An improved theoretical procedure for the pore-size analysis of activated carbon by gas adsorption

Amorphous carbon materials play a vital role in adsorbed natural gas (ANG) storage.One of the key issues in themore prevalent use of ANG is the limited adsorption capacity,which is primarily determined by the porosity and surface characteristics of porous materials.To identify suitable adsorbents,we need a reliable computational tool for pore characterization and,subsequently,quantitative prediction of the adsorption behavior.Within the framework of adsorption integral equation (AIE),the pore-size distribution (PSD) is sensitive to the adopted theoretical models and numerical algorithms through isotherm fitting.In recent years,the classical density functional theory (DFT) has emerged as a common choice to describe adsorption isotherms for AIE kernel construction.However,rarely considered is the accuracy of the mean-field approximation (MFA) commonly used in commercial software.In this work,we calibrate four versions of DFT methods with grand canonical Monte Carlo (GCMC) molecular simulation for the adsorption of CH4 and CO2 gas in slit pores at 298 K with the pore width varying from 0.65 to 5.00 nm and pressure from 0.2 to 2.0 MPa.It is found that a weighted-density approximation proposed by Yu (WDA-Yu) is more accurate than MFA and other non-local DFT methods.In combination with the trapezoid discretization of AIE,the WDA-Yu method provides a faithful representation of experimental data,with the accuracy and stability improved by 90.0％ and 91.2％,respectively,in comparison with the corresponding results from MFA for fitting CO2 isotherms.In particular,those distributions in the feature pore width range (FPWR)are proved more representative for the pore-size analysis.The new theoretical procedure for pore characterization has also been tested with the methane adsorption capacity in seven activated carbon samples.     

Monte Carlo模拟对称型三嵌段共聚高分子的吸附

The adsorption behavior of symmetric triblock copolymers, Am/2BnAm/2, from a nonselective solvent at solid-liquid interface has been studied by Monte Carlo simulations on a simple lattice model. Either segment A or segment B is attractive, while the other is non-attractive to the surface. Influences of the adsorption energy,bulk concentration, chain composition and chain length on the microstructure of adsorbed layers are presented.The results show that the total surface coverage and the adsorption amount increases monotonically as the bulk concentration increases. The larger the adsorption energy and the higher the fraction of adsorbing segments, the higher the total surface coverage is exhibited. The product of surface coverage and the proportion of non-attractive segments are nearly independent of the chain length, and the logarithm of the adsorption amount is a linear function of the reciprocal of the reduced temperature. When the adsorption energy is larger, the adsorption amount exhibits a maximum as the fraction of adsorbing segment increases. The adsorption isotherms of copolymers with different length of non-attractive segments can be mapped onto a single curve under given adsorption energy. The adsorption layer thickness decreases as the adsorption energy and the fraction of adsorbing segments increases, but it increhses as the length of non-attractive segments increases. The tails mainly govern the adsorption layer thickness.     

用于吸附过程开发的从分子水平级开始的多尺度模拟

This article presents a multiscale simulation approach starting at the molecular level for the adsorption process development. A grand canonical Monte Carlo method is used for the prediction of adsorption isotherms of methanol on an activated carbon at the molecular level. The adsorption isotherms obtained in the linear region （or adsorption constant） are exploited as a model parameter required for the adsorption process simulation. The adsorption process model described by a set of partial differential equations （PDEs） is solved by using the conservation element and solution element method, which produces a fast and an accurate numerical solution to PDEs. The simulation results obtained from the adsorption constant estimated at the molecular level are in good agreement with the experimental results of the pulse response. The systematical multiscale simulation approach addressed in this study may be useful to accelerate the adsorption process development by reducing the number of experiments.     

两液层体系传质引发的Marangoni效应的数值模拟

The Marangoni effect induced by mass transfer at the interface between two immiscible liquids displays important influence on laboratory and industrial operation of solvent extraction. A systematic numerical study of the two-dimensional Marangoni effect in a two liquid layer system was conducted. The linear relationship of the inter- facial tension versus the solute concentration was incorporated into a mathematical model accounting for liquid flow and mass transfer in both phases. The typical cases analyzed by Sternling ＆ Scriven （AIChE J., 1959） using the linear instability theory were simulated bv the finite difference method and good agreement between the theory and the numerical simulation was observed. The simulation suggests that the Marangoni convection needs certain time to develop sufficiently in strength and scale to enhance the interphase mass transfer, the Marangoni effect is dynamic and transient, and remains at some stabilized level as long as the mass transfer driving force is kept con- stant. When certain level of shear is imposed at the interface as in most cases of practical significance, the Maran- goni effect is suppressed slightly but progressively as the shear is increased gradually. The present two-dimensional simulation of the Marangoni effect provides some insight into the underlying mechanism and also the basis for further theoretical study of the three-dimensional Marangoni effect in the real world and in chemical engineering applications.     

Investigation on a vertical radial flow adsorber designed by a novel parallel connection method

Due to the increasing global demand for industrial gas,the development of large-scale cryogenic air separation systems has attracted considerable attention in recent years.Increasing the height of the adsorption bed in a vertical radial flow adsorber used in cryogenic air separation systems may efficiently increase the treatment capacity of the air in the adsorber.However,uniformity of the flow distribution of the air inside the adsorber would be deteriorated using the height-increasing method.In order to reduce the non-uniformity of the flow distribution caused by the excessive height of adsorption bed in a vertical radial flow adsorber,a novel parallel connection method is proposed in the present work.The experimental apparatus is designed and constructed;the Computational Fluid Dynamics (CFD) technique is used to develop a CFD-based model,which is used to analyze the flow distribution,the static pressure drop and the radial velocity in the newly designed adsorber.In addition,the geometric parameters of annular flow channels and the adsorption bed thickness of the upper unit in the parallelconnected vertical radial flow adsorber are optimized,so that the upper and lower adsorption units could be penetrated by air simultaneously.Comparisons are made between the height-increasing method and the parallel connection method with the same adsorber height.It is shown that using the parallel connection method could reduce the difference between the maximum and minimum radial static pressure drop by 86.2％ and improve the uniformity by 80％ compared with those of using the height-increasing method.The optimal thickness ratio of the upper and lower adsorption units is obtained as 0.966,in which case the upper and lower adsorption units could be penetrated by air simultaneously,so that the adsorbents in adsorption space could be used more efficiently.     

分子模拟二氧化碳在狭缝炭微孔中的吸附

Both the grand canonical Monte Carlo and molecular dynamics simulation methods are used to investigate the adsorption and diffusion of carbon dioxide confined in a 1.86 nm slit carbon pore at 4 temperatures from subcritical (120 K) to supercritical (313 K) conditions. Layering transition, capillary condensation and adsorption hysteresis are found at 120 K. The microstructure of carbon dioxide fluid in the slit carbon pore is analyzed. The diffusion coefficients of carbon dioxide parallel to the slit wall are significantly larger than those normal to the slit wall.     

CO2和N2吸附的研究札记

Experiments were made for the adsorption of CO2 and N2 on typical adsorbents to investigate the effects of porous structure and surface affinity of adsorbents as well as those of adsorption temperature and pressure that might cause the variation of adsorption mechanism. It is shown that polar surface tends to enlarge the adsorption difference between CO2 and N2, and the difference is more sensitive to temperature than the adsorbents with non-polar surface. The adsorbents with non-polar surface are not much sensitive to the effect of water vapor, though the water vapor interferes the separation remarkably. The separation coefficient linearly increases with the micropore volume per unit surface area of activated carbons, but no rule is shown on mesoporous silicon materials. The function of adsorption mechanism on the separation is not as much as expected.     

蛋白质在温度响应性PNIPAAm接枝硅胶表面吸附和解吸过程:分子模拟和实验研究(英文)

Poly(N-isopropylacrylamide)(PNIPAAm) grafted onto silica,which may be used for reverse phase chromatography(RPC),was simulated and synthesized for protein separation with temperature-triggered adsorption and desorption.Molecular dynamics simulation at an all-atom level was performed to illustrate the adsorption/desorption behavior of cytochrome c,the model protein,on PNIPAAm-grafted-silica,a temperature responsive adsorbent.At a temperature above the lower critical solution temperature(LCST),the PNIPAAm chains aggregate on the silica surface,forming a hydrophobic surface that is favorable for the hydrophobic adsorption of cytochrome c,which has a high exposure of hydrophobic patches.At temperatures below the LCST,the PNIPAAm chains stretch,forming hydrophilic surface due to hydrogen bonding between PNIPAAm and surrounding water.Desorption of cytochrome c on the PNIPAAm-grafted-silica surface occurs as a result of competition with water,which forms hydrogen bonds with the protein.The conformational transitions of both cytochrome c and PNIPAAm are monitored,providing molecular insight into this temperature-responsive RPC technique.PNIPAAm-grafted-silica beads were synthesized and used for the adsorption and desorption of cytochrome c at approximately 313 K and 290 K,respectively.The experimental results validate the molecular dynamics simulation.In comparison to conventional RPC,using temperature as a driving force for RPC reduces the risk of protein denaturation caused by exposure to chaotropic solvents.Moreover,it simplifies the separation process by avoiding the buffer exchange operations between the steps.     

表面活性素集成生产过程(Ⅲ)吸附柱的模型

The study is focused on modeling of separation process and optimization.An adsorption separation process is simulated.The surfactin production process by Bacillus subtilis ATCC 21332 followed by surfactin adsorption in a fixed-bed column packed with commercial active carbon is studied in laboratory.The adsorption column achieves high surfactin recovery(94%)by up-flow methanol elution at 25°C.The adsorption column is simulated with a complex one-dimensional plug flow dispersion model coupled with nonlinear adsorption equilibrium,based on the assumption that the adsorption of surfactin is monomolecular layer and no micelle is formed.The molecular diffusion coefficient of surfactin in water solution with electric neutrality is estimated to be 0.428×10 -5 cm 2 ·s -1 by molecular dynamics simulation.The model developed can describe the complex interplay of adsorption kinetics,fluid dynamics,and mass-transfer phenomena based on the assumption of no radial temperature and concentration gradients,and is of adequate precision.The work involved in this paper is valuable for the optimization of the production process of surfactin.     

正烷烃在硅石和ZSM-5与沸石上的吸附平衡

The adsorption equilibria of n-heptane, n-octane and n-nonane on silicalite and ZSM-5 have been measured in the temperature range of 373.15--473.15K under low pressure (0---5.332kPa). All the experimental data can be represented by a generalized characteristic curve of the extended adsorption potential theory utilizing the parameter of the initial heat of adsorption, which is estimated reasonably by a new approach.     

Theory of chromatography of ring-shaped block copolymers

A theory is developed for describing liquid chromatography of ring diblock and multiblock copolymers. The chromatographic behavior of ring block copolymers at different adsorption interactions is analyzed theoretically and compared with that of linear block copolymers; typical chromatograms are simulated by using the theory. In particular, it is shown that under the critical interaction condition for one block the chromatographic retention of a ring diblock copolymer is dependent of the length of the ‘critical’ block; this behavior differs qualitatively from that of a linear diblock copolymer. Ring copolymers are always more retained than linear ones, therefore such copolymers can be separated. Especially good separation of heterogeneous ring and linear block copolymers is predicted at near-critical interaction conditions. According to the theory, ring diblocks and multiblocks can be separated as well, if one component of a copolymer is adsorbing, while the other one is not adsorbing.     

共聚高分子吸附的Monte Carlo模拟

用MonteCarlo方法对选择性溶剂中两嵌段共聚高分子在固液界面的吸附进行了模拟 ,获得了吸附等温线以及吸附层厚度、链附着率、表面覆盖率、链节浓度分布等表征吸附层结构的信息 ,同时模拟获得了固液界面区吸附构型大小及分布等表征高分子构型的微观信息 ,考察了吸附性链节A所受的对比排斥能、链组成以及体相浓度等因素对这些参数的影响 .     

Spontaneous origination of chirality in melts of diblock copolymers with rigid and flexible blocks

A self-consistent mean-field computer simulation of ordering in melts of diblock copolymers consisting of flexible and rigid rodlike blocks is performed. A three-dimensional model is considered, and a corresponding algorithm for solving mean-field equations in sequential and parallelized versions is developed. The coexistence of microphase separation and orientational ordering gives rise to the appearance of new types of spatial arrangements. In particular, phases with the cubic symmetry and the morphology of hexagonally arranged chiral cylinders are found. The transition of achiral cylinders to chiral cylinders in the melt of achiral diblock copolymers consisting of rigid and flexible blocks is revealed for the first time. The origination of chirality is due to the presence of rigid blocks in the system and orientational interactions between them. With a decrease in temperature, microphase separation caused by incompatibility of chemically different blocks initially occurs in these systems. As a result, the hexagonally ordered structure in which rigid blocks are concentrated in cylindrical microdomains arises. A further decrease in temperature results in the involution of cylindrical microdomains and the formation of a helical structure. To quantify the degree of chirality, a new pseudoscalar index, depending on the linear-scale parameter for which the chirality is studied, is suggested.     

Controlling the phase behavior of block copolymers via sequential block growth

Block copolymers remain one of the most extensively investigated classes of polymers due to their abilities to self-organize into various nanostructures and modify polymer/polymer interfaces. Despite fundamental and technological interest in these materials, only a handful of experimental phase diagrams exist due to the laborious task of preparing such diagrams. In this work, two copolymer series are each synthesized from a single macromolecule via sequential living anionic polymerization to yield molecularly asymmetric diblock and triblock copolymers systematically varying in composition. The phase behavior and morphology of these copolymers are experimentally interrogated and quantitatively compared with predictions from mean-field theories, which probe copolymer phase behavior beyond current experimental conditions.     

Influence of block lengths and symmetries of block copolymers on phase behavior of polymer A/polymer B/block copolymer ternary blends

Monte Carlo simulations were used to investigate the compatibilizing effects of diblock copolymers in A/B/A-B diblock copolymer ternary blends and triblock copolymers in A/B/triblock copolymer ternary blends, respectively. The volume fraction of homopolymer A was 19% and was the dispersed phase. The simulation results show that diblock copolymers with longer A-blocks are more efficient as compatibilizers, and symmetric triblock copolymers with a shorter middle block length are easily able to bridge each other through the association of the end blocks. This kind of triblock copolymers have relatively high ability to retard phase separation as compatibilizers.     

Acrylic block copolymer analysis by adsorption chromatography with evaporative light scattering detection

Block copolymers of methyl methacrylate and butyl methacrylate are characterized using silica adsorption chromatography with a toluene-2-butanone solvent gradient. Evaporative light scattering is the detection method for these otherwise hard to detect acrylic polymers. The homopolymer contamination in the diblock materials are well resolved from the diblock in the separation. The major separation mechanism is composition controlled, but the results also indicate a block length dependency on the separation mechanism.     

Controlled release of bovine serum albumin using MPEG–PCL diblock copolymers as implantable protein carriers

MPEG–PCL diblock copolymers consisting of methoxy polyethylene glycol (MPEG) and poly(?‐caprolactone) (PCL) as drug carriers were synthesized by ring‐opening polymerization. It is possible to control the balance between hydrophilic and hydrophobic by changing the MPEG and the ratio of ?‐CL to MPEG. Implantable wafers were easily fabricated by the direct compression method after physical mixing of diblock copolymers and bovine serum albumin–fluorescein isothiocyanate (BSA‐FITC) as a model protein drug. The BSA release from wafers prepared by MPEG–PCL diblock copolymers were higher than that from PCL with the physical blending of MPEG. The wafers prepared by a variety of MPEG–PCL diblock copolymers exhibited the controlled BSA release profiles with a dependence on MPEG–PCL diblock copolymer compositions. In addition, the changing of MPEG and PCL molecular weights within MPEG–PCL diblock copolymer controlled the initial burst of BSA. We confirmed that the diblock copolymers could be served as protein delivery carrier in implantable wafer form. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102:1561–1567, 2006     

Sphere-forming diblock copolymers confined in a square well

By means of the mean-field dynamic density functional theory (DDFT), the self-assembly behavior of the sphere-forming diblock copolymers confined in the square well has been studied systematically. Various novel structures are predicted, such as a string of spheres, stacked square bricks, a row of elongated potato-like micelles rotated with respect to each other, a single helix, two staggered helices, well-ordered square array of cylinders, highly ordered body-center-cubic (BCC) spheres, highly ordered face-center-cubic (FCC) spheres and concentric layers. Those morphologies provide the theoretical evidence for the experimental observations. Moreover, some interesting phenomena are found, such as the decrease of the number of layers with the increase of the surface field and the formation of the soft spheres, which can be observed in the experiments. Furthermore, a new order parameter is defined to characterize the degree of micro-phase separation for each kind of microscopic structure, and the key factor in affecting micro-phase separation is discussed.     

Orientational ordering in blends of flexible and rigid diblock copolymers

A self-consistent mean-field computer simulation of ordering in blends of flexible and rigid rodlike diblock copolymers is performed. Each type of rigid block is selective toward one of the types of flexible blocks and is incompatible with the other. Flexible diblocks form a predominant component of the blend with parameters providing formation of a stable hexagonal morphology. As a result, orientational ordering is induced in the blend by a minor admixture of rigid diblocks, while the type of ordering is determined by the composition of the rigid diblocks. Three different types of orientational ordering, which having a clearly pronounced electromagnetic analogy, are established during variation in the composition of the rigid diblocks. Near a cylindrical micelle of the hexagonal morphology, the vector fields of primary directions of the orientational-order-parameter tensors for the three orientation types are highly similar to the magnetic field of the infinitely long rectilinear direct current, the cylindrical solenoid magnetic field with a constant linear current density, and the electrostatic field of a uniformly charged straight line, respectively.     

Cooperative surface-induced self-assembly of symmetric diblock copolymers confined films with embedded nanorods

The self-assembly of symmetric diblock copolymers confined films with embedded nanorods is investigated by the self-consistent field (SCF) theory. We obtain some phase diagrams as a function of film thickness H and nanorod diameter D. The increase in preferential nanorod diameter D can promote the formation of incomplete cylindrical and spherical structures near the film surfaces, and can also induce complete lamellar, cylindrical and spherical structures in the interior. The formation of these induced self-assembled structures is due to the competition between inner surface confinement (two parallel surfaces) and outer surface confinement (nanorods). This investigation can provide some insights into the self-assembly of diblock copolymers with complex confinements.