COMPARATIVE STUDY OF THE EVAPORATION CAPACITY OF THE CONVENTIONAL AND JET SPOUTED BED DRYER FOR LIQUID MATERIALS C.R.F. SOUZA and W. P. OLIVEIRA Faculdade de Ciências Farmacêuticas de Ribeirão Preto/USP, Via do Café S/N., 14040-903, Ribeirão Preto, SP, Brazil, Email: wpoliv@fcfrp.usp.br, FAX: (16)6331941, Brazil. ABSTRACT This work presents a comparative study of the evaporation capacity of the conventional and Jet spouted bed dryers. The conventional spouted bed (CSB), has an inlet orifice diameter of 33 mm, diameter of the cylindrical column of 150 mm and height of 400 mm. Connected to the cylindrical column there are a conical base with included angle of 40 o or 60 o. The jet spouted bed (JSB), consist of a conical base with included angle of 38 o, inlet orifice diameter of 60 mm and upper diameter of 340 mm. Two distinct inert material were used: glass beads and teflon beads. Distilled water was used as standard liquid in the evaporation tests. The variables analysed were the inlet gas temperature, (80 to 150 o C), the static bed height, ( 7 to 14 cm), the relation between the feed flow rate of the spouting gas by the feed flow at minimum spouting, Q/Q ms (1.4 and 1.85) and the conical base angle, (40 o and 60 o ). The results showed that the JSB with equal to 7 cm was the most efficient configuration. INTRODUCTION The drying of liquid materials, such as pastes, suspensions and solutions is an important application of spouted beds. This process has already been applied to the drying of several chemical and biological products (REGER et al., 1967; PHAM, 1983; SCHNEIDER and BRIDGWATER, 1989; MARKOWSKI, 1992; RUNHA et al, 2001). This equipment has low installation and operational costs and shows elevated heat and mass transfer rates, reflected in higher drying rates. In the original design, the spouted bed consists of a cylindrical column connected to a conical base, know as conventional spouted bed (CSB). Currently, there are several others configurations of this equipment. However, an overview of the literature shows that there is a lack of information regarding physical modelling of this process without which the design and construction of this equipment becomes a difficult task. This occurs due the fact the studies already developed are specific for a determined type of equipment and type of material. The development of comparative studies between the several dryer s configurations are extremely important to identify the configuration presenting the best performance in an operation of drying of liquid materials. Due to, this work was developed with the objective to compare the evaporation capacity of three distinct configurations of spouted bed dryers: the conventional spouted bed with included angle of 40 o ; the conventional spouted bed with included angle of 60 o and the jet spouted bed (JSB). Materials MATERIALS AND METHODS Two distinct inert materials were employed in this work: glass beads with mean diameter of 2.8mm and concave-cylindrical particles of polytetrafluorethylene, (Teflon ), with a mean diameter of 5.45 mm. The physical characteristics of these materials are presented in Table 1 where m p is the mean unitary mass, d p is the mean particle diameter, p is the density, S is the surface area and is the shape factor. In the evaporation tests distilled water was used as standard liquid (SPITZNER NETO, 1997).
Equipment Jet spouted bed: The jet spouted bed consisted of a conical base with an inlet orifice diameter of 60 mm, included angle of the conical base of 38º, upper diameter of 340mm and height of 520mm. All parts were constructed in stainless steel. The figure 1 show a schematic diagram of the jet spouted bed dryer used. Conventional spouted bed: This equipment consists of a conical base with an inlet orifice diameter of 33mm. Connected to the conical base there are a cylindrical column with diameter of 150mm and height of 400mm. The upper part of the equipment is constituted by another cone. Two different conical bases, respectively, with included angle of 60 o and 40 o were used in the evaporation tests. Table 2 presents the main dimensions of the equipaments used. Table 1: Physical properties of the inert material (Teflon and glass beads). Material m p0 (mg) d p0 (mm) p0 (g/cm 3 ) S (cm 2 /g) (-) Teflon 183.0 5.45 2.16 5.27 0.96 Glass 35.3 2.8 2.50 24.0 1.0 Table 2: Main characteristics of the spouted bed dryers used in this work. Equipment Conventional Spouted Bed (CSB) Jet Spouted Bed (JSB) Main Dimensions d c = 150 mm = 40 e 60 H t = 400 mm d i = 33 mm d c = 340 mm = 38 H t = 520 mm d i = 60 mm The fluiddynamic and temperature data were obtained with the aid of an acquisition data system (acquisition data board PCL 711-S ADVANTECH and a temperature reading board, PCL 789D), installed in a Pentium 166 running the software LABTECH. Thermocouples, pressure transducers and a system to measure the air humidity (psychrometer), were employed in the equipment instrumentation. Experimental procedure Figure 1: Schematic diagram of the Jet Spouted Bed (JSB). Fluiddynamic characterization of the equipment: The global parameters of the spouted beds, P m, P j and Q ms, were determined through measurements of pressure drops in the bed as a function of gas flow rate introduced (MATHUR et al., 1974). Determination of evaporation capacity of the dryer, W máx : First, the maximum evaporation capacities in the dryers were determined. The operation started with the introduction of a given load of inert material into the equipment. Spouting occurred by injecting air at the base of the bed. With the establishment of the spout, the inlet air was heated to the desired temperature. Measurements of the outlet gas temperature were carried-out in order to detect when the process reaches the steady state. Attained the steady state, the feed of distilled water was started, after which measurements of the inlet gas temperature,
P (N/m 2 ), of the effluent gas temperature, T go, and of the dry bulb temperature and moist bulb temperature, T bs and T bu, respectively, were carried out. From the measurements of the dry and moist bulb temperature, the outlet gas humidity could be determined, through the utilization of the software DRYPAK v. 3, developed by PAKOWISK (1996). The obtained data were used to: - Determination of the time where the process attained the steady state and, - Determination of the evaporation capacity of the dryers by two criterions: the gas humidity saturation criterion and by the development of instabilities during the operation. The procedure used for the estimation of the evaporation capacity by the gas humidity saturation criteria was adapted from SPITZNER NETO, (1997). The variables analysed were the type of the inert material (glass and Teflon beads); the static bed height, (7-14cm); the inlet gas temperature, (80-150 o C); the ratio between the gas flow introduced in the system relative to minimum spouting, Q/Q jm and the equipment configuration. Table 3 presented variables and ranges used. Equipment Conventional Spouted Bed (CSB) Jet Spouted Bed (JSB) Table 3: Variables and ranges used. Inert Material Glass Teflon Teflon Variable Range Unity T ge Q/Q ms T ge Q/Q ms 7-14 40-60 80-150 1.4 and 1.85 7-14 38 80-150 1.4 and 1.85 cm degree C - cm degree C - Comparison criterions: To compare the performance of the spouted bed dryers during operations of evaporation of distilled water were used the following parameters: mass of the evaporated water - Volumetric evaporation rate, Ev = ; [kg/m 3.s] Particle volume - Ratio between the mass of evaporated water relative to the feed mass flow-rate of gas = W max /W g ; [ - ] - Ratio between the mass of evaporated water relative to the surface particle s area = W max /Ap; [kg/m 2.s] - Ratio between the mass of water evaporated relative to the mass of the inert material into the bed = W max /M 0, [1/s] These parameters were proposed by MARKOWSKI (1992) to compare the drying performance of the Jet spouted bed with spray dryers. RESULTS AND DISCUSSION Fluiddynamic characterization of the dryers Graphs similar to presented in the literature were obtained in the fluiddynamic tests performed with glass and with teflon beads. Figure 2 show a typical fluiddynamic result obtained for the jet spouted bed with teflon beads. From the results obtained was possible to estimate the experimental values of the maximum pressure drop, P m, of the pressure drop of stable spouting, P s, and of the minimum spouting flow rate, Q ms. Table 4 show the results obtained for all spouted bed configurations studied. These fluid fluiddynamic data were used to select the operational conditions utilized in the evaporation tests. 3500 3000 2500 2000 1500 1000 500 increasing the gas flow decreasing the gas flow 14.0 cm JSB 7.0 cm 0 0.0 0.5 1.0 1.5 2.0 2.5 Q (m 3 /min) Figure 2: Fluiddynamic result for the JSB with teflon beads.
- T go ( o C) Tgi - Tgo o C Table 4: Experimental results of the minimum spouting flowrate, (Q ms ), of the minimum spouting velocity (U ms ), of the maximum pressure drop ( P m ) and of the pressure drop of stable spouting ( P j ). EQUIPAMENT J S B C S B MATERIAL Teflon beads Glass beads Teflon beads (graus) (cm) Q ms (m 3 /min) Ums (m/s) P m (N/m 2 ) Ps (N/m 2 ) 40 7.0 0.65 12.67 1150 350 40 10.5 1.00 19.49 1850 700 40 14.0 1.30 25.34 2350 750 60 7.0 0.73 14.23 950 300 60 10.5 1.35 26.31 1650 650 60 14.0 1.50 29.22 2150 850 40 7.0 0.40 7.79 2200 750 40 10.5 0.58 11.30 2450 950 40 14.0 0.80 15.59 3200 1250 60 7.0 0.50 9.75 1650 600 60 10.5 0.90 17.54 2350 900 60 14.0 1.00 19.49 2900 1600 38 7 0.9 5.3 1350 740 38 14 1.4 8.25 2700 1450 Determination of the steady state regime Graphs relating ( T go ) as functions of the operation time were constructed in order to estimate the instant were the dryers attained the stationary condition. The Figure 3 presents typical graphs obtained. From these Figures it can be observed that the steady state regime is established near 500 seconds of the drying operation. Only the data obtained after 500 seconds were employed for the estimation of the parameters used to compare the dryer s configurations. 90 80 70 60 50 40 30 20 10 0 (a) W H20 = 5.0 g/min W H20 = 10.0 g/min W H20 = 15.0 g/min W H20 = 20.0 g/min W H20 = 25.0 g/min 0 200 400 600 800 1000 90 (b) W H2O = 10 g/min 80 70 60 W H2O = 20 g/min W H2O = 25 g/min W H2O = 30 g/min W H2O = 35 g/min 0 200 400 600 800 1000 (s) (s) Figure 3: -T go as a function of the processing time having the feed flow rate of the distilled water in the bed as a parameter, for the CSB with 60 o (a) and for JSB (b) with =80 o C, = 7.0 cm and Q/Q jm = 1.85. Determination of the steady state regime Graphs relating ( T go ) as functions of the operation time were constructed in order to estimate the instant were the dryers attained the stationary condition. The Figure 3 presents typical graphs obtained. From these Figures it can be observed that the steady state regime is established near 500 seconds of the drying operation. Only the data obtained after 500 seconds were employed for the estimation of the parameters used to compare the dryer s configurations. 50 40 30 20 10 0
Y g,sat -Y go (W máx /W g ) sat Comparison between the criterions of the saturation and instabilities in the bed The flow rate where the outlet gas humidity equalled to saturation humidity was assumed as the evaporation capacity by the gas humidity saturation criterion. This parameter was determined through linear equations fitted to the graphs relating the difference similar those presented in Figure 4. The flow-rate where drops of water arise in the cyclone or in the flanges of the equipment was assumed as the instability criterion for the estimation of the evaporation capacity. To compare the results obtained by the two criterions was constructed the Figure 5. This Figure show that the instabilities in the dryer arise at flow-rates lower than that obtained by the saturation criterion. This behaviour is an indication that the temperature profiles is not uniform inside the bed, with the occurrence of cold zones, facilitating the condensation of the water vapour into the bed before the effluent gas reach the saturation. 0,05 0,04 0,03 = 80 o C = 115 o C = 150 o C Fitting Linear 0.060 +20% +50% 0.040 0,02 0.020 0,01 0,00 0 20 40 60 80 100 W H2 O (g/min) Figure 4: Typical graph used for the determination of the evaporation capacity of the spouted bed dryer (CSB with =14cm, Q/Q jm =1.4 and = 60 o ) 0.000 0.000 0.020 0.040 0.060 (W máx /Q) inst Figure 5: Comparison between the values of (W máx /W g ) obtained by the saturation and by the stability criterions. The Figures 6 (a) and (b) presents, respectively, experimental results of the relation between the evaporation capacity by the mass feed flow-rate of the spouting gas, W máx /W g, and of the volumetric evaporation rate, E v, as a function of the inlet gas temperature, system configuration and type of inert material, for = 7 cm and Q/Q jm = 1.4. From these Figures it can be observed that the JSB presented the best performance among the configurations studied, for = 7 cm and Q/Q jm = 1.4. It can be observed also that the higher Ev values were obtained for the experiments carried out with Teflon beads as inert material. For = 14 cm, the best performance was verified for the CSB with included angle of 40 o. However, for this bed height, the ratio /d i is bigger than 2, therefore, higher than the recommended range for the establishment of the JSB regime. The Figures 7 (a) and 7 (b) show, respectively, experimental results of the relation between the evaporation capacity by the mass feed flow rate of the spouting gas, W max /W g, and of the volumetric evaporation rate, E v, as a function of the inlet gas temperature, system configuration, type of inert material and static bed height, for Q/Q jm = 1.4. From these Figures, it can be observed that higher values of W max /W g and of the E v were obtained for the static bed height of 7 cm. From the results obtained in all experiments carried out, was constructed the Table 5, where can be observed the effect of the variables analysed on the comparison parameters: W max /W g, E v, W max /Ap e W max /M 0.
W máx /W g Ev (Kg/m 3.s) W máx /W g Ev (Kg/m 3.s) 0.055 0.050 CSB Teflon CSB Vidro = 40 o CSB Teflon JSB Teflon CSB Vidro = 60 o ( a ) 5.0 4.0 CSB Teflon CSB Vidro = 40 o CSB Teflon JSB Teflon CSB Vidro = 60 o ( b ) 0.045 0.040 0.035 0.030 0.025 3.0 2.0 0.020 0.015, Q/Q gjm =1.4 80 100 120 140 160 ( o C) 1.0, Q/Q jm =1.4 80 100 120 140 160 ( o C) Figure 6: Experimental results of the relation between the evaporation capacity by the feed flow rate of the spouting gas, W max /W g and of the volumetric evaporation rate, E v, as a function of the inlet gas temperature, system configuration and type of the inert material, for = 7 cm and Q/Q jm = 1.4. 0.060 H=14cm CSB Teflon 60 o 0.055 H=14cm CSB glass 60 o H=14cm JSB Teflon 0.050 (a) 5 4 H=14cm CSB Teflon 60 o H=14cm CSB glass 60 o H=14cm JSB Teflon (b) 0.045 0.040 0.035 0.030 0.025 0.020 3 2 1 0.015 80 100 120 140 160 80 100 120 140 160 ( o C) ( o C ) Figure 7: Experimental results of the relation between the evaporation capacity by the feed flow rate of the spouting gas, W max /W g and of the volumetric evaporation rate, E v, as a function of the inlet gas temperature, system configuration, type of the inert material and static bed height for Q/Q jm = 1.4. Table 5: Effect of the studied variables on the comparison parameters of the dryers. VARIABLE W max /M 0 (1/s) W max /Ap (Kg/m 2.s) W max /W g (Kg/Kg) Ev (Kg/m 3.s) Air Consumption (Kg/Kg H 2 O) Q/Q jm Increase parameter Decrease the parameter no effect in the parameter
CONCLUSIONS From the exposed the following conclusions could be shown: - The experimental values of the evaporation capacity determined by the gas saturation humidity criterion are higher than estimates obtained by the instability criterion. - The drying of liquid materials in spouted beds occurs in stationary conditions 500 seconds after the start of the liquid feeding to the bed. - An increment in the temperature of the inlet gas produces an elevation in the experimental values of the relation between the evaporation capacity by the feed flow rate of the spouting gas introduced to the system. - The performance of the JSB with = 7cm was better than the CSB configurations investigated. - No significant differences between the experimental values of the relation between the evaporation capacity by the feed flow rate of spouting gas obtained for the CSB were observed for the tests carried out with glass and with Teflon beads. - Higher values of the evaporation capacity were obtained for the tests carried out with teflon beads. NOTATION d c = column diameter, [mm] d i = inlet orifice diameter, [mm] d po = inert material diameter, [mm] = static bed height, [cm] m po = mass of the inert material, [M] S = surface area of the inert material, [cm 2 /g] T bs = dry bulb temperature, [ 0 C ] T bu = moist bulb temperature, [ 0 C ] = inlert gas temperature, [ 0 C ] T go = outlet gas temperature, [ 0 C ] U jm = minimun spouting velocity, [m/s] Q = flow rate of the spouting gas, [m 3 /min] Q ms = minimum spouting flowrate, [m 3 /min] W máx = evaporation capacity, [g/min] Wg = mass flow rate of the gas, [g/min] Y ar = umidade de saída do ar, [ - ] Y sat = saturation humidity of the efluent gas, [ - ] Pj = pressure drop of the stable spouting, [N/m 2 ] Pm = maximum pressure drop, [N/m 2 ] = shape factor, [ - ] = included angle of the conical base, [degree] = processing time, [ min] po = densidade das partículas, [g/cm 3 ] ACKNOWLEDEGEMENTS We would like to express our gratitude to Foundation for Research Support of the São Paulo State (FAPESP) for a fellowship for the first author and for the financial aid. REFERENCES BARRET, N. e FANE, A., (1990); Drying liquid materials in a spouted bed, Drying 89, New York, Hemisphere Publ, Co., v. B, p. 415-420.
MARKOWSKI, A.S., (1992); Drying Characteristics in a Jet-Spouted Bed Dryer, Can. J. Chem. Eng., v. 70, n. 5, p. 938-944. MARKOWSKI, A.S., (1993); Quality Interaction in a Jet Spouted Bed Dryer for Bio-Products, Drying Tech., v. 11, n. 2, p. 369-387. MATHUR, K.B. e EPSTEIN, N., (1974); Dynamics of spouted beds, Adv. Chem.Eng., v. 9, p. 111-191. PHAM, Q.T., Behavior of a conical spouted bed dryer for animal blood, Can. J. Chem. Eng., v. 61, p. 426-434, 1983. PAKOWSKI, Z. (1996), drypakv.3 - Program for psychrometric and drying computations, users manual, janeiro. PATEL, K. et al, (1993); Spouting behavior of wet solids, Drying 86, New York, Hemisphere Publ. Co., v.1, p. 415-420, 1986. In: RODRIGUES, C.C. Análise da secagem de suspensões em leito de jorro com partículas inertes, São Carlos, SP, PPG-EQ, UFSCar, 112p., (Dissertação). RÉ, M.I.; FREIRE, J.T., (1988); Drying of paste-liquid material in spouted beds. International Drying Symposium, Op119, Versailles. REGER, E.O., ROMANKOV, P.G. e RASHKOVSKAYA, N.B., Drying of paste-like materials on inert bodies in a spouting bed, Zh, Prikl. Khim. (Leningrad), v. 40, n. 10, p. 2189-2191, 1967. REYES, A.E.S., (1994); Secagem de suspensões em leito de jorro cônico, Rio de Janeiro, RJ, COPPE- UFRJ, 184p., (Tese). RUNHA, F.P., et al, Production of Dry Extracts of Medicinal Brazilian Plants by Spouted Bed Process: Development of the Process and Evaluation of Thermal Degradation During Operation, Trans IChemE, v. 79, Part C, September, 2001. SCHNEIDER, T. e BRIDGWATER, J. Drying of solutions and suspensions in spouted beds, Drying 89, Hemisphere Publ. Co., New York, USA, p.421-425, 1989. SPITZNER NETO, P.I., (1997); Secagem de pastas em leito de jorro, PPG-EQ/UFSCar, São Carlos, SP, 197p., (Tese).