Question

Which of the following problems is encountered in spray drying due to partial recycling of exhaust air ?

• A. Increased protein levels
• B. Increased powder solubility levels
• C. Increased humidity levels
• D. Increased salt levels

Hint:

Recycling exhaust air saves energy but increases the humidity of the drying air. This reduces the driving force for evaporation, which can cause powder sticking inside the drying chamber.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
In a spray dryer, hot, dry air transfers heat to the liquid droplets and carries away the evaporated water as moisture.
This moist air leaves the drying chamber as exhaust air.

Step 2: Detailed Explanation:
To recover heat and improve thermal efficiency, some spray drying plants partially recycle the exhaust air back into the air heater and drying chamber.
However, this exhaust air carries a high load of water vapor evaporated from the milk.
Recycling this air back into the system leads to several operational challenges:
- Increased Humidity Levels (C): The absolute humidity of the drying air increases over time.
- This higher humidity reduces the vapor pressure gradient between the wet milk droplets and the drying air, slowing down the drying rate.
- Slow drying can result in the powder particles remaining wet, sticky, and forming deposits on the chamber walls.
- It also increases the residual moisture content of the final powder, reducing its shelf life.
This recycling does not affect the chemical levels of protein, salt, or powder solubility directly, but the increased humidity is a major process control problem.

Step 3: Final Answer:
Partial recycling of exhaust air in spray drying increases the humidity levels of the drying air.