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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