Heat Of Liquefaction Research Articles

Nucleation mechanism of methane heterogeneous condensation under different driving forces

Clarifying alkane gas condensation characteristics is crucial to designing and optimizing liquefaction heat exchangers, which imposes higher demands on the microscopic understanding of alkane heterogeneous condensation. Thus, in this study, the condensation process of methane nucleation under different driving forces is investigated by molecular dynamics (MD) simulations. The dynamics characteristics of nucleation in methane heterogeneous condensation are analyzed by varying initial gas-phase pressure, cold wall temperature, and ethane content. The results showed that increasing the initial gas-phase pressure enhances intermolecular interactions, which alter the cluster formation path and significantly increase the methane gas nucleation rate from 1.997 × 1033/(m3·s) to 1.068 × 1034/(m3·s). While lowering the cold wall temperature promotes condensation by weakening the thermal motion of the gas molecules. Once condensation nuclei form in the system, the lower temperature conditions from lowering the cold wall temperature result in a higher growth rate of the clusters. Additionally, the addition of easily condensable component ethane can improve the condensation nucleation characteristics of low-saturated methane gas, facilitating the liquefaction of methane. This study aims to provide a microscopic understanding of the advancement of gas liquefaction technology by investigating the heterogeneous nucleation of alkane under different conditions from a microscopic perspective.

Catalyst filled heat exchanger for hydrogen liquefaction

Cryogenic hydrogen liquefaction plants have a uniquely designed plate fin heat exchanger (PFHE) in which the fin channels are filled with an ortho-para hydrogen conversion catalyst (O-P catalyst). This PFHE can save liquefaction energy by reducing the heat of liquefaction. Despite the unique heat exchanger design, there are few studies on the thermohydraulic performance of the PFHE for the hydrogen liquefaction process. In this study, the thermohydraulic performance of a PHFE filled with an O-P catalyst is investigated in two steps. In the first step, pressure drop experiments are performed using a cylinder filled with commercial O-P catalyst. The pressure drop is approximately five times lower than that of the Ergun's equation. Therefore, we propose a new correlation for the porous channel filled with the O-P catalyst and apply it in the numerical research, which is the second step. According to the results, the difference in Nu/f1/3 between the new empirical correlation and the Ergun's equation is up to 1.6 times, and the exergy efficiency of the CPFHE is underestimated by Ergun's equation. Furthermore, we report the impact of the design parameters on the heat transfer and pressure drop on a PFHE filled with catalyst. This CPFHE is a counter-flow heat exchanger and includes a porous hot laminar stream and two cold laminar streams through the plain-fin channel. The design parameters are selected based on the fin height and fin spacing. Among the various fin channel designs, the most effective one involves a fin height of 4 mm and a fin spacing of 0.7 mm. Interestingly, at the same Re number, the variation in Nu/f1/3 is almost twice the variation in the fin spacing. Based on these results, we discuss the impact of the design parameters on the PFHE performance, using the performance evaluation plot. Therefore, this study is expected to provide insights and guidance for designing PFHEs in hydrogen liquefaction plants.

Equilibrium and heat of water vapor adsorption on the surface of natural lignocellulose materials

Water vapor is to be separated from industrial gaseous streams at reduced costs. Natural lignocellulose materials have demonstrated a high water adsorption capacity and great potential in industrial gas dehydration, specifically natural gas dehydration. In this work, flax shives, a representative of abundant lignocellulose materials generated as by-products of agriculture industry, was chosen as a model to investigate their water adsorption. The focus of this work is on analyzing the surface morphology and the chemistry of the material, and investigating the effects of key parameters on the equilibrium, isotherms, surface affinity, and heat of water adsorption. The results showed that total operating pressure had the most significant effect on the equilibrium, followed by the temperature. The shape of water adsorption isotherm was changed from type III to type I as the total operating pressure was decreased and temperature was increased. The Anderson, Toth, and multi-site Langmuir models successfully described the isotherms obtained at various operating conditions. The modeling results were further used to determine the standard heat of adsorption, isosteric heat, heat of individual sites, and excess heat of liquefaction that the second and subsequent adsorbed layers released. In addition, the material surface was characterized by FE-SEM and FTIR, and the water transport through the material’s structure was visualized using optical microscopy images and video. These findings contributed to the fundamental science of water adsorption and natural lignocellulose materials and provided information for the design of drying processes based on adsorption and related interface sciences.

Modifying the BET model for accurately determining specific surface area and surface energy components of aggregates

When adopting the vapor adsorption method to measure the surface energy components of aggregates, a critical step is to determine the specific surface area (SSA) using the traditionally-used BET (T-BET) model. However, this model is demonstrated to be valid only for a limited relative pressure range of 0.05–0.40. Furthermore, the determined total SSA from this model is arbitrarily utilized to calculate the spreading pressure of probe vapors on aggregates, which does not comply with the multilayer adsorption theory. All these would definitely lead to the erroneous values of SSA and surface energy components of aggregates.To overcome these deficiencies, this study develops a modified BET (M−BET) model in the light of the multilayer adsorption theory, based on which five steps are designed to determine the SSA and the surface energy components for aggregates. The vapor adsorption tests of three probe vapors are performed on four types of aggregates. The SSA, spreading pressure and surface energy components are then separately calculated using the two BET models. It is observed that the linear fit of M-BET model applies satisfactorily to the entire adsorption isotherm with the R2 value larger than 0.98. The adsorption heat of the probe vapor in the second to the ith layer is demonstrated to be less than the heat of liquefaction, which accounts for the derivations of the data points of the T-BET model from linearity over the limited pressure range. The accompanying calculation results of the M-BET model reveal significant differences from those of the T-BET model, while the separation of SSA of the non-zero adsorption site from the total SSA also produces a non-negligible effect on the calculated surface energy components, identifying the significance of accurate measuring the SSA. Thus, in the vapor adsorption method it is imperative to employ the M-BET model to determine the SSA of the non-zero adsorption site so as to obtain the accurate surface energy components for aggregates.

Enhanced Adsorption of Organic Compounds over an Activated Carbon Cloth by an External-Applied Electric Field

Adsorption of pollutants on activated carbon is an effective air pollution control technique. In this study, a strong and non-uniform electric field was applied over an activated carbon fiber cloth. The adsorption kinetic of several organic compounds (Acetone, Acetaldehyde, Benzene, Cyclohexane, Ethanol, Methyl Ethyl Ketone, Toluene, 1-Propanol) on the activated carbon cloth was evaluated in the presence and in the absence of an electric field. Results suggest that its application enhances the adsorptive process. A linear correlation was found between such enhancement and the specific heat of liquefaction of the organic compounds.

Adsorption of water and methanol on highly graphitized thermal carbon black: The effects of functional group and temperature on the isosteric heat at low loadings

Isosteric heats of water and methanol adsorption on highly graphitized thermal carbon black have been studied using grand canonical Monte Carlo simulation to investigate the effects of the concentration of functional groups and temperature on the isosteric heats observed experimentally. The heat at zero loading is associated with the interaction between an adsorbate and the functional group. The variation in the heat with loading is due to a combination two effects: (1) Saturation of the functional groups by adsorbate molecules. (2) Adsorption at the convex boundary of the clusters, resulting in a lower number of neighbours for each new molecule adsorbed onto the clusters compared to the bulk liquid. The isosteric heat versus loading reaches a minimum and then increases again as the number of interactions with neighbouring molecules increases, and finally reaches the heat of liquefaction when the very large clusters approach liquid-like behaviour.

Programming MIL-101Cr for selective and enhanced CO2adsorption at low pressure by postsynthetic amine functionalization

MIL-101Cr fully or partially (p) postsynthetically modified with nitro (-NO2) or amino (-NH2) groups was shown to be a robust, water stable, selective and enhanced carbon dioxide (CO2) adsorption material with the amine-functionality. The highly microporous amine-modified frameworks (up to 1.6 cm(3) g(-1) total pore volume) exhibit excellent thermal stability (>300 °C) with BET surface areas up to 2680 m(2) g(-1). At 1 bar (at 273 K) the gases CO2, CH4 and N2 are adsorbed up to 22.2 wt%, 1.67 wt% and 2.27 wt%, respectively. The two amine-modified MIL-101Cr-NH2 (4) and MIL-101Cr-pNH2 (5) showed the highest gas uptake capacities in the series with high ratios for the CO2 : N2 and CO2 : CH4 selectivities (up to 119 : 1 and 75 : 1, respectively, at 273 K). Comparison with non-modified MIL-101Cr traces the favorable CO2 adsorption properties of MIL-101Cr-NH2 (4) and MIL-101Cr-pNH2 (5) to the presence of the Lewis-basic amine groups. MIL-101Cr-NH2 (4) has a high isosteric heat of adsorption of 43 kJ mol(-1) at zero surface coverage and also >23 kJ mol(-1) over the entire adsorption range, which is well above the heat of liquefaction of bulk CO2. Large CO2 uptake capacities of amine-functionalized 4 and 5, coupled with high adsorption enthalpy, high selectivities and proven long-term water stability, make them suitable candidates for capturing CO2 at low pressure from gas mixtures including the use as a CO2 sorbent from moist air.

Highly stable nanoporous covalent triazine-based frameworks with an adamantane core for carbon dioxide sorption and separation

Adamantane substituted with two to four 4-cyanophenyl groups was used for preparation of a new series of robust Porous Covalent Triazine-based Framework (PCTF) materials. Novel adamantane PCTFs were synthesized in good yields (>80%) by the trimerization reaction of 1,3-bis-, 1,3,5-tris- and 1,3,5,7-tetrakis(4-cyanophenyl)adamantane, respectively, in the presence of ZnCl2 (Lewis acid condition) and CF3SO3H (strong Bronsted acid condition). From N2 adsorption isotherms, the Lewis acid condition gives higher surface areas than the strong Bronsted acid condition. The amorphous nano- to microporous frameworks (>50% micropore fraction) exhibit excellent thermal stabilities (>450 °C) with BET surface areas up to 1180 m2 g−1. A very similar ultramicropore size distribution between 4 and 10 A was derived from CO2 adsorption isotherms with a “CO2 on carbon based slit-pore model”. At 1 bar the gases H2 (at 77 K), CO2 (at 273 and 293 K) and CH4 (at 273 K) are adsorbed up to 1.24 wt%, 58 cm3 g−1 and 20 cm3 g−1, respectively. Gas uptake increases with BET surface area and micropore volume which in turn increase with the number of cyano groups in the monomer. From single component adsorption isotherms, IAST-derived ideal CO2:N2, CO2:CH4 and CH4:N2 selectivity values of up to 41 : 1, 7 : 1 and 6 : 1, respectively, are calculated for p → 0 at 273 K. The adamantane PCTFs have isosteric heats of adsorption for CO2 of 25–28 kJ mol−1 at zero loading and most of them also >25 kJ mol−1 over the entire adsorption range which is well above the heat of liquefaction of bulk CO2 or the isosteric enthalpy of adsorption for CO2 on activated carbons.

Water vapor sorption and free volume in the aromatic polyamide layer of reverse osmosis membranes

Thin film composite membranes consist of an ultra thin active layer of polymer that governs the membrane's salt rejection and water permeation properties. However, the fragility of the ultrathin layer makes it challenging to select a technique capable of differentiating between the properties of this layer from the supporting layer. In this study, we isolated enough active layer material to characterise the water vapour sorption and free volume cavity size as functions of water activity. The sorption data were modelled using the Guggenheim–Anderson–De Boer (G.A.B.) isotherm and from this the number of sorption sites for water was calculated to be 189×1019 per gram for the active layer from a commercial Dow Filmtec SW30 membrane and 188×1019 per gram for an aromatic polyamide material prepared in house. The activation energy for diffusion of water through the active layer of SW30 was also evaluated and found to be 6.95kcal/mol, lower than the heat of liquefaction of water. This suggests that water does not permeate as individual molecules in the vapour state, but rather as clusters of water molecules. The free volume cavity size in the active layer as a function of moisture uptake was also investigated and the results were explained using a pore filling and pore swelling mechanism. This study sheds light on the mechanisms of water entry into the active layer, water transport through the active layer, and the corresponding response of the polymer chains, thereby giving critical insight for the development of more novel systems.

OPTIMIZATION OF THE C3MR CYCLE WITH GENETIC ALGORITHM

The aim of this paper is thermodynamic simulation and optimization of the C3MR system with Genetic Algorithm. For this purpose, in the first step, the Peng Robinson equation of state is simulated with a code in MATLAB and then used for simulating thermodynamic properties of natural gas and refrigerants that are used in the cycle. Following that the cycle is thermodynamically simulated and composite curves for subcooling and liquefaction heat exchangers are plotted. If composite curves in heat exchangers approach, the total power will decrease. Then, the total power used by the compressors is calculated. In the next step, the thermodynamic modeling is linked with Genetic Algorithm and the total power consumed by compressors is defined as objective function. The best value resulted from optimization has 23% lower power than the base design. In addition, heat exchange curves closed together.

Surface Structure, Hydration and Cationic Sites of Nanohydroxyapatite

In this work, we studied the surface/water interaction properties of a pure calcium hydroxyapatite (HA) and their modifications as a consequence of the partial Ca2+/Mg2+ (MHA) substitution by means of IR spectroscopy and microcalorimetry of adsorbed water. IR data indicated that water molecules in direct contact with the surface of HA are coordinated to surface cations and experience H-bond significantly stronger than in liquid water. The heats of adsorption associated to such interactions are very high, being twice-triple the heat of liquefaction of water. Interestingly, water experiences H-bond higher than in its bulk liquid state also in the second layer. Finally the entering in the material of Mg2+ ions was shown to significantly affect the affinity of the material toward water and the properties of its hydration layers.

On Inverse Adsorption Chromatography. 2. Determination of Isotherms and Heats of Adsorption as well as Energy Distributions of Adsorption Sites

The isotherms and heats of adsorption, as well as the energy distributions of adsorption sites for pyridine at 100 °C and benzene at 40 °C on the silica surface, were determined by the method of inverse gas chromatography (IGC). These characteristics are usually obtained from IGC after integration of its direct result: the derivative of the adsorption isotherm. It is shown that the constant of integration plays a critical role in obtaining the isosteric heats of adsorption and the energy distributions of adsorption sites. The method of extrapolation is shown herein to be most appropriate for obtaining this constant. The modified ECP method, which takes account of longitudinal diffusion and kinetics of adsorption, was developed in the companion paper (Bakaev, V. A. J. Phys. Chem. C 2007, 111, 7463−7472) and was applied here to the pyridine/silica and benzene/silica systems. It is shown how to experimentally determine the longitudinal diffusion coefficient and the kinetics of adsorption coefficient. The IGC method probes only a small fraction of all of the adsorption sites on a surface. These are the strongest adsorption sites whose adsorption energy in the case of pyridine is almost 6 times larger in magnitude than the heat of liquefaction.

Variable temperature FT-IR studies on hydrogen adsorption on the zeolite (Mg,Na)-Y

Variable-temperature infrared spectroscopy was used for the thermodynamic studies on the adsorption of hydrogen on the zeolite (Mg,Na)-Y. Adsorption renders the H H stretching mode infrared active, and simultaneous measurement of IR absorbance and hydrogen equilibrium pressure, over a range of temperature, allowed adsorption enthalpy and entropy to be determined. The standard adsorption enthalpy and entropy resulted to be Δ H° = −18.2(±0.8) kJ mol −1 and Δ S° = −136(±10) J mol −1 K −1, respectively. The adsorption enthalpy is substantially higher than the hydrogen liquefaction heat, which suggests that magnesium-containing porous materials are potential candidates in the search for suitable adsorbents for reversible hydrogen storage.

Evaluation of an Effective Gas Storage Amount of Latent Nanoporous Cu-Based Metal−Organic Framework

The supercritical N2, O2, and CH4 adsorption isotherms on a Cu-based metal−organic framework, [Cu(4,4‘-bipyridine)2(BF4)2]n, over the temperature range of 196−303 K were measured by a gravimetric method. The N2 and O2 adsorption isotherms at 196 K showed a vertical adsorption accompanied by a rectangular hysteresis loop. A method of evaluating the absolute adsorption amount of these gases was developed taking into account the intermolecular interactions for the buoyancy-mediated method. The isotherms of the absolute adsorption amount were obviously different from the isotherms of the surface-excess mass adsorption at 196 K. We also obtained isotherms of the absolute adsorption amount of CH4 at different temperatures. The isosteric heat of adsorption from the isotherms of the absolute adsorption amount of CH4 is larger than the liquefaction heat of CH4 at its boiling point by more than 3 kJ mol-1.

Thermodynamic Study of Water Adsorption in High-Silica Zeolites

The joint use of microcalorimetric and computational approaches has been adopted to describe H2O interaction with cus Al(III) Lewis and Si(OH)+ Al- Brønsted acidic sites within H-BEA and H-MFI zeolites (both with approximately 6 Al/unit cell). Adsorption data obtained at 303 K were compared to experimental model systems, such as all-silica zeolites, amorphous silica, and silico-alumina, transition alumina. In parallel, ab initio molecular modeling was carried out to mimic, in a cluster approach, Lewis and Brønsted acidic sites, as well as a variety of Si-OH species either with H-bonding interacting (nests and pairs) or isolated. H-BEA and H-MFI water affinity values were found to be almost equivalent, in both quantitative and energetic terms, in that dominated by Al-containing sites population, more than by nanocavity topology or by acidic site nature. Both H-zeolites, saturated with approximately 5 Torr of H2O vapor, bind approximately 4 H2O per Al site, almost one of which is tightly bound and not eliminated by RT pumping-off. A 160 < q(diff) < 80 kJ/mol interval was measured for the adsorption up to 1H2O/Al. The zero-coverage heat of adsorption (q0 approximately 160 kJ/mol, for both H-zeolites) was assigned to H2O/Lewis complex formation, which dominates the early stage of the process, in agreement with the ab initio computed H2O/Lewis sites binding energy. The rather broad q(diff) interval was interpreted as due to the simultaneous adsorption of H2O on both structural Brønsted sites and strongly polarized H2O already adsorbed on Lewis sites. For this latter species, BE = 74 kJ/mol was computed, slightly higher than BE = 65 kJ/mol for H2O/Brønsted sites interaction, showing that H2O coordinated on cus Al(III) Lewis sites behaves as a structural Brønsted site. The investigated all-silica zeolites have been categorized as hydrophilic in that the measured heat of adsorption (100 < q(diff) < 44 kJ/mol) was larger than the heat of liquefaction of water (44 kJ/mol) in the whole coverage examined. Indeed, polar defects present in the hydrophobic Si-O-Si framework do form relatively stable H2O adducts. Crystalline versus amorphous aluminosilicate q(diff) versus n(ads) plots showed that the measured adsorption heat is lower than expected, due to the extraction work of Al atoms from the amorphous matrix to bring them in interaction with H2O. On the contrary, such an energy cost is not required for the crystalline material, in which acidic sites are already in place, as imposed by the rigidity of the framework. Modeling results supported the experimental data interpretation.

Study of the single-stage mixed-gases refrigeration cycle for cooling temperature-distributed heat loads

The single-stage mixture refrigeration cycle is the simplest of numerous existing mixture refrigeration cycles. In this paper, detailed study was made on the use of a single-stage closed throttling refrigeration cycle with multicomponent zeotropic mixtures for cooling temperature-distributed heat loads. The heat capacity balance of the heat exchanger with temperature-distributed heat loads was calculated with an equation of state. Because of the important contribution of specific heat capacity from latent heat, the result shows that with optimal mixtures, the single-stage cycle can provide excellent performance for cooling temperature-distributed heat loads associated with gas liquefaction. The optimization result was also compared with other published results of calculations. A small-scale prototype based on the single-stage cycle was also developed and tested, which is driven by an oil-lubricated air-conditioning compressor with a nominal input power of 1.1 kW. Electrical resistances were used to simulate the temperature-distributed heat loads along the heat exchanger. The temperature-distributed heat loads were just used to simulate the gas liquefaction heat loads. Preliminary experimental results of 0.77 liter liquid methane per 1.2 kW·h and 1.3 liter liquid CF 4 per 1.2 kW·h were achieved. The significance of using single-stage mixture refrigeration cycle is that the configuration of the refrigeration system becomes much simpler than the traditional liquefaction system, while maintaining a high efficiency.

Thermodynamics of hydrogen adsorption on the zeolite Li-ZSM-5

Thermodynamic characterisation of the adsorption process (at a low temperature) of dihydrogen on the zeolite Li-ZSM-5 was carried out by means of variable-temperature infrared spectroscopy, with the simultaneous measurement of temperature and equilibrium pressure. Adsorption renders the H–H stretching mode infrared active, at 4092 cm −1 . The standard adsorption enthalpy and entropy resulted to be ΔH 0=−6.5(±0.5) kJ mol −1 and ΔS 0=−90(±5) J mol −1 K −1 , respectively. The adsorption enthalpy is significantly larger than the liquefaction heat, and this fact renders Li-ZSM-5 a potential cryoadsorbent for hydrogen storage.

Surface Functionalization and Stabilization of Mesoporous Silica Spheres by Silanization and Their Adsorption Characteristics

Siliceous MCM-48 spheres are tailored with silane coupling agents, n-octyldimethylchlorosilane (C8) and (3-aminopropyl)triethoxysilane (APTS). The changes in adsorption features of water and acetaldehyde with the modifications are elucidated by aid of adsorption calorimetry. The MCM-48 surface exhibits a hydrophobic feature after the C8 treatment, and the hydrophobicity is retained by repetition of water sorption, although the surface of crude MCM-48 easily changes to hydrophilic by water adsorption. The APTS treatment drastically increases an adsorptivity of acetaldehyde vapor. The adsorbed amount of acetaldehyde is saturated at ca. 5 mmol/g for crude MCM-48; however, that for the APTS-treated sample is three times higher at equilibrium pressure (Pe) of ca. 6 kPa and monotonically increases at Pe > 6 kPa. Differential heats of adsorption during the mesopore filling of acetaldehyde are 53−70 kJ/mol, which is much higher than the heat of liquefaction (27.2 kJ/mol) of acetaldehyde, suggesting that chemical reactions of adsorbed acetaldehyde take place on the surface.

Calorimetric and IR spectroscopic study of the interaction of NH3 with variously prepared defective silicalites

Abstract Nature, population and acidic properties of the hydroxylated species (hydroxyl nests) present in the nanocavities of variously prepared defective silicalites have been characterised by the adsorption of NH 3 , monitored through the combined use of microcalorimetry and IR spectroscopy. The experimental data (enthalpy of adsorption, stretching frequency of the O–H bonds perturbed by the interaction with NH 3 ) have been compared with ab initio computational results (binding energy, red shift of the stretching frequency) and were found to be in good agreement. The presence of rings of mutually interacting silanols has been postulated (on the basis of both experimental and computational results) in the case of chemically very pure, defective silicalites. The dimension of the rings depends on the preparation conditions. A perfect silicalite sample, taken as hydrophobic reference material, was found to be virtually unreactive towards NH 3 , in that it interacts very scarcely with ammonia, showing a heat of adsorption much lower than the heat of liquefaction of NH 3 itself.

Thermal behavior of a pharmaceutical solid acetaminophen doped with p-aminophenol.

Thermal behavior of a series of acetaminophen (APAP) doped with p-aminophenol (PANP) was studied by differential scanning calorimetry (DSC) to determine whether it exhibited a eutectic system. Within the temperature range of 120 to 200 degrees C, accurately weighed (1-2 mg) samples sealed in hermetic pans were calorimetrically scanned with a low scanning rate of 1 degrees C/min. The mixture formed a single eutectic with the composition ratio APAP/PANP of 0.6/0.4 at a temperature of 138 degrees C, where it liquefied. Melting began as early as at the eutectic point, which was below the melting temperature of APAP (169 degrees C). The melting point as well as heat of APAP fusion was depressed with the increase in doped PANP. It was postulated that there might be a deficit heat of APAP fusion in APAP doped with PANP, which was coincident with the heat consumed by early liquefaction. The deficit heat was used to correct fraction molten in the van't Hoff law of purity determination. It was found that the purity determination of APAP doped with PANP was comparable to the UV-spectroscopic method up to the maximum doped PANP level of 8 mol percent. It was concluded that DSC was able to approach early heat of liquefaction of APAP doped with PANP. The van't Hoff law may be applicable to the determination of APAP with the presence of PANP as a eutectic impurity.