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• A phenomenological design approach is constructed for the design of CLC-PPBR at/above pilot scale. • Macroscopic behaviors of the heat/reaction front are introduced to depict the dynamic operation of the reactors. • Bed size must conform to certain limitations to fulfill reasonable operational stability and cost-effectiveness. • Effects among design parameters are studied to optimize the preliminary design scheme for the 1.5 MW th reactor. The parallel packed bed reactor (PPBR) is a promising route for industrial application of chemical looping combustion. This paper introduces a phenomenological approach for the rational design of PPBR, serving as a manner to efficiently obtain comprehensive design results. Based on the macroscopic propagation of heat and reaction fronts, it elucidates the dynamic heat/mass transfer within the bed. Subsequently, it is applied to design a 1.5 MW th reactor. The risk of carbon deposition in the bed is firstly investigated from a thermodynamic perspective. Then, the study reveals that the height-to-diameter ratio of the reactor must meet certain constraints to ensure stable and cost-effective operation, and these constraints are extended to the scale-up process. Sensitivity analysis is utilized to investigate the effect of key parameters on the design schemes and performance. Effects of particle size on intra-particle gas diffusion is analyzed. Finally, detailed design schemes and performance of the reactor are determined. [ABSTRACT FROM AUTHOR]

An accurate and self-consistent methodology for mass transport of multi-component mixtures in multi phase media is a necessity for a proper description of complex physical and chemical processes in reactors such as catalytic packed beds. In this regard, a novel methodology has been developed to describe and couple underlying transport phenomena in fluid and porous media as well as at the solid-fluid interface. The methodology is symmetric as it treats all components in a mixture equally. The Maxwell-Stefan equations are symmetrically formulated, discretized conservatively and coupled with a compressible flow solver for the fluid part. The Dusty Gas Model is applied inside porous media by developing a self-consistent and robust numerical formulation. A ghost-cell Immersed Boundary Method is used to capture the physics at the solid-fluid interface with the implementation of a novel symmetric non-singular mass flux formulation. Several test cases are established to demonstrate the accuracy and robustness of the newly developed symmetric methodology in this paper. These test cases can be used as benchmark for the future development of symmetric methodologies for multicomponent systems in multi phase media. • A novel computational methodology has been developed to couple transport phenomena in fluid-solid multicomponent media. • The developed methodology is symmetric as all components in a mixture are treated equally. • A symmetric and unconditional stable formulation for the Dusty Gas Model has been developed. • A novel non-singular mass flux formulation has been developed and embedded in the ghost-cell Immersed Boundary Method. • Several test cases are designed for verification and validation of the developed methodology. [ABSTRACT FROM AUTHOR]

Mass transfer modeling is important to design and scale-up gas–liquid reactors in the research of gas–liquid mass transfer intensification. The parameters of bubbles in reactors, including size, shape and dynamics, are the keys in mass transfer modeling. The digital image analysis technique is convenient and efficient and has been widely used to measure bubble parameters. In this paper, we have proposed a flexible method for measuring the bubble parameters, which consists of establishing a template database for difform bubbles, splitting contours for every overlapped single bubble in bubble swarm, and reconstructing the segmented bubbles. The contour splitting algorithm is based on a new concave point extraction algorithm proposed in this paper. Based on the template database that we establish, the proposed reconstruction method can reconstruct segmented bubbles with different shapes and measure their important parameters effectively, such as area, perimeter, major/minor axis, roundness, etc. Experimental results show that for dense bubble images, our proposed method can segment and reconstruct bubbles accurately from overlapping bubbles and reliable in most cases. [ABSTRACT FROM AUTHOR]

• The constants in the turbulent mass transport two-equation model are derived by the renormalization group method. • A grid turbulent laser induced fluorescence experiment is carried out to verify the reliability of these constants. • The turbulent mass transport two-equation model can obtain more reliable concentration distributions without adjusting the model constants. Turbulent mass transport is an essential phenomenon in chemical reactors. A two-equation model of turbulent mass transport has been proposed for less numerical simulation error in previous work. However, there are still some model constants that have undetermined values introduced by empirical modeling rules. In the present paper, the renormalization group (RNG) method is applied to the turbulent mass transport process. Through the RNG procedure, the small scale turbulent components in Navier-Stokes and convection diffusion equations can be eliminated and combined into large scale sources. The corresponding model constants are derived and applied to a grid turbulent diffusion simulation case. In this paper, the grid turbulence diffusion experiment with laser induced fluorescence (LIF) is carried out to verify the simulation results. A satisfactory agreement of concentration distribution between simulation and experiment indicates that the model constants obtained by the RNG method are reliable. [ABSTRACT FROM AUTHOR]

Abstract: Design and analysis of photoreactors is significantly more challenging than conventional reactors due to participation of radiation in chemical reactions. This problem is further compounded in case of photocatalytic reactors because of presence of photocatalytic particles, which not only produce complex light scattering effects but, in case of slurry systems, also act as an additional phase, the hydrodynamics of which is essential to characterize for evaluating the phase distribution of photocatalyst particles without which it is not possible to calculate the light intensity distribution. This then necessitates the use of a computational fluid dynamics (CFD)-based simulation approach which can simultaneously take into account the hydrodynamics of multiple phases, light intensity distribution and reaction kinetics. This paper presents a sequential review of all steps for CFD simulations of photocatalytic reactors. The hydrodynamic modeling has been considered first with an emphasis on the Eulerian–Eulerian model because of its ability to handle large-scale photocatalytic reactor systems with only relatively moderate computational resources. This has been followed by a review of lamp emission models, which in CFD models are used as boundary conditions for solving the radiation transport equation (RTE). Before discussing the kinetics of photocatalytic reactors, a review of numerical models for solving the RTE has also been presented for both slurry and immobilized reactor systems. Finally, the paper discusses important factors for setting up the boundary conditions for CFD modeling of photocatalytic reactors. [Copyright &y& Elsevier]

Abstract: This paper presents phase diagrams for the cavitation bubble dynamics in a dual frequency ultrasonic processor employing organic medium. These phase diagrams depict spatial dispersal of the cavitation intensity (as indicated by peak temperature and pressure reached in the bubble at transient collapse) as a function of two design parameters, viz. ratio of frequencies of two ultrasound waves and phase difference between the waves. n-hexane has been chosen as model organic solvent with cavitation bubbles made of argon. The choice of the design parameters for simulations has been made on the basis of two criteria, viz. maximum total energy dissipation and most uniform energy dissipation in the processor. Simulations of cavitation bubble dynamics have been made for harmonic and ultra-harmonic coupling of two ultrasound waves with base frequency of 20kHz and coupling frequency ratio in the range of 1.5–5. The range of phase difference between waves is 0°–270°. The phase diagrams clearly indicate the regions of constructive and destructive interference between the waves. For similar maxima and minima of acoustic pressure amplitudes, higher cavitation intensity was seen for lower frequency ratios. Moreover, in the phase diagrams, the span of parameter space of constructive interference was higher for ultra-harmonic coupling. However, due to higher vapor pressure and lower density of n-hexane, the peak temperatures attained in the bubble, even for the conditions of maximum constructive interference between two ultrasound waves, were relatively lower than those observed for water. Thus, merits of dual frequency ultrasound field for augmentation of cavitation intensity are not so marked for organic medium. Despite this limitation, the phase diagrams presented in this paper could form a useful tool for optimization of the dual frequency ultrasound processor. [Copyright &y& Elsevier]

Abstract: This paper proposes that a diesel oxidation catalyst (DOC) operating within the fully turbulent flow regime is an efficient means of reducing the contents of particulate matter in the exhaust gases. The suggested mode of operation is in contrast to the fact that the DOCs are typically operated within the laminar flow regime. In the paper, the particle trapping efficiency and pollutant conversion in turbulent ceramic DOCs are calculated using both mass-transfer correlations available in the literature and computational fluid dynamics (CFD). It is shown that a turbulent DOC substantially increases the removal of small particulates from the exhaust gases. This indicates the potential of the aftertreatment system to comply with the forthcoming number-based emission legislations on particulate matter. In addition, the turbulent DOC can be used to optimize the overall performance of a combined system consisting of a DOC and a diesel particulate filter. [Copyright &y& Elsevier]

Graphical abstract Highlights • 3D Eulerian-Lagrangian model on particle circulations in CLC DCFB was developed. • Particle Size Distribution (PSD) was introduced in the current models. • Flow patterns of particles with a wide size distribution in DCFB were revealed. • Effects of operating conditions on the particle circulation behaviors were studied. Abstract Understanding the particle dynamics is of paramount importance to the design, operation and optimization of looping combustion reactors. In this paper, a three-dimensional Eulerian-Lagrangian model was developed to study the particle flow in a dual circulating fluidized bed consisting of an air reactor and a fuel reactor. Based on the Multi-Phase Particle-In-Cell (MP-PIC) scheme, the turbulent gas motion was simulated by the large eddy simulation (LES) and the particle flow simulated by the discrete particle method, with the particle size distribution (d p = 0–108 μm) being fully taken into account. The effects of particle size, particle density, initial bed inventory and fluidizing gas flow rate were numerically studied. The results show that small particles mainly distribute in the fuel reactor while large particles tended to stay in the air reactor, leading to an unfavorable influence on the efficiency of metal oxide. The increase of particle density led to the increase of global and internal circulation rates. A high initial bed inventory would result in a high particle concentration and lower axial velocities of particles. When increasing the fluidizing gas flow rate in the fuel reactor, the internal particle circulation rate increased gradually; however, the global particle circulation rate decreased. The results should be useful for the design and control of dual circulating fluidized bed, looping combustion reactors. [ABSTRACT FROM AUTHOR]

Highlights • The relative position of each feed influences the dimensionality of the system. • Independent feeds reveal achievable distinct pockets of state space. • Multiple-feed AR consists of smaller single-feed ARs. Abstract Much of the existing literature on attainable region (AR) theory has focused on finding it from a single feed. Yet, many industrial processes involve multiple feed streams and thus the total set of achievable states within a reactive process is affected by the number and type of feeds available. In this paper, we describe conditions for which multiple feeds change the dimension of the space belonging to the AR, allowing for an expansion of the region that would otherwise be infeasible from a single feed. Under specific conditions, we describe how multiple feeds may only alter the shape and size of the AR whilst preserving its dimension to that of a single feed AR. We determine the maximum number of the parallel reactors required to achieve these new states. This is obtained from the reaction stoichiometry and feeds alone, without consideration of reaction kinetics. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Lagrangian Mixing Simulation (LMS) method to characterise mixing is developed. • LMS is applied over a 2D CFD simulation in a CIJ reactor. • LMS results were used to characterise mixing: calculation of interfacial area generation and segregation scales. • LMS is able to simulate and compute even the smallest mixing scales. • LMS has short CPU time when compared to CFD mixing simulations. Abstract This paper introduces Lagrangian Mixing Simulation (LMS), which is a new method that uses particles to track the flow front between two fluids in a mixer. The line formed by an ensemble of particles enables the calculation of the instantaneous rate of interfacial area generation between the fluids and the computation of segregation scales. LMS uses the Eulerian velocity field from CFD simulation, which has a spatial resolution determined from the flow dynamics, for the simulation of mixing at scales that are several orders of magnitude below the CFD gridscale. Since only the interface between the two fluids is simulated, the LMS is orders of magnitude faster than the flow field simulation. In this work, LMS is applied to 2D CFD simulations of the flow in Confined Impinging Jets (CIJ) reactors for Re = 100, 300 and 500. LMS shows that the interfacial area generation is exponential for chaotic flow regimes (Re = 300 and 500) and linear for steady flows (Re = 100). Moreover, LMS is able to simulate segregation scales smaller than 10−8 m, i.e., LMS result spans and overcomes the entire range of spatial scales that have physical meaning. [ABSTRACT FROM AUTHOR]

Toluene is a kind of volatile organic compounds usually present in industrial emissions. In this work, the rigorous model for species concentration distribution prediction proposed in part I was applied in simulating the biological degradation process of toluene in a bubble column (BC) reactor. We show in the present paper that the species concentration, turbulent mass diffusivity and velocity distributions in the bubble column bioreactor can be obtained simultaneously by applying the proposed model. On the basis of the simulations, the analogy between the turbulent mass diffusivity and turbulent viscosity for the bubble column reactor was investigated. The results showed that this analogy may depend on the rate of chemical or bio-chemical reaction in the bubble columns and remain for the case with a lower reaction rate. [ABSTRACT FROM AUTHOR]

Abstract: Miniaturization of plasma reactors has the potential of low power operation. In general, the electric field strength in the gap between two electrodes increases proportionate to inverse of the gap width, so that it is possible to overcome the first ionization potential of the gas with a low voltage. However, plasmas are extinguished primarily by recombination at the walls. Wall collisions are enhanced by the greater surface area to volume ratio in microchannels, which also increases proportionate to the inverse of the gap width. If the plasma were well mixed, then the plasma creation in the bulk would be balanced by extinction at the wall, providing no particular advantage with regard to low voltage/low power operation. However, the plasma is transferred from the bulk to the wall by ambipolar diffusion. If the operation of the plasma microreactor is essentially transient or batch, whether or not the reaction kinetics are comparable to or faster than ambipolar diffusion determines if there is a regime of operation in which a low voltage plasma discharge can generate a high yield of product. In this paper, this question is investigated with regards to the ozone formation reaction and a particular design of a microchannel plasma reactor, with parameters so chosen to arguably achieve low voltage operation. The focus of this paper is the simulation of the kinetics of the plasma reactions leading to ozone formation, which shows a time to completion that is comparable (10−2 s) or faster than the estimate of ambipolar diffusion time at these length scales. Preliminary results of a microchip reactor are consistent with this prediction. [Copyright &y& Elsevier]

Horizontal stirred bed reactors for polypropylene production have a typical axial powder mixing pattern which is unique for polyolefin gas-phase reactors. The polymer, although adhering to the catalyst particles, has a shorter mean residence time and a broader residence time distribution than the catalyst. Both polymer and catalyst residence time distributions, strongly depend on the temporal catalyst activity profile. The powder mixing pattern in a horizontal stirred bed reactor results from two transport effects: the continuously increasing powder net flow in the downstream direction caused by the particle growth, and the simultaneous stirring flows with equal intensity in the up- and downstream directions. Both together lead to a residence time distribution of the reacting catalyst particles between fully backmixed and plug flow. A modelling approach is proposed which describes the axial flow contributions of stirring and polymerisation separately, and which is more accurate and flexible than simple cells-in-series models used in most papers so far. For large-scale reactors, residence time distributions are predicted from experimental pulse–response curves obtained in a miniaturised mixing model without reaction. Residence time distributions equivalent to “three to five” well-mixed reactors in a series reported in the literature are experimentally accessible only with difficulties under polymerising conditions. With the modelling approach proposed in this paper, the “reacting” RTD can be calculated on the basis of the cold-flow experimental data. We compare the proposed modelling concept with simple cells-in-series models by simulating catalyst yields, particle size distributions and catalyst transitions using two model catalysts with different activity profiles. The selection of the right modelling approach has indeed a significant influence on the simulation results, especially if the throughput or the axial velocity profile is changing and the catalyst is not deactivating. [Copyright &y& Elsevier]

The control of tubular fixed-bed reactors with highly exothermic reactions is approached from a passivity-based control perspective. The proposed controller solves dynamic tracking of the reactor exit conversion and stabilization of the reactor temperature by exploiting the passivity properties of the process. The model-based control structure proposed in this paper provides a suitable framework for developing the passivity-based control law and the state predictor. The integrated controller is designed and its performance in the face of parameter variation and model uncertainty is tested by numerical simulation. Digital simulation on an industrial phthalic anhydride fixed-bed reactor shows that the proposed control scheme can give satisfactory dynamic tracking ability and disturbance rejection performance, which is robust in the presence of process variation and model uncertainty. This paper provides a basic insight into the characterization and solution of control problems that are particular to tubular fixed-bed reactor systems and constitutes the application of passivity-based control theory to complex chemical processes. [Copyright &y& Elsevier]

Abstract: This paper describes the development of a microchannel-based Sabatier reactor for applications such as propellant production on Mars or space habitat air revitalization. Microchannel designs offer advantages for a compact reactor with excellent thermal control. This paper discusses the development of a -based catalyst using powdered form and its application and testing in a microchannel reactor. The resultant catalyst and microchannel reactor demonstrates good conversion, selectivity, and longevity in a compact device. A chemically reacting flow model is used to assist experimental interpretation and to suggest microchannel design approaches. A kinetic rate expression for the global Sabatier reaction is developed and validated using computational models to interpret packed-bed experiments with catalysts in powder form. The resulting global reaction is then incorporated into a reactive plug-flow model that represents a microchannel reactor. [Copyright &y& Elsevier]

Computer-based simulations of a reverse-flow reactor should be carried out till the attainment of the so-called cyclic steady state. Usually this state is achieved by the method of direct dynamic simulations. In the paper of Unger et al. (Comput. Chem. Eng. 21 (1997) 5167.) special approaches, making use of various minimization algorithms based most often on Newton algorithms, are proposed. In the present paper one deals with a comparison and an appraisal of these methods, applied to the reverse-flow catalytic combustion of methane that occurs in coal-mine ventilation air. [Copyright &y& Elsevier]

• A simple 3D reconstruction algorithm based on laser scanning is developed. • Bubble slice images are exploited to reconstruct the 3D structures of bubbly flow. • Velocity, size and void fraction of bubbles at various flow rates are investigated. Bubbly flow is frequently observed in chemical industries such as reactors and steam boilers, where flow parameters of bubble velocity, size, and void fraction are required to guide process safety. In this paper, a laser scanning-based three-dimensional reconstruction algorithm is introduced to achieve simple and effective measurements in vertical bubbly flow. A series of slice images are captured by a high-speed camera, and a modified two-dimensional cross-correlation algorithm is proposed to achieve bubble extraction and matching. Based on the reconstructed flow structures of bubbly flow, bubble velocities and their distribution in different scanning cycles are measured. Statistically, bubble sizes are mainly concentrated within 1.8–2.2 mm. The void fractions are calculated for various gas flow rates with relative errors primarily within ±10%. The relative errors of the average void fraction are less than 8% by comparing the experimental and theoretical results, indicating a higher reconstruction precision. [ABSTRACT FROM AUTHOR]

In this paper, we study the optimal control problem for a continuous stirred tank reactor (CSTR) that represents a reaction of the type “ A → product”. The reactor dynamics is described by a nonlinear system of ordinary differential equations controlled by two inputs: the inlet concentration and the inlet temperature. We formulate the problem of maximizing the average product of this reactor for a fixed consumption of the input component over a period of time. This kind of isoperimetric optimal control problem is analyzed by using the Pontryagin maximum principle with Lagrange multipliers. We show that the optimal controls are bang-bang and propose an upper bound for the number of switchings for the linearized problem with periodic boundary conditions. Numerical simulations confirm that our control strategy can be used to improve the reactor performance over a specified period of time in comparison to the steady-state operation. [ABSTRACT FROM AUTHOR]

• A simple practical control structure for preventing batch reactor runaways is demonstrated. • Ramping the setpoint of the reactor temperature controller achieves effective management of reactor temperatures during the period when reactant concentrations are large. • The setpoint ramp is paused when cooling demand becomes too large. • Using slow ramps result in very small reductions in productivity in terms of batch time. Batch reactors can present challenging dynamic control problems because of the large nonlinear changes in process parameters during the batch cycle. Large reactant concentrations at the beginning of the batch produce fast reaction rates and large heat-transfer duties if the reactor temperature is increased too rapidly from its low initial temperature. Exothermic reactions can lead to reactor runaways that exceed maximum temperature/pressure safety constraints. Several papers have studied sophisticated techniques for detecting the potential of an impending runaway and taking corrective action. A complex stability criterion embedded in a model predictive control structure is a recent suggestion for handling the problem. This paper discusses a simple practical technique that has been used in industry for decades. The setpoint of a temperature/pressure controller is ramped up at a reasonable rate, but if the demand for coolant increases too much, the ramp is stopped until the demand for control effort drops down to a normal level. [ABSTRACT FROM AUTHOR]

This paper regards the experimental study on the polymerization of vinyl chloride droplets in quiescent state using microreactors. As presented in a previous work performed with methyl methacrylate monomer, this work presents results on the reaction kinetics of vinyl chloride (VCM) polymerization monitored by Raman spectroscopy and on the evolution of VCM droplets as captured by charge-coupled device (CCD) camera in a microcapillary. Different experimental recipes were proposed using commercial initiators in order to compare the system performance when initiated with monofunctional and bifunctional peroxides. For the first time in the open literature the evolution of VCM droplets is shown up to high conversions ( > 90 % ) in quiescent state. It is also shown how the pressure drop can modify the PVC particle morphology at the end of polymerization. Finally, it is shown that the Raman technique is able to monitor the reaction kinetics at different conditions, being possible to identify four different characteristic stages during the vinyl chloride polymerization reactions. [ABSTRACT FROM AUTHOR]

The paper is devoted to the mathematical modeling of the dynamic behavior, which was observed experimentally during the study of methane oxidation over the two Pd foils coupled via the gas phase in a continuous stirred tank reactor (CSTR). The essential difference in the catalytic activity of the two identical Pd foils is simulated. Mathematical models, describing the reaction of methane oxidation over one Pd foil and over two identical pieces of the large foil divided in half are presented. At low oxygen pressure the symmetric steady state in the model for the two foils is stable and similar to that in the model of the single foil of a double area. With the inlet oxygen pressure increase the symmetric steady state becomes unstable via the pitch-fork bifurcation, and the two stable asymmetric states emerge. With a further oxygen pressure increase the state of one (active) foil approximately approaches the state of a single foil in the reactor with one foil, while the reaction rate on the other (passive) foil drastically decreases demonstrating that under some conditions only one foil can be active in a reactor with the two identical foils. The ‘anti-phase’ and the ‘in-phase’ oscillations of the reaction rate on two foils are also obtained in the simulations at low and high oxygen pressure in the agreement with the experimental observations. [ABSTRACT FROM AUTHOR]

The focus of this paper is especially on the role of combination termination in the context of 3-dimensional population balance modeling of trivariate chain length/number of branch points/number of combination points distribution for low-density Polyethylene. Termination by combination is a well-known termination mechanism in radical polymerization and regarding low-density Polyethylene many authors have assumed this mechanism to be present. The ratio between disproportionation and combination termination in the previous studies varies, but usually the two mechanisms are thought of equal importance. Full 3-dimensional population balance equations are solved by a combination of the 2-dimensional Galerkin–finite element method and the pseudo distributions method to estimate chain length/branching/number of combination points distributions. Termination by combination is treated in a different scheme than the framework employed for other kinetic reactions. Good agreement with Monte Carlo results has been observed for low-density Polyethylene-like kinetics. [ABSTRACT FROM AUTHOR]

Dynamic optimization solutions largely rely on the accuracy of the underlying mathematical models. However, these models only represent an approximation of the real dynamic process and their predictions are dependent on a set of parameter values. These parameter values can be hard to estimate exactly (e.g., thermal conductivity) or vary over time (e.g., due to fouling) potentially leading to hazardous situations when applying a model based optimal solution. Robust dynamic optimization deals with the uncertainty related to these parameters in order to quantify their effect and deliver safer (i.e., more robust) operating conditions. This paper discusses a computationally efficient robust dynamic optimization approach based on the Sigma Point method and shows how it outperforms a linearization-based method for a nonlinear dynamic chemical vapor deposition reactor case-study with multiple uncertainties. Moreover, by accounting for uncertainty a trade-off between process safety and performance of the reactor is introduced. This aspect is cast in a multi-objective dynamic optimization framework. In particular, it is illustrated how an increasing robustness (i.e., process safety) induces a worsening of other investigated objective functions and results in robustified Pareto sets. [ABSTRACT FROM AUTHOR]

In this paper a new approach is presented for synthesis of adiabatic reactor networks (ARNs) using genetic algorithm (GA) coupled with quasi linear programming (LP) method. In most cases reactor network (RN) synthesis leads to mixed integer non-linear programming (MINLP) problems or complicated non-linear programming (NLP) problems. In the suggested method to prevent complexity, GA is used to find the best configuration while continuous variables are handled using a quasi LP formulation. In quasi LP model continuous variables are regrouped into (a) normal variables such as concentration, temperature and flow rate (b) variables which make the equations nonlinear such as conversion in each reactor, split ratio of streams, recycles and exit temperatures from coolers or heaters. To optimize continuous variables the quasi LP consists of two nested loops. In the outer loop variables of group (b) are optimized while variables of group (a) are obtained using known values of group (b) in the inner loop. Therefore the complex MINLP or NLP model is converted to a quasi LP which is much easier to solve. Although in the RN only continuous stirred tank reactors (CSTR) and plug flow reactors (PFR) are used. The cases studied showed that this method can reach near optimum and in some cases better solutions compared to the literature. [ABSTRACT FROM AUTHOR]

A full extensive 2-dimensional (2D) model of low-density Polyethylene chain length/branching distribution is provided in this paper. 2D Population Balance Equations are solved by the Galerkin-finite element method to obtain the distributions, accounting for 2D ‘topological scission’; a manner to approximate random scission by an estimated fragment length distribution. Explicit assumptions have been made concerning the redistribution of branch points on scission fragments. The calculations without scission show that in the 2D case a finer grid of chain length is required in comparison to the 1-dimensional case because of the narrow shape of the branching distribution at the given chain length. With a fine grid, results are in exact agreement to Monte Carlo simulations, which demonstrates the success of the implementation. In the present implementation it was necessary to assume a fragment length independent redistribution of branch points, as was done in an earlier 2D model by Krallis et al. (2007) and Krallis and Kiparissides (2007) . We demonstrated that the shape of the branching density distribution is sensitive to this assumption. [ABSTRACT FROM AUTHOR]

This paper demonstrates the integration of process safety concepts (e.g., reactive chemicals, inherently safer design) with classical reaction engineering/system fundamentals (e.g., first-principles reactor modeling, system dynamics and stability) for identifying economically viable and safe continuous stirred tank reactor (CSTR) designs for scale-up of batch-wise fine chemical and pharmaceutical syntheses. As an illustrative and industrially relevant example, the homogeneous catalytic exothermic N-oxidation of alkylpyridines is investigated using a jacketed CSTR model. Reactor steady-state multiplicity and oscillation phenomena, which may bring about serious incidents, are analyzed based upon system equations. Reactor design and operation conditions as well as start-up strategies and worst-case scenarios are systematically investigated to address process safety concerns and balance reactor efficiency. [ABSTRACT FROM AUTHOR]

This paper is devoted to the prediction of self-ignition and quenching limits in a catalytic micro-burner in the context of a zero-dimensional approach that was originally developed by Semenov and Zel’dovich for the analysis of self-accelerating chemical reactions and critical phenomena in an ideal homogeneous chemical reactor. We formulated the analytical criteria for self-ignition and quenching in the catalytic micro-burner, and the sensitivity equations that show the effect of the variations of geometrical, chemical and regime parameters on the boundary of the operating regimes. We compare the theoretical self-ignition and quenching limits with those known in the literature for the CH 4 –air and CH 4 /H 2 –air mixtures obtained on the micro-structured burner with a Pt-LaMnO 3 catalytic honeycomb. Agreement was found between the theoretical and experimental results. Our semi-empirical analysis explains qualitatively the different effects of hydrogen addition to a methane–air mixture on self-ignition and the quenching limits that were observed in the analyzed experiments, and predicts these limits for the operation regimes that were not tested in the experiments. We perform the sensitivity analysis of the self-ignition problem, which shows how small variations in the geometrical, chemical and thermodynamic parameters influence the minimal ignition temperature in the analyzed experiments with the CH 4 –air and CH 4 /H 2 –air mixtures, and propose a way to perform sensitivity analysis for the quenching problem. [ABSTRACT FROM AUTHOR]

This paper describes the use of a two dimensional heterogeneous computational model for the catalytic reverse flow reactor for the combustion of lean methane mixtures. A uniform design method is used to generate a table of 41 operating points. The results from these points were used to generate a simple empirical model for the reactor at a single specified value of the superficial gas velocity. Stability maps that show the envelope for operation that satisfy a minimum level of conversion and a maximum operating temperature are generated using the empirical model. These maps delineate all possible combinations of methane concentration, switch time and mass extraction rate that are capable of giving stable operating conditions that meet these two criteria. The model is potentially useful in model predictive control of the reactor. [ABSTRACT FROM AUTHOR]

The chemical system of monomolecular reactions among n species, first analysed in the classical paper by Wei and Prater (1962) was revisited. It was shown that using symmetry relationships, based on all the equilibrium constants and n ( n − 1 ) / 2 known dependences, all remaining kinetic dependences can be calculated. A new method of closing the mass balance for such a complex chemical system was developed, using only measurements of one or a few species, and based on symmetry relations. A new method of testing completeness of a kinetic description was proposed, using the properties of the trace sum of components, i.e., the sum of all concentration dependences, each of which is started from maximal initial concentration. [ABSTRACT FROM AUTHOR]

This paper presents the results of the characterization of a new method for quantification of the micromixing efficiency in a viscous medium in stirred tank reactors. The used method is inspired by a chemical system based on two competitive reactions, the iodide–iodate reaction system ( Villermaux–Dushman method), whose products selectivity is a measure of the mixing efficiency. The approach proposes a new protocol based on a phosphate buffer and the use of HydroxyEthylCellulose (HEC, 720, 000 g/mol) as an inert viscosifying agent, exhibiting various interesting properties (no disturbance of the spectrophotometric measure, low cost, low required amount, eco-friendly product). This agent is almost Newtonian up to 0.25 wt% in water and becomes non-Newtonian above 0.5 wt%. By means of its use, the viscosity of the reactive medium can be raised by around two orders of magnitude while adding less than 1 wt% HEC to the water solution. Preliminary micromixing results of moderate viscous solutions (50 mP as with HEC 0.5 wt%) in a stirred tank reactor are presented. [ABSTRACT FROM AUTHOR]

Periodic operation of a non-isothermal CSTR subject to a single input modulation is analyzed considering a n th order reaction and using the nonlinear frequency response (NFR) method, introduced in our previous publications. The method is based on deriving the asymmetrical second order frequency response function (FRF) and analyzing its sign. In Part I of this paper, the periodic modulations of inlet concentration or flow-rate of the reaction stream were investigated. In this Part II, periodic operations are analyzed for modulations of the temperature of the inlet reaction stream or the temperature of the heating/cooling fluid. Conditions that need to be fulfilled in order to achieve process improvement through periodic operation are identified. The method is applied for the same numerical example taken from literature which was used in Part I. The results obtained by the NFR method are compared with the results of numerical simulations. Good agreement is obtained, except for forcing frequencies close to the resonant frequency. In these cases, even the sign of the DC component was not predicted correctly. [ABSTRACT FROM AUTHOR]

Periodic operations of a non-isothermal CSTR with n-th order reaction, subject to a single input modulation, is analysed using the nonlinear frequency response (NFR) method, introduced in our previous publications. The method is based on deriving the asymmetrical second order frequency response function (FRF) and analysing its sign. In Part I of this paper, periodic operation with modulation of the inlet concentration or flow-rate of the reaction stream is analysed. As a result, conditions regarding the reaction order, process parameters and frequency of the input modulation are identified that need to be fulfilled in order to achieve process improvement through the periodic operation compared to conventional steady state operation. The method is applied for a numerical example from literature and the results obtained by the NFR method are compared with the results of numerical simulation. Good agreement is obtained, except for imposed forcing frequencies close to the resonant frequency and high forcing amplitudes. [ABSTRACT FROM AUTHOR]

This paper proposes a risk-based process performance assessment methodology using loss functions. The proposed method helps to overcome the existing challenges in assessing impacts of deviations of process variables on safety and economy of a process operation. The inverted Beta loss function is used to incorporate the effects of process deviations on the safety and quality losses. The demand rate adjustment factor is used to model the effect of process deviations on the failure probability of safety systems. The probability of a failed process state due to abnormal events is continuously updated based on the current value of the characteristic variables. The use of the loss function approach in combination with probability updating provides a continuously revised risk estimation. Such a real-time risk profile provides a leading performance indicator for decision-making at an operational level. As an example, a temperature surge in a continuous stirred tank reactor is used to demonstrate the efficacy of the proposed methodology. [ABSTRACT FROM AUTHOR]

Abstract: This paper presents numerical study of co- and counter-flow arrangements for catalytic plate reactors (CPR). CH4 steam reforming coupled with CH4 oxidation is simulated using detailed surface reaction mechanisms. Effect of inlet velocities to the reforming channel, oxidation channel, and material properties of the plate on the resulting plate temperature and CH4 conversions is studied. The simulation results agree very well with an industrial scale reformer unit and calculations are further carried out to evaluate the number of CPRs and stacks required to replace and industrial unit. [Copyright &y& Elsevier]

Abstract: Gas–solid fluidized-bed reactors are complex systems that involve multiple interrelated phenomena. One key phenomenon that affects reactor performance is the change in gas volumetric flow. The volumetric flow in a fluidized-bed reactor can change with height due to reaction, feed distribution, change of phase, selective removal/addition of species and changes in temperature and pressure. This paper presents results from an experimental program and gives a qualitative comparison with modeling results obtained with computational fluid dynamics (CFD). A high-pressure two-dimensional column was used to investigate the effects of sudden increase/decrease in pressure on fluidization behavior by recording multiple bubble properties using high-speed video recording. A CFD model was developed to investigate the same variations in process conditions. Changing the volumetric flow can radically alter the bed hydrodynamics. The CFD code is able to predict the experimental trends correctly. [Copyright &y& Elsevier]

In this paper, scalar mixing by several arrays of vortex generators mounted inside a circular pipe is investigated using numerical simulations. Two flow configurations are studied in which the arrays are in-line and rotated periodically by an angle of 90°. Each vortex generator creates a pair of streamwise vortices which enhances the mixing process in the flow cross-section. It is shown that the alternate configuration, in which the vortex generators are rotated periodically by an angle of 90°, enhances the mixing process relative to the in-line one due to the generation of chaotic advection flow, while in the in-line configuration the flow is regular and the mixing process is only caused by the convective motion of the longitudinal vortices. Both Eulerian and Lagrangian analyses are used to investigate the chaotic behavior. From the Poincaré sections, the alternate rotation of the vortex generators is found to better disperse the fluid particles in the flow cross-section, while in the in-line array the particles are trapped into the vortex core. The Lagrangian study shows that initially close fluid particle paths exhibit an exponential remoteness in the alternate configuration, a sign of chaotic advection flow. This chaotic advection enhances the stretching and folding of the fluid particles which are responsible for mixing in laminar flows. The proposed flow configuration can be used as a multifunctional heat exchanger/reactor for industrial applications such as in chemical reaction and food processing. [ABSTRACT FROM AUTHOR]

Abstract: Water splitting under sunlight illumination in the presence of semiconductor photocatalyst is a very promising way to produce clean hydrogen fuel. Solar hydrogen can be obtained in two routes: photoelectrochemical (PEC) water splitting based on immobilized photocatalysts in thin films and photocatalytic (photochemical) water splitting based on powder photocatalysts in slurry system. Over the past several decades, tremendous research work has been devoted to exploring new semiconductor materials suitable for PEC and photochemical systems and understanding the underlying mechanism of the water splitting process. However, much less attention has been paid to the design of photocatalytic reaction systems or reactors, which is indeed critically important for the overall solar energy conversion performance. This paper summarizes the basic working mechanisms of both PEC and photochemical systems, and gives an overview of a variety of photoreactor design and development. [Copyright &y& Elsevier]

• Quantifying forced periodic operation of chemical reactor. • Analysis by the Nonlinear Frequency Response method. • Methanol synthesis in isothermal and isobaric lab-scale CSTR. • Single input modulations (partial pressures of all reactants and volumetric flow-rate) • No significant improvement could be obtained with single input modulations. In this two-part paper a comprehensive study of the potential to improve performance criteria of a methanol synthesis reactor through forced periodical operations is presented. The study uses the Nonlinear Frequency Response method, a powerful analytical and approximate tool which gives an answer whether and under which conditions certain periodic operation would lead to improvement of process performance. To demonstrate the method, isothermal and isobaric methanol synthesis in a lab-scale CSTR is considered. In Part I, the analysis is performed for single input modulations. Partial pressures of each reactant in the feed stream and the total inlet volumetric flow-rate are considered as possible modulated inputs. The results show that modulations of single inputs essentially do not provide potential for significant improvements. In Part II, the study will be extended to analysis of periodic operations with simultaneous modulations of two inputs and conditions offering significant performance enhancements will be identified. [ABSTRACT FROM AUTHOR]

Abstract: The problem of modeling a class of two-phase moving-bed tubular gasification reactors by means of a lumped (finite-dimensional) representation is addressed in this paper. A model is designed, as simple as possible, in the light of a specific – experimental, equipment, operation, monitoring or control – design task and the uncertainty of the underlying kinetics and transport parameters. First, the enforcement of quasi-steady state (QSS) gas-phase assumptions and stoichiometric considerations followed by spatial finite-difference approximation plus interpolation leads to a representation of the tubular reactor as a train of N continuous stirred tank reactors (CSTRs). Then, the number of tanks and their volumes are chosen according to the modeling objective. The proposed approach is illustrated with a case example, studied before with experiments and PDE-based simulations, finding that the dynamics of the tubular reactor can be modeled with three CSTRs (9-ODE). [Copyright &y& Elsevier]

Abstract: The crystal product quality, functionality and properties such as flow characteristics are greatly influenced by the crystal size and shape distributions. Hence, prediction and control of crystal size and shape distributions are important from the viewpoint of smooth operation and product quality control. The contribution of this paper is twofold: firstly, it describes the modelling and simulation aspects for prediction of crystal shape distribution in the presence of crystal growth modifiers (CGMs). Secondly, a control setup is proposed that manipulates the CGM concentration in the crystallizer in order to achieve the desired crystal shape. The effect of CGM on crystal shape is modelled using a morphological population balance model (PBM) in combination with Kubota–Mullin's model for the pinning mechanism of CGM molecules on the crystal face to take into account their possible hindering effect on crystal growth (Kubota and Mullin, 1995; Kubota, 2001). Subsequently, a simple feedback control configuration is implemented in the proposed control setup which manipulates the CGM concentration profile in the crystallizer. The control setup uses a hybrid continuous-batch operation of the crystallizer, in which the solid phase is operated in a batch mode, whereas the liquid phase is operated in a continuous mode maintaining a constant volume in the reactor. This hybrid operation allows the dynamic control of the CGM concentration during the batch. Using potassium dihydrogen phosphate (KDP) as a model system, the simulation results demonstrate that, in principle, the crystal shape distribution can be controlled by manipulating the CGM concentration profile. [Copyright &y& Elsevier]

Abstract: This paper deals with the modeling and optimization of a photochemical reactor for the production of hydroxyl radicals. These radicals can be used in advanced oxidation processes for water purification. The UV-lamps used in the photochemical reactor lead to a relatively high energy and operating cost. Minimization of these costs is possible via a model-based optimization of the reactor geometry. First, a model is built taking into account the reaction kinetics, the reactor geometry and the radiation intensity distribution in the reactor. Afterwards, two different configurations of photochemical reactors are optimized. The first modeled reactor configuration is a single-lamp reactor and consists of a cylindrical UV-lamp that is present inside a quartz sleeve, outside of which the water flows between it and the external reactor wall (annular channel reactor). A rigorous optimization problem is formulated consisting of the calculation of the optimal reactor geometry (length and radius) in such a way that, for a given reactor volume and a given flow rate of the water to be treated, the mean outlet concentration of an organic pollutant is minimized. By solving this optimization problem, it has been shown that an optimal reactor length exists. If the absorption coefficient of the water or the reaction rate constant increases, the optimal reactor length increases as well. The second modeled reactor configuration is similar to the first one but now contains multiple lamps positioned symmetrically in a circular pattern (i.e. multi-lamp reactor). The optimization problem consists of the calculation of the optimal lamp position inside the reactor in such a way that the mean outlet concentration of an organic pollutant is minimized. The lamp position is given by the radial distance between the centers of the reactor and a lamp. Again a well-posed optimization problem is obtained and an optimal lamp position exists. If the number of lamps or the reaction rate constant increases or the absorption coefficient of the water decreases, the optimal distance between the centers of the reactor and the lamp increases. [Copyright &y& Elsevier]

Abstract: Mixed electronic and ionic conductors are of potential interest in improving the efficiency of high temperature processes that require oxygen. One of the primary industrial applications for these conductors is in the separation of oxygen from air for the production of hydrogen through the partial oxidation of methane (POM) or oxy-combustion at high temperatures. This brief review provides a methodical approach for the selection of a membrane material for the POM applications. From a practical point of view, this paper focuses on perovskite or perovskite-derived structures. The primary performances and characteristics of the published data on perovskite materials are compared, such as the oxygen semi-permeation, the coefficient of thermal expansion, and the chemical stability at high temperature under a reducing atmosphere. The most promising membrane materials and key structural parameters are identified and discussed. [Copyright &y& Elsevier]

Abstract: This paper describes an efficient method for the functionalizing of multi-walled carbon nanotubes (MWCNT) using oscillatory flow mixing (OFM). A 3l batch oscillatory flow reactor (OFR) was designed and constructed for pilot scale functionalization of MWCNT in order to potentially improve their compatibility within a thermoplastic polyphenylene sulphide (PPS) matrix. The OFM batch reactor consisted of a jacketed cylindrical vessel with a vertical axial oscillator that contained a series of baffled mixing plates. MWCNTs dispersed in dimethylformamide (DMF) were introduced into the reactor and a two stage reaction for functionalizing MWCNTs with PPS compatible groups was carried out under oscillation of baffles at elevated temperatures. Fluid mixing observations in the reactor showed that MWCNTs formed a uniform dispersion of aggregated flocs before and during the functionalization reaction. On completion of the reaction and cessation of the oscillation, the aggregated flocs of MWCNT rapidly sedimented at the bottom of the reactor; hence could be collected as a concentrated mass thereby facilitating the separation of functionalized MWCNTs from the solvent. The functionalized MWCNTs were dried and then characterized by transmission electron microscopy, infrared spectroscopy as well as thermal gravimetric analysis in order to investigate the extent of MWCNT functionalization. The characterization results confirmed the effective and relatively uniform functionalization of the MWCNTs despite formation of aggregates, indicating that OFM provides a viable approach for functionalizing MWCNTs. [Copyright &y& Elsevier]

Abstract: Sorption enhanced methane reforming (SER), employing a CaO-based solid as a high temperature CO2 sorbent, is generally considered to be a promising route for H2 production. In this paper we present a dynamic pseudo-homogeneous model to describe the operation of a packed bed reactor in which the SER reaction is carried out under adiabatic conditions. This reactor can be implemented according to several process schemes, including a novel Ca/Cu looping process for hydrogen generation with inherent CO2 capture. The proposed SER model is based on the well-established principles of gas–solid contact and heat transfer in fixed-bed reactors and on the kinetic expressions published in the literature that describe the main reactions involved in the process. The resulting model describes the transient performance of the SER reaction and confirms the theoretical viability of this critical reaction stage in a large scale H2 production facility. It is demonstrated that the SER process can yield a CH4 conversion and H2 purity of up to 85% and 95%, respectively, under operating conditions of 923K, 3.5MPa, a steam/carbon ratio of 5 and a space velocity of 3.5kg/m2 s. [Copyright &y& Elsevier]

Abstract: The dynamic optimisation and optimal control of innovative cyclic chemical processes are challenging because these problems involve apart from temporal periodicity constraints often also Partial Differential Equation (PDE) models and multiple objectives. To allow a fast and efficient generation of the Pareto set, this paper presents a deterministic approach that integrates accurate scalarisation methods for multi-objective optimisation as Normal Boundary Intersection (NBI) and (Enhanced) Normalised Normal Constraint (E)NNC, with a fast direct multiple shooting approach for optimal control. The feasibility of the proposed approach is presented for a reverse flow reactor involving conflicting conversion and energy objectives. In addition, advantages of the NBI and (E)NNC methods over the classic Weighted Sum of the individual objectives have been illustrated. [Copyright &y& Elsevier]

Abstract: The high water recycling ratio used in some industries can cause nutrients to accumulate in the system, leading to thick, rapidly growing biofilms inducing detrimental effects. Nowadays, chemical treatments are widely used but the increasingly restrictive environmental regulations induce the necessity to take the environmental aspect into account in the choice of a suitable treatment. With this in mind, this study analyses a method for controlling thick, dense biofilm development by applying synergistic actions having low environmental impact, i.e., an enzymatic and a mechanical (shear stress) treatment well suited to industrial water networks. For that purpose, biofilms were grown on plastic plates set in a Couette–Taylor reactor (CTR) that was inoculated using a “white water” sampled from a paper industry network. Development of the biofilms was controlled by applying a constant continuous feeding with a high COD/N ratio of 20, a well-defined shear stress and an external aeration, those conditions leading to a well-controlled G-value and a γ S/O2 value close to 1, and consequently, to the development of thick mainly aerobic heterotrophic biofilms representative of that encountered in high loaded industrial water networks. Optimal operating conditions for biofilm treatments were determined considering the penetration time for the enzymatic treatment taking into account both internal and external mass transport (using Biot number calculation). In addition, an optimal shear stress increment of 2.5Pa was selected in order to maximise biofilm detachment while avoiding compression phenomena due to the mechanical treatment. The combined treatment led to an increase of 80% in biofilm mass removal (COD) compared to the enzymatic treatment alone and removed a large part of the basal layer of the biofilm, 80% reduction being observed in the support coverage. [Copyright &y& Elsevier]

Abstract: In this paper, the mass transfer phenomenon during hydrate formation from methane is explained. It is demonstrated that during the hydrate formation of methane, the mass transfer in the liquid side of the contact area of gas–liquid controls the gas consumption. Two types of experiments (isochoric and isobaric) have been conducted in a stirred vessel to indicate that the gas uptake rate is independent of the number of hydrate particles and their total area. Data shows that the mass transfer coefficient is a function of pressure and temperatures. [Copyright &y& Elsevier]

Abstract: The saddle-focus bifurcation and chaotic behavior of a continuous stirred tank reactor (CSTR) with two PI controllers (PI1 and PI2) to produce propylene glycol is analyzed in this paper. First, a simplified reactor model is used to show that disturbances in the inlet flow rate and variation of the parameters in the PI2 controller may give rise to a homoclinic orbit. Then, a complete reactor model is considered, for which the analysis of the saddle-focus bifurcation around the homoclinic orbit demonstrates the presence of chaotic dynamics. The chaotic behavior is corroborated from the sensitive dependence, Lyapunov exponents, power spectral density, Poincaré sections and bifurcation diagrams. Once the chaotic dynamics has been reached, it is possible to modify the strange attractor by changing the proportional PI2 constant, which in turn allows improving the reactor performance in chaotic regime. The results of the analytical calculations are verified throughout full numerical simulations. [Copyright &y& Elsevier]

Abstract: In this paper, the dimensionless reaction rate density of a two-dimensional channel with catalyst on its boundary was maximized by optimizing its aspect ratio H/L. Order of magnitude expressions in the limit of small H/L and in the limit of large H/L were developed and then intersected to predict the optimal aspect ratio and maximum reaction rate. Numerical simulations were performed and the results corroborated very well with the scale analysis. The results showed that the optimal design of microreactors depends on the Schmidt number (Sc), the Bejan number (Be) and the Catalyst number (Ct). Correlations based on those numbers are proposed. The analysis was performed for steady-state laminar flows, using the dilute species assumption and a first order kinetic expression. [Copyright &y& Elsevier]

Abstract: Uniform and efficient particulate–liquid contact is of critical importance in solid–liquid systems. Typically, complete homogeneity of the particulate suspension is desired, but not achieved. In mechanically agitated contactors, the critical impeller speed for off-bottom suspension, N js, and cloud height are used to assess solids distribution across the axial axis as an indicator of suspension efficiency. Further, various, but inconsistent, mixing indices have been proposed previously to quantify homogeneity. In this paper, the definition of mixing indices is extended to describe homogeneity in terms of the average of the standard deviation in solids concentration across the axial and radial directions as well as across the overall reactor. These mixing indices are quantified using data collected using electrical resistance tomography (ERT) in which an unstructured mesh matching the vessel geometry was used during reconstruction. The average standard deviation of solids concentrations in each volume element across four planes and seven annuli provided the axial (MIz) and radial (MIr) mixing indices. Combined data across the 28 regions provided the overall mixing index (MIo). These indices were compared across three particle size distributions, three solids concentrations and five impeller speeds. The data demonstrate inhomogeneity in solids concentration across both the radial and axial directions as a function of operating conditions. For the coarse and medium particle fractions (600–850μm and 425–600μm), MIo is dominated by MIz whereas MIr dominated MIo for the fine particle fraction (106–212μm). In all cases, homogeneity of suspension decreases with decreasing solids concentration across the range 5–20% by volume. Homogeneity improves with increasing impeller speed, up to approximately 0.8N js for the medium and coarse fractions. Little dependence of mixing index on impeller speed was observed for the fine fraction across the range 0.5N js to 1.4N js. The limited dependence of homogeneity on impeller speeds above 0.8N js suggest significant energy savings can be achieved without loss of suspension by operating at impeller speeds below N js. [Copyright &y& Elsevier]