Study on Continuous Fluidized Bed Drying Using Terminal Velocity Difference for Separating Dry from Wet Particles

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1 Proceedings of the International Conference on Fluid and Thermal Energy Conversion 2006 FTEC 2006 Jakarta, Indonesia, December 10 14, 2006 ISSN Study on Continuous Fluidized Bed Drying Using Terminal Velocity Difference for Separating Dry from Wet Particles Haryadi (1), Aryadi Suwono (2), and Toto Hardianto (2) (1) Mechanical Engineering Dept., Politeknik Negeri Bandung, INDONESIA (2) Mechanical Engineering Dept., Institut Teknologi Bandung, INDONESIA Contact Person: Haryadi, Mechanical Engineering Dept., Institut Teknologi Bandung, Bandung, 40132, INDONESIA Phone: , Fax: , haryadi331@students.itb.ac.id Abstract Coal, peat and biomass are the next potential energy resources after petroleum. Indonesian potential coal reserve is 57,6 billion tons, 60% of which is low rank coal, while Indonesia has the third largest peat reserve area, and as tropical country with abundance of sunlight, Indonesia is also a large biomass producer. The previous three energy resources have to be upgraded, to become well usable. One of the upgrading processes, well known because of its advantages is fluidized bed drying. In this work, fluidized bed drying process using terminal velocity difference for separating dry from wet particle was investigated. A continuous fluidized bed drier apparatus based on terminal velocity difference for separating dry from wet particle was designed for peat drying purpose in laboratory scale.the apparatus was constructed, and tested. The tests showed that the apparatus stability could be maintained in wide range operating conditions. Keywords: fluidized bed, continuous, terminal velocity, terminal velocity difference. 1 BACKGROUND The use of solid fuel becomes more important, due to oil shortage. Indonesian coal reserve is 57,6 billion tons, and 60% of them are so called low rank coal (LRC), with heating value less than 5500 kcal/kg [1]. Indonesia has 27 million hectares [2] third largest peat area in the world, with 7.62 billion tons of dry peat proven reserve [3]. As tropical country with abundance of sunlight, Indonesia is also a large biomass producer. The three energy resources have to be upgraded, to become well usable. Among upgrading process needed for heating value improvement is drying, and fluidized bed drying is well-known process because of it advantages

2 Haryadi, A. Suwono, and T. Hardianto Fluidized bed drying has many special characteristic, both advantages and drawback. Generally, the advantages of fluidized bed drying are as follows [4]: 1. High mass and heat transfer rates between drying media and particles because of random particle movements, as can be seen from bubbles and clouds movements. 2. Interphase gas exchange causes rapid and intense mixing. This condition makes fluidized bed almost isotherm, and easier to control. 3. Because solid particles flow smoothly likes fluid, the operation can be carried out continuously and controlled automatically. Besides of some advantages, fluidized bed drying has some drawback as follows [4]: 1. Rapid mixing in the bed causes differences resident time between particles. 2. Bed and pipes wall erosion and abrasion are caused by high and non uniform velocity of fluid and particles. 3. Particles agglomeration possibility, especially when the moisture content of particles is high. In the conventional fluidized bed drying method, drying products are not well separated from wet particles. An outlet port is placed in a certain height of the fluidized bed wall, thus there are some possibilities for wet particles to be mixed with drying product. The present work is aimed for the possibility of using terminal velocity difference to take the cyclone function in separating drying product from raw wet material just in the bed. 2 THE CONCEPT OF DESIGN The drying apparatus was designed as a laboratory scale size, continuously operated, and using hot air as drying medium. The hot air is also used as separating medium of dry particles (product) from wet particles. The separation of wet and dry particles is based on terminal velocity differences. For the same size and dry density, a particle which has greater moisture content, has greater terminal velocity. To separate dry from wet particles, the superficial velocity should be less than terminal velocity of wet particle and greater than terminal velocity of drying product with expected moisture content. This condition will make the drying product entrained in drying medium, but still leaving the wet particles in the bed. The next step is separating drying product from drying medium. This step also employs terminal velocity, by expanding cross section area in the freeboard. The cross section area of freeboard should make superficial velocity of drying air in this area less than the terminal velocity of drying product, which causes drying product falls on a receiving tray. The ideal conditions of drying and separating processes in the bed are: uniform size, shape, dry density of particles, superficial velocity and drying air properties through out cross sectional area of the bed. The deviation from these ideal conditions causes a non uniform moisture content of drying product. 3.1 General Description 3 THE DRYING APPARATUS DISCRIPTION By concept of design, a continuous fluidized bed apparatus consists of fluidized bed, in which the particles are dried and dry particles are separated from wet particles; freeboard in which dry particles are disengaged from drying medium. Some other supporting accessories are needed to make the drying system runs well, i.e.: cyclone, air heater, blower, plenum, distributor, screw conveyor, hopper, tray, and vibrator. Schematic drawing of the drying apparatus is showed by Figure 1. The blower drives fresh air to the air heater. The suction side of the blower is equipped with a damper to control drying air flow rate. The air heater is finned tube heater, with the heating steam is in the tube side. Drying air temperature 169 2

3 Study on Continuous Fluidized Bed Drying Using Terminal Velocity Difference for Separating Dry from Wet Particles from air heater is controlled by steam flow rate. The hot air enters the plenum, and than passes a distributor plate to the bed. Drying air outlet Cyclone Freeboard Fluidized bed System boundary Hopper Screw conveyor Tray Vibrator Air Heater Blower Peat outlet Distributor Plenum Figure 1 Schematic diagram of drying apparatus Raw material goes into the bed through a hopper. A screw conveyor conveys raw material to the bed. Dry particles with terminal velocity less than superficial velocity is entrained drying medium to the freeboard. As freeboard cross sectional area greater than bed one, superficial velocity decreases into below the terminal velocity of the dry particles, then the dry particles fall into the tray either the bed. The later is entrained again by drying air until falls into the tray. Drying product pile on the tray closes the freeboard and tray gap, blocks drying air from leakage to the atmosphere. Before any drying product pile present on the tray, this gap closes firmly. The gap size is such that the drying product pile be able to prevent drying air from leak to the atmosphere, in the same time, the gap let the drying product flows pass through. The raw material should continuously flows into the bed and drying product should leave the bed continuously too. The material handling should let drying process operates continuously. 3.2 Feeding System For a continuous drying process, the feeding system have to ensure raw materials reach the drying station continuously, while the drying medium and other material involve in the process are isolated from the atmosphere. Thus, the feeding system is designed with following conditions: continouously operating, no drying air leak from the bed, and variable feeding rate. In additional, the system should be easy manufactured and economicaly affordable. From these conditions, a screw conveyor with multiple pulley variable speed has been chosen as feeding system. The feeding rate was 0.25 to 0.75 kg/min of raw material. The scew was 100 mm outer diameter, 50 mm pitch, and 45 0 pitch angle. In additonal, the hopper neck gap can be varied. 3.3 Discharge System The discharge system is responsible for removing the drying product from the drying product apparatus. The discharge mechanism should prevent drying air from leakage to atmosphere, be adaptable to the material flow properties change. Flow properties of material are affected by average and distribution of particle diameter, sphericity, even moisture content

4 Haryadi, A. Suwono, and T. Hardianto From these conditions, vibrating tray was chosen as the discharge system. The basic principle is: if the material is assumed as fluid, head in the inner side of tray caused by material pile, equals to the head loss of the material passes through the gap. If this condition can be achieved the material flow system will steady. This mechanism is showed in Figure 2. The alteration of material flow properties can be anticipated with the size of gap opening, which can be done manually or automatically. Gap Regulator Tray Excentric Roll Spring Figure 2 The Discharge system 4 EXPERIMENT A drying apparatus based on the concept explained has been made. The bed diameter and height are 30 cm. The apparatus was used for peat drying. The aim of the experiments is to have knowledge about the fluidized bed stability, the working of feeding and discharge system, at design operating conditions and some various operating conditions arround them. The design condition are : 0.5 kg/min feeding rate, laju 0.14 m 3 /s drying air flow rate, and 80 0 C drying air temperature. The experiment was started with the design condition. Feeding rate was varied to 50% above and 50% below design condition. To aim better knowledge, feeding rate was also varied to 25% above and 25% below design condition. Drying air flow rate was varied to 0.13 m 3 /s which is bottom level of Pemberton criterion, and 0.15 m 3 /s which is maximum of the blower capacity. Temperature of drying air was varied in 10 0 C step till 50 0 C. The temperature of drying air was not varied above, because of peat self ignition possibility. Peat average diameter, d p, was 0.92 mm, with 70% moisture content (wet basis). 5 THE DATA AND ANALYSIS 5.1 The Drying Process Hold up for constant feeding rate 0.5 kg/min and constant drying air temperature 80ºC is showed by Figure 3. The bed tends to lean as drying air temperature and flow rate increased, or feeding rate decreased. If the hold up is steady, then the bed is stable. Hold Up (kg) Drying Air Temp. (ºC) Hold Up (kg) Feeding Rate (kg/min) Figure 3 Hold up for constant feeding rate 0.5 kg/min and constant drying air temperature 80ºC 169 4

5 Study on Continuous Fluidized Bed Drying Using Terminal Velocity Difference for Separating Dry from Wet Particles Stability of continuous fluidized bed drying strongly depends on superficial velocity and entrainment. Entrainment depends on superficial velocity, terminal velocity, mass fraction of entrained species, bed height and diameter. While particle terminal velocity affected by moisture content, which is affected by mass and heat transfer. Interdependent of all of these variables creates bed stability. In this work, the bed stability was evaluated experimentally. From macroscopic view, if entrainment rate is less than needed (feeding rate minus nett vaporization rate), the bed hold up will increase. If the blower has enough static pressure, entrainment rate will increase, and equilibrium will be reached. But if the blower static pressure cannot overcome the material hold up, the bed will lock drying air eventually. If entrainment rate is greater than needed, the bed will lean. The drying process will not efficient. This phenomenon is showed by Figure 4 and 5. The low hold up at low feeding rate caused low vaporization rate and high VMR Vaporization Rate (kg/min) Vaporization Rate (kg/min) Drying Air Temp. (ºC) Feeding Rate (kg/min) Figure 4 Vaporization rate for constant feeding rate 0.5 kg/min and constant drying air temperature 80ºC VMR Drying air temp. (ºC) VMR Feeding Rate (kg/min) Figure 5 VMR for constant feeding rate 0.5 kg/min and constant drying air temperature 80ºC The evaluation of superficial velocity can be carried out in minimum fluidization velocity based. According to Pemberton, entrainment phenomena can be seen at superficial velocity from 10 to 15 times of minimum fluidization velocity [5]. The drying experiment was carried out at 10.4 to 13.5 of peat raw material minimum fluidization velocity, or 12.6 to 30.1 of drying product minimum fluidization velocity. In this condition, separation dry particles from wet particles were done. From terminal velocity point of view, superficial velocity must be grater than terminal velocity of drying product. In all experiment variants, superficial velocities were greater than terminal velocity of drying product designed, and lower than raw material terminal velocity; and the continuity of drying 169 5

6 Haryadi, A. Suwono, and T. Hardianto process could be maintained. Due to uncertainty of which species of particles, minimum fluidization velocity is based on; the design of fluidized bed drying is more reliable based on terminal velocity A fluidized bed drying stability is characterized by hold up stability with minor disturbances toward the bed. Some of these disturbances are alterations of fluidization parameters, feeding rate and drying air temperature alteration, and particle agglomeration. The agglomeration occurred at the screw conveyor and in the bed. The bed stability was more vulnerable at high feeding rate, low drying air flow rate or temperature drying air. To minimize the disturbance effect, peat which adhered to screw had to be removed, and feeding had to be done carefully. Drying process at design condition showed a good stability and high humidity of exit drying air. It is recommended that the drying process should be carried out at design condition. 5.2 Feeding System There were some feeding difficulties found during experiment. Bridging often occurred at the hopper. Bridging occurs because of high particle cohesiveness and adhesiveness between particle and hopper wall. This is because of improper particles size and shape, high particles moisture content, and improper hopper material or geometry. Some improvements ware recommended: 1. Using higher hopper inclination angle and enlarge hopper neck. 2. Using less sticky material for hopper wall and screw conveyor, such as stainless steel. 3. Install anti bridging equipment, such as: vibrator, agitator, and two-shaft type constant feeder. 4. Keep screw conveyor clean. Remove adhered peat on the crew. 5.3 Discharge System An improvement of the discharge system was needed. The former tray design is showed by Figure 6a. Although the height of peat pile exceeded front wall and the tray was vibrated, the peat did not flow and leave the tray. Figure 6a Former tray design Figure 6b Final tray design To make peat flows, all front wall of tray was removed as can be seen at Figure 6b. With this design, the peat flew continuously. The use of vibrating tray for large scale dryer needs to be evaluated, because of inertia effect. If mass of tray (and peat) increase, inertia effect will increase, and the tray will be more difficult to vibrate. Another method for discharge system is using hopper-like receiver instead of tray. The discharging of drying product can be done with rotary discharger at the bottom

7 Study on Continuous Fluidized Bed Drying Using Terminal Velocity Difference for Separating Dry from Wet Particles 6. CONCLUSIONS AND RECOMMENDATIONS 6.1 Conclusions From the analysis towards the design and experiment of continuous fluidized peat drying, the following conclusions were drawn: 1. The drying experiment can be carried out at 10.4 to 13.5 of peat raw material minimum fluidization velocity, or 12.6 to 30.1 of drying product minimum fluidization velocity. In these conditions, separation dry particles from wet particles could be carried out. 2. A design method of continuous fluidized bed drying process has been proposed. The design principles are: at the dying bed the superficial velocity should be greater than terminal velocity of drying product and lower than raw material, and at the disengaging bed (freeboard) bed the superficial velocity should be lower than terminal velocity of drying product. 3. It is recommended that the drying process should be carried out at design conditions. 6.2 Recommendations Concerning what has been achieved in this work, the following points were recommended for future works: 1. The feeding system needs some improvements to avoid bridging at the hopper. The improvements include hopper geometrical change and using less sticky material. 2. The use of vibrating tray for large scale dryer needs to be evaluated, because of inertia effect. Another method for discharge system is using hopper-like receiver instead of tray. The discharging of drying product can be carried out with rotary discharger at the bottom. 3. The uniformity moisture content of drying product of this drier needs to be investigated and compared with the conventional fluidized bed drier. REFERENCES [1] [2] International Peat Society (IPS), Peat as a Resource, [3] Tarsis A. Dinarna, Untung Trimo, Tobing, SM, Hasil-hasil Kegiatan Inventarisasi Sumber Daya Gambut dan Bitumen Padat Tahun 2002, Kumpulan Makalah Kolokium Hasil-hasil Kegiatan Lapangan Direktorat Inventarisasi Sumber Daya Mineral Tahun Anggaran 2002, Direktorat Inventarisasi Sumber Daya Mineral, Departemen Energi dan Sumber Daya Mineral, Bandung, pp , 2003 [4] Kunii, D., Levenspiel, O., Fluidization Engineering, 2nd edition. Butterworth-Heinemann, New York, [5] Pell, M., Gas Fluidization, Elsevier, London, 1990 [6] Donker, A.., Design of an Experimental Steam-Fluidised bed Dryer of Coal, Delft University of Technology and Institute of Technology Bandung, Bandung, [7] Hovmand, Svend, Fluidized Bed Drying, Handbook of Industrial Drying Mujumdar, Arun S., Editor,, 2nd edition, vol. I, Marcel Dekker, Inc., New York, pp , [8] Pell, M., Dunson, James B., Gas Solid Operations and Equipment, Perry, Chemical Engineering Handbook, The McGraw-Hill Companies, Inc., New York, [9] Majumdar, Arun S., Menon, Anilkumar S., Drying of Solids: Principles, Classification and Selection of Dryers, Handbook of Industrial Drying, Mujumdar, Arun S., Editor, 2nd edition, vol. II, Marcel Dekker, Inc., New York, pp. 1 39, [10] Molnar, Karoly, Experimental Techniques in Drying, Handbook of Industrial Drying, Mujumdar, Arun S., Editor, 2nd edition, vol. II, Marcel Dekker, Inc., New York, pp , [11] Srinivasakannan, C., Balasubramanian, N., A Simplified Approach to the Drying of Solids in A Batch Fluidized Bed ; Brazilian Journal of Chemical Engineering, Vol. 19, No. 3, pp ,

8 Haryadi, A. Suwono, and T. Hardianto [12] Suyitno, Model Dinamik dan Kaji Eksperimental Pengeringan Batubara Peringkat Rendah Indonesia dalam Fluidized Bed Menggunakan Uap Superpanas, Institut Teknologi Bandung, Bandung, [13] Wimmerstedt, Roland, Drying Peat and Biofuels, Handbook of Industrial Drying, Mujumdar, Arun S., Editor, 2nd edition, vol. II, Marcel Dekker, Inc., New York, pp , [14] Andriesse, J.P., Nature and Management of Tropical Peat Soils, FAO Food and Agriculture Organization of the United Nations, Rome, [15] Ribberink, Sjoerd Jan, Modeling of Superheated Steam Fluidized Bed Drying, Delft University of Technology, Delft,

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