Selection of competitive adsorption additives to relieve product inhibition of maleic acid hydrogenation in proton exchange membrane flow cell reactor: A molecular dynamics simulation

Adsorption of products on carbon carriers and the sluggish mass transport of products in the catalytic layer lead to product inhibition in proton exchange membrane flow cell reactor (PEMFCR), which seriously decreases hydrogenation performance. Here, via molecular dynamics simulations, an alleviative mechanism of competitive adsorption additives on product inhibition is proposed by choosing biomass derivatives maleic acid (MA) as hydrogenation model compound and ethanol (EtOH) as an additive model. The enhanced solvation and competitive adsorption synergistically promote the outflow of product succinic acid (SA) from the catalytic layer. Strong hydrophobic-hydrophobic interactions between EtOH and SA promote solvation and solubility of SA in the solvent water, and meanwhile the competitive adsorption steric hindrance of EtOH on adsorption interfaces weakens van der Waals interactions between SA and carbon carriers. Accordingly, the main structural characteristics that influence competitive adsorption of additives, such as, amphiphilic structure, polarity, molecular weight, and size of hydrophobic moiety, are exacted for selection of favorable additives. Among the studied water miscible alcohol and ketone additives, isopropanol (iPrOH) is proved to be favorable, which is further verified by the highest increase (64.3%) of hydrogenation conversion of MA in experiments. This work could provide theoretical guidance for selecting high-performance additives, intensifying heterogeneous hydrogenation in PEMFCR.     

Combining electrochemical hydrogen separation and temperature vacuum swing adsorption for the separation of N2, H2 and CO2

In biohydrogen processes, H₂ is often present in gas mixtures with CO₂ and N₂. Therefore, we present a separation process that combines electrochemical hydrogen separation (ECHS) and amine-based temperature vacuum swing adsorption (TVSA). Such a separation process does not require the compression of the bulk feed. For the ECHS, a trade-off between cell potential, i.e. power consumption, and stack size, i.e. investment costs, existed. The optimal cell voltage laid in the range of 0.1–0.2 V resulting in costs of 0.37 €/kg_H2. Equivalently, the energy consumption and throughput for the TVSA must be balanced. The optimal desorption pressure and temperature of 0.025 bar and 100 °C resulted in costs of 2.30 €/kg_H2. Thus, the total costs for the separation process were 2.67 €/kg_H2, which are in the range of the separation costs of a binary H₂/CO₂ mixture with a similar H₂ feed fraction.     

H2 and CO production through coking wastewater in supercritical water condition: ReaxFF reactive molecular dynamics simulation

The degradation mechanism of benzo[a]pyrene (BaP), a representative component of coking wastewater, and the pathway for the production of H₂ and CO in supercritical water have been investigated via ReaxFF reactive molecular dynamics simulations. The BaP molecules in the SCWG, SCWPO and SCWO systems show different degradation pathways. The maximum H₂ yield is obtained at the oxygen ratio of 0.2. There are three routes for the generation of H₂ molecules and production from H radical-rich water is the main route. CO molecules are formed by the CC bond breakage and CO bond breakage in the reforming fragments. There is a time delay between the fuel gas generation reaction and the side reactions due to the change of the instantaneous concentrations of H₂ and CO, providing a possible pathway to increase the amount of the produced fuel gases by designing a suitable reactor and recovering the gas fuel in time. Finally, kinetic behaviors of coking wastewater have been analyzed.     

The effect of temperature and topological defects on H2 adsorption on carbon nanotubes

Physisorption of molecular hydrogen on pristine single-walled carbon nanotube and three types of topologically defected nanotubes (Stone-Wales, vacancy and interstitial defects) at different temperatures 77, 300 and 600 K has been investigated via molecular dynamics simulation. The interatomic interactions (covalent bonds) between the carbon atoms within the nanotube wall were modeled by the well-known bond order Tersoff potential. The applied intermolecular forces are modeled using the modified form of the well-known Lennard-Jones potential based on the nanotube curvature. The adsorption/desorption cycle was followed by increasing the operating temperature under the pressure of 1 bar. The simulation results of exposing 6.5%wt of H₂ on defected and pristine (3,3) nanotubes reveal that the highest and lowest adsorption energies and storage capacities are obtained from the nanotubes with interstitial and vacancy defects, respectively.     

不同复合页岩模型内气体竞争吸附特性的分子模拟

构建了高岭石-干酪根IID、蒙脱石-干酪根IID和方解石-干酪根IID三种复合模型,采用巨正则蒙特卡罗方法(GCMC)和分子动力学方法(MD),从分子层面研究了CH4与注采气体(CO2、N2)在复合页岩材料中的吸附情况,分析了不同模型对不同气体的吸附能力.结果 表明:在高岭石-干酪根IID复合模型中三种气体的吸附量为m...     

CH4/H2S在TiO2纳米通道中吸附分离的分子动力学模拟

采用分子动力学模拟方法研究了CH_4/H_2S混合流体在TiO_2纳米通道中吸附分离的过程。结果显示,TiO_2纳米通道对H_2S的吸附分离能力大于其对CH_4的吸附分离能力。当体系施加电场时能够一定程度提高纳米通道对H_2S的吸附分离能力,但是随着电场增强,分离能力提升不大,表明电场的影响是有限的。而通道内壁TiO_2粒子振动条件下吸附分离效果也比壁面粒子固定不动时的效果好。根据研究,建议在一定的电场强度下采用TiO_2纳米通道分离吸附CH_4/H_2S混合流体,降低天然气中H_2S的浓度。     

Experiments on supercritical CO2 adsorption in briquettes

CO₂ storage in deep coal seams is one of the safe methods to mitigate the effects of greenhouse. In the present work, experiments of supercritical CO₂ adsorption were performed at 35℃, 45℃, and 55℃ on briquettes, the porosity of which can be controlled. Curves of CO₂ adsorption increased at three rates at pressures ranging from 8 to 13 MPa, and the adsorption decreased linearly with the increase of temperatures from 35℃ to 55℃. The adsorption of supercritical CO₂ was fitted best with Brunauer, Emmett, and Teller model compared with the Langmuir model and Dubinin? Radushkevich model.     

Electronic structure calculations and molecular dynamics simulations of hydrogen adsorption on Beryllium doped complexes

Interaction of molecular hydrogen with Be and Be₂ decorated acetylene is studied using first principles calculations and molecular dynamics simulations. C₂H₂Be and C₂H₂Be₂ complex can interact with maximum of two and four H₂ molecules respectively thereby showing respective H₂ uptake capacity of 10.3 and 15.5 wt % and is well above the target set by U.S. department of energy. Temperature dependent Gibbs free energy corrected adsorption energy shows that H₂ adsorption on C₂H₂Be is energetically favorable below 250 and 200 K and 1 atm. pressure at MP2/6-311++G** and CCSD(T)/6-311++G** level whereas it is energetically favorable on C₂H₂Be₂ at entire temperature and pressure range considered here from 50 K to 400 K and 50 atm to 400 atm. The desorption temperature obtained for C₂H₂Be(2H₂) and C₂H₂Be₂(4H₂) complexes at MP2/6-311++G** level using van't Hoff equation is 281 and 638 K respectively. Molecular dynamics simulations performed at different temperature show that all the adsorbed H₂ molecules remain adsorbed during the simulations on a complex at temperature T only if Gibbs free energy corrected H₂ adsorption energy at that temperature T is positive. H₂ adsorption on Be decorated ethylene is also studied and the results are compared with Be decorated acetylene.     

Adsorption mechanism and improvements of the adsorption equation for adsorption refrigeration pairs

An improved adsorption model for adsorption refrigeration pairs such as activated carbon–methanol and zeolite–water is suggested based upon normal Dubinin adsorption equations. This model has been verified by various experimental results. Copyright © 1999 John Wiley & Sons, Ltd.     

Single- and double-bed pressure swing adsorption processes for H2/CO syngas separation

Pressure swing adsorption (PSA) technology is an effective method to extract hydrogen from synthesis gas (syngas) and purify the produced hydrogen. The dynamic adsorption models for syngas (H₂/CO 70/30 mol%) treatment by single- and double-bed PSA systems with zeolite 5A were developed. The breakthrough curves of the single-bed hydrogen purification PSA system were studied. Subsequently, the performance of the single- and double-bed PSA cycles was studied. The models were built and implemented using the Aspen Adsorption platform. After model validation and successful simulation of the breakthrough curves in the single-bed model, the simulation of five- and six-step PSA cycles in the single-bed and double-bed models, respectively, were carried out. A parametric study of both single- and double-bed models was then carried out. The results reveal that the simulated breakthrough curves agree with the experimental curves very well. The parametric study shows that, with certain range of 1.38 × 10⁻⁶ to 2.08 × 10⁻⁶ kmol/s for feed flow rate, the adsorption time of 240–360 s for single-bed and 180–300 s for double-bed, a lower feed flow rate and shorter adsorption time leads to higher purity, lower recovery, and lower productivity. For the double-bed PSA model, the influence of the pressure equalization time, with the range of 5–40 s, on the PSA process was also studied. It can be found that, as the pressure equalization time increased, better purity and recovery but lower productivity were obtained. The results show that, at a feed flow rate of 1.58 × 10⁻⁶ kmol/s, the recovery and productivity of the double bed are higher by 11% and 1 mol/kg/h, respectively, than those of the single bed.     

Br改性Mg-MOF-74吸附分离CO2性能研究

金属有机骨架材料Mg-MOF-74因不饱和金属位的存在具有低压下较高的CO_2吸附量,且具有化学表面可修饰、可调控孔径等特点。基于密度泛函理论和巨正则蒙特卡罗方法对Mg-MOF-74进行官能团Br改性,发现Br改性使得苯环附近产生更强的静电势梯度,增强了骨架原子和极性CO_2分子间的相互作用,利于CO_2在骨架孔道内的吸附。但Br的引入带来了骨架自身比表面积、孔体积的下降,不利于在高压区CO_2吸附。φ(CO_2)∶φ(N2)=15∶85条件下,Br改性使得骨架对混合气体中CO_2分离比相比改性前提高了近64%。在含湿条件下(φ(CO_2)∶φ(N2)∶φ(H2O)=15∶84∶1),Br改性使得H2O质量吸附量大大下降,低压下的分离比得到提高。     

Molecular dynamics simulation of H2 in amorphous polyethylene system: H2 diffusion in various PE matrices and bubbling during rapid depressurization

Polyethylene (PE) is a candidate liner material for Type IV storage devices. In this case, all-atom molecular dynamics simulations are employed to study the properties of Polyethylene with the presence of H₂, including tensile properties, glass transition, diffusion in different PE and bubbling during rapid depressurization. The presence of H₂ deteriorates the polyethylene matrix's tensile performance and decreases the glass transition temperature. The branch, side chain and small molecules promote the diffusion of H₂ in the amorphous region by introducing more free volume below Tg. With a sufficient length, the length of polymer chain has minor effect on the diffusion of H₂. Graphene, as a 2D reinforcement, could decrease the diffusion of H₂ but suffers from poor interfacial bonding. Finally, H₂ bubble(s) formed from the over-saturated H₂ molecule and were observed in both the exclusive and free volume and stabilised at low pressure during rapid depressurization. According to the result obtained in this work, branchless HDPE is expected to give superior performance while the viscosity, which is important during processing, could be tailored by molecular weight. Processing technique leads to orientation is preferred, such as injection moulding.     

Research on the reverse flotation of kaolinite from fine coal on basis of adsorption behavior

The strong hydrophilic properties of both sub-bituminous coal and kaolinite make it difficult to separate by direct flotation. In this paper, the removal of kaolinite from fine sub-bituminous coal was investigated by reverse flotation tests using N, N-dimethyl dodecyl amine (DRN₁₂) as a kaolinite collector. In addition, the adsorption behavior of DRN₁₂ on coal and kaolinite surfaces was also studied to explore its interaction mechanism The experimental results showed that the beneficiation of kaolinite from the raw coal was effective only in the acid pulp with DRN₁₂ less than 1.5kg/t. Moreover, in acid solutions, DRN₁₂ preferentially adsorbs on kaolinite surface by electrostatic force, and the adsorption capacity of DRN₁₂ on kaolinite surface was much higher than on coal, which caused an increase of kaolinite hydrophobicity and floatability.     

Boron substitution effect on adsorption of H2 molecules on organometallic complexes

Hydrogen adsorption properties of Be/Sc doped pentalene complexes are investigated using second ordered Møller-Plesset method (MP2). In order to study the boron substitution effect, pentalene is further modified by substituting two and four boron atoms for carbon atoms at different positions and named as TBP1 and TBP2 for two boron atom substituted structures and FBP1 and FBP2 for four boron atom substituted structures. Two H₂ molecules get adsorbed on each Be doped complex and having 3.25, 3.31, 3.31, 3.38 and 3.38 wt% H₂ uptake capacity for C₈H₆Be₂, TBP1Be₂, TBP2Be₂, FBP1Be₂ and FBP2Be₂ complexes respectively. All Sc doped pentalene and boron substituted pentalene complexes can interact with nine H₂ molecules except TBP2Sc₂ complex. The TBP2Sc₂ complex can adsorb eight H₂ molecules. The H₂ uptake capacity is found to be 8.63, 8.73, 7.84, 8.83 and 8.83 wt% for C₈H₆Sc₂, TBP1Sc₂, TBP2Sc₂, FBP1Sc₂ and FBP2Sc₂ complexes respectively. Gibbs free energy corrected adsorption energy plots show that the H₂ adsorption on all Be doped complexes is possible at all temperatures and pressures considered here. The TBP1Sc₂ complex seems to be more promising hydrogen storage material among all Sc doped complexes over a wide range of temperature and pressure. The H₂ desorption temperatures obtained for the Be doped complexes are higher than the Sc doped complexes. Stability of the complexes is predicted with the help of the gap between the highest occupied molecular orbitals and the lowest unoccupied molecular orbitals.     

Molecular dynamics simulations on the effect of temperature and loading in H2 exohedral adsorption on and SWCNTs

The adsorption of H₂ on single walled carbon nanotubes is investigated as a function of temperature, H₂ loading and diameter of the nanotubes. The physisorption phenomenon is simulated by extensive equilibrium molecular dynamics. The applied intermolecular forces are modeled using the modified form of the well-known Lennard-Jones potential based on the tube curvature. The simulation results of exposing different H₂ loadings on (3,3) and (9,9), at 77,300 and 600 K, under moderate pressure of 10 bar, show that the amount of adsorption is strongly influenced by the applied temperature, and that the adsorption energy is higher for nanotubes with smaller diameters. Moreover, analyzing the deformation of the nanotube adsorbents during the simulation time indicates that increasing the operating temperatures not only decreases the amount of adsorption monotonically but also imposes more nanotube distortions.     

A novel process of cyclic adsorption and plasma-desorption for ppm-level CO removal from H2-rich stream

A cyclic adsorption and plasma-desorption (CAPD) process is applied to remove ppm-level carbon monoxide (CO) from the H₂-rich stream for the first time. In the CAPD process, CO is first selectively adsorbed on CuCl/γ-Al₂O₃ till adsorption saturation; Then, the adsorbed CO is in-situ desorbed by plasma generated in dielectric barrier discharge (DBD) and the adsorbent is regenerated simultaneously. The effects of discharge power (P_dis) and flow rate of discharge gas (F_D) on performances of plasma-desorption are investigated. The time to reach desorption of 50% (t₅₀) and 90% (t₉₀) significantly decreases with P_dis but is weakly correlated to the F_D. At the investigated conditions, t₅₀ and t₉₀ are lower than 2.6 min and 4.5 min, respectively, which indicates that plasma-desorption features high desorption rate. In addition, desorption efficiency of plasma-desorption is higher than 98%. Moreover, CAPD process stability is preliminarily confirmed by six consecutive adsorption-desorption cycles.     

喷枪结构对低温超音速火焰喷涂颗粒飞行特性的影响

建立了低温超音速火焰喷涂传热和流动模型,对喷涂过程的焰流和颗粒的运动加热历程进行了模拟分析.研究了三种不同结构的喷枪对焰流速度与温度分布、不同粒径颗粒飞行特性的影响.模拟结果表明,粒径为20μm的Cu颗粒在撞击基板时能达到临界速度,且温度低于熔点,有利于沉积并减少了颗粒氧化;枪管的扩张率对喷涂颗粒的速度影响不大,而对颗粒温度的影响较大;延长扩张段的长度代替平直枪管有利于在保证颗粒速度的同时提高颗粒的温度.     

Experimental study on the interactions of supercritical CO2 and H2O with anthracite

To better understand the mechanism of interactions of supercritical CO₂ (SCCO₂) and H₂O, untreated, SCCO₂ treated and SCCO₂-H₂O treated anthracites were adopted to analyze changes in pore structure, adsorption capacity. Observations from experimental data reveal that mineral and other substances in the fractures and matrix of coal body are dissolved and mobilized by the carbonic acid formed from the mixture of SCCO₂ and H₂O, which may contribute to smaller pore development and enhance the adsorption capacity. These findings may provide new insights into effective and safe storage of CO₂ in coal reservoir.     

Electrochemical investigations on CO2 reduction mechanism in molten carbonates in view of H2O/CO2 co-electrolysis

In order to reduce carbon dioxide emission, one solution is to convert into valuable chemicals or fuels, e.g. transforming CO₂ into CO by electrochemical reduction. Thus, this greenhouse gas could be re-used in particular as syngas (CO + H₂) by co-electrolysis of CO₂/H₂O. High temperature electrolysis cells can be the best energetic devices to produce such syngas. In particular, molten carbonates are known to solubilize CO₂ very significantly higher than other solvents. Therefore, it is compulsory to investigate and understand the mechanism of CO₂ reduction in such media to consider its further use and valorisation. The present study is a critical approach aiming at elucidating the mechanisms for CO₂ electroreduction, using an inert Pt electrode in the molten eutectic Li₂CO₃–K₂CO₃ (62-38 mol%), at 650 °C, under different partial pressures of CO₂. Complementary electrochemical techniques, including sweep square-wave voltammetry and relaxation chronopotentiometry, were carried out. Their combination allowed us to evidence that the electroreduction of CO₂ into CO is feasible in oxo-acidic conditions, involving a diffusion-limited quasi reversible system in a one electron-step.