L.S. Kershenbaum

Modelling of an indirect internal reforming solid oxide fuel cell

Creation of an autothermal system by coupling an endothermic to an exothermic reaction demands the matching of the thermal requirements of the two reactions. The application under study is a solid oxide fuel cell (SOFC) with indirect internal reforming (IIR) of methane, whereby the endothermic steam reforming reaction is thermally coupled to the exothermic oxidation reactions. A steady-state model of an IIR-SOFC has been developed to study the mismatch between the thermal load associated with the rate of steam reforming at typical SOFC temperatures and the local amount of heat available from the fuel cell reactions. Results have shown a local cooling effect, undesirable for ceramic fuel cells, close to the reformer entrance. The system behaviour towards changes in catalyst activity, fuel inlet temperature, current density, and operating pressure has been studied. Increasing the operating pressure is shown to be an effective way of reducing both the local cooling caused by the reforming reactions and the overall temperature increase across the cell. Simulations for both counter-flow and co-flow configurations have been performed and compared.

Implementation of an inverse-model-based control strategy using neural networks on a partially simulated exothermic reactor

Recently, the use of control strategies based upon inverse process models for non-linear systems has been found promising. The requirement of a true analytical inverse can be avoided when neural network models are used; they have the ability to approximate both the forward and the inverse system dynamics. Although many simulation studies have illustrated the use of neural network inverse models for control, very few on-line applications have been reported. This paper describes a novel implementation of a neural network inverse-model based control method on a experimental system—a partially simulated reactor, designed to test the use of such non-linear algorithms. The implementation involved the control of the reactor temperature in the face of set point changes and load disturbances despite the existence of significant plant/model mismatch. Comparison was also made with conventional PID cascade control in several cases. The results obtained show the capability of these neural-network-based controllers and, incidentally, point out the differences between simulation studies and on-line experimental tests. Since the system in this study was only mildly non-linear, in some cases, the performance was comparable to that achieved by classical controllers while in other cases an improved control was achieved.

Combined reaction and separation in pressure swing processes

Abstract Theoretical studies of a novel reactor which combines pressure swing adsorption (PSA) and chemical reaction are presented; such a reactor is referred to as a pressure swing reactor, or PSR. The design is based on a conventional two-bed PSA process, in which many of the usual cycle configurations for operation are possible, e.g. simple and purge cycles. Each bed contains a mixture of an active catalyst for reaction, and a selective adsorbent for the adsorption of one or more of the reaction species. Theoretical calculations predict that such a process may lead to greater conversions than conventional steady flow reactors, and thus allow a lower temperature of operation for a desired conversion. Furthermore, for equilibrium reactions of the general formaA ⇌bB +cC, and especially when an adsorbent can be chosen such that the adsorption equilibrium constants are in the sequenceHB > [HA,HC], possible improvements over equilibrium conversions are indicated. In this work, theoretical investigations have concentrated on three types of reversible reaction schemes; isomerisation, dissociation/disproportionation, and dehydrogenation; potential advantages of a PSR have been illustrated for these. As a test case, experimental investigations have concentrated on the dehydrogenation reaction of methylcyclohexane to toluene, for which a Pt Al2O3 catalyst was found to have suitable activity at temperatures as low as 450 K. Pulse chromatography experiments have been carried out to scan the high temperature (400 K to 700 K) adsorption properties of methylcyclohexane, toluene and hydrogen on some commercial adsorbents; clay-based adsorbents were found to be particularly suitable for this case, yielding the desired sequence of adsorption strengths.

The operation and modelling of a periodic, countercurrent, solid—liquid reactor

Abstract Several models are proposed for the Cloete—Streat stage-wise solid—liquid reactor in which the solids are periodically transferred from stage to stage countercurrent to the net flow of liquid. The behaviour predicted by the models is compared with experimental data on an ion exchange reaction system. The results illustrate a range of operating conditions for which unsteady-state operation (with periodic solid transfer) is superior to continuous steady-state operation. The various models differ in the treatment of the composition distribution of the ion exchange resin. They include (a) a discretisation of the distribution; (b) use of the continuous distribution function and solution of the resulting hyperbolic partial differential equations; and (c) approximation of the state of the resin by the leading moments of the distribution.

Enhancement of catalytic reaction by pressure swing adsorption

Abstract A theoretical study of an adsorptive reactor which combines multibed pressure swing adsorption and chemical reaction is presented; such a reactor is referred to as a pressure swing reactor, or PSR. Studies have concentrated on an asymptotic case in which there is the ideal propagation of concentration waves within the reactor beds; the method of characteristics was employed in the solution of the governing PSR equations. The studies assessed the effects of operating conditions, and cycle configurations, on the PSR performance. Calculations indicate enhanced reactant conversion when compared to conventional steady state plug flow operation. In particular, for some reversible reactions, substantial improvements over equilibrium yields have been calculated. For example, for the dissociation reaction 2A ⇔ B + C, and where B is the only adsorbing component, approximately two-fold improvements over the equilibrium yield of product B have been predicted. Such reaction enhancement can be attributed to the limitation of the backward reaction, which results from the separation of the product species B and C. In addition to the method of characteristics, a cells-in-series method for the asymptotic case has been developed, and found to yield calculations consistent with the method of characteristics solutions. In a third numerical approach, the spatial discretisation technique of orthogonal collocation on finite elements was applied to the governing PSR equations, and the resulting system of ordinary differential equations solved by a standard integration algorithm. In this case, many of the simplifying model assumptions were relaxed, allowing, for example, the simulation of a non-isothermal PSR with finite mass transfer rates. One practical significance of reaction enhancement by pressure swing adsorption is a lower temperature of operation than in a conventional reactor. This would lead to savings in the energy requirements of the reactor, and limit the rate and degree of catalyst deactivation due to coke deposition or sintering.

The effect of deactivation of a V2O5/TiO2 (anatase) industrial catalyst on reactor behaviour during the partial oxidation of o-xylene to phthalic anhydride

Abstract It is well known that during the partial oxidation of o -xylene to phthalic anhydride, on a V 2 O 5 /TiO 2 (anatase) catalyst under industrial conditions, the catalyst can experience both reversible and irreversible deactivation. Experimental evidence presented here suggests that the reversible deactivation can be attributed to the deposition of some carbonaceous compounds. Experiments were carried out in both a microreactor and an industrial-scale pilot-plant reactor. Catalyst samples from the microreactor were analysed by elemental C–H–N analysis, X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). These analyses suggest that the decrease in the disappearance rate of o -xylene at high o -xylene concentrations, which the standard redox model cannot predict, is most likely to be caused by the deposition of carbonaceous compounds rather than by the over-reduction of the catalyst. Two types of reversible deactivation are postulated from the experimental results: (1) by easily removable and (2) by strongly adsorbed carbonaceous compounds. Experiments on the pilot-plant reactor exhibited some unusual dynamic behaviour such as multiple steady-state operation, travelling hot spots and decreased catalyst activity, following an attempted reactivation process at a low temperature. These were all found to be consistent with a model based upon the postulated deactivation mechanism and kinetics; corresponding models based upon constant activity profiles could not reproduce the observed results.

Adsorption in non-isobaric fixed beds

Abstract The paper examines the effect of finite pressure gradients on the dispersion of a pulse of an adsorbable gaseous species as it flows through a fixed bed of adsorbing spherical particles. It is shown that, even for significant deviations from isobaric conditions within the bed, there is an asymptotic linear relationship between the first moment of the response (the mean retention time) and the reciprocal of the exit flow rate, a characteristic of the isobaric situation. However, unlike the isobaric case, extrapolation of the asymptotic linearity to infinite flow rate leads to a non-zero first moment at that point. Furthermore, extrapolation to zero first moment corresponds to a hypothetical flow rate which is dependent only on the parameters of the packed bed and the viscosity of the carrier gas and, specifically, is independent of the nature of the adsorbed species. The theory has been confirmed by a series of chromatographic adsorption experiments under non-isobaric conditions. Subsequently, the same experimental data have been employed in conjunction with the theory to obtain adsorption equilibrium constants for four normal alkanes (propane, butane, pentane, and hexane) on alumina in the temperature range 50–200°C with an uncertainty of ±3%.

Improving catalyst structures and reactor configurations for autothermal reaction systems: application to solid oxide fuel cells

Abstract Creation of an autothermal system by coupling an endothermic reaction (such as steam reforming) to an exothermic oxidation reaction requires the matching of the thermal requirements of the two reactions. The application under study is a solid oxide fuel cell (SOFC) with indirect internal steam reforming of methane, whereby the endothermic reforming reaction in thermally coupled to the exothermic oxidation reaction in a single unit. However, such coupling is not easy to achieve because of the mismatch between the thermal load associated with the rate of steam reforming at typical SOFC temperatures and the local amount of heat available for this purpose from the fuel cell reactions. Two possible methods of achieving such coupling at SOFC operating conditions are the use of catalysts with non-uniform distribution of active metal within the inert support, and/or the introduction of a diffusive barrier placed near the outer surface of the catalyst. Optimum distributions of active catalyst and effective pore sizes within a reforming catalyst were determined which maintain a desired rate of reaction despite a halving of the intrinsic catalyst activity because of coking. It is found that for a spherical pellet, an “egg-yolk” distribution of active catalyst coupled with a diffusion barrier placed in the outer regions of the pellet lead to the desired performance. Catalyst pellets with this formulation have been fabricated and tested.

Model predictive control of reactor temperature in a CSTR pilot plant operating at an unstable steady-state

Abstract The main purpose of this work is to investigate the feasibility of on-line experimental implementation of a Receding Horizon Non-linear Model Predictive Control technique to stabilise an unstable steady state. Experiments were carried out in a pilot plant system which realistically simulates an exothermic reaction taking place in a CSTR, with and without imposed disturbances in the feed flowrate. The controller has been devised by using the recently available software packages gPROMS and gOPT for simulation and optimisation, respectively. The configuration implemented allows for the testing of the nonlinear Receding Horizon controller performance both by simulation and experimentally on-line. It has been applied to a first order irreversible exothermic reaction in a pilot scale PARSEX (PARtially Simulated EXothermic) system with all the relevant flow rates and temperatures measured. The results obtained for the controlled system at these operating conditions demonstrate excellent control behaviour with stabilised, reactor operation even in the presence of step changes in the feed flowrate. Moreover, it was feasible to carry out the optimal control calculations in real time on a SPARC workstation.

Analytical basis for separation enhanced reaction in continuous flow processes

Abstract Modified conversion and reaction extent parameters are used to investigate the effect of simultaneous reaction and adsorption in a well-mixed, steady-state, continuous flow reactor. The analytical method generates conditions in which reactant conversion, product yield or product selectivity exceed that of an equivalent adsorbent-free system. The conditions are shown to be functions of effective reaction Damkohler numbers and the adsorption parameters of the various reaction species. The method is specifically applied to the linear reaction schemes A ⇌ B and A→B→C, and to the general scheme aA+bB+⋯⇌qQ+sS+⋯. The results provide a simple quantitative method for selecting favourable catalysts and adsorbents for such adsorptive reactors, and for choosing ideal contact times between the solid and fluid phases when mass transfer limitations are of importance. The analysis is also applied to systems involving solvent-based extraction for reaction enhancement. In this case, the effective reaction Damkohler numbers are shown to involve absorption parameters based on Henry’s law (gas–liquid systems) or partition parameters (liquid–liquid systems).