Activated carbon adsorption isotherms

The two binders, the ENG and the thermoplastic polymer, are inert concerning the methane adsorption. The methane quantity delivered depends only on the activated carbon adsorption isotherm and its apparent density in the adsorbent composite block (Table 1). For pure methane, the quantity delivered at 298K between 3.5 and 0.1 MPa is equal to 89 (v/v). 

Properties of Activated Carbon Adsorption Isotherm Models Design Consideration of PAC Systems Regeneration... 

Adsorption. Adsorption (qv) is an effective means of lowering the concentration of dissolved organics in effluent. Activated carbon is the most widely used and effective adsorbent for dyes (4) and, it has been extensively studied in the waste treatment of the different classes of dyes, ie, acid, direct, basic, reactive, disperse, etc (5—22). Commercial activated carbon can be prepared from lignite and bituminous coal, wood, pulp mill residue, coconut shell, and blood and have a surface area ranging from 500—1400 m /g (23). The feasibiUty of adsorption on carbon for the removal of dissolved organic pollutants has been demonstrated by adsorption isotherms (24) (see Carbon, activated carbon). Several pilot-plant and commercial-scale systems using activated carbon adsorption columns have been developed (25—27). 

Adsorption This is the most widely used of the physical-chemical treatment processes. It is used primarily for the removal of soluble organics with activated carbon serving as the adsorbent. Most liquid-phase-activated carbon adsorption reactions follow a Freundlich Isotherm [Eq. (25-21)]. 

Adsorption — An important physico-chemical phenomenon used in treatment of hazardous wastes or in predicting the behavior of hazardous materials in natural systems is adsorption. Adsorption is the concentration or accumulation of substances at a surface or interface between media. Hazardous materials are often removed from water or air by adsorption onto activated carbon. Adsorption of organic hazardous materials onto soils or sediments is an important factor affecting their mobility in the environment. Adsorption may be predicted by use of a number of equations most commonly relating the concentration of a chemical at the surface or interface to the concentration in air or in solution, at equilibrium. These equations may be solved graphically using laboratory data to plot "isotherms." The most common application of adsorption is for the removal of organic compounds from water by activated carbon. 

Whittaker, K. F., 1980, Adsorption of selected pesticides by activated carbon using Isotherm and continuous flow column system, Ph.D. Thesis, Purdue Unlveslty, West Lafayette, Indiana. 

FIGURE 6.17 (a) Adsorption isotherm of N2 at 77 K on an active carbon ( , adsorption branch , desorption... 

Adsorption. Organic compounds are adsorbed on activated carbon and synthetic resins (eg, XAD-2 and XAD-4, Rohm and Haas Co.). This technique depends on the properties of the compound being removed and the regenerative capabiHty of the adsorbent. The EPA has developed carbon-adsorption isotherms for various toxic organic compounds, and the results are shown in Table 7 (36). The following compounds are not adsorbed on activated carbon acetone cyanohydrin, butylamine, choline chloride, cyclohexylamine, diethylene glycol, ethylenediamine, hexamethylenediamine, morpholine, and triethanolamine. 

The secondary effluent from a biological treatment plant is treated with the activated carbon adsorption process and allowed to arrive at eqnilibrinm. The equilibrium data in terms of phenol are given below. Determine the constants for the Langmuir and Freundlich isotherms. 

Figure 22.5 Ad(de)sorption characteristics of activated carbons (a) isotherms for adsorption of pure COj on various activated carbons, (b) isotherms for adsorption of various gases on the BPL carbon, (c) desorption of pure COj from various adsorbents.

Consider the following application of fixed-bed, activated carbon adsorption for the control of VOC emissions. An industrial waste gas consists of 0.5 vol% acetone in air at 300 K and 1 atm. It flows at the rate of 2.3 kg/s through a fixed bed packed with activated carbon. The bed has a cross-sectional area of 5.0 m2 and is packed to a depth of 0.3 m. The external porosity of the bed is 40%, its bulk density is 630 kg/m3, and the average particle size is 6 mm. The average pore size of the activated carbon particles is 20 A, the internal porosity is 60%, and the tortuosity factor is 4.0. A Langmuir-type adsorption isotherm applies with qm = 0.378 kg VOC/kg of carbon, K = 0.867 kPa-1. At the break point, the effluent concentration will be 5% of the feed concentration. Calculate ... 

Tables XLIV to XLVI show the carbon adsorption data for treating resorcmol, vanillin and salicylic acid with Norit PAC 20B at resorcmol concentration of 100 mg/l using Langmun, Fremdlich and BET isotherms. These are representative samples for the other activated carbons adsorption data. Tables XLVII to XCDC show other adsorption data described by the Langminr isotherm for the six activated carbons—Norit PAC 20B, Norit E Supra USP, Darco KB, Norit Daico S-51 and Hydrodarco C at...

The Langmufr and Freundlich isotherms appear to be better than the BET isotherm m the analysis of activated carbons adsorption capabiMes. For the BET isotherm, the saturated concentration of solute value, Cs, is necessary m the computation of X/m values while this is not needed in the former cases. The introduction of negative values of X/m at some low targeted TOC concentrations of organic compounds also makes the use of BET isotherm unfavorable. 

FIGURE 3.21 Composite isotherms on Spheron-6 from glycol-water binary solutions. (After Puri, B.R., in Activated Carbon Adsorption, I.H. Suffet and M.J. McGuire, Eds., Ann Arbor Science Publishers, Ann Arhor MI, 1981, p. 353. With permission.)... 

FIGURE 8.16 Adsorption and desorption isotherms of CCI4 at 35°C on activated carbons, Adsorption = open desorption = closed. (After Puri, B.R., Arora, V.M., and Verma, S.K., J. Indian Chem. Soc., 56, 802, 1979. With permission.)... 

The book has been written with a view to equip the surface scientists (chemists, physicists, and technologists) with the surface processes, their energetics, and with the adsorption isotherm equations, their applicability to and deviations from the adsorption data for both gases and solutions. To carbon scientists and technologists, the book should help understand the parameters and the mechanisms involved in the activated carbon adsorption of organic and inorganic compounds. The book thus combines in one volume the surface physical and chanical structure of activated carbons, the surface phenomenon at soUd-gas and solid-liquid interfaces, and the activated carbon adsorption of gaseous adsorbates and solutes from solutions. 

It is well known that both the adsorbate and the adsorbent properties play a very important role in activated carbon adsorption. Adsorption is a manifestation of complicated interactions among the three components involved, that is, the adsorbent, the adsorbate, and the solvent. Normally, the affinity between the adsorbent and the adsorbate is the main interaction force controlling adsorption. However, the affinity between the adsorbate and the solvent (i.e., solubility) can also play a major role in adsorption. Hydrophobic componnds have low solubility and tend to be pushed to the adsorbent surface and hence are more adsorbable than hydrophilic compounds. Meanwhile, we know that phenolic compounds with different fnnctional groups can lead to different solubility, which may lead to different oligomerization extent. Therefore, the adsorption behavior of phenolic componnds with different fnnctional gronps has to be nnderstood. As illustration, we consider the interpretation of experimental isotherms by Ln and Serial (2007) for the adsorptive capacity of five different phenolics on GAC F400 and two ACFs, ACC-10 and ACC-15, under both anoxic and oxic conditions (Fignre 6.2). 

Breakthrough curves can be considered as the last of the essential characterizations of an activated carbon. Equilibrium isotherm data provide information of the capacity of a carbon. Next, the kinetics of the adsorption processes must be known, giving information of the rates at which adsorptives are adsorbed by the adsorbent. Finally, the performance of a carbon (so characterized) in an industrial situation can be simulated by making use of the breakthrough curves. 

The isotherm (I) represents microporous materials, generally found in zeolites and activated carbons. The isotherm (II) is the multilayer physisorption on a flat surface (usually nonporous). The isotherms (III) and (V) are characteristic of gas-solid weak interactions, and the isotherm (IV) is the most frequent in heterogeneous catalysts, representing multilayer adsorption and capillary condensation in mesoporous materials. The isotherm (VI) shows the behavior of nonporous materials, energy uneven. 

When calcium is used in the preparation of activated carbons it is found that the adsorption capacity increases in both carbon series with the extent of burn-off. However changes in the porosity of the activated carbons with burn-off differ considerably in the presence of calcium, mainly for CO, activation. Figure 1 (a and b) for carbon A and Figure 2 (a and b) for carbon B show the remarkable effect of the catalyzed carbon-CO, activation. The adsorption isotherms shapes are very different from those found for the uncatalyzed activation. Isotherms are a combination of type I and II in contrast to the well defined type I isotherms obtained for the uncatalyzed CO, activation. Carbon A2 and B2 behave differently (Figure 1 b and 2b) probably due to their different initial porosity and calcium contents. In any case, catalytic activation in CO, gives rise to a noticeable development of mesoporosity and, as a result, a much wider pore size distribution. Mercury porosimetry. Figure 3 (a and b), show the very different pore size distributions obtained by catalytic activation with calcium mesoporosity development is very noticeable in agreement with the N, adsorption data. 

Fig. 5.14 Adsorption isotherms of water on carbon in (a) to f) with corresponding isotherms of nitrogen in (a), (c) and (J), and of benzene in (f>). (a) Charcoal (b) active carbon AY8 (c) charcoal A (J) charcoal (e) a coal tar pitch kilned at 1200°C (/) a charcoal (S600H). (Redrawn from the diagrams in the original papers.)...

Fig. 1. Fquilihrium isotherms for adsorption on activated carbon at 298 K showing the effect of surface modification (2). —, SO2 -...

Isotherm Models for Adsorption of Mixtures. Of the following models, all but the ideal adsorbed solution theory (lAST) and the related heterogeneous ideal adsorbed solution theory (HIAST) have been shown to contain some thermodynamic inconsistencies. References to the limited available Hterature data on the adsorption of gas mixtures on activated carbons and 2eohtes have been compiled, along with a brief summary of approximate percentage differences between data and theory for the various theoretical models (16). In the following the subscripts i and j refer to different adsorbates. 

Typical adsorption isotherms for light hydrocarbons on activated carbon prepared from coconut shells ate shown in Figure 11 (46). The polarizabihties and boiling points of these compounds increase in the order... 

Fig. 11. Adsorption isotherms for hydrocarbons on activated coconut-sheU carbon at 25°C (46). 0> Adsorption A, desorption. To convert kPa to mm Hg,...

Many simple systems that could be expected to form ideal Hquid mixtures are reasonably predicted by extending pure-species adsorption equiUbrium data to a multicomponent equation. The potential theory has been extended to binary mixtures of several hydrocarbons on activated carbon by assuming an ideal mixture (99) and to hydrocarbons on activated carbon and carbon molecular sieves, and to O2 and N2 on 5A and lOX zeoHtes (100). Mixture isotherms predicted by lAST agree with experimental data for methane + ethane and for ethylene + CO2 on activated carbon, and for CO + O2 and for propane + propylene on siUca gel (36). A statistical thermodynamic model has been successfully appHed to equiUbrium isotherms of several nonpolar species on 5A zeoHte, to predict multicomponent sorption equiUbria from the Henry constants for the pure components (26). A set of equations that incorporate surface heterogeneity into the lAST model provides a means for predicting multicomponent equiUbria, but the agreement is only good up to 50% surface saturation (9). 

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