Michiel T. Kreutzer

Mass transfer characteristics of three-phase monolith reactors

The mass transfer characteristics of monolith reactors in gas–liquid–solid applications have been investigated. A model is proposed which gives correlations based on di2erent mass transfer steps for Taylor 4ow in capillaries. The fast hydrogenation of � -methylstyrene over Pd monolith catalysts of di2erent cell densities (200–600 cpsi) was used as a test reaction in a pilot reactor. The reaction could be carried out in the internal and external mass transfer limited regime, as has been veri6ed by a change in activation energy plots. Depending on the operating conditions, the overall mass transfer group kova ranged from 0.5 to 1: 5s −1 , which shows that excellent mass transfer can be achieved for gas–liquid–solid reactions in monolith reactors. ? 2001 Elsevier Science Ltd. All rights reserved.

Monolithic catalysts as efficient three-phase reactors

Monolithic reactors are in many cases an attractive alternative to conventional multi-phase reactors. Advantages are the low-pressure drop, the absence of a need for a catalyst separation, and the large geometrical surface area. The main disadvantage is, however, the lack of practical experience with monolithic reactors. A pilot-scale study has been carried out to overcome this disadvantage and to evaluate the performance of a monolithic reactor. The results of this study have demonstrated that a higher productivity or a higher selectivity can be obtained, when a monolithic reactor is used instead of a trickle-bed reactor.

Hydrodynamic aspects of the monolith loop reactor

Abstract In this paper, the so-called monolith loop reactor is introduced. In this reactor, liquid is circulated over a monolith catalyst using a pump while gas-phase circulation is obtained by the pressure difference across the reactor. A model is derived to calculate the liquid hold-up inside the monolith as a function of the liquid circulation flow rate. Gas–liquid mass-transfer measurements in a monolith are compared to a model for capillaries. A combination of the hydrodynamic and the mass transfer models leads to predicted kLa values as a function of the power input. The performance of the monolith loop reactor is compared to bubble columns and stirred-tank reactors.

Gas–liquid mass transfer of aqueous Taylor flow in monoliths

Abstract The gas–liquid mass transfer of a monolith operating in the Taylor flow regime is presented. Mass transfer measurements are compared with a literature model derived for single capillaries. The comparison resulted in a prediction of the unit cell length ( gas bubble + liquid slug ) . Independent measurements of the liquid slug length showed that the predicted unit cell length is close to the measured ones. This leads to the conclusion that mass transfer models for single capillaries may indeed be used for monoliths. Additionally, it is shown that the liquid slug length may also be estimated from pressure drop measurements.

Monolithic catalysts as more efficient three-phase reactors

Abstract Monolithic reactors are an attractive alternative to conventional multi-phase reactors. Advantages are the low pressure drop, the absence of a need for a catalyst separation, and the large geometrical surface area. The main disadvantage is, however, the lack of practical experience with monolithic reactors. A pilot-scale study has been carried out to evaluate the performance of a monolithic reactor on a larger scale. The results of this study have demonstrated that a higher productivity can be obtained for a monolithic reactor compared to a trickle-bed reactor for a solid catalysed gas–liquid reaction that is mass transfer limited in the gas-phase reactant. A higher selectivity has been demonstrated for the selective hydrogenation of benzaldehyde towards benzylalcohol when a monolithic reactor is used instead of a trickle-bed reactor as a result of the narrower residence-time distribution.

Is a monolithic loop reactor a viable option for Fischer-Tropsch synthesis?

A Monolithic Loop Reactor design for Fischer-Tropsch synthesis with a production capacity of 5000 ton middle distillates per day (about 40000 bbl/day) is presented. The required volume, 3350 m 3 , is competitive with conventional reactors, while eliminating disadvantages of existing reactor types such as catalyst attrition and separation, backmixing and large diffusion distances. Although the kinetic expressions used in this study do not allow calculating the selectivity precisely, all important conditions, low temperature rise and constant H 2 /CO ratios, are met to ensure high selectivity towards heavy hydrocarbons.

Mechanism of Laccase–TEMPO-Catalyzed Oxidation of Benzyl Alcohol

The oxidation of benzyl alcohol by air, catalyzed by the organocatalyst TEMPO and the enzyme laccase has been investigated. To establish the kinetically significant pathways and corresponding kinetic parameters, a series of experiments is conducted with synthesized stable oxidized and reduced forms of the organocatalyst, the oxoammonium cation, and hydroxylamine. The time course of TEMPO and its oxidized and reduced derivatives is monitored off line by a combination of GC analysis, UV/Vis spectroscopy, EPR spectroscopy, and FTIR spectroscopy. TEMPO is found to be regenerated through noncatalyzed comproportionation of the oxoammonium cation with hydroxylamine. A kinetic model is presented based on the experimentally determined kinetically significant pathways. The time dependences of the concentrations of the three redox states of TEMPO and benzyl alcohol are adequately described by the model. The results provide new leads for the development of a practical process for a combined laccase–TEMPO‐catalyzed selective oxidation of alcohols.

All-aqueous core-shell droplets produced in a microfluidic device

We present a microfluidic method to compartmentalize aqueous polymer solutions within water-in-water microdroplets, which are continuously generated without using organic solvents or surfactants. Phase separation inside the drops yields all-aqueous core-shell structures (water-in-water-in-water), as we demonstrate using the aqueous two phase system of polyethylene glycol and dextran.

Heterogeneously Catalyzed Continuous-Flow Hydrogenation Using Segmented Flow in Capillary Columns.

Segmented flow in standard GC capillary columns, with a heterogeneous Pd catalyst on the walls, gave rapid information about catalytic processes in them. The residence time and conversion was monitored visually, greatly simplifying bench-scale optimization. Examples show the benefits of the elimination of pore diffusion and axial dispersion. Further, we demonstrated how to quickly identify deactivating species in multistep synthesis without intermediate workup.

Slow growth of the Rayleigh-Plateau instability in aqueous two phase systems

This paper studies the Rayleigh-Plateau instability for co-flowing immiscible aqueous polymer solutions in a microfluidic channel. Careful vibration-free experiments with controlled actuation of the flow allowed direct measurement of the growth rate of this instability. Experiments for the well-known aqueous two phase system (ATPS, or aqueous biphasic systems) of dextran and polyethylene glycol solutions exhibited a growth rate of 1 s(-1), which was more than an order of magnitude slower than an analogous experiment with two immiscible Newtonian fluids with viscosities and interfacial tension that closely matched the ATPS experiment. Viscoelastic effects and adhesion to the walls were ruled out as explanations for the observed behavior. The results are remarkable because all current theory suggests that such dilute polymer solutions should break up faster, not slower, than the analogous Newtonian case. Microfluidic uses of aqueous two phase systems include separation of labile biomolecules but have hitherto be limited because of the difficulty in making droplets. The results of this work teach how to design devices for biological microfluidic ATPS platforms.