M.E.E. Abashar

Investigation of low temperature decomposition of ammonia using spatially patterned catalytic membrane reactors

A mathematical model is used to simulate a bench-scale membrane reactor for the decomposition of ammonia over Ni/Al2O3 catalyst. Since the reaction is equilibrium limited the removal of the product hydrogen by the membrane derives the thermodynamic equilibrium. For further displacement of the thermodynamic equilibrium, auxiliary methanation reactions over Ni/Mg Al2O4 catalyst are used to remove part of hydrogen. The two catalysts are loaded together in different pattern configurations. The purpose of this study is to investigate the effect of catalyst patterns on ammonia decomposition. Optimal conditions are observed and explanations offered. An effective length criterion for the optimal conditions is presented. The results show that the pattern strategies have substantial improvement in the reactor performance in terms of high conversions, low temperatures and reduced mass of the catalyst used. The investigation, although is restricted to two catalyst layers, has uncovered a part of the rich characteristics of this system.

Steam reforming and methanation effectiveness factors using the dusty gas model under industrial conditions

Abstract A rigorous mathematical model based on the dusty gas model is developed to investigate the phenomenon of diffusion and chemical reaction in porous catalyst pellets for the steam reforming and methanation reaction under industrial conditions. Two simplified Fickian type diffusion reaction models are also developed and their results are compared with the results of the rigorous model. Parametric investigations on the single catalyst pellet are carried out using the dusty gas model as well as the simplified models in order to obtain a deeper insight into these complex catalytic gas-solid systems and to explore regions of applicability of the simplified models. The phenomenon of negative effectiveness factors is also investigated.

On the chaotic behaviour of forced fluidized bed catalytic reactors

Abstract The response of a non-isothermal fluidized bed catalytic reactor with consecutive exothermic reactions to high frequency forcing is investigated. The present investigation concentrates on the effect of the forcing amplitude on the behaviour of the system at a number of chosen positions of the centre of forcing. The behaviour of the system over the wide range of forcing amplitudes covered is found to be very sensitive to the position of the centre of forcing relative to the homoclinical orbit (infinite period bifurcation point) of the unforced system. A new type of intermittency with two laminar phases of different periodicities has been uncovered, analysed and is shown to form a transitional region between two ordinary intermittencies each with its single laminar channel. Different mechanisms of the transition between chaotic regions and periodic windows has been indentified and analysed in some details, these mechanisms include: period doubling, period halving, period adding and tangent bifurcation. The system exhibited one-dimensional stroboscopic maps which resemble the logistic map, the Rose-Hindmarsh model maps as well as the experimental one-dimensional map of bromide concentration for the Belousov-Zhabotinsky reaction in a continuous stirred tank reactor.

Investigation of Methane–Steam Reforming in Fluidized Bed Membrane Reactors

The theme of this work is to investigate the performance of cocurrent and countercurrent fluidized bed membrane reactors for steam reforming of methane. A mathematical model is used to simulate the experimental data and to explore the potential application of high flux membranes to enhance the methane conversion in fluidized bed membrane reactors. It has been shown that complete conversion of methane is achieved by implementing high flux membranes. The influence of some key parameters on the reactors performance has been reported. It was found that in most cases of large bed lengths, the countercurrent configuration is superior to cocurrent configuration and more sensitive to the parameters changes. However, for short bed lengths at relatively low temperatures the cocurrent configuration is superior to countercurrent configuration.

Integrated catalytic membrane reactors for decomposition of ammonia

A model is presented for the simulation of an integrated catalytic membrane reactor for the removal of toxic ammonia traces from coal gasification streams. Heat integration and equilibrium shifting are achieved by introducing integrated reaction network and various catalyst pattern strategies. Different configurations of spatially patterned catalytic beds (layers) have been investigated. Two important reactions were considered namely, the decomposition of ammonia as a primary reaction and methanation reactions as secondary reactions. An effective length criterion is used to evaluate the performance of the reactor. Optimal conditions were observed and explanations offered. The results show substantial improvement in the reactor performance in terms of high conversions, low temperatures and reduced mass of the catalyst used. The investigation, although is restricted to two catalyst layers, has uncovered some of the rich characteristics of this system.

Chaotic behaviour of periodically forced fluidized-bed catalytic reactors with consecutive exothermic chemical reactions

Abstract The chaotic behaviour of the fluidized-bed catalytic reactor with exothermic consecutive reactions is investigated for the case where the dynamics of the product is very fast and the feed temperature is periodically forced. The investigation concentrates on the less studied case when the centre of forcing is close to the homoclinical orbit of the autonomous system and preliminary results for the effect of the distance between the centre of forcing and the homoclinical orbit are presented. It is shown that period doubling and chaos occur for very small amplitudes of forcing when the centre of forcing is close to the homoclinical orbit. A new type of intermittency with two laminar phases of different periodicities has been uncovered and analysed. The mechanisms for the transition between chaotic regions and periodic windows have been identified and they include period doubling, period halving, period adding and tangent bifurcation. It has been shown that some chaotic regions separate windows which are different in periodicity by one period (period adding), while other chaotic regions separate windows having the same periodicity. The system exhibited one-dimensional stroboscopic maps which resemble a number of the well-known maps of simpler systems.

Bifurcation, instability and chaos in fluidized bed catalytic reactors with consecutive exothermic chemical reactions

Abstract A two-phase model for the non-isothermal fluidized bed catalytic reactor with consecutive exothermic reactions has been used to investigate the bifurcation, instability and chaotic characteristics of this industrially important unit. The investigation, although in a restricted region of the parameter space, has uncovered a good part of the rich dynamic characteristics of this system, including; 2n and 2nk period doubling sequences leading to chaos, banded and fully developed chaos, interior crises, tangent bifurcation leading to intermittency, periodic windows interrupting chaotic regions and alternating periodic-chaotic sequences. Within the chaotic region a new type of periodic windows (termed periodic horns) which differ quantitatively and qualitatively from ordinary periodic windows have been discovered.

Chaotic behaviour of fluidized-bed catalytic reactors with consecutive exothermic chemical reactions

A two-phase model for the non-isothermal fluidized-bed catalytic reactor with consecutive exothermic reactions has been used to investigate the bifurcation, instability and chaotic characteristics of this unit. The investigation, although in a restricted region of the parameters space, has uncovered a good part of the rich dynamic characteristics of this system, including 2n and 2nk period-doubling sequences leading to chaos, banded and fully developed chaos, interior crises, tangent bifurcation leading to intermittency, periodic windows interrupting chaotic regions and alternating periodic-chaotic sequences. Within the chaotic region a new type of periodic windows (termed periodic horns) which differ quantitatively and qualitatively from ordinary periodic windows has been discovered. Lyapunov exponents are also computed for the chaotic attractors in order to prove that they are truly chaotic and not strange non-chaotic attractors.

Mathematical modelling of diffusion and reaction for gas-solid catalytic systems with complex reaction networks. Negative effectiveness factors

The concept of effectiveness factor (@h) as a measure of diffusional limitations for gas-solid catalytic reactions has gone a long way since the time of Thiele. Multiple steady states giving rise to multiple values of (@h) for the same bulk conditions, and @h values greater than unity have been widely reported in the literature in the last three decades. In this paper an interesting phenomenon associated with the effectiveness factors (@h) for industrial gas-solid catalytic reactions is reported, that is the possible occurrence of negative values of @h for certain intermediate components. This physically means that diffusional resistance can also reverse the direction of the net production or consumption of intermediate components in consecutive and/or reversible reaction networks. It is shown both numerically and analytically that the results represent real physical phenomenon and not artefacts resulting from numerical problems or model simplifications. Two industrially important reactions are considered, namely, the steam reforming of natural gas which is a highly endothermic reaction and the partial oxidation of O-xylene to phthalic anhydride which is a highly exothermic reaction.

Application of heat interchange systems to enhance the performance of ammonia reactors

Abstract A rigorous heterogeneous model for adiabatic ammonia reactors is used to explore the application of internal heat exchange theory to cross the adiabatic reaction equilibrium values and to improve the conversion in ammonia reactors. The mathematical model is developed for adiabatic ammonia converters and the resulting two-point boundary value differential equation for the catalyst particles is solved using the orthogonal collocation method. Two simple configurations of reactors with heat interchangers are implemented. An industrial ammonia reactor having three adiabatic beds with Montecatini Edison catalyst and interstage cooling is used as the basis for comparison. The comparison shows that an increase of 13.37% of the overall ammonia conversion is possible. The results presented in this study reveal the potential application of energy optimization and integration in the ammonia industry.