Heat pump vacuum evaporators have been used for decades for wastewater and industrial effluents treatment applications.
In this article we will present the features, working principle and applications of heat pump evaporators.
What Is a Heat Pump Vacuum Evaporator?
A Heat Pump Vacuum Evaporator is a device used to turn the liquid form of a chemical substance, such as water, into a vapor, by using the heat pump technology for the evaporation and the subsequent condensation process.
Heat pump evaporators are used in a wide range of industrial sectors to treat industrial wastewater. It represents a clean, safe and very versatile technology with low management costs, which in most cases serves as a zero-discharge treatment system.
Heat pump vacuum evaporators can treat an aqueous waste stream efficiently and simply without using reagents. They are highly effective even when conventional technologies are not viable. Using a vacuum evaporator, waste effluent is transformed into two streams: one, highly concentrated (perhaps even dry) and the other high-quality water. Evaporators work under vacuum conditions to reduce the boiling temperature of the liquid effluent and lower energy consumption.
What Is the Working Principle of a Heat Pump Vacuum Evaporator?
The working principle of a heat pump vacuum evaporator can be described as follows.
The liquid to be concentrated is conveyed to the evaporation boiler maintained under vacuum by an ejector.
The degree of vacuum obtained allows the liquid to reach the boiling point at temperatures between 28 and 40° C.
The steam produced then passes into the condensation section to return to the liquid state, however, without the salts that deposit in the boiling chamber. When a high salt concentration has been reached in the latter, the boiler must be drained.
The evaporation process and the subsequent condensation process are carried out by a heat pump which can be described as follows.
A compressor compresses the vapor, also increasing the temperature, until it begins to condense at a temperature of 45-50 °C. Condensation, which takes place in coils placed inside the boiler, generates the heat that is transferred from the vapor to the liquid which begins boiling at a low temperature.
After having supplied heat, the condensed fluid is further cooled by means of a heat exchanger with forced ventilation. We will therefore have that the distilled vapor condenses giving heat to the gas which evaporates and returns as such to the compressor for a new condensation and evaporation cycle.
Finally, the condensate is collected in a tank where a pump is installed that provides the driving force to the ejector to maintain the vacuum necessary for boiling at low temperatures.
What Are the Advantages of a Heat Pump Vacuum Evaporator?
In a heat pump vacuum evaporator, the use of the heat pump for the evaporation and condensation processes is the most economically advantageous, compared to other technologies.
With the heat pump evaporation system it is possible to concentrate the solutions of 10-20 times the initial volume, depending on the type of wastewater treated. For example, this means that starting from a wastewater volume of 1000 liters, after concentration, there will be a final volume of up to 50 liters in typical applications.
Furthermore, YASA ET systems are equipped with all the automatisms and safety devices that make their operation completely automatic and reliable, which is normally a 24h continuous cycle.
Heat Pump Evaporator System Components
Heat pump evaporator components include: boiling chamber, demister, blower, heat exchanger, distillate tank, heat recovery system, pumps and piping (as shown in the picture below).
The system consists of a heating and evaporation boiler for the liquid to be concentrated and a steam cooling and condensation section, a vacuum forming unit inside the concentrator and a refrigeration circuit that supplies heat for the evaporation and at the same time condensation frigories of the evaporated product; this circuit is called a "heat pump".
These are electric vacuum evaporators that operate at a temperature of 40°C and provide a significant reduction in the COD of the distillate and a low amount of discharge concentrate. The technology has a low maintenance cost, is fully automated and ensures consistent distillate quality by providing complete separation of metals and surfactants.
Heat Pump Vacuum Evaporators Applications
Heat pump vacuum evaporators are used in many industrial applications, such as in metalworking and surface treatment factories.
Other applications for heat pump evaporators are:
Oily emulsions, coolant fluids, mold release agents
Purging of compressors, water from washing floors
Water from washing tanks and reactors (in the chemical, pharmaceutical, cosmetic and perfumery industries)
Working and wash baths in galvanic processes and surface treatments
Graphic arts waste (e.g. cleaning water and paint)
Water treatment plant reject (e.g. reverse osmosis and demineralizers)
Leachate from municipal solid waste dumps
Digestate from biogas generation plants
Food and beverage industry
Paper, minerals and extraction industries
Functioning And Operation of Heat Pump Evaporators
The operation of a heat pump vacuum evaporator is based on bringing the effluent to its boiling point, which is around 40°C when working under vacuum conditions. When the effluent begins to boil in the evaporator boiler tank, the steam condenses and is withdrawn from the system as more effluent is fed into the boiler tank.
The effluent being fed is preheated by a heat pump so the evaporation process continues.
The operation of the heating system to evaporate the effluent is based on the refrigeration cycle of a gas in a closed circuit. The refrigeration gas is compressed by a compressor, as a result of which its temperature and pressure increase. It then circulates through the heat exchanger of the evaporator itself, heating the feed. In the heat pump evaporator as the system operates under vacuum, the boiling temperature is around 40 ºC. The refrigeration liquid leaves the evaporator’s exchanger and is decompressed and cooled using an expansion valve.
Passage through a second heat exchanger (the condenser) causes the vapor formed in the evaporator to condense and its temperature to increase immediately prior to passing through the compressor again, thus repeating the cycle. The same refrigeration fluid allows the feed to be evaporated and the vapor generated to be condensed, therefore the system does not require any other heating or refrigeration source. This means that the process is highly advantageous from an economic and management viewpoint.
What Is the Energy Consumption and Cost of a Heat Pump Evaporator?
Energy consumption in a heat pump evaporator is reduced to around 130W per litre (about 5 times lower compared to atmospheric evaporation) as most of the energy (latent heat) is reused for new vapor production.
Also, electrical energy consumption depends from the compression of the intermediate fluid and it represents the power energy to effect re-use.
When the wastewater flow is higher than 1 ton/hour, a multi effect evaporator can be manufactured to reduce operating costs.
How Does a Multiple Effect Evaporator Work?
A multiple effect evaporator exploits vapor produced within the first boiling chamber to produce new vapor in a second chamber with a lower temperature and pressure. This vapor recycle can be effected several times in a cascade (multiple effect) sequence on the basis of the first step of fluid heating temperature and of the fluid cooling step of the last process.
Can I Use a Heat Pump Evaporator For My Wastewater Treatment Application and System?
A heat pump evaporator can be used for the treatment of many wastewaters. As a preliminary step, it’s important to test the effluent treatability by evaporation and proceed with careful chemical analysis.
YASA ET designs and manufactures evaporation systems for companies all around the world for their industrial effluents and wastewater treatment problems.
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