Adsorption dynamics

It is a mass transfer between a mobile, solid, or liquid phase, and the adsorption bed packed in a reactor. To carry out adsorption, a reactor, where a dynamic adsorption process will occur, is packed with an adsorbent [2], The adsorbents normally used for these applications are active carbons, zeolites and related materials, silica, mesoporous molecular sieves, alumina, titanium dioxide, magnesium oxide, clays, and pillared clays. 

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Most dynamic adsorption data are obtained in the form of outlet concentrations as a function of time as shown in Figure 18a. The area iebai measures the removal of the adsorbate, as would the stoichiometric area idcai, and is used to calculate equiUbrium loading. For constant pattern adsorption, the breakthrough time and the stoichiometric time ( g), are used to calculate LUB as (1 — (107). This LUB concept is commonly used... 

Because of this heat generation, when adsorption takes place in a fixed bed with a gas phase flowing through the bed, the adsorption becomes a non-isothermal, non-adiabatic, non-equilibrium time and position dependent process. The following set of equations defines the mass and energy balances for this dynamic adsorption system [30,31] ... 

The adsorption of hydrocarbons by activated carbon is characterized by the development of adsorption isotherms, adsorption mass and energy balances, and dynamic adsorption zone flow through a fixed bed. 

Adsorption for gas purification comes under the category of dynamic adsorption. Where a high separation efficiency is required, the adsorption would be stopped when the breakthrough point is reached. The relationship between adsorbate concentration in the gas stream and the solid may be determined experimentally and plotted in the form of isotherms. These are usually determined under static equilibrium conditions but dynamic adsorption conditions operating in gas purification bear little relationship to these results. Isotherms indicate the affinity of the adsorbent for the adsorbate but do not relate the contact time or the amount of adsorbent required to reduce the adsorbate from one concentration to another. Factors which influence the service time of an adsorbent bed include the grain size of the adsorbent depth of adsorbent bed gas velocity temperature of gas and adsorbent pressure of the gas stream concentration of the adsorbates concentration of other gas constituents which may be adsorbed at the same time moisture content of the gas and adsorbent concentration of substances which may polymerize or react with the adsorbent adsorptive capacity of the adsorbent for the adsorbate over the concentration range applicable over the filter or carbon bed efficiency of adsorbate removal required. 

In a GAC column, dynamic adsorption occurs along an adsorption wave front where the impurity concentration changes. 

T. Austad, S. Ekrann, I. Fjelde, and K. Taugbol. Chemical flooding of oil reservoirs Pt 9 Dynamic adsorption of surfactant onto sandstone cores from injection water with and without polymer present. Colloids Surfaces, Sect A, 127(l-3) 69-82, 1997. 

Surfactant Transport in Porous Media Dynamic Adsorption/Desorption Equilibria... 

Also, other dependent variables associated with CO2-foam mobility measurements, such as surfactant concentrations and C02 foam fractions have been investigated as well. The surfactants incorporated in this experiment were carefully chosen from the information obtained during the surfactant screening test which was developed in the laboratory. In addition to the mobility measurements, the dynamic adsorption experiment was performed with Baker dolomite. The amount of surfactant adsorbed per gram of rock and the chromatographic time delay factor were studied as a function of surfactant concentration at different flow rates. 

Results and Discussion on Dynamic Adsorption Measurements. Baker dolomite was used to study the dynamic adsorption experiment. The computed porosity of the rock was 24%. One concentration below the CMC of AEGS, one at CMC, and two concentrations above CMC were chosen to measure the adsorption of this surfactant with Baker dolomite. The mass of surfactant adsorbed per gram of rock is plotted as a function of flow rate in a semi-log plot in Figure 9. 

The slopes of the peaks in the dynamic adsorption experiment is influenced by dispersion. The 1% acidified brine and the surfactant (dissolved in that brine) are miscible. Use of a core sample that is much longer than its diameter is intended to minimize the relative length of the transition zone produced by dispersion because excessive dispersion would make it more difficult to measure peak parameters accurately. Also, the underlying assumption of a simple theory is that adsorption occurs instantly on contact with the rock. The fraction that is classified as "permanent" in the above calculation depends on the flow rate of the experiment. It is the fraction that is not desorbed in the time available. The rest of the adsorption occurs reversibly and equilibrium is effectively maintained with the surfactant in the solution which is in contact with the pore walls. The inlet flow rate is the same as the outlet rate, since the brine and the surfactant are incompressible. Therefore, it can be clearly seen that the dynamic adsorption depends on the concentration, the flow rate, and the rock. The two parameters... 

Trogus, F., Sophany, T., Schechter, R.S., Wade, W.H. "Static and Dynamic Adsorption of Anionic and Nontonic Surfactants," SPE paper 6004, 1976 SPE Annual Technical Conference and Exhibition, New Orleans, October 3-6. 

Type Dynamic Adsorption Coefficient (am STP Air/g) Temperature (°C) Reference... 

The dynamic adsorption coefficient can be used to gauge the performance of a charcoal bed under various conditions. 

Carbon type Given a fixed set of conditions (ie. flow rate, temperature, mass of charcoal, humidity) the type of carbon demonstrating the highest dynamic adsorption coefficient will be identified. 

Flow rate The system must be capable of processing a moderate volume of air per unit time to be of practical use. However, the empirical formulations for the dynamic adsorption coefficient described in this paper are valid only for a certain range of conditions. Experiments will be performed to identify the flow rate/flow channel diameter combination beyond which the formulations are no longer valid. 

Contaminants Indoor contaminants are expected to compete for adsorption sites on the charcoal. We will experimentally find the effect that these contaminants have on the dynamic adsorption coefficient and on the life-time of the charcoal bed. Since the number of radon atoms in even the most seriously contaminated houses is very small, decay product buildup is not expected to pose a significant problem. 

The feasibility of building a radon removal apparatus will depend on finding the parameters which will give the highest dynamic adsorption coefficient under operational conditions. However, it is recognized that real world constraints like initial costs, operating costs, and physical size may force the ideal parameters to be compromised. Devices of this type are intended for use in private homes where these constraints cannot be ignored. 

Adams, R.E., W.E. Browning, Jr., and R.D. Ackley, Containment of Radioactive Fission Gases by Dynamic Adsorption, Industrial and Engineering Chemistry, 51 1467-1470 (1959). 

Strong, K.P. and D.M. Levins, Dynamic Adsorption of Radon on Activated Carbon, Proceedings of the 15th DOE Nuclear Air Cleaning Conference, CONF-780819,627-639 (1978). 

Third, we assume the adsorption process is dynamic. Adsorption and desorption occur to equal and opposite extents at equilibrium, provided the mass of material adsorbed remains constant. 

Very recently, experiments using new techniques have been performed by Lodewyckx et al. [4], X-ray microtomography coupled with image analysis allows visualising dynamic adsorption of organic vapour and water vapour on activated carbon. Figure 17.3 in [4] shows profiles inside the bed at different times. It is remarkable that the fronts seem to be of constant pattern shape. 

Considering first adsorption of metal ions or neutral species directly on the snbstrate, there are a nnmber of possible mechanisms for this process. Most simply, there will an equilibrinm between metal species in solution and a solid snr-face leading to dynamic adsorption of the metal. Adsorption of metal ions onto solid snrfaces has been extensively studied, to a large extent becanse of the nse of oxide snrfaces to adsorb heavy metal ions and remove them from solntion (see Ref. 55 for an example and list of other references on this snbject). This adsorption may go even farther with ion exchange between the solntion metal ions and ions in the substrate (again, glass is a good example of where this may occnr). 

The most important property of adsorbent materials, the property that is decisive for the adsorbent s usage, is the pore structure. The total number of pores, their shape, and size determine the adsorption capacity and even the dynamic adsorption rate of the material. Generally, pores are divided into macro-, rneso- and micropores. According to IUPAC, pores are classified as shown in Table 2.2. 

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