Dramatic efficiency gains through improved heat exchangers: the case of cassava flash drying in Nigeria

01.06.2015
Flash dryers form a significant component of the cassava processing industry in Nigeria, with approximately 150 units installed; however, many are no longer operational because of the poor margins, in part due to the relatively high cost of oil and low efficiency levels. Fuel typically comprises 30 per cent of the total production cost. This paper outlines the results of performance testing at processors, and the resultant data for specific energy consumption is given, ranging from a worst of 14.2 MJ/kg to an improved 2.9 MJ/kg. Cashew nut shells (CNS) and palm kernel shells (PKS) are a common waste product in Nigeria, and therefore an ideal alternative fuel. Local fuels were analysed for the net calorific value (NCV), since there is a significant variation in this for PKS due to method of processing. NCV (dry basis) of PKS ranged from 10.7 to 17.6 MJ/kg, and for CNS 19.9–23.9 MJ/kg. A retrofit heat exchanger was developed and results from trials and subsequent installations at SMEs over a 15-month period are reviewed. An economic and carbon emission assessment of the heat exchanger is given, with fuel cost reductions of 90 per cent.