DESIGN OF GRAIN DRYER

Transcription

1 DESIGN OF GRAIN DRYER Viboon Thepent Part 1 CAPACITY CALCULATION I. Basic Condition 1. Object Efficient usage of husk (Minimize the capacity as much as possible for economical operation) 2. System adopted Cyclonic Rice Husk Burner and LSU Dryer 3. Materials (received) Paddy 1000 ton (for 28 days) Corn 250 ton (for 10 days) 4. Usage of hot-air for the final drying by LSU Dryer and for pre-drying in storage-bin II. Required Colorific Calculation 1. Paddy Drying To be dried by LSU Dryer (Continuous-flow type) Air Volume 500 m 3 /min Air Temperature (for drying) 80 o C Ambient Temperature 30 o C, 60 % RH Dryer s Calorific Efficiency 85 % Use of Psychrometric Chart: Point 1 Ambient air 30 o C, 60 % RH: Reading Enthalpy (h 1 ) 71 kj/kg Dry Air Point 2 Drying air temperatures for paddy 80 o C (Heating Process) Reading Enthalpy (h 2 ) 125 kj/kg Dry Air Reading Specific Volume of Air m 3 /kg Dry Air Required Heat 500 m 3 /min m 3 /kg x (125 71) kj/kg 60 s/min = 439 kw ( kj/s) Or Using the Formula q = m C p (T 2 T 1 ) Where q = Required Heat, kw m = Mass flow rate of drying air, kg Dry Air/s

2 C p = Specific Heat of Air, kj/kg o C T 2, T 1 = Temperature of Drying and Ambient Air, o C Required Heat 500 m 3 /min m 3 /kg x 1.0 kj/kg o C x(80-30) o C s/min = 478 kw ( kj/s) (A) 2. Corn Drying To be dried by LSU Dryer Air Volume 500 m 3 /min Air Temperature (for drying) 100 o C Ambient Temperature 30 o C, 60 % RH Dryer s Calorific Efficiency 85 o C Use of Psychrometric Chart: Point 1 Ambient air 30 o C, 60 % RH: Reading Enthalpy (h 1 ) 71 kj/kg Dry Air Point 2 Drying air temperatures for paddy 100 o C (Heating Process) Reading Enthalpy (h 2 ) 148 kj/kg Dry Air Reading Specific Volume of Air 1.08 m 3 /kg Dry Air Required Heat 500 m 3 /min 1.08 m 3 /kg x (148 71) kj/kg 60 s/min = 594 kw ( kj/s) Or Using The Formula Required Heat 500 m 3 /min 1.08 m 3 /kg x 1.0 kj/kg o C x(100-30) o C s/min = 635 kw ( kj/s) (B) Husk s Combustion Calorie Find the size of Rice husk Burner: Required Rice Husk 594 kw 15,500kJ/kg(husk) 0.85 x 3600 = 162 kg/hr 1. Select Rice Husk Burner: Husk Burner Type Cyclonic Rice Husk Burner

3 Capacity 200 kg/hr (Actual required 184 kg/hr) Efficiency 80% Combustion Calorie of Husk 15.5 MJ/kg (15,500 kj/kg) Effective Calorie per sec 200 kg/hr x 15,500 kj/kg x s/hr = 689 kw (kj/s) (C) (C)/(A) = 1.44, and (C)/(B) = 1.09 It is cleared that the husk burner has enough Capacity for operation of dryer III. Husk Consumption 1. Husk Consumption For paddy drying 0.2 ton/hr x 24 hr x 28 days = ton (D) For Corn drying 0.2 ton/hr x 24 hr x 10 days = 48.0 ton (E) Total = ton (F) Husk consumption will be ton IV. Operation of Husk Tank Calculate Tank s Holding Capacity: Husk Consumption per day 0.2 ton/hr x 24 hr = 4.8 ton (G) Required Rice Husk Tank s Holding Capacity for 7 days stock = 4.8 ton/day x 7 days = 33.6 ton (H) Required Tank volume = 33.6ton/0.12ton/m 3 = 280 m 3 NOTE: Specific heat of air: 1.0 kj/kg o C Bulk density of husk: 120 kg/m 3 Recovery of husk: 23% (on dried paddy)

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5 Part 2 DESIGN OF HUSK-FIRE FURNACE Preliminary combustion studies of rice husk in a pot furnace indicated an optimum rate of combustion to be 70 kg husk/m 2 hr with 60 percent excess air. The following considerations were incorporated in designing a husk-fired furnace: 1. Setting up a mixing chamber adjoining the furnace, in which the missing of the products of the products of combustion with ambient air should take place in order to attain the necessary temperature off the gas-air mixture 2. Arresting the flying ash and sparks from going into the drying chamber. 3. An arrangement permitting the rapid change in the direction of the flue gases either to the chimney or to the drying chamber. 4. The furnace should ensure the best combustion of the fuel, as the appearance of smoke or soot in the products of combustion may cause not only the lowering of efficiency of the furnace but also deterioration in the quality of dried grain. 5. Convenience and simplicity of maintenance should be taken into account. 6. It should preferably be a portable unit. Based on the preliminary combustion studies and the fuel properties of rice husk, a box type furnace for supplying 1,680 cubic meters per hour (1,000 cfm) at 70 0 C to C was designed, fabricated and tested at the Post-harvest Engineering Research Group. The furnace is equipped with an inclined grate (45 0 angle, 0.5 m 2 ) consisting of the cast-iron bars in a staircase fashion. At the bottom of the inclined grate is a horizontal revolving grate which disposes off the accumulated ash at a certain interval. In between the combustion space comprising the inclined and horizontal grates and the outlet for the flue gases, there is a curtain wall throughout the width of the furnace. The height of the curtain wall is a little over the outlet so that no amount of un burnt husk or fly ash goes into the chimney. The husk is fed at the top of the inclined grates with the help of feeding roller mounted in the hopper and powered with a 1/8 hp motor. The husk is spread in a thin layer throughout the width of the furnace and flows down the inclined grate by its gravity while combustion takes place. The burnt husk or ash is disposed off intermittently by rotating the horizontal grate. The suction

6 end of the blower is connected with the outlet of the furnace. A secondary inlet to the blower is made to bring in the ambient air the mixture of the air and the flue gases at a required temperature is supplied by the blower either to the drying chamber or the chimney. With the feed rate of 11 kg-hr of husk, the supply of 1,680 cubic meters per hour (1,000 cfm) of heated air flue gas mixture can be maintained at C. The furnace provides a perfect combustion with no traces of smoke in the flue gas.the flue-gas analysis shows about 3 percent CO 2, 16 percent O 2 O percent CO and the rest is inert nitrogen. It may be added that the gas-air mixture is nearly as good as the heated ambient air for drying purposes and has no bad consequences on the dried paddy. To calculate the mass of drying air m da let us proceed from the composition of husk Husk Actual Moisture and ash free basis Water content 13.23% - Ash content 18.18% - Carbon content 29.50% 43.01% Hydrogen content 5.44% 7.93% Nitrogen content 0.46% 0.67% Oxygen content 33.19% 48.39% Composition of air by volume N 2 = 78.03% O 2 = 20.99% CO 2 = 0.03% Hydrogen = 0.01% By wt O 2 = 23% of air On a mole basis for 100 Kg dry ash free husk the composition would by Carbon, C = 43.01/12 = 3.58 kg mole H 2 = 7.93/2 = 3.97 kg mole N 2 = 0.67/28 = 0.02 kg mole O 2 = 48.39/32 = 1.51 kg mole

7 Estimation of air requirement on theoretical basis- Husk has C, H, N, and O 2 as per composition given earlier C = 29.50%, C + O 2 CO x 32 = kg of oxygen/kg of husk H = 5.44%, 2H 2 + O 2 2H 2 O 5.44 x 32 = kg of oxygen/kg of husk N = 0.46%, N + O 2 NO x 32 = kg of oxygen/kg of husk Total = kg of oxygen/kg of husk Approximately 37% of it is present in husk i. e kg O 2 is 23% of air by wt, the quantity of air/kg of husk = 100 x = 5.36 kg Husk feed rate is 87 kg/hr of Rate of air Q da = 87 kg/hr x 5.36 kg of dry air = 466 kg of dry air/hr; 7.77 kg/min or 0.13 kg/s = m 3 /hr = 7.54 m 3 /min = m 3 /s In practice, however, excess air from % is to be supplied for proper combustion.

8 Table Calorific Value of Selected Agricultural Residues Material Source Ash Content Gross Calorific Value (oven dry) (%) (MJ/kg) Alfalfa straw (1) 6.0% 18.4 Almond shell (1) 4.8% 19.4 Cassava stem (2) 18.3 Coconut shell (3) 0.8% 20.1 Coconut husk (3) 6.0% 18.1 Cotton stalks (1) 17.2% 15.8 (4) 3.3% 17.4 Groundnut shells (1) 19.7 (4) 4.4% 20.0 Maize stalks (1) 6.4% 18.2 (4) 3.4% 16.7 Maize cobs (1) 1.5% 18.9 (4) 1.8% 17.4 Olive pits (1) 3.2% 21.4 Pigeon pea stalks (4) 2.0% 18.6 Rice straw (5) 15.2 (4) 19.2% 15.0 Rice husks (5) 15.3 (4) 16.5% 15.5 (1) 14.9% 16.8 Soybean stalks (2) 19.4 Sunflower straw (1) 21.0 Walnut shells (1) 1.1% 21.1 Wheat straw (1) 18.9 (4) 8.5% 17.2 Sources: 1. Kaupp and Goss (1981) 2. Saunier et al (1983) 3. Kjellstrom (1980) 4. Pathak and Jain (1984) 5. OTA (1980)