has been found in particular that the presence of the water in the mixture enhances the methanol permeability, due to the positive effects of matrix plasticization and favourable energetic interactions. In turn, by considering water permeability data in the presence of a poorly permeating component such as glycol, it can be concluded that also water permeation is enhanced by the presence of methanol, although to a lower extent. The molar equilibrium uptake of pure water and pure methanol were also measured in separate sorption experiments and result the same for both components, while the estimated diffusion coefficient of water is 3.1–3.5 times that of methanol. The activation energies for the relevant transport properties in the range between 35 and 65 Cwere calculated, both for pure penetrants as well as for the penetrants in mixture, thus determining the deviations from the pure component behaviors

Reactive ionic liquid media for the separation of propylene/propane gaseous mixtures

Designed room temperature ionic liquids (RTILs) containing silver salt are presented as reactive media in separating propylene/propane gas mixtures. Solubilities of propylene and propane in the reactive media, silver tetrafluoroborate (AgBF4) dissolved in 1-butyl-3-methylimidazolium tetrafluoroborate (BmimBF4) and N-butyl-4-methylpyridinium tetrafluoroborate (BmpyBF4), were investigated as a function of silver ion concentration, temperature, and pressure. Equilibrium data were obtained working in a temperature range between 278 and 318 K and at pressures up to 6 bar. Propylene absorption was chemically enhanced in the silver-based RTILs and was considerably higher than that in the standard RTILs. Absorption of propane in the silver-based RTILs is based on physical interactions only. A simple mathematical model based on the formation of complex species with different stoichiometry has been developed in order to describe the total propylene absorption, and the model was validated with experimental data obtained working with different concentrations of silver salt (between 0.1 and 1 M). The model parameters, equilibrium constants (KEq,1 f(T) and KEq,2 f(T)), and enthalpies of complexation (¿Hr,1, ¿Hr,2) were obtained. Thermal stability of the silver ions was analyzed and to be found dependent on the silver salt concentration. Complete regeneration of the reaction media was possible at a temperature of 313 K and 20 mbar of pressure

Reactive ionic liquid media for the separation of propylene/propane gaseous mixtures

Designed room temperature ionic liquids (RTILs) containing silver salt are presented as reactive media in separating propylene/propane gas mixtures. Solubilities of propylene and propane in the reactive media, silver tetrafluoroborate (AgBF4) dissolved in 1-butyl-3-methylimidazolium tetrafluoroborate (BmimBF4) and N-butyl-4-methylpyridinium tetrafluoroborate (BmpyBF4), were investigated as a function of silver ion concentration, temperature, and pressure. Equilibrium data were obtained working in a temperature range between 278 and 318 K and at pressures up to 6 bar. Propylene absorption was chemically enhanced in the silver-based RTILs and was considerably higher than that in the standard RTILs. Absorption of propane in the silver-based RTILs is based on physical interactions only. A simple mathematical model based on the formation of complex species with different stoichiometry has been developed in order to describe the total propylene absorption, and the model was validated with experimental data obtained working with different concentrations of silver salt (between 0.1 and 1 M). The model parameters, equilibrium constants (KEq,1 f(T) and KEq,2 f(T)), and enthalpies of complexation (¿Hr,1, ¿Hr,2) were obtained. Thermal stability of the silver ions was analyzed and to be found dependent on the silver salt concentration. Complete regeneration of the reaction media was possible at a temperature of 313 K and 20 mbar of pressure

Screening of RTILs for propane/propylene separation using COSMO-RS methodology

The quantum chemical COSMO-RS method was applied to analyze the equilibrium solubility of gaseous propane and propylene in a set of different room temperature ionic liquids in order to contribute to the design of olefin/paraffin separation technologies based on reactive absorption onto ionic liquids with dissolved silver salts media. First of all, the predictive capability of COSMO-RS was evaluated through a comparison of estimated values with a wide range of solubility experimental data; next a further optimization step based on the comparative analysis of predicted and experimental values of the Henry’s law constant of each solute in different ILs was developed to improve the accuracy of the calculations. Afterwards, the optimized COSMO-RS approach was applied to select the most suitable RTILs for C3H6/C3H8 separation based on driving a computational screening of 696 RTILs. Results highlighted that small and symmetric fluorinated inorganic anions such as View the MathML sourcePF6- or View the MathML sourceBF4- provide lower solubilities for both hydrocarbons, but on a whole this results in higher separation selectivities. With regard to the structure of the cation, ILs based on imidazolium, pyridinium and pyrrolidinium cations provide similar properties. However ILs based on monosubstituted butyl ammonium have much lower solubilities for both gases and at the same time higher equilibrium selectivities for propylene. Also it was gathered that less and shorter alkyl chains in the cation also improve the selective separation of these mixtures. Finally, as the separation process is enhanced by the presence of Ag+ cations into the ionic liquid, the solubility of 8 commercially available silver salts was qualitatively related to the excess enthalpy of Ag+-IL in solution predicted by COSMO-RS in order to select a silver salt suitable to be dissolved in the selected RTIL. Thus, this work reports for the first time the use of a predictive tool in order to facilitate the design of innovative separation processes by reactive absorption in a Ag+-IL mediaThis research has been funded by the Spanish Ministry of Science and Innovation (Project CTQ2008-00690/PPQ