Equilibrium Limit Research Articles

Enhanced catalytic activity of Ni on η-Al2O3 and ZSM-5 on addition of ceria zirconia for the partial oxidation of methane

Nickel supported on η-Al2O3 and ZSM-5(80) catalysts with and without the addition of ceria-zirconia, were prepared by co-precipitation and wet impregnation methods and used for the low temperature catalytic partial oxidation of methane (CPOM). The catalysts were tested under reaction temperatures of between 400 and 700°C with a WHSV of 63,000mLg−1h−1. The activity of the catalyst was found to be dependent on the support and preparation method. The optimum catalyst composition of those tested was 10% Ni on 25%CeO2-ZrO2/ZSM-5(80), prepared by co-precipitation, where the reaction reached equilibrium conversion at 400°C (T50%<400°C), which is one of the lowest temperatures reported to date. Further increases in temperature led to improved selectivity to CO reaching 60% at 600°C. Although the observed kinetics were found to be controlled by strong adsorption of CO at lower temperature, this was an equilibrium limitation with longer time on stream experiments showing no decrease in the catalyst activity over 25h at 400°C.

Simulation of novel process of CO2 conversion to liquid fuels

Carbon dioxide utilization by conversion with hydrogen into liquid fuels was simulated based on the experimental data of a novel process using CHEMCAD. A detailed kinetic model of the novel iron-based spinel catalyst that included reverse water gas shift (RWGS), Fischer-Tropsch synthesis (FTS), C5+ hydrocarbons and oxygenates, oligomerization of olefins, as well as hydrogenation of light olefins (C2C4) was employed. The RWGS reaction rate was significantly inhibited by steam produced in the process because of the chemical equilibrium limitation and apparent strong adsorption. Therefore, periodical water removal is critical in the process, which required operation in several reactors in series or in a reactor with recycle. Those system configurations were examined and compared over a range of temperatures, pressures, weight hourly space velocities and carbon dioxide with hydrogen feed ratios. The other aspect of this process which has a significant impact on performance is the oligomerization of light olefins. Both reactors-in-series and single reactor with recycle improved dramatically the productivity and the selectivity to C5+.

Enhanced effect of plasma on catalytic reduction of CO2 to CO with hydrogen over Au/CeO2 at low temperature

In terms of the reaction of CO2 reduction to CO with hydrogen, CO2 conversion is very low at low temperature due to the limitation of thermodynamic equilibrium (TE). To overcome this limitation, plasma catalytic reduction of CO2 to CO in a catalyst-filled dielectric barrier discharge (DBD) reactor is studied. An enhanced effect of plasma on the reaction over Au/CeO2 catalysts is observed. For both the conventionally catalytic (CC) and plasma catalytic (PC, Pin=15 W) reactions under conditions of 400°C, H2/CO2=1, 200 SCCM, GHSV=12,000 mL•g−1cat•h−1, CO2 conversions over Au/CeO2 reach 15.4% and 25.5% due to the presence of Au, respectively, however, those over CeO2 are extremely low and negligible. Moreover, CO2 conversion over Au/CeO2 in the PC reaction exceeds 22.4% of the TE conversion. Surface intermediate species formed on the catalyst samples during the reactions are determined by in-situ temperature-programmed decomposition (TPD) technique. Interestingly, it disclosed that in the PC reaction, the formation of formate intermediate is enhanced by plasma, and the acceleration by plasma in the decomposition of formate species is much greater than that in the formation of formate species on Au/CeO2. Enhancement factor is introduced to quantify the enhanced effect of plasma. Lower reactor temperature, higher gas hourly space velocity (GHSV), and lower molar ratio of H2/CO2 would be associated with larger enhancement factor.

Enzyme kinetics for the transesterification of ethyl butyrate with enzyme beads, coated packing and ultrasound assistance

The use of immobilized enzymes as renewable biocatalysts is gaining attention in the scientific community. The immobilization of enzymes is necessary to avoid a complex separation and recovery of the enzymes and to also apply enzymes in an Enzymatic Reactive Distillation (ERD). The ERD helps to further overcome the equilibrium limitations of transesterification reactions by means of in-situ product separation. Potential mass transfer limitations caused by the immobilization can be overcome by ultrasound (US) application to form the US-assisted ERD (US-ERD).In order to evaluate the application of ERD and US-ERD this article presents the experimental investigation of the transesterification of ethyl butyrate catalyzed by the enzyme Candida antarctica lipase B for two different types of immobilizate (coated packing and enzyme beads) with and without US-assistance. Low frequency US tends to destroy the immobilizates whereas high frequency US can increase the rate of an enzymatic reaction by 50% for the coated packings. Enzyme beads do not show diffusion limitation whereas mass transfer limitations for the coated packing can be reduced by US application. A kinetic model is identified for each of the immobilizates with and without US application and the parameters of the models accurately fit to experimental data.

Dynamical mechanisms of phase-2 early afterdepolarizations in human ventricular myocytes: insights from bifurcation analyses of two mathematical models.

We investigated mechanisms of phase-2 early afterdepolarization (EAD) by bifurcation analyses of human ventricular myocyte (HVM) models. EAD formation in paced HVMs basically depended on bifurcation phenomena in non-paced HVMs, but was strongly affected by intracellular ion concentrations in stationary and dynamic states. EAD generation did not necessarily require IKs.

Conversion enhancement for acetalization using a catalytically active membrane in a pervaporation membrane reactor

Glycerol ketals are often considered as important bio-based diesel additives or perfume substances which could be synthesized from the acetalization of glycerol and cyclohexanone, however, acetalization is a typical example of reactions controlled by thermodynamic equilibrium. In this study, a catalytically active membrane was prepared by immersion phase inversion and was used for the conversion enhancement of present reaction by continuous removal of by-product water in a pervaporation membrane reactor. SEM images showed that a highly porous “sponge-like” catalytic layer immobilized with catalyst Zr(SO4)2·4H2O was uniformly coated on a polyvinyl alcohol/polyethersulfone pervaporation membrane. Synthesis performances in batch reactor, inert membrane reactor and catalytically active membrane reactor were compared, showing that no equilibrium limitation for glycerol conversion was observed in both the inert membrane reactor and catalytically active membrane reactor. The effect of various operational parameters on the synthesis performance in the catalytically active membrane reactor were investigated, showing that higher temperature and A/V ratio favored better enhancement in glycerol conversion since an acceleration of water removal rate was induced. Optimal catalyst loading and initial mole ratio was also determined for better synthesis performance. Under optimized conditions, the glycerol conversion reached 93% and a conversion enhancement of approximately 52% was achieved when compared to equilibrium conversion at 75°C.

Iron and oxygen isotope fractionation during iron UV photo-oxidation: Implications for early Earth and Mars

Banded iron formations (BIFs) contain appreciable amounts of ferric iron (Fe3+). The mechanism by which ferrous iron (Fe2+) was oxidized into Fe3+ in an atmosphere that was globally anoxic is highly debated. Of the three scenarios that have been proposed to explain BIF formation, photo-oxidation by UV photons is the only one that does not involve life (the other two are oxidation by O2 produced by photosynthesis, and anoxygenic photosynthesis whereby Fe2+ is directly used as electron donor in place of water). We experimentally investigated iron and oxygen isotope fractionation imparted by iron photo-oxidation at a pH of 7.3. The iron isotope fractionation between precipitated Fe3+-bearing lepidocrocite and dissolved Fe2+ follows a Rayleigh distillation with an instantaneous 56Fe/54Fe fractionation factor of +1.2‰. Such enrichment in the heavy isotopes of iron is consistent with the values measured in BIFs. We also investigated the nature of the mass-fractionation law that governs iron isotope fractionation in the photo-oxidation experiments (i.e., the slope of the δ56Fe–δ57Fe relationship). The experimental run products follow a mass-dependent law corresponding to the high-T equilibrium limit. The fact that a ∼3.8 Gyr old BIF sample (IF-G) from Isua (Greenland) falls on the same fractionation line confirms that iron photo-oxidation in the surface layers of the oceans was a viable pathway to BIF formation in the Archean, when the atmosphere was largely transparent to UV photons.Our experiments allow us to estimate the quantum yield of the photo-oxidation process (∼0.07 iron atom oxidized per photon absorbed). This yield is used to model iron oxidation on early Mars. As the photo-oxidation proceeds, the aqueous medium becomes more acidic, which slows down the reaction by changing the speciation of iron to species that are less efficient at absorbing UV-photons. Iron photo-oxidation in centimeter to meter-deep water ponds would take months to years to complete. Oxidation by O2 in acidic conditions would be slower. Iron photo-oxidation is thus likely responsible for the formation of jarosite–hematite deposits on Mars, provided that shallow standing water bodies could persist for extended periods of time.The oxygen isotopic composition of lepidocrocite precipitated from the photo-oxidation experiment was measured and it is related to the composition of water by mass-dependent fractionation. The precipitate-fluid 18O/16O isotope fractionation of ∼+6‰ is consistent with previous determinations of oxygen equilibrium fraction factors between iron oxyhydroxides and water.

Optimization Design for Hub-and-Spoke Container Shipping Network Considering CO2 Emissions

With the development of global economic integration, the container shipping has become the main transportation of world shipping industry. Shipping companies usually employ hub-and-spoke shipping network in order to chase for optimal economic benefits. The CO 2 emissions system of global maritime has a direct effect on shipping company’s design for hub-and-spoke container shipping network. With the constraints of container flow equilibrium and hub port capacity limit, a hub-and-spoke container shipping network optimization model by considering CO 2 emissions is established. Under the measurement of container transportation costs, harbor dues and CO 2 emissions costs, the shipping company reconsider the optimal shipping routes and make decision of hub ports to be calling. The decision made by the company should aim to reduce the total shipping costs. Lagrangian relaxation algorithm is applied to solve the model. Numerical experiments show that the model and algorithm are effective. After analyzing the impact of hub ports to hub-and spoke network design, we get to the conclusion that, CO 2 emissions costs are negatively correlated with utilization ratio of hub port. And hub port capacity limit also affects the port selection decision making of shipping companies. The hub-and-spoke container shipping network optimization model by considering CO 2 has a great impact for the decision making and route optimization of the shipping companies in the future.

In situ gasification Chemical-Looping Combustion of coal using limestone as oxygen carrier precursor and sulphur sorbent

In-situ Gasification Chemical-Looping Combustion (iG-CLC) burning coal is achieving a great interest due to the possibility of using low cost oxygen carriers such as CaSO4. The Limestone Chemical Looping Combustion process (LCL-C™) registered by Alstom Power Inc. proposes the use of a continuous CaCO3 feeding together with coal to produce CaSO4 as an oxygen carrier via sulphur retention. The operation is similar to what happens in a circulating fluidized bed boiler burning coal. In the present research work, the study of thermodynamic equilibrium limitations together with mass and enthalpy balances have been carried out for a CLC system in order to investigate whether the LCL-C™ process is a promising and energy efficient option to carry out the coal combustion with CO2 capture and in-situ desulphurisation. So, no limitations were found to transfer the required oxygen from air to fuel using sulphated limestone as oxygen carrier for whatever coal used. The selection of a suitable oxygen carrier to fuel ratio mainly depends on the sulphur content of the coal used. In addition, no drawback referred to thermal integration in the system has been detected. Thus, operation at 950°C in the fuel reactor to avoid SO2 release via side reactions, and 1050°C in the air reactor is feasible. Likewise, experimental tests have been performed in a thermogravimetric analyser to evaluate the capability of a limestone to be sulphated and to transfer oxygen. A value of the oxygen transport capacity of about 16.7wt.% was obtained. This value is four times higher than that of others typical inexpensive oxygen carriers published in literature.

Sensitivity of Computational Rotor Dynamics Towards the Empirically Estimated Lubrication Gap Clearance of Foil Air Journal Bearings

AbstractThis contribution is concerned with the computational analysis of a rigid rotor supported by means of two self‐acting foil air journal bearings. Even though the overall equation system is thereby typically written in a nondimensional form, prior knowledge about realistic value ranges of occurring dimensionless numbers is required in order to parameterize and interpret such simulations correctly. Unlike all other quantities, the nominal lubrication gap clearance between the rotating journal and the undeformed foil structure is reported to be only poorly known. Thus, even in the light of an advanced understanding of the bearing rotor system's fundamental behavior, the quantitative reproduction and prediction of experimental results by means of computational analysis need to be viewed critically. In this study, the sensitivity of numerical results towards the assumed nominal lubrication gap clearance will be investigated. To this end, the stability of the system is considered and the characteristics of occasionally observed equilibrium points and limit cycles are addressed. (© 2016 Wiley‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Electrochemical impedance spectroscopy study of Concanavalin A binding to self-assembled monolayers of mannosides on gold wire electrodes

The interactions of the lectin Concanavalin A (Con A) with self-assembled monolayers (SAMs) of thiolated mono-, di-, and tri-mannosides were studied on the surface of gold wires using electrochemical impedance spectroscopy (EIS). The SAMs of mannosides were prepared either pure or along with thiolated triethylene glycol (TEG) at different molar ratios (1:1, 1:2, 1:4, 1:9, and 1:19) to better understand and optimize the interaction conditions. The charge-transfer resistance of the [Fe(CN)6]3−/4− redox probe was compared before and after the interaction at different concentrations of Con A to determine the equilibrium dissociation constant (Kd) and limit of detection (LOD). Values of Kd were found in the nanomolar range showing multivalent interactions between mannosides and Con A, and LOD was found ranging from 4 to 13nM depending on the type of mannoside SAM used. Analysis using the Hill equation suggests negative cooperativity in the binding behavior. Peanut agglutinin was used as a negative control, and cyclic voltammetry was used to further support the experiments. We have found that neither the pure nor the widely dispersed monolayers of mannosides provide the conditions for optimal binding of Con A. The binding of Con A to these SAMs is sensitive to the molar ratio of the mannoside used to prepare the SAM and to the structure of the mannoside. A simple cleaning method has also been shown to regenerate the used gold wire electrodes. The results from these experiments contribute to the development of simple, cheap, selective, and sensitive EIS-based bioassays, especially for lectin–carbohydrate interactions.

Thermal fluctuations in a system of microcavity exciton‐polariton condensations

AbstractThe exciton‐polariton condensation scenario in a microcavity matter–light system at finite temperature is investigated in the framework of the unrestricted Hartree–Fock approximation applying the two‐dimensional exciton‐polariton model. Treating electron–hole Coulomb interaction and matter–light coupling effects on an equal footing, we analyze the temperature influence on the excitonic and photonic order parameters. In the thermal equilibrium limit, the Kosterlitz–Thouless type phase transition of the exciton‐polariton condensation has been found. Depending on detuning and excitation density one finds that the system either favors the photonic or excitonic condensate. The effect of temperature on a crossover from a BCS‐excitonic to a BEC‐polaritonic condensed state then is discussed.

Subthreshold and suprathreshold vibrational resonance in the FitzHugh-Nagumo neuron model.

We study the subthreshold and suprathreshold vibrational resonance in the FitzHugh-Nagumo neuron model. For the subthreshold situation, two cases where the stationary states are equilibrium point and limit cycle are considered, where different natures of vibrational resonance are observed via theoretical and numerical methods. Especially when the frequency of the high-frequency driving force is near the so-called canard-resonance frequency, the firing rate can be significantly enhanced at the presence of noise. For the suprathreshold situation, we show that the local maxima of the response amplitude are located at the transition boundaries of different phase-locking patterns. The minimal required forcing amplitudes of high-frequency signal of the firing onset are just multiples of the spiking frequency. Furthermore, phase portraits and time series show that the presence of the global maxima of the response results from not only the suprathreshold but also the subthreshold phase-locking modes. In spite of the distinct characteristics for two stationary states on subthreshold oscillation, the suprathreshold vibrational resonance showed no qualitative difference between the two cases.

Unifying framework for modeling swelling soil behaviour

The destructive effect of expansive soils on infrastructure is one of the classical issues associated with unsaturated soils. Given the continuing challenges associated with swelling soils there is still a need for a practical analysis method. The swelling equilibrium limit (SEL) provides a concept for modeling expansive soil behaviour. The SEL is an upper bound swelling limit, which depends on the soil’s initial state and the boundary conditions upon wetting. The SEL is obtained experimentally from swelling tests performed by applying a range of swelling scenarios from rigid boundary condition to free swelling without confinement. In this paper, improvements to the triaxial swelling apparatus are reported, SELs for two new swelling soils are characterized, and a framework for SEL prediction is developed. The SEL framework is developed to relate SEL parameters to index properties as well as the soil’s initial volume. SEL parameters are correlated to liquid limit, plasticity index, free swell potential, and specific volume. The relationships are used to estimate the SEL for Regina clay, which is shown to agree with experimental data.

Hydrogen production from heavy fraction of bio-oil using iron-based chemical looping process: Thermodynamic simulation and performance analysis

The purpose of this work is to investigate the overall performance of the iron-based chemical looping hydrogen (CLH) production from the heavy fraction of bio-oil (HFB). The multiple ASPEN models are employed to simulate a two-stage fluidized bed reduction reactor considering the thermodynamic equilibrium limit. Several important factors are discussed to determine the suitable reactor operation conditions for the process simulation of two operation modes. The results show HFB CLH process has a maximum hydrogen efficiency of 73.1% (LHV) and a total thermal efficiency of 59.2% (LHV) at the cost of decreasing the CO2 capture efficiency under the condition of supplementary firing. Within the complete self-sustaining operation range, the highest hydrogen thermal efficiency is 57.8% (LHV) corresponding to a total thermal efficiency of 58.3% (LHV) and a CO2 capture efficiency of nearly 100%. This study indicates the CLH process has significant advantages over the conventional coal gasification (CG) for hydrogen production owing to its high energy conversion efficiency and high-efficient CO2 capture with low cost.

Esterification of glycerol from biodiesel production to glycerol carbonate in non-catalytic supercritical dimethyl carbonate.

Conversion of glycerol from biodiesel production to glycerol carbonate was studied by esterification with dimethyl carbonate in a non-catalytic supercritical condition. It was found that in a non-catalytic supercritical condition, glycerol at higher purity gave higher yield of glycerol carbonate at 98 wt% after reaction at 300 °C/20–40 MPa/15 min. The yield of glycerol carbonate was observed to increase with molar ratio, temperature, pressure and time until a certain equilibrium limit. The existence of impurities such as water and remnants of alkaline catalyst in crude glycerol will direct the reaction to produce glycidol. Although impurities might not be desirable, the non-catalytic supercritical dimethyl carbonate could be an alternative method for conversion of glycerol from biodiesel production to value-added glycerol carbonate.Graphical abstractPlausible reaction scheme for conversion of glycerol to glycerol carbonate in non-catalytic supercritical dimethyl carbonate.Electronic supplementary materialThe online version of this article (doi:10.1186/s40064-016-2643-1) contains supplementary material, which is available to authorized users.

Modeling and operability of DME production from syngas in a dual membrane reactor

In this research, a conventional shell and tube reactor is supported by the hydrogen and vapor membrane tubes for water vapor removal and hydrogen permeation in the single step DME synthesis process. The proposed dual membrane structure is heterogeneously modeled based on the mass and energy conservation laws at steady state condition. To prove the accuracy of the developed model and assumptions, the simulation results of the conventional DME reactor are compared with the available data. Then, the performance and operability of the proposed structure is compared with the hydrogen membrane, water membrane and conventional reactors. The simulation results show that the DME production capacity in the dual membrane reactor is improved about 17.2% compared to the conventional process. Higher DME production rate due to shift the thermodynamic equilibrium limitations, and lower CO2 production are the main advantages of the proposed reactor.

Upper-bound solutions for the face stability of a shield tunnel in multilayered cohesive–frictional soils

Tunneling in urban shallow soft ground is more frequently being carried out using the shield method. An important problem in the stability analysis of a tunnel face is to provide enough support pressure to the excavation face. Among the analytical approaches used to investigate the stability of a tunnel face, limit equilibrium methods or limit analysis methods are widely used. For cohesive–frictional soils, the results obtained from experimental tests and numerical simulations indicate that the failure soil in front of the tunnel face demonstrates two main features, i.e., a shear failure band in the lower part and a pressure arch effect in the upper part. Compared to the limit equilibrium methods, the limit analysis methods have a strictly theoretical basis and provide better results. To better interpret failure features, a new three-dimensional failure mechanism is proposed to analyze the limit support pressure of the tunnel face in multilayered cohesive–frictional soils based on limit analysis methods. The new failure mechanism is composed of five truncated cones that represent the shear failure band and a distributed force acting on the truncated cones that represents the pressure arch effect. The distributed force is calculated using Terzaghi earth pressure theory. The limit support pressures obtained from the new failure mechanism and the existing approaches are compared, and the new mechanism of this paper provides relatively satisfactory results for limit support pressures.

Characterization and evaluation of poly(ether sulfone) membranes in biodiesel production using liquid–liquid film reactors

Biodiesel production in liquid–liquid film reactors integrated with membranes (LLFRM) has been investigated to obtain a high quality biodiesel with high process productivity. This device takes advantages of both technologies: the high productivity of the liquid–liquid film reactor (LLFR) and the continuous product removal with the poly(ether sulfone) hollow fiber membranes (PES-HFM). This intensified process was evaluated in order to overcome problems associated to chemical equilibrium limitations. Initially, permeabilities of single compounds involved in biodiesel production through PES-HFM were experimentally determined. Afterwards, permeabilities of the same compounds in the reactive mixture were tested. Finally, a LLFR integrated with PES-HFM (LLFRM) was designed, constructed and tested for the methanolysis of Jatropha oil. It was found that single components follow the Darcy’s law, where high viscosities produce low permeabilities. The permeability of the compounds in the reactive mixture was lower than the obtained for single ones, indicating possible mass transfer limitations. The LLFRM was able to remove the mixture glycerol-methanol selectively. The maximum conversion and yield in the reactor were 85% and 63%, respectively, using a reaction-separation zone of 30% Lmem/Lmax at molar ratio methanol to oil of 9:1.

A kinetic reaction model: Decay to equilibrium and macroscopic limit

We propose a kinetic relaxation-model to describe a generation-recombination reaction of two species.The decay to equilibrium is studied by two recent methods [9,13] for proving hypocoercivity of the linearized equations.Exponential decay of small perturbations can be shown for the full nonlinear problem.The macroscopic/fast-reaction limit is derived rigorously employing entropy decay, resulting in a nonlinear diffusion equation for the differenceof the position densities.