Sections
- Definition
- Basic operating principles of Vacuum Evaporation
- Advantages of vacuum evaporation
- Types of vacuum evaporation processes
Definition
Thermal distillation consists of the separation of two or more liquids that are mixed or of a solute and its solvent, by applying sufficient energy to cause boiling.
With this boiling, the more volatile components of the mixture turn into a gaseous state and can subsequently be condensed apart from the initial mixture.
The usual objective of thermal separation is the removal of impurities that are dissolved in water and that prevent it from being reused or returned to a natural environment.
Separation by evaporation has been extensively used over time and has evolved into different techniques that have particular characteristics and different applications, such as water treatment, wastewater treatment, solute recovery, or liquid purification, among others.
In this article, we will focus on analyzing vacuum evaporation as a process for industrial wastewater treatment.
Basic operating principles of vacuum evaporation
To achieve the phase change that allows the separation of the solute and the solvent, heat must be supplied to the mixture so that the liquid part evaporates and separates from the solid part. This heat can be generated in several ways:
- Directly by heating a container that holds the sample.
- Indirectly using steam as a heat transfer agent: vacuum evaporation
In the case of vacuum evaporation, a boiler is used to heat water until it evaporates. This steam is directed to a chamber where it will transfer that heat to the mixture that is to be separated.
On the other hand, to facilitate the evaporation of the solvent, vacuum generating systems can be used, subjecting the mixture to be separated to pressures lower than atmospheric pressure. This reduces the boiling point of the liquids and also increases the efficiency of the heat transfer phenomenon in the system.
When considering the use of a thermal separation process for a specific liquid mixture, the following points must be taken into account:
- The thermal sensitivity of the solution.
- The potential corrosion to the materials.
- The concentration and other physical characteristics.
- The potential appearance of scaling.
Regarding the thermal sensitivity of the solution, it is noteworthy the importance of working at low temperatures in cases where the properties of the solution may be altered by temperature.
This is very common in protein elements, which can denature with increasing temperature. The separation by volatilization of the solvent to concentrate the solute allows the application of vacuum to thus lower the volatilization temperature of the solvent and eliminate the risk of altering the properties of the food solution.
Corrosion of the materials that make up the evaporator can occur if the feed liquid is considerably aggressive towards the materials from which the evaporator is constructed. Currently, all evaporators are built with stainless steel, graphite, nickel, copper, and some alloys with special resistance to corrosion, so the spectrum of potentially treatable solutions by thermal separation is very broad.
Advantages of vacuum evaporation
Separation by vacuum evaporation serves to separate incoming water into two parts: one part with water that has a low concentration of dissolved contaminants and another part with a liquid condensate with a high content of the same contaminants.
To achieve this, the water is transformed into vapor, separating it at that moment from the contaminating materials that are dissolved in it, and this vapor is transported to a chamber where it is cooled to reconcentrate the water free of contaminants.
Thus, this procedure is one of the most efficient for industrial effluent treatment, as it allows for the effective separation of contaminants found in water based on the relatively low volatility of salts compared to water. Thanks to evaporation, substances such as dissolved solids can be eliminated, although compounds with a boiling point similar to or close to that of water, such as alcohol, may not be separated.
It is an essential technology for companies that want to implement a zero discharge system.
After an evaporation process, very high percentages of distilled water (95%) are obtained and a very small amount of reject (5%) to be managed. This reject is so small due to the high concentration of waste achieved in the process. Thanks to this, industries that need to treat medium and large flows can benefit from significant savings, as the volume of waste that must be sent for management is considerably reduced.
It is also a very suitable technology for producing high-quality water that many industries need to incorporate into their production processes.
Advantages of vacuum evaporators:
- High quality of the distillate.
- It is possible to recover up to 97% of clean water.
- Allows for the reuse of treated waters.
- Can treat the most complex effluents.
- Low electricity consumption.
- Flexible and compact design of the machines.
- It is an easy-to-use technology and requires little maintenance.
- High reduction and concentration of liquid waste.
Another notable aspect of vacuum evaporators is their versatility and the large number of occasions they can be applied (as long as the results justify the investment needed for their installation, as they are not the most economical technology). Vacuum evaporators are especially suitable for the separation and treatment of:
- Dissolved hydrocarbons in contaminated waters.
- Oily emulsions.
- Landfill leachate treatment.
- Galvanic metal cleaning rinse waters.
- Degreasing waters.
- Waters with a high content of oily substances.
- Waters with a high content of heavy metals.
- Waters with a high content of dissolved salts.
It is common to complete a vacuum evaporation process with other wastewater treatment technologies, which can be applied beforehand (membranes, physicochemical processes, etc.), subjecting the effluent to a pretreatment that facilitates the evaporation process, or afterwards if a greater concentrate is desired. In this second case, the most suitable technology is crystallizers, which can be used in two ways:
- Crystallizer used as a final stage after a classic evaporation process.
- Evaporator and crystallizer integrated into a single unit that combines both processes. This solution is suitable for small and difficult-to-treat flows.
Depending on the composition of the wastewater to be treated, an evaporative crystallization process allows for the separation of its components and the recovery of secondary products, which can be reused or sold. This happens with oil from oily waters, which can be sold as a secondary product with a water content of less than 5%, or with the recovery of aluminum hydroxide, which can later be used as a chemical product, to name a few examples.
Types of vacuum evaporation processes
The different types of vacuum evaporation that we can find are:
Multi-Stage Evaporation
It is widely used in the industrial field and consists of heating the feed liquid in a container and then conducting the water through a heating pipe system in which part of the water turns into vapor. It then passes to another container where the pressure and temperature are such that part of the hot water suddenly turns to vapor, leaving a concentrated liquid residue that feeds the next stage.
After this, the vapor is allowed to cool until it liquefies again and is then collected free of impurities. The process is then repeated in another stage. After a certain number of stages, water that has been distilled repeatedly very quickly is obtained, which therefore contains very few dissolved contaminants.
This type of evaporation operates at temperatures between 90º and 120º.
Multiple Effect Evaporation
It consists of heating the feed water by utilizing the residual heat from already treated waters and conducting it to a series of tanks where it arrives hot but still in liquid state. In these tanks, the water is distributed in thin films to facilitate evaporation by reducing pressure. The phenomenon of progressive pressure reduction allows the feed water to undergo liquefaction and evaporation processes continuously without the need to add heat to the system.
These processes work at temperatures around 70º.
Vapor Compression Evaporation
It consists of evaporating water by supplying heat from vapor compression, instead of transferring heat through direct contact with a hot solid body. These types of plants are designed to operate by reducing the boiling point of water through pressure reduction.
The compressor creates a vacuum at one end of a container from which it extracts the formed water vapor, but at the other end, it compresses the formed vapor and condenses it inside some tubes. The water falls onto these hot tubes and evaporates. Subsequently, by compressing the vapor and bringing it into contact with the feed water, the evaporation of the water and the removal of salts into a highly concentrated brine is achieved.
