Adsorption and desorption kinetics in activated carbon

Modelling of fixed bed adsorption and desorption kinetics for gas separation and purification is based on the correct representation of mass transport mechanisms on the single-pellet scale. In the case of adsorption it is commonly assumed that pore mass transport within the adsorbent particle is rate limiting. It has been questioned whether this is also true for desorption. Here the exchange step between the adsorbed state on the solid surface and the mobile state within the pore fluid was considered as being the dominant kinetic resistance. Therefore, experimental and theoretical investigations of single component adsorption and desorption of water vapour, n-hexane, cyclohexane and tetrachloroethylene on single pellets of activated carbon were performed. From the results it can be concluded that for most cases of physical gas-phase adsorption as well as desorption the overall kinetics is controlled by mass transport within the pore system of the adsorbent. The exchange step between the adsorbed phase and the fluid phase happens infinitely fast justifying the assumption of adsorption equilibrium at every position along the pellet radius.     

Adsorption and electrothermal desorption of organic vapors using activated carbon adsorbents with novel morphologies

Novel morphologies of activated carbons such as monolith, beads and fiber cloth can effectively capture organic vapors from industrial sources. These adsorbent materials are also unique because they can undergo direct electrothermal regeneration to recover the adsorbed organic vapors for potential re-use. This investigation compares and contrasts the properties of these adsorbents when using electrothermal-swing adsorption. The adsorption systems consisted of an organic vapor generation system, an electrothermal-swing adsorption vessel, a gas detection unit, and a data acquisition and control system. The activated carbon monolith (ACM) had the lowest pressure drop, highest permeability, highest electrical resistivity and lowest cost as compared to the activated carbon beads (ACB) and the activated carbon fiber cloth (ACFC). ACB had the largest throughput ratio and lowest length of unused bed as compared to the other adsorbents. However, ACFC had the largest adsorption capacity for toluene when compared to ACM and ACB. ACFC was also faster to regenerate and had a larger concentration factor than ACM and ACB. These results describe relevant physical, electrical, adsorption and cost properties for specific morphologies of the adsorbents to more effectively capture and recover organic vapors from gas streams.     

Adsorption and desorption kinetics for hydrophilic and hydrophobic vapors on activated carbon

Adsorption dynamics are of fundamental importance in applications of adsorbents in real situations. The adsorption/desorption characteristics of a series of adsorbates, with varying hydrophilic/hydrophobic and structural characteristics, for activated carbon BAX950, were investigated for temperatures in the range 288-323 K. These data provide a comprehensive kinetic study of adsorption/desorption for an activated carbon. The results are discussed in relation to the adsorbent pore structure and functional group concentration, adsorptive structure and adsorption mechanism. The study provides evidence for a compensation effect where activation energy and ln(pre-exponential factor) parameters obtained from the Arrhenius equation exhibit a linear correlation.     

Multicomponent sorption kinetics of hydrocarbons in activated carbon: Simultaneous desorption and displacement

Binary desorption and displacement kinetics of ethane and propane in Ajax activated carbon are studied in this paper to further understand the multicomponent adsorption of light hydrocarbons onto activated carbon. The experimental data are collected using a differential adsorption bed over a range of temperatures, bulk concentration combinations, particle sizes and shapes. A macropore, surface and micropore diffusion (MSMD) model recently proposed by Hu and Do (Chem Eng Sci (1993) 48 1317) is used in the analysis of experimental data. This model takes into account the concentration dependency of the surface diffusivity. Both diffusions of free and adsorbed species are allowed for in the theory. The adsorbed species are assumed to diffuse in the particle as well as in the microparticle (grain) coordinates. An imaginary gas-phase concentration concept is used to calculate the local diffusion flux of the adsorbed species inside the microparticle. The local multicomponent adsorption equilibrium at any point within the particle is calculated using the ideal adsorbed solution theory (IAST), with the single-component adsorption equilibrium data described by a Unilan equation.     

Adsorption and desorption characteristics of 2-methyl-4-chlorophenoxyacetic acid onto activated carbon

Adsorption and desorption characteristics of the 2-methyl-4-chlorophenoxyacetic acid (MCPA) from aqueous solution onto the activated carbon (GAC, F-400) were studied. Adsorption equilibrium capacities of the MCPA increased with decreasing pH and temperature of the solution. Adsorption equilibrium of the MCPA could be represented by the Sips equation. The internal diffusion coefficients were determined by comparing the experimental concentration decay curves with those predicted from surface diffusion model and pore diffusion model. The adsorption model based on the linear driving force approximation (LDFA) was used for simulating the adsorption behavior of the MCPA in a fixed bed. Over ninety five percent desorption of the MCPA could be obtained using distilled water.     

Adsorption of mixed polycyclic aromatic hydrocarbons in surfactant solutions by activated carbon

The adsorption behavior of three polycyclic aromatic hydrocarbons (PAH) in TX100 solution by activated carbon was studied aiming at surfactant recovery. Adsorption experiments were conducted in batch and column models. The adsorption data fitted well with the Langmuir isotherm model, Dubinin–Radushkevich isotherm models and pseudo-second-order kinetics model. Regarding column adsorption, the exhausting time of TX100 was shorter than the breakthrough time of PAH in the fixed AC column. This finding indicates the feasibility of reusing surfactants from soil-washing solutions. The total costs can reduce about $ 0.57 per 10 L washing solution with AC adsorption.     

活性炭对苯酚的吸附研究

进行了活性炭处理含苯酚废水的应用研究,考察了影响苯酚吸附效果的因素。确定了处理水中苯酚的最佳条件:吸附平衡时间为30 min;最佳pH值为6左右;苯酚初始浓度为10 mg/L;投炭量为20～25 mg/L;苯酚的吸附率高达97.4%;温度对苯酚吸附率的影响不明显。研究了活性炭对苯酚的吸附动力学特性,分别用伪一级动力学方程、伪二级动力学方程、修正伪一级动力学方程和颗粒内扩散模型进行拟合,对于不同浓度的苯酚废水都只有伪二级动力学方程拟合程度比较高,伪二级动力学方程更为真实地反映苯酚在活性炭上的吸附机理。     

Adsorption characteristics of bamboo activated carbon

Dye is difficult to remove from aqueous solution with common adsorbents due to its large molecular size. Mesoporous bamboo activated carbon is utilized in the adsorption of Black 5, Red E and phenol. The adsorption performance of the carbon is experimentally examined along with the characterization of the adsorbent. The comparison of adsorption capacity of the bamboo activated carbon with that of coconut activated carbon and carbon cryogel indicates that the large volume of mesopore in the carbon helps the expansion of adsorption capacity. Microscopic observation, the measurement of pore characteristics and fitting to the adsorption isotherms are conducted in the characterization of the bamboo activated carbon.     

Adsorption of hydrocarbons on CO2-reacted activated carbons

A series of carbons covering the burn-off range 5–82% has been obtained by slow gasification at 1098 K in CO₂ of carbonized almond shells. The adsorption of hydrocarbons with different molecular sizes (benzene, n-butane, iso-butane, cyclohexane, 2,2-dimethylbutane, iso-octane) has shown the evolution in the development of porosity upon increasing gasification. This development is very uniform except for the narrow microporosity for which the widening predominates above 30% burn-off; for supermicropores, mesopores and macropores, their ratio to the total porosity is kept almost constant in the range 16–82% burn-off. Carbons with low burn-off exhibit a molecular sieve effect toward 2,2-dimethylbutane and iso-octane and also present low pressure hysteresis for iso-octane; both characteristics disappear upon further gasification.     

Adsorption equilibria of two halogenated carbons on a carbon fiber

Chlorofluorocarbons (CFCs) are presently considered to be the prime contribution to stratospheric ozone depletion. In this study, a microporous activated carbon felt (KF-1500) was used as a useful adsorbent for adsorption of trichlorofluoromethane and methylene chloride. The isotherms of these organic vapors on a microporous carbon felt were measured and the equilibrium parameters based on Dubinin-Radushkevich correlation were evaluated. Besides, the isosteric heats of adsorption were evaluated from the equilibrium data. It was found that equilibrium data for this carbon felt were well described by Dubinin-Radushkevich correlation.     

Adsorption/desorption of phenols onto granular activated carbon in a liquid–solid fluidized bed

The adsorption/desorption of phenols in aqueous solution onto coconut‐shell granular activated carbon (GAC) in a liquid–solid fluidized bed adsorber approaching saturation capacity was investigated. Experiments were carried out using a 20 mm id adsorber under a variety of operating conditions including GAC particle sizes (0.937, 1.524 mm), GAC mass (12, 24 g), influent phenol concentration (0.367–1.071 mmol dm⁻³), surface loading of GAC (2.0, 2.5 mmol g⁻¹) and liquid flow rate (0.15, 0.2, 0.35 dm³ min⁻¹). The effect of repetitive adsorption/desorption cycles on the adsorption capacity has also been examined for phenol/GAC systems. The model based on the external mass transfer with film‐surface diffusion, surface adsorption equilibrium and internal mass transfer was proposed to simulate the breakthrough curves of the phenol adsorption/desorption process. Using the experimentally measured Langmuir isotherm equilibrium parameters in the model has been found to describe reasonably well the experimental results. © 1999 Society of Chemical Industry     

Adsorption of weak and non-electrolytes by activated carbon

Four activated carbons, one of them commercially available Darco active carbon and the remaining three from a series of coconut charcoals steam activated to varying degrees, are used to study the adsorption of weak and non-electrolytes. Introduction of —CI into the CH₃COOH molecule increases adsorption onto activated carbon while —OH and —NH₂ have the opposite effect. Substitution in the benzene ring shows that adsorption from aqueous solutions is in the order —NH₂, >—OH, >CO₂, >(—OH + —COOH). Effect of polarity of solvent on adsorption capacities is studied in H₂O, CHCl₃ and C₆H₆. Substitution of —Cl into the CH₃COOH molecule invariably increases the adsorption irrespective of the polarity of the solvent. On introducing a specific group in the benzene ring the adsorption is in the order: aniline ? phenol > benzoic acid > salicylic acid in H₂O medium but in CHCl₃ and C₆H₆ media aniline ? phenol > salicylic acid > benzoic acid. However, on comparing the results of individual adsorbates in three media, generally the magnitude of adsorption is H₂O > C₆H₆ ? CHCl₃. Adsorption of lower aliphatic acids (formic to caproic) from aqueous solutions increases regularly as one ascends the homologous series—a behaviour known as Traube's rule; however, of all the surface area available for N₂ adsorption, only a fraction of it is available for adsorption of the aliphatic acids.     

Adsorption of benzoyl peroxide on activated carbon

The kinetics of adsorption of benzoyl peroxide from solutions in ethanol and n-hexane on coconut shell activated carbon was studied. It was found that the kinetics of adsorption of benzoyl peroxide is described by a pseudo-first-order rate equation. The adsorption isotherm of benzoyl peroxide at its equilibrium concentrations from 0.07 to 1.25 g/L in solution is adequately described by the Langmuir equation. It was established that, in a solution in ethyl alcohol, the degradation of benzoyl peroxide with the formation of ethyl benzoate occurred along with adsorption.     

Adsorption of gold-thiourea complex on activated carbon

The adsorption kinetics of the gold-thiourea complex on activated charcoal was studied. Experimental runs were made in a batch reactor at constant temperatures and stirred by a magnetic system. Samples were taken at different periods of time and were analyzed for gold in a Perkin-Elmer Model 360 atomic absorption spectrophotometer. The samples were prepared with synthetic solution of gold with thiourea, sulfuric acid, and iron sulfate with concentrations similar to the ones in the leaching process. The gold adsorbed on activated charcoal was 99% at 20°C. The adsorption behavior was characterized by the Freundlich model at different temperatures (20°C, 35°C and 50°C).     

Adsorption of ethyl benzoate on activated carbon

The kinetics of adsorption of ethyl benzoate from solutions in ethanol on coconut shell activated carbon was studied. It was found that the kinetics of adsorption of ethyl benzoate can be described by pseudofirst-order and pseudo-second-order rate equations with the same accuracy. It was established that the adsorption isotherm at a concentration to 2 g/L in the initial solution is adequately described by the Langmuir equation.     

Adsorption of gas mixture on activated carbon

Equilibrium adsorptions of H₂ and CH₄ from their mixture on three commercial activated carbons are measured by analyzing the breakthrough curves of the more strongly adsorbed CH₄ at 25 and 204°C and pressures up to 1000 psia. Three methods of predicting adsorption of mixture based on single-gas isotherms are used and compared with experimental data: ideal adsorbed solution theory, modified Polanyi-Dubinin potential theory and loading ratio correlation. None of the methods seems to have a distinct advantage over the others. However the loading ratio correlation is the only non-iterative method.     

CS_2在超高交联树脂上的吸附脱附特性研究

研究超高交联树脂对二硫化碳的吸附与脱附性能。以静态法测定了二硫化碳在吸附树脂上的吸附等温线,采用Dubinin-Radushkevich(D-R)方程和Langmiur方程对吸附平衡数据进行拟合,显示D-R方程拟合效果更好,R2均大于0.99;研究了不同温度(303,313,323 K)下二硫化碳的穿透曲线,结果表明,温度对于树脂吸附二硫化碳具有显著影响,323 K时吸附穿透时间比303 K时减少44.08%;热脱附实验显示,当温度上升到363 K时,吸附树脂能完全再生,且随着温度的升高,其再生速率加快。     

粘胶基活性炭纤维对甲苯的吸附及再生

在自制的实验装置上,研究了粘胶基活性炭纤维(ACF)对甲苯气体的吸附及其吸附饱和后采用高温水蒸气解吸、再生。正交实验表明,甲苯气体流量、浓度和粘胶基ACF的填充高度对吸附过程都有显著的影响。最佳操作参数为:温度13.0℃、粘胶基ACF用量1.50 g、填充高度15 cm、甲苯气体流量0.8 m3/h、甲苯进口平均浓度89.7 mg/m3,脱附平均温度136.0℃时,粘胶基ACF对甲苯气体吸附再生效果较好。验证了粘胶基ACF的吸附量与气体进口浓度成正比,与气体流量成反比关系。