Prediction of breakthrough curves for adsorption on activated carbon fibers in a fixed bed

A new model is proposed to describe the removal of volatile organic compounds (VOC) from a gas stream passing through a bed packed with activated carbon fibers (ACFs). Toluene was used as the test compound. Both pore diffusion and surface diffusion are considered in the model. The equilibrium behavior is shown to fit the Dubinin–Radushkevich isotherm with the values of parameters K and W₀ of 1.101 × 10⁻⁹ and 57.73 kg/m³, respectively. The experimental results show that this model can predict VOC breakthrough curve very well.     

A systematic investigation of the overall rate coefficient in the Wheeler-Jonas equation for adsorption on dry activated carbons

Modelling of the adsorption rate coefficient, k_v of the Wheeler-Jonas equation for estimating the service life of packed carbon beds, is addressed. Current methods for extracting k_v from experimental breakthrough data include approaches that introduce easily propagated errors. The weaknesses of these approaches are analyzed, and a calculation based on multiple points on the breakthrough curve is suggested. Experimental breakthrough data for a representative set of volatile organic compounds (VOCs) has been measured. A systematic investigation of factors influencing k_v, including adsorbate and carbon properties, adsorbate inlet concentration and flow velocity is performed. It is found that flow velocity and carbon particle size have the largest influence, followed by adsorbate properties related to the adsorption capacity. A simple linear empirical model for k_v, including air flow velocity, carbon particle size, and dielectric constant of the adsorbate, is presented. The model is based on the breakthrough range up to 20% of the inlet concentration, for which k_v is shown to be almost constant. The range of the model covers breathing rates valid for a respirator at different work loads. The model can be used to estimate the adsorption rate coefficient for specific carbon particle sizes and various VOCs present in workplace environments.     

Removal of metal ions from aqueous solution by adsorption onto activated carbon cloths: adsorption competition with organic matter

Activated carbon cloths are recent adsorbents whose adsorption properties are well known for monocomponent solutions of organics or metal ions. However, to treat wastewaters with these materials, their performance has to be determined in multicomponent solution. This work studies adsorption competition between metal ions (Cu²⁺, Pb²⁺) and organic matter (benzoic acid). The first part investigates adsorption equilibrium of monocomponent metal ions solutions and shows the dependence of adsorption capacities on adsorbent porosity and metal ions chemical properties (molecular weight, ionic radius and electronegativity). The influence of pH is also demonstrated. The second part focuses on adsorption competition: (1) between both metal ions (a decrease of adsorption capacities is observed, whose value is related to adsorption kinetics of metal ions); (2) between metal ions and organic matter, in solution or adsorbed onto the activated carbon cloth (a strong influence of pH is shown: when benzoic acid is under benzoate form, in both cases adsorption is increased due to the formation of ligands between adsorbed benzoate ions and metals).     

Effect of air oxidation of Rayon-based activated carbon fibers on the adsorption behavior for formaldehyde

The tailoring of pore surface chemistry of activated carbon fibers is shown to be an effective method for improving the adsorption efficiency of various volatile chemical compounds (VOCs). An oxidation treatment with air resulted in a significant increase in the adsorption capacities and breakthrough time for Rayon-based activated carbon fibers (ACFs) in removal of formaldehyde. The porous structure parameters of Rayon-based ACFs were determined with standard nitrogen adsorption analysis. The pore surface chemistry of samples under study was analyzed by Fourier Transform Infrared spectra (FTIR). Thus to some extent, the relationship between the adsorption properties, porous structure and pore surface chemistry was revealed.     

Quantification and application of skew of breakthrough curves for gases and vapors eluting from activated carbon beds

Vapor and gas breakthrough curves for packed activated carbon beds are often assumed to be symmetrical, when they are actually more often skewed. This skew explains why adsorption rate coefficients calculated at differing breakthrough fractions may not agree. Three extensive databases of breakthrough curves were analyzed to quantify this skew and the effects of relative humidity (preconditioning and use) on it. Skew results for varieties of chemicals and carbons agreed well and were combined to get a quadratic expression for a defined skew parameter. This expression was combined with a previous observation of the effect of breakthrough fraction on calculated rate coefficient. The combination allows estimation of an adsorption rate coefficient at a desired breakthrough fraction from a rate coefficient known (experimentally or by calculation) at another breakthrough fraction. A sample calculation is given.     

The role of adsorbent pore size distribution in multicomponent adsorption on activated carbon

The impact of adsorbent pore size distribution (PSD) on adsorption mechanism for the multi solute system was evaluated in this study. Anoxic and oxic adsorption equilibrium for the single solute (phenol), binary solute (phenol/2-methylphenol) and ternary solute (phenol/2-methylphenol/2-ethylphenol) systems on one granular activated carbon (GAC) F400 and two types of activated carbon fibers (ACFs), namely, ACC-10 and ACC-15, were determined. F400 has a wide PSD, while ACC-10 and ACC-15 have narrow PSD and their critical pore diameters are 8.0 Å and 12.8 Å, respectively. In single solute adsorption, the increase of adsorptive capacity under oxic conditions as compared to anoxic ones was related to the PSD of the adsorbent. Binary solute adsorption on ACC-10 and ternary solute adsorption on ACC-15 indicated no impact of the presence of molecular oxygen on the adsorptive capacity and the adsorption isotherms were well predicted by the ideal adsorbed solution theory (IAST). Significant differences between oxic and anoxic isotherms were noticed for other multicomponent adsorption systems. The narrow PSD of ACFs was effective in hampering the oligomerization of phenolic compounds under oxic conditions. Such a phenomenon will provide accurate predictions of fixed bed adsorbers in water treatment systems.     

Characterization of the surfaces of single-walled carbon nanotubes using alcohols and hydrocarbons: a pulse adsorption technique

A pulse mass analyzer was used to study the vapor phase adsorption of organic compounds on single-walled carbon nanotubes and chemically modified/oxidized SWCNTs. The change in mass of a packed bed of adsorbent held at 200 °C was observed following the injection of a pulse of an organic compound from the series: ethanol, iso-propanol, cyclohexane, cyclohexene, benzene, or n-hexane. The relative strength of adsorption was obtained by the mass increase resulting from injection of the pulse and by the time required for desorption. This time was broken into the transit time to reach the end of the bed and the half-time for return from peak to baseline. Hexane was the most strongly held compound of the organic sequence. Oxidative purification of a raw nanotube sample produced a less hydrophobic surface. The effect of the purification was reversed by thermolysis at 700 °C, which removed oxygenated functional groups and increased the affinity for hydrocarbons. The amorphous carbon associated with the raw nanotube sample is a strong adsorbent for hydrocarbons. By comparison, an activated carbon had a greater affinity for hydrocarbons than any of the nanotube samples.     

Prediction of adsorption equilibrium using a modified D-R equation: organic-water vapor mixtures on BPL carbon

Methods to describe and predict adsorption equilibrium using the D-R equation are presented in this paper. We focus on adsorption of organic-water vapor binary mixtures on BPL activated carbon. By adjusting the partial pressure of the organic component in the binary mixture, the effects of the presence of water vapor on adsorption of the organic component are quantitatively expressed using one term in a modified form of the D-R equation. The developed models have broad application ranges that are not limited by organic type and the loadings of the organic and water vapor. They can be used for coadsorption of water-immiscible or water-miscible organic compounds with water vapor on BPL activated carbon in the cases of small or large organic loading with low or near-saturation relative humidity. Compared with other methods for the adsorption of organic-water vapor binary mixtures, our models are simple and do not require iteration.     

The effect of airflow pattern on filter breakthrough in physical adsorption

The Wheeler-Jonas (WJ) model for prediction of the protection capacity of organic vapor filters under a fixed airflow was extended to breathing-simulation, pulsating flow. Breakthrough curves of dimethyl-methyl phosphonate (DMMP) and decane were measured under fixed flow and sinusoidal flow. A linear dependence of ln(C_X/C₀) on the breakthrough time (t_B) was observed in all the experiments, indicating that the concepts of critical bed weight (W_C) and dynamic adsorption capacity (W_E) as defined by the WJ model are applicable to pulsating flow as well. W_E was found to be almost unchanged by the flow pattern, whereas W_C was considerably larger (by 7-44%) at pulsating flow compared to fixed flow with the same average rate. Thus, shifting from fixed flow to pulsating flow may shorten t_B significantly. The effect of the flow type on t_B increases with the ratio of the critical weight to the total bed weight. For a high protection level (C₀/C_X=60?000), the protection capacity of personal NBC canisters was reduced by up to 15% upon shifting from fixed to pulsating flow.     

Prediction of adsorption equilibrium using a modified D-R equation: pure organic compounds on BPL carbon

A method for predicting adsorption equilibrium using a modified Dubinin-Radushkevich (D-R) equation is presented in this paper. We focus on adsorption of pure organic compounds on BPL-activated carbon. We introduce a new variable k_v, a volume adjusting coefficient, and simply use V_b, the constant molar volume of the adsorbate at its normal boiling temperature, instead of V_m, the temperature-dependent molar volume of the adsorbate at the adsorption temperature. The model parameters in the modified D-R equation for an adsorbate are predicted from V_b. Overall, the modified D-R equation gives more accurate results than the traditional one.     

Using the modified Wheeler-Jonas equation to describe the adsorption of inorganic molecules: chlorine

Very little has been published on the subject of chlorine gas adsorption on activated carbon. In this work we studied breakthrough of chlorine on canisters filled with both impregnated and non-impregnated activated carbons. The results show that the Wheeler-Jonas equation, normally used for the physisorption of organic vapours, is also applicable to the adsorption of chlorine. Hence it can be used to extrapolate laboratory results to real life situations. It can also help to establish the nature of the chlorine adsorption process. The available data do not permit a conclusive statement on the exact reaction mechanism but it appears that moisture, micropore volume and surface complexes all have an impact on this process. Apparently neither the Cu-Cr complexes, nor the TEDA, of the impregnated carbon have a significant influence on the adsorption behaviour.     

Adsorption of zinc, cadmium and mercury ions from aqueous solutions on an activated carbon cloth

The adsorption of zinc, cadmium and mercury ions from aqueous solutions on an activated carbon cloth was studied as a function of their concentrations and solution pH. For that purpose, pertinent adsorption isotherm data was collected at different pH values. The amount of adsorbed zinc and cadmium ions increases while the amount of adsorbed mercury remains constant with an increase in the equilibrium pH of the solution. The adsorption mechanisms of metal ions on activated carbon cloth are discussed. Under the conditions investigated, these primarily involve adsorption of monovalent cations (Zn and Cd) or precipitation of metal hydroxides (Cd and Hg) on the activated carbon cloth tested.     

A small-angle neutron scattering study of activated carbon fibers

Phenolic resin based activated carbon fibers are widely used as filter materials in industry. We studied the fiber type ACFY-0204-3-18 which is applied to recover 2-propanol from air. The fibers were characterised by measuring the chemical composition, the apparent density, the wide angle X-ray diffraction data, and the sorption isotherms for nitrogen at 77.4 K and 2-propanol at 308.2 K. The porous structure was studied by small-angle neutron scattering. The two-dimensional contour patterns show an anisotropic intensity distribution I(q) at scattering vectors |q|<0.4 nm⁻¹. In this q-range I(q) is perpendicular to the fiber axis due to a refraction of the neutrons by the fibers, and I(q) increases abruptly with decreasing q. At |q|>0.4 nm⁻¹ scattering is isotropic and I(q) shows a weak interference peak due to the presence of micropores. The mean pore size of ∼2 nm was determined by fitting a monodisperse and a polydisperse Percus-Yevick hard sphere model to the experimental I(q) data. The microstructure of ACFY differs basically from the microstructure of polyacrylonitrile and pitch based carbon fibers.     

Pore size distribution model derived from a modified DR equation and simulated pore filling for nitrogen adsorption at 77 K

The Dubinin-Radushkevich (DR) equation has been used extensively to characterize microporous materials. However, it assumes a continuous pore filling mechanism and does not reduce to Henry’s law at low adsorptive pressures. In this study, we modify the DR equation by introducing adsorption density and the correlation between pore filling pressure and critical pore size for N₂ adsorption at 77 K. This modified DR equation is used as the local isotherm in the generalized adsorption isotherm (GAI) to interpret N₂ adsorption isotherm at 77 K. To solve the GAI, a modified normal distribution is developed and used as a distribution function of pore size on the domain [0, ∞). Pore size distribution (PSD) analyses of several adsorbent samples are carried out using this model. The results are found comparable with other popular PSD methods (e.g. MP, JC, HK and DFT).     

Development of thermally conductive packing for gas separation

A new highly thermally conductive composite material made of activated carbon in-situ activated within a consolidated expanded natural graphite (CENG) matrix is developed in the specific case of gas separation processes. According to the particular chosen testing case of CO₂/N₂ mixture, the microporous characteristics of the adsorbent fraction have been selected and the various properties of the whole material studied. Activated carbon effective densities, overall thermal conductivities and external heat transfer coefficients up to 650 kg×m⁻³, 32 W×m⁻¹×K⁻¹ and 3000 W×m⁻²×K⁻¹ were obtained, respectively. Two macroscopic external shapes were realized, namely a corrugated-sheets static mixer and a multi-axial-diffuser cylindrical monolith which will be further tested in a pilot equipment for comparison with conventional adsorbents.     

Flue gas desulphurization by activated carbon fibers obtained from polyacrylonitrile by-product

By pyrolysis of a polyacrylonitrile textile by-product, subsequent activation by CO₂ and treatment (at high temperature) with a N₂ flow containing a low percentage of O₂ or of NH₃, three carbonaceous matrices are obtained having a high surface area and surface sites with basic characteristics. The SO₂ sorption properties of these carbon samples (in the temperature range between 100 and 160 °C) from gaseous mixtures having a similar composition to flue gases, seems to be promoted by nitrogen bonded to carbon. The SO₂ adsorbed by the carbons can be divided, by suitable extraction with distilled water, into: (i) desorbable, such as SO₂ or H₂SO₃, (ii) desorbable, such as SO₃ or H₂SO₄, (iii) non-desorbable. Following 10 SO₂ adsorption and desorption cycles, the surface area values of the activated carbons remain practically constant, while both the content of the acidic surface sites and the amount of non-desorbable SO₂ increase; this results in the decrease in the SO₂ carbon sorption property seeming to be even more marked for the carbon sample containing nitrogen.     

Estimation of activated carbon adsorption efficiency for organic vapours: I. A strategy for selecting test compounds

Strategies for developing quantitative structure–affinity relationships (QSAfR) for the prediction of break-through performance of 31 chlorinated hydrocarbons on activated carbon have been studied. Two different approaches for the selection of a limited set of compounds for modelling were evaluated through the predictive power of the resulting QSAfR models. When the model was based on a training-set selected without a rational strategy, the developed QSAfR model showed poor predictive performance. Accordingly, such models have a limited capability to produce information concerning the important adsorbate related parameters influencing adsorption. By using a strategy where multivariate data analytical techniques are used in conjunction with statistical experimental design to select a balanced set of compounds for break-through performance evaluation, it was possible to develop QSAfR models with high predictive capability.     

Physical properties and biological effects of activated carbon fibers treated with the herbs

Activated carbon fibers (ACFs) were treated with various medicinal materials. The physical properties and biological effects of these ACFs were examined. In order to investigate the properties, adsorption isotherms, BET surface area and pore analysis for the herb material treated ACFs must be obtained. The pore size distribution of the samples was obtained by a regularization method on the basis of the non-local and smoothed density function theory approximation support these findings. To investigate the surface of treated ACFs, the surface morphology of the resulting samples by S.E.M. show that many of the treated herbs were distributed irregularly on the surface of the fiber. In results of biological effects in the treated herbs, most all of samples showed excellent antibacterial effects after 200-400 min.