The advantage of backward feed multiple effect evaporators over forward feed units is that

ANS:C - most concentrated liquid is at highest temperature.

In a multiple effect evaporator system, several evaporator stages are used to progressively concentrate a liquid by evaporating its solvent. In a backward feed configuration, the concentrated liquid from each stage is fed back to the previous stage, but at a higher temperature. Here's how it works:

1. Initial Stage: The feed enters the first (initial) evaporator stage, where it is partially evaporated. The vapor from this stage heats the next stage.
2. Subsequent Stages: The partially concentrated liquid from the initial stage is fed to the second stage, where further evaporation occurs. The concentrated liquid from this stage is then fed back to the first stage, but at a higher temperature due to heat exchange with the vapor from the second stage. This process continues for each subsequent stage, with the temperature of the concentrated liquid increasing with each stage.
3. Final Stage: The most concentrated liquid, having undergone evaporation in all stages, is at the highest temperature among all the liquid streams in the system.

This arrangement ensures that the most concentrated liquid, which is typically the product desired from the evaporation process, is maintained at the highest temperature. This is advantageous because it allows for efficient heat transfer throughout the system, facilitating further concentration without excessive energy input and minimizing the risk of thermal degradation for heat-sensitive materials.