Showing papers in "Chemical Engineering Science in 2012"

Demand reduction in building energy systems based on economic model predictive control

Abstract: This paper proposes and demonstrates the effectiveness of an economic model predictive control (MPC) technique in reducing energy and demand costs for building heating, ventilating, and air conditioning (HVAC) systems. A simulated multi-zone commercial building equipped with of variable air volume (VAV) cooling system is built in Energyplus. With the introduced Building Controls Virtual Test Bed (BCVTB) as middleware, real-time data exchange between Energyplus and a Matlab controller is realized by sending and receiving sockets. System identification is performed to obtain zone temperature and power models, which are used in the MPC framework. The economic objective function in MPC accounts for the daily electricity costs, which include time-of-use (TOU) energy charge and demand charge. In each time step, a min–max optimization is formulated and converted into a linear programming problem and solved. In a weekly simulation, a pre-cooling effect during off-peak period and a cooling discharge from the building thermal mass during on-peak period can be observed. Cost savings by MPC are estimated by comparing with the baseline and other open-loop control strategies. The effect of several experimental factors in the MPC configuration is investigated and the best scenario is selected for future practical tests.

Thin film composite forward osmosis membranes based on polydopamine modified polysulfone substrates with enhancements in both water flux and salt rejection

Abstract: A novel approach has been demonstrated to improve the separation performance of thin film composite (TFC) forward osmosis (FO) membranes, which were interfacial polymerized on the surface functionalized porous polymeric substrates. In the new approach, top surface of the polysulfone (PSf) substrates were modified by a novel bio-inspired polymer polydopamine (PDA) through the oxidant-induced dopamine polymerization in a Tris-buffer solution at pH=8.5 in air, over which m-phenylenediamine (MPD) and trimesoyl chloride (TMC) were employed as the monomers for the interfacial polymerization reaction to form a polyamide (PA) rejection layer. This new scheme has revealed that it is possible to fabricate novel TFC-FO membranes with significantly improved water permeability and salt rejection properties simultaneously compared with those constructed on pristine PSf substrates. A high Jw/Js of about 20 l/g (or a low Js/Jw of about 0.05 g/l) can be achieved by using a 2 M NaCl as the draw solution and deionized water as the feed solution in a testing configuration where the active layer facing the draw solution at 23 °C. The PDA surface modification step plays a positive role in the fabrication of TFC-FO membranes, which is realized by producing a hydrophilic smooth membrane surface with smaller surface pores and a narrower pore size distribution for the interfacial polymerization reaction, as well as improving the hydrophilicity of the pore wall inside the substrate. Furthermore, the coated PDA layer could actively interact with TMC monomer during the interfacial polymerization, which may be favorable for the formation of a better quality PA layer with a high salt rejection.

Exact analytical solutions for heat and mass transfer of MHD slip flow in nanofluids

Abstract: This paper is devoted to the heat and mass transfer characteristics of the magnetohydrodynamic nanofluids flow over a permeable stretching/shrinking surface. The hydrodynamic as well as thermal slip conditions are accounted for. The novelty is that derivation of exact analytical solutions are aimed for different water-based nanofluids containing Cu, Ag, CuO, Al2O3, and TiO2. Results of the present analysis in the absence of hydrodynamic and thermal slip for the stretching sheet are in excellent agreement with those available in the literature. The velocity and temperature profiles, skin friction coefficient and Nusselt number are easily examined and discussed via the closed forms obtained. A rescaling of the governing equations is also proposed, which enables one to interpret the physical coefficients in terms of the published data.

Enhanced rate of gas hydrate formation in a fixed bed column filled with sand compared to a stirred vessel

Abstract: The performance of two gas/liquid contact modes was evaluated in relation to the rate of gas hydrate formation. Hydrate formation experiments were conducted for several gas mixtures relevant to natural gas hydrate formation in the earth (CH4, CH4/C3H8, CH4/C2H6 and CH4/C2H6/C3H8) and two CO2 capture and storage (CO2, CO2/H2/C3H8). One set of experiments was conducted in a bed of silica sand, saturated with water (fixed fed column) while the other experiment was conducted in a stirred vessel for each gas/gas mixture. Both sets of experiments were conducted at a constant temperature. The rate of hydrate formation is customarily correlated with the rate of gas consumption. The results show that the rate of hydrate formation in the fixed bed column is significantly greater and thereby resulted in a higher percent of water conversion to hydrate in lesser reaction time for all the systems studied.

Economic Optimization of a Lignocellulosic Biomass-to-Ethanol Supply Chain

Abstract: This paper presents an optimization study of the net present value of a biomass-to-ethanol supply chain in a 9-state region in the Midwestern United States. The study involves formulating and solving a mixed integer linear programming (MILP) problem. A biochemical technology is assumed for converting five types of agricultural residues into ethanol utilizing dilute acid pretreatment and enzymatic hydrolysis. Optimal locations and capacities of biorefineries are determined simultaneously with biomass harvest and distribution. Sensitivity analysis is performed to elucidate the impact of price uncertainty on the robustness of the supply chain and whether or not the proposed biorefineries will be built or will fail financially after being built.

Droplet formation and breakup dynamics in microfluidic flow-focusing devices: From dripping to jetting

Abstract: Droplet formation and breakup dynamics in either dripping or jetting regimes in microfluidic flow-focusing devices were investigated experimentally. More viscous liquids were dispersed into less viscous liquids in square microchannels with 600 and 400 μm wide, respectively. In the dripping regime, for low viscosity ratio of both phases, the variation of the minimum width of the dispersed thread with the remaining time could be scaled as a power–law relationship with exponent related to initial conditions, and the droplet size could be correlated with the flow rate ratio of both phases and the capillary number of the continuous phase; while for high viscosity ratio of both phases, the dispersed thread experiences a linear thinning procedure, and the droplet size can be scaled with the Weber number of the continuous phase. In the jetting regime, the stable jet width was affected by the viscosity ratio and flow rate ratio of both phases. And the relationship between the droplet size and the flow rate ratio of both phases could be scaled as a power–law relation with the exponent dependent on the viscosity ratio of both phases. The analysis of the mode of the maximum instability of the oil jet indicated that the most unstable instability is related to the viscosity ratio of both phases and is almost independent on the flow rate ratio of both phases.

Modeling and control of a solar thermal power plant with thermal energy storage

Abstract: Dynamic simulation results for a thermal energy storage (TES) unit used in a parabolic trough concentrated solar power (CSP) system are presented. A two-tank-direct method is used for the thermal energy storage. While previous works have been focused largely on controlling the outlet temperature of the solar collector as a single unit, this work emphasizes the storage component, its interaction with the other components of the system, and how it can be leveraged to control power output in addition to collector outlet temperature. The use of storage gives the system the ability to provide power at a constant rate despite significant disturbances in the amount of solar radiation available. It can also shift times of power generation to better match times of consumer demand. By contrast, a CSP system without storage undergoes large fluctuations in power output, particularly during intermittent cloud cover. Adding a storage system increases the solar share of the power plant by as much as 47% for a base load thermal power output of 1 MW. This reduces the supplementary fuel requirement by as much as 43%.

Direct numerical simulation of flow and heat transfer in dense fluid-particle systems

Abstract: In this paper a novel simulation technique is presented to perform Direct Numerical Simulation (DNS) of fluid flow and heat transfer in dense fluid–particle systems. The unique feature of our fluid–solid coupling technique is the direct (i.e., implicit) incorporation of the boundary condition (with a second-order method) at the surface of the particles at the level of the discrete momentum and thermal energy equations of the fluid. Contrary to lattice Boltzmann or other commonly used immersed boundary implementations, our method does not require using any effective diameter. A fixed (Eulerian) grid is utilized to solve the Navier–Stokes equations for the entire computational domain. Dissipative particle–particle and/or particle-wall collisions are accounted via a hard sphere discrete particle approach using a three-parameter particle–particle interaction model accounting for normal and tangential restitution and tangential friction. Following the detailed verification of the method several dense multi-particle systems are studied in detail involving stationary arrays of particles and fluidized particles.

A nonlinear kernel Gaussian mixture model based inferential monitoring approach for fault detection and diagnosis of chemical processes

Abstract: A nonlinear kernel Gaussian mixture model (NKGMM) based inferential monitoring method is proposed in this article for chemical process fault detection and diagnosis. Aimed at the multimode non-Gaussian process with within-mode nonlinearity, the developed NKGMM approach projects the operating data from the raw measurement space into the high-dimensional kernel feature space. Thus the Gaussian mixture model can be estimated in the feature space with each component satisfying multivariate Gaussianity. As a comparison, the conventional independent component analysis (ICA) searches for the non-Gaussian subspace with maximized negentropy, which is not equivalent to the multi-Gaussianity in multimode process. The regular Gaussian mixture model (GMM) method, on the other hand, assumes the Gaussianity of each cluster in the original data space and thus cannot effectively handle the within-mode nonlinearity. With the extracted kernel Gaussian components, the geometric distance driven inferential index is further derived to monitor the process operation and detect the faulty events. Moreover, the kernel Gaussian mixture based inferential index is decomposed into variable contributions for fault diagnosis. For the simulated multimode wastewater treatment process, the proposed NKGMM approach outperforms the ICA and GMM methods in early detection of process faults, minimization of false alarms, and isolation of faulty variables of nonlinear and non-Gaussian multimode processes.

Theoretical and experimental analysis of droplet evaporation on solid surfaces

Abstract: The evaporation of sessile drops is central to a number of important processes, including printing, washing and coating. In this paper, the evaporation of water sessile droplets on hydrophobised silicon wafers and Teflon was analysed from theoretical and experimental perspectives. The contact angle, volume and base radius of the water droplets as a function of time were determined using tensiometry. The theoretical analysis showed different evaporative flux phenomena for acute and obtuse contact angles. The non-linear evolution of residual droplet volume, contact angle and base radius are solved and depend on the hydrophobicity of the solid surface and droplet dimension. Good agreement between the theoretical and experimental results was observed during pinning and depinning stages of evaporation. It was shown that the surface roughness, hydrophobicity and the contact angle hysteresis significantly influenced the evaporation of sessile drops and need to be considered when quantifying the evaporation process.

Study on the mechanism of droplet formation in T-junction microchannel

Abstract: In the present paper, the mechanism of droplet formation in immiscible liquid/liquid T-junction system has been studied numerically and analytically. Different droplet stream patterns such as long slugs, small spheres, stable stream connected to a jet and finally parallel flow are observed, which are conventionally called squeezing, dripping and jetting regimes appearing under appropriate numerical setups. We emphasize that the effects of surface force and viscous force in this system alternatively dominate the droplets stream, and the periodically varied pressure and local velocity ensure the generation of uniform droplets. In particular, a two-dimensional lubrication analysis is applied to the confined continuous flow between dispersed emerging phase and the channel wall, which verifies that the pressure buildup upstream facilitate the long slugs formation. The numerical results and theoretical analysis hold a good agreement with the experimental measurements.

Continuous processing of low-density, microcellular poly(lactic acid) foams with controlled cell morphology and crystallinity

Abstract: Poly(lactic acid) (PLA) represents perhaps the most viable environmentally-sustainable alternative to petrochemical-based plastics. This paper reports the continuous processing of PLA foams with a microcellular structure, a high expansion ratio, and varied microcell morphology and crystallinity. The extrusion process, which can be easily scaled-up, takes advantage of the tailored physical properties of PLA and the plasticizing effect of the supercritical blowing agent. Three grades of PLA with different molecular weight and branching topology are used. The processing parameters are optimized based on the well-characterized thermal and rheological properties of PLAs and diffusion properties of PLA/CO2 mixture. In general, melt strength governs cell morphology, with cell density, closed-cell content, and expansion ratio increasing as a function of both molecular weight and branching density. Influences of shearing and dissolved-CO2 on crystallization of PLA are characterized and they are believed to induce crystallinity in the foams. In the case of branched PLA, crystallization allows high-expansion-ratio microcellular foams to be stably produced over a wide temperature window. By controlling crystallinity, foams with similar cell morphology but varied mechanical properties and surface gloss are also produced. X-ray diffraction of the foams confirms that crystallization is governed by shearing in the die, and the crystallites are mainly of α-form.

Olefin/paraffin separation using membrane based facilitated transport/chemical absorption techniques

Abstract: Seeking alternative olefin/paraffin separation techniques have been attracting great interest due to the high operating and capital costs of current commercially practiced separation processes. An olefin/paraffin separation scheme using membrane based facilitated transport/chemical absorption techniques offers great advantages such as reducing the operating and capital costs, as well as eliminating the operating drawbacks occurring in absorption processes such as flooding, foam formation, etc. Consequently, the utilization of a facilitated transport/chemical absorption scheme with a suitable membrane system for olefin/paraffin separation would be an attractive option for the replacement of the current separation technologies. A comprehensive review is presented for application of membranes for light olefin/paraffin separation. This article covers all types of membrane based facilitated transport/chemical absorption techniques which include various conventional liquid membranes configurations, membrane contactors and more advanced solid membrane electrolytes. The performance evaluation, shortcomings, and advantages are discussed in details.

Application of PVDF membranes in desalination and comparison of the VMD and DCMD processes

Abstract: Membrane distillation (MD) is a promising desalination technology that may help to resolve the fresh water shortage. Hydrophobic flat-sheet polyvinylidene fluoride (PVDF) membranes were used for desalination by direct contact MD (DCMD) and vacuum MD (VMD) processes. The effect of the operating conditions, such as the feed temperature, flow velocity on both the hot and cold sides and the concentration of the feed solution were investigated. During the DCMD processing of a 35 g/L aqueous NaCl solution, with a hot side temperature of 73 °C and a cold distillate water temperature of 25 °C, the permeate flux was as high as 18.9 kg/m 2 h with a NaCl rejection of 99.8%. A permeate flux of 22.4 kg/m 2 h and a NaCl rejection of 99.9% were attained during the VMD process at the same feed temperature and a downstream pressure of 31.5 kPa. Furthermore, there was hardly any wetting during 6 h desalination test. A dusty gas model was used to successfully predict the permeating flux in both the DCMD and VMD processes, and the mass transfer mechanisms through the membrane were determined. Comparison of the flux and the thermal efficiency in the DCMD and VMD processes under the same conditions demonstrated that this type of thin flat-sheet membrane, which had a high surface hydrophobicity and a sponge-like cross-section, was more suitable for use in the VMD process than in the DCMD process.

Effects of additives on dual-layer hydrophobic–hydrophilic PVDF hollow fiber membranes for membrane distillation and continuous performance

Abstract: The advantages of the implementation of dual-layer hydrophobic–hydrophilic hollow fiber membranes for membrane distillation (MD) have been highlighted in this work. The effects of incorporating methanol as a non-solvent additive and self-synthesized fluorinated silica (FSi) particles as a hydrophobic modifier on the resultant membrane morphology and MD performance were investigated. Employing a 3.5 wt% sodium chloride solution at 80 °C, the highest direct contact membrane distillation (DCMD) flux of 83.40±3.66 kg/(m2 h) and separation factor higher than 99.99% were attained for the membrane spun with methanol additive. Moreover, the stability of the dual-layer hydrophobic–hydrophilic hollow fiber membrane has been demonstrated through continuous DCMD experiments for 5 days. The separation factor was maintained higher than 99.99% for the membrane spun with methanol additive, verifying the suitability of the dual-layer hydrophobic–hydrophilic hollow fiber membrane configuration for desalination processes. The morphological transformation of the outer membrane surface from a porous agglomerated globule structure into a denser interconnected globule structure may be accounted for by the stability improvement of the membrane spun with methanol additive. On the other hand, it was found that an enhanced hydrophobicity of the membrane spun with FSi particles did not result in an improvement of the membrane stability. The existence of the hydrophilic hydroxyl group on the FSi particle surface may favor the occurrence of membrane wetting.

Determination of breakage rates using single drop experiments

Abstract: An evaluation of several breakage rates from the literature based on single drop experiments was carried out. This data was collected in a single drop breakage cell under turbulent conditions, comparable to those in a stirred tank. For a constant initial diameter and flow velocity at least 750 single drops have been investigated to measure the breakage time and probability, using high-speed imaging. These results were used for the determination of breakage rates by the product of the inverse of breakage time and the breakage probability. The same subdivision was carried out for the literature models. These differentiations in the analysis showed that published models for the breakage probability are more or less similar and in good agreements with the experimental results. Proposed approaches for the breakage time are contrary. The experiments support the assumption of some researchers that the breakage time rise with increasing drop diameter. The magnitude of the predicted values of the breakage time for all kind of models is one or more magnitudes higher than experimental results in this study and from literature. Furthermore the influence of the physical properties, like viscosity or interfacial tension, is only poorly reflected in the available models. These analysis results lead to an improved breakage time model, which takes into account different breakage mechanisms and the influence of viscosity and interfacial tension. Combined with a breakage probability from literature, this new model leads to an excellent prediction of the breakage rate for the investigated single drops.

A new view on the metastable zone width during cooling crystallization

Abstract: The Metastable Zone Width (MSZW) is the difference between the saturation temperature and the temperature at which crystals are detected under constant cooling rate. The MSZW is conventionally treated as a reproducible, deterministic, volume independent property and is commonly used to characterize crystal nucleation and to determine the crystallization process operation window. In this paper we investigate the volume dependency of the MSZW both experimentally and theoretically. MSZW measurements were performed for paracetamol–water and isonicotinamide–ethanol model systems at four different volumes between 500 mL and 1 L. A stochastic model developed based on the Single Nucleus Mechanism and a deterministic population balance were used to theoretically study the effect of volume on MSZW. It was experimentally observed that the MSZW is not a reproducible point at small volumes but a spread which increases roughly inversely proportional to the volume. The dependency on volume of the MSZW cannot be explained by the deterministic population balance model but can be explained by the stochastic model based on the single nucleus mechanism. The knowledge of the MSZW behaviour at a certain volume and nucleation rate would help in identifying a process operating window limited by the saturation temperature on one side and the smallest MSZ limit on the other for a particular concentration. The knowledge of the process window at given conditions of volume and concentration would lead to better crystalline product quality as controlled seeding can be performed without the influence of crystals appearing as a result of a primary nucleation event.

Modeling the carbonator of a Ca-looping process for CO2 capture from power plant flue gas

Abstract: The calcium-looping process is a promising technique for CO2 capture from coal-fired power plants and for reducing GHG emissions from the power generation sector. This paper presents a calculation model of the carbonator, the key reactor of the Ca-looping process, where CO2 is captured as a result of its reaction with CaO. The model presented is based on the Kunii–Levenspiel theory for circulating fluidized bed and on the recent findings on the properties of CaO as a CO2 sorbent, while taking into account the effects of coal ash and sulfur species. This model can be used for process optimization and for the prediction of the performance of power plants based on the Ca-looping process. Also presented in this paper are the results of a sensitivity analysis of the primary parameters that influence the performance of the carbonator. These results confirm the feasibility of the Ca-looping process with reactors of reasonable size for industrial applications and highlight the importance of the properties of the Ca-based sorbent as they highly affect the carbonator's performance.

Experimental analysis of forced convective heat transfer in novel structured packed beds of particles

Abstract: Followed by the numerical study of Yang et al. (2010) , the macroscopic hydrodynamic and heat transfer characteristics in some novel structured packed beds are experimentally studied in this paper, where the packings of ellipsoidal or non-uniform spherical particles are investigated for the first time with experiments and some important results are obtained. For present experiments, the interstitial heat transfer coefficient in the packed bed is determined using an inverse method of transient single-blow technique. The effects of packing form and particle shape are carefully investigated and the experimental and numerical results ( Yang et al., 2010 ) are also compared in detail. Firstly, it is discovered that, the computational method reported by Yang et al. (2010) might be appropriate for heat transfer predictions in structured packings, while it might underestimate the friction factors, especially when the porosity is relatively low. Secondly, it is found that, the traditional Ergun's and Wakao's equations might overpredict the friction factors and Nusselt numbers for the structured packings, respectively, and some experimental modified correlations are obtained. Furthermore, it is also revealed that, both the effects of packing form and particle shape are significant to the macroscopic hydrodynamic and heat transfer characteristics in structured packed beds. With proper selection of packing form, such as simple cubic packing (SC) and particle shape, such as ellipsoidal particle, the pressure drops in the structured packed beds can be greatly reduced and the overall heat transfer performances will be improved. These experimental results would be reliable and useful for the optimum design in industry applications.

MP-PIC simulation of CFB riser with EMMS-based drag model

Abstract: MP-PIC (multi-phase particle in cell) method combined with the EMMS (energy minimization multiscale) drag force model was implemented with the open source program MFIX to simulate the gas-solid flows in CFB (circulating fluidized bed) risers. Calculated solid flux by the EMMS drag agrees well with the experimental value; while the traditional homogeneous drag over-predicts this value. EMMS drag force model can also predict the macro- and meso-scale structures. Quantitative comparison of the results by the EMMS drag force model and the experimental measurements show high accuracy of the model. The effects of the number of particles per parcel and wall conditions on the simulation results have also been investigated in the paper. This work proved that MP-PIC combined with the EMMS drag model can successfully simulate the fluidized flows in CFB risers and it serves as a candidate to realize real-time simulation of industrial processes in the future. (C) 2012 Elsevier Ltd. All rights reserved.

Multidimensional population balance model for the simulation of turbulent gas–liquid systems in stirred tank reactors

Abstract: The prediction of turbulent gas–liquid systems, and in general of multiphase flows, has been historically performed with the implicit assumption of separately considering fluid dynamics issues from the evolution of the dispersed phase (i.e., the gas bubbles). These two aspects can be simultaneously accounted for by means of a multidimensional population balance model, able to describe the interactions between the continuous liquid phase and the gas bubbles, as well as the interactions among different gas bubbles (e.g., coalescence and break-up), both in terms of momentum and mass coupling. The model has to quantify the effects of these interactions on the population of dispersed bubbles and has to estimate the distributions of bubble velocity, size and composition, as well as the state of the continuous phase. A novel approach based on the direct quadrature method of moments is here formulated, tested on several simplified cases and adopted to describe the evolution of the gas bubble in a realistic gas–liquid stirred tank reactor. Results are eventually validated through comparison with experimental data from the literature.

Synthesis of CaO-based sorbents through incorporation of alumina/aluminate and their CO2 capture performance

Abstract: CaO-based CO 2 sorbents derived from various calcium and aluminum precursors were prepared by a wet mixing method and characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and N 2 adsorption–desorption techniques. The as-prepared sorbents consisted of active CaO and inert support materials that could be Al 2 O 3 , Ca 12 Al 14 O 33 or Ca 9 Al 6 O 18 , depending on calcium and aluminum precursors used during the preparation process. A formation mechanism for the inert support materials was proposed. Compared to pure CaO, most of the synthetic CaO-based sorbents showed much higher CO 2 capture capability and stability over multiple carbonation/calcination cycles, which was ascribed to the relatively high specific surface area of the sorbents, the bimodal pore-size distribution with a fair number of small pores, and the inert support material that can effectively prevent or delay sintering of CaO particles. Among these synthetic sorbents the CaO–Ca 9 Al 6 O 18 sorbent with a CaO content of 80 wt% (weight fraction) derived from calcium citrate and aluminum nitrate exhibited the best performance for CO 2 capture, and can be expected to be applied in the sorption-enhanced steam methane reforming process.

Layered two- and four-bed PSA processes for H2 recovery from coal gas

Abstract: Huge amounts of global warming gas emissions have prompted interest in the recovery of H 2 from off-gases in the iron and steel industries. Pressure swing adsorption (PSA) processes with layered beds packed with zeolite 5A and activated carbon were applied for H 2 recovery from coal gas with relatively low H 2 concentrations (H 2 /CO 2 /CH 4 /CO/N 2 ; 38/50/1/1/10 vol.%). Breakthrough curves in the layered bed showed behavior results between the zeolite 5A bed and the activated carbon bed. The bed with the higher zeolite ratio produced H 2 of higher purity in the PSA operation, but recovery loss became more significant with its increasing ratio. The variation of purity and recovery by operating variables were more significant in the two-bed PSA process than they were in the four-bed PSA process. The purity in the two-bed PSA varied asymptotically according to P/F ratio in the range of 0.1–0.3, while purity variation in the four-bed PSA process was almost linear. The zeolite layer in the two-bed PSA process worked as a separator of N 2 , while that in the four-bed PSA process worked as a purifier of N 2 . The four-bed PSA process could produce H 2 with a purity of 96–99.5% and a recovery of 71–85% with N 2 as the major impurity. The dynamics of the breakthrough and H 2 PSA processes were studied using a non-isothermal dynamic model.

A comprehensive frictional-kinetic model for gas–particle flows: Analysis of fluidized and moving bed regimes

Abstract: A comprehensive frictional-kinetic model for collisional and frictional gas–particle flows is presented. The model treats gas and particles as a continuum. The kinetic-collisional stresses are closed using kinetic theory of granular flows (KTGF). The frictional stresses are based on inertial number dependent rheology and dilation laws. From these laws the frictional normal and shear stresses are derived. These individual contributions to the solids stress tensor are treated additively, which requires a modification of the radial distribution function in the frictional regime. The presented model is validated for both, i.e. frictional and collisional dominated, flow regimes: (1) the collisional-frictional gas–particle flow in multiple-spout fluidized beds is studied and (2) the friction dominated discharge of particles from a rectangular bin is considered. In case of the multiple-spout fluidized beds the numerical simulations show excellent agreement with the experimental data of van Buijtenen et al. (2011) . The numerical results demonstrate that the presented model is a substantial improvement compared to the coupled CFD-DEM simulations of van Buijtenen et al. (2011) and to the Princeton model ( Srivastava and Sundaresan, 2003 ). In case of the discharge of particles the model predicts height-independent mass flow rates and stagnant shoulders in the corners of the bin. For the computed discharge rates excellent correlation with measurements (relative error e 2.5 % ) for three different particle diameters is obtained. In contrast, the Princeton model yields relative errors up to e = 41.5 % . Finally, the computed solids velocities near the exit orifice show good agreement with experimental particle tracking (PT) results as well.

Numerical simulation of species transfer across fluid interfaces in free-surface flows using OpenFOAM

Abstract: This paper presents the Continuous-Species-Transfer (CST) method, which enables interface capturing techniques – a group among Computational Multi-Fluid Dynamics (CMFD) methods that rely upon a smooth interface representation – to deal with species transfer. In this study we examine realistic species transfer across fluid interfaces, taking into account both the steep interfacial concentration gradients (at high Schmidt numbers) and the sharp interfacial concentration jump (at high Henry coefficients due to different species solubilities). Thus, the main objective is to establish the CST method for species transfer across fluid interfaces of arbitrary morphology in free-surface flows at high viscosity and density ratios. Detailed numerical simulations of single rising bubbles have been performed at high resolutions. Results were compared to experimental data and correlations derived thereof.

Catalytic conversion of lignocellulosic biomass to fuels: Process development and technoeconomic evaluation

Abstract: Levulinic acid (LA) has been identified as a platform chemical, which can be produced from lignocellulosic materials and transformed into liquid fuels, fuel additives and even other specialty chemicals. These conversions have been made possible through recent advances in heterogeneous catalysis. Taking advantage of novel chemistries and catalytic materials, we have developed a LA-based strategy to convert lignocellulosic biomass into liquid hydrocarbon fuels. To assess the economic potential of this approach, a process synthesis effort supported by detailed process simulation and capital/operational cost calculations has been undertaken. Furthermore, we study different feedstocks and perform sensitivity analysis studies for several process and economic parameters. Finally, we present the results of an energy efficiency analysis and discuss biomass transportation aspects.

Assessment of drag models in simulating bubbling fluidized bed hydrodynamics

Abstract: The hydrodynamic behavior of gas–solid flow is investigated in a 2-D bubbling fluidized bed reactor filled with 530 μm particles. The Computational Fluid Dynamics (CFD) is used to simulate the complex transient behavior of gas–solid flow. The CFD simulation of the bed hydrodynamics is based on the concept of Euler–Euler two-fluid model in combination with Kinetic Theory of Granular Flow (KTGF). In the present study, four different drag models are used to determine the drag force between the two phases and the results are compared. It is observed that the drag model has a significant effect on the simulation of gas–solid flow. The Gidaspow drag model and Syamlal–O'Brien model could predict the core-annulus structure of the bed very well. In comparison, the energy minimization multi-scale (EMMS) model and McKeen model cannot clearly predict the core-annulus structure of the flow. The Gisadpow model was found to provide better agreement with the experimental results of time-averaged particle velocity. On the other hand, the Syamlal–O'Brien model and EMMS model predicted the time-averaged granular temperature comparatively well. The effect of turbulence modeling on the flow behavior is also studied by incorporating the RNG k – e turbulence model. The results showed that the effect of turbulence modeling on the bed hydrodynamics is not very significant.

Unsupported MoS2 and CoMoS2 catalysts for hydrodeoxygenation of phenol

Abstract: The structural properties and catalytic functionality of two different morphologies of unsupported MoS 2 catalysts were investigated systematically. Hydrodeoxygenation (HDO) of phenol was used as the model reaction to examine the effect of the catalyst structure on the hydrogenolysis and hydrogenation activity and selectivity. The unsupported MoS 2 , with amorphous and highly bent multi-layer structures, was much more active than the highly crystalline structured MoS 2 and resulted in direct oxygen elimination. Co promoter showed a strong effect on the activity and selectivity of Mo sulfides. Temperature programmed reduction (TPR) exhibited the close interaction between Co and Mo in the bimetallic sulfide catalysts resulting in shift of TPR peaks to lower temperatures. The addition of promoter also affected the textural properties of amorphous Mo sulfide by decreasing the surface area, changing the pore characteristics and shifting the pore-size distribution towards smaller sizes. The growth of MoS 2 crystallized particles was inhibited when Co was incorporated. The enhanced catalytic activity observed with the promoter addition was essentially due to the enhancement of the rate of direct-deoxygenation route. The present study indicated that the activity and selectivity for phenol HDO can be controlled by tailoring the morphology and the promoter of the unsupported MoS 2 catalysts.

Large-scale DNS of gas-solid flows on Mole-8.5

Abstract: Direct numerical simulation (DNS) for gas-solid flow is implemented on a multi-scale supercomputing system-Mole-8.5 featuring massive parallel GPU-CPU hybrid computing, for which the lattice Boltzmann method (LBM) is deployed together with the immersed moving boundary (IMB) method and discrete element method (DEM). Numerical schemes and their GPU parallelism strategy are described in detail, where more than 40-fold speedup is achieved on one Nvidia C2050 GPU over one core of Intel E5520 CPU in double precision, and nearly ideal scalability is maintained when using up to 672 GPUs. A two-dimensional suspension with 1,166,400 75-mu m solid particles distributed in an area of 11.5 cm x 46 cm, and a three-dimensional suspension with 129,024 solid particles in a domain of 0.384 cm x 1.512 cm x 0.384 cm are fully resolved below particle scale and distinct multi-scale heterogeneity are observed. The simulations demonstrate that LBM-IMB-DEM modeling with parallel GPU computing may suggest a promising approach for exploring the fundamental mechanisms and constitutive laws of complex gas-solid flow, which are, so far, poorly understood in both experiments and theoretical studies. (C) 2011 Elsevier Ltd. All rights reserved.

Cluster characteristics of Geldart group B particles in a pilot-scale CFB riser. II. Polydisperse systems

Abstract: Experiments with a focus on the impact of polydispersity on clustering characteristics (namely, appearance probability, duration, and frequency) of Geldart Group B particles in a circulating fluidized bed (CFB) riser have been performed. Three mixtures are considered: (i) a density-difference binary mixture, with species of different material density (ρs) but similar particle sizes (dave), (ii) a size-difference binary mixture, with species of different dave but similar material density ρs, and (iii) a continuous particle size distribution (PSD). Local cluster information spanning the entire riser was obtained over a range of operating conditions using a fiber optic probe. Results show that cluster trends for the binary mixtures are similar to those reported in the companion work for monodisperse materials (Chew et al., this issue) on two counts. First, local riser position has a significant influence on all three cluster characteristics, while effects of operating condition and material type are secondary. Second, among the three cluster characteristics, the cluster appearance probability is most influenced by local position, and least affected by operating condition and material type. Furthermore, the density-difference binary mixture exhibits a distinctly lower cluster duration than either of its constituent components. In contrast, the size-difference binary mixture has a cluster duration profile that mimics one constituent component, while the frequency profile mimics the other. Comparing the two binary mixtures at any riser location, the density-difference binary mixture has lower cluster duration and higher frequency than the size-difference binary mixture regardless of local position. Finally, with respect to the continuous PSD, which was investigated under a wider range of operating conditions, the effect of operating condition is more apparent. This deviation may be due to an inherent behavioral difference between binary mixture and continuous PSD and/or to the wider range of operating conditions examined.