Capillary Condensation Mechanism Research Articles

Condensation heat transfer characteristics of flue gas moisture recovery using ceramic membranes

The use of a ceramic membrane condenser (TMC) can help recover moisture from flue gas, resulting in a reduction in white smoke, smog, and resource conservation. Although several studies have investigated TMC heat and mass transfer and water vapor condensation mechanisms, few have analyzed the effect of membrane area on thermal mass performance. This paper presents a comparative study of ceramic membranes with different pore sizes to address this gap. In this study, the influence of the condensation mechanism along the flow direction on the heat transfer is innovatively analyzed by combining the mass transfer characteristics. The results show that capillary condensation is more pronounced in high-temperature flue gas, which improves heat transfer efficiency and results in a higher wall temperature rise at the flue gas outlet. The capillary condensation mechanism can also increase the condensation depth, facilitating the removal of water vapor. In particular, in the treatment of 10000 m3/h flue gas, the maximum water recovery rate of the 0.4 nm pore size ceramic membrane is 26.6 kg/(m2·h), which is 27.9% higher than that of the 1 μm pore size ceramic membrane. However, it is important to note that nanomembranes cost 4.62 times more than micro membranes under the same flue gas conditions.

Zeolites and superporous hydrogels-based hybrid composites as solid desiccants to capture water vapors from humid air

This work reports the synthesis and use of zeolites based super-porous hydrogel composites (SPHCs) as solid polymer desiccants to capture water vapors from moist air. The SPHCs were composed of super-porous hydrogels (SPHs) of acrylic acid and methacrylamide (i.e. P(AA-co-MAM)-SPHs) as polymer matrix and AQSOA type zeolites, i.e. AQSOA-Z01, AQSOA-Z02 and AQSOA-Z05 zeolites as reinforcing materials. Different structural and morphological properties of SPHCs were characterized using techniques like XRD, SEM, FTIR and TGA-DTG. Desiccant properties of P(AA-co-MAM)-SPH and SPHCs were explored in terms of adsorption isotherm and kinetics. All the desiccants exhibited type-III adsorption isotherm, suggesting water vapors’ adsorption via capillary condensation mechanism and correlated well with GAB and FHH isotherm models. P(AA-co-MAM)-SPH exhibited maximum adsorption capacity of 0.72 gw/gads. However, after reinforcing different zeolites, adsorption capacity increased to 0.80, 0.84 and 0.91 gw/gads in SPHC/AQSOA-Z01, SPHC/AQSOA-Z02 and SPHC/AQSOA-Z05, respectively at 90% relative humidity and 25 °C. Further, the adsorption kinetics data correlated well with linear driving force and intraparticle diffusion models and the adsorption equilibrium was achieved much quicker at 70% relative humidity as compared to 90% relative humidity. SPHCs showed much better re-use efficiency as compared to parental SPH, therefore, reinforcement of zeolites improved the re-use efficiency of P(AA-co-MAM)-SPH.

On the Universality of Capillary Condensation and Adsorption Hysteresis Phenomena in Ordered and Crystalline Mesoporous Materials

AbstractUsing adsorption experiments for argon over a wide range of temperatures from above the triple point (i.e., 87 K) down to 65 K, coupled with advanced molecular simulation studies, the underlying mechanism of capillary condensation and hysteresis in a mesoporous metal‐organic framework (MOF), MIL‐101(Cr) is explored. The results obtained on the mesoporous MOFs are compared with the experimental and molecular simulation studies of argon adsorption and phase behavior in mesoporous molecular sieves (e.g., MCM‐41 silica) of similar pore size and over the same temperature range (65–87 K). The studies provide insights into the nature of confinement effects on the adsorption and phase behavior of fluids in mesoporous MOFs. The obtained results suggest that certain features of adsorption, capillary condensation, and associated hysteresis phenomena in mesoporous MOFs and in mesoporous molecular sieves are governed by the same underlying physics, despite the striking differences in pore structure and chemical composition. Hence, the findings are of importance for advancing characterization methodologies based on gas adsorption, but also for predicting the behavior of mesoporous MOFs in various potential applications.

Inhibition of 2-phenyl imidazoline on chloride-induced initial atmospheric corrosion of copper by quartz crystal microbalance and electrochemical impedance

Initial atmospheric corrosion of clean copper and NaCl-deposited copper was investigated by quartz crystal microbalance and electrochemical impedance spectroscopy. 2-phenyl imidazoline (2-PI) exhibits increasing inhibition efficiency with temperature on chloride-induced atmospheric corrosion of Cu. However, on the NaCl-deposited Cu, adsorption amount of 2-PI is much higher than that on clean Cu possibly due to that rough corrosion products entrap lots of 2-PI molecules. The adsorption of 2-PI on corroded Cu is suggested to obey a capillary condensation and entrapment mechanism, but in dry argon atmosphere, it obeys a simple pseudo first-order model for lack of corrosion products.

Capillary condensation mechanism for gas transport in fiber reinforced poly (ether-b-amide) membranes

One of the most important shortcomings of the polymeric membranes in comparison with inorganic membranes is the trade-off between permeability and selectivity. Gas permeability in inorganic membranes follows different mechanisms including capillary condensation mechanism. The aim of present study was improving the polymeric membranes performance by introducing capillary condensation mechanism for the gas transport similar to that observed in the inorganic membranes. Capillary condensation of gases needs a continuous diffusion pathway, therefore in the present study short glass wool fibers (with a micro size in diameter and millimeter size in length) were chosen as a proper alternative instead of nanoparticles. Short glass wool fibers present proper dispersion in the Pebax matrix as a result of its good compatibility with polymer segments. Gas permeability behavior in such composite membranes was assisted using pure CO2, CH4 and N2 gases. Obtained results demonstrated that gas permeability behavior in the short fiber reinforced composite membranes is depends on the fiber content. At lower content of the fiber, such reinforcement causes simultaneous increment in gas permeabilities in a way that 62.37%, 32.12% and 50.71% increase in permeability was observed for CO2, CH4, and N2 respectively. But at higher content due to formation of more continuous interface, capillary condensation was appeared for more condensable gas (CO2) which results in 17.65% reduction in its permeability. Based on the results the using of short fiber instead of nanoparticles in polymeric membranes can be more beneficial in performance and economic point of view.

Thermochemical characterizations of high-stable activated alumina/LiCl composites with multistage sorption process for thermal storage

High-stable activated alumina/LiCl composites with multistage sorption process were fabricated to store low-temperature heat below 120 °C. The worried issue of solution leakage was solved by controlling its salt content below the threshold value. The microstructure was observed by transmission electron microscopy (TEM). Research on nitrogen adsorption suggested that because of the impregnated salt, composite sorbents presented different pore structure from pure activated alumina. Based on the measured results of water isotherms, the sorption equilibrium states represented by the mass concentration of the inside LiCl solution were evaluated. Moreover, the multistage sorption process-physical adsorption, chemical adsorption and liquid-gas solution absorption were quantitative analyzed based on the capillary condensation mechanism and hydrous reaction equation. Sorption kinetics proved that the sorption capacity of composite sorbents was significantly improved compared with pure activated alumina. Sorption energy storage density was obtained by the TGA/DSC measurement. Overall, AA/LiCl composite sorbent with salt content of 14.68% was selected as the optimistic AA/LiCl composite sorbent with a sorption capacity of 0.45 g/cm3 (0.41 g/g) and an energy storage density of 318.3 kWh/m3 (1041.5 kJ/kg) at the condition of 20 °C, 80% RH with a charging temperature of 120 °C.

Fluids in nanospaces: molecular simulation studies to find out key mechanisms for engineering

We have analyzed various phenomena that occur in nanopores, focusing on elucidating their key mechanisms, to advance the effective engineering use of nanoporous materials. As ideal experimental systems, molecular simulations can effectively provide information at the molecular level that leads to mechanistic insight. In this short review, several of our recent results are presented. The first topic is the critical point depression of Lennard-Jones fluid in silica slit pores due to finite size effects, studied by our original Monte Carlo (MC) technique. We demonstrate that the first layers of adsorbed molecules in contact with the pore walls act as a “fluid wall” and impose extra finite size effects on the fluid confined in the central portion of the pore. We next present a new kernel for pore size distribution (PSD) analysis, based entirely on molecular simulation, which consists of local isotherms for nitrogen adsorption in carbon slit pores at 77 K. The kernel is obtained by combining grand canonical Monte Carlo (GCMC) method and open pore cell MC method that was developed in the previous study. We show that overall trends of the PSDs of activated carbons calculated with our new kernel and with conventional kernel from non-local density functional theory are nearly the same; however, apparent difference can be seen between them. As the third topic, we apply a free energy analysis method with the aid of GCMC simulations to investigate the gating behavior observed in a porous coordination polymer, and propose a mechanism for the adsorption-induced structural transition based on both the theory of equilibrium and kinetics. Finally, we construct an atomistic silica pore model that mimics MCM-41, which has atomic-level surface roughness, and perform molecular simulations to understand the mechanism of capillary condensation with hysteresis. We calculate the work required for the gas–liquid transition from the simulation data, and show that the adsorption branch with hysteresis for MCM-41 arise from spontaneous capillary condensation from a metastable state.

A new model for water adsorption in porous ceramics

This research proposes a new isotherm model that is derived for adsorption of clustering and condensable vapors on mesoporous ceramics. The adsorption characteristics of water vapor on both experimental data and isotherm model were investigated over the relative pressure ranging from 0 to 0.95. The applicability of the model was evaluated by significance of fitting parameters. For each adsorption isotherm there are five parameters (m, qf, Kf, qm and Km). The comparison of modeling fit with experimental data was used as the methodology for selecting the most informative and the best-fitting model. The experimental testing results showed that this model is able to describe all possible behaviors of the water adsorption isotherm displaying type IV classification. The model is based on the forming and growth of the water molecule clusters and capillary condensation mechanism in mesopore. The findings suggest a significant tool for understanding the behavior of water adsorption in humidity-control porous ceramics.

Unusual mechanism of capillary condensation in pores modified with chains forming pillars

Density functional approach is applied to study the phase behavior of Lennard-Jones(12,6) fluid in pillared slit-like pores. Our focus is in the evaluation of phase transitions in fluid adsorbed in the pore of a fixed width. If the length of pillars is sufficiently large, we observe additional phase transitions of the first and second order due to the symmetry breaking of the distribution of chain segments and fluid species with respect to the slit-like pore center. Re-entrant symmetry changes and additional critical, critical end points and tricritical points then are observed. The scenario of phase changes is sensitive to the energy of fluid-solid interaction, the amount, and the length of the pillars. Quantitative trends and qualitative changes of the phase diagrams topology are examined depending on the values of these parameters.

Argon and krypton adsorption on templated mesoporous silicas: molecular simulation and experiment

In this work we report molecular simulation results for argon and krypton adsorption on atomistic models of templated mesoporous silica materials. These models add atomistic levels of detail to mesoscale representations of these porous materials, which were originally generated from lattice Monte Carlo simulations mimicking the synthesis process of templated mesoporous silicas. We generate our atomistic pore models by carving out of a silica block a ‘mathematically-smooth’ representation of the pores from lattice MC simulations. Following that procedure, we obtain a model material with mean mesopore and micropore diameters of 5.4 nm and 1.1 nm, respectively (model A). Two additional model materials were considered: one with no microporosity, and with mesopores similar to those of model A (model B), and a regular cylindrical pore (model C). Simulation results for Ar and Kr adsorption on these model materials at 77 K and 87 K shows that model A provides the best agreement with experimental data; however, our results suggest that fine-tuning the microporosity and/or the surface chemistry (i.e., by decreasing the density of OH groups at the pore surface) of model A can lead to better agreement with experiments. The filling of the mesopores in model materials A and B proceeded via a classical capillary condensation mechanism, where the pores fill at slightly different pressures. This observation contrasts with what was observed in our previous study (Coasne, et al. in Langmuir 22:194–202, 2006), where we considered atomistic silica mesopores with an important degree of surface roughness at length scales below 10 A, for which we observed a quasi-continuous mesopore filling involving intermediate phases with liquid-like “bridges” and gas-like regions. These results suggest that pore surface roughness, and other morphological features such as constrictions, play an important role in the mechanism of adsorption and filling of the mesopores.

Flux decline during ultrafiltration of protein-fatty acid mixtures

Proteins frequently co-exist with fatty acids and have specific binding interactions, forming complexes which can affect membrane performance. While specific and non-specific binding to proteins occurs at lower concentrations of fatty acid, at higher concentrations, excess fatty acids can produce additional interactions by adsorption in the protein cake or in the membrane pores. The effect of the fatty acid on ultrafiltration is multifold. The fatty acids can behave as a hydrophobic solute at low pH and a charged highly soluble solute at high pH. Using caprylic acid and bovine serum albumin (BSA) as a model fatty acid and protein, this study examines the flux decline caused by protein-fatty acid mixtures in the pH range of 3–10 and the potential effect of capillary condensation of fatty acids on membranes with proteins present as co-foulants. The flux and retention effects of increasing excess of fatty acids were particularly examined with respect to potential removal of residual fatty acids in the presence of proteins for food or environmental applications. Experiments on flux were carried out by diafiltration of solutions containing BSA and caprylic acid, using a 30 kDa polyethersulphone membrane at 50 kPa. BSA-fatty acid binding parameters were evaluated in bulk and compared with fatty acid flux and retention through BSA pre-fouled membranes. The fatty acid retention by the protein deposit exceeded specific and non-specific binding sites by several orders of magnitude, thus supporting a capillary condensation mechanism. Despite the high retention of fatty acid, the flux decline did not become significant until at high fatty acid concentrations relative to the saturation, suggesting that condensation occurred initially in small and dead end pores which did not carry the bulk of the flow. The possible effect of the protein conformation on this process is also discussed.

Adsorption-based porosimetry using capacitance measurements

It is shown that the dependence of capacitance of an Al/por-Si/Si structure on relative humidity is represented as an isothermal curve of physical adsorption. An analysis of this dependence makes it possible to determine the total porosity, the effective fraction of the oxide phase in por-Si, and the ratio between the volumes of micropores (the monomolecular and polymolecular adsorption of water which results in filling of their volume) and mesopores (which are filled according to the mechanism of capillary condensation). The size distribution for mesopores was obtained.

Molecular mechanism of capillary condensation of acetonitrile vapor on MCM-41 with the aid of a time-correlation function analysis of IR spectroscopy

The adsorption isotherm and IR spectra of acetonitrile adsorbed on MCM-41 (pore-width=3.2 nm) were measured at 303 K. The adsorption isotherm had a sharp jump at P/ P 0=0.3 without adsorption hysteresis. The CN stretching ν 2-band of adsorbed acetonitrile had two-component α- and β-bands at 2263 and 2254 cm −1, respectively, assigned to hydrogen-bonded molecules on surface hydroxyls of MCM-41 and physisorbed molecules in mesopores whose walls are coated with hydrogen-bonded molecules, respectively. The β-band was analyzed with a time correlation function, giving a reorientation time τ. The τ value of the adsorbed molecule before capillary condensation was smaller than that of the bulk liquid, indicating a gas-like state. After capillary condensation, τ was the same as for the bulk liquid.

Capillary condensation in structured pores

We investigate capillary condensation in slit-like pores with structured walls, described by a simple lattice gas model based on the mean-field approximation. Most theoretical workers did not go beyond very simple capillary models with perfect symmetry, thus being unable to map experiments in real pores with rather complex structure. In this paper we deal with chemical inhomogeneity, produced by a periodic wall potential, that causes fluid attraction and repulsion altering along the walls. We observe a new mechanism of capillary condensation, characterized by a splitting of the equilibrium ‘‘gas–liquid’’ transition, and we concentrate on the crucial role the typical length for the wall inhomogeneity, λ, plays compared with the pore width, H. Our microscopic results are confirmed by a macroscopic analysis that leads to a modified Kelvin equation, giving a quantitative prediction of capillary condensation in structured pores. Furthermore our model explains the nature of the critical isochore as measured in mesopore condensation experiments by Thommes and Findenegg.

Separation of dilute organic/water mixtures with asymmetric poly(vinylidene fluoride) membranes

An in-depth investigation of integral asymmetric poly(vinylidene fluoride) (PVDF) membranes has been carried out for the extraction of polar and non-polar organic compounds from dilute organic-in-water feed solutions. Membrane performance for low and high-boiling non-polar organic feed components was excellent, with separation factors as high as 4900 and high organic transmembrane fluxes. Polar organic feed components such as ethanol and acetone were also separated effectively from water but the separation factors were lower than non-polar organics. There was no change in membrane performance when either the dense or porous face of a PVDF membrane contacted the feed solution as long as the feed solution flow rate was sufficiently high. The effect of membrane preparation conditions, such as casting solution composition, air humidity and temperature during film drying, and the molecular weight of PVDF, on membrane performance was quantified. Although the water flux through the resulting films changed significantly, the organic (benzene) flux was essentially independent of the fabrication method. Variations in casting conditions also changed the mean diameter of a small number of pores in the dense layer of PVDF membranes. Transmembrane water fluxes during benzene/water separations correlated with increasing pore size, indicating that such pores were providing pathways for water movement across the hydrophobic PVDF dense layer. Based on benzene swelling and diffusivity measurements in homogeneous PVDF films, pores in the dense layer of an asymmetric membrane control permeate enrichment by either a membrane distillation, pore flow, or capillary condensation mechanism. The total time for asymmetric membrane casting was reduced from 69–72 min to 7 min, while maintaining a high organic (benzene) separation factor with only a small drop in transmembrane benzene flux.

Thermodynamic and structural properties of physisorbed phases within the model mesoporous adsorbent M41S (pore diameter 2.5 nm)

An M41S sample with a pore diameter of 2.5 nm has been characterised by the physisorption of various probe molecules (N 2, CO, D 2, CH 4, CD 4, Ar and Kr). This gives rise to a distinct step in the adsorption isotherm as if a capillary condensation mechanism occurs for these adsorbates. However, not all of the isotherms present a hysteresis in the desorption branch of the isotherm as would be expected for a capillary condensation-type mechanism. Thermodynamic (isothermal microcalorimetry) studies reveal an output of differential enthalpy around |1–2.5 kJ mol −1| above the enthalpy of liquefaction during this step. Furthermore, in the case of krypton, a “solidification” is suspected. A structural (neutron diffraction) study however, indicates that the adsorption of deuterium and methane is characterised by short range order, even at 3 K.

Preparation and gas permeation properties of ceramic-silicalite membranes

Zeolites, up until now, have been utilized for discontinuous separation and catalytic processes. To achieve continuous separation processes zeolites have to be used in the form of a membrane. The special properties of zeolites are high thermal resistance, insolubility, and very well defined pores of molecular size. Membrane separation with zeolites gives the possibility of very high separation factors because of their regular pore structure. For zeolites as membranes, hydrophobic zeolites with a three-dimensional channel network are preferred. It has been shown that this zeolite layer can be grown on top of a porous ceramic membrane disc. Best results have been achieved with the ZSM 5 zeolite on a ceramic support containing alumina [1]. First, Suzuki [2] reported and patented gas permeation through pure and crosslinked zeolite layers, the other membrane applications were discovered for zeolite membranes. Zeolite layers were developed for the separation of strong and weak sorbents using a capillary condensation mechanism [3]. Pervaporation of the ethanol/water system was successfully achieved [4]. In this work we describe an optimized in situ synthesis of a zeolite layer on top of a ceramic support. A new calcination process, derived by thermo-gravimetry, reduces the stress during removal of the template. The results of the permeation of pure gases and vapours were compared with gas/ vapour mixtures at different concentrations and temperatures. Fig. 1 shows a schematic of the nucleation and growth of the zeolite layer. The aim is a thin dense defect-free zeolite layer; therefore small well crosslinked crystals are needed. Zeolite synthesis as normally described in the literature has been modified for these special demands.

Influence of surface energetical heterogeneity on capillary condensation in slit-like pores: a Monte Carlo study

The results of a Monte Carlo simulation of capillary condensation in slit-like model pores with walls composed of parallel strips characterized by different surface potential strengths are presented. The mechanism of capillary condensation in such nonuniform pores is discussed and compared with the results obtained by other methods. It is demonstrated that for sufficiently high differences in the gas-solid interactions in different parts of a pore, the formation of a liquid phase proceeds in two steps, corresponding to condensation in different parts of the pore. The first step, precondensation, leads to the formation of a thin liquid layer over the pore part with higher energy of adsorption. The second step of capillary condensation appears to be very similar to the condensation observed in pores closed at one end.

Water adsorption equilibria on microporous carbons correlated using a modification to the Sircar isotherm

Water vapor adsorption equilibria are measured for several unimipregnated and impregnated microporous carbons, including BPL, ASC, ASC-TEDA and a polymeric beaded carbon. Both the shape of the isotherms and the presence of hysteresis are characteristics of the capillary condensation mechanism. Sircar's isotherm expression, which incorporates capillary condensation with the Kelvin equation, has been modified to correlate the data for both adsorption and desorption. Two gamma distribution functions are required to represent the pore size distribution for the activated carbons while only one gamma distribution function is required to represent the pore size distribution for the polymeric beaded carbon. The pore distributions that result from correlating the isotherm data are consistent with the known structure of these adsorbents.

Adsorption equilibria of trimethylamine on porous adsorbents.

Adsorption equilibria of trimethylamine as the offensive odor substance was measured by a gravimetric method to elucidate the mechanism of its adsorption on activated carbon, silicate, and zeolite. The mechanism of adsorption was discussed on the basis of application of the Dubinin-Astakhov equation to the adsorption isotherms and relation between the characteristic energy (E), exponent (n) and pore size distribution. The Dubinin-Astakhov equation was well applicable to the adsorption isotherms of activated carbon, silicate, and zeolite. The exponent (n) of activated carbon, silicate, and zeolite was 2, 3, and 3, respectively. It was confirmed that the adsorption of trimethylamine on the adsorbents resulted in the volume filling of their micropores by the mechanism of capillary condensation, and that the two or three degrees of freedom in translation of trimethylamine was lost by its adsorption on the micropores or ultramicropores, respectively. The amount of trimethylamine adsorbed on the microporous adsorbents was determined by their micropore volume of pore radius less than about 20 Å.