Measuring desorption rate

Fuller et al. (1993) measured desorption rates for As(V) from ferrihydrite, aged for 24 h, as a result of increasing pH. Arsenate was first equilibrated with ferrihydrite for 144 h at pH 8.0 The molar ratio of As(V) adsorbed to Fe in ferrihydrite was 0.10 (Fig. 7). Desorption was initiated by rapidly increasing the pH to 9.0. Within a few hours, the molar ratio of As(V)/Fe had decreased to about 0.08. The rate of desorption then slowed as the rate became limited by diffusion of As(V) from pores within mineral aggregates. Within 96 h, the concentration of As(V) still adsorbed was only about 5% greater than the adsorbed concentration of As(V) determined in a separate adsorption experiment at pH 9.0. Similar desorption behavior for As(V) from ferrihydrite were measured by Fuller and Davis (1989). 

FIG. 16 Variation of the steady-state rate of production, Pcoj, with Pco in the NO + CO lattice gas model with NO desorption (rate d o = 0.5), and CO desorption at various rates (shown). The inset shows the reaction rate measured experimentally at 410 K. (From Ref. 81.)... 

Wetness of a metal surface The lime of wetness of the metal surface is an exceedingly complex, composite variable. It determines the duration of the electrochemical corrosion process. Firstly it involves a consideration of all the means by which an electrolyte solution can form in contact with the metal surface. Secondly, the conditions under which this solution is stable with respect to the ambient atmosphere must be considered, and finally the rate of evaporation of the solution when atmospheric conditions change to make its existence unstable. Attempts have been made to measure directly the time of wetness , but these have tended to use metals forming non-bulky corrosion products (see Section 20.1). The literature is very sparse on the r61e of insoluble corrosion products in extending the time of wetness, but considerable differences in moisture desorption rates are found for rusted steels of slightly differing alloy content, e.g. mild steel and Cor-Ten. 

The required distribution of initial populations ntu can be obtained in the following manner (32). Let us consider a system with Ed mi = 20 kcal/ mole and Ed max = 45 kcal/mole. Assuming that kd = 1013 sec-1 and x = 1, we can calculate theoretical desorption rates dnai/dt for Ed = 20, 21, 22,..., 45 kcal/mole as a function of nBOi. With increasing temperature, 25 values of dnjdt are measured at temperatures corresponding to Ed of 20, 21, 22,. . ., 45 kcal/mole. Since the total desorption rate at any moment must be equal to the sum of the individual desorption processes, we obtain 25 linear equations. Their solution permits the computation of the initial populations of the surface sites in the energy spectrum considered, i.e. the function n,oi(Edi). From the form of this function, desorption processes can be determined which exhibit a substantial effect on the experimental desorption curve. 

Equation (1) can be ignored. Assuming further that the desorption rate is fast compared to the Ra half lives, then the ( Ra/ Ra) ratios in the groundwater and adsorbed on surfaces (and so in the mobile Ra pool) are equal. In this case, the measured groundwater ( Ra/ Ra) ratio reflects the ratio of the supply rates of Ra and Ra, which is equal to the ratio, adjusted for any differences in the distributions of and... 

Figure 1.70 shows the three times repeated measurement of desorption rates, without pressure control to demonstrate the reproducibility, and two measurements where the main drying has been pressure-controlled at 0.36 and 0.21 mbar. The process conditions for these five measurements correspond with those in Table 1.10.1. 

By barometric temperature measurements (BTM) and the measurements of the desorption rate (DR) the influence of varied drying conditions can be seen and analyzed. Figure 1.71 compares four different test runs ... 

Fig. 1.71. Synopsis of Tict and desorption rates (DR) of the two tests in Fig. 1.63 (1) and Fig. 1.64 (4) and comparison with two other tests (2) carried out as (1) but with activated pressure control at 0.36 mbar and (3) only one tray used (instead of three trays in Fig 1.63), which has been placed at such a slope that the thickness of the product has been 0.5 cm at one side and 0.9 cm at the other. The course of the pressure (see Figs. 1.63 and 1.64) permits quantitative judgment of the SD. The DR data measure, independent of the chosen process data, the amount of desorbed water per hour in % of the solid content. It is visible, that a DR value of 5 %/h in test (4) is reached in 6.2 h, in test (2) in 10.2 h, in test (1) in 13.5 h, but in test (3) the time cannot be estimated. Because of the unequal product thickness, the DR values can change (9.5 h), the desorption process is not uniform for such a product.

The measuring of desorption rates can be used, as the above examples show, to determine the amount of desorbable water if the following prerequisites are fulfilled ... 

By using 22.4 103 L mbar corresponding to 18 g HzO, mbar L can be converted into g. This relationship is accurate enough as the temperature of the water vapor depends on several factors and will also be modified by a change of Tsh The desorption process can be best illustrated by using the desorption rate (DR), which measures the desorbed amount of water in % of the solids of the product per hour. 

Measurements of the desorption rate (DR) require three conditions ... 

In a freeze drying plant automated in this way, the desorption rates and the desorbable water content (in % of solids) can be measured, calculated and documented. 

In addition to the stepwise mechanism, Dautzenberg and Platteeuw proposed another platinum-catalyzed cyclization mechanism (23). This might correspond simply to a disguised stepwise aromatization where the further reaction of unsaturated intermediates is very rapid compared with their desorption. Thus, hydrogen pressure would govern the probability of desorption versus further reaction. Since the cyclization of triene is irreversible, a very low steady-state surface triene concentration must be sufficient to ensure a measurable reaction rate. 

DSC tests show a substantial reduction of the hydrogen desorption onset (red circles) (T J and peak (T ) temperatures due to the catalytic effects of n-Ni as compared to the hydrogen desorption from pure MgH also milled for 15 min. (Fig. 2.57). It is interesting to note that there is no measurable difference between spherical (Fig. 2.57a) and fdamentary (Fig. 2.57b) n-Ni, although there seems to be some effect of SSA. We also conducted desorption tests in a Sieverts apparatus for each SSA and obtained kinetic curves (Fig. 2.58), from which the rate constant, k, in the JMAK equation was calculated. The enhancement of desorption rate by n-Ni is clearly seen. At the temperature of 275°C, which is close to the equilibrium at atmospheric pressure (0.1 MPa), all samples desorb from 4 to 5.5 wt.% within 2,000 s. 

While microscopic techniques like PFG NMR and QENS measure diffusion paths that are no longer than dimensions of individual crystallites, macroscopic measurements like zero length column (ZLC) and Fourrier Transform infrared (FTIR) cover beds of zeolite crystals [18, 23]. In the case of the popular ZLC technique, desorption rate is measured from a small sample (thin layer, placed between two porous sinter discs) of previously equilibrated adsorbent subjected to a step change in the partial pressure of the sorbate. The slope of the semi-log plot of sorbate concentration versus time under an inert carrier stream then gives D/R. Provided micropore resistance dominates all other mass transfer resistances, D becomes equal to intracrystalline diffusivity while R is the crystal radius. It has been reported that the presence of other mass transfer resistances have been the most common cause of the discrepancies among intracrystaUine diffusivities measured by various techniques [18]. 

Adsorption and desorption reactions of protons on iron oxides have been measured by the pressure jump relaxation method using conductimetric titration and found to be fast (Tab. 10.3). The desorption rate constant appears to be related to the acidity of the surface hydroxyl groups (Astumian et al., 1981). Proton adsorption on iron oxides is exothermic potentiometric calorimetric titration measurements indicated that the enthalpy of proton adsorption is -25 to -38 kj mol (Tab. 10.3). For hematite, the enthalpy of proton adsorption is -36.6 kJ mol and the free energy of adsorption, -48.8 kJ mol (Lyklema, 1987). 

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