S.S.E.H. Elnashaie

On the non-monotonic behaviour of methane—steam reforming kinetics

Abstract A parameteric study on the rate expression recently developed by Xu and Froment (1989a, A.I.Ch.E.J. 35 , 88-96) is carried out over a wide range of parameters. The analysis of this rate expression has shown a non-monotonic dependence of the reaction rate upon steam partial pressure which explains the contradictions between the rate expressions a available in the literature, that is the prediction of positive, as well as negative effective reaction order with respect to steam. A simplified pseudo-homogeneous model with a constant effectiveness factor, with Xu and Froments rate expression was used to investigate the implications of the non-monotonic kinetics on the performance of steam reformers. The main implication was also checked using a heterogeneous model that represents very closely industrial steam reformers.

Novel circulating fast fluidized-bed membrane reformer for efficient production of hydrogen from steam reforming of methane

The coupling of steam reforming and oxidative reforming of methane for the efficient production of hydrogen is investigated over Ni/Al2O3 catalyst in a novel circulating fast fluidized-bed membrane reformer (CFFBMR) using a rigorous mathematical model. The removal of product hydrogen using palladium hydrogen membranes “breaks” the thermodynamic equilibrium barrier exists among the reversible reactions. Oxygen can be introduced into the adiabatic CFFBMR for oxidative reforming by direct oxygen (or air) feed and through dense perovskite oxygen membranes. The simulations show that high productivity of hydrogen can be obtained in the CFFBMR. The combination of these two different processes does not only enhance the hydrogen productivity but also save the energy due to the exothermicity of the oxidative reforming. Based on the preliminary investigations, four parameters (number of hydrogen membranes, number of oxygen membranes, direct oxygen feed rate and steam-to-carbon feed ratio) are carefully chosen as main variables for the process optimization. The optimized result shows that the hydrogen productivity (moles of hydrogen produced per hour per m3 of reactor) in the novel CFFBMR is about 8.2 times higher than that in typical industrial fixed-bed steam reformers.

Simulation of steam reformers for methane

A model is developed for industrial steam reformers for both top fired and side fired furnaces. The catalyst tube model is a one-dimensional heterogeneous model with intra-particle diffusional resistances. The two point boundary value differential equations of the catalyst pellets are solved using a modified novel orthogonal collocation technique to obtain the effectiveness factor variation along the length of the reactor. The side fired furnace equations are algebraic equations, the top fired furnace equations are two-point boundary value differential equations which are solved using the orthogonal collocation technique. A recently developed more general rate expression is used. The model performance is checked against industrial steam reformers. The model is used to investigate the effect of various parameters on the behaviour of the catalyst tubes and the furnace. The effectiveness factor variation along the length of the catalyst tube is also analysed.

Simultaneous production of styrene and cyclohexane in an integrated membrane reactor

Abstract A rigorous heterogeneous model is used to study the performance of the membrane catalytic reactor for the dehydrogenation of ethylbenzene to styrene. The mathematical model is extended to simulate a novel hybrid configuration which is composed of two catalytic sections separated by hydrogen selective composite membrane for hydrogen separation. One side of the reactor is a dehydrogenation section in which ethylbenzene is dehydrogenated to styrene, while the other catalytic side is a hydrogenation section in which benzene is catalytically converted to cyclohexane. The continuous removal of hydrogen from the dehydrogenation section leads to the shift of equilibrium conversion in this section, thus higher styrene yield is obtained. Detailed parametric investigation has been carried out for the membrane reactor and the hybrid reactor configurations. The effect of co-current and counter-current flow pattern is investigated. Superior performance in terms of ethylbenzene conversion enhancement far above the equilibrium value was observed in the hybrid reactor configuration. The selectivity and styrene yield considerably exceed the industrial values.

Digital simulation of industrial fluid catalytic cracking units—IV. Dynamic behaviour

Abstract A relatively rigorous dynamic model is developed for industrial FCC units and is used to investigate the dynamic characteristics of a typical industrial type IV FCC unit. The investigation covers: numerical difficulties associated with the digital dynamic simulation of this system, extreme sensitivity of the dynamics to initial catalyst activity in the reactor, effect of model dimensionality on the reliability of the model predictions, the behaviour of the system with switching control around the middle unstable steady state as well as the behaviour of the closed-loop system with simple feedback proportional control.

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.

Simulation for steam reforming of natural gas with oxygen input in a novel membrane reformer

The performance of a novel circulating fast fluidized bed membrane reformer (CFFBMR) is investigated using a reliable mathematical model. The removal of product hydrogen using hydrogenpermselective membranes ‘‘breaks’’ the thermodynamic equilibrium in the reversible system and makes it possible to operate the process at lower temperatures. The oxidative reforming of a part of the feed methane by oxygen input into the reformer using direct feed or through oxygen-permeable membranes supplies the heat needed for the highly endothermic steam reforming of methane. The combination of the exothermic oxidative reforming and endothermic steam reforming not only produces high yield hydrogen but also makes it possible to operate the CFFBMR under autothermal conditions. The novel configuration is a highly efficient hydrogen producer with minimum energy consumption. The simulation results show that the hydrogen productivity (moles of hydrogen produced per hour per m 3 of reactor) of the CFFBMR is about 8 times that in an industrial fixed bed and 112 times that in a bubbling fluidized bed membrane reformer. D 2003 Published by Elsevier Science B.V.

Exploration and exploitation of bifurcation/chaotic behavior of a continuous fermentor for the production of ethanol

Abstract A four-dimensional model for the anaerobic fermentation process, developed and used earlier to simulate the oscillatory behavior of an experimental continuous stirred tank fermentor is utilized in the present investigation to explore the static/dynamic bifurcation and chaotic behavior of this fermentor, which is shown to be quite rich. The present investigation is a prelude to the experimental exploration of bifurcation and chaos in a membrane fermentor. Dynamic bifurcation (periodic attractors) as well as period doubling sequences leading to different types of periodic and chaotic attractors have been uncovered. It is fundamentally and practically important to discover the fact that in some cases, periodic and chaotic attractors have higher ethanol yield and production rate than the corresponding steady states.

Digital simulation of industrial fluid catalytic cracking units: bifurcation and its implications

Abstract The mathematical model for fluid catalytic cracking units developed earlier (Elnashaie, S.S.E.H. and El-Hennawi, I.M., 1979, Chem. Engng Sci. 34 , 1113–1121) has been used, after introducing some modifications, to simulate two industrial units. The model results were found to be in good agreement with the plant data. It was found that the plants operate at the middle unstable steady state and the yield can be increased to its maximum value by manipulating some of the operating variables. A parametric study has been conducted to investigate the effect of some parameters on the operating middle steady states, bifurcation behaviour and gasoline yield.

Modeling and basic characteristics of novel integrated dehydrogenation — hydrogenation membrane catalytic reactors

Abstract Considering a catalytic hydrogenation reaction in the shell side instead of the inert sweep gas extends, one step forward, the idea of using catalytic reactors with hydrogen selective membranes for ‘breaking’ the thermodynamic barrier of dehydrogenation reactions towards novel configurations of integrated membrane catalytic reactors (IMCRS). An important member of the class of IMCRS is the integrated catalytic dehydrogenation—hydrogenation (ICDH) reactor, which offers a good number of advantages through its different possible configurations, e.g. co-current, counter-current. Rigorous mathematical models are excellent tools for the exploration of the basic characteristics of such novel configurations. The specific system considered in this paper is the simultaneous catalytic dehydrogenation of ethylbenzene and hydrogenation of benzene coupled through the hydrogen selective membranes. Rigorous models are developed and used to explore the basic characteristics of a number of configurations for a number of catalysts and membranes. The results show that considerable increase in styrene productivity can be achieved in addition to the extra product from the hydrogenation section.