The aim of this study is to compare pore structure characteristics of two industrial catalysts de... more The aim of this study is to compare pore structure characteristics of two industrial catalysts determined by standard methods of textural analysis (physical adsorption of nitrogen and mercury porosimetry) and selected methods for obtaining parameters relevant to transport processes (multicomponent diffusion and permeation of gases). The Mean Transport Pore Model (MTPM) described diffusion and permeation and the Dusty Gas Model (DGM) described permeation; models (represented as a boundary value problem for a set of ordinary differential equations) are based on Maxwell—Stefan diffusion equation and Weber permeation law. Parameters of models are material constants of the porous solid and, thus, do not depend on conditions under which the transport processes take place. Both catalysts were monoor bidispersed with mean pore radii about 70 nm and 2000 nm; diffusion and permeation measurements were performed with four inert gases (H2, He, N2, and Ar).
The new experimental way was proposed and verified in this study to get the associated transport ... more The new experimental way was proposed and verified in this study to get the associated transport parameters (r)ip, (r)ip from permeation measurements on the three industrial catalysts under nonstationary conditions in the modified Wicke—Kallenbach cell. These parameters were determined by the fitting to the theoretical model response curves by the Marquardt—Levenberg optimization method. The confidence intervals of the associated transport parameters were evaluated on the basis of the Beale criterion. The width of confidence intervals was discussed in terms of different mech anisms of mass transport. The advantages of the new experimental setup and calculation method were compared with the latter approach, too.
The goal of our contribution was to develop a metho d providing morphological (microstructural) d... more The goal of our contribution was to develop a metho d providing morphological (microstructural) descriptors whose values would ma tch total porosity and specific surface obtained other independent methods. The model obtained from limited amount of statistical information, accessible by image analys is of cross-sections, offers an attractive way for the prediction of equilibrium and transport phenomena in natural and man-made macroporous media.
The aim of this study is to compare pore structure characteristics of two industrial catalysts de... more The aim of this study is to compare pore structure characteristics of two industrial catalysts determined by standard methods of textural analysis (physical adsorption of nitrogen and mer-cury porosimetry) and selected methods for obtaining parameters relevant to transport processes (multicomponent diffusion and permeation of gases). The Mean Transport Pore Model (MTPM) described diffusion and permeation and the Dusty Gas Model (DGM) described permeation; models (represented as a boundary value problem for a set of ordinary differential equations) are based on Maxwell—Stefan diffusion equation and Weber permeation law. Parameters of models are material constants of the porous solid and, thus, do not depend on conditions under which the transport processes take place. Both catalysts were mono-or bidispersed with mean pore radii about 70 nm and 2000 nm; diffusion and permeation measurements were performed with four inert gases (H2, He, N2, and Ar). The industrial application of porous...
Results from stochastic reconstruction of porous solids and from a direct comparison of calculate... more Results from stochastic reconstruction of porous solids and from a direct comparison of calculated and experimental effective transport properties are presented. Eight porous solids of different microstructures were selected to evaluate the performance of two reconstruction methods based on simulated annealing. The common method was constrained by the two-point probability function and the lineal-path function for the void phase, whilst
ABSTRACT Recently, purification of raw biogas has attracted a great attention of research communi... more ABSTRACT Recently, purification of raw biogas has attracted a great attention of research community. Possibility of effective waste utilization for cheap biofuel production has enormous ecological and economical benefits. In this work a water-swollen thin film composite membrane for effective separation of carbon dioxide from methane in biogas is presented. The hydrophilic reverse osmosis membrane creates under water vapor condensing conditions a thin selective separation layer in which dissolved feed gaseous mixture, except methane. Water vapor saturated biogas was brought into a contact with cooled membrane in order to keep the selective skin layer. Sweeping gas N-2 is used to keep constant driving force and to control partial water evaporation from the membrane. Under the described experimental conditions was obtained retentate stream containing 95 vol.% of methane. (c) 2012 Elsevier B.V. All rights reserved.
Particle & Particle Systems Characterization, 2006
ABSTRACT The permporometry technique, common in the fields of thin filters, membranes etc., was e... more ABSTRACT The permporometry technique, common in the fields of thin filters, membranes etc., was extended to porous solids with far larger thicknesses. The basic idea of permporometry is the controlled expulsion of a suitable liquid from pores of different sizes by increasing the gas pressure difference across the tested porous solid. The gas flow rate through liquid-free pores then corresponds to the amount of pores with different sizes. Two modes of measurement were suggested and tested. The contribution of the Knudsen flow mechanism was taken into account in addition to the usually anticipated viscous gas flow mechanism.It was confirmed that both measurement modes give consistent pore size distributions (PSD) of flow-through pores. Permporometry PSDs were compared with results from mercury porosimetry and pseudo-steady state permeation. It appears that permporometry should be considered as a standard textural characterization technique suitable for the prediction and simulation of gas flow through porous media in all processes where gas transport takes place.
ABSTRACT The pore structures (microstructures) of two metallic filters were reconstructed using t... more ABSTRACT The pore structures (microstructures) of two metallic filters were reconstructed using the stochastic reconstruction method based on simulated annealing. The following microstructural descriptors were included in the description of the real microstructures: the two-point probability function, the lineal-path functions for the void or solid phases, i.e. simulated annealing was constrained by all low-order statistical measures that were accessible through the analysis of images of polished sections. An effect of the microstructural descriptors on the course of reconstruction was controlled by modifying two parameters of the reconstruction procedure [1]. Their values resulted from repeated reconstruction of two-dimensional microstructures in such a way that the reference (experimental) and calculated two-point cluster functions deviated negligibly. It was tacitly assumed that the parameters adjusted during two-dimensional reconstruction had the same influence on the formation of the three-dimensional microstructures. Since connectivity of phases is a critical property of the stochastically reconstructed media, clusters of pore and solid voxels were determined using the Hoshen-Kopelman algorithm. It was found that the solid phase formed one large cluster in accordance with the physical feasibility. The void phase created one large cluster and a few small clusters representing the isolated porosity. The percolation properties were further characterised using the local porosity theory [2]. Effective permeability of the replicas was estimated by solving the Stokes equation for creeping flow of an incompressible liquid in pore space. Calculated permeability values matched well their experimental counterparts.
Collection of Czechoslovak Chemical Communications, 1998
ABSTRACT A spontaneous temporary rise in the pressure gradient through countercurrent binary tran... more ABSTRACT A spontaneous temporary rise in the pressure gradient through countercurrent binary transport of inert gases in porous bidispersed alumina was observed in diffusion cell of the Wicke-Kallenbach type. Experimental pressure responses to gas step changes were detected and fitted to a set of partial differential equations. These were based on the transport models Mean Transport Pore Model (MTPM) and Dusty Gas Model (DGM). The dynamic set transport parameters 〈 r 〉, 〈 r 2〉 and ψ for both models were obtained using an optimization algorithm and discussed. It was found that the values of the transport radius 〈 r 〉 are in a good agreement with the mean radius obtained from the measurement of alumina texture.
ABSTRACT Effective diffusion coefficients for perylene and vanadyl porphyrine in acetone and six ... more ABSTRACT Effective diffusion coefficients for perylene and vanadyl porphyrine in acetone and six polystyrenes with molecular weight in the range 1200–19,000 g/mol in tetrahydrofuran were determined from the responses of a chromatographic column packed with porous glass (pore diameter 11 nm). The contribution of axial dispersion was evaluated from the measurements with polystyrenes which cannot penetrate the pores. To obtain the diffusion time constants a newly developed method for matching the experimental peaks in the time domain was utilized. The steric factors, S (partition coefficients), of the solutes evaluated from the first moments of the column responses were used for the determination of ratios of solute and pore sizes λ. The enhanced drag coefficient, which characterizes the interaction of the solute with the pore wall, changes with (1 − λ)1.34. Thus, the obtained tortuosity of the porous glass (2.71) is well within the expected interval. The effective diffusion coefficients are related to the bulk diffusivities of the solute—solvent pair, , as D = ε(1 − λ)3.34/q.
Two types of permeation cells and measurement methods for determination of transport characterist... more Two types of permeation cells and measurement methods for determination of transport characteristics of porous solids were compared. The dynamic method is based on unsteady permeation of single gases in a porous medium. The pseudostationary method uses equilibration of a small gas pressure difference across the porous medium. For evaluation of data from the dynamic cell full and simplified solutions are formulated and compared. Transport parameters of the porous solid are obtained by nonlinear matching of data to theory. Both the full and simplified descriptions of the pressure transients give comparable transport characteristics. From the pseudostationary cell data the transport parameters are obtained by simple linear regression, again in good agreement with the dynamic method. In both cases it is possible to determine the significance of Knudsen and viscous gas transport mechanisms. Permeation measurements were performed with hydrogen, helium, nitrogen, and argon on one monodispersed and one bidispersed type of porous pellet.
Two ex-situ and one in-situ semi-pilot plant UCG experiments in the experimental mine Barbara wer... more Two ex-situ and one in-situ semi-pilot plant UCG experiments in the experimental mine Barbara were performed with hard coal and lignite samples. To evaluate the influence of the UCG process on the textural properties of surrounding strata and coals, samples from various locations of the coal seam and the stratum samples before and after the UCG process were collected. Mercury porosimetry, helium pycnometry, and physical adsorption of nitrogen were used for the determination of textural properties of samples. Permeation gas transport was modelled based on the knowledge of the real structure characteristics of the stratum samples by the Mean Transport-Pore Model (MTPM). Influence of the individual texture and transport parameters on the post processing gas transport through porous strata with respect to the variability of their possible values was also evaluated.
The aim of this study is to compare pore structure characteristics of two industrial catalysts de... more The aim of this study is to compare pore structure characteristics of two industrial catalysts determined by standard methods of textural analysis (physical adsorption of nitrogen and mercury porosimetry) and selected methods for obtaining parameters relevant to transport processes (multicomponent diffusion and permeation of gases). The Mean Transport Pore Model (MTPM) described diffusion and permeation and the Dusty Gas Model (DGM) described permeation; models (represented as a boundary value problem for a set of ordinary differential equations) are based on Maxwell—Stefan diffusion equation and Weber permeation law. Parameters of models are material constants of the porous solid and, thus, do not depend on conditions under which the transport processes take place. Both catalysts were monoor bidispersed with mean pore radii about 70 nm and 2000 nm; diffusion and permeation measurements were performed with four inert gases (H2, He, N2, and Ar).
The new experimental way was proposed and verified in this study to get the associated transport ... more The new experimental way was proposed and verified in this study to get the associated transport parameters (r)ip, (r)ip from permeation measurements on the three industrial catalysts under nonstationary conditions in the modified Wicke—Kallenbach cell. These parameters were determined by the fitting to the theoretical model response curves by the Marquardt—Levenberg optimization method. The confidence intervals of the associated transport parameters were evaluated on the basis of the Beale criterion. The width of confidence intervals was discussed in terms of different mech anisms of mass transport. The advantages of the new experimental setup and calculation method were compared with the latter approach, too.
The goal of our contribution was to develop a metho d providing morphological (microstructural) d... more The goal of our contribution was to develop a metho d providing morphological (microstructural) descriptors whose values would ma tch total porosity and specific surface obtained other independent methods. The model obtained from limited amount of statistical information, accessible by image analys is of cross-sections, offers an attractive way for the prediction of equilibrium and transport phenomena in natural and man-made macroporous media.
The aim of this study is to compare pore structure characteristics of two industrial catalysts de... more The aim of this study is to compare pore structure characteristics of two industrial catalysts determined by standard methods of textural analysis (physical adsorption of nitrogen and mer-cury porosimetry) and selected methods for obtaining parameters relevant to transport processes (multicomponent diffusion and permeation of gases). The Mean Transport Pore Model (MTPM) described diffusion and permeation and the Dusty Gas Model (DGM) described permeation; models (represented as a boundary value problem for a set of ordinary differential equations) are based on Maxwell—Stefan diffusion equation and Weber permeation law. Parameters of models are material constants of the porous solid and, thus, do not depend on conditions under which the transport processes take place. Both catalysts were mono-or bidispersed with mean pore radii about 70 nm and 2000 nm; diffusion and permeation measurements were performed with four inert gases (H2, He, N2, and Ar). The industrial application of porous...
Results from stochastic reconstruction of porous solids and from a direct comparison of calculate... more Results from stochastic reconstruction of porous solids and from a direct comparison of calculated and experimental effective transport properties are presented. Eight porous solids of different microstructures were selected to evaluate the performance of two reconstruction methods based on simulated annealing. The common method was constrained by the two-point probability function and the lineal-path function for the void phase, whilst
ABSTRACT Recently, purification of raw biogas has attracted a great attention of research communi... more ABSTRACT Recently, purification of raw biogas has attracted a great attention of research community. Possibility of effective waste utilization for cheap biofuel production has enormous ecological and economical benefits. In this work a water-swollen thin film composite membrane for effective separation of carbon dioxide from methane in biogas is presented. The hydrophilic reverse osmosis membrane creates under water vapor condensing conditions a thin selective separation layer in which dissolved feed gaseous mixture, except methane. Water vapor saturated biogas was brought into a contact with cooled membrane in order to keep the selective skin layer. Sweeping gas N-2 is used to keep constant driving force and to control partial water evaporation from the membrane. Under the described experimental conditions was obtained retentate stream containing 95 vol.% of methane. (c) 2012 Elsevier B.V. All rights reserved.
Particle & Particle Systems Characterization, 2006
ABSTRACT The permporometry technique, common in the fields of thin filters, membranes etc., was e... more ABSTRACT The permporometry technique, common in the fields of thin filters, membranes etc., was extended to porous solids with far larger thicknesses. The basic idea of permporometry is the controlled expulsion of a suitable liquid from pores of different sizes by increasing the gas pressure difference across the tested porous solid. The gas flow rate through liquid-free pores then corresponds to the amount of pores with different sizes. Two modes of measurement were suggested and tested. The contribution of the Knudsen flow mechanism was taken into account in addition to the usually anticipated viscous gas flow mechanism.It was confirmed that both measurement modes give consistent pore size distributions (PSD) of flow-through pores. Permporometry PSDs were compared with results from mercury porosimetry and pseudo-steady state permeation. It appears that permporometry should be considered as a standard textural characterization technique suitable for the prediction and simulation of gas flow through porous media in all processes where gas transport takes place.
ABSTRACT The pore structures (microstructures) of two metallic filters were reconstructed using t... more ABSTRACT The pore structures (microstructures) of two metallic filters were reconstructed using the stochastic reconstruction method based on simulated annealing. The following microstructural descriptors were included in the description of the real microstructures: the two-point probability function, the lineal-path functions for the void or solid phases, i.e. simulated annealing was constrained by all low-order statistical measures that were accessible through the analysis of images of polished sections. An effect of the microstructural descriptors on the course of reconstruction was controlled by modifying two parameters of the reconstruction procedure [1]. Their values resulted from repeated reconstruction of two-dimensional microstructures in such a way that the reference (experimental) and calculated two-point cluster functions deviated negligibly. It was tacitly assumed that the parameters adjusted during two-dimensional reconstruction had the same influence on the formation of the three-dimensional microstructures. Since connectivity of phases is a critical property of the stochastically reconstructed media, clusters of pore and solid voxels were determined using the Hoshen-Kopelman algorithm. It was found that the solid phase formed one large cluster in accordance with the physical feasibility. The void phase created one large cluster and a few small clusters representing the isolated porosity. The percolation properties were further characterised using the local porosity theory [2]. Effective permeability of the replicas was estimated by solving the Stokes equation for creeping flow of an incompressible liquid in pore space. Calculated permeability values matched well their experimental counterparts.
Collection of Czechoslovak Chemical Communications, 1998
ABSTRACT A spontaneous temporary rise in the pressure gradient through countercurrent binary tran... more ABSTRACT A spontaneous temporary rise in the pressure gradient through countercurrent binary transport of inert gases in porous bidispersed alumina was observed in diffusion cell of the Wicke-Kallenbach type. Experimental pressure responses to gas step changes were detected and fitted to a set of partial differential equations. These were based on the transport models Mean Transport Pore Model (MTPM) and Dusty Gas Model (DGM). The dynamic set transport parameters 〈 r 〉, 〈 r 2〉 and ψ for both models were obtained using an optimization algorithm and discussed. It was found that the values of the transport radius 〈 r 〉 are in a good agreement with the mean radius obtained from the measurement of alumina texture.
ABSTRACT Effective diffusion coefficients for perylene and vanadyl porphyrine in acetone and six ... more ABSTRACT Effective diffusion coefficients for perylene and vanadyl porphyrine in acetone and six polystyrenes with molecular weight in the range 1200–19,000 g/mol in tetrahydrofuran were determined from the responses of a chromatographic column packed with porous glass (pore diameter 11 nm). The contribution of axial dispersion was evaluated from the measurements with polystyrenes which cannot penetrate the pores. To obtain the diffusion time constants a newly developed method for matching the experimental peaks in the time domain was utilized. The steric factors, S (partition coefficients), of the solutes evaluated from the first moments of the column responses were used for the determination of ratios of solute and pore sizes λ. The enhanced drag coefficient, which characterizes the interaction of the solute with the pore wall, changes with (1 − λ)1.34. Thus, the obtained tortuosity of the porous glass (2.71) is well within the expected interval. The effective diffusion coefficients are related to the bulk diffusivities of the solute—solvent pair, , as D = ε(1 − λ)3.34/q.
Two types of permeation cells and measurement methods for determination of transport characterist... more Two types of permeation cells and measurement methods for determination of transport characteristics of porous solids were compared. The dynamic method is based on unsteady permeation of single gases in a porous medium. The pseudostationary method uses equilibration of a small gas pressure difference across the porous medium. For evaluation of data from the dynamic cell full and simplified solutions are formulated and compared. Transport parameters of the porous solid are obtained by nonlinear matching of data to theory. Both the full and simplified descriptions of the pressure transients give comparable transport characteristics. From the pseudostationary cell data the transport parameters are obtained by simple linear regression, again in good agreement with the dynamic method. In both cases it is possible to determine the significance of Knudsen and viscous gas transport mechanisms. Permeation measurements were performed with hydrogen, helium, nitrogen, and argon on one monodispersed and one bidispersed type of porous pellet.
Two ex-situ and one in-situ semi-pilot plant UCG experiments in the experimental mine Barbara wer... more Two ex-situ and one in-situ semi-pilot plant UCG experiments in the experimental mine Barbara were performed with hard coal and lignite samples. To evaluate the influence of the UCG process on the textural properties of surrounding strata and coals, samples from various locations of the coal seam and the stratum samples before and after the UCG process were collected. Mercury porosimetry, helium pycnometry, and physical adsorption of nitrogen were used for the determination of textural properties of samples. Permeation gas transport was modelled based on the knowledge of the real structure characteristics of the stratum samples by the Mean Transport-Pore Model (MTPM). Influence of the individual texture and transport parameters on the post processing gas transport through porous strata with respect to the variability of their possible values was also evaluated.
Uploads
Papers by V. Hejtmánek