Bernard Allen Toseland

Kinetics and modelling of dimethyl ether synthesis from synthesis gas

The kinetics of the dual catalytic methanol and dimethyl ether (DME) synthesis process over a commercial CuO/ZnO/Al2O3 (methanol forming) and a γ-alumina (dehydration) catalyst have been investigated at 250°C and 5 MPa using a gradientless, internal-recycle-type reactor. A kinetic model for the combined methanol+DME synthesis based on a methanol synthesis model proposed by Vanden Bussche and Froment (1996) J. Catal., 161, 1–10) and a methanol dehydration model by Bercic and Levec (1993) Ind. Engng Chem. Res., 31, 1035–1040) has been tested using results obtained from a wide range of CO2 : CO feed ratios. Results at different COx : H2 ratio and catalyst loading ratios were also obtained. Catalyst deactivation was observed during DME synthesis at high space velocities and a large ratio of dehydration catalyst.

Kinetic understanding of the chemical synergy under LPDMETM conditions—once-through applications

The single-step process for conversion of syngas to DME gives higher conversion than the syngas-to-methanol process. This arises because of a synergy among the three simultaneous reactions, methanol synthesis, methanol dehydration and water gas shift, in the process. This paper analyzes the role each reaction plays in the synergy using both experimental and simulated results. Under typical industrially relevant conditions using a commercially available catalyst system, the reaction system is far away from the thermodynamic equilibrium, and the rates of the reactions are the prime determinant in realizing the beneficial effects of the synergy. Furthermore, feed gas composition strongly affects the potential benefit of the synergy on the process. The implications of this understanding for research and process applications are discussed.

Gas holdup distributions in large-diameter bubble columns measured by computed tomography

Abstract Using the computed tomography (CT) and computer automated radioactive particle tracking (CARPT) facilities at the Chemical Reaction Engineering Laboratory (CREL), time-averaged gas holdup distributions and liquid recirculation velocities were measured in a 44 cm diameter bubble column for air–water and air–drakeoil systems at 2, 5, and 10 cm/s superficial gas velocities, which cover bubbly, transition and churn-turbulent flow regimes, respectively. Gas holdup was found to increase only slightly with the increase in axial distance from the distributor, but increased significantly with the increase in superficial gas velocity, as expected. A lower gas holdup was observed in the air–drakeoil system than in the air–water system. This could be predominantly attributed to the formation of large bubbles in the former case due to the higher viscosity of drakeoil (approximately 0.03 Pas (=30 cp)). At high superficial gas velocities, the time-averaged cross-sectional gas holdup distributions were almost symmetric for both air–water and air–drakeoil systems. However, at 2 cm/s superficial gas velocity, an asymmetry in the holdup distribution was observed, which manifested itself in an asymmetric liquid recirculation pattern. At all gas velocities, the radial gas holdup distribution for the air–water system was steeper than that for the air–drakeoil system, yielding steeper radial liquid velocity profiles. Comparison of the gas holdup obtained in the 44 cm diameter column and that obtained in a 10 cm diameter column is discussed.

Hydrodynamics of churn turbulent bubble columns: gas–liquid recirculation and mechanistic modeling

Abstract A phenomenological (mechanistic) model has been developed for describing the gas and liquid/slurry phase mixing in churn turbulent bubble columns. The gas and liquid phase recirculation rates in the reactor, which are needed as inputs to the mechanistic reactor model are estimated via a sub-model which uses the two-fluid approach in solving the Navier–Stokes equations. This sub-model estimates the effective bubble diameter in the reactor cross-section and provides a consistent basis for the estimation of the volumetric mass transfer coefficients. The strategy for the numerical solution of the sub-model equations is presented along with the simulation results for a few cases. The overall reactor model has been tested against experimental data from radioactive gas tracer experiments conducted at the Alternate Fuels Development Unit (AFDU), La Porte, TX under conditions of methanol synthesis.

Comparison of single- and two-bubble class gas–liquid recirculation models — application to pilot-plant radioactive tracer studies during methanol synthesis

Abstract Radioactive gas tracer measurements conducted during liquid-phase methanol synthesis from syngas in a pilot-scale slurry bubble column at the alternate fuels development unit (AFDU), La Porte have been compared with simulations from two mechanistic reactor models — single-bubble class model (SBCM) and two-bubble class model (TBCM). The model parameters are estimated from an independent sub-model gas and liquid recirculation, and the long-time-averaged slip velocity between the gas and liquid/slurry in the column center can be as high as 50– 60 cm / s depending on the operating conditions. Comparison of experimental data with simulation results from the two models indicates that accurate description of interphase gas–liquid mass transfer is crucial to the reliable prediction of tracer responses. Coupled with a correct description of gas and liquid recirculation, the models presented here provides a simple and fundamentally based methodology for design and scale-up of bubble column reactors.

A novel mechanism of catalyst deactivation in liquid phase synthesis Gas-to-DME reactions

Commercial methanol synthesis catalysts and γ-alumina are stable when used separately in a slurry phase reactor for the methanol synthesis and the methanol dehydration reaction, respectively. However, when they are used as a physical mixture for our slurry-phase synthesis gas-to-DME process, both catalysts deactivate rapidly. Traditional causes of catalyst deactivation, including hydrothermal sintering, leaching, coking, and poisoning, were investigated and subsequently ruled out. Detrimental interaction between the two catalysts was identified as the cause of the rapid, simultaneous deactivation of both catalysts. Intimate solid-state contact between the two catalysts is necessary for this interaction to take place. Most likely, the interaction is due to the migration of Zn-and Cu-containing species from the methanol catalysts to γ-alumina. Although not discussed in this paper, a proprietary dual catalyst system with good stability has been developed.

Chapter 1 – Measurement techniques for local and global fluid dynamic quantities in two and three phase systems

Publisher Summary This chapter discusses the measurement techniques for local and global fluid dynamic quantities in two and three phase systems. The emphasis is on techniques that can be utilized under conditions of interest in practice, such as high pressure and temperature and large solids holdup. Gas holdup and solids concentration measurement methods can be classified into two categories: those providing an overall or global measurement and those that provide a local or point measurement. The global measuring techniques yield information on the line, area or volume averaged gas or solids holdup. The volume averaged or the overall fractional holdup of a phase is defined as the fraction of the volume of the multiphase dispersion that is occupied by that phase. In general, the measurement of the overall holdup is relatively simple. It provides information regarding what fraction of the system volume is occupied by the phase of interest. Measurement of the phase fraction at a point is in itself not of much use unless the distribution of such point measurements is obtained in space. The knowledge of the volume occupied by the phase of interest as well as the volume of the expanded multiphase dispersion suffices for the needed calculation. For three phase systems no single method can provide both the solids and overall gas holdup. The expansion of solids (when larger solid particles are used) because of the fluidization by either liquid or gas can be measured in a manner similar to the bed expansion method.

Measurement techniques for local and global fluid dynamic quantities in two and three phase systems

Available measurement techniques for evaluation of global and local phase holdups, instantaneous and average phase velocities and for the determination of bubble sizes in gas-liquid and gas-liquid-solid systems are reviewed. Advantages and disadvantages of various techniques are discussed. Particular emphasis is placed on identifying methods that can be employed on large scale, thick wall, high pressure and high temperature reactors used in the manufacture of fuels and chemicals from synthesis gas and its derivatives.