ANS:B - highly viscous

The term "highly viscous" refers to fluids that have a high resistance to flow, often due to internal friction between their molecules. These fluids typically have a thick, syrupy consistency and may flow very slowly. Using open pan evaporators for highly viscous solutions can be challenging due to several reasons:

1. Flowability: Highly viscous solutions may not flow easily within the evaporator, leading to poor circulation and uneven concentration. This can result in inefficient heat transfer and incomplete evaporation.
2. Heat Transfer: The thick consistency of highly viscous solutions can create a barrier to heat transfer, reducing the efficiency of the evaporation process. Heat transfer coefficients may be lower in highly viscous fluids compared to less viscous fluids, further complicating the concentration process.
3. Handling: Handling highly viscous solutions can be more difficult and require specialized equipment and procedures. Open pan evaporators, which rely on the natural circulation of the solution, may not be well-suited for processing highly viscous fluids.

Instead of open pan evaporators, solutions that are highly viscous are often processed using specialized equipment designed specifically for handling viscous fluids, such as scraped surface evaporators or thin film evaporators. These types of evaporators provide better control over the flow and heat transfer characteristics, allowing for efficient concentration of highly viscous solutions. Therefore, the statement "Open pan evaporators are preferred to be used when the solution to be concentrated is highly viscous" is not accurate.