Heat of desorption

Fig. XVIII-11. Calorimetric differential heat of adsorption of H2 on ZnO. Dashed line differential heat of desorption. (From Ref. 104.)...

However, it is not practical to set the gas temperature in steady state without equally setting the temperature of the surface and bulk phases hounding the gas. Consideration of the response of the system as a vacuum environment can then provide a sufftciendy precise prediction of the pressure P and the surface coverage 9 at temperature Tfor molecules of a known species in a known state on a known surface. For example, an isotherm is estabhshed between the surface of the condensed and the gaseous phases, depending, eg, on the heat of desorption. For submonolayer coverage on a... 

Dj IE, ratio of a crack is held constant but the dimensions approach molecular dimensions, the crack becomes more retentive. At room temperature, gaseous molecules can enter such a crack direcdy and by two-dimensional diffusion processes. The amount of work necessary to remove completely the water from the pores of an artificial 2eohte can be as high as 400 kj/mol (95.6 kcal/mol). The reason is that the water molecule can make up to six H-bond attachments to the walls of a pore when the pore size is only slightly larger. In comparison, the heat of vaporization of bulk water is 42 kJ /mol (10 kcal/mol), and the heat of desorption of submonolayer water molecules on a plane, soHd substrate is up to 59 kJ/mol (14.1 kcal/mol). The heat of desorption appears as a exponential in the equation correlating desorption rate and temperature (see Molecularsieves). 

Figure 228 shows the examples of liquid and solid open sorption storage systems. In both cases the Desorption is activated by an hot air stream carrying the heat of desorption. For the solid a packed bed of adsorbent pellets and for the liquid solution a reactor are blown through, leaving the packed bed dry and the solution concentrated. 

Fig. 3.4. Heat of desorption for CO on single crystal platinum as a function of coverage (taken from [83]).

Isothermal operation in a fixed bed may be achieved in a well-cooled laboratory column and also in large-scale equipment if the concentration of adsorbate is low and the release of heat of adsorption is not great. A third and rather specialised situation in which isothermal conditions may exist is that in which a component is adsorbed on to a surface already covered with a second component. If this second component is displaced by the first, its heat of desorption will consume the heat released when the adsorption of the first component occurs. 

The origin of the relationship between Kp and temperature in Eq. (YY) can be seen by reexamining equations (RR) and (SS), which show that Kp is directly proportional to bL, i.e., to the ratio of the rate constants for adsorption and desorption, k, k, . The rate constant for desorption from the surface, k, can be expressed as a function of the heat of desorption, A7/d (Adamson, 1982) ... 

Plots of log P vs 1/T for several R2C07-H systems are shown in Figures 3 and 4. The AH values, giving the heats of desorption of hydrogen from the compound, were obtained from the slopes of the curves in Figures 3 and 4 and are listed in Table II. 

Table II. Heats of Desorption of Hydrogen from R2C07-H Systems...

Berlin, E., Kliman, P. G., Anderson, B. A. and Pallansch, M. J. 1971. Calorimetric measurement of the heat of desorption of water vapor from amorphous and crystalline lactose. Therm. Acta 2, 143-152. 

Several predictive approaches are available for estimating the fraction of POPs sorbed onto aerosols. The Junge-Pankow model assumes that gaseous POPs are adsorbed onto active sites on the aerosol. Key parameters in the model are particle surface area per unit volume of air (0), the liquid-phase vapor pressure of the compound ( P ) and a factor (c) which depends on the excess heat of desorption from the particle surface. 

The heat of desorption is also given by the same difference between levels E and A and is, consequently, equal to the heat of adsorption ... 

In this case cesium is desorbed in the ionic form, the heat of desorption being no more than... 

In case an activation energy is noticed the heat of desorption is higher than the heat of adsorption. A glance at Fig. 10 shows that the speed of the desorption reaction will be governed by the difference in height between level E and intersection point S, which is the sum of the heat of adsorption (A to E) and the activation energy (A to S). 

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