微纳多孔结构中稀薄气体流动渗透率的解析型预测模型

针对多孔结构内气体表观渗透率受稀薄效应的影响而显著高于其固有渗透率的现象,从孔隙尺度流线的几何拓扑特性出发,提出了利用固有渗透率、孔隙率、弯曲度和收缩-扩张因子来表示多孔结构的有效孔隙尺寸的方法,并将该有效孔隙尺寸与经典的稀薄气体管道流动模型相结合,理论推导出一种新的多孔结构稀薄气体渗透率模型。利用该模型,可以在孔隙几何结构和物性状态已知的条件下对气体的表观渗透率进行预测。随后,通过高精度的直接模拟Monte Carlo方法(DSMC)对提出模型的准确性进行验证。通过对Knudsen数在0.01~10范围、孔隙率在0.17~0.90范围、不同气体工质以及多种有序性孔隙形式下的气体流动过程进行数值模拟表明,所提出的理论模型与模拟数据的平均偏差小于10%。     

微纳多孔结构中稀薄气体流动渗透率的解析型预测模型

针对多孔结构内气体表观渗透率受稀薄效应的影响而显著高于其固有渗透率的现象,从孔隙尺度流线的几何拓扑特性出发,提出了利用固有渗透率、孔隙率、弯曲度和收缩-扩张因子来表示多孔结构的有效孔隙尺寸的方法,并将该有效孔隙尺寸与经典的稀薄气体管道流动模型相结合,理论推导出一种新的多孔结构稀薄气体渗透率模型。利用该模型,可以在孔隙几何结构和物性状态已知的条件下对气体的表观渗透率进行预测。随后,通过高精度的直接模拟Monte Carlo方法(DSMC)对提出模型的准确性进行验证。通过对Knudsen数在0.01~10范围、孔隙率在0.17~0.90范围、不同气体工质以及多种有序性孔隙形式下的气体流动过程进行数值模拟表明,所提出的理论模型与模拟数据的平均偏差小于10%。     

The permeability of fibrous porous media

The literature was searched for experiments and theories related to low-Reynolds-number flow through fibrous porous media, particularly highly porous structures. Experimental data were found for a wide range of materials, from polymer chains to fiberglass, and the results collapse reasonably well when the appropriate dimensionless co-ordinates are employed. Of the theories, accurate solutions of Stokes equation are available for regular arrays of parallel rods, either aligned with or normal to the flow. For irregular arrays and three-dimensional media, approximate permeabilities can be calculated from several flow models.     

Temperature-responsive permeability of porous PNIPAAm-g-PE membranes

Grafting a temperature-responsive polymer, poly(N-isopropylacrylamide) (PNIPAAm), onto porous polyethylene (PE) membranes by UV irradiation was investigated. A wide range of graft yields (5–449%) was achieved by varying irradiation time (20–240 min) and monomer concentration (1.2–3.6 wt %). Characterization by XPS and SEM shows that the graft polymers are located both on the external surfaces as well as inside the pores of the membranes. Diffusional permeation experiments show that two distinct types of temperature responses were observed, depending on the graft yield; permeability increases with temperature in low graft yield membranes, while permeability decreases with temperature in high graft yield membranes. A mechanism explaining the dual valve functions of the graft membrane is proposed based on the location of the graft polymers on the membrane. It was also observed the permeability response exhibits a maximum with permeant molecular weight. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 70: 2133–2142, 1998     

The mechanism of porous membrane formation from polymer solutions

Translated from Khimicheskie Volokna, No. 5, pp. 26–28, September–October, 1991.     

Estimation and determination of the permeability of porous ceramics

The possibility of estimating the permeability of porous ceramic materials both with mercury and with gas liquid pore diagrams and based on model concepts concerning the porosity and average size of the constituent particles in the material was demonstrated. It was shown that the permeability of a porous material and the average pore radius can be directly determined with the linear segment of the reverse GLP curve and the capillary sinuosity can be calculated with it.     

Modification of porous polyacrylonitrile membrane

The effects of the modification of polyacrylonitrile ultrafiltration membranes with sodium hydroxide are discussed. The NaOH-induced hydrolysis of nitrile groups on the membrane surface results in membranes with decreasing pore diameter. The average pore diameter changes from 2.6 to 0.6 nm in the progress of hydrolysis. The modified membranes are less prone for protein deposition. Fouling causes a pore diameter reduction of 80% for the untreated and 20% for the modified membranes. Modification results also in the creation of the membranes working in the nanofiltration mode. The unmodified membrane does not reject salt while a membrane immersed in the modification bath is capable of rejecting about 50% of calcium carbonate.     

A single parameter equation for the prediction of multicomponent vapor-liquid data from binary isobaric data

A single parameter Wilson equation that is suitable for use with isobaric systems is presented. Parameters for both the original and modified equations are obtained from isobaric binary data and from infinite dilution activity coefficients and then used to predict multicomponent vapor-liquid data for systems up through a quinary. Satisfactory results were obtained from the use of infinite dilution activity coefficients and the sensitivity of the final result to the accuracy of these end points is discussed.     

Density prediction of multicomponent aqueous solutions from binary data

An isopycnotic (equal density) relation has been developed for non-interacting solutes in multicomponent aqueous solutions. The relation can be used to estimate ternary densities from binary data. Predicted and observed values for 26 systems at 25°C are compared, usoing Young's rule, the isopycnotic, and other methods. Young's rule and the isopycnotic methods give relatively better predictions (<0.2% error) even when applied to concentrated solutions.     

Toward a continuous synthesis of porous carbon xerogel beads

A continuous process for producing porous carbon xerogel beads has been developed. It consists in injecting a pre‐cured aqueous solution of resorcinol and formaldehyde on top of a column filled with hot oleic acid. The latter is pumped on the top of the column and fed at the bottom, generating an upward flow that can be adjusted to match the terminal velocity of the settling beads. Thus, the bead residence time in the column can be adjusted to match the gelation time, allowing the beads to solidify before reaching the bottom of the vessel. The obtained beads are subsequently dried and pyrolyzed. The developed experimental setup proved the continuous synthesis of porous carbon beads is possible. Nevertheless, the shaping process caused various texture changes of the porous carbon, which mainly yields macropores instead of micro and mesopores. This process also leads to the build‐up of a denser skin around the beads. © 2018 American Institute of Chemical Engineers AIChE J, 64: 1049–1058, 2018     

Effect of membrane preparation conditions on solute permeability in chitosan membrane

Chitosan membrane was prepared in various conditions and diffusive permeabilities of theophylline and vitamin B₁₂ were investigated. The membrane preparation procedure consists of dissolving chitosan in acid solution, cast on the glass plate, drying the dope solution, and immersion of the plate in the gelating agent. Effects of the kinds of acids to dissolve chitosan, chitosan concentration, drying time of the dope solution, and the kinds of the gelating agent on the membrane structure and performance were studied in detail. With increasing the chitosan concentration, the solute permeability decreased, while the selectivity of theophylline to vitamin B₁₂ increased. The membrane changed from the wholly porous structure to the asymmetric structure by the increase of the chitosan concentration. Furthermore, the use of ethanol as the gelating agent brought about the wholly porous structure with the high permeability and the low selectivity. The asymmetric structure and the wholly dense structure were obtained in the cases of the gelating agents, such as aqueous NaOH solution and dimethyl sulfoxide (DMSO), respectively. Thus, the membrane structure can be controlled by the kinds of gelating agent. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 73: 2715–2725, 1999     

Determination of Darcian permeability of porous material by infrared spectrometry

The active cooling of aerospace structures can be performed by the use of porous materials. It requires characterizing its permeability to predict the cooling efficiency by means of Computational Fluid Dynamics codes. The Darcian term is generally deduced experimentally from a relationship between the mass flow rate and the pressure drop through the porous media. Due to thermo-chemical process involved in the cooling, the permeability can change. It is currently not possible with common techniques to determine these variations during the flight. This paper presents a novel approach taking advantage of well known flow behaviour in chemical reactor engineering in order to propose a real-time in situ quantification of the Darcian permeability. The residence time distribution is analyzed thanks to tracer injection and to associated experimental measures of Infra-Red signals. The IR peak characteristics (height, width, surface, rising and falling gradients) and time delays are analyzed and correlated to physical parameters (mass flow rate, injected mass of tracer, properties of fluids). The peak height and surface and the rising gradient vary linearly in the same sense as the injected tracer mass while the falling gradient varies in the opposite sense. Both gradients decrease with a mass flow rate increase. The time delay between injection and detection of the tracer is quite constant except when changing the fluid nature. A design of experiments allowed estimating quantitatively the influence of each physical parameter on the optical one of the IR signal. Thanks to this first understanding, the Darcian permeability is linked to the observed IR signal. A linear regression is proposed with the peak width which is judged to be the most relevant parameter. Finally, an analytical approach is developed to fit ordinary differential equations to IR peak measures and to correlate the parameters of the law to the Darcian permeability. Several laws (linear, logarithmic and power) are proposed depending on the parameter but the linear regression involving the coefficient noted ?? ₁ is the most promising. One of the advantages of this method is to be able measuring the permeability during the flight and not only on-ground with specific test facility.     

Air plasma treatment of polyacrylonitrile porous membrane

The effect of plasma treatment of porous polyacrylonitrile membranes is discussed. It is shown that air plasma increases the surface polarity at low-energy treatment and gradually diminishes it when the energy is increased. When the plasma is set at energies of 12, 30 or 60 W, etching of polymers dominates. Pore diameters of the membranes increase and their distribution functions become broader. When plasma of higher energies (120 or 180 W) is used, the effect of changes of the pore distribution disappears. Membranes modified with plasma of 60, 120 or 180 W do not differ in their porous structure. A simple method of an evaluation of pore diameter distribution function is presented.     

介尺度视角下的电催化：从界面、隔膜到多孔电极

电催化过程是时空多尺度的复杂系统,探究认识多孔电极电催化体系中不同层次和尺度下的介尺度行为,对进一步强化电催化反应以及物质扩散传递,提高效率减低能耗具有重要的意义。围绕多孔电极、电催化剂以及隔膜,对多级孔电极结构的构筑、电催化剂表界面活性位调控、介尺度结构可控制备策略和隔膜结构调控中存在的介尺度现象以及介尺度效应进行了详细的综述,发现电催化体系中各层次之间的介尺度行为可以用于优化、调控、指导多孔电极、电催化剂以及隔膜的设计与制备,为丰富和完善电化学催化体系提供了新思路和新角度。     

Analytical expressions for predicting permeability of bimodal fibrous porous media

Pressure drop is one of the most important characteristics of a fibrous media. While numerous analytical, numerical, and experimental published works are available for predicting the permeability of media made up of fibers with a unimodal fiber diameter distribution (referred to as unimodal media here), there are almost no easy-to-use expressions available for media with a bimodal fiber diameter distribution (referred to as bimodal media). In the present work, the permeability of bimodal media is calculated by solving the Stokes flow governing equations in a series of 3-D virtual geometries that mimic the microstructure of fibrous materials. These simulations are designed to establish a unimodal equivalent diameter for the bimodal media thereby taking advantage of the existing expressions of unimodal materials for permeability prediction. We evaluated eight different methods of defining an equivalent diameter for bimodal media and concluded that the area-weighted average diameter of Brown and Thorpe [2001. Glass-fiber filters with bimodal fiber size distributions. Powder Technology 118, 3-9], volume-weighted resistivity model of Clague and Phillips [1997. A numerical calculation of the hydraulic permeability of three dimensional disordered fibrous media. Physics of Fluids 9 (6), 1562-1572], and the cube root relation of the current paper offer the best predictions for the entire range of mass (number) fractions, 0?n_c?1, with fiber diameter ratios, 1?R_cf?5, and solidities, 5?α?15.     

Characterisation of porous solids by simplified gas relative permeability measurements

A novel design for the application of a simplified experimental procedure, capable of providing satisfactory relative permeability data for porous media at low relative pressures, is presented. The technique avoids the two-component mixing complications as well as the problems arising from the sample macroscopic inhomogeneity. The latter has been reduced with the aid of a variable pressure compaction procedure. Thus, it becomes easier to utilise the powerful relative permeability technique for the characterisation of the structure of porous solids. Emphasis is given to the combination of the relative permeability data obtained with an analytical approach based on Effective Medium Theory for the calculation of pore connectivity.     

Control of the permeability of a porous media using a thermally sensitive polymer

Experiments explore the reduction in permeability of a porous bead pack when a suspension of thermally responsive polymer is injected and the temperature then increased above the thermal activation temperature. The change in permeability is greater with higher polymer concentration, provided that the ionic concentration of the solution is sufficient for floc formation. The time for activation of the blocking effect is within tens of seconds to minutes of when the polymer solution is heated. This is consistent with the timescale for diffusion‐limited aggregation, although the detailed value depends on the geometry and polymer concentration. Dynamical experiments demonstrate that once the porous media is blocked, adding additional polymer has no effect. The mechanism for permeability reduction may be modeled in the context of a pore‐network model, and we build a simple model to illustrate the permeability reduction as a function of the fraction of pores links which are blocked. © 2014 American Institute of Chemical Engineers AIChE J, 60: 1193–1201, 2014     

Determination of optimized membrane area from long-term flux decline data

Due to membrane fouling, long-term flux decline follows a general trend: it starts with a steep decline followed by a gradual decrease and approaches its final value asymptotically. Coupled with the concentration effect, our data indicate that this flux behavior can be represented with the equation F = f(t) (k₁ 1n r + k₂) with f(t) = a₀ + exp [a₁ + a₂t + a₃t²] where t is time, a₀, a₁, a₂, a₃, k₁, and k₂ are constants and r is the concentration ratio.

To take advantage of early high flux, ultrafiltration is usually discontinued after a period of time t_u, to be replaced by membrane cleaning period of time t_c. Only then the ultrafiltration cycle is repeated. The total membrane area required for multistage ultrafiltration can be shown as

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where Q₀ is the ultrafiltration capacity, n is the number of stages, and rr_i is the concentration ratio at stage i. An optimization method can be utilized to determine minimum value of A_T. Results will be presented for the case of ultrafiltration of palm oil mill effluent.     

On the apparent permeability of a porous layer backed by a perforated plate

Two-dimensional slow viscous flow from a fluid reservoir, through a porous layer and then through a perforated plate is studied assuming Stokes flow in the fluid reservoir and Darcy flow within the porous medium. It is first shown that the coupled Stokes/Darcy problem can be reduced to a Darcy problem when the various length scales are constrained such that Darcy's law is appropriate to describe flow in the porous layer in the vicinity of the perforations of the plate. The apparent permeability of the porous layer is studied as a function of the (uniform) thickness of the layer, and as a function of the size and spacing of the performations in the plate. The apparent permeability is shown to be significantly lower than the intrinsic permeability of the porous layer when the layer is sufficiently thin. Closed-form expressions for the apparent permeability are derived using conformal transformation techniques. We then present a model of particle deposition onto the perforated plate. The growth of the porous layer resulting from the deposition is studied, as is the evolution of its apparent permeability.