Showing papers on "Mixing (process engineering) published in 2008"

Solid-liquid two phase flow

Abstract: This book is an undertaking of a pioneering work of uniting three vast fields of interfacial phenomena, rheology and fluid mechanics within the framework of solid-liquid two phase flow. No wonder, much finer books will be written in the future as the visionary aims of many nations in combining molecular chemistry, biology, transport and interfacial phenomena for the fundamental understanding of processes and capabilities of new materials will be achieved. Solid-liquid systems where solid particles with a wide range of physical properties, sizes ranging from nano- to macro- scale and concentrations varying from very dilute to highly concentrated, are suspended in liquids of different rheological behavior flowing in various regimes are taken up in this book.Interactions among solid particles in molecular scale are extended to aggregations in the macro scale and related to settling, flow and rheological behavior of the suspensions in a coherent, sequential manner. The classical concept of solid particles is extended to include nanoparticles, colloids, microorganisms and cellular materials. The flow of these systems is investigated under pressure, electrical, magnetic and chemical driving forces in channels ranging from macro-scale pipes to micro channels. Complementary separation and mixing processes are also taken under consideration with micro- and macro-scale counterparts. It is up-to-date including emerging technologies. It provides coherent, sequential approach. It presents wide scope: microorganisms, nanoparticles, polymer solutions, minerals, wastewater sludge, etc.It covers all flow conditions, settling and non-settling particles, non-Newtonian flow, etc. It includes processes accompanying conveying in channels, such as sedimentation, separation, mixing.

Exhaust emission control device

Abstract: An object of the invention is to provide an exhaust emission control device which eliminates the use of a heat-retention structure using heat-insulating material such as glass wool so as to attain substantial lessening in production cost. A discharge end of a gas gathering chamber is connected to an entry end of a mixing pipe so as to encase the entry end of the mixing pipe and close an opened end face in a required spaced-apart relationship. Moreover, a side surface of the entry end of the mixing pipe adjacent to the discharge side of the particulate filter is formed with an opening; and gas guide passages are formed in the gas gathering chamber by guide fins for tangential introduction of all of the exhaust gas from the discharge side of the particulate filter to the opening. An extra space isolated from the gas guide passages is ensured in the gas gathering chamber as a heat-retention chamber surrounding the entry end of the mixing pipe.

Review on Mixing Characteristics in Solid‐Liquid and Solid‐Liquid‐Gas Reactor Vessels

Abstract: Mechanically agitated reactors with single and multiple impeller systems are used in the industry for the various three-phase mixing processes such as crystallization, fermentation, and hydrogenation, etc. The paper reviews the experimental work reported in the literature along with different techniques used for the measurement of the specific quantities such as minimum or critical impeller speed for solid suspension. The work critically surveys the literature and makes specific recommendations for the use of appropriate correlations and conditions to be used for the success of such equipment. This assessment will put all the relevant literature on a common footing and will help to validate work reported earlier.

Hydrodynamic and solid residence time distribution in a circulating fluidized bed: Experimental and 3D computational study

Abstract: Vertical profiles of local pressure, horizontal profiles of net vertical solid mass flux, and residence time distributions (RTD) of the solid phase are experimentally assessed in the riser of a small scale cold circulating fluidized bed of 9 m high having a square cross section of 11 cm × 11 cm. Air (density 1.2 kg/m 3 , dynamic viscosity 1.8 × 10 −5 Pa s) and typical FCC particles (density 1400 kg/m 3 , mean diameter 70 μm) are used. The superficial gas velocity is kept constant at 7 m/s while the solid mass flux ranges from 46 to 133 kg/m 2 s. The axial dispersion of the solid phase is found to decrease when increasing the solid mass flux. Simultaneously, 3D transient CFD simulations are performed to conclude on the usability of the eulerian–eulerian approach for the prediction of the solid phase mixing in the riser. The numerical investigation of the solid mixing is deferred until later since the near-wall region where the solid phase downflow and mixing are predominant is not well predicted in spite of well-predicted vertical profiles of pressure.

Effect of Mixing and Other Operating Parameters in Sol-Gel Processes

Abstract: In this work the effect of mixing on a sol-gel process is quantitatively investigated. Titanium dioxide synthesis from titanium tetra-isopropoxide is used as a test reaction. Solutions of titanium alkoxide in isopropyl alcohol and water in isopropyl alcohol are mixed in a special mixing device (i.e., vortex reactor) at different mixing rates, and the effect of mixing is quantified and compared with the effect of the other relevant operating parameters, namely the water to alkoxide, acid to alkoxide ratios, and alkoxide initial concentration. Dynamic light scattering, specific surface area measurement through nitrogen adsorption, X-ray diffraction, and field emission scanning electron microscopy are employed to determine particle size distribution, morphology, and crystallite size of the different particulate products (i.e., sols, gels, powders) obtained before and after thermal treatments under different synthesis conditions. A factorial design is used to plan the experimental campaign and results show that the role of mixing cannot be neglected. Moreover results show that mixing can be actively used to control the final product characteristics and must be taken into account when a process is transferred from the laboratory to the industrial scale. Eventually a scale-up criterion based on our previous work will be discussed.

Electrical properties from 10−3 to 10+9 HZ−−Physics and chemistry

Abstract: In dry materials, physical factors control all electrical properties. The addition of a polar liquid solvent such as water or alcohol adds a host of solvent‐rock chemical interactions. These chemical interactions range from oxidation‐reduction corrosion, cation exchange, and clay‐organic processes at frequencies below 1 Hz to diffusion‐limited relaxation around colloidal particles at frequencies up to 100 MHz. Most mixing formulas are based upon physical mixing of noninteracting materials, and they fail when chemical processes appear. If the specific chemical processes are identifiable, combined physical and chemical mixing formulas must be used. The simplest systems to model are noninteracting physical mixtures of solvents with pure silica sand. The most complicated systems are mixtures of solvents with chemically surface‐reactive materials like clays and zeolites.

Method and apparatus to produce synthesis gas via flash pyrolysis and gasification in a molten liquid

Abstract: Disclosed are a method and a corresponding apparatus for converting a biomass reactant into synthesis gas. The method includes the steps of (1) heating biomass in a first molten liquid bath at a first temperature, wherein the first temperature is at least about 100° C., but less than the decomposition temperature of the biomass, wherein gas comprising water is evaporated and air is pressed from the biomass, thereby yielding dried biomass with minimal air content. (2) Recapturing the moisture evaporated from the biomass in step 1 for use in the process gas. (3) Heating the dried biomass in a second molten liquid bath at a second temperature, wherein the second temperature is sufficiently high to cause flash pyrolysis of the dried biomass, thereby yielding product gases, tar, and char. (4) Inserting recaptured steam into the process gas, which may optionally include external natural gas or hydrogen gas or recycled syngas for mixing and reforming with tar and non-condensable gases. (5) Further reacting the product gases, tar, and char with the process gas within a third molten liquid bath at a third temperature which is equal to or greater than the second temperature within the second molten liquid bath, thereby yielding high quality and relatively clean synthesis gas after a relatively long residence time needed for char gasification. A portion of the synthesis gas so formed is combusted to heat the first, second, and third molten liquid baths, unless external natural or hydrogen gas is available for this use.

Measuring Mixing Time in the Agitation of Non‐Newtonian Fluids through Electrical Resistance Tomography

Abstract: Electrical resistance tomography (ERT), which is a non-invasive and robust measurement technique, was employed to visualize, in three dimensions, the concentration field inside a cylindrical mixing vessel equipped with a radial-flow Scaba 6SRGT impeller. The ability of ERT to work in opaque fluids makes this technique very attractive from an industrial perspective. An ERT system with a 4-plane assembly of peripheral sensing rings, each containing 16 electrodes, was used to measure the mixing time in agitation of xanthan gum solution which is a pseudoplastic fluid with yield stress. An image reconstruction algorithm was used to generate images of the tracer distribution within the sensing zone. In this study, the effect of impeller speed, fluid rheology, power consumption, and Reynolds number on the mixing time was investigated.

Process for producing rubber composition for tire tread

Abstract: A process for producing a rubber composition for tire treads which comprises a step of master batch mixing comprising mixing 20 to 150 parts by mass of silica (B), 1 to 30 parts by mass of a silane coupling agent (C) and a compound promoting reaction of silica and a silane coupling agent (D) with 100 parts by mass of a rubber component (A) comprising at least one of natural rubber and synthetic diene-based rubbers; and a step of final mixing comprising mixing a master batch obtained in the step of master batch mixing, a vulcanizing agent and a vulcanization accelerator. The rubber composition obtained in accordance with the above process increases abrasion resistance and also improves processability and productivity in the steps of mixing and extrusion.

Optimised Mixture Formation for Diesel Fuel Processing

Abstract: Mixture formation plays an important role in the diesel reforming process. It is important to maintain proper O2/C and H2O/C ratios to avoid hot spots and coking. Fuel must be completely evaporated before entering the reaction zone in order to prevent catalyst damage by coking. Computational fluid dynamics (CFD) is used to optimise the mixing process. Turbulent mixing, diesel spray injections and evaporation and simplified chemical reactions have been calculated. This revealed critical parts of the existing construction. However, experimental verification is necessary. To identify thermodynamic conditions for a possible carbon formation process, experiments with idealised model fuels as well as with real diesel fuel were carried out. Flow visualisation experiments serve for the verification of the CFD simulations. Quartz glass reactors as models of the reformers were operated under real mixing temperatures (400 °C) to observe the effect of the flow profile on fuel sprays. Experiments with coloured fuels were used to visualise the flow and concentration profiles in the mixing chamber. Results were compared with CFD models. Two patented reformers were designed as a result of the CFD optimisation. These were operated for 500 h and 1,000 h respectively with a commercially available diesel, showing very promising results.

Water-absorbing agent and production method thereof

Abstract: The present invention provides a water absorbing agent, and a method for producing the water absorbing agent. The water absorbing agent includes water absorbent resin particles, an organic acid and/or salt thereof having carbon number of 10 or more and not more than 30 in its molecule, and a water-soluble polyvalent cation. The method includes the step (i) of mixing the water absorbent resin particles, the organic acid and/or salt thereof having carbon number of 10 or more and not more than 30 in its molecule, and the water-soluble polyvalent cation with one another.

CFD Modelling and Simulation of Jet Mixed Tanks

Abstract: Mixing is one of the common unit operations employed in chemical industries. Conventional mixers are equipped with impellers but are expensive for mixing in large storage tanks and underground tanks. Jet mixers have become an alternative to impellers for over 50 years in the process industry. For the design of jet mixers, much experimental work has been done and many correlations have been proposed. However, these correlations are case specific and are not valid for any generic mixer. In order to establish a generic model for jet mixers, Computational Fluid Dynamics (CFD) is employed to understand the mixing phenomena and to find out the time required for mixing in a jet mixed tank. The 3D simulations are carried out using the CFD package FLUENT 6.2 to generate data and validate them against experimental results. The turbulence model engaged is the standard k-e model, which is found to show good agreement between the experimental results and simulated results for Reynolds number of 10000 and above...

Gas Back-Mixing in a Two-Dimensional Baffled Turbulent Fluidized Bed

Abstract: The effect of louver baffles on gas back-mixing was investigated in a large-scale two-dimensional fluidized bed of fluid catalytic cracking (FCC) particles with helium as tracer gas. The axial gas dispersion coefficient of the baffle-free fluidized bed first increased with increasing superficial gas velocity and then decreased after reaching a maximum near the onset of the turbulent flow regime. “Gulf streaming” of emulsion flow in the baffle-free bed determined the lateral profiles of tracer gas concentration. Solids back-mixing flux was greatly reduced by the addition of a layer of louver baffles, while solids mixing above the baffle layer was enhanced. The modified baffled fluidized bed with multilayer louver baffles not only provided a high efficiency of gas−solids contacting but also greatly suppressed the back-mixing of both gas and solids.

Population balance equations’ application in rotating fluidized bed polymerization reactor

Abstract: Gas phase olefin polymerizations are now widely achieved in fluidized bed reactors. In fluidized bed poly-olefin reactors, small catalyst particles (20–80 μm) are introduced into the bed, and when exposed to the gas flow (monomer), polymerization occurs. At early stage of polymerization, the catalyst particles fragment into a large number of small particles then the polymer particles grow continuously, reaching a typical size of 1000–3000 μm. A successful analysis of this process not only should account for the kinetics of the polymerization but also should include the particles mixing and particle size distribution in the reactor. Rotating fluidized bed reactors are the promising process to have a better control on the particle size distribution, particle separation and increasing the reactor efficiency. Due to the high rotational acceleration (e.g. 14 “g”) that can be imposed in these kinds of reactors, our preliminary results showed that the amount of throughput, i.e. monomer flow rate, can be increased without worrying of changing the fluidization regime from well mixed condition to slugging, so the production rate and in consequence the polymerization yield will increase. In this study the population balance approach is used to describe the evolution and growth of the particle size in gas–solid rotating fluidized bed olefin polymerization reactors along with CFD using Fluent program. The SMM (standard method of moments) method and QMOM (quadrature method of moments) method are used to solve the population balance equations; these are coupled with the conservation equations of mass and momentum for the gas and solid phases. Simulations have been performed with; a) constant particle growth rate and b) variable particle growth rate that is a function of polymerization reaction rate.

Fluidization, Mixing and Segregation of a Biomass-Sand Mixture in a Fluidized Bed

Abstract: Fluidization, mixing and segregation of a biomass-sand mixture in a 3D gas-fluidized bed have been investigated by means of visual observation, pressure fluctuation analysis and the bed-frozen method. Three types of mixtures are considered, in which biomass is a thin long stalk, and sand belongs to the Geldart B category. Experiments are carried out in a segmented fluidized bed equipped with multiple pressure transducers. Three initial packing conditions and two experiment procedures are used. The fluidization velocity varies to cover a wide range. Results show that in the local fluidization region, the mixing and segregation patterns are sensitive to the initial packing condition. In the case of a fully segregated state with biomass at the bottom, the bed inversion can be significantly observed due to the great segregation tendency of biomass. Further analyses indicate that the mixing ratio exerts a subtle influence on the competition between mixing and segregation by disturbing the coalescence and break-up of the bubble. In addition, the pressure fluctuation signal proves to be helpful in understanding the dynamic features of the phenomenology.

Mathematical model for mixing reactants in a capillary microreactor by transverse diffusion of laminar flow profiles.

Abstract: Transverse diffusion of laminar flow profiles (TDLFP) was recently suggested as a generic approach for mixing reactants inside a capillary microreactor. Conceptually, solutions of reactants are injected inside the capillary by high pressure as a series of consecutive plugs. Because of the laminar nature of the flow inside the capillary, the nondiffused plugs have parabolic profiles with predominantly longitudinal interfaces between the plugs. After the injection, the reactants are mixed by transverse diffusion across the longitudinal interfaces. TDLFP-based mixing is still in its infancy as only the principle was proved. Here, we develop the theory of TDLFP and introduce a dimensionless parameter, York number, which can be used in predicting the quality of TDLFP-based mixing. The theory uses a single simplifying assumption that the longitudinal diffusion is negligible; this assumption is readily satisfied. We then develop a numerical model of TDLFP and use it to simulate the concentration profiles of three reactants mixed by TDLFP in the capillary. The correlation between the York number and quality of mixing is analyzed. Two ways of improving the quality of TDLFP-based mixing are suggested and studied: (i) increasing the longitudinal interface between the plugs by a long last plug of a solvent and (ii) "shaking" the injected reactants by a series of alternating negative and positive pressure pulses. The developed theory and computational simulation of TDLFP will stimulate the practical use of capillary microreactors.