International Journal of Chemical Engineering

Flow Reversal of Fully Developed Mixed Convection in a Vertical Channel with Chemical Reaction

Habibis Saleh, Ishak Hashim, and Sri Basriati — Sun, 12 May 2013 10:53:17 +0000

The present analysis is concerned with the criteria for the onset of flow reversal of the fully developed mixed convection in a vertical channel under the effect of the chemical reaction. The governing equations and the critical values of the buoyancy force are solved and calculated numerically via MAPLE. Parameter zones for the occurrence of reversed flow are presented. The exothermic chemical reaction is found to enhance the flow reversal and made flow reversal possible for symmetrical walls temperature.

Pilot-Scale Removal of Trace Steroid Hormones and Pharmaceuticals and Personal Care Products from Municipal Wastewater Using a Heterogeneous Fenton’s Catalytic Process

George Tangyie Chi, John Churchley, and Katherine D. Huddersman — Mon, 15 Apr 2013 10:06:21 +0000

The pollution of water sources by endocrine disrupting compounds (EDCs) and pharmaceutical and personal care products (PPCPs) is a growing concern, as conventional municipal wastewater treatment systems are not capable of completely removing these contaminants. A continuous stir tank reactor incorporating a modified polyacrylonitrile (PAN) catalyst and dosed with hydrogen peroxide in a heterogeneous Fenton’s process was used at pilot scale to remove these compounds from wastewater that has undergone previous treatment via a conventional wastewater treatment system. The treatment system was effective at ambient temperature and at the natural pH of the wastewater. High levels of both natural and synthetic hormones (EDCs) and PPCPs were found in the effluent after biological treatment of the wastewater. The treatment system incorporating the modified PAN catalyst/H2O2 decomposed >90% of the EDCs and >40% of PPCPs using 200 mgL−1 H2O2, 3 hr residence time. The estrogenic potency EE2-EQ was removed by 82.77%, 91.36%, and 96.13% from three different wastewater treatment plants. BOD was completely removed (below detection limits); 30%–40% mineralisation was achieved and turbidity reduced by more than 68%. There was a <4% loss in iron content on the catalyst over the study period, suggesting negligible leaching of the catalyst.

MHD Heat and Mass Transfer of Chemical Reaction Fluid Flow over a Moving Vertical Plate in Presence of Heat Source with Convective Surface Boundary Condition

B. R. Rout, S. K. Parida, and S. Panda — Thu, 11 Apr 2013 10:59:01 +0000

This paper aims to investigate the influence of chemical reaction and the combined effects of internal heat generation and a convective boundary condition on the laminar boundary layer MHD heat and mass transfer flow over a moving vertical flat plate. The lower surface of the plate is in contact with a hot fluid while the stream of cold fluid flows over the upper surface with heat source and chemical reaction. The basic equations governing the flow, heat transfer, and concentration are reduced to a set of ordinary differential equations by using appropriate transformation for variables and solved numerically by Runge-Kutta fourth-order integration scheme in association with shooting method. The effects of physical parameters on the velocity, temperature, and concentration profiles are illustrated graphically. A table recording the values of skin friction, heat transfer, and mass transfer at the plate is also presented. The discussion focuses on the physical interpretation of the results as well as their comparison with previous studies which shows good agreement as a special case of the problem.

Hybrid Multiphase CFD Solver for Coupled Dispersed/Segregated Flows in Liquid-Liquid Extraction

Kent E. Wardle and Henry G. Weller — Sun, 24 Mar 2013 17:32:58 +0000

The flows in stage-wise liquid-liquid extraction devices include both phase segregated and dispersed flow regimes. As a additional layer of complexity, for extraction equipment such as the annular centrifugal contactor, free-surface flows also play a critical role in both the mixing and separation regions of the device and cannot be neglected. Traditionally, computional fluid dynamics (CFD) of multiphase systems is regime dependent—different methods are used for segregated and dispersed flows. A hybrid multiphase method based on the combination of an Eulerian multifluid solution framework (per-phase momentum equations) and sharp interface capturing using Volume of Fluid (VOF) on selected phase pairs has been developed using the open-source CFD toolkit OpenFOAM. Demonstration of the solver capability is presented through various examples relevant to liquid-liquid extraction device flows including three-phase, liquid-liquid-air simulations in which a sharp interface is maintained between each liquid and air, but dispersed phase modeling is used for the liquid-liquid interactions.

Evidence for PAH Removal Coupled to the First Steps of Anaerobic Digestion in Sewage Sludge

Mon, 18 Mar 2013 11:33:28 +0000

Anaerobic degradation of polycyclic aromatic hydrocarbons has been brought to the fore, but information on removal kinetics and anaerobic degrading bacteria is still lacking. In order to explore the organic micropollutants removal kinetics under anaerobic conditions in regard to the methane production kinetics, the removal rate of 12 polycyclic aromatic hydrocarbons was measured in two anaerobic batch reactors series fed with a highly loaded secondary sludge as growth substrate. The results underscore that organic micropollutants removal is coupled to the initial stages of anaerobic digestion (acidogenesis and acetogenesis). In addition, the organic micropollutants kinetics suggest that the main removal mechanisms of these hydrophobic compounds are biodegradation and/or sequestration depending on the compounds.

Advances in Computational Fluid Dynamics

Sun, 17 Mar 2013 13:21:21 +0000

Large Eddy Simulation for Dispersed Bubbly Flows: A Review

M. T. Dhotre, N. G. Deen, B. Niceno, Z. Khan, and J. B. Joshi — Thu, 14 Mar 2013 18:21:25 +0000

Large eddy simulations (LES) of dispersed gas-liquid flows for the prediction of flow patterns and its applications have been reviewed. The published literature in the last ten years has been analysed on a coherent basis, and the present status has been brought out for the LES Euler-Euler and Euler-Lagrange approaches. Finally, recommendations for the use of LES in dispersed gas liquid flows have been made.

The Application of Response Surface Methodology for Lead Ion Removal from Aqueous Solution Using Intercalated Tartrate-Mg-Al Layered Double Hydroxides

Yamin Yasin, Maszlin Mohamad, and Faujan B. H. Ahmad — Mon, 04 Mar 2013 09:14:47 +0000

Layered double hydroxide intercalated with tartrate (tartrate-Mg-Al) was used as an adsorbent to remove lead ions from aqueous solutions. The effects of various optimization parameters such as contact time, solution pH, lead ion concentrations, and adsorbent dosage were investigated by the use of Response Surface Methodology (RSM). The Response Surface Methodology (RSM) based on a four-level four-variable Central Composite Rotatable Design (CCRD) was employed to evaluate the interactive effects of the various optimization parameters. The parameters were contact time (6–10 h), solution pH (1–3), adsorbent dosage (0.06–0.1 g), and lead ion concentrations (10–30 mg/L). The percentage of lead ions removal for each of the parameters studied was determined by Inductively Coupled Plasma-Optical Emission Spectrophotometer. Simultaneously by increasing contact time and amount of dosage of tartrate-Mg-Al used the percentage of lead ions removal from aqueous solution will increase; however, the percentage removal decreases with an increase in pH and concentrations of lead ions. The experimental percentage removal recorded under optimum conditions was compared well with the maximum predicted value from the RSM, which suggest that Central Composite Rotatable Design of RSM can be used to study the removal of lead from aqueous solution by the use of tartrate-Mg-Al as an adsorbent.

CFD Study of Industrial FCC Risers: The Effect of Outlet Configurations on Hydrodynamics and Reactions

Tue, 25 Dec 2012 15:27:07 +0000

Fluid catalytic cracking (FCC) riser reactors have complex hydrodynamics, which depend not only on operating conditions, feedstock quality, and catalyst particles characteristics, but also on the geometric configurations of the reactor. This paper presents a numerical study of the influence of different riser outlet designs on the dynamic of the flow and reactor efficiency. A three-dimensional, three-phase flow model and a four-lump kinetic scheme were used to predict the performance of the reactor. The phenomenon of vaporization of the liquid oil droplets was also analyzed. Results showed that small changes in the outlet configuration had a significant effect on the flow patterns and consequently, on the reaction yields.

Effect of Contaminants on the Gas Holdup and Mixing in Internal Airlift Reactors Equipped with Microbubble Generator

Surya K. Pallapothu and Adel M. Al Taweel — Wed, 14 Nov 2012 09:29:52 +0000

The impact of contaminants on the gas holdup and mixing characteristics encountered in internal airlift reactors was investigated using a 200 L pilot scale unit equipped with a two-phase transonic sparger capable of generating microbubbles. Small dosages of a cationic surfactant (0–50 ppm of sodium dodecyl sulfonate (SDS)) were used to simulate the coalescence-retarding effect encountered in most industrial streams and resulted in the formation of bubbles that varied in size between 280 and 1,900 μm. Gas holdups as high as 0.14 were achieved in the riser under homogeneous flow regime when slowly coalescent systems were aerated at the relatively low superficial velocity of 0.02 ms−1, whereas liquid circulation velocities as high as 1.3 ms−1 were achieved in conjunction with rapidly coalescent systems at the same superficial velocity. This excellent hydrodynamic performance represents a 5-fold improvement in the riser gas holdup and up to 8-fold enhancement in the liquid circulation velocity and is expected to yield good mixing and mass transfer performance at low energy dissipation rates.

Ion-Exchange Membranes

Yoshinobu Tanaka, Seung-Hyeon Moon, Victor V. Nikonenko, and Tongwen Xu — Sun, 04 Nov 2012 15:29:30 +0000

Advances in Mixing Technology: Recent Advances in Mixing Research and Development

Shunsuke Hashimoto, Naoto Ohmura, See-Jo Kim, and Shaliza B. Ibrahim — Sun, 04 Nov 2012 11:25:50 +0000

Enthalpy-Entropy Compensation in Polyester Degradation Reactions

Adam Al-Mulla — Tue, 30 Oct 2012 08:23:04 +0000

In an earlier work the author had studied the degradation kinetics of polyethylene terephthalate (PET), polytrimethylene terephthalate (PTT), and polybutylene terephthalate (PBT) under nonisothermal conditions in air and N2 at heating rates of 5, 10, 15, and 20°C/min. In this paper the kinetic degradation parameters of PET, PTT, and PBT were estimated using the Coats-Redfern method for two different weight loss regions ranging from 2–8% (Zone I) and 8–40% (Zone II). A comparative analysis of the enthalpy-entropy compensation effect for these polyesters in air and N2 is presented. A linear relationship was found to exist between entropy and enthalpy values. The following criteria were applied to establish an enthalpy-entropy compensation effect and to check the presence of an isokinetic temperature: (a) Exner’s plot of log versus log , and (b) Krug et al. linear regression of ΔH versus ΔG. By the use of the latter two methods, varying isokinetic temperatures were obtained. These temperatures were not in the range of the experimental work conducted, indicating that these systems do not display compensation phenomena.

Numerical Studies of the Gas-Solid Hydrodynamics at High Temperature in the Riser of a Bench-Scale Circulating Fluidized Bed

Fri, 12 Oct 2012 16:22:00 +0000

The hydrodynamics of circulating fluidized beds (CFBs) is a complex phenomenon that can drastically vary depending on operational setup and geometrical configuration. A research of the literature shows that studies for the prediction of key variables in CFB systems operating at high temperature still need to be implemented aiming at applications in energy conversion, such as combustion, gasification, or fast pyrolysis of solid fuels. In this work the computational fluid dynamics (CFD) technique was used for modeling and simulation of the hydrodynamics of a preheating gas-solid flow in a cylindrical bed section. For the CFD simulations, the two-fluid approach was used to represent the gas-solid flow with the k-epsilon turbulence model being applied for the gas phase and the kinetic theory of granular flow (KTGF) for the properties of the dispersed phase. The information obtained from a semiempirical model was used to implement the initial condition of the simulation. The CFD results were in accordance with experimental data obtained from a bench-scale CFB system and from predictions of the semiempirical model. The initial condition applied in this work was shown to be a viable alternative to a more common constant solid mass flux boundary condition.

The Effects of Mixing, Reaction Rates, and Stoichiometry on Yield for Mixing Sensitive Reactions—Part II: Design Protocols

Syed Imran A. Shah, Larry W. Kostiuk, and Suzanne M. Kresta — Tue, 09 Oct 2012 11:42:39 +0000

Competitive-consecutive and competitive-parallel reactions are both mixing sensitive reactions where the yield of desired product depends on how fast the reactants are brought together. Recent experimental results have suggested that the magnitude of the mixing effect may depend strongly on the stoichiometry of the reactions. To investigate this, a 1D, dimensionless, reaction-diffusion model was developed at the micromixing scale, yielding a single general Damköhler number. Dimensionless reaction rate ratios were derived for both reaction schemes. A detailed investigation of the effects of initial mixing condition (striation thickness), dimensionless reaction rate ratio, and reaction stoichiometry on the yield of desired product showed that the stoichiometry has a considerable effect on yield. All three variables were found to interact strongly. Model results for 12 stoichiometries are used to determine the mixing scale and relative rate ratio needed to achieve a specified yield for each reaction scheme. The results show that all three variables need to be considered when specifying reactors for mixing sensitive reactions.

Ion Transport through Diffusion Layer Controlled by Charge Mosaic Membrane

Akira Yamauchi — Mon, 24 Sep 2012 15:32:28 +0000

The kinetic transport behaviors in near interface of the membranes were studied using commercial anion and cation exchange membrane and charge mosaic membrane. Current-voltage curve gave the limiting current density that indicates the ceiling of conventional flux. From chronopotentiometry above the limiting current density, the transition time was estimated. The thickness of boundary layer was derived with conjunction with the conventional limiting current density and the transition time from steady state flux. On the other hand, the charge mosaic membrane was introduced in order to examine the ion transport on the membrane surface in detail. The concentration profile was discussed by the kinetic transport number with regard to the water dissociation (splitting) on the membrane surface.

Bubble Dynamics of a Single Condensing Vapor Bubble from Vertically Heated Wall in Subcooled Pool Boiling System: Experimental Measurements and CFD Simulations

Arijit A. Ganguli, Aniruddha B. Pandit, and Jyeshtharaj B. Joshi — Sun, 16 Sep 2012 10:20:16 +0000

Bubble dynamics of a single condensing vapor bubble in a subcooled pool boiling system with a centrally heated cylindrical tank has been studied in the Rayleigh number range 7.9×1012

CFD Simulation of Annular Centrifugal Extractors

S. Vedantam, K. E. Wardle, T. V. Tamhane, V. V. Ranade, and J. B. Joshi — Thu, 06 Sep 2012 11:45:35 +0000

Annular centrifugal extractors (ACE), also called annular centrifugal contactors offer several advantages over the other conventional process equipment such as low hold-up, high process throughput, low residence time, low solvent inventory and high turn down ratio. The equipment provides a very high value of mass transfer coefficient and interfacial area in the annular zone because of the high level of power consumption per unit volume and separation inside the rotor due to the high g of centrifugal field. For the development of rational and reliable design procedures, it is important to understand the flow patterns in the mixer and settler zones. Computational Fluid Dynamics (CFD) has played a major role in the constant evolution and improvements of this device. During the past thirty years, a large number of investigators have undertaken CFD simulations. All these publications have been carefully and critically analyzed and a coherent picture of the present status has been presented in this review paper. Initially, review of the single phase studies in the annular region has been presented, followed by the separator region. In continuation, the two-phase CFD simulations involving liquid-liquid and gas-liquid flow in the annular as well as separator regions have been reviewed. Suggestions have been made for the future work for bridging the existing knowledge gaps. In particular, emphasis has been given to the application of CFD simulations for the design of this equipment.

Zwietering's Equation for the Suspension of Porous Particles and the Use of Curved Blade Impellers

S. Ibrahim, S. N. Jasnin, S. D. Wong, and I. F. Baker — Mon, 03 Sep 2012 14:24:32 +0000

The minimum speed for just-suspension, 𝑁js, of porous palm shell-activated carbon (PSAC) particles has been determined in a 15 cm diameter cylindrical tank using a 6-curved blade (6CB) impeller, compared to a 6-blade downpumping mixed-flow (6MFD) impeller and a Rushton turbine (6DT). The particles size ranged from 0.75–1.00 mm, 1.00–1.40 mm, and 1.40–2.36 mm with concentrations between 0 and 5% by weight. The 6CB being a radial impeller performed similarly to 6DT in terms of speed and power requirement at just-suspension, and particles distribution on the base. The 6MFD, with power requirement 100% to 200% less than the radial impellers, was the most efficient for suspending the particles, as usually reported for the range of solid concentrations used here. Specific power per unit mass for all three impellers showed reduction towards minima as the concentration of particles increased. The geometric factor, 𝑆, values agreed reasonably with published data, when the particle density was adjusted taking into account water filling the pores of the submerged activated carbon. This result means that Zwietering’s equation can be used to predict suspension for porous particles with adjustment to the particle density. 𝑆 values for curved-blade impellers are presented for the first time.

CFD Modeling of Solid Suspension in a Stirred Tank: Effect of Drag Models and Turbulent Dispersion on Cloud Height

Thu, 30 Aug 2012 08:56:51 +0000

Many chemical engineering processes involve the suspension of solid particles in a liquid. In dense systems, agitation leads to the formation of a clear liquid layer above a solid cloud. Cloud height, defined as the location of the clear liquid interface, is a critical measure of process performance. In this study, solid-liquid mixing experiments were conducted and cloud height was measured as a function operating conditions and stirred tank configuration. Computational fluid dynamics simulations were then performed using an Eulerian-Granular multiphase model. The effects of hindered and unhindered drag models and turbulent dispersion force on cloud height were investigated. A comparison of the experimental and computational data showed excellent agreement over the full range of conditions tested.

Enhancing Ion Transfer in Overlimiting Electrodialysis of Dilute Solutions by Modifying the Surface of Heterogeneous Ion-Exchange Membranes

Thu, 16 Aug 2012 13:32:54 +0000

The desalination of dilute NaCl solutions with heterogeneous Russian commercial and modified ion-exchange membranes was studied in a laboratory cell imitating desalination channels of large-scale electrodialysers. The modification was made by casting a thin film of a Nafion-type material on the surface of cation-exchange membrane, and by processing with a strong polyelectrolyte the surface of anion-exchange membrane. It was shown that the modifications resulted in an increase of mass transfer coefficient and in a decrease in water splitting rate, both by up to 2 times. The effect of mass transfer growth is explained by higher surface hydrophobicity of the modified membrane that enhances electroconvection. The decrease in water splitting rate in the case of cation-exchange membrane is due to homogenization of its surface layer. In the case of anion-exchange membrane the effect is due to grafting of quaternary ammonium bases onto the original membrane surface layer. The suppression of water splitting favors development of electroconvection. In turn, intensive electroconvection contributes to deliver salt ions to membrane surface and thus reduces water splitting.

A Priori Direct Numerical Simulation Modelling of the Curvature Term of the Flame Surface Density Transport Equation for Nonunity Lewis Number Flames in the Context of Large Eddy Simulations

Mohit Katragadda and Nilanjan Chakraborty — Wed, 15 Aug 2012 14:25:15 +0000

A Direct Numerical Simulation (DNS) database of freely propagating statistically planar turbulent premixed flames with Lewis numbers Le ranging from 0.34 to 1.2 has been used to analyse the statistical behaviours of the curvature term of the generalised Flame surface Density (FSD) transport equation, in the context of the Large Eddy Simulation (LES). Lewis number is shown to have significant influences on the statistical behaviours of the resolved and sub-grid parts of the FSD curvature term. It has been found that the existing models for the sub-grid curvature term 𝐶sg do not capture the qualitative behaviour of this term extracted from the DNS database for flames with Le<<1. The existing models of 𝐶sg only predict negative values, whereas the sub-grid curvature term is shown to assume positive values within the flame brush for the Le=0.34 and 0.6 flames. Here the sub-grid curvature terms arising from combined reaction and normal diffusion and tangential diffusion components of displacement speed are individually modelled, and the new model of the sub-grid curvature term has been found to capture 𝐶sg extracted from DNS data satisfactorily for all the different Lewis number flames considered here for a wide range of filter widths.

Effect of Mixing on Microorganism Growth in Loop Bioreactors

A. M. Al Taweel, Q. Shah, and B. Aufderheide — Tue, 14 Aug 2012 14:28:43 +0000

The impact of mixing on the promotion of microorganism growth rate has been analyzed using a multiphase forced-circulation pipe-loop reactor model capable of identifying conditions under which it is possible to convert natural gas into Single-Cell Protein. The impact of mixing in the interphase mass transfer was found to exert a critical role in determining the overall productivity of the bioreactor, particularly at the high cell loadings needed to reduce the capital costs associated with the large-scale production needed for the production of relatively low-value SCP in a sustainable manner.

Gas-Solid Reaction Properties of Fluorine Compounds and Solid Adsorbents for Off-Gas Treatment from Semiconductor Facility

Wed, 08 Aug 2012 11:17:33 +0000

We have been developing a new dry-type off-gas treatment system for recycling fluorine from perfluoro compounds present in off-gases from the semiconductor industry. The feature of this system is to adsorb the fluorine compounds in the exhaust gases from the decomposition furnace by using two types of solid adsorbents: the calcium carbonate in the upper layer adsorbs HF and converts it to CaF2, and the sodium bicarbonate in the lower layer adsorbs HF and SiF4 and converts them to Na2SiF6. This paper describes the fluorine compound adsorption properties of both the solid adsorbents—calcium carbonate and the sodium compound—for the optimal design of the fixation furnace. An analysis of the gas-solid reaction rate was performed from the experimental results of the breakthrough curve by using a fixed-bed reaction model, and the reaction rate constants and adsorption capacity were obtained for achieving an optimal process design.

Analysis of Hydrodynamics of Fluid Flow on Corrugated Sheets of Packing

Kumar Subramanian and Günter Wozny — Thu, 26 Jul 2012 11:15:57 +0000

Modelling of the hydrodynamics behaviour of the liquid on the corrugated sheets of packing is studied using three-dimensional, volume-of-fluid (VOF) model that is incorporated in Ansys Fluent 12.0. The flow of three different liquids with different physical properties is modelled. A domain of corrugated sheets of packing resembling the real structured packing with little modifications in the elementary geometry is constructed using ICEM CFD 12.0. The quantitative comparisons of the wetting behavior from the simulations are in good agreement with experiments. Further, the study has been extended to understand the influence of the second corrugated sheet on the flow behavior. The contours from the simulations indicate the liquid hold-up in the crimp of two corrugated sheets, and these results are in good agreement with the earlier experimental studies performed using X-ray tomography in the literature. The result from the simulation shows that even for the high flow rate of around 811 mL/min for silicon-oil (DC5), only 60% of the corrugated sheet has been wetted. Hence, the efficiency of the existing packing can be further increased by increasing the wetted area in the corrugated sheet of packing.

Oxygen Absorption into Stirred Emulsions of n-Alkanes

Thanh Hai Ngo and Adrian Schumpe — Wed, 25 Jul 2012 08:42:32 +0000

Absorption of pure oxygen into aqueous emulsions of n-heptane, n-dodecane, and n-hexadecane, respectively, has been studied at 0 to 100% oil volume fraction in a stirred tank at the stirring speed of 1000 min−1. The volumetric mass transfer coefficient, 𝑘𝐿𝑎, was evaluated from the pressure decrease under isochoric and isothermal (298.2 K) conditions. The O/W emulsions of both n-dodecane and n-hexadecane show a 𝑘𝐿𝑎 maximum at 1-2% oil fraction as reported in several previous studies. Much stronger effects never reported before were observed at high oil fractions. Particularly, all n-heptane emulsions showed higher mass-transfer coefficients than both of the pure phases. The increase is by upto a factor of 38 as compared to pure water at 50% n-heptane. The effect is tentatively interpreted by oil spreading on the bubble surface enabled by a high spreading coefficient. In W/O emulsions of n-heptane and n-dodecane 𝑘𝐿𝑎 increases with the dispersed water volume fraction; the reason for this surprising trend is not clear.

Investigation of the Pulsed Annular Gas Jet for Chemical Reactor Cleaning

Wed, 18 Jul 2012 11:20:32 +0000

The most economical technology for production of titanium dioxide pigment is plasma-chemical syntheses with the heating of the oxygen. The highlight of the given reaction is formation of a solid phase as a result of interactions between two gases, thus brings the formation of particle deposits on the reactor walls, and to disturbing the normal operation of the technological process. For the solving of the task of reactor internal walls cleaning the pulsed gaseous system was suggested and investigated, which throws circular oxygen jet along surfaces through regular intervals. Study of aerodynamic efficiency of the impulse system was carried by numerical modeling and experimentally with the help of a specially created experimental facility. The distribution of the pulsed flow velocity at the exit of cylindrical reactor was measured. The experimental results have shown that used impulse device creates a pulsed jet with high value of the specified flow rate. It allows to get high velocities that are sufficient for the particle deposits destruction and their removal away. Designed pulsed peelings system has shown high efficiency and reliability in functioning that allows us to recommend it for wide spreading in chemical industry.

The Effects of Mixing, Reaction Rates, and Stoichiometry on Yield for Mixing Sensitive Reactions—Part I: Model Development

Syed Imran A. Shah, Larry W. Kostiuk, and Suzanne M. Kresta — Tue, 17 Jul 2012 13:43:15 +0000

There are two classes of mixing sensitive reactions: competitive-consecutive and competitive-parallel. The yield of desired product from these coupled reactions depends on how fast the reactants are brought together. Recent experimental results have suggested that the mixing effect may depend strongly on the stoichiometry of the reactions. To investigate this, a 1D, dimensionless, reaction-diffusion model at the micromixing scale was developed. Assuming constant mass concentration and mass diffusivities, systems of PDE's were derived on a mass fraction basis for both types of reactions. Two dimensionless reaction rate ratios and a single general Damköhler number emerged from the analysis. The resulting dimensionless equations were used to investigate the effects of mixing, reaction rate ratio, and reaction stoichiometry. As expected, decreasing either the striation thickness or the dimensionless rate ratio maximizes yield, the reaction stoichiometry has a considerable effect on yield, and all three variables interact strongly.

Recent Advances in Valorization Methods of Inorganic/Organic Solid, Liquid, and Gas Wastes

Licínio M. Gando-Ferreira, Faïçal Larachi, and Santiago Esplugas — Mon, 16 Jul 2012 13:36:49 +0000

Characterization of Minimum Impeller Speed for Suspension of Solids in Liquid at High Solid Concentration, Using Gamma-Ray Densitometry

Rouzbeh Jafari, Philippe A. Tanguy, and Jamal Chaouki — Sun, 15 Jul 2012 13:32:49 +0000

The successful design and operation of Liquid-Solid (LS) and Gas-Liquid-Solid (GLS) stirred tank reactors requires an accurate determination of the level of solid suspension needed for the process at hand. A poor design of the stirred tank to achieve optimum conditions and maintain the system under these conditions during operation may cause significant drawbacks concerning product quality (selectivity and yield) and cost. In this paper, the limitations of applying conventional measurement techniques for the accurate characterization of critical impeller speed for just off-bottom suspension (𝑁JS) at high solid concentrations are described. Subsequently, the Gamma-Ray Densitometry technique for characterizing 𝑁JS is introduced, which can overcome the limitations of previous experimental techniques. The theoretical concept of this method is explained, and experimental validation is presented to confirm the accuracy of the Gamma-Ray Densitometry technique. The effects of clearance, scale, and solid loading on 𝑁JS for several impellers are discussed. Experimental 𝑁JS values are compared with correlations proposed in the literatures, and modifications are made to improve the prediction. Finally, by utilizing the similarity to the incipient movement of solid particles in other systems, a theoretical model for 𝑁JS prediction is presented.