No description available.

In this work, multiple kinetic models have been screened as potential candidates for simulating the performances of three oxidative coupling of methane (OCM) catalysts. Two of the proposed models were subjected to testing and optimization. The types of models screened covered both kinetic and microkinetic type models, i.e., radical omitting and radical considering. Some of the models only accounted for catalytic heterogeneous pathways, while others have expanded on the homogeneous gas-phase mechanism of the OCM reaction. The optimization process was carried out in MATLAB® R2020a using an error minimization tool. The range of experimental conditions examined was as follows: 740–800◦C, 100 kPa, 2–4 CH4/O2 ratio, 1–6 gcat h molC –1 spacetime. The results show successful optimization of both models as well as discrepancies in terms of their performances in predicting experimentally obtained values of CH4 and O2 conversions, as well as selectivities towards COx and C2+ products. While a kinetic model served as an easy option to optimize, it expressed limits in terms of achievable performance, mainly failing to simulate experimental runs conducted at low spacetimes. A microkinetic model on the other hand, managed to simulate all experimental conditions, with less accuracy towards COx species and much greater computational demand.

M.S.

Over 50 million children attend public elementary and secondary schools in the United States each day. Children spend a substantial portion of their waking hours in school and consume one-third to one-half of their daily calories there, making schools a promising site to influence dietary quality and potentially the risk of childhood obesity. Important policy revisions have been implemented in the National School Lunch Program (NSLP) to improve the nutritional quality of school meals. In 2010, the Healthy, and Hunger-Free Kids Act updated the NSLP standards. The revised nutrition standards required schools to increase the availability of fruits, vegetables, whole grains, and fat-free and low-fat fluid milk in school meals; reduce the level of sodium, saturated fat and trans fat; and meet the nutritional needs of school children within their calorie requirements. About 60% of elementary children participate in the NSLP at least once per week, with the remaining 40% of children bringing a packed lunch from home. While school lunches are guided by national standards and regularly monitored to ensure standards are maintained, the remaining 40% are not guided by national standards. The ultimate purpose of this research was to assess the current school lunch environment and increase the overall nutritional quality of elementary lunches. Data collection procedures included school and packed lunch observations, elementary parent questionnaires, and a pilot intervention to assist elementary parents in providing healthier packed lunches. Results from these studies provide insight on the nutritional differences in school and packed lunches, provide knowledge concerning the parental motivations and barriers to participating in the NSLP or packing lunch for a child, and contribute to the limited research on effective modalities for assisting parents in providing healthier lunches. This research has significant implications public policy and provides valuable information for health professionals, researchers, food service directors, parents, and elementary school administrators to encourage NLSP participation and/or develop interventions which assist parents in packing healthier lunches. Interventions can take the form of marketing strategies to potentially influence NSLP participation, interventions to increase the nutritional value of packed lunches, and/or improvements in school wellness policies. / Ph. D.

Gas absorption may be defined as the transfer from a gas to a liquid of one or more components of a gaseous mixture by means of solution in a liquid. This is accomplished by bringing the two phases together, with a large amount of interfacial surface, in an economical manner for commercial use. Much is lacking in knowledge of the theory of gas absorption. This lack of knowledge and the wide variety of purposes and specifications have caused many types of absorption equipment to be designed, built and used. The objective in all causes has been to increase the surface of contact between the gas and the liquid at low installation cost and with low operating costs. The most common type of absorption equipment is the packed tower. It consists, essentially, of a vertical shell filled with an inert packing material. Absorption is accomplished by countercurrent operation, the solvent being allowed to trickle down over the packing material while the gas passes up through the wetted packing. The tower packing is the fundamental consideration in improving the efficiency of the equipment. A packing material should have a low resistance to gas flow so as to give low maintenance costs, a high absorption capacity and low costs of installation. In any case, the packing that most completely fills the requirements should be used. It is the purpose of this investigation to make a study of the characteristics of operation of copper knit-metal cloth, in comparison to Raschig rings, when used absorption tower packings. / Master of Science

The High Pressure Test Unit (HPTU) forms part of the Pebble Bed Modular Reactor (PBMR) Heat Transfer Test Facility (HTTF). One of the test sections that forms part of the HPTU is the Braiding Effect Test Section (BETS). This test section allows for the evaluation of the so–called ‘braiding effect’ that occurs in fluid flow through a packed pebble bed. The braiding effect implies an apparent enhancement of the fluid thermal conductivity due to turbulent mixing that occurs as the flow criss–crosses between the pebbles. The level of enhancement of the fluid thermal conductivity is evaluated from the thermal dispersion effect. The so–called thermal dispersion quantity r K is equivalent to an effective Peclet number eff Pe based on the inverse of the effective thermal conductivity eff k . This thesis describes the experiments carried out on three different BETS test sections with pseudo–homogeneous porosities of 0.36, 0.39 and 0.45, respectively. It also provides the values derived for the enhanced fluid thermal conductivity for the range of Reynolds numbers between 1,000 and 40,000. The study includes the following: * Compilation of a literature study and theoretical background. * An uncertainty analysis to estimate the impact of instrument uncertainties on the accuracy of the empirical data. * The use of a Computational Fluid Dynamics (CFD) model to simulate the heat transfer through the BETS packed pebble bed.* Application of the CFD model combined with a numerical search technique to extract the effective fluid thermal conductivity values from the measured results. * The assessment of the results of the experiments by comparing it with the results of other investigations found in the open literature. The primary outputs of the study are the effective fluid thermal conductivity values derived from the measured data on the HPTU plant. The primary variables that were measured are the temperatures at radial positions at different axial depths inside the bed and the total mass flow rate through the test section. The maximum and minimum standard uncertainties for the measured data are 10.80% and 0.06% respectively. The overall effective thermal conductivities that were calculated at the minimum and maximum Reynolds numbers were in the order of 1.166 W/mK and 38.015 W/mK respectively. A sensitivity study was conducted on the experimental data and the CFD data. A maximum uncertainty of 5.92 % was found in the calculated effective thermal conductivities. The results show that relatively high values of thermal dispersion quantities or effective Peclet numbers are obtained for the pseudo–homogeneous packed beds when compared to randomly packed beds. Therefore, the effective thermal conductivity is low and it can be concluded that the radial mixing in the structured packing is low relative to the mixing obtained in randomly packed beds. / Thesis (M.Ing. (Mechanical Engineering))--North-West University, Potchefstroom Campus, 2011.

The High Pressure Test Unit (HPTU) forms part of the Pebble Bed Modular Reactor (PBMR) Heat Transfer Test Facility (HTTF). One of the test sections that forms part of the HPTU is the Braiding Effect Test Section (BETS). This test section allows for the evaluation of the so–called ‘braiding effect’ that occurs in fluid flow through a packed pebble bed. The braiding effect implies an apparent enhancement of the fluid thermal conductivity due to turbulent mixing that occurs as the flow criss–crosses between the pebbles. The level of enhancement of the fluid thermal conductivity is evaluated from the thermal dispersion effect. The so–called thermal dispersion quantity r K is equivalent to an effective Peclet number eff Pe based on the inverse of the effective thermal conductivity eff k . This thesis describes the experiments carried out on three different BETS test sections with pseudo–homogeneous porosities of 0.36, 0.39 and 0.45, respectively. It also provides the values derived for the enhanced fluid thermal conductivity for the range of Reynolds numbers between 1,000 and 40,000. The study includes the following: * Compilation of a literature study and theoretical background. * An uncertainty analysis to estimate the impact of instrument uncertainties on the accuracy of the empirical data. * The use of a Computational Fluid Dynamics (CFD) model to simulate the heat transfer through the BETS packed pebble bed.* Application of the CFD model combined with a numerical search technique to extract the effective fluid thermal conductivity values from the measured results. * The assessment of the results of the experiments by comparing it with the results of other investigations found in the open literature. The primary outputs of the study are the effective fluid thermal conductivity values derived from the measured data on the HPTU plant. The primary variables that were measured are the temperatures at radial positions at different axial depths inside the bed and the total mass flow rate through the test section. The maximum and minimum standard uncertainties for the measured data are 10.80% and 0.06% respectively. The overall effective thermal conductivities that were calculated at the minimum and maximum Reynolds numbers were in the order of 1.166 W/mK and 38.015 W/mK respectively. A sensitivity study was conducted on the experimental data and the CFD data. A maximum uncertainty of 5.92 % was found in the calculated effective thermal conductivities. The results show that relatively high values of thermal dispersion quantities or effective Peclet numbers are obtained for the pseudo–homogeneous packed beds when compared to randomly packed beds. Therefore, the effective thermal conductivity is low and it can be concluded that the radial mixing in the structured packing is low relative to the mixing obtained in randomly packed beds. / Thesis (M.Ing. (Mechanical Engineering))--North-West University, Potchefstroom Campus, 2011.

No description available.

This thesis constitutes fundamental experimental and computational investigations on gasification and combustion in a packed bed of biomass. Packed bed gasification-combustion in counter-current mode is used in two applications -(1) Gasifier stove in reverse downdraft mode (or equivalently, top-lit updraft mode) that constitutes the idea behind efficient and clean burning domestic stoves. (2) Combustion-on moving grate for boiler application, studied widely in Europe. While a large part of the present study is around domestic stoves, a crucial part of the study aims to address the second application as an extension of the approach taken in the first part to clarify conflicting conclusions of earlier studies and explain the aero-thermochemical behavior over the entire range of superficial velocities, V s (this is velocity of air through the empty cross section of the reactor). Operational differences between the two applications lie in the range of superficial velocity -3.5 to 6 cm/s for domestic stoves and 15 to 30 cm/s for grate combustion. Lower values of Vs are chosen for domestic stoves to limit the particulate emissions; higher values of V s for combustion-on-grate to maximize the conversion rate. Present work deals with a fan based gasifier stove, Oorja, built by BP, India (currently transferred to FEPL, Pune) and disseminated to over 400,000 households between 2005 and 2009. The technology was developed at CGPL, IISc and transferred to BP for commercialization. Work reported in this thesis was started to resolve issues of higher CO emissions in char mode operation and occasional smoking during transition from flaming to char mode. The contribution of the thesis is split into two parts. (a) Use of the principles of gasification to improve the performance of the stoves to the highest possible level, balancing between efficiency and ash fusion issues for domestic and industrial applications and (b) fundamental studies to unravel the flame structure in the two-phase gasification-combustion process over the entire range of Vs. Improving the stove performance It has been known that in most free-convection based stoves, like three stone fire and others developed over the last two decades, the amount of energy extracted from the stove by a cooking pot, usually measured as water boiling efficiency, is between 15 to 35 % with CO emissions of more than 1.5 g/MJ. Oorja stove had demonstrated water boiling efficiency of 50 % and CO emissions of 0.75 g/MJ. Operational issues noticed in the field provided an opportunity to further improve the performance by conducting detailed thermo-chemical studies. Towards this, the components of water boiling efficiency in different phases and from different modes of heat transfer were determined. Optimizing the ratio of air flow rate between combustion air from top and gasification air through the grate (denoted by R) was the key to improving the performance. The maximum water boiling efficiency obtained was 62% with 0.53 g/MJ CO for a 320 mm diameter vessel; under these conditions, the first phase, termed flaming mode, involving pyrolysis-gasification-gas phase combustion contributed 45% to the total efficiency and 0.4 g/MJ CO at R = 4.8 and the second phase, termed char mode, involving char surface oxidation-gasification-gas phase combustion contributed 17% and 0.13 g/MJ CO at R = 1.9. Under optimal air flow conditions, efficiency depends on the size of the vessel used; reactive flow calculations were performed with fast chemistry (using mixture fraction approach) in a zone that includes the free space of the combustion chamber and the vessel to obtain the heat transfer efficiency and bring out the effect of vessel size. Experiments aimed at evaluating the performance of the stove on either side of stoichiometry, revealed that while the stove could be operated on the rich side, it was not possible to operate it on the lean side -it was always tending towards the stoichiometric point with enhanced power. Computational studies showed that increased air flow from the top caused enhanced recirculation around the fuel bed bringing more oxygen that reacted closer to the surface and transferred additional heat enhancing the pyrolysis rate, explaining the observed shift towards stoichiometry. An examination of literature showed that the energy balance for stoves had long remained unexplained (unaccounted losses in stoves were up to 40 %). Using the different components of efficiency obtained from experiments and computations, a heat balance was established to within 5%. This vast improvement in the heat balance is due to the fact that the unaccounted loss in the earlier estimates was essentially due to poor combustion, but was not so recognized. The very significant increase in combustion efficiency in this class of stoves allowed the possibility of estimating other components reasonably accurately. This is a direct consequence of the two stage gasification-combustion process yielding steady flow of gases which contain 80% (gasification efficiency) of the input energy enabling near-stoichiometric combustion with the help of controlled supply of combustion air. Fundamental studies Experiments with wood chips (615 kg/m3) and pellets (1260 kg/m3) showed that particle density has no effect on single particle and packed bed combustion in flaming mode beyond the role played through the surface energy balance (involving the product of fuel density and propagation rate, ˙r). Same is true for single char particles. A transport controlled combustion model taking into account the ash build up over the char surface confirmed this behaviour and showed that the phenomenon follows d2 law, where d is the equivalent diameter of the fuel particle, consistent with the experimental results. But packed bed of char particles showed distinct dependence on particle density. Differences were traced to poor thermal environment faced by low density wood char pieces compared to pellet char leading to variations in the volumetric heat release rate. A composite picture of the operational behaviour of the packed bed flame propagation was obtained from the measurements of exit gas composition, bed temperature, temperature of gas phase and condensed phase surface using 100 µm thermocouples, O 2 drop across the flame front using lambda sensor as a function of Vs. The packed bed studies were conducted in insulated steel and glass reactors. These studies clearly showed distinctive regimes in the bed behavior. In the first regime from Vs = 3 to 17 cm/s, (a) the propagation rate increases with Vs, (b) the fractions of CO, H2 are at least 10%, CH4 drops from 3 to 1%, (c) the oxygen fraction is near zero, (d) the gas phase temperature in the bed is constant at about 1600 K, (e) the condensed phase surface temperature increase from 850 K to 1600 K and (f) oxygen fraction drops from 0.21 to 0.0 within a single particle depth and coincides with the gas phase ignition. The inferences drawn from these data are that (i) the process is diffuusion controlled (ii) the bed operates in fuel rich mode, (iii) char participates only in reduction reactions. In the second domain from V s = 17 cm/s up to about 50 cm/s, (a) the propagation rate is nearly constant (b) the mass fractions of CO and H2 drops to less than 5%, CH4 decreases further, (c) oxygen fraction remains near zero, (d) CO 2 increases, (e) gas phase and surface temperatures are nearly equal and increase from 1600 K to 2200 K and match with the equilibrium temperature at that equivalence ratio, (f) oxygen fraction drops from 0.21 to 0 in one particle depth like in the first regime indicating diffuusion limitedness in this regime as well, (g) unlike in the first regime, volatiles from biomass are convected up to the next layer suppressing a local flame and char oxidation dominates. Beyond Vs = 50 cm/s, the propagation ceased to occur. The precise value of the extinction V s depended on the rate of increase of Vs in this range. A faster change initiated the extinction earlier. Observations showed that extinction began at some location around the periphery and spread laterally. Extinction at one layer was adequate to complete the extinction process. To explain the observed behaviour a simple zero-dimensional model tracking the heating of a fresh biomass particle upstream of the propagating flame front because of radiative heat transfer was set up. This equation was coupled with the equation for single particle flaming combustion to explain the behavior in the first regime. In order to explain the observed flattening of propagation rate in the second regime, it was found essential to account for the effect of the ash layer building on the oxidizing char particle and the temperature dependence of ash emissivity, on the radiative heat transfer to fresh biomass. The results of the model coupled with the experimental data from all sources on a corrected propagation rate vs. V s showed a universal behaviour that is considered a very important recognition of the packed bed propagation behaviour. Combining theory and experiments was essential to explain the extinction. The features are: (a) the presence of ash layer over the surface is shown to be responsible for maintaining a steady char conversion in a single particle at low stream speeds, (b) the feature that the ash layer would be blown away at stream velocities of 2.5 to 3 m/s in a single particle combustion, (c) with V s close to 50 cm/s, local velocities of air flow through the bed can reach 2 to 3 m/s, this value being sensitive to the bed arrangement (with slight shifting or settling of one particle leading to increase of the local velocity at the periphery). Thus, the high local speeds of flow through the bed (more than 2 m/s) was considered responsible for removal of ash layer such that radiation losses would be dominant and cause local extinction of the reaction front at the char surface. Thus, this study has led to a comprehensive understanding of the gasification-combustion behavior of packed bed in stoves and on grates. It has also led to the evolution of parameters for obtaining high efficiency and low emissions (HELE) from stoves -both domestic and industrial. Most interestingly, it has resulted in recognition of an universal behavior of flame propagation rate through packed bed of biomass.

Non-thermal plasma generated in a dielectric barrier packed-bed reactor has been used for the remediation of chlorinated volatile organic compounds. Chlorinated VOCs are important air pollutant gases which affect both the environment and human health. This thesis uses non-thermal plasma generated in single and multiple packed-bed plasma reactors for the decomposition of dichloromethane (CH2Cl2, DCM) and methyl chloride (CH3Cl). The overall aim of this thesis is to optimize the removal efficiency of DCM and CH3Cl in air plasma by investigating the influence of key process parameters. This thesis starts by investigating the influence of process parameters such as oxygen concentration, initial VOC concentration, energy density, and plasma residence time and background gas on the removal efficiency of both DCM and CH3Cl. Results of these investigations showed maximum removal efficiency with the addition of 2 to 4 % oxygen to nitrogen plasma. Oxygen concentrations in excess of 4 % decreased the decomposition of chlorinated VOCs as a result of ozone and NOx formation. This was improved by adding an alkene, propylene (C3H6), to the gas stream. With propylene additives, the maximum remediation of DCM was achieved in air plasma. It is thought that adding propylene resulted in the generation of more active radicals that play an important role in the decomposition process of DCM as well as a further oxidation of NO to NO2. Results in the single bed also showed that increasing the residence time increased the removal efficiency of chlorinated VOCs in plasma. This was optimized by designing a multiple packed-bed reactor consisting of three packed-bed cells in series, giving a total residence time of 4.2 seconds in the plasma region of the reactor. This reactor was used for both the removal of DCM, and a mixture of DCM and C3H6 in a nitrogen-oxygen gas mixture. A maximum removal efficiency of about 85 % for DCM was achieved in air plasma with the use of three plasma cells and the addition of C3H6 to the gas stream. Nitrogen oxides are air pollutants which are formed as by-products during the decomposition of chlorinated VOCs in plasmas containing nitrogen and oxygen. Results illustrate that the addition of a mixture of DCM and C3H6 resulted in the formation of the lowest concentration of nitric oxide, whilst the total nitrogen oxides concentrations did not increase. A summary of the findings of this work is presented in chapter eight as well as further work. To conclude, the maximum removal efficiency of dichloromethane was achieved in air plasma with the addition of 1000 ppm of propylene and the use of three packed-bed plasma cells in series. The lowest concentration of nitric oxide was formed in this situation.