Fluidized-bed reactor modeling for production of silicon by silane pyrolysis

An ideal backmixed reactor model (CSTR) and a fluidized bed bubbling reactor model (FBBR) were developed for silane pyrolysis. Silane decomposition is assumed to occur via two pathways: homogeneous decomposition and heterogeneous chemical vapor deposition (CVD). Both models account for homogeneous and heterogeneous silane decomposition, homogeneous nucleation, coagulation and growth by diffusion of fines, scavenging of fines by large particles, elutriation of fines and CVD growth of large seed particles. At present the models do not account for attrition. The preliminary comparison of the model predictions with experimental results shows reasonable agreement. The CSTR model with no adjustable parameter yields a lower bound on fines formed and upper estimate on production rates. The FBBR model overpredicts the formation of fines but could be matched to experimental data by adjusting the unkown jet emulsion exchange efficients. The models clearly indicate that in order to suppress the formation of fines (smoke) good gas-solid contacting in the grid region must be achieved and the formation of the bubbles suppressed

Czochralski crystal growth: Modeling study

The modeling study of Czochralski (Cz) crystal growth is reported. The approach was to relate in a quantitative manner, using models based on first priniciples, crystal quality to operating conditions and geometric variables. The finite element method is used for all calculations

Looking Inside the Mind of Millennial Students: What Do They Know or Not Know about Learning

Metacognition, or insight into one’s own learning process, may be critical for academic success. In this presentation, we cover some of the key metacognitive processes, discussing the areas in which college students often struggle. We address the questions of whether these metacognitive deficiencies are related to brain maturation, developmental stages or educational shortcomings and whether there are any metacognitive issues that are unique to millennial students (i.e., generational effects). Finally, we consider whether millennial characteristics should influence the ways in which we attempt to teach metacognitive skills to the current cohort of students

NOVEL DESIGN OF MULTIPHASE REACTORS FOR BIOMASS-TO-LIQUID SYNTHESIS

Generation of liquid fuels from renewable sources such as biomass has been practiced since early 1900’s. In view of the skyrocketing oil prices and the depleting reserves of natural gas, it has gained further interest. The Fischer-Tropsch synthesis is one of the main processes considered. It involves the reaction of syngas in presence of a catalyst to produce liquid fuels. Syngas sources are numerous ranging from waste gasification, anaerobic digestion to clean coal. The reactor of choice for gas to liquid conversion is a slurry bubble column. Although, these multiphase reactors offer several advantages including good heat and mass transfer, ease of construction and operation, the absence of moving parts, one of their main disadvantages is the difficulties associated with the scale-up. The latter is due to complex phases’ interactions and significant backmixing of phases. In general, the scaling rules are derived from mass and momentum balances resulting in dimensionless hydrodynamic numbers. For a proper scaling these numbers should be kept constant, together with dimensionless geometric numbers in order to ensure both dynamic and geometrical similarity. With the complex nature of the flow in these systems, this becomes very hard to achieve since it may result in the need for matching a large numbers of dimensionless quantities. Hence, different routes to provide a firm scale-up methodology are needed. Controlling the effect of scale using heat exchanging internals by means of reactor compartmentalization is proposed in this study. The details of this methodology can be summarized as follows: a) The large reactor diameter is subdivided into similar, vertical compartments by means of the cooling tubes; b)The compartments are to have a diameter similar to that of a small scale column on which investigations can be (have been) performed; and c) The various hydrodynamic parameters within each compartment are to be compared with those measured in a bubble column of the same diameter. Preliminary results show that radial gas holdup profiles inside the compartments exhibited similar behavior as inside a solid wall column, with a close agreement between resulting gas holdup profile inside the single tube bundle compartment and data obtained in 6 inches steel bubble column

Quantification of Solids Flow in a Gas-Solid Riser: Single Radioactive Particle Tracking

Solids in Risers of Circulating Fluidized Beds (CFB) Exhibit Local Backflow and Recirculation. Measurement of the Concentration-Time Response to an Impulse Injection of Tracer, Even at Two Elevations Cannot Determine the Residence Time Distribution (RTD) of Solids Uniquely. Hence, Evaluation of RTD in Risers from Conventional Tracer Responses is Difficult and Often Not Possible. in Addition, Estimating the Solids Circulation Rate in These Closed Loop Systems, is a Non-Trivial Problem. in This Work, a Single Radioactive Particle in the CFB Loop is Tracked during its Multiple Visits to the Riser And, by Invoking Ergodicity, Solids Circulation Rate, Accurate Solids RTD and Additional Information on the Solids Flow Pattern in the Riser Are Estimated. a Calibration Curve Was Established for the overall Solids Mass Flux as a Function of Superficial Gas Velocity. a Second Peak in the Probability Density Function (PDF) of the Solids RTD Curve in the Riser Was Observed for Operating Conditions in the Fast-Fluidization Regime. © 2004 Elsevier Ltd. All Rights Reserved

The principles of molecular gel formation

Molecular gels are associated with the formation of strongly anisotropic structures at low volume fractions (less than 1 wt%) that induce solid-like mechanical properties. Low molecular weight gelators based on aromatic short peptide derivatives have been shown to self-assemble into fibrous networks featuring highly ordered molecular packing. The building units of these structures are individual molecules experiencing hydrogen bonding and π-π stacking interactions, making them distinct from typical gels formed by aggregation of colloidal particles or crosslinking of polymer chains. The remarkable structural and mechanical properties of these materials offer a wide range of potential applications. Despite a surge in scientific publications on a variety of molecular gelators over the past decade, the mechanism and fundamental thermodynamic principles of molecular gel formation remain poorly understood. The aim of this thesis is to address these issues by a thorough and systematic experimental characterization of a model molecular gelator, fluorenylmethoxycarbonyl diphenylalanine (Fmoc-FF). The nature of the gel transition, as well as the relationship between composition, dynamics, structural and mechanical properties are discussed within the framework of current soft matter theories. The experimental observations reveal that the formation of the gel is a result of the system undergoing an equilibrium first order phase transition and a generalized phase diagram is developed

The technique of quick designing of casting panel for plastic gear wheel injection

Sadašnji tehnološki nivo strojarstva zahtijeva aktivnu primjenu softvera u svim stadijima izrade nekog strojarskog projekta – proizvoda. Izraz proizvod u strojarstvu se danas odnosi i na projektiranje i na stvarnu izradu tog proizvoda. ProEngineer, kao osnovni softver ovoga rada je, sa svojim interaktivno povezanim modulima, potpuno funkcionalan alat za rješavanje i najsloženijih strojarskih zadataka u prihvatljivom vremenskom razmaku. Pojam prihvatljivog vremena proizvodnje i izrade proizvoda uključuje rješavanje problema s ciljem dobivanja finalnog proizvoda, bilo kojeg oblika, uz optimiziranje svih stadija planiranja, analize, razvoja i proizvodnje. Brzina rješavanja konstrukcijskog zadatka je uvjet postojanja profitabilne proizvodnje u strojarstvu, a ovaj bi rad to trebao dokazati. Ranije je rečeno da je ovaj rad zasnovan na primjeni interaktivno povezanih modula softvera za konstruiranje ljevačkog panela za ubrizgani plastični zupčanik.The current technological level of mechanical engineering sequentially requires an active software application in all construction stages of certain mechanical projects – products. When we say product today in engineering we refer to the design of a product, as well as to the actual construction. ProEngineer, as basic software of this thesis, with its interactively connected modules, is a completely functional tool for solving even the most complex tasks of mechanical engineering in acceptable time frame. The notion of acceptable construction time frame and mechanical design production includes problem solving aiming at constructing the final product, in any shape, tending to optimize all stages of planning, analysis, development and production. The speed of construction task solving conditions the existence of profitable production in mechanical engineering, which should be proved through the work under this title. It is said previously that the foundation of this thesis is the application of interactively joined software modules for constructing a casting panel for injected plastic gear wheel

High-Pressure Trickle-Bed Reactors: A Review

A Concise Review of Relevant Experimental Observations and Modeling of High-Pressure Trickle-Bed Reactors, based on Recent Studies, is Presented. the Following Topics Are Considered: Flow Regime Transitions, Pressure Drop, Liquid Holdup, Gas-Liquid Interfacial Area and Mass-Transfer Coefficient, Catalyst Wetting Efficiency, Catalyst Dilution with Inert Fines, and Evaluation of Trickle-Bed Models for Liquid-Limited and Gas-Limited Reactions. the Effects of High-Pressure Operation, Which is of Industrial Relevance, on the Physicochemical and Fluid Dynamic Parameters Are Discussed. Empirical and Theoretical Models Developed to Account for the Effect of High Pressure on the Various Parameters and Phenomena Pertinent to the Topics Discussed Are Briefly Described

Using a Fiber-Optic Probe for the Measurement of Volumetric Expansion of Liquids

A Fiber-Optic Probe is Developed for the Fast, In-Situ Measurement of Volumetric Expansion of Multiphase and Multicomponent Systems. an Experiment with the Binary Mixtures of CO2-Toluene and CO2-Ethanol Was Conducted to Demonstrate the Usefulness of the Fiber-Optic Probe in Accurately Tracking the Isothermal Volumetric Expansion as a Function of Pressure. in the 1-L Autoclave that Has Been Used, the Probe Was Shown to Detect the Liquid Level Height within a Precision of 0.35% of the Total Height of the Vessel. the Results for the Volumetric Expansion of Toluene and Ethanol with CO2 Correlate Well with Those Found in the Literature. the Probe itself Can Be Used Up to Pressures of 140 Barg and Temperatures of 120°C. © 2007 American Chemical Society

Discriminating Trickle-Flow Hydrodynamic Models: Some Recommendations

The Forecasting Ability of Five One-Dimensional (1-D) Two-Fluid Phenomenological Models for Liquid Holdup and Two-Phase Pressure Drop in Trickle-Flow Reactors Was Evaluated using the Most Comprehensive Trickle-Flow Regime Database. All of These Models, Namely, the Permeability Model, the Slit Model, the Extended Slit Model, the 1-D CFD Model, and the Double-Slit Model Can Be Used to Predict Liquid Holdup. among Them, the Permeability and the Slit Models, Because of a Much Simpler Structure, Are Recommended. the Extended Slit Model based on Iliuta Et Al. (Ind. Eng. Chem. Res. 1998, 37, 4542) Shear and Slip Constitutive Relationships Can Be Employed for Two-Phase Pressure Drop Predictions. When the Knowledge of Wetting Efficiency Becomes Essential at Very Low Liquid Flow Rates, the Double-Slit Model is Recommended