The most important unit operation in the processing of cassava roots to value-added products is drying. Dryers are used to convert cassava mash to floury/powdery products. Among the different dryers, flash dryers have the shortest residence time of drying, are the most economical and widely used drying system for solids that have been dewatered or inherently have low moisture content. Thus, it's suitability for the production of starch, high-quality cassava flour (HQCF) and powdered fufu. Many flash dryer designs based on efficiency, product quality and throughput have been developed by fabricators in Nigeria. Year 2000 was the first time a flash dryer (owned by Femtex Starch factory in Lagos) was tested by IITA for drying HQCF. The result was encouraging, and it led to the subsequent technical support to equipment fabricators in Nigeria for the widespread fabrication and its use for drying of cassava to HQCF in Nigeria. Since 2004, flash dryers have been fabricated commercially and installed in Nigeria and other African countries (Ghana, Tanzania, Madagascar, Malawi, Zambia, etc.) by subsequent projects, government institutions, and the private sector.
The wet product enters the drying duct of a flash dryer at the feeding point and is entrained by the hot airstream. Both negative and positive pressure conveying systems are used in pneumatic dryers. In a negative-pressure system, the blower is located downstream of the material separator, while in a positive-pressure system the blower is located upstream of the material feeding point. During transport, heat is transferred from the hot air to the particles through convection, drying the material as it moves. At the end of the drying duct, a cyclone is commonly used to separate the entrained dry product from the drying air. The large surface area of the product particles results in high drying rates and short residence times and this allows heat sensitive materials to be dried at relatively high temperatures, without overheating taking place.
Flash dryers are used to dry granular materials in the chemical, pharmaceutical and food industries.
A flash dryer is composed of a feeding point, drying duct through which cassava grits are carried by hot airstream and are simultaneously dried. An air blower pulls or pushes the hot air through the drying duct. A cyclone separates the dried material from the drying hot air.
A flash dryer (pneumatic dryer) has to be placed in a proper factory building or similar space with a complement of other cassava processing machines. Fresh roots are processed into wet but free-flowing granules to be dried in the dryer. After drying, the grits are milled into flour.