Modification And Testing Of Biomass Dryer

2.4.2    Deep Bed Drying
The deep-bed resent complex problems due to the multitude of interdependent and continuously changing temperature and moisture profiles generated in both air and grain masses as well as inside individual kernels.  In an attempt to characterize these phenomena, three distinct approaches have been used namely analytical, empirical and theoretical.   
The drying air carries away moisture from the grain.  As the air-absorb moisture, its temperature fall and its ability to pick u more moisture decreases.  A batch type dryer can be considered t be made u if a group of thin layers of parboiled paddy stacked upon one another with the drying air flow up through the stacks.
Thus, the moisture removed from one article affect the state of the air surrounding other adjacent articles in subsequent layers.  The mathematical simulation of such a dryer would lead to complex equation of momentum, heat and mass transfer since each is continuously exposed to continuously changing environmental condition.
Three distinct layers exist within deep-bed drying proceeds as follows:

The three distinct layer are:-  The drying zone, the dry zone and the wet or undried zone.  These zones progressively form the button of the bed to the top.  The rate of movement of the drying front will depend on factors such as grain depth, grain initial moisture, air flow, grain temperature and relative humidity.
2.5    Factors affecting rate of drying
Drying rate is the rate at which water is been removed from agricultural material per unit time
    Various important factors that affect drying can be classified into three general categories.
i)    Crop parameters
ii)    Air parameters
iii)    Dryer parameters
2.5.1    Crop Parameters
i)    The initial moisture content of thee crop:-  If the initial moisture content of crop is high or low, this will affect the energy and period of drying of such crop.
ii)    The final moisture content of the crop:-  This will also affect the rate of drying in terms of energy supplied for the period of drying etc.  
     iii)    The size of crop:-  This will also affect the rate of drying, small size dry faster than big sizes.
iv)    Shape of the crop:-  This also affect the rate of drying which can either be a faster or slower rate depending on the shape.
v)    The porosity of the crop:-  This also affect the rate of drying, a porous material will dry faster than a compact material because a porous material will allow easy evaporation than the compact ones.
vi)    The solute concentration in the crop:-  if the solute concentration is high, the rate of drying is slow but if low, it means there are much water than the rate of evaporation/drying is high.
vii)    The surface area:-  A crop that has a broad surface area will dry faster than a narrow surface area
viii)    The extent of cracking of the kernel, a kernel that is prone to cracking will require a lower rate of drying than the one that is not prone to cracking.
2.5.2    Air Parameters
i)     Air Temperature:-  The higher the temperature, the faster the rate of drying and at a short length of time.  At high temperature, the air is dry and has less moisture content.  Hence, it will be able to absorb much water when ass into a stream of crop to be dried.  A lower temperature result in a slower rate of drying and a long period of drying because the air temperature is moist, it has little ability to absorb water.  Hence the rate and length of during is affected.
ii)    Air relative humidity:-  The dryness of air is termed relative humidity, the lower the humidity, the dryer the air and such air have a high capacity to hold extra water vapor.  A lower humidity causes faster drying rate and a shorter period time but a high leads to a lower drying rate and longer period of time.
iii)  Air Flow Rate:- The volume of air passing through the crop will cause even and higher or non-uniform and lower drying rate.  If the volume is high, it leads to faster, uniform drying and short time but when thee volume is small, then the drying rate quality of drying and time will be affected.
2.5.3    Dryer Parameters
i)   Type of dryer will also affect the drying, sun drying or natural method has a slower rate of drying and longer time when compared to mechanical method of drying.
ii)   Feed rate of the crop:-  This is the thickness (depth) of crop in the dryer.  A thin layer dries faster than a deep layer.
ii)   Heat losses in dryer by radiation and convection.  Dryer that is not well insulated to reverent heat losses will send much time and the drying rate will be slower than a well insulated dryer.  (Onipede, 2015).
2.6    Review of Dryers
    The summary of researchers that have worked on dryers are reviewed below:
    Reyes et al(2014)developed hybrid solar dryer for dehydration of tomatoes.  In this dryer, the air temperature is increased to 118OC above the ambient temperature.  The various drying curves are obtained at different operation conditions and this curves exhibit good adjustment with empirical models of age such as modified are, the result obtained by this tomatoes is rehydrated into + 2% for all runs, in less than 20 minutes and the energy is save u to 6.6 – 12.5%
    Mohapatra et al (2013) developed a natural convection grain dryer for drying paddy and other cereals by use of thermal energy effectively.  Average drying temperature of paddy is between 500C to 580C.  For drying 100Kg of paddy the drying efficiency of the system is estimated as 50.4%.  A 20kg of paraffin wax grade II was used as a phase change material for constant rate of drying.  The moisture content of the product is reduced from 30.1% to 14.6%.
    Mursalim et al., (2002) developed natural convection solar dryer for drying cashew nut in shells (CNS).  The dryer could heat dry air up to 780C temperature and also high quality standard product was obtained through this process.