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Experimental and Theoretical Studies of VOC Adsorption on Acid-Activated Bentonite in a Fixed-Bed Adsorber
Abstract
In this work, the adsorption of toluene onto acid-activated bentonite in a fixed bed using an inverse gas chromatography was investigated. Adsorbent was prepared according to an optimized activation process. Experimental and theoretical studies were established to evaluate the removal efficiency of toluene by adsorption on acid-activated bentonite and to predict kinetics parameters. A suitable adsorption model has been developed to simulate the measured data based on linear driving force approximation. The fourth-order Runge−Kutta method was used to integrate the partial differential equations, and the resulting functions were simultaneously solved to obtain the breakthrough profiles. Theoretical predictions from the model were compared with column adsorption data to ensure the validity of the model. Adequate agreement between simulations and experimental data was reached.
... Four parameters are known to strongly influence the structural properties and can affect significantly the required results: the temperature, the contact time, the solid-to-liquid ratio and mass percentage of acid. These parameters are the main factors that delimit the extent of acid-activation and thus, the induced physicochemical and structural changes [13][14][15]. As a consequence of these structural changes, acid-treatment can extend the fields of application of these porous solids such as in the control of atmospheric pollution [15]. ...
... These parameters are the main factors that delimit the extent of acid-activation and thus, the induced physicochemical and structural changes [13][14][15]. As a consequence of these structural changes, acid-treatment can extend the fields of application of these porous solids such as in the control of atmospheric pollution [15]. This work is a continuation of numerous studies carried out in the field of surface and colloid science and dealing with clay minerals [15,[29][30][31][32][33][34][35]. ...
... As a consequence of these structural changes, acid-treatment can extend the fields of application of these porous solids such as in the control of atmospheric pollution [15]. This work is a continuation of numerous studies carried out in the field of surface and colloid science and dealing with clay minerals [15,[29][30][31][32][33][34][35]. Natural green clay mineral collected from south of Tunisia was treated with sulfuric acid in order to prepare suitable adsorbent for several organic compounds. ...
In this study, natural clay minerals with green appearance were treated with sulfuric acid. Mass percentage of acid (wt%), temperature (T), contact time (t) and liquid-to-solid mass ratio (R) are used as the prevailing factors that determine the extent of acid-activation. The values of these factors range from 15–50%, 60–90 °C, 1.5–6 h and 4–7, respectively. The study has focused on the structural changes as well as textural characteristics of the clay. Three activated clay samples were prepared under different treatment conditions. The samples were characterized using X-ray powder diffraction (XRD), fourier transform infrared (FTIR), scanning electron microscope (SEM), chemical analysis and N2 adsorption techniques. Characterization of the treated clay minerals exhibited significant structural changes to a greater extent of acid-activation, from being partially crystalline to being amorphous silica. The surface area and total pore volume of clay increased proportionally with the level of acid treatment. The average pore diameter behaved differently. During the strong acid treatment, a large increase in pore volume and the enlargement of the pore size distribution were observed. This suggests that considerable structural changes and partial destruction may have occurred in this condition. The removal of methylene blue, used as cationic dye, from aqueous solution by the batch adsorption technique on three prepared acid-activated clay samples was studied. The Langmuir model was found to agree well with the experimental data.
... Volatile organic compounds (VOCs), including kinds of organic chemical, are one of the most important pollutants emitted from anthropical activities such as industrial processes. 1,2 Most of them are hazardous to the environment, contributing to serious problems such as destruction of the ozone layer, photochemical smog and global warming. Also, long-term exposure to VOCs is harmful to human health, and may cause deformity, pathogenic and even fatal diseases. ...
... 2,3,5 The most important issue in adsorption technology is the choice of proper adsorbents, and an ideal adsorbent for VOCs adsorption is expected to possess a large adsorption capacity, favorable hydrophobic performance and an easy regeneration property. 1,7 Until now, activated carbon has been most widely used as an adsorbent for its low cost and good adsorption capacity. 7,8 However, some intrinsic weaknesses such as its hydrophilic properties and pore blocking cause problems in various operational conditions. ...
... 7,8 However, some intrinsic weaknesses such as its hydrophilic properties and pore blocking cause problems in various operational conditions. 1,7 Thus, a new kind of adsorbent possessing high surface area and good hydrophobicity, as well as high thermal stability is in great need for VOCs removal. 8 In recent years, a new kind of polymeric adsorbent, namely hyper-cross-linked polymers, which were rst synthesized by Davankov,9 have gained great attention for the removal of organic compounds from industrial effluents. ...
A series of hyper-cross-linked polymers (HCPs) was synthesized via a post-cross-linking reaction using low-cross-linked polydivinylbenzene (PDVB) as precursor and 4,4′-bis(chloromethyl)biphenyl (BCMBP) as cross-linking reagent. The effects of various amounts of BCMBP on the structures of the hyper-cross-linked polymers were discussed. The resultant typical HCP-1.3 possesses hierarchical micro/meso pore structures and a high stability up to 400 °C. The increased specific surface area and total pore volume of HCP-1.3, which reach 1231 m2 g−1 and 1.99 cm3 g−1, respectively, imply the high cross-linking ability of BCMBP. In order to study the adsorption properties of HCP-1.3, benzene and water were selected as typical adsorbates for their non-polar and polar characteristics. The static adsorption amount of benzene on HCP-1.3 is 30.0 mmol g−1, which is two times higher than the corresponding adsorbed water amount, suggesting a potential hydrophobic property of HCP-1.3. Dynamic adsorption was performed with a fixed-bed column to simulate the real situation under dry and 30% relative humidity conditions. The results show the dynamic adsorption amount under the 30% relative humidity condition is as high as 90% of the value under dry conditions, implying HCP-1.3 would be a promising adsorbent for VOC adsorption under both dry and humid conditions.
... The decreases in the breakthrough time and adsorption capacity were ascribed to the enhanced overall mass transfer coefficient at higher temperatures and the exothermic adsorption characteristics. The adsorption process agrees well with previous VOC adsorption studies [44,45]. Although the adsorption capacity at 200 • C was low, it was very high at 120 • C. Previous studies usually investigated the adsorption performance of VOCs under temperatures below 100 • C. ...
This study focused on the elimination of chlorobenzene by dual adsorption/catalytic oxidation over activated carbon fibers (ACFs) loaded with transition metal oxides (TMOs). The TMOs were successfully loaded on the ACFs by the incipient wetness impregnation method, which has the advantages of easy preparation, low cost, and size uniformity. The removal effects for chlorobenzene (CB) were investigated on pristine ACFs and TMOs@ACFs in a fix-bed reactor. The adsorption/catalytic oxidation experiments result demonstrated that ACFs can be used as a very efficient adsorbent for the removal of low-concentration CB at the low temperature of 120 °C; the breakthrough time of CB over pristine ACFs can reach 15 h at an inlet concentration of 5000 ppmv and space velocity of 20,000 h−1. As the bed temperature rose above 175 °C, the CB removal mainly contributed to the catalytic oxidation of MnO2; a preferable CB removal ratio was achieved at higher temperatures in the presence of more MnO2. Therefore, CB can be effectively removed by the dual adsorbent/catalyst of MnO2@ACF at the full temperature range below 300 °C.
... With the development of the global economy and population growth, the energy crisis and environmental pollution have increasingly become the two most concerned focuses, and sustainable development has become the goal pursued by countries for economic development. In recent years, the phenomenon of haze has become more frequent, and VOCs, as a photochemical reaction and an important precursor of PM2.5, have become a hot issue in society [1,2]. Air pollutants VOCs are mainly derived from the production process emissions of major industries. ...
In this paper, open-loop desorption experiments were carried out by hot gas sweeping. The effects of different concentrations of methylene chloride and temperatures in the sweeping gas on the desorption process were investigated from three perspectives: average desorption rate ( V ¯ 30 , V ¯ 9 ), instantaneous desorption rate (V max ) and desorption kinetics. It was concluded that the maximum instantaneous desorption rate could better represent the desorption rate required by the experiment at different sweeping concentrations and temperatures. The maximum instantaneous desorption rate V max increased with the rising temperature and decreased with the increasing concentration of inlet methylene chloride. The concluded law was the same as the average desorption rate ( V ¯ 30 ) of 30 min. Thereinto, the temperature had a significant influence on V max . As the temperature increased, the concentration of inlet methylene chloride also inhibited the maximum instantaneous desorption rate V max more obviously. This provided important reference data for the optimization design of desorption equipment in the activated carbon adsorption process.
... Adsorption capacity (mg of toluene/ g of PFAB/C/A) significantly [24]. On the other hand, Amari et al. [49] performed the adsorption tests in a packed bed column at different temperatures and concluded that at high temperatures, the adsorption capacity relatively decreased to small equilibrium constants and an earlier breakthrough curve occurred, attributed to increasing micropore and macropore diffusion with temperature. The reverse trend observed in the breakthrough curves in the tests of Talmoudi et al. [50] was attributed to the combined effects of increased pore diffusion but decreased equilibrium or saturation capacity at high temperature. ...
Furfuryl alcohol (C 5 H 6 O 2 ) (FA) was used as the precursor for the synthesis of approximately 1.0 mm sized polymeric beads (PFAB) via suspension polymerization. The polymeric beads were carbonized and activated to synthesize porous carbon beads (PFAB/C/A) as an efficient adsorbent for gaseous volatile organic compounds (VOCs). Surface characterization tests revealed the material to be predominantly microporous with the specific surface area measured to be ∼446 m ² /g. Raman measurements revealed a graphitic characteristics of PFAB/C/A. Adsorption tests were performed in a fixed tubular packed bed adsorber under different operating conditions: amounts of adsorbents (2–6 g), gas flow rates (0.2–0.4 standard cc per min), adsorption temperatures (40–60 °C), VOC concentrations (2000–53,300 ppm) and types of VOCs (toluene and benzene). The tests revealed high adsorption capacities of the synthesized material i.e., 515 and 350 mg/g for toluene and benzene, respectively at 50 °C. This study has clearly shown that the biomass-based environmentally benign FA can be a potential alternative precursor to the synthetic petro-based polymers presently used for preparing carbon-based adsorbents.
... The mass transfer rate equation was established by the Linear Driving Force (LDF) model, which is frequently and successfully used for analysis adsorptive process in adsorption column e.g. Sircar et al. (2000) [11] and Amari et al. (2010) [12] .The mass transfer rate equation can be written as ...
Silica gel has a potential ability to adsorb water vapor and contaminant together, so it has been focused recently in indoor air environment. In this paper, the performance of silica gel adsorbing trace gas pollutions in isothermal column is studied through theoretical methods. The model of the adsorption column system was set up based on mass conservation, linear driving force model and Henry's Law. An implicit differential method was used to solve the model in the Matlab environment and to obtain the breakthrough curves. Seven simulation experiments were conducted to discuss the effect of overall mass transfer coefficient, Henry parameter and gas velocity on adsorption process. From the result it indicated that the overall mass transfer coefficient impacts the breakthrough point enormously. Henry parameter reflects the adsorption capacity of adsorbent particle, high Henry constant affords long life-span of adsorbent. Furthermore, high efficient and long-term use of adsorbent cannot obtain without reasonable gas velocity.
... Volatile organic compounds (VOCs) such as benzene, toluene, alkanes and amines can readily evaporate into the atmosphere and become hazardous air pollutants. [1][2][3] The environmental concerns associated with these compounds create an urgent need for new functional materials for the elimination of air pollutants. [4][5][6][7] Among them, porous organic frameworks (POFs) have attracted great attention because of their high surface area and good chemical stability. ...
Triptycene-based porous organic frameworks (POFs) with high BET (Brunauer-Emmett-Teller) surface areas and tailored micropore environments were prepared using a facile external knitting strategy. The functionalized POFs exhibited very high aliphatic amine vapor uptakes of up to 5 times their own weight and more than 30 times selectivity for aliphatic amines over n-hexane; thus, they provide new opportunities for environment-related applications. © 2016 The Society of Polymer Science, Japan (SPSJ) All rights reserved.
Conjugated microporous polymers (CMPs) have attracted extensive attention due to their permanent porosity, large specific surface area, unique thermal and chemical stability, and highly delocalized π-πconjugated skeleton. These outstanding characteristics give them a great potential for the applications in adsorption science and many other fields. This review summarizes the recent significant developments and breakthroughs in the synthesis of CMPs and their applications in the adsorption of a variety of compounds including gases, organic pollutants, heavy metal ions, etc. This review also highlights the efforts that the researchers have taken in the improvement of the adsorption performance of the CMPs. Meanwhile, the challenges that CMPs face in the field of adsorption are also discussed. Finally, the possible application directions of CMPs in the field of adsorption are also anticipated.
Controlling excessive biomass is essential for maintaining the long-term stable operation of gas biofilters. Current methods based on physical and chemical technologies can reduce removal efficiency. In this study, we developed a new method using enzymatic-quorum quenching (QQ) to simultaneously control biomass and maintain a stable removal efficiency. A 70-d biofilter operation showed that clogging was controlled with the application of enzymatic-QQ, thereby maintaining a stable operation performance. The pressure drop of the biofilter with QQ enzyme (QQBF) was 75 Pa m⁻¹, while that of the control biofilter (BF) was 156 Pa m⁻¹. Moreover, the biomass accumulation of QQBF was only 130.58 kg m⁻³, while that of BF was 218.50 kg m⁻³. Further analysis indicated that enzymatic-QQ decreased biofilm adhesion strength. Accordingly, the biofilm inside the QQBF detached more easily from the filler surface than that in the BF. The 16S rDNA gene sequencing results suggested that the enzymatic-QQ regulated the related functional genes, thereby changing the biofilm characteristics. Moreover, the reduced biofilm thickness and lower inactivated microbes in QQBF enhanced the metabolic activity. Overall, our results demonstrated that QQ is a promising technology for regulating biofilm characteristics and controlling the clogging of gas biofilters.
Volatile organic compounds (VOCs) emitted from electronic industry may pose severe problems to human health and the environment. In the present study, the VOCs adsorption on commercial activated carbon fibers (ACFs) in a fixed-bed column was investigated by continuous mode as a function of inlet flow rate (100–500 L/h), inlet VOCs concentration (200–400 mg/m³), adsorption temperature (15–35 °C) and bed height (120–240 mm). Afterwards, the adsorption behavior and mechanism of VOCs on ACFs were quantified by the adsorption kinetic, equilibrium, thermodynamic models. To further quantify the adsorption performance in the fixed bed column, an axial dispersion model was proposed to model the breakthrough curves. The results indicated that the adsorption process conformed to the Langmuir isotherm and pseudo-first-order kinetic equation, indicative of physical adsorption. Moreover, the adsorption capacity increased with the increase of VOCs concentration and bed height, but decreased with the increase of inlet flow rate and adsorption temperature. Interestingly, the axial dispersion model can well predict the breakthrough curve and visualize the temporal and axial distribution of VOCs adsorption in the fixed-bed column by integrating Matlab ode15s solver with lsqcurvefit function, from which both the overall mass transfer and axial dispersion coefficient were also derived. The model herein may facilitate the process design of VOCs adsorption and precisely predict the adsorption performance of a scaled-up adsorption fixed bed.
Aromatics cause environmental pollution and seriously threaten human health. Currently, adsorption is regarded as an economical and practical method for aromatics removal. This work aims to explore the adsorption characteristics of benzene, toluene, p-xylene on activated carbon (AC) by the Grand Canonical Monte Carlo method (GCMC). The influence of functional groups (-H and –OH), the density, mesopore size of AC, and classes of aromatics on the adsorption characteristics was analyzed. It was found that –H and –OH functional groups could increase the isosteric heat of aromatics on AC and increase the adsorption capacity in low pressure. As the density of AC increases, the free volume inside AC decreases, and isosteric heat of adsorption increases obviously, resulting in a decrease in saturated adsorption capacity and the saturated adsorption pressure, respectively. The aromatics molecular diameter and mesopore size have a noticeable effect on the adsorption behavior. As the molecular diameter increases, the saturated adsorption pressure decreases. When the pressure is high enough, the mesopores could enhance the adsorption capacity of AC dramatically.
Clay minerals are natural products with very high absorbent, ion-exchange and catalytic properties; chemical nature and pore structure have an important impact on these properties. In this paper, it is proposed to modify the Bentonite clay of Kosovo (Goshica region) by acid and alkali activation to be used for oil recycling. Acid and alkali activation was performed in ratios (10, 30 and 50%) in order to see the effect of acid and alkali treatment on the properties of Goshica Bentonite. The samples were characterized using X-ray powder diffraction (XRD), Fourier transform infrared (FTIR), structural and chemical analysis. Physio-chemical analysis was performed to find out the best activation treatment in order to increase the bentonite´s adsorption and ions exchange capacity. Significant changes were observed in the original pore structure. All the conclusions drawn correlate well with the amount of activator used. This study will provide valuable data on the effect of acid and alkali activated bentonite for the treatment and recyclability of UMO, which is essential for local industry.
Separation or purification is one of the difficult problems in the petrochemical industry. To help solve the difficulty of separation or purification for C2H2/CO2 and C2H
n
/CH4 in the chemical industry, we synthesized a new metal-organic framework (MOF), [Ni(dpip)]·2.5DMF·H2O (1), by a bipyridyl-substituted isophthalic acid ligand. The MOF includes two types of one-dimensional (1D) tubular channels with different sizes and porous environments. The unique tubular channels lead to not only remarkable gas sorption capacity of C2H4, C2H2, and CO2, but also good selectivity for C2H2/CH4, C2H2/CH4, CO2/CH4, and C2H2/CO2, as demonstrated by single-component sorption isotherm results, ideal adsorbed solution theory calculations, and dynamic breakthrough curves. Grand canonical Monte Carlo (GCMC) simulation reveals preferential adsorption sites in the MOF for CO2, C2H2, and C2H4. The MOF also exhibits an obvious size-selective absorption effect on vapor molecules.
To help address efficient separation of C2H
n
light hydrocarbons and C2H2/CO2 in the chemical industry, the self-assembly via an azolate-carboxylate ligand and Co(II) ion gave rise to a new porous MOF material, [Co(btzip)(H2btzip)]·2DMF·2H2O (1) (H2btzip = 4,6-bis(triazol-1-yl)isophthalic acid). In the MOF, the pores are modified by rich uncoordinated triazolyl Lewis basic N atoms and acidic -COOH groups, which strengthen interactions with C2H
n
hydrocarbons and CO2 molecules, leading to high adsorption amounts for C2H2, C2H4, C2H6, and CO2 and remarkable separation efficiency for C2H
n
-CH4, CO2-CH4, and C2H2-CO2 mixtures, as confirmed by breakthrough experiments on the realistic gas mixtures. The MOF also reveals outstanding selective adsorption ability for benzene/toluene, methanol/1-propanol, methanol/2-propanol, and 2-propanol/1-propanol isomers. Molecular simulations disclose the different adsorption sites in the MOF for various adsorbates.
A novel microporous ZIF-67 membrane coating on paper-like stainless steel fibers (ZIF-67 membrane/PSSF) was successfully synthesized by a wet layup papermaking-sintering method followed by the seed-induced growth method. The effects of key factors such as crystallization temperature, synthesis time and Hmim/Co²⁺ (2-methylimidazole/cobalt chloride) molar ratio on the morphology structure of the samples were studied by the modern characterization techniques including SEM, EDX, TEM, XRD, BET, FTIR and TGA, respectively. The experimental result indicated that the as-synthesized ZIF-67 membrane/PSSF was extremely compact with the average membrane thickness of 4.8 μm. And the sample exhibited high BET surface area (440.9 m²/g), uniform pore size (1.881 nm) and large pore volume (0.2096 cm³/g). The adsorption breakthrough experiment of toluene was performed on different bed reactors filled with ZIF-67 membrane/PSSF and granular ZSM-5 zeolites, respectively. The experiment data was well fitted by Yoon–Nelson model, where the values of rate constant in the fixed bed with ZIF-67 membrane/PSSF increased by 32.2% compared with that of commercial ZSM-5 zeolites at the flow rate of 100 mL/min. And the bed pressure drop and length of unused bed (LUB) were dramatically reduced by 39.6% and 16.8% in 3 cm ZIF-67 membrane/PSSF fixed bed. In conclusion, mass transfer efficiency was significantly enhanced based the structured ZIF-67 membrane/PSSF fixed bed, and this new material is a promising adsorptive material for VOCs removal.
The copper-benzene-1,3,5-tricarboxylate (Cu-BTC) attracts increasing attention because of its large BET surface area available for the adsorption of gases. However, low stability in water creates a barrier for its potential applications. In this study, graphene oxide (GO) and reduced graphene oxide (RGO) are adopted to modify the Cu-BTC. The influence of GO and RGO on the resulting BET surface area, pore diameter distribution, morphology and yield of the Cu-BTC are revealed. It is found that the adsorption abilities of the resulting composites for the polar and nonpolar gas molecules are closely related to the presence at surface of the functional groups of the used graphene samples, which provides a potential method to achieve selective adsorption. Furthermore, the preparation processes of the GO/Cu-BTC and RGO/Cu-BTC exert a significant effect on the performances of the resulting product, which is also studied in detail.
A low-cost and scalable strategy for the synthesis of the hierarchically porous and hydrophobic adsorbent for volatile organic compounds removal remains a challenge. In this work, a novel silicon/silicon oxide (Si/SiOx) composite with hierarchical porosity and hydrophobicity was successfully designed from natural palygorskite via a one-pot synthesis strategy, which could be promising in the scalable production due to the cost-effective precursor and simple procedure. The insufficient reduction of palygorskite during the molten salt-assisted magnesiothermic reaction led to the formation of hydrophobic crystalline silicon and the maintenance of amorphous silica framework. The results showed that the Si/SiOx composite possessed the large specific surface area (~307 m² g⁻¹) and hierarchically porous structure (i.e., micro-, meso-, and macropores). As adsorbent for benzene molecules, the Si/SiOx composite exhibited much better static and dynamic benzene adsorption capacities (585.7 and 316.2 mg g⁻¹, respectively) than raw palygorskite (265.7 and 166.1 mg g⁻¹, respectively). The excellent benzene adsorption performance of the Si/SiOx composite could be attributed to the large specific area (offering ample adsorption sites) and hierarchical porosity (facilitating the diffusion/mass transfer process). Moreover, the Si/SiOx composite maintained a high adsorption capactiy for benzene under humidity condition (238.7 mg g⁻¹, with the adsorption efficiency of 76%). Our work provided a universal one-pot method from clay mineral to the hierarchically porous and hydrophobic Si/SiOx composite, and this novel product could be used as an attractive adsorbent for the practical removal of volatile organic compounds.
In this study, interactions between the superalkali species Li3F2 and four volatile organic compounds (VOCs), methanol, ethanol, formaldehyde, and acetaldehyde, are assessed using the CBS‐QB3 composite model. Adiabatic ionization energy (AIE), adiabatic electron affinity (AEA), binding energy (BE), charge transfer (∆q), and highest occupied molecular orbital and lowest unoccupied molecular orbital (HOMO‐LUMO) gaps have been computed. Stronger interactions are observed between Li3F2 and the aldehydes than alcohols. The smaller aldehyde show a larger BE with Li3F2 than the bigger aldehyde. However, alcohol clusters do not show this trend due to their weak interactions (low BEs). Both the alcohol clusters increase their binding energies as they become cations. This unexpected behavior is explained based on molecular orbital arguments.
Cost-effective adsorbents for volatile organic compounds elimination were synthesized at room temperature. New kinds of external cross-linkers were developed and used in the one-step Friedel-Crafts reaction. The synthesized polymers exhibit a high surface area (BET surface area up to 1345 m² g⁻¹), large pore volume, superhydrophobic nature and excellent adsorption capacity for benzene, which is one of the highest to date among the reported adsorbents. It also possesses super preferential selectivity towards benzene in a high humid gas stream. Therefore, this kind of material is a promising adsorbent for air purification and environmental protection.
Soil vapor extraction (SVE) is a common remediation practice typically implemented without a rigorous design process because of insufficient or unspecific design data. Field observations have shown that the mass of contaminant removed by SVE often tails off over time. Significant time has been spent modeling SVE to gain a better understanding of the governing processes and the cause of tailing. Studies have shown that improper mass transfer coefficients affect modeling accuracy. As a result, considerable effort has been spent studying the mass transfer coefficients directly related to the non-aqueous-phase liquid (NAPL), with less emphasis on aqueous air and sorbed-aqueous mass transfer coefficients, despite affecting the observed tailing behavior. Accordingly, a laboratory and modeling study was undertaken with toluene to determine the appropriate aqueous air and sorbed-aqueous mass transfer coefficients. SVE column data generated in laboratory experiments were used to back-calculate the mass transfer coefficients by using FRACMAT, a numerical model. The developed experimental protocol allowed the placement of toluene contamination in the unsaturated soil environment without the development of a NAPL phase. The data generated by the SVE colunm with soils with organic matter and without organic matter, showed that the aqueous-air mass transfer coefficient was exponential, with the aqueous concentration the independent variable. For the zero to moderate organic matter content soils tested, the aqueous-air mass transfer coefficient varied from 1 s(-1) to 0.001 s(-1). Some sorbed contamination was also observed, requiring a sorbed-aqueous mass transfer coefficient. Numerical modeling with FRACMAT showed that the best sorbed-aqueous mass transfer coefficient was a constant value of 0.01 s(-1). The aqueous-air mass transfer coefficient was observed to be the controlling rate limitation in SVE when no NAPL was present in the soil with the zero to moderate organic matter content soil. Studies with silty loam soil showed that additional mass transfer resistances occurred, which could be attributed to the increase in organic matter content and decrease in particle size. DOI: 10.1061/(ASCE)EE.1943-7870.0000405. (C) 2011 American Society of Civil Engineers.
Basing on series of porous carbon models, systematic Monte Carlo studies on the adsorption of acetonitrile (as a simple representative of polar volatile organic compounds) were performed. An influence of porosity and chemical composition of carbon surface on CH3CN adsorption was studied and it was shown that both the factors influenced the adsorption mechanism. A decrease in the pore size and the introduction of oxygen surface groups led to a rise in adsorption energy and to an increase in the filling of accessible volume in the low-pressure part of isotherm. However, from the practical point of view, it is easier to increase the adsorption by introducing polar groups on the carbon surface than by modifying the porosity. Supplementary_Material.zip.
Conjugated microporous polymers (CMP) synthesized through homocoupling polymerization reaction were investigated as adsorbent for the removal of volatile organic compounds from gas streams. The nitrogen adsorption and desorption data revealed that the resultant CMPs possess micro/mesoporous structure. The results of the static water vapor adsorption experiment indicated that CMP-50 had a more hydrophobic surface than the commercial activated carbon (AC). The adsorption and desorption performance of benzene on CMP-50 under static and dynamic conditions were investigated, comparing with AC. It was found that CMP-50 had high adsorption capacity under wet condition (RH=80%) and fast adsorption kinetics. Moreover, the presence of water vapor in the gas stream had little effect on the adsorption capacity. Taken together, it is expected that this kind materials (such as CMP-50) would be a promising adsorbent for the removal of VOCs vapor from the humid gas streams.
The combined concentrator/oxidizer system has been proposed as an effective physical-chemical option and proven to be a viable solution that enables Volatile Organic Carbons (VOCs) emitters to comply with the regulations. In this work, a field scale honeycomb zeolite rotor concentrator combined with a recuperative oxidizer was developed and applied for the treatment of the VOC waste gas. The research shows the following: (1) for the adsorption rotor, zeolite is a more appropriate material than Granular Activated Carbon (GAC). The designing and operation parameters of the concentrator were discussed in detail including the size and the optimal rotation speed of rotor. Also the developed rotor performance's was evaluated in the field; (2) Direct Fired Thermal Oxidizer (DFTO), Recuperative Oxidizer (RO), Regenerative Thermal Oxidizer (RTO) and Regenerative Catalytic oxidizer (RCO) are the available incinerators and the RO was selected as the oxidizer in this work; (3) The overall performance of the developed rotor/oxidizer was explored in a field scale under varying conditions; (4) The energy saving strategy was fulfilled by reducing heat loss from the oxidizer and recovering heat from the exhaust gas. Data shows that the developed rotor/oxidizer could remove over 95% VOCs with reasonable cost and this could be helpful for similar plants when considering VOC abatement.
A novel adsorption model based on multi-layer adsorption reactions and the heterogeneity of adsorbents is developed, which is more general and provides a more complete insight into the adsorption phenomena than Do and Do's model. There are many essential properties (such as layer thickness, layer coverage, effective coverage, deactivated fraction, isosteric heat and equilibrium constants of adsorption) available in this proposed multi-layer adsorption model but not available in the traditional models. The fitting results of the proposed model fit the literature data better than the results from Do and Do's model. The optimal parameters of this multi-layer adsorption model for various samples of activated carbon and zeolite were obtained. The effects of the polarity and molecular weight on the interaction between adsorbate and adsorbent, as well as both adsorbate and adsorbate in the adsorption and desorption processes were also investigated.
In this work, the removal of tetrachloroethylene (PCE) over faujasite-type zeolites exchanged with various cations was investigated under static and dynamic conditions. The nature of the non-framework cation appeared as a predominant parameter, in terms of adsorbate−adsorbent interactions as well as adsorbent micropore volume. Indeed, a molecular simulation has highlighted the interaction between cations and Cl atoms. Moreover, it was observed that the nature of the cation strongly influenced the adsorbent behavior in the PCE removal process. Cationic zeolites appeared as efficient materials for the removal of PCE with good adsorption capacities, in particular, the sodium-exchanged X and Y zeolites. The presence of steam in the gas feed was determined to have a strong negative effect on the performance of the cationic zeolites. However, the addition of 3A sieve in a double bed with NaY zeolite appeared to be a good system. The complete regeneration of the system was obtained at relatively low temperature (180 °C), which allowed it to be still performing after several adsorption/desorption cycles.
Adsorption of toluene onto acid activated clay was carried out. Modified clay was prepared by acid attack (H2SO4) on raw material. Response surface methodology based on a 2-level, 4-varlables central composite orthogonal design was used to evaluate the effects of important parameters on the adsorption of toluene on to activated clay. Temperature (53.8-96.2°C), contact time (0.57-6.93 h), mass ratio of liquid/solid (3.38-7.62) and strength of acid (7.75-57.24%) were chosen as process variables for the optimization. Of these parameters, temperature reaction and time had greater impact on toluene adsorption than did the other parameters. Analysis of variance (ANOVA) shows a good agreement between theoretical analysis and experimental data. The validity of model is verified by an experiment at the optimum conditions. The optimum conditions for the maximum adsorption of toluene onto activated clay are: temperature of 96.2°C, a contact time of 6.93 h, a liquid/solid ratio of 5.98 and strength of acid of 32.94%. Since the predicted values and the actual experimental value obtained for the maximum adsorption of toluene are within 95% confidence intervals, the final model is considered valid and has satisfactory predictive ability.
The optimum conditions for acid activation of a clay were determined in order to bleach olive oil. The methodology of experimental research, with an orthogonal central composite design, has been used in this study and the selected variables were temperature, reaction time, initial acid concentration and liquid/solid weight ratio. The last is the most significant factor in the activation process followed in decreasing order of importance, by time, temperature and initial acid concentration. Good agreement between theoretical analysis and experimental results was obtained. The quality of the bleached oil is similar to that of commercial oil. Finally, the low cost of the activated clay makes it economically very attractive.
A predictive mathematical model for fixed-bed adsorber design that incorporates both liquid and solid phase resistances to mass transfer is verified for four different single-component systems under normal operating conditions. The adsorbates chosen for model verification were selected on the basis of representing a broad range of diffusive and adsorptive properties. Mass transfer limitations for the adsorbates vary from primarily intraparticle mass transfer resistance to primarily liquid-phase mass transfer resistance, and adsorption capacities vary over an order of magnitude. The general agreement between model predictions and fixed-bed performance under a wide variety of conditions confirms the model validity for predicting single-component adsorber behavior. Related papers by the writers describe model development, parameter estimation, and multicomponent model verifications.
Longitudinal dispersion in a bed of non-porous particles has been measured for the flow of air, traced by a pulse of argon. Dispersion coefficients have been calculated from response curves measured using an ionisation detector situated in the gas stream. The results indicate that, at low Reynolds numbers ( = udpe/ν), the dispersion is effected by molecular diffusion, but at high Reynolds numbers eddy diffusion predominates. The validity of the dispersion mechanism is verified and it is shown that the results can be expressed in the following form
A method is shown which permits a test of various empirical diffusion equations giving the rate of the mean internal concentration change dq̄/dt as function of the mean internal concentration q̄ and the surface concentration q. (Formula Presented) It is shown that, under the conditions occurring in chromatographic columns with substances following linear or moderately curved adsorption or exchange isotherms (Kd < 3), where conditions are maintained close to equilibrium, eqn. (A) is almost perfect, (B) is fully adequate, while (C) and (D) are not. For strongly curved isotherms (3 < Kd < 100), the gradient of the fronts of advancing solute is very steep, and during the passage of such a boundary the particles are far from equilibrium with the contacting solution. Under these conditions eqn. (D) offers by far the best representation. For almost irreversible exchange (K d > 100), eqn. (C) is superior to (A), (B) and (D). It can be shown, however, that when applied to the practical chromatographic break-through curves, the difference between the four equations, except for irreversible exchange, is hardly big enough to warrant the use of equations more complex than (B).
The adsorption properties of different ordered mesoporous materials, for the removal of volatile organic compounds (VOCs) in air, have been studied by means of a thermal programmed desorption technique. Toluene, isopentane, and water have been used as adsorbates, whereas a variety of mesostructured materials (MCM-41, Al-MCM-41, and SBA-15) have been selected as adsorbents. As expected, Al-MCM-41 is a hydrophilic material, adsorbing high amounts of water due to the presence of acid sites. On the contrary, pure silica MCM-41 and SBA-15 samples exhibit hydrophobic properties, and they are capable of adsorbing toluene and isopentane in significant amounts. Among these materials, MCM-41 prepared by a sol−gel route [MCM-41(H-sg)] presents the best combination of hydrophobic character and hydrocarbon adsorption, being a potentially interesting adsorbent for the removal of VOCs from humid gases. This behavior has been related to the high structural disorder and low condensation degree of the silica walls present in this material.
The objective of this paper is to provide the basic tools necessary to guide the optimal design of monolithic adsorbents. Previous work has concentrated on optimizing monolith manufacturing processes and experimental studies have suggested that the mass transfer performance of the monolithic form might be inferior to that of the more traditional packed bed form. In this paper, the classical linear driving force approximation, along with the parabolic concentration gradient assumption, is applied to a number of simple geometries. Transformation of square, rectangular, triangular and hexagonal geometries to an equivalent hollow cylinder on the basis of equal volume and equal internal surface area, facilitates use of the linear driving force analytical solution for a cylinder in order to guide the design of the more complex monolith geometries. Taking channel mass transfer performance and pressure gradients into consideration as well, the analyses indicate that regular hexagonal channels offer the best compromise on overall performance, with minimization of the wall thickness being the key design objective. Use of the algebraic design equations for the circular channel provides an excellent approximation for the regular hexagon and thus design work can be carried out with the former and simpler geometry. The engineering challenge now becomes one of manufacturing monoliths with appropriately thin walls. A challenge for the future is to obtain the full numerical solutions for the square, rectangular, triangular and hexagonal geometries.
The effect of non-uniform flow distribution on the performance of a tubular fixed bed adsorber was investigated numerically for a linear adsorption isotherm and constant total volume flow. Quite large effects with regard to the radial dependence of breakthrough curves were found for a laboratory adsorber of = 21. Since, however, flow profiles depend on the ratio, the effects will be smaller for larger values of this ratio. This poses some questions concerning the similarity between small- and large-scale equipment.
Acid activation is a common chemical modification of clays, usually bentonites, with a hot solution of a mineral acid (typically HCl or H 2SO4), and it is used for both scientific and industrial purposes. The aim is to obtain partly dissolved material of increased specific surface area, porosity and surface acidity. The product consists partly of the remains of the starting mineral and partly of an amorphous, porous, protonated and hydrated silica phase with a three-dimensional cross-linked framework. Illites containing non-swelling interlayer spaces dissolve more slowly than smectites but the chemical composition of the layers has a greater effect on the process than swellability. The dissolution rate of organoclays decreases with the size of organic cations. Acid activation modifies principal clay properties and thus also their industrial applications. The extent of clay mineral dissolution is derived by many methods, such as chemical analysis, IR or NMR spectroscopy, X-ray diffraction, thermal analysis and microscopic investigation.
The optimum conditions for acid activation of a clay were determined in order to bleach olive oil. The methodology of experimental research, with an orthogonal central composite design, has been used in this study and the selected variables were temperature, reaction time, initial acid concentration and liquid/solid weight ratio. The last is the most significant factor in the activation process followed in decreasing order of importance, by time, temperature and initial acid concentration. Good agreement between theoretical analysis and experimental results was obtained. The quality of the bleached oil is similar to that of commercial oil. Finally, the low cost of the activated clay makes it economically very attractive.
Attention is focused on the effect of the heat of adsorption and of the radial maldistribution on the toluene adsorption on activated carbon in a fixed-bed adsorber. A two-dimensional (2D), non-equilibrium, non-isothermal model is proposed to describe the heat and mass transfer. The influence of maldistribution on the local gas concentration, the toluene loading and the temperature in the adsorber are shown. Taking the effect of the heat of adsorption and the effect of maldistribution into account the predicted sorption behavior is in good agreement with the experimental results. For practical use the influence of adsorption heat on the adsorption is negligible at low concentrations.
An easily applied method for predicting binary gas-phase diffusivities is based on the use of special diffusion volumes coupled with extensive experiment and nonlinear least squares analysis of the data. Comparison with eight other correlations demonstrates the relative reliability and simplicity of the new method.
Adsorption of ethyl acetate, a toxic volatile organic compound, on activated carbon is studied. Adsorption isotherms at temperatures of 30, 45, and 55 °C are obtained. Langmuir isotherm parameters, saturation capacity, and heat of adsorption are found from the experimental data. The overall mass-transfer coefficient of ethyl acetate is evaluated using the uptake curve method. A mathematical model, with consideration of nonisothermal and nonadiabatic processes without axial dispersion of mass and energy and on the basis of the linear driving force approximation, well-predicted the experimental breakthrough data. Results show that the isotherms are of Brunauer type I and well-fitted with the Langmuir isotherm model. The saturation capacity of ethyl acetate on activated carbon is 0.73−0.487 kg/kg in the temperature range of 303−328 K. The heat of adsorption both by van't Hoff and by Clausius−Clapeyron equations are on the order of 105 J/kg. The overall mass-transfer coefficient is on the order of 10-4 s-1.
Mass transfer factors associated with the vaporization of water and heavy hydrocarbons from the surface of porous spheres, 0.0721 to 0.370 inch in diameter, have been established in the low Reynolds number region (3 < NRe < 230) taking into account a correction for axial mixing. The carrier gas used was air which flowed through packed and dispersed beds of these spheres. To produce a dispersed bed, porous spheres saturated with a liquid were placed in a matrix of dry glass spheres. The results for both types of beds have been correlated with the Reynolds number to produce a single relationship of ε¡d vs. NRe.
Outdoor smog chamber photooxidations to determine the molecular composition of secondary organic aerosol (SOA) from toluene, m-xylene, p-xylene, ethylbenzene, m-ethyltoluene, p-ethyltoluene, and 1,2,4-trimethylbenzene in sunlight-irradiated hydrocarbon−NOx mixtures are reported. Gas-phase mechanisms leading to the observed products are proposed. Unsaturated anhydrides (2,5-furandione, 3-methyl-2,5-furandione, 3-ethyl-2,5-furandione) are predominant components of aerosol from all the aromatics, an observation that is consistent with gas-phase aromatic mechanisms involving ring fragmentation. Saturated anhydrides were also detected in significant quantities, which could result from the hydrogenation of the furandiones in sunlight in the particle phase.
Experimental breakthrough results of methane, ethane and propane in activated carbon and silica gel obtained over a wide range of gas compositions, bed pressures, interstitial velocities, and column temperatures were analyzed using a dynamic, nonisothermal, nontrace column breakthrough model. A linear driving force (LDF) approximation is used for particle uptake, and the Langmuir-Freundlich isotherm represents adsorption equilibrium. The LDF mass-transfer-rate coefficient (and, hence, effective particle diffusivity) and column-wall heat-transfer coefficient were determined. The results show that hydrocarbon transport in the activated carbon particles used is essentially by Knudsen and surface flow, while for the silica gel used the transport is primarily by Knudsen flow. For activated carbon, the experimentally derived LDF coefficients for all three sorbates are well correlated using an average effective diffusivity value. With regard to heat transfer, the column-wall Nusselt number is approximately constant for the range of Reynolds numbers considered. Simulations of multicomponent breakthrough in the activated-carbon bed based on independently measured single-component kinetic parameters and the extended Langmuir-Freundlich isotherm agree very well with experimental results. The computational efficiency gained by adopting the simpler extended Langmuir isotherm model is also investigated.
Experimental and theoretical studies were made on the adsorption of benzene, toluene, p-xylene, and their binary and ternary mixtures by activated carbon in an isothermal condition of 303 K. Experimental isotherms for pure components were measured by two kinds of experimental techniques: the static volumetric method and breakthrough data analysis. Breakthrough data analysis shows that both the extended Langmuir equation and the ideal adsorbed solution theory could explain mixture isotherm data with good accuracy. In addition, a mathematical model was developed to simulate the column dynamics of pure and mixed-vapor adsorption systems. To represent the mass-transfer rate inside the adsorbent particle, the linear driving-force approximation model was applied. By optimizing the pure- and the binary-component breakthrough curve data with a dynamic model, an empirical correlation was proposed to represent the overall mass-transfer rates for the BTX vapor-activated carbon system. Results with this correlation, which is a function of the partial pressures for each adsorbate and interstitial bulk fluid velocities, showed that the prediction agrees well with the experimental data of ternary breakthrough curves.
An experimental and theoretical study is performed for bulk separation of gas mixture by pressure swing adsorption, a process widely used commercially for gas purification. By cycling the pressure of a bed of activated carbon between 3 and up to 500 psig (0.021 to 3.445 MPa), at the ambient temperature, a 50/50 H2/CH4 mixture is separated into two products with well over 90% purity and recovery at high throughputs. All process characteristics can be predicted by a pore-diffusion model. A fundamental understanding of the function of each step in the cyclic process is given.
To illustrate the application of the method to the prediction of the transient response of an adsorption column, the authors have solved a complicated linear problem to which an exact analytic solution has recently been derived. A similar nonlinear problem could be solved in exactly the same way, and the property of linearity is of no special advantage to the collocation method. However, the availability of an exact solution provides a convenient check on the accuracy of the collocation solution.
With the passage of the Clean Air Act Amendment (CAAA) of 1990, solvent emission regulations continue to become ever more stringent. An ongoing and increasingly strong incentive exists to develop and apply to the solvent emission control industry more efficient and cost effective control technologies.
This paper presents a new technology used for solvent separation and concentration, hydrophobic zeolite adsorbent. Because of its unique physical properties, solvent control systems using hydrophobic zeolite may exhibit separation capacity equal to or better than systems using activated carbon without requiring much of the support equipment. Though activated carbon remains the adsorbent of choice for many applications, hydrophobic zeolite represents an efficient alternative for many niche applications. Extensive tests have proven its capabilities and the results are presented here.
Also presented is the modular solvent concentrator (MSC) designed to handle up to 50,000 scfm (79,000 Nm3/h)—up to 50% more air flow as current concentrators—in the same area. The MSC is flexible enough to handle any currently available adsorbent. Several case studies are presented to illustrate use of hydrophobic zeolite as well as the MSC.
Acid-activation of bentonite was optimised to prepare an effective adsorbent of toluene. The activated bentonite was obtained with a specific surface area of 195 m2/g, a pore volume of about 0.46 cm3/g and a most frequent pore size of 62 Å. Compared to the raw bentonite, the adsorption of toluene onto acid-activated bentonite was increased from 66 mg/g to 197 mg/g. Vapor–solid adsorption isotherms of toluene were measured at 120 °C, 140 °C, 160 °C and 182 °C using an inverse gas chromatography. The experimental data were correlated with different adsorption isotherm models such as Langmuir, Freundlich, Langmuir–Freundlich and Toth models. Only the Langmuir–Freundlich equation provided good fit to the experimental data.
An extensive series of layered minerals including montmorillonite was studied to determine if the fundamental physicochemical
properties responsible for pigment adsorption could be identified. Samples were subjected to a uniform preparation regimen
to eliminate such secondary effects as particle size, moisture content, level of activation and degree of washing. By doing
so, it has been possible to show that both carotene and chlorophyll adsorptions can be described by a rather simple model
that employs both surface acidity and pore volume as key variables. Further, it was found that this model provides an adequate
fit to the actual data only when specific regions for these variables are considered (i.e., concentration of sites of strong
surface acidity, not just total surface acidity; pore volume in region 50\s-200 Å, not just total pore volume).
The results presented in the literature, which attempt to elucidate the mechanisms by which triglyceride oils are bleached by earths, are reviewed. The impact of this work and how the mechanistic proposals affect changes in oil properties are considered, with particular emphasis on the needs of the palm oil processor. Important properties include color, metals and phosphorus content and oxidative stability of the oil.
Investigations made in our own laboratories have been aimed at elucidating the effect of varying physical and chemical properties of the bleaching earth on the quality of bleached and deodorized oils. Techniques used in this work are pore-size distribution, surface area, scanning and transmission electron microscopy and a variety of chemical and X-ray analysis methods.
The ability to vary such parameters in montmorillonite clays by alteration of process conditions to give materials with specific performance characteristics is demonstrated.
Comparisons are made between acid-activated montmorillonites and other clay types.
The mechanism of bleaching by a nonswelling montmorillonite clay activated at various acid concentrations was studied in the
bleaching of palm oil. Montmorillonite clay was activated by 2 parts of H2SO4 at concentrations of 10-40%. Chemical composition, bleaching ability, specific surface area and phosphorus content were studied.
The study showed that an initial increase in bleaching ability by clay activated by an increasing addition of H2SO4 was due to acid leaching of organic matter and impurities in the clay. The consequence of acid leaching in this case tends
to expose active sites for adsorption. Acid leaching also removed Al3+, causing charge deficiency in the clay lattice and, hence, promoting the adsorption properties of the clay. A drop in bleaching
efficiency at higher additions of H2SO4 was observed. This was due to exces-sive acid leaching of Al3+, causing collapse of the clay lattice struc-ture.
Many mass transfer kinetic models are used to study varying pressure steps of a pressure swing adsorption (PSA) cycle, namely pressurization and blowdown steps. It is shown that the choice of an appropriate model to account for intra-particle diffusional limitations is essential to simulate accurately PSA processes. It has been demonstrated that, besides mathematical approximations (parabolic profile within the particle), a very important factor that can affect simulation results remarkably is the correctness of the mass transfer kinetic model when regarded as being a mass balance for the adsorbent particle. In fact, neglecting intra-particle gas phase leads to erroneous simulations. Some models widely used in the literature, such as the classic linear driving force (LDF) and the solid diffusion models prove inadequate. It is recommended to use pore diffusion model as well as a new version of the solid diffusion model to get reliable predictions.
Soils surrounding industrial sites or at locales where industrial chemicals are utilized, frequently become contaminated through unsuitable discharge of potentially hazardous organic compounds. The fate and transport of these chemicals must be sufficiently understood to predict detrimental environmental impacts and to develop technically and economically appropriate remedial action to minimize environmental degradation. Improving our understanding of the processes involved in organic pollutant vapor transport is important because the gas phase is often the most mobile, and therefore most potentially hazardous phase. In order to gain a better understanding of the basic kinetic processes affecting soil adsorption/desorption of volatile organic compounds (VOC's) in the vapor phase, we conducted VOC adsorption/desorption experiments using oven-dry clay minerals. Transient, isothermal, gravimetric sorption experiments using volatile organic compounds (VOC's) acetone, benzene and toluene onto pure clay minerals obtained from Ward's Scientific (kaolinite, illite, and Ca-montmorillonite) suggest a biphasic sorption mechanism on these minerals. Experimental results indicate that hydrophobic sorption onto oven-dry clay minerals with negligible soil organic matter is controlled by rates of inter-particle Fickian diffusion mechanisms, intra-particle Fickian diffusion mechanisms, and sorption kinetics. Using an analytical solution to Fick's Second Law where sorption is partitioned into macroscopic and microscopic domains, each with unique diffusion time constants, enables precise prediction of experimental sorption observations. Correlation coefficients of 0.999 were found between the parameter optimized analytical solution and very large sets of experimental data. Macroscopic diffusion coefficients ranged from 10−2 to 10−4 cm2/min, while microscopic diffusion coefficients ranged from 10−12 to 10−17 cm2/min. Sorption rates suggest that significant fractions of VOC's adsorb onto surfaces other than the external mineral surfaces. The fraction of mass sorbed on the mineral surface ranged from 30% to 75% of the equilibrium sorbed mass. Equilibrium sorption mass correlates approximately to mineral specific surface area and, similarly, the fraction of `irreversible' sorption appears to be proportional to the specific surface area of the clay mineral. Finally, the experimental results suggest that sorption processes (once the sorbate has diffused to the sorbent) are not instantaneous. Due to the experimental inter-particle length scales and intra-particle diffusion processes, diffusional time constants associated with each process during sorption experiments were generally found to be within two orders of magnitude. Such close agreement would not be expected if particles larger than a few micrometers (i.e., aggregates) comprised part of the sorbing soil system. Although the experiments were conducted using oven-dry clays, it is hoped that these results offer an improved conceptual model of clay mineral sorption processes to better explain field observations of VOC persistence in the subsurface.
Adsorption dynamics were investigated in a laboratory scale fixed bed column, functioning under three different non-isothermal conditions: adiabatic, near adiabatic and non-adiabatic. Axial and radial temperature profiles were registered, as well as a corresponding breakthrough curves at the column exit. Experimentally it has been demonstrated that the thermal effect of adsorption leads to deformation of the temperature profiles along the column. This directly affects the total amount adsorbed in the bed and breakthrough at the exit, an effect which is different for the different non-isothermal conditions. A two-dimensional mathematical model for description of non-isothermal adsorption was developed, including the effects of the radial temperature gradients. A biporous structure of the adsorbent particles is assumed and the heat effect on the equilibrium is taken into account. Good agreement is shown between experimental and theoretical results, when the mathematical model accounts for the radial thermal conduction and thermal flow through the wall.
Volatile Organic Compounds (VOCs) are among the most common air pollutants emitted from chemical, petrochemical, and allied industries. VOCs are one of the main sources of photochemical reaction in the atmosphere leading to various environmental hazards; on the other hand, these VOCs have good commercial value. Growing environmental awareness has put up stringent regulations to control the VOCs emissions. In such circumstances, it becomes mandatory for each VOCs emitting industry or facility to opt for proper VOCs control measures. There are many techniques available to control VOCs emission (destruction based and recovery based) with many advantages and limitations. Therefore, deciding on a particular technique becomes a difficult task. This article illustrates various available options for VOCs control. It further details the merits, demerits and applicability of each option. The authors hope that this article will help in critically analysing the requirements and accordingly decide on the appropriate strategy to control VOCs.
A new state-of-the-art indoor environmental chamber facility for the study of atmospheric processes leading to the formation of ozone and secondary organic aerosol (SOA) has been constructed and characterized. The chamber is designed for atmospheric chemical mechanism evaluation at low reactant concentrations under well-controlled environmental conditions. It consists of two collapsible 90 m3 FEP Teflon film reactors on pressure-controlled moveable frameworks inside a temperature-controlled enclosure flushed with purified air. Solar radiation is simulated with either a 200 kW Argon arc lamp or multiple blacklamps. Results of initial characterization experiments, all carried out at ∼300–305 K under dry conditions, concerning NOx and formaldehyde offgasing, radical sources, particle loss rates, and background PM formation are described. Results of initial single organic–NOx and simplified ambient surrogate–NOx experiments to demonstrate the utility of the facility for mechanism evaluation under low NOx conditions are summarized and compared with the predictions of the SAPRC-99 chemical mechanism. Overall, the results of the initial characterization and evaluation indicate that this new environmental chamber can provide high quality mechanism evaluation data for experiments with NOx levels as low as ∼2 ppb, though the results indicate some problems with the gas-phase mechanism that need further study. Initial evaluation experiments for SOA formation, also carried out under dry conditions, indicate that the chamber can provide high quality secondary aerosol formation data at relatively low hydrocarbon concentrations.
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The pore-opening size of MCM-41 is tailored to be in the microporous region using a chemical vapor deposition technique for selective tailoring. Although the pore opening is narrowed, the internal pore body of MCM-41 remains unchanged so the pore volume retains a substantial portion (80%) of its original value. The adsorption equilibrium of nitrogen and benzene in the modified MCM-41 shows a type I isotherm, which significantly improves the adsorption performance of MCM-41 for low-concentration volatile organic compounds. The adsorption kinetics of benzene in the modified MCM-41 is also studied.
From experimental results of adsorption of volatile organic compounds (VOCs) on zeolite, we propose simulations of the breakthrough curves based on the Linear Driving Force model. Experiments were run on fixed beds of hydrophobic commercial zeolites. Pollutants chosen are from several chemical classes with different polarities. A good agreement between experimental and numerical results is found when an adjustable value of the internal mass-transfer coefficient is used. A constant value of effective diffusivity is found independent of the nature and the amount of VOCs adsorbed. A relation linking intrapellet mass-transfer coefficient and equilibrium constant is proposed, including the average effective diffusivity, to make predictions of breakthrough curves for any kind of volatile organic pollutant in gaseous effluents.
On the basis of experimental breakthrough curves of lead ion adsorption on ETS-10 particles in a fixed-bed column, we simulated the breakthrough curves using the two-phase homogeneous diffusion model (TPHDM). Three important model parameters, namely the external mass-transfer coefficient (k(f)), effective intercrystal diffusivity (D(e)), and axial dispersion coefficient (D(L)), were optimally found to be 8.33x10(-5) m/s, 2.57x10(-10) m(2)/s, and 1.93x10(-10) m(2)/s, respectively. A good agreement was observed between the numerical simulation and the experimental results. Sensitivity analysis revealed that the value of D(e) dictates the model performance while the magnitude of k(f) primarily affects the initial breakthrough point of the breakthrough curves.
On the Mechanism of the Montmorillonite Acid Activation, I. Degradation of Ca-Montmoril-lonite
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Fijal, J.; Klapyta, Z.; Zietkiewicz, J.; Zyla, M. On the Mechanism of the Montmorillonite Acid Activation, I. Degradation of Ca-Montmoril-lonite Structure. Mineral. Pol. 1975, 6, 29–41.
Gé de la ré chimique conception et fonctionnement des ré, Technique et documentation
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Villermaux, J. Gé de la ré chimique conception et fonctionnement des ré, Technique et documentation; Lavoisier: Paris, 1982.