Gas Hydrate Formation by Methane‐Helium Mixtures

Equilibrium conditions for gas hydrates formed by methane‐helium mixtures with helium concentrations of 31.9, 63.9, and 74.6 mol.‐% have been studied at pressures up to 160 bar. The data obtained indicate that in the studied range of helium concentrations and pressures helium hardly contributes to the stability of the gas hydrate formed. The shift of equilibrium conditions to lower temperatures and higher pressures is caused by dilution of methane in the gas phase and, consequently, the decrease in the chemical potential of methane in the gas phase.     

Low‐Temperature Pretreatment of Lignocellulosic Biomass for Enhanced Biogas Production

Readily available lignocellulosic biomass as substrate for biogas plants is gaining popularity amongst biogas plant operators. Results of low‐temperature pretreatment (light cooking) of wheat straw to remove waxes and prepare the biomass for microbial action are described. Benefits of light cooking are low thermal energy demand and low investment cost compared to conventional techniques such as steam explosion. The novelty lies in utilizing the low temperature range 25–100 °C for pre‐soaking the biomass. Two different types of wheat straws were pretreated at varying temperatures and sizes. The results were compared with Buswell's equation for theoretical maximum biomethane yield. Compared to untreated straw, pre‐soaking leads to a significantly higher methane yield. Size reduction combined with light‐cooking does not affect the methane yield in the same manner as pre‐soaking of the biomass.     

Modeling Oxygen Gas Diffusion Electrodes for Various Technical Applications

In gas diffusion electrodes (GDEs), electrocatalysts are in contact with gas and electrolyte ensuring a large active three‐phase boundary. GDEs are used for important technical applications in energy transformation and chemical synthesis. This review gives an introduction into the vast range of existing models for GDEs and their specific purpose, with an emphasis on oxygen reduction electrodes. After introducing the processes occurring in GDEs, modeling approaches are described according to their dimensionality (from 0D to 3D to multiscale) and perspectives for future research are discussed.     

Performance Comparison of Different Separation Systems for H2 Recovery from Catalytic Reforming Unit Off‐Gas Streams

The performance of pressure swing adsorption (PSA), membrane separation, and gas absorption systems for H₂ recovery from refinery off‐gas stream was studied by simulation‐based data. The PSA process was simulated using adsorbents of silica gel and activated carbon for removing heavy and light hydrocarbons. The mole fraction profiles of all components and the relationship between hydrogen purity and recovery as a function of feed pressure were examined. The solution‐diffusion model was applied for modeling and simulation of a one‐stage membrane process. The gas absorption process with a tower tray was simulated at sub‐zero temperature and the correlation between hydrogen purity and recovery as a function of tower pressure and temperature was evaluated at different solvent flow rates.     

SO2 Removal from Gas Streams by Ammonia Scrubbing: Process Optimization by Response Surface Methodology

Reduction of the SO₂ content by ammonia scrubbing in the incinerator of the Isfahan Refinery complex was investigated. An experimental continuous setup was designed to study the underlying process parameters affecting SO₂ capture from air. The effects of various parameters was analyzed by a systematic experimental design based on response surface methodology with central composite design. The developed response surface model was found to be useful and robust to predict the degree of desulfurization of ammonia wet flow gas disulfurization reactors. The ammonia concentration had the most significant influence on the efficiency of the desulfurization process. The square of temperature affected the efficiency more than the temperature, whereas the gas flow rate had a minor influence on the separation efficiency.     

Energetic Optimization of the Three-Stage Gas Permeation Process for the Upgrading of Biogas

The mass balances of countercurrent gas permeation modules for the separation of multicomponent gas mixtures are formulated as a boundary value problem, using the ideal gas law and isobaric isothermal stages. The balances are extended for the common three-stage gas permeation process for the upgrading of biogas. The solution is calculated with MATLAB's BVP4C boundary value solver. The process is optimized by means of the FMINCON SQP-algorithm towards lowest power consumption. Parameter variations are conducted, highlighting the impact of product and lean-gas quality setpoints, biogas composition, membrane surface and selectivity, permeate compression, and choice of compressor model on the energetic optimum.     

Influence of Reaction Conditions on the Conversion of Methane‐Rich Gases to Fischer‐Tropsch Products

Evidence is provided that stable operation of a microstructured reactor for steam‐assisted catalytic partial oxidation (sCPOX) and its subsequent coupling with a Fischer‐Tropsch synthesis (FTS) reactor is possible at pressures up to 25 bar. The product composition of the sCPOX was determined and subsequently used as feed composition for a downstream FTS reactor to prove the possibility of coupling with syngas generation. After stable operation was proven in both setups, they were coupled and operated together, feeding the product gas stream of the sCPOX to the FTS. In addition, the negative influence of sulfur in the sCPOX‐gas feed was evaluated.     

Synthesis Gas Production Configurations for Gas‐to‐Liquid Applications

Different syngas configurations in a gas‐to‐liquid plant are studied including autothermal reformer (ATR), combined reformer, and series arrangement of gas‐heated reformer and ATR. The Fischer‐Tropsch (FT) reactor is based on a cobalt catalyst and the degrees of freedom are steam‐to‐carbon ratio, purge ratio of light ends, amount of tail gas recycled to synthesis gas (syngas) and FT synthesis units, and reactor volume. The production rate of liquid hydrocarbons is maximized for each syngas configuration. Installing a steam methane reformer in front of an ATR will reduce the total oxygen consumption per barrel of product by 40 % compared to the process with only an ATR. The production rate of liquid hydrocarbons is increased by 25.3 % since the flow rate of the purge stream for the ATR is the highest one compared to other configurations and contains mainly CO₂.     

Adsorption-Reaction Processes for the Removal of Hydrogen Sulphide from Gas Streams

The use of porous materials for the removal of traces of hydrogen sulphide from gas streams was investigated in this study. It was found that microporous carbons had the highest removal efficiency. This was due to the fact that carbons have extra potential for the oxidation of hydrogen sulphide while other sorbents remove the gas only by adsorption. The oxidative removal efficiency of microporous carbons was found to be a function of water vapour pressure, oxygen concentration and temperature. The mechanism of the reaction is discussed. © 1997 SCI.     

Development of a Model for Water Scrubbing‐Based Biogas Upgrading and Biomethane Compression

Biomethane is an easily storable, renewable energy source that is applicable to the sectors electricity, heat, and transport. It is mostly obtained from biogas by different upgrading technologies. At present, the most common technology is water scrubbing because of its reliability and simplicity. A methodology for designing as well as for evaluating and optimizing water scrubbing plants including biomethane compression is introduced. To demonstrate possible applications of this methodology, a zero‐emission water scrubbing process characterized by under‐pressure regeneration of washing water is modeled and investigated by a sensitivity analysis.     

A Two‐dimensional Network Study on Oxygen Transport in Porous Gas Diffusion Layer

Oxygen transport in the porous gas diffusion layer (GDL), which is generally characterised by the oxygen effective diffusivity, is of great importance for the performance of proton exchange membrane fuel cells (PEMFCs). The determination of the oxygen effective diffusivity is challenging due to the complex structure of the porous GDL samples. In the present study, a two‐dimensional network consisting of arms and nodes is adopted to illustrate how oxygen effective diffusivity is affected by the GDL structure under the condition with/without water invasion. Water permeation in the network is simulated using the invasion percolation algorithm and oxygen transport in the arms is described by Fick's law. The simulation results reveal that oxygen effective diffusivity under dry condition decreases with increase in the network heterogeneity. With water permeation, the oxygen effective diffusivity goes to zero even though water saturation is rather less than unity. The critical water saturation, above which the oxygen effective diffusivity becomes zero, is found to decrease with increasing heterogeneity. To enhance oxygen transport, four different modified networks are introduced in the present study. It is found that the network with large arms in oxygen transport direction has the best oxygen and water transport properties.     

Selective Removal of H2S from Gas Streams with High CO2 Concentration Using Hollow‐Fiber Membrane Contractors

Selective and simultaneous separation of H₂S and CO₂ from CH₄ was accomplished in a hollow‐fiber membrane contactor (HFMC). The absorption of both H₂S and CO₂ using an aqueous solution of methyldiethanolamine (MDEA) was almost complete and acid gases were totally removed. Despite the large difference between H₂S and CO₂ concentrations, the rate of H₂S absorption was not significantly influenced by CO₂ absorption. The independent effect and interactions of several process variables on the separation performance of H₂S and CO₂ were investigated. The results indicated that the membrane contactor could be a highly efficient choice for removal of almost all H₂S in the presence of a large CO₂ content even at high gas/liquid flow ratio. The selectivity of H₂S was about three times higher compared to the conventional absorption packed towers.     

Effects of Contaminants on the Mass‐Transfer Characteristics of a Two‐Impinging‐Streams Gas‐Liquid Reactor

The mass‐transfer characteristics of a new type of two‐impinging‐streams reactor (TISR) was studied by means of sodium sulfite solution as the liquid phase and air as the gas phase, in the presence and absence of various types of surface‐active agents (SAAs). The influences of anionic, cationic, and nonionic SAAs on the specific interfacial area and overall volumetric mass‐transfer coefficient obtained in the TISR were investigated. It was found that the presence of a little amount of the above‐mentioned contaminants increases the specific interfacial area and decreases the overall volumetric mass‐transfer coefficient. On the basis of the experimental results obtained for various types of SAAs, correlations were derived for the interfacial area as well as the Sherwood number for the liquid phase in terms of Froude, Reynolds, Schmidt, and Morton numbers.     

Extraction of Lipids from Aqueous Protein‐Rich Streams using Near‐Critical Dimethylether

An extraction process for obtaining lipids from aqueous protein/lipid mixtures has been developed using near‐critical dimethylether (DME) as the solvent. The process has been demonstrated on fresh and reconstituted egg yolks, and on selected dairy by‐product streams. The lipid yield is dependent on the processing temperature, solids content and feed ratio of solvent to liquid. The extent of protein denaturation is also dependent on these parameters. The phase equilibria of DME‐water‐phospholipid mixtures are presented with modeling of the DME‐water system using the Peng‐Robinson equation of state.     

Comparison of Large‐Scale Production Methods of Fe2O3/Al2O3 Oxygen Carriers for Chemical‐Looping Combustion

Oxygen carrier (OC) particles for chemical‐looping combustion (CLC) may be produced in large scale by a number of methods such as freeze granulation, spray drying, impregnation, and mechanical mixing methods. To select the most appropriate technology for large‐scale preparation, the four preparation methods were adopted to prepare Fe₂O₃/Al₂O₃ OCs and compared with each other in terms of productivity, preparation period, physical and chemical characterization, and reactivity in CLC of lignite. Freeze granulation and spray drying methods were found to be more suitable for large‐scale production of OCs for CLC. The results of the comparative studies may provide guidelines for selecting appropriate methods for preparing OCs on industrial scale.     

Fabrication of Homogenous Polymer‐Zeolite Nanocomposites as Mixed‐Matrix Membranes for Gas Separation

Interfacial void‐free mixed‐matrix membranes (MMMs) of polyimide (PI)/zeolite were developed using 13X and Linde type A nano‐zeolites and tested for gas separation purposes. Fabrication of a void‐free polymer‐zeolite interface was verified by the decreasing permeability developed by the MMMs for the examined gases, in comparison to the pure PI membrane. The molecular sieving effect introduced by zeolite 13X improved the CO₂/N₂ and CO₂/CH₄ selectivity of the MMMs. Separation tests indicated that the manufactured nanocomposite membrane with 30 % loading of 13X had the highest permselectivity for the gas pairs CO₂/CH₄ and CO₂/N₂ at the three examined feed pressures of 4, 8 and 12 atm.     

Sulfur Removal from Low‐Sulfur Gasoline and Diesel Fuel by Metal‐Organic Frameworks

Several materials in the class of metal‐organic frameworks (MOF) were investigated to determine their sorption characteristics for sulfur compounds from fuels. The materials were tested using different model oils and common fuels such as low‐sulfur gasoline or diesel fuel at room temperature and ambient pressure. Thiophene and tetrahydrothiophene (THT) were chosen as model substances. Total‐sulfur concentrations in the model oils ranged from 30 mg/kg (S from thiophene) to 9 mg/kg (S from tetrahydrothiophene) as determined by elementary analysis. Initial sulfur contents of 8 mg/kg and 10 mg/kg were identified for low‐sulfur gasoline and for diesel fuel, respectively, by analysis of the common liquid fuels. Most of the MOF materials examined were not suitable for use as sulfur adsorbers. However, a high efficiency for sulfur removal from fuels and model oils was noticed for a special copper‐containing MOF (copper benzene‐1,3,5‐tricarboxylate, Cu‐BTC‐MOF). By use of this material, 78 wt % of the sulfur content was removed from thiophene containing model oils and an even higher decrease of up to 86 wt % was obtained for THT‐based model oils. Moreover, the sulfur content of low‐sulfur gasoline was reduced to 6.5 mg/kg, which represented a decrease of more than 22 %. The sulfur level in diesel fuel was reduced by an extent of 13 wt %. Time‐resolved measurements demonstrated that the sulfur‐sorption mainly occurs in the first 60 min after contact with the adsorbent, so that the total time span of the desulfurization process can be limited to 1 h. Therefore, this material seems to be highly suitable for sulfur reduction in commercial fuels in order to meet regulatory requirements and demands for automotive exhaust catalysis‐systems or exhaust gas sensors.     

Ruthenium Supported on Nitrogen‐Doped Carbon Nanotubes for the Oxygen Reduction Reaction in Alkaline Media

Nitrogen‐doped carbon nanotubes (N‐CNTs) prepared via thermal chemical vapor deposition (CVD), were used to support ruthenium (Ru) nanoparticles made using a microwave assisted reduction technique. The amount of Ru deposited on N‐CNTs was varied between 0 and 10 wt.%. The activity of the prepared nanocatalysts toward the oxygen reduction reaction (ORR) was characterized using the rotating disk electrode and voltammetry techniques. The ORR activity was higher at lower concentrations of Ru on N‐CNTs. The four electron pathway of ORR is more favorable on 2Ru/N‐CNTs and 5Ru/N‐CNTs than 10Ru/N‐CNTs.