Adsorption controls

Adsorption-Control Equipment If a gas stream must be treated for a short period, nsnally only one adsorption unit is necessary, provided, of course, that a sufficient time interval is available between adsorption cycles to permit regeneration. However, this is usually not the case. Since an nninternipted flow of treated gas is often required, it is necessary to employ one or more units capable of operating in this fashion. The units are designed to handle gas flows without interruption and are charac terized by their mode of contact, either staged or continuous. By far the most common type of adsorption system used to remove an objectionable pollutant from a gas stream consists of a number of fixed-bed units operating in such a sequence that the gas flow remains nninternipted. A two- or three-bed system is nsn ly... 

Wayner et al. (1976) developed a simple procedure to obtain the heat transfer coefficient for the interline region of an adsorption controlled wetting film. Xu and Carey (1990) developed an analytical model to predict the heat transfer characteristics of film evaporating on a microgroove surface. 

Since the recombination step (c) does not principally differ from a recombination of two H or D atoms to the respective hcmonuclear imole-cule there is no reason to assume a special activation barrier for a H and a D atom to recombine to the HD molecule. Therefore the rate of the HD production is solely determined by the rates of adsorption of H and D, respectively (as long as the reaction is adsorption-controlled, i.e., at hi enou tenperatures), or by the rate of desorption of HD (provided the reaction is desorpticai-oontrolled, i.e., at low temperatures). If wie deal with the first case only we may w/rite ... 

Fig. 7 Deca-BDE amount in the abiotic and adsorption control for an initial concentration of 2 mg L-1 (left) and 10 mg L-1 (right)...

Sol id Sol utions. The aqueous concentrations of trace elements in natural waters are frequently much lower than would be expected on the basis of equilibrium solubility calculations or of supply to the water from various sources. It is often assumed that adsorption of the element on mineral surfaces is the cause for the depleted aqueous concentration of the trace element (97). However, Sposito (Chapter 11) shows that the methods commonly used to distinguish between solubility or adsorption controls are conceptually flawed. One of the important problems illustrated in Chapter 11 is the evaluation of the state of saturation of natural waters with respect to solid phases. Generally, the conclusion that a trace element is undersaturated is based on a comparison of ion activity products with known pure solid phases that contain the trace element. If a solid phase is pure, then its activity is equal to one by thermodynamic convention. However, when a trace cation is coprecipitated with another cation, the activity of the solid phase end member containing the trace cation in the coprecipitate wil 1 be less than one. If the aqueous phase is at equil ibrium with the coprecipitate, then the ion activity product wi 1 1 be 1 ess than the sol ubi 1 ity constant of the pure sol id phase containing the trace element. This condition could then lead to the conclusion that a natural water was undersaturated with respect to the pure solid phase and that the aqueous concentration of the trace cation was controlled by adsorption on mineral surfaces. While this might be true, Sposito points out that the ion activity product comparison with the solubility product does not provide any conclusive evidence as to whether an adsorption or coprecipitation process controls the aqueous concentration. 

Often it is advisable to utilize the slopes and intercepts of such plots as well as the above described curvature of the data. Kabel and Johanson (Kl), for example, could eliminate an adsorption-controlling model with only 70 %... 

Adsorption controlled with dissociation k(x2 - Xi/K) [l+KiXi + (K2IKx3) +K3x3y... 

Adsorption mechanisms represent probably the most important interaction phenomena exerted by solid surfaces on the environmental fate of organic pollutants [65, 127-130]. Adsorption controls the quantity of free organic components in solution and thus determines their persistence, mobility, and bioavailability. The extent of adsorption depends on the amount and properties of both solid phase-humic substances (SPHS) and organic pollutants. Once adsorbed on an SPHs >an organic pollutant may be easily desorbed, desorbed with difficulty, or not at all. Thus sorption phenomena may vary from complete reversibility to total irreversibility. 

In weak-strong separations, one component does not strongly adsorb but has a high diffusivity the other component is slow but strongly adsorbs. In this case, the separation mechanism exhibited is either diffusion-controlled permeation or adsorption-controlled permeation, depending on the operating conditions [39]. 

From this point of view, a brief comparison of acyloxylation of cis- or irany-stilbenes in electrochemical and chemical conditions is also relevant. Oxidation of cis- or irany-stilbene at the platinum anode in the presence of acetic or benzoic acid gives predominantly meyo-diacylates of hydroxy-benzoin or, if some water is present, t/treo-monoacylate. None of the stereoisomeric erythro-mono-acylate and rac-diacylate were obtained in either case. There was no evidence of isomerization of cis- to trany-stilbene nnder the electrolytic conditions employed (Mango and Bonner 1964, Koyama et al. 1969). The sequence of reaction steps in Scheme 2.27 was proposed. Adsorption-controlled one-electron oxidation of the snbstrate takes place. Then the cis-stilbene cation-radical interact with acetate to form an oxonium ion. The phenyl groups in the oxoninm adopt the trans mntnal disposition which is thermodynamically preferential. The trany-benzoxoninm ion is the common intermediate for conversions of both cis- and trany-stilbenes and, of conrse, for all the final prodncts (Scheme... 

Mensah, P. Gainer, J. L. Carta, G. Adsorptive Control of Water in Esterification with Immobilized Enzymes I. Batch Reactor Behavior. Biotechnol. Bioeng. 1998, 60 (4), 434-444. 

When the adsorption rate is slowest in all steps (2nd step), the reaction is under the adsorption-controlled condition. That is, the total reaction rate is equal to the adsorption rate as shown below ... 

Hall SB, Khudaish EA, Hart AL. Electrochemical oxidation of hydrogen peroxide at platinum electrodes. Part I. An adsorption-controlled mechanism. Electrochimica Acta 1998, 43, 579-588. 

Dependence of peak currents (Jp) on the scan rate (v) enables distinguishing between diffusion currents (Ip v1/2), adsorption controlled currents (Ip v), and kinetic currents (Ip 4= /(v)). 

The numerous reasons which can account for various deviations from the ideal FFF retention theory were discussed in the corresponding sections. Here, additional problems are treated which can complicate FFF measurements and significantly distort the results obtained. General requirements for a successful FFF measurement include precise flow control and flow rate precise temperature measurement precise determination of t0 and tr correct relaxation procedure control of sample overloading and integrity and control of mixed normal and steric retention effects as well as wall adsorption control. Some of these complications cannot be avoided so one must correct for these effects, usually in a sem-iempirical and partially very complicated fashion. 

In an effort to develop a rapid and efficient scheme for immobilizing nucleic acids onto carbon screen-printed transducers, many authors took advantage of the strong adsorptive accumulation of these biomolecules at the screen-printed carbon surfaces. In particular, many of them reported that electrochemical adsorption (adsorption controlled by a positive potential) enhances the stability of the probe and this technique was preferentially chosen. 

The development of assay techniques that have convenience of solid-phase hybridization and are rapid and sensitive will have a significant impact on diagnostics and genomics [3]. In this respect, SPE genosensors have several advantages they are safe because they are disposable, they are reproducible, they are inexpensive, and the overall procedure is quite fast. In this respect, electrochemical adsorption (adsorption controlled by a positive potential) is an easy to perform and rapid way of immobilization. The method does not require special reagents or nucleic acid modifications. 

Rate expressions of this form were derived for calcite precipitation with = 1 (Nancollas and Reddy, 1971 Reddy and Nancollas, 1971), and with mj = 0 and = 0.5 (Sjoberg, 1976 Kazmierczak et al, 1982 Rickard and Sjoberg, 1983 Sjoberg and Rickard, 1983). Rate equations such as (57) wherein rates are linearly proportional to AG close to equilibrium have been attributed to adsorption-controlled growth (Nielsen, 1983 Shiraki and Brantley, 1995). Such rate models have been used by some researchers to model dissolution and precipitation of quartz over a wide range in temperature and pressure (Rimstidt and Barnes, 1980) however, it has been pointed out that this has only been confirmed with experiments at high temperature (Dove, 1995). 

Another example where molecular sieving and preferential adsorption control the separation is the separation of CO2/CH4 in CHA and DDR zeolite membranes [43,81]. The separation of CO2 from natural gas is important during its transport and combustion because of the acidic and corrosive character of CO2 in the presence of humidity and moreover, it reduces the methane number (similar to the octane number in gasoline) which is important for getting high efficiency during combustion. 

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