Q1: Heat sensitive materials can be concentrated in an evaporator employing

ANS:A - vacuum

Heat-sensitive materials can be concentrated in an evaporator by employing a vacuum. When operating under vacuum conditions, the boiling point of the liquid decreases, allowing evaporation to occur at lower temperatures. This helps prevent thermal degradation of heat-sensitive materials by reducing the exposure to high temperatures. High pressure and high residence time in the evaporator may not necessarily be suitable for concentrating heat-sensitive materials. High pressure can increase the boiling point of the liquid, requiring higher temperatures for evaporation, which could potentially lead to thermal degradation. Similarly, high residence time may increase the exposure of the material to heat, also risking degradation. So, among the options provided, vacuum is the most suitable method for concentrating heat-sensitive materials in an evaporator.