M. van Sint Annaland

Multiscale modeling of gas-fluidized beds

Numerical models of gas-fluidized beds have become an important tool in the design and scale up of gas-solid chemical reactors. However, a single numerical model which includes the solid-solid and solid-fluid interaction in full detail is not feasible for industrial-scale equipment, and for this reason one has to resort to a multiscale approach. The idea is that gas-solid flow is described by a hierarchy of models at different length scales, where the particle-particle and fluid-particle interactions are taken into account with different levels of detail. The results and insights obtained from the more fundamental models are used to develop closure laws to feed continuum models which can be used to compute the flow structures on a much larger (engineering) scale. Our multi-scale approach involves the lattice Boltzmann model, the discrete particle model, and the continuum model based on the kinetic theory of granular flow. In this chapter we give a detailed account of each of these models as they are employed at the University of Twente, accompanied by some illustrative computational results. Finally, we discuss two promising approaches for modeling industrial-size gas-fluidized beds, which are currently being explored independently at the Princeton University and the University of Twente.

Supercritical shallow granular flow through a contraction: experiment, theory and simulation

Supercritical granular flow through a linear contraction on a smooth inclined plane is investigated by means of experiments, theoretical analysis and numerical simulations. The experiments have been performed with three size classes of spherical glass beads, and poppy seeds (non-spherical). Flow states and flow regimes are categorized in the phase space spanned by the supercritical Froude number and the minimum width of the contraction. A theoretical explanation is given for the formation of steady reservoirs in the contraction observed in experiments using glass beads and water. For this purpose, the classical, one-dimensional shallow-water theory is extended to include frictional and porosity effects. The occurrence of the experimentally observed flow states and regimes can be understood by introducing integrals of acceleration. The flow state with a steady reservoir arises because friction forces in the reservoir are much smaller than in other parts of the flow. Three-dimensional discrete-particle simulations quantitatively agree with the measured granular flow data, and the crucial part of the theoretical frictional analysis is clearly confirmed. The simulations of the flow further reveal that porosity and frictional effects interact in a complicated way. Finally, the numerical database is employed to investigate the rheology in a priori tests for several constitutive models of frictional effects.

Modelling of a reverse flow catalytic membrane reactor for the partial oxidation of methane

Gas-To-Liquid (GTL) processes have great potential as alternative to conventional oil and coal processing for the production of liquid fuels. In GTL-processes the partial oxidation of methane (POM) is combined with the Fischer-Tropsch reaction. An important part of the investment costs of a conventional GTL-plant is related to cryogenic air separation. These costs could be substantially reduced by separating air with recently developed oxygen perm-selective perovskite membranes, which operate at similar temperatures as a POM reactor. Integration of these membranes in the POM reactor seems very attractive because oxygen reacts at the membrane surface resulting in a high driving force over the membrane increasing the oxygen permeation.Because the POM-reaction is only slightly exothermic, the natural gas and air feed have to be preheated to high operating temperatures to obtain high syngas yields and because the Fischer-Tropsch reactor operates at much lower temperatures, recuperative heat exchange is essential for an air-based POM process. External heat transfer at elevated temperatures is expensive and therefore recuperative heat exchange is preferably carried out inside the reactor, which can be achieved with the reverse flow concept. To combine the POM reaction, air separation and recuperative heat exchange in a single apparatus a novel, multi-functional reactor is proposed, called the Reverse Flow Catalytic Membrane Reactor (RFCMR). In this reactor a relatively uniform temperature profile should be established at the membrane section and the temperature fronts should be located in the inert in- and outlet sections.To study the RFCMR concept, reactor models have been developed assuming a shell-and-tube geometry, based on models that are commonly used to describe conventional reverse flow reactors. Simulations of the novel reactor concept revealed that a small amount of methane has to combusted on the air side to create the reverse flow behaviour. Also a small amount of steam has to be injected distributively along the perovskite membrane section to maintain the centre of the reactor at nearly isothermal conditions. With these modifications it was found that the desired temperature profile could indeed be created in the RFCMR and that high overall syngas yields can be achieved.

The clustering morphology of freely rising deformable bubbles

We investigate the clustering morphology of a swarm of freely rising deformable bubbles. A three-dimensional Voronoi analysis enables us to distinguish quantitatively between two typical preferential clustering configurations: a regular lattice arrangement and irregular clustering. The bubble data are obtained from direct numerical simulations using the front-tracking method. It is found that the bubble deformation, represented by the aspect ratio χ , plays a significant role in determining which type of clustering is realized: nearly spherical bubbles form a regular lattice arrangement, while more deformed bubbles show irregular clustering. Remarkably, this criterion for the clustering morphology holds for different diameters of the bubbles, surface tensions and viscosities of the liquid in the studied parameter regime. The mechanism of this clustering behaviour is most likely connected to the amount of vorticity generated at the bubble surfaces.

Advanced reactor concepts for oxidative coupling of methane

ABSTRACT Oxidative coupling of methane (OCM) has been investigated as an interesting way to obtain higher hydrocarbons from natural gas. The aim of this article is to evaluate the reactor concepts for oxidative coupling of methane, from the 1980s through the current state of the art, giving a general insight into the reactor engineering possibilities and perspectives of application of OCM in large scale reactors. The concepts were classified according to the type of reactor bed, the heat management system, the oxygen feeding policy, the degree of integration with separation units, the relative cost, and the current demonstration on industrial scale.

Chapter 23 Multi-level computational fluid dynamics models for the description of particle mixing and granulation in fluidized beds

Publisher Summary To model gas-fluidized bed granulation processes, a multi-level modeling approach has been adopted in this chapter. The idea of this approach is to use different levels of modeling, each level developed to study phenomena that occur at a certain length scale. Information obtained at the level of small length scales can be used to provide closure information at the level of larger length scales. The interaction between the gas and the particles is another important aspect in the continuum and the Discrete Element Model (DEM), which requires closures. There are a number of semi-empirical closure relations available, which despite their widespread application contain a large uncertainty, rendering accurate prediction of the overall bed behavior difficult. Techniques, such as the lattice Boltzmann model (LBM) can be used to validate and eventually improve these closure relations. In LBM the flow around small ensembles of particles can be modeled without making prior assumptions, hence gas-particle interactions can be quantified. This chapter focuses on the levels of the DEM and the continuum model. A detailed theoretical description of these models is given and the predictive capabilities of these models are illustrated here with a few examples.

Membrane reactors for autothermal reforming of methane, methanol, and ethanol

This chapter discusses the application of membrane reactors for hydrogen production through autothermal reforming (ATR) reactions, with particular attention to the ATR of methane as fossil fuel and methanol and ethanol as biofuels. First the concept of ATR is explained, the catalysts used for such reactions are reported, and the traditional reactors are discussed. Afterwards, the membrane reactor concepts are discussed, and two possible configurations, namely the fluidized bed and the packed bed configuration, are discussed and compared. Modeling aspects of both reactors are introduced. Finally, the recent advances in membrane reactors for these reactions and future trends are discussed in the chapter.