Adsorbents temperature effects

Heats of Adsorption. Temperature effects were determined by measuring adsorption at three temperatures. As seen from TABLE IV, the K values vary with temperature such that for butylate, K increases with temperature, while for alachlor and metolachlor, K decreases with temperature. These results indicate that butylate becomes more adsorbed to Keeton soil as the temperature increases while alachlor and metolachlor become less adsorbed as temperature increases. In order to obtain a quantitative measure of these effects, heats of adsorption (AH) were calculated as described previously in the Materials and Methods section (equation 3). TABLE IV contains values for the average molar distribution constants (Kd) for butylate, alachlor, and metolachlor which were plotted vs the inverse temperatures (1/°K) to obtain the AH s shown in Figure 3. 

All heat evolutions which occur simultaneously, in a similar manner, in both twin calorimetric elements connected differentially, are evidently not recorded. This particularity of twin or differential systems is particularly useful to eliminate, at least partially, from the thermograms, secondary thermal phenomena which would otherwise complicate the analysis of the calorimetric data. The introduction of a dose of gas into a single adsorption cell, containing no adsorbent, appears, for instance, on the calorimetric record as a sharp peak because it is not possible to preheat the gas at the exact temperature of the calorimeter. However, when the dose of gas is introduced simultaneously in both adsorption cells, containing no adsorbent, the corresponding calorimetric curve is considerably reduced. Its area (0.5-3 mm2, at 200°C) is then much smaller than the area of most thermograms of adsorption ( 300 mm2), and no correction for the gas-temperature effect is usually needed (65). 

Because carbon has a natural affinity for adsorption of heavy hydrocarbon species and polar molecules, CMS membranes need to be used at a sufficiently high temperature to eliminate contribution/interference of the adsorption. In contrast, strong adsorption of heavier molecules may be used to separate those species by adsorption as discussed earlier by the SSF mechanism (Rao and Sircar, 1993b). The SSF carbon membranes typically have pore dimensions much greater than those needed for CMS membranes since the separation is based on the adsorbed species effectively blocking permeation of other components (Fuertes, 2000). Carbon membranes are resistant to contaminants such as H2S and are thermally stable and can be used at higher temperatures compared to the polymeric membranes. For the synthesis gas environment, the hydrothermal stability of carbon in the presence of steam will be a concern limiting its operation temperature. 

A common cause of baseline drift is a slow elution of substances previously adsorbed on the column. A column cleanup procedure may be in order, or it may need to be replaced. This problem may also be caused by temperature effects in the detector. Refractive index detectors are especially vulnerable to this. In addition, a contaminated detector can cause drift. The solution here may be to disassemble and clean the detector. 

Slow elution of chemicals adsorbed on the column, 2) temperature effects, such as with the refractive index detector, and 3) a contaminated detector... 

Pollution Control. Zeolite adsorbents can effectively remove pollutants such as S02, H2S, and NO from industrial off-gas streams at near ambient temperature (54-57). Since water vapor usually exists along with these acidic compounds, an acid-stable or acid-resistant zeolite adsorbent is necessary for a long service life. Union Carbide announced three new processes for pollution control recently. They are the PuraSiv-Hg process for mercury vapor removal, the PuraSiv-N process for NO removal from nitric acid plant off-gas, and the PuraSiv-S process for S02 removal from... 

Recent work by Selwood (9), based on changes in the magnetization of nickel during chemisorption of ethylene, indicates that ethylene is associatively adsorbed on bare nickel. He suggests that the discrepancy between this result and the dissociative chemisorption indicated by the infrared experiments is due to factors such as the relative activity of the sample surfaces and temperature effects caused by the heat of chemisorption. Low-temperature infrared experiments in which ethylene is studied at —78° C. are expected to provide evidence on the importance of the above factors in determining the course of ethylene chemisorption. 

N diffuses into the structural pores of clinoptilolite 10 to 10 times faster than does CH4. Thus internal surfaces are kinetically selective for adsorption. Some clino samples are more effective at N2/CH4 separation than others and this property was correlated with the zeolite surface cation population. An incompletely exchanged clino containing doubly charged cations appears to be the most selective for N2. Using a computer-controlled pressure swing adsorption apparatus, several process variables were studied in multiple cycle experiments. These included feed composition and rates, and adsorber temperature, pressure and regeneration conditions. N2 diffusive flux reverses after about 60 seconds, but CH4 adsorption continues. This causes a decay in the observed N2/CH4 separation. Therefore, optimum process conditions include rapid adsorber pressurization and short adsorption/desorp-tion/regeneration cycles. 

The calorimetric measurements in metal oxide-aqueous electrolyte solution systems are, beside temperature dependence of the pzc measurements, the method for the determination of the enthalpy of the reaction in this system. Because of the low temperature effects in such systems they demand very high precision. That is why these measurements may be found only in a few papers from the last ten years [89-98]. A predominant number of published measurements were made in the special constricted calorimeters (bath type), stirring the suspension. The flow calorimeters may be used only for sufficiently large particles of the solid. A separate problem is the calculation of the enthalpy of the respective reactions from the total heat recorded in the calorimeter. A total thermal effect consists of the heat of the neutralization in the liquid phase, heat connected with wetting of the solid, heat of the surface reaction and heat effects caused by the ion solvation changes (the ions that adsorb in the edl). Considering the soluble oxides, one should include the effects connected with the transportation of the ions from the solid to the solution... 

In the application of a gravimetric technique, (ii) to (viii) must be taken into account and also special attention must be given to the control and measurement of the adsorbent temperature and to the assessment of the buoyancy corrections. Thermal transpiration effects should be allowed for if volumetric or gravimetric measurements are made at low pressure. 

It has been known that condensation pressure depends on the adsorbate, temperature, pore size, and geometry of sorbent. As increasing temperature, the capillary condensation pressures is increased and adsorption capacity is decreased. This fact implies that adsorption and desorption can be easily achieved by only little adjustment of pressure and temperature. Therefore the effective removal of VOC can be done by pressure swing adsorption (PSA) or thermal swing adsorption (TSA) processes. 

In fact studying each adsorbate separately shows that the temperature effect is not well described by this method. As an illustration, figure 2 presents the DR plot of the nitrogen data. 

Table 2 presents the structural characteristics of the adsorbent obtained from a DR plot treatment of the characteristic curves of each adsorbate The fact that the temperature effect is well described leads to more stable values of the DR plot slope and to lower values of the average deviation. As a consequence, the H values does not fluctuate in the same way as in table 1. However, the obtained values are too low Besides, such stable H values do not prevent the fluctuations of Vdr Such fluctuations can be attributed to the fact that the characteristic curves are located in a Srcf area which seems to be translated of 10000 J mof from the y-axis whereas the first method provides nearly-zero Srcf values This is not surprising... 

The first method gives a poor representation of the temperature effect with, as consequence, the impossibility to use a unique characteristic curve for the characterization procedure. This curve is not even unique for a given adsorbate. 

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