Vapor Compression Distillation
Vapor Compression Distillation
Mechanical energy provides a source of thermal energy for condensation processes similar to MED.
Mechanism
Vapor Compression Distillation, or VCD, is often used together with Multi-effect Distillation for large applications or alone when used in smaller settings.
The key difference that makes VCD unique is that the compression of vapor is utilized to change the boiling point of water. A quick refresher of physics and the ideal gas law PV = nRT reminds us pressure and temperature are proportional. At a constant volume, an increase in pressure leads to an increase in temperature (Loverude et. al. 2001). VCD plants, therefore, use compression of vapor as a source of evaporating heat.
Plants are divided into stages and may contain one in smaller plants or more in larger ones. VCD is simple and reliable compared to other methods of desalination. It is used most often in industries, resorts, and drilling sites where fresh water is not available. Depending on the plant, pressure may reach up to 300 pounds per square inch!
Benefits
1) the operating costs are low compared to multi-stage or multi-effect flash distillation systems
2) the equipment is smaller than the multi-stage flash or multi-effect flash distillation systems
Cons
1) maintenance on compressors and heat exchangers is greater than those of other systems
2) energy consumption is high
3) capital costs are high
National Science Foundation: Office of Polar Programs. "Sea water desalination system, McMurdo" 1993.
Sources
Loverude, Michael E., Christian H. Kautz, and Paula R. L. Heron. "Student understanding of the first law of
thermodynamics: Relating work to the adiabatic compression of an ideal gas" University of Washington.
Seattle, WA. 14 September 2001. http://chaos.fullerton.edu/~mloverud/AJPthermo.pdf
thermodynamics: Relating work to the adiabatic compression of an ideal gas" University of Washington.
Seattle, WA. 14 September 2001. http://chaos.fullerton.edu/~mloverud/AJPthermo.pdf
National Science Foundation: Office of Polar Programs. "Sea water desalination system, McMurdo" 1993.
UNEP - International Environmental Technology Centre. United Nations Environment Programme. "Source Book of Alternative Technologies for Freshwater Augmentation in Latin America and the Caribbean" 1997.
Vapor compression
There exist two vapor compressor (VC) processes. The first configuration is mechanical vapor compression (MVC), in which a mechanical compressor is used.
The second is thermal vapor compression (TVC), in which a thermal compressor or ejector is used to increase the vapor’s pressure. Both types are widely used.
The fundamental concept of this process is inherently simple, in that after vapor has been
produced it is then compressed to increase its pressure and consequently it's saturation temperature before it is returned to the evaporator as the heating vapor for the evaporation of more liquid.
Technology description
The main equipment used in the VC compression process are the evaporator, the compressor, pumps, and the heat exchanger.
In this process, the feed water is preheated in a heat exchanger or a series of heat exchangers by the hot discharge of the brine blow down and the distillate.
The hot feed water enters the evaporator (any type of evaporator can be used), where it is heated up to its boiling point and some of it is evaporated. The vapor formed in the evaporator goes to the compressor where its pressure and consequently its saturation temperature are raised.
The power consumption of the compressor, and therefore the efficiency of the process, is dependent on this pressure difference. Thus the compressor represents the main energy consumer in the system.
The compressed vapor is then fed back into the evaporator to be condensed, providing the thermal energy to evaporate the applied seawater on the other side of the tubes. The process has the potential of delivering high-performance ratios due to the effective recirculation of the latent heat around the plant. By increasing the surface area and lowering the compression ratio (temperature difference) it is possible to decrease the energy consumption of the process. The distilled water produced by this condensation leaves the plant through the pre-heaters as the product water.
Extra care is required with the control of the brine level in the evaporator and the proper
maintenance of the compressor. Some manufacturers use compressors that rotate at very high speeds. Operation at low temperatures minimizes the formation of scaling and corrosion of materials.
Technology Deployment
Vapor compression plants have been in use since the end of the 19th century. The vapor
compression process is usually used for small and medium scale water desalination units in a range of 20-2500 m3/d. Many applications for this process have been found. Because of its compactness, ease of operation and transportability, military versions have been developed.
The process is mainly used in Western countries. 20% of the total installed capacity is in USA,
13% in the Middle East, and 22% in Europe.
Manufacturing
Concerning VC manufacturing, 48% of the capacity was sold by European companies, 32% by US companies and 18% by an Israeli company.
Economics
Capital and energy costs are significant factors in the determination of the total water production cost. The energy demand mainly required to drive the vapor compressor motor. Its operation and maintenance sometimes covers half of the total operating and maintenance cost. However, the energy requirements of VC plants have been reduced (from 20 kWh/m3) and currently range between 8 to 12 kWh/m3 - with the potential for further reduction.
Comments
Post a Comment