Vacuum evaporation is a technique that represents a major breakthrough in the treatment of liquid effluents as it allows effluents that cannot viably be treated using physicochemical or biological techniques to be treated in a clean, efficient, safe and compact manner. Vacuum evaporation results in a dramatic reduction in the volume of liquid waste (with the resulting savings in waste management), the concentration of corrosive or scale-producing waste, reuse of the water recovered and the implementation of a zero-waste system, amongst many other advantages.
Evaporation is a unit operation that consists of concentrating a solution by eliminating the solvent by boiling. In this case, it is performed at a pressure lower than atmospheric pressure. Thus, the boiling temperature is much lower than that at atmospheric pressure, thereby resulting in notable energy savings.
Evaporation is an operation that is controlled by the rate of heat transfer, and the evaporation rate depends on the following factors:
1. Temperature difference between the heating agent and the liquid to be evaporated.
The boiling temperature of the liquid to be evaporated increases as it becomes more concentrated. However, as the process is conducted under vacuum, the temperature difference between the heating agent and the liquid to be evaporated is greater as the boiling temperature if the mixture is much lower than that corresponding to atmospheric pressure. Higher temperature differences lead to higher evaporation rates.
2. Exchange area
The effective exchange area depends on the geometry of the equipment and phenomena inherent to concentration of the solution, such as the deposition of solids or crust formation on the exchange surface. Larger areas lead to a higher heat-exchange capacity and higher evaporation rate.
3. Overall heat-transfer coefficient (U)
This coefficient depends on the physical properties of the fluids concerned (heating agent and liquid to be evaporated), the materials of the walls at which heat exchange occurs, the design and geometry of the equipment, and flow parameters (fluid circulation rates, etc.). Higher values for this coefficient imply a greater ease of heat exchange in the equipment.
4. Properties of the liquid to be evaporated
The viscosity, possibility of foam formation, ability to corrode, etc. all have practical effects on the rate of heat transfer.
The key parameter when designing the evaporator is the exchange area required for evaporation. Both mass and energy balances must be considered when calculating this area. Thus, for an evaporator into which a flow F is fed and two flows (the concentrate S and distillate E) are removed, as shown in the figure:
The following mass balances must be considered:
Overall mass balance:
F = E + S
V = C
Mass balance for the solute:
F xF = S xS
And the following energy balances:
V HV + F hF = C hC + E HE + S hS
Q = V HV – C hC = V (HV – hC) = U A >T
where Q is the heat flow transmitted via the heating surface of the evaporator, U the overall heat-transfer coefficient, A the area required for evaporation, and >T the temperature difference between the heating agent and the liquid to be evaporated.
One of the factors that results in important operational difference between different vacuum evaporators is the type of technology used to heat the effluent to be evaporated, an aspect that also affects operating costs. Thus, we can find:
1. Heat pump vacuum evaporators
Operation of this system is based on the refrigeration cycle of gas contained in a closed loop. 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. 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.
It is an ideal technology for treating not particularly high flows of corrosive, scale-producing, or viscous liquids. Its operation typically requires an energy consumption of 130-170 kWh per cubic meter distillate.
2. Mechanical vapor recompression vacuum evaporators
This technology is based on recovery of the heat of condensation of the distillate as a heat source for evaporating the feed. To this end, the temperature of the vapor generated upon evaporation is increased by mechanical compression. Upon passing through the exchanger of the evaporator itself, this compressed, and therefore superheated, vapor has two effects: (1) it heats the liquid to be evaporated and (2) it condenses, thereby reducing the need for a refrigeration fluid.
It is a very efficient and competitive evaporation system, with an energy consumption of around 50-60 kWh per cubic meter of distillate obtained.
3. Multiple-effect vacuum evaporators
This technology comprises a series of mutually connected evaporators in which the vacuum steadily increases from first to last. This means that, in principle, the boiling temperature decreases, thus allowing the vapor generated in an evaporator (or effect) to be used as heating fluid in the following effect.
Its main advantage with respect to a single evaporator is the saving in both heating fluid and refrigeration fluid. This is one of the economically most competitive options for treating high flows.
In summary, vacuum evaporation allows the treatment of flows which, as a result of their composition, characteristics, or management complexity, cannot be treated using conventional physicochemical techniques. In addition, with a reduced energy consumption, this technique allows the volume of waste generated to be significantly reduced, a significant flow of water to be recovered for reuse, and the implementation of a zero-waste system with readily assumable economic cost.
Although these systems are simple to operate, it is essential that the choice and design of the most suitable equipment for specific needs is performed by a team of experts in this technology.