Modélisation des phénomènes de désactivation des catalyseurs à base de cobalt utilisés dans différent réacteurs de synthèse Fischer-Tropsch

La désactivation reste un enjeu important lors de la synthèse Fischer-Tropsch, car il limite la vie des catalyseurs, ainsi que leurs productivités catalytiques. Elle peut être liée à certains mécanismes selon la littérature. Le frittage a été proposé comme la source principale de désactivation initiale, et avec le cokage comme phénomène responsable de la désactivation à long-terme dans ce travail. Le but de cette thèse est de développer les modèles mécanistiques capables de prédire le changement d’activité catalytique des catalyseurs FT à base de cobalt avec le temps. Dans la première étape, le changement des propriétés physico-chimiques des particules avec le temps est considéré. Un modèle de frittage est développé, qui inclut l’effet d’accélération de l’eau par formation d’une couche d’oxyde de cobalt à la surface. Ce mécanisme nous permet de lier l’agglomération des particules à certaines conditions opératoires, notamment le rapport molaire de H2O/H2. Nous avons aussi développé un mécanisme pour l’empoisonnement des sites catalytiques par dépôt de carbone pour la désactivation à long-terme. Ce mécanisme permet d’évaluer le changement de fraction des sites libres avec le temps, ainsi que les fractions molaires de CO, H2, et H2O.Ces deux modèles microscopiques sont ensuite intégrés dans les modèles des réacteurs à lit fixe et slurry pour coupler les propriétés des catalyseurs et l’activité catalytique. L’effet des conditions opératoires sur la taille des cristallites, la fraction des sites actifs et la conversion sont considérés. Les modèles sont ensuite employés dans les réacteurs de laboratoire pour s’accorder avec les résultats expérimentaux.Catalyst deactivation remains a major challenge in Fischer-Tropsch synthesis; as it reduces the catalyst lifetime as well as its productivity. Deactivation can be attributed to certain mechanisms according to the literature. Sintering is proposed in this work to be responsible for the initial deactivation whereas coking is suggested to be the main cause of long-term deactivation. The final objective of this thesis is to develop the mechanistic models which could predict the extent of catalyst deactivation with time. In the first step, the change in the catalyst physico-chemical properties with time on stream is considered. A three-step sintering model is proposed which involves the effect of water acceleration through the formation of surface cobalt oxide layer. This mechanism allows correlating the crystallites growth with certain operating conditions especially the H2O/H2 molar ratio inside the reactor. We have also developed a mechanism for the active site poisoning by carbon deposition for the long-term deactivation. This mechanism helps to evaluate the change in the active sites coverage with time as well the CO, H2, and H2O mole fractions. The two microscopic models are then integrated in the reactor models in order to correlate the change in the catalytic activity with the catalyst properties. We have developed the models dedicated to fixed bed and slurry reactors. The effect of operating conditions on the crystallite size, active sites fraction, and conversion is considered by the simulations. The models are then employed in the laboratory scale reactors to fit the experimental data and to optimize the deactivation constants

Combined pinch and exergy analysis of an ethylene oxide production process to boost energy efficiency toward environmental sustainability

Ethylene oxide production process is one of the highest energy consumers in chemical industry, and therefore even a slight improvement in its overall efficiency can have a significant impact on the sustainability of the process. Efficiency improvement can be carried out using the exergy-aided pinch analysis outlined in this paper. The overall exergy loss distribution in different unit operations of an ethylene oxide process was first evaluated and mapped out in the form of "visualized exergetic process flowsheet". An initial analysis of the four main functional blocks of the process showed that the exothermic reaction block contained the largest exergy loss (6043 and 428 kJ/kg of internal and external losses, respectively) which can be reduced by isothermal mixing, as well as increasing reaction temperature and reduction in pressure drop. The absorption block was also estimated to have the second highest contribution with total exergy losses of 3640 kJ/kg which were mainly due to the cooling column. These losses were then recommended to be reduced by improvements in the concentration and temperature gradients along the tower. Following the block-wise analysis, exergy analysis was then carried out for individual unit operations in each block to pinpoint the main sources of thermal exergetic inefficiency. Thermal solutions to reduce losses were also proposed in accordance with the identified sources of inefficiency, leading to a comprehensive list of cold and hot process streams that could be introduced to reduce losses. Finally, pinch analysis was brought into action to estimate the minimum energy requirements, to select utilities, and to design heat exchanger network. Thus, the methodology used in this work took advantage of both exergy and pinch analyses. The combined thermal-exergy-based pinch approach helped to set energy targets so that all the thermal possible solutions supported by exergy analysis were considered, preventing exclusion of any hot or cold process stream with high potential for heat integration during pinch analysis. Results indicated that the minimum cold utility requirement could be reduced from 601.64 MW (obtained via conventional pinch analysis) to 577.82 MW through screening of streams by the combined methodology

Exergy aided pinch analysis to enhance energy integration towards environmental sustainability in a chlorine-caustic soda production process

This paper presents a case study on the improvement of energy integration in a chlorine-caustic soda process based on the main sources of thermal exergy losses. Exergy analysis has been performed to pinpoint the causes of thermal exergetic inefficiency. Thermal solutions have been then developed, leading to a comprehensive list of cold and hot process streams. Finally, pinch analysis has been brought into action to estimate the minimum energy requirement, to select utilities and to design heat exchanger network. As a result, the combined methodology followed here takes advantages of both exergy and pinch analyses. This bilateral thermal-exergy-based pinch approach helps to set energy targets in a way that all the possible thermal solutions supported by exergy analysis are considered, including all hot and cold process streams that have a high potential for heat integration during pinch analysis. To demonstrate this, energy targeting through conventional pinch analysis leads to 7.74 MW and 13.00 MW of hot and cold utility energy demand, respectively. These figures change to 8.17 MW and 0.40 MW of hot and cold utility energy demand, respectively through streams screening by the combined methodology. (C) 2017 Elsevier Ltd. All rights reserved

Diagnosis of an alternative ammonia process technology to reduce exergy losses

Ammonia production through more efficient technologies can be achieved using exergy analysis. Ammonia production is one of the most important but also one of most energy consuming processes in the chemical industry. Based on a panel of solutions previously developed, this study helps to identify potential areas of improvement using an exergy analysis that covers all aspects of conventional ammonia synthesis and separation. The total internal and external exergy losses are calculated as 3,152 and 6,364 kJ/kg, respectively. The process is then divided into five main functional blocks based on their exergy losses. The reforming block contains the largest exergy loss (3,098 kJ/kg) and thus the largest potential for improvement including preheating cold feed through an economizer, developing technology towards isobaric mixing, and pressure drop reduction in the secondary reformer as the main contributors to the irreversibility (1,302 kJ/kg) in this block. The second largest exergy loss resides in the ammonia synthesis block (3,075 kJ/kg) where solutions such as reduced temperature rise across the compressor, proper compressor isolation, reducing undesired components such as argon in the reactor feed, and using lower temperatures for reactor outlet streams, are proposed to decrease the exergy losses. Throttling process in the syngas separator is the key contributing mechanism for the irreversibility (1,635 kJ/kg exergy losses) in the gas upgrading block. The exergy losses in the residual ammonia removal block (833 kJ/kg exergy losses) are mainly due to the stripper and the absorber column where a modified column design might be helpful. The highest exergy loss in the preheating block belongs to the compressors (518 kJ/kg exergy losses) where a lower inlet temperature and better system isolation could help to reduce losses

Cleaner production of purified terephthalic and isophthalic acids through exergy analysis

The purified terephthalic and isophthalic acids production process was improved through the exergy analysis approach demonstrated in this work. The overall exergy losses and low-exergy-efficient units were first identified and presented using visualised exergetic flowsheets. Recommendations were then proposed to reduce losses based on the main cause(s) of irreversibility. Three out of the five constituent blocks contained the highest exergy losses. The oxidation block was the main player where it was suggested that using several reactors in series with gradually decreasing temperatures could lower losses. The product refining block had the second-largest irreversibilities, where improving coolers' performances were recommended. The crude terephthalic acid crystallisation block was the third-largest loss producer, where isothermal and isobaric mixing in the solvent dehydrator was suggested to reduce losses. The approach used in this work can be adapted to improve the energy footprint of other chemical processes

Exergy analysis as a scoping tool for cleaner production of chemicals:a case study of an ethylene production process

High energy consumption is one of the main challenges of the chemical industry. The energy footprint of most processes can, however, diminish through the solutions presented in the current paper, leading to cleaner ways of producing chemicals. Ethylene production process is selected as a case study to demonstrate the approach as it is one of the most energy consuming processes in the chemical industry. The study involves an exergetic diagnosis and does not only find the low-exergy-efficient unit operations, but also proposes tools to improve these units based on their key sources of irreversibility. For the ethylene production process, this is conducted by first splitting the flowsheet into four main functional blocks (namely cracking, compression, refrigeration, and separation and purification) according to their exergy losses. This results in identifying the cracking block as the most inefficient block with more than 45% of total exergy losses and thus the first block to be improved so that overall losses reduce (examples of which include increasing the number of furnace tubes while reducing their lengths). Although the compression block is-found to have the lowest contribution to internal exergy losses, the inefficient unit operations such as the water cooler (with an exergy loss of 214 4J/kg) can still be improved through solutions such as system isolation. The refrigeration block is also shown to have the second highest exergy losses with its ethylene and propylene compressors being the main contributors. Solutions are again provided to improve the block performance with specific focus on intercooler design improvement and system isolation. Finally, exergy losses in the purification and separation block are identified to be mainly due to demethanator, deethanator, and ethylene column where modifications in column design might be helpful as concentration and temperature gradients along the towers are the main sources of exergy losses. The approach used in the current study can also be applied to other chemical processes and the findings suggest that even for a well-developed process technology, there is still opportunity for thermodynamically justifiable energy efficiency improvements. Therefore, it is important for process developers to continuously revisit existing processes, in order to ensure lessons learned in one area can be applied to another one. Using a panel of solutions, which has been constructed from a number of previous case studies helps to make this approach more systematic and user-friendly. (C) 2016 Elsevier Ltd. All rights reserved

Modélisation des phénomènes de désactivation des catalyseurs à base de cobalt utilisés dans différent réacteurs de synthèse Fischer-Tropsch

La désactivation reste un enjeu important lors de la synthèse Fischer-Tropsch, car il limite la vie des catalyseurs, ainsi que leurs productivités catalytiques. Elle peut être liée à certains mécanismes selon la littérature. Le frittage a été proposé comme la source principale de désactivation initiale, et avec le cokage comme phénomène responsable de la désactivation à long-terme dans ce travail. Le but de cette thèse est de développer les modèles mécanistiques capables de prédire le changement d activité catalytique des catalyseurs FT à base de cobalt avec le temps. Dans la première étape, le changement des propriétés physico-chimiques des particules avec le temps est considéré. Un modèle de frittage est développé, qui inclut l effet d accélération de l eau par formation d une couche d oxyde de cobalt à la surface. Ce mécanisme nous permet de lier l agglomération des particules à certaines conditions opératoires, notamment le rapport molaire de H2O/H2. Nous avons aussi développé un mécanisme pour l empoisonnement des sites catalytiques par dépôt de carbone pour la désactivation à long-terme. Ce mécanisme permet d évaluer le changement de fraction des sites libres avec le temps, ainsi que les fractions molaires de CO, H2, et H2O.Ces deux modèles microscopiques sont ensuite intégrés dans les modèles des réacteurs à lit fixe et slurry pour coupler les propriétés des catalyseurs et l activité catalytique. L effet des conditions opératoires sur la taille des cristallites, la fraction des sites actifs et la conversion sont considérés. Les modèles sont ensuite employés dans les réacteurs de laboratoire pour s accorder avec les résultats expérimentaux.Catalyst deactivation remains a major challenge in Fischer-Tropsch synthesis; as it reduces the catalyst lifetime as well as its productivity. Deactivation can be attributed to certain mechanisms according to the literature. Sintering is proposed in this work to be responsible for the initial deactivation whereas coking is suggested to be the main cause of long-term deactivation. The final objective of this thesis is to develop the mechanistic models which could predict the extent of catalyst deactivation with time. In the first step, the change in the catalyst physico-chemical properties with time on stream is considered. A three-step sintering model is proposed which involves the effect of water acceleration through the formation of surface cobalt oxide layer. This mechanism allows correlating the crystallites growth with certain operating conditions especially the H2O/H2 molar ratio inside the reactor. We have also developed a mechanism for the active site poisoning by carbon deposition for the long-term deactivation. This mechanism helps to evaluate the change in the active sites coverage with time as well the CO, H2, and H2O mole fractions. The two microscopic models are then integrated in the reactor models in order to correlate the change in the catalytic activity with the catalyst properties. We have developed the models dedicated to fixed bed and slurry reactors. The effect of operating conditions on the crystallite size, active sites fraction, and conversion is considered by the simulations. The models are then employed in the laboratory scale reactors to fit the experimental data and to optimize the deactivation constants.LILLE1-Bib. Electronique (590099901) / SudocSudocFranceF

Performance Comparison of Three Common Proton Exchange Membranes for Sustainable Bioenergy Production in Microbial Fuel Cell

AbstractProton exchange membranes (PEMs) have essential role in the performance of microbial fuel cells (MFCs). They act as a separator and separate anode and cathode compartments and they also transfer protons between anode and cathode. In this study three types of PEMs (Nafion 112, SPEEK and Nafion 117) have been applied to MFC and the amount of produced bioenergy with the feed of a wastewater in 5000 m/l chemical oxygen demand (COD) have been reported. It has been observed that the MFC working with Nafion 117 as separator produced the highest power among the other MFCs. Also It was found that the produced power was 179.7 mW/m2 for Nafion 117 while it was 126.1 for SPEEK and 19.7 for Nafion 112. Moreover it has been concluded that the low power production of Nafion 112 was due to the diffusion of oxygen from the cathode chamber to the anode chamber that disturb the microorganism's metabolism for degradation of organic compounds. Generally we have found a new economic PEM for using in MFCs

Mass transfer limitation in different anode electrode surface areas on the performance of dual chamber Microbial Fuel Cell.

In this study, the effect of different electrode surface areas on the performance of dual chamber Microbial Fuel Cells (MFC) was investigated. Four different electrodes with 12, 16, 20 and 24 cm2 surface areas were tested in an MFC system. The 20 cm2 electrode generated an output power of 76.5 mW/m2 was found to be the highest among all the electrodes tested. This might be due to better interactions with microorganism and less mass transfer limitation. In addition, this indicates that the chances for attachment of bacteria and generation of electricity in larger electrode surface areas might be limited by mass transport and by higher surface area. The output power generation was then followed by the 16, 12 and 24 cm2 electrodes which generated 69.6, 64.7 and 61.25 mW/m2 electricity, respectively