Question

• At least two application of evaporation in a bioengineering process, explained, with biography please

Answer 1

Evaporation is a process in which a liquid gets converted into gas. The thermal motion of a molecule of liquid must be sufficient to overcome the surface tension and evaporate, that is, its kinetic energy must exceed the work function of cohesion at the surface. Evaporation therefore, proceeds more quickly at higher temperature, at higher flow rates between the gaseous and liquid phase and in liquids with lower surface tension (i.e. higher vapor pressure). Since only a small proportion of the molecules are located near the surface and are moving in the proper direction to escape at any given instant, the rate of evaporation is limited. Also, as the faster-moving molecules escape, the remaining molecules have lower average kinetic energy, and the temperature of the liquid thus decreases. This phenomenon is also called evaporative cooling. This is the reason that evaporating sweat cools the human body.

In the context of thermal process engineering, evaporation is understood to be the separation of a solvent from a solution. An example of a solution is salt water, in which salt (the dissolved solid) is present in the solvent, i.e. water. The addition of heat exclusively evaporates the pure solvent (water in this example) from the solution and carries it away. The remaining solution thus has a higher concentration of dissolved solid than before the addition of heat.

Applications of evaporation in a bioengineering process: The aim of evaporation can be to obtain the solvent, to create a concentrate solution or to precipitate the dissolved solid by crystallisation.

1. Increasing the concentration of solutions i.e., enzymes, salts, alkalis, acids, plastic solutions, fruit and vegetable juices, milk etc.

2. Obtaining products i.e., sugar from juices, salt from brine, drinking water from sea water.

3. Fabric Cooling by Water Evaporation

Clothing can provide safety and comfort for persons exposed to both cold and hot thermal environments. To assess the potential impact of clothing moisture and wetness on fabric cooling, a series of wind-tunnel tests was conducted to quantify the evaporative cooling capacity of selected fabric samples. Single-layer cotton, polyester, nylon and silk were evaluated. The results showed that onset and magnitude of evaporative cooling was determined by the amount of water contained in a fabric sample. The results also showed that an exposed “skin” exhibited more cooling when covered with a fabric than when it was not. The information obtained helps better understand the evaporative cooling process for fabrics and assist in the selection of garment materials that optimize worker comfort and safety.