A. Evaporation via Mechanical Vapour Recompression
Method diagram
Method and installation description
The aim of evaporation is to concentrate dissolved pollution and to distil purified water from wastewater. The technique described below is based on the principle of mechanical vapour recompression, possibly in combination with falling film evaporation. A circulation pump transports the influent to the top section of the drum, where the water is distributed across the heat elements. Part of the wastewater evaporates on the outer surface of the heat element. The created vapour is passed through a compressor to increase the pressure somewhat, and is then guided to the inner surface of the heat element where in condenses. Condensation energy is then transported to the wastewater-side of the heat element, and the clean condensate is collected. The concentrated wastewater flows to the bottom of the drum where it is transported by the concentrate pump. The material used for the heat element is a thin, non-corrosive elastic film made from polymers or solid metals.
Specific advantages and disadvantages
The purified water is of a high quality and can be reused in the process or discharged into surface water. All non-volatile substances can be separated from the distillate flow. The heat elements are modular; in other words, the installation can easily be expanded and elements can be replaced per module. Volatile compounds cannot be separated in this installation and need a separate (post) treatment. Under certain conditions, a number of salts may also be encountered in the distillate flow.
The remaining residual flow is greatly enriched by salts and non-volatile compounds. It is often impossible to discharge this flow and the flow needs to be processed externally. It is also possible to further concentrate and dry this flow.
Considering the high investment costs, this technique is only feasible with very low capacities. Corrosion could be a problem when salt concentrations are increased.
Application
Evaporation is used in various industrial production processes. In the food industry, MVR can be used for volume reduction – for example, when increasing the concentration of milk or fruit juice.
Only a limited number of references are available for treating wastewaters. For example, a milk company treats a volume of 375 m3 wastewater per day. The second example is of a chemical company, with a wastewater volume of 30 m3/h.
Other sectors where MVR may be implemented include: The pharmaceutical industry, surface treatment of metals, petrochemicals, and paint, varnish and printing ink production.
Boundary conditions
Evaporation takes place in a "vacuum" at 120-200 mbar. Due to the drop in pressure, the boiling point drops to 50-60°C. The capacity of already implemented vaporisers lies between 12 and 900 ton/day. Multiple vaporisers could possibly be placed in parallel set-up to realise a larger capacity. It is important to remove corrosive and volatile compounds, like ammoniac, from the wastewater.
Effectiveness
MVR is used for the evaporation of water. Depending on the influent and type of pollution, the effectiveness lies around 99%.
Support aids
An anti-foam dose could possibly be needed.
Environmental issues
Concentrate (sludge) is released as by-product.
Costs
MVR installations are accompanied by high investment costs, though the instillation can still be economically viable. The yield is often determined by the following parameters:
- Energy costs;
- Opportunity to reduce discharge levies and water collection by reusing water;
- Cost price for disposing of the condensate (or returns).
Comments
An extensive preliminary study is needed to correctly dimension the installation.
Complexity
No data available.
Level of automation
No data available.
References
- Brauns E. and Bernard De Jonghe B., Towards recuperating Process water via Vaporisation?, Het Ingenieursblad, 11-12/2004, 2004
- EIPPCB, Reference Document on BAT in Common Waste Water and Waste Gas Treatment / Management Systems in the Chemical Sector, draft February 2009 (revision upon release)
- Koistinen, Peter R., Treatment of industrial effluents and landfill leachates using new low cost evaporation technology with polymeric heat transfer surfaces, Hadwaco Ltd Oy, Helsinki (FIN)
- Woelders, J.A., Sicherwasserreinigung und Konzentratrückfuhrung in den Niederlanden
B. Vacuum Evaporation
Method diagram
Method and installation description
Evaporation is a technique that can be used to reduce the volume of waste(water) flows. Existing evaporation technology is very advanced and works via the recuperation of latent vaporisation heat[1] in the form of condensation heat. Thus, for many years already, evaporation is not seen as technology with high energy consumption, which was the case in the past.
Vacuum evaporation involves using a vacuum vaporiser with heat pump. Compared to atmospheric evaporation, this provides the following savings:
- Rinse water does not need to be heated to ca. 100°C; the evaporation process can take place when wastewater is at normal temperature;
- All latent evaporation heat is recovered. Instead of simply discharging evaporated water into the atmosphere, it is condensed; the released heat is then used to allow the evaporation process to continue;
- There is less loss of heat to the environment;
- In a number of cases, the installation can be realised using cheaper materials.
Specific advantages and disadvantages
The purified water is of a high quality and can be reused in the process or discharged into surface water. All non-volatile substances can be separated from the distillate flow. The heat elements are modular; in other words, the installation can easily be expanded and elements can be replaced per module. Volatile compounds cannot be separated in this installation and need a separate (post) treatment.
Due to concentration increases of, for example, chlorides, one must be alert to the risk of corrosion. High performance materials are often needed to produce the evaporation installation.
Application
The technique is used for wastewater with low volumes and high concentrations. Here are a few examples of evaporation being implemented:
- As pre-treatment for increasing concentration of waste(water) flows, to reduce the volume of waste(water) that needed to be disposed of.
- Further concentration of half-concentrates, which are released in the surface treatment of metals:
- Further thickening the concentrate from membrane filtration, e.g. from degreasing baths, with the aim of more economical removal;
- Concentration of static rinse baths in electrolysis processes. In this case, the distillate can be reused. In some cases, vacuum evaporation is also aimed at recovering the electrolyte or the metal (e.g. iron).
- Acid recovery from rinse water after electro-chemical polishing.
Other sectors where vacuum evaporation may be implemented include: The pharmaceutical industry, petrochemicals, and paint, varnish and printing ink production.
Boundary conditions
Evaporation takes place in a "vacuum" at 50-70 mbar. Due to the drop in pressure, the boiling point drops to 30-35°C. The capacity of already implemented vaporisers lies between 12 and 900 ton/day. Multiple vaporisers could possibly be placed in parallel set-up to realise a larger capacity. It is important to remove corrosive and volatile compounds, like ammoniac, from the wastewater.
Effectiveness
Vacuum evaporation is used for the evaporation of water. Depending on the influent and type of pollution, the effectiveness lies around 99%.
Support aids
An anti-foam dose could possibly be needed.
Environmental issues
Concentrate (sludge) is released as by-product.
Costs
Despite the sizeable package of energy-saving measures, vacuum evaporation remains an energy-intensive treatment. Typical consumption can be 0.25 kWh per litre of evaporated water. This is higher for non-free-flowing or concentrated liquids, by up to 0.45 kWh/l.
The investment costs for a vacuum evaporation installation with heat pump made from stainless steel, amount to ca € 50.000 for an installation of 70 l/h; ca. € 155.000 for an installation of 300 l/h. Designs in special steel types or titanium are around 50% more expensive. For example, an installation made from inox 316 for a volume of 300 l/h, is accompanied by an investment cost of € 245.000.
In addition to energy costs, one must also consider maintenance costs (regularly cleaning heat exchangers, replacement parts, etc.).
In return for these heavy investments, one could benefit from cost-saving in terms of recovered chemicals, costs avoided for wastewater purification, costs avoided for purchasing water and the production of demi-water. These vary greatly from case to case.
Comments
None.
Complexity
It is easy to operate a vacuum evaporation installation.
Level of automation
Industrial installations have a high degree of automation.
References
- EIPPCB, Reference Document on BAT in Common Waste Water and Waste Gas Treatment / Management Systems in the Chemical Sector, draft February 2009 (revision upon release)
- Koistinen, Peter R., Treatment of industrial effluents and landfill leachates using new low cost evaporation technology with polymeric heat transfer surfaces, Hadwaco Ltd Oy, Helsinki (FIN), 1998.
- Jacobs A., Goovaerts L. and Vrancken K., Best Available Techniques (BAT) for surface treatments on metals and plastics, June 2008
- VITO-SCT, revision of technical notes WASS, 2008
Version February 2010
[1] Quantity of energy in heat form that is needed to subject a substance to a phase transfer at constant temperature and pressure.