Adsorbent transfer control

Solutions are provided for external mass-transfer control, intraparticle diffusion control, and mixed resistances for the case of constant Vf and F0 in = FVi out = 0. The results are in terms of the fractional approach to equilibrium F = (ht — hf)/(nT — nf), where hf and are the initial and ultimate solute concentrations in the adsorbent. The solution concentration is related to the amount adsorbed by the material balance - (hi - nf )M,Ay. 

Equation (17) represents the general case of uptake under complete heat transfer control and equation (18) gives the corresponding adsorbent temperature profile. Furthermore, if kg is large compared to h, (s << 1), equation (10) can be approximated as p 3 s and equations (17) and (18) reduce to n... 

Figure 10.11 Schematic representation of mass-transfer controlled reaction by an ionic species formed through an adsorbed intermediate.

The properties of the surface layers have a strong effect on the deposition process. The driving force of the electrochemical reaction is the potential difference over the electrochemical double layer. Adsorption of species can change this potential. For example, the additives used in electrodeposition adsorb in the Helmholtz layer. They can change the local potential difference, block active deposition sites, and so on. The thickness of the diffusion layer affects the mass-transfer rate to the electrode. The diffusion layer becomes thinner with increasing flow rate. When the diffusion layer is thicker than the electrode surface profile, local mass-transfer rates are not equal along the electrode surface. This means that under mass-transfer control, metal deposition on electrode surface peaks is faster than in the valleys and a rough deposit will result. 

Although Chen et al. focused on CO oxidation in gas turbine exhausts with noble metal catalysts, much of the deactivation data that they presented is also relevant to oxidation of VOCs in other air pollution control applications. They reported that 100 to 200 ppm SO2 in the exhaust will require 150 to 200 C higher catalyst temperatures for the same CO conversion as that without SO2. However, above -350 C the effect of SO2 disappears with these catalysts because the CO reaction rate becomes mass-transfer controlled. The inhibition by SO2 is attributed to the strong adswption of the sulfur compounds on both the catalyst and carrier, limiting adsorption of CO. These adsorbed sulfur compounds can be removed with time and high temperatures in the absence of SO2, restoring catalyst activity. 

In the case of a desorption process all these resistances must be overcome in the reverse order. At first the heat of desorption to be added results in a detachment of the molecules which pass then through the micro- and macroporous system and finally through the concentration boundary layer into the bulk fluid around an adsorbent pellet. The heat of adsorption (in most cases exothermic) and the heat of desorption (endothermic as a rule) lead to the result that these processes cannot be carried out in an isothermal field. The increase of temperature of the adsorbent by adsorption and the decrease of temperature of the sohd phase are the reason that the driving force is reduced and the mass transfer is retarded. It can happen that the mass transfer rates of adsorptives with great heats of adsorption result in such tem-peratrue changes that additional adsorptive can only be adsorbed after a removal of heat combined with a temperatrrre loss. The kinetics in the adsorber is limited by heat transfer (heat transfer controlled). 

The extreme case of complete heat transfer control for COj-SA is illustrated in Figure 6.15. For this system diffusion is much faster and even in relatively large crystals the uptake rate is controlled by heat transfer. Uptake curves are essentially independent of crystal size but vary with sample size due to changes in the effective heat capacity and external area-to-volume ratio for the sample. Analysis of the uptake curves according to Eq. (6.70) yields consistent values for the overall heat capacity (34 mg sample C 0.32 and 12.5 mg sample 0.72 cal/g deg.). The variation of effective neat capacity with sample size arises from the increasing importance of the heat capacity of the containing pan when the adsorbent weight is small. 

FIGURE 6.20. Test of the spinning basket adsorber. Overall rate coefficients for adsorption of propane on Linde 5A at 54 C were measured at several speeds of rotation in the presence of 500 Torr of either iQH,) or SFj. These species are both too large to penetrate the sieve. The uptake rate correlates with the gas phase molecular diffusivity indicating external mass transfer control. (Data of Taylor.< °>)... 

Pumpdown time from Very High Vacuum (VHV) to UHV is the longest. This arises because in this regime, it is the removal rate of surface adsorbates, which controls the pressure. Adsorbates develop when any metal surface is exposed to atmospheric gases. Indeed, under ambient conditions, it can take weeks to pump a vacuum chamber from atmosphere to UHV, irrespective of the pumping system used. Note This is also the reason why samples are not transferred direcUy from the atmosphere into analysis chambers. 

Figure 4.8 Experimental uptake curves for CO2 on 5A zeolite demonstrating the limiting behaviour of heat transfer control. Adsorption temperature 273 K. Figures on curves represent various adsorbate pressures which relate to differing effective heat capacities. Curve 1,4.3 -3.6 torr curve 2, 20-17 torr curve 3,68 - 63 torr curve 4,234 - 204 torr.

Fig. 6. Concentration profiles through an idealized biporous adsorbent particle showing some of the possible regimes. (1) + (a) rapid mass transfer, equihbrium throughout particle (1) + (b) micropore diffusion control with no significant macropore or external resistance (1) + (c) controlling resistance at the surface of the microparticles (2) + (a) macropore diffusion control with some external resistance and no resistance within the microparticle (2) + (b) all three resistances (micropore, macropore, and film) significant (2) + (c) diffusional resistance within the macroparticle and resistance at the surface of the...

Advances in fundamental knowledge of adsorption equihbrium and mass transfer will enable further optimization of the performance of existing adsorbent types. Continuing discoveries of new molecular sieve materials will also provide adsorbents with new combinations of useflil properties. New adsorbents and adsorption processes will be developed to provide needed improvements in pollution control, energy conservation, and the separation of high value chemicals. New process cycles and new hybrid processes linking adsorption with other unit operations will continue to be developed. 

Reaction kinetics at phase houndaiies. Rates of adsorption and desorption in porous adsorbents are generally controlled by mass transfer within the pore network rather than by the kinetics of sorption at the surface. Exceptions are the cases of chemisorption and affinity-adsorption systems used for biological separations, where the kinetics of bond formation can be exceedingly slow. 

The reaction kinetics approximation is mechanistically correct for systems where the reaction step at pore surfaces or other fluid-solid interfaces is controlling. This may occur in the case of chemisorption on porous catalysts and in affinity adsorbents that involve veiy slow binding steps. In these cases, the mass-transfer parameter k is replaced by a second-order reaction rate constant k. The driving force is written for a constant separation fac tor isotherm (column 4 in Table 16-12). When diffusion steps control the process, it is still possible to describe the system hy its apparent second-order kinetic behavior, since it usually provides a good approximation to a more complex exact form for single transition systems (see Fixed Bed Transitions ). 

Cobalt(II) complexes of three water-soluble porphyrins are catalysts for the controlled potential electrolytic reduction of H O to Hi in aqueous acid solution. The porphyrin complexes were either directly adsorbed on glassy carbon, or were deposited as films using a variety of methods. Reduction to [Co(Por) was followed by a nucleophilic reaction with water to give the hydride intermediate. Hydrogen production then occurs either by attack of H on Co(Por)H, or by a disproportionation reaction requiring two Co(Por)H units. Although the overall I easibility of this process was demonstrated, practical problems including the rate of electron transfer still need to be overcome. " " ... 

Lastly, we have shown that transport of ions across the double layer is facilitated by water via proton transfer and that the barrier for the reduction of O2 is controlled by electron transfer that occurs as the proton moves close to the adsorbed O2 to form a reactive center. Electron transfer appears to occur before the actual formation of the 00 H bond. 

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