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Physical Aspects of Scraped Heat Exchanger Crystallizers: An Application in Eutectic Freeze Crystallization
No full textOne of the most important natural resources is fresh water. An unfortunate issue is however, the low availability and high demand of fresh water world-wide. From the total resources industry consumes about 20 % (1200 km3 yearly) [UN, 2007]. Industrial aqueous waste streams are therefore widely occurring, and their reuse becomes obligatory. For the recovery of water from dilute solutions multistage flash evaporation has been used for a long time. Reverse osmosis has proven to be a better alternative energy-wise, but is only a good option up to a solute concentration of about 5% above which the required pressure becomes too high. Conventional separation processes then become too expensive, and the residual often large waste streams are collected in ponds for further atmospheric drying or discharged in surface waters. For the treatment of more concentrated waste waters novel separation techniques therefore had to be developed where besides pure water, one or more pure inorganic or/and organic compounds were obtained, and where only a minor bleed stream remains for e.g. incineration or disposal. If indeed additionally one or more pure secondary raw materials can be produced for reuse or with a market value, such a separation process becomes economically feasible, and has a fair chance to be implemented. Eutectic Freeze Crystallization fits this requirement. In this crystallization process, that operates under eutectic conditions, pure ice and a second pure solid compound such as a salt or an acid, are simultaneously formed in case of a binary aqueous solution. For a more complex solution both pure ice and a mixture of compounds or in subsequent steps pure ice and one of the pure compounds are formed. Since ice floats to the top of the vessel and the other compound(s) present in the solution settle(s) to the bottom, the separation of the ice from the salts is facilitated. For the first prototypes of EFC crystallizers horizontal heat exchanger disks were used with holes to allow transportation of crystals through these holes during their segregation. EFC research continues to improve these designs and create new ones that minimize energy consumption by cutting energy losses to the environment, by increasing the heat fluxes by improved scraper designs, and recently by a radical change in heat exchanger geometry to improve the gravitational separation efficiency. This research focuses on physical phenomena that take place inside a common cooling crystallizer or inside a eutectic freeze crystallizer with a scraped heat exchanger surface with the aim to improve the scraper and the crystallizer design. One of the limiting factors for heat transfer in EFC processes is the formation of an insulating ice scale layer on the heat exchanger surface. In spite of the fact that scrapers are used to increase the heat transfer at the solution side, as well as to avoid the formation of an insulating ice scale layer, such scrapers are only effective in keeping the surface free from ice up to a certain temperature difference between the coolant and the solution. Earlier experiments have also demonstrated that for a horizontal heat exchanger surface that was evenly cooled underneath and evenly scraped at the top, scaling still preferentially starts at specific areas of the surface. Since ice scale formation at a certain spot on the heat exchanger surface is a consequence of the local temperature difference between the surface and the solution, this implies that the temperature at the surface differs locally. These variations in local temperatures are most likely induced by the flow field of the liquid insidethe crystallizer. In chapter 2 the temperature distribution across the horizontal heat exchanger surface at the bottom of the crystallizer was therefore measured by Liquid Crystal Thermometry and the local heat transfer distribution was calculated, while the fluid velocity inside the crystallizer was directly visualized by Stereoscopic PIV measurements. Liquid Crystal Thermometry revealed that the temperature on the heat exchanger surface was not uniformly distributed with temperature differences larger than 4 oC. Such differences are sufficient to explain the local variation in scaling tendency. Experimental results showed that in the middle of the scraped heat exchanger area the heat transfer is larger than around the shaft and close to the crystallizer wall. A strong secondary fluid flow in this crystallizer geometry was shown by stereoscopic PIV measurements. This flow pattern in the crystallizer is undoubtedly related to the differences in local heat flux, but the local heat fluxes could not directly be explained without a more detailed knowledge of the flow field close to the HE surface in the scraper region. For more detailed information on the flow pattern between the scrapers and thus more closely to the HE surface we had to rely on computational flow simulations, because this region was not accessible for PIV measurements. The turbulent flow in the total scraped HE crystallizer was therefore computationally resolved by Large Eddy Simulations using a Lattice Boltzmann scheme as reported in chapter 3. The simulations were validated by comparison with the PIV measurements performed previously. The LES results could explain the radial local heat transfer distribution along the HE surface. The local heat flux is higher in the middle region of the HE surface between the shaft and the outer wall because the fluid descends directly from the bulk, and its residence time near the HE surface is low. The lower local heat flux coefficient around the shaft is because the fluid is largely in solid body rotation. Close to the outside wall, a worse situation for heat flux is provoked by a static vortex that brings the fluid into a segregated area. These fluid flow results explain the local heat flux distribution measured by the TLC experiments. The shape of the scraper has a large impact on the turbulent fluid flow and related behavior of the scraped off ice particles in the direct vicinity of the HE surface. For a well-designed scraper the ice particles have to be transported up by the flow into the solution, and should not be redirected to the surface. In chapter 4 the liquid flow is visualized by dye injection and the particles are monitored directly for two types of commonly used scraper geometries. In conjunction with this experimental work direct numerical simulations of the two phase (solid-liquid) flow system were done. The particle-flow simulations used Lattice-Boltzmann discretization to solve the Navier-Stokes equations, and a Lagrangian approach to particle tracking. The simulation results were in qualitative agreement with the experimental visualizations of the same system, and provided good insight in the behaviour of the particles and the fluid flow induced by the two scraper geometries. Only drag and gravity forces acting on the particles were taken into account. Furthermore particles collided elastically with the HE surface and the walls defining the moving scraper. Our numerical approach without particle-particle collisions provided satisfactory results for the streamlined scraper because this shape did not accumulate the particles during its scraper action. For the vertically oriented scraper this simulation approach failed due to the build up of particles as could be observed from the visualisation experiments. It was therefore considered to be essential to include particle-particle collisions in order to predict accumulation of particles. When this was implemented the simulations were in agreement with the results of the visualization experiments. This type of simulations demonstrates that scrapers can be primarily designed based on numerical predictions. In chapter 5 a novel laboratory scale scraped cooled wall crystallizer with 130 liter capacity, two internal vertical cylindrical heat exchangers, and a total heat transfer area of 2.94 m2.m-3 crystallizer volume was designed. This crystallizer was tested for the recovery of Na2CO3.10H2O and ice from an industrial aqueous sodium carbonate/sodium bicarbonate solution. Compared with earlier designs this new design showed a high improvement in gravitational separation and ice scaling removal, and high heat transfer fluxes were achieved between the heat exchanger and the solution up to 10.5 kW.m-2. A cone installed in the top section effectively directed the ice crystals towards the outlet. The total solid content in the crystallizer was up to 36 wt% under steady state conditions, with average crystal sizes for ice of 100-200 mm and 50-120 mm for Na2CO3.10H2O. Impurities present in the solution, most likely the molybdate ions changed the habit of the Na2CO3.10H2O crystals from pseudo hexagonal for a pure system to lozenges. In Chapter 6 the performance was tested of a scaled up version of the new type of crystallizer with two heat exchanger modules stacked on top of each other. This crystallizer was directly connected to a continuous carbonate/bicarbonate containing blow-down stream in an industrial plant. The EFC process was effectively operated at a heat transfer of 5 kWm-2 without any ice scaling. Sodium carbonate decahydrate and pure ice were continuously produced at -3.8 oC by operating within the metastable zone width of bicarbonate. The crystals showed good filterability with a molybdenum content below 1 ppm. At -4.0 °C bicarbonate started to co-crystallize. The bimodal size distribution of the mixture resulted in poor filtration and purity of the salt product. Because of the necessity of an accurate phase diagram for the process described in chapter 6, the eutectic solubility lines of the ternary system NaHCO3-Na2CO3-H2O were determined experimentally and calculated with the extended UNIQUAC model in chapter 7. The extended UNIQUAC model describes the experimental data quite well. A special crystallizer was built and a procedure developed to accurately measure the eutectic solubility lines where ice and salt coexist in equilibrium with the solution. Anhydrous NaHCO3 and Na2CO3.10H2O were the only two types of crystals present in equilibrium with ice crystals in the ternary system. The anhydrous NaHCO3 crystals were needle shaped with lengths between 5 and 10 ?m, that were agglomerated into particles of about 100 ?m to 300 ?m, while the Na2CO3.10H2O were hexagonally shaped with sizes between 100 and 500 ?m. Chapter 8 focuses at the interface between a scale layer that grows upon a cooling surface and the solution. Scale formation on heat exchangers is not only happening in EFC processes or for ice, but is frequently encountered in many cooling crystallization processes. Crystallization is mostly an exothermic process. According to irreversible thermodynamics, the heat released during crystallization at the interface is distributed to both the liquid and solid phases which are in contact with the interface also for isothermal conditions. The excess entropy production rate for heat and mass transport into, out of and across the interface was used to define the fluxes and forces of the system. The method describes the interface as a separate (two-dimensional) phase in local equilibrium. Coupled heat and mass flux equations from non-equilibrium thermodynamics were defined for crystal growth and the temperature jump at the interface of the growing crystal. For theoretical and experimental simplicity the growth of MgSO4·7H2O on a cold surface was taken as an example. For a growth rate of 2.33×10-3 mol.m-2.s-1, around 20-30% of the heat of crystallization is calculated to be transferred back into the liquid side. The interface resistivity to mass transfer is 1.26×103 J.m2.s.K-1.mol-2, while the interface resistivity to heat transfer is 2.1×10-7 m2.K-1.W-1.Process and EnergyMechanical, Maritime and Materials Engineerin
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
koamabayili/VECTRON-author-checklist: VECTRON author checklist
No full textWe have done our best to complete the author checklist relating to the use of animals in the hut study. Note that the objective for the hut study was to evaluate the IRS treatment applications for residual efficacy against Anopheles mosquitoes, including the local An. coluzzii mosquito population. Cows were only used to attract mosquitoes into the huts and no tests were carried out directly on the cows. The author checklist is intended for use with studies where experiments are carried out on animals, which is why we have had such difficulty in completing this for the hut study, as many of the questions do not relate to how the cows were used
Author-wise bibliometric analysis based on entropy.
No full textAuthor-wise bibliometric analysis based on entropy.</p
Author Under Sail The Imagination of Jack London, 1893-1902
No full textIn Author Under Sail, Jay Williams offers the first complete literary biography of Jack London as a professional writer engaged in the labor of writing. It examines the authorial imagination in London's work, the use of imagination in both his fiction and nonfiction, and the ways he defined imagination in the creative process in his business dealings with his publishers, editors, and agents. In this first volume of a two-volume biography, Williams traverses the years 1893 to 1902, from London's "Story of a Typhoon" to The People of the Abyss. The Jack London who emerges in the pages of Author Under Sail is a writer whose partnership with publishers, most notably his productive alliance with George Brett of Macmillan, was one of the most formative in American literary history. London pioneered many author models during the heyday of realism and naturalism, blurring the boundaries of these popular genres by focusing on absorption and theatricality and the representation of the seen and unseen. London created an impassioned, sincere, and extremely personal realism unlike that of other American writers of the time. Author Under Sail is a literary tour de force that reveals the full range of London as writer, creative citizen, and entrepreneur at the same time it sheds light on the maverick side of machine-age literature.Intro -- Title Page -- Copyright Page -- Dedication -- Contents -- Acknowledgments -- Introduction -- 1. Spirit Truth -- 2. From Absorption to Theatricality and Back Again -- 3. "I Will Build a New Present" -- 4. Sons as Authors -- 5. Fathers as Publishers -- 6. The Daughter as Author -- 7. Lovers as Authors -- 8. At Sea with the Family -- 9. Yellow News, Yellow Stories -- 10. The Return Home -- Notes -- Bibliography -- Index -- About Jay WilliamsIn Author Under Sail, Jay Williams offers the first complete literary biography of Jack London as a professional writer engaged in the labor of writing. It examines the authorial imagination in London's work, the use of imagination in both his fiction and nonfiction, and the ways he defined imagination in the creative process in his business dealings with his publishers, editors, and agents. In this first volume of a two-volume biography, Williams traverses the years 1893 to 1902, from London's "Story of a Typhoon" to The People of the Abyss. The Jack London who emerges in the pages of Author Under Sail is a writer whose partnership with publishers, most notably his productive alliance with George Brett of Macmillan, was one of the most formative in American literary history. London pioneered many author models during the heyday of realism and naturalism, blurring the boundaries of these popular genres by focusing on absorption and theatricality and the representation of the seen and unseen. London created an impassioned, sincere, and extremely personal realism unlike that of other American writers of the time. Author Under Sail is a literary tour de force that reveals the full range of London as writer, creative citizen, and entrepreneur at the same time it sheds light on the maverick side of machine-age literature.Description based on publisher supplied metadata and other sources.Electronic reproduction. Ann Arbor, Michigan : ProQuest Ebook Central, YYYY. Available via World Wide Web. Access may be limited to ProQuest Ebook Central affiliated libraries
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