van M Martin Sint Annaland

A kinetic rate expression for the time-dependent coke formation rate during propane dehydrogenation over a platinum alumina monolithic catalyst.

Coke formation rates under propane dehydrogenation reaction conditions on a used monolithic Pt/y-Al2O3 catalyst have been experimentally determined in a thermogravimetric analyser (TGA) as a function of time on stream covering wide temperature and concentration ranges. For relatively short times on stream, especially at low temperatures and low propylene concentrations, a remarkable initial quadratic increase has been observed in the coke formation rates versus time with a high apparent propylene reaction order. After longer times on stream the coke formation rate decreases to a constant residual coke growth above approximately 12 wt.% coke content. The experimental data have been successfully described by a kinetic rate expression based on a mechanistic dual coke growth model. In this model it has been assumed that initially coke precursor is formed via a propylene oligomerisation process, explaining the observed auto-catalysis for short times on stream.

A novel reverse flow reactor coupling endothermic and exothermic reactions: an experimental study

A new reactor concept is studied for highly endothermic heterogeneously catalysed gas phase reactions at high temperatures with rapid but reversible catalyst deactivation. The reactor concept aims to achieve an indirect coupling of energy necessary for endothermic reactions and energy released by exothermic reactions, without mixing of the endothermic and exothermic reactants, in closed-loop reverse flow operation, i.e. with incorporation of regenerative heat exchange inside the reactor via periodic gas flow reversals. In a small laboratory scale reactor the concept of this `reaction coupling reverse flow reactor? (RCRFR) has been investigated experimentally for the propane dehydrogenation coupled with methane combustion over a monolithic catalyst, aiming for a proof of principle. Despite the inherently and inevitably large influences of radial heat losses on the axial temperature profiles in a laboratory scale reactor, as shown with some experiments with propane and methane combustion in reverse flow without propane dehydrogenation reaction steps, the experimental results show that indeed endothermic and exothermic reactions can be integrated inside the reactor together with recuperative heat exchange. The periodic steady state was easily obtained without any problems associated with process control. Furthermore, intermediate flushing with nitrogen between the propane dehydrogenation and methane combustion steps could be safely omitted. However, it was necessary to reduce the oxygen concentration during the methane combustion steps in order to avoid too high temperatures due to local combustion of carbonaceous products in the washcoat deposited during the preceding propane dehydrogenation reaction step. Propane dehydrogenation experiments in a reactor filled entirely with active catalyst demonstrated the seriousness of `back-conversion?, a term used to indicate the loss of propane conversion due to propylene hydrogenation because of the low exit temperatures. Experiments performed in a reactor with inactive sections flanking the active catalyst section at both ends showed that the back-conversion could be effectively counteracted.

A novel reverse flow reactor coupling endothermic and exothermic reactions: Part II: Sequential reactor configuration for reversible endothermic reactions

The new reactor concept for highly endothermic reactions at elevated temperatures with possible rapid catalyst deactivation based on the indirect coupling of endothermic and exothermic reactions in reverse flow, developed for irreversible reactions in Part I, has been extended to reversible endothermic reactions for the sequential reactor configuration. In the sequential reactor configuration, the endothermic and exothermic reactants are fed discontinuously and sequentially to the same catalyst bed, which acts as an energy repository delivering energy during the endothermic reaction phase and storing energy during the consecutive exothermic reaction phase. The periodic flow reversals to incorporate recuperative heat exchange result in low temperatures at both reactor ends, while high temperatures prevail in the centre of the reactor. For reversible endothermic reactions, these low exit temperatures can shift the equilibrium back towards the reactants side, causing ‘back-conversion’ at the reactor outlet. The extent of back-conversion is investigated for the propane dehydrogenation/methane combustion reaction system, considering a worst case scenario for the kinetics by assuming that the propylene hydrogenation reaction rate at low temperatures is only limited by mass transfer. It is shown for this reaction system that full equilibrium conversion of the endothermic reactants cannot be combined with recuperative heat exchange, if the reactor is filled entirely with active catalyst. Inactive sections installed at the reactor ends can reduce this back-conversion, but cannot completely prevent it. Furthermore, undesired high temperature peaks can be formed at the transition point between the inactive and active sections, exceeding the maximum allowable temperature (at least for the relatively fast combustion reactions). A new solution is introduced to achieve both full equilibrium conversion and recuperative heat exchange while simultaneously avoiding too high temperatures, even for the worst case scenario of very fast propylene hydrogenation and fuel combustion reaction rates. The proposed solution utilises the movement of the temperature fronts in the sequential reactor configuration and employs less active sections installed at either end of the active catalyst bed and completely inactive sections at the reactor ends, whereas propane combustion is used for energy supply. Finally, it is shown that the plateau temperature can be effectively controlled by simultaneous combustion of propane and methane during the exothermic reaction phase.

A novel reverse flow reactor coupling endothermic and exothermic reactions. Part I: comparison of reactor configurations for irreversible endothermic reactions

A new reactor concept is studied for highly endothermic heterogeneously catalysed gas phase reactions at high temperatures with rapid but reversible catalyst deactivation. The reactor concept aims to achieve an indirect coupling of energy necessary for endothermic reactions and energy released by exothermic reactions, without mixing of the endothermic and exothermic reactants, in closed-loop reverse flow operation. Periodic gas flow reversal incorporates regenerative heat exchange inside the reactor. The reactor concept is studied for the coupling between the non-oxidative propane dehydrogenation and methane combustion over a monolithic catalyst. Two different reactor configurations are considered: the sequential reactor configuration, where the endothermic and exothermic reactants are fed sequentially to the same catalyst bed acting as an energy repository and the simultaneous reactor configuration, where the endothermic and exothermic reactants are fed continuously to two different compartments directly exchanging energy. The dynamic reactor behaviour is studied by detailed simulation for both reactor configurations. Energy constraints, relating the endothermic and exothermic operating conditions, to achieve a cyclic steady state are discussed. Furthermore, it is indicated how the operating conditions should be matched in order to control the maximum temperature. Also, it is shown that for a single first order exothermic reaction the maximum dimensionless temperature in reverse flow reactors depends on a single dimensionless number. Finally, both reactor configurations are compared based on their operating conditions. It is shown that only in the sequential reactor configuration the endothermic inlet concentration can be optimised independently of the gas velocities at high throughput and maximum reaction coupling energy efficiency, by the choice of a proper switching scheme with inherently zero differential creep velocity and using the ratio of the cycle times. In this first part, both the propane dehydrogenation and the methane combustion have been considered as first order irreversible reactions. However, the propane dehydrogenation is an equilibrium reaction and the low exit temperatures resulting from the reverse flow concept entail considerable propane conversion losses. How this `back-conversion? can be counteracted is discussed in part II Chemical Engineering Science, 57, (2002), 855?872.

Computational fluid dynamics for dense gas-solid fluidized beds: a multi-scale modeling strategy

Dense gas-particle flows are encountered in a variety of industrially important processes for large scale production of fuels, fertilizers and base chemicals. The scale-up of these processes is often problematic and is related to the intrinsic complexities of these flows which are unfortunately not yet fully understood despite significant efforts made in both academic and industrial research laboratories. In dense gas-particle flows both (effective) fluid-particle and (dissipative) particle-particle interactions need to be accounted for because these phenomena to a large extent govern the prevailing flow phenomena, i.e. the formation and evolution of heterogeneous structures. These structures have significant impact on the quality of the gas-solid contact and as a direct consequence thereof strongly affect the performance of the process. Due to the inherent complexity of dense gas-particles flows, we have adopted a multi-scale modeling approach in which both fluid-particle and particle-particle interactions can be properly accounted for. The idea is essentially that fundamental models, taking into account the relevant details of fluid-particle (lattice Boltzmann model) and particle-particle (discrete particle model) interactions, are used to develop closure laws to feed continuum models which can be used to compute the flow structures on a much larger (industrial) scale. Our multi-scale approach (see Fig.1) involves the lattice Boltzmann model, the discrete particle model, the continuum model based on the kinetic theory of granular flow, and the discrete bubble model. In this paper we give an overview of the multi-scale modeling strategy, accompanied by illustrative computational results for bubble formation. In addition, areas which need substantial further attention will be highlighted.

Heat transfer in a membrane assisted fluidized bed with immersed horizontal tubes

The effect of gas permeation through horizontally immersed membrane tubes on the heat transfer characteristics in a membrane assisted fluidized bed operated in the bubbling fluidization regime was investigated experimentally. Local time-averaged heat transfer coefficients from copper tubes arranged in a staggered formation with the membrane tubes to the fluidized bed were measured in a square bed (0.15 m x 0.15 m x 0.95 m). Glass particles (75-110 micrometer) were fluidized with air distributed via a porous plate, where the ratio of gas fed or removed through the membrane bundles and the porous plate distributor was varied. The experimental results revealed that high gas permeation rates through the membranes strongly decreased the heat transfer coefficient at high superficial gas velocities for tubes at the top of the tube bundle, which was attributed to the reduced mobility and increased bubble hold up and/or dilution of the emulsion phase, reducing overall heat capacity.In the design of membrane assisted fluidized beds care must be taken to include the effect of gas addition or withdrawal through the membranes on the required heat transfer surface area.

Safety analysis of switching between reductive and oxidative conditions in a reaction coupling reverse flow reactor

A new reverse flow reactor is developed where endothermic reactants (propane dehydrogenation) and exothermic reactants (fuel combustion) are fed sequentially to a monolithic catalyst, while periodically alternating the inlet and outlet positions. Upon switching from reductive to oxidative conditions hydrocarbons come into contact with air. Due to mixing in the monolith channels and in the inlet sections, combustible gas mixtures can be formed. In this work the effects during reaction phase switching are studied by detailed numerical simulations and some qualitative experiments. Due to the reverse flow concept and the use of a monolithic catalyst switching between oxidative and reductive conditions can be carried out without intermediate flushing with inert gases, if proper precautions are taken.

Effect of superficial gas velocity on the particle temperature distribution in a fluidized bed with heat production

The hydrodynamics and heat transfer of cylindrical gas–solid fluidized beds for polyolefin production was investigated with the two-fluid model (TFM) based on the kinetic theory of granular flow (KTGF). It was found that the fluidized bed becomes more isothermal with increasing superficial gas velocity. This is mainly due to the increase of solids circulation and improvement in gas solid contact. It was also found that the average Nusselt number weakly depends on the gas velocity. The TFM results were qualitatively compared with simulation results of computational fluid dynamics combined with the discrete element model (CFD-DEM). The TFM results were in very good agreement with the CFD-DEM outcomes, so the TFM can be a reliable source for further investigations of fluidized beds especially large lab-scale reactors

A Review on Recent Patents on Chemical and Calcium Looping Processes

Chemical and calcium looping processes are interesting concepts for power production with integrated CO2 capture. In this review, recent patents on both chemical and calcium looping processes are discussed after an introduction to the carbon capture and sequestration problem. Novel process concepts on chemical looping and calcium looping are described in detail. Chemical looping combustion is mainly considered as an alternative to oxy-fuel processes, while calcium looping can be applied to retrofit existing power plants or for novel plants based on the reforming of fossil fuels to hydrogen. As such calcium looping processes are foreseen to be applied first in industry. Most of the patents deal with interconnected fluidized bed systems, while also alternative concepts are discussed in this review.

A critical comparison between the wave model and the standard dispersion model

For the description of the phenomena occurring in packed-bed reactors a wave model (WM) has been proposed by Kronberg and Westerterp (Chem. Eng. Sci. 54 (1999) 3977) as an alternative to the standard dispersion models (SDM). In this work the WM is investigated. The capabilities of the WM have been tested on the basis of three industrially important reactions: (1) partial oxidation of methanol to formaldehyde; (2) synthesis of vinyl acetate from acetic acid and acetylene and (3) methanation of carbon dioxide. The predictions of the WM have been compared with the predictions of the SDM and experimental data taken from literature. In case of moderate reaction rates very good agreement was found between the predictions of both models and the experimental data. In case of highly exothermic reactions with steep temperature and concentration profiles the SDM fails to describe the experimental data, whereas the WM gives a good agreement with the experiments.