A review on the Tray Dryer System for Agricultural Products Anju Katiyar,K. Sudhakar Energy Centre, Maulana Azad National Institute of Technology, Bhopal (M.P) 462051, India Abstract Tray dryer is widely used in agricultural drying because of its capability to dry products irrespective of time and weather conditions. One more benefit is that agricultural waste can be used as fuel in such system like in biomass dryer. However, the greatest drawback of the tray dryer is uneven drying because of poor airflow distribution in the drying chamber that can be removed by implementing some modification in the dryer design. This paper discusses several designs of tray dryer system for drying different agricultural products and its performance. Most of the dryer systems have been developed are using solar energy because the systems run at low operating cost but they are not reliable with respect to weather condition. Computational fluid dynamics simulation is also reviewed in this paper. I INTRODUCTION In our country agriculture represents the biggest part of the economy. Despite these large numbers, national food production still does not meet the needs of the population because of the lack of appropriate preservation and storage systems. Sun drying is the cheapest method of food preservation but it cause contamination of crop by dust, insect attack and infection by microbes. So different drying methods are implemented to ensure continuous high quality food supply. Among all methods the tray dryer is the most extensively used because of its simple and economic design. The food is spread out on trays at an acceptable thickness so that the product can be dried uniformly. Heating may be produced by hot air stream across the trays, conduction from heated trays, or radiation from heated surfaces. In a tray dryer,more products can be loaded as the trays are arranged at different levels. The key to the successful operation of the tray dryer is uniform airflow distribution over the trays. Sapto et al. [1]designed tray dryer in solidwork and analysis and simulation wass performed using Ansys Fluent i.e. a computational fluid dynamics software. He analyzed heat transfer and temperature distribution, pressure, air flow and turbulence to predict the efficiency of the tray dryer. E. Akpinar et al. [2] performed experiments on the single layer drying behaviour of potato slices in a convective cyclone dryer and also performed mathematical modeling by using single layer drying models in the literature. Drying experiments of potato slices with the thicknesses of 12.5 mm and 8 mm were conducted at inlet temperatures of drying air of 60, 70 and 80 C and with drying air velocities of 1 and 1.5m/s. It was concluded that potato slices with thickness of 12.5mm would dry perfectly in the range of 460 740 min, while those with thickness of 8 mm would dry in the range of 280 520 min in these drying conditions in the convective type cyclone dryer. Fig 1. Experimental Set Up of convective cyclone tray dryer(2) P. Srinivasa Rao[3] et al. developed a recirculatory dryer with a central air distribution system to solve problem of of non-uniform drying along the length of the trays. The dryer was tested with blanched potato chips. At a constant air flow rate of 1.5 m^3/min and 65 c temperature, it took
about 3 h time to reduce the moisture content from 856.94% to 9.98%.The heat utilization factor and thermal heat efficiency of the developed dryer were found to be18.94% and 22.16% respectively. Mortaza Aghbashlo[7] et al. presented the energy and exergy analysis of drying process in a semi-industrial continuous band dryer. He perfomed experiments on thin layer drying of carrot slices with thickness of 5 mm, at air temperatures of 50, 60 and 70 C, drying air mass flow rates of 0.61, 1.22 and 1.83 kg/s and feeding rates of 2.98 x 10 4, 3.48 x 10 4 and 4.16 x 10 4 kg/s. He evaluated the effects of drying variables on weight loss of dried products, energy utilization, energy utilization ratio, exergy loss and exergy efficiency. An amount of 250 g of fresh material was used on each band for drying experiments and the weight loss of dried samples were found to be in the range of 51.6 84.4% of initial weight. The exergy loss and exergy efficiency were found to be in the range of 0.6677 14.1577 kj/s and 0.5527 0.9329, respectively. Fig 2. Complete assembly of recirculating tray dryer (3) Kevin Cronin & Stephen Kearney [4] constructed, a model of a tray dryer to investigate the variability in the dehydration of vegetables. It was solved using the Monte Carlo technique by describing the initial moisture content and the drying rate with probability distribution functions. He conducetd a series of experiments to provide constants and statistical data for the model and to validate it. The utility of this approach is illustrated by applying the probabilistic drying model to quantibing and optimising the final moisture content distribution. Y. Soysal and S. Oztekin[5] designed and did performance and economic analysis of a heated-air tray dryer designed for medicinal and aromatic plants. He conducted six drying tests with varying loading density on Mentha piperita and Hypericum perforatum. In this drying process the product moisture contents was reduced from 59 to 80% to 15% which took 6-9 h for the temperature of the drying air was controlled at 46C during drying experiments. This dryer was successfully used to dry 145 kg of M. piperita and 120 kg of H. perforatum in each drying batch. The payback period of the dryer is estimated to be less than 2 months for M. piperita drying and less than 0.5 months for H. perforatum drying. Dionissios P. Margaris[6] et al. presented the numerical simulation inside complex geometry drying spaces having hundreds of trays. He evaluated velocity components due to the change of the flow direction inside the investigated domain using a five-hole tube connected with a scanivalve device and a differential pressure transducer. From the comparison between the measured data and the predicted results from the numerical simulation of an identical arrangement he conclusion that the standard k e is the most adequate turbulence model. Fig 3. Spherical pressure tube with five holes (6) H. Umesh Hebbar et al.[8] developed a combined infrared and hot air heating system for drying of vegetables. A conveyorised drying system having three chambers was fitted with mid-infrared heaters for radiative heating. Through-flow hot air circulation was also provided for convective mode heating. The system was designed to operate under infrared, hot air and combination mode independently. The performance evaluation studies indicated that combination drying of carrot and potato at 80 C with air at a velocity of 1m/s and temperature of 40 C reduced the drying time by 48%, besides consuming less energy 63% compared to hot air heating. The energy utilization efficiency of the dryer was estimated to be 38% for both carrot and potato drying. Fig 4. Combined infrared and hot air tray dryer(8) Gikuru Mwithiga et al.[9] investigated the effect of air temperature and sample thickness on the drying kinetics of kale using a convective air dryer at a fixed airflow rate of 1 m/s and drying air temperatures of 30, 40, 50 and 60 C. The sliced kale leaves were dried in wire trays in 10, 20, 40 and 50 mm thick layers. The drying rate increased with drying air temperature but decreased with layer thickness. The effective diffusivity for 10 mm thick layers was found
to increase with the drying air temperature and ranged between 14.9 and 55.9 x 10 10 m2/s. He found the Modified Page to be marginally better than the other four models in estimating the drying curve over the experimental temperature range. M. Carsky [10] investigated the drying dynamics of lemon peels and showed a design strategy of an industrial scale dryer based on laboratory and pilot plant tests. For performing laboratory drying tests, he crushed peels into three different sizes (3, 6, and 9 mm) provided drying times (15, 20, and 40 min at 150C, and 30, 35, and 60 min at 100 C) to achieve the required final moisture content of peels (10%). He chose a fluidised bed dryer because of particle agglomeration and relatively long drying times. Milind V. Rane[11] developed a liquid desiccant-based dryer that has higher energy efficiency compared to conventional hot air based drying systems used in industrial and agricultural sectors. Two-stage regeneration of the liquid desiccant, an aqueous solution of calcium chloride, was used to improve the energy efficiency. Carryover of liquid desiccant into the process and/or regenerated air streams was eliminated with the novel contacting device, which has 120 185% greater surface density compared to conventional packing. He presented the experimental results of a liquid desiccant-based dryer, designed for a paper tray drying application.the average specific moisture extraction rate (SMER) of the liquid desiccant-based dryer was experimentally found to be 1.5kg/kWh of heat. Energy savings and resultant reduction in CO2 emissions is about 56% compared to conventional hot air based dryer. Aphirak Khadwilard et al.[12] proposed modified hot air dryer by leaf stove for banana drying. He evaluated the efficiency of the leaf stove and the performance of MHAD_LS. The MHAD_LS system consisted of a drying chamber, heat exchanger in dryer, leaf stove and control system for using the speed adjustment on leaf feed motor and air feed motor to serve the thermal load. System efficiency was 18.68%. The recycle air ratio was 90%. It was found that the product capacity of 30 kg with the initial moisture at 250.10 %dry basis and the resulting final moisture at 63 %dry basis used time 22 hours. The leaf amount of Indian almond for using in the experiment was 44.96 kg, the total electric power consumption was 3.64 kwh and an average drying rate was 0.73 kgwater h- 1. M.J. Urbicain et al.[13] designed a semi-continuous rotary drier to simulate the effect of the air recirculation rate on the unit performance, in particular time taken and the heat requirements to attain a given final solid water content. It consisted of a rotary drying device made of two concentric wire mesh cylinders in a closed box where hot air is blown. The drier cage had specially designed vanes and baffles to induce the solid circulation. In this work the air heat and mass balances equations, together with the drying velocity equation are solved so that the complete operation can be simulated for any working time, if the initial conditions are known. Dionissios P. Margaris[14] designed and constructed a laboratory drying unit to evaluate the essential drying characteristics of various fruits and vegetables. He did experiments in the case of hot air drying of Sultana grapes to calculate moisture content, drying rates and relative moisture content.he used thin-layer model and Page s equation for modelling the drying of Sultana grapes up to the water moisture content to attain the shelf stability. Neslihan Colak[15] evaluated performance of a single layer drying process of green olives in a tray dryer using exergy analysis method. He tested green olive at four different drying air temperatures (40,50, 60 and 70 C) and a constant relative humidity of 15%. He found temperature of 70 C and a drying air mass flow rate of 0.015 kg/s with 0.0004 kg/s of olive as conditions for maximum exergy efficiency. The exergy efficiency values were found to be in the range of 68.65% 91.79% from 40 C to 70 C with drying air mass flow rates of 0.01 kg/s 0.015 kg/s. N. Tippayawong[16] evaluated different methods for improvement of energy utilization and reduction of energy cost per unit product mass in traditional longan fruit drying.he evaluated performance of novel forced draft, recirculating air dryer in terms of specific energy utilisation, thermal efficiency and operating cost. He found that the new dryer yielded an average thermal efficiency of 29%, compared to 19% for the existing design. For the same mass of dried longan fruit produced, specific energy utilisation, fuel cost and operating cost were reduced by more than 42%, 45% and 27%, respectively. It was due to enhanced heat transfer from free to forced convection, heat recovery via hot air recycling, better thermal insulation, and better temperature and humidity control. Fig 5. A new dryer with forced convection and air recirculation(16) Shrikant Baslingappa Swami[17] studied drying characteristics of bori nuggets using hot air temperature of 30, 50 and 70 C and air flow rates of 0.6, 1.0 and 1.4 m/s. The drying constants obtained from Lewis equation decreased from 0.457 to 0.139, 0.514 to 0.324 and 0.735 to 0.487 (rp 0.975; MSE 6 0.0082) for 30, 50 and 70 C, respectively, while the air velocity decreased from 1.4 to
0.6 m/s in each case.the superimposed contour plots suggest optimum drying air temperature between 59 and 63 C and air velocity between 0.9 and 1.05 m/s for attaining minimum values of yellowness index (44) hardness (9.0 9.5 N), cooking time (8.0 8.4 min) and drying time (250 275 min). Fig 6. Schematic of the experimental drying set-up(17) II CONCLUSION The objective of this review is to analyse various designs implemented to improve tray dryer performance, increase quality of dried product and produce uniform drying. In recirculatory dryer with central air distribution system problem of non-uniform drying along the length of the trays is solved. By combining infrared and hot air heating system for drying of vegetables, it was found that drying time was reduced. The rotating/moving tray dryer also produced more uniform drying The drying chamber configuration can be optimized by CFD simulation to predict the airflow distribution throughout the drying chamber. CFD nowadays has become a valuable tool for engineering design and analysis of solving complex fluid flow and mass transfer phenomena, aiding in the better design of tray dryers and produce high quality of dried product. References: 1. W. w. Sapto, C.Y.Wong, A. M. Kamarul, A. Nurul Hidayah CFD Simulation for tray dryer optimization, ISSN:1985-3157 Vol. 5 No.2 July- December(2011) 2. E.Akpinar,A.Midilli,Y. Bicer Single layer drying behaviour of potato slices in a convective cyclone dryer and mathematical modeling, Energy Conversion and Management 44 (2003) pp. 1689 1705 3. Shawik Das, Tapash Das, P.Srinivasa Rao, R.K. Jain Development of an air recirculating tray dryer for high moisture biological materials, Jounal of Food Engineering 50 (2110), pp. 223-227 4. Kevin Cronin & Stephen Kearney Monte Carlo Modelling of a Vegetable Tray Dryer, Journal of Food Engineering 35 (1998) pp. 233-250 5. Y. Soysal, S. OGztekin Technical and Economic Performance of a Tray Dryer for Medicinal and Aromatic Plants, J. agric. Engng Res. (2001) 79 (1), pp. 73-79 6. Dionissios P. Margaris, Adrian-Gabriel Ghiaus Dried product quality improvement by air flow manipulation in tray dryers, Journal of Food Engineering 75 (2006), pp. 542 550 7. Mortaza Aghbashlo, Mohammad Hossien Kianmehr, Akbar Arabhosseini Performance analysis of drying of carrot slices in a semiindustrial continuous band dryer, Journal of Food Engineering 91 (2009),pp. 99 108 8. H. Umesh Hebbar, K.H. Vishwanathan, M.N. Ramesh Development of combined infrared and hot air dryer for vegetables, Journal of Food Engineering 65 (2004) pp. 557 563 9. Gikuru Mwithiga, Joseph Ochieng Olwal The drying kinetics of kale (Brassica oleracea)in a convective hot air dryer, Journal of Food Engineering 71 (2005) 373 378 10. M. Carsky Design of a dryer for citrus peels, Journal of Food Engineering 87 (2008),pp. 40 44 11. Milind V. Rane a, S.V. Kota Reddy, Roshini R. Easow Energy efficient liquid desiccant-based dryer, Applied Thermal Engineering 25 (2005), pp.769 781 12. Aphirak Khadwilard and Phairoach Chunkaew Modified Hot Air Dryer by Leaf Stove for Banana Drying, Energy Procedia 9 ( 2011 ),pp. 344 350 13. A.H. Pelegrina, M.P. Elustondo, M.J. Urbicain Rotary semi-continuous drier for vegetables: e ect of air recycling, Journal of Food Engineering 41 (1999), pp. 215-219 14. Dionissios P. Margaris, Adrian-Gabriel Ghiaus Experimental study of hot air dehydration of Sultana grapes, Journal of Food Engineering 79 (2007), pp. 1115 1121 15. Neslihan Colak, Arif Hepbasli Performance analysis of drying of green olive in a tray dryer, Journal of Food Engineering 80 (2007),pp. 1188 1193 16. N. Tippayawong, C. Tantakitti, S. Thavornun, V. Peerawanitkul Energy conservation in drying of peeled longan by forced convection and hot air recirculation, bio systems engineeing 104(2009),pp. 199 204r 17. Shrikant Baslingappa Swami, S.K. Das, B. Maiti Convective hot air drying and quality characteristics of bori:traditional Indian nugget prepared from black gram pulse batter, Journal of Food Engineering 79 (2007),pp. 225 233 18. S. Misha, S. Mat, 1,2 1 1M.H. Ruslan, 1K. Sopian and 1E. Salleh Review on the Application of a
Tray Dryer System for Agricultural Products, World Appl. Sci. J., 22 (3): 424-433, 2013