The system is employed for evaporation of water ; crystallization /separation of dissolved inorganic salts like Sodium Sulphate / Sodium Chloride / Manganese Sulphate etc.and drying of these solids. Additionally water is recovered making this system also a Zero Discharge System.
FILTRATION STAGE : Pre filtration of effluent stream to remove suspended solids. Pressure leaf filters ( PLF/CD ) with mechanised discharge are employed for this operation.
EVAPORATION STAGE : Multiple Effects Evaporator are used to get the required final concentration of liquid stream. Steam economy of high order is achieved because of use of Multiple Effect Evaporatos. Thermo Compressor is also employed to get additional steam economy. Concentrations upto 20% are achieved by using conventional Forced Circulation Evaporators. Higher concentration is obtained in Crystallizing Evaporator ( Oslo type ). Chance of choking of the tubes of the Calendria because of settling of solids, is minimized because solids get separated in the settler below the Vapour Liquid Separator. The upper layer, which is a weak solution, is recirculated through Calendria through axial flow pumps, avoiding any chances of settling of solids inside the tubes.
THICKENING & CENTRIFUGING STAGE : The concentrated liquid is allowed to further settle in a thickening tank with low RPM agitation on the bottom side. The settled mass is then centrifuged in a pusher type Centrifuges to separate associated mother liquor and a wet cake.
DRYING STAGE : In this stage the wet cake from the centrifuge is fed continuously to either Rotary Cascade Dryer, Drum Dryer, Paddle Dryer or Flash Dryer as per the final product quality requirements and layout of the plant etc.
Advantages of Oslo type Evaporators over conventional Forced Circulation Evaporators :-
- These are primarily used for materials having either flat or reverse solubility curve. This means the solubility of the material decreases as there is rise in temperatures and this ultimately results in salting of the materials after reaching the saturation level. This type of material tends to salt out inside the calendria , choking the tubes in case of conventional evaporators . In Oslo type Evaporator the salting of solids takes place in the settler, outside the calendria.
- The calendria and the pump will be handling the weaker solution so that there is no possibility of salting of crystals in regime of heat exchanger.
- This type of evaporator can also be employed for separating sticky solids from water.
- Due to incorporation of axial flow pumps, the crystal breakage is minimal and pumping cost is far lower than conventional centrifugal pumps used in forced circulation evaporator systems. Net NPSH requirements of pumps is very low in these systems resulting in lower power consumption.
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