ANS:B - vacuum

For concentrating an aqueous solution of a material like anhydrous Na2SO4, whose solubility decreases with a rise in temperature, the most suitable evaporator is a vacuum evaporator. Explanation:

1. Solubility Decrease with Temperature: In the case of materials like anhydrous Na2SO4, where solubility decreases with an increase in temperature, it is preferable to operate the evaporator at lower temperatures to prevent precipitation and maintain a concentrated solution.
2. Vacuum Evaporator: A vacuum evaporator operates under reduced pressure conditions, which lowers the boiling point of the solution. By operating at lower temperatures due to reduced pressure, the risk of temperature-induced precipitation of solutes with decreasing solubility can be minimized.
3. Benefits of Vacuum Evaporation:
   • Lower operating temperatures: The reduced pressure in a vacuum evaporator allows for lower boiling points, reducing the risk of thermal degradation or precipitation of solutes sensitive to temperature.
   • Improved efficiency: Vacuum evaporation can enhance heat transfer rates and overall evaporator efficiency by maintaining a larger temperature gradient between the heating medium and the solution.
   • Energy savings: Operating at lower temperatures reduces energy consumption compared to high-temperature evaporators.
4. High Pressure Evaporator: High-pressure evaporators typically operate at higher temperatures, which may exacerbate the issue of solubility decrease with temperature for materials like anhydrous Na2SO4. Therefore, high-pressure evaporators are not suitable for concentrating solutions where solubility decreases with temperature.
5. Other Options: Backward feed evaporators do not directly address the issue of solubility decrease with temperature. While they may offer certain operational advantages in specific situations, they do not specifically address the solubility-temperature relationship.

In summary, for concentrating aqueous solutions of materials like anhydrous Na2SO4 with decreasing solubility at higher temperatures, a vacuum evaporator is the most suitable choice as it allows for operation at lower temperatures, reducing the risk of precipitation.