Waec Model Questions Vol1 2017 Agricultural Science Question 2

A farmer harvests 500 kg of fresh cassava roots containing 60 % moisture (wet basis). The roots must be dried to 12 % moisture for safe storage. The drying process is carried out in two stages: Stage 1: The kiln operates at 30 °C and 40 % relative humidity. Under these conditions the equilibrium moisture content (EMC) of cassava is 20 % (wet basis) and the drying constant k₁ = 0.5 % h⁻¹. The farmer dries the roots until the moisture content reaches 25 %. Stage 2: The roots are then transferred to a second dryer operating at 45 °C and 20 % relative humidity, where EMC = 10 % and k₂ = 0.8 % h⁻¹. Drying continues until the moisture content reaches the target 12 %. Assuming the moisture loss follows the exponential model M(t) = EMC + (M₀ - EMC)·e^{‑k t}, where M is moisture content (wet basis) and t is time in hours, calculate the total drying time required to reach 12 % moisture. Give your answer to one decimal place in hours.