Multiple adsorbates

Although most appHcations of fixed bed have multiple adsorber beds to treat continuous streams, batch operation using a single adsorber bed is an alternative. For purification appHcations, where one vessel can contain enough adsorbent to provide treatment for days, weeks, or even months, the cost savings and simplicity often justify the inconvenience of stopping adsorption treatment periodically for a short regeneration. 

Typically, adsorption isotherms are generated using a batch experiment at a fixed temperature and a fixed feed composition. These experiments include exposing a known amount of adsorbent to a known concentration of adsorbate at a constant temperature. Once equilibrium is established, the net adsorbate concentration in the liquid is measured. This process is repeated at multiple adsorbate concentrations and temperatures. A plot of adsorbate loading (g adsorbate/g adsorbent) versus adsorbate concentration reveals the adsorption isotherm with the shape of the isotherm determining the suitability of a particular adsorbent for a particular system [20]. 

An atom or molecule that approaches the surface of a solid always experiences a net attractive potential ). As a result there is a finite probability that it is trapped on the surface and the phenomenon that we call adsorption occurs. Under the usual environmental conditions (about one atmosphere and 300 K and in the presence of oxygen, nitrogen, water vapor and assorted hydrocarbons) all solid surfaces are covered with a monolayer of adsorbate and the build-up of multiple adsorbate layers is often detectable. The constant presence of the adsorbate layer influences all the chemical, mechanical and electronic surface properties. Adhesion, lubrication, the onset of chemical corrosion or photoconductivity are just a few of the many macroscopic surface processes that are controlled by the various properties of a monolayer of adsorbates. 

Since the previously stated limitations on vessel size were still in effect, it was decided to pursue a roate using multiple adsorber beds in a highly integrated system. It was necessary to define integrated high-capacity systems with at least 80% product recovery and to devise control schemes for such systems. 

For multiple adsorbing species, this equation becomes... 

A periodic adsorption process is operated as a series of sequential steps on one or more a otbers packed with either single or multiple layers of adsorbent. Although the operation of each bed is batch-wise, the whole system is continuous owing to the use of multiple adsorbers operated in phased cycles. The traditional approach for CSS determination is to execute a dynamic simulation of the process, starting with a specified set of initial conditions and then simulated over a large number of cycles until a CSS condition is eventually obtained with a pre-defined criteria, i.e., the cycle initial state at to must be identical with the cycle end state at tn (see Figure 1). 

Multiple Adsorbates Adsorption isotherms are defined for individual adsorbate species. Collective parameters like DOC, phenols, and humic acids can only be used empirically as adsorbate parameters. Adsorption isotherms with collective parameters cannot be used for simple mechanistic interpretation of the data, even if these data can be fitted to such equations (Tomaic and Zutic, 1988). 

Radke and Prausnitz (1972) have provided a theoretical treatment on isotherms of multiple adsorbates. 

Most wastewater contain compounds that may mutually enhance, interfere, or act independently in the adsorption process. Factors affecting overall adsorption of multiple adsorbates include the relative molecular size and configuration, the relative adsorptive affinities, and the relative concentration of the solutes. 

The dynamic tracer pulse technique used in this work facilitates the study of BV and how BV might be altered because of high levels of organic compounds or humidity. Based on competitions of the various, selected probes, cursory information about surface sites can be obtained for a prospective adsorbent such information is especially useful for multifunctional polymers. This technique can also permit the fine tuning of the environmental collection strategy through the examination of retentions on mixed adsorbents or multiple adsorbent beds. The use of dynamic TPC with the polyimides demonstrated that these materials are more sensitive, in terms of BV of the tested probe molecules, to the effects of humidity than is Tenax-GC. 

Examples of studies of specific adsorption are presented in Tables 4.1 (small cations), 4.2 (small anions), 4.4 (organic compounds). 4.5 (surfactants), and 4.6 (polymers). Studies presenting new experimental data are compiled together with discussions, interpretations and model calculations based on experimental results taken from other publications. The tables are sorted primarily by the chemical formula of the adsorbent, according to the rules outlined in the Introduction, and then by the adsorbate. The entries with a single adsorbate are followed by entries with multiple adsorbates (for the same adsorbent) sorted by the name or chemical formula of the first adsorbate, then of the second adsorbate, etc. Each Table is accompanied by an alphabetical index of adsorbates. 

Any condensable, inert gas can be used for BET measurements, but the preferred gases are nitrogen and krypton. Nitrogen is used for most samples that exhibit surface areas of 2 m /g or greater, but materials with smaller surface areas should be measured using krypton. The gas to be adsorbed (the adsorbate) is mixed with an inert, noncondensable, carrier gas (usually helium). A range of 5-30% adsorbate in carrier gas is commonly used, and the use of multiple adsorbate gas levels in a BET determination is recommended. 

Third, diffusion is often an integral part of the dynamics in real systems under non-UHV and even under UHV conditions. These involve nonzero coverages of multiple adsorbates, nonequilibrium surface structures of adsorbates, and other complications. It is clearly important to understand the simplest case of single-atom diffusion on a surface since this will underlie the more complex situations. A number of important conclusions have been learned from studies of atomic diffusion (1) lattice motion substantially increases the quantum mechanical diffusion constant as shown in Fig. 44 by... 

Adsorption isotherms are generally presented for a single component, but many applications involve multicomponent mixtures. The Langmuir isotherm is easily modified for multiple adsorbates by adding terms to the denominator ... 

Constant in equation for adsorption isotherm iCj, K2, for multiple adsorbates... 

The behaviour of a multi-component gas mixture in a microporous adsorbent is important both theoretically and practically in separation and catalytic processes. Detailed theoretical analysis of the frequency response for the diffusion of multiple adsorbates in microporous material crystals have been... 

A preliminary analysis (see Figure 6.21) leads to the observation that there is large water desorption at 180 and 470°C. The two peaks probably express a dehydration followed by a dehydroxylation of the oxide. The m/e = 48 ratio value corresponds to the carbon dioxide, which is desoibed at 180, 410 and 610°C. Desorption of oxygen leads to a much more intense and complex spectrum. It reaches its maximum at 520°C, and also presents many oscillations that show the presence of multiple adsorbed species with different energies. 

Due to the inefficient packing of the rigid struts, CMPs have an open structure even after removal of all solvents. This open structure leads to high surface areas and pore volumes. Surface areas of CMPs are most often reported as Brunauer-Emmett-Teller (BET) surface areas, which takes into account the adsorption of multiple adsorbate layers. 

O Shea, J.D., and Liapis, A.I., Evaluation of simple and complex models for mass transfer in the non-isothermal gas adsorption of multiple adsorbates in a single adsorbent particle, Chem. Eng. Res. Des., 68(3), 242-250... 

To achieve a higher purification effect, a column can use multiple adsorbent materials placed as individual layers. In the case of a single adsorbent, the relative strength of adsorption of gases is listed in Table 9.9. 

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