MASS AND ENERGY ANALYSIS OF PNEUMATIC DRYING SYSTEM AND DEVELOPMENT OF IMPROVED DRYER FOR HIGH QUALITY CASSAVA FLOUR

Abstract

Cassava plays a vital role in the food security of the rural economies of sub-Saharan African countries. However, improved processing technologies are needed for cassava transformation to advance to the next stage of industrial raw material. Drying which is an essential unit operation in the production of high quality cassava flour (HQCF) is the highest energy consuming operation, and its control is crucial for ensuring that the product produced is of high quality. A pneumatic dryer is the most suitable and most widely used drying system for such product as HQCF. However, the use of locally fabricated pneumatic drying system is a major challenge for the production of HQCF, owing to the associated problems with those dryers. Consequently, mass and energy balance analysis is required for better design and optimization of pneumatic dryer. Survey of HQCF processing centers was carried out in order to identify the challenges being encountered in the usage of pneumatic dryer for production of HQCF. Thereafter, four design models of locally fabricated flash dryer for HQCF drying were evaluated. The flash dryer models were; positive six cyclones (+6C), positive single cyclone (+1C), negative single cyclone (-1C) which uses black oil and negative single cyclone (-1C) which uses diesel for firing burner. The dryers were subsequently modified based on minimum air using the sorption parameters of cassava. The dryer models were assessed based on energy efficiency, specific energy consumption, thermal efficiency, heat losses to the ambient and heat losses via air outlet. A pneumatic drying system based on positive, single cyclone system for HQCF was designed and fabricated. The pneumatic drying system consists of pre – heating and drying section. The survey result showed that most of the processors (95%) employed pneumatic dryer for drying HQCF and 80% of these pneumatic dryers were locally fabricated. The evaluation study shows that the highest energy efficiency was recorded for the positive single cyclone vii system dryer model 2 (+1C) which increased from 63.27 to 78.55% while its specific energy consumption was reduced from 3.79 to 3.06 MJ/kg after modification. Thus, it is established that the positive single cyclone system had the best drying performance. Also, the modification reduces the fuel consumption by 22%, 14%, 14% and 16% for the dryer models (+6C) (+1C) (-1C) and (-1C) respectively. The maximum performance efficiency of 86.56% and capacity of 350 kg/h respectively was recorded for the pre – heating unit of the developed pneumatic drying system, while the minimum moisture content of the cassava flour samples obtained was 22.12% w/w. The energy efficiency and thermal efficiency increased with increased temperature with the maximum values of 83.21% and 78.61%, respectively at the temperature of 150 ⁰C. In addition, the minimum values for specific energy consumption and final moisture content of sample obtained were 2.82 MJ/kg and 10.98% respectively at the drying temperature of 150 ⁰C for the drying unit of the developed pneumatic drying system. The result obtained show that the drying system could be very useful where considerable quantities of cassava is to be processed into high quality cassava flour in a limited time, hence, it is recommended for adoption and immediate.