Adsorbate loading

At low adsorbate loadings, the differential heat of adsorption decreases with increasing adsorbate loadings. This is direct evidence that the adsorbent surface is energetically heterogeneous, ie, some adsorption sites interact more strongly with the adsorbate molecules. These sites are filled first so that adsorption of additional molecules involves progressively lower heats of adsorption. 

In Figure 5, the heat of adsorption of CO2 increases slightly at the higher adsorbate loadings. This increase is due to the increasing self-potential contribution at the higher loadings. 

Fig. 18. (a) Time trace of adsorbate composition in an adsorber effluent during adsorption, (b) Adsorbate loading along the flow axis of an adsorber during... 

Equilibrium Considerations - Most of the adsorption data available from the literature are equilibrium data. Equilibrium data are useful in determining the maximum adsorbent loading which can be obtained for a specific adsorbate-adsorbent system under given operating conditions. However, equilibrium data by themselves are insufficient for design of an adsorption system. Overall mass transfer rate data are also necessary. 

The equilibrium adsorption characteristics of gas or vapor on a solid resemble in many ways the equilibrium solubility of a gas in a liquid. Adsorption equilibrium data are usually portrayed by isotherms lines of constant temperature on a plot of adsorbate equilibrium partial pressure versus adsorbent loading in mass of adsorbate per mass of adsorbent. Isotherms take many shapes, including concave upward and downward, and S-curves. Equilibrium data for a given adsorbate-adsorbent system cannot generally be extrapolated to other systems with any degree of accuracy. 

Several useful methods are available for extrapolating equilibrium data for a given system to various temperatures and pressures. One convenient method is by use of a reference substance plot. Here, the adsorption equilibrium partial pressure of the adsorbate is plotted against a pure substance vapor pressure, preferably that of the adsorbate. If logarithmic coordinates are used on both axes, lines of constant adsorbent loading, isosteres, are linear for most substances. Therefore, only two datum points are required to establish each isostere. 

Typically, adsorption isotherms are generated using a batch experiment at a fixed temperature and a fixed feed composition. These experiments include exposing a known amount of adsorbent to a known concentration of adsorbate at a constant temperature. Once equilibrium is established, the net adsorbate concentration in the liquid is measured. This process is repeated at multiple adsorbate concentrations and temperatures. A plot of adsorbate loading (g adsorbate/g adsorbent) versus adsorbate concentration reveals the adsorption isotherm with the shape of the isotherm determining the suitability of a particular adsorbent for a particular system [20]. 

The two adsorbent chambers contain the zeolitic adsorbent, the liquid xylenes and p-diethylbenzene desorbent. Proper loading of the adsorbent into the large diameter vessels in industrial production plants is of critical importance to maximize adsorbent mass in the fixed vessel volume and not generate low and high density areas within the adsorbent bed. Density inconsistencies could adversely affect liquid flow distribution and thereby have a detrimental effect on the performance of the process. Adsorbent loading methods are a matter of proprietary know how of the technology licensors. However, Seko has published a paper on the practical matters involved in an actual problem case [20]. 

X Adsorbent loading, kg of adsorbate/kg of adsorbate-free adsorbent Xf Adsorbent loading in equibbrium with adsorption feed, kg/kg molar loading, kmol/kg in Eq. (9.14)... 

Alternatively, an appropriate isotherm expression may be substituted in Equation 1 to obtain an explicit relationship between the isosteric heat of adsorption and the adsorbent loading (3). The validity of these calculated heats is based upon the assumptions that the differential adsorption... 

Adsorbent Loading solvent Relative intensity (°/o) Chemical shift (ppm from TMS) Linewidth (Hz)... 

Where T is the temperature of the adsorbent mass at radius r and time t. n is the adsorbate loading per unit weight of the adsorbent at radius r and time t. q is the isosteric heat of adsorption, p, c and k are, respectively, the density, the heat capacity and the effective thermal conductivity of the adsorbent mass. 

The adsorbent is at equilibrium under local conditions of P and T. Thus for a differential test where the changes in the adsorbent temperature and the adsorbate loading are small, one may write ... 

The asterick indicates that the quantities are evaluated under equilibrium conditions, n and na are, respectively, the initial and the final equilibrium adsorbate loadings during the test. 

The average adsorbate loading in the adsorbent mass, n(t), can be obtained by ... 

The rate of uptake slows down considerably as p decreases, p affects the slopes of both the initial and the later portions of the uptake curve. This shows that uptake rate is very sensitive to the adsorbate loading and the heat of sorption. For a Langmurian system, p may be written as ... 

Figure 16 Simulated temperature profiles along a reactor with and without "desorptive" cooling at various times for the oxidation of CO on a Pt catalyst with water vapor desorption from 3A zeolite in a fixed bed comprising equal proportions of catalyst and adsorbent. The solid curves give the simple regenerative behavior and the dotted curves describe the desorptively cooled case. Initial reactor temperature is 125°C, initial adsorbent loading 0.12 kg/kg, inlet CO-concentration 0.2 mol/l, gas loading 6000 h-1.

Figure 5 shows the adsorbent loading, Q, as a function of C°, based on data at various pHs. The result suggests that the neutral form of berberine was adsorbed on XAD-7. This result was consistent with the result that adsorption onto XAD-7 was due to hydrophobic interaction. In Fig. 5, the adsorption isotherm for berberine was of the Langmuir isotherm type. This result suggested that berberine might be adsorbed onto XAD-7 form a monolayer. 

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