Effect of Drying Condition on Natural Block Rubber: Simulation and Experiments

Transcription

1 Effect of Drying Condition on Natural Block Rubber: Simulation and s Tasara, J. 1, Tirawanichakul, S. 1* and Tirawanichakul, Y. 1 Department of Chemical Engineering, Faculty of Engineering, Department of Physics, Faculty of Science, Prince of Songkla University, Hat Yai, Songkhla, Thailand Supawan.t@psu.ac.th* Abstract The main objectives of this research were to study effect of drying conditions on natural rubber (NR) producing Standard Thai Rubber grade (STR ) and were to develop mathematical models for block rubber drying by using the empirical equation and diffusion model. The experimental set-up of NR producing STR rubber was carried on drying temperatures of 15 14C, inlet air flow rates of.7 and.5 m/s with the block rubber bed-depths of.1-.3 m. The average initial moisture content of samples was in the ranges of 35-5% dry basis while the final moisture content was fixed at 5% dry basis. The experimental results were compared to the mathematical results and it indicated that the simulated results using empirical drying equation were in good agreement with the experimental data. However, the results from moisture diffusion model were also adequate to predict the drying kinetic of experiments. As the results, the simulated specific energy consumption relatively depended on the bed depth of block rubber, initial moisture content and air flow rate. At the low specific energy consumption for drying system could be achieved by drying with recycling of exhaust air. The recommended exhaust air recycling for STR block rubber drying process was in range of 8-95%. Keywords: Energy consumption, Fixed-bed drying, Mathematical diffusion model, STR 1. Introduction Thailand is currently the largest natural rubber (NR) producing and exporting country in the world. About 9% of NR produced was exported in commercial forms as follows: Ribbed Smoked Sheet (RSS), various grades of Standard Thai Rubber (such as STR 5, STR 1 and STR ) [3], rubber concentrated latex, and miscellaneous other forms. The proportion of each product is 35, 35, 8 and %, respectively []. Nowadays, the NR ribbed smoke sheets have a downward trend in production whereas the various STR grades and rubber concentrated latex will be increasingly produced especially on the STR block rubber (the lowest grade block rubber export). This STR is normally made of scrap rubber and low quality of rubber sheet and there are many processes for making these scrap rubber before drying. Most of STR rubber factories have high energy consumption among production line such as cleaning, granulating, creping, shredding, hammer milling, preheating, drying and packaging etc. Thus the energy management in each process should be concerned. However a few researches in energy management have been reported. Drying is a principal process in producing STR block rubber and requires much more energy than other processes, which result in wasting energy and increased investment. There have been very few studies relating STR block rubber drying and theirs production process development. So, design of this drying system has become an important consideration for the processing operation. Development of mathematical drying model should been 185

2 investigated because it can be used for predicting the evolution of moisture transfer and energy consumption of drying system including of quality of product. Even the STR block rubber drying has done on deep-bed drying but it actually composes of many thinlayers and drying phenomena occurs in each layer until all of NR bed depth was already dried. Understanding of thin-layer drying kinetic must be performed. Therefore, the objectives of this research were to drying experiment under different drying condition. Development of the mathematical model and simulation crumb rubber drying process was presented and used for prediction of the optimum condition. In addition, drying model using the empirical equation and diffusion model was comparative studied.. al set-up.1 Materials Fresh crumb rubber was provided by the STR rubber factory at the Southland Resource Co., Ltd., Surajthani provinces, Thailand. The drying of block rubber was carried out in a fixed-bed dryer with dimensions of.35 x.7 x.8 m 3. The dryer basically consisted of the electrical heater unit of.4 kw, a centrifugal fan driven by a. hp motor, temperature controller unit and a cubic shape drying chamber.. al procedure The experimental set-up of block rubber producing STR rubber was carried on drying temperatures of 15 14C, inlet air flow rates of.7 and.5 m/s with the block rubber beddepths of.1-.3 m. The average initial moisture content of samples was in the ranges of 35-5% dry basis while the final moisture content was of 5% dry basis. The inlet and outlet drying air temperature, ambient air temperature and the grain temperature in each rubber bed depth were measured by K-type thermocouple connected to a data logger with an accuracy of +1C. The moisture contents of rubber were determined by standard method of the Association of Official Agricultural Chemists [1]. Figure 1 illustrated the schematic diagram of experimental set-up. The moisture content was represented by means of duplicate. After drying, the dried STR rubber was qualitative analyzed for the compliance with the STR standard. The tests were determined at the laboratory of the Southland Resource Co., Ltd. Fresh sample: initial moisture content 35-5% dry basis. Air velocity of.7 and.5 m/s. Two-stage drying strategy Sample was put into the stainless steel basket Bed depth of.1,. and.3 m The 1 st stage: Inlet air direction from top to bottom of basket. Fixed drying temperatures of 1 and 13C. Drying times of and 3 min. The nd stage, Inlet air direction from bottom to top of basket. Fixed drying temperatures of 15 and 11C. Drying times of and 3 min. Final moisture content of dried crumb rubber of 5% dry basis. Fig. 1: Schematic diagram and Illustrative of experimental set-up 186

3 .3 Mathematical Drying Model To determine drying kinetic of crumb rubber, development of the mathematical drying model was based on a near-equilibrium drying model and the crumb rubber bed was composed of series of thin layers. The major assumption was that there exists thermal equilibrium between grain and drying air and the outlet drying air condition from the first thin layer was used as the input drying air condition for the next layer and the same calculation procedure is made for all thin layers with advancing time step. In addition, the moisture is transferred by liquid diffusion and the shrinkage is negligible during drying. The main equations of the model are listed as follows: (a) Energy balance for a thin layer: The sum of changes of the internal energy in the control volume of a thin bed and enthalpy of air flowing is equal to zero. The following equation can thus be written as follows: T eq C a C v W T C pw Rθ (1) C C W C R a v where C = specific heat capacity, kj/kg- C T = air temperature, C = grain temperature, C W = absolute humidity, kg H O/kg dry air R = ratio of dry grain mass to dry air mass, kg dry matter/kg dry air ρ pw Δx R G Δt () pw a pw m = dry rubber mass, kg m = air flow rate, kg/h h fg = specific evaporated enthalpy, kj/kg with subscripts: a = dry air v = water vapor pw = wet rubber = before drying eq = equilibrium (b) Mass balance for a thin layer: The mass transfer between hot air inlet and crumb rubber can be written as follows: W f M in M f R W (3) (c) Thin-layer drying equation: The equilibrium moisture content equation proposed by Tirawanichakul et al. [4] can be written as follows: B 1 RH exp ATM eq (4) where A = B = The following empirical thin-layer drying equation proposed by Tirawanichakul and Tirawanichakul [4] has been used to describe the drying rate of a thin layer of block rubber. It can be written as below: (M t - M eq ) MR A exp( Bt) (5) (M - M ) in eq when A =.1T +.57T B =.49674T T The diffusion equation for an infinite slap-shaped bed is used for calculating the moisture ratio as follows: MR when MR = moisture ratio M t = moisture content at drying time, % dry-basis M eq = equilibrium moisture content, % dry-basis M in = initial grain moisture content, decimal dry basis T = temperature in C t = drying time, min A, B = constant value (d) Specific energy consumption equation: The calculated the electrical energy at fan and thermal energy. It can be written as below: Q h And 8 π p 1 (p 1) π X [ ]exp[ ] (p 1) 4 c v W T T m c a b (7) E fan ΔP m / 1ρ a η f η m (8) (6) 187

4 Specific energy consumption (SEC) can be calculated as follow:.6 E Q h / t fan SEC (9) M w when Q h = rate of heat consumption of heater, kw = rate of the electricity consumption of E fan fan, kw M w = weight of water evaporated out from the rubber, kg P = total pressure drop in system, Pa η f = efficiency of fan, % η m = efficiency of motor, % m = mass flow rate of dry air, kg-dry air/s 3. Results and Discussions 3.1 Evolution of the moisture content Fig shows the evolution of moisture content to during drying period of mathematical drying model was developed and compared with the experimental results. It can be observed that the simulation results were in good agreement with the experimental results. The simulation using theoretical thin layer drying equation gave better results than those of using empirical thin layer drying equation. Moisture content (%d.b Simulation: Empirical Simulation: Diffusity Fig.: Comparison of moisture prediction using the empirical equation and diffusion model; inlet temperature of 14 C with min. and 11 C with 13 min., fraction of air recycled of 95%, bed depths of 1 cm and airflow rate of.7 m/s. The evolution of moisture content of simulation results using the diffusion model show in Fig 3, the results showed that the moisture content rapidly decreased in the first stage to be due to using high temperature and gradually decreased in the second stage. The time taken to decrease the moisture content until it became constant of the high drying temperature was less than when using lower This evolution of the moisture content corresponded to that of the rubber Moisture content (%d.b m. from base.4 m. from base.6 m. from base.8 m. from base.1 m. from base Fig.3: Evolution of moisture content of simulation results; inlet temperature of 14 C with min. and 11 C with 13 min., fraction of air recycled of 95%, bed depths of 1 cm and airflow rate of.7 m/s. 3. Evolution of the temperature The temperature of each rubber layer gradually increased when the drying time increased for every different drying strategy. At approximately 6 min drying time, there were not many differences in the rubber temperatures among each drying strategy which were in the range of 8-95 C. The results indicated that high drying temperature resulted in more evenly heat distribution in the rubber bed than using lower drying Fig.4 shows the evolution of drying air temperature according to time recorded during one of the experimental results. The result show that the temperature of each rubber layer increased rapidly (11-13 C) because of used the high drying temperature and approached the same temperature (11 C) in the second stage that used lower drying 188

5 Drying temperature ( C) m. from base.4 m. from base.6 m. from base.8 m. from base.1 m. from base Fig.4: Evolution of drying air temperature of simulation results; inlet temperature of 14 C with min. and 11 C with 13 min., fraction of air recycled of 95%, bed depths of 1 cm and airflow rate of.7 m/s. 3.3 Quality Test of block rubber To determine the quality of crumb rubber, the dried samples were analyzed following STR method. The results show that qualities of dried sample shown in Table 1 were satisfied compared to all standard criteria. It found that the dry rubbers obtained from all experiment were passed the standard test. 3.4 Specific energy consumption (SEC) Table showed the comparison of specific energy consumption between experiments and simulations of various drying strategies (drying conditions are recycled air of 95% and air flow rate of.5 and.7 m/s, initial moisture content of 4 to 55 % dry basis and final moisture content of. to.5% dry basis). The prediction of specific energy consumption from the model has a similar trend to the experimental values. Table 1: Chemical quality testing of crumb rubber producing STR Test No. Bed depths (cm.) Drying condition Quality of Block Rubber (STR ) Air velocity (m/s) Temperature ( C) Stage 1 Stage % Dirt % Ash %VM %N %PO %PRI N/A N/A N/A N/A N/A N/A Note: Standard STR block rubber must be limited as follows: % Dirt.16%; ASH.8%; VM.8%; N.6%; PO>3%; PRI>4%. N/A means no test applicable. Dirt = dirt material or impurity inside block rubber; VM = volatile matter; PO = initial plasticity; PRI = plasticity ratio index Table : Comparison of specific energy consumption between experiments and simulations of various drying strategies (drying conditions are recycled air of 95% and air flow rate of.5 and.7 m/s, initial moisture content of 4 to 55 % dry basis and final moisture content of. to.5% dry basis). Temperature Initial Bed SEC (MJ/kg of water evaporated) Test ( C) Air velocity moisture depths No. (cm.) Stage Stage Stage Stage (m/s) content Simulation Simulation 1 1 (%d.b.) (Empirical) (diffusivity)

6 Fig 5 and 6 showed the comparison of specific energy consumption between experiments and simulations of various drying strategies. Fig 5 indicates that the trend of the specific energy consumption agreed reasonably well with the experimental data. Specific energy consumption (MJ/kg HO) Test No. Simulation : Empirical equation Simulation:diffusion model Fig.5: Comparison of the specific energy consumption from experimental and simulated ;initial moisture content of 4 to 55% dry basis and final moisture content of.5% dry basis, fraction of air recycle of 95% and airflow rate of.7 m/s. final moisture content of.5% dry basis, bed depth 1-3 cm and airflow rate of.5 m/s of block rubber. A simulated result showed the specific energy consumption was decreased when the bed depth was increased. 4. Conclusion The following conclusions were drawn from this study: 1. The moisture content of the rubber rapidly decreased when using high drying temperature but gradually decreased when using lower drying The initial moisture content had only small effect compared to drying. The simulation using theoretical thin layer drying equation gave better results than those of using empirical thin layer drying equation. 3. The simulation prediction indicated that the rubber bed depth had significant effects on the specific energy consumption. Decreasing of specific energy consumption was as a result of these drying conditions: high rubber bed depth. Specific energy consumption (MJ/kg HO) Simulation : Empirical equation Simulation:diffusion model Test No. 5. Acknowledgement The authors would like to express their sincere thanks to the Thailand Research Fund, agricultural and Seafood Product and Technology for SME OTOP Research Unit (ASPT), Thailand Research Fund, Graduate School Scholarship, Department of Chemical Engineering Faculty of Engineering and Department of Physics Faculty of Science, Prince of Songkla University for their financial support and this presentation. Fig.6: Effect of the rubber bed thickness (.1-.3 m) on the specific energy consumption of the drying. Conditions: initial moisture content of 4-48 % dry basis, final moisture content of.5 % dry basis, air flow rate of.5 m/s. Fig 6 showed the effect of bed depth on specific energy consumption. The simulated drying conditions are determined under the condition of air recycle of 95%, initial moisture content of 4 to 55% dry basis and 6. References [1] The Association of Official Agricultural Chemists (AOAC), Official methods of analysis (17 th ed.), Washington, D.C., USA,. [] Cousin, B., Benet, J.C. and Auria, R, al Study of the Drying of a Thin Layer of Natural Crumb Rubber, J. International of Drying Technology, Vol. 1, No.6, pp , [3] Rubber Research Institute. Department of Agriculture, Volume 6, June,. 19

7 [4] Tirawanichakul S. and Tirawanichakul, Y. Comparison and Selection of EMC Desorption Isotherms for Crumb Rubber. PSU-UNS International Conference on Engineering and Environment - ICEE- 5, Novi Sad 19-1 May, 5, No. T1-1.1, pp.1-4,