An experimental study of fluidized bed drying of granular materials

Fluidizacija je proces kojim se sloj čestica čvrste tvari u dodiru s fluidom koji ga prostrujava dovodi u stanje slično kapljevini koja vrije. čestice se pri tom kreću u određenom volumenu, a nastala mješavina i sama poprima neka svojstva fluida. U fluidiziranom sloju dolazi do intenzivnog miješanja čestica u krutoj fazi te čestica i fluida zbog čega se ostvaruje veliki intenzitet prijenosa topline i tvari kao i niz još drugih prednosti. Prva značajna primjena fluidizacije bila je u kemijskoj industriji 40-ih godina dvadesetog stoljeća kada se počela koristiti za katalitičko krekiranje teških hidrokarbonata u sloju katalitičkih čestica. \No vrlo brzo se primjena počela širiti i na druge fizikalne operacije i kemijske reakcije tako da se danas fluidizacija intenzivno koristi u farmaceutskoj, prehrambenoj, kemijskoj, naftnoj i ostalim granama industrije. Uz širenje primjene započeto je i značajnije istraživanje fluidizacije, pa su u razdoblju između 60-ih i 80-ih godina postavljene fundamentalne zakonitosti ponašanja plinskih fluidiziranih slojeva, a izdano je i mnoštvo literature koja se bavi miješanjem čestica, mjehurima, te prijenosom topline i tvari u sloju. Obzirom da je fluidizacija vrlo interesantna za termotehniku kao perspektivni način poboljšanja izmjene topline između stijenke i fluida, ona predstavlja jedno od polja istraživanja Katedre za tehničku termodinamiku Fakulteta strojarstva i brodogradnje. Istraživanja su započela tijekom 80-ih godina (doktorski rad prof. Galovića pod nazivom „Prilog izmjeni topline između fluidizirane kupke i uronjene čvrste stijenke“), a nastavljena su u okviru Znanstvenih projekata „Intenziviranje izmjene topline na izmjenjivačkim površinama (1996-2002.) i „Izmjena topline i tvari pri kondenzaciji, ishlapljivanju i sušenju“ (2002-2005.) prijavljenih pri MZOš. U tom razdoblju objavljeno je niz radova u različitim publikacijama, a treba istaknuti da su istraživanja bila ne samo teorijska nego i eksperimentalna na mjernoj liniji projektiranoj i postavljenoj u prostoru Laboratorija za tehničku termodinamiku. Ovaj rad se nastavlja na ta istraživanja (poglavito na magistarski rad autora pod nazivom „Analiza prijelaza topline između fluidiziranog sloja i uronjene zavojnice“), a nastao je u okviru projekta „Intenziviranje izmjene topline i tvari pri kondenzaciji, ishlapljivanju i sušenju“.The experimental investigation of the fluidized bed drying of the selected grains, pearly barley and wheat, was performed with the following objectives: - to define their drying kinetics (drying curves, periods of drying, influence of process parameters on the drying kinetics), and to quantify the intensity of heat and mass transfer in a fluidized bed; - to present the obtained results in the form of non-dimensional correlation functions that are structured by the appropriate number of relevant non-dimensional numbers commonly cited in the literature in view of such problems; - the obtained results needs to be critically analyzed and compared with the results of similar recent studies of other researchers, and also compared with drying rates achieved by conventional drying methods. The cylindrical fluidization column with perforated plate as air distributor was used in this study. In the experiments, moisture content, relative humidity and temperature of the inlet and exit air, moisture content of the inlet and exit particles, and air mass flow are measured. To determine the mass of dried specimens (fluidized particles), and thus to determine the mass of evaporated moisture, the precision balance was used. Since the results are reduced to a mass of dry matter, for complete removal of moisture from the samples and to obtain the mass of bone dry solid, standard laboratory furnace was used. The experimental investigation resulted in the following: 1. The minimum fluidization velocity was obtained by measuring bed pressure drop across a bed of wetted grains with a moisture content of about 25% d.b. and bed height 130 mm. The measured minimum fluidization velocities were 0,9 m/s for pearl barley, and 0,85 m/s for wheat. The range of air velocities for the fluidization was selected based on these results: 1,0 m/s, 1,07 m/s and 1,15 m/s. Despite a somewhat higher velocity than the minimum, initial grains moisture content was usually higher than 25% and grains are sticky and cohesive, so the bed started to fluidize only after a certain time when the particles are sufficiently dried. 2. The experiments were conducted at three inlet air temperatures (35 °C, 42 °C and 50 °C) and three drying air velocities (1,0, 1,07 and 1,15 m/s) and moisture contents data with time (drying curves) was obtained. Based on the measured moisture contents, drying rates were calculated and plotted as a function of the drying time. The measured values of moisture content are presented in dimensionless form in order to be able to analyze the effect of temperature and air velocity on drying time for different values of initial moisture content of grains and air inlet conditions. 3. The change of fluidized bed temperature with time was also plotted. Temperatures were measured at three bed heights: 45, 75 and 105 mm above the distributor, and they are good indicator of the fluidization quality because in fully fluidized bed temperatures should be completely uniform. 4. Nondimensional correlation functions for the investigated fluidization and temperature ranges were obtained by means of mathematical computation. 5. The results were analyzed and compared with results of similar recent studies of other authors and with the results of conventional grain drying method. Based on the obtained results and the experimental-computational analysis, the following conclusions were carried out: 1. Grain drying curves have a characteristic curved shape, so no constant rate period is observed in the course of drying, and drying takes place in the falling rate period. Surface moisture evaporates very quickly, so drying takes place inside the grains and the drying process is mostly controlled by internal mass transfer parameters (diffusion). 2. The inlet air temperature has a dominant influence on the drying rate, thus the drying time is greatly reduced at higher temperatures. The air velocity range in this study was relatively small, only 15%, but it can be concluded that the air velocity is very important in the initial stage of drying when the grain surface is partially or fully saturated, and when the maximum drying rates are achieved due to very intense evaporation of surface moisture. By increasing the air velocity the amount of bubbles in the bed is also increased, so the mixing of particles is greatly enhanced. Also, higher air velocities ensures the supply of fresh air with low humidity because it is very important that in the initial stage air is not saturated with moisture and thus reduce the drying rate. In a later stage of drying, when diffusion of moisture from inside the grain controls the drying rate, the influence of air velocity becomes much smaller. At lower moisture content of air, the difference of water vapour partial pressures in air over the saturated grain surface is increased, resulting in greater drying rates. The initial moisture content of the grains hase an important effect on the drying rate. Grains with higher initial moisture content have greater drying rates in the initial stage of drying, but in the later stage moisture diffusivity is also increased with the moisture content. The influence of bed height and particle size of grains on the drying rates was not studied in this research, but it can be found from the literature that drying rate decreases with increasing bed height and particle size. 3. Nondimensional correlation functions for the investigated fluidization and temperature ranges were obtained by means of mathematical computation. The equation for pearl barley is valid for the range of nondimensional temperatures: 1,70 ≤ T* ≤ 48, the range of Fourier numbers: 10 ≤ Fo ≤ 226, and the range of Reynolds numbers: 207 ≤ Re ≤ 269. The equation for wheat is valid for the range: 1,82 ≤ T* ≤ 42,08, 4 ≤ Fo ≤ 90 and 226 ≤ Re ≤ 288. It must be emphasized however that those correlation functions are based on Page empirical model and as such are valid only for the material being dried, the applied process parameters and the specific fluidization column geometry. 4. Drying rates from this research are compared with ones from commercial batch grain dryer. Drying rates in these conventional dryers are much smaller compared with rates in fluidized bed dryers with large horizontal and vertical moisture gradients. On the contrary, grain moisture content in fluidized bed is very uniform if fully fluidized. 5. The obtained results and conclusions in this research are consistent with similar recent studies by other authors