Processing Desorption

Of course, Eq. 11.73 also implies that the reverse process (desorption) is generally endothermic by roughly Q + 5NkBT/2. 

These observations lead one to believe that all graphite oxidation reactions at temperatures above 1400° C. are controlled by an identical process—desorption of surface oxides. This should apply at least for C + C02 and C -f H20. 

The same procedures and calculations, described in the previous section on pyridine desorption, can be applied to the natural desorption of water from silica as a function of temperature.32 The thermogram of silica (figure 5.19) shows two distincts areas. Water desorption in the region (298-423 K) is due to a dehydration process, desorption in the region (423-1173 K) is due to a dehydroxylation process. 

The process by which certain porous solids bind large numbers of molecules to their surfaces is known as adsorption. Not only does it serve as a separation process, but it is also a vital part of catalytic-reactionprocesses. As a separation process, adsorptionis used most oftenfor removal of lo w-concentrationimpurities and pollutantsfrom fluid streams. It is also the basis for cliromatography. In surface-catalyzedreactions, the initial step is adsorptionof reactant species the final step is the reverse process, desorption of product species. Since most industrially important reactions are catalytic, adsorption plays a fundamental role in reaction engineering. 

Hemoproteins are a broad class of redox-proteins that act as cofactors, e.g. cytochrome c, or as biocatalysts, e.g. peroxidases. Direct ET between peroxidases such as horseradish peroxidase, lactoperoxidase," or chloropcroxidasc"" and electrode surfaces, mainly carbonaceous materials, were extensively studied. The mechanistic aspects related with the immobilized peroxidases on electrode surfaces and their utilization in developing biosensor devices were reviewed in detail. The direct electrical contact of peroxidases with electrodes was attributed to the location of the heme site at the exterior of the protein that yields close contact with the electrode surface even though the biocatalyst is randomly deposited on the electrode. For example, it was reported " that non-oriented randomly deposited horseradish peroxidase on a graphite electrode resulted in 40-50% of the adsorbed biocatalyst in an electrically contacted configuration. For other hemoproteins such as cytochrome c it was found that the surface modification of the electrodes with promoter units such as pyridine units induced the binding of the hemoproteins in an orientation that facilitated direct electron transfer. By this method, the promoter sites induce a binding-ET process-desorption mechanism at the modified electrode. Alternatively, the site-specific covalent attachment of hemoproteins such as cytochrome c resulted in the orientation of the protein on the electrode surfaces and direct ET communication. ... 

Adsorption is very important for the biological properties of the chemicals. Many soil pesticides may be applied in higher quantities when the soil has strong adsorption properties. Adsorption inactivates and makes toxicants less harmful and reduces leakage, but on the other hand, it can make the pesticides more recalcitrant to microbial degradation. The adsorption process is quite fast, and often less than an hour is needed to produce equilibrium. The opposite process, desorption, takes longer and sometimes a low residue is bound irreversibly. 

A second important surfactant characteristic is its adsorption onto the pore walls of reservoir rock. Perhaps as a consequence of the size of the surfactant molecules, adsorption equilibrium is not immediate but requires appreciable time. The reverse process, desorption into a lower-concentration solution, is even slower. Given a long enough period of contact, surfactant molecules can be expected to adsorb onto, and desorb from, the internal walls of a porous rock according to an equilibrium isotherm resembling the Langmuir curve. However, in many laboratory experiments and at the displacement front in the field such equilibrium may not be attained. 

Mubeena A. et al. (2007) Moringa Oleifera pods Organic aromatic compounds Lower than Ih 50-100 mmol.g ion-exchange mechanism, pseudo-first kinetic model, exothermic and spontaneous process, desorption by methanol wash... 

The regeneration step is the key to the implementation of the anion exchange system on the commercial scale. Desorption studies help to evaluate the nature of adsorption process. Desorption experiments were performed using different regenerating agents such as 1M NaCl, 1 M Na2S04,1 M N32C03,1 M NaOH, 1 M FECI and even 1 M KSCN. As previously stated [2, 15,20,23,25-31], the aqueous solutions mentioned above were ineffective for the dyes removal from the resin phase. 

Following the scrubbing of the inert carrier gas the absorbent and absorbate are collected as a solution at the base of the column. A secondary process, desorption, is then required to achieve separation and liberate the absorbate. This separation is usually achieved by atmospheric pressure or vacuum distillation. Therefore the choice of absorbent is critical to ensure the materials have significant differences in boiling point and do not form an azeotrope. 

Absorption is favored at raised pressure and low temperature. Therefore, the reverse process, desorption, favors low pressure and high temperature. With desorption the absorbed component is removed from the absorbent the solvent is degassed and regenerated before reuse. On the whole, desorption is carried out in four ways, which can either be applied individually or in combination ... 

After this stage, injection was switched back from dyed brine to brine and the brine/oil flow rate ratio was increased to determine the next point on the relative permeability curve. In this process desorption of the dye took place. The amount of dye desorbed was determined colorimetrically in the effluent brine. 

REVERSIBILITY OF THE PROTEIN ADSORPTION PROCESS DESORPTION AND EXCHANGE... 

Figure 15. Variation of the chemical composition of well crystalline LADH-Cl during desorption (d) and sorption (s) processes. Desorption conditions T-348K t-0.5h liq/sol. ratio-10 [LiCl]-0g/l (1), 0.25g/l (2). Sorption conditions T-363K t-lh [LiCl)-60g/l.

Sorbent material Chemical elements Surface area (BET, mVg) Porosity , Pore Volume (Vp / cm / g) Main use Technical Processes Desorption Enthalpies (kj/mol)... 

Adsorption kinetics on activated carbon, as well as on the surface of any other adsorbent, is controlled by the capture velocity of substance by adsorbent surface and by the velocity of the reverse process—desorption. The value is proportional to the number of collisions between molecules of adsorbate Vj with a unit of adsorbent surface, the free area fraction of this surface (1 - 0), and the portion of molecules a, which are able to be adsorbed on this surface. The value Vj is proportional to the number of molecules V2, which are desorbed from adsorptive layer with filling degree 0. As a result, the rate of adsorption is given by... 

TG-FTIR has become quite a popular, versatile, cost-effective and informative instrument for modern polymer analysts concerned with thermal decompositions, oxidation processes, desorption behaviour, effectiveness of additives, aging processes, characterisation of raw materials and detection of residues. The growth rate of TG-FTIR instrumentation currently exceeds that of TG-MS. 

The Gibbs equations derived for fiee, S/L, and S/G interfaces provide a uniform picture of physical adsorption however, they eannot give information on the structure of energy [i.e., we do not know how many and what kind of physieal parameters or quantities influenee the energy (heat) processes connected with the adsorption]. As it is well known these heat proeesses ean be exactly measured in a thermostat of approximately infinite eapacity. This thermostat eontains the adsorbate and the adsorptive, both in a state of equilibrium. We take only the isotherm proeesses into account [i.e., those in whieh the heat released during the adsorption process is absorbed by the thermostat at eonstant temperature dT = 0) or, by eonverse processes (desorption), the heat is transferred fi om the thermostat to the adsorbate, also at eonstant temperature]. Under these conditions, let <7n -mol adsorptive be adsorbed by the adsorbent and, during this process, an... 

Desorption Ifom a homogeneous sirrface is characterized by the existence of a characteristic time T such that the degree of advancement of the process ( desorption kinetics ) is negligible for tjx 0.3, is nearly completed for tjx 0.3, and occurs significantly for tjx 1. If desorption is a thermally activated process, the characteristic time x depends on the activation energy for desorption E as... 

If adsorption is a reversible process (i.e. backward process—desorption, passes through exactly the same states), the rates of both processes can be described using the same equation ... 

However, in contrast to adsorption which may or may not be activated process, desorption is always activated, with a minimum activation energy denoted as activation energy for desorption (AE ). The rate constant for desorption can be expressed by Arrhenius equation ... 

Temperature-swing process Desorption Reactivation Steam desorption or inert gas desorption at tenqier-atures of < 500 C Partial gasification at 800 to 900 C with steam or other suitable oxidants Solvent recovery, process waste gas cleaniq) AU or nic confounds adsorbed in gas cleaning applications Reprocessing of desorbate Post-combustion, if required scrubbing of flue gas generated... 

The Amazon and Fly River (Papua New Guinea) estuaries serve as good examples of the five features outlined above. The salinity distribution of dissolved Nd (<0.22 pm) in fig. 12 is representative of the other bivalent lanthanides. This figure contains data for both surface and deep waters the bottom part B is an expanded scale for S >3 samples. The Fly and Amazon data sets have Nd-salinity distributions for surface waters which are remarkably similar and exhibit the same two major features (1) removal of dissolved lanthanides in the low (0-5) salinity region and (2) desorption in the seaward region. Hence, it appears that the removal process and the release process (desorption) are decoupled. 

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