Principles of Operation
The ENCON CFE Evaporator is a forced circulation flash evaporator utilizing boiler steam.
Evaporation
Evaporation is a process where water is removed from a feed stream to produce a concentrated discharge stream and a separate water stream.
For a typical evaporator, the feed solution is heated to reduce the required heat input. This is accomplished by utilizing a feed preheat exchanger that transfers sensible heat from the hot boiler condensate to the cooler feed stream.
The heated feed stream is mixed with the evaporator liquid and this mixture is then heated by the main heat exchanger. This heat exchanger utilizes boiler steam to provide the energy required for evaporation.
Heat from the main heat exchanger is used to boil the evaporator liquid, producing a concentrated liquid stream and a water vapor stream. Then, the vapor can be discharged as is or condensed into liquid water.
Forced Circulation Flash Evaporator
A forced circulation flash evaporator utilizes a circulation pump to create a large circulation stream, and a pressure-reducing device (typically an orifice plate) to allow for flashing of the hot circulating stream.
The heated feed stream (see above) is mixed with the large circulating stream, which flows through the main heat exchanger. The design pressure of the circulation loop is high enough to suppress boiling of the liquid in the main heat exchanger.
An orifice plate is installed between the heat exchanger and the separation tank. The pressure downstream of the orifice is low enough to allow the hot liquid to boil, and the resulting liquid and vapor then flow to the separation tank.
The advantage of the forced circulation flash evaporator design is that no boiling takes places in the heat exchanger, and the wastewater is circulated at a high rate. These conditions combine to significantly reduce the tendency to foul or scale the evaporator. They also reduce down time for cleaning and prevent reduced evaporation rates.
Process Description
- Process wastewater is fed by the feed pump through the feedstock heat exchanger and into the
circulating stream. The feedstock heat exchanger is used to heat the wastewater by transferring sensible
heat from the hot condensate to the cooler feed. - The circulation pump circulates wastewater from the separation tank through the main heat exchanger,
to the orifice plate, and back into the separation tank. The latent heat from the compressed vapor is
transferred to the wastewater via the main heat exchanger. - An orifice plate is used to reduce the pressure of the circulating stream. The downstream pressure is
low enough to allow flashing of the circulating stream into liquid and vapor components. - The liquid and vapor then flow to the separation tank where they are separated. The liquid steam exits
the tank at the bottom and flows back to the recirculation pump. The vapor stream exits the tank at the
top and flows to the either atmosphere or to a condenser. - A mist pad is provided at the top of the separation tank to remove small droplets of liquid from the
vapor. - The boiler provides steam to the main heat exchanger, where it transfers its latent heat to the
wastewater in the recirculation loop. - High temperature condensate exits the main heat exchanger and flows to the condensate tank, where
any remaining vapor is separated. The hot condensate is then pumped to the feedstock heat exchanger,
where it transfers sensible heat to the incoming feed wastewater. - Upon reaching steady-state at the target concentration, the concentrated wastewater is purged from the
circulation loop, using the residue valve.