Patent Publication Number: US-8974643-B2

Title: Method and device for purifying a liquid

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application, as a national state application, claims the benefit of PCT Application No. PCT/NL2008/000013, filed Jan. 16, 2008 and foreign priority to Netherlands Application No. 1033253, filed Jan. 18, 2007, which are both incorporated herein by reference. 
     The present invention relates to a method for purifying a liquid comprising liquid particles and residual particles. It is hereby possible to generate substantially pure water from for instance seawater. 
     A known system for desalinating seawater, and thereby converting it into fresh water, is membrane distillation. Here salt water is heated, wherein the water evaporates and passes through a membrane, after which the water vapour condenses on a relatively cool condensation surface and relinquishes the heat to the salt water to be heated. The condensation is then discharged as substantially pure fresh water. 
     The use of membranes is a costly aspect of the whole purification process. Not only are the membranes expensive to produce and purchase, the membranes are also susceptible to contamination, whereby they begin to operate less efficiently. An additional problem is that many membranes are essentially sensitive to temperature and, particularly at higher temperatures, become less efficient in use. The evaporation of the liquid further requires a great deal of energy. 
     The present invention has for its object to provide a method for purifying a liquid, wherein the purification is performed in a more efficient manner. 
     The present invention provides a method for purifying a liquid comprising liquid particles and residual particles, comprising the steps of:
         heating the liquid with liquid particles and residual particles for purifying;   carrying the liquid in the form of liquid droplets into a purification space;   applying a similar electric charge to the liquid droplets and to a condensation surface;   evaporating liquid particles in the purification space;   condensing the evaporated liquid particles on the condensation surface;   discharging the condensation and the unevaporated residual particles separately.       

     Liquid can evaporate efficiently by heating the liquid for purifying and distributing or vapourizing this heated liquid in a purification space. Applying electric charge to the liquid droplets achieves that the droplets become unstable. Once the critical point has been passed, these droplets are then distributed as a type of mist of charged particles. These particles have a diameter in the order of magnitude of 10 μm or even smaller. The process (“Electro-Spraying”) of vapourizing the particles can also be repeated. Liquid evaporates from the particles. An efficient evaporation process is hereby obtained. The evaporated liquid will then seek a surface on which to condense. Applying a charge to the condensation surface similar to that on the residual particles achieves that the residual particles of the liquid are repelled by the condensation surface and only the evaporated liquid particles will condense on this condensation surface. The condensation surface will here have a lower temperature than the particles. A substantially pure liquid flow is obtained by then collecting the condensation and discharging it separately. The unevaporated residual particles are discharged as a residual flow. The applying of said charge has the result, among others, that no membrane is required to separate the evaporated liquid particles from the residual particles. An advantageous process is hereby obtained, the operation of which is not diminished through use as a result of for instance contamination. In an advantageous embodiment the liquid is seawater which is purified into fresh water suitable for consumption. 
     In a preferred embodiment according to the invention the evaporated liquid particles are guided to the condensation wall. 
     By guiding the liquid particles to the condensation wall a greater part of the liquid for purifying is separated from the residual particles. This guiding can take place by making use of an extra gas flow and/or a pressure difference. A greater output can hereby be realized from the process. An additional advantage is that the residual flow will comprise fewer liquid particles and will therefore be smaller, with for instance a higher concentration of salt. 
     In a preferred embodiment according to the invention the step of heating a liquid comprises of carrying the liquid though a heat exchanger and further heating the liquid with an external source. 
     Owing to the use of a heat exchanger to heat the liquid use can be made of the energy released from, among others, the condensation process and/or the remainder of the residual flow. An external energy source is necessary for the purpose of supplying extra energy in order to maintain the process. In an advantageous embodiment this energy source is embodied as an additional heat exchanger. Energy in the form of heat can be supplied to the liquid in relatively simple manner by using a heat exchanger. The heat exchanger can for instance obtain this heat in the form of a residual energy originating from another process preferably located nearby. Use can herein also be made for instance of solar energy. The heating of the liquid preferably takes place in counter-flow to the evaporation, condensation and/or discharge. The use of the energy required for the process is hereby managed most efficiently. A positive charge is further preferably applied to both the liquid droplets and the condensation surface. The most efficient use is hereby made of properties of the substances involved. 
     In a preferred embodiment according to the invention the residual particles are repelled by the condensation surface and attracted by a discharge wall to which a charge opposite to that on the condensation wall is applied. 
     By applying the charge to the liquid and to the condensation surface the residual particles in the liquid are repelled by this condensation surface. The direction of the flow of residual particles can further be directed by the use of a discharge wall to which a charge opposite to that on the condensation wall is applied. The residual particles are hereby attracted and the purification of the liquid is further improved. The charge can be applied by making use of one or more power sources. The discharge wall can for instance also be provided with a charge by an process of induction. 
     In an advantageous preferred embodiment according to the invention the residual flow is used for energy generation by making use of the pressure difference between salt and fresh water. 
     A membrane potential is set by for instance making use of an ion-selective membrane which separates salt water from fresh water. This potential amounts to about 80 mV. The small voltage difference can optionally be increased by placing a plurality of compartments in series in order to use a greater voltage difference, whereby electricity can be generated. By using the highly concentrated salt residual flow for this purpose a relatively high voltage difference can be realized using a membrane, whereby electricity can be generated in more efficient manner by making use of this residual flow. 
     The present invention further also relates to a device for purifying a liquid. The device provides the same effects and advantages as those stated with reference to the method. In an advantageous embodiment of the device according to the invention a double-walled cylinder is provided as heat exchanger and condensation surface. Through the use of a double-walled cylinder the incoming, liquid flow can for instance be heated by the outer edge, after which evaporation, condensation and discharge of the separated flows takes place through the centre of the cylinder. The incoming liquid flow can of course also be guided through the centre of the cylinder and the discharge through the outer edge. An advantageous device can be realized in efficient manner with such a double-walled cylinder. 
     In a further preferred embodiment the liquid for purifying is heated to about 200° C. in order to intensify the evaporation step. 
     By heating the liquid for purifying above 100° C., for instance by applying a higher pressure, the heat content of the liquid flow is increased. A greater part of the water will hereby evaporate in the evaporation step. The optimum temperature depends on the specific process conditions, such as magnitude of flow rates, heat-exchanging surfaces and so on. The temperature of the liquid for purifying will preferably be brought above 100° C., to about 125° C., more preferably to about 150° C., and most preferably to 200° C. or even higher. It will be apparent that, when the temperatures are too high, among other things the heat loss will become so great that the process will progress less efficiently. 
    
    
     
       Further advantages, features and details of the invention are elucidated on the basis of a preferred embodiment, wherein reference is made to the accompanying drawings, in which: 
         FIG. 1  shows a schematic view of a known principle for purifying liquid; 
         FIG. 2  shows a schematic view of the method according to the invention; and 
         FIG. 3  shows a schematic view of an embodiment according to the diagram of  FIG. 2 ; and 
         FIG. 4  shows an alternative embodiment, connected in parallel, of the purification according to the invention. 
     
    
    
     A device  2  for purifying a liquid ( FIG. 1 ) takes up a salt flow  4 , such as seawater. Such a flow  4  comprises about 35 g of salt per liter of salt water. On the inlet side this salt water has a temperature of for instance 25° C. In a non-porous tube  6  the liquid is heated to a temperature of about 80° C. of the outgoing flow  8 . This flow  8  is further heated by a heating element  10 , and this results in an extra-heated flow of about 95° C. The extra-heated flow  12  then enters a porous tube  14 . Due to the high temperature water will evaporate and come to lie on a relatively cold condensation surface  20  as water vapour flow  18  through openings  16  in the membrane or porous tube  14 . This pure condensed water is discharged for further use as pure water flow  22 . The residual particles of the liquid flow which do, not pass through openings  16  in, the membrane or porous tube  14  are discharged as a residual flow  24 . During the passage of water vapour flow  18  to condensation surface  20  a small underpressure can optionally be employed in order to facilitate this passage. 
     Device  26  according to the invention ( FIG. 2 ) takes up and carries salt water  28  through tube  30 . The water flow is herein heated from about 25° C. of ingoing flow  28  to about 80° C. of outgoing flow  32 . This flow  32  is then further heated by heating element  34  to form extra-heated flow  36 . This extra-heated flow  36  has a temperature of about 95° C. Flow  36  is carried to a sprayer or vaporizer  38 . Sprayer  38  is provided with a needle-shaped outflow (not shown). The sprayer is connected to a charge source  40  via connection  42 . Charge source  40  provides the liquid in the sprayer with a charge by applying an electric voltage. Flow  36  is then distributed over droplet flows  44  by sprayer  38 . Water from these droplets can evaporate and is carried by fan  47  in the direction of condensation surface  48  as water vapour flow  46 . Condensation surface  48  is provided with the same charge as sprayer  38  by the charge source  50  by means of connection  52 . Only the flow of water vapour  46  will condense onto condensation surface  48  and the residual particles will be discharged as residual flow  54 . The residual particles are attracted by a plate  56 , which is provided with a charge by source  58  by means of connection  60 . The charge on plate  56  is opposite to the charge of condensation wall  48 . Alternatively, the charge on plate  56  can be obtained by induction. Flow  44  can be directed by making use of a fan  49 . Use can here be made of air or another gas. 
     Due to the high temperature of droplet flow  44  water can evaporate and then condense onto the cold surface  48 . This process takes place in a space  61 . The heat released during this condensation is used to heat ingoing flow  28 . The outgoing flows have a temperature of about 28° C. In the shown embodiment about 10% of the water will for instance evaporate in a single purification step, depending of course on the process parameters. 
     A device  62  for purifying a liquid ( FIG. 3 ), as according to the operation shown in  FIG. 2 , is formed by a double-walled cylinder  64 . This cylinder  64  has an outer wall  66  and an inner wall  68 . An ingoing flow  70  formed by seawater flows through a passage  72  formed by outer wall  66  and inner wall  68 . Outgoing flow  74  is further heated by a heating element  76 , resulting in an extra-heated flow  78 . Hot flow  78  is carried to sprayer  80 , which distributes flow  78  as a droplet flow  82 . Water can evaporate from this flow  82  and forms a flow of water vapour  84  which will condense on the relatively cold inner wall  68 . The condensation is collected and carried further as pure water flow  86 . The remaining particles are carried further as residual flow  88 . Sprayer  80  is connected to charge source  90  via connection  92 . Inner wall  68  is connected to voltage source  94  via connection  96 . Residual flow  88  is collected by collecting wall  98 , which is connected to a power source  100  via connection  102 . 
     The ingoing flow is heated from a temperature of 25° C. to about 80° C. at outgoing flow  74 . Due to the additional heating the extra-heated flow  78  has a temperature of about 95°. The heating from 25° C. to 80° C. can be achieved by making use of the released condensation heat and the cooling of the flows on the inner side of the double-walled cylinder. Flows  86  and  88  cool from 95° C. close to sprayer  80  to about 28° C. when leaving device  62 . Power sources  90  and  94  apply the same charge to sprayer  80  and inner wall  68 , wherein inner wall  68  functions as condensation surface. Discharge wall  98  is on the other hand provided with an opposite charge by power source  100 . The residual particles are hereby attracted by this wall and will not deposit against condensation wall  68 . Pure separation of residual flow  88  and pure water flow  86  can hereby be realized. Flows can optionally be forced by fans and/or for instance pressure differences. 
     In an alternative embodiment of the purifying process the device  104  ( FIG. 4 ) is provided with a number of plates  106 . An ingoing flow  108  is carried alternately between these plates or tubes/cylinders  106 . This flow is heated by the released condensation heat in the purification spaces provided between ingoing flows  108 . Ingoing flow  108  is carried to sprayer  116  by means of conduits  110 , via one or more heating elements  112  and conduits  114 . This sprayer  116  is provided with charge from a charge source  118  via connection  120 . The sprayers can herein be provided with charge from one charge source  118  or each be provided with their own source. The vapourized flow  122  is directed by a fan  124 . Liquid will evaporate and come to lie in a flow  126  on plates  106 , where condensation takes place on the relatively cold surface. Plates  106  are provided with charge from energy source  128  via connection  130 . The condensation is discharged with condensation flow  132 . Residual flow  134  comes to lie on plate  136 , which is provided with a charge from energy source  138  via connection  140 . The further operating principle is the same as that in the above discussed embodiments. 
     The present invention is in no way limited to the above described preferred embodiment; the rights sought are defined by the following claims, within the scope of which many modifications can be envisaged. It is thus possible for instance to combine the power sources into one source, such as power sources  90  and  94  of  FIG. 3 . It is further possible to make use of the earth&#39;s electric field, for instance by bringing sprayer  80  to the voltage of higher air layers. In order to apply a voltage use can for instance also be made of a Kelvin pot, wherein falling water droplets are used to generate voltage differences, making use of electrostatic induction which occurs between two mutually connected and oppositely charged sub-systems. Although the charge can be applied to the liquid droplets in sprayer  38 , it is for instance also possible to envisage applying the charge to the vapourized flow. In further directing or guiding of the flows, such as for instance flow  44  and/or  46 , use can be made of underpressure in addition or as alternative to the use of the fans. It is also possible for instance to use the residual flow to generate electricity by means of “Reverse Electro-Dialysis” by bringing salt and fresh water into contact by means of ion-selective membranes. The residual flow can also, be used in a different way for energy generation, such as for instance by means of “pressure-retarded osmosis”. Other methods are otherwise also possible. This energy can then for instance be used again for heating elements  76  in order to realize an even more advantageous purification therewith. It is further possible to envisage bringing for instance flow  36  to a high pressure in order to enable the evaporation process to then take place efficiently. In a possible alternative configuration of the system a plurality of purifying processes are performed in parallel. This gives the overall system a greater capacity. Use can also be made here of opposite charges in different parallel sub-systems. This has the additional advantage that an equal number of similarly charged particles are obtained. In a possible alternative configuration a plurality of purifying steps is carried out sequentially and/or iteratively. Purer flows can hereby be obtained. A combination of these configurations is of course also possible.