Modelling and simulation of a direct synthesis of dimethyl ether (DME) in a tubular reactor with a hybrid catalyst bed
Advancing the chemical engineering fundamentals
Chemical Reaction Engineering (T2-2P)
Keywords: simulation, zeolite, hybrid catalyst bed, dimethyl ether
The modern method of manufacturing methyl ether uses a hybrid catalyst bed. The method allows for a simultaneous synthesis of methanol, and for its dehydration. This way of proceeding is also advantageous from the viewpoint of reaction thermodynamics. The literature coverage is poor in areas of modelling and simulation of reactors for dehydration of methanol, and in the area of direct synthesis of DME from synthesis gas. In the majority of publications, authors use the mathematical model of the process to estimate parameters of kinetic equations. Missing is the in-depth analysis of the influence of individual technological parameters on the process path.
For a reaction system describing a direct synthesis of methyl ether from carbon dioxide and hydrogen:
CO2+3H2 <-> CH3OH + H2O (1a)
2CH3OH <-> CH3OCH3 + H2O (1b)
CO2 + H2 <-> CO H2O (1c)
a mathematical model was created of an adiabatic tubular reactor with a plug flow. It was assumed that reference reagents are DME and carbon dioxide. For carbon dioxide a change in number of moles was established, as concluded from reactions (1a) and (1c). Using the definition of the reaction progress number running numbers of moles of dimethyl ether and carbon dioxide were established. Defined was the dwell time, t and then a dimensionless time related to the maximum dwell time: t/tmax Using these equations a differential control volume was derived. The mass balance for individual reference compounds and energy balance were written in the form of four ordinary differentil equations.
Finding a solution for the system of equations required selecting kinetic equations of methanol synthesis (1a), a reaction of a non-selective addition of hydrogen to carbon dioxide (1c) and a reaction of dimethyl ether synthesis (1b). Used here was the established in the Ph.D. work (Anna Ptaszek, Modelling of DME direct synthesis, Gliwice, 2004) kinetic equation of methanol dehydration on a zeolite catalyst. Based on the available literature, selected were a methanol synthesis kinetics and kinetics of a reaction (1c) given by Vanden Bussche and Froment (J. Catal. 1996, 161, 1). Integrations of the system of equations were performed for two sets of substrate compositions: y0H2 = 0.8, y0CO2 = 0.2 and y0H2 = 0.6, y0CO2 = 0.2, y0CO = 0.2 and under the following conditions: initial temperture = 500 K and pressure changing from 1 to 5 MPa.
The performed computations showed many interesting effects and the possibility of the optimisation of process.
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Presented Tuesday 18, 13:30 to 15:00, in session Chemical Reaction Engineering (T2-2P).
