Adsorption equilibria temperature effect

With high concentrations, heat effects in the chromatographic column may be important. This would require the simultaneous application of an energy balance and the introduction of a term reflecting the influence of temperature on the adsorption equilibrium. 

Time-depended behavior of Cu + ion adsorption was measured by varying the equilibrium time between in the range of 0.5-72 h. The percentage adsorption of Cu + ions plotted in Fig. 28.2 as a function of contact time. The percentage adsorption of Cu + indicates that the equilibrium between the Cu + ions and sumac leaves was attained 4 h. Therefore, 4 h stirring time was found to be appropriate for maximum adsorption and was used in all subsequent measurement. The effect of temperature and pH the adsorption equilibrium of Cu + on sumac leaves was investigated by varying the solution temperature from 283 to 303 and pH from 6 to 10. The results are presented in Fig. 28.3. The results indicated that the best adsorption results were obtained at pH 8 at 293 K. 

More recently, Kander and Paulaitis (16) have studied the adsorption of phenol onto activated carbon and measured its sorption equilibria from dense C02. These researchers found that temperature controlled the adsorption equilibria and that phenol uptake was negligibly effected by changes in the gas phase density. Such a result indicates that factors other then a solute s solubility in a dense gas play a key role in defining the adsorption equilibrium which accompany such processes. 

The study of adsorption kinetics of a surfactant on the mineral surface can help to clarify the adsorption mechanism in a number of cases. In the literature we found few communications of this kind though the adsorption kinetics has an important role in flotation. Somasundaran et al.133,134 found that the adsorption of Na dodecylsulfonate on alumina and of K oleate on hematite at pH 8.0 is relatively fast (the adsorption equilibrium is reached within a few minutes) as expected for physical adsorption of minerals with PDI H+ and OH". However, the system K oleate-hematite exhibits a markedly different type of kinetics at pH 4.8 where the equilibrium is not reached even after several hours of adsorption. Similarly, the effect of temperature on adsorption density varies. The adsorption density of K oleate at pH 8 and 25 °C is greater than at 75 °C whereas the opposite is true at pH 4.8. Evidently the adsorption of oleic acid on hematite involves a mechanism that is different from that of oleate or acid soaps. 

The technique of gas adsorption manometry is now probably the most widely used it is simple and effective since the pressure transducer provides all the information required to determine the adsorption isotherm. Thus, the pressure and temperature of each dose of gas are measured and the gas is allowed to enter the adsorption bulb. After adsorption equilibrium has been established, the amount adsorbed is calculated from the change in pressure. The most critical features of adsorption manometry are summarized in the following checklist, with more detailed comments given in Section 3.4. 

It has been already discussed (e.g. Eq, (3.18)) that the PZC can be considered as a linear function of log A of a surface reaction responsible for proton adsorption and dissociation. Once this reaction is defined, the standard thermodynamic approach (Section 2.II) can be applied, and the temperature effect on the equilibrium constant can be calculated by combining Eqs, (2.20) and (2,24). 

Some papers report that low temperatures favor the CO2 reduction in comparison with ambient temperature. " Enhancement of the CO2 solubility in electrolyte solutions as well as favorable adsorption equilibrium of CO2 may lead to effective reduction of CO2. 

Not all adsorption beds will develop stable MTZs. One requirement for stability (i.e., the MTZ reaches a limiting size) is that the equilibrium line must be favorable. In the case of a single adsorbate isothermally removed from a non-adsorbable component, the curve of loading as a function of composition must be concave downward in the region of loading below the stoichiometric point to be favorable. This effect is described in more detail in Section 7.9. In non-isothermal adsorption it is possible for the temperature effects to cause a favorable isotherm to become an unfavorable equilibrium line. This was discussed previously in the context of the crossover ratio R. 

The temperature effect on the dynamic adsorbent capacity is shown in Figure 6. Although the rate of adsorption increases with temperature the total amount adsorbed decreases, as it does in the case of static equilibrium. Possibly, the higher effective volume of molecules at higher temperatures decreases the number of molecules that can be accommodated per unit volume of the adsorbent. 

Raising the temperature, the adsorption equilibrium between hydrogen and carbon monoxide, jointly adsorbing on platinum, shifts in favor of hydrogen adsorption. This raises the highest admissible threshold concentration of carbon monoxide. The effect could be seen in fuel cells with phosphoric acid electrolyte, which work at temperatures of about 180-200°C and admit carbon monoxide concentrations in hydrogen as high as 100 ppm, despite the fact that platinum catalysts are used. 

Adsorption phenomena are classified in two types physical adsorption and chemical adsorption. In the chemical adsorption exists an effective exchange of electrons between the solid and the adsorbed molecule, causing the following formation of a single layer on the solid surface, irreversibility, and great force of attraction between the adsorbent and adsorbate. For this reason this type of adsorption is favored by increased temperature and pressure. The physical adsorption is a reversible phenomenon, where usually observed the deposition of more than one layer of adsorbate on the adsorbent surface. The forces acting on the physical adsorption are the Van Der Waals forces. The energy released is relatively low and rapidly reaches equilibrium. In this process, the temperature increase is detrimental to the adsorption efficiency. 

Azeotrope formation and selectivity reversal appear to be fairly common features of binary adsorption equilibrium behavior but are not predicted by most of the simpler models such as Eqs. (4.11) or (4.16). Such behavior is predicted by Eq. (3.102) when the effective molecular volumes of the components are different and the component with the smaller volume also has the smaller Henry constant but only at relatively high sorbate concentrations. The experimental data for C2H4-CjHg in 5A show that selectivity reversal occurs even at relatively low loading (high temperatures) and is much more... 

In an adiabatic adsorption column the temperature front generally travels at a velocity which is different from the velocity of the primary mass transfer front and, since adsorption equilibrium is temperature dependent, a secondary mass transfer zone is established coincident with the thermal front. In a system with finite heat loss from the column wall one may approach either the isothermal situation with a single mass transfer zone or the adiabatic situation with two mass transfer zones, depending on the relative rates of heat generation and dissipation from the column wall. In the former case the effect of finite heat transfer resistance is to widen the mass transfer zone relative to an isothermal system. 

Poisoning effects are often correlated with the poison concentration in the feed stream, which, of course, is the important parameter in practical operation. However, in a more detailed analysis this approach can hardly be justified for other than isothermal tests in gradientless reactors. The adsorption equilibrium depends on temperature and composition of the gas phase which varies through the reactor as well as within the single catalyst pellet. Therefore it appears more rational to correlate the deactivation with the amount of poison present on the catalyst rather than... 

The effect of temperature on adsorption equilibrium Is worth mentioning. Rise of temperature decreases the extent of adsorption. This is in spite of the fact that surface tension decreases with a rise in temperature. The reason behind this observation is that molecular cohesion, a fundamental cause of adsorption, is disrupted by thermal agitation. For example, when iodine is added to starch solution, a blue colour appears due to formation of starch iodide. However, upon heating, the blue colour disappears due to the failure of iodine to remain adsorbed on the starch particles at higher temperatures. 

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