A fundamental study on the removal of air pollutants (sulfur dioxide,nitrogen dioxide and carbon dioxide) by adsorption on activated carbon

Gravimetrically measured adsorption and desorption dynamics of sulfur dioxide, nitrogen dioxide and carbon dioxide on a commercial activated carbon are interpreted by a single-particle model based on three transport processes: macropore, micropore and sorbed-phase diffusion. Additional phenomena, concentration-dependent sorbed-phase diffusivity and sorbent non-isothermality, are incorporated to expand the applicability of the model. The dynamic sorption behaviour of all three gases is adequately described, without resorting to a different particle tortuosity factor for each sorbate. The value of the tortuosity factor (8) and the extracted diffusion coefficients are consistent with literature values. The affinity of the activated carbon for the adsorbates is, in increasing order, CO₂ < SO₂ < NO₂, while the extracted diffusion coefficients show the reverse trend, NO₂ < SO₂ < CO₂.     

Selective transport of CO2, CH4, and N2 in coals: insights from modeling of experimental gas adsorption data

Selective adsorption and transport of gases in coal are important for natural gas recovery and carbon sequestration in depleted coal seams for environmental remediation. Gases are stored in coal mainly in the adsorbed state. In this study, the interaction energies of adsorbates (CO₂, CH₄, and N₂) and micropores with various widths are investigated using a slit-shape pore model. The experimental adsorption rate data of the three gases conducted on the same coal sample are numerically simulated using a bidisperse model and apparent diffusivities of each adsorbate in the macropore and micropore are determined. The results indicate that the relative adsorbate molecule size and pore structure play an important role in selective gas adsorption and diffusion in micropores. Generally, the microporous coals diffusion is activated and the apparent micropore diffusivities of gases in coal decrease strongly with increase in gas kinetic diameters. Apparent micropore diffusivity of CO₂ is generally one or two order of magnitude higher than those of CH₄ and N₂ because their kinetic diameters have the relation: CO₂ (0.33 nm)<N₂ (0.36 nm)<CH₄ (0.38 nm). In contrast to theoretical values, apparent macropore diffusivity of CO₂ is also larger than those of CH₄ and N₂, suggesting that coal has an interconnected pore network but highly constricted by ultra micropores with width <∼0.6 nm. It is also found that the apparent diffusivity strongly decreases with an increase in gas pressure, which may be attributed to coal matrix swelling caused by gas adsorption. Therefore, rigorous modeling of gas recovery and production requires consideration of specific interaction of gas and coal matrix.     

Kinetics of carbon dioxide sorption on potassium-doped lithium zirconate

Potassium-doped lithium zirconate (Li₂ZrO₃) sorbents with similar crystallite but different aggregate sizes were prepared by a solid-state reaction method from mixtures of Li₂CO₃, K₂CO₃, and ZrO₂ of different particle sizes. Carbon dioxide sorption rate on the prepared Li₂ZrO₃ sorbents increases with decreasing sorbent aggregate size. It is the size of the aggregate, not the crystallite, of Li₂ZrO₃ that controls the sorption rate. Temperature effect on CO₂ sorption is complex, depending on both kinetic and thermodynamic factors. A mathematical model based on the double-shell sorption mechanism was established for CO₂ sorption kinetics and it can fit experimental data quite well. Above 500°C, the rate-limiting step of CO₂ sorption is the diffusion of oxygen ions through the ZrO₂ shell formed during the carbonation reaction. Oxygen ion conductivities in the ZrO₂ shell were obtained by regression of the experimental CO₂ uptake curves with the model and are consistent with the literature data.     

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.     

Diffusion anomaly and blind pore character in carbon molecular sieve membrane

The structure of a carbon molecular sieve (CMS) membrane is characterized by the through pores and blind pores with non-linear sorption isotherms inside. Time-lag analysis was conducted for gas permeation in such a structure and mathematical formulations were derived for two cases. It is found that the pressure dependence of the time lag is dominated by the ratio of sorption affinities in the two types of pores. The experimental permeation data of pure component CO₂ and N₂ measured on a CMS membrane were used to validate the model. It is found that, for the adsorbing species (CO₂), the model is able to well describe the diffusion anomalies over a wide range of permeation pressure, while for the weakly adsorbing species (N₂), the model is inadequate to cope with the anomalies at the low end of permeation pressure.     

Temperature- and pressure-dependent transient analysis of single component permeation through nanoporous carbon membranes

Using a transient analysis of permeation, i.e. the time lag method, as a function of temperature and pressure, it is shown that each of the parameters needed to fully evaluate adsorption and diffusion can be obtained in situ. These experiments were conducted on a supported tubular nanoporous carbon membrane, 5.1 cm² in area and prepared by ultrasonic deposition of poly(furfuryl alcohol) onto a porous stainless steel support. The permeation experiments were conducted at temperatures ranging from 25 to 225°C and over a range of pressures from 100 to 700 kPa. Under these conditions the fluxes ranged from 10⁻⁷ to 10⁻⁴ mol/m²/s with permeances ranging between 10⁻¹² and 10⁻⁹ mol/m²/s/Pa. Heats of adsorption were found to be 2.5, 2.21, 3.05 and 2.52 kcal/mol for N₂, O_2, Ar and CO₂, respectively, and generally lower than those reported for granular nanoporous carbons. The apparent activation barriers to diffusion were also found to be low at 2.06, 5.87, 4.12, 5.89 and 2.19 kcal/mol for He, N₂, O₂, Ar and CO₂. These results point to the presence of two parallel pathways for transport — the major one through the nanopores but a second through a few defect pores. Assumed to be on the order of 50 nm in diameter, these defects were calculated to represent a total area fraction of 3.43×10⁻⁹.     

Comparison of DFT characterization methods based on N2, Ar, CO2, and H2 adsorption applied to carbons with various pore size distributions

Adsorption isotherms of N₂, Ar, CO₂, and H₂ were measured on selected activated carbons representing various pore size distributions. Isotherm data were analyzed using the DFT method. A good agreement was obtained between N₂ and Ar results in a wide range of pore sizes. Both N₂ and Ar results agree with CO₂ analysis at 273 K in the range of small micropores where CO₂ analysis is most applicable. Also, in this range of pores the analysis of H₂ adsorption data at 77 K gives results which are consistent with the results obtained from the more conventional vapor-phase adsorptives. However, the range of PSDs calculated from H₂ data extends to pore sizes smaller than those that can be characterized by other adsorbates.     

Diffusion and sorption for carbon dioxide in Kapton at extremely low pressure

Pressure-dependent solubility and diffusion coefficients for carbon dioxide in glassy polymers have been well described using the “dual sorption and transport model.” However, the plastisization effect by high-pressure carbon dioxide seems to promote the pressure dependence of the sorption and transport coefficients. To avoid the relaxation process by the plastization which is superimposed on the diffusion process, the diffusion and sorption of carbon dioxide were measured at extremely low pressure (below 1 cmHg). Linear isotherms observed for CO₂ sorption into Kapton were interpreted in terms of the dual model equation at extremely low pressure. From the permeation curve of the Kapton/CO₂ system, the diffusion and permeation coefficients were obtained according to the usual manner, and both coefficients were independent of pressure. Sorption and transport parameters were obtained from sorption isotherms and average values of the permeation coefficients. The parameters thus obtained were substituted in an approximated dual sorption and transport equations at extremely low pressure and the pressure independence of the diffusion and permeation coefficients were sufficiently reproduced. It is a good technique to experiment at such extremely low pressure when the validity of the dual model is evaluated. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 69: 1013–1017, 1998     

Behaviors and kinetic models analysis of Li4SiO4 under various CO2 partial pressures

The absorption behaviors of Li₄SiO₄ sorbent under various CO₂ partial pressures and temperatures were investigated through numerical and experimental methods. It was found that Li₄SiO₄ showed poor absorption capacity at high temperatures (>525°C) under CO₂ partial pressure of 5066 Pa. This phenomenon was explained by the thermodynamic results from FactSage5.5 software. Meanwhile, a modified Jander‐Zhang model based on the double‐shell structure of the Li₄SiO₄ sorbent was developed to describe the absorption kinetic behaviors of CO₂ on Li₄SiO₄. The results showed that the modified Jander‐Zhang model could fit the kinetic experimental data well. Furthermore, the influence of steam on CO₂ absorption was also analyzed by the modified Jander‐Zhang model. The results showed that the activation energy in the absorption process with steam was smaller than that without steam, which indicated that the presence of steam could promote the CO₂ diffusion in product layer, therefore, improving the sorption capacity. © 2017 American Institute of Chemical Engineers AIChE J, 63: 2153–2164, 2017     

Ab initio molecular orbital study of the mechanism of SO2 oxidation catalyzed by carbon

Ab initio molecular orbital calculations were performed on the possible pathways of the carbon-catalyzed oxidation of SO₂ by O₂/H₂O to form sulfuric acid. Both zigzag and armchair edge sites of graphite, with and without surface oxide, were considered as the possible active sites. For the sites with oxide, both isolated and twin oxides were included. MO calculations at the B3LYP/6-31G(d)//HF/3-21G(d) level were used for calculating the energies of SO₂ adsorption, oxidation and hydration. Based on these calculations, three viable pathways emerged, and all three took place on the zigzag edge sites. Hence the armchair sites were not viable sites. On the bare surface, the only possible pathway involved the formation of a sulfurous acid intermediate. Thus, SO₂ was first adsorbed on the bare zigzag sites, followed by reaction with H₂O to form H₂SO₃, which was further oxidized by O₂ to form the end product. On the zigzag edge site with isolated oxide, both pathways with either SO₃ or H₂SO₃ as the intermediate are possible. Chemisorption on the edge sites containing twin oxides was not viable. This latter result explains the seemingly conflicting results in the literature regarding the dependence of SO₂ adsorption (and oxidation) on the amount of surface oxygen.     

Analysis of CO2 sorption/desorption kinetic behaviors and reaction mechanisms on Li4SiO4

The CO₂ sorption/desorption kinetic behaviors on Li₄SiO₄ were analyzed. The theoretical compositions of the sorption/desorption reactions were calculated using FactSage. The sorption/desorption process on Li₄SiO₄ was investigated by comparing the shrinking core, double exponential, and Avrami–Erofeev models. The Avrami–Erofeev model fits the kinetic thermogravimetric experimental data well and, together with the double‐shell mechanism, clearly explains the sorption/desorption mechanism. The sorption process is limited by the rate of the formation and growth of the crystals with double‐shell structure consisting of Li₂CO₃ and Li₂SiO₃. The whole desorption process is found to be controlled by the rate of the formation and growth of the Li₄SiO₄ crystals. Furthermore, the influence of steam on the CO₂ sorption process was analyzed. It has been observed that the presence of steam enhance the mobility of Li⁺ and, therefore, the rate of diffusion control stage. © 2012 American Institute of Chemical Engineers AIChE J, 59: 901–911, 2013     

A kinetic model of nano‐CaO reactions with CO2 in a sorption complex catalyst

This article describes the reactive kinetics of nano‐CaO with CO₂ in a sorption complex catalyst. Based on an observation of nano‐CaO reaction with CO₂ has a fast surface reaction regime and followed by a slow diffusion‐controlled regime, a criterion has been proposed to divide the fast surface reaction regime and the slow diffusion‐controlled reaction regime. The kinetics of the fast surface reaction was studied, and a new ion reaction mechanism was proposed. A surface reaction‐controlled kinetic model with a Boltzmann equation, X = X_u?X_u/[1+exp((t?t₀)k/X_u)], was developed. Experiments using nano‐CaO to react with CO₂ in a fast surface reaction regime within a sorption complex catalyst were performed using thermogravimetric analysis at 773–873 K under a N₂ atmosphere with 0.010–0.020 MPa CO₂. The activation energy of the kinetic model for carbonation is 30.2 kJ/mol, and the average relative deviation of the sorption ratio is less than 9.8%. © 2011 American Institute of Chemical Engineers AIChE J, 2012     

Sorption of carbon dioxide on activated carbons: Effect of the heat of sorption during kinetic measurements

The temperature changes of the adsorbent particles during ad(de)sorption of CO₂ on BPL activated carbon and MSC V molecular sieve carbon were measured using a volumetric sorption apparatus. The tests showed that the adsorbent temperature and the adsorbent loading for both carbons rose (or fell) almost instantaneously to a plateau level after the gas-solid contact was made and then they slowly decreased (or increased) to, respectively, the starting temperature and the final equilibrium loading value at the starting temperature. It was found that the adsorbents were very close to equilibrium with the instantaneous gas phase pressure and the adsorbent temperature at all times during the kinetic runs. Neither carbons exhibited internal pore diffusion resistance to sorption of CO₂. Heat transfer from or to the carbons was the controlling factor in determining the uptake or desorption curves.Equilibrium sorption isotherms for CO₂ on BPL and MSC V carbons were also measured at different temperatures.     

Effects of adsorbate properties on adsorption mechanism in a carbon molecular sieve

The adsorption characteristics of six pure components (N₂, O₂, Ar, CO, H₂, and CH₄) on a CMS were studied over a wide pressure range up to 15 arm by using a volumetric method. Despite only relatively small differences in the kinetic diameters of the probe molecules used, very large differences in the magnitude of apparent time constants were observed. The adsorption kinetic characteristics of six components on the CMS were affected by the relative importance of atomic/molecular size, shape, and polar properties. Especially, the interaction properties of adsorbate molecules were proposed as an important factor to estimate the relative adsorption rate.     

Gas transport properties of polyaniline membranes

The transport properties of He, H₂, CO₂, O₂, N₂, and CH₄ gases in solvent cast, HCl doped, and undoped polyaniline (PANi) membranes were determined. Measurements were carried out at 40 psi pressure from 19°C to 60°C. An excellent correlation was found between the diffusion coefficients and the molecular diameters of gases. The solubility coefficients of gases were found to correlate with their boiling points or critical temperatures. The sepa-ration factors for CO₂/N₂ and CO₂/CH₄ are dominated by the high solubility of CO₂. These correlations enable us to predict the permeability, diffusion, and solubility coefficients of other gases. After the doping-undoping process, the fluxes of gases with kinetic diameters smaller than 3.5 Å increased but those of larger gases decreased. This results in a higher separation factor for a gas pair involving a small gas molecule and a larger one. © 1996 John Wiley & Sons, Inc.     

Gas sorption in poly(butylene terephthalate). I. Influence of gas molecules

The sorption of CO₂ and the noble gases Ne and Ar in semicrystalline glassy poly(butylene terephthalate) (PBTP) films was measured by the gravimetric method with a recording microbalance at 298 K. The sorption of CO₂ was found to be significantly higher than that of Ne and Ar. This is attributed to a specific interaction between CO₂ and PBTP. The sorption isotherm for CO₂ was analyzed by the dual-mode sorption model, while the sorption behaviors for Ne and Ar did not follow this model. Their sorption isotherms can be described by the sorption model developed here for the noble gases in PBTP. A critical adsorption pressure p^* that is dependent mainly on the relative size of the frozen microvoids and the noble gas atoms is defined in this model. The Langmuir adsorption for describing the sorption in this gas/polymer system in addition to Henry's solubility happens just above this p^*, whereas only Henry's solubility takes place below p^*. © 1993 John Wiley & Sons, Inc.     

Reduction of SO2 on different carbons

The reduction of SO₂ on four carbons (graphite, charcoal, activated carbon and coke) was studied under steady-state conditions and when the kinetics was chemically controlled in a reactor operated under differential conditions. The reaction showed second-order kinetics: first order with respect to carbon and first order with respect to the partial pressure of SO₂. The reactivity of the different carbons, as measured by the second-order rate constants, followed the sequence of decreasing crystallinity: graphite<coke (7.34)<coke (11.73)<charcoal. The difference in reactivity between graphite and charcoal was determined by ΔH^≠, while for cokes it increased with the ash content because of a favorable ΔS^≠. The main reaction products for all carbons were CO₂ and sulfur in the ratio 2:1, considering the sulfur as S₂, which was shown to be formed through the same path. CO, COS, and CS₂ were also detected, and the product distribution depended on the carbon and whether the reaction was diffusion controlled or chemically controlled. Analysis of product ratios strongly suggested that CO, COS and CS₂ were produced from consecutive reactions of the primary products. CO was formed from CO₂ by a slow Boudouard reaction that occurred partially and under conditions of non-equilibrium. Complexed sulfur reacted with CO to form COS and CS₂. There was an interaction between the active site of reduction and the site where sulfur is inserted.     

Effects of carbon nanofiber on the dissolution and diffusion of CO2 in polypropylene nanocomposites

The effects of nanofiller with elongated structure on the dissolution and diffusion behaviors of CO₂ in polypropylene (PP)/carbon nanofiber (CNF) composites were investigated in this work. The solubility of CO₂ in PP and PP composites containing 5 wt% and 10 wt% CNF was measured by using magnetic suspension balance (MSB) combined with the experimental swelling correction by using a self-designed high-temperature and -pressure view cell at the temperatures of 200 and 220 °C and pressures up to 20 MPa. The diffusion coefficient of CO₂ in PP and PP composites was also determined from the sorption line at CO₂ pressures ranging from 5 to 10 MPa. It was found that the solubility and diffusivity of CO₂ in PP/CNF composites increased with increasing the filler content, which should be mainly attributed to the change of the distribution of free volume in the polymer matrix besides the small amount of adsorption capacity of CO₂ in CNF. A modified Henry model incorporated with Langmuir adsorption factor was proposed to correlate the solubility of CO₂ in the PP/CNF composites with an average relative deviation less than 3%. A new model based on free volume theory incorporated with the diffusion driving force factor was established to correlate the experimental diffusion coefficient of CO₂ in the PP/CNF composites within an average relative deviation of 2%.     

Synthetic 6FDA-ODA copolyimide membranes for gas separation and pervaporation: Functional groups and separation properties

Copolyimides were prepared from one-step polymerization of 6FDA and ODA with four diamines DBSA, DABA, DAPy and DANT as the third monomers. Polymers were characterized with GPC, FTIR, NMR, DSC and TGA. Surface free energies and membrane-water interfacial free energies were calculated from contact angles, and results indicated that DABA, DBSA and DAPy moieties helped to increase the hydrophilicity. Gas permeation was measured for N₂, O₂, H₂, He and CO₂. Linear moiety contribution method was proposed to study the moiety effects on gas selectivities. The selectivities of O₂/N₂, H₂/N₂, and He/N₂ were greatly affected by the steric effects from the monomer moieties, but permeation of CO₂ was controlled by its solubility in polymers as well as the interactions with the functional groups. Water permeation and dehydration of isopropanol were carried out in pervaporation processes. Concentration coefficients were proposed for feed concentration effects on permeation flux, and permeation activation energies were used to study the temperature dependence of flux. Linear moiety contribution method was applied to quantitatively compare the effects of monomer moieties. Functional groups could change the sorption and diffusion properties in pervaporation, and moiety contribution factors indicated that feed concentrations mainly affected sorption properties, whereas temperatures influenced diffusion properties.     

SO2 removal from flue gas by activated semi-cokes: 1. The preparation of catalysts and determination of operating conditions

The objective of this work was to use waste semi-coke as the raw material to prepare catalysts of industrial-scale size for SO₂ removal from flue gas and to find the optimal preparation methods. Results showed that lignite semi-coke was a suitable raw material, and that the catalyst, prepared by pre-activating in an autoclave, oxidizing with HNO₃, loading with CuSO₄ and finally calcining at 700 °C, exhibited the best desulfurizing property with a sulfur retention of about 9.6% SO₂/100 gC at a reaction temperature of 90 °C. Also, the effects of H₂O content in the flue gas, reaction temperature and space velocity on the desulfurizing property were investigated to determine optimum operating conditions. An H₂O content of 7% was appropriate for catalysts in this work. In the temperature range 80-120 °C, the catalyst showed good performance for SO₂ removal and was gradually deactivated at temperatures above 120 °C. Space velocity exhibited an optimal value of 830 h⁻¹. The kinetic behavior varied with space velocity and the desulfurizing property was controlled by diffusion at space velocities below 830 h⁻¹, and controlled by adsorption or catalytic reaction at space velocities above 830 h⁻¹.