Estudo termodin��mico da precipita����o de parafinas em petr��leos brasileiros

Um dos grandes desafios enfrentados pela ind��stria do petr��leo �� reduzir o impacto causado pela cristaliza����o indesej��vel de hidrocarbonetos paraf��nicos de elevada massa molar em tubula����es e equipamentos de produ����o. A cristaliza����o de parafinas em petr��leo �� normalmente detectada atrav��s da determina����o da temperatura inicial de aparecimento de cristais (TIAC), que pode ser estimada atrav��s de modelagem termodin��mica com base na composi����o do petr��leo. Os objetivos deste trabalho s��o: estudar os principais modelos termodin��micos adotados para descrever a precipita����o de parafinas; verificar a validade desses modelos para os petr��leos brasileiros e determinar qual modelo �� o mais adequado para esses ��leos. Para tanto, tr��s formas de c��lculo da raz��o entre as fugacidades das fases s��lida e l��quida e cinco modelos para calcular os coeficientes de atividade dos componentes em cada fase s��o aplicados aos dados de composi����o de vinte e tr��s petr��leos brasileiros. Os resultados mostram que o modelo ideal de m��ltiplas fases s��lidas e o modelo de Escobar-Remolina geram valores bastante abaixo da TIAC experimental. Para os modelos de ��nica fase s��lida ideal, de Won e de Coutinho, foi poss��vel observar que: a) a grande maioria dos erros �� negativa; b) que estes se distribuem melhor em torno de zero quando se utiliza a correla����o de Coutinho para o c��lculo dos valores de i; c) os valores de erro m��dio para os modelos de Coutinho, de Won e ideal com ��nica fase s��lida se equivalem, qualquer que seja o modelo utilizado para o c��lculo de i, exceto para o caso em que todos os compostos presentes na fase l��quida podem precipitar; d) os valores obtidos atrav��s do modelo de Coutinho apresentam erro sistem��tico em rela����o ao modelo ideal de ��nica fase s��lida; e) as diferentes formas de se calcular a raz��o entre as fugacidades da fase s��lida e l��quida (i) influenciam fortemente a capacidade preditiva dos modelos, o que n��o era esperado; f) o perfil do primeiro cristal formado nos petr��leos �� influenciado pelas mol��culas mais pesadas presentes nos res��duos, o que mostra a necessidade de se desenvolver metodologias precisas e robustas de caracteriza����o de res��duos; g) a inclus��o de uma estimativa para a composi����o dos res��duos efetivamente melhorou o desempenho dos modelos em petr��leos m��dio; h) em petr��leos pesados, houve um aumento do erro de previs��o da TIAC devido �� pouca ou nenhuma quantidade de parafinas nos res��duos desses ��leos. A necessidade de uma melhor caracteriza����o dos res��duos de petr��leos �� corroborada pelo fato da TIAC calculada pelos modelos ser, via de regra, mais baixa que a TIAC experimental e pela melhora no desempenho dos modelos quando se estimou a composi����o dos res��duos, em petr��leos m��dios

Equil��brio l��quido-l��quido entre ��cido ac��tico, ��gua e fen��licos: experimental e modelagem

Com o passar do tempo, a popula����o mundial vem se conscientizando mais sobre problemas ambientais. Isso fez surgir uma demanda por tecnologias novas que possam se encaixar no cen��rio de sustentabilidade. Instabilidades frequentes no cen��rio pol��tico-econ��mico global acabam por elevar o pre��o do barril do petr��leo. Assim a ind��stria qu��mica come��a a buscar por alternativas que tenham a mesma versatilidade do petr��leo. Dentre as op����es de combust��veis renov��veis, destaca-se o bio-��leo de pir��lise. Seu interesse em estudos cient��ficos vem do fato de poder-se utilizar do rejeito de processos como mat��ria prima, n��o necessitando competir por espa��o com a planta����o de alimentos. Sua composi����o pode ser representada por ��cidos e fen��is. Em especial destacamos o ��cido ac��tico e fen��is oxigenados como m-cresol, o-cresol, p-cresol e guaiacol por estarem presentes em grandes quantidades. Sua separa����o das fra����es menos polares do bio-��leo pode ser realizada por meio de extra����o com ��gua que �� um reagente abundante e de baixo custo. O conhecimento das propriedades desses componentes puros �� amplamente dispon��vel na literatura, por��m dados de composi����es das fases corrosivas, como misturas tern��rias de ��gua-��cido ac��tico-m-cresol/o-cresol/p-cresol/guaiacol nas temperaturas de (298,15, 310,65 e 323,15) K s��o escassos. Devido a isso, o uso de modelos termodin��micos para simula����o do comportamento desses sistemas torna-se interessante. Todavia, quando s��o testadas as capacidades dos modelos cl��ssicos, como o UNIFAC e NRTL, percebe-se que os mesmos n��o conseguem reproduzir o comportamento da binodal dos componentes corrosivos. Sendo assim, essa disserta����o investigou solu����es para melhorar a descri����o desses sistemas, assim como obteve dados experimentais para tais sistemas de misturas tern��rias de ��gua-��cido ac��tico-m-cresol/o-cresol/p-cresol/guaiacol nas temperaturas de (298,15, 310,65 e 323,15) K; desenvolveu-se uma metodologia para estimar par��metros do modelo NRTL a partir de dados de composi����o da binodal e verificou-se a possibilidade de utilizar o modelo UNIFAC para prever o comportamento de equil��brio de fases. Como resultado foram obtidos 314 novos dados experimentais, obtiveram-se par��metros para o modelo NRTL que conseguem reproduzir com acur��cia a forma da binodal com a metodologia proposta e verificou-se a necessidade de aperfei��oamento no estudo do modelo UNIFAC para otimizar sua utiliza����o na previs��o do comportamento dos sistemas estudados

Estudo do equil��brio l��quido-l��quido e da estabilidade de sistemas envolvendo biodiesel, ��lcoois e ��gua

O estudo da estabilidade de emuls��es de biocombust��veis destaca-se, dentre outros aspectos, pela necessidade de identifica����o das caracter��sticas destas. Esta necessidade refor��a-se pela dificuldade de detec����o de biodiesel e suas misturas quando est��o fora de especifica����o, al��m de se aplicar aos processos de separa����o e purifica����o do biodiesel ap��s a transesterifica����o. O entendimento deste problema poder�� ser alcan��ado a partir de estudos de estabilidade associados aos modelos termodin��micos de equil��brio l��quido-l��quido (modelo NRTL) em sistemas tern��rios, que ser��o a base da modelagem proposta neste trabalho. O levantamento de dados envolve t��cnicas de observa����o direta em titula����o, e ensaios de estabilidade e tensiometria, com algumas altera����es das t��cnicas cl��ssicas. Desta forma, o objetivo principal do trabalho �� propor uma maneira de identificar e caracterizar as interfaces das emuls��es formadas e estudar a estabilidade das fases envolvidas. Al��m disso, estuda-se a introdu����o de alco��is como agentes tensoativos para viabilizar a utiliza����o de misturas de combust��veis, mesmo na regi��o de mais de uma fase. Para o desenvolvimento do trabalho, emuls��es envolvendo os constituintes b��sicos ser��o combinadas entre si em diferentes propor����es. A etapa inicial constitui-se de um mapeamento das misturas e propor����es que formam as emuls��es, baseadas nas curvas de equil��brio. Em seguida, faz-se o estudo de caracteriza����o e estabilidade destes sistemas. Com estas informa����es �� poss��vel mapear o sentido de incremento de tens��o na interface e instabilidade da separa����o de fases das emuls��es em rela����o aos diagramas tern��rios correspondentes. Por fim, prop��e-se uma estrat��gia para estimar os par��metros termodin��micos do NRTL para as esp��cies presentes nos sistemas estudados e prever o comportamento dos mesmos e de sistemas similares. Este modelo matem��tico proposto tem como objetivo complementar a t��cnica experimental adotada para constru����o das curvas de equil��brio, sendo bem sucedido

Experimentos num��ricos para deposi����o de parafinas com modelo Multis��lido

Este trabalho objetiva a constru����o de estruturas robustas e computacionalmente eficientes para a solu����o do problema de deposi����o de parafinas do ponto de vista do equil��brio s��lido-l��quido. S��o avaliados diversos modelos termodin��micos para a fase l��quida: equa����o de estado de Peng-Robinson e os modelos de coeficiente de atividade de Solu����o Ideal, Wilson, UNIQUAC e UNIFAC. A fase s��lida �� caracterizada pelo modelo Multis��lido. A previs��o de forma����o de fase s��lida �� inicialmente prevista por um teste de estabilidade termodin��mica. Posteriormente, o sistema de equa����es n��o lineares que caracteriza o equil��brio termodin��mico e as equa����es de balan��o material �� resolvido por tr��s abordagens num��ricas: m��todo de Newton multivari��vel, m��todo de Broyden e m��todo Newton-Armijo. Diversos experimentos num��ricos foram conduzidos de modo a avaliar os tempos de computa����o e a robustez frente a diversos cen��rios de estimativas iniciais dos m��todos num��ricos para os diferentes modelos e diferentes misturas. Os resultados indicam para a possibilidade de constru����o de arcabou��os computacionais eficientes e robustos, que podem ser empregados acoplados a simuladores de escoamento em dutos, por exemplo.

Controlled growth of aligned ZnO nanowires

The growth of vertically aligned zinc oxide nanowires (ZnO NW) using a simple vapor deposition method system is reported. The growth properties are studied as a function of the Au catalyst layer thickness, pressure, deposition temperature, and oxygen ratio. It was found that the diameter and density of the nanowires is controlled mostly by the growth temperature and pressure. The alignment of the nanowires depends on a combination of three factors including the pressure, temperature and the oxygen ratio. Our results implicates the growth occurs by a vapor liquid solid (VLS) process [1].

Mechanics of capillary forming of aligned carbon nanotube assemblies.

Elastocapillary self-assembly is emerging as a versatile technique to manufacture three-dimensional (3D) microstructures and complex surface textures from arrangements of micro- and nanoscale filaments. Understanding the mechanics of capillary self-assembly is essential to engineering of properties such as shape-directed actuation, anisotropic wetting and adhesion, and mechanical energy transfer and dissipation. We study elastocapillary self-assembly (herein called "capillary forming") of carbon nanotube (CNT) microstructures, combining in situ optical imaging, micromechanical testing, and finite element modeling. By imaging, we identify sequential stages of liquid infiltration, evaporation, and solid shrinkage, whose kinetics relate to the size and shape of the CNT microstructure. We couple these observations with measurements of the orthotropic elastic moduli of CNT forests to understand how the dynamic of shrinkage of the vapor-liquid interface is coupled to the compression of the forest. We compare the kinetics of shrinkage to the rate of evporation from liquid droplets having the same size and geometry. Moreover, we show that the amount of shrinkage during evaporation is governed by the ability of the CNTs to slip against one another, which can be manipulated by the deposition of thin conformal coatings on the CNTs by atomic layer deposition (ALD). This insight is confirmed by finite element modeling of pairs of CNTs as corrugated beams in contact and highlights the coupled role of elasticity and friction in shrinkage and stability of nanoporous solids. Overall, this study shows that nanoscale porosity can be tailored via the filament density and adhesion at contact points, which is important to the development of lightweight multifunctional materials.

Vertically oriented epitaxial germanium nanowires on silicon substrates using thin germanium buffer layers.

We demonstrate a method to realize vertically oriented Ge nanowires on Si(111) substrates. Ge nanowires were grown by chemical vapor deposition using Au nanoparticles to seed nanowire growth via a vapor-liquid-solid growth mechanism. Rapid oxidation of Si during Au nanoparticle application inhibits the growth of vertically oriented Ge nanowires directly on Si. The present method employs thin Ge buffer layers grown at low temperature less than 600 degrees C to circumvent the oxidation problem. By using a thin Ge buffer layer with root-mean-square roughness of approximately 2 nm, the yield of vertically oriented Ge nanowires is as high as 96.3%. This yield is comparable to that of homoepitaxial Ge nanowires. Furthermore, branched Ge nanowires could be successfully grown on these vertically oriented Ge nanowires by a secondary seeding technique. Since the buffer layers are grown under moderate conditions without any high temperature processing steps, this method has a wide process window highly suitable for Si-based microelectronics.

Improvement of morphology, structure, and optical properties of GaAs nanowires grown on Si substrates

We investigate vertical and defect-free growth of GaAs nanowires on Si (111) substrates via a vapor-liquid-solid (VLS) growth mechanism with Au catalysts by metal-organic chemical vapor deposition (MOCVD). By using annealed thin GaAs buffer layers on the surface of Si substrates, most nanowires are grown on the substrates straight, following (111) direction; by using two temperature growth, the nanowires were grown free from structural defects, such as twin defects and stacking faults. Systematic experiments about buffer layers indicate that V/III ratio of precursor and growth temperature can affect the morphology and quality of the buffer layers. Especially, heterostructural buffer layers grown with different V/III ratios and temperatures and in-situ post-annealing step are very helpful to grow well arranged, vertical GaAs nanowires on Si substrates. The initial nanowires having some structural defects can be defect-free by two-temperature growth mode with improved optical property, which shows us positive possibility for optoelectronic device application. ��2010 IEEE.

III-V compound semiconductor nanowires

InP and GaAs based nanowires were grown epitaxially on InP or GaAs (111)B substrates by metalorganic chemical vapor deposition via vapor-liquid-solid (VLS) mechanism. In this report, I will give an overview of nanowire research activities in our group. In particular, the effects of growth parameters for InP and GaAs nanowires on the crystal quality have been studied in detail. We demonstrated the ability to obtain defect-free GaAs nanowires and control the crystal structure of InP nanowires, ie, WZ or ZB, by choosing a combination of growth parameters, such as temperature, V/III ratio and nanowire diameter. �� 2009 IEEE NANO Organizers.

Growth behavior of epitaxial semiconductor axial nanowire heterostructures

In this paper, we demonstrate the key issues of axial nanowire heterostructures, such as, the fundamental criteria for formation and failure of axial nanowire heterostructures via vapor-liquid-solid mechanism and lateral misfit strain relaxation in these structures. We show the failure of axial nanowire heterostructures by growing InAs axially on GaAs nanowires, and the lateral misfit strain relaxation by axial growth of GaSb on GaAs nanowires. �� 2008 IEEE.

Understanding the kink formation in GaAs/InAs heterostructural nanowires

The kinks formation in heterostructural nanowires was observed to be dominant when InAs nanowires were grown on GaAs nanowires. Nanowires were grown through vapor-liquid-solid (VLS) mechanism in an MOCVD (metalorganic chemical vapor deposition) reactor. GaAs nanowires were grown in [1 1 1 ]B direction on a GaAs (1 1 1 )B substrate. When InAs nanowires grown on the GaAs nanowires, most of the InAs nanowires changed their growth directions from [1 1 1 ]B to other ���111���B directions. The kinks formation is ascribed to the large compressive misfit strain at the GaAs/InAs interface (7.2% lattice mismatch between GaAs and InAs) and the high mobility of indium species during MOCVD growth. The in-depth analysis of the kinks formation was done by growing InAs for short times on the GaAs nanowires and characterizing the samples. The hindrance to compressively strain InAs to form coherent layers with GaAs pushed the InAs/Au interfaces to the sides of the GaAs nanowires growth ends. New InAs/Au interfaces have generated at the sides of GaAs nanowires, due to lateral growth of InAs on GaAs nanowires. These new interfaces led the InAs nanowires growth in other ���111���B directions. The morphological and structural features of these heterostructural kinked nanowires were characterized using scanning electron microscopy (SEM) and transmission electron microscopy (TEM) techniques. �� 2006 IEEE.

Effect of channel surface wettability and temperature gradients on the boiling flow pattern in a single microchannel

The objective of this paper is to investigate the effects of channel surface wettability and temperature gradients on the boiling flow pattern in a single microchannel. The test section consists of a bottom silicon substrate bonded with a top glass cover. Three consecutive parts of an inlet fluid plenum, a central microchannel and an outlet fluid plenum were etched in the silicon substrate. The central microchannel had a width of 800 mu m and a depth of 30 mu m. Acetone liquid was used as the working fluid. High outlet vapor qualities were dealt with here. The flow pattern consists of a fluid triangle (shrinkage of the liquid films) and a connected long liquid rivulet, which is generated in the central microchannel in the timescale of milliseconds. The peculiar flow pattern is formed due to the following reasons: (1) the liquid rivulet tends to have a large contact area with the top hydrophilic channel surface of the glass cover, but a smaller contact area with the bottom silicon hydrophobic surface. (2) The temperature gradient in the chip width direction at the top channel surface of the glass cover not only causes the shrinkage of the liquid films in the central microchannel upstream, but also attracts the liquid rivulet populated near the microchannel centerline. (3) The zigzag pattern is formed due to the competition between the evaporation momentum forces at the vapor-liquid interfaces and the force due to the Marangoni effect. The former causes the rivulet to deviate from the channel centerline and the latter draws the rivulet toward the channel centerline. (4) The temperature gradient along the flow direction in the central microchannel downstream causes the breakup of the rivulet to form isolated droplets there. (5) Liquid stripes inside the upstream fluid triangle were caused by the small capillary number of the liquid film, at which the large surface tension force relative to the viscous force tends to populate the liquid film locally on the top glass cover surface.

Molecular dynamics study on the phase diagrams of linear and branched chain molecules

The particle transfer molecular dynamics is used to study the phase equilibria of linear and branched chain molecules. The scaling of the critical temperature versus chain length is obtained and the critical densities are found to decrease with increasing chain length, which are in agreement with the results of experiment and theory. The phase diagrams of the linear and the branched chain molecules nearly overlap with each other. Moreover, the radial distribution functions of linear and branched chain molecules in gas phase are very similar, but in the liquid phase, they are different for different kinds of chains.

������MC-ICP-MS������������������������������������������������������������������

Based on previous studies, boron can be separated from aqueous samples with Amberlite IRA-743 resin. Experiments on the elute temperature, elute volume and the dynamic resin exchange capacity have been performed in this study. Results show that the dynamic exchange capacity of the resin is 4.2mg B/g and at room temperature, boron fixed on the resin within this capacity level can be extracted quantitatively by using 5ml 2%HNO3. A new procedure has been developed for the measurement of boron isotope ratios in water samples using a Neptune MC-ICP-MS, after resolving the memory effect, which is a key problem, and investigating the impacts of mass bias and Si matrix effect. Using this method, it usually takes 20 min to perform one measurement on 0.1ppm boron solution with a precision of 0.23��� (SD). If the relative deviation between a sample and the standard is large, the washout time needs to be doubled to achieve a higher precision. ��11B values of water samples from Yangbajing geothermal field vary from -10.53 to -9.13���. Owing to the large difference B concentration and the small B isotope difference between deep geothermal water and surface water, B isotope ratios of the shallow geothermal fluids are dominated by the deep end member rather than the shallower one in the mixing process. As a consequence, ��11B-B relation is indicative basically of a dilution process. Vapor-liquid separation and calcite scaling also greatly influence B isotope fractionation. ��11B values of water samples from Dagejia geothermal field are from -15.98��� to -11.67���. Boron in Changma River near the field has two sources, freshwater lakes (Dajiamang Lake and Canke Lake) and geothermal waters. Finally, a preliminary discussion is included on boron geochemical characteristics of the salt lakes in Shuanghu area and other geothermal fields, to provide information for future studies on boron isotope geochemistry of geothermal systems and salt lakes in Tibet.