Water vapour, diffusion

Table 7.5b. Permeability coefficient P for the permeation of water vapour (quantity of diffused water vapour measured in mass units) in HDPE pipes...

In this derivation, the diffusion coefficient which is used is really a parameter, since it is not certain which gas diffusion rate is controlling, that of hydrogen into a pore, or that of water vapour out of die pore. The latter seems to be the most probable, but the path of diffusion will be vety tormous drrough each pore and drerefore the length of the diffusion path is ill-defined. 

For Fe in steam, water vapour or COj below 570°C, a two-layered Fej04 layer is observed, the inner layer growing by Oj diffusion inwards. Similarly, Potter and Mann reported the formation of a duplex Fej04 layer during the oxidation of mild steel in steam between 300°C and 550°C. 

Rahmel and Tobolsk have shown that the iron core below triplex scales of Fe0/Fe304/Fe203 grown in Oj + water vapour at 950°C become enriched in H, indicating that water vapour can penetrate relatively thick wustite scales. Cracks are a valid path for such diffusion but penetration can occur even when the oxide can deform plastically and there is no evidence of cracking. ... 

Storage rooms are similar in principle to packages, but the rate of entry of moisture is less predictable. Replacement of the air and diffusion of water vapour will have a considerable effect on the atmosphere with building materials other than glass and metals, and will vary markedly with weather conditions. 

Mass transfer may take place from a mixture of gases, such as the condensation of water from moist air. In this instance, the water vapour has to diffuse through the air, and the rate of mass transfer will depend also on the concentration of vapour in the air. In the air-water vapour mixture, the rate of mass transfer is roughly proportional to the rate of heat transfer at the interface and this simplifies predictions of the performance of air-conditioning coils [1,5, 9]. 

Fig. 13. Plot of variations of activation energy ( /kJ mole"1) with water vapour pressure (PHjO/Torr) for dehydration of calcium sulphate. Data from Ball et al. [281,590, 591] who discuss the significance of these kinetic parameters. Dehydrations of CaS04 2 H2O, nucleation ( ), boundary (o) and diffusion (e) control Q-CaSC>4 5 H2O, diffusion control, below (X) and above (+) 415 K j3-CaS04 5 H20, diffusion control ( ).

This deceleratory reaction obeyed the parabolic law [eqn. (10)] attributed to diffusion control in one dimension, normal to the main crystal face. E and A values (92—145 kJ mole-1 and 109—10,s s-1, respectively) for reaction at 490—520 K varied significantly with prevailing water vapour pressure and a plot of rate coefficient against PH2o (most unusually) showed a double minimum. These workers [1269] also studied the decomposition of Pb2Cl2C03 at 565—615 K, which also obeyed the parabolic law at 565 K in nitrogen but at higher temperatures obeyed the Jander equation [eqn. (14)]. Values of E and A systematically increased... 

Kodama and Brydon [631] identify the dehydroxylation of microcrystalline mica as a diffusion-controlled reaction. It is suggested that the large difference between the value of E (222 kJ mole-1) and the enthalpy of reaction (43 kJ mole-1) could arise from the production of an amorphous transition layer during reaction (though none was detected) or an energy barrier to the interaction of hydroxyl groups. Water vapour reduced the rate of water release from montmorillonite and from illite and... 

An open bowl, 0.3 m in diameter, contains water at 350 K evaporating into the atmosphere, if the air currents are sufficiently strong to remove the water vapour as it is formed and if the resistance to its mass transfer in air is equivalent to that of a 1 mm layer for conditions of molecular diffusion, what will be the rate of cooling due to evaporation The water can be considered as well mixed and the water equivalent of the system is equal to 10 kg. The diffusivity of water vapour in air may be taken as 0.20 ctn2/s and the kilogram molecular volume at NTP as 22.4 in3. 

The diffusivity of water vapour in air is 2.4 x It) 5 nr/s and the mass transfer resistance is equivalent to that of a stagnant gas film of thickness 0.25 mm. Neglect the effects of bulk flow. 

If this reaction is implemented at temperatures where the iron yielded is a solid, a sectioned, fractionally reacted iron oxide might appear as shown in Figure 3.26. As shown, for the hydrogen to reach the iron oxide with which it reacts, it has to diffuse through a layer of iron (mostly porous). In addition, the water vapour produced as a consequence of the reaction must be transported away from the iron-iron oxide interface by diffusion. Failing this, there will be accumulation of water vapour at the interface which will permit equilibrium point to be attained and the reaction ceases from further occurring. 

During the constant rate period, it is assumed that drying takes place from a saturated surface of the material by diffusion of the water vapour through a stationary air film into the air stream. Gilliland(8) has shown that the rates of drying of a variety of materials in this stage are substantially the same as shown in Table 16.1. 

Masking of some of the cell wall hydroxyl content occurs, which can be a contributory factor in reducing water vapour sorption. The efficiency of OH masking depends upon the efficiency of diffusion of the cell waU bulking agent. 

By determining the water vapour and oxygen permeability of the free films as well as the water solubility in the coatings, the coefficients of diffusion of water could be established. 

Table I. Oxygen and water vapour permeabilities (P02 PH2O) with mean deviation (m.d.), water solubility (S1J20) and diffusion coefficient (Djj o) at 21°C.

Dyrstad, K., Veggeland, J., and Thomassen, C. (1999), A multivariate method to predict the water vapour diffusion rate through polypropylene packaging, Int. J. Pharm., 188,... 

The above material is expected to have excellent diffusion resistance to gases such as sulfur dioxide, nitrogen oxides and saturated water vapour and outstanding chemical resistance to mineral acids, bases, aqueous phases and polar solvents and weatherproof. 

Excellent diffusion resistance to gases such as sulfur dioxide, nitrogen oxides and saturated water vapour ... 

Most common moisture probes are prone to contamination. The most serious causes of the latter are exposure to CI2, NH3 or wet acids. Common causes of errors in hygrometer readings are too infrequent calibration and the use of the wrong materials for sampling pipework. The allowable concentration of water vapour is so small in some processes that diffusion of water vapour through the walls of... 

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