Comment on “Simulation and experimental verification of the filtration and filter cake compression model”

Comment on “Simulation and experimental verification of the filtration and filter cake compression model” by Kocurek and Palica [Kocurek, J., Palica, M., 2005. Simulation and experimental verification of the filtration and filter cake compression model. Powder Technology 159 (1) 17-26.].     

Mixing in a vibrated granular bed: Diffusive and convective effects

In this study, Discrete Elementary Method (DEM) is employed to simulate the motion and mixing behavior of granular materials in a three-dimensional vibrated bed, which is energized by vertical sinusoidal oscillations under different vibrating conditions. With frictional sidewalls, the convection flow is a very important phenomenon in the vibrated granular bed. The influence of vibrating conditions, including vibration acceleration and frequency, on the formation of symmetric convection flow is investigated in this study. In order to characterize the convective flow and the diffusive motion of granular materials, the dimensionless convection flow rate, J_conv, and the vertical self-diffusion coefficient, D_yy, are defined, respectively. Péclet number, Pe, is employed to characterize the ratio of the convective flow to the diffusive motion in vertical direction. The role of Pe in the formation of symmetric convection flow is discussed in detail. Moreover, the top-bottom initial loading pattern of two groups of glass beads with different colors is employed to investigate mixing behavior of granular materials. The well-known Lacey index is employed as the mixing degree, M, to quantify the mixing quality. The mixing rate is calculated from a least-square fit using the time evolution of M. The simulation results demonstrate that the mixing rates increase with increasing J_conv and D_yy in exponential relations.     

Sorption and Permeation of Aqueous Alkyl Piperazines through Hydrophilic and Organophilic Membranes: A Transport Analysis

ABSTRACT

A detailed analysis of separation of N-methyl piperazine (NMP), N-ethyl pipera-zine (NEP), and water was undertaken by the pervaporation technique. A systematic study of sorption and permeation of the aqueous alkyl piperazines through poly(dimethylsiloxane) (PDMS), styrene-butadiene rubber (SBR), PDMS filled with zeolites NaX and silicalite (SA-5), polyimide (PI), and poly(acrylonitrile-co-acrylic acid) (PAN-co-AA) was carried out at different concentrations and temperatures. Organophilic membranes showed higher selectivity toward alkyl piperazines during sorption, but permeation was in favor of water. Hydrophilic membranes, however, showed higher affinity toward water during both sorption and permeation. PI membrane showed higher selectivity for water than PAN-co-AA. A model was used to estimate the diffusion coefficients of the various permeants. It was found that the transport selectivity for water in organophilic membranes was due to high diffusion selectivity (for water) although sorption selectivity favored the piperazines.     

Continuous chromatographic processes with a small number of columns: Comparison of simulated moving bed with Varicol,PowerFeed, and ModiCon

The Simulated Moving Bed process and its recent extensions called Varicol, PowerFeed and ModiCon are studied, in the case where a small number of columns are used, i.e. from three to five. A multiobjective optimization approach, using genetic algorithms and a detailed model of the multicolumn chromatographic process, is applied to optimize each process separately, and allow for comparison of the different operating modes. The non-standard SMB processes achieve better performance than SMB, due to the availability of more degrees of freedom in the operating conditions of the process, namely the way to carry out asynchronous switches for Varicol, and the different flow rates and feed concentration during the switching interval for PowerFeed and for ModiCon, respectively. We also consider the possibility of combining two non-standard operating modes in a new hybrid process, and evaluate also in this case the possible performance. Finally, a critical assessment of the results obtained and of the potential for practical implementation of the different techniques is reported. This paper is dedicated to Professor Hyun-Ku Rhee on the occasion of his retirement from Seoul National University. The importance of the seminal papers of Professor Rhee on first order partial differential equations and the theory of multicomponent chromatography (H.-K. Rhee, R. Aris, N. R. Amundson,Philos. Trans. Roy. Soc. London,A267 (1970) 419–455;A269 (1971) 187–215) for the SMB design and optimization tools (Triangle Theory) that the authors have developed over the years cannot be overestimated.     

Ablation and thermal degradation behaviour of a composite based on resol type phenolic resin: Process modeling and experimental

During atmospheric re-entry, ballistic or space vehicle is subjected to severe aerodynamic heating and its successful return through the Earth's atmosphere depends largely on the provision that is made for reducing aerodynamic heat transfer to its structure. For this purpose ablative heat shield is normally used which undergoes physical, chemical, and mostly endothermal transformations. These transformations produce new liquid or gas phases which are subsequently injected into the environment.Mass and energy balance equations have been solved in order to model the ablation and thermal degradation behaviour of an ablative composite. A method to determine and calculate some of the parameters in the ablative equation is proposed from the simultaneous thermal gravity and differential scanning calorimetry analysis techniques.The objective of this work is to model ablating, charring and thermal degradation behaviour of a heat shield resol-type phenolic resin/asbestos cloth composite in oxyacetylene flame test. This requires solving serious heat transfer equations with moving boundaries. Explicit forward finite difference method (FDM) is used for heat transfer calculation. Moving boundaries are fixed by the Landau transformation. The ablation equation has been solved numerically.Temperature distribution through the composite thickness, temperature of moving surface, and the rate of moving boundary changes are evaluated. The results are in a good agreement with the experimental data obtained from oxyacetylene flame tests. The model can successfully be used for both material selection and thickness calculation in the design of thermal protection shields.     

Effect of temperature on gas adsorption and separation in ZIF-8: A combined experimental and molecular simulation study

In this work, the effect of temperature on adsorption of CO₂, CH₄, CO, and N₂ and separation of their binary mixtures in ZIF-8 were investigated using experimental measurements combining with molecular simulations. The results show that for pure gas adsorption, the effect of temperature is large when strong adsorption occurs, mainly due to the variation of the interaction energy between adsorbate molecules with temperature; while for gas mixtures, systems with large selectivity are more sensitive to temperature. In addition, this work shows that temperature influences the working capacity of CO₂ in temperature swing adsorption (TSA) process with the interplay of pressure, which should be considered in the design of TSA process in practical applications.     

Semi-batch extraction of anthocyanins from red grape pomace in packed beds: experimental results and process modelling

A semi-batch extraction process of anthocyanins was studied in a packed bed. Methanol was used as solvent and the raw material studied consisted of skins of the tempranillo grape, which was obtained from the pomace from red wine vinification. The results show large diffusional effects due to strong control from the mass transfer. The best results were obtained using a high temperature and a high flow rate. A penetration model is proposed in terms of a bed of spherical particles. The model provides a value for the diffusion coefficient of the solute within the solid matrix. It can be seen that the adjustment of the model is satisfactory and that it is able to predict, to a reasonable extent, the yield of the extraction process.     

Early development of properties in a cement paste: A numerical and experimental study

This paper presents the results of an experimental and numerical study of the properties of a high-performance Portland cement paste (w/c ratio 0.37; 5% silica fume) cured at 20 °C in sealed conditions for 5 days. Properties such as electrical conductivity, strength, stiffness, porosity, Vicat penetration, and autogenous deformation were measured and modelled. The kinetics of hydration was studied by means of isothermal calorimetry. The numerical simulations were performed with CEMHYD3D, developed at the National Institute of Standards and Technology, and HYMOSTRUC, developed at the Delft University of Technology. The results of the simulations were compared with experimental data, and the match was good. Clear correlations were found among electrical conductivity, autogenous shrinkage, and connectivity of solids.     

Polymerisation of methyl methacrylate in a pilot-scale tubular reactor: modelling and experimental studies

A pilot-scale tubular reactor fitted with in-line static mixers is experimentally and theoretically evaluated for the polymerisation of methyl methacrylate (MMA). A non-isothermal and non-adiabatic axially dispersed plug-flow model is used to describe the flow characteristics of the reactor. The model is applied to the polymerisation of a concentrated MMA solution (up to 72% (v/v)). Key model parameters were attained through independent bench and pilot-scale experiments. Measured monomer conversions and polymer molecular weight were accurately predicted by model simulation. The presence of static mixers is shown to give near-ideal plug-flow operation for the experimental conditions of this study. Furthermore, an approximately four-fold increase in overall heat transfer coefficient is indicated due to the radial mixing incited by the mixers. Studies also demonstrated the importance of inhibitor kinetics on the dynamic and steady-state performance of the reactor.     

Fluidized bed ash cooler used in a circulating fluidized bed boiler: An experimental study and application

The performance of a fluidized bed ash cooler was studied using a cold experiment bench. The air flow rate, particle size of the solids and air distributor type are considered to be the key parameters of the operation of a fluidized bed ash cooler (FBAC). It was found that the amount of fluidizing air directly affects the conveying of ash and that they have a quadratic relationship. An acceptable particle size distribution for the solids is d₅₀ < 450 μm. Furthermore, the experiment reveals that the height of the weir in the FBAC does not affect the conveying of the ash flow. The influence of injected air on the transportation of ash is not significant. Optimal air nozzle dimensions are presented.     

Flow regimes in a spout-fluid bed: A combined experimental and simulation study

Spout-fluid beds find a widespread application in the process industry for efficient contacting of large particles with a gas. However, detailed understanding of the complex behavior of these systems is lacking, which leads to significant scale-up problems in industry. In this paper we report results of a combined experimental and simulation study on the various regimes, which can be encountered during spout-fluid bed operation.A regime map for a 3D spout-fluid bed was composed employing spectral analysis of pressure drop fluctuations and fast video recordings. In addition 3D Euler-Lagrange computations were performed to assess the capability of the model to reproduce the experimentally observed flow regimes.The influence of the drag closure on the model results was assessed and the influence of the computational grid was studied using a new method for the implementation of the two-way coupling, which is proposed in this paper.For most regimes our model is able to predict the appropriate regime. The frequency, at which the largest power is found, is overpredicted by the model. Contrary to the experimental observations, our model did not predict any large slugs in the slugging bed regime.The remaining differences between the simulated and experimentally observed bed behavior is most likely related to the representation of the effective fluid-particle interaction in our model, which relies on local spatial homogeneity.     

Hydrodynamics of a dual fluidized bed gasifier. Part II: simulation of solid circulation rate, pressure loop and stability

This paper focuses on the determination of the solids circulation of a CFB gasification system with a dual fluidized bed concept, and the distribution of the solid hold up under different fluidization conditions. A mathematical model of the riser was designed and implemented in a model of a dual fluidized bed system. This model contains routines for calculation of each section of the dual fluidized bed system. The behaviour of the system was analysed regarding changes in solid inventory and variations of geometry. A diagram is presented which allows an illustration of the influence of changes in the dual fluidized bed system configuration on the resulting stable operation points. Analysis concerning the effect of counter pressure on the combustion and gasification side confirms the role of the seal loop in stabilizing the operation of the gasification system.     

Simulation and experimental studies on fluidization properties in a pressurized jetting fluidized bed

The influence of pressure on the bubble size and average bed voidage has been investigated experimentally and computationally in a circular three-dimensional cold-flow model of pressurized jetting fluidized bed of 0.2 m i.d. and 0.6 m in height with a central jet and a conical distributor, which roughly stands for the ash-agglomerating fluidized bed coal gasifier. The pressurized average bed voidage and bubble size in the jetting fluidized bed were investigated by using electrical capacitance tomography (ECT) technique. The time-averaged cross-sectional solids concentration distribution in the fluidized bed was recorded. The influence of pressure on the size of bubble and the average bed voidage in a pressurized fluidized bed was studied. Both experimental and theoretical results clearly indicate that there is, at the lower pressure, a small initial increase in bubble size decided by voidage and then a decrease with a further increase in pressure, which proves the conclusion of Cai et.al. [P. Cai, M. Schiavetti, G. De Michele, G.C. Grazzini, M. Miccio, Quantitative estimation of bubble size in PFBC, Powder Technology 80 (1994) 99-109]. At higher pressure, bubbles become smaller and smaller because of splitting. The average bed voidage increases gradually with the pressure at the same gas velocity. However, there is a disagreement between the experimental results and simulation results in the average bed voidage at the higher gas velocity, especially at the higher pressure. It suggests that the increase in density of gas with pressure may result in the drag increase and the drag model needs to be improved and revised at higher pressure.     

Adsorption of propane, propylene and isobutane on a metal-organic framework: Molecular simulation and experiment

The separation of propane/propylene mixtures is the most energy-intensive operation practiced in the petrochemical industry. Adsorptive processes are currently viewed as a promising alternative to cryogenic distillation for the separation of these mixtures. In this paper, we explore the possibility of using a new metal-organic framework material, CuBTC, in adsorptive separation processes, particularly in a simulated moving bed (SMB) context using isobutane as a potential desorbent. A gravimetric method has been used to measure the adsorption equilibrium isotherms of propylene, propane and isobutane onto a commercial CuBTC powder over a temperature range from 323 to 423 K and pressures up to 100 kPa. These were complemented by a detailed experimental characterization of the structure of CuBTC using XRD and SEM techniques. Comparison of experimental isotherms with grand canonical Monte Carlo simulations in CuBTC showed that propane adsorption occurs preferentially in small octahedral pockets, while isobutane is excluded from these pockets due to its bulky structure. Propylene was seen to interact strongly with unsaturated metal sites, due to specific π‐Cu bonds. These interactions significantly enhance the affinity of this MOF for unsaturated hydrocarbons. Furthermore, in a range of temperatures and pressures, the affinity of CuBTC for isobutane is intermediate to that of propane and propylene. Our results suggest that CuBTC-isobutane is a very promising adsorbent-desorbent pair for use in SMB processes for propane/propylene separations.     

Biomass steam gasification in fluidized bed of inert or catalytic particles: Comparison between experimental results and thermodynamic equilibrium predictions

In order to improve the understanding of biomass gasification in a bed fluidized by steam, the thermochemical equilibrium of the reactive system was studied. The equilibrium results were compared to LGC experimental results, obtained by the gasification of oak and fir in a laboratory-scale fluidized bed of different catalysts: sand, alumina, and alumina impregnated with nickel.The research was completed by a study of the influence on the equilibrium of additional parameters such as the quantity of steam, the pressure or the kind of biomass. Those results of simulation may be used for evaluating the limits of actual reactors.The following conclusion may be drawn from all the results:

The thermodynamic equilibrium state calculated is far away from the experimental results obtained on sand particles.

The steam to biomass ratio, between 0.4 and 1 kg_steam/kg_{dry biomass}, has a strong influence on the gas mixture composition.

The temperature increase and the use of catalyst allow producing a gas mixture with a high content of hydrogen and carbon monoxide. The H₂:CO ratio may reach values greater than 3.

The use of catalyst allows the system to get closer from the equilibrium, especially for the nickel based catalyst.

     

Mixing and segregation in a bidisperse gas–solid fluidised bed: a numerical and experimental study

In many industrial applications of dense gas–solid fluidised beds, mixing and segregation phenomena play a very important role. The extent of mixing and segregation is strongly influenced by the bubble characteristics. Therefore, the extent of mixing and segregation, induced by a single bubble injected in a monodisperse and bidisperse fluidised bed at incipient fluidisation conditions and in freely bubbling fluidised beds has been studied both with well-defined experiments and with a 3D Euler–Lagrangian model. Particle image velocimetry (PIV) was successfully applied to obtain the ensemble averaged particle velocity profile in the vicinity of the bubble in dense gas–solid fluidised systems.

The bubble size of a single injected bubble in a fluidised bed at minimum fluidisation conditions calculated with a 3D discrete particle model (DPM) depended strongly on the selected gas-particle drag model. The widely used Ergun equation combined with the Wen and Yu [Powder Technol. 98 (1998) 38; Chem. Eng. Sci. 47 (1992) 1913] relations overpredicted the bubble size due to an overprediction of the drag force. The DPM with the drag model proposed by Koch and Hill [Annu. Rev. Fluid Mech. 33 (2001) 619], based on Lattice–Boltzmann simulations, gave much better agreement with the experimental findings.

The segregation rates in a bidisperse freely bubbling fluidised bed predicted by the DPM agreed very well with the experimentally measured segregation rates by Goldschmidt [M.J.V. Goldschmidt, Hydrodynamic modelling of fluidised bed spray granulation, PhD thesis, Twente University, 2001].     

An experimental study on the primary fragmentation and attrition of limestones in a fluidized bed

In this paper, an experimental study on the primary fragmentation and attrition of 5 limestones in a fluidized bed was conducted. The intensity of fragmentation and attrition were measured in the same apparatus but at different fluidizing velocities. It was found that the averaged size of the particles decreased by about 10-20% during the fragmentation process. The important factors for particle comminution include limestone types, heating rate, calcination condition and ambient CO₂ concentration. Fragmentation mainly occurred in the first a few minutes in the fluidized bed and it was more intense than that in the muffle furnace at the same temperature. The original size effect was ambiguous, depending on the limestone type. The comminution caused by attrition mainly occurred during calcination process rather than sulphation process. The sulphation process was fragmentation and attrition resisted. The attrition rate of sulphate was similar to that of lime in trend, decaying exponentially with time, but was one-magnitude-order smaller than that of lime. Present experimental results indicate that fragmentation mechanism of the limestone is dominated by CO₂ release instead of thermal stress.     

Simulation and analysis of industrial crystallization processes through multidimensional population balance equations. Part 1: a resolution algorithm based on the method of classes

In order to obtain constant solid properties with particles exhibiting a low order of symmetry, it is necessary to monitor and to control several distributed parameters characterising the crystal shape and size. A bi-dimensional population balance model was developed to simulate the time variations of two characteristic sizes of crystals. The nonlinear population balance equations were solved numerically over the bi-dimensional size domain using the so-called method of classes. An effort was made to improve usual simulation studies through the introduction of physical knowledge in the kinetic laws involved during nucleation and growth phenomena of complex organic products. The performances of the simulation algorithm were successfully assessed through the reproduction of two well-known theoretical and experimental features of ideal continuous crystallization processes: the computation of size-independent growth rates from the plot of the steady-state crystal size distribution and the possibility for MSMPR crystallizers to exhibit low-frequency oscillatory behaviours in the case of insufficient secondary nucleation.