Adsorption systems, model

Adsorption beds of activated carbon for the purification of citric acid, and adsorption of organic chemicals by charcoal or porous polymers, are good examples of ion-exchange adsorption systems. Synthetic resins such as styrene, divinylbenzene, acrylamide polymers activated carbon are porous media with total surface area of 450-1800 m2-g h There are a few well-known adsorption systems such as isothermal adsorption systems. The best known adsorption model is Langmuir isotherm adsorption. 

Table 4.2. The basic parameters of the models under discussion for some adsorption systems. The dephasing (2f 4)) and the relaxation (2T 3)) contributions to the full spectral linewidth for local vibrations as well as the...

Most publications dealing with chromatographic reactors focus on theoretical issues of this very complex system. Models of different complexity were derived and used to predict the behavior of chromatographic reactors. Such models typically take into consideration different types of mass transfer, adsorption isotherms, flow profiles, and reactions. A general scheme of these models, not including the reaction, is presented in Fig. 4. There are also several review papers... 

Sheindorf, C., Rebhun, M., and Sheintuch, M. Organic pollutants adsorption from multicomponent systems modeled by Freundlich type isotherm. Water Res., 16(3) 357-362, 1982. 

Model 4 system described contains steel pipes. Model 10 system described is backwashable. Systems include aU piping and manual valves to comprise a complete adsorption system, enabling all operations. Equipment costs include drawings, technical submittals, and provision of an operation and maintenance manual. Freight cost may need to be added for some models. 

Theoretical quantum-chemical study of pyridine adsorption at Hg electrode (including its charged surface) has been described by Man ko et al. [137,138]. An ab initio Hartree-Fock-Roothaan method has been employed. The electrode was modeled as a planar seven-atomic Hg-7 cluster. The deepest minimum of the total energy of the adsorption system was found for positive charge density and Py interacting with the metal through the lone electron... 

Evaluations of an integrated adsorption system were also conducted. In this system, by varying the pH conditions, the dissolved organics (model compounds) are separated into fractions by isolation onto Amberlite XAD-8, AG MP-50 cation-exchange resin, and graphitized carbon black. The procedure is based on the separation of organic solutes into hydrophobic and hydrophilic neutral, acidic, and basic fractions. 

Stewart and Hack (5.) have presented operating characteristics of pressure swing adsorption systems for reducing impurities in a hydrogen stream from 40 vol percent to 1 ppm. Impurities included ammonia, water, methane, carbon monoxide, carbon dioxide, nitrogen, and several hydrocarbons. In this study heatless adsorption is used to separate hydrogen sulfide-hydrogen mixtures and the experimental results are compared with theoretical models. 

Ayoob, S., Gupta, A.K. and Bhakat, P.B. (2007) Analysis of breakthrough developments and modeling of fixed bed adsorption system for As(V) removal from water by modified calcined bauxite (MCB). Separation and Purification Technology, 52(3), 430-38. 

A final area of difficulty is in the application of data analysis to specific models of adsorption isotherms. This difficulty results from the fact that different models for adsorption isotherms generate plots of surface versus dissolved concentration that have characteristic shapes. If a plot of observational data results in a curve with a shape similar to that generated by a model, this result is often taken as proof that the particular model applies. Unfortunately, this assumption has been made for situations where many of the basic requirements of the model are violated in the system under study. The Langmuir adsorption isotherm model has suffered considerable abuse by geochemists in this regard. It should be remembered that "shapes" of adsorption isotherms are far from proof that a specific model applies. 

The polarization of the organic molecule is more significant in the case of adsorption on the octahedral side (see Figs. 13.5 and 13.6). The adsorption energies obtained at the ONIOM(B3LYP/6-31G(d,p) PM3) level of theory and using large models of the mineral for the adsorption systems of GB and GD on the octahedral surface of dickite, are about 16 and 15 keal/mol, respectively. In the case of... 

The strategy used to reach an accurate interpretation of the STM experimental results is to, whenever possible, perform DFT calculations on appropriate model adsorption systems and then to use the optimized DFT structures as input for subsequent STM image simulations [1]. We now briefly describe the details of both of these types of calculation. 

The number of transitions or mass transfer zones provides a direct measure of the system complexity and therefore of the ease or difficulty with which the behavior can be modeled mathematically. It is therefore convenient to classify adsorption systems in the manner indicated in Section V.B. It is generally possible to develop full dynamic models only for the simpler classes of systems, involving one, two, or at the most three transitions. 

Many adsorption systems can be shown to correspond either to a localized adsorption model or to a two-dimensional gas. However, there must also be a large number of adsorbate-adsorbent pairs for which the potential energy functions will have variations intermediate between the extremes required for the limiting models. The formal equations applicable to these systems are included in the theory presented here, and furthermore, it appears to be feasible to carry out specific computations of the isotherms and heats for such systems up to moderate coverages (11, 24). 

Because the CO/nickel system was one of the first investigated and because the adsorbate resonances are energetically separated from the metal d-band, several groups have developed models for the bonding mechanism based on the assumption that the adsorbate spectral function is revealed directly by the photoelectron spectrum. This has led to a proposed re-assignment in the CO/nickel spectra, and indeed in other spectra of CO adsorption systems, as we see in the following. 

With the aid of fast computers it is now possible to predict the adsorption and pore filling of simple molecules by model adsorbents (Cracknell et al., 1995 Nicholson, 1996 Gubbins, 1997 Steele and Bqjan, 1997 Nicholson and Pellenq, 1998 Steele and Bojan, 1998 Ravikovitch, Haller and Neimark, 1998). The various computational procedures that have been developed in recent years are based on the statistical mechanics of confined fluids. Before any attempt can be made to solve the equations defining the locations, configurations and movement of individual molecules, it is necessary to specify the exact nature of the adsorption system. 

The integral molar quantities are of importance for modelling adsorption systems or in the statistical mechanical treatment of physisorption. For example, they are required for comparing the properties of the adsorbed phase with those of the bulk... 

The retention model by Cecchi and co-workers also quantitatively faced the prediction of the retention behavior of neutral and zwitterionic analytes in IPC. According to the electrostatic models, at odds with clear experimental data [1,50,52,53], the retention of a neutral solute is not dependent on the presence and concentration of a charged IPR in a chromatographic system. Equation 3.23 is very comprehensive if Ze is zero [50], it simplifies since ion-pairing does not occur (C2= C3 = 0). Adsorption competition models the retention patterns of neutral analytes in IPC and the slight retention decreases of neutral analytes with increasing HR concentration may be quantitatively explained [50,53]. 

Perfect fit was observed at experimental isotherms of different adsorption systems in large ranges of temperature and pressure [20,21]. Shown in Fig.l and Fig.2 are only examples. The model for absolute adsorption isotherm was obtained basing on the data at relatively low pressure, but the model fits the data at high pressure as well. It is concluded that the adsorption mechanism of supercritical adsorption does not change as pressure increases although maximum or even negative (excess) adsorption was observed. 

Modeling in multicomponent adsorption systems is an extension to that of single component adsorption. Many models have been reported in the literature for the prediction of concentration versus time decay curves in single component batch adsorption stem. However, there are very few research papers on the topic of multicCHnponent mass transport studies for liquid phase adsorption, therefore, it is a valuable contribution and novel development to adsorption research. 

The second-order kinetic model [15] is expressed as t/q=l/k2qe +t/qe (2) where k2 (g/mg/min) is the rate constant of second order adsorption. The calculated q values agree very well with the experimental data. This indicates that the adsorption system belongs to the second order kinetic model. The second order rate constants were in the range 0.61-0.07g/mg/min. Similar phenomena have been observed in the adsorption of Congo red on coir pith carbon [ 16]. 

The adsorption dynamics of binary and ternary hydrogen mixture in activated carbon and zeolite SA bed was studied by experimentally and theoretically through breakthrough and desorption experiments. Energy balance is an essential element for accurate adsorption process modeling in case of bulk separation. Especially in ternary system, sinusoidal... 

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