Capillary moisture

The reagent must be carefully protected from moisture as it is comparatively easily hydrated to the acid, m.p. 216-218° (sealed capillary tube). Dilute aqueous solutions of an alcohol should be treated with solid potassium carbonate and the alcohol layer used for the test. 

The key to understanding dewatering by air displacement is the capillary pressure diagram. Figure 6 presents an example typical for a fine coal suspension there is a minimum moisture content, about 12%, called irreducible saturation, which cannot be removed by air displacement at any pressure and a threshold pressure, about 13 kPa. 

In green wood, the cell walls are saturated, whereas some cell cavities are completely filled and others may be completely empty. Moisture ia the cell walls is called bound, hygroscopic, or adsorbed water. Moisture ia the cell cavities is called free or capillary water. The distiaction is made because, under ordinary conditions, the removal of the free water has Htde or no effect on many wood properties. On the other hand, the removal of the cell wall water has a pronounced effect. 

Capillary Suction Processes. The force needed to remove water from capillaries increases proportionately with a decrease in capillary radius, exceeding 1400 kPa (200 psi) in a 1-p.m-diameter capillary. Some attempts have been made to use this force as a way to dewater sludges and cakes by providing smaller dry capillaries to suck up the water (27). Sectors of a vacuum filter have been made of microporous ceramic, which conducts the moisture from the cake into the sector and removes the water on the inside by vacuum. Pore size is sufficiently small that the difference in pressure during vacuum is insufficient to displace water from the sector material, thus allowing a smaller vacuum pump to be effective (126). 

Critical moisture content is that obtained when the constant rate period ends and the falling rate periods begin. Second critical moisture content specifies that remaining in a porous material when capillary flow dominance is replaced by vapor diffusion. 

In porous and granular materials, Hquid movement occurs by capillarity and gravity, provided passages are continuous. Capillary flow depends on the hquid material s wetting property and surface tension. Capillarity appHes to Hquids that are not adsorbed on capillary walls, moisture content greater than fiber saturation in cellular materials, saturated Hquids in soluble materials, and all moisture in nonhygroscopic materials. 

Bound moisture in a solia is that hquid which exerts a vapor pressure less than that of the pure hquid at the given temperature. Liquid may become bound by retention in small capillaries, by solution in cell or fiber walls, by homogeneous solution throughout the sohd, and by chemical or physical adsorption on solid surfaces. 

Capillary Flow Moisture which is held in the interstices of solids, as liquid on the surface, or as free moisture in cell cavities, moves by gravity and capiUarity, provided that passageways for continuous flow are present. In diying, liquid flow resulting from capiUarity appUes to liquids not held in solution and to aU moisture above the fiber-saturation point, as in textiles, paper, and leather, and to all moisture above the equiUbrium moisture content at atmospheric saturations, as in fine powders and granular solids, such as paint pigments, minerals, clays, soU, and sand. 

In diying solids it is important to distinguish between hygroscopic and nonhygroscopic materials. If a hygroscopic material is maintained in contact with air at constant temperature and humidity until equilibrium is reached, the material will attain a definite moisture content. This moisture is termed the equilibrium moisture content for the specified conditions. Equilibrium moisture may be adsorbed as a surface film or condensed in the fine capillaries of the solid at reduced pressure, and its concentration will vaiy with the temperature and humidity of the surrounding air. However, at low temperatures, e.g., 15 to 50°C, a plot of equilibrium moisture content versus percent relative humidity is essentially independent of temperature. At zero humidity the equilibrium moisture content of all materials is zero. 

Surface evaporation can be a limiting factor in the manufacture of many types of products. In the drying of paper, chrome leather, certain types of synthetic rubbers and similar materials, the sheets possess a finely fibrous structure which distributes the moisture through them by capillary action, thus securing very rapid diffusion of moisture from one point of the sheet to another. This means that it is almost impossible to remove moisture from the surface of the sheet without having it immediately replaced by capillary diffusion from the interior. The drying of sheetlike materials is essentially a process of surface evaporation. Note that with porous materials, evaporation may occur within the solid. In a porous material that is characterized by pores of diverse sizes, the movement of water may be controlled by capillarity, and not by concentration gradients. 

The problems experienced in drying process calculations can be divided into two categories the boundary layer factors outside the material and humidity conditions, and the heat transfer problem inside the material. The latter are more difficult to solve mathematically, due mostly to the moving liquid by capillary flow. Capillary flow tends to balance the moisture differences inside the material during the drying process. The mathematical discussion of capillary flow requires consideration of the linear momentum equation for water and requires knowledge of the water pressure, its dependency on moisture content and temperature, and the flow resistance force between water and the material. Due to the complex nature of this, it is not considered here. 

Bound moisture This is moisture retained within the solid such that it exerts a vapour pressure less than that of free solvent (Figure 4.24). Such solvent may be adsorbed on the surface, retained in capillaries or within cells or occlusions of liquor. The latter can be difficult to remove without resorting to high temperatures, which may damage the crystals. 

Carbon and Hydrogen.—Carbon compounds are frequently inflammable, and when heated on platinum foil take fipe or char and burn away. A safer test is to heat the substance with some easily reducible metallic oxide, the oxygen of which forms carbon diovide with the carbon present. Take a piece of soft glass tube about 13 cm. (5 in.) long, and fuse it together at one end. Heat a gram or two of fine copper oxide in a porcelain crucible for a few minutes to drive off the moisture, and let it cool in a desiccator. Mix it with about one-tenth of its bulk of powdered sugar in a mortar. Pour the mixture into the tube, the open end of which is now drawn out into a wide capillary and oeni. at the same time into the form Fig. i. 

Critical relative humidity The primary value of the critical relative humidity denotes that humidity below which no corrosion of the metal in question takes place. However, it is important to know whether this refers to a clean metal surface or one covered with corrosion products. In the latter case a secondary critical humidity is usually found at which the rate of corrosion increases markedly. This is attributed to the hygroscopic nature of the corrosion product (see later). In the case of iron and steel it appears that there may even be a tertiary critical humidity . Thus at about 60% r.h. rusting commences at a very slow rate (primary value) at 75-80% r.h. there is a sharp increase in corrosion rate probably attributable to capillary condensation of moisture within the rust . At 90% r.h. there is a further increase in rusting rate corresponding to the vapour pressure of saturated ferrous sulphate solution , ferrous sulphate being identifiable in rust as crystalline agglomerates. The primary critical r.h. for uncorroded metal surfaces seems to be virtually the same for all metals, but the secondary values vary quite widely. 

Capillary condensation The vapour pressure above a concave meniscus of water is less than that in equilibrium with a plane water surface. It is therefore possible for moisture to condense in narrow capillaries from an atmosphere of less than 100% r.h. 

Corrosion in soil is aqueous, and the mechanism is electrochemical (see Section 1.4), but the conditions in the soil can range from atmospheric to completely immersed (Sections 2.2 and 2.3). Which conditions prevail depends on the compactness of the soil and the water or moisture content. Moisture retained within a soil under field dry conditions is largely held within the capillaries and pores of the soil. Soil moisture is extremely significant in this connection, and a dry sandy soil will, in general, be less corrosive than a wet clay. 

It is best prepd by mixing 543 parts of 72% perchloric ac with 45.7 parts of 100% perchloric ac. As it is very hygroscopic, it must be sealed in capillaries or otherwise protected from moisture (Ref 27) Qf -19.7kcal/mole (Ref 20). It is... 

In the lower region of the unsaturated zone, immediately above the water table, is the capillary fringe, where water is drawn upward by capillary attraction. Above the capillary fringe, moisture coats the solid surfaces of the soil or rock particles. If the liquid coating becomes too thick to be held by surface tension, a droplet will pull away and be drawn downward by gravity. The fluid can also evaporate and move through the air space in the pores as water vapor. 

A typical profile of the pressure potential of soil moisture tested by a tensometer across the free-water surface shows a negative pressure (lower than atmosphere pressure) in the capillary zone (Figure 18.2). The negative pressure in the capillary zone indicates that the capillary zone belongs to the unsaturated zone. 

In an unsaturated zone, the capillary force becomes predominant, and the pressure gradient becomes a suction gradient. Hydraulic conductivity is no longer constant, but is a function of the water content or suction, which is greatest in value when the soil is saturated and decreases in value steeply when the soil water suction increases and the soil loses moisture. 

Performance data Percolation is being measured with a lysimeter connected to flow monitoring systems, soil moisture is being measured with water content reflectometers, and soil matric potential and soil temperature are being monitored with heat dissipation units. From November 1999 to July 2002, the capillary barrier cover system had a cumulative percolation of 0.5 mm. Total precipitation was 837 mm over the 32-month period. Additional field data were collected through 2005. 

---