Today three technologies are dominating the international market. These are Multi-Stage Flash (MSF), Multiple-Effect Distillation (also Multi Effect Distillation or MED) and Reverse Osmosis (RO).
Looking into a global installed capacity in 2016 of about 90 mio m³/d app. 90% of this water is produced by these three technologies.
Multi-Stage Flash (MSF) takes app 20%, Multiple-Effect Distillation (also Multi Effect Distillation or MED) app. 5% and Reverse Osmosis (RO) app 65% of this.
The number of installed plants differs very much because a single MSF plant today easily does produce 60.000 m³/d versus a RO plant very often produces 5.000 m³/d or less. Today more then 50 different desalination technologies are classified. Find out 27 of them here (LINK)
On the left hand side there is a process sketch of a MSF process published 23.-26 05 1934 by Mr. R. Blaum presenting this at the Schiffstechnischen Gesellschaft.
MSF 1934 Germany
RO Plant -Spain
Reverse Osmosis (RO)
Reverse Osmosis (RO) is an osmotic process that uses a semi-permeable membrane to separate water from dissolved matter. Applying a pressure greater than the osmotic pressure (determined by the TDS), feed water is forced through RO-membranes which hold back diluted ions and only allow water molecules to pass through. This divides the feed water into a brine stream with a high TDS concentration and permeate stream with pure water (Fichtner, 2011), (Trieb, 2007). Reverse Osmosis plants are used as well for seawater as for brackish water desalination (Fichtner, 2011). As the driving force of the process is the pressure, which rises with increasing feed water salinity, a suitable high-pressure pump is needed. And due to the fact that RO membranes typically are very sensitive to organic matter, several pre-treatment steps like chlorination, cartridge filtration, dissolved air floatation and ultra-/ microfiltration are necessary (Fichtner, 2011). The main advantages of reverse osmosis desalination are the wide range of feed water quality, the flexibility of the location because of independence of adjacent power plants and the scalability (The World Bank, 2012). On the other hand, the membranes high susceptibility for fouling and the subsequent comprehensive need for pre-treatment as well as the complex setup and the required skills for the staff are the main disadvantages (The World Bank, 2012). On the one hand, current research and development focus on the improvement of the membrane technologies (e.g. incorporation of nanocomposites, large diameter spiral wound elements, low bio-fouling feed spacers). On the other hand, efforts are made to improve the overall process performance (better energy recovery, application of renewable energies, new chemical products for anti-scaling and membrane cleaning, reducing maintenance efforts) (Peñate & García-Rodríguez, 2012).
MED-TVC Plant - UAE
Multiple-Effect Distillation (MED)
Multiple-Effect Distillation (also Multi Effect Distillation or MED) is a thermal distillation technology based on evaporation and condensation processes in multiple stages, called effects, that is mainly used for desalination. In most MED plants, the seawater enters all the effects in parallel and is raised to the boiling point after being preheated on tubes. The seawater is either sprayed or otherwise distributed onto the surface of the evaporator tubes in a thin film to further rapid boiling and evaporation. The tubes are heated by steam from a boiler or some other source, which is condensed on the opposite side of the tubes (inside). The condensate from the boiler steam is recycled to the boiler for reuse. Only a portion of the seawater applied to the tubes in the effects evaporates. The remaining feed water is collected and fed to the last effect, from where it is removed by a brine pump. The tubes in the various effects are heated in turn by the vapours arose from the previous effect. This vapour is condensed to a fresh water product, while giving off heat to evaporate a portion of the seawater feed in the effects. This continues for several effects, with 4 or 16 effects being found in a typical large plant. The remaining seawater of each effect flows to the next effect through pipes by gravity. Generally, these plants are powered by low temperature heat leading to Top Brine Temperatures (TBT) of 55-70°C and are combined with mechanical vapour compressors or thermal vapour compressors. The number of effects directly correlating with the Performance Ratio (PR). In contrast to this, the PR is not significantly influenced by TBT. Depending on the TBT, pre-treatment of the feed and usage of anti-scalants is necessary. Furthermore, corrosion problems limit the usage of cheap construction materials.
MSF Plant - KSA
Multi-Stage Flash (MSF)
Multi-Stage Flash (MSF) evaporation desalination plants are based on flash evaporation in a cascade of stages (effects) at different pressure levels. In the brine-recirculation mode, the brine is recirculated through the system.
In the last stage of an MSF plant, the heated feed water is led into the pressure vessel where it partially evaporates in a flashing process due to the pressure decline. The vapour is used to preheat the feed water. The part of the brine that does not evaporate is sucked into the next stage which has a lower pressure. There, the same flashing and evaporation procedure takes place again, but at lower pressure and thus lower saturation temperature.
In this way, the feed that is used to condense the steam at every stage from the lowest pressure at the first stage to the highest pressure at the last stage is heated up continuously. The result is a low mean temperature difference between the feed water and the condensing steam, although the heat transfer is of type latent-sensible. Thus, the exergy losses are low.
The circulation of the brine mainly reduces costs for pre-treatment, but also reaches higher operation flexibility and better thermal efficiency. At the same time complexity and costs for components, construction, and maintenance rise.
MENA Regional Water Outlook – Part II – Desalination Using Renewable Energy. Final Report of Study, Stuttgart, Germany; Fichtner (Ed.) (2011) Trieb, F.: Concentrating Solar Power for Seawater Desalination. Final report of AQUA-CSP Study, Stuttgart, Germany; German Aerospace Center (DLR) (2007) Renewable Energy Desalination: An Emerging Solution to Close the Water Gap in the Middle East and North Africa. MENA Development Report, Washington, D.C., USA; The World Bank (Ed.) (2012) Peñate, B.; García-Rodríguez, L.: Current trends and future prospects in the design of seawater reverse osmosis desalination technology. Desalination 284, pp. 1-8 (2012)
Engineering of MED, MED-TVC and MVC Desalination Plants. DME-Seminar (S-005-2011), September 27th – 28th, 2011, Essen, Germany; DME GmbH (Ed.) (2011) Gebel, J.; Yüce, S.: An Engineer’s Guide to Desalination. VGB PowerTech Service GmbH (2008)
Gebel, J.; Yüce, S.: An Engineer’s Guide to Desalination. VGB PowerTech Service GmbH (2008)