1. INTRODUCTION

Evaporation is the removal of solvent as vapor from a solution, slurry or suspension of solid in a liquid. The aim is to concentrate a non-volatile solute, such as organic compounds, inorganic salts, acids or bases from a solvent . Common solutes are caustic soda, caustic potash, sodium sulfate, sodium chloride, phosphoric acid, and urea. The most common solvent in most of the evaporation systems is water.

Evaporation differs from the other mass transfer operations such as distillation and drying. In distillation, the components of a solution are separated depending upon their distribution between vapor and liquid phases based on the difference of relative volatility of the substances . Removal of moisture from a substance in presence of a hot gas stream to carry away the moisture leaving a solid residue as the product is generally called drying. Evaporation is normally stopped before the solute starts to precipitate in the operation of an evaporator.

Invention of evaporators: Norbert Rillieux is famous for his invention of the multiple effect pan evaporator for sugar refining process in 1881. Rillieux was born in New Orleans, Louisiana in 1806. He used the steam generated from one pan to heat the sugar juice in the next pan for energy efficient means of water evaporation.

2. TYPE OF EVAPORATORS

Evaporator consists of a heat exchanger for boiling the solution with special provisions for separation of liquid and vapor phases. Most of the industrial evaporators have tubular heating surfaces. The tubes may be horizontal or vertical, long or short; the liquid may be inside or outside the tubes.

2.1. Short-Tube Vertical Evaporators

Short-tube vertical evaporators are the oldest but still widely used in sugar industry in evaporation of cane-sugar juice. These are also known as calandria or Robert evaporators. This evaporator was first built by Robert . It became so common in process industry that this evaporator is sometimes known as standard evaporator .