Desorption models

Jain, A.K., Hudgins, R.R. and Silveston, P.L., "Adsorption/ Desorption Models How Useful to Predict Catalyst Behavior under Transient Conditions", paper submitted to Seventh North American Meeting, The Catalysis Society, Boston, 1981. 

In the previous sections, the use of surfactants to increase the rate of desorption of hydrophobic organic contaminants was discussed. For the current study, several different surfactants were tested to determine whether the rate of TCE desorption from a peat soil could be increased. The effects of the surfactants on the rate of TCE desorption was tested using a continuous-flow stirred-tank reactor (CFSTR) methodology. The observed data were simulated using a distributed-rate kinetic desorption model. The parameters determined from the model simulation were then use to discern the effects of the surfactants on the rate of TCE desorption from the peat soil. The experimental methodology and the modeling procedure are now described in detail. 

Selim, H, M., Davidson, J. M., and Mansell, R. S. (1976a). Evaluation of a two-site adsorption-desorption model for describing solute transport in soils. Proc. Summer Comput. Simul. Conf. pp. 444-448. 

Geochemical models of sorption and desorption must be developed from this work and incorporated into transport models that predict radionuclide migration. A frequently used, simple sorption (or desorption) model is the empirical distribution coefficient, Kj. This quantity is simply the equilibrium concentration of sorbed radionuclide divided by the equilibrium concentration of radionuclide in solution. Values of Kd can be used to calculate a retardation factor, R, which is used in solute transport equations to predict radionuclide migration in groundwater. The calculations assume instantaneous sorption, a linear sorption isotherm, and single-valued adsorption-desorption isotherms. These assumptions have been shown to be erroneous for solute sorption in several groundwater-soil systems (1-2). A more accurate description of radionuclide sorption is an isothermal equation such as the Freundlich equation ... 

To design large scale supercritical desorption processes is necessary to understand in which way dynamic desorption is influenced by process variables as mass transfer effects and equilibrium considerations. The governing equilibrium in all desorption processes is the adsorption equilibrium and a description of this equilibrium is essential in all desorption models and design equations [3]. 

Later on, Iribarne and Thomson proposed a different mechanism for the production of gas-phase ions from charged droplets [27,28]. Interestingly, the motivation for their studies was far removed from a concern for the needs of mass spectrometry. Instead, it stemmed from their interest in charged droplets as a possible source of ions in the atmosphere. They proposed a model for such ion formation based on the idea that, on charged droplets that were small enough, evaporation could make the surface field sufficiently intense to lift solute ions from the droplet into the ambient gas before the Rayleigh limit is reached. This model is nowadays usually referred to as ion desorption model (IDM). 

Kinetic regime. The first mechanism corresponds to the so-called kinetic regime (Popel 1994, Grigorenko et al. 1998) which is similar to the adsorption/desorption model (Blake 1993). Contrary to macroscopic hydrodynamic models, the adsorption/desorption model is based on the hypothesis that the motion of the triple line is ultimately determined by the statistical kinetics of atomic or molecular events occurring within the three-phase zone (Samsonov and Muravyev 1998). Such processes may be limiting at the very early stages of spreading of low... 

The texture and morphology of the synthesized objects were studied by the set of the instrumental methods XRD, FT-IR, SEM, TEM, BET, N2 adsorption-desorption. Model reaction of thiophene hydrodesulfurization (HDS) was used to investigate the chemical mobility of Ni-containing particles obtained by different ways. 

Organic inhibitors in the nickel bath also influence the texture of nickel deposits. The inhibition effects are related to their molecular structure [6.69]. In the presence of brightners with unsaturated ethylenic or acetylenic compounds, the [110] texture is preferentially formed. With aryl-sulfonic compounds used as leveling agents, the [100] or [211] textures are favored. The modification of the crystal growth has been interpreted by an adsorption-hydrogenation-desorption model. The nature and the strength of a bond between a metallic surface and an adsorbed species depend on the... 

The isothermal and non-isothermal effectiveness factors for the single unimolecular irreversible reaction. Equilibrium adsorption-desorption model with linear isotherm For simplicity of presentation and without any loss of generality we consider here the case where the bulk temperature and concentration are taken as the reference temperature and concentration. In this case the boundary conditions (5.127) become at fu= 1.0... 

The adsorption-surface reaction-desorption model used consists of the following 13 steps ... 

From the above equations, models of different degrees of sophistication can be obtained. The simplest physically realistic one is that based on the assumption of equilibrium adsorption-desorption with linear Langmuir adsorption isotherm, which will be presented first, followed by the general non-equilibrium adsorption-desorption model. 

The equations can be put in a dimensionless form in a manner similar to the case of the equilibrium adsorption-desorption model, but using more parameters. 

Qualitative and quantitative models of adsorption kinetics of surfactants and polymers are described in this chapter. A comprehensive presentation of the most developed physical model, the difRision-controlled adsorption and the desorption model, is given and different methods of solving the resulting differential equations are discussed (Miller Kretzschmar 1991). A direct numerical integration enables us to consider any type of adsorption isotherm relating the surfactant bulk concentration with the adsorbed amount at the interface. 

In the following sections, we will first consider metal—silicon and metal/III—V compound systems, covering electronic and chemical interface effects. We will then present a general treatment of the adsorption-desorption models which have been proposed, including some discussion of the interpretation of thermal desorption spectra, and finally we will discuss to what extent the two sets of data can be reconciled. We will not be concerned with metal films in device fabrication, nor the formation of additional phases by heat treatment. We shall only deal with interactions and interface chemistry which is directly relevant to adsorption-desorption behaviour. 

In the framework of the Langmuir adsorption-desorption model, all O atoms approaching the fraction 1 - 0 of the caibon surface not covered by C 0 complexes immediately react, forming the complexes. The kinetic equation for the fraction 6 can be written as (Lieberman Lichtenberg, 1994)... 

As a final example in this chapter, a study involving the sorption—desorption modeling of uranium (VI) onto lanthanum monophosphate, LaP04 [29], can be... 

The overall equilibrium constant is 0.589. The feed consisted of the azeotropic mixture ethanol-water, containing 13.5 mole % water. Water is not adsorbed on the catalyst Estimate the parameters of the adsorption, surface reaction, and desorption models, using conversion as the regression variable. Comment on the feasibility for the estimation of the parameters. Which model is the best On what basis ... 

Although the kinetic data derived in these experiments can be well represented by a singular second order desorption model, closer inspection of the data presents evidence for three different, and successive second order... 

A adsorption-desorption model was proposed to explain pulse duration behavior for protein molecules, and the statistical analysis of Ai and t for the preceding three protein molecules showed they were distinguished. Further, tiny vibrations around the average steady-state current value revealed that the protein molecules engage in repeated adsorption/desorption events to/from the nanotube wall as they translocate through the detection zone. Additionally, it is believed that the rate of physical adsorption/desorption events for the smaller protein molecules to the membrane was larger than for the larger protein molecules. 

Figure 2.21. Desorption of ions from charged droplets into the gas phase (a) charge-residue model (b) ion-desorption model (From ref. 81.)...

The kinetic constants ki and k2 are defined according to ki = kj 0g. Hence also and Q2 pressure dependent. The ciarves in Fig.4 are simulated with this desorption model, the agreement with experimental points is quite satisfactory. 

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