Step 1: Understanding the Concept:
Drying is the removal of relatively small amounts of water or other liquids from a solid material by evaporation.
Step 2: Key Formula or Approach:
Analyze the energy and material flow. Drying is a coupled phenomenon.
Step 3: Detailed Explanation:
Drying involves two simultaneous processes:
\begin{enumerate}
Heat Transfer: Energy (usually from hot air) is transferred to the solid to provide the latent heat of vaporization required to evaporate the moisture.
Mass Transfer: The evaporated moisture must move from the interior of the solid to the surface, and then from the surface into the surrounding gas stream.
\end{enumerate}
Because neither can happen without the other in a typical dryer, it is classified as a "simultaneous heat and mass transfer" operation.
Step 4: Final Answer:
Drying involves the simultaneous transfer of heat and mass.