Methane storage in wet activated carbon: Studies on the charging/discharging process

The dynamic behavior of charging/discharging methane onto/from water-preloaded activated carbon was studied at different conditions. It was shown that methane hydrate could form quickly in the porous space of carbon at the condition of 275 K and pressures beginning with 4.12 MPa. The stored methane could be continuously released at a constant flowrate for the whole discharging process. The packing density of 0.6 g cm⁻³ seemed optimal for the wet carbon tested, which yielded 152 V/V of released methane at charging pressure of 8 MPa. The thermal effect observed on the charging/discharging process was low and did not affect the effective storage capacity.     

Modified activated carbons for catalytic wet air oxidation of phenol

This study aims at testing several activated carbons for the catalytic wet air oxidation (CWAO) of phenol solutions. Two commercial activated carbons were used both as received and modified by treatment with either HNO₃, (NH₄)₂S₂O₈, or H₂O₂ and by demineralisation with HCl. The activated carbons were characterised by measuring their surface area, distribution of surface functional groups and phenol adsorption capacity. The parent and treated activated carbons were then checked for CWAO using a trickle bed at 140 °C and 2 bar of oxygen partial pressure. The treatments increase the acidic sites, mostly creating lactones and carboxyls though some phenolic and carbonyl groups were also generated. Only (NH₄)₂S₂O₈ treatment yields a significant decrease in surface area. CWAO tests show that catalytic activity mainly depends on the origin of the activated carbon. The modifications generally had a low impact on phenol conversion, which correlates somewhat with the increase in the acidity of the carbons. Characterisation of the used activated carbon evidences that chemisorbed phenolic polymers formed through oxidative coupling and oxygen radicals play a major role in the CWAO over activated carbon.     

Methane storage capacities and pore textures of active carbons undergoing mechanical densification

Chemically activated anthracites with pore textures optimised for methane storage were densified by mixing suitable granular fractions, and the prepared mixtures were submitted to increasingly high mechanical pressures (up to 220 kg cm⁻²). The effect of densification was investigated and discussed. It was shown that, while the apparent densities increase significantly, compressing of the adsorbent has no effect on the adsorption capacities. This was confirmed by the measurement of surface areas and micropore volumes using several probe molecules; the microporosity of the material undergoing compression was found to be generally unchanged, whatever the applied pressure and whatever the burn-off. Densification of the material decreases the amount of compressed gas within the storage vessel and, as a result, the global mass storage capacities were found to decrease with the compacting force. Consequently, it was shown that because the density and mass capacities vary in opposite directions, the volume storage capacity should not be extrapolated from measurements made on uncompacted material. An optimal compaction pressure close to 100 kg cm⁻², corresponding to maxima of volume storage capacities of methane, was evidenced and found as 193 V/V stored and 163 V/V deliverable at 3.5 MPa and 20 °C.     

Thermal modeling of activated carbon based adsorptive natural gas storage system

A homogeneous, isotropic porous matrix of activated carbon inside a portable steel cylinder is considered as the adsorption bed for natural gas (NG) which is idealized as pure methane for the purpose of simulation. The heat and fluid flow inside the porous adsorption bed are modeled using a volume averaging technique and Darcy-Brinkman formulation. The effective thermal conductivity of the activated carbon-methane system is calculated as a function of uptake according to the Luikov model. Heat generation due to the exothermic process of adsorption is considered. The governing equations are solved using an implicit finite volume method for the given boundary conditions. Three different models of adsorption are considered, namely (i) a no-flow model, (ii) flow model with uniform adsorption and (iii) a flow model with local adsorption. For each of these models, transient temperature profiles in the adsorption bed during the charging process are obtained, and the corresponding mass adsorption potentials are calculated. Parametric studies are performed to investigate the effects of gas inlet temperature and rate of charging on the maximum bed temperature and the time required to fill the cylinder.     

Improved methane storage capacities by sorption on wet active carbons

The possibility of storing large amounts of natural gas within wet active carbons is examined. The sorption isotherms of methane at 2 °C and up to 8 MPa are built for four carbonaceous materials. Three of them originate from the same precursor (coconut shell), are physically activated at various burn-offs and are mainly microporous. The fourth material is a highly mesoporous chemically activated pinewood carbon. These adsorbents are wetted with a constant weight ratio water/carbon close to 1. The resulting isotherms all exhibit a marked step occurring near the expected formation pressure of methane hydrates, thus supporting their occurrence within the porous materials. The amount of gas stored at the highest pressures investigated then ranges from 6 to 17 mol/kg of wet adsorbent (i.e., corresponding to 10-36 mol/kg of dry carbon), depending on the material. The results are discussed on the basis of the known pore texture of each adsorbent, and stoichiometries of the formed hydrates are calculated. Considerations about adsorption/desorption kinetics and metastability are also developed.     

Removal of sulfur compounds from utility pipelined synthetic natural gas using modified activated carbons

Synthetic natural gas (SNG), which is produced from petroleum and distributed via pipeline in Honolulu by The Gas Company, was analyzed using a gas chromatograph equipped with a sulfur chemiluminescence detector (GC/SCD). Hydrogen sulfide (H₂S), methyl mercaptan (MM), ethyl mercaptan (EM), dimethylsulfide (DMS), dimethyl disulfide (DMDS), tetrahydrothiophene (THT), ethyl disulfide (EDS), and one unidentified compound (UN1) were detected. Among these sulfur compounds, THT is added as an odorant and was present in the highest concentration.A commercial activated carbon (Calgon OLC plus 12X30) was modified by oxidation and impregnation methods and the resulting materials were evaluated for their ability to adsorb sulfur compounds present in SNG. The evaluation results indicate that all of the modification methods can improve the retention of individual sulfur compounds or the total sulfur capacity compared with the untreated virgin carbon. It is also found that activated carbons impregnated with metal impurities have different selectivity for sulfur compounds. Cu and Zn loaded carbons had the highest capacity for H₂S removal, Fe loaded carbon was more efficient for DMS removal (the most difficult S compound to remove), and carbon oxidized by HNO₃ was the best for THT removal.Based on these findings, a composite sorbent consisting of Cu loaded and Fe loaded carbons was designed and tested. The test results indicate that the composite sorbent had improved performance in the removal of individual sulfur compound. A linear programming model was used to design a composite sorbent optimized to minimize the required sorbent mass based on a 1-kW scale fuel cell system service target. Validation tests showed that the optimized sorbent required less of the individual modified carbon components than when they were individually used for the same sulfur removal target.     

Composites of expanded natural graphite and in situ prepared activated carbons

Activated carbons present very low thermal conductivity (0.2 W m⁻¹ K⁻¹) responsible for severe thermal limitations inducing low overall adsorption and desorption kinetics as well as secondary reaction products or hazards in industrial applications. The in situ activation of various precursors within a consolidated expanded natural graphite matrix led to composites of high thermal conductivities (from 1 to 32 W m⁻¹ K⁻¹) and high effective adsorbent level (80 wt.%). Preparation protocols adapted to the different precursors and activation agents are proposed and a wide range of microporous characteristics (from molecular sieve to super-adsorbent) was obtained.     

Optimal design of an activated carbon for an adsorbed natural gas storage system

The Dubinin-Astakhov equation was used to determine the influence of the microporous characteristics of activated carbon on the performances of both charge and discharge of an ANG system. From the dynamic performance criterion as a function of total microporous volume (W_o), average micropore width (L_o), and micropore size dispersion (n), it is possible to identify the optimal activated carbon for methane storage under dynamic conditions by way of the heat and mass transfer limitations. This study shows that the activated carbon must be conductive (with an average micropore width of 1.5 nm) for the charge step only, permeable and sufficiently conductive for the discharge process (with an average micropore width of 2.5 nm). The well-known Maxsorb activated carbon shows the better performance. This theoretical investigation has been validated by experimental results.     

In vitro adsorption study of fluoxetine in activated carbons and activated carbon fibres

We study the in vitro adsorption of fluoxetine hydrochloride by different adsorbents in simulated gastric and intestinal fluid, pH 1.2 and 7.5, respectively. The tested materials were two commercial activated carbons, carbomix and maxsorb MSC30, one activated carbon fibre produced in our laboratory and also three MCM-41 samples, also produced by us. Selected samples were modified by liquid phase oxidation and thermal treatment in order to change the surface chemistry without significant modifications to the porous characteristics. The fluoxetine adsorption follows the Langmuir model. The calculated Q₀ values range from 54 to 1112 mg/g. A different adsorption mechanism was found for the adsorption of fluoxetine in activated carbon fibres and activated carbons. In the first case the most relevant factors are the molecular sieving effect and the dispersive interactions whereas in the activated carbons the mechanism seams to be based on the electrostatic interactions between the fluoxetine molecules and the charged carbon surface. Despite the different behaviours most of the materials tested have potential for treating potential fluoxetine intoxications.     

Thiouracil modified activated carbon as a sorbent for some precious and heavy metal ions

A new sorbent is prepared by modifying activated carbon obtained from apricot stones with 2-hydroxy-2-mercaptopyrimidine (2-thiouracil). The products have been characterized as to their surface area, pore volumes, content of the functional groups with basic and acidic properties, IR spectra, sulfur content. Their sorption properties with respect to some precious (Au(III), Ag(I), Pt(II)) and heavy (Cu(II), Mn(II), Ni(II), Hg(II)) metals are investigated. The sorption is studied as a function of pH (in the interval 1÷8) and stirring time (1÷24 h). The static sorption capacities towards the metal ions are determined under optimum conditions at room temperature. The new sorbent containing thiouracil is appropriate for simultaneous preconcentration of the precious and heavy metal ions within a wide pH range. It shows a significantly higher capacity for the investigated metal ions than the original carbon.     

Tailoring activated carbons for enhanced removal of natural organic matter from natural waters

Several pathways have been employed to systematically modify two granular activated carbons (GACs), F400 (coal-based) and Macro (wood-based), for examining adsorption of dissolved natural organic matter (DOM) from natural waters. A total of 24 activated carbons with different physical and chemical characteristics was produced. The impact of carbon treatment on the DOM adsorption was examined by conducting isotherm experiments at a neutral pH using the modified carbons and a DOM isolated from the influent to Myrtle Beach drinking water treatment plant in South Carolina (USA). Adsorption of the DOM by two activated carbon fibers, with relatively uniform pore size distributions, showed that only pores with widths larger than 1 nm were accessible to the DOM macromolecules. Increases in the carbon supermicropore and mesopore volume (i.e., >1 nm) increased the DOM uptake, if the surface chemistry was favorable. The isotherms normalized on a surface area basis showed the significance of carbon surface chemistry on the DOM uptake. At neutral pH, adsorption of negatively charged DOM molecules was favored by basic and positively charged surfaces, while the DOM uptake was minimized when the surface had acidic characteristics. High temperature ammonia treatment of oxidized carbons considerably enhanced the DOM uptake, mainly due to the increase in accessible surface area and surface basicity. Iron-impregnated carbons indicated an enhanced affinity of iron-laden carbon surface toward the DOM species, if the surface was not negatively charged.     

Comparison of parameters calculated from the BET and Freundlich isotherms obtained by nitrogen adsorption on activated carbons: A new method for calculating the specific surface area

The empirical linear relationship between the BET surface area S_BET and the Freundlich constant K_F, calculated from nitrogen adsorption isotherms of activated carbons, S_BET = a₀K_F is mathematically demonstrated. This correlation exists in the relative pressure domain in which the BET equation is valid, whatever the value of c for the BET equation and for values of the Freundlich exponent, , between 0 and 0.2. This study allows to determine the correlation factor a₀ = 1/a with . From this result, a new expression, depending of and K_F, can be deduced for calculating the specific surface area.     

A novel method to synthesize super-activated carbon for natural gas adsorptive storage

The design and scale-up preparation of adsorbent is the key to commercialize the adsorptive storage technology of natural gas (ANG). The super activated carbon might be potential to ANG because of its special porosity. The traditional approach to prepare the super activated carbon is the chemical activation using alkali hydroxides as activation agent. Of difficult is how to control of micropore diameter and volume while maintain the super high surface area. Coupling approach to prepare super activated carbon from coke or coal was firstly achieved and the mechanism was tentatively postulated. It is found that activated carbon prepared by coupling approach bears developed micropore structure, which is produced by depth activation catalyzed by chemical agent at moderate temperature and might be feasible for adsorption storage of natural gas, and considerable mesopore structure, which is formed by width activation catalyzed by chemical agent and/or steam at elevated temperature and feasible for desorption process of ANG. The activation mechanism of coupling approach is actually two-step activation, depth activation at moderate temperature and width activation at elevated temperature. It means that the coupling of chemical activation with physical activation is a potential approach to prepare the adsorption materials for natural gas storage.     

Influence of oxidation process on the adsorption capacity of activated carbons from lignocellulosic precursors

A set of activated carbon materials non-oxidised and oxidised, were successfully prepared from two different lignocellulosic precursors, almond shell and vine shoot, by physical activation with carbon dioxide and posterior oxidation with nitric acid. All samples were characterised in relation to their structural properties and chemical composition, by different techniques, namely nitrogen adsorption at 77 K, elemental analysis (C, H, N, O and S), point of zero charge (PZC) and FTIR. A judicious choice was made to obtain carbon materials with similar structural properties (apparent BET surface area ∼ 850-950 m²g⁻¹, micropore volume ∼ 0.4 cm³g⁻¹, mean pore width ∼ 1.2 nm and external surface area ∼ 14-26 m²g⁻¹). After their characterisation, these microporous activated carbons were also tested for the adsorption of phenolic compounds (p-nitrophenol and phenol) in the liquid phase at room temperature. The performance in liquid phase was correlated with their structural and chemical properties. The oxidation had a major impact at a chemical level but only a moderate modification of the porous structure of the samples. The Langmuir and Freundlich equations were applied to the experimental adsorption isotherms of phenolic compounds with good agreement for the different estimated parameters.     

Natural gas storage on activated carbon modified by metal oxides

The surface of activated carbon has been modified with metal oxides in order to improve the adsorption capacity for methane. Three metal oxides were used: MgO, Fe₂O₃, and NiO. The results indicate that the adsorption capacity can be marginally improved by doping small amount of metal oxides. Excess loading of metal oxides would result in the blockage of pores and hence reduce the surface area. By modifying the surface with metal oxides, although the adsorption capacity of methane on activated carbon cannot be significantly improved, faster thermal diffusion was observed for the increase of desorption capacity.     

Production of fibrous activated carbons from natural cellulose (jute, coconut) fibers for water treatment applications

Different fibrous activated carbons were prepared from natural precursors (jute and coconut fibers) by physical and chemical activation. Physical activation consisted of the thermal treatment of raw fibers at 950 °C in an inert atmosphere followed by an activation step with CO₂ at the same temperature. In chemical activation, the raw fibers were impregnated in a solution of phosphoric acid and heated at 900 °C in an inert atmosphere. The characteristics of the fibrous activated carbons were determined in the following terms: elemental analysis, pore characteristics, SEM observation of the porous surface, and surface chemistry. As the objective of this study was the reuse of waste for industrial wastewater treatment, the adsorption properties of the activated carbons were tested towards pollutants representative of industrial effluents: phenol, the dye Acid Red 27 and Cu²⁺ ions. Chemical activation by phosphoric acid seems the most suitable process to produce fibrous activated carbon from cellulose fiber. This method leads to an interesting porosity (S_BET up to 1500 m² g⁻¹), which enables a high adsorption capacity for micropollutants like phenol (reaching 181 mg g⁻¹). Moreover, it produces numerous acidic surface groups, which are involved in the adsorption mechanisms of dyes and metal ions.     

Adsorption capacity and removal efficiency of heavy metal ions by Moso and Ma bamboo activated carbons

In order to understand the adsorption capacity and removal efficiency of heavy metal ions by Moso and Ma bamboo activated carbons, the carbon yield, specific surface area, micropore area, zeta potential, and the effects of pH value, soaking time and dosage of bamboo activated carbon were investigated in this study. In comparison with once-activated bamboo carbons, lower carbon yields, larger specific surface area and micropore volume were found for the twice-activated bamboo carbons. The optimum pH values for adsorption capacity and removal efficiency of heavy metal ions were 5.81–7.86 and 7.10–9.82 by Moso and Ma bamboo activated carbons, respectively. The optimum soaking time was 2–4 h for Pb²⁺, 4–8 h for Cu²⁺ and Cd²⁺, and 4 h for Cr³⁺ by Moso bamboo activated carbons, and 1 h for the tested heavy metal ions by Ma bamboo activated carbons. The adsorption capacity and removal efficiency of heavy metal ions of the various bamboo activated carbons decreased in the order: twice-activated Ma bamboo carbons > once-activated Ma bamboo carbons > twice-activated Moso bamboo carbons > once-activated Moso bamboo carbons. The Ma bamboo activated carbons had a lower zeta potential and effectively attracted positively charged metal ions. The removal efficiency of heavy metal ions by the various bamboo activated carbons decreased in the order: Pb²⁺ > Cu²⁺ > Cr³⁺ > Cd²⁺.     

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.