Methanol-to-olefins in a fluidized bed: Experimental and modelling study

In this thesis the Methanol To Olefin (MTO) reaction has been studied in fixed bed and fluid bed with emphasis on the fluid bed. The catalyst used in this study was based on a ZSM-5 zeolite and was modified with phosphorous to reduce methane formation. The work has been divided into three parts: first the methanol to DME reaction is studied in fluidized bed. Second the MTO process is investigated in fixed bed and last kinetic studies of the MTO reaction is conducted in fluidized bed and a model for the complex reaction is proposed.

The methanol to DME reaction has been studied in a 4.6 mm inner diameter quartz fluidized bed reactor to investigate the reaction kinetics and effect of mass transfer on the reaction.

Two different reactor models have been used for the fluid bed during the kinetic analysis. A two phase fluid bed model with interphase mass transfer and a n-CSTR in series model. A kinetic model for the methanol to DME reaction have been proposed and compared to the literature model of Berĉiĉ and Levee, 1992.

The analysis showed that the proposed kinetics with the nCSTR's in series model gave the best fit to the experimental data and that the predictions of the data at the highest methanol concentrations was excellent. The n-CSTR reactor model gave the best result for both the proposed model and the literature model although the fluid bed model was not significantly worse.

The study showed that kinetic experiment in a small scale fluid bed has advantages for exothermic reactions since uncertainties regarding hotspots or temperature gradients can be eliminated. To be able to use the n-CSTR's in series reactor model the gas velocities have to be kept relatively low. Given the restraints on gas velocities and catalyst inventory the conversion range at a given temperature can be limited.

The methanol to olefin reaction has been studied in small diameter fixed bed reactors with different diameters. Ethylene production paralleled the aromatics formation suggesting that ethylene are formed by splitting off from aromatics (mainly xylenes and trimethylbenzenes) which is in accordance with labelling studies of Svelle et al., 2006. Methane predominated the products at low oxygenate conversion and high WHSV which suggest that methane is formed from methanol and/or DME and not from reactions by higher hydrocarbons.

Deactivation of the catalyst was highly dependent on the space velocity with increasing deactivation as the WHSV was increased. As a result the methanol capacity of the catalyst was dependent on the feed rate. The feed composition also has a high influence on the deactivation and product distribution. Low partial pressure of methanol favours olefin production but at the same time decreases the methanol capacity. This contradicts the findings of Chen et al., 2000 who found that deactivation was based on methanol converted and temperature and not WHSV.

The third part on the thesis treats the MTO reaction in fluidized bed.

A kinetic model has been proposed which is based on the hydrocarbon pool mechanism where the olefins are produced through reversible reactions with a larger hydrocarbon species. Methane is considered to be formed from DME in accordance to the findings from fixed and fluidized bed. Aromatics are considered as C9H12 and paraffins as propane both are formed directly from propylene.

For the MTO reaction in a fluidized bed, the two phase model was chosen as the hydrodynamic model. This was because the bed height was generally higher than in the experiments with methanol dehydration resulting in larger bubbles and since the gas velocity spanned a larger range. Further, the two phase model did not perform significantly worse than the n-CSTR model for the methanol dehydration. The model predicts the light olefins fractions well but some problems were observed with the paraffin and C₆₊ fractions. The latter because aromatic, paraffins and olefins are lumped together which have different reaction patterns. (Abstract shortened by UMI.)