Drying velocity

For drying of wet material, experimental and calculated mass loss curves of the water for different gas temperatures are shown In Fig.7. To avoid intaraction with the pyrolysis process, a gas temperature of 150 C has been chosen for the drying of wood. At this tempo-ature the drying velocity is low, thus wood in the expo-iments contained only a small amount of water. Because pyrolysis of wood starts at temp atures of about 200 C, wet slate particles have b n used for drying experiments at higher temperatures. 

Agar-maltodextrin slabs were taken as models to evaluate the influence of the processing conditions over drying velocity (Deg), microstructure generation of the surface in terms of FDbcm and shrinkage. Empirical equations and their coefficients (Table 32.1) corresponding to the fitting polynomial (Equation 32.2 to Equation 32.4) were determined to carry out... 

Furthermore, the following relation between the drying velocity and air flow holds ... 

When spraying solutions of substances of low molecular weight, a first drying step occurs above the solubility limit of the solute, and the drop reduces in size until the solubility limit is reached. For this first step, which is usually called the first or constant rate period (CRP), the drying velocity, that is, the solvent mass evaporated per unit time, is given by the vapor pressure of the solution at the drop surface, p s, and the vapor pressure in the vicinity of the particle, pv.oo. The vapor pressure at the surface depends - assuming a well-mixed state within the droplet - on the drop temperature and on the water activity within the solution. The surface temperature remains low in the CRP as the solvent, due to its heat of evaporation Ah uses up the sensible heat (expressed by the specific heat capacity Cp of the air-vapor mixture) transferred to the particle by the gas in a hot atmosphere. The particle surface temperature Ts is more or less close to the wet bulb temperature of pure water, depending on the water activity in the case of dissolved matter (see Eq. 5.44 in Volume 1 of this series). The dependence of the vapor pressure of the solvent on the surface temperature Ts may be expressed by the Antoine or Clausius-Clapeyron equation, as... 

Pig. 22. Schematic representation of typical pressure drop as a function of superficial gas velocity, expressed in terms of G = /9q tiQ, in packed columns. O, Dry packing , low Hquid flow rate I, higher Hquid flow rate. The points do not correspond to actual experimental data, but represent examples. 

Pressure filters can treat feeds with concentrations up to and in excess of 10% sohds by weight and having large proportions of difficult-to-handle fine particles. Typically, slurries in which the sohd particles contain 10% greater than 10 ]lni may require pressure filtration, but increasing the proportion greater than 10 ]lni may make vacuum filtration possible. The range of typical filtration velocities in pressure filters is from 0.025 to 5 m/h and dry sohds rates from 25 to 250 kg nY/h. The use of pressure filters may also in some cases, such as in filtration of coal flotation concentrates, eliminate the need for flocculation. 

Web Heat-Set Publication and Commercial Inks. Almost all heat-set inks are now printed on web offset presses, and are based on vehicles containing synthetic resins and/or some natural resins. These are dissolved in hydrocarbon solvent fractions which are specially fractionated for use in the ink industry. They vary in boiling range between 180 and 300 °C. Small percentages of alkyd resins (qv) may be contained in these inks. They dry in less than one second by means of solvent evaporation in a heatset oven. These ovens utilize high velocity hot air to raise the web temperature to 120-150 °C. 

A typical coating composition for the CF component is shown in Table 12. It is dried in a high velocity air oven at 93°C. 

After the SO converter has stabilized, the 6—7% SO gas stream can be further diluted with dry air, I, to provide the SO reaction gas at a prescribed concentration, ca 4 vol % for LAB sulfonation and ca 2.5% for alcohol ethoxylate sulfation. The molten sulfur is accurately measured and controlled by mass flow meters. The organic feedstock is also accurately controlled by mass flow meters and a variable speed-driven gear pump. The high velocity SO reaction gas and organic feedstock are introduced into the top of the sulfonation reactor,, in cocurrent downward flow where the reaction product and gas are separated in a cyclone separator, K, then pumped to a cooler, L, and circulated back into a quench cooling reservoir at the base of the reactor, unique to Chemithon concentric reactor systems. The gas stream from the cyclone separator, M, is sent to an electrostatic precipitator (ESP), N, which removes entrained acidic organics, and then sent to the packed tower, H, where SO2 and any SO traces are adsorbed in a dilute NaOH solution and finally vented, O. Even a 99% conversion of SO2 to SO contributes ca 500 ppm SO2 to the effluent gas. 

Many factors affect dry deposition, but for computational convenience air quaUty models resort to using a single quantity called the deposition velocity, designated or to prescribe the deposition rate. The deposition velocity is defined such that the flux T of species i to the ground is... 

To model convection drying both the heat transfer to the coated web and the mass transfer (qv) from the coatiag must be considered. The heat-transfer coefficient can be taken as proportional to the 0.78 power of the air velocity or to the 0.39 power of the pressure difference between the air in the plenum and the ambient pressure at the coatiag. The improvement in heat-transfer coefficients in dryers since the 1900s is shown in Figure 20. The mass-transfer coefficient for solvent to the air stream is proportional to the heat-transfer coefficient and is related to it by the Clulton-Colbum analogy... 

Convection heat transfer is dependent largely on the relative velocity between the warm gas and the drying surface. Interest in pulse combustion heat sources anticipates that high frequency reversals of gas flow direction relative to wet material in dispersed-particle dryers can maintain higher gas velocities around the particles for longer periods than possible ia simple cocurrent dryers. This technique is thus expected to enhance heat- and mass-transfer performance. This is apart from the concept that mechanical stresses iaduced ia material by rapid directional reversals of gas flow promote particle deagglomeration, dispersion, and Hquid stream breakup iato fine droplets. Commercial appHcations are needed to confirm the economic value of pulse combustion for drying. 

Gas impingement from slots, orifices, and nozzles at 10—100 m/s velocities is used for drying sheets, films, coatings (qv), and thin slabs, and as a secondary heat source on dmm dryers and paper (qv) machine cans. The general relationship for convection heat transfer is (13,14) ... 

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