Fluidisation behaviour of cylindrical bean particulates were studies and empirical models were presente

Bhandari, Bhesh R.

Senadeera, Wijitha

Wijesinghe, Bandu

Young, Gordon

English

Name not available

Fluidisation behaviour of cylindrical green bean particulates in drying

Queensland University of Technology ePrints Archive

               This is the author-manuscript version of this work - accessed from   http://eprints.qut.edu.au  Senadeera, Wijitha and Bhandari, B. R. and Young, G. and Wijesinghe, B. (1998) Fluidisation behaviour of cylindrical green bean particulates in drying. Tropical Agricultural Research 10:pp. 192-202.  Copyright 1998 (please consult author)             1 1 FLUIDIZATION BEHAVIOUR OF CYLINDRICAL GREEN BEAN PARTICULATES DURING DRYING  Wijitha Senadeera1, Bhesh R. Bhandari1, Gordon Young1 and Bandu Wijesinghe2  1 Food Science and Technology School of Land and Food University of Queensland Gatton College, QLD 4345, Australia  ABSTRACT. Changes in fluidization behaviour of green bean particulates with change in moisture content during drying were investigated using a fluidized bed dryer. Fluidization behaviour was characterised for cylindrical shape particles with three length diameter-ratios; 1:1, 2:1 and 3:1.  All drying experiments were conducted at 50 + 2 0C and 13 + 2 % RH using a heat pump dehumidifier system. Fluidization experiments were undertaken for the bed heights of 100, 80, 60 and 40 mm and at 10 moisture content levels.  Data were analysed using SAS, and an empirical relationship of the form Umf = A + B e -Cm was developed for change of minimum fluidization velocity with moisture content during drying. Also a generalized equation was used to calculate minimum fluidization velocity.   INTRODUCTION  The use of fluidization is one of main applications in drying of agro-food materials. When an air stream is passed through a free flowing material resting on a permeable support, the bed starts to expand when a certain air velocity is reached. The superficial velocity of the air at this stage is the minimum fluidization velocity, with continual increase in air velocity, a stage is reached where the pressure across the fluidized bed drops rapidly, and the product is carried along the air stream. The velocity of air at this stage is called terminal velocity. During fluidizing operations the superficial velocity of the air should remain between minimum fluidization velocity and terminal velocity.  Fluidized bed drying has been recognized as a gentle, uniform drying, down to a very low residual moisture content, with a high degree of efficiency (Borgotte et al., 1981). This is a very convenient method for heat sensitive food materials as it prevents them from overheating (Gibert et al., 1980, Giner and Calvelo, 1987). The fluidized bed drying for granular materials is now established (Butler, 1974, Masters, 1992) and many driers are operating throughout the world in the chemical  _________________________________________________________________  2 Centre for Food Technology, Queensland Department of Primary Industries      Hamilton, QLD 4007, Australia     2 2 and food industries. The properties of particulate materials relate to the type of fluidization technique (Shilton and Niranjan, 1993). The application of this technique is best suited to smaller and spherical particles. The disadvantages of this method include entrainment of friable solids by the gas and limited application to larger and poorly fluidized materials. Simultaneous moisture removal, shrinkage and structural changes are common in drying operations. These changes affect the physical properties of the agro-food materials and hence influence fluidization behaviour (Senadeera et al., 1998)  The Ergun equation (Ergun, 1952) is the widely accepted model to determine minimum fluidization velocity of a fluid to fluidize the particle (Kunii and Levenspiel, 1969; Zenz and Harbor, 1971; Michelis and Calvelo, 1994)  : ( ) ( )1 − −ε ρ ρmf s f gpmffmfmfpmfmfmfduduφρεεφµεε 23232 )1(75.1)()1(150−+−=          (1)  The Ergun equation was used to calculate minimum fluidization velocity of baker’s yeast (Egerer et al., 1985), peas (Rios et al., 1984) and diced potato and potato strips (Vazquez and Calvelo, 1980; Vazquez and Calvelo, 1983 ). An equation similar to Ergun was valid for peas (Michelis and Calvelo, 1994).  The values for velocity obtained by the Ergun equation are mostly reliable for spherical and relatively small particles. Most agro-food particulates however  comprise of various shapes and sizes, and consist of larger particles. Therefore, the minimum fluidization values obtained from Ergun equation does not conform to the experimental values (Mclain and McKay, 1980, 1981a, 1981b; McKay et al., 1987)  The Ergun equation consists of viscous and kinetic energy terms (1st and 2nd LHS part of the equation 1). In the case of larger particles at higher Reynolds numbers (Re > 1000) the fluidization behaviour was mainly governed by the kinetic energy term in the Ergun equation. Hence the Ergun equation can be simplified to (Kunii and Levenspiel, 1969) :    u2mfp s ffmfd g=−φ ρ ρρε23175.( )   (2) For wide variety of systems it was found that value 1 143φ ε mf ≅  (Wen and Yu, 1966) and a generalized equation can be applied to predict Umf for larger particles when Re > 1000.    u2mf = dgp s ff( ).ρ ρρ−24 5    (3) There is a continuous change in physical properties of the particulates during drying, which also changes the fluidization behaviour of the particles. It is important to understand these changes, so that the air-flow during drying can be controlled to achieve an optimum fluidization.    3 3 The objective of this study is to study the continuous change in minimum fluidization velocity for a given shape of food material (green bean in this case) during drying and relate this to moisture content by a suitable model, and compare the minimum fluidiztion velocity with the generalized model.  MATERIALS AND METHODS  Fresh green beans Phasecolus vulgaris of the variety Labrador was used for producing cylindrical particles. Beans were purchased from the same supplier to maximize reproducibility of results. Care was taken when selecting the size of beans to obtain a consistent diameter of near 10 mm. Size was measured using a vernier caliper with an accuracy of 0.05mm. Initially both ends of the beans were cut and only the middle portions, which resemble a cylindrical shape, were used to produce the required samples. Samples were prepared at three length to diameter ratios of 1:1, 2:1 and 3:1, and each experiment was repeated three times. After cutting, beans were kept in a plastic container in a cold room at 40 C for more than 12 hours before experimentation to stabilize the moisture content.  Drying experimentation was carried out using a heat pump dehumidifier system at a temperature of 50+ 20 C and relative humidity of 13 %. Materials were placed inside the drying system on mesh trays and stacked vertically to achieve maximum exposure to the air-flow. During drying, samples were taken from the dryer at ten arbitrary moisture levels. These samples were collected into a sealed container and used for fluidizing experiments and physical measurements.  All fluidization trials were conducted in a batch type flexi-glass fluidizing column of 185mm inside diameter and 1m long (Fig. 1). The hot air was taken from a heat pump dehumidifier system coupled to the dryer. Hot air entered the material bed through a perforated plate with circular holes of 1mm diameter (18 holes/ cm2). An even air distribution was achieved by placing another perforated plate (with 10 mm diameter holes with a diametral pitch of 40 mm in concentrically arranged holes), 10mm vertically below the perforated plate. Air flow entering the dryer was varied by means of varying the incoming flow to the fan and velocity of the incoming air was read from a digital manometer (EMA 84, range 0 - 10kPa ) connected to a pitot tube (Dwyer DS-300). Pressure drop across the bed was measured by a U-tube manometer connected to the drying chamber below the air distributor plate and above the bed of bean samples. In order to determine the optimum bed height for improved fluidization, bed heights of 100, 80, 60 and 40 mm were used. Used samples were collected in a separate container and reused for the drying experimentation.   To determine the particle density, a known number of particles were weighed by a Sartorious electronic balance, and the volume was measured by the difference in meniscus levels before and after immersion of particles in liquid paraffin in a measuring cylinder. The difference in meniscus levels was measured by a vernier caliper (accuracy 0.05mm). This value was used to calculate the equivalent diameter of the particle, which was used in the generalized equation (Equation 3). Moisture content was determined by measuring the loss in weight of finely chopped samples held at 700 C and -13.3 Kpa vaccum for 24 hours ( AOAC, 1995).    4 4                                                                  U-tube                                                                 drying chamber                                                                 material                                                                 porous plate                                                                 air distributor plate                                                                 pitot tube                                                                               from heat pump Figure 1 A fluidized bed experimental setup  RESULTS AND DISCUSSION  Modelling of minimum fluidization velocity with change in moisture content  The behaviour of minimum fluidization velocity is given in Figure 2 for different length to diameter ratios and different bed heights. Slugging and channelling were a common phenomena at the initial higher moistures for every L:D ratio. It was difficult to achieve good fluidization at initial moisture levels. This was more evident when the L/D ratio increased. Visual observation of the bed at an instance of fluidization after bed expansion was the criteria considered to categorise minimum fluidization. Also this value was compared with graphical variation, the  pressure drop of the bed with velocity. Both observed and graphical values were identical.  Using Statistical Analysis System (SAS), a model which correlate with a L:D ratio of 1:1 was fitted to the variation of minimum fluidization velocity with moisture during drying (Figure 3). The data were best fitted to the model Umf = A + B e-Cm and its parameters are shown in the Table 1 for different bed heights. The same model was used for other ratios (2:1, 3:1). They were poorly correlated for L:D = 2:1 and 3:1. It should be noted that for L:D = 2:1 and 3:1, the slugging and channelling were  commonly observed phenomena (results not shown).  Table 1.  Parameters for Equation Umf = A + B e -Cm for variable bed heights  Bed height A B C r2 MAE% 100 mm 2.3541 0.8825 0.0017 0.91 3.8973 80 mm 2.2990 0.8514 0.0015 0.91 4.0703 60 mm 2.0793 0.7097 0.0019 0.86 4.6296 40 mm 2.1202 0.7691 0.0016 0.86 4.5223      5 5 At initial moisture values, minimum fluidization occurs together with channelling and slugging. This were more pronounced in larger L:D ratios. As moisture was reduced, the quality of fluidization improved reducing slugging and channelling. Good fluidization was observed at 32% (wb) for L:D ratio 3:1, 52 % (wb) for L:D ratio 2:1 and 60 % (wb) for L:D ratio 1:1. There was an increase in minimum fluidization velocity at very low moistures (< 4.1 % db), this can be probably attributed to an increase in the particle density due to shrinkage. This is also supported by the generalized equation (Equation 3). Mean absolute error percentage (MAE%) (Equation 4) was calculated according to the methods given by Mayer and Butler (1993) for different L:D ratios and are given in Table 1.  [ ]MAE% 100 ( y y / y ) / ni i i= −Σ $ $     (4)  Umf calculation based on dimensional changes during drying  The Generalized model was used to calculate the predicted values of minimum fluidization velocity. For all three L:D ratios, this generalized model gave underestimated values except at lower moisture levels. The predicted versus observed plots of minimum fluidization velocity are presented in Figure 4.              Table 2. Mean Absolute Error % for predicted versus observed                                     Minimum fluidization values  L:D MAE% 1:1 9.54 2:1 7.66 3:1 5.32  The MAE% values were less than 10 % (Table 2), indicating that the use of these models can be satisfactorily applicable (Kleijn, 1987) to predict the minimum fluidization velocity of green bean particulates, with reasonable accuracy.   6 6  Figure 2. Fluidization behaviour of beans      Figure 3. Fluidization models L:D=1:1                   (a) 40 mm (b) 60 mm                                     (a) 40 mm (b) 60 mm                   (c) 80 mm (d) 100 mm                                   (c) 80 mm (d) 100 mm                (ο L:D=1:1   L:D=2:1 ∆ L:D=3:1)                   (ο experimental  model) (a )00 .511 .522 .50 4 0 0 8 0 0 1 2 0 0m o is tu re  %  (d b )umf (m/s)(b )00 .511 .522 .50 4 0 0 8 0 0 1 2 0 0m o is tu re  %  (d b )umf (m/s)(c )00 .511 .522 .50 4 0 0 8 0 0 1 2 0 0m o is tu re  %  (d b )umf (m/s) (d )11 .522 .50 4 0 0 8 0 0 1 2 0 0m o is tu re  %  (d b )umf (m/s)(a )00 .511 .522 .50 4 0 0 8 0 0 1 2 0 0m o is tu re  %  (d b )umf (m/s)(b )00 .511 .522 .50 4 0 0 8 0 0 1 2 0 0m o is tu re  %  (d b )umf (m/s)(c )00 .511 .522 .50 4 0 0 8 0 0 1 2 0 0m o is tu re  %  (d b )umf (m/s)(d )00 .511 .522 .50 4 0 0 8 0 0 1 2 0 0m o is tu re  %  (d b )umf (m/s)  7 7  Figure 4. Predicted versus observed plots for different L:D ratios                                [Note: Predicted values were calculated using u2mf  =                               dgp s ff( ).ρ ρρ−2 4 5, and dotted line represents y= x, which                                 helps to determine over or under estimation by the model]         L:D=1:100.511.522.50 0.5 1 1.5 2 2.5Umfobserved (m/s)Umf predicted (m/s)L:D=2:100.511.522.50 0.5 1 1.5 2 2.5Umfobserved (m/s)Umf predicted (m/s)L:D=3:100.511.522.50 0.5 1 1.5 2 2.5Umfobserved (m/s)Umf predicted (m/s)  8 8 CONCLUSIONS  The minimum fluidization velocity decrease as the drying proceeded. The minimum fluidization velocity of green bean with change in moisture can be predicted with an empirical model Umf = A + B e - Cm with a satisfactory fit for L:D = 1:1. Further, the calculated Umf using a generalized equation u2mf  =  d gp s ff( ).ρ ρρ−2 4 5, based on the dimensional changes of the product during drying can be applied to predict minimum fluidization velocity for all L:D ratios.  Further investigation of relationship between L/D change with moisture and bed height with minimum fluidization velocity is necessary.   NOMENCLATURE  A constant B constant C constant d equivalent diameter  (m) D diameter                 (m) g acceleration due to gravity               (m/s2) L length    (m) m moisture content (dry basis) (kg/kg db) Re Reynolds number u velocity                  (m/s) y value  φ sphericity ε porosity ρ density    (kg/m3) µ viscosity                 (N s/m2)  Superscripts  ∧ predicted value   Subscripts  f fluid  i integer mf minimum fluidization n no of observations p particle s solid    REFERENCES   9 9  AOAC. (1995). Official Methods of Analysis, 16th edition, Association of Official Analytical Chemists, washington DC.  Borgolte, G. and Simon, E. J. (1981). Fluid bed processes in the manufacture of snack products, CEB review for Chocolate. Confectionary and Bakery. (62): 7- 8, 10.  Egerer, B. Zimmermann, K. and Bauer, W. (1985). Flow and fluidization behaviour of yeasts in gas/ solid fermentation and drying. IchemE symposium series, (91): 257 - 269.  Ergun, S. (1952). Fluid flow through packed columns. Chemical Engineering Progresses. 48(2): 89 110.  Gibert, H., Baxerres, J. L., and Kim, H. (1980). Blanching time in fluidized beds. pp. 75- 85. In: Linko, P., Malkki, Y.,Olkku, J. and Larinkari J. (Ed) Food Process Engineering; 1: Food Processing Systems, Applied Science Publishers, London.  Giner, S. A. and Calvelo, A. (1987). Modeling wheat drying in fluidized beds. Journal of Food Science. 52(5): 1358- 1363.  Kleijnen, J. P. C. (1987). Statistical tools for simulation practitioners. Marcel Dekker. NY.  Kunii, D. and Levenspiel, O. (1969). Fluidization Engineering. John Wiley & Sons Inc. NY.  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Engineering and Food, Vol 2: Processing Applications. Elsevier Applied Science Publishers. London.    10 10 SAS. (1985). User’s Guide: Statistics. 5th edition. SAS Institute Inc. Cary. NC.  Senadeera, W., Bhandari, B. R., Young, G. and Wijesinghe, B. (1998). Change of physical properties of green beans and its influence on fluidization. pp. 1139- 1146. In  Akritidis, C. B., Marinos-Kouris, D. and Saravakos, G. D. (Ed). Drying’98, Volume B: Proceedings of the 11th International Drying Symposium, Halkidiki, Greece.  Ziti Editions (Pub). Thessaloniki, Greece.  Shilton, N. C. and Niranjan, K. (1993). Fluidization and its applications to food processing. Food Structure. 12: 199 - 125.  Vazquez, A. and Calvelo, A. (1983). Gas-particle heat transfer coefficient for the fluidization of different shaped foods. Journal of Food Science. 48: 114 - 118.  Vazquez, A. and Calvelo, A. (1980). Gas particle heat transfer coefficient in fluidized pea beds. 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A generalized method for predicting the minimum fluidization velocity.

Blanching time in fluidized beds.

Change of physical properties of green beans and its influence on fluidization.

Flow and fluidization behaviour of yeasts in gas/ solid fermentation and drying.

Fluid bed processes in the manufacture of snack products, CEB review for Chocolate. Confectionary and Bakery.

Fluid flow through packed columns.

Fluidisation and hydraulic transport of carrot pieces.

Fluidization and its applications to food processing.

Fluidization Engineering.

Gas particle heat transfer coefficient in fluidized pea beds.

Gas-particle heat transfer coefficient for the fluidization of different shaped foods.

Importance of proper design of the air distributor plate in a fluidized bed system. AIchE symposium series.

Longitudinal dispersion coefficients for the continuous fluidization of different shaped foods.

Modeling wheat drying in fluidized beds.

New developments of fluidization in the IQF food area.

Official Methods of Analysis, 16th edition, Association of Official Analytical Chemists,

Fluidisation behaviour of cylindrical green bean particulates in drying

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