Phases of the Drying Process

In phase III, bound liquid such as solvents bound as solvates or water bound as hydrate is 

In the third phase, bound liquids, that is, solvates bound in solvates or water bound in hydrates, are removed, the drying is discussed in detail below. 

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Setting of ink n. The initial phase of the drying process by slight absorption of the vehicle by the paper, wherein printed sheets, though not fully dry, can be handled without smudging. 

Three phases of the drying process are distinguished I, II, and III. In phases I and II, the mother liquor, first as bulk and second as adhering liquid, is evaporated. This evaporation occurs at the temperature and pressure close to the equilibrium vapor pressure of the mother phase (see Figure 14.8). 

Therefore, product temperature should be monitored closely to control the fluidized bed drying process. During fluid-bed drying, the product passes through three distinct temperature phases (Fig. 21). At the beginning of the drying process, the material heats up from the ambient temperature to approximately the wet-bulb temperature of the air in the dryer. This temperature is maintained until the granule moisture content is reduced to the critical level. At this point, the material holds no free surface water, and the temperature starts to rise further. 

The absorption of excess exudate not only avoids tissue maceration but also removes exotoxins or cell debris that may retard growth or extend the inflammatory phase of the healing process. The balance between humidity and absorption is critical and excessive wick-ing must be avoided to prevent drying and necrosis. 

The measured electric field wilt rise sharply as free solvent becomes exhausted and the microwaves attempt to couple with the ever-decreasing solvent load (or product load). At this time the temperature may also sharply rise. Therefore, it is imperative to cut back on the amount of microwave energy going into the chamber when the end of the steady state phase is reached. By proper instrumentation of the dryer careful monitoring of the drying process should allow the operator to prevent overheating of the product. The relationship between the electric field and temperature can be determined experimentally on small batches and ultimately be used to control the drying process. 

If vacuum is applied, the phase boundary between liquid and vapour is reached at a certain pressure and water starts to vaporize. To prevent a loss of temperature, heat -the so-called heat of evaporation - has to be introduced from the outside. If all water has been evaporated, a rise in temperature will ensue that can be taken as an indicator for the end of the drying process. Although this method is gentle, the material to be dried is puffed up and structural and cell deformations follow. 

Figure 14.6 Van Tent s view of the drying process for a latex film. As in Figure 14.1, loss of water from the dispersion is accompanied by the particles coming closer together at successive times ti.tj. In model (b), a discrete aggregated phase forms at the air-water interface above the fluid (Uspersion. In the fully packed dispersion (t4), 2A refers to the distance between successive layers of latex particles. (Taken from ref. [12c].)...

These results give rise to a picture of the drying process like that drown in Figure 14.6 [12c]. In this picture one sees an ordered dispersion in the bulk with a particle separation of 13 run. and a flocculated phase at the air-water intoface. According to van Tent, as water is lost, fire particle density in the bulk remains constant, but the floe layer at the surface becomes tbidmr until it reaches the substrate. 

This image has similarities to that presented CroU in his modd of the drying process. CroU envisions the top l er to be transparent and essentially dry, but with a percolation network of tir pores. This sits atop the flocculated phase, which in turn rests upon tire aqueous dispeisian. Tent s results require the flocculated phase to be wet, witii die particles separated by a water-swollen hydrophilic membrane. If, howevo-, there were a diin transparmt layer at the top which grew steadUy in thickness, the changing interference between reflections at the upiper and lower interfaces would almost certainly be doected in die UV-visible transmission measurements. 

Drying is a separation process that converts a wet solid, semisolid, or liquid feedstock into a solid product by evaporation of the liquid into a vapor phase via heating. Essential features of the drying process are phase change and production of a solid, dried product. 

Three-dimensional steady-state calculations of drying process in vertical pneumatic dryer were performed by [21], The theoretical model is based on two-phase Eulerian-Lagrangian approach and incorporates advanced drying kinetics for wet particles. The model was utilized for simulation of the drying process of wet PVC and silica particles in a large-scale vertical pneumatic dryer. 

Figure 5.18 Schematic illustration of the drying process, (a) before evaporation occurs, the meniscus is flat, (b) Capillary tension develops in liquid as it stretches to prevent exposure of the solid phase, and network is drawn back into liquid. The network is initially so compliant that little stress is needed to keep it submerged, so the tension in the liquid is low, and the radius of the meniscus is large. As the network stiffens, the tension rises, and at the critical point (end of the constant rate period), the radius of the meniscus drops to equal the pore radius, (c) During the falling rate period, the liquid recedes into the gel. (From Ref. 36.)...

It is equal to the tension under the liquid-vapor meniscus [Eq. (5.35)]. The capillary tension in the liquid imposes a compressive stress on the solid phase, causing conttaction of the gel. As outlined later, the capillary tension is smaller than the maximum value during most of the drying process. 

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