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Adsorption: Journal of the International Adsorption Society (v.18, #2)
Bifunctional activated carbon with dual photocatalysis and adsorption capabilities for efficient phenol removal by Wei Li; S. Liu (pp. 67-74).
Bifunctional activated carbons (AC) with the abilities of both photocatalysis and adsorption were fabricated via the sol–gel route combined with hydrothermal treatment and N2 reactivation method. TiO2 was located mainly at the entrance of the surface macropores of AC. Under UV light irradiation, efficient removal of phenol was realized by combination of adsorption and photocatalytic degradation for the obtained bifunctional materials. In insufficient light or dark, phenol removal occurred mainly through adsorption. The prepared bifunctional carbon with a mass ratio of 50 TiO2 per AC ratio exhibited high efficiency for phenol removal. The total phenol removal capacity of 50TiO2/AC was almost 5 times of that of pure AC and 6 times of that pure TiO2 after 10 cycles. The prepared bifunctional carbons possess the advantages of high pollutant removal capability and good recyclability, making them promising for the efficient treatment of lightly polluted aqueous solutions.
Keywords: Photocatalysis; Adsorption; Activated Carbon; Titanium dioxide; Bifunctional carbon
Adsorption and diffusion of Xenon in a granulated nano-NaY zeolite by Amir Charkhi; Mohammad Kazemeini; Seyyed Javad Ahmadi; Sareh Ammari Allahyari (pp. 75-86).
Henry’s law constant and crystal diffusivity of xenon in the granulated nano-NaY zeolite were measured by the pulse gas chromatography method. For this purpose the moments of response peaks of xenon were analyzed. The effect of extra column parts of the utilized chromatographic system was also considered by analyzing the moments of the response peak which was obtained by pulse injection of inert gas of helium into the carrier gas of nitrogen. In addition, the measurement of average velocity of the carrier gas regarding the pressure drop in the extra column parts of the system attributed to precise results. By carrying out the experiments at various temperatures in the range of 30–110 °C the heat of adsorption and activation energy of crystal diffusivity were estimated. In order to find the binder effect on the adsorption of and diffusion into granules, the aforementioned parameters were also measured for the binderless granules of macron sized NaY zeolite. Results revealed that although the adsorption of xenon on the binder of bentonite was negligible, the diffusion resistance created by this binder was significant such that the effective crystal diffusivity in the granules with 25 % binder was determined to be 96 percent lower than the granules with no binder.
Keywords: Crystal diffusivity; Henry’s law constant; Pulse chromatography; Nano sized; NaY zeolite; Xenon
Numerical study of nitrogen desorption by rapid oxygen purge for a medical oxygen concentrator by Siew Wah Chai; Mayuresh V. Kothare; Shivaji Sircar (pp. 87-102).
Efficient desorption of selectively adsorbed N2 from air in a packed column of LiX zeolite by rapidly purging the adsorbent with an O2 enriched gas is an important element of a rapid cyclic pressure swing adsorption (RPSA) process used in the design of many medical oxygen concentrators (MOC). The amount of O2 purge gas used in the desorption process is a sensitive variable in determining the overall separation performance of a MOC unit. Various resistances like (a) adsorption kinetics, (b) column pressure drop, (c) non-isothermal column operation, (d) gas phase mass and thermal axial dispersions, and (e) gas-solid heat transfer kinetics determine the amount of purge gas required for efficient desorption of N2. The impacts of these variables on the purge efficiency were numerically simulated using a detailed mathematical model of non-isothermal, non-isobaric, and non-equilibrium desorption process in an adiabatic column.The purge gas quantity required for a specific desorption duty (fraction of total N2 removed from a column) is minimum when the process is carried out under ideal, hypothetical conditions (isothermal, isobaric, and governed by local thermodynamic equilibrium). All above-listed non-idealities (a–e) can increase the purge gas quantity, thereby, lowering the efficiency of the desorption process compared to the ideal case. Items (a–c) are primarily responsible for inefficient desorption by purge, while gas phase mass and thermal axial dispersions do not affect the purge efficiency under the conditions of operation used in this study.Smaller adsorbent particles can be used to reduce the negative effects of adsorption kinetics, especially for a fast desorption process, but increased column pressure drop adds to purge inefficiency. A particle size range of ∼300–500 μm is found to require a minimum purge gas amount for a given desorption duty. The purge gas requirement can be further reduced by employing a pancake column design (length to diameter ratio, L/D<0.2) which lowers the column pressure drop, but hydrodynamic inefficiencies (gas mal-distribution, particle agglomeration) may be introduced. Lower L/D also leads to a smaller fraction of the column volume that is free of N2 at the purge inlet end, which is required for maintaining product gas purity.The simulated gas and solid temperature profiles inside the column at the end of the rapid desorption process show that a finite gas-solid heat transfer coefficient affects these profiles only in the purge gas entrance region of the column. The profiles in the balance of the column are nearly invariant to the values of that coefficient. Consequently, the gas-solid heat transfer resistance has a minimum influence on the overall integrated N2 desorption efficiency by O2 purge for the present application.
Keywords: Desorption by purge; Medical oxygen concentrator; Model simulation; Non-isothermal; Non-isobaric; Non-equilibrium
Effects of organic additives on crystallization process and the adsorption performances of zeolite A by Hui Sun; Benxian Shen (pp. 103-111).
The crystallization of zeolite A was monitored by measuring the adsorption capacities of synthetic products. The influences of organic additives on the crystallization process and adsorption performances of zeolite were investigated. SDS (sodium dodecyl sulphonate), TWEEN (Tween-80), and PEG (poly(ethylene glycol)) shorten the induction period by reducing the interfacial energy while SCMC (sodium carboxymethylcellulose) can prolong the induction period by increasing the interfacial energy. TEA (triethanolamine) can also suppress the nucleation through reducing the effective supply of aluminum. All the organic additives but SCMC diminish the rate of crystal growth. CTAB (cetyltrimethylammonium bromide) causes the destruction of crystal structure and reduce the concentration of OH− ions. As a result, the rate of crystal growth is significantly reduced. Meanwhile, PAM (poly(acrylamide)), SDS, TWEEN, HMTA (hexamethylenetetramine), and PEG increase the viscosities of synthesis systems, thus, diminish the growth rate. PAM restrains the transformation of zeolite A crystal into hydroxysodalite one, therefore, tremendously improves the stability of crystals of zeolite A. In addition, PAM can promote the rates of n-hexane adsorption on zeolite 5A because of the impact of PAM on the crystal-size distributions of zeolite 5A.
Keywords: Crystallization; Zeolite A; Adsorption performance; Organic additive
Pore wall thickness and interpore influence on adsorption of alkanes in carbons using explicit pore models by Sebastião M. P. Lucena; Daniel V. Gonçalves; Paulo G. M. Mileo; Célio L. Cavalcante Jr. (pp. 113-119).
In modeling of activated carbons, the pores are often assumed to be slit-shaped formed of a constant number of graphene layers. X-ray diffraction studies show that micropores are formed between stacks of different numbers of graphene layers. In this study, we investigate, through the grand canonical Monte Carlo method, the influence on the adsorbed alkanes densities of pore walls with different graphene layers thickness and the related interpore adsorbate interaction when the pore wall has only one graphene layer. All studies of thickness and interpore interaction to date were performed using the Steele 10-4-3 potential model. Instead of Steele model, we propose explicit models made up of graphene layers of discrete carbon atoms. We also investigated the sensitivity of the system to the cut-off and solid-fluid parameter. With our explicit model we found that the influence of the number of carbon layers is not significant for n>2 as previously observed by Steele model—DFT studies. The system was also insensitive to cut-off and well deep parameter variations. A new pore model with an extra dummy graphene wall was proposed to investigate the interpore interaction. The interpore interaction study with the alkanes series C1 to C4 shows that the retention capacity of heavier alkanes is the same whether for activated carbons with few layers (stronger interpore interaction) as for carbons with two or more layers (stronger solid-fluid interaction) assuming negligible surface mediation. The explicit models proposed can be successfully used in the elaboration of virtual porous carbon models to reproduce wall thickness and interpore adsorbate interactions phenomena.
Keywords: Activated carbon; Explicit models; Monte Carlo Simulation; Alkane adsorption
Sorption equilibria of CO2 on silica-gels in the presence of water by Juanjuan Zheng; Yaping Zhou; Yong Ting Zhi; Wei Su; Yan Sun (pp. 121-126).
The sorption equilibria of carbon dioxide on three types of silica gel (SG) with different pore size distributions in the presence of water were studied experimentally using a volumetric method at 275 K with pressures from 0 to 3.7 MPa. Both the pore size distribution of the silica gel and the quantity of pre-sorbed water impact the formation of the CO2 hydrates. For wet silicon gel A(SG-A) with water loading ratio of 0.75, the highest CO2 sorption was about 2.5 mmol of CO2 per gram of dry sorbent at 275 K. Similarly, the highest sorption was about 2.7 mmol for wet SG-B with R w =0.81. However, CO2 hydrate did not form on the wet surface of SG-C due to its large pore sizes.
Keywords: Carbon dioxide; Hydrate; Silica gel; Adsorption; Sequestration
Adsorption of aromatic trace compounds from organic solvents on activated carbons—experimental results and modeling of adsorption equilibria by Theo Gräf; Christoph Pasel; Michael Luckas; Dieter Bathen (pp. 127-141).
Liquid phase adsorption is an important process for the removal of trace compounds from liquid matrices. Until today, research on liquid phase adsorption is less substantial than work on other thermal separation processes. The description of relevant mechanisms and interactions is difficult mainly because of lacking experimental data. This paper presents extensive isotherm measurements for the adsorption of organic trace compounds from organic solvents on activated carbons. A systematic variation of molecular structure of adsorptives and solvents enabled the identification of main structural factors dominating adsorption in these systems. The factors are polarity, extension and density of π electrons and sterical complexity. An analysis of the measured isotherms revealed incremental effects of functional groups and structural elements being characteristic for the adsorption capacities on activated carbons. Three consecutive empirical prediction models of adsorption equilibria are developed and compared. The empirical Freundlich equation appeared to be best suited for fitting the experimental data. The models apply an incremental concept permitting the calculation of adsorption isotherms on the basis of the structural increments of solvent and adsorptive molecules. The three models have a different extent of underlying data, a different number of parameters and a different range of application. The experimental data are predicted with satisfying accuracy for many engineering applications. The most sophisticated model has the most extensive range of application and manages on the smallest number of parameters.
Keywords: Liquid phase adsorption; Activated carbon; Isotherm modeling; Incremental methods
Fixed bed adsorption of CO2/H2 mixtures on activated carbon: experiments and modeling by Nathalie Casas; Johanna Schell; Ronny Pini; Marco Mazzotti (pp. 143-161).
We present breakthrough experiments in a fixed bed adsorber packed with commercial activated carbon involving feed mixtures of carbon dioxide and hydrogen of different compositions. The experiments are carried out at four different temperatures (25 °C, 45 °C, 65 °C and 100 °C) and seven different pressures (1 bar, 5 bar, 10 bar, 15 bar, 20 bar, 25 bar and 35 bar). The interpretation of the experimental data is done by describing the adsorption process with a detailed one-dimensional model consisting of mass and heat balances and several constitutive equations, such as an adsorption isotherm and an equation of state. The dynamic model parameters, i.e. mass and heat transfer, are fitted to one single experiment (reference experiment) and the model is then further validated by predicting the remaining experiments. Furthermore, the choice of the isotherm model is discussed. The assessment of the model accuracy is carried out by comparing simulation results and experimental data, and by discussing key features and critical aspects of the model. This study is valuable per se and a necessary step toward the design, development and optimization of a pressure swing adsorption process for the separation of CO2 and H2 for example in the context of a pre-combustion CO2 capture process, such as the integrated gasification combined cycle technology.
Keywords: Pre-combustion CO2 capture; PSA; Activated carbon; Breakthrough experiments; Simulations