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20320140501017
1. International Journal JOURNAL OF ADVANCED RESEARCH Technology (IJARET), INTERNATIONAL of Advanced Research in Engineering and IN ENGINEERING ISSN 0976 – 6480(Print), ISSNAND – 6499(Online) Volume 5, Issue 1, January (2014), © IAEME 0976 TECHNOLOGY (IJARET) ISSN 0976 - 6480 (Print) ISSN 0976 - 6499 (Online) Volume 5, Issue 1, January (2014), pp. 154-163 © IAEME: www.iaeme.com/ijaret.asp Journal Impact Factor (2013): 5.8376 (Calculated by GISI) www.jifactor.com IJARET ©IAEME EXPERIMENTAL STUDY OF COCOA DRYING KINETICS WITH A VIEW TO MODELLING A DRYER Abraham Kanmognea*, a Oumarou Hamandjodaa, Jean Nganhoua, Bienvenu Kenmeugnea Laboratoire d’Energétique (Energy Systems Laboratory), Ecole Nationale Supérieure Polytechnique, P.O. Box 8390 Yaounde, Cameroon ABSTRACT Kinetic drying measurements make it possible to determine the influence of various parameters (temperature, thickness of the layer of the product, speed of drying air) on the behaviour of the product when it is being dried and thus deduce from it a characteristic curve of the product including the maximum parameters. In this paper, we have studied through experiment the drying of cocoa in several layers and defined the correlations that make it possible to find the characteristic cocoa drying curve for a group of given parameters. The graphic determination of the characteristic drying curve made it possible to obtain results that conform to the direct method. Tests were conducted in the Laboratoire d'énergétique (Energy Systems Laboratory) of the Ecole Nationale Supérieure Polytechnique of Yaounde (ENSPY). The atmospheric conditions in the laboratory are relatively constant: average room temperature of 28°C ; average humid temperature of 23°C ; average absolute humidity: 15,5 g water/kg dry air; average relative humidity of 65% Keywords: drying kinetics, parameter, cocoa, characteristic curve, laboratoire d’énergétique (energy systems laboratory). 1. INTRODUCTION The economy of developing countries is based generally on agriculture and particularly on cash crops such as: coffee, cotton, cocoa etc… In sub-Saharan Africa, cocoa occupies a pride of place in the exports of many countries of the sub-region. However, in the world market the price is compatible with the quality of marketable cocoa. Cocoa producing countries must take special measures to ensure that the product put in the market is of quality. Drying seems like an economic and essential means adapted to the improvement of the quality of cocoa and its preservation. Cocoa dried up to 7% water content allows for its proper preservation [1]. In Cameroon, losses after harvest 154
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International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 5, Issue 1, January (2014), © IAEME due to the drying of cocoa are estimated at 15 % [2]. The building of appropriate dryers aimed at transforming cocoa with a view to improving its quality, its preservation requires a perfect knowledge of its behaviour during drying. Knowledge of the behaviour of solid products during drying is an essential element in calculating the dimension or size of a dryer and/or defining its conditions of use [3]. At the Energy Systems Laboratory (LAEN) of the Ecole Nationale Supérieure Polytechnique (ENSP), research works have been undertaken to study experimentally the kinetics of the drying of cocoa. It is impossible to estimate the drying curve of a product by a calculation that would take into account the influence of phenomena (transfer of water and heat in the product, deformation of the product) that are described in the current state of knowledge in the domain of drying. The only means to know the kinetics of drying is thus to measure it [4]. The purpose of this study consist in making measurements of loss of mass of cocoa in a test section (small scale dryer) in the laboratory and drawing or plotting the drying kinetics with a view to modelling a large scale dryer. 2. MATERIAL AND METHOD 2.1 Principle A quantity of cocoa that has a determined density or a thickness is placed in the test section, under a hot current air of fixed temperature and known speed. The product is weighed at regular time intervals. These measurements make it possible to obtain the variation of the mass of the product depending on time and thus the variation of the water content of the product depending on time. 2. 2 Description of the device A thermostatically-controlled drying vein makes it possible to have an air flow that has well controlled areothermic characteristics [5]. The entire device assembled on a frame comprises (figure 1) • • • • • • • • An axial fan driven by a 750 watts engine An alternostat that allows for the regulation of the heating power of resistances A Plexiglas test section of 125 mm x 125 mm and 200 mm long A convergent at the entrance of the sheath to ensure proper canalization of air flow A diaphragm placed at the air outlet that enables the regulation of its deficit, Three deflectors installed respectively downstream of the heating battery, between the heating battery and the vein upstream of the test section for a better distribution of the speed of air at the section of the vein, A thermometer that gives digital readings to ensure a constant indication of the temperature in the test section, 10,5 cm x 10,5 cm x 4 cm 10,5 cm x 10,5 cm x 3 cm et 10,5 cm x 10,5 cm x 2 cm aluminium plated trays introduced in the vein and connected to an electronic balance of precision ± 0,01 g by a screw-nut system. 155
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International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 5, Issue 1, January (2014), © IAEME Diaphragm Thermometer Vein Resistances chauffantes Sheath Fan Balance Figure 1: Drying test bench 2. 3 Measurement devices The installation is rounded off with a certain number of measuring devices: • A Platinium Pt sensor electronic thermometer 100 (- 100 °C to 500°C) makes it possible to measure the dry air temperature in the test section, a Sartorius electronic balance is used to measure mass losses during the drying process. The measurements are made with the help of intermittent weighing hours, • A TESTO 425 thermo-anemometer measures the speed and air temperature in the test section. Measurement takes place inside the vein. Air speed can be adjusted by creating pressure drops on the diaphragm located at the fan outlet. • An H27O electronic thermo-hygrometer to measure the room temperature and relative humidity of the ambient air. • A Chromel-Alumel type thermocouple connected to a Digi-Sense electronic thermometer provides temperature control in the vein. • A drying oven at 103 ° C ± 2 ° C to determine the dry mass of cocoa. [6] 2.4 Experiment Method Sample Preparation After opening the pod, the cocoa beans are put into clusters in a one-litre plastic bowl. They are then covered with banana leaves. After 3 to 7 days of fermentation, the beans are placed in the aluminium tray with a dimension of 10.5 cm x 10.5 cm and a height of 4, 3 or 2 cm, which is put into the test section. At the same time, other beans are placed in three cups that are tared beforehand. The samples are ready for testing in the drying oven. 156
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International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 5, Issue 1, January (2014), © IAEME Experiment Protocol The air speed is set using a diaphragm and the heating system is adjusted to the desired temperature using an alternostat. The product to be dried is arranged in a thick layer in the tray, placed in the test section, parallel to the flow of hot air. The mass of the sample, the ambient relative humidity and the room temperature measurements are taken at regular time intervals (15 m at the beginning of the test and 30 to 45 mm towards the end of the test). During the tests, temperature control in the test section is done by using the platinum sensor electronic thermometer. The control of the drying air-speed is done through a thermo-anemometer. The three previous cups are placed in the drying oven after weighing. Weighing of the cups is done after every 24 hours. At the end of 72 hours, the weight of each cup becomes constant and we can then determine the dry mass of cocoa which enable us to determine the initial water content of the cocoa. The experimental curves were obtained following the test plan shown in Table 1. Test n° 1 2 3 4 5 6 7 8 9 Table 1: Parameter values for the various tests T (°C) V (m.s-1) e (mm) Test n° T (°C) V (m.s-1) e (mm) 0.25 10 0.25 0.5 20 11 0.5 20 1.0 12 1.0 0.25 13 0.25 40°C 55°C 0.5 30 14 0.5 30 1.0 15 1.0 0.25 16 0.25 0.5 40 17 0.5 40 1.0 18 1.0 3. RESULTS AND DISCUSSION 3.1 Determination of the equilibrium water content Xeq The dry mass is used to calculate the equilibrium water contents on a dry basis (Xeq) of each sample under each condition set in advance using the following relations: X0 = m0 −1 ms (1) X eq = m (1 + X 0 ) −1 m0 (2) where m is the sample mass, X0 the initial water content of the sample and m0 its initial mass. 3.2 Determination of the water content X(t) An m0 mass of the fresh product is placed on a support which mass M0 is determined by weighing. The whole mass (M+m)0 is measured before putting into the drying section. The mass value (M+m)i is then measured at different times ti until the product reaches the final desired water content. This is calculated as follows: 157
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International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 5, Issue 1, January (2014), © IAEME X i = mi − ms ms (3) plus ms dry mass of the sample. Figure 2 shows the variation in the cocoa water content depending on the drying time for a 3 cm-layer thickness. Figure 2 : Experimental curves for cocoa drying of a 3 cm-layer 3.3 Determination of the drying speed The drying curves are determined from measuring the sample mass variation over time. They represent either variations in the average water content (X) depending on time, or the drying speed ( − dX ) depending on (X). Determination of curve ( − dX )=f(X) is obtained by calculating the dt dt derivative directly from the experimental points using the following formulae: 1 ( X i +1 − X i ) ( X i − X i −1 ) dX − + =− i ≠ 0, n 2 ti +1 − ti ti − ti −1 dt X i (5) X1 − X 0 dX − =− t1 − t0 dt X 0 (6) X n − X n −1 dX − =− tn − tn −1 dt X n Where n is the number of experimental points, Xi is the instant average water content. The humidity transfer between the air and the product can be represented in a curve describing the evolution in water content depending on time or evolution in drying speed depending on the water content. Influence of temperature on drying speed Figure 3 shows drying kinetics at different temperatures. An increase in air temperature causes an increase in drying speed including a drop in drying time. 158
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International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 5, Issue 1, January (2014), © IAEME 1 X (kg.kgms-1) 0.8 X 40° C 0.6 X 55° C 0.4 0.2 0 0 20 40 60 80 t (h) Figure 3: Influence of temperature on Cocoa drying speed (V = 0,5 m.s-1, layer thickness = 3 cm) Influence of air speed on drying speed An increase in air speed causes an increase in drying speed. Influence of Cocoa layer thickness on drying speed The influence of the product’s thickness is obtained by modifying from one test to the other the product’s loading density and characteristics of the air remained constant. Figure 4 presents this effect on the growth of the average water content that drops as time changes. The product’s drying speed drops as the product’s layer thickens. 1 0.8 e = 20 mm X/X0 0.6 e = 30 mm e = 40 mm 0.4 0.2 0 0 10 20 30 40 t (h) Figure 4: Influence of Cocoa layer’s thickness on drying speed (V=1 m.s-1, T = 55°C) 159 50
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International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 5, Issue 1, January (2014), © IAEME 3.4 Modelling drying kinetics Several theories and models have been formulated to illustrate drying kinetics and to understand the physical laws that control transfers. The complexity of the mechanisms employed and the variable nature of the products (type, form, and physical properties) hamper the formulation of a unique model that can represent all situations. The drying characteristics curve method (CCS) consists in representing drying kinetics in the following form: Vr = V = f (φ) (7) X − X eq Where: φ = X 0 − X eq (8) V0 And: Vr is reduced drying speed V0 is initial drying speed (kgeau.kgms-1.h-1) Xeq is product’s water content in harmony with its milieu (kgeau.kgms-1) X is product’s average water content (kgeau.kgms-1) X0 is initial water content (kgeau.kgms-1) Direct Method This method is based on the processing of raw experimental data, which helps to determine the value of V0 following the drying curve ( − dX )=f(X). This method assumes that at the start of the dt drying, the product’s humidity level is below or equal to the critical humidity. The function f(φ) is a function that, in principle, is characteristic of the environment under study. It can be a polynomial function or power function. To process our data, we will use the following power characteristics curve: f(φ)=φα (9) From the curve m=f(t), we will determine the curve X=f(t) and the curve: V=( − dX )=f(X) (10) dt Equations 7, 8 and 12 will give the following formula: X − X eq X − X 0 Vr = V = f X 0 − X eq = X 0 − X eq V0 α (11) Where α is positive or nil. The process to determine α is as follows: points (X, V) and values X0, V0, Xeq are known for each test and are used to calculate Vr and φ at instant t. For each experimental point we deduce an instant value αt as follows: 160
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International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 5, Issue 1, January (2014), © IAEME αt = [ Ln ( Vr ) Ln (φ) ] (12) For a test performed on a specific product under defined drying conditions, the average value αi is calculated using the formula: P αi = 1 ∑ α t P (13) i =1 Where P is the number of points per test. The average value retained for the α parameter is calculated using the formula: K α = 1 ∑ αi K (14) i =1 Where K is the number of the drying test conducted. This final α value, characteristic of the product, does not depend on the test conditions or a group of test parameters. The values from Xeq to 40°C and to 55°C are determined by desorption isotherms at 40 and 55°C [8]. The estimated values of the initial rate of drying V0 and the exponent α are shown in table 2. The average values of α obtained for the different thicknesses are: α20µµ = 1.61; α30µµ = 1.47 ανδ α40µµ = 1.485 Table 2: Values of α, characteristic parameter of the characteristic curves of cocoa Test n° V0 (kg.kgms-1.s-1) Test n° V0 (kg.kgms-1.s-1) α α -5 -5 1 16,6.10 1,77 10 1,3.10 1,28 2 8.10-5 1,58 11 1,8.10-5 1,51 1,55 3 8,3.10-5 12 3,7.10-5 1,25 4 5,3.10-5 1,45 13 3,3.10-5 1,41 -5 5 7,5.10 1,64 14 1,6.10-5 1,45 -5 -5 6 3,2.10 1,71 15 2,4.10 1,53 1,72 7 2.10-5 16 2,2.10-5 1,82 8 3,8.10-5 1,23 17 1,3.10-5 1,49 -5 -5 1,52 9 3,5.10 18 1.10 1,48 The model presented by formula (9) is used to describe the change in the reduced speed depending on the reduced water content. It makes it possible to increase the water content value depending on time from the following correlation: V (1 − α ) X = X eq + ( X 0 − X eq )1−α − 0 ( X 0 − X eq )α 1 (1−α ) [9] t (15) Considering the different α values, the modelled curves agree with the experimental results as exemplified in figures 5 representing the simulated experimental curve corresponding to test n°6. 161
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International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 5, Issue 1, January (2014), © IAEME Figure5: Comparison of the theoretical model with experimental results: example of the change in the reduced rate of drying for test n°6 Graphical determination of the characteristic drying curve We sketched the experimental curves (Vr,Xr) for different drying parameters and identified the value that is closest to the experimental points in order to deduce the expression of the water content depending on time. Figure 6 shows curves for three values of α and the experimental points for a cocoa layer thickness of 4 cm. Some dispersion is observed in the experimental points as compared to the identified theoretical patterns. The identification is quite precise when the experimental points obtained are considered for Xr values lower than 0.4. Figure 6: Graphic determination of parameter α The average value of α determined using the direct method is 1.485 and the value of α determined using the graphic method is 1.55, which clearly conforms to the former value. 162
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International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 5, Issue 1, January (2014), © IAEME 4. CONCLUSION A series of experimental drying curves for different operating conditions (air speed, air temperature, cocoa layer thickness) were sketched. An equation deduced from these curves enables the identification for each product layer thickness a characteristic drying graph. Based on the correlations obtained, a good reconstitution of the results of the experiments can be made. Thus, we can calculate the drying time, an important parameter for the sizing of dryers, as per the conditions adopted. BIBLIOGRAPHY [1] [2] [3] [4] [5] [6] [7] [8] [9] Kanmogne A., 2003, Contribution à l’étude du séchage du cacao au Cameroun. Conception, réalisation et modélisation d’un séchoir adapté aux conditions locales. Thèse de Doctorat Ph.D, Université de Yaoundé I, 142 p. Kanmogne A., Jannot Y. et Nganhou J., 2012, Description concise et analyse des systèmes utilisées dans la région Sud du Cameroun pour le cacao. Tropicultura, 2012, 30, pp. 94-102. J.J. Bimbenet, 1978. Le séchage dans les industries agricoles et alimentaires. Cahier du GIA, SEPAIC, Paris. Daudin J.D., Bimbenet J.J., Détermination expérimentale du comportement des produits solides lors du séchage par entraînement. Ind. Alim. Agric. , 99e année, 4, pp.226-235, 1982. Ahouannou C., 2001. Etude du séchage de produits agroalimentaires tropicaux : application au manioc, gingembre, gombo et piment rouge. Thèse de Doctorat, Université Nationale du Bénin, 219 p. Abraham Kanmogne, Yves Jannot et Jean Nganhou, 2013. Ameliorating the energy performance of electric dryer for agro-food product. Case study: the dryer of transformation unit for cassava at Pouma in Cameroon. International Journal of Advance Research of Engineering and Technology (IJARET), ISSN 0976-6480 (print), ISSN 0976-6499 (online) Volume 4, Issue 7, November-December 2013. Helene Desmorieux, 1992. Le séchage en zone subsaharienne : une analyse technique à partir des réalités géographiques et humaines. Thèse de Doctorat de l’Institut Polytechnique de Loraine. Abraham Kanmogne, Yves Jannot, Bernard Lips et Jean Nganhou, 2012. Sorption Isotherms and drying characteristic curve of fermented cocoa. International Journal of Science and Technology, ISSN (online) 2250-141X, pp19-31. Ahouannou C., Jannot Y., Lips B et Lallemand A., 2000. Caractérisation et modélisation du séchage de trois produits tropicaux : manioc, gingembre et gombo. Sciences des aliments, 20(2000) 413-432. 163
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