Binder jetting is an additive manufacturing process that layer by layer builds a 3D model by selectively binding regions of powder using binder deposited though an inkjet printhead. The process offers several advantages over other additive manufacturing processes including fast build rates, vast material selection, decreased cost, and part resolution. The main disadvantage of binder jetting is poor mechanical properties, stemming from a poor understanding of the process physics. Porosity in final parts is not uncommon, but there is little understanding of where the porosity originates. The purpose of this thesis is to report the investigation of increased powder bed cohesion and its effects on part formation, part properties, and printing parameters in binder jetting. The interaction between binder and powder is complex. Binder exiting the printhead impacts the powder bed at speeds up to 10 m/s. The kinetic energy carried by the droplet disperses into the powder bed on impact, causing some powder particles to eject from the bed and other particles to rearrange within the bed. The particle ejection and rearrangement is theorized to be the physical cause of porous regions in binder jetted parts. This work uses a method called pre-wetting to introduce small amounts of moisture into the powder bed to effectively increase the cohesive forces between powder particles. Increased cohesion makes particle ejection and rearrangement during the powder/binder interaction more difficult. A method of accomplishing pre-wetting was developed and achieved successful moisture delivery using water and a water/tri-ethylene glycol mixture. Printed lines were used to characterize moisture content and study its effects on line formation and saturation levels. Low levels of moisture were shown to perform the best. Particle ejection and rearrangement was shown to decrease with moisture addition. Pre-wetting was shown to eliminate the defect known as balling, increasing the parameters known to successfully print lines. Water was identified as a poor substance for pre-wetting due to rapid evaporation, but tri-ethylene glycol/water solutions succeeded in proper moisture delivery. Saturation levels in lines decrease with added moisture and part dimensions increase. high-speed x-ray imaging verified pre-wetting reduction in particle ejection and rearrangement as well as supply some preliminary understanding of void formation during the printing process. The first few layers of the binder jetting process have been shown to increase in surface roughness values when compared to the undisturbed powder bed. This is likely due to a balling-like effect seen in layers. The effects of pre-wetting on layer and multi-layer formation were studied. Pre-wetting reduced the surface roughness levels in printed layers to the levels near the levels seen in undisturbed powder beds. In contrast, saturation levels in layers and multi-layers increased in value above those found in parts printed into dry powder, giving indication that porous regions within bound parts are being eliminated. Layer and multi-layer parts showed increased part dimensions with the addition of moisture. Overall, pre-wetting was shown to greatly reduce the effects of the binder/powder interaction and results strongly suggest that pre-wetting mitigates defect creation during the printing process. Further research should include testing of thicker multi-layer parts to study how saturation trends continue with increased layer numbers. In-process drying should be used in conjunction with pre-wetting in multi-layer parts to determine its effects on saturation levels and part dimensions. Post processing should be done to partially sinter, or infiltrate multi-layer parts created with and without pre-wetting to analyze porosity.

Trickle flow is encountered in a variety of process engineering applications where gas and liquid flow through a packed bed of stationary solid. Owing to the complexities of three interacting phases, a fundamentally exhaustive description of trickle flow hydrodynamics has not been achieved. A complicating factor in describing the hydrodynamics is the fact that the hydrodynamic state is dependent not only on the present operating conditions but also on their entire history, including fluid flow rate changes and pre-wetting procedures. This phenomenon is termed hydrodynamic multiplicity and is the subject of this work. Hydrodynamic multiplicity greatly complicates both the experimental investigation into the behaviour of a trickle flow column and the theoretical modelling of the observed behaviour. Broadly speaking, this study addresses hydrodynamic multiplicity on three levels. First, a conceptual framework is proposed that can be used to study hydrodynamic multiplicity with limited resources. It is based on the absolute limiting values that the hydrodynamic parameters can adopt for a certain set of conditions, and encompasses both flow rate hysteresis loops and pre-wetting procedures. There are 5 such hydrodynamic modes. When the existing literature is critically evaluated in light of this framework, it is established that the reported experimental studies have not addressed all the issues. Previous modelling attempts are also shown to be unable to qualitative explain all the existing data. Moreover, authors have suggested different (and often contradictory) physical mechanisms responsible for hydrodynamic multiplicity. Secondly, an experimental investigation intended to supplement the existing literature and illustrate the utility of the proposed framework is launched. This includes bed-scale measurements of liquid holdup, pressure drop and gas-liquid mass transfer for a variety of conditions including different flow rates, pressures, particle shapes, particle porosity and surface tension. The second part of the experimental effort uses radiography and tomography in new ways to visualise the temporal and spatial characteristics of the different hydrodynamic modes. The tomographic investigation incorporates advanced image processing techniques in order to culminate in a pore-level evaluation of the hydrodynamic modes that reveals additional features of hydrodynamic multiplicity. Thirdly, the experimental insights are condensed into a set of characteristic trends that highlight the features of hydrodynamic multiplicity. A pore-level capillary mechanism is then introduced to qualitatively explain the observed behaviour. The mechanism shows how the differences in advancing and receding contact angles and the characteristics of the packed structure (or pore geometries) are ultimately responsible for the observed hydrodynamic multiplicity behaviour. Lastly, the effect of hydrodynamic multiplicity on trickle bed reactor performance is discussed. It is established experimentally that depending on the reaction conditions, different modes yield optimal performance. The idea of optimizing the performance by manipulating the hydrodynamic state is introduced. In totality, this work advances the understanding of trickle flow hydrodynamics in general and hydrodynamic multiplicity in particular. / Thesis (PhD (Chemical Engineering))--University of Pretoria, 2008. / Chemical Engineering / unrestricted

Hydrodynamic multiplicity in the trickle flow, or low interaction, regime is a well documented phenomenon. Multiple hydrodynamic states are often presented in the form of hysteresis loops where the hydrodynamic parameter studied are shown as a function of the operating history of the bed, i.e. liquid and gas flow rates. In extreme cases the lower leg, representing an increase in liquid flow rate on a pre-wetted and drained bed, is commonly referred to as the Levec mode. The upper extreme, referred to as the Kan-liquid mode, represents a decrease in liquid flow rate after operation in the high interaction regime. The many reported studies investigating liquid-solid mass transfer in trickle beds have generally used either the dissolution or electrochemical techniques. Numerous researchers have used their data to develop correlations predicting solid-liquid mass transfer coefficients. Most of these studies do not specify the multiplicity mode of operation. Only two studies (Sims et al. (1993) and Van der Merwe, Nicol&Al-Dahhan (2008)) use both the Levec and Kan-liquid operating modes. Both of these studies suggest that solid-liquid mass transfer also exhibit multiplicity behaviour although the trends suggested or speculated differ from each other. Sims et al. (1993) found that a Kan-liquid operated bed will outperform a Levec operated bed; however in contrast to this Van der Merwe et al. (2008) speculated that a Levec operated bed is better suited for liquid limited reactions due to enhanced liquid-solid mass transfer in the Levec mode as a result of faster interstitial velocity. This study showed that solid-liquid mass transfer coefficients, measured with both the dissolution and electrochemical technique, show multiplicity behaviour. Two distinct operating regions were found, which corresponds to the Levec and Kan-liquid modes. Measurements taken using the electrochemical technique yielded solid-liquid mass transfer coefficients larger than those measured using the dissolution method. The experimental results agree with the trend found by Sims et al. (1993) but the mass transfer coefficients in this study were significantly lower. Additionally it was shown that the difference in mass transfer coefficients, in the two modes, cannot be explained by merely compensating for the differences in wetting efficiency and interstitial velocity, suggesting that the Levec mode has a larger percentage of stagnant or poorly irrigated zones. It was also shown that mass transfer coefficients measured at the top of the column is higher than those measured at the bottom, suggesting that the flow structure is changing as a function of axial length. Lastly, with regards to electrochemical measurements of liquid-solid mass transfer, it was shown that measurements using a single particle electrode compared well to that of a multiple packing electrode. / Dissertation (MEng)--University of Pretoria, 2009. / Chemical Engineering / unrestricted

Abstract The old corrugated container (OCC) recycling process produces pulp suspension from recovered packaging material with sufficient strength and cleanliness to meet regulatory requirements for produced packaging material. Pulping is the first unit operation in the OCC recycling process and the most energy-intensive unit operation, accounting for 20–30% of the total energy used in the process, if dispersion is not used. From the viewpoint of improving the energy efficiency of the OCC recycling process, the pulping stage is an important research target. The approach of this thesis work was to experimentally study practical possibilities to decrease the energy requirements of the OCC pulping process by understanding disintegration mechanisms and pulping fundamentals. The pulping process was conceptually divided into three separate phases: wetting, bulk disintegration, and residual disintegration. The aim was to elucidate the effect of each phase on disintegration kinetics and specific energy consumption of pulping, and to discuss how those phases could be intensified to minimize energy consumption. The results suggest that forced wetting of OCC material by mechanical pressing increases the disintegration rate owing to thorough impregnation of water into the OCC material, which efficiently reduces the material’s wet strength by cutting hydrogen bonds. The method can potentially reduce the energy requirement of the pulping process by about 30%. Bulk disintegration by falling the material in high-consistency drum pulping was emulated in a specially designed device. The results reveal that the energy needed for material disintegration depends significantly on the material’s wet strength. Pulping energy consumption can be lowered in the residual disintegration phase by separating the easily disintegrated weaker materials in an early stage from the drum and continuing pulping of the remaining non-disintegrated material (Fractional pulping). This setup led to estimated energy savings of around 20%. During the thesis work, the authors developed an analysis device that photographs flakes separated from the pulp and determines flake size distribution by image analysis. The device revealed the difference between the low- and medium-consistency OCC pulping mechanisms. / Tiivistelmä Pahvin kierrätysprosessissa kerätyistä pakkausmateriaaleista valmistetaan uusiomassaa, joka käytetään uuden pahvin valmistukseen. Kierrätysprosessin ensimmäinen yksikköoperaatio on pulpperointi eli pahviraaka-aineen kuiduttaminen sulpuksi. Pulpperointi on usein koko kierrätysprosessin energiankulutukseltaan suurin yksikköprosessi, joka voi viedä jopa 20 – 30 % koko prosessin energiankulutuksesta. Tämän vuoksi pulpperoinnin energiankulutuksen pienentämiseen tähtäävä tutkimus on erittäin tärkeää. Työn tarkoituksena oli tutkia kokeellisin menetelmin mahdollisuuksia vähentää pulpperoinnin energiankulutusta. Tavoitteena oli saada ymmärrystä pulpperoinnin hajotusmekanismeista ja pulpperointiin vaikuttavista tekijöistä. Tässä työssä pulpperointi jaettiin kolmeen vaiheeseen: pahvin vettymiseen, varsinaiseen bulkkihajotukseen ja jälkihajotukseen. Tarkoituksena oli havainnollistaa jokaisen vaiheen vaikutusta materiaalin hajotuskinetiikkaan ja pulpperoinnin energiankulutukseen. Lisäksi tavoitteena oli pohtia mahdollisia tapoja tehostaa jokaista vaihetta ja näin minimoida energiankulutusta. Tulosten perusteella pahvin pakotettu kostuttaminen mekaanisella puristuksella kasvattaa hajotusnopeutta johtuen vedestä, joka kastelee materiaalin läpikotaisin. Tämä pienentää materiaalin märkälujuutta katkomalla kuitujen välisiä vetysidoksia. Menetelmän avulla pulpperoinnin energiankulutusta pystytään vähentämään jopa 30 %. Rumpupulpperin bulkkihajotusta jäljiteltiin tarkoitukseen suunnitellulla pudotuslaitteella. Tulokset paljastivat, että energiatarve materiaalin hajottamiseen on verrannollinen materiaalin märkälujuuteen. Pulpperoinnin energiankulutusta jälkihajotuksessa voidaan pienentää ns. fraktioivalla pulpperoinnilla, jossa pulpperin kuormaa kevennetään poistamalla nopeasti hajoava heikkolujuuksinen materiaali mahdollisimman aikaisin, jonka jälkeen pulpperointia jatketaan hajoamattomalla materiaalilla. Tällaisella menetelmällä voidaan parhaimmillaan säästää 20 % rumpupulpperoinnin energiankulutuksesta. Työn aikana kehiteltiin myös kuvantamiseen perustuva analyysimenetelmä, jolla voidaan analysoida hajoamattoman materiaalin kokojakaumaa. Analyysimenetelmä paljasti hajotusmekanismieron matala- ja keskisakeuspulpperoinnin välillä.

Submitted by Maicon Juliano Schmidt (maicons) on 2015-04-09T19:46:05Z No. of bitstreams: 1 Rafaela Gava Padovan.pdf: 7081051 bytes, checksum: 9e1501c4f4ebed520c7882be21164038 (MD5) / Made available in DSpace on 2015-04-09T19:46:05Z (GMT). No. of bitstreams: 1 Rafaela Gava Padovan.pdf: 7081051 bytes, checksum: 9e1501c4f4ebed520c7882be21164038 (MD5) Previous issue date: 2013-09-05 / CAPES - Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / FINEP - Financiadora de Estudos e Projetos / Os agregados reciclados de concreto, de um modo geral, apresentam elevada absorção de água, em função da sua maior porosidade. Esta absorção resulta em alterações no teor de água das misturas, e têm implicação direta sobre as propriedades microestruturais dos materiais à base de cimento, não sendo diferente com o agregado reciclado. Contudo, tais alterações ainda são pouco conhecidas nestes materiais. Esta pesquisa possui o objetivo de determinar a influência do processo de pré-molhagem do agregado no comportamento de concretos desenvolvidos através da incorporação de agregado graúdo reciclado de concreto (AGRC) em substituição ao agregado graúdo natural (AGN). O teor de substituição do AGRC em relação ao AGN utilizado foi de 50% em relação ao volume do concreto. Previamente à utilização do AGRC, foi realizada uma pré-molhagem do material com percentuais na ordem de 40, 60, 80 e 100% da água relativa à absorção total do resíduo. Foram adotadas relações água/cimento iniciais de 0,45; 0,55 e 0,65. O abatimento foi fixado em 100 ± 20 mm, realizando-se uma compensação de água da mistura no teor necessário para atingir este abatimento. Num segundo grupo, a menor quantidade de água utilizada na produção dos concretos sem aditivo foi adotada para a produção dos mesmos concretos, com percentuais de pré-molhagem de 40, 60 e 80%, utilizando-se aditivo superplastificante ao invés de água para que fosse atingido o abatimento fixado. Verificou-se a influência da pré-molhagem do AGRC sobre as propriedades do concreto fresco, através da determinação da consistência ao longo do tempo; e do concreto endurecido, através da resistência à compressão e absorção de água por capilaridade dos concretos, e a alteração de porosidade de amostras de argamassa extraídas dos concretos frescos após diferentes tempos de contato com o agregado reciclado. Os resultados obtidos indicam que a trabalhabilidade e a resistência a compressão dos concretos com AGRC são pouco influenciadas pelo teor de pré-molhagem, enquanto que a permeabilidade a água e o teor de vazios existente na argamassa da mistura sofrem influencia significativa: nos concretos com AGRC sem aditivo, há pequenas variações de permeabilidade e porosidade, sendo que o teor de pré-molhagem de 80% resulta em concretos menos porosos. Entretanto, nos concretos com AGRC e aditivo superplastificante, o menor teor de pré-molhagem testado (40%) se revela a melhor opção, pois a água de mistura é absorvida pelos poros vazios do agregado reciclado, o que diminui a porosidade da argamassa, e consequentemente a capilaridade do concreto. / Recycled aggregates have high water absorption properties due to its high porosity. It changes the water content of the mixtures, and has implications on the microstructural properties of cement-based materials as those who uses recycled aggregate; however, little is known about these materials. This research has the objective of determining the influence of aggregate pre-wetting in new concrete made with recycled coarse aggregate concrete (RCAC) replacing natural aggregate. The substitution ratio of natural coarse aggregate (NCA) in relation to RCAC was 50% relative to the volume of the concrete. Prior to employing RCAC, it was pre-wetted with pre-wetting levels of 40, 60, 80 and 100% of the total water absorption of the residue. Water to cement ratios were initially determined to be 0.45, 0.55 and 0.65. The slump was set at 100 ± 20 mm, performing a compensation on the water content of the mixture required to get this slump. In a second group, the smallest amount of water used in the concrete’s production without additive was set with saturation levels of 40, 60 and 80%, using a superplasticizer instead of water to achieve the required slump. The recycled concrete produced was found to influence the pre-wetting RCAC on the properties of fresh concrete, through determination of consistency over time, compressive strength and durability and water absorption by capillarity. Mortar samples were obtained from the extraction of recycled concrete. These samples were analyzed for porosity. The results indicate that the workability and the compressive strength of concretes with AGRC are little influenced by the amount of pre-wetting, while the water permeability and the void content of the mixture existing in the mortar suffer significant influence: in the concrete with no additive AGRC there are small variations in porosity and permeability, and in the amount of pre-wetting results in 80% less porous concrete. However, in concrete with AGRC and superplasticizer, the lowest level of pre-wetting tested (40%) proves the best option, because the mixing water is absorbed by the empty pores of recycled aggregate, which reduces the porosity of the mortar, and consequently the capillarity of hard concrete.