Question

Which of the following statements regarding multiple effect evaporators is/are TRUE?

• A. The pressure of the effect with fresh steam is the lowest for both forward feed and backward feed.
• B. Backward feed is preferred over forward feed for cold feed.
• C. Backward feed is preferred over forward feed for highly viscous concentrated product.
• D. The temperature of the effect with fresh steam is the lowest for both forward feed and backward feed.

Hint:

When dealing with multiple effect evaporators, adjusting feed flow direction based on the feed properties and desired concentration levels can significantly enhance efficiency and prevent operational issues.

Answer 1

The Correct Option is B, C

Multiple effect evaporators are used extensively in industries that require the concentration of solutions by evaporating the solvent, typically water. They utilize steam efficiently by cascading it through several effects where the temperature and pressure decrease sequentially. 
Step 1: Analysis of Backward Feed for Cold Feed:
Backward feed is preferred when the feed is cold, as it minimizes thermal shock and avoids excessive heating of the cold feed. In this configuration, the cold feed enters the last effect (with the lowest temperature), reducing the chances of thermal degradation or fouling. 
Step 2: Backward Feed for Highly Viscous Products:
Backward feed is advantageous for processing highly viscous solutions because the feed enters the last effect where the temperature is lowest and the viscosity is consequently reduced, making it easier to pump and handle. As the solution moves to hotter effects (where temperature is higher), its viscosity further decreases, improving heat transfer and reducing the risk of fouling.