Evaporation and evaporators
Vaporisation is the change of a liquid substance to a gas or a vapor. There is fundamentally no difference between the terms gas and vapor, but gas is used commonly to describe a substance that appears in the gaseous state under standard conditions of pressure and temperature, and vapor to describe the gaseous state of a substance that appears ordinarily as a liquid or solid.
When heat is added to a liquid at its boiling point, with the pressure kept constant, the molecules of the liquid acquire enough energy to overcome the intermolecular forces that bind them together in the liquid state, and they escape as individual molecules of vapor until the vaporization is complete. The temperature of a boiling liquid remains constant until all of the liquid has been converted to a gas.
For each substance a certain specific amount of heat must be supplied to vaporize a given quantity of the substance. This amount of heat is known as the latent heat of vaporization of the substance. The quantity of heat applied for each gram (or each molecule) undergoing the change in state depends on the substance itself. For example, the amount of heat necessary to change one gram of water to steam at its boiling point at one atmosphere of pressure, i.e., the heat of vaporization of water, is approximately 540 calories.
The thermal motion of a molecule overcomes the surface tension of the liquid and it evaporates, that is, its kinetic energy exceeds the work function of cohesion at the surface.
When the temperature is maintained at the new point for a short time, a new equilibrium is soon established.
The pressure exerted by the vapor of a liquid in a confined space is called its vapor pressure. It differs for different substances at any given temperature, but each substance has a specific vapor pressure for each given temperature. At its boiling point the vapor pressure of a liquid is equal to atmospheric pressure. For example, the vapor pressure of water, measured in terms of the height of mercury in a barometer, is 4.58 mm at 0°C and 760 mm at 100°C (its boiling point).
The factors which influence evaporation are:
- Concentration of the substance evaporating in the air. If the air already has a high concentration of the substance evaporating, then the given substance will evaporate more slowly.
- Concentration of other substances in the air. If the air is already saturated with other substances, it can have a lower capacity for the substance evaporating.
- Temperature of the substance. If the substance is hotter, then evaporation will be faster.
- Flow rate of air. This is in part related to the concentration points above. If fresh air is moving over the substance all the time, then the concentration of the substance in the air is less likely to go up with time, thus encouraging faster evaporation. In addition, molecules in motion have more energy than those at rest, and so the stronger the flow of air, the greater the evaporating power of the air molecules.
- Inter-molecular forces. The stronger the forces keeping the molecules together in the liquid of solid state the more energy that must be input in order to evaporate them.
Evaporation units (or evaporators) use the evaporation principle for the treatment of process water, waste water and water based waste. The tipical liquid treated is an aqueous waste with organic and inorganic pollutants having a concentration not greater then 100 g/L.
Different types of evaporators can face different water treatment problem with different performances.
The productive capacity varies from 0.15 to 60 tons/day and the all the models can concentrate the wastewater as a pumpable fluid, easily reaching a concentrate with 30% of TDS, with low power consumption.
Evaporators can also treat pre-concentrated or scaling liquids, or concentrated acids and extremely corrosive liquids. They use heat pump, hot water or steam, or Mechanical Vapor Recompression (MVR), with natural or forced circulation.
Some examples of evaporator applications concern the separation of water from:
- diluted galvanic baths to recover substances and active ingredients
- oil emulsions to recover oil
- degreasing baths rich in soap and detergents
- concentrated acid baths or highly corrosive solutions
- photographic developing baths
- concentrated saline solutions
- landfill leachate and drippings from waste stocking and disposal
- bilge waters
- rinsing waters containing exhausted inks
- synthesis intermediates and waste waters of the chemical, cosmetic and pharmaceutical industry
Examples of application
Zero discharge in aluminium poly-chloride production
Motivated by environmental authorities and by higher disposal costs, a Spanish company, located in an industrial area not far from Barcelona, developed in 2001 a plant (see Fig. 1) designed to treat the acid eluates of the resins used to process the wastewater of their production line.
The purpose of the plant was to recover the aluminium dissolved in the wastewater coming from offset plates printing plates.
The evaporator supplied was a heat pump vacuum evaporator with intermediate fluid: the material in contact with the concentrated waste and condensate is PTFE-coated steel and heat exchange is achieved in two PVDF and silicon carbide exchangers.
A year after start-up, analytical results confirmed the initial expectations of the end product - a constant density solution as demanded by the market for chemicals for water treatment. The distillate recycling for the production of the regeneration baths allowed to have local permission for the expansion of production since a zero discharge was obtained.
In the past, the company disposed of waste consisting of a solution of HCl and AlCl3 with a high annual cost and significant storage problems.
Currently, this cost item has been eliminated and, together with the limited plant management costs, it is able to sell the concentrated product on a stable basis, guaranteeing an annual income.
Treatment through vacuum evaporation of wastewater from engraving process
In the rotogravure process, the printing elements are engraved in a groove-shape on a cylinder. Then, the cylinder is immerged into the ink, which fills the engraved drawing, to transfer later the drawing directly to the paper.
The cylinder usually has an iron core and, before the engraving, is nickel plated, electrolitically copper plated and rectified. Then the cylinder is subject to the engraving and to the chromium plating before being placed on the rotary press. The engraving occurs by exposing the cylinder surface, on which a photosensitive gelatine has been spread, to a laser beam. The laser burns the gelatine and creates a groove, which will be then subjected to the corrosive action of a ferric perchloride solution.
It is the most important phase of the process. As soon as it is over, the cylinder is washed, in order to remove the traces of ferric perchloride and of the exceeding gelatine, dried and chromium plated.
The wastewaters produced during this process are split in four streams: rinsing waters from the nickel plating, from the copper plating, from the engraving and from the chromium plating. All these wastewaters are aqueous solutions with an acidic pH, a COD higher than 8000 ppm and a high content of heavy metals (nickel, chromium, copper and of other chemical species like chlorides and sulphates).
In particular, the engraving waters have a COD higher than 1500 ppm, an acidic pH lower than 1.5, a high contents of iron and chlorides and a conductivity higher than 80000 µS/cm, index of a high salinity. The iron and chlorides concentration, coming from the ferric perchloride, used in the engraving is critical for this kind of wastewaters.
The high concentration of heavy metals and specially the high contents of chlorides have made the traditional anti-pollution techniques not successful.
In the treatment process, the four streams are stored in a tank in order to homogenize them.
Then the wastewaters are subjected to a pH adjustment, by adding caustic soda 30%, in order to neutralize part of the acidity and to reach a 5.5 pH.
The evaporation occurs by means of two heat pump vacuum evaporators.
These evaporators produce two streams: a distillate, continuously produced, and a concentrate, automatically discharged in a discontinuous way.
The distillation yield is about 90%. The distillate is recycled in the process.
The concentrate, which is stored in a proper tank, is disposed of, in accordance with the regulations in force.
The plant is able to treat 4200 m3/year of waste waters.
Check also our infrared evaporator.
Please feel free to contact Lenntech for any further information about evaporators.