2007 Jan - June - DETAILS
Strongly exothermic heterogeneously catalysed gas phase reactions are commonly carried out in multitubular reactors. The inherent limitations of recuperative heat removal lead to hot-spot formation and can even cause reactor runaway. Apart from recuperative cooling via the reactor walls, convective, reactive or regenerative cooling principles may be exploited. The last, somewhat neglected, concept entails heat removal through the regenerative heating of the fixed-bed followed by a subsequent cooling phase.
Desorptive cooling represents a new hybrid reactive-regenerative process, with desorption of an inert providing the .reactive. contribution. Initial studies on a test reactor using CO- Oxidation as a strongly exothermic test system demonstrated the principle feasibility of integrating the desorption process into the catalytic fixed-bed. The heat of reaction liberated from the catalyst pellet is locally dissipated by the desorption of an inert component from adjacent adsorbent particles (see fig. 1), thus leading to quasi-isothermal conditions within the bed. This concept permits operation in a simple adiabatic reactor without internal cooling surfaces. As a result, cycle times in a technically relevant range and much longer than those for simple regenerative processes were attained. Further work has identified considerable potential for extending cycle times up to an hour or more: bed structuring and adsorptive profiling in the inlet to achieve more uniform utilisation of cooling capacity. In particular the latter proved to be an effective and feasible strategy for exploiting the concept to its full. While thermal desorption alone provides inadequate cooling capacity, ramping adsorptive partial pressure avoids initial subcooling and leads to an isothermal temperature profile (see fig.2). Furthermore, the regeneration procedure was modelled in order to establish overall process parameters for a full cycle and the importance of the reaction kinetics for the breakthrough behaviour and the self-regulating character of the process were identified. Present work is focussing on scale-up to pilot plant dimensions and the application of desorptive cooling to the industrial relevant selective hydrogenation of acetylene to ethylene.
The presentation focuses on the most recent results from a research programme supported by the German Research Foundation (DFG). Following an introduction to the desorptive cooling process with all the relevant design parameters, a flexible dynamic mathematical model, which is able to simulate various reactor configurations will be described. Two-dimensional simulations were found to be necessary to achieve reasonable agreement with bench-scale experimental results. The degrees of freedom available in the design of desorptive cooling systems will be discussed. In addition, the optimisation of key process parameters, such as the distribution of catalyst and adsorbent along the bed, the regulation of operating parameters, such as the inert level in the feed and the partial pressure ramping for effective cooling with maximum cycle times and acceptable conversion rates and selectivities will be addressed.
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We visualize simplified dense granular flows using a fluorescent refractive index matched liquid technique to measure motion of the grains in the bulk, and further compare it with those near the sidewalls. First, we discuss granular flow inside a silo away from the side walls to compare and contrast their fluctuation properties with simple liquids in equilibrium. The fluctuation properties of the particles are observed to be independent of the flow rate, and therefore the interstitial fluid has no significant effect on the grain fluctuations over the range studied in these experiments. We find that the correlations in the granular fluctuations as measured by the mean square displacements and the velocity auto-correlation function are remarkably similar to that observed in elastic hard sphere liquids. The observed correlations are qualitatively different at the boundaries. We will then briefly discuss a study of gsranular bed erosion by a fluid flow to give us new insight concerning the relationship between hydrodynamic stress and surface granular flow.
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With size-dependent properties that are vastly different from those of corresponding bulk solids, nanoparticles (NPs) can be made in a variety of compositions, sizes, and even shape, as a result of the advances made in synthesis chemistry over the last two decades. The full potential of NPs (and NP-containing materials) for applications remains unrealized, though, due to critical knowledge gaps in our ability to exploit their nanoscale properties and our ability to scale up their production in an economical and environmentally friendly manner. My research group?s efforts focus on filling in these ?gaps? through the rational design of application-specific, NP-based materials and the development of scalable synthesis techniques thatlead to these materials.
In this talk, I will discuss our successful efforts in inducing the formation of microcapsule structures out of NPs and polymers, and our current work in understanding the NP assembly process. We recently discovered that, under specific solution conditions, cationic polyelectrolytes can induce negatively-charged silica NPs to form micron-sized hollow spheres rather than the randomly structured precipitate that would ordinarily result from flocculation. The synthesis conditions (room temperature, atmospheric pressure, near-neutral pH, water solvent, rapid formation) allow water-soluble compounds to be encapsulated easily and without damage, and the microcapsule formation process to be potentially scaled up.
Trichloroethene, an industrial solvent, is one of the most hazardous pollutants in US groundwaters. Palladium (Pd) catalysts are known to catalyze the hydrodechlorination of trichloroethene in water, at room temperature, and in the presence of hydrogen. We recently discovered that palladium-on-gold nanoparticles (Pd/Au NPs) can be two orders of magnitude more active than Pd supported on alumina on a per-Pd gram basis. I will discuss our progress in understanding how the gold enhances the Pd catalytic activity so dramatically, and the applicability of this catalyst for the water-phase hydrodechlorination of other chlorinated compounds, as well as efforts to immobilize these NPs onto solid supports.
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Confinement in nanopores can alter fluid structure and induce phase transitions. Surface force experiments which probe the structure and dynamics of liquids confined in nanopores reveal a sharp increase in the confined fluid viscociy for non-poloar fluids, whereas for confined water and electrolytes the viscosity is bulk-like. We have used molecular dynamics simulations to to investigate the effect of confinement on the solubility of electrolyte solutions (NaCl). The solubility of NaCl in water confined in graphitic nanopores ranging from 8-20 Angstroms in width is investigated. Upon adding salt to water, a solubility limit is defined based the rapid decrease in the hydration number obtained from the ion-water radial distribution functions. Results shows a significant effect of confinement with the solubility of NaCl dropping by almost a factor of 2 when water is confined to pores of width 8 Angs. A sharp increse in solubility is observed when the pore width is increased. These changes in solubility are rationalized with the structuring and layering of water in the nanopore. With advances in technology, this study is relevant to our understanding of biological system and tranport in nano-devices.
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Over the last fifteen years both experimental and computational studies of turbulent jets and plumes subjected to off-source volumetric heating have been reported. The results shed light on a long standing counter intuitive finding, from field observations, of the entrainment characteristics of clouds. The talk will describe this work and discuss the present status on understanding of the fluid dynamics of clouds.
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The flow of a gas (saturated or dry) through a porous medium, which is partially occupied by a liquid phase, causes evaporation. The latter occurs, even if the inlet gas is fully saturated, due to volume expansion. This process, referred to as flow-through drying, is important in a variety of natural and industrial applications, such as convective drying, fuel cells and natural gas production, which is the context of this work. In this study, a mathematical model is developed to understand the process and to predict drying rates and the evolution of liquid saturation profiles. The model includes the effects of gas compressibility and capillarity. Compressibility effects account for the evaporation into the saturated gas phase, while the capillary pressure gradients cause the liquid flow that leads to spreading of the saturation profile. Two important parameters, a normalized viscous pressure drop across the medium and capillary .wicking. number, control the two respective regimes. Capillary-driven flow from regions of high saturation to regions of low saturation leads to more uniform saturation profiles or spreading drying fronts. The results are compared against experimental data obtained by X-ray imaging.
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Complex fluids and soft condensed matter are a major area of academic and industrial research today. Existence of relevant length and time scales, larger than the atomistic scales, are common features of such systems. Dissipative particle dynamics(DPD) is one of the techniques used to simulate the system with large length and time scales which are typically inaccessible in conventional molecular dynamics simulation. The major advantage of DPD is the preservation of hydrodynamics. The method has been successfully applied to simulate Poiseullie flow, and colloidal solutions. It has also been applied to capture complex phases e.g. micelles, reverse micelles, bilayers, vesicles,etc. that formed in oil-water-surfactant systems. In the present work, Dissipative particle dynamics is applied to study the surfactant bilayer phase. The phase transition from gel phase to liquid crystalline phase is successfully captured.
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During Photocatalysis (PC), an advanced oxidation process, the photocatalytic activity depends on the ability of the catalyst to create electron-hole pairs on irradiation with UV, which generate free radicals that are able to undergo secondary reactions. Role of holes and electrons during photocatalysis by Combustion Synthesized TiO2 (CS-TiO2): Both electrons and holes that are generated during PC, produces OH radicals (identified to be the key species for the oxidative degradation of the substrates). Our interest was to decouple the role of holes and electrons towards PC which can help us to identify the key species and also to understand the retardation effects in PC if those key species are suppressed in some way (like e-s can be suppressed if cations are present). It has been proved that the direct oxidation (by hole) is negligible compared to indirect oxidation (by OH radical). Efficiency of CS-TiO2 for polymer scission under simultaneous ultraviolet and ultrasound irradiation: Illustration of ternary breakage of linear chain in the combined system (in contrast to binary breakage in UV or US system alone). Photocatalytic activity of CS TiO2 for metal reduction: The variation of the photo reduction rate of Cu (II) and Cr (VI) correlates with the variation of the driving force required for their reduction by the photo generated electrons. Effect of particle size of GdCoO3 on their photocatalytic activity: The photocatalytic degradation studies carried out using 3, 12 and 200 nm sized GdCoO3 oxides, on dyes, phenols and substituted phenols indicate favorable activity. The intermediates formed during the photocatalytic reactions were analyzed and possible reaction mechanisms have been proposed. The results of the present study indicate that the degradation appear to be faster in the presence of 3 nm GdCoO3 compared to P-25 Degussa TiO2 catalyst.
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Polymeric gas separation membranes are often subjected to the simultaneous and competing effects of compaction (creep) and plasticization. Consequently, the membranes often evidence performance decline over long times. Since compaction is a viscoelastic response, its deleterious effects should be exacerbated at elevated temperatures, especially close to Tg. On the other hand, elevated temperature separations offer significant advantages in many chemical processes such that a growing number of applications require membranes that can operate at high temperatures and in chemically challenging environments. Characterization of the simultaneous mechanical and transport responses under such conditions becomes important for developing strategies to optimize the long-term mechanical stability and permselectivity of polymeric membranes. We have undertaken a comprehensive investigation of the simultaneous creep/transport behavior of dense polymeric films, and describe the results of systematic experiments conducted on poly(methyl methacrylate) (PMMA) and polybenzimiadazole (PBI) films in pressurized nitrogen and carbon dioxide environments. In general, both PMMA and PBI films evidenced negligible flux decline over time scales in which creep was significant. Furthermore, the thickness-corrected permeability showed a pronounced time dependent decrease whose magnitude seems comparable to the compressive strain. We have also demonstrated for the first time the applicability of the time-temperature superposition principle to predict the long-term permeability of PMMA films under inert nitrogen. Under CO2 pressure, the PMMA and PBI films evidenced swelling at low temperatures and relatively high compressive strains at high temperatures. Overall, the results provide an improved understanding of the relationship between mechanical and transport behavior at elevated temperatures.
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Human immunodeficiency virus (HIV) infection is an important cause of morbidity and mortality today. Current therapies prolong disease progression in HIV infected individuals but are unable to eradicate the virus. HIV invariably develops resistance to administered drugs and renders long term therapy futile. One reason for the rapid emergence of drug resistance is the high rate of recombination during HIV replication. Recombination occurs when the enzyme reverse transcriptase switches templates between the two RNA strands contained in a virion to produce a proviral DNA. The probability of the emergence of recombinant forms of HIV, which may be resistant to multi-drug therapy, is high when heterozygous virions carrying two distinct RNA strands infect target cells. Sources of heterozygous virions are cells multiply infected by different kinds of HIV virions. The dynamics of the emergence of recombinant strains remains poorly understood. We develop a mathematical model that describes HIV dynamics with multiple infections of cells and recombination. Mimicking recent experiments, we consider infection of target cells with two distinct kinds of viruses. Cells coinfected with both the kinds of viruses yield heterozygous virions, which present the necessary substrate for recombination to produce progeny genomes that are a mosaic of the two parent genomes. We develop a probabilistic description of template switching during reverse transcription and predict the dynamics of the emergence of recombinant strains. Our model captures key experimental observations and establishes a framework for timing the emergence of multi-drug resistance in HIV infected individuals undergoing therapy.
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Nucleation is one of the primary mechanisms of a first order phase transition. Examples include condensation, vaporization, crystallization etc. In spite of the importance of nucleation, our understanding of nucleation remains incomplete. Although we have a qualitative understanding of the mechanism of nucleation, no satisfactory theory exists for accurately predicting the nucleation rate. This is because nucleation is an activated process and its rate depends on the nature and properties of the critical nucleus. Direct observation of the critical nucleus is very difficult since (i) it is microscopic is size consisting of few hundreds of particles, (ii) its formation is very rare (typically 101 to 106 nuclei per cm3 per second) and (iii) its lifetime is very short. However, these characteristics of the critical nucleus make it an interesting problem to study via molecular simulations. Liquid mixtures are interesting because they are generally harder to crystallize than a pure liquid. They also show much more variety in phase behavior as compared to pure fluids. Their phase diagrams vary from simple spindle shaped structure to complex ones showing azeotropes, eutectic points, peritectic points, compound solid formation etc. Thus under varying compositions a liquid mixture can form crystals with that not only vary in composition but also in structure. I have performed Monte Carlos simulations of crystal nucleation to study the effect of these complex phenomena on the nucleation barrier and structure of critical nucleus[1]. Our calculations indicate that fractionation of species upon crystallization increases the difficulty of crystallization of fluid mixtures and in absence of fractionation (azeotropic conditions) the nucleation barrier is comparable to pure fluids. I have also studied crystal nucleation in liquid mixtures that can form substitutionally ordered solids. In such systems which also show solid-solid phase separation, we find that the phase that nucleates is the one whose equilibrium composition is closer to the composition of the fluid phase. My work on adsorption involves the study of liquid phase adsorption of linear alkanes in zeolites via molecular simulations. Molecular simulations have become an important tool for understanding adsorption in zeolites. However, bulk of molecular simulations of adsorption have focused mainly on adsorption from the gas phase. Liquid phase adsorption differs from gas phase adsorption due to the high density of the adsorbates inside the zeolites which results in the intermolecular forces among adsorbate molecules playing a significant role in the adsorption mechanism. Although in principle, grand canonical Monte Carlo (GCMC) simulation of an adsorbed phase in contact with a liquid or gas phase is similar, there are several challenges in practice. As mentioned above, liquid phase adsorption is characterized by high density of adsorbates inside the zeolite. Hence insertions and deletions of molecules become inefficient via convention GCMC techniques resulting in poor sampling averages. The sampling efficiencies can be improved significantly through the use of advanced simulation techniques such as configurational biasing. I have applied these techniques to study adsorption of alkanes in "cage-like" Zeolite A (LTA-5A) [2]. The n-alkanes were modeled as chain molecules using a united atom forcefield. Analysis of molecular-level conformations, siting and packing show that alkanes adopt highly coiled configurations to fit inside the supercages of Zeolite A. The simulations show how size and packing constraints affect the maximum loading of alkanes in Zeolite A. The simulations also reveal the role of diffusion while measuring the maximum loading of alkanes from experiments.
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Since the components in the human cells are in nanoscale range, nanotechnology is poised to significantly affect medicine. Recent benefits to medicine have come from the use of nanoparticles, which are tiny particles about 200-fold smaller than the diameter of a human hair. For example, nano-taxol, recently approved for breast cancer, provides significantly better treatment with reduced toxicity. Due to their versatility in targeting tissues, accessing deep molecular targets, and controlling drug release, nanoparticles are helping address challenges in the delivery of both modern and conventional drugs.
Gupta*s research is in the area of the production of pharmaceutical nanoparticles. His research group at Auburn University has developed SAS-EM process (USPatent 6,620,351) to produce nanoparticles of controllable size. The drug compound is first dissolved in a solvent, and then precipitated extremely rapidly, yet controllably, using supercritical CO2 to produce nanoparticles of desired size. The technology has now been adopted by the pharmaceutical industry for commercial production. In many of the medical applications, these nanoparticles can used directly via oral, injection, inhalation, dermal, etc. routes. His research group is also utilizing the nanoparticles to produce smart drug formulations. For example, sustained-release medicine, where one dose is enough for the whole treatment; magnetically responsive drug nanoparticles for targeted delivery to the disease site, avoiding the side effects; nanoparticle delivery to the brain for treatment of cancer, memory loss, Alzheimer*s, etc.
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