MVC Evaporator Process Description
Principles of Operation
The ENCON MVC Evaporator is a forced circulation flash evaporator utilizing mechanical vapor compression. By using this design, the ENCON MVC Evaporator allows for minimal scaling, maximum uptime, and low operating costs.
- What is 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 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 can utilize steam, electricity, hot oil, or other forms of available energy.
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.
- What is a Forced Circulation Flash Evaporator?
The heated feed stream 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.
- What is Mechanical Vapor Compression?
In a Mechanical Vapor Compression (MVC) evaporator heat is transferred to the circulating stream by condensing vapor from the compressor(s). Mechanical vapor compression (increasing the temperature and pressure of the vapor) requires significantly less energy than producing steam at the desired conditions from liquid water. In the ENCON MVC Evaporator, the vapor generated from the circulating stream contains a large amount of energy in the form of latent heat.
This vapor cannot be utilized "as is" because it is at the same temperature as the boiling wastewater. A higher temperature will be required to allow the main heat exchanger to function properly.
In order to make the vapor usable, it is compressed by the vapor compressor. Compressing the vapor raises its pressure (and hence its saturation temperature) to a point that it produces the desired heat transfer in the main heat exchanger. This allows for recycling the energy contained by the vapor for use in the main heat exchanger, which greatly improves energy efficiency.
Process Description (refer to diagram below)
- 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 recirculation 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 vapor compressor(s).
- A mist pad is provided at the top of the separation tank to remove small droplets of liquid from the vapor.
- The vapor compressor compresses the vapor (raising the temperature and pressure), and sends the vapor 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 recirculation loop, using the residue valve. Depending on the energy balance, energy can be added to the system by electric heaters / process steam or excess energy can be removed from the system by the steam relief valve.