Drying Kinetic of Pepper Corns in a Fluidized-bed with Helical ...

The 2 nd Joint International Conference on “Sustainable Energy and Environment (SEE 2006)”E-059 (P) 21-23 November 2006, Bangkok, ThailandDrying Kinetic of Pepper Corns in a Fluidized-bed with Helical Distributor PlateAmnart Boonloi * and Pongjet PromvongeFaculty of Engineering, King Mongkut’s Institute of Technology Ladkrabang, Bangkok 10520, ThailandAbstract: This paper presents the characteristic study of drying pepper corns in a fluidized-bed with typical/helical distributor plates.The experiment has been carried out in a drying bed having 0.13 m. inner diameter and 1 m. height and the helical distributor plate isalso used to produce swirling flow in the bed. In each experimental run, the pepper corns with initial moisture content of about 80%w.b. of 0.2 kg is loaded into the bed with inlet air temperature of 80°C and varying air inlet velocities to be 1.34, 1.65 and 2.06 m/s.Drying time and weight of pepper corns are measured in 10 minute intervals for 3 hours for each test of both beds. The experimentalresults show that use of different velocities leads to no significant effect on drying rate for the fluidized-bed with typical distributorplate but provides a substantial influence for the swirling drying method of the fluidized-bed with helical distributor plate. Incomparison between both beds, the swirling drying bed performs better than the typical fluidized-bed with a reduction of drying timeat about 30%Keywords: Fluidized-bed, Drying Rate, Helical Distributor Plate1. INTRODUCTIONDrying is the method for food preservation for a long time. The desiccated product can be kept for a long period without rot.Therefore, there are many studies about drying technique up to date. Drying is the process in removing moisture from product byevaporation. Generally, hot air is widely used as media of drying process. Heat is transferred to the product while the moisture isremoved. The rate of drying process is up to the temperature difference between hot air and the product, relative humidity and airvelocity. Basically, high temperature air with low relative humidity is preferable for quick drying process. However, the maximumtemperature of hot air is limited by the quality and application of the product. The highest temperature of hot air is chosen in eachapplication because the higher air temperature, the quicker in drying and the lower in operation cost.Although the most common types of hot air dryers used for commercial vegetable dehydration are tray and tunnel dryers, dryingin fluidized beds offers an interesting alternative. A fluidized bed dryer provides smooth, clean and efficient drying to low moisturecontent due to uniform temperature distribution and a large exchange area between solid and gas [1]. Various researches such asMulet et al. [2], Berna et al. [3,4], and Rossello et al. [5] have examined the effect of air velocity on drying kinetics in the drying ofvarious food products. They stated that this effect was negligible at high air velocities and the critical air velocity value at whichdrying rate is not affected was found between 1 and 1.5 m/s.The aim of this study is to measure the drying rate of pepper corn using a fluidized bed technique with helical distributor plate andto investigate the effects of specific air velocity on the drying rate.2. THEORETICAL ANALYSISMoisture is the quantity of water in product. There are two basis in specifying moisture content, wet-weight basis ( m w ) and dryweightbasis ( m d ).Wherew − dm w =w(1)andw − dm d =d(2)wherew = the total mass of sample as it is (kg)d = the mass of the dry matter (kg).The dry-weight basis is widely used in organic drying analysis because the dry weight of the matter is almost constant duringdrying process.3. EXPERIMENTAL APPARATUSA schematic diagram of the overall experimental set-up is shown in figure 1. A schematic diagram of an overall experimental setupconsisted of (1) Fluidized bed drying chamber (2) sample-containing bowl (3) weight digital (4) electric air heater unit (5) blower(6) hot wire anemometer (7) orifice meter with inclined manometer (8) digital manometer and (9) control valveCorresponding author:ao_amnart@yahoo.com1

The 2 nd Joint International Conference on “Sustainable Energy and Environment (SEE 2006)”E-059 (P) 21-23 November 2006, Bangkok, ThailandFig. 1 Schematic diagram of experimental setup3.1 Fluidized bed dryerA diagram of the fluidized bed dryer, which was used in the experiment, is shown in Fig. 1. It possesses an acrylic plastic bedcompartment of a circular section with the inner diameter of 130 mm and the height of 1000 mm. Air distribution plate, fitted at thebottom, was a perforated plate with 908 holes, 3.2 millimeters in diameter of each hole, where the total hole area was 32 % of totalplate area. For fluidized bed drying, the most significant component is the drying column. Therefore, the thermal balance is derivedby applying mass and energy balances to the drying column in batch fluidization shown in Fig.2 (a). The drying process in a batchfluidizedbed is modeled by assuming a perfect mixing of particles. The process occurs during an isobaric process due tosimultaneous heat and mass transfer between hot air and corn.(a) fluidized-bed with typical distributor plate(b) fluidized-bed with helical distributor plateFig.2 Bed column with various dryers3.2 Fluidized bed with Helical Distributor PlateThe apparatus setup of fluidized bed with helical distributor plate was the same rig as the fluidized-bed dryer but the inlet airdirection was different. In the case of dryer with helical distributor plate, the inlet air was tangentially injected into the rig. Theschematic diagram of fluidized bed with helical distributor plate as can be seen in Fig. 2(b). The swirling flow was generated by ahelical distributor placed at the bottom of the rig. Therefore, the air was spiraled upward as swirling flow. The helical plate provided2

The 2 nd Joint International Conference on “Sustainable Energy and Environment (SEE 2006)”E-059 (P) 21-23 November 2006, Bangkok, Thailanda flow direction with 30˚ inclination from the horizontal plane as can be seen in Fig. 3. The superficial air velocity in the bed is set to1.34, 1.65 and 2.06 m/s for all experiments.Fig.3 Characteristic of helical distributor plate3.3 Corn grainIn the experiment, the pepper corns were drying media. The pepper corns ware loaded into the test section at 0.2 kilograms eachwith 80% humidity (wet basis)3.4 ProcedureDrying time and weight of pepper corns are measured in 10 minute intervals for 3 hours for each test of both techniques. A 3-hpblower coupled to an inverter for controlling the airflow rate was used to supply air which is preheated with a 7.5 kW electric heaterinserted at the plenum chamber. The electric heater provided hot air of a maximum temperature of 120 o C and the heating took about15 minutes for the bed to raise its temperature until 80 o C. In experiments, the temperature of the bed was measured by means oftype-K thermocouples. The measurement of fluidizing airflow rate was taken using an orifice meter and the minimum fluidizationvelocity was measured in the usual way as the point of interaction between two straight lines on a plot of velocity at the inletdistributor plates and pressure drop across the bed4. RESULTS AND DISCUSSIONIn the experimental results, the effect of the superficial air velocity on drying rate is investigated using various drying techniquesas can be seen in Figs. 4-6. In the first part of the study, experiments were performed to compare the drying rates of variedsuperficial air velocity (inlet air temperature of 80°C). In the second part of the study, experiments were performed to determine theeffects of inlet air velocity (1.34, 1.65, 2.06 m/s). Finally, the third part of the study was devoted to the study of the effects of variousdrying techniques (fluidized bed with/without helical distributor plates)4.1 Effect of superficial air velocity on drying rateDrying rate for the fluidized-bed with air inlet velocities of 1.34, 1.65 and 2.06 m/s is presented for the bed with a typicaldistributor as seen in Fig.4. It is found that the moisture content of pepper corns is highly reduced for a drying time period of 0 - 100min. and then gradually decreases for the rest of drying times for all velocities. The decreases in the moisture content with drying timeare nearly the same for all velocities especially at higher velocity. This indicates that the reduction of the moisture content of peppercorns would not be effected by air velocity used. Fig. 5 shows the variations of the moisture content of pepper corns with drying timefor the fluidized-bed with a helical plate. In the figure, the moisture content of pepper corns is decreased rapidly with drying time forthe first half of drying and the drying rate is high at the beginning of drying and then tends to reduce slightly for the second half ofdrying. The moisture content reduces with the rise of the drying air velocity. This points out that the drying air velocity has asignificant effect on the drying of pepper corns, especially at higher air inlet velocity. The drying rate of the swirling technique isfound to be better than the typical fluidized bed technique.100Moisture Content (%)806040201.34 m/s1.65 m/s2.06 m/s00 20 40 60 80 100 120 140 160 180 200Drying Time (min)Fig. 4 Variation of moisture content of pepper corns with drying time for using typical distributor plate3

The 2 nd Joint International Conference on “Sustainable Energy and Environment (SEE 2006)”E-059 (P) 21-23 November 2006, Bangkok, Thailand100Moisture Content (%)806040201.34 m/s1.65 m/s2.06 m/s00 20 40 60 80 100 120 140 160 180 200Drying Time (min)Fig. 5 Variation of moisture content of pepper corns with drying time for using helical distributor plate4.2 Comparison between fluidized bed techniques with typical/helical distributor platesFig.6 showed a plot at a particular air flow rate for both the techniques. The results show that the drying time is obviously different forusing air inlet velocity at 2.06 m/s for both techniques. The drying rate of the fluidized bed with the helical distributor plate is better thanfluidized bed with typical distributor plate.10080Typical Distributor PlateHelical Distributor PlateMoisture Content (%)60402000 20 40 60 80 100 120 140 160 180 200Drying Time (min)Fig. 6 Comparison between drying pepper corns in the typical bed and the bed with helical plate at air velocity of 2.06 m/s.5. CONCLUSIONSExperimental study of moisture content reduction of drying pepper corns is carried out in a fluidized-bed with a helical distributorplate. The experimental results show that the use of different velocities leads to no significant effect on drying rate for the fluidizedbedwith typical distributor plate but provides substantial influence for the swirling drying method of fluidized-bed with helicaldistributor plate. The swirling drying bed performs better than the typical fluidized-bed which can reduce drying time at some 30%.6. REFERENCES[1] M.S. Nargal and B. Oraikul, Effect of some physical and chemical pre-treatment on improvement of drying characteristics ofhash-brown potatoes, J Food Science Technology, 33, 436-439 (1996).[2] A. Mulet et al., Effect of air flow rate on carrot drying’, Drying Technology, 5, 25-258 (1987).[3] A. Berna et al., Drying kinetics of apricots’ In: Spiess WEL, Schubert H: editors. Engineering and food, Physical properties andprocess control, London: Elsevier Applied Science, 1, 628-636 (1990).[4] A Berna et al., Drying kinetics of Majorcan seedless grape variety, In: Mulumdar AS, editor, Drying’91, Amsterdam: ElsevierScience Publishers, 455-462 (1991).[5] A. Rossello et al., Simple mathematical model to predict the drying rates of potatoes, J Agric Food Chemical, 40, 2374-2378(1992).4