MED - Multi-effect desalination
Process
Steam is passing inside the tubes and cold sea water flowing outside the tubes, steam condenses as distillate (freshwater) inside the tubes passing through series of cells During this process sea water warm up and partially evaporate as well as recovering the heat (latent heat). Seawater due to evaporation getting concentrated flowing down to the bottom where vapor raised at a lower temperature than the heating system. But still it is being used as heating media for the next effect to repeat the process, the decreasing pressure allows brine and distillate to pass to the next cell for flashing producing additional vapors which condense into distillate in the next cell.
Repeating the same process in a series of effects called Multiple Effect Distillation. In the last cell where steam condenses on a shell and tubes heat exchanger acting as distillate condenser which is cooled by sea water. At the outset of the condenser, some of the warm sea water being used for the makeup of the unit and the rest is rejected. Distillate and brine are flowing from cell to cell until the end where collected by the pumps. The thermal efficiency of this system can be calculated by the distillate produced (in kilos) per one kilo of steam entered into the system and is called as Gain Output Ratio (GOR) (see sketch 2 with 3 effects).
The GOR can be enhanced by the addition of thermo-compressor between one of the cells, by heating with LP or MP steam the static compressor used the vapor raised in one cell and recycle to get higher pressure vapors used as heating media for the first one Sketch 3 shows the arrangement of evaporator with thermo-compression (MED-TVC). When the GOR of the evaporator in sketch 2 would be in the range of 3 (using LLP steam) the GOR of sketch 3 would rather each 6 (using LP or MP steam). When no steam is available, it is still possible to use the MED process with a Mechanical Vapour Compressor (MED-MVC). In such case, the vapor is recycled from the cold cell to the hot one by means of a centrifugal compressor driveby an electrical engine (Sketch 4).The electrical consumption of such a system in the range of 8 to 15 kWh/m3. Due to a current limitation in compressors technology the maximum capacity of MED-MVC units is 5000 m3/day.
Alfa Laval Multi-Effect Distillation (MED) equipment converts seawater to high purity water for use in power plants, the oil & gas industry, solar desalination, on cruise liners and in industries and domestic water production.
Pioneer within thermal desalination
A multi-effect desalination plant is an important component in modern power- and process plants which secure a reliable and cost-effective supply of freshwater by using sea water distillation based on waste heat or low-pressure steam.
For modern seawater desalination systems, the MED is state of the art in thermal desalination. The distillate from MED plants is high purity water with low dissolved solids content. Further water demineralization downstream of the MED plants is then achieved with significantly reduced chemical consumption and less regeneration of the ion exchange columns.
Custom-designed units
Our product range covers capacities up to 10,000 m3/day per unit. Based on standard components and a modular concept, each unit is custom-designed for each particular installation.
Features
MEP offers high fresh water capacity with small installation footprint and a low weight, a feature particularly valuable in the offshore industry as well as in other marine and land-based installations where weight and space can be limiting factors whereas easy installation is of high importance.
The MEP design (MED with titanium plate evaporators) offers more features and benefits as for example:
- Titanium plate type evaporators for a long lifetime without corrosion
- 100% performance recovery after cleaning
- Lowest total water production costs
- Highest availability
· Simple and fully automatic operation & easy maintenance
· High-quality distillate for boiler feed, process water, and potable water
- How it works
The process is based on the evaporation of seawater. This is done by moving the vacuum steam generated by the external energy source over a heating surface to heat the seawater feed on the opposite side of the heat exchanger plates. A part of the seawater flow is evaporated to pure water vapor. The water vapor is then utilized as evaporation energy for the subsequent effect, after which the same process is repeated several times corresponding to the number of effects.
The water vapor generated from the last effect is condensed in the main system condenser cooled by seawater. A dedicated PLC (programmable logic control) system ensures that the desalination unit operates fully automatically with a short start-up period to the required fresh water production.
Special features of MED plants with titanium plates
- Corrosion resistant due to high-grade titanium evaporators
- Compact design with low weight and footprint due to the high thermal efficiency of the PHE design
- Controlled thin falling film avoiding dry spots, thereby avoiding scaling
- The short residence time of the media in a PHE unit resulting in faster process response and less scaling
- Easy maintenance - CIP cleaning, as well as fully mechanical cleaning, is possible
Multi-effect Distillation
Mechanism
Steam condenses inside the effects is the drinking water, and a vacuum pump controls ambient pressure in the effects. Heat exchangers come in different types and the most common consist of horizontal tubes with a falling film. The final condenser is itself a heat exchanger in which incoming seawater is preheated by the condensate energy while the condensate is cooled by the seawater. This circulation of condensate increases the
efficiency as well since energy needed to heat the seawater is acquired from within the
system. Operating temperatures run around 70 degrees Celsius.
History
Multi-effect Distillation (MED) was initially used by the juice industry to evaporate juice to get sugar and salts. The process also giving distillate which can be used for drinking. So the same process is being used for seawater desalination. Due to the high-temperature scaling problem arises and later on the new process developed to be able to operate at a lower temperature to minimize scaling and other problems,
Nowadays MED is being used Caribbean, India, Canary Island and the United Arab Emirates
Locations that use MED include the Caribbean, India, Canary Islands, and the United Arab giving production ranging from 0.5 to 5 million gallons per day. The capacity depends on the number of effects usually 8 to 16 which means the increase of the heat transfer area and ultimately results in high production.
MED process has increased and a number of new designs have been developed and production capacity up to 5000 m3/d have been constructed. However, small single and multiple effect units are more common.
MED process is primarily used in the former USSR, which accounts for 39% of the global MED
installed capacity, 10% in the Caribbean islands, 7.2% in the USA, and 12.7% in Europe [4].
Multiple effect distillation
Multiple effect distillation (MED or ME) was the first process used for seawater desalination. It is widely used in the chemical industry where the process was originally developed
The MED process is similar to the MSF process since it also operates in part by flashing. Moreover, in this process, the majority of the distillate is produced by boiling.
MED, like MSF, takes place in a series of vessels (effects) and uses the principle of reducing the ambient pressure in the various effects. This permits the feed water to undergo multiple boiling without supplying additional heat after the first effect.
The incoming feed water is pumped into the plant through a number of preheaters located in each effect (evaporators) in order to raise its temperature. After passing through the last of these, the feed enters the “ top” effect where the heating steam from a boiler or other source raises its temperature to the saturation temperature for the effect pressure. The second and the subsequent effects use steam produced by the previous effects.
In some modern MED plants, the feed water is sprayed onto the surface of the evaporator tubes to form a thin film to promote rapid boiling and evaporation. The vapor produced then goes, in part, to heat the incoming feed and in part to provide the heat supply for the second effect which is at lower pressure and receives its feed from the brine of the first effect.
From the second effect, the vapor itself is condensed (product water) while at the same time giving up the heat to evaporate a portion of the remaining feed water in the next effect. This process is repeated all the way down the plant. The effects are gradually operated at lower temperatures. This is accomplished by maintaining the effects at successively lower pressure (or higher vacuum by means of an air ejector).
Most of the new MED plants have been built around the concept of operating at lower
temperatures. Some of the more recent plants have been built to operate with top temp. (in the first effect) around 70 C0 reducing the potential for scaling within the plant.
There are a number of MED variants. These variations depending on the combinations of heat
transfer configurations and flow sheet variations (horizontal/vertical, recirculation/once through, etc.), that may be selected independently. Thus, leading to a larger number of possible combinations. Some heat transfer surface configurations are:
ƒ multi-effect submerged the tube
ƒ multi-effect vertical tube climbing-film
ƒ multi-effect vertical tube falling-film
ƒ multi-effect horizontal tube falling-film
MED plants tend to have a smaller number of effects than MSF stages. Usually, 8-16 effects are used in typical large plants, due to the relation of the number of effects with the performance ratio (which cannot exceed the number of effects of the plant).
As in an MSF plant, special attention is required concerning the operating temperature of the sea water and brine in each effect to avoid scaling and corrosion of materials.
Technology deployment
MED has been widely used for industrial applications (e.g. for sugar production by juice
distillation) and for salt production by seawater distillation. Some of the early water distillation plants used the MED process, but this process was displaced by the MSF units because of cost factors, fewer operating problems, and their apparent higher efficiency.
Manufacturing
A number of European companies are developing innovative MED designs which offer lower
energy consumptions than conventional MSF. The desalination market is particularly conservative and there is a reluctance to move from the well-proven MSF design.
Economics
The cost of a MED plant heavily depends on the performance ratio. Capital and energy costs are significant factors (Table 2.5). The main energy requirement is thermal energy. For a plant operating with a performance ratio equal to 8, the thermal energy consumption is around 290 kJ/kg of produced water. Electrical energy demand is low around 2.5-3 kWh/m3.
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