Bound moisture drying

The plug-flow FB drying concept is particularly suitable for drying bound moisture from heat-sensitive materials since the residence time is controlled within narrow limits. In the typical polymer application, this means that the bound moisture can be removed from the polymer product at the lowest possible product temperature. 

Care must be exercised in drying bound moisture under such conditions since the product temperature can rise rapidly. If the feed moisture or rate is susceptible to random variation or different products are to be dried in the same dryer, a suitable control system sensitive to the product surface temperature rise is recommended strongly. 

The plate dryer is limited in its scope of apphcations only in the consistency of the feed material (the products must be friable, free flowing, and not undergo phase changes) and diying temperatures up to 320°C. Applications include speci ty chemicals, pharmaceuticals, foods, polymers, pigments, etc. Initial moisture or volatile level can be as high as 65 percent and the unit is often used as a final dryer to take materials to a bone-dry state, if necessary. The plate dryer can also be used for heat treatment, removal of waters of hydration (bound moisture), solvent removal, and as a product cooler. 

As the bound moisture is removed and the material is heated to a temperature of 58 to 62 °C in the falling-speed drying section, the content of methylol groups in batches Nos. 1,2, and 3 slightly increases, which may be due to the effect of agglomerating additives. 

The free moisture in a particle is the moisture in excess of the equilibrium moisture and may consist of unbound and some bound moisture. Only free moisture can be removed by evaporation during spray drying. 

The presence of unbound moisture in the droplet means that the drying proceeds at a constant high rate as long as the moisture diffusion within the droplet is able to maintain saturated surface conditions. When the diffusional and capillary flows can no longer maintain these conditions, a critical point is reached and the drying rate will decline until equilibrium moisture content is reached. The evaporation of bound moisture is strongly dependent on the nature of the solid matter in the spray droplet. 

In the drying of materials it is necessary to remove free moisture fi om the surface as well as bound moisture fi-om the interior. The drying characteristics of wet solids can be described by plotting the rate of drying against the corresponding moisture content. A typical drying curve is shown in Fig. 1 and it can easily be seen that this is subdivided into four distinct sections ... 

Evaporative cooling—when drying a solid with free or bound moisture, the effect of a phase change from the liquid state to the vapor state removes energy from the liquid-solid mass. This results in a reduction of temperature in a nonadiabatic operation, whereas in an... 

Bound moisture Constant rate drying period Dew point... 

Drying commonly describes the process of thermally removing volatile substances (moisture) to yield a solid product. Moisture held in loose chemical combination, present as a liquid solution within the solid or even trapped in the microstructure of the solid, which exerts a vapor pressure less than that of pure liquid, is called bound moisture. Moisture in excess of bound moisture is called unbound moisture. 

Nonhygroscopic capillary-porous media, such as sand, crushed minerals, nonhygroscopic crystals, polymer particles, and some ceramics. The defining criteria are as follows. (1) There is a clearly recognizable pore space the pore space is filled with liquid if the capillary-porous medium is completely saturated and is filled with air when the medium is completely dry. (2) The amount of physically bound moisture is negligible that is, the material is nonhygroscopic. 

While the convective heat flux expressiou is straightforward, the expression for drying rate needs explanation. The drying rate can be calculated from this formula, when drying is controlled by gas-side resistance. The driving force is then the difference between absolute humidity at equilibrium with solid surface and that of bulk gas. When solid surface is saturated with moisture, the expression for F is identical to Equation 3.48 when solid surface coutains bound moisture, F will result from Equation 3.46 and a sorption isotherm. This is in essence the so-called equilibrium method of drying rate calculation. 

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