Importance of energy management in desiccated coconut industry

Energy flow rates across commoniy used desiccators and associated heat transfer equipment are quantified. It is seen that approximately 726 percent excess heat energy over the actual requirement is presently used for drying in desiccators. Approximately 49 percent of heat energy is exhausted to the atmosphere through the chimney and approximately 23 percent along with the desiccator exhaust gases. Approximately 40 percent of energy could be Saved by introducing recirculation techniques of heat recovery. This saving represents about 83.2 million kwh of energy for 1981 alone, which is an equivalent of 4.5 percent of the total electrical energy generated in Sri Lanka in 1981. I N T R O D U C T I O N The commonly used process of manufacture of desiccated coconut is a batchwise process. Two types of drying equipment presently in operation in Sri Lanka, namely, desicca tors which are loaded and unloaded manually and dryers with a semi automatic arrangement for loading and unloading. These are designed for indirect heating from flue gases obtained by either combustion of firewood or oil firing. The ambient air at approximately 30% is sent through a heat exchanger. Combustion gases leaving the furnace enters the heat exchanger as die heating medium. Indirect heat transfer from combustion gases to the desiccator air increases the temperature of the desiccator air. Used combustion.gases leave the heat exchanger via a chimney as a flue gas. The average drying temperature inside the desiccator is around 88*C (190 F ) . Under these conditions the drying process takes approximately 45 minutes. The normal inlet temperature of air is approximately I30°C (266*F) and the outlet temperature approximately 88° C (19CTF). In 1982 seventy five factories were engaged in the manufacture of desiccated coconut in Sri Lanka. The overall production capacity of these factories in 1982 was around 60,000 metric tonnes of desiccated coconut estimated on the basis of an 8-hours working day and 200 work ing days per year (Ministry of Coconut Industries, 198-2). 19 Energy management in the desiccated coconut industry The average consumption of firewood per tonne of desiccated coconut is approximatly 1.05 tonnes (3.5 yards) (Nattandiya Coconut Producers Co-op Society Ltd.. 1983) representing in overall requirement of about 63.000 tonnes of firewood per year. The following section presents an energy analysis of the production process. E N E R G Y A N A L Y S I S Excess Energy The average moisture content of shredded kernel prior to desiccation is 43 percent on wet basis. The average moisture content to which the shredded kernel is dried within the desiccator is 3 percent on a wet basis. The average temperature within the desiccator is 88° C. The theoretical heat requirement for heating shredded kernel from an ambient temperature of 30°C to 88°C and for the removal of moisture from 43 percent to 3 percent is 2.227 x 10 6 kJ tonne of desiccated coconut. (See Appendix 1). The process of drying of desiccated coconut usually required approximately 1.05 tonnes of firewood per tonne of desiccated coconut. The average moisture content of firewood in the western coastel belt of Sri Lanka between Chilaw and Colombo where the desiccated coconut mills are usually situated was found to be 13.77 percent on dry basis. The variation of calorific value of firewood with the moisture content is shown in Fig. 1. The nett calorific value of firewood at 13.77 percent moisture content is 17.235 kJ/kg (7420 Btu/lb). The actual heat generated in the process of drying one tonne of desiccated coconut is therefore 18.405 x 10 6 kJ. The energy generated in excess of the requirement for drying one tonne of desiccated coconut is therefore 16.178 x 10 6 kJ. This represents an energy generation of 726.45 percent in excess of the theoretical requirement. (See Appendix 2) A P P R O P R I A T I O N E N E R G I E S Energy Balance Across Desiccator The average drying temperature within a desiccator is 88°C. If complete mixing is assumed within a desiccator, the exit air stream from the desiccator may be assumed to leave at a temperature of 88° C. The average flow rate of air through adesiccatoris estimated lo be 7400 kg air per hour (See Appendix 3). The rate of moisture removal by this stream is 77.23 kg/hr. Estimated temperature of air inlet to the desiccator is 130.0° C (See Appendix 3). Taking the datum as the ambient conditions for energy measurements, the energy flow rates per tonne of desiccated coconut at the inlet and exit of the desiccator are 67.67 x I 0 5 kJ respectively.