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Adsorption: Journal of the International Adsorption Society (v.9, #4)
Mechanisms of Molecular Mobility of Oxygen and Nitrogen in Carbon Molecular Sieves by Dongmin Shen; Martin Bülow; Norberto O. Lemcoff (pp. 295-302).
Molecular mobility of oxygen, O2, and nitrogen, N2, in Carbon Molecular Sieves, CMS, was investigated using the Frequency Response, FR, technique to identify mass-transfer mechanisms and related kinetic time constants. The FR data showed that O2 mobility in four types of CMS was dominantly controlled by surmounting surface-barrier resistances, whereas the mobility of both O2 and N2 in pellets of a fifth CMS type obeyed the Fickian diffusion model. Temperature and pressure dependences of surface-barrier penetration time constants were obtained for O2 and N2 in several of those CMS materials. The kinetic time constants of surface-barrier penetration were related to Langmuir-type rate constants, which indicates that kinetic behavior of O2 therein could also be interpreted in terms of a Langmuir-kinetics equation.
Keywords: oxygen; nitrogen; carbon molecular sieve; sorption; molecular mobility; surface barrier; Langmuir kinetics; frequency response technique; piezometric technique
Assessment of Hydrophobic-Hydrophilic Properties of Microporous Materials from Water Adsorption Isotherms by João Pires; Moisés L. Pinto; Ana Carvalho; M.B. de Carvalho (pp. 303-309).
The characterization of the hydrophobic-hydrophilic properties of different types of microporous materials, namely activated carbons, pillared clays and zeolites, was made by the determination of water adsorption isotherms. The data were analysed by the Dubinin and Astakhov (D-A) equation. The use of the E parameter of the D-A equation as a measure of the hydrophobic-hydrophilic character is proposed. When the information obtained from the E parameter is compared with the information that can be obtained from other parameters used in the literature to characterize the hydrophobicity of materials, it is found that the former is more sensitive and is more directly related with the shape of the adsorption isotherms which, ultimately, is the more direct manifestation of the hydrophobic-hydrophilic properties of a given material.
Keywords: hydrophobicity; water-adsorption; zeolites; pillared clays; activated-carbons
A Study of the Influence of Hydrophobicity of Activated Carbon on the Adsorption Equilibrium of Aromatics in Non-Aqueous Media by Falk Ahnert; Hassan A. Arafat; Neville G. Pinto (pp. 311-319).
The effect of hydrophobicity on the adsorption of aromatics on metal-free activated carbons was studied. Adsorption isotherms for phenol, aniline, benzene, and xylene were generated in cyclohexane and heptane media, using seven carbons with different surface heterogeneity. The hydrophobicity of these carbons was probed using flow microcalorimetry (FMC). Surface polarity and solvent and adsorbate hydrophobicity were found to influence the adsorption capacity. For adsorbates that do not form hydrogen bonds with oxygen on the carbon surface, higher surface acidity lowers adsorption capacity due to increased polarity. In contrast, for adsorbates that can form hydrogen bonds with surface oxygen, the capacity is enhanced at higher surface acidities. A higher solvent hydrophobicity was found to decrease capacity for all the aromatic adsorbates studied, except at high surface polarity, where the effect of the solvent was found to be minimal.
Keywords: activated carbon; adsorption; aromatics; isotherms
Propane–Propylene Binary Adsorption on Zeolite 4A by Carlos A. Grande; Carlos Gigola; Alírio E. Rodrigues (pp. 321-329).
In this work we report new experimental data of pure and binary adsorption equilibrium of propane and propylene on zeolite 4A at 423 and 473 K. The pressure range studied was 0–500 kPa, which is the entire pressure range used in PSA–VSA (Pressure–Vacuum Swing Adsorption) units. The amount adsorbed of propane is much higher than that reported in previous literature. Propane diffusivity was estimated from uptake curves in the linear isotherm region. Adsorption of propane was extremely slow and equilibrium was established only after three days of adsorbate–adsorbent contact. The IAST (Ideal Adsorbed Solution Theory) using the Generalized Dubinin model to describe the pure propylene equilibrium and the Langmuir model for propane predicted with acceptable accuracy the binary adsorption data. Alternatively, the multisite model of Nitta was used to fit pure component isotherms and used in the IAST. Predictions were worse than those with the other strategy.
Keywords: binary adsorption; equilibria; propane and propylene; kinetics
Sorption Behaviour of Molybdenum on Different Antimonates Ion Exchangers by I.M. El-Naggar; E.A. Mowafy; G.M. Ibrahim; H.F. Aly (pp. 331-336).
Various antimonate compounds are well known as important inorganic ion exchangers, since they resist radiation and chemical degradation and also exhibit selectivities towards different cations. Ceric, silicon, titanium and ferric antimonates were prepared as inorganic ion exchangers. Characterization of these materials has been described using different techniques, including thermal analysis, surface area measurements, X-ray diffraction and IR-spectroscopy. In batch distribution experiments the influence of HNO3 molarity and Mo concentration for Mo sorption on different matrices is described in terms of their retention capacities and distribution coefficients.The selectivities of these exchangers towards molybdenum are in the order: CeSb > SiSb > FeSb > TiSb.
Keywords: antimonates; sorption; ion exchange; selectivity; molybdenum
Pressure-Swing Adsorption Using Layered Adsorbent Beds with Different Adsorption Properties: I—Results of Process Simulation by Yaping Lü; Shain-Jer Doong; Martin Bülow (pp. 337-347).
A simulation study was conducted on layered-bed pressure-swing adsorption, PSA, processes with adsorbents that differ in their adsorption properties. As an example, an oxygen, O2, vacuum-swing adsorption, VSA, process was analyzed to investigate relationships between process performance and adsorption properties of the adsorbents used. For two adsorbents with identical nitrogen-to-oxygen, N2/O2, selectivity but different N2 and O2 capacities, placing the high-capacity adsorbent at the product end and the low-capacity adsorbent at the feed end of the adsorption bed gives a better performance than the case of reversing layering of these adsorbents. However, for two adsorbents with different values of N2/O2 selectivity but identical N2 capacity, changing the bed-layer configuration does not show a significant difference in O2-VSA performance. The advantages of layering a high-capacity adsorbent on product end of the bed are demonstrated by an examination of the N2-loading difference in a VSA cycle. The modeling study also reveals an effect of cycle features (e.g., equalization step) on the effectiveness of using layered-bed configurations in VSA/PSA processes. It suggests that layering appropriately two adsorbents with different adsorption properties could result in better VSA/PSA-process performance than using a single-layer bed with either of the two adsorbents.
Keywords: VSA/PSA processes; performance simulation; layered beds; positioning of adsorbents; variation in capacity and selectivity
Experimental Results and Analysis for Adsorption Ice-Making System with Consolidated Adsorbent by S.G. Wang; R.Z. Wang; J.Y. Wu; Y.X. Xu (pp. 349-358).
An adsorption ice-making machine has been built with a single consolidated adsorber and activated carbon-methanol pair. A consolidated adsorbent block made of activated carbon mixed with a binder with good heat transfer properties has been developed and implemented in the adsorber. The design is focused on the adsorber consisting of copper finned tubes and carbon blocks. Experimental tests have been performed suitable for ice making. This paper describes the experimental results of such an ice-maker operating with an intermittent cycle and a cycle time of 35 minutes. The thermal conditions used to test the cycle are: 115°C heat source, 22°C heat sink, the evaporator temperature corresponding to the chilled ethylene glycol temperature is −7°C. At this evaporating pressure, the mass transfer resistance controls the adsorption process. Test results show that the COP reaches 0.07 whereas the SCP (specific cooling power) is 11 W kg−1 activated carbon. A two-bed adsorptive prototype ice-making machine operating with a heat and mass recovery cycle has also been made for onboard adsorption refrigeration in fishing boats. Good performances have been achieved due to improved mass transfer and the new ice maker can produce 18–20 kg h−1 of flake ice at mean temperature of −7°C.
Keywords: adsorption; consolidated block; activated carbon; ice-maker