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- Crawford, James
(författare)
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A model of pH and redox buffer depletion in waste landfills
- 1999
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The geochemical processes governing the release of heavymetal contaminants from solid waste materials have been studiedexperimentally and modelled using a mass balance approach. Theaim of the modelling work was to develop calculation tools thatcan be used to predict the release rate of heavy metals fromwaste heaps or landfills. The leaching models are based uponthe observation that the pH and redox state of the water incontact with waste are the two master variables that controlheavy metal mobility in aqueous environments. The modelsdeveloped are intended to be simple and sufficiently flexiblethat they may be adapted to different waste types withreasonable ease.The depletion rates of pH and redox buffering constituentsin the waste material are calculated by coupling a descriptionof the geochemical processes occurring within the waste withthe in- and out-transport of reactive chemical species. Themodels of waste leaching require input data to describe thereactive properties of the waste. These data may be obtainedfrom simple laboratory experiments such as pH static titrationsand reducing capacity measurements.The simulation results indicate that oxidation of organicmaterial can improve the quality of solid wastes by way ofcarbonation processes that transform highly soluble oxide,hydroxide, and silicate forms of Ca into calcite. Calcitebuffers leachate at a circumneutral pH level that is optimalfor the minimisation of heavy metal leaching. Oxidation oforganic material can occur under oxic conditions, or underanoxic conditions using sulphate as an oxidant. Under theanoxic conditions that prevail in a fully water saturatedleaching environment, reduction of sulphate also promotes theformation of insoluble heavy metal sulphide minerals. It isconcluded that disposal of waste materials in a fully watersaturated environment is the best way of minimisingenvironmental impact from these potentially hazardousmaterials.Keywords: Leaching, chemical weathering, acidneutralisation, solid waste, combustion residues, pH buffering,redox buffering, geochemical modelling, reactive transport
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- Ganemi, Bager
(författare)
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Zeolite ZSM5 catalysts for abatement of nitrogen oxide
- 1999
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Airborne pollutants from the combustion of fossil fuels area global problem. Emission of nitrogen oxides (NOx) is increasing with the worldwide increase in theuse of energy. Atmospheric and photochemical reactions linknitrogen oxides to hydrocarbons and tropospheric ozone. Theemission of NOxhas to be tackled urgently in order to limit theharmful effects of anthropogenic activity on theenvironment.The subject of this thesis is catalytic nitrogen oxideabatement through direct decomposition and reduction by methaneover ion-exchanged zeolite ZSM5. The work covers catalyticconversionand surface intermediates, including correlationswith the level of exchanged Cu2+cations and Ni2+or Pd2+co-cations. Special attention is given to thealuminium content of the support and changes in structuralparameters. It was found that NOxconversion over cation-exchanged ZSM5 is stronglyinfluenced by the ion-exchange procedure and by the abovematerial parameters.Characterization of Cu-ZSM5 reveals that approximately twomolecules of water per Cu2+ion desorb at temperatures between 150 and 350oC, in addition to the conventional dehydration atlower temperatures. The desorbed water comes from thedecomposition of Cu(OH)2. Decomposition of hydroxylated copper ions resultsin the formation of Cu2+-O-Cu2+dimers, which are suggested to be the activesites for catalytic decomposition of NO.Acid sites are important for the dispersion of copper ionson the catalyst surface. Acid sites are also important for theinteraction between copper species and the zeolite. Increasedacidity leads to a stronger interaction between the exchangedcation and the framework, i.e. the exchanged cations becomemore resistant to mobility. The stronger bond between theexchanged cations and lattice oxygen also preventsdealumination of the catalyst and decreases the thermalexpansion at higher temperatures.The temperature of dehydroxylation of acid sites on H-ZSM5overlaps with the light-off temperature for NO reduction overCu-ZSM5. Bridged nitrato groups ligated to Cu2+-O-Cu2+dimers act as site blockers below the light-offtemperature. At the light-off temperature zeolite latticevibrations destabilize surface nitrates and open the sites forcatalytic reactions via short-lived N2O3intermediates.The same lattice movementsdecompose OH-groups on the H-form of the zeolite and it wassuggested that zeolite ZSM5 should be noted mainly for itsflexibility rather than its narrow channels with strongelectrostatic fields or metal exchange sites with opencoordination.Keywords:NO, NO2, Decomposition, Reduction, Methane, ZSM5,Ion-exchange, Cu, Pd, Ni
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- Lindberg, Magnus
(författare)
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On the influence of micromixing in reaction crystallization
- 1999
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In crystallization, the mean crystal size is governed by thetotal number of crystals sharing the total mass produced in theprocess. In reaction crystallization, it is believed that asignificant part of the crystals is formed during the localcontacts between reactant solutions close to the feed point. Inthe ultimate stage of the mixing processes, small vorticesbring the reactant solutions into contact in a lamellarstructure of thin fluid films which are strained. The aim ofthe present work is to analyse the local solute concentrationgradients that develop during the life time of the smallestvortex as well as the crystal size distribution that may begenerated as a result of crystal nucleation and growth in thevortex. The investigation has been carried out by mathematicalmodelling of the physical situation in the locally occurringlamellar structure of reactant solution films. The chemicalreaction is assumedto be instantaneous, and the subsequentcrystallization to be fast and to occur via the formation of asparingly soluble solute molecule.Three models of increasing complexity have been developed.All models consider mass transfer and instantaneous reaction instrained fluid reactant films. Model I assumes that thegeometry of the fluid films is semi-infinite. In model II, alamellar structure of finite reactant films is considered. Themost complex model, model III, is an extension of model IItaking also the crystallization phenomena intoconsideration.The effects on the locally occurring crystallization ofdifferent parameters such as reactant concentrations,diffusivities, specific local energy dissipation rate andcrystallization kinetics are examined. Simulations of model IIIshow that the mean size and number concentration generallyincrease with decreasing specific local energy dissipationrate. The simulations also suggest that the size of thecrystals leaving the vortex is often governed by the growthrate and not by nucleation and mass constraints, and that themean size may be larger than the limiting size for Ostvaldripening in the bulk. The size distribution is usually quitenarrow. The range of total crystal number concentration is ofthe same order of magnitude as that reported in the literature.The neglect of the detailed conditions at the feed point inreaction crystallization of a molecular compound may not bejustified.It is found that due to the very non-linear dependence ofnucleation rate on supersaturation, the maximum supersaturationoccurring in the vortex correlates well with the total numberconcentration of crystals formed during the local contact. Themaximum product concentration depends in a complex way on thediffusivities, reactant concentrations, film thicknesses anddegree of consumption. However, the results show that themaximum product concentration during a local contact can beestimated by either of the two simpler models, even thoughneither of these models considers crystallization. Model IIestimates the maximum product concentration at negligibleconsumption and model I at a high degree of consumption.Based on calculations using models I and II of the behaviourof the maximum product concentration, recommendations arepresented for how to choose reactant concentrations in batchand semi-batch reaction crystallization processes in order tominimise the locally occurring maximum product concentration.In addition, a new methodology for carrying out semi-batchreaction crystallization processes is suggested: the"programmed feed concentration" concept. By starting with a lowfeed reactant concentration and then continuously increasingit, the local supersaturation is lowered considerably,especially during start up of the process.Keywords:Reaction crystallization, Precipitation,Micromixing, Modelling, Programmed feed concentration
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