MeOH to DME in bubbling fluidized bed: Experimental and modelling

Abstract

Methanol dehydration over a ZSM-5 containing catalyst was studied in a fluidized bed reactor. At temperatures ranging from 250 to 325 • C, methanol conversion varied from 30% at a contact times of 0.14 s and approached 100% of the equilibrium conversion at a contact time starting from 10 s. Sequential and parallel reactions were negligible at low temperatures while hydrocarbon formation became appreciable at 325 • C. Online gas analysis by mass spectrometry provided real-time measurements at a frequency of 4.4 Hz that allowed for fast determination of steady-state conditions. Gas phase residence time distribution (RTD) measurements indicated that axial dispersion was essentially negligible at short contact times with a shallow bed of catalyst. With longer residence times, the flow pattern could be approximated by six continuously stirred-tank reactors (CSTR) in series. Both the simple 1D hydrodynamic model and a detailed multi-zone fluidized model were used to interpret the experimental data to derive a kinetic expression for the dehydration of methanol to di-methyl ether (DME). The expression includes the reverse reaction that is most often neglected in the literature. The reaction data were best fit with the kinetics based on the 1D model. The fluidized bed is a viable reactor type for kinetic measurements of highly exothermic reactions where hotspots and radial and axial temperature gradients are problematic in fixed beds.La déshydratation du méthanol sur un catalyseur contenant du ZSM-5étaitétudiée dans un réacteurà lit fluidisé.À des températures allant de 250à 325 • C, la conversion du méthanol variaità partir de 30%à un temps de contact de 0,14 s et approchait de 100% de la conversion de l'équilibreà un temps de contact commençantà partir de 10 s. Les réactions séquentielles et parallèlesétaient négligeablesà des températures basses, alors que la formation d'hydrocarbure devenait appréciableà 325 • C. Une analyse du gaz en ligne par spectroscopie de masse a fourni des mesures en temps réelà une fréquence de 4,4 Hz, ce qui permettait une détermination rapide de conditions d'état permanent. Les mesures de la distribution des temps de séjour (DTS) de la phase gazeuse ont indiqué que la dispersion axialeétait essentiellement négligeableà des temps de contact courts avec un lit peu profond de catalyseur. Avec des temps de séjour plus longs, on pourrait approximer le modèle d'écoulement par six réacteursà cuve agitée en continu en série.À la fois le modèle hydrodynamique 1-D simple et un modèle fluidisé multizones détaillé ontété utilisés pour interpréter les données expérimentales afin de trouver une expression cinétique pour la déshydratation du méthanol en méthoxyméthane. L'expression comprend la réaction inverse qui est très souvent négligée dans la littérature. Les données de réaction concordaient le mieux avec la cinétique fondée sur le modèle 1-D. Le lit fluidisé constitue un type de réacteur viable pour les mesures cinétiques de réactions très exothermiques pour lesquelles des points chauds ainsi que des...

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