Abstract:
An evaporation device has an inner tube and an outer tube. The inner tube is made of a hydrophilic membrane, such as DutyionT. The outer tube is preferably made of a material that readily absorbs solar radiation and is a good heat conductor. There is a gap between the inner tube and the outer tube for the flow of air. The inner tube contains a flow of impure water. The hydrophilic membrane allows water to pass to the outside of the inner tube as vapor, but prevents impurities from passing through. Air flowing in the gap takes up the water vapor and humidified air exits the evaporation device. This humidified air is subsequently cooled to collect the vapor and provide purified water, e.g. at a condenser.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application is a National Phase Application of International Application No. PCT/GB2008/003356, filed Oct. 3, 2008, which claims priority to Great Britain Patent Application No. 0719390.7 filed Oct. 4, 2007, which applications are incorporated herein fully by this reference. 
     FIELD OF THE INVENTION 
     The invention relates to the purification of water and, in particular, to an evaporation device for use in purifying water. 
     BACKGROUND TO THE INVENTION 
     There is an increasing demand for water suitable for drinking and irrigation. Consequently there is an increasing demand for purifying impure water, such as sea water. 
     German Patent Application No. 10230668 describes a scheme for purifying “raw” water in which air is humidified as it passes across an arrangement of rods while the raw water is dripped over the surface of the rods. A condenser is then used to extract purified water from the humidified air. 
     European Patent Application No. 1 362 833 A2 discloses a water purification apparatus comprising a water source and a hydrophilic membrane. The hydrophilic membrane allows water to pass through the membrane as a vapour, and prevents impurities passing through. That patent specification contains a description of suitable hydrophilic materials, and also the results of experiments conducted in using bags made from hydrophilic material for irrigating plants. 
     SUMMARY OF THE INVENTION 
     According to a first aspect of the invention there is provided an evaporation device for use in purifying water, comprising a first tube arranged within a second tube, wherein the first tube comprises a hydrophilic membrane, and wherein a gap is provided between the first and second tubes for the flow of air. 
     Therefore the invention provides an evaporation device (or, alternatively, a pervaporation device) having an inner tube comprising a hydrophilic membrane, arranged inside an outer tube, with a gap between the tubes for the flow of air. In use, impure water in the inner tube can pervaporate through the inner tube and humidify air in the gap. The use of an inner tube arranged within an outer tube channels the air close to the hydrophilic membrane to provide efficient humidification. It also provides a structure that can be readily heated by solar radiation incident on the outer tube, in particular to heat the air in the gap. This enhances the convection of the air through the device and results in warmer air, which is able to carry a greater amount of water vapour than colder air, being present in the gap. This improves the humidification capacity of the device. 
     In one embodiment, the first tube is arranged substantially coaxially with the second tube. Such an arrangement can provide efficient humidification and is simple to manufacture. In other embodiments, at least a portion of the first tube may be coiled or arranged in a zig-zag configuration. The coil is preferably substantially coaxial with the second tube. Likewise, the zig-zag would usually be arranged centrally in the outer tube. The inner tube itself may be smooth or corrugated. Generally, arrangements that can provide a large surface area of the hydrophilic membrane, whether by coiling, zig-zags, corrugation or otherwise, provide more efficient humidification. 
     The gap may contain means for creating turbulence in air flowing through it. The turbulence causes drag, causing the air to spend longer in the device. So, the air has more time to take up pervaporated water molecules from the surface of the hydrophilic membrane. Overall, take up of moisture by the air is thereby promoted. The means for creating turbulence may comprise fins attached to either or both of the outside of the first tube and the inside of the second tube. Additionally or alternatively, either or both of the outside of the first tube and the inside of the second tube could be rough or undulating, e.g. corrugated or threaded. 
     The second tube may comprise metal, for example copper. This improves absorption and conduction of heat from the outside of the second tube to the inside of the second tube, thereby helping to optimise heat transfer to the air in the gap. 
     The inside of the second tube may be coated with a black substance. This improves radiation of heat from the inside surface of the second tube to the air in the gap. 
     The first and second tubes are preferably arranged not to touch each other in the gap, for example by suspending one or both of the tubes, or by employing a spacer. This feature facilitates air flow over the whole outer surface of the first tube. 
     The evaporation device may comprise an evaporation module for housing the evaporation device. The evaporation module may comprise an elongate housing with an outwardly curved window. This serves to improve the collection of solar radiation, by acting in a similar manner to a greenhouse. This can further improve heating of the evaporation device and, in particular, air in the gap. 
     According to a second aspect of the invention, there is provided a system for purifying water, comprising: 
     a water inlet channel for supplying impure water; 
     an evaporator for humidifying air by pervaporation using the impure water supplied by the water inlet channel; and 
     a condenser for condensing purified water from the air humidified by the evaporator, the condenser being cooled by the impure water supplied by the water inlet channel before it is supplied to the evaporator. 
     Also, according to a third aspect of the present invention, there is provided a method of purifying water, comprising: 
     supplying impure water through an inlet channel; 
     humidifying air with an evaporator by pervaporation using the impure water supplied by the water inlet channel; and 
     condensing purified water from the humidified air with a condenser that is cooled by the impure water supplied by the inlet channel before it is supplied to the evaporator. 
     In this way, the impure water used for humidification may also be used for effecting condensation. This avoids the need for separate supplies of water for the evaporator and the condenser. Importantly, the impure water is heated during the condensation process, with a result that the water supplied to the evaporator is warmer than if it were supplied to the evaporator directly. This has the benefit of facilitating humidification and condensation with effectively no net increase in the overall energy requirements of the system. 
     The water inlet channel may comprise heating means for heating the impure water after it is supplied to the condenser and before it is supplied to the evaporator. Heating the impure water can improves humidification. However, the condensation is still effected at the temperature at which the impure water is initially supplied, prior to heating the impure water by the heating means. Another advantage of this arrangement is that convection of the impure water in the heating means it can facilitate flow of the impure water through the water inlet channel. However, in some embodiments, for example where there is a large head between the source of the impure water and an outlet from the system, this may not be sufficient to facilitate flow of the impure water through the water inlet. So, a pump may be provided for propelling water through the water inlet channel. 
     The heating means may be arranged to employ solar energy, which enables low cost operation and a low environmental impact. 
     The water inlet channel may comprise a sediment trap. This feature reduces the likelihood of sediment inhibiting the flow of impure water in the system. 
     The evaporator may comprise the evaporation device described above. The first and second tubes may be arranged substantially vertically with the air inlet below the air outlet. This arrangement facilitates the flow of air in the gap by convection. 
     However, the system may comprise forcing means for forcing the air through the gap. For example a fan may blow or suck the air. This feature can increase the volume of air that is humidified and enhance the take up of the water vapour by the air. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will now be described, by way of example, only, with reference to the accompanying drawings, wherein: 
         FIG. 1  is a perspective view of a first preferred embodiment of an evaporation device; 
         FIG. 2  is a cross-section through the evaporation device of  FIG. 1 , along the line X-X; 
         FIG. 3  is a cross-section through the evaporation device of  FIG. 1 , along the line Y-Y; 
         FIG. 4  is a perspective view of a second preferred embodiment of an evaporation device; 
         FIG. 5  is a perspective view of a third preferred embodiment of an evaporation device; 
         FIG. 6  is a schematic block diagram of a water purification system; 
         FIG. 7  is a perspective view of an evaporator module; and 
         FIG. 8  is a cross-section through the evaporator module of  FIG. 7 , along line Z-Z. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to  FIGS. 1 to 3 , there is illustrated an evaporation device  100  having an inner tube  10  and an outer tube  20 . The inner tube  10  is made of a hydrophilic membrane, such as Dutyion™. Suitable hydrophilic membranes are described in EP 1 362 833 A2. The outer tube  20  is preferably made of a material that readily absorbs solar radiation and is good heat conductor, such as a metal, for example copper, although other materials, such as plastics, are suitable. The inside  22  of the outer tube  20  may be coated with a black substance, such as a black paint, to encourage the radiation, inside the outer tube  20  towards the inner tube  10 , of heat which has been absorbed by the outer tube  20 . 
     There is a gap  30  between the inner tube  10  and the outer tube  20  for the flow of air. In  FIGS. 1 to 3 , the inner tube  10  and the outer tube  20  are arranged substantially coaxially to facilitate the flow of air, although this is not essential. Preferably the inner tube  10  does not touch the outer tube  20 , thereby enabling air to flow in the gap all around the inner tube  10 ; this may be implemented, for example, by providing a spacer  40 . Another suitable means of arranging that the inner tube  10  and outer tube  20  do not touch is to suspend the inner tube  10  inside the outer tube  20  by a cord or a wire. 
     In use, the inner tube  10  contains water, which may be impure water such as sea water. One end  12  of the inner tube  10  may be connected to a source of such water via a pipe, for which purpose a connector (not illustrated in  FIGS. 1 to 3 ) may be provided at the end  12 . Furthermore, water may be channelled away from the end  14  of the inner tube  10  via a further pipe, for which purpose a connector (not illustrated in  FIGS. 1 to 3 ) may be provided at the end  14 . 
     The inner tube  10  and outer tube  20  illustrated in  FIGS. 1 to 3  have a circular cross-section, but this is not essential. A square, or other shape, cross-section would be suitable. 
     It is advantageous if the air flow within the gap  30  between the inner tube  10  and the outer tube  20  is turbulent, as this facilitates the take up of moisture into the air from the hydrophilic membrane of the inner tube  10 . The turbulent air flow causes drag and this drag causes the air to take longer to pass through the gap  30 , thereby giving the air longer to humidify. So, the evaporation device  100  may include means for creating turbulence, such as obstacles in the air path. Such obstacles may be provided on the outside of the inner tube  10 , on the inside of the outer tube  20 , or on both. In the embodiment illustrated in  FIGS. 1 to 3 , the evaporation device  100  has fins  50  arranged on the inside of the outer tube  20  for creating turbulence. 
     In second and third preferred embodiments, illustrated in  FIGS. 4 and 5 , the inner tube  10  is not arranged coaxially with the outer tube  20  but is, respectively, coiled or has a zig-zag configuration within at least part of the outer tube  20 . These configurations allow the inner tube  10  have to a greater surface area of hydrophilic membrane for a given length of outer inner tube  20 , which improves humidification of the air flowing in the gap  30 . Other configurations of the inner tube  10  may be used. Whatever configuration is used, it is considered useful to ensure that there are no cusps in the inner tube  10 , as water droplets tend to form in or near the cusps in the hydrophilic membrane, which is undesirable. 
     Referring to  FIG. 6 , there is illustrated a water purification system  500  comprising an evaporation device  100  as described above with reference to  FIGS. 1 to 5 . One or more such evaporation devices  100  may be used; in  FIG. 6  an evaporator  510  comprising a bank of five evaporation devices  100  is illustrated. To enable modular assembly of such a bank of evaporation devices  100 , the inner tube  10  of each evaporation device  100  may be provided with a snap-fit connector which may be self opening so that the evaporation device  100  automatically becomes part of the water circuit when it is added to the bank, and self closing when it is removed from the bank. Preferably, each evaporation device  100  is arranged with its outer tube  20  substantially vertical, in order to facilitate the flow of air upwards in the gap  30  by convection. 
     The water purification system  500  also comprises a water inlet channel  520  for conveying impure water to the inside of the first tube  10  of each evaporation device  100 . As the impure water is vaporised through the hydrophilic membrane in the evaporation device  100  and humidifies the air in the gap  30 , replacement impure water enters the first tube  10  through the water inlet channel  520 . In the evaporator  510  illustrated in  FIG. 6 , the first tube  10  of each evaporation device  100  is coupled in parallel to the water inlet channel  520 . 
     The water purification system  500  also comprises an air inlet  512  for supplying air  516  to the gap  30  of each evaporation device  100 . When each evaporation device  100  is arranged with its outer tube  20  substantially vertical, the air inlet  512  is preferably at the lower end of the evaporation device  100 , because air in the gap  30  will naturally rise by convection when heated. Such an air inlet  512  may be provided by installing the lower end of each evaporation device  100  above ground level. An air outlet  514  for extracting air  518  from the gap  30  of each evaporation device  100  is provided at the upper end of each evaporation device  100 . 
     A condenser  530  condenses water vapour from the humidified air, thereby forming purified water. Preferably the condenser  530  is arranged above the evaporator  510  so that the humidified air can readily pass into the condenser  530 . In other embodiments, an air path, such as ducting or such like, may be provided between the evaporator  510  and the condenser  530  to channel the humidified air from the evaporator  510  to the condenser  530 . The purified water may be extracted from the condenser  530  by way of an outlet  535 . 
     The evaporation devices  100  may be arranged to be heated by solar energy. This is absorbed by the outer tubes  20  of the evaporation devices  100  and then, in turn, heats the air within the gaps  30  and, to a lesser degree, the impure water within the inner tubes  10 . The pervaporation process is improved by this heating as there is a greater amount of evaporative energy available. Similarly, overall humidification is improved by the heating because warm air can hold more water vapour than cold air. Also, as the air in the gaps  30  is heated, air flows by convection, drawing in un-humidified air through the bottom of the evaporation devices  100  and causing more air to be humidified. 
     As illustrated in  FIG. 6 , the condenser  530  may be located in the water inlet channel  520  such that the impure water passes through the condenser  530  on its way to the evaporator  510 . At the condenser  530 , the impure water would normally be relatively cold, as it may be drawn from the sea or a ground water supply. It can therefore operate to keep the condenser  530  cool and facilitate the condensing. The temperature of the impure water is also raised in the condenser  530  before it reaches the evaporator  510 . This has the benefit of further heating the evaporator  510  and thereby improving the pervaporation process and humidification, for the same reasons as stated above. 
     The water inlet channel  520  comprises an optional heater  540  for heating the impure water prior to supplying the heated impure water to the evaporator  510 . This can also improve the pervaporation process and humidification of the air, as stated above. The heater  540  may be, for example, a solar heat sink operating from solar energy thereby reducing the environmental impact of the system  500 . If a heater  540  is employed to heat the impure water and also the impure water is passed through the condenser  530 , the heater  540  should be located downstream of the condenser  530 , such that the condenser  530  receives the coolest possible impure water. Heating the water in the heater  540  can also provide convection in the impure water in the part of the inlet channel  520  inside the heater  540 , sufficient to cause flow of the water through the channel  520 . Even without the heater, water heated elsewhere in the system will aid water circulation by convection. 
     The air inlet  512  comprises an optional fan  550  for forcing air through the gap  30 . The fan  550  may be arranged to blow or suck the air through the gap  30 . In this way a greater flow rate of air may provided, which can improve take up of the water vapour by the air. 
     The water inlet channel  520  comprises an optional sediment trap  560  for reducing the likelihood of sediment inhibiting the flow of impure water in the system  500 . The sediment trap  560  is preferably located upstream of the condenser  530 , heater  540  and the evaporator  510 . 
     Also, in the embodiment illustrated in  FIG. 6 , there is a water outlet channel  570  for extracting impure water from the inner tube  10  of each evaporation device  100  in the humidifier  510 . This enables a flow of impure water to be established, which enables freshly heated impure water to flow into each evaporation device  100 , thereby bringing the stated benefits of heated water, and also reducing the likelihood of sediment accumulating in the evaporator  510  or in the constituent parts of the water inlet channel  520 , which could reduce efficiency of the system  500 . Optionally, a pump  580  may be provided for pumping the impure water through the water inlet channel  520  to the water outlet channel  570 , thereby improving the flow of the impure water. The pump  580  may be solar powered to provide low cost operation and low environmental impact. Water emerging from the water outlet channel  570  may be returned to the source  590  of impure water, such as the sea, a well, or, a reservoir. 
     Optionally, impure water from the water outlet channel  570  may coupled back to the water inlet channel  520 , with only the water lost through pervaporation needing to be replaced from the source of impure water. In this way impure water is re-used, which reduces the requirement for a supply of replacement impure water, making the system  500  suitable for purifying water in locations that do not have a plentiful supply of impure water. 
     Referring to  FIGS. 6 and 7 , in another preferred embodiment of the invention, the evaporation devices  100  may be housed in evaporation modules  600 . Each module  600  comprises a housing  610  with a window  620 . The housing  610  is elongate and can accommodate one or more evaporation devices  100 . It is preferably made of an insulating material. 
     The window  620  is preferably curved outwardly from the housing  610 . This helps to maximise the amount of solar radiation collected by the module  600 . The inside surface of the window  620  preferably has a coating that reduces the amount of solar radiation that escapes from the module  600  through the window  620 . 
     In use, the module  600  may be mounted in a position where it receives a large amount of solar radiation, e.g. on the outside of a building. Alternatively, it may be portable. The received solar radiation is collected in the module  600  due to the insulative nature of the housing  610  and the inward reflectiveness of the window  620 , e.g. like an efficient greenhouse. Heating of the evaporation device(s)  100  in the module  600  by solar radiation is therefore improved. 
     The evaporator  510  of the system  500  illustrated in  FIG. 6  may comprise one or more evaporation modules  600 . The number of evaporation modules  600  can be varied in the same way as the number of evaporation devices  100 . 
     The system  500  may also be adapted in other ways to suit the prevailing circumstances in different locations. For example, different forms of condenser  530  may be employed. In a cold region of the world, a sheet of glass, for example part of a building, cooled by the ambient temperature may be sufficient for performing the required condensation, whereas in other locations having an ample supply of solar energy, a powered condenser using active cooling, for example actively cooled fins, may be more appropriate. 
     Reference in the specification and claims to humidification (or equivalently hydration), purification and heating are not intended to signify any predetermined, respectively, humidity, purity and temperature, but merely signify an increase in, respectively, humidity, purity and temperature.