The preferred feeding scheme for evaporation of viscous solution in a multiple effect evaporator is backward feeding.

Explanation:

Introduction to Multiple Effect Evaporator:
A multiple effect evaporator is a thermal evaporation process that uses multiple stages of evaporation to concentrate a solution or extract a desired product. It is commonly used in industries such as food processing, chemical processing, and pharmaceuticals.

Viscous Solutions:
Viscous solutions have a high viscosity, which means they have a thick and sticky consistency. Examples of viscous solutions include syrup, honey, and molasses. These solutions require special handling during the evaporation process due to their high viscosity.

Forward Feeding:
In forward feeding, the feed solution is introduced into the first effect of the evaporator and flows in the same direction as the vapor flow. This means that the feed enters at the lowest temperature and concentration and moves towards the highest temperature and concentration.

Backward Feeding:
In backward feeding, the feed solution is introduced into the last effect of the evaporator and flows in the opposite direction to the vapor flow. This means that the feed enters at the highest temperature and concentration and moves towards the lowest temperature and concentration.

Reason for Backward Feeding:
The preferred feeding scheme for evaporation of viscous solutions in a multiple effect evaporator is backward feeding. This is because viscous solutions have a higher boiling point and require higher temperatures for evaporation. By introducing the feed solution into the last effect, which operates at the highest temperature, the solution is immediately exposed to the required high temperature for evaporation.

Advantages of Backward Feeding:
- Efficient Energy Utilization: Backward feeding allows for the maximum utilization of the available heat energy. The heat is effectively transferred from the higher temperature effects to the lower temperature effects, resulting in energy savings.
- Better Heat Transfer: Backward feeding enables better heat transfer between the feed solution and the heating medium. This is because the feed solution is exposed to higher temperatures initially, which promotes better heat transfer and faster evaporation.
- Reduced Scaling and Fouling: Viscous solutions are prone to scaling and fouling due to their high concentration. Backward feeding helps to reduce scaling and fouling by exposing the solution to higher temperatures initially, which helps prevent the precipitation of solids and the formation of deposits on heat transfer surfaces.

Overall, backward feeding is the preferred feeding scheme for evaporation of viscous solutions in a multiple effect evaporator due to its advantages in energy utilization, heat transfer, and prevention of scaling and fouling.