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Desalination of sea and brackish water has become a necessity in many arid and semiarid regions. Due to the fast growing population and a correspondingly high water demand in these regions the few water sources often get brackish or contaminated.
Waste and contamination of water sources are increasingly creating serious problems also in the northern hemisphere, where water supply never used to be a problem.
In most cases, the water supply in dry regions is realized using desalination plants with daily fresh water outputs between 1000 and 100,000 m3 (RO, MSF).
The water supply system using big plants running with fossile energies, however, is not always an economic and ecologic solution /1, 2/:
Due to the fact that big regions rely on the function of one big plant breakdowns are of concern for many people.
The presented technology is able to supply clean water with plant capacities up to 20 m3/d by avoiding the mentioned disadvantages of conventional technologies.
The membrane distillation (MD)-plant can be driven by solar heat or waste heat and is therefore ideal to deliver water at remote areas with poor infrastructure (Fig. 1):
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It is possible to concentrate aqueous solutions of non-volatile dissolved substances by microporous membranes impermeable for water but permeable for water vapour. Driving force for this "membrane distillation" is a vapour pressure difference on both sides of the membrane due to a corresponding temperature gradient across the membrane.
The distillation is performed at ambient pressure and at a maximum temperature of 80°C (175°"F). Operating costs are extremly low because the process can be driven by low temperature heat sources eg. solar heat or waste heat from diesel engines /3/.
The system is employing spiral wound desalination modules. Inside the distillation modules a thin microporous hydrophobic PTFE-membrane is used with pore diameters between 0.051 and 0.2 m. This material shows the surprising property of allowing easy passage of water vapour, but of completely blocking the flow of liquid water. The high surface tension of water prevents the passage of liquid water through the sub-micron pores up to a pressure of typically 0.5 MPa (72.5 psi).
In the process one surface (hot side) of the flat sheet membrane is in contact with the process solution while the opposite surface (cold side) is in contact with distillate. Thus the diffusion gap between evaporating and condensing surfaces is reduced to the thickness of the membrane that is only about 30 mm. Wth an actual pore fraction of 80% high specific evaporation rates are possible.
The recovery of the heat of condensation is done by utilizing the heat of condensation to preheat the feedwater.
Fig. 2 shows schematically the principle of a membrane distillation module with integrated recovery for the heat of condensation.
Cold feedwater (temperature Tl) enters the module and is progressively heated by the hot condenser sheet, so that it emerges from flow channel 1 (heat recovery channel) on a significantly higher temperature level (temperature T2).
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Before the feedwater reenters the module into flow channel 2, the
temperature has to be elevated from T2 to T3 using an external heat source.
The distillation takes place from flow channel 2 across the membrane into
flow channel 3. The feed water is gradually loosing heat and is getting
concentrated. The temperature difference between flow channel 2 and 3 is
the driving force for the process and is maintained along the whole channel
length. The concentrate emerges therefore with a higher temperature than
the incoming feed. The distillate is collected in flow channel 3 and emerges
from the module almost at ambient temperature (between Tl and T4). The
spiral wound design of the module (Fig. 3) allows high recovery rates of
latent heat, eliminates the need for thermal insulation and mechanical
support and performs as a compact and resistant unit.
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Experimental desalination modules for membrane distillation have been
developed by several companies. Due to high manufacturing costs, poor
distillate output and material failures a functional, economic and reliable
module system has not been available so far /4, 6/.
When we started a R&D-program for membrane distillation modules in 1989, materials technology was more advanced and highly resistant materials were available at more reasonable prices.
The whole module construction has been optimized in terms of
Failures became less probable due to a new construction of the flow channels for feed water and distillate, that keeps pressure losses at a minimum /5/.
Module performance is constantly tested until the present day.
In case there is a high amount of waste energy available, it is possible to achieve high specific membrane flow rates by using a 2-channel module with a countercurrent flow of distillate and concentrate. High heat transfer rates result in a 3 to 5 times higher distillate output compared to modules with integrated recovery of the heat of condensation. These modules have been developed recently and will be available soon.
Because of low process temperatures solar energy and waste energy can be used to run the plant /3, 4/. The operational efficiency and the long term behaviour of the process for the seawater desalination has been proven in pilot installations on the Canary islands and on the island of Ibiza.
In order to test the module performance and to optimize the operation under real conditions a prototype plant has been tested on the island of Ibiza/Spain since May 1993 (Fig. 4, 5).
| DESALINATION UNIT |  |
| plant dimensions | 2.5 x 0.8 x 1.5 m (8.3 x 2.6 x 5 ft) |
| number of modules | 4 |
| feed water flow | 0.8 - 1.7 m3/h (3.5 - 7.5 gpm) |
| distillate conductivity | <10 mS |
| distillate flow | 40-85 I/h (0.17 - 0.40 gpm) |
| brine temperatures | 60 - 80 "C (140 - 175°F) |
| energy consumption | 150 - 200 kWhlm3 (570 - 750 Wh/gal) |
| energy source | solar heat, waste heat |
| SOLAR UNIT | |
| vacuum flat plate collector area | 51m2 |
| storage collector area | 45m2 |
In order to keep heat losses for plant start up to a minimum a 24-h-operation is necessary. This also improves the plant utilisation and reduces material stress through stop and go operation. The energy to run the plant during the times without solar radiation is delivered from a storage collector with a water capacity of 10 m3.
To show the simple design of the membrane distillation systems a flow diagram (Fig. 7) for a single module unit is added. Instead of the solar collector the unit can also be combined with a waste heat source.
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Numerical codings of Fig. 7:
1. Filter
2. Magnetic Valve
3. Pressure Reducing Valve
4 . Flow Meter
5. Pressure Indicator
6. Heat Exchanger
7. Conductivity Meter
8. Temperature Indicator
9. Expansion Container
10. Safety Valve
11. Solar Pump
12. Degasifier
13. Insulation
Components of the desalination circuit:
The desalination plant is a compact unit with outer dimensions of about (1.O x I.5 x 0.8m) (3.3 x 5 x 2.6 ft).
Module design:
Spiral wound module cylinder: dimensions: height 500 mm (20 inch), diameter 460 mm (18 inch)
... membrane area: 10m 2 (108 ft2) ... condenser area: 10m2 (108 ft2)
Operational data
Prepositions:
| feed flow | 300500 I/h(l.I - 2 gpm) |
| distillate flow | 15-25 l/h(0.05 - 0.09 gpm) |
| distillate quality | <10mS/cm |
| feed water temperatures | IO-40 °C (50-100°F) |
| brine temperatures | 60-80°C( 140 - 175°F) |
| temperature level of heat source | 70-90°C (l60 - 195°F) |
| average input of thermal energy | 150-200 Wh/l(570-750 Wh/gal) |
| electric energy demand | none (except for the magnetic valve) |
Fig. 8 shows a data evaluation sheet for a solar driven single module plant.
The upper diagram displays the development of the temperatures Tl -
T4 (corresponding to Fig. 2 and Fig. 3) during a sunny day. In the diagram
below the corresponding distillate production is displayed. Because of the
high heat capacity of solar collector, piping and the module the start of the
distillate production is delayed. For that reason 24 h-operation at fixed flow
rates is in most cases the more economic alternative.
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Solar loop:
...type of solar installation: flat plat collector
Assumptions for the calculation of the necessary collector area:
...daily mean solar radiation: 5 kWh/m2 (0.5 kWh/sqft)
...efficiency of the collector: 35 % between 65°C (150°F) and 95°C (200°F) collector fluid temperature
...operating hours: 6.5 h per day
...resulting collector area: 13m3 (140 sqft)
...electric energy demand: approx. 100 W (for pump and control)
...production of boiler feed water
...production of ultrapure water for use in medical, pharmaceutical or electronic industry
...generation of pure water for rinsing in surface treatment technology
...recycling of process solutions by concentration
...treatment of contaminated fluids (poisonous, radioactive) /3, 4/.
The industrial applicability for the treatment of process solutions has been proven in our plant for nickel electroplating in Dresden/Germany. The membrane distillation is being optimized for the production of boiler feed water, the recycling of electrolytes and for the production of distilled water for rinsing
Small simple desalination plants operating independent from the electric grid are either not available or not economic at all. The process of membrane distillation allows the effective use of low temperature heat sources like solar energy or waste energy from engines for small to medium scale desalination.
Although the process of membrane distillation is known since over 30 years cost-effective desalination modules have not been available so far.
In order to achieve an effective membrane distillation process spiral wound modules have been developed and optimized during a 6-year R&D program. The modules are designed as compact units with integrated recovery of the heat of condensation, allowing a highly efficient use of low temperature heat sources.
In case there is a high amount of free waste energy available the heat recovery is not needed and flow rates 3-5 times higher can be obtained. Desalination modules without integrated recovery of latent heat have been developed, too and will be available soon.
Individual plant sizes are available with product water output ranging from 0.l (25 gal) to 5m3 (1300 gal) per day.
/1/ E. Delyannis. V. Belessiotis Solar desalination, is it effective? Desalination and Water Reuse Vol. 414
/2/ J. Manwell, J. McGowan, Recent renewable energy driven desalination system research and development in North America Desalination, 94 (I 994) p. 229-241
/3/ K. Schneider, T. van Gassel, Membrandestillation Chem.-lng.-Tech. 66 (1984) Nr. 7, S. 514-521
/4/ N. Kjellander, Design and fold test of a membrane distillation system for seawater desalination Desalination, 61 (1987) p. 237- 243
/5/ C. Biir, LIevelopment, design and testing of a solar driven desalination Plant by membrane distillation, Phase II BMFF-Report Ref.-No. 032 9085 A
/6/ E. Delyannis. Status of solar assisted desalination: A Review Desalination, 67 (1967) p. 3-19