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Orientador: José Roberto Nunhez / Dissertação (mestrado) - Universidade Estadual de Campinas, Faculdade de Engenharia Química / Made available in DSpace on 2018-08-21T00:15:32Z (GMT). No. of bitstreams: 1 MorenoCardenas_Sebastian_M.pdf: 3545307 bytes, checksum: 797cf7a13df284eafac584f66c5d0826 (MD5) Previous issue date: 2012 / Resumo: Processos comerciais de hidrotratamento (HDT) normalmente operam em regime de leito gotejante "Trickle bed" em concorrente com fluxos descendentes de gás e de líquido, sobre um leito de partículas de catalisador onde ocorrem as reações. É bem conhecido que a remoção de enxofre é fortemente inibida pelo efeito de adsorção competitiva de H2S nos sítios ativos do catalisador. Como consequência disso, é importante manter a concentração de H2S tão baixa quanto possível nestes processos durante a reação para obter um produto com baixo teor de enxofre na saída do reator. De acordo com Ancheyta et al.(2007), um perfil mais conveniente de concentração seria obtido com um reator operando em contracorrente,. Por exemplo, poderia se introduzir a carga na parte superior e o H2 na parte inferior do reator. Assim, na parte inferior do reator, a concentração de H2S é menor e a concentração do H2 é maior, promovendo taxas mais elevadas de reação. Os principais problemas da operação em contracorrente são uma menor eficiência de contato líquido-gás e dificuldade de prevenção de inundação do reator. Com o objetivo de estudar estes processos com mais detalhes, um estudo fluidodinâmico tridimensional do reator em regime de gotejamento "trickle bed" em contracorrente foi realizado com o uso de técnicas de CFD (Fluidodinâmica Computacional), com ênfase na determinação da perda de carga e da distribuição do hold-up de líquido no leito. Um modelo com o uso de CFD tridimensional baseado na abordagem Euleriana - Euleriana foi utilizado para modelar a hidrodinâmica do leito do reator de leito gotejante operando em contracorrente. Os termos de fechamento interfases foram estimados pelo modelo de balanço de forças apresentado por Attou, A et al. (1999), o qual foi usado e validado por Gunjal e Ranade (2007). O perfil de distribuição radial da porosidade foi estimado com o uso da distribuição de Klerk (2003). O modelo foi resolvido com o uso do software comercial ANSYS-CFX 13. Neste documento pode-se encontrar uma revisão bibliográfica do CFD aplicado a reatores de leito gotejante. Posteriormente são mostrados o desenvolvimento matemático e método numérico empregado, e finalmente os resultados dos casos de estudo, os quais forneceram informação de distribuição do líquido, perfis de velocidades, queda de pressão e hold-up do líquido para reatores isotérmicos de leito fixo em escala laboratório (0,0254 m de diâmetro e 0,3 m de comprimento) escoando concorrente e contracorrente / Abstract: Commercial HDT processes usually operate in a trickle-bed regime, with co-current downward flow of gas and liquid over a randomly fixed bed of catalyst particles while reactions take place. It is well known that sulfur removal is strongly inhibited by the competitive adsorption effect of H2S at the sulfided active sites of the catalyst. As a consequence, it is important in these process to maintain the concentration of H2S as low as possible during the reaction to achieve a low sulfur content product at the outlet of reactor. According to Ancheyta et al. (2007), a more convenient profile of H2S concentration can be provided by operating the reactor in countercurrent mode, for instance, introducing the feed at the top and H2 at the bottom of the reactor. Thus, in the lower reactor, the H2S concentration is lower and H2 is greater, promoting higher reaction rates. The main problems of the operation counter are a lower efficiency of gas-liquid contact and difficulty in preventing flooding conditions. In order to investigate the above processes in more detail, a three-dimensional fluid-dynamic study of a countercurrent trickle bed reactor was carried out using CFD (Computational Fluid Dynamics) techniques with emphasis on determining the pressure drop and the distribution of liquid hold-up in the bed. A computational fluid dynamics (CFD) tridimensional model based on an Eulerian - Eulerian multiphase approach was used to model the hydrodynamics of the pseudo two-phase flow in a trickle bed reactor (TBR) in counter-current operation. The closure terms for phase interactions have been addressed by adopting the fluid-fluid interfacial force balance concept (Attou et al. 1999), which was used and validated to simulate co-current HDT reactors by Gunjal e Ranade (2007). Radial variation of porosity was estimated using the Klerk (2003) distribution. Above set of model equations were implemented in commercial software ANSYS-CFX Release 13. In this document it could be found a literature review of CFD models applied to trickle bed reactors, later a mathematical modeling and numerical method to solve the transport equations and finally the results of study cases, which provided information about phase distribution, velocity profiles, pressure drop liquid hold up for a isothermal trickle bed reactor at laboratory scale (0.0254 m diameter and 0.3 length) in co-current and counter-current operation / Mestrado / Desenvolvimento de Processos Químicos / Mestre em Engenharia Química

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"Actual models for fixed-bed reactor modeling make this assumption that temperature is uniform, or at least symmetric, within the catalytic pellets. However, if this holds true for large beds (tube-to-particle diameter ratio N greater than 10), it appears that for small N tubes (N = 3-10) that wall effects cannot be neglected anymore. A large temperature gradient appears in the near wall region. Hence for a particle at the wall a variation in temperature of up to 50¢ªC was noticed. This temperature change was investigated, and it has been noticed that the proximity to the wall, but also to a low velocity region could explain a maximum in temperature. Furthermore, species concentration discrepancies were also notice. An adiabatic run was made to show that these were not due to heated wall effects. Instead it appeared that these concentration variations are due to both their proximity to a low flow region and to a confined area. Hence incoming diffusion in these zones appeared to be lower than for the rest of the surface. We also could notice a strong impact of the flow on the temperature patterns in the near wall regions. Hence in our case, it appeared that the 4 holes geometries allowed a better flow in front the particle at the flow, and therefore better transport phenomena. On the contrary, the full cylinder geometry tend to block the flow, consequently temperature on the wall particles were hotter than what they were with the 4 holes cylinder geometry. A study of the diffusion within the catalytic particles was also conducted. Hence, the Maxwell-Stefan, the dusty gas and the binary friction models were implemented in Fluent. The goal here is to refine step by step the diffusion model used. First products and reactants molar fluxes were assumed to be proportional. The next step was to compute the actual molar fluxes; however this added one more parameter to converge; that is the diffusion coefficient. Finally the assumption of negligible pressure variation within the pellets was dropped. Unfortunately, the implementation into Fluent was not successful, and few possible reasons were given. "

No abstract available. / Thesis (Ph.D.)-University of Natal, Durban, 1973.

Thesis (M.S.) -- Worcester Polytechnic Institute. / Keywords: fixed bed; heat transfer. Includes bibliographical references (p. 67-71).

Effective treatment of wastewater is an important process in reducing the environmental impact of industry and human activity. Although conventional water treatment systems can adequately remove the principle components of waste (i.e. substances that can be represented the majority of biological and chemical oxygen demand) several materials are poorly or slowly removed. Tertiary treatment polishing processes are therefore required to remove these contaminants to ensure complete wastewater treatment. This thesis reports investigations made using film reactors that are used to remove recalcitrant materials such as ammonia- nitrogen and endocrine disrupters that although present in low concentrations, if left untreated can have a strong impact on the environment. Film reactors potentially offer several process advantages over conventional activated sludge treatments systems as they allow very long residence time and contact with high concentrations of fixed microbes with the low concentrations of pollutants so enhancing kinetic performance and efficiency of the process. Two reactor configurations, a partial fixed bed (PFBR) and moving bed biofilm reactors (MBBR) were investigated. A thirty liter reactor with a working volume of 16 liters was constructed and contained fixed microbial films on either free suspended or fixed beds plastic packing (K2 AnoxKaldnes). The investigation of ammonia-nitrogen oxidation showed that after a suitable acclimation period (2 weeks) that ammonia was oxidise rapidly reducing the feed concentrations of 35 mg/L to < 2 mg/L in the effluent. To assess the performance for ammonia-nitrogen removal the reactors operated for long periods (up to 3 months) with continuous feed using the reactor in either PFBR or MBBR modes in addition of 17alpha-ethynylestradiol (EE2) and mestranol (MeEE2), the endocrine disrupting compounds commonly found in municipal wastewater. These substances is derived from a synthetic hormones if found in the natural environment can reduced the productivity of the fish as this can cause feminization in aquatic organisms with disastrous consequences on fish populations. The MBBR and PFBR systems were used to investigate the co-metabolism of ammonia-nitrogen, 17alpha-ethynylestradiol (EE2) and mestranol from model waste water feed containing 35 mg/L of ammonia-nitrogen and 100 mug/L of 17alpha-ethynylestradiol (EE2) and mestranol (MeEE2). A kinetic analysis of the systems were made and for the PFBR reactor, the specific growth rate, mumax of 7.092 d-1 with saturation constants, Ks of 1.574 mg/L. The kinetic analysis for the MBBR system was 6.329 d-1 for the mumax with the K.S of 0.652 mg/L. When the PFBR was used removal of EE2 represents 70% MeEE2 was removed. MBBR were shown to be more effective and efficient in removing ammonia-nitrogen reducing the levels under good conditions to > 2 mg/L while the PFBR could also achieve 2 mg/L. The MBBR system was also more competent in the removal of 17alpha-ethynylestradiol (EE2) and mestranol compared to PFBR. This work demonstrates that there are considerable advantages to using thin film reactors as polishing step for the tertiary treatment of waste waters when to compared to other processes in reducing the inorganic pollutants as endocrine disrupting compounds. The significance of these results is discussed in this context.

To handle a great demand for biomass, alternative biomasses beyond stem wood are being introduced into the solid fuel combustion market, fuels with generally higher (&gt;0,5 wt-%) ash content and different fuel ash compositions compared to stem wood, such as forest residue, bark, grass and straw. Unfortunately, combustion of these alternative fuels often causes more ash related problems such as fouling, slagging and higher particle emissions compared to combustion of stem wood. Many research studies have been conducted regarding ash melting and ash sintering in biomass combustion. However, literature discussing slagging of biomass ash is rather scarce, especially relating to fixed bed combustion. The majority of the biomass fuels available on the market today are phosphorus-poor and this thesis emanates from those. The overall objective was to obtain knowledge of slag formation in fixed-bed combustion of phosphorus-poor biomass, based on bench- and full-scale experiments, chemical analysis of produced ash fractions, chemical equilibrium calculations, viscosity estimations and statistical evaluations.   This thesis investigates slagging of [phosphorus-poor] biomass in fixed bed combustion. 85 fuels and 10 different burner/boiler technologies were utilized. The results in this thesis highlight the importance of the ash forming elements Si, Ca, K and Alin the slag formation process in fixed bed combustion of phosphorus-poor biomass. Increased Ca/Si ratios in the fuel reduce slag formation due to formation of more temperature stable phases, i.e. Ca/Mg-oxides and/or formation of carbonate melts with lower viscosity (not sticky) that are less prone to forming slag. A high Al/Si ratio increases the possibility of forming solid and thermally stable K−Al silicates that can reduce slag formation.   The fraction of ash melt, along with viscosity, are critical for slag formation and these parameters vary between different fuels. Four classes were defined according to their slagging potential; 1) No slag: fuel composition and the bottom ash contains low Si and K contents and higher Ca content. Fuel examples: non-contaminated stem- and pulpwood/energy wood, 2) Minor slagging tendency: fuel compositions show increased Si compared to non-slagging fuels and the bottom ash contains lower Ca, but increased Si content and approximately unchanged K content compared to the former category. Fuel examples: stem wood, bark and logging residue with increased Si-content due to light contamination. 3) Moderate slagging tendency: fuel composition contains further increased Si content. Increased share of formed silicate melt and higher viscosity (more sticky) compared to minor slagging fuels. Fuel examples: mostly contaminated woody fuels and grass and straws with relatively high amount of Ca. and 4) Major slagging tendency: Fuel composition contains high Si and K content. Sticky K-silicates causes major increase in slagging tendency. Fuel examples: different types of grass and straw fuels.   The burner/boiler technology can affect whether slagging will induce major problems in the burner or not. However, long residence times and high temperatures for the combustion residues in the hot part of the fuel bed are technical prerequisite for increased slag formation.   This thesis developed two qualitative fuel indices for predicting slagging in fixed bed combustion of phosphorus-poor biofuels – one index for fraction of fuel ash that forms slag and one index for sintering category of the formed slag. Both novel indices deliver acceptable results and are more reliable than previous indices found in the literature. Importantly, the fraction of fuel ash that forms slag index outperforms the sintering category for qualitative prediction of the problematic slagging potential of a certain fuel. Additional work is needed to further widen the compositional range as well as to fine tune the indices’ boundaries.

No / In this paper, a model of catalytic naphtha reforming process of commercial catalytic reforming unit of Kaduna Refining & Petrochemical Company (KRPC) is adopted and simulated using the gPROMS software, an equation-oriented modelling software. The kinetic and thermodynamic parameters and properties were obtained from literature. The model was used to monitor the behaviour of the temperature and concentrations of parafins, naphthenes and aromatics with respect to the changing heights of the reactors. A comprehensive sensitivity analysis of the product quality (Aromatics) and product yield, reformate, lighter gases and hydrogen yields is performed by varying the operating conditions of the reaction and the following conclusions were made. It was found that the production of aromatics, hydrogen yield, lighter gases and coke on catalyst increase with increasing temperature of the reaction while the reformate yield decreases with the increasing temperature and vice versa. The aromatics, hydrogen yield, coke on catalyst and lighter gases decrease with increasing pressure while the reformate yield decreases with decreasing pressure and vice versa. Hydrogen-hydrocarbon ratio (HHR) affects the product quality slightly by increasing the reformate and hydrogen yield and decreasing the aromatics slightly as well decreasing the coke on catalyst.

Fixed bed reactors are used in many different chemical processes, and are a very important part of chemical industry. To model fixed beds we must have a good qualitative understanding of heat transfer in them. Fixed bed models have been developed for high tube-to-particle ratio (N) beds. Modeling of low tube-to-particle beds (3 ¡ÃƒÅ“ N ¡ÃƒÅ“ 8), that are used in extremely exo- and endothermic processes in tube-and-shell type reactors, is complicated, due to the presence of wall effects across the entire radius of the bed. Heat transfer is one of the most important aspects. To obtain accurate models of heat transfer we need to study the physical mechanisms involved especially in the wall vicinity using CFD as a non intrusive tool to collect numerical data. An extra heat transfer resistance is always present near the wall. This is caused by three mechanisms which happen in the wall vicinity. The change of porosity which leads to a change of bed conductivity, the damping of mixing due to the lateral displacement of fluid, the presence of a laminar (viscous) sublayer at the wall. Many authors have been working on how to model the extra resistance near the wall. The main previous approach was to introduce a lumped parameter hw (heat transfer coefficient) which idealizes these three contributions to the extra heat resistance to be at the wall. Our approach will be to keep the parameter hw which will now represent only the viscous boundary layer idealized at the wall, and we are going to incorporate velocity and porosity profiles in the energy equation. In this way we will able to get rid of artificial parameters using the true conductivity of the bed, and the real velocity profile. So we need to study separately each contribution of the different physical mechanisms to clearly understand what happens in the wall vicinity. For this CFD will be a very powerful tool. How CFD models flow near the wall must be understood before starting simulations. Two main approaches for wall bounded flows are available and will be studied: either solve all way down to the wall, or bridge numerical values from the core of the bed to the wall using semi-empirical formulas called wall functions. These methods will be studied and compared. Also with CFD it is possible to run simulations without conduction in the bed, and so, study radial fluid displacement only and obtain reduced velocity profiles. Using the meshing it is also possible to get a very accurate porosity profile. These profiles will be combined in a simplified fixed bed model which will be used to predict temperature profiles. These may then be compared to the full CFD energy solution and to experiment to test the model.