Theodore T. Tsotsis

Statistical and continuum models of fluid-solid reactions in porous media

In this review, we discuss past theoretical works on fluid-solid reactions in a porous medium. Such reactions are often accompanied by a continuous alteration of the pore structure of the medium, and at high conversions they exhibit percolation-type behavior, i.e. the solid matrix of the medium and/or the fluid phase lose their macroscopic connectivity. These phenomena are, therefore, characterized by a percolation threshold which is the volume or area fraction of a phase (solid or fluid) below which that phase exists only in isolated clusters or islands. Important classes of such processes are acid dissolution of a porous medium and gas—solid reactions with pore volume growth, e.g. coal gasification, and with pore closure, e.g. lime sulfation, and catalyst deactivation. These processes are characterized by continuous changes in the pore space as a result of a chemical reaction. We also consider here other processes such as the flow of fines, stable emulsions and solid particles in a porous medium which also alter the structure of the pore space, but by physical interaction of the particles and the solid surface of the pores. In this review we compare two different modelling approaches to reactions accompanied by structural changes. First we review the continuum approach, which is based on the classical equations of transport and reaction supplemented with constitutive equations describing the effect of structural changes on reaction and transport parameters. We then outline the relevant concepts, ideas and techniques of percolation theory and the statistical physics of disordered media, and review their application to the phenomena mentioned above. In particular, we emphasize the fundamental role of connectivity of the porous medium in such phenomena. Since in both approaches one needs to estimate the effective transport properties of the porous medium that is undergoing continuous change, we also review continuum and statistical methods of estimating the effective transport properties of disordered porous media.

Electrochemical reaction dynamics: a review

Abstract We review here the status of research on the dynamics of electrochemical reactions. The electrodissolution of metals, cathodic deposition, and electrocatalytic reactions are discussed. We treat dynamic behavior such as periodic oscillations, bifurcations, chaos, and spatial patterns and also the status of mechanisms and mathematical models, which explain the dynamics.

Catalytic membranes and membrane reactors

Introduction. Catalytic Membrane Separation Processes. Pervaporation Membrane Reactors. Membrane Bioreactors. Modelling of Membrane Reactors. Economic and Technical Feasibility Issues of Membrane Reactor Processes. Conclusions. Index.

A study by in situ techniques of the thermal evolution of the structure of a Mg–Al–CO3 layered double hydroxide

Abstract Several in situ techniques have been used to investigate the thermal evolution of the structure of a Mg–Al–CO 3 layered double hydroxide (LDH) under an inert atmosphere. Based on the results of the study, a model is proposed to describe the structural evolution of the Mg–Al–CO 3 LDH. According to this model as the temperature is increased, loosely held interlayer water is lost in the temperature range of 70–190°C, but the LDH structure still remains intact. The OH − group, likely in a Al–(OH)–Mg configuration, begins to disappear at 190°C, and is completely lost at 280°C; a gradual transformation of the LDH structure begins in the same range of temperatures. The OH − group, likely in a Mg–(OH)–Mg configuration, begins to disappear at 280°C and is completely lost at 405°C; a gradual degradation of the LDH structure is observed in the same range. Although some CO 3 2− loss is observed at lower temperatures, its substantial loss begins at 410°C, and is completed at 580°C. At these temperatures the material becomes an amorphous metastable, mixed solid oxide solution.

A percolation model of catalyst deactivation by site coverage and pore blockage

Abstract The problem of catalyst deactivation by active site poisoning and pore blockage, under globally kinetic control, is analyzed. The catalyst pore space is represented by a three-dimensional network of interconnected pores. As a result, the effect of morphological properties of the catalyst pore space, i.e., its geometry (pore size distribution) and topology (connectedness), on the deactivation process is investigated, for the first time, simultaneously. The concepts of percolation theory, a modern theory of statistical physics of disordered media, are employed to show that both single-pore and bundle of parallel pore models perform rather poorly and that the interconnectivity of the pores plays a fundamental role in the overall catalytic behavior. The extension of the model to more complicated systems is also discussed.

A continuous pervaporation membrane reactor for the study of esterification reactions using a composite polymeric/ceramic membrane

The esterification reaction between acetic acid and ethanol was studied in a continuous flow pervaporation membrane reactor utilizing a polymeric/ceramic composite membrane. For a range of experimental conditions reactor conversions were observed which are higher than the corresponding calculated equilibrium values. This is due to the ability of the membrane to remove water, a product of the reaction. A theoretical model has been developed which gives a reasonable fit of the experimental results.

A high temperature catalytic membrane reactor for propane dehydrogenation

Abstract High temperature catalytic membrane reactors are attracting renewed interest. This surge in research activity is motivated by the development of good quality ceramic membranes. In this paper, results are presented of a study of the propane dehydrogenation reaction in a membrane reactor utilizing a sol-gel alumina membrane for feed mixtures containing significant amounts of propylene and hydrogen. Yields to propylene are reported which are higher than the corresponding equilibrium yields at the same temperature and pressure conditions.

A high temperature catalytic membrane reactor for ethane dehydrogenation

A high temperature catalytic membrane reactor, containing a Pt impregnated alumina ceramic membrane tube in a shell-and-tube configuration, was used to study dehydrogenation reactions. Experiments in this membrane reactor in the temperature range of 450–600°C, with the ethane dehydrogenation reaction to produce ethylene, show reactor conversions up to 6 times higher than equilibrium conversions. This shift in equilibrium is due to the selective permeation of one of the reaction products, i.e. hydrogen, according to Knudsen diffusion. In the experiments we have utilized a trans-membrane pressure difference and an inert sweep gas on the low pressure side of the membrane.

Design issues of pervaporation membrane reactors for esterification

Esterification reactions are typically limited by thermodynamic equilibrium, and face challenges with product purification. Commercially, they are carried out using either large excess of one of the reactants, or by removing through reactive distillation one of the products. The former is a relatively inefficient approach because it requires a large reactor volume. As a result reactive distillation, which favorably shifts equilibrium through the removal of one of the products, is becoming more common in plant-scale production. It is, however, an energy-demanding operation and is not recommended when dealing with temperature-sensitive chemicals or biocatalysts. The aforementioned difficulties have motivated efforts for the development of other coupled reactive/separation processes. Pervaporation membrane reactors (PVMR), in particular, are receiving increased attention as a potentially competitive alternative to reactive distillation. In this paper, we present a model that we have developed to describe PVMR behavior. The simulation results of the model have been validated with experimentally observed pervaporation membrane reactor conversions. The model is used to describe a number of alternative PVMR configurations and analyze the factors that affect and optimize their performance.

Packed bed catalytic membrane reactors

Abstract Membrane reactor studies of the ethane dehydrogenation and methane steam reforming reactions are discussed. A general catalytic packed bed membrane reactor model is also presented.