Littérature scientifique sur le sujet « Absorption/desorption kinetic »

Li2MgN2H2 can reversibly store more than 5.5 wt% hydrogen. However, the high activation energy of hydrogen desorption poses a kinetic barrier for low-temperature operation. In this work, the composition of the Li–Mg–N–H system has been modified by the partial substitution of Mg or Li by Na. The changes in structure and hydrogen absorption/desorption kinetics have been investigated. It was found that the peak temperature for hydrogen desorption was decreased by ∼10 °C, and that the hydrogen absorption/desorption isotherms were also significantly changed. Furthermore, the activation energy calculated by the Kissinger’s approach was reduced after the substitution of Mg or Li by Na. In addition, the different dehydrogenation structures were detected at different molar ratios of Mg, Li, and Na.

The kinetics of the title process is approximated by differential equations based on kinetic and equilibrium data for carbon dioxide. The course of pH after a sudden change of the concentration of CO2 in the gas is calculated by numerical integration. The course of pH during absorption of CO2 is different from that during desorption. The course of pH during desorption calculated on the assumption that the rate of the noncatalysed hydration of CO2 is sufficient to ensure chemical equilibrium is in good agreement with experimental data from the literature. During absorption of CO2 in a solution of hydrogen carbonate, the chemical reaction rate is sometimes insufficient to ensure chemical equilibrium prior to pH measurement.

In the most recent years, MgH2has attracted considerable attention for reversible hydrogen storage purposes because of a large 7.6 w% H-uptake, single plateau reaction at low pressure and abundance of metal. If the Mg ↔ H reactions take place at rather high temperature (> 300°C), the kinetic remains very low. However, early transition metal based additives (Ti, V, Nb...) improve dramatically the kinetics of hydrogen absorption/desorption, while having no essential impact on the reversible sorption capacity. Systematic analysis of many experimental data led to question chemical, physical, mechanical... parameters contributing significantly to improve the kinetics of absorption/desorption. Besides, results of theoretical and numerical computation enlighten the impact of structural and mechanical parameters owing to the local bonds of Mg/MgH2with of TM elements, in terms of total energy and electronic structure. More specifically, we found highly relevant to consider 1 - the impact of the crystallite sizes of Mg and the TM-phase, 2 - the role of internal and external stresses, as well as 3 - the role of texture on the kinetics of hydrogen absorption/desorption. Apart the previous considerations, we like to underline the role of specific TM in trapping intermediately hydrogen thus forming TMHxprior initiating the Mg ↔ MgH2nucleation process.

In the current study, Twin Parallel Channel Angular Extrusion (TPCAE) as a developed SPD processing technique is used to improve the hydrogen storage properties of AZ91 cast alloy. The processing is conducted at different temperatures, ranging from 340 °C down to 200 °C. The hydrogen absorption and desorption tests are conducted kinetically at three different temperatures, using a Sievert-type apparatus. Remarkable improvement in the absorption kinetic is achieved as a result of the TPCAE processing. A maximum absorption capacity of 6.1 wt.% within a time span of 2000 s is achieved for the sample with three passes of processing complemented at 250 °C. Also, the kinetic of dehydrogenation is improved significantly and complete desorption at 350 °C is achieved for all the processed samples within a time span of maximum 2500 s. By calculating the activation energy of hydrogenation and evaluating the microstructure changes, it is found that implementing sufficient thermomechanical work level along with applying the last pass of the process at lower temperature results in a reduction of the activation energy and improvement of the hydrogenation kinetic.

Transition metal catalysts are particularly effective in improving the kinetics of the reversible hydrogen storage reaction for light metal hydrides. Herein, K2MoO4 microrods were prepared using a simple evaporative crystallization method, and it was confirmed that the kinetic properties of magnesium hydride could be adjusted by doping cubic K2MoO4 into MgH2. Its unique cubic structure forms new species in the process of hydrogen absorption and desorption, which shows excellent catalytic activity in the process of hydrogen storage in MgH2. The dissociation and adsorption time of hydrogen is related to the amount of K2MoO4. Generally speaking, the more K2MoO4, the faster the kinetic performance and the shorter the time used. According to the experimental results, the initial dehydrogenation temperature of MgH2 + 10 wt% K2MoO4 composite is 250 °C, which is about 110 °C lower than that of As-received MgH2. At 320 °C, almost all dehydrogenation was completed within 11 min. In the temperature rise hydrogen absorption test, the composite system can start to absorb hydrogen at about 70 °C. At 200 °C and 3 MPa hydrogen pressure, 5.5 wt% H2 can be absorbed within 20 min. In addition, the activation energy of hydrogen absorption and dehydrogenation of the composite system decreased by 14.8 kJ/mol and 26.54 kJ/mol, respectively, compared to pure MgH2. In the cycle-stability test of the composite system, the hydrogen storage capacity of MgH2 can still reach more than 92% after the end of the 10th cycle, and the hydrogen storage capacity only decreases by about 0.49 wt%. The synergistic effect among the new species MgO, MgMo2O7, and KH generated in situ during the reaction may help to enhance the absorption and dissociation of H2 on the Mg/MgH2 surface and improve the kinetics of MgH2 for absorption and dehydrogenation.

Hydride-forming alloys are currently considered reliable and suitable hydrogen storage materials because of their relatively high volumetric densities, and reversible H2 absorption/desorption kinetics, with high storage capacity. Nonetheless, their practical use is obstructed by several factors, including deterioration and slow hydrogen absorption/desorption kinetics resulting from the surface chemical action of gas impurities. Lately, common strategies, such as spark plasma sintering, mechanical alloying, melt spinning, surface modification and alloying with other elements have been exploited, in order to overcome kinetic barriers. Through these techniques, improvements in hydriding kinetics has been achieved, however, it is still far from that required in practical application. In this review, we provide a critical overview on the effect of mechanical alloying of various metal hydrides (MHs), ranging from binary hydrides (CaH2, MgH2, etc) to ternary hydrides (examples being Ti-Mn-N and Ca-La-Mg-based systems), that are used in solid-state hydrogen storage, while we also deliver comparative study on how the aforementioned alloy preparation techniques affect H2 absorption/desorption kinetics of different MHs. Comparisons have been made on the resultant material phases attained by mechanical alloying with those of melt spinning and spark plasma sintering techniques. The reaction mechanism, surface modification techniques and hydrogen storage properties of these various MHs were discussed in detail. We also discussed the remaining challenges and proposed some suggestions to the emerging research of MHs. Based on the findings obtained in this review, the combination of two or more compatible techniques, e.g., synthesis of metal alloy materials through mechanical alloying followed by surface modification (metal deposition, metal-metal co-deposition or fluorination), may provide better hydriding kinetics.

The most attractive way to storage hydrogen safely and economically is in metal hydrides. In particular, magnesium has attracted much interest since their hydrogen capacity exceeds that of known metal hydrides. One of the approaches to improve the kinetic is addition of metal oxide. In this paper, we tried to improve the hydrogen absorption properties of Mg. The effect of transition oxides, such as Nb2O5 on the kinetics of the Mg hydrogen absorption reaction was investigated. MgHx-Nb2O5 composites have been synthesized by hydrogen induced mechanical alloying. The powder synthesized was characterized by XRD, SEM, EDX, BET and simultaneous TG/DSC analysis. The hydrogenation behaviors were evaluated by using an automatic Sievert’s type PCT apparatus. Absorption/desorption kinetics and PCI of MgHx catalyzed with 5wt.%Nb2O5(as-received), 5wt.%Nb2O5(30min. milled) are determined at 423, 473, 523, 573, 623K.

The first part of this thesis deals with hydrogen storage materials, in view of their applications as promising energy carriers. One of the main open problems with these materials is: how can their decomposition temperature be lowered, when hydrogen is wanted to be released, so as to improve the energy efficiency of the process. A possible answer is given by joint decomposition of two or more hydrides, if very stable mixed compounds are formed (‘hydride destabilization’). Aiming at this result, the new hydride composite 2LiBH4-Mg2FeH6 was considered, it was synthesized, and its thermodynamic and kinetic properties were investigated. In the second part of this thesis work lithium oxide materials, of relevant interest for applications to batteries, were investigated. The chemical lithiation reaction of niobium oxide was considered, as equivalent to the electrochemical process of lithium insertion on discharging a Nb2O5 cathode vs. a metal Li anode. Thus, the Li2Nb2O5 compound was synthesized by reaction of monoclinic a-Nb2O5 with n-butyllithium.This material was investigated by neutron powder diffraction (D2B equipment at ILL, France) and its structure was Rietveld refined in space group P2 to wRp=0.045, locating the Li atoms inserted in the a-Nb2O5 framework.

Le premier objectif dans cette thèse est d'étudier le phénomène de diffusion radiale dans un polymère élastomérique (EVA) de forme sphérique en tenant compte du changement de dimension : gonflement durant l'étape d'absorption et rétrécissement durant l'étape de désorption, modélisation et expérimentation. Le second objectif concerne l'étude des différents paramètres qui influent sur les cinétiques de transfert et les profils de concentration dans différents cas. Plusieurs variables sans dimensions sont considérées : l'expansion volumique, l'expansion linéaire, la diffusivité, le coefficient de transfert convectif et le rayon de la sphère

Magnesium is considered as one of the more promising candidates for hydrogen storage, primarily because of its abundance and the high hydrogen capacity of MgH2, magnesium dihydride (7.6 wt%). Unfortunately, practical applications of MgH2 are limited by poor hydrogen sorption kinetics and high thermodynamic stability, resulting in slow charging and the need for high temperatures to release the hydrogen. Methods to improve the sorption kinetics of MgH2, through ball-milling, alloying and introducing small amounts of additives are currently under investigation. The aim of this work was to investigate the effect of different transition metal oxides, as well as other catalysts, on the hydrogen sorption kinetics, hydrogen release temperature and cycling stability of MgH2. The rate of MgH2 absorption is determined by the physisorption and dissociation of molecular hydrogen, diffusion through the hydride layer and then nucleation of the hydride. Whereas desorption is determined by nucleation of metal phase, diffusion of atomic hydrogen through the metal and hydride, and recombination to form molecular hydrogen at the surface before dissociation of the molecule form the surface. The addition of small amounts of additives (< 10 mol%) to MgH2 during milling have been shown to have a significant effect on the kinetics of absorption and desorption of hydrogen. In addition, the effect of oxygen as a component of these additives has been extensively studied. Although a surface layer of MgO is known to slow the diffusion of hydrogen into metal, niobium oxide, Nb2O5, is one of the best additives for kinetic improvement of MgH2. It has been suggested that higher valance oxides have a greater effect on the kinetics; also recent studies have indicated that the formation of magnesium-niobium ternary oxide compound may be responsible for the enhancement, however the exact mechanism by which Nb2O5 enhances the kinetics is still unclear. Various transition metal oxides have proved to be effective additives for hydrogen absorption/desorption of Mg-based hydrides e.g., Nb2O5 and TiO2. In this work organometallic additives based on transition metal oxides (Ti and V) and halides have been chosen, some of which are liquid at room temperature. These additives were chosen to extend the oxide valency to higher values, provide a comparison between liquid and solid oxide additives, and compare a non-oxide transition metal additive (Ti-based chloride). Nb2O5 was used as the benchmark for comparisons. A range of different amounts of transition metal (Ti and V) oxides and other additives, both liquid and solid, were ball milled with pre-milled MgH2 under different gas environments and the hydrogen sorption behaviour of the ball milled composite samples was investigated using a TPD/TDS Sieverts apparatus. A key difference from previous work in this area was the use of organo-metallic compounds, some of which were liquid at room temperature. Samples were characterized using X-Ray Diffraction, and, where feasible, Scanning Electron Microscopy and Raman Spectroscopy before and after recording the hydrogen sorption behaviour. A Sievert apparatus was improved, then modified to include TPD and TDS capability to determine the hydrogen uptake and release and to perform all these measurements in one consistent environment. With the aim of understanding what effect the ball milling gas environment might have on the MgH2 sorption kinetics, two different gases, Ar and H2, were used. MgH2, with and without additives (C60, TTIP (Titanium TetraIsoPropoxide) and Nb2O5) were milled under the two different gases. In most cases the milling gas had little to no effect on desorption kinetics, but a noticeable effect in the absorption uptake was observed - by up to 2 wt% - depending on the gas used. To understand the role of an organo-metallic oxide liquid additive on MgH2 sorption kinetics, a systematic survey was performed by milling up to 2 mol% of TTIP with MgH2 and the results compared to composite samples with Nb2O5 as the additive. TTIP was found to be equally as effective as Nb2O5 with superior hydrogen capacity, and, just as for Nb2O5, only a small amount of additive, 0.5 mol% of TTIP was found to be sufficient for kinetics enhancement. Interestingly, 0.5 mol% TTIP hand-mixed with the pre-milled MgH2 was also found to be effective for desorption, but not for the absorption kinetics. To further investigate the effect of organo-metallic liquid oxides, particularly transition metal oxides, on the enhancement of the sorption kinetics of MgH2, a range of liquid oxides (Ti-based and V-based oxides) were milled with MgH2 and compared to powder oxides. Ti-based oxides were found to have superior desorption enhancement, with the liquids performing better than the powders. The V-based oxides (all liquids) showed faster absorption and higher uptake when compared to Ti-based oxides. The Ti-chloride based organo-metallic additive was investigated to compare a non-oxide transition metal additive to the oxide additives. It was found that a small amount of 1 mol% of additive milled for short time (1 h) had the best desorption of all the additives investigated in this work - but with quite poor absorption. Overall, this project established that the use of transition metal oxides as additives has a great impact on improving the sorption kinetics of MgH2. The TTIP additive produced results at least as good as the benchmark additive Nb2O5, but with the significant advantage of being able to be mixed with MgH2 without ball-milling. The Ti and V based oxides additives were also shown to be effective, Ti-based achieving better desorption enhancement, whereas V-based oxide samples had faster absorption and higher uptake. It was also determined the ball milling gas environment can have a significant effect on the sorption kinetics, but the effect depended on the additive. The difference in the effect of additives on desorption and absorption cycles confirms the need to study combinations of additives for optimal overall benefit.
Thesis (PhD Doctorate)
Doctor of Philosophy (PhD)
School of Environment and Sc
Science, Environment, Engineering and Technology
Full Text

Thermochemical heat storage (TCS) is a thermal energy storage (TES) technology used to store thermal energy for later use. TCS can provide heating or cooling services from intermittently available thermal energy, often low grade waste heat. The system studied here stores and releases the energy in the form of chemical energy by utilizing reversible chemical reactions. TCS has potential to reduce greenhouse gas emissions, increase infrastructure system efficiency, lower society-wide energy system costs and by that contribute to sustainable development. This thesis is part of a joint TCS research project named Neutrons for Heat Storage (NHS), involving three research institutes. The project is funded by Nordforsk and KTH Royal Institute of Technology. KTH´s objective in the NHS project is to design, build and operate a bench-scale TCS system using strontium chloride (SrCl2) and ammonia (NH3) as a solid-gas reaction system for low-temperature heat storage (40-80 ℃). Here, absorption of NH3 into SrCl2⋅NH3 (monoammine) to form SrCl2⋅8NH3 (octaammine) is used for heat release, and desorption (of NH3 from SrCl2⋅8NH3 to form SrCl2⋅NH3) for heat storage. Prior to this thesis project, this TCS system, as well as its reactor+heat exchanger (R-HEX) units, were numerically designed at KTH, and the R-HEX units were manufactured. This system is now being built at the laboratory of Applied Thermodynamics and Refrigeration division at the Department of Energy Technology, KTH. The initial system is comprised of a shared storage tank, expansion valve, ammonia meter and an R-HEX (absorption path); and an R-HEX, ammonia meter, gas cooler, compressor, condenser, and the storage tank (desorption path), to accommodate absorption, desorption, and NH3 storage. This thesis was originally planned to include commissioning, operation and experimental data acquisition, and performance evaluation of this system. However, due to various delays and shortcomings discovered at the beginning of the project, its objectives were then redefined to review the system and its components and propose necessary design adaptations of the initially designed (and partially built) system. This thesis project was partly a joint project, where Harish Seetharaman performed various tasks in the overarching NHS project as part of his own thesis project, performed alongside the work described in this report. For various information and results, referring to Harish´s report therefore will be necessary. A literature review of the research into SrCl2-NH3 systems was conducted, with emphasis on performance evaluation, kinetics, and reaction paths. TES performance evaluation is discussed concerning the TCS key performance indicators, with the 2018 IEA's Annex 30 as a guideline and 2013 IRENA´s E17 technology brief as a comparative reference. Much progress and refinement has been made in the 5-year span between the publications of these documents, but some adaptations and interpretations still need to be made to the Annex 30 approach for a good approach to a TCS system of similar nature as the one studied in this report. Review of the latest research on the kinetics and reaction path of the SrCl2-NH3 reaction pair revealed that the 100-year-old single-line-and-path reaction expression is an oversimplification of the actual chemistry. The reaction path seems to be dependent on the kinetics of the reaction, and varies with heating rate, temperature, and pressure. Various literature was found and compared, which show that the reaction enthalpies and entropies are not settled science. This demonstrates the necessity for further research into the SrCl2-NH3 reaction pair before application-scale product design and commercialization can take place. A comprehensive equipment and system review was conducted, whereby multiple issues were found and addressed, that if gone unnoticed, would have caused difficult setbacks for the project.  Consequently, the previous purchased ammonia flow meters and ammonia compressor, were exchanged for new and better suiting equipment. The original ammonia flow meters were undersized due to miscalculations of converting flow units of NLPH (Normal Liters Per Hour) to the project units of g/s, while wrongly using the density of compressed ammonia to convert to g/s, instead of it at the defined normal conditions. Furthermore, these flow meters were of the wrong type, as they had no digital output for data acquisition. The original compressor was also severely undersized, only capable of evacuating 7-14% of the expected maximum desorption flow. This was due to a similar miscalculation during conversion of NLMP (Normal Liters Per Minute) to g/s, as well as an unrequested compressor stroke reduction. New solutions and additional equipment were then required to accommodate the operational limitations discovered in the final chosen equipment and system configuration. These include limiting the compressor inlet pressure to a maximum of 1.1 bar(a); avoiding risk of NH3 condensation at them inlets of the new mass flow meters and compressor; and maintaining the flow meter and compressor inlet temperatures below 40 °C. The pressure limitations required considerable design adaptations. Firstly, an ammonia by-pass is introduced to keep feeding ammonia into the compressor during low desorption flows. The inlet pressure limitation necessitated active pressure management in the form of pressure reduction valves, which were thus introduced. Secondly, the condensation regulation and temperature management required a new approach, as the cooling and condensation temperatures in the original design were too low, causing risks of far too low temperature and pressure in the desorption path, as well as counter-acting simultaneous heating and cooling between the condenser and the storage tank heating sleeve. As a solution, a shunt pump is proposed, where constant cooling water temperature provides condensation on a tight temperature range using an infinite cold wall approach. Along with reviewing the equipment and the system design, new procedures concerning investigating and confirming homogeneous heat transfer properties of the reactors are proposed. Furthermore, improvements are suggested concerning the commissioning of the experimental rig, that include equipment testing with N2-gas and stepwise changes in temperature in sequential cycles to gain a good understanding of the likely behaviors of the system before it is run at the extremes of the operating range. In conclusion, a new and improved process flow diagram, showing all these adaptations, additions, and changes from the original diagram is presented herein as the final key contribution to the overarching NHS-project. This is complemented with an instruction manual, to allow the next researchers a smooth continuation, in terms of the system build, and later commissioning and operation. Finally, some suitable next steps in the project are suggested. These include a conceptualization of descriptive functions for the performance and behavior of the specific system and reactors. These functions are proposed with temperature and pressure as independent variables, as these are two main variables influencing the kinetics of the reaction in the given system. As no experimental data exists yet, the form of the proposed functions is generic. Furthermore, a suggestion is made for a future adaptation for achieving the phase change from NH3(g) to NH3(l) (which is the storage form of ammonia in the system) by deep cooling at the desorption pressure, resulting in only a liquid pump being required to raise the pressure of the NH3(l) in the storage tank.
Termokemisk energilagring (TCS) är en teknik inom termisk energilagring (TES) som används för att lagra termisk energi för senare bruk. TCS kan tillhandahålla värme och kyla från periodvis tillgänglig termisk energi, ofta lågtemperatur spillvärme. Systemet lagrar energin som kemisk energi genom att använda reversibla kemiska reaktioner och massaseparation av reaktions-produkterna. TCS har potential att minska utsläppet av växthusgaser, öka effektiviteten av system i vår infrastruktur, minska energikostnader i samhället och därmed bidra till hållbar utveckling. Detta exjobbsprojekt är en del av ett gemensamt TCS-forskningsprojekt som heter Neutrons for Heat Storage (NHS), där tre forskningsinstitut deltar. Projektet är finansierat av Nordforsk och Kungliga Tekniska Högskolan. KTH:s mål med NHS-projektet är att projektera, bygga, samt driva ett TCSsystem i bänkskala med strontiumklorid (SrCl2) och ammoniak (NH3) som ett fast-gasreaktionssystem för lågtemperaturvärmelagring (40-80 ℃). Här används absorption av NH3 till SrCl2⋅NH3 (monoammin) för att bilda SrCl2⋅8NH3 (oktaammin) för värmeurladdning och desorption (av NH3 från SrCl2⋅NH3 till SrCl2⋅NH3) för värmelagring. Innan detta exjobbsprojekt började hade detta TCS-system, samt systemets reaktor+värmeväxlare (R-HEX) enheter varit numeriskt projekterad vid KTH, och R-HEX-enheterna hade redan tillverkats. Detta system byggs nu på laboratoriet för Avdelningen för tillämpad termodynamik och kylning vid Institutionen för Energiteknik, KTH. Det initiala systemet består av en gemensam lagringstank, expansionsventil, ammoniakmätare, och en R-HEX (systemets absorptionssida) och en R-HEX, ammoniakmätare, gaskylare, kompressor, en kondensor, och en gemensamma lagringstanken (desorptionssidan), for att rymma absorption, desorption (samtidigt) och NH3-lagring. Exjobbsprojektet var ursprungligen planerat att inkludera driftsättning, drift och experimentdatainsamling samt utvärdering av systemet. På grund av olika förseningar och brister som upptäcktes i projektet, omdefinierades projektets mål och består nu av att granska systemet och, samt att föreslå nödvändiga designanpassningar av det ursprungligen konstruerade systemet och dess komponenter. Projektet var delvis ett gemensamt arbete, där Harish Seetharaman utförde olika uppgifter i det övergripande NHS projektet som en del av sitt eget exjobbssprojekt. För olika uppgifter och resultat kommer det därför att vara nödvändigt att hänvisa till Harishs rapport. Litteraturstudié av forskningen kring SrCl2-NH3 system genomfördes, med betoning på prestandautvärdering, kinetik och reaktionsvägar. Prestandautvärdering av TES system diskuteras angående TCS-nyckelindikatorer, med 2018 års IEA:s Annex 30 som riktlinje och IRENA:s E17 Teknologi-sammandrag från 2013 som en referens. Många framsteg och förbättringar har gjorts under femårsperioden mellan dessa publikationer, men vissa anpassningar och tolkningar måste fortfarande härledas till metoderna i Annex 30 för att få ett bra förhållningssätt till ett TCS-system av liknande karaktär som det som studeras i detta projekt. Granskning av den senaste forskningen avseende reaktionskinetik och reaktionsvägar för SrCl2-NH3 reaktionsparet visade att det hundraåriga enkellinje-och-reaktionsväg-formuleringen är en förenkling av den faktiska kemin. Reaktionsvägen verkar beroende av reaktionens kinetik och varierar med uppvärmnings-takten, temperaturen och även trycket. Olika litteratur jämfördes som visar att reaktionsentalpierna och entropierna inte är fastställd vetenskap. Detta visar behovet av ytterligare forskning avseende SrCl2-NH3 innan produktdesign och kommersialisering i applikations-skala kan utföras. En omfattande granskning av systemet och dess komponenter genomfördes, där flera problem hittades och åtgärdades. Om dessa problem hade gått obemärkt förbi skulle det ha orsakat svåra bakslag för projektet. Följaktligen byttes de tidigare köpta ammoniakflödesmätarna ut till nya och en ammoniakkompressor byttes ut mot en ny, för tillämpningen bättre anpassad. De ursprungliga ammoniak-flödesmätarna var underdimensionerade pga. felberäkningar i omvandling av flödesenheter för NLPH (normal liter per timme) till projektenheterna g/s. Samtidigt var densiteten av komprimerad ammoniak felaktigt använt för omvandling till g/s, istället för densiteten vid de definierade normala förhållandena; 1 bar (a) och 20 ° C. Dessutom var dessa flödesmätare av fel typ, eftersom de inte hade någon digital utgång för datainsamling. Den ursprungliga kompressorn var också kraftigt underdimensionerad, endast kapabel att evakuera 7-14% av det förväntade maximala desorptionsflödet. Detta berodde på en liknande felberäkning vid konvertering av NLPM (normal liter per minute) till g/s, samt en oönskad kompressorslagsminskning. Nya lösningar och ytterligare utrustning krävdes för att tillgodose de operativa begränsningar som upptäcktes i den slutgiltigt valda utrustningen och systemutformningen. Dessa inkluderar: begränsa kompressorns inloppstryck till maximalt 1,1 bar(a); undvika risk för NH3 kondens i de nya massflödesmätarna och kompressorn; samt bibehålla flödesmätarens och kompressorns inloppstemperaturer under 40 °C. Tryckbegränsningarna krävde omfattande projekteringsanpassningar. För det första införs en ammoniak-by-pass för att fortsätta mata ammoniak till kompressorn under låga desorptionsflöden. Inloppstrycksbegränsningen nödvändiggjorde aktiv tryckhantering i form av tryckreduceringsventiler. För det andra krävde kondensregleringen och temperaturhanteringen en ny strategi, eftersom kyl- och kondenseringstemperaturerna i den ursprungliga utformningen var för låga. Detta orsakade risker för alldeles för låg temperatur och tryck på desorptionssidan, samt samtidigt motverkande uppvärmning och kylning av kondensorn och förvaringstankens värmehylsa. Som en lösning föreslås en shunt där konstant kylvattentemperatur ger kondens i ett tätt temperaturintervall med en oändlig kallväggsinriktning. Tillsammans med granskning av utrustningen och systemutformningen föreslås nya tillvägagångssätt för undersökning och bekräftelse av reaktorers förmodade homogena värmeöverförings-egenskaper. Dessutom föreslås förbättringar av idrifttagningen av den experimentella riggen, som inkluderar utrustningstestning med N2-gas och stegvisa temperaturförändringar i sekventiella körningar för att få en god förståelse för systemets troliga beteenden innan det körs i ytterligheterna av systemts arbetsområde. Sammanfattningsvis presenteras ett nytt och förbättrat processflödesdiagram, som visar alla utförda anpassningar, tillägg och ändringar från det ursprungliga diagrammet, som är avhandlingsprojektets huvudbidrag till det övergripande NHS-projektet. Detta kompletteras med en bruksanvisning för att smidigt fasa in kommande forskare avseende systemets konstruktion, driftsättning, och drift. Slutligen föreslås några lämpliga kommande steg i projektet. Dessa inkluderar en konceptualisering av beskrivande funktioner för prestanda och beteende av det specifika systemet och reaktorer. Dessa funktioner föreslås med temperatur och tryck som oberoende variabler, eftersom dessa är två huvudvariabler som påverkar reaktionens kinetik. Eftersom inga experimentella data ännu finns, är formen för de föreslagna funktionerna generisk. Vidare ges förslag om framtida anpassning för att uppnå fasändringen från NH3(g) till NH3(v) (som är lagringsformen för NH3 i systemet) genom djup nedkylning vid desorptionstrycket, vilket resulterar i att endast en vätskepump krävs för att höja trycket för NH3(v) i lagringstanken.

Manganese oxide catalysts supported on Al2O3, ZrO2, TiO2 and SiO2 supports were used to study the effect of support on ozone decomposition kinetics. X-ray diffraction (XRD), in-situ laser Raman spectroscopy, temperature programmed oxygen desorption, surface area measurements, extended and near edge x-ray absorption fine structure (EXAFS and NEXAFS) showed that the manganese oxide was highly dispersed on the surface of the supports. EXAFS spectra suggest that the manganese active centers on all of the surfaces were surrounded by five oxygen atoms. These metal centers were of a mononuclear type for the Al2O3 supported catalyst and multinuclear for the other supports. NEXAFS spectra for the catalysts showed a chemical shift to lower energy and an intensity change in the L-edge features which followed the trend Al2O3 > ZrO2 > TiO2 > SiO2. The trends provided insights into the positive role of available empty electronic states required in the reduction step of a redox reaction. The catalysts were tested for their ozone decomposition reactivity and reaction rates had a fractional order dependency (n < 1) with ozone partial pressure. The apparent activation energies for the reaction was low (3-15 kJ/mol). The support influenced the desorption step (a reduction step) and this effect manifested itself in the pre-exponential factor of the rate constant for desorption. Trends for this pre-exponential factor correlated with trends in NEXAFS features and reflected the ease of electron donation from the adsorbed species to the active center.
Ph. D.

Nitrogen-alloyed austenitic stainless steels are becoming increasingly popular, mainly due to their excellent combination of strength and toughness. Nitrogen desorption to the atmosphere during the autogenous welding of these steels is often a major problem, resulting in porosity and nitrogen losses from the weld. In order to counteract this problem, the addition of nitrogen to the shielding gas has been proposed. This study deals with the absorption and desorption of nitrogen during the autogenous arc welding of a number of experimental stainless steels. These steels are similar in composition to type 310 stainless steel, but with varying levels of nitrogen and sulphur. The project investigated the influence of the base metal nitrogen content, the nitrogen partial pressure in the shielding gas and the weld surface active element concentration on the nitrogen content of autogenous welds. The results confirm that Sievert's law is not obeyed during welding. The weld nitrogen content increases with an increase in the shielding gas nitrogen content at low nitrogen partial pressures, but at higher partial pressures a dynamic equilibrium is created where the amount of nitrogen absorbed by the weld metal is balanced by the amount of nitrogen evolved from the weld pool. In alloys with low sulphur contents, this steady-state nitrogen content is not influenced to any significant extent by the base metal nitrogen content, but in high sulphur alloys, an increase in the initial nitrogen concentration results in higher weld nitrogen contents over the entire range of nitrogen partial pressures evaluated. A kinetic model can be used to describe nitrogen absorption and desorption during welding. The nitrogen desorption rate constant decreases with an increase in the sulphur concentration. This is consistent with a site blockage model, where surface active elements occupy a fraction of the available surface sites. The absorption rate constant is, however, not a strong function of the surface active element concentration. Alloys with higher base metal nitrogen contents require increased levels of supersaturation prior to the onset of nitrogen evolution as bubbles. These increased levels of supersaturation for the higher-nitrogen alloys is probably related to the higher rate of nitrogen removal as N2 the onset of bubble formation. Given that nitrogen bubble formation and detachment require nucleation and growth, it is assumed that a higher nitrogen removal rate would require a higher degree of supersaturation. Nitrogen losses from nitrogen-alloyed stainless steels can be expected during welding in pure argon shielding gas. Small amounts of nitrogen can be added to the shielding gas to counteract this effect, but this should be done with care to avoid bubble formation. Supersaturation before bubble formation does, however, extend the range of shielding gas compositions which can be used. Due to the lower desorption rates associated with higher surface active element concentrations, these elements have a beneficial influence during the welding of high nitrogen stainless steels. Although higher sulphur contents may not be viable in practice, small amounts of oxygen added to the shielding gas during welding will have a similar effect.
Dissertation (PHD)--University of Pretoria, 2004.
Materials Science and Metallurgical Engineering
unrestricted

Oil leakage during the exploration, production, and transportation of crude oil is a significant issue worldwide because crude oil spills severely impact the physical and chemical properties of the surrounding soil. A range of remediation methods for oil-contaminated soil is recommended, consisting of sand washing, bioremediation, electro-kinetic sand remediation, and thermal desorption; however, none are cost-effective. To find a suitable alternative remediation method, oil-contaminated sand utilisation in construction was considered. Several researchers found that oil contamination generally has an adverse effect on the mechanical properties of sand, but certain levels of contamination have beneficial effects on some of the important properties of the sand and its produced concrete. This chapter reviews the main sources of oil contamination and the existing remediation methods for this waste material. It analyses the different factors that affect the properties of oil-contaminated sand and concrete, including the type of crude oil and permeability of sand, like its properties, absorption, chemical composition, and spillage quantity. Furthermore, the intensive evaluation results of light crude oil effects on the geotechnical properties of fine sand, cement mortar and concrete were presented. Potential applications for oil-contaminated sand were also identified for the re-use of this material in engineering and construction.

Abstract Betavoltaics are direct conversion energy devices that are ideal for low, micropower and long-lasting, uninterruptable applications. Betavoltaics operate similarly to photovoltaics where a radioisotope irradiates beta particles into a semiconductor p-n junction that converts the kinetic energy into electrical energy. Betavoltaics are limited by their power output from the radioiso-tope. The source density can be increased by the selection of solid-state substrates. While solid-state substrates can be selected from simulations, the viability of the substrate to absorb tritium has to evaluated. The development of a hydrogen loading system was performed to evaluate different film types to understand how they perform during the hydrogen/tritium loading process. The hydrogen loading system utilizes the Sievert method, where the initial pressure and volume is constant and pressure drop in the system is used to determine hydrogen uptake of a film substrate. The procedures of the hydrogen loading system are detailed. To test the procedures of the hydrogen loading system, old, palladium films were loaded. Results show uptake of hydrogen by the thin palladium films, as well as cycles of hydrogen absorption and desorption. Hydrogen loading of palladium was compared to a prior result and was shown to have similar results.

3D dynamic models are developed for polymer electrolyte fuel cells (PEFCs) and hydrogen tanks, respectively. In the fuel cell model, we consider the major transport and electrochemical processes within the key components of a single PEFC that govern fuel cell transient including the electrochemical double-layer behavior, mass/heat transport, liquid water dynamics, and membrane water uptake. As to modeling hydrogen tanks, we consider a LaNi5-based system and develop a general formula that describes hydrogen absorption/desorption. The model couples the hydride reaction kinetics and mass/heat transport. The dynamic characteristics of the PEFC and hydrogen tank, together with the possible coupling of the two systems, are discussed.

Increased research into the chemistry, physics and material science of hydrogen cycling compounds has led to the rapid growth of solid-phase hydrogen-storage options. The operating conditions of these new options span a wide range: system temperature can be as low as 70K or over 600K, system pressure varies from less than 100kPa to 35MPa, and heat loads can be moderate or can be measured in megawatts. While the intense focus placed on storage materials has been appropriate, there is also a need for research in engineering, specifically in containment, heat transfer, and controls. The DOE’s recently proposed engineering center of expertise underscores the growing understanding that engineering research will play a role in the success of advanced hydrogen storage systems. Engineering a hydrogen system will minimally require containment of the storage media and control of the hydrogenation and dehydrogenation processes, but an elegant system design will compensate for the storage media’s weaker aspects and capitalize on its strengths. To achieve such a complete solution, the storage tank must be designed to work with the media, the vehicle packaging, the power-plant, and the power-plant’s control system. In some cases there are synergies available that increase the efficiency of both subsystems simultaneously. In addition, system designers will need to make the hard choices needed to convert a technically feasible concept into a commercially successful product. Materials cost, assembly cost, and end of life costs will all shape the final design of a viable hydrogen storage system. Once again there is a critical role for engineering research, in this case into lower cost and higher performance engineering materials. Each form of hydrogen storage has its own, unique, challenges and opportunities for the system designer. These differing requirements stem directly from the properties of the storage media. Aside from physical containment of compressed or liquefied hydrogen, most storage media can be assigned to one of four major categories, chemical storage, metal hydrides, complex hydrides, or physisorption. Specific needs of each technology are discussed below. Physisorption systems currently operate at 77K with very fast kinetics and good gravimetric capacity; and as such, special engineering challenges center on controlling heat transfer. Excellent MLVSI is available, its cost is high and it is not readily applied to complex shape in a mass manufacture setting. Additionally, while the heat of adsorption on most physisorbents is a relatively modest 6–10kJ/mol H2, this heat must be moved up a 200K gradient. Physisorpion systems are also challenged on density. Consequently, methods for reducing the cost of producing and assembling compact, high-quality insulation, tank design to minimize heat transfer while maintaining manufacturability, improved methods of heat transfer to and from the storage media, and controls to optimize filling are areas of profitable research. It may be noted that the first two areas would also contribute to improvement of liquid hydrogen tanks. Metal hydrides are currently nearest application in the form of high pressure metal hydride tanks because of their reduced volume relative to compressed gas tanks of the same capacity and pressure. These systems typically use simple pressure controls, and have enthalpies of roughly 20kJ/mol H2 and plateau pressures of at most a few MPa. During filling, temperatures must be high enough to ensure fast kinetics, but kept low enough that the thermodynamically set plateau pressure is well below the filling pressure. To accomplish this balance the heat transfer system must handle on the order of 300kW during the 5 minute fill of a 10kg tank. These systems are also challenged on mass and the cost of the media. High value areas for research include: heat transfer inside a 35MPa rated pressure vessel, light and strong tank construction materials with reduced cost, and metals or other materials that do not embrittle in the presence of high pressure hydrogen when operated below ∼400K. The latter two topics would also have a beneficial impact on compressed gas hydrogen storage systems, the current “system to beat”. Complex hydrides frequently have high hydrogen capacity but also an enthalpy of adsorption >30kJ/mol H2, a hydrogen release temperature >370K, and in many cases multiple steps of adsorption/desorption with slow kinetics in at least one of the steps. Most complex hydrides are thermal insulators in the hydrided form. From an engineering perspective, improved methods and designs for cost effective heat transfer to the storage media in a 5 to 10MPa vessel is of significant interest, as are materials that resist embrittlement at pressures below 10MPa and temperatures below 500K. Chemical hydrides produce heat when releasing hydrogen; in some systems this can be managed with air cooling of the reactor, but in other systems that may not be possible. In general, chemical hydrides must be removed from the vehicle and regenerated off-board. They are challenged on durability and recycling energy. Engineering research of interest in these systems centers around maintaining the spent fuel in a state suitable for rapid removal while minimizing system mass, and on developing highly efficient recycling plant designs that make the most of heat from exothermic steps. While the designs of each category of storage tank will differ with the material properties, two common engineering research thrusts stand out, heat transfer and structural materials. In addition, control strategies are important to all advanced storage systems, though they will vary significantly from system to system. Chemical systems need controls primarily to match hydrogen supply to power-plant demand, including shut down. High pressure metal hydride systems will need control during filling to maintain an appropriately low plateau pressure. Complex hydrides will need control for optimal filling and release of hydrogen from materials with multi-step reactions. Even the relatively simple compressed-gas tanks require control strategies during refill. Heat transfer systems will modulate performance and directly impact cost. While issues such as thermal conductivity may not be as great as anticipated, the heat transfer system still impacts gravimetric efficiency, volumetric efficiency and cost. These are three key factors to commercial viability, so any research that improves performance or reduces cost is important. Recent work in the DOE FreedomCAR program indicates that some 14% of the system mass may be attributed to heat transfer in complex hydride systems. If this system is made to withstand 100 bar at 450K the material cost will be a meaningful portion of the total tank cost. Improvements to the basic shell and tube structures that can reduce the total mass of heat transfer equipment while maintaining good global and local temperature control are needed. Reducing the mass and cost of the materials of construction would also benefit all systems. Much has been made of the need to reduce the cost of carbon fiber in compressed tanks and new processes are being investigated. Further progress is likely to benefit any composite tank, not just compressed gas tanks. In a like fashion, all tanks have metal parts. Today those parts are made from expensive alloys, such as A286. If other structural materials could be proven suitable for tank construction there would be a direct cost benefit to all tank systems. Finally there is a need to match the system to the storage material and the power-plant. Recent work has shown there are strong effects of material properties on system performance, not only because of the material, but also because the material properties drive the tank design to be more or less efficient. Filling of a hydride tank provides an excellent example. A five minute or less fill time is desirable. Hydrogen will be supplied as a gas, perhaps at a fixed pressure and temperature. The kinetics of the hydride will dictate how fast hydrogen can be absorbed, and the thermodynamics will determine if hydrogen can be absorbed at all; both properties are temperature dependent. The temperature will depend on how fast heat is generated by absorption and how fast heat can be added or removed by the system. If the design system and material properties are not both well suited to this filling scenario the actual amount of hydrogen stored could be significantly less than the capacity of the system. Controls may play an important role as well, by altering the coolant temperature and flow, and the gas temperature and pressure, a better fill is likely. Similar strategies have already been demonstrated for compressed gas systems. Matching system capabilities to power-plant needs is also important. Supplying the demanded fuel in transients and start up are obvious requirements that both the tank system and material must be design to meet. But there are opportunities too. If the power-plant heat can be used to release hydrogen, then the efficiency of vehicle increases greatly. This efficiency comes not only from preventing hydrogen losses from supplying heat to the media, but also from the power-plant cooling that occurs. To reap this benefit, it will be important to have elegant control strategies that avoid unwanted feedback between the power-plant and the fuel system. Hydrogen fueled vehicles are making tremendous strides, as can be seen by the number and increasing market readiness of vehicles in technology validation programs. Research that improves the effectiveness and reduces the costs of heat transfer systems, tank construction materials, and control systems will play a key role in preparing advanced hydrogen storage systems to be a part of this transportation revolution.