Drying characteristics of Air Dried Sheet (ADS) Rubber

Transcription

1 Paper Code: ee11 characteristics of Air Dried Sheet (ADS) Rubber Y. Tirawanichakul 1*, W. Suchonpanit 2 and S. Tirawanichakul 3 1 Plasma and Energy Technology Research Laboratory, Department of Physics, Faculty of Sciences, Prince of Songkla University, Songkhla 911 Thailand 2 Energy Technology Research Center and 3 Department of Chemical Engineering Faculty of Engineering, Prince of Songkla University, Songkhla 911 Thailand * yutthana.t@psu.ac.th Abstract The research is aimed to study effect drying conditions on kinetics and quality of air dried sheet (ADS) rubber. The four different drying strategies were carried on by combined green house and natural convention (GH-NC) drying, green house (GH) drying, combined open sun and natural convention (OS-NC) drying and solar drying. Initial moisture content of fresh rubber sheet was varied between 3 and 5% dry-basis and dried samples had final moisture content of.5% dry-basis. From the experiments, it was found that falling drying rate stages. ADS rubber was dried by solar drying better and faster to combined GH-NC drying, combined OS-NC drying and green house drying, respectively The efficiency of solar collector obtained was 37.9 %. This drying technology is suitable for small-scale process of dried ADS rubber in the southern area of Thailand Keyword: Natural rubber, Moisture content, Solar drying, Air dried sheet (ADS) rubber 1

2 Paper Code: ee11 1. Introduction Rubber is the most important economic agricultural crops in Thailand because of its largest natural rubber export product. Natural rubber (NR) was roughly harvested about of million rais and raw production of rubber sheet was about of 3.17 million tons [1]. NR products are mostly delivered in form of cream concentrated latex, air dried sheet (ADS) rubber, ribbed smoked sheets (RSS) rubber and block rubber etc. Like most agricultural products including all natural rubber export is hygroscopic and the storage environment could adversely affect its quality [7]. Understanding Moisture and heat transfer between products and air surrounding are essential, especially in tropical climate. of rubber is mainly carried out by traditionally under open sun. Sun drying represents a low cost processing technique to preserve rubber. Natural sun drying has been used since time immemorial for agricultural products. Open sun drying has limitation to control the drying process and parameter, weather uncertainties, high labor costs, required large drying area, mixing with dust and other foreign materials and so on. However, open sun drying is widely practiced in tropical and subtropical countries to preserve agricultural products, where solar radiation is convenient [1]. Basunia and Abe [5] conducted experiments on thin layer solar drying of rough rice in natural convection and determined the drying rate by using the Page equation. Manohar and Chandra [4] studied the drying process in green house type solar dryer using natural as well as forced ventilation and the drying data were represented with the Page drying equation. Yaldiz et al. [7] presented various mathematical models of thin layer solar drying of sultana grapes on the basis of regression analysis of the experimental data. Sodha et al. [6] presented an analytical model of open sun drying and a cabinet type solar dryer. The model was used to predict the hourly variation of crop temperature and rate of moisture evaporation under constant and falling rates of drying. Solar thermal technologies have been used in various applications either, as natural convective type dryers, or with forced ventilation, in the drying of coffee, paddy, cassava, bananas, mango, medicinal plant and herbs [2]. Solar drying can be used in most areas but how quickly the sample dries is affected by many variables, especially the amount of sunlight and relative humidity. Typical drying times depend on sun, air movement, humidity and the type of food to be dried. Thus, the objectives of this research are aimed to study effect drying conditions on kinetics and quality of air dried sheet (ADS) rubber. 2. Material and Method 2.1 Materials The fresh natural rubber sheets were provided from Biochemistry research laboratory (BRL), Faculty of Science Prince of Songkla University. The dimension of sample rubber sheet of BRL was about of 75 cm 2 correlated to their weight of kg. The moisture content of samples was determined following by ASAE, 1982 method [3] 2.2 equipment Fig. 1 illustrated the green house dryer. The dimension of drying room was about 1x1.2x1.5 m 3. All sides of dryer were made of black plastic sheet for acting as solar collector whilst the translucent plastic sheet was used for making roof. At the bottom and top of dryer there were two inlet and two outlet air tubes for heating and exhausting air, respectively. Fig. 1. Schematic diagram of green house drying system Fig. 2. Schematic diagram of solar drying system 2.3 experiment conditions were carried on as follows: green house (GH) drying, green house and natural convection (GH-NC), open sun drying (OS) and solar drying. To confirm long life storage of dried rubber sheet, desired final moisture content of dried rubber sheet was approximately.5% dry-basis [1]. The bulk temperatures, dry bulb, wet bulb of ambient air temperature and temperature inside drying chamber were measured by K-typed thermocouple, which was 2

3 Moisture Ratio, MR Moisture Ratio, MR continuously monitored by a wisco data logger with an accuracy of.1ºc. The relative humidity was determined by dry and wet bulb temperatures. The moisture content was determined by ASAE method [3]. 2.4 kinetic and efficiency of solar collector (a) kinetic The moisture content is normally shown in terms of moisture ratio (MR). The MR value is defined as ratio of moisture difference between real time moisture and equilibrium moisture content to moisture difference between initial moisture content and equilibrium moisture content. The following equation was written as below: (1) where MR is moisture ratio (dimensionless), M i is initial moisture content (% dry-basis), M t is average moisture content at drying time (% dry-basis), rate was calculated by moisture content transfer per drying time, calculation in equation (2). M t MR M i where W d is weight of dry matter in product (kg), t is drying time (h), M i is initial moisture content (% drybasis) and M f is final moisture content (% dry basis) (2) (b) Efficiency of solar collector The calculation of the efficiency of the solar collector can be calculated from [2]. solarcolle ctor (3) where q is cabinet heat gain (kj), I = solar radiation incident on solar collector (kj/m 2 ) and A s = solar collector area (m 2 ) q q IAs where m a is the amount of air into the solar collector (kg), C p is specific heat of air (kj/kg๐c), T i is air inlet temperature ( ๐ C) and T is air outlet temperature ( ๐ C). ma m C a p M eq M eq (4) (5) where ρ is density of air drying (kg/m3), V is average velocity at inlet solar collector (m/s), A is sectional area of air flow channels of solar collector (m) and t is time (s). T T i VAt M M i f rate W t d 2.5 Quality analysis (a) Measurement of shrinkage The percentage of shrinkage of sample rubber was determined by an average of that measured with vernier calipers with an accuracy of ±.5 mm and the percentage of shrinkage was defined as follows: (H Shrinkage (%) (6) where H initial is geometric mean thickness of the rubber sheet at the beginning of the drying experiment and H final is geometric mean thickness of the rubber sheet at the end of the drying experiment 3. Results and Discussion 3.1. kinetic Fig. 3, 4 and 5 show the moisture ratio during drying time at various drying strategies GH (32.8ºC, RH= 68%) GH (36.1ºC, RH= 65.6%) Solar (44.5ºC,RH= 47%) Time (h) Fig. 3. Evolution of the moisture ratio of ADS rubber using combined a green house and conventional drying initial H H final Open sun 12hr (26.8 ºC, RH= 58%) + Convectional (28.1ºC, RH= 6%) Open sun 24hr (27.4ºC, RH= 55%) + convectional (28.9ºC, RH= 55%) Open sun 48hr (27.8ºC, RH= 55%) + Convectional (29.5ºC, RH= 53%) Time (h) final ) 1 Fig. 4. Evolution of moisture ratio of ADS rubber using combined open sun and conventional drying. Fig. 3 showed that the drying of solar drying faster period than GH drying. Fig. 4 showed that temperature in the combined OS and conventional drying was found that the moisture ratio is similar. However, the drying time at high temperatures faster than at low temperatures. Paper Code-3

4 Solar radiation (W/m 2 ) Moisture Ratio, MR Temperature ( C).6.4 GH (36.1ºC, RH= 65.6%) Open sun 12hr (26.8 ºC, RH= 58%) + Convectional (28.1ºC, RH= 6%) GH 24hr (31.5ºC, RH= 6%) + Convectional (28.7ºC, RH= 55%) Solar (44.5ºC,RH= 47%) to the air is heated in a plate collector and then passed through the drying cabinet Time (h) Fig. 5. Evolution of moisture ratio with drying time of ADS rubber at any drying conditions. Fig. 5 comparison of moisture ratio between experimental of ADS rubber at any drying conditions, showed that the effect of energy used in drying rubber during temperature 26-45ºC was found that solar drying alone can reduce the moisture content of materials to be faster than other condition. Comparison of drying with the drying conditions in both the case of a drying one step and two steps, all conditions showed that factors affecting the drying temperature, drying rate and dried at high temperatures used in drying time is shorter than the drying temperature range under all sources of heat. At the beginning of drying time, moisture ratio of rubber decreased rapidly because the main part of moisture content of sample exists 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 do not only occur at the surface of rubber sheet but also they stimulate inside the rubber sheet. However, moisture inside rubber sheet moves to surface slower than the movement from the surface of rubber sheet to ambient environment. rate will be relative lowers compared to the beginning of drying time. Additionally, the moisture content of sample is decreased easily when the thickness of rubber sheet decrease. Consequently, at a higher drying air temperature, rate of moisture removal became relatively faster than those of a lower temperature. These drying curves are typical equations for predicting drying kinetics of grain kernel and food stuff, corresponding to the previous works [9]. Fig. 6 shows hourly plots of average temperatures records of drying cabinet and plate collector. It can be seen that average temperatures of drying cabinet and plate collector vary depend on solar radiation. The average temperatures of a plate collector were higher than the average temperatures of a drying cabinet due 2 1 Ta Tcollector Tmid Trubber Ttop Tbottom Texit 6:14 AM 8:9 AM 1:4 AM 12: PM 1:55 PM 3:5 PM 5:45 PM Time Fig. 6. Hourly variations of ambient air temperature, average temperatures of drying cabinet and plate collector on a typical day (15/7/211) in Songkhla province :43 AM 9:7 AM 11:31 AM 1:55 PM 4:19 PM 6:43 PM Time Fig. 7. Hourly variations of solar radiation intensity on a typical day (15/7/211) in Songkhla province Quality analysis of rubber sheet (a) (b) (c) Fig. 8. Visual characteristics of dried rubber sheet (a) combined OS-NC drying (27.8 C, 29.5 C) (b) solar drying (44.5 C) and (c) GH drying (36.1 C). Considering the visual characteristic of the drying at inlet air temperatures of 27-45ºC, it reported that before starting the experiment, granules were dispersed uniform in its texture. After drying, the dried rubber texture seemed to be bright and then became uniform gel on its texture as revealed in Fig. 8(a) and (b). Except greenhouse drying, dried rubber sheet texture appears to be sticky in Fig. 8(c). Paper Code-4

5 Paper Code: ee11 temp. & RH ( C, %) Table 1 Physical drying characteristics of ADS rubber in experiment time rate Shrinkage (h) (kg/h) (%) Green House (GH) 36.1, Dense/ Physical drying characteristics Brightness and Initial Final Sticky/ yellow Bubble 32.8, Sticky/ yellow No Green House + Natural Convention (GH-NC) (31.5, 6),(28.7, 55) Dense/ Bright gel/ yellow No (35, 67), (29.3,51.5) Bright gel/ yellow No Open Sun + Natural Convention (OS-NC) 12hr (26.8, 58), (28.1,6) Dense/ Bright gel/ yellow No 24hr (27.4, 55), (28.9, 55) Bright gel/ yellow No 48hr (27.8, 55), (29.5, 53) Bright gel/ yellow No Solar drying 44.5, Dense/ Bright gel/ yellow No No 4. Conclusions The results can be concluded as the follows: 1. The drying rate increased with increase of inlet drying temperature. 2. The drying time of rubber sheet was affected by sheet thickness and drying temperature 3. At the beginning of drying time, the moisture ratio of sample rapidly increased, moisture ratio decreased with the increasing of drying time 4. ADS rubber was dried by solar drying better and faster to combined GH-NC drying, combined OS- NC drying and green house drying, respectively 5. The efficiency of solar collector obtained was 37.9 %. Acknowledgement 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 under Faculty of Science, Department of Chemical Engineering under Faculty of Engineering Prince of Songkla University Thailand for their grant and financial support. References [1] Agricultural Statistics of Thailand 29, Ministry of Agriculture & Co-Operatives, Bangkok, Thailand, 29. [2] A.O. Fagunwa, O. A. Koya, and M.O. Faborode, Development of an Intermittent Solar Dryer for Cocoa Beans. Agricultural Engineering International: the CIGR Ejournal. Manuscript number 1292, XI. July, 29. [3] ASAE standard, Moisture Measurement, the 2 9th ed., St. Joseph, Miami, USA, [4] K.R, Manohar, P. Chandra, of agricultural produce in a greenhouse type dryer. Int Agric Eng J 9(3)(2) [5] M.A, Basunia, T. Abe, Thin layer solar drying characteristics of rough rice under natural convection. J Food Eng 47(21) [6] M.S. Sodha, A. Dang, P.K. Bansal, S.B. Sharma, An analytical and experimental study of open sun drying and a cabinet type dryer. Energy Convers Mgmt 25(3)(1985) [7] O. Yaldiz, C. Ertekin, H.I. Uzun, Mathematical modeling of thin layer solar drying of sultana grapes. Energy 26(21) [8] S. Cenkowski, D.S., Jayas, D. Hao, Latent heat of vaporization for selected foods and crops. Canadian Agricultural Engineering, 34(1992) [9] S. Tirawanichakul, Y. Tirawanichakul, Comparison and Selection of EMC Desorption Isotherms for Crumb Rubber. Proc. in PSU-UNS Inter. Conf. on Eng. and Envir.: ICEE 25, Novi Sad, Serbia-Montenegro, May 18-2( 25). [1] W. Szulmayer, From sun drying to solar dehydration I. Methods and equipments. Food Technology in Australia 23(1971)