A.A.C.M. Beenackers

Kinetics and Selectivity of the Fischer–Tropsch Synthesis: A Literature Review

A critical review of the kinetics and selectivity of the Fischer–Tropsch synthesis (FTS) is given. The focus is on reaction mechanisms and kinetics of the water–gas shift and Fischer–Tropsch (FT) reactions. New developments in the product selectivity as well as the overall kinetics are reviewed. It is concluded that the development of rate equations for the FTS should be based on realistic mechanistic schemes. Qualitatively, there is agreement that the product distribution is affected by the occurrence of secondary reactions (hydrogenation, isomerization, reinsertion, and hydrogenolysis). At high CO and H2O pressures, the most important secondary reaction is readsorption of olefins, resulting in initiation of chain growth processes. Secondary hydrogenation of α-olefins may occur and depends on the catalytic system and the process conditions. The rates of the secondary reactions increase exponentially with chain length. Much controversy exists about whether these chain-length dependencies stem from differe...

KINETICS OF LOW-PRESSURE METHANOL SYNTHESIS

The kinetics of low-pressure methanol synthesis, starting from CO, CO2 and hydrogen over a commercial CuZnAl catalyst, were studied in a spinning basket reactor at p = 15–50 bar and T = 210–245°C. The results show that methanol can be formed from both CO and CO2. Besides these two reactions the water-gas-shift reaction takes place. Based on these three reactions and a dual-site adsorption mechanism, 48 kinetic rate models are derived. Hydrogen is believed to adsorb dissociatively. The experimental results support this assumption. Based on χ2-statistics and consistency tests a final kinetic rate model is selected. This kinetic model gives a significantly better agreement with the experimental results than kinetic models taken from recent literature.

Mass transfer in gas-liquid slurry reactors

A critical review is presented on the mass transfer characteristics of gas?liquid slurry reactors. The recent findings on the influence of the presence of solid particles on the following mass transfer parameters in slurry reactors are discussed: volumetric gas?liquid mass transfer coefficients (kLa, kGa), liquid-side mass transfer coefficients (kL and kS) and specific gas?slurry contact area (a). The second part of this paper reviews the recent progress in our knowledge and understanding of the enhancement of gas?slurry mass transfer due to the presence of solids. Five different cases are distinguished, i.e. ? enhanced mass transfer by physical adsorption on small particles. ? enhanced mass transfer by fast homogeneous reactions in the slurry, due to inert particles, ? enhanced mass transfer by homogenous reaction in the liquid with dissolving particles, ? enhanced mass transfer due to reactive particles and ? enhanced mass transfer due to catalytic particles in heterogeneous reactive systems. Prospective areas for additional research are identified.

Development of a new photocatalytic reactor for water purification

The purification of water by heterogeneous photocatalysis is one of the most rapidly growing areas of interest to both research workers and water purification plants. Recent literature has demonstrated on a laboratory scale the potential of this promising technology to completely destroy organic pollutants dissolved or dispersed in water into harmless substances. However, to date no viable pilot plant exists using this technology. New reactor design ideas are necessary that must be able to address the two most important parameters, namely, light distribution inside the reactor through the absorbing and scattering liquid to the catalyst, and providing high surface areas for catalyst per unit volume of reactor. In this paper, a new reactor design addressing the solution to both the above problems is proposed for water treatment. The reactor consists of several hollow tubes coated on its outside surface with the catalysts. The hollow tubes have been employed as a means of light delivery to the catalyst. Experiments performed in a reactor containing 54 densely packed hollow tubes of 0.006 m diameter showed promising results. The new reactor aims at developing a technical solution to the design of a commercial photocatalytic reactor. # 1998 Elsevier Science B.V.

Experimental comparison of three reactor designs for photocatalytic water purification

The photocatalytic degradation of formic acid was compared for a suspended system, an immobilized system with a coated wall and an immobilized system packed with coated glass beads. The quantum yields found for the three systems are comparable. Mass transfer limitations only occurred in the tubular reactor with the catalyst coated on the wall. The addition of air directly in this reactor increased the degradation rate by decreasing the mass transfer limitation. In the packed-bed reactor, two bead diameters were compared. The activity of these packed-bed reactors appeared to be comparable for low amounts of titanium dioxide, whereas the packed-bed reactor with the large beads shows a higher activity for high amount of catalyst.

Heterogeneous hydrogenation of vegetable oils: A literature review

Abstract Hardening of vegetable oils is reviewed from an engineering point of view. The present review focuses on kinetics of the hydrogenation and relevant transport and adsorption steps. It aims to contribute to accelerate new research to improve substantially on selectivities in general and a decrease of trans fatty acid content in particular. From a comprehensive literature review, we concluded the absence of reliable, mechanistically based kinetic rate expression. Moreover, transport limitations, both intraparticle and interfacial, cannot be excluded from the vast majority of available experimental data. Therefore, future research should focus on the development of intrinsic kinetic rate expressions, which may subsequently contribute to develop new and improved hydrogenation catalysts.

Modeling the photocatalytic degradation of formic acid in a reactor with immobilized catalyst

A kinetic model for the photocatalytic degradation of formic acid in an immobilized system is presented, including the dependency of the reaction rate on the concentration of formic acid and oxygen, the catalyst layer thickness and the light flux. In the system some external mass transfer limitation occurs which is included in the modeling with experimentally determined values for the mass transfer coefficient of both formic acid and oxygen. The model describes the measurements well. The degradation rate appears to depend linearly on the light intensity. The adsorption of formic acid and oxygen on the catalyst layer appears to play an important role in the degradation rate.

Enhancement of gas-liquid mass transfer by a dispersed second liquid phae

Abstract From experiments in a stirred cell contactor with flat gas-liquid interface, it is shown that the rate of mass transfer of oxygen into an aqueous solution can be enhanced by the presence of small amounts (0.5–1 vol%) of a dispersed organic phase (e.g. hexadecane, decane, decanol or cyclohexane), provided that the droplet diameter is in the order of or smaller than the film thickness ( D/k L ) and the solubility of oxygen in the organic phase is larger than in water. A simple model is proposed that predicts the experimentally observed enhancement factors with reasonable accuracy.

Comparison of the efficiency of immobilized and suspended systems in photocatalytic degradation

The photocatalytic degradation of formic acid in suspended and immobilized systems, with and without oxygen addition, are compared. In the immobilized system, oxygen addition to the reactor appeared to increase the efficiency, not only because oxygen acts as an efficient electron scavenger, but also due to increased mass transfer in this two-phase reactor. This immobilized system had an efficiency comparable to that of the suspended system. The addition of oxygen to the immobilized system appeared to increase the quantum yield with a factor 4, whereas the addition of oxygen to the suspended system hardly had any effect.

Kinetics of the three phase methanol synthesis

The kinetics of the three-phase methanol synthesis, starting from carbon monoxide, carbon dioxide and hydrogen over a commercial Cu-Zn-Al catalyst suspended in squalane as the slurry liquid, were studied in a well-mixed, agitated slurry reactor at p = 15 – 40 bar and T = 210 – 260 °C. A kinetic model is selected, based on a dual-site adsorption mechanism and three reactions: methanol from CO, methanol from CO2 and the water-gas-shift reaction. It turns out that methanol from CO2 is the most important reaction under three-phase conditions. This kinetic model gives a good agreement with the experimental results.