Simultaneous etherification of C4 and C5 iso-olefins with ethanol over acidic ion-exchange resins for greener fuels

Abstract

The use of tertiary alkyl ethers as gasoline components has been gaining relevance in the last decades because they can enhance the gasoline octane rating at the time that they reduce the environmental impact of combustion processes, being therefore considered as environmentally friendly additives. Methyl tert-butyl ether (MTBE) and ethyl tert-butyl ether (ETBE) are the most produced ethers at industrial scale. More recently, ETBE market is acquiring a predominant role compared to MTBE, due to environmental concerns associated with MTBE usage and because ETBE can be manufactured from bioethanol, what confers a bioether character. Apart from MTBE and ETBE, other ethers as tert-amyl methyl ether (TAME) and tert-amyl ethyl ether (TAEE) are interesting alternatives since they can be manufactured from C5 reactive olefins present in gasoline that present drawbacks as high blending vapor pressure or high potential of tropospheric ozone formation. Similar to ETBE, TAEE can be considered as a bioether since bioethanol can be used as reactant. In this sense, ETBE and TAEE can be envisaged as the most promising alternatives as gasoline additives to fulfill the progressively stricter legislation while being compatible with current engines and therefore, a plausible solution at medium-short term.

The optimization and integration of chemical processes is an incentive for industrial plants since several benefits can be obtained as savings in operating and maintenance costs. The simultaneous production of several ethers in the same reaction unit can be a clear example of such technology. A simultaneous etherification unit yielding a mixture of ETBE and TAEE would be one of the most interesting configurations. To the best to our knowledge, such a process has not been yet studied at bench scale. The present PhD thesis is focused on the study of the feasibility of producing ETBE and TAEE as one pot synthesis in the same reaction unit since the industrial interest of such ethers implies a realistic possibility of implementation in the near future.

The main topics covered and assessed throughout the present manuscript are: the comparison of different chemical pathways to evaluate the process feasibility, the optimization of experimental conditions that maximize etherification yields, the study of the effect of water presence on the ethanol used as reactant, the characterization of potential byproducts and how to avoid side reactions, the study of the chemical equilibrium and the implicit thermodynamics of involved reactions, the assessment of several catalysts in order to find the best catalyst and the catalytic properties influencing the observed catalytic activity, the study of the intrinsic kinetics of the main reactions involved in order to find a reliable kinetic model, the study of the adsorption equilibrium of involved species in a potential catalytic surface and finally, the study of the catalyst deactivation process caused by the presence of acetonitrile in the feed stream.

As for the main results, the optimum chemical pathway for producing ETBE and TAEE has been found. Reactants equilibrium conversions and selectivity have been obtained over a wide range of experimental conditions and etherification yields have been empirically modeled and optimized to obtain the experimental conditions that maximize them. Equilibrium constants and thermodynamic state functions of main reactions involved have been experimentally determined, resulting that main etherification reactions are all of exothermic nature. Amberlsyt™35 has been obtained as the best catalyst among evaluated ones. The relation between acid capacity and volume of swollen polymer have been found as the catalyst properties with more influence on the observed catalytic activity. Reliable kinetic models including activation energies and adsorption parameters have been obtained for the main reactions. An Eley-Rideal mechanism has been deduced as the most probable for the reaction system studied. Adsorption equilibrium constants of species on Amberlsyt™35 have been determined in liquid and gas phase. Adsorption of olefins and ethers are comparable and notably lower than that of alcohols. Also diffusivities of species have been estimated. Finally, the catalyst poisoning is enhanced by the concentration of acetonitrile and temperature and a first order kinetic law has been found to describe better the deactivation process.

El uso de éteres terciarios como aditivos para las gasolinas ha estado ganando relevancia en las últimas décadas ya que además de aumentar el número de octano de la mezcla, reducen el impacto medioambiental generado por la quema del combustible. La optimización e integración de procesos químicos es un incentivo para muchas plantas industriales ya que se pueden obtener cuantiosos beneficios en los costes de operación y mantenimiento. La producción simultanea de varios éteres terciarios en la misma unidad podría considerarse un ejemplo claro de esta tecnología. Una unidad de eterificación simultanea de isobuteno e isoamilenos con etanol para producir una mezcla de etil terc-butil éter (ETBE) y terc-amil etil éter (TAEE) es una configuración industrial muy interesante y que hasta el momento no se ha planteado ni estudiado a escala de laboratorio. La presente tesis doctoral se basa en el estudio de la viabilidad de la producción simultánea de ETBE y TAEE en la misma unidad de reacción ya que el interés industrial y medioambiental la hace una alternativa plausible y realista que podría ser una realidad industrial a corto medio plazo.

Los principales temas abordados en la tesis son: la comparación de diferentes rutas químicas con el objetivo de elucidar la ruta más verde desde un punto de vista medioambiental; la optimización de las condiciones experimentales que permiten maximizar el rendimiento a éteres; el estudio del efecto de agua como potencial impureza del etanol a usar como reactante; el estudio de las constantes de equilibrio y propiedades termodinámicas de las reacciones implicadas; la evaluación de varios tipos de catalizadores con el objetivo de encontrar un catalizador potencial para el proceso e indagar en las propiedades morfológicas que más influencia presentan sobre la actividad catalítica observada; un estudio cinético exhaustivo con el fin de elucidar información importante sobre la cinética del proceso y los posibles mecanismos de reacción implicados en el mismo; el estudio de la adsorción de las especies implicadas sobre un catalizador potencial con el fin de evaluar las constantes de equilibrio y propiedades de adsorción termodinámicas de los diferentes compuestos químicos; y finalmente, el estudio de la desactivación del catalizador por la presencia de posibles venenos en las corrientes de alimento de reactantes.