The passive separation of a binary mixture of spherical particles is accomplished using a laborat... more The passive separation of a binary mixture of spherical particles is accomplished using a laboratory scale quasi two-dimensional inclined board such that gravity alone drives the flow of the mixture through a static array of obstacles. Experimental results compare well with simulations both qualitatively and quantitatively. An increase in separation is observed for increasing board length, whereas a decrease in separation is observed as the solid fraction (area coverage) of particles increases. This work demonstrates the possibility of designing green technology for solid-solid separations by taking advantage of particle properties that aid naturally occurring segregation. A probability based model is suggested as a way to predict the viability of separation between particle types as a function of particle size and coefficient of restitution. It should be noted that size separation is achieved despite peg spacings that are larger than both particles in a mixture. This article is protected by copyright. All rights reserved.
The interaction between catalytic nanoparticles (NPs) and their supports, which are often amorpho... more The interaction between catalytic nanoparticles (NPs) and their supports, which are often amorphous oxides, has not been well characterized at the atomic level, although it is known that, in some cases, NP–support interactions dominate the catalytic activity of the system. Furthermore, there is a lack of understanding of how support preparation affects both the stability of the NP (resistance to sintering) and the catalytic activity. We present first-principles density functional theory (DFT) calculations on amorphous silica supported Pt NPs of various sizes. Our calculations predict that support preparation methods that lead to higher hydroxyl density when NPs are deposited on the support will lead to higher resistance to sintering. We find that the total charge on supported NPs, which can affect catalyst activity, depends linearly on the number of Pt–silica bonds formed during NP deposition. The number of bonds between an NP of a known geometry and the silica support with a known hydroxyl density can be...
ABSTRACT An accurate description of metal nanoparticle (NP)?support interactions is required for ... more ABSTRACT An accurate description of metal nanoparticle (NP)?support interactions is required for designing and optimizing NP catalytic systems because NP?support interactions may significantly impact NP stability and properties, such as catalytic activity. The ability to calculate NP interactions with amorphous supports, which are commonly used in industrial practice, is hampered because of a general lack of accurate atomically detailed model structures of amorphous surfaces. We have systematically studied relaxation processes of Pt13 NPs on amorphous silica using recently developed realistic model amorphous silica surfaces. We have modeled the NP relaxation process in multiple steps: hard-sphere interactions were first used to generate initial placement of NPs on amorphous surfaces, then Pt?silica bonds were allowed to form, and finally both the NP and substrate were relaxed with density functional theory calculations. We find that the amorphous silica surface significantly impacts the morphology and electroni
The passive separation of a binary mixture of spherical particles is accomplished using a laborat... more The passive separation of a binary mixture of spherical particles is accomplished using a laboratory scale quasi two-dimensional inclined board such that gravity alone drives the flow of the mixture through a static array of obstacles. Experimental results compare well with simulations both qualitatively and quantitatively. An increase in separation is observed for increasing board length, whereas a decrease in separation is observed as the solid fraction (area coverage) of particles increases. This work demonstrates the possibility of designing green technology for solid-solid separations by taking advantage of particle properties that aid naturally occurring segregation. A probability based model is suggested as a way to predict the viability of separation between particle types as a function of particle size and coefficient of restitution. It should be noted that size separation is achieved despite peg spacings that are larger than both particles in a mixture. This article is protected by copyright. All rights reserved.
The interaction between catalytic nanoparticles (NPs) and their supports, which are often amorpho... more The interaction between catalytic nanoparticles (NPs) and their supports, which are often amorphous oxides, has not been well characterized at the atomic level, although it is known that, in some cases, NP–support interactions dominate the catalytic activity of the system. Furthermore, there is a lack of understanding of how support preparation affects both the stability of the NP (resistance to sintering) and the catalytic activity. We present first-principles density functional theory (DFT) calculations on amorphous silica supported Pt NPs of various sizes. Our calculations predict that support preparation methods that lead to higher hydroxyl density when NPs are deposited on the support will lead to higher resistance to sintering. We find that the total charge on supported NPs, which can affect catalyst activity, depends linearly on the number of Pt–silica bonds formed during NP deposition. The number of bonds between an NP of a known geometry and the silica support with a known hydroxyl density can be...
ABSTRACT An accurate description of metal nanoparticle (NP)?support interactions is required for ... more ABSTRACT An accurate description of metal nanoparticle (NP)?support interactions is required for designing and optimizing NP catalytic systems because NP?support interactions may significantly impact NP stability and properties, such as catalytic activity. The ability to calculate NP interactions with amorphous supports, which are commonly used in industrial practice, is hampered because of a general lack of accurate atomically detailed model structures of amorphous surfaces. We have systematically studied relaxation processes of Pt13 NPs on amorphous silica using recently developed realistic model amorphous silica surfaces. We have modeled the NP relaxation process in multiple steps: hard-sphere interactions were first used to generate initial placement of NPs on amorphous surfaces, then Pt?silica bonds were allowed to form, and finally both the NP and substrate were relaxed with density functional theory calculations. We find that the amorphous silica surface significantly impacts the morphology and electroni
Soft-sphere particle dynamics simulations have found wide use in recent years. One application fo... more Soft-sphere particle dynamics simulations have found wide use in recent years. One application for which this technique is particularly well suited is that of granular mixing. Particle properties can be varied on a particle- by-particle basis and detailed mixed structures are easily captured and visualized. While this method has proven to be quite versatile, it is computationally intensive. Current particle dynamics simulations in the literature generally handle 3000-5000 particles, whereas a typical industrial mixer may easily contain as many as 109 particles - six orders of magnitude more. However, in certain circumstances - such as in a tumbler mixer, where the bulk of the particle motion consists of a solid body rotation - it is not necessary to explicitly calculate the motion of all of the particles. By combining particle dynamics and geometrical insight, in essence, by focusing the particle dynamics simulation only where it is needed, a new hybrid method of simulation, which is much faster than a conventional particle dynamics method, can be achieved. This technique can yield more than an order of magnitude increase in computational speed, allowing simulations of the order of 104 particles, while maintaining the versatility of a particle dynamics simulation. Two applications of this technique are presented: a tumbler operated in the avalanching regime and a continuously flowing tumbler.
Mixing of granular solids is invariably accompanied by segregation, however, the fundamentals of ... more Mixing of granular solids is invariably accompanied by segregation, however, the fundamentals of the process are not well understood. We analyze density and size segregation in a chute flow of cohesionless spherical particles by means of computations and theory based on the transport equations for a mixture of nearly elastic particles. Computations for elastic particles (Monte Carlo simulations), nearly elastic particles, and inelastic, frictional particles (particle dynamics simulations) are carried out. General expressions for the segregation fluxes due to pressure gradients and temperature gradients are derived. Simplified equations are obtained for the limiting cases of low volume fractions (ideal gas limit) and equal sized particles. Theoretical predictions of equilibrium number density profiles are in good agreement with computations for mixtures of equal sized particles with different density for all solids volume fractions, and for mixtures of different sized particles at low volume fractions (nu{\textless}0.2), when the particles are elastic or nearly elastic. In the case of inelastic, frictional particles the theory gives reasonable predictions if an appropriate effective granular temperature is assumed. The relative importance of pressure diffusion and temperature diffusion for the cases considered is discussed. (c) 1999 American Institute of Physics.
We consider the mixing of similar, cohesionless granular materials in quasi-two-dimensional rotat... more We consider the mixing of similar, cohesionless granular materials in quasi-two-dimensional rotating containers by means of theory and experiment. A mathematical model is presented for the flow in containers of arbitrary shape but which are symmetric with respect to rotation by 180° and half-filled with solids. The flow comprises a thin cascading layer at the flat free surface, and a fixed bed which rotates as a solid body. The layer thickness and length change slowly with mixer rotation, but the layer geometry remains similar at all orientations. Flow visualization experiments using glass beads in an elliptical mixer show good agreement with model predictions. Studies of mixing are presented for circular, elliptical, and square containers. The flow in circular containers is steady, and computations involving advection alone no particle diffusion generated by interparticle collisions show poor mixing. In contrast, the flow in elliptical and square mixers is time periodic and results in chaotic advection and rapid mixing. Computational evidence for chaos in noncircular mixers is presented in terms of Poincare ́ sections and blob deformation. Poincare ́ sections show regions of regular and chaotic motion, and blobs deform into homoclinic tendrils with an exponential growth of the perimeter length with time. In contrast, in circular mixers, the motion is regular everywhere and the perimeter length increases linearly with time. Including particle diffusion obliterates the typical chaotic structures formed on mixing; predictions of the mixing model including diffusion are in good qualitative and quantitative in terms of the intensity of segregation variation with time agreement with experimental results for mixing of an initially circular blob in elliptical and square mixers. Scaling analysis and computations show that mixing in noncircular mixers is faster than that in circular mixers, and the difference in mixing times increases with mixer size.
An important industrial problem that provides fascinating puzzles in pattern formation is the ten... more An important industrial problem that provides fascinating puzzles in pattern formation is the tendency for granular mixtures to de-mix or segregate. Small differences in either size or density lead to flow-induced segregation. Similar to fluids, noncohesive granular materials can display chaotic advection; when this happens chaos and segregation compete with each other, giving rise to a wealth of experimental outcomes. Segregated structures, obtained experimentally, display organization in the presence of disorder and are captured by a continuum flow model incorporating collisional diffusion and density-driven segregation. Under certain conditions, structures never settle into a steady shape. This may be the simplest experimental example of a system displaying competition between chaos and order.
Particulate systems have proven difficult to probe experimentally in many instances. Simulations ... more Particulate systems have proven difficult to probe experimentally in many instances. Simulations of granular flows, and mixing flows in particular, provide a useful means of studying particulate behavior. Mixing flows generate large scale patterns and structures which can be easily visualized. Thus, mixing studies provide a means of indirectly examining granular flows. In this paper we review recent computational studies of tumbler mixing, focusing on two very different, yet complementary, techniques: Particle Dynamics and Lagrangian Simulation. We discuss mixing in different tumbler geometries, as well as segregation and cohesive effects. (C) 2000 Elsevier Science S.A.
Cohesive forces between grains can arise from a variety of sources - such as liquid bridge (capil... more Cohesive forces between grains can arise from a variety of sources - such as liquid bridge (capillary) forces, van der Waals forces, or electrostatic forces - and may play a significant role in the processing of fine and/or moist powders. While recent advances have been made in our understanding of liquid-induced cohesion at the macroscopic level, in general, it is still not possible to directly connect this macroscopic understanding of cohesion with a microscopic picture of the particle properties and interaction forces. In fact, conventional theories make no attempt to distinguish between these modes of cohesion, despite clear qualitative differences (lubrication forces in wet systems or electrostatic repulsion are two good examples). In this work, we discuss several discrete characterization tools for wet (cohesive) granular material with simple, physically relevant interpretations. We examine the utility of these tools, both computationally and experimentally, by exploring a range of cohesive strengths (from cohesionless to cohesive) in several prototypical applications of solid and gas-solid flows. Copyright {\textcopyright} 2002 by ASME.
The challenges associated with the building of chemical engineering curricula in the University o... more The challenges associated with the building of chemical engineering curricula in the University of Pittsburgh, keeping in view the demands of the technological advancement, are discussed. The updated version of the curriculum should prepare students for todays engineering economy, while enabling them to maintain versatility through life-long learning and continuing education. It is observed that the reform of undergraduate chemical engineering instruction is overdue and the curriculum has lagged behind research and industry. It is suggested that to allow the curriculum to grow organically and add courses wherever and whenever needed or desired.
Slow granular flows play an important role in industries ranging from food to pharmaceuticals to ... more Slow granular flows play an important role in industries ranging from food to pharmaceuticals to ceramics, and have been an area of active research in recent years. In contrast, heat transfer in flowing particulate systems has received relatively little attention. In this work, we employ a computational technique that couples the Discrete Element Methods (DEM), Computational Fluid Dynamics (CFD), and heat transfer calculations to simulate realistic heat transfer in a rotary kiln. Our results suggest a novel transition in heat transfer regime as the conductivity of the particle changes. At low particle conductivities, the heat transfer is dominated by gas-solid conduction; however, at higher particle conductivities solid-solid conduction plays the dominant role. The impact that this transition has on the importance of particle mixing will be the focus of our talk.
Rotating viscous flows have been shown to contain regions of chaos and regions of regularity, whi... more Rotating viscous flows have been shown to contain regions of chaos and regions of regularity, which can serve as attractors for non-Brownian particles. This study details experimental observations of non-neutrally buoyant spherical particles spontaneously migrating (across fluid streamlines) to such regular locations within the toroidal flow between parallel flat disks in a stirred tank flow. These experimental results are also compared to two different versions of continuum-based Lagrangian advection model, based on the Basset-Boussinesq- Oseen (BBO) equation. The first model is an approximate 2D analytical solution which shows an inward migration within the cell flow structure for particles both heavier and lighter than the continuous phase. The second model is a discrete version of the BBO equation which includes a numerical analysis of the experimental flow field. We show that the character of the flow in these toroids, located above and below each impeller in a simple mixing tank, has a dramatic effect on the ultimate equilibrium location of the particles. Depending on flow/particle conditions the asymptotic migration position varies between multi-period islands as well as the center of the toroid.
The challenge of developing a better Chemical Engineering Curriculum, or any Engineering curricul... more The challenge of developing a better Chemical Engineering Curriculum, or any Engineering curriculum, is to build it such that it prepares students for the engineering economy of today, while enabling them to maintain versatility through life-long learning and continuing education. Toward this end, the National Science Foundation (NSF) funded a number of Coalitions to study "best practices" in Engineering Education. Overwhelmingly, these Coalitions have favored active-learning activities and integration of complementary subjects. With funding from the NSF, we have expanded the recommendations of the Coalitions and developed an integrated Chemical Engineering curriculum that spans the Sophomore to Senior years. To implement the integrated curriculum, we are using block scheduling, a technique with a strong literature base in K-12 education. In a nutshell, through block scheduling multi-semester courses are delivered in a single-semester course. The result has been the Six Pillars of Chemical Engineering. These courses have considerably longer contact hours than a traditional University course so that: (1) students may gain systems insight through integration of their core knowledge across traditional course and discipline boundaries; (2) the instructors have the time to include truly multi-scale (from molecular to continuum to macroscopic) descriptions of Chemical Engineering content; and (3) the instructors have the flexibility to accommodate diverse learning styles and incorporate active learning more effectively. In this paper, we outline the design of our Six Pillars and highlight preliminary results from the pilot of one Pillar: Transport Phenomena. By using a suite of assessment techniques including concept maps, concept inventories, and surveys, we hope to produce a truly validated success story that can serve as a model that is applicable for all engineering disciplines.
We use a perturbed streamfunction to analyze a dilute suspension of slightly non-neutrally buoyan... more We use a perturbed streamfunction to analyze a dilute suspension of slightly non-neutrally buoyant solid spheres as they migrate across the curved fluid streamlines of a viscous cellular flow. This is done by incorporating particle-fluid interactions into a continuum-based Lagrangian advection model derived from the Basset-Boussinesq-Oseen (BBO) equation. The results demonstrate significant interplay between the underlying fluid structure and non-trivial equilibrium locations of the non-Brownian particles. We also evaluate the effect of the Saffman lift force on the lateral migration of the solid spheres.
Chemical engineers who enter the marketplace today are facing a vastly different reality than tho... more Chemical engineers who enter the marketplace today are facing a vastly different reality than those who started their careers even five years ago. Keith Watson, (Senior Director, Strategic Marketing, Dow Chemical Company) noted in 2011, "The attributes needed to compete for employment in the modern chemical industry have changed. However, the curriculum at most traditional Western universities does not necessarily reflect these new dynamics." The majority of chemical engineering programs today do not leave room within their curriculum for students to be able to adequately explore the concept of chemical product design and how novel ideas can become the basis for new businesses. In fact, out of the 158 ABET accredited chemical engineering programs in the US, only 25 offer chemical product design classes. This state of affairs presents a stark contrast with mechanical, industrial, and even bioengineering programs, where product design has been a routine part of the curriculum for decades. In response to this need, the chemical engineering program at the University of Pittsburgh has taken the initiative to re-design its chemical product design senior level course and expand upon it to create a three-year, chemical "Product Innovation Sequence". This course sequence will start with required courses in both the sophomore and junior year followed by a senior year elective for those students who are particularly interested in this field of study. The novel nature of this curriculum is found in its coupling of scaffolding techniques to encourage students to build and develop their chemical product design skills progressively as they go through the course sequence, the experiential nature of the final senior level prototyping course and the effort to provide mentorship opportunities between students in different years of the course sequence. One of the key features of this new experiential product innovation sequence for chemical engineers is the showcasing of the role of the customer within the design process which can often be an afterthought in engineering design. In contrast, it is the beginning of thought in this new product design sequence. Specifically, the sophomore level class focuses exclusively on concepts related to the front-end portion of chemical product design including: customer identification and needs, brainstorming and decision making processes. Subsequently, the junior level course in the sequence focuses on the fundamentals necessary to perform chemical product design including formulations, heuristics and life cycle analysis for the development of more sustainable products. This course also includes elements important to small business development such as intellectual property, commercialization plans and how to deliver a business pitch. Finally, in the senior level prototyping course, students will actually be given the opportunity to create a physical prototype of their product and work alongside a faculty mentor on the development of their own business model. This course sequence provides a "safe" environment for chemical engineering students to get a real taste of what starting your own business might be like prior to entering into the marketplace. It is believed that this first-of-itskind "Product Innovation Sequence" will build not only a culture of entrepreneurship that permeates all levels of education (from sophomores to graduate student TAs to faculty), but also produce a new generation of alumni better equipped to work in today's marketplace, whether they choose to work within an existing company or venture out on their own. {\textcopyright} American Society for Engineering Education, 2014.
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