multi effect distillation - desalación · 2017. 4. 14. · multi effect distillation med was the...
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MULTIEFFECTDISTILLATION
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MEDwastheseawaterdesalinationpioneersystemFirstPlantswereconstructedin1930inSaudi
Arabia,withonly2or3effects.
Apartofthehighenergyconsumption,theyhadbigoperationalproblemslike:scaling,corrosion,andproductcontaminationbywaterdrops
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Mul$pleEffectEvapora$on
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Mul$ple-effectdis$lla$on
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Inthissystem,vaporflowsinsidethetubes.
Refrigerationseawaterflowsoutsidethetubes.Fullactionhappensintwosidechambers
It’sjustthecontraryasMSF
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MED PROCESS
MED, like MSF, takes place in a series of vessels (effects) and uses the principles of condensation and evaporation at reduced ambient pressure in the various effects. The main difference between MSF and MED is in the method of evaporation and heat transfer. In a MED plant, evaporation is from a seawater film in contact with the heat transfer surface, whereas in the MSF plant only convective heating of seawater occurs within the tubes and evaporation is from a flow of brine “flashing” in each stage to produce vapor.
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The unit is essentially made of two parts: an evaporator and a condenser. The evaporator is constituted by a tube bundle heat exchanger, a brine accumulation zone (in the bottom of the chamber) and a vapour zone. In this latter zone a demister is used in order to avoid brine droplets entrainment in the vapour flow towards the condenser. The condenser is a simple heat exchanger which is used to condense the vapour produced, but also to preheat the feed brine before it enters the evaporator. This is the simplest distillation process, not very much used in the desalination industry, due to its low efficiency.
Single Distillation
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46 Flow sheets: Once through
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MED PROCESS SCHEMATIC
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MEDPROCESS
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Feed seawater passes through the condenser, where it is
preheated by the condensation heat of the vapor, which is condensing
on the other side of tube bundle;
MED Process Description
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Then the feed water passes through a number of pre-heaters
located in each effect (evaporators) in order to raise its temperature.
MED Process Description
www.prodes-project.org
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• 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.
MED Process Description
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• The preheated feed enters the evaporator and it sprayed onto
the surface of the evaporator tubes to form a thin film to promote
rapid boiling and evaporation.
MED Process Description
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• Only a portion of the sea water applied to the tubes in the first effect is evaporated. The vapour 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 vapour itself is condensed (product water) while at
the same time giving up 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).
• The heating steam is condensed inside the evaporator and is re-circulated to a boiler for regeneration;
• Concentrated brine is collected in the bottom of the unit and then is discharged as blow-down.
www.prodes-project.org
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Three arrangements have evolved for MED process. They are based primarily on the arrangement of the heat exchanger tubing: - Horizontal tube arrangement - Vertical tube arrangement - Vertically stacked tube bundles
Most modern large MED plants have a horizontal tube configuration (HTE), with the feed water film sprayed over the outside of the tubes and condensation inside. The horizontal tube design uses a system of spray nozzles or perforated trays to distribute the feed water evenly over the heat transfer tubes. In the vertical tube or VTE configuration this is reversed, with the feed water flowing down the inside of the tubes.
www.prodes-project.org
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MED horizontal tube arrangement MED vertical tube bundle arrangement
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MED cross flow plant internal layout
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MEDInternallayout
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MED plants tend to have 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.
The performance ratio of the MED plant is approximately equal
to the number of effects minus 1 : (N-1) For an 8:1 performance ratio
plant, the number of effects needed in a MED plant would be 9. This is
much lower than in an equivalent MSF plant. The smaller number of
effects in MED plants contributes to savings in capital cost compared
with MSF.
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MED Process Characteristics
Source: Bureau of Reclamation
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COMPARISON MSF vs MED
MED has a higher efficiency than MSF due to : - A better use of the temperature difference.
- A better heat transfer coefficient.
-A lower pumping power.
-A lower working temperature (less risk of corrosion and scaling))
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Prohibitive Zone
Impact of the variation of operating temperature on the energy consumption of the MED Process
To eliminate the possibility of scale formation, commercial MED desalination plants are currently operating with TBT up to 65 oC .
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LET Proprietary Courtesy of Leon Awerbuch. Used with permission.
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MED Materials of Fabrication
Source: Bureau of Reclamation
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Process/energy type MED MED - MSF VC RO TVC
2.5-3.5 2.5-3.5 _ _ Steam pressure, ata 0.2 - 0.4
Electric energy equivalent, kWhr/m3
4.5 9.5-11* 9.5-11.0 _ _
Electric consumption, 1.2--1.8 kWhr/m3
1.2--1.8 3.2-4.0 8.5 3.5-5.0
Total electric energy equivalent, kWhr/m3
5.2-6.3 10.7-12.8 12.7-15 8.5 3.5-5.0
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Distillation Processes Comparison
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