Isotherm Adsorption Behavior and Infrared Drying Kinetics of Air Dried Sheet (ADS) Rubber

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1 104 The 5 th PSU-UNS International Conference on Engineering and Technology (ICET-011, Phuket, May -3, 011 Prince of Songkla University, Faculty of Engineering Hat Yai, Songkhla, Thailand 9011 Isotherm Adsorption Behavior and Infrared Drying Kinetics of Air Dried Sheet (ADS Rubber Jatarut Tasara 1, Waiyarat Suchonpanit, Supawan Tirawanichakul 3* and Yutthana Tirawanichakul 4 1,, 3 Prince of Songkla University, Faculty of Engineering, Thailand 4 Energy Technology Research Center, Prince of Songkla University, Faculty of Sciences, Thailand *Authors to correspondence should be addressed via Supawan.t@psu.ac.th Abstract: Equilibrium moisture contents (EMC of natural rubber (NR were evaluated using the gravimetric-static method among surrounding temperature of C correlated to surrounding relative humidity (RH ranging of 10-90%. The results showed that equilibrium moisture content decreased with increasing temperature at constant relative humidity. The Modified Oswin model was found to be the most suitable for describing the relationship among equilibrium moisture content, relative humidity and temperature. The batch drying experiments were operated at drying air temperatures between 39. and 64.1 C for infrared drying at IR Intensity of 4,000 W, thickness of rubber sheet of 3-4 mm. Rubber sheet was dried from initial moisture content of 30 to 50% drybasis to 1±0.5% dry-basis. The drying rate increased with increase of inlet air temperature, experimental results using Modified Henderson and Pabis model was the best fitting model for describing infrared drying behavior of rubber. Additionally, rubber sheet was dried by natural convection drying took longer period than those of IR drying. Additionally, drying at a low drying temperature has a low specific energy consumption compared to those drying of high temperature. To determine qualities following the highest standard Thai rubber (STR5L grade, the results showed that quality of samples was acceptable in market level. Key Words: ADS/ EMC/ Infrared/ Quality 1. INTRODUCTION Natural rubber (NR is the most important economic crops in Thailand, which is reported as the largest exporter of rubber in 008. Moreover, NR was roughly harvested about of million and then applied to raw production (rubber sheet about 3.17 million tons whereas in Indonesia and Malaysia, defined as the second and later NR exporters, they were of.9 and 7 million tons, respectively [1]. NR products can be exported in form of concentrated latex, air dried sheet (ADS rubber, rubber smoked sheets (RSS and block rubber etc. Nonetheless, the thermo-physical properties are still investigated further. Pahlevanzadeh and Yazdani [6] revealed that the equilibrium moisture contents of almond were determined using the gravimetric-static method at 15, 30, 55 and 75 C for powder and 15, 55 and 75 C for nut state of almond for water activity (aw ranging from 0.11 to 7. At a given aw, the results indicated that the moisture content decreased with increasing temperature. The experimental sorption curves were then described by the BET, GAB, Henderson, Oswin, Smith and Halsey models. A nonlinear regression-analysis method was used to evaluate the constants of equations. The GAB model was found to be the most suitable for describing the sorption curves; the monolayer-content values for the sorption at different temperatures are calculated. Also, the isosteric heats of adsorption of water were detected as a function of moisture content from the equilibrium data at different temperatures using the Clasius- Clapeyron equation. Besides, Naon et al. [3] reported the drying of natural rubber and developed the mathematical model to predict the feasible conditions for drying. The dryer dimensions L x W x H: 3 x 1 x m, the air speeds of 0-5 m/s. and the air temperatures of C were applied. The relative humidity was studied in the range of %. It was found that the drying rates were various with temperature, drying time and relative humidity. The heat transfer coefficient corresponded to the material temperature and the air temperature. Each

2 105 parameter obtained from the thin layer drying was taken to predict thick layer drying rate using the mathematical model. It indicated that the model results were in good agreement with the experimental results. The final moisture after drying was defined as %. In addition, when compared this model to the semiautomatic Trolly dryer, usually used in the industry, that result was agree with these criteria such as humidity, temperature, relative humidity and drying energy. Meeso et al. [7], moreover, investigated the effect of FTIR irradiation on paddy moisture reduction and milling quality after fluidized bed drying. The gain results exposed that the moisture reduction inclined with the declined of the load of irradiative chamber. The amount of radiation energy absorbed by paddy was directly proportional to the of water content, containing inside the paddy kernel. Therefore, the objectives of this research are to quantify experimental equilibrium moisture content of NR, determine the most suitable model describing the isotherms, develop infrared drying system for ADS drying and study the performing system by comparing with natural drying.. MATERIALS AND METHODS.1 Materials The fresh natural rubber sheets were granted from sources: (1 Rubber sheet using infrared drying was derived from the agriculture cooperation at Hat Yai, Songkhla province, Thailand. The dimension of sample rubber was 40 x 90 cm and thickness was about 3-5 mm with -1.5 kg weight. ( Rubber sheet was determined by EMC donated from the department of biochemistry, faculty of Science, Prince of Songkla University, Hat Yai Campus. The dimension of sample rubber was 70x50 cm and kg weight.. Equilibrium moisture content (EMC These works here observed the equilibrium moisture contents (EMC by ascertaining the natural rubber (NR utilizing the gravimetric- static method. Six saturated salt solutions for achieving an EMC stage used in this experiments such as KNO 3, NaCl, Mg (NO 3 6H O, MgCl 6H O, (NH 4 SO 4 and LiCl. All of the saturated salt solutions provided relative humidity surrounding of %. During experiments, the rubber sheet samples and saturated salt solutions were put in the airtight vials. These vials were placed into incubator for controlling the temperature of 40, 50, 55 and 60 C to obtain dry matter weight. After few weeks, this specimen was in an equilibrium state with the saturated salt solution. This state was acknowledged when three consecutive weight measurements showing a difference lower than 01 g. Then, it was taken to analyse moisture content by following ASAE method []. The sample was operated by means of triplication. For this study, the six models for predicting EMC were chosen to fit the experimental data, including various temperatures of 40 to 60C and the relative humidity of 10 to 90 %. Formulated functions of relative humidity, temperature and EMC conditions are written following Table 1. Table 1. Different model for determination the equilibrium moisture content (EMC Model Equation No. Modified Oswin [9] (1 D RH Meq (A BT CT E 1 RH Modified Henderson [9] ( 1/C ln(1 RH M eq A(T B Smith [10] (3 Meq A B ln(1 RH Modified Smith [10] (4 M eq A [BT CT ln(1 RH] Modified Halsey [5] (5 1/D - A M eq (BT CT ln RH GAB [8] (6 ABC(1 RH M eq (1- B(1- RH(1- B(1- RH BC(1- RH where M eq is the equilibrium moisture content (% drybasis, T is the absolute temperature (K, RH is the relative humidity, A, B, C, D and E are the constant value..3 Drying equipment A dryer with infrared heat source is showed in Fig. 1. The infrared heater with power of 500 x 8 W was supplied by convectional supplying of 0 V AC. The walls were cored composing micro fiber with thickness of 0 mm to proven heat loss. Furthermore, the drying air temperature was restrained by a PID controller with an accuracy of 1C. Fig. 1. Schematic diagram of infrared drying system The drying temperature was measured by K-type thermocouple connected to a data logger with an accuracy of +1C. Spacing gap between samples and IRsources was fixed at 15 cm throughout the experiments. The major specifications of the developed dryer were provided in Table.

3 106 Table. Specification of the infrared dryer Description Specification Overall dimension x 1. x m 3 No. of heating chamber 1 Wavelength of radiation -1 m. No. of IR heater 8 Power input for IR heating 4 kw Distance between the sample 15 cm and the heater.4 Drying experiment For IR and natural drying, the initial moisture contents of sample rubber were operated in range of % dry-basis. The thickness of sample rubber was about 3-5 mm with -1.5 kg weight. The experiment was dried at constant temperature ranging from C. The desired final moisture content was about 1 0.5% dry-basis. The temperatures of rubber sheet were recorded at the 7 levels on rubber sheet. The bulk temperatures, dry bulb, wet bulb of ambient air temperature and temperature inside drying chamber were measured by K-type thermocouple, which was continuously monitored by a Wisco data logger with an accuracy of 0.1C. The relative humidity was analysed by the dry and wet bulb temperatures. The moisture content was determined by ASAE method [] at the laboratory..5 Drying kinetic and mathematics model Relationship between moisture ratios and drying time normally is determined by three major mathematical drying models as following: (1 simulation including heat and mass transfer, ( the diffusion model and (3 the empirical or semi-empirical model mostly developed from experimental data. The first and second model also called as theoretical model and semi theoretical model. Both of them are solved by complicated mathematical analysis. The rest of model is quite suitable for the experimental data but it has limitation for practice owing to it is in-situ condition of each experiment. To simplify the predicting evolution of moisture ratio in this work, the empirical equations are developed by assumption of moisture ratio (MR as function of drying time and eventually the simplified equation can be noted as: M M t eq MR f(t (7 M M i eq where MR is moisture ratio (dimensionless, M t is average moisture content at drying time (% dry-basis, M i is initial moisture content (% dry-basis, M eq is equilibrium moisture content (% dry-basis, t is drying time (h. To predict the evolution of moisture transfer against drying time, moreover, the mathematical modeling of thin layer drying equations was formulated. These empirical models and semi-theoretical model were developed from experimental results using the statistical nonlinear regression and the best fitting empirical model were demonstrated in Table 3. Table 3. Different empirical model for rubber sheet drying [4] Model Equation No. Page (8 MR exp( kt n Newton (9 MR exp( kt Two term exponential (10 MR a exp( kt (1a exp( kat Henderson and Pabis (11 MR a exp( kt Logarithmic (1 MR a exp( kt b Modified Page (13 MR exp( (kt n Modified Henderson and Pabis (14 MR = a exp(-kt + b exp(-k 1 t + c exp(-k t Two term (15 MR a exp( kt b exp(-k t 1 Verma et al. (16 MR a exp( kt (1 a exp(- k1t Midilli et al. (17 MR a exp( kt n bt where MR is the moisture content (% dry-basis, t is drying time (h, and a, b, c, k, k 1 and n are the constant value. From the thin layer drying equations and geometric shapes, it can be written in a differential equation of moisture diffusion that is the ratio of moisture to form slab sheets. So, it uses the five-term equation to the drying equation (18: Dt 1 9 Dt 1 5 Dt exp exp exp 8 l 9 l 5 l MR ( Dt 1 81 Dt exp exp 49 l 81 l where D is the effective diffusion coefficient (m /h, t is the drying time (h, l is the thickness of the slab sheet (0339 m. An effective diffusion coefficient (D is namely described by the Arrhenius type equation as [11]: E D Dexp RT (19 where D ' is the Arrhenius factor of the heterogeneous solid (m /h, E a is the activation energy (kj/mol-k, R is the universal gas constant (8.314 kj/mol-k, T abs is absolute temperature (K. The constant values in these models were estimated by the non-linear regression analysis from the experimental data. The suitability of the equation was evaluated and compared using the coefficient of a abs

4 Equilibrium moisture content (%dry-basis 107 determination (R and root mean square error (RMSE, which indicated the fitting ability of a model to a data set for selecting the best equation to describe the experimental data. This equation was written following: N (Data predicted,i Data experiment,i i1 RMSE (0 N where Data predicted is the predicted moisture with models, Data experiment is the experimental moisture and N is number of data points..6 Quality analysis and specific energy consumption (SEC of rubber sheet The quality of sample rubber was tested after drying to check the compliance with the STR 5L standard; by measuring %DIRT, %ASH, %VM, %N, %P O, %PRI and Color. The percentage shrinkage of sample rubber was determined by an average of that measured with vernier calipers with an accuracy of ± 5 mm and was calculated following this equation: (H Shrinkage (% (1 where H initial, H final are the geometric mean thickness of the rubber sheet at the beginning and at the end of the drying experiment, respectively. The energy consumption and the drying rate of the drying were recorded by watt-hour meter and then reckoned following equation ( and (3, respectively. Drying rate initial 3.6P SEC (M M W i H H ( (3 where M i is the initial moisture content of rubber sheet (% dry basis, M f is the final moisture content of rubber sheet (% dry basis, W d is mass of dry sample (kg and P is the amount of energy used (kilowatt - hours. final f final 100 (Mi - M f W Drying time d d 3. RESULTS AND DISCUSSION 3.1 Equilibrium moisture content (EMC The six widely recommended models, which were investigated for rubber sheet, were given in Table 1. In addition, the constant values that were estimated for each of the models, and the indices for estimating the errors associated with the models, which were the coefficient of fit (R and RMSE, are shown in Tables 4. From the observing study, the Modified Oswin model had a good relation to the experimental values, corresponding to its higher R and lowest RMSE value when compared to other models. The R and RMSE value of Modified Oswin model was and 0057 (Table 4. Furthermore, the data in Fig. also illustrated the impact of RH on EMC of rubber sheet at surrounding temperature of 40 C and relative humidity of 10-90% that the EMC increased with increasing of RH at constant temperature. Of these, the Modified Oswin model had well correlated to the experimental values. 0. Experiment 40 C Modified Oswin Modified Henderson Modified Smith Modified Halsey Relative Humidity (% Fig.. Comparison equilibrium moisture content between experimental data and expected data of NR at relative humidity of 10-90% and temperatures of 40 C On the other hand, Fig. 3 elucidated that the influence of RH on EMC of rubber sheet at surrounding temperature of C and relative humidity of 10-90% and these results clearly showed the EMC decreased with increasing of temperature at constant RH. Table 4. Results models for the determination of the equilibrium moisture content (EMC of rubber sheet Model Arbitrary constants A B C D E R RMSE Modified E Oswin [9] Modified Henderson [9] Smith [10] Modified E Smith [10] Modified Halsey [5] GAB [8]

5 Moisture Ratio, MR Equilibrium moisture content (%dry-basis Modified Oswin, 40 C Modified Oswin, 50 C Modified Oswin, 55 C Modified Oswin, 60 C 108 the proposed Modified Henderson and Pabis model could predict the moisture ratio in good agreement with the experimental results for infrared drying. Indeed, rubber sheet was dried by natural drying, which took longer times than drying with infrared Relative Humidity (% Fig. 3. Comparison equilibrium moisture content of NR at relative humidity of 10-90% and temperatures of C 3. Drying kinetic and mathematics model The comparison of moisture ratio between experimental and predicted value of infrared drying at various inlet drying air temperatures and natural drying was carried out in Fig. 4. This information argued that 0. Experiment 39. C Experiment 44.0 C Experiment 6.9 C Natural 9.8 C Model 39. C Model 44.0 C Model 6.9 C Drying time (h Fig. 4. Drying curves of Infrared drying (Modified Henderson and Pabis model and natural drying experimental and predicted, using the different air temperature values Table 5. Results of statistical analyses on the modeling of rubber sheet drying with temperature of 39. to 64.1 C (Infrared Drying [4] Model Constant R RMSE Page n = k = exp( /T Newton k = exp( /T Two term exponential a = k = exp( /T Henderson and Pabis a = k = exp( /T Logarithmic a = b = 747 k = exp( /T Modified Page n = k = exp( /T Modified Henderson and Pabis a = b = c = k 1 = 008exp( /T k = exp( /T = exp( /T k 3 Two term a = b = k 1 = exp( /T k = exp( /T Verma et al. a = k 1 = exp( /T k = exp( /T Midilli-Kucuk a = b = 005 n = k = exp( /T where T is the absolute temperature (K, and a, b, c, n, k, k 1, k and k 3 are constant values

6 Moisture Ratio, MR Moisture Ratio, MR Moisture Ratio, MR 109 More studied on this research (Fig. 5, its data observed the evolution of moisture content for IR dried rubber sheet with initial moisture contents of % dry-basis. The sample was dried by IR radiation (4000 W with constant temperatures of 44.0 C whereas in control using ambient air ventilation as dryer. All drying curves unequivocally revealed that drying rate of sheet rubber was in the falling rate period while the moisture content was exponentially decreased with elapsed drying time. 0. IR 4000W, sheet 1, Min= 53.0 % d.b., H= 3.46 mm IR 4000W, sheet, Min= 54.1 % d.b., H= 3.40 mm IR 4000W, sheet 3, Min= 54.3 % d.b., H= 3.44 mm IR 4000W, sheet 4, Min= 56.0 % d.b., H= 3.9 mm Natural 9.8 C, Min= 57.6 % d.b., H= 3.4 mm Drying time (h Fig. 5. Evolution of moisture ratio of drying rubber with drying air temperature of 44.0 C initial moisture content of % dry-basis, thickness (H of mm IR intensity of 4000 W At the beginning of drying time, the moisture ratio of sample rapidly decreased since the main part of moisture content of sample existed around the exterior surface, thus allowing the easier water removal without any interference of disordered void spaces inside sample. At nearly end of drying period, heat and mass transfer did not only occur at the surface of rubber sheet but also they stimulated inside the rubber sheet. Nevertheless, the moisture inside rubber sheet moved to surface slower than the movement from the surface of rubber sheet to ambient environment. Drying rate will be relative lowers compared to the beginning of drying time. Additionally, the moisture content of sample was easily decreased when the thickness of rubber sheet reduced. A thickness of rubber sheet is decreased if the moisture inside rubber can evaporate easier than the infrared hot air. Consequently, at a higher drying air temperature, rate of moisture removal became relatively faster than those of a lower temperature. These drying curves were typical equations for predicting drying kinetics of grain kernel and food stuff, corresponding to the previous works [1]. 3.3 Moisture diffusion coefficient of rubber Experimental drying a thin layer of rubber was dried in the drying rate decreased. The equation for the effective diffusion coefficient of the rubber can be written in the form equation exponential. An effective diffusion coefficient (D is a function of rubber with temperature (T and can express the effective diffusion coefficient in the mathematical equation (4. D = ( * exp( /rt (4 where R =0.95 When calculating the effective diffusion coefficient, it was found that the effective diffusion coefficient of the rubber decreased with increasing time and temperature. 3.4 Effect of temperature and thickness 0. IR 4000W, 51. C, Min= 36.7 %d.b., H= 3.5 mm IR 4000W, 64.1 C, Min= 38.7 %d.b., H= 3.34 mm Drying time (h Fig. 6. Effect drying air temperature of rubber sheet at drying temperatures of 51.C and 64.1C (initial moisture contents of % dry-basis, IR intensity of 4000 W 0. IR 4000W, sheet 1, Min= 3.0% d.b., H= 3.14 mm. IR 4000W, sheet, Min= 36.5 % d.b., H= 3.16 mm. IR 4000W, sheet 3, Min= 38.4% d.b., H= 3.33 mm. IR 4000W, sheet 4, Min= 39.9 % d.b., H= 3.36 mm Drying time (h Fig. 7. Effect thickness of rubber sheet at drying temperatures 51.C (initial moisture content of % dry-basis, thickness of mm. IR intensity of 4000 W Further investigating, Fig. 6 performed the evolution of moisture ratios among drying time at inlet air temperatures of 51.C and 64.1C. At the drying air temperature of 64.1C, the reduction of moisture was faster than that of a low drying air temperature 51.C. Moreover, the initial moisture content of the rubber had low relatively effect on decreasing rate of moisture content compared to drying temperature and thickness of sheet rubber. Next experiment (Fig. 7., they kept going on observing the effect of thickness (fixed drying air at 51.C on the moisture ratios, which were initially moisture contents of % dry-basis, sheet thickness of mm. Here obtained results present that the rate of moisture transfer trended to be decreased when the thickness of sample was increased, thus IR can penetrate into sample.

7 Quality analysis and specific energy consumption (SEC of rubber sheet Table 6. Physical drying characteristics and specific energy consumption (SEC in experiment Drying Drying Drying Physical drying characteristics SEC Exp temp. time rate no. Shrinkage Brightness and color Bubble (kj/kg H O ( C (h (kg/h (% Initial Final evap Bright gel/ yellow color No Bright gel/ yellow color No Bright gel/ yellow color No Bright gel/ dark yellow Little Dense/ color white Sticky/ yellow color Much color Bright gel/ dark yellow Much 145 color 7 Natural 9.8 C Bright gel/ yellow color No - (a (b (c Fig. 8. Visual characteristics of dried rubber sheet (a IR heating 44.0 C (b IR heating 64.1 C and (c natural drying 9.8 C Considering the visual characteristic of the drying at inlet air temperatures of ºC, it reported that before starting the experiment, white granules were dispersed uniform in its texture. After drying at low inlet air temperatures of C, the dried rubber texture seemed to be bright and then became uniform gel on its texture as revealed in Fig. 8 (a and (c. On the other hand, when drying at high inlet air temperature of ºC, dried rubber sheet texture appeared to be darker and stickier than those obtained of low inlet air temperature (Fig. 8 (b. Obviously seen in high inlet drying air temperature, the visual observation showed that mostly texture of the dried rubber contained a small amount of bubble granule on sheet. As mentioned earlier, these results were corresponding to chemical quality test as shown in Table 7. Also, the quality of dried rubber sheet was determined by following with STR 5L standard methods because this sample was the grade in marketing. Of these, all data imply that this research using IR source as low drying temperature might be another approaches to be applied for preparing a good quality drying rubber sheet. Table 7. Chemical quality testing of rubber sheet with STR 5L standards Exp. Drying Chemical quality testing no temp. ( C % Dirt % Ash % VM % N P O % PRI Colors Drying time (h Natural 9.8 C Note: Standard STR 5L block rubber must be limited as follows: %Dirt 4 %, %ASH 0 %, %VM 0 %, %N 0 %, %P O 35 %, %PRI 60 %, Color CONCLUSIONS The results can be concluded as the followings: 1. The equilibrium moisture content (EMC of the natural rubber sheet increase when the relative humidity increases in the temperature range of C. The appropriate mathematical model equation for the best describing experimental results was the Modified Oswin EMC model.. The drying rate increased with increase of inlet air temperature and the experimental results was wellpredicted by the Modified Henderson and Pabis thinlayer drying model. 3. Effective moisture diffusion coefficient exponential related to drying temperature and can be written as Arrhenius equation.

8 The drying time of rubber sheet was affected by sheet thickness, drying temperature and IR intensity level. 5. To standardize ADS quality, the quality analysis was followed by STR 5L rubber grade, conclusion stated that the quality of ADS rubber was in the same standardized level as STR5L. 5. ACKNOWLEDGEMENTS The authors wish to express their sincere thank to the National Research University Project of Thailand's Office of the Higher Education Commission, the Graduate school scholarship, Department of Physics, Department of Biochemistry., Faculty of Science, Department of Chemical Engineering., Prince of Songkla University Thailand for their grant and financial support in this work. 6. REFERENCES [1] Agricultural Statistics of Thailand 009, Ministry of Agriculture & Co-Operatives, 009, Bangkok, Thailand. [] ASAE standard, Moisture Measurement", the 9 th ed., 198, St. Joseph, Miami, USA. [3] B. Naon, G. Berthomieu, J.C. Benet, and C. Saix, Improvement of Industrial Drying of Natural Rubber through Analysis of Heat and Mass Transfer, J. Drying Technol., 1995, Vol. 13, pp [4] C. Ertekin, and O. Yaldiz, Drying of Eggplant and Selection of a Suitable Thin Layer Drying Model, J. Food Eng., 004, Vol. 63, pp [5] G.D. Halsey, Physical Adsorption on Non-uniform Surfaces, J. Chem. Phys., 1948, Vol. 16, pp [6] H. Pahlevanzadeh, and M. Yazdani, Moisture Adsorption Isotherms and Isosteric Energy for Almond, J. Food Process Eng., 005, Vol. 8, pp [7] N. Meeso, A. Nathakaranakule, T. Madhiyanon, and S. Soponronnarit, Influence of FIR Irradiation on Paddy Moisture Reduction and Milling Quality after Fluidized Bed Drying, J. Food Eng., 004, Vol. 65, pp [8] N.D. Menkov, Sorption Equilibrium Moisture Content of the Seeds of Several Tobacco Varieties, J. Agric. Eng. Res., 1999, Vol. 7, pp [9] S. Achaviriya, and S. Soponronnarit, Studies of Parameter for the Analysis of Papaya Glace Drying, J. Agric. Sci., 1990, Vol. 4, pp [10] S.E. Smith, Sorption of Water Vapor by Proteins at High Polymers, J. Am. Chem. Soc., 1947, Vol. 69, pp [11] S. Soponronnarit, Drying Grains and Some Types of Foods. 7 th ed. King Mongkut s University of Technology Thonburi, Bangkok, Thailand, 1997, pp [1] S. Tirawanichakul, and Y. Tirawanichakul, Comparison and Selection of EMC Desorption Isotherms for Crumb Rubber, Proc. of PSU-UNS Inter. Conf. on Eng. and Envir. 005: ICEE 005, Novi Sad, Serbia-Montenegro, Novi Sad, Serbia- Montenegro, May 18-0, 005.