Search

The traditional concept of the Thiele or Weisz modulus can fail to assess the relevance of pellet-scale transport limitations in the case of heterogeneously catalyzed gas-phase reactions such as oxidative coupling of methane (OCM). Irreducible transport limitations occur when surface-produced gas-phase intermediates (e.g., radicals) react fast in the pores of catalyst pellets, causing strong pellet-scale concentration profiles that directly affect the product distribution. To assess the effect of the pellet diameter on pellet-scale gradients of reactants and intermediates during OCM, a pellet-scale model is used in combination with detailed kinetic models for the gas (57 species, 317 reactions) and surface chemistry (11 species, 26 reactions). On the basis of a preliminary analysis using a skeleton reaction mechanism a so-called Boudart number, Phi(Bou), is proposed for assessment of the potential relevance of transport limitations during this type of heterogeneously catalyzed gas-phase reactions. The effect of pellet-scale gradients on product yields and selectivities is evaluated using a rigorous 1D heterogeneous reactor-scale model in combination with the above-mentioned detailed kinetic models for the gas and surface chemistry. Pellet-scale diffusion limitations are found to have a significantly negative effect on C-2 selectivities during OCM with the applied kinetics. When the Weisz-Prater criterion is satisfied for reactants and products, pellet-scale gradients of the intermediates are responsible for this drop in selectivity. For larger pellet diameters, the drop in selectivity is due to diffusion limitations of the C-2 products and the corresponding increased importance of secondary total oxidation reactions.

A lateral flow reactor (LFR) is a low pressure drop fixed bed reactor especially suitable for end-of-pipe treatment, for example, the Shell DeNO(x) system for selective catalytic reduction of NOx. Inherently, an LFR is prone to flow maldistribution, which in turn may negatively affect conversion. A detailed study of the hydrodynamics and overall reactor performance of an LFR is presented. The characterization of the flow patterns is carried out in detail with a 2D computational fluid dynamics (CFD) model. In addition, a "double 1D" hydrodynamic model is developed and validated against the CFD results. This computationally much more efficient model is extended with catalytic reaction kinetics, accounting for intraparticle mass transfer limitations. LFR efficiency can be defined as the ratio of the amount of catalyst needed in a fixed bed with uniform distribution over the amount of catalyst needed in the LFR to reach the same overall conversion. Despite the inherently limited flow uniformity, this efficiency of the LFR remains higher than 95% as long as the flow uniformity is above 50%, that is, for nearly all practical cases.

Understanding ignition and extinction behavior is of crucial importance for oxidative coupling of methane (OCM). Therefore, for the first time the bifurcation behavior of OCM has been investigated while considering both homogeneous gas phase reactions and heterogeneous reactions using a detailed microkinetic model. Three different adiabatic reactor models are considered: a plug flow reactor (PFR), a continuously stirred tank reactor (CSTR) and a lumped thermal reactor (LTR) model. The latter represents the limiting case with zero backmixing (cf. PFR behavior) for species and perfect thermal backmixing (cf. CSTR behavior). For homogeneous processes this reactor type could for example be realized by adding a high thermal conductivity inert to the reactor tubes, for catalytic processes a high thermal conductivity catalyst could be used. The bifurcation behavior in these reactor types is compared with a focus on methane conversion, C 2 yields and their dependence on operating conditions such as inlet composition, inlet temperature and space time. Steady state multiplicity is observed for adiabatic CSTR and LTR models. This multiplicity of steady states is not observed for isothermal reactor models, indicating that it is caused solely by thermal backmixing and is not related to chemical feedback features such as autocatalysis. The start-up procedures or initial conditions determine the actual steady state that is obtained. Among the three investigated reactor types, a LTR shows the highest product yields and the lowest extinction temperatures, which allows autothermal operation at a much lower inlet temperature compared to a PFR and CSTR. For OCM without catalyst, autothermal operation on the ignited branch at ambient inlet temperatures and reasonable space times is only possible by using methane-to-oxygen ratios below 3 leading to low selectivities. For catalytic OCM compared to OCM without catalyst, the range for autothermal operation is much broader and it is much easier to find feasible operating conditions allowing autothermal operation at ambient inlet temperatures. By operating a LTR on the ignited branch at ambient inlet temperature of 300 K, methane-to-oxygen ratio CH 4 :O 2 = 6, space time V/F CH4,0 = 0.02 s, bulk density of Sn-Li/MgO = 1000 kg cat /m 3 and pressure P = 1 bar, overall C 2 selectivities (i.e. sum of ethane, ethylene and acetylene selectivity) of 80% can be obtained at methane conversions as high as 30%.

Oxidative coupling of methane (OCM) is considered one of the most promising routes to directly convert methane into more valuable hydrocarbons. The uncertain economics related to the tradeoff between conversion and C2 selectivities is the primary reason why OCM is currently not industrially applied. In the last decades, numerous studies have focused on developing a viable catalyst that has the potential to improve the low C2 yields. But is the primary issue of OCM truly a catalyst problem? Because of the high exothermicity of the OCM process, thermal effects and path dependence are dominating in all OCM reactors of practical importance. Furthermore, irreducible diffusion limitations exist on the pellet scale. Understanding how to exploit these mass and heat transfer effects by reactor engineering is a prerequisite for the breakthrough of OCM. In this work an overview is given of criteria used to assess mass and heat transfer resistances, parametric sensitivity and runaway in catalytic packed bed reactors. The importance of mass transport limitations, runaway and/or ignition for OCM, either on the pellet scale or on the reactor scale, is shown in several examples. Both simple and advanced reactor concepts are discussed with a focus on their heat and mass transfer characteristics. Clear progress has been made in the past lustrum on all these fronts and it seems that research is on the verge of a real breakthrough to make OCM happen on an industrial scale.

An adequate use of gas pulsation can create an ordered, dynamically structured bubble flow in a bed of Geldart B particles. A structured bed is more homogeneous, responds to external control and is scalable. While earlier studies have focused on describing the self-organization of the gas bubbles, the solid mixing and gas–solid contact patterns have remained unclear. In this work, the solids circulation and mixing behavior in structured and unstructured beds at various pulsation frequencies are compared with a traditional fluidized bed operation. The degree of lateral mixing is hereby quantified through an effective lateral dispersion coefficient extracted from CFD-DEM (discrete element modelling) simulations in a thin fluidized bed system. Mixing shows major quantitative and qualitative differences amongst the investigated cases. The coordinated motion of the gas bubbles wraps the solid flow into a series of compartments with minimal interaction, whereby effective lateral dispersion coefficients are an order of magnitude lower than in an unstructured operation. More importantly, unlike a traditional bed, dispersion in a structured bed is driven by advection and is no longer a diffusive process. Compartmentalization decouples the time scales of micro- and macromixing. Every pulse, the compartments rearrange dynamically, causing a level of local axial mixing that is scale-independent. While further work is necessary to fully understand the compartmentalization at a larger scale, the circulation described here indicates that a dynamically structured bed can provide a tight control of mixing at low gas velocities and a narrower distribution of stresses in the solid phase compared to traditional devices.

Vortex reactors take advantage of the synergy between enhanced heat and mass transfer rates and multifunctional phenomena at different temporal and spatial scales. Proof-of-concept experiments with our novel and innovative STAtor-Rotor VOrtex Chamber (STARVOC) confirm its advantageous features for the sustainable production of chemicals and fuels. STARVOC is a high-g contactor that uses carrier flow (gas or liquid) tangential injection to drive a rotor attached to low-friction bearings. The vortex chamber inside the rotor contains a secondary phase or phases, such as a solids bed, a liquid layer, or a suspension. Carrier fluid passes through the perforated rotor wall and contacts a densely and uniformly distributed secondary phase with enhanced slip velocities. Experiments focused on pressure profiles, rotor angular velocity, and solids azimuthal velocity. With air as the carrier fluid and different solid particle beds as the secondary phase, STARVOC reached bed azimuthal velocities up to four-fold compared to those reached in Gas-Solid Vortex Units with fully static geometry. These results show its potential to improve interfacial heat and mass transfer rates and take advantage of flow energy and angular momentum. Due to its process intensification capabilities, STARVOC is a promising alternative for the state-of-the-art chemical industry.

Recently computational fluid dynamics (CFD) has been successfully applied for the evaluation of the start-of-run performance of three-dimensional (3D) coil geometries in steam cracking reactors. However, determining the full economic potential of a coil involves tracking its performance throughout the run and not only at start-of-run. Therefore in this work a novel method has been developed that allows to assess the most debated characteristic of these 3D coil geometries, i.e. the potential extension of the run length in combination with the evolution of the product yields during the time on stream. An algorithm based on dynamic mesh generation is presented for simulating coke formation in 3D steam cracking reactor geometries, tracking the apparent geometry deformation caused by the growing coke layer. As a proof-of-concept, a Millisecond propane cracker is simulated over the first days of its run length, and this for three different coil designs: a bare tube, a finned tube and a continuously ribbed reactor design. Our simulations show that the ribbed reactors overall outperform the others although in these enhanced tubular geometries the growth of the coke layer is far from uniform. Because of this, the reactor geometry will change over time, which will in turn influence the fluid dynamics, product yields and successive coke formation substantially.

Most olefins (e.g., ethylene and propylene) will continue to be produced through steam cracking (SC) of hydrocarbons in the coming decade. In an uncertain commodity market, the chemical industry is investing very little in alternative technologies and feedstocks because of their current lack of economic viability, despite decreasing crude oil reserves and the recognition of global warming. In this perspective, some of the most promising alternatives are compared with the conventional SC process, and the major bottlenecks of each of the competing processes are highlighted. These technologies emerge especially from the abundance of cheap propane, ethane, and methane from shale gas and stranded gas. From an economic point of view, methane is an interesting starting material, if chemicals can be produced from it. The huge availability of crude oil and the expected substantial decline in the demand for fuels imply that the future for proven technologies such as Fischer-Tropsch synthesis (FTS) or methanol to gasoline is not bright. The abundance of cheap ethane and the large availability of crude oil, on the other hand, have caused the SC industry to shift to these two extremes, making room for the on-purpose production of light olefins, such as by the catalytic dehydrogenation of propane.

Two grass-roots light olefins recovery sections for a methanol to olefins reactor effluent are compared in terms of energy efficiency, robustness, capital expenditure and operational expenditure. As the methanol to olefins process is characterized by a higher propene to ethene ratio and a lower light ends content compared to the steam cracking process, a novel absorption based separation section for the methanol to olefins process was proposed by Wison Engineering, replacing the traditional cryogenic demethanizer column by the combination of a precutting column and an absorption column. Pinch analysis showed that the heat integration potential for both separation sections is nearly identical, but the absorption configuration operates at a higher thermodynamic efficiency. The cryogenic separation section has a higher robustness when the amount of light ends in the reactor effluent varies. The capital expenditure of the absorption configuration is up to 14% lower and the operational expenditure is up to 9% lower compared to the cryogenic configuration, depending on the light ends content in the reactor effluent, leading to more favorable economic key performance indicators. The novel absorption configuration is an interesting alternative to the traditional cryogenic configuration for a methanol to olefins separation section.

Biomass fast pyrolysis has been considered one of the best alternatives for the thermal conversion of biomass into bio-oil. This work introduces a new reactor technology for biomass fast pyrolysis, the Gas-Solid Vortex Reactor (GSVR), to obtain high bio-oil yields. The GSVR was designed to decrease the residence time of the pyrolysis vapors; thus, the secondary cracking reactions are reduced, to enhance the segregation of the char and the unreacted biomass and to improve the heat transfer rate. Biomass fast pyrolysis experiments have been carried out for the first time in a Gas-Solid Vortex Reactor (GSVR) at 773 K, using softwood (pine) and hardwood (poplar) as feedstock. Char yields as low as 10 wt. % in the GSVR were comparable to those reported for the same feedstocks processed in conventional fluidized bed reactors. The yields of non-condensable gases in the range of 15–17 wt. % were significantly lower than those reported for other commonly used biomass fast pyrolysis reactors. Two-dimensional gas chromatography (GC × GC) revealed noticeable differences at the molecular level between the bio-oils from the GSVR and bio-oils from other reactors. The aromatics in the pine bio-oil consist almost entirely (85 wt. %) of guaiacols. For poplar bio-oils no predominant group of aromatics was found, but phenolics, syringols, and catechols were the most pronounced. The experimental results highlight the advantages of the GSVR for biomass pyrolysis, reaching stable operation in around 60 s, removing the formed char selectively during operation, and enabling fast entrainment of pyrolysis vapors. Results indicate a great potential for increasing yield and selectivity towards guaiacols in softwood (e.g., pine) bio-oil. Likewise, decreasing pyrolysis temperature could increase the yield of guaiacols and syringols in hardwood (e.g., poplar) bio-oil.

Streamwise periodic boundary conditions (SPBCs) have been successful in reducing the computational cost of simulating high aspect ratio processes. Extending beyond the classic assumptions of constant property flows, a novel approach incorporating non-equilibrium kinetics was developed and implemented for the simulation of an industrial propane steam cracker. Comparison with non-periodic benchmarks provided validation as relative errors on the main product yields were consistently below 1% for different reactor configurations. A further order-of-magnitude reduction of the radial errors on product concentrations was obtained via an intuitive correction method based on the concept of local fluid age. The computational speedup achieved through application of SPBCs was a factor 16–250 compared to the non-periodic simulations. The presented methodology thus serves as a quick screening tool for the development of novel reactor designs and unlocks the potential for using more elaborate kinetic models or a more fundamental approach toward turbulence modeling. VC 2016 American Institute of Chemical Engineers AIChE J, 63: 1715–1726, 2017 Keywords: CFD, periodic boundary conditions, heat & mass transfer, reactor design, steam cracking

The process intensification abilities of gas–solid vortex units (GSVU) are very promising for gas–solid processes. By working in a centrifugal force field, much higher gas–solid slip velocities can be obtained compared to gravitational fluidized beds, resulting in a significant increase in heat and mass transfer rates. In this work, local azimuthal and radial particle velocities for an experimental GSVU are simulated using the Euler–Euler framework in OpenFOAM and compared with particle image velocimetry measurements. With the validated model, the effect of the particle diameter, number of inlet slots and reactor length on the bed hydrodynamics is assessed. Starting from 1g-Geldart-B type particles, increasing the particle diameter or density, increasing the number of inlet slots or increasing the gas injection velocity leads to an increased bed stability and uniformity. However, a trade-off has to be made since increased bed stability and uniformity lead to higher shear stresses and attrition.

Processes requiring intensive interfacial momentum, mass and heat exchange between gases and particulate solids can be greatly enhanced by operating in a centrifugal field. This is realized in the Gas-Solid Vortex Reactor (GSVR) with centrifugal accelerations up to two orders of magnitude higher than the Earth's gravitational acceleration. Here, the flow patterns of two 1g-Geldart B-type particles are experimentally assessed, over the gas inlet velocity range 82–126 m s−1, in an 80 mm diameter and 15 mm height GSVR. The particles are monosized aluminum spheres of 0.5 mm diameter, and walnut shell in the sieve fraction 0.50–0.56 mm and aspect ratio 1.3 ± 0.2. Two dimensional Particle Image Velocimetry combined with Digital Image Analysis and pressure measurements revealed that periodic fluctuations in solids azimuthal and radial velocity between gas inlet slots are strongly related to the average solids azimuthal velocity and bed uniformity. Aluminum particles feature steeper changes in azimuthal velocity and more attenuated changes in radial velocity than walnut shell particles. Within the assessed gas inlet velocity range the solids bed of aluminum exhibits average azimuthal velocities and bed voidages 40–50% and ≈10% lower than those of walnut shell. The aerodynamic response time of the particles, i.e. ρsdp2/18μg, emerged as an important parameter to assess the influence of the carrier gas jet on the radial deflection of the particles and the interaction solids bed-outer wall. Too low aerodynamic response time relates to nonuniformity in bed voidage due to solids radial velocity fluctuations. Excessive aerodynamic response time indicates low solids azimuthal velocities due to solids bed-outer wall friction.

Helical inserts or baffles in tubular membranes can provide flux increases without additional energy usage. However, the optimal shape of these turbulence-enhancing structures remains debated, with many experimental studies being performed, though few full-scale applications reported. A model-based approach can assist experimental work in getting detailed insights into the hydrodynamic phenomena near the membrane surface. This creates the opportunity for model-based design of such membranes or an optimization of operating conditions. Both the fluid flow and particle behavior were simulated in a single Pentair X-Flow Compact Helix membrane with helical ridge, and results were compared to those of a plain tubular membrane. Higher mean wall shear stresses were found for the membrane with helical ridge as well as a wider distribution of wall shear stresses, which promote fouling mitigation. Within a shadow zone behind the helical ridge, wall shear stress was, however, slightly lower compared to the plain memb...

Using detailed kinetic models in computational fluid dynamics (CFD) simulations is extremely challenging because of the large number of species that need to be considered and the stiffness of the associated set of differential equations. The high computational cost associated with using a detailed kinetic network in CFD simulations is why one-dimensional simulations are still used, although this leads to substantial differences compared to reference three-dimensional simulations. Therefore, a methodology was developed that allows one to use detailed single-event microkinetic models in CFD simulations by on the fly application of the pseudo-steady-state assumption to the radical reaction intermediates. Depending on the reaction model size, a speedup factor of more than 50 was obtained compared to the standard ANSYS Fluent routines without losing accuracy. As proof of concept, propane steam cracking in a conventional bare reactor and a helicoidally finned reactor was simulated using a reaction model contain...

The process intensification possibilities of a gas–solid vortex reactor have been studied for biomass fast pyrolysis using a combination of experiments (particle image velocimetry) and non-reactive and reactive three-dimensional computational fluid dynamics simulations. High centrifugal forces (greater than 30g) are obtainable, which allows for much higher slip velocities (>5 m s–1) and more intense heat and mass transfer between phases, which could result in higher selectivities of, for example, bio-oil production. Additionally, the dense yet fluid nature of the bed allows for a relatively small pressure drop across the bed (∼104 Pa). For the reactive simulations, bio-oil yields of up to 70 wt % are achieved, which is higher than reported in conventional fluidized beds across the literature. Convective heat transfer coefficients between gas and solid in the range of 600–700 W m–2 K–1 are observed, significantly higher than those obtained in competitive reactor technologies. This is partly explained by reducing undesirable gas–char contact times as a result of preferred segregation of unwanted char particles toward the exhaust. Experimentally, systematic char entrainment under simultaneous biomass–char operation suggested possible process intensification and a so-called “self-cleaning” tendency of vortex reactors.