Absorption and Desorption

Percentage moisture content of a fiber is the percentage of the total weight of the fiber which is due to the moisture present, and is obtained fiom the following formula  

Percentage moisture content Conditioned weight - weight x 100% 

The percentage moisture content will always be the smaller of the two values. 

Absorption denotes the dissolution of gaseous substances (absorptives) in a liquid (absorbent, solvent, washing agent). The reversed process is called desorption. 

In most cases absorption units consist of an absorption step and a regeneration step. Regeneration enables the internal recycling of the absorption agent as shown in Fig. 5.3-1. 

In principle, all separation techniques can be applied to the regeneration of the loaded absorbents. Often used are flash, stripping with inert gases or steam and distillation. Very effective are combinations of different regeneration processes (Fig. 5.3-2), e.g., flash and distillation. 

Absorption processes are often technically realized in packed columns or, more scarcely, in tray columns. For chemical absorption, however, many other types of equipment are used in industiy, e.g., spray tower, venturi scmbber, bubble columns, etc. 

Absorption equilibrium is preferably described by Henry s law since often, but not always, the absorptives are above their critical temperatures  

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Extensive work has been done on corrosion inhibitors (140), activated carbon use (141—144), multiple absorption zones and packed columns (145,146), and selective absorption and desorption of gas components (147,148). Alkan olamines can also be used for acid gas removal in ammonia plants (149). 

Three commercial processes that use these various hot carbonate flow arrangements are the promoted Benfield process, the Catacarb process, and the Giammarco-Vetrocoke process (26—29). Each uses an additive described as a promoter, activator, or catalyst, which increases the rates of absorption and desorption, improves removal efficiency, and reduces the energy requirement. The processes also use corrosion inhibitors, which aHow use of carbon—steel equipment. The Benfield and Catacarb processes do not specify additives. Vetrocoke uses boric acid, glycine, or arsenic trioxide, which is the most effective. 

Chl-Hung Shen and George S. Springer, Moisture Absorption and Desorption of Composite Materials, Journal of Composite Materials, January 1976, pp. 2-20. 

In the classical set-up of bulk liquid membranes, the membrane phase is a well-mixed bulk phase instead of an immobilized phase within a pore or film. The principle comprises enantioselective extraction from the feed phase to the carrier phase, and subsequently the carrier releases the enantiomer into the receiving phase. As formation and dissociation of the chiral complex occur at different locations, suitable conditions for absorption and desorption can be established. In order to allow for effective mass transport between the different liquid phases involved, hollow fiber... 

Sherwood, T. R and F. A. L. Holloway, Performance of Packed Towers—Experimental Studies of Absorption and Desorption, Trans. Amer. Inst. Chem. Eng., 36, 21 (1940). 

A typical sorption experiment involves exposing a polymer sample, initially at an equilibrium penetrant concentration of c to a bathing penetrant concentration of Ci. The weight gain or loss is then measured as a function of time. The term sorption used in this context includes both absorption and desorption. The sorption is of the integral type if c° = 0 in the case of absorption or if cf = 0 in the case of desorption. Details of the experimental setup for the sorption measurement are discussed elsewhere [4],... 

Both reduced absorption and desorption curves have a linear region extending up to a MJM value of 0.6. Should D be an increasing function of Ci, this linear region can be further extended. 

The reduced absorption and desorption curves do not depend on the thickness of the polymer film. 

Various anomalous diffusional behaviors have been observed and documented for both sorption and permeation experiments. Detailed discussions of these anomalies can be found elsewhere [12,35,36], Here only a brief summary of major findings is given. First, for the sorption anomaly, it has been observed that the reduced sorption curve has a distinctive thickness dependence. In this case, the reduced absorption and desorption curves obtained at various thick-... 

Choi and Funayama [19] also measured sodium atom emission from sodium dodecylsulfate (SDS) solutions in the concentration range of 0.1-100 mM at frequencies of 108 kHz and 1.0 MHz. The sodium line intensity observed at 1 MHz was nearly constant in the concentration range from 3 to 100 mM and was considerably higher than that at 108 kHz. This frequency dependence of the intensity is opposite that for NaCl aqueous solution. The dynamical behavior of the absorption and desorption of surfactant molecules onto the bubble surface may affect the reduction and excitation processes of sodium atom emission. This point should be clarified in the future. 

Air temperatures in arid zones are generally high and show significant daily variations. Because of the absence of a protective vegetative cover and a specific thermal absorption and desorption effects due to surface or rock colors or slope aspect, air temperatures of 35-45°C may reach peaks up to 50-60°C or more on rock surfaces. 

Hydrogen Absorption and Desorption Properties of Sodium Alanates... 

Fig. 5. Absorption and desorption kinetics for MgH2 with ceramic catalyst addition at 300°C.

Figure 5 shows the absorption and desorption kinetics of nanocrystalline MgH2 with ceramic catalysts at 300°C. The BCN catalyst addition was very effective to enhance the desorption kinetics in comparison to pure MgH2 and SDC added composite. 

Table 1. Absorption and desorption kinetics of MgH2 with ceramic catalysts at 300°C.

The materials to be used should be inert. This means absorption and desorption should be avoided. There should be no reaction with the sample, and the material should not change the odour. Some examples of materials to be used for the probe, the tubes and the valves are ... 

A linearization of the steady-state concentration gradient could be demonstrated by relating the depth to the weight of the tissue, removed per piece of adhesive tape. However, large errors, especially, within the first tapes, cast doubt over these findings [127, 128], The procedure is time-consuming and artifacts, due to absorption and desorption of moisture, formulation excipients, or sebaceous lipids, are likely. 

Fernandez and Sanchez [18] investigated the kinetics of hydrogen absorption and desorption by activated magnesium powder (several cycles of hydrogen absorp-tion/desorption at 375°C) using a volumetric technique. They pointed out that for-mation/decomposition of metal hydrides comprises a number of steps taking place in series transport to the surface, dissociation, H chemisorption, surface-... 

Thermodynamic parameters of the Mg6H powder synthesized for 150 h (Tables 2.15 and 2.17) were investigated by PCT curves for absorption and desorption at various temperatures as shown in Fig. 2.43a. The powder after such a long reactive... 

As shown in Fig. 2.43b, the enthalpy of absorption and desorption calculated from the Van t Hoff plots using the mid-plateau pressures of PCT curves in Fig. 2.43a, which are listed in Table 2.18, is equal to -72 and 83 kJ/mol, respectively. The value of entropy is 138 and 151 J/mol K for absorption and desorption, respectively. The enthalpy value for absorption is very close to the values found in the literature for MgHj as discussed in Sect. 2.1.2 and 2.1.3. Surprisingly, however, the enthalpy of desorption at 83 kJ/mol is much greater than the former and also greater than the enthalpy of desorption of the as-received and activated MgH as shown in Fig. 2.11. The coefficients of fit are excellent and give good credibility to the obtained values. 

Imamura et al. [137] studied absorption of Mg ball-milled with graphite and benzene as milling additives. In a Sieverts-type apparatus, the mixture after 20 h milling was able to absorb at 180°C. This in itself is nothing outstanding because milled, activated, and cycled MgH can also absorb at 200°C (Fig. 2.22a). Bouaricha et al. [138] also studied absorption of a Mg -i- graphite mixture at 300°C, which showed much better kinetics than that of just milled Mg. A number of researchers studied both absorption and desorption [25, 139-141] but unfortunately, all desorption studies were conducted in vacuum. However, it must be pointed out that even under vacuum conditions, desorption kinetics were no better than those obtained with a number of other additives discussed earlier in the text. The lowest desorption temperature applied was 290°C. 

J.F. Ferndndez, C.R. Sanchez, Rate determining step in the absorption and desorption of hydrogen by magnesium, J. Alloys Compd. 340 (2002) 189-198. 

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