Investigations On The Flow Behaviour And Three-Phase Methanol Synthesis In A Trickle-Bed Reactor With Low Wettability

In order to improve the catalyst utilization, complete wetting of the catalyst in trickle-bed reactors is usually desired. However, partial wetting would be more beneficial to improve the reactor performance in case of gas-limited reactions where gas reactants can access the catalyst pores from the externally dry area. In order to decrease the external wetting fraction, one effective way is to change the wettability of the catalyst. Effect of the catalyst wettability on the flow behavior in trickle-bed reactors was investigated. To overcome the heat transfer problem encountered in methanol synthesis in the fixed-bed reactor, a three-phase reaction system was designed by introducing an inert liquid medium, 1-hydroxyethyl-3-methylimidazolium nitrate, which is a room temperature ionic liquid with good thermal stability and hydrophilic property. To prevent wetting of the catalyst by the liquid phase, the catalyst was transformed into hydrophobic through silanization of the support.In this paper, effects of catalyst wettability on the pressure drop and liquid holdup, which are the most important parameters that affect the designing and operation of the trickle-bed reactor, were investigated. The hydrophobic packing was obtained from alumina through surface modification by grafting with trimethoxy (methyl) silane. Hysteresises in pressure drop and liquid holdup were observed no matter the packing was hydrophilic or hydrophobic. However, the hysteresis was less pronounced in the hydrophobic column. In addition, difference in the shapes of the pressure drop profiles indicated that the liquid flow texture was different in these two columns. In the hydrophilic bed, the liquid flow texture varied from rivulets or channels to films when the liquid flow rate increased, while film flow was predominant as the liquid flow rate decreased. In contrast, the liquid flowed in rivulet form in the hydrophobic bed despite the liquid flow rate was increasing or decreasing.Catalyst wetting efficiency in the trickle-bed reactor was measured by dye adsorption method. The catalyst wetting efficiency increases in a very moderate way with liquid flow rate in hydrophilic bed while it does not vary with the liquid flow rate in the hydrophobic bed. Effect of the wettability on the liquid phase residence time distribution, which is closely interlinked with reaction conversion and interfacial mass transfer, was also investigated. The wettable packing gives strongly asymmetric curves, especially at low and moderate liquid flow rates. With a non-wettable packing, the cures are symmetric and the reduced cures are practically the same for all liquid flow rates.Liquid dispersion is one of the crucial parameters affecting the design of liquid distributor. Effect of wettability on the liquid spreading was studied in a laboratory scale column equipped with a liquid collector to measure the liquid flux pattern. With the point source, steady state of the liquid distribution was obtained after 30 cm due to the stronger capillary force in the hydrophilic bed. However, the distance needed to arrive steady state is about 40-50 cm in hydrophobic bed. A stochastic model was developed to describe the liquid radial dispersion in a trickling flow reactor, which had been conventionally modeled by the effective diffusion model. As a transfer process, the matrix of transfer probability was derived based on the coordinate of the particle network in the cross section of the column. With this transfer probability matrix and the initial liquid distribution, liquid distribution in the downstream could be predicted.Hydrodynamic in gas-liquid co-current trickle-bed reactors was simulated with the Eulerian-Eulerian two-fluid CFD approach. As film flow was assumed in the two-fluid model, better agreement with experimental data was obtained for the hydrophilic bed as compared with the hydrophobic one. This can be ascribed to that the liquid flow texture in the hydrophobic column deviates from the film flow, which is assumed in the two-fluid model.To overcome the heat transfer problem encountered in methanol synthesis under high syngas concentrations in the gas phase, a three-phase non-wetted catalytic system was established by introducing an inert liquid medium into the fixed-bed reactor. To form a repellent interface between the liquid and the solid, the catalyst support was modified into hydrophobic, and the liquid medium was selected from a room temperature ionic liquid, which are emerging as a new family of environmentally benign solvents alternative to the conventional organics for most catalytic reactions due to their nonvolatility, with hydroxyl groups. The liquid-solid contact angle was measured to be 106°. On the other hand, the ionic liquid selected has higher solubility of methanol as compared with traditional solvents.With the catalyst prepared and solvent selected as mentioned above, the catalyst surface will no longer be wetted by the liquid. Instead, the catalyst will be covered by the gas film, which leads to the decrease of the mass transfer resistance. For methanol synthesis in a trickle-bed reactor with this non-wettable catalyst-solvent system, the reaction rate could reach 60%-70% of that obtained under gaseous condition, while it was only 10%-20% for the wetted one. This implies that almost gas-phase reaction was conducted with the non-wetted catalyst.