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A closed system is a natural physical system that does not allow transfer of matter in or out of the system, although – in the contexts of physics, chemistry, engineering, etc. – the transfer of energy (e.g. as work or heat) is allowed. Physics In classical mechanics In nonrelativistic classical mechanics, a closed system is a physical system that doesn't exchange any matter with its surroundings, and isn't subject to any net force whose source is external to the system. A closed system in classical mechanics would be equivalent to an isolated system in thermodynamics. Closed systems are often used to limit the factors that can affect the results of a specific problem or experiment. In thermodynamics Thermodynamic system In thermodynamics, a closed system can exchange energy (as heat or work) but not matter, with its surroundings. An isolated system cannot exchange any heat, work, or matter with the surroundings, while an open sys

Autogenous Shrinkage and Hydration Kinetics of SH-UHPFRC under Moderate to Low Temperature Curing Conditions

Mohammadhadi Kazemi Kamyab

Strain Hardening Ultra High Performance Fiber Reinforced Concrete (SH-UHPFRC) were used successfully over the last 8 years in numerous cast in place rehabilitation applications as long lasting waterproofing layers (20 to 30 mm thickness) and, combined with rebar, to increase the structural load bearing capacity of existing bridge decks or building slabs (50 to 70 mm thickness). In composite applications on reinforced concrete substrates, SH-UHPFRC provide a deformation capability (strain hardening) larger than their free shrinkage which makes localized macro cracking at service state very unlikely. On the other hand some specific conditions of microclimatic state of the substrate (moisture and thermal conditions) and climatological (temperature and humidity) of the site such as casting at low temperatures (winter conditions) might increase the autogenous shrinkage and thus eigenstresses of the freshly cast SH-UHPFRC and hinder the development of its tensile strength or/and deformability. Combination of these specific circumstances could compromise the SH-UHPFRC protective functions due to the occurrence of localized macro cracks, making the objective of using SH-UHPFRC as a onetime intervention strategy in rehabilitation application obsolete. Advancement in hydration at early age leads to formation of partially emptied capillary spaces contributing to self-desiccation, decrease of the relative humidity and increase of the capillary depression. Since water is under depression, the solid porous material is under compression forcing the material to shrink. From the aforementioned, it can be deduced that the autogenous shrinkage is a force driven phenomenon: a force is applied on the ageing viscoelastic porous skeleton and the apparent structural response of the bulk cementitious material is the autogenous shrinkage strain. Based on the aforementioned two levels of investigation were considered in this thesis: I. Material level: ageing viscoelastic porous skeleton of hydrating cementitious material II. Structural level: autogenous shrinkage development under the action of its driving forces The objective of this thesis was to study the development of the autogenous shrinkage and its relation with the development of microstructure and mechanical properties in SH-UHPFRC cured at moderate and low temperature conditions (1 to 20°C). Focus was put on: (1) kinetics and magnitude of hydration of silica fume and cement, (2) activation energy of the processes, (3) stiffness (E modulus and Poisson ratio) development, and (4) states of water and pore development. Finally a cross link analysis of information obtained from material and structural level was carried out to better highlight the mechanisms and driving forces of autogenous shrinkage. CM22-TKK, a SH-UHPFRC developed at MCS for rehabilitation applications, on the basis of CEMTECmultiscale® fibrous mixes, was used. Silica fume is one of the major constituents of CM22-TKK (26% by mass of cement). Extent of the silica fume pozzolanic reaction significantly affects the pore size distribution of the UHPFRC matrix which then influences the development of the relative humidity, self-desiccation and finally autogenous deformation. Detailed experimental investigation performed showed that only maximum 8% by mass of cement out of 26% silica fume used in the recipe reacts in moderate and low temperature curing conditions. Finally, a model of silica-fume degree of reaction as a function of curing temperature (5 to 250 °C) was proposed on the basis of a detailed literature survey of existing works with low W/B mixes. The activation energy of the thermo-mechanical properties was evaluated for thermo (cumulative heat of hydration) and mechanical properties development (such as dynamic Elastic modulus, dynamic Poisson’s ratio and compressive strength). A single activation energy Ea of 27.4 KJ/mol could successfully model the activation of all properties. Since maturity was not a proper reference to compare the autogenous deformations at various temperatures, a more fundamental reference such as degree of hydration was used. Experimental investigation was carried out using different techniques such as isothermal calorimetry and 29Si solid NMR to evaluate the development of the degree of hydration of cement and silica fume in UHPFRC at moderate and low temperature curing conditions. The ultimate degree of hydration of cement in CM22-TKK for a closed system was 0.34. There was a close correspondence between the experimental results and the predictions of the volumetric phase distribution models of Waller (0.28) and Jensen (0.32). Finally, the De Schutter et al. 1996 model was used to describe the mechanical properties (Emodulus and Poisson ratio) as function of degree of hydration. Nondestructive and non-invasive 1H-NMR was used to evaluate volumetric phase distribution development at 20°C, especially the classification of the water types (capillary, gel and C-S-H interlayer water) which brought fully new and valuable information about the hydration kinetics and pore size development. The results were also compared with Jensen volumetric phase distribution model to check the advantages and limitation of both approaches. Finally experimental investigation was carried out to follow the development of autogenous shrinkage and eigenstresses (in case of restrained shrinkage) of CM22-TKK cured in isotherm conditions (1 to 20°C). Eigenstresses were obtained and used to highlight the effect of the viscous response of the material at the different temperatures investigated. The previously mentioned results on the kinetics of the silica fume reaction, cement hydration and evolution of states of water and development of stiffness were then put into perspective with the trends observed on the autogenous shrinkage and eigenstresses development.

EPFL2013

Enhanced biofuel production from microalgae by adding CO2 to stimulate lipid biosynthesis (biodiesel) and by hydrothermal processing (syngas)

Gabriela Anca Haiduc, Christian Ludwig, David Pauli, Jean-Paul Schwitzguebel, Frédéric Vogel

Introduction: Microalgae cultures represent an attractive source of biomass for biofuel and chemicals production since they can have higher energy yields per area than conventional biomass crops and can be grown on marginal land using waste, saline, or brackish water. Their use can thus avoid the food-fuel competition for land, resulting in major economic and social benefits. At the present stage however, major developments are needed for making microalgae-to-biofuels processes technologically and economically feasible. One substantial technical challenge is the development of cost-effective processes for efficient conversion of microalgae biomass to biofuels. Our research aims to enhance the production of biodiesel and of bio-Synthetic Natural Gas (SNG) from microalgae. Experimental : Several species of microalgae are grown at laboratory scale to compare the yield of biomass and lipid production. In addition to the potential use of oil for biodiesel production, we are currently working towards demonstrating the technical and economical feasibility of an innovative process, “SunCHem”, for SNG production via hydrothermal (HT) processing of microalgae. The process is envisioned as a closed-cycle with respect to nutrients, water and CO2, that are separated and reused for microalgae growth, resulting in unprecedented energy efficiency for both algae and fuel production. The possible effect of increasing injection of CO2 in the growth medium is under investigation, with the aim to mitigate CO2 and enhance biomass production. Results and Discussion: Before the feasibility of the process can be demonstrated several key challenges stemming from its closed-nature need to be tackled. One major challenge is the recycling of nutrient-rich effluents, which upon continuous operation might become enriched in potential toxicants that in turn affect the growth of algae. We demonstrated that the presence of trace metals (Ni, Cr) in the recycled effluent as a result of reactor wall corrosion and catalyst leaching resulted in adverse effect on the growth of microalgae. The effect was directly proportional to the heavy metal concentration, and for the highest concentrations tested (Ni 25 ppm or Cr 5 ppm) complete inhibition of cell growth was observed. It is known that algae, like many other organisms, can adapt to extreme environments, including those with heavy metal contamination, and resulting mutants or strains should be more suited to survive under those particular conditions. The potential of developing and using metal-tolerant mutant microalgae as a biomass source for the SunCHem process is under investigation. Experiments are undertaken to develop Cr- and Ni- tolerant mutants by selective breeding and then comparing the mutants and the wild-type strains for their sensitivity to the heavy-metal contaminated HT effluent and the resulting biomass yields obtained in each case. It is hoped that the use of the metal-tolerant mutants or strains will contribute to the cost-effectiveness of the process by offering an efficient alternative to the use of the more costly setups otherwise needed for effluent clean-up, thus providing a way to avoid the reduction in the biomass yield upon continuous operation of the SunCHem process. On the other hand, metal-tolerant and accumulating microalgae could be used to treat industrial wastewater. Conclusions: Another aspect of utmost importance is the evaluation of the availability for algae of the nutrients contained in the recycled effluents from the process. To this end, microalgae growth experiments will be performed using as sole nutrient source the salt brine delivered from the HT step. The biomass composition with and without recycled input from the hydrothermal process will be characterized. These tests will provide information on the recycling efficiency and speciation of nutrients and will help to identify potential nutrient management issues. If there is significant loss of a given key nutrient, it will have to be replaced in the bioreactor. A candidate for loss through volatilization could be N. Subsequent studies will explore the role of growth media and especially that of nutrients such as N, P, and minerals. Algae growth experiments will be performed using different synthetic media compositions and varying nutrient levels to assess the tolerance of algae to changes in nutrient level and quality. The issue is of special relevance as the possibility of recycling and reusing all the nutrients, including N and P, is a sine qua non condition for closing the loop in the process.

2010

Storage of Renewable Energy by Reduction of CO2 with Hydrogen

Elsa Callini, Marco Roman Holzer, Jianmei Huang, Shunsuke Kato, Philippe Mauron

The main difference between the past energy economy during the industrialization period which was mainly based on mining of fossil fuels, e.g. coal, oil and methane and the future energy economy based on renewable energy is the requirement for storage of the energy fluxes. Renewable energy, except biomass, appears in time- and location-dependent energy fluxes as heat or electricity upon conversion. Storage and transport of energy requires a high energy density and has to be realized in a closed materials cycle. The hydrogen cycle, i.e. production of hydrogen from water by renewable energy, storage and use of hydrogen in fuel cells, combustion engines or turbines, is a closed cycle. However, the hydrogen density in a storage system is limited to 20 mass% and 150 kg/m(3) which limits the energy density to about half of the energy density in fossil fuels. Introducing CO, into the cycle and storing hydrogen by the reduction of CO, to hydrocarbons allows renewable energy to be converted into synthetic fuels with the same energy density as fossil fuels. The resulting cycle is a closed cycle (CO2 neutral) if CO, is extracted from the atmosphere. Today's technology allows CO, to be reduced either by the Sabatier reaction to methane, by the reversed water gas shift reaction to CO and further reduction of CO by the Fischer Tropsch synthesis (FTS) to hydrocarbons or over methanol to gasoline. The overall process can only be realized on a very large scale, because the large number of by-products of FTS requires the use of a refinery. Therefore, a well-controlled reaction to a specific product is required for the efficient conversion of renewable energy (electricity) into an easy to store liquid hydrocarbon (fuel). In order to realize a closed hydrocarbon cycle the two major challenges are to extract CO, from the atmosphere close to the thermodynamic limit and to reduce CO2 with hydrogen in a controlled reaction to a specific hydrocarbon. Nanomaterials with nanopores and the unique surface structures of metallic clusters offer new opportunities for the production of synthetic fuels.

Schweizerische Chemische Gesellschaft2015