Intensificación en la separación de mezclas olefina/parafina

Resumen

Light olefins such as ethylene and propylene are important chemical precursors that play an essential role in many chemical syntheses and industrial processes. Common low-temperature distillation technology for separation of olefins from paraffin streams represents one of the most important, but also the most costly, processes in the petrochemical industry. Development of economically viable propylene/propane separation processes is becoming increasingly important, but it is extremely challenging due to the physico-chemical similarities between those two molecules. Among a number of alternatives separation processes, reactive absorption of olefins from an olefin/paraffin mixture using a silver salt solution as absorption liquid may be attractive in this respect. In this thesis, firstly, a brief, outlook overview of the currently light olefin industry and their importance in the modern society is presented, emphasizing the problematic in the propylene/propane gas mixture separations. With the potential use of the room temperature ionic liquids as sustainable reactive media, a suitable gas-liquid contactor and the reactive absorption process perspective, the framework of this thesis in which this novel research is conducted can be established as the process intensification strategy to reach an efficient propylene/propane mixture separation. Room temperature ionic liquids (RTILs) are liquid organic salts, which generally consist of an organic cation and either an inorganic or organic anion. Among other properties, RTILs are non volatile and can be considered as designer solvents. Chapter 2 deals with the equilibrium absorption isotherms of propane and propylene gases in the selected RTILs containing silver ions in order to improve the absorption capacity of propylene and consequently the C3H6/C3H8 selectivity. The individual and gas mixture absorption measurements have been performed in a stainless steel autoclave for gas-liquid contacting at temperatures between 278 K and 318 K and pressures up to 7 bar. The effect in the propylene absorption of different operational variables, namely, silver ion concentration, temperature, and pressure has been consequently analyzed and compared for two RTIL with different structure of the ionic liquid cation. A comparison between Ag+-RTIL and aqueous solutions as reaction media has been established observing a much higher efficient the Ag+-RTIL system. Moreover, the development of a simple mathematical model based on the formation of complex species with different stoichiometry has been presented in order to describe the total propylene absorption. The model parameters, equilibrium constants (KEq,1 f(T) and KEq,2 f(T)) and enthalpies of complexation (¿Hr,1, ¿Hr2,,) were obtained. Reaction media Ag+-RTILs have been successfully used for the experimental measurements after having been completely regenerated by dissociating the propylene from the metal salt complexes using a combination of increased temperature up to 313 K and lower pressure down to 20 mbar of pressure. Thermal stability of the silver ions was analyzed and found to be dependent on the silver salt concentration. With a favourable environmental situation from the thermodynamic point of view, Chapter 3 studies the kinetics of absorption of propylene in Ag+-RTIL media. A kinetic study was carried out in a stirred cell reactor, operated in batch mode using the decreasing pressure method. The volumetric mass transfer coefficient of C3H6 in the liquid phase determined in silver-free RTIL solutions, the enhancement factors due to the chemical reaction under different silver salt concentrations, and the physico-chemical parameters necessary to predict the absorption rate have been experimentally determined. Attending to the experimental measurements and the kinetic regime discrimination criteria, it is possible to assume instantaneous chemical reaction between propylene and silver ions, at silver concentrations higher than [0.25 M]. Therefore, the rate of absorption 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. Once proved the viability of the separation of olefin/paraffin mixtures employing the reactive absorption process with Ag+-RTILs as an efficient reaction media, this dissertation, in Chapter 4, proposes, in order to carry out the separation, two different gas-liquid contactors with high specific area, a membrane contactor and a semibatch stirred tank reactor. The influence of operation variables such as gas feed composition (30%-70% v/v of C3H6), solvent type (aqueous solutions or ionic liquid) and silver salt concentration on the absorption flux of propylene at room temperature has been studied experimental and theoretically trough the mass balances to both contactors obtaining the characteristic parameters of the systems. Finally a comparison in terms of propylene fluxes and overall mass transport coefficient, Koveralla, has been developed, concluding that from the kinetic point of view the stirred tank reactor performed better than the membrane contactor, for the contactors characteristics and experimental operating conditions . However, selecting the right gas-liquid membrane contactor and the optimal operational conditions, this technology can provide an encouraging alternative to the current separation processes, intensifying the process and reducing the separation effective costs in a sustainable way.