1,721,003 research outputs found
DFT Study of Copper-Nickel (111) Catalyst for Methane Dry Reforming
Get PDFNo one can deny that the increasing energy demand -due to world population booming- and climate change are two major challenges facing humanity in the current century. Climate change phenomenon is basically related to green house gases (GHGs) emissions which result in increasing temperature of earth. Among GHGs, COV2 is the major contributor in global warming while CHV4 is considered a major energy source as the main component of natural gas. Dry reforming of methane (DRM) achieves utilization of both COV2 with CHV4 by producing syngas which can be converted into valuable compounds. Thus, DRM is a currently a hot subject in both industrial catalysis and environmental research. The applicability of DRM in industry is hindered by its high energy demand and coke formation on catalyst surface which leads to rapid catalyst deactivation. Nickel catalyst is well-known for an activity comparable to those of the expensive and abundant noble metals. However, pure Ni catalyst can suffer from severe coke formation at the elevated temperatures required for DRM reaction. To reduce coke formation, Nibimetallic catalysts are examined as they have shown reasonable activity and reduced carbon deposition. Several nickel-transition metals bimetallic catalysts showed their potential for coke resistance and improved activity. While the synergetic effects of CoNi-bimetallic catalyst is found to be due to its oxophilicty, Ni-Fe catalyst activity is attributed to redox system formation. However, Cu behavior of coke resistance and activity enhancement is still not well-defined at molecular level at the time of this study.
This study uses DFT (Density Functional Theory) computational methods to evaluate DRM reaction on Ni2Cu (111) bimetallic catalyst. The study will reveal how different species of elementary reactions are adsorbed on catalyst surface, explore the reaction mechanism and investigate the role of atomic oxygen as well as hydroxide species in carbon removal and catalyst stability in presence of Cu in the Ni-Ni network. The results will also explain the dominant reaction pathway by calculating the activation energy barriers of different elementary reactions and contribute to design of new stable and coke-resistant catalyst that can be used for DRM on the industrial scale
Modeling Catalyst Activity and Selectivity for the Gas to Liquid Technology (GTL)
Get PDFThe activity and selectivity of catalytic systems used in gas to liquid (GTL) technology have been studied. In the activity study, seven catalytic systems that were used in the dry reforming of methane have been analyzed. The generalized power law expression (GPLE) model was used to fit the activity profile and predict mechanism of catalyst deactivation. The first and second order GPLE fit well to the experimental data with regression factor (R2 ) ranged between 0.95 and 0.99. Also, it was possible to deconvolute the deactivation mechanism into two main causes, fast deactivation by sintering and the slow deactivation by carbon deposition. In the selectivity study, a detailed kinetics model was developed to estimate the product distribution of the cobalt catalyst in the supercritical fluid phase of the Fischer-Tropsch synthesis reaction (SCF-FTS) up to carbon number 15. The adopted mechanism to describe the reaction network is the alkyl mechanism. Six experimental runs were conducted, corresponding to three temperature levels of experimental data (230���, 240��� and 250���), three total pressures (45 bar, 65 bar, and 80 bar) to capture the critical and near critical condition, (H2:CO=2:1) and gas hourly space velocity (GHSV) of 500 (1/h).
To estimate the model parameters a genetic algorithm code was developed in MATLAB. The model results showed that the maximum mean absolute relative residual (MARR) was 35.32%. Moreover, the model was able to predict the n-paraffin formation rate and Anderson-Schulz-Flory (ASF) product distribution with acceptable range of error
Simulation of Fischer-Tropsch Fixed-Bed Reactor in Different Reaction Media
Get PDFThe continuous increase in the global demand for a cleaner energy source has instigated much interest in converting natural gas to ultra-clean fuels and value-added chemicals. Fischer-Tropsch synthesis (FTS) is a key technology for converting syngas, produced from coal, biomass or natural gas, into a variety of hydrocarbon products. Although this technology has been around for decades, commercial development remains relatively slow and limited to use of few reactor configurations (e.g. fixed-bed reactor and slurry-bubble column reactor).
On the lab-scale, supercritical solvents were utilized in FTS as a reaction media since they have the advantages of both the gas-phase reaction (fixed-bed reactor) and the liquid-phase reaction (slurry-bubble column reactor), while simultaneously overcoming their limitations. This work focuses on modeling the behavior in the reactor bed (���macro-scale��� assessment) and then zooming into the catalyst pellet itself (���micro-scale��� assessment).
The aim of this research is to simulate the heat and mass transfer behavior inside the reactor bed, identify typical conditions that look at the existence and absence of both mass and heat transfer limitations, and to quantify the role of the main controlling parameters on the overall behavior of the reactor bed and on the catalyst effectiveness factor. An often used mathematical model of the fixed-bed reactor was applied to simulate the concentration and temperature profile simultaneously based on the appropriate mass and heat balances at both scales. A second-order ordinary differential equation was used for a spherical pellet in the radial coordinate for both mass and heat balances, while a one-dimensional steady state pseudo heterogeneous model was used for the reactor bed modeling in the axial direction. In addition, in both models the mass balance equation was expressed in terms of fugacity to account for the non-ideal behavior of the reaction mixture in the SCF-FTS. The thermodynamic properties of the mixture were estimated using the Soave-Redlich-Kwong equation of state (SRK-EOS).
The simulation results of this study showed a high temperature rise in the gas- phase FTS relative to that in the SCF-FTS under a comparable reaction conditions. Carbon monoxide conversion was considerably higher in the SCF compared to the gas- phase reaction. The effect of the particle size on the overall catalyst effectiveness factor was also investigated in both reaction media
Removal of Mercury From Water Using Iron(II) Sulphide Nanoparticles and Ultrafiltration Membrane
Get PDFIn this study, reactive nanoparticulate FeS was used to remove Hg(II) from water with an ultrafiltration system. A dead-end ultrafiltration (DE/UF) system was developed to remove Hg(II)- contacted FeS from water in the presence of 0.01M anions (Cl- , NO3 - , SO4 2- ) and 1 mg/L HA in non-stirred mode using regenerated cellulose membrane. The DE/UF stirred mode was applied to evaluate the ���shear effect��� on the rejection of Hg-contacted FeS. Batch tests reveal that complete Hg(II) removal was achieved in 10 minutes in the presence of anions and 60 minutes in the presence of HA. A cross-flow ultrafiltration (CF/UF) system was implemented to examine continuous removal of Hg-contacted FeS in the presence of 0.01 M anions using 1000 kDa polyethersulfone membrane. Experimental results showed that in the presence of anions, higher Hg(II) removal was observed compared to Hg(II) and FeS alone with slight decrease in pH and increased flux decline. The highest Hg(II) removal was achieved in the presence of HA with no pH effect despite significant impact on membrane permeability and slight Fe released during the desorption tests. The DE/UF stirred mode system exhibited reduced cake formation leading to less flux decline. In terms of membrane pore size, 100 and 300 kDa exhibited significant flux recovery despite greater flux decline compared to 30 kDa. Overall, the developed ultrafiltration systems produced chemically stable Hg-contacted FeS particles that can be reused and disposed safely in the environment. In the DE/UF system nonstirred mode, Hg-contacted FeS achieved complete additional Hg(II) removal. However, the DE/UF stirred mode and the CF/UF system exhibited decreased additional removal capacity. These could be due to chemical variations in the FeS particles caused by the shear effect and tangential flow on the Hg(II)-contacted FeS. SEM/EDS analyses demonstrate that the Hg loading on the membrane was higher in the presence of humic acid and anions. These findings present fundamental data that could be applied in the advancement of Hg(II)-contaminated water treatment using low cost FeS adsorbents and can serve as a guideline for continuous treatment of other toxic inorganic chemicals
Utilization of Supercritical Fluids in the Fischer-tropsch Synthesis over Cobalt-Based Catalytic Systems
Get PDFFischer-Tropsch synthesis (FTS) holds great potential for the production of ultra-clean transportation fuels, chemicals, and other hydrocarbon products through the conversion of readily available syngas (CO/H2) from abundant resources (coal, natural gas, and biomass). Utilization of supercritical phase in FTS as a medium that has superior properties (liquid-like density and heat capacity, and gas-like diffusivity) represents a new challenge to the 80-years old FTS technology. The objective of this research is to establish optimum operating conditions for FTS within the supercritical region that would maximize the production of value added chemicals and middle distillate hydrocarbons (gasoline fuel, jet fuel, and diesel fuel fractions) and at the same time minimize the production of methane and carbon monoxide. Chapters 3-5 of this dissertation are designed to examine the effects of supercritical fluid (SCF) (n-pentane or hexane) on FTS over an alumina supported cobalt catalyst in a fixed-bed-reactor. The influence of reaction conditions (such as temperature (210-260 ?C), pressure (20-80 bar), syngas feed ratio (H2/CO ratio of 0.5-2), contact time and space velocity (50-150 sccm/gcat)) on the FTS activity, selectivity, and hydrocarbon product distributions in the supercritical fluids (SCF) media was studied. Our results show that the adjustable thermophysical properties of the SCF significantly impact the FTS reaction performance and in most cases the SCF-FTS operations yield higher activity and better selectivity towards the most desired products compared to conventional gas-phase FTS operations. An excellent opportunity to maximize the production of desired fuel fractions, through a simple tuning process of the reaction environment from liquid-like properties to vapor-like properties, can be achieved in the SCF-FTS conditions as discussed in Chapter 4. An approach to understand the enhanced chain growth probability in SCF-FTS conditions is reported in Chapter 5. This phenomenon was attributed to the enhanced ?-olefins incorporation in the chain growth process. Chapter 6 covers a preliminary examination of the kinetics of the FTS reactions under high-pressure high-temperature conditions in both conventional gas-phase FTS and supercritical hexanes FTS (SCH-FTS). Our findings illustrate that the classical surface reaction kinetics model fails to predict the rates in the SCH-FTS. Our findings also show that the cobalt-based catalytic systems show excellent stability in terms of activity and selectivity as well as their structure under the SCF-FTS conditions for relatively long time-on-stream (up to 13 days). The influence of the cobalt-based catalyst characteristics on the FTS performance in both SCH-FTS and conventional gas-phase FTS is addressed in Chapters 7 and 8
A Path to the Formulation of New Generations of Synthetic Jet Fuel Derived from Natural Gas
Get PDFCharacterization of jet fuels obtained from sources other than crude oil is a modern area of research that is developing continuously to replace available petroleum-based fuels with ���drop-in��� alternative fuels. Therefore, reliable composition-property relations are developed to correlate the hydrocarbon compositions of formulated synthetic fuels with their properties to be certified for aviation commercial use.
Intensive studies have been initiated at Texas A&M University Qatar in collaboration with industry and academia to study synthetic jet fuels derived from natural gas. These studies are being implemented at its Fuel Characterization Lab where the most advanced testing equipment is used and strict Quality Management and safety systems are followed.
This study is divided into two tracks. The first track is focused on conducting experimental investigations using in-house formulated synthetic jet fuels derived from natural gas via Gas-to-Liquid technology and Fischer-Tropsch chemistry. Throughout this research work, these fuels will be referred to as Synthetic Paraffinic Kerosene (SPK). These experimental investigations activities are composed of three phases: the first phase focuses on the influence of SPK building blocks (paraffinic hydrocarbons) on fuels��� properties, the second phase concerns evaluating the role of aromatics and cyclo-paraffins on properties, and the third phase studies the influence of mixing SPK with conventional Jet A-1 derived from crude oil. All of the aforementioned experimental investigations are aimed at building an experimental data bank to assist the efforts of the formulation of new generations of SPKs that meet aviation industry standards. On the other hand, the second track is directed towards the development of mathematical correlations for four properties of high importance to SPK certification. These correlations aim at optimizing fuel composition whereby major physical/chemical properties of ASTM D1655 are met at the lowest cost of composed fuel.
The primary findings of this study showed that GTL derived SPK paraffinic constituents can improve certain properties while affecting others negatively, and emphasizing the necessity of aromatics in improving specific properties. Further studies compensating the absence of aromatics and sulfur through blended Jet A-1 revealed a practical solution through jet fuels optimization based on cost and technical effective manners
Utilizing modeling tools to design a reactor and a catalyst for dry reforming of methane
Get PDFDry reforming of methane (DRM) is a catalytic reaction in which two greenhouse gases (CO2 and CH4) are converted to synthesis gas (a mixture of CO and H2), an important precursor to produce various chemical products. DRM is highly attractive due to its ability to convert greenhouse gases; however more research is required to address its process challenges: (a) high energy requirement, (b) low synthesis gas quality, and (c) catalyst deactivation due to carbon formation. Nickel (Ni) catalyst is widely used for methane reforming and thus suitable for DRM as well. However, it is prone to carbon formation due to reactions like methane decomposition and Boudouard reaction causing deactivation. A novel bimetallic nickel-copper (Ni-Cu) catalyst was previously developed in our research group at an atomistic scale using density functional theory (DFT) approach to address the Ni catalyst���s carbon formation challenge. The Ni-Cu catalyst provides significant carbon resistance and superior stability compared to the conventional Ni catalyst. The catalyst���s performance was proven and validated experimentally in our laboratory���s state-of-the-art bench-top reactor.
The scope of this thesis is to explore the scalability of the novel Ni-Cu catalyst using a mathematical modeling approach. The approach comprises of utilizing an existing one-dimensional (1-D) pseudo-homogeneous reactor bed model supported by lumped kinetics of a network of complex reactions that take place during DRM. This model was updated by including accountability of carbon formation and advancing further to account for detailed transport properties. The kinetics of Ni to Ni-Cu catalyst were scaled using a novel approach utilizing DFT and results in providing predictions for the bulk-scale kinetics performance in terms of carbon formation rates.
The experimental validation of the model was first tested using a conventional monoatomic Ni catalyst then later extended to predict the performance of the Ni-Cu catalyst. The 1-D model yielded results that match within an error margin of 5% with experimental data especially on CH4 conversions. The developed 1-D model serves as a tool to predict the performance of the Ni-Cu catalyst at various reactor scales and to conduct future optimization and process intensification studies for DRM process by maximizing feed conversions
An Energy Integrated Approach to Design Supercritical Fischer-Tropsch Synthesis Products Separation and Solvent Recovery System
Get PDFExtensive research has been done in the advancement of gas-to-liquid (GTL) technology for producing a cleaner source of energy through the conversion of natural gas into ultra-clean fuels and value-added chemicals. The Fischer-Tropsch (FT) synthesis, which is a catalytic process that converts synthesis gas (or syngas, which is a mixture of CO and H2) into longer chain hydrocarbons is considered to be the heart of the GTL process. Conventional FT processes are currently utilizing two most common types of reactors: the multi-tubular fixed bed reactor (in which the reaction takes place in a gas phase medium) and the slurry bubble column reactor (where the reaction takes place in a liquid phase medium). However, they possess heat transfer and mass transfer limitations, respectively.
In order to avoid the challenges, the application of a supercritical fluid (SCF) solvent in the Fischer-Tropsch synthesis was introduced. The SCF-FT process, in essence, combines the benefits of the two major reactor technologies used in conventional GTL processes due to the SCF’s gas-like diffusivity, liquid-like solubility and heat transfer.
The SCF-FT synthesis involves co-feeding the SCF solvent along with the syngas into the reactor at a specific solvent to syngas ratio (set as 3:1 in this work). Introducing the supercritical solvent (which was selected to be n-hexane in this work) requires adjustments in the SCF-FT products’ separation sequence due to the significantly large amount of solvent available in the process. The major additional costs associated with the SCF-FT synthesis is in the product separation and solvent recovery. For SCF-FT to be adopted on a large-scale, the economics from operation under high pressure supercritical conditions must exceed the additional cost required for the separation of the solvent.
The aim of this work is to construct an optimum separation design to target the separation of synthetic crude oil (or syncrude) obtained from SCF-FT synthesis while recovering the supercritical solvent. Aspen Plus® was used as the process simulator to determine the energy consumption and quantify the sensitivity of the various parameters on the solvent recoverability, purity, product yield, and operation feasibility while comparing it to the typical FT process.
Three separation sequences were developed using existing GTL plants as references. The three scenarios were compared with regards to their energy requirements. The simulation results showed that despite the addition of a large amount of solvent, the separation of the products, water, and the recovery of the solvent was achieved
Experimental Investigation on the Effect of Pore Structure on Capillary Trapping in 3D Printed Porous Media
Get PDFAs the global population increases with science and technology advancements, the energy demand continues to grow, which presents a generational challenge between energy production and climate change. Understanding capillary trapping in porous media and its implications would aid in meeting the increasing energy demands through enhanced oil recovery and reducing the carbon dioxide concentration in the atmosphere through CO_2 sequestration. Capillary trapping is a phenomenon caused by the displacement of the non-wetting phase by the wetting phase in the reservoir. Capillary trapping dictates the efficiency of enhanced oil recovery (EOR) and CCS.
The objective of this work was to study the relationship between the pore structure ��� in terms of the average coordination number and the average aspect ratio ��� and capillary trapping.
It was found that the average aspect ratio and average coordination number of a system directly influence capillary trapping. The higher the average aspect ratio, the higher the capillary trapping due to the difference in size between the pore and the surrounding throats. When the pore is significantly bigger in size than the throats, the non-wetting phase gets trapped in the pore due to the snap-off effect. On the contrary, capillary trapping decreases with the increase in coordination number value due to more throats connected to each pore, making mobility easier. The aspect ratio has more effect on the capillary trapping when compared to the coordination number. Additionally, this study shows that the higher the pore structure parameter, the higher the capillary trapping at higher initial gas (non-wetting phase) saturations. Based on the results of this work, the pore structure parameter is recommended to be extracted using an extraction model that utilizes the maximal axis ball algorithm when compared to the medial axis algorithm. Moreover, an empirical equation, including the effect of pore structure, was formulated to estimate the trapping capacity of any water-wet system. The formulated equation was tested using one extraction method and showed significant agreement to two widely used capillary trapping estimation models
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