Velocity of adsorption

There are, however, a number of criticisms of these theories. Beidler argued that, as Renqvist had assumed that the magnitude of response is proportional to the amount of stimulant adsorbed per unit time, it is evident that, at equilibrium, the net velocity of adsorption is zero. It would follow that taste intensity should be zero, and the receptors completely adapted. However, Beidler showed that the receptors do not adapt completely, but reach a steady level of response that is consistent for the duration of stimulation. Therefore, he concluded that human taste-adaption is dominated by events in the central nervous system, and not by the peripheral receptor. The same facts also prove Lasarefl s assumption to be incorrect, as his experimental data also depended on a change in adaption that is not seen at the receptor level. 

The Langmuir adsorption isotherm is based on the characteristic assumptions that (a) only monomolecular adsorption takes place, (b) adsorption is localised and (c) the heat of adsorption is independent of surface coverage. A kinetic derivation follows in which the velocities of adsorption and desorption are equated with each other to give an expression representing adsorption equilibrium. 

The velocity of adsorption depends on (a) the rate at which gas molecules collide with the solid surface, which is proportional to the pressure (b) the probability of striking a vacant site (l — VIVm), and (c) an activation term exp [-E/RT], where E is the activation energy for adsorption. 

In the absence of redistribution of molecules on the surface the velocity of the adsorption process is determined solely by the distribution function of activation energies. The velocity of adsorption on sections of the surface characterized by the activation energy E is expressed in terms of the change in the fractional surface coverage 8(E,t) with the time t by... 

In adsorption with redistribution in which there is no functional relationship between E and Q, it is assumed that the velocity of redistribution is larger than the velocity of adsorption. Consequently the various sections of the surface will be occupied in the order of decreasing heats of adsorption and at all times all sections will be involved in the adsorption irrespective of the magnitude of their activation energy. However, due to the exponential dependence of the velocity of adsorption upon the activation energy the character of the adsorption will be determined principally by the processes occurring upon sections with minimal values of E. These sections form on the graph p(E), as shown in Fig. 8, a relatively narrow vertical band the position of which determines the... 

An increase in temperature accelerates the velocity of adsorption by decreasing the viscosity of the liquid, stepping up the diffusion of solute molecules, and hastening the displacement of pre-adsorbed gases from the pores of the carbon. 

Fig. 4 has also been pointed out by Taylor and Liang (8a). Measurement of the velocity of adsorption of a given gas on a clean surface AB at temperature Ti would record the amount of gas adsorbed with time on area A only. At temperature Tt the measurement would record the rate of adsorption on area B, The two velocities would actually have no relation to each other. They should not be used to determine activation energies by means of the equation... 

There is some debate about the affinity of proteins to hydrophobic surfaces. Some authors sustain the hydrophobic affinity theory [36], while others prefer the hydrophilic affinity theory [37]. The results obtained from studies carried out with culture medium supplemented with 10% fetal calf serum (PCS) suggested that the complex mixture of proteins present in PCS presented a higher affinity for the hydrophiUc surfaces. Furthermore, the Wa values suggested that the mixture of proteins adsorbed better on hydrophiUc surfaces, though the type of protein and their adsorption speed onto the surfaces is still unknown. However, the use of dynamic contact angle techniques (as has been confirmed with other materials) could help identify the velocity of adsorption and the munber of steps of adsorption, desorption, and/or ad-sorption/desorption that lead to the final interaction between the substrate and the proteins in the culture medium (Fig. 4). The contact angles obtained for the two different fluids are shown in Table 5. 

From this point of view, we are studying the extraction of uranium from sea water. It is one of the most important subject in the establishment of the extraction process to develop the excellent adsorbent with the selectivity and high velocity of adsorption for uranium. 

In the adsorption experiments, different wettabilities of the resins were observed, the resin with high velocity of adsorption of uranium was wetted easily with sea water. In the adsorption of uranium the wettability of the resin is of great importance. 

Examples of calculation for adiabatic adsorption by means of the above set of equations are given in Fig. 8.8 for adsorption of hydrocarbon gas on activated carbon columns. Effect of heat generation on the shape of adsorption front is clearly shown. By assuming that the equilibrium constant varies with keeping the other parameters constant, changes in thermal waves are also illustrated, i.e. when velocity of adsorption front is slower than that of the thermal wave which is generated at the adsorption front, the thermal wave proceeds in front of the adsorption front while in the opposite case the formation of adsorption front is greatly affected by the temperature increase in the bed. 

Eqs. (I l-S) and (11-6) give the velocities of adsorption front during high pressure adsorption and low pressure desorption, vh and vl denote linear gas velocities at high pressure and low pressure flow. 

The solution to this model for a deep bed indicates an increase in velocity of the fluid-phase concentration wave during breakthrough. This is most dramatic for the rectangular isotherm—the instant the bed becomes saturated, the fluid-phase profile Jumps in velocity from that of the adsorption transition to that of the fluid, and a near shocklike breakthrough curve is obseived [Coppola and LeVan, Chem. Eng. Sci.,36, 967(1981)]. 

The relationship between adsorption capacity and surface area under conditions of optimum pore sizes is concentration dependent. It is very important that any evaluation of adsorption capacity be performed under actual concentration conditions. The dimensions and shape of particles affect both the pressure drop through the adsorbent bed and the rate of diffusion into the particles. Pressure drop is lowest when the adsorbent particles are spherical and uniform in size. External mass transfer increases inversely with d (where, d is particle diameter), and the internal adsorption rate varies inversely with d Pressure drop varies with the Reynolds number, and is roughly proportional to the gas velocity through the bed, and inversely proportional to the particle diameter. Assuming all other parameters being constant, adsorbent beds comprised of small particles tend to provide higher adsorption efficiencies, but at the sacrifice of higher pressure drop. This means that sharper and smaller mass-transfer zones will be achieved. 

Fig. 17.7. Fluidised bed adsorption of G3PDH from milled yeast homogenate onto zirconia-silica Cibacron Blue. The feedstock (20% w/v) was fed to the bead mill at a rate 4.05 dm3-h 1, which corresponded to a linear flow velocity of 250 cm-h 1 within the BRG contactor with a settled bed height of 21 cm. The disrupted baker s yeast homogenate from the bead mill was applied to the integrated fluidised bed directly and terminated when C/Ca = 0.65.

Gas, cells, 464, 477, 511 characteristic equation, 131, 239 constant, 133, 134 density, 133 entropy, 149 equilibrium, 324, 353, 355, 497 free energy, 151 ideal, 135, 139, 145 inert, 326 kinetic theory 515 mixtures, 263, 325 molecular weight, 157 potential, 151 temperature, 140 velocity of sound in, 146 Generalised co-ordinates, 107 Gibbs s adsorption formula, 436 criteria of equilibrium and stability, 93, 101 dissociation formula, 340, 499 Helmholtz equation, 456, 460, 476 Kono-walow rule, 384, 416 model, 240 paradox, 274 phase rule, 169, 388 theorem, 220. Graetz vapour-pressure equation, 191... 

In the fluidized bed process, attrition caused dry sorbent to be carryover. This mainly occurred in the early stage of fluidization and was highly affected by gas velocity. The amount of attrition of molecular sieve 5 A and molecular sieve 13X were larger than those of activated carbon and activated alumina. In addition, percentage losses of adsorption capacities of molecular sieve 5A and molecular 13X were 14.5% and 13.5%, whereas those of activated carbon and activated alumina were 8.3% and 8.1%, respectively. This is because retention time of molecular sieve 5A and molecular 13X decreased due to elutriation of particle generated from attrition. Also, Ka of activated alumina and activated carbon were the lower than those of Molecular sieve 13X and 5A. Consequently, molecular sieve 5A and molecular 13X could cause high maintenance cost for dry sorbent and problems in the operation of fluidized bed process. 

In model equations, Uf denotes the linear velocity in the positive direction of z, z is the distance in flow direction with total length zr, C is concentration of fuel, s represents the void volume per unit volume of canister, and t is time. In addition to that, A, is the overall mass transfer coefficient, a, denotes the interfacial area for mass transfer ifom the fluid to the solid phase, ah denotes the interfacial area for heat transfer, p is density of each phase, Cp is heat capacity for a unit mass, hs is heat transfer coefficient, T is temperature, P is pressure, and AHi represents heat of adsorption. The subscript d refers bulk phase, s is solid phase of adsorbent, i is the component index. The superscript represents the equilibrium concentration. 

Chemically pure semiconducor materials can absorb only those photons, the energy hv of which exceeds the band gap E . Therefore, E. value determines the "red boundary of the light that is used in photocatalytic action of these materials. By way of example. Table 1 presents the values of Eg and the corresponding values of boundary wave length Xg= hc/E (where c is the velocity of light) for some semiconductor and dielectric oxides [2]. However, a semiconductor PC can be sensitized to light with X> by chemical modifications of its surface layer or adsorption of certain molecules on its surface, provided that such treatment creates additional full or empty electron levels in the band gap of the semiconductor material. 

Room temperature CO oxidation has been investigated on a series of Au/metal oxide catalysts at conditions typical of spacecraft atmospheres CO = 50 ppm, COj = 7,000 ppm, H2O = 40% (RH) at 25 C, balance = air, and gas hourly space velocities of 7,000- 60,000 hr . The addition of Au increases the room temperature CO oxidation activity of the metal oxides dramatically. All the Au/metal oxides deactivate during the CO oxidation reaction, especially in the presence of CO in the feed. The stability of the Au/metal oxide catalysts decreases in the following order TiOj > FejO, > NiO > CO3O4. The stability appears to decrease with an increase in the basicity of the metal oxides. In situ FTIR of CO adsorption on Au/Ti02 at 25 C indicates the formation of adsorbed CO, carboxylate, and carbonate species on the catalyst surface. 

The first possibility is that the attractive potential associated with the solid surface leads to an increased gaseous molecular number density and molecular velocity. The resulting increase in both gas-gas and gas-wall collision frequencies increases the T1. The second possibility is that although the measurements were obtained at a temperature significantly above the critical temperature of the bulk CF4 gas, it is possible that gas molecules are adsorbed onto the surface of the silica. The surface relaxation is expected to be very slow compared with spin-rotation interactions in the gas phase. We can therefore account for the effect of adsorption by assuming that relaxation effectively stops while the gas molecules adhere to the wall, which will then act to increase the relaxation time by the fraction of molecules on the surface. Both models are in accord with a measurable increase in density above that of the bulk gas. 

The performance of adsorption processes results in general from the combined effects of thermodynamic and rate factors. It is convenient to consider first thermodynamic factors. These determine the process performance in a limit where the system behaves ideally i.e. without mass transfer and kinetic limitations and with the fluid phase in perfect piston flow. Rate factors determine the efficiency of the real process in relation to the ideal process performance. Rate factors include heat-and mass-transfer limitations, reaction kinetic limitations, and hydro-dynamic dispersion resulting from the velocity distribution across the bed and from mixing and diffusion in the interparticle void space. 

EOF when no disturbing processes such as wall adsorption occurs. The practically obtained migration time of the neutral marker (Teof) can be utilized to calculate the velocity of the EOF ... 

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