Question

In spray drying process, during which of the following periods, much of the drying occurs?

• A. Constant rate period
• B. First falling rate period
• C. Second falling rate period
• D. Third falling rate period

Hint:

Constant rate = Free water evaporation (Fast). Falling rate = Bound water diffusion (Slow). Most of the "work" is done in the constant rate phase.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Drying kinetics describe how the moisture content of a product changes over time.
The process is generally divided into two main phases: the Constant Rate Period and the Falling Rate Period(s).

Step 2: Detailed Explanation:
In spray drying, milk is atomized into millions of tiny droplets. This creates an enormous surface area for evaporation.
1. Constant Rate Period (Option A): During this initial stage, the surface of the droplet is completely saturated with water. Evaporation occurs as if from a free water surface. As long as the surface remains wet, the drying rate remains constant and very high. This is where the bulk of the moisture (much of the drying) is removed—typically from 90% moisture down to the critical moisture content. The temperature of the droplet stays low (at the wet-bulb temperature) due to evaporative cooling.
2. Falling Rate Period (Options B, C, D): This begins when the moisture can no longer migrate to the surface fast enough to keep it saturated. A "crust" or "skin" starts to form on the particle. Drying slows down significantly because the water must now diffuse through the solid matrix. This stage is used to remove the final few percentages of moisture to reach the desired 3-4% level in the powder.
Because the surface area is so high and the initial moisture is so accessible, the Constant Rate Period is the most productive phase of a spray dryer.

Step 3: Final Answer:
Much of the drying occurs during the Constant rate period.