Johan J. Heiszwolf

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.

New non-traditional multiphase catalytic reactors based on monolithic structures

Also in multiphase applications, monolithic catalytic reactors have a large potential. In many respects they outperform the conventional reactors such as slurry and trickle bed reactors. Monoliths can play a role in process intensification because they allow exceptionally large rates and selectivities. They can also be used in multifunctional reactors.

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.

Gas and liquid phase distribution and their effect on reactor performance in the monolith film flow reactor

Nuclear magnetic resonance imaging (MRI) has been applied to study the phase distribution in the monolith film flow reactor. The accumulation of the liquid in the corners of the square channel with an arc-shaped gas–liquid interface has been determined. The average liquid saturation is in good agreement with model calculations. Non-uniformities of the liquid distribution over the four corners of the square channel were apparent, besides the maldistribution over the cross-section of the monolith. Computational fluid dynamics (CFD) calculations applying the measured liquid distribution predict a broadening of the residence time distribution and a shorter break-through time, due to the maldistribution, which is in very good agreement with experimental results. The impact on the modeled gas–liquid mass transfer performance seems to be negligible, due to the nearly linear relation between kGLaV and uLs.

Selection and development of a reactor for diesel particulate filtration

Abstract The diesel engine is an efficient power generator but its exhaust gas needs to be cleaned. A diesel particulate filter, in fact a multi-phase multi-purpose chemical reactor for environmental protection, can lower the emission of diesel soot particles. The aim is to develop a diesel particulate filter that is robust, dependable, energy efficient and resistant to plugging. Moreover, ultrafine particles should be trapped and no poisoning gases should be emitted. A strategic approach is used for the design of a diesel particulate filter, inspired by the method of Krishna and Sie (1994) Chemical Engineering Science 49, 4029–4065, for multiphase reactor selection. The method involves three strategic levels for reactor selection: catalyst design, heat and mass injection and dispersion, and hydrodynamic flow regime; in this paper, the emphasis is on hydrodynamics. This approach has led to the design of a novel filter type for diesel soot filtration: the turbulent precipitator with foam collector plates. In this filter the gas flow is divided over two zones with different hydrodynamic characteristics: fast gas flow in an open channel and slow gas flow in stagnant zones. The open channel enables low pressure drop and prevents plugging, the stagnant zones enable deposition of diesel soot particles and, if desired, the placement of the catalytic material. The results for two different geometries of the turbulent precipitator are presented, they indicate that 90 ppi ceramic foam collector plates perform the best and that it is possible to tune different turbulent precipitators for different diesel engines. Computational fluid dynamics can be used to optimize the turbulent precipitator because it identifies the two hydrodynamic zones in the filter.

Gas–liquid mass transfer in an internally finned monolith operated countercurrently in the film flow regime

Gas-liquid mass transfer has been determined in internally finned monoliths (IFM) by desorbing oxygen from a falling water film. Measurements were performed for various monolith lengths to quantify the inlet effects caused, among others, by the development of the concentration profile. The local volumetric mass transfer coefficients were found to vary from approximately 0.04 (s -1 ) at the inlet to about 0.01 (s -1 ) in the downstream sections. A theoretical model has been derived, assuming that the water flow can be treated as a film falling down in a corner. By averaging the radial mass transfer perpendicular to the film surface, mass transfer can be predicted within reasonable accuracy. On comparing the film in the IFM with a film along a vertical plane, it was found that in an IFM the concentration profile develops faster while mass transfer is less effective in the developed sections.

Pressure Drop of Taylor Flow in Capillaries: Impact of Slug Length

In a single capillary, the frictional two-phase pressure drop in Taylor flow has been measured using various liquids, and a correlation to predict the friction factor has been developed. A carefully designed inlet section for the capillary allowed the independent variation of gas bubble and liquid slug length. Gas and liquid superficial velocities were varied in the range 0.04–0.3 m/s. If the slug length was lower than 10 times the capillary diameter, the frictional pressure drop in the liquid slug increased drastically from the single phase limit (f = 16/Re). The slug length dependence is caused by a larger contribution to the pressure drop of the end effects near the bubble caps. Increased pressure drop at the ends of the slug is caused by two separate effects: (1) near the bubbles the circulation inside the liquid slug induces extra friction, and (2) the difference in curvature of the gas-liquid interface at the front and at the rear of the bubble gives rise to extra pressure drop. The use of different liquids allowed the independent variation of the Reynolds number Re and the Capillary number Ca, and an expression for the frictional pressure drop as a function of Re, Ca and the slug length was developed. The results of this work allow the determination of slug length from pressure drop measurements in closed equipment where the slug length cannot otherwise be measured easily. The applicability of the pressure drop model to estimate mass transfer is demonstrated by combined pressure drop and gasliquid measurements in a monolith, which is essentially an array of capillary channels.© 2003 ASME