Multiple - Effect Evaporator

Chlorine demand in wastewater is due to algae, bacteria, iron, manganese, hydrogen sulfide, COD, BOD, etc. The selection of a gas chlorinator is dependent upon the flow rate of water to be treated, and the chlorine demand of the water. Chlorine gas is supplied in steel cylinders, as a compressed gas in a liquid state. The pressure in the cylinder is dependent upon the temperature of the cylinder. As a safety feature, the cylinders are fitted with fusible plugs that will melt at about specific temperature. This is done to prevent the cylinder from exploding if it is in a fire situation.

In a multiple-effect evaporator, water is boiled in a sequence of vessels, each held at a lower pressure than the previous one. Because the boiling point of water decreases as pressure decreases, the vapour boiled off in one vessel can be used to heat the water in the next vessel, and only the first vessel (at the highest pressure) requires an external source of heat such as fresh steam. In multiple effect evaporator, generally steam is used as the heating media. Two most commonly used types of Evaporators for RO reject are a) Falling Film Evaporator and b) Forced Circulation Evaporator.

A) Falling Film Evaporator:


In this type of evaporator, liquid flows downwards in the form of a film along with the vapours in parallel flow. It is ideally suited to heat sensitive non-salting and non-crystallizing solutions. In falling film evaporators, velocity of liquid through tubes is maintained below 1.5 m/s. In these evaporators sufficient wetting of the heating surface is very important for the heat economy and trouble free operation.

B) Forced Circulation Evaporator:


It is ideally used when the boiling of feed liquid on the heating surfaces is to be avoided due to the salting and fouling characteristics of the liquid. In forced circulation evaporator, velocity of liquid through tubes is maintained in the range of 2 m/s to 3 m/s. This liquid is partially evaporated when the pressure is reduced in the vapor-liquid separator to the corresponding boiling temperature. High -volume, low - head axial flow pumps are used for recirculating the crystal - laden slurry in the evaporator.