Fixed bed reactors are an essential part of the chemical industry as they are used in a wide variety of chemical processes. To better model these systems a more fundamental understanding of the processes taking place in a fixed bed is required. Fixed bed models are traditionally based on high tube-to particle diameter ratio (N) beds, where temperature and flow profile gradients are mild and can be averaged. Low-N beds are used in extremely exo- and endothermic processes on the tube side of tube and shell type reactors. In these beds, heat transfer is one of the most important aspects. The importance of accurate modeling of heat transfer and its dependence on accurate modeling of the flow features leads to the need for studying the phenomena in these low-N beds in detail. In this work a comparative study is made of the influence of spherical and cylindrical packing particle shapes, positions and orientations on the rates of heat transfer in the near-wall region in a steam reforming application. Computational Fluid Dynamics (CFD) is used as a tool for obtaining the detailed flow and temperature information in a low-N fixed bed. CFD simulation geometries of discrete particle packed beds are designed and methods for data extraction and analysis are developed. After conceptual and quantitative analysis of the simulation data it is found that few clear relations between the complex phenomena of flow and heat transfer can be easily identified. Investigated features are the orientations of the particle in the flow, and many design parameters, such as the number and size of longitudinal holes in the particle and external features on the particle. We find that many of the investigated features are related and their individual influences could not be isolated in this study. Some of the related features are, for example, the number of holes in the particle design and the particle orientation in the flow. Some general conclusions could be drawn. External features on the particles enhance the overall heat transfer properties by better mixing of the flow field. When holes are present in the cylindrical particle design, heat transfer effectiveness can be improved with fewer larger holes. After identifying the packing-related features influencing the near-wall heat transfer under steam reforming conditions, an attempt was made to incorporate the steam reforming reaction in the simulation. In the initial attempts the reaction was modeled as an energy flux at the catalyst particle surfaces. This approach was based on the abilities of the CFD code, but turned out not accurate enough. Elimination of the effects of local reactant depletion and the lack of solid energy conduction in the catalyst particles resulted in an unphysical temperature field. Several suggestions, based on the results of this study, are made for additional aspects of particle design to be investigated. Additionally, suggestions are made on how to incorporate the modeling of a reaction in fixed bed heat transfer simulations.

Modeling of fluid flow, heat transfer and reaction in fixed beds is an essential part of their design. This is especially critical for highly endothermic reactions in low tube-to-particle diameter ratio (N) tubes, such as methane steam reforming (MSR) and alkane dehydrogenation as two important commercial reactions. The modeling of fixed bed reaction is available in literatures with lots of assumptions. However, there is a need for implementing the reaction conditions with diffusion aspects on a real fixed bed reactor without assuming any pseudo conditions. Computational fluid dynamics (CFD) has been found as a suitable tool by many researchers to simulate fixed beds. CFD can simulate complex geometry of randomly-packed tubes, and provides us with more fundamental understanding of the transport and reaction phenomena in reactor tubes. CFD can be used to obtain detailed three-dimensional velocity, species and temperature fields that are needed to improve engineering approaches. Previously, the geometry of 120-degree wall segment (WS) of the whole reactor tube has been studied in our group. Previous works have introduced the coupling of gas flow and resolved species and temperature gradients inside pellets by CFD for methane steam reforming (MSR) and propane dehydrogenation (PDH) without considering deactivation. The deactivation of catalysts due to carbon formation is an important problem in industry, such as steam reforming and the catalytic dehydrogenation of alkanes, which are both strongly endothermic reactions. Many researches were carried out to study the effect of carbon formation and catalyst deactivation on the reactor performance. The local carbon deposition on catalysts can cause particle breakage and strongly decrease reaction rates. Catalyst deactivation in heated tubes removes the heat sink and can result in local hot spots that weaken the reactor tube. This is particularly a problem for a low tube-to-particle diameter ratio fixed bed reactor. A 3D resolved CFD model simulation was used to study the local details of carbon deposition in which the reactions and deactivation take place inside the catalytic solid particles. CFD simulations of flow, heat transfer, diffusion and reaction were carried out using the commercial CFD code FLUENT/ANSYS 6.3 in a 3D 120-degree periodic wall segment with N=4. The mesh used boundary layer prism cells at both the inside and outside particle surfaces and at the tube wall. These reactions were represented in the solid particles using user-defined scalars to mimic species transport and reaction, with user-defined functions supplying reaction rates. Diffusion in the particles was modeled by Fick's law using an effective diffusivity, given by Hite and Jackson's approximation of the Dusty Gas Model. The transient developments of particle internal gradients and carbon accumulation have been studied for the early stages of deactivation. Carbon concentration is initially strongest close to the surface and in the high temperature regions of the catalysts and affected by the wall heat flux. Deactivation of the endothermic reactions causes a slow increase in the average catalyst temperature. The second stage of the research was the verification of our CFD reaction model with experimental data under reacting conditions. The highly endothermic commercial methane steam reforming (MSR) reaction was studied experimentally in a fixed bed reactor. The temperature contributions inside catalyst particles were measured. The MSR reaction showed strong effects on the temperature profile along the reactor. Then, a CFD model was used to predict temperature profiles under MSR reaction conditions. Comparison of CFD and experimental data showed very good qualitative as well as quantitative agreement for temperature inside catalyst particles at different inlet gas temperatures. The last stage was to develop a fundamental energy equation without introducing new adjustable parameters to study heat transfer in fixed beds. In the past, many researchers have been carried out to simulate the heat transfer in fixed bed reactors by using kr (effective thermal conductivity) and hw (heat transfer coefficient). But the classical model with kr and hw cannot give a correct T(r) near tube wall, where deactivation is strongest. Therefore we need a better model which can represent the near wall heat transfer more accurate. CFD modeling was used to develop pseudo-continuum model for T(r) using Vr(r,z) and Vz(r). To get better temperature at the wall vicinity close to the physical reality. In this model radial thermal conductivity was obtained from Zehner-Schlünder model. The convection heat transfer was calculated in the 2D flow fluid from the CFD run. Results were obtained for Reynolds numbers in the range 240€“1900. The accuracy of the new model has been validated by analytical solution. The temperature calculated by the new velocity field pseudohomogenous energy equation showed reasonable quantitative agreement with values predicted by the CFD model.

As biomass and municipal solid wastes become increasingly viable fossil fuels alternatives, more researches are being conducted to improve on the processes for their conversion to energy or energy carriers. Gasification is one of such processes and it forms the core of this project. This project presents a specified design of a small-scale fixed-bed reactor for the purpose of investigating the gasification processes of biomass and municipal solid wastes. Gas extraction ports are evenly distributed along the height of the reactor to extract product gases and accommodate thermo-couples for temperature measurement. A cyclone separator was incorporated to clean the gases as well as extract bio-oil from the gases. / Program: Masterutbildning i energi- och material

Concentrated animal feeding operations such as cattle feedlots and dairies produce a large amount of manure, cattle biomass (CB), which may lead to land, water, and air pollution if waste handling systems and storage and treatment structures are not properly managed. However, the concentrated production of low quality CB at these feeding operations serves as a good feedstock for in situ gasification for syngas (CO and H2) production and subsequent use in power generation. A small scale (10 kW) countercurrent fixed bed gasifier was rebuilt to perform gasification studies under quasisteady state conditions using dairy biomass (DB) as feedstock and various air-steam mixtures as oxidizing sources. A DB-ash (from DB) blend and a DB-Wyoming coal blend were also studied for comparison purposes. In addition, chlorinated char was also produced via pure pyrolysis of DB using N2 and N2-steam gas mixtures. The chlorinated char is useful for enhanced capture of Hg in ESP of coal fired boilers. Two main parameters were investigated in the gasification studies with air-steam mixtures. One was the equivalence ratio ER (the ratio of stochiometric air to actual air) and the second was the steam to fuel ratio (S:F). Prior to the experimental studies, atom conservation with i) limited product species and ii) equilibrium modeling studies with a large number of product species were performed on the gasification of DB to determine suitable range of operating conditions (ER and S:F ratio). Results on bed temperature profile, gas composition (CO, CO2, H2, CH4, C2H6, and N2), gross heating value (HHV), and energy conversion efficiency (ECE) are presented. Both modeling and experimental results show that gasification under increased ER and S:F ratios tend to produce rich mixtures in H2 and CO2 but poor in CO. Increased ER produces gases with higher HHV but decreases the ECE due to higher tar and char production. Gasification of DB under the operating conditions 1.59<ER less than6.36 and 0.35<s:f>less than0.8 yielded gas mixtures with compositions as given below: CO (4.77 - 11.73 %), H2 (13.48 - 25.45%), CO2 (11-25.2%), CH4 (0.43-1.73 %), and C2H6 (0.2- 0.69%). In general, the bed temperature profiles had peaks that ranged between 519 and 1032 degrees C for DB gasification.

This dissertation explores catalyst technology for the production of renewable liquid fuels via thermo-chemical conversion of biomass derived syngas. Fischer-Tropsch synthesis is a process for converting syngas, i.e. a mixture of CO and H2, into energy rich long chain hydrocarbons and oxygenated compounds. This synthesis process involves a number of elementary reactions leading to an array of polymeric products. The economic operation of an FTS process lie in the interplay of both catalyst and reactor design. In relation to catalysis, the nature of chemisorbed species, and the fractional availability of active metal sites determines rate, conversion and yield. Similarly, reactor design decides the operational envelope and determines the economics of an FTS process. Eggshell cobalt catalysts are used in CO hydrogenation reactions due to their ability to maximize the use of precious cobalt metal. The thickness of the shell can be utilized to control the product yield and distribution. In this study, during catalyst synthesis stage, metal-support interaction has been exploited to control the thickness and hence, the product distribution. The catalysts are prepared using precipitation of cobalt nitrate (dissolved in ethanol) on silica support. The metal deposition rate and the location are controlled through optimized non-polar solvent imbibing, followed by water addition to a Co(NO3)2-ethanol solution and hydrolysis by urea. The eggshell coating thickness (in the absence of restricting solvent) onto silica gel substrate was modeled via theoretical equations and experimentally verified during catalyst preparation through microscopic analysis of catalyst samples. Bulk precursor solution properties such as viscosity and surface tension along with substrate properties such as tortuosity are analyzed and included in the theoretical analysis for tailoring the catalyst eggshell thickness. Polar and non-polar solvent interactions with silica gel are exploited during cobalt precipitation to control the eggshell thickness. The catalyst samples were characterized using hydrogen chemisorption studies. The catalyst was tested in a fixed bed tubular bench scale reactor using research grade synthetic feed gases (H2:CO being 2:1). Products were analyzed in a GC column fitted with flame ionized detector and the results were compared with Anderson-Schulz-Flory distribution. Liquid product analysis validated the approach used for eggshell catalyst design and synthesis. The impact of solvent and calcination conditions, on the performance of eggshell catalysts was examined. Solvents such as water and alcohol attach to the silanol groups on the silica gel surface and compete with metal salts during ion exchange and adsorption. The solution properties impact metal dispersion and interaction with metal support. The calcination conditions (static versus dynamic, oxidizing versus reducing atmosphere) also have an impact on metal dispersion and support interaction. Ethanol proved to be a better solvent for enhancing the dispersion due to its surface wetting properties. Direct reduction in dynamic hydrogen provided gradual decomposition of the cobalt precursor thus reducing agglomeration. Both the use of water as a solvent and a static air environment during calcination led to lower dispersion. The back reaction of calcination products (especially H2O) and agglomeration due to thermal expansion were competing phenomenon in a static oxidizing environment. Catalyst characterization revealed that the latter effect was pre-dominant. Catalyst performance testing was first done with pure gases (H2 & CO) in a fixed bed reactor. Additionally, to examine the technological feasibility and economic viability of producing liquid fuels from biomass via the thermo-chemical route, laboratory scale testing was done using syngas produced by gasification of pine chips. The pine chips were gasified in a tubular entrained flow gasifier operated at MSU and supplied in cylinders. The raw biomass syngas was treated using a series of adsorbents to remove tar, water and other impurities. This pre-treated gas was subjected to Fischer-Tropsch Synthesis (FTS) in a bench scale fixed bed reactor using the eggshell cobalt catalyst developed in our laboratory. Hydrogen was added to attain the 2:1 stoichiometric ratio required for the FTS reaction. The product gases were analyzed using an FTIR gas cell while liquid product was analyzed using a GC/MS HP-5 column. The eggshell catalyst produced fuel preferentially in the range of middle distillates. The activity of FTS catalyst under biomass derived syngas was lower when compared to that under pure surrogates (H2/CO) due to the presence of inert components (such as methane) in the biomass derived syngas To complement the experimental study, a comprehensive model of FTS catalytic process was developed. This included both catalyst and a fixed bed reactor model. While modeling a catalyst pellet, intra-particle diffusion limitation was taken into account. For a spherical 2mm pellet, eggshell morphology provided highest activity and selectivity. The reactor model was developed by coupling intra-pellet model with inter-pellet model via reaction term. The entire process operation starting with gas injection was considered. Presence of radial temperature profile, due to wall cooling, was confirmed by Mears criterion. Thus for a fixed time duration, a 2-dimensional reactor model, with respect to temperature and concentration, was developed. The safe operational envelopes for a fixed bed reactor, using cobalt catalyst, was narrow 473 < T < 493. The extent of catalyst pore fill changed (i) the radial thermal conductivity (ii) the overall temperature and concentration profile across the bed and (iii) the limits of safe operation without reaction runaway. Finally, hydrocarbon product selectivity also varied during startup. While the catalyst pores were being filled, effluent product mainly composed of lighter, more volatile components. Once the pores are filled, heavier products started to trickle down the bed. The economics of a large scale production of liquid fuels using this technology was explored using a CHEMCAD model of a large scale process for producing liquid fuel from biomass, a sensitivity study was conducted to determine key process parameters Two different gasification technologies were compared, one that uses only biomass (BTL process) and a second process that supplements the biomass feed with natural gas for meeting energy and hydrogen needs (BGTL process). The basis for the design was 2000 metric tons of dry biomass feed per hour. The breakeven price for synthetic crude oil was estimated at $106/bbl. for the BTL plant, and $88/bbl. for a natural gas assisted BGTL plant using current market prices for raw materials utilities and capital equipment. With the increasing availability, and falling prices of natural gas, the reforming of natural gas will provide a bridge solution in the short term for economical natural gas assisted BTL conversion, thus making it competitive in marketplace.

The increasing yield of municipal solid waste (MSW) is one of the main by-products of modern society. Among various MSW treatment methods, plasma gasification in a fixed-bed melting reactor (PGM) is a new technology, which may provide an efficient and environmental friendly solution for problems related to MSW disposals. General objectives of this work are to develop mathematical models for the PGM process, and using these models to analyze the characteristics of this new technology. In this thesis, both experimental measurement and numerical analysis are carried out to evaluate the performance of both air gasification and air&amp;steam gasification in a PGM reactor. Furthermore, parameter studies were launched to investigate the effect of three main operation parameters: equivalence ratio (ER), steam feedstock mass ratio(S/F) and plasma energy ratio (PER). Based on the above analysis, the optimal suggestions aiming at providing highest syngas calorific value, as well as system energy efficiency, are given. Six experimental tests were conducted in a demonstration reactor. These tests are classified into two groups: air gasification (case 1 and 2) and air&amp;steam gasification (case 3 to 6). In all these cases, the plasma gasification and melting of MSW produced a   syngas with a lower heating value of 6.0-7.0 MJ/Nm3. By comparing the syngas yield and calorific value, the study found out that the steam and air mixture is a better gasification agent than pure air. It is also discovered that the operation parameters seriously influence the operation of the PGM process. A zero-dimensional kinetic free model was built up to investigate the influence of operation parameters. The model was developed using the popular process simulation software Aspen Plus. In this model, the whole plasma gasification and melting process was divided into four layers: drying, pyrolysis, char combustion&amp;gasificaiton, and plasma melting. Mass and energy balances were considered in all layers. It was proved that the model is able to give good agreement of the syngas yield and composition. This model was used to study the influence of ER, S/F and PER on average gasification temperature, syngas composition and syngas yield. It is pointed out that a common problem for the PGM air gasification is the incomplete char conversion due to low ER value. Both increasing plasma power and feeding steam is helpful for solving this problem. The syngas quality can also be improved by reasonably feeding high temperature steam into the reactor.   In order to provide detailed information inside the reactor, a two-dimensional steady model was developed for the PGM process. The model used the Euler-Euler multiphase approach. The mass, momentum and energy balances of both gas and solid phases are considered in this model. The model described the complex chemical and physical processes such as drying, pyrolysis, homogeneous reactions, heterogeneous char reactions and melting of the inorganic components of MSW. The rates of chemical reactions are controlled by kinetic rates and physical transport theories. The model is capable of simulating the pressure fields, temperature fields, and velocity fields of both phase, as well as variations of gas and solid composition insider the reactor. This model was used to simulate both air gasification and air&amp;steam gasification of MSW in the PGM reactor. For PGM air gasification, simulated results showed that when ER varies from 0.043 to 0.077, both the syngas yield and cold gas efficiency demonstrated a trend of increasing. This is explained mainly by the increase of char conversion rate with ER. However, the increase of ER was restricted by peak temperature inside the fixed-bed reactor. Therefore, it is not suggested to use only air as gasification in the PGM process. The influence of plasma power is not obvious when PER varies from 0.098 to 0.138.  The positive influences of steam addition on cold gas efficiency and syngas lower-heating-value are confirmed by the simulation results of PGM air&amp;steam gasification. The main effect of steam addition is the rouse of water shift reaction, which largely accelerates the char conversion and final yields of hydrogen and carbon dioxide. The effect of steam injection is affected by steam feeding rate, air feeding rate and plasma power. Based on the above modeling work, Interactions between operation parameters were discussed. Possible operation extents of operation parameters are delimitated. The optimal points aiming at obtaining maximum syngas LHV and system CGE are suggested.

Orientador: Kamal Abdel Radi Ismail / Tese (doutorado) - Universidade Estadual de Campinas, Faculdade de Engenharia Mecânica / Made available in DSpace on 2018-08-20T09:32:00Z (GMT). No. of bitstreams: 1 Moraes_RaykleisonIgordosReis_D.pdf: 4245831 bytes, checksum: 07427cc907cc6cf8690c510c3dd14a1c (MD5) Previous issue date: 2012 / Resumo: O presente trabalho consiste na investigação numérica e experimental do carregamento de um leito fixo com material de mudança de fase no interior das cápsulas esféricas. O modelo matemático é baseado na equação da energia e a solução obtida com a técnica numérica em diferenças finitas com o esquema da malha móvel. O acoplamento do fluido de trabalho com as cápsulas foi feita utilizando o balanço de energia com o material de mudança de fase, obtendo a temperatura em função do tempo e da posição no leito. O modelo desenvolvido mostra os efeitos da temperatura de entrada, vazão mássica, diâmetro da cápsula esférica, material das cápsulas, tempo de carregamento e altura do armazenado. A Validação e otimização dos resultados foi feita com base nos resultados experimentais apresentados. As equações empíricas apresentadas podem ser ferramentas úteis como forma alternativa nos projetos de armazenadores com erro médio de ± 11% e a incerteza nos dados experimentais para a temperatura é de ± 0,5oC. Todos os resultados são analisados e discutidos / Abstract: The present work is to investigate numerical and experimental and the charging of fixed bed PCM within the spherical capsules. The mathematical model is based on the energy equation and the solution obtained with the numerical technique of finite difference scheme with mesh mobile. The coupling of the working fluid with the capsules was conducted using the energy balance of the phase change material, obtaining the temperature versus time and bed position. The model shows the effects of the inlet temperature, mass flow rate, diameter of the spherical capsule, the capsule material, charging time and the stored height. Validation and optimization of the results was based on experimental results. The empirical equations presented can be useful tools as an alternative projects in storages with average error of 11% and the uncertainty of the experimental data for temperature is 0.5oC. All results are analyzed and discussed / Doutorado / Termica e Fluidos / Doutor em Engenharia Mecânica

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Ligninolytic fungi are well known for their ability to degrade a wide range of xenobiotics contaminating the environment, including synthetic industrial dyes. In this work Pleurotus ostreatus was used for decolorization of a synthetic textile dye Remazol Brilliant Blue R (RBBR). To set up a model fungal "fixed-bed" bioreactor the fungus was immobilized on a polyurethane foam and artificially contaminated with a model bacterium Rhodococcus erythropolis. The development of bacterial contamination can be expected during a real application of fungal bio filters in wastewater treatment. The main aim of the work was to study interspecies interactions in the model bioreactors during the dye decolorization. Ligninolytic enzyme activities were followed in the bioreactor cultures as markers of fungal biodegradation ability. In contrast to the controls, no bacterial growth was observed in the P. ostreatus bioreactor culture liquid. The results showed that fungal laccase, pH of the culture liquid, and glucose consumption by the fungus had no effect on the bacterial growth. However, 4*105 - 1,3*106 CFU/ml of R. erythropolis was detected to be associated with the fungal solid support. The presence of these bacteria had no effect on the decolorization performance of the bioreactors. Dye decolorization efficiency...

Parmi les réacteurs de génération IV, le GFR et le VHTR utilisent l'hélium comme caloporteur. Il est nécessaire de contrôler sa qualité chimique. Outre des impuretés radiochimiques et particulaires, il peut contenir H2, CO, CH4, CO2, H2O, O2 et des composés azotés, et doit être purifié en permanence. Au CEA, un pilote permet d'étudier cette purification, réalisée en trois étapes: oxydation de H2 et CO sur CuO, puis deux étapes d'adsorption. L’objectif est de fournir une analyse détaillée des deux premières étapes, en les mettant en œuvre à l'échelle du laboratoire. On montre à l’aide d’une première modélisation que la consommation du lit de CuO est totale, et en régime chimique. Les particules de CuO sont constituées de grains d'environ 200 nm de diamètre. Un deuxième modèle, défini à l'échelle de ces grains, permet de trouver des résultats en accord avec les précédents. Un facteur d'échelle lié à la géométrie entre les constantes cinétiques issues des deux types de modélisation a été mis en évidence. Une compétition entre les réactions d'oxydation du CO et de H2 a été observée. Les énergies d'activation des ces réactions sont de l'ordre de 30 kJ.mol-1. La réaction CO/CuO est favorisée. La simulation numérique du déroulement simultané des deux réactions montre qu'il faut envisager une adsorption préférentielle de CO sur le CuO. Dans le cas de l'étape d'adsorption sur tamis moléculaire de CO2 et H2O, une méthodologie similaire a été mise en œuvre. Les isothermes expérimentales obtenues sont de type Langmuir. Les courbes de percée en sortie d'adsorbeur ont montré que le comportement global du lit était correctement représenté par le modèle retenu / In GEN IV studies on future fission nuclear reactors, two concepts using helium as a coolant have been selected: GFR and VHTR. Among radioactive impurities and dusts, helium can contain H2, CO, CH4, CO2, H2O, O2, as well as nitrogenous species. To optimize the reactor functioning and lifespan, it is necessary to control the coolant chemical composition using a dedicated purification system. A pilot designed at the CEA allows studying this purification system. Its design includes three unit operations: H2 and CO oxidation on CuO, then two adsorption steps. This study aims at providing a detailed analysis of the first and second purification steps, which have both been widely studied experimentally at laboratory scale. A first modelling based on a macroscopic approach was developed to represent the behaviour of the reactor and has shown that the CuO fixed bed conversion is dependent on the chemistry (mass transfer is not an issue) and is complete. The results of the structural analysis of the solids allow considering the CuO as particles made of 200 nm diameter grains. Hence, a new model at grain scale is proposed. It is highlighted that the kinetic constants from these two models are related with a scale factor which depends on geometry. A competition between carbon monoxide and hydrogen oxidation has been shown. Activation energies are around 30 kJ.mol-1. Simulation of the simultaneous oxidations leads to consider CO preferential adsorption. A similar methodology has been applied for CO2 and H2O adsorption. The experimental isotherms showed a Langmuir type adsorption. Using this model, experimental and theoretical results agree