Evaporation and crystalization are common operations throughout many industries. Without these processes, some industries might not be able to achieve the quality and production results they need for either a concentrated product or a properly treated waste stream. A number of products and chemicals refined today would be unattainable, and a host of process by-products would be left unutilized if it were not for these processes, however, a significant obstacle is the energy use required.
To evaporate one kilogram of water at atmospheric conditions, about 2300 kJ of energy is required. If this energy were generated using a boiler fired by industrial fuel oil, it would require on the order of 70 liters of fuel oil for each ton of water evaporated, and result in some 194 kg of CO2 emissions. However, there are fortunately more efficient means of attaining the required energy.
The MVC Process
While there are several methods for generating the energy required of carrying out an evaporation or crystallization process, including Multiple Effect Evaporation (MEE) and Thermal Vapor Compression (TVC), typically none are as efficient as Mechanical Vapor Compression (MVC), and thus MVC is usually the preferred technology for most users. In the MVC process, the vapor compressor is the key component in providing the energy required for evaporation. In many cases, the vapor compressor is driven by means of electrical energy from the user’s electrical grid through a motor, although there are other suitable sources of energy such as lower grade plant steam through a turbine or lesser fuel quantities through an engine.
In most evaporator designs, the feed stream is evenly distributed onto heat transfer surfaces on the process side of the heat exchanger. Evaporation takes place when the feed stream is heated to boiling by a heat source on the opposite side of the heat exchanger. In the MVC cycle, it is the water vapor that is evaporated off of the feed stream that is pulled through the vapor compressor and thereby the temperature and pressure of the vapor is increased to be used as the heating medium on the opposite side of the heat exchanger by releasing its latent heat of vaporization. As the vapor loses its heat energy in the heat exchange, it condenses as distilled water on the distillate side of the heat exchanger. Likewise, as the feed stream loses water due to evaporation, concentrated dissolved solids of product or waste results on the process side.
In a waste water evaporator, for example, the concentrated solids may be dumped or dried while the condensate produced can be reused as process water. On the other hand, for someone producing a concentrated product such as milk powder, the product concentrate from the evaporator may be further processed and refined, while the condensate that results will be used for other plant purposes or disposed of.