Chemical Engineering - Mass Transfer - Discussion

If the steam is at high temperature or low pressure. For this small amount of steam also have large latent heat. By using small quantity steam we can get unit quantity of distillate.

We required low quantity of steam when decreasing the surrounding pressure by which boiling point come at low temperature.

Decreasing pressure decreases b.p of mixture and thus increases the temperature difference between mixture b.p. and steam temperature, So that large amount of latent heat is available. High temperature steam also increases the temperature diff and hence more evaporation.

In steam distillation we are decreasing partial pressure of component, total pressure remains constant.

In steam distillation, the amount of steam required per unit quantity of distillate depends on the ratio of the vapor pressures of the components.

Lowering the total pressure reduces the boiling temperature and increases the relative volatility of the immiscible components, thereby reducing the steam required per unit distillate.

Raising the temperature (at the same pressure) does not reduce the steam requirement; In fact, steam distillation is usually carried out at the lowest possible temperature to avoid decomposition and excess steam use.