Drying kinetics

M. J. Pikal, S. Shah, M. L. Roy, and R. Putman, The secondary drying stage of freeze drying drying kinetics as a function of shelf temperature and chamber pressure, Int. J. Pharm., 60, 203-217 (1990). 

Pikal, M. J., Shah, S., Roy, M. L., Putman, R. The secondary stage of freeze-drying Drying kinetics as a function of temperature and chamber pressure. Intern. Journ. of Pharmaceutics, 60, p. 203-217. Elsevier Science Publishers B. V. (Biomedical Division) 1990... 

J Wolff, E., Gibert, H. Rudolf, F Vacuum freeze-drying kinetics and modeling of a liquid in a vial. Chem. Eng. Process., 25 (3), p. 153-158, 1989... 

Van Nieuwenhuijzen, N.H., Zareifard, M.R., and Ramaswamy, H.S. 2001. Osmotic drying kinetics of cylindrical apple slices of different sizes. Dry. Technol. 19, 525-545. 

Efremov, G. and Kudra, T., Calculation of the effective diffusion coefficients by applying a quasi-stationary equation for drying kinetics. Drying Tech., 22 (2004) 2273-2279. 

Madhiyanon, T. and Soponronnarit, S., High temperature spouted bed paddy drying with varied downcomer air flows and moisture contents effects on drying kinetics, critical moisture content, and milling quality. Drying Tech., 23 (2005) 473-495. 

Niamnuy, C. and Devahastin, S., Drying kinetics and quality of coconut dried in a fluidized bed dryer, /. Food Eng., 66 (2005) 267-271. 

Krokida, M. K., Karathanos, V. T., Maroulis, Z. B., Marinos-Kouris, D. (2003). Drying kinetics of some vegetables. 

Carter, E.J.V., Paredes, G.E., Beristain, C.l. and Tehuitzil, H.R. (2001) Effect of foaming agents on the stability, rheological properties, drying kinetics and flavour retention of tamarind foam mats. Food Research International 34, 587-598. 

TABLE 14.1 Spherical Green Body Drying Kinetics by Mass... 

Even though one mass transfer mechanism can usually be invoked to approximate the drying kinetics at any particular time, in reality several mechanisms may occur simultaneously. For this reason it may be difficult or impossible to accurately model drying kinetics in the falling rate period. 

Innovations in particle engineering have been complemented with advances in the understanding of the particle formation process, which has motivated research into the physical and chemical mechanisms that control the drying kinetics and particle formation and many aspects of the drying of droplets in an air stream in small scale spray dryers... 

Bennett DB, K. BT, Snyder HE, Platz RM, Inventors Nektar Therapeutics, Inc., assignee. Spray drying process control of drying kinetics. US patent 20030044460, 2000. 

For a pure water drop, the driving force for drying is the difference between the vapor pressure of water and the partial pressure of water in the gas phase. The rate of drying is proportional to this driving force please see the discussion on drying kinetics later in this chapter. 

There are several advantages to the dynamic vapor sorption device. First, any humidity value can be dialed in, whereas salt solutions are not available for every humidity value and some are quite toxic. Second, since the weight is monitored as a function of time, it is clear when equilibrium is reached. The dynamic devices also give the sorption/desorption rates, although these can easily be misused (see the drying kinetics section later). The salt solution method, on... 

Example 16 Air Drying of a Thin Layer of Paste Simulate the drying kinetics of 100 Um of paste initially containing 50 percent moisture (wet-basis) with dry air at 60°C, 0 percent relative humidity air at velocities of 1, 10, or 1000 m/s (limiting case) and at 60°C, 0 percent relative humidity air at 1 m/s. The diffusion coefficient of water in the material is constant at 1 x 10" ° mVs. The length of the layer in the airflow direction is 2.54 cm. 

The drying kinetics (rate of drying, and hence required drying time) also depend strongly on solids properties, particulany particle size and porosiW. The surface area/mass ratio and the internal pore structure control the extent to which an operation is diffusion-limited, i.e., diffusion into and out of the pores of a given solids particle, not through the voids among separate particles. 

Again, these methods take no account of the actual drying kinetics of the particle, which are included in the next section. 

These models use experimental data from drying kinetics tests in a laboratory, pilot-plant or full-scale dryer, and are thus more accurate and reliable than methods based only on estimated drying kinetics. They treat the dryer as a complete unit, with drying rates and air velocities averaged over the dryer volume, except that, if desired, the dryer can be subdivided into a small number of sections. These methods are used for layer dryers (tray, oven, horizontal-flow band, and vertical-flow plate types) and for a simple estimate of fluidized-bed dryer performance. For batch dryers, they can be used for scale-up by refining the scoping design calculation. 

The mass and heat balance equations are the same for any type of dryer, but the particle transport equation is completely different, and the heat- and mass-transfer correlations are also somewhat different as they depend on the environment of the particle in the gas (i.e., single isolated particles, agglomerates, clusters, layers, fluidized beds, or packed beds The mass-transfer rate from the particle is regulated by the drying kinetics and is thus obviously material-dependent (at least in falling-rate drying). 

Application of concept of characteristic drying curve A linear-falling rate curve implies the following equation for the drying kinetics ... 

---