Title of Invention

A DEVICE FOR EVAPORATION AND CONDENSATION OF WATER FROM AN AQUEOUS LIQUID

Abstract

The present invention relates to a device for evaporation and condensation of water from an aqueous liquid by a temperature difference or concentration difference, which device comprises a layer-structure having a layer comprising a vapour channel 15 and a liquid channel 12 for the aqueous liquid; the vapour channel 15 is bounded at a first vapour channel side by a water permeable membrane 13 bounding simultaneously a second liquid channel side for generating vapour at the surface of the membrane 13; the vapour cnannel15 is bounded at a second vapour channel side, opposite said first vapour channel side, by a water impermeable foil 16, 17 providing a condensation surface in order to condense vapour evaporated at the surface of the membrane 13; and the layer structure comprises a multiple of said layers; characterised in that the liquid channel of a lower one of said layers is bounded at a first liquid channel side, opposite said second liquid channel side, by the water impermeable foil 16 of an adjacent higher, bne of said layers; and the first liquid channel side of the top layer of said layer structure is a water impermeable solar collector absorber sheet 11.

Full Text

Regenerative membrane purification device. The present invention relates to invention provides a device for evaporation and condensation of water from an aqueous liquid by means of a temperature difference and/or concentration difference, which device comprises (a) a vapour chamber which is or can be brought in contact with a condensation compartment wherein at least part of the water vapour from the aqueous liquid condenses, and wherein inside of said vapour chamber is located (b) an evaporation compartment having at least a wall with an outer surface and comprising a closed, hollow and water permeable membrane. The invention specifically relates to a device and a method for enabling the evaporation of water from a waste-containing or salt-containing aqueous liquid and the condensation of the resulting water vapour by means of a temperature difference and/or concentration difference between this aqueous liquid and a surface for condensation of the water vapour. Here the terms "evaporation" and "condensation" depict net vapour fluxes from a liquid interface. "Evaporation" designates the non-equilibrium situation that more solvent molecules leave the solution than that enter the solution. Likewise the term "condensation" means that more molecules enter the solution than that leave the solution. The deviation from non-equilibrium (net evaporation or condensation) can be enforced e.g. by raising/lowering the solution's temperature and/or lowering/raising the vapour pressure. For evaporation, energy is required to increase the energy level of the molecules in order to escape from the solution. As the energy level increase is associated with an increase in intermolecular distance (lower density), rather than in an increase in temperature, the supplied energy is termed "latent", rather than "sensible". In nature, solar irradiation is used for evaporation of water from oceans and seas (increase of latent heat contents). When the water vapour condenses, latent heat is released again. In industry, the efficiency of the water purification process is an important factor. Here the efficiency is defined as the latent heat flux, associated with the process output, in relation to the process energy input. A well-known measure to increase the efficiency of the purification process, is to reuse the latent heat -released during condensation of the solvent vapour- for evaporation of another amount of solvent. When the latent heat is temporarily stored before re-usage, the process is termed "regenerative", when it is re-used without intermediate storage, the process is termed "recuperative". In this patent application, both processes will be designated as "regenerative". The non-prepublished international patent application PCT/EP01/00421 describes a device and a method for enabling the evaporation of water from a waste-containing or salt-containing aqueous liquid and the condensation of the resulting water vapour by means of a temperature difference and/or concentration difference between this aqueous liquid and a surface for condensation of the water vapour. According to this application the device comprises: (a) a vapour chamber that can be brought in contact with a condensation compartment wherein at least part of the water vapour from the aqueous liquid condenses, and wherein inside of said vapour chamber is located (b) an evaporation compartment having a least a wall with an outer surface and comprising a closed, hollow water permeable membrane, and which vapour chamber has an outer surface that comprises a substantially water-impermeable insulation skirt with an inner and an outer surface, such that there is a gap between the inner surface of the insulation skirt and the outer surface of the evaporation compartment, and wherein the vapour chamber has a lower surface of an active width (w) which is at least 10% of the effective diameter (d) of the evaporation compartment. The crux of the invention according to PCT/EPO1/00421 resides, int. al., in the fact that when the device is in operation there is a temperature difference and/or concentration difference between an aqueous liquid and a condensation compartment, which can be a supporting material, and the surface of such a supporting material below the vapour chamber will have a significantly lower temperature than the upper part of the irrigation system and in particular the aqueous liquid contained in the evaporation compartment. Due to the resulting vertical temperature difference and/or concentration difference, condensation occurs in the lower part of the device and above all in the condensation compartment, such as on the surface of the supporting material. In this way water is distilled by using a membrane, a process that is generally referred to as membrane distillation. Thus, it has surprisingly been found that a highly efficient condensation is possible without any external cooling device being necessary. It is stressed that the term "lower surface" in this invention means the surface that is not directed to the heat source. The term "upper surface" means the surface that is directed to the heat source. Although the device according to PCT/EP01/00421 is effective and efficient, the device does not effectively use the energy generated by the condensation of the water vapour. Therefore it is an object of the present invention to further improve the device according to PCT/EPO1/00421. Moreover it is an object of the present invention to provide a method and a device, which are suitable for direct solar heating. That object is achieved in that the evaporation compartment is adapted to receive heat by means of direct solar radiation, the device comprising means for regeneration of the latent heat, wherein latent heat released during condensation of water vapour is used to heat the aqueous liquid. The membrane that is suitable for the present invention is water permeable and preferably salt resistant. Salt can be transported through the membrane as long as it is dissolved in water. Further, it is noted that the outer surface of the membrane is in contact with an evaporation chamber. The chamber is filled with a carrier gas, such as air, or alternatively is a vacuum. When the membrane is salt resistant, this means that the membrane polymer does not degrade under the influence of warm aqueous salt solutions. More preferably, the membrane is a homogeneous non-porous hydrophilic membrane. The membrane preferably is made of the material described in WO00/28807. The condensation compartment can be a vessel, container, basin, or the like. It is advantageous when the vapour chamber comprises means for obtaining a convection stream to enhance the evaporation from the evaporation compartment and/or the transport of the water vapour from the vapour chamber to the condensation compartment, such as a ventilator, fan, and the like. It is noted that WO0072947 discloses a method and a device for the purification of a liquid by means of membrane distillation, in particular for the production of desalinated water from seawater or brackish water. In the method according to this patent application relatively warm stream of water passes over a porous membrane. Vapour will flow via the pores of the membrane to the other side of said membrane. Thereafter said vapour will condensate on a relatively cool condenser surface forming a distillate stream, wherein said appreciable proportion of the latent heat will be transferred to the feed stream. According to WO0072947 a device for performing the method consists of a number of segments connected to one another and each segment is made up of layers of essentially parallel non-porous fibre membranes for the feed stream and layers of essentially parallel porous fibre membranes for the retentate stream, wherein a or each layer of porous fibre membranes is arranged between two successive layers of non-porous membranes. This arrangement is required in order to assure the transfer of the latent heat. Since the layers of porous and non-porous membranes are arranged in successive order, in order to form a stack of membranes, the device according to WO0072947 is not suitable for direct heating by means of the sun. The solar heat would not only increase the temperature of the porous membranes, but also the temperature of the non-porous membranes, diminishing the temperature difference between the two types of membranes which is needed for the process. Therefore the device uses an external heat-device, in order to heat up the fluid outside the stack of membrane-layers before returning the fluid to the porous membranes. The article "Performance and analysis of a multiple-effect solar still utilising an internal multi-tubular heat exchanger for thermal energy recycle", ISES Solar World Congress 1999, Vol. Ill, Mink e.a. discloses a solar still, which is suitable for direct heating by means of the sun, wherein regeneration of latent heat is used to improve the efficiency of the device. According to Mink fluid to be purified is introduced in the system by means of a non-porous feed line. The fluid is preheated in a black, serpentine feed line by solar heat. The fluid exits the feed line and flows onto a black textile wick. The liquid then evaporates under the influence of the solar heat from this wick. At least a part of the so formed water vapour will condensate on a condensation surface, positioned underneath the serpentine feed line. The latent heat will be released during the condensation and will preheat the liquid inside the feed line. In this way a certain regeneration of the latent heat is achieved. A disadvantage of the system according to Mink is that the purification of the liquid is not accomplished by means of a membrane, but solely by means of evaporation and condensation from a textile wick. According to the invention it is possible that the device comprises a feedline for feeding the aqueous liquid towards the evaporation compartment, wherein at least the end part of the feedline extends through the condensation compartment, part of the exterior of the feedline providing a condensation surface. In a preferred embodiment it is possible that heating means are provided for supplying heat to the feedline, the heating means being positioned downstream of the condensation surface, wherein the evaporation compartment is positioned in a first part of the vapour room and wherein the condensation surface of the feedline is positioned in a second part of the vapour room. Thereby it is possible that the first part of the vapour room is adapted to receive heat by means of direct solar heating. It is preferred that the device is enclosed by means of a vapour-tight enclosure, the enclosure being made of a translucent material. According to the invention it is possible that the first and second parts of the vapour room are separated by means of a separation wall. Thereby it is possible that the separation wall is formed as a concentrating mirror, adapted to focus solar heat onto the evaporation compartment. In a preferred embodiment the non-condensable gasses are removed from the vapour chamber. Because of this measure the transport of fluids in the device is improved. According to the invention it is possible that the device comprises ventilation means, such as a fan, in order to urge vapour in the vapour room towards the condensation compartment. A preferred embodiment of the invention is characterised in that it comprises at least a first and a second evaporation compartment, wherein a first part of the exterior of the second evaporation compartment provide a condensation surface in order to condense vapour evaporated by means of the first evaporation compartment. In the device according to the invention the membrane material preferably is spectral selective. This can be achieved by applying a special coating on the membrane material. This coating could at the same time be used as reinforcement for the membrane. Alternatively a dye can be added to the membrane material during the production thereof. It is possible that the device comprises a layer-structure, each layer comprising a liquid channel bounded of an water impermeable foil at a first side thereof and by means of a water permeable membrane at the other side thereof, and a vapour channel bounded at a first side thereof by means of said water permeable membrane and at bounded at the other side of the vapour channel by means of a water impermeable foil of an adjacent layer. Further it is possible that the evaporation compartment is defined by means of a first hollow member, such as a cylinder, and a second hollow member, such as a second cylinder, positioned inside the first member, wherein the interior of the first member and the exterior of the second member define a space for containing the aqueous liquid, and wherein a first element of the exterior of the first member and the interior of the second member provides an evaporation compartment, the second element of the exterior of the first member and the interior of the second member providing a condensation compartment. The invention also relates to a method for the condensation of water from an aqueous liquid by means of a temperature difference and/or concentration difference. The present invention will be clarified, reference being made to the appending drawing, wherein: Fig. 1 shows the basic principle of continuously regeneration of latent heat. Fig. 2 shows the basic principle of stepwise regeneration of latent heat. Fig. 3 shows the device according to the invention enclosing the basic principle of continuously (gradual) regeneration. Fig. 4 relates to preferred embodiment of the present invention, showing a continuously regenerative solar still using a membrane, solar energy absorber and forced feed water convection. Fig. 5 shows a continuously regenerative device with internal heat exchanger and forced feed water convection. Fig. 6 shows a continuously regenerative device on the basis of irradiative heat supply or internal heat exchanger, and forced vapour circulation and/or vapour compression. Fig. 7 shows a stepwise regenerative device coupled to a solar collector. Fig. 8 shows the device enclosing the basic principle of stepwise regeneration. Fig. 9 shows a stepwise regenerative device using flat stacked membranes and external heat supply. Fig. 10. shows the spiral configuration for a stepwise regenerative system. Figs. 11 A-and 1 IB show two embodiments wherein the device comprises a first hollow element and a second hollow element positioned in the second hollow element. In figs. 12a and 12b two possible embodiments are shows of a multi stage system wherein a first device according to the invention is coupled to a further device. Fig. 1 shows the basic principle of continuously regeneration of latent heat. Fig. 2 shows the basic principle of stepwise regeneration of latent heat. This device according to Fig. 3 uses the regeneration principle according to Fig. L The device resembles a semi-permeable membrane 1 (optionally reinforced by means of fabric layer), which absorbs solar radiation and evaporates a solvent from the contained contaminated solution. The envelope 3 can be shaped tubular or bag-box-wise. The solvent vapour which is contained inside a casing flows upward by means of natural convection (or propelled by means of a fan or compressor (not shown)). The casing 3, which can be tubular, bag- or box-wise shape is transparent at least at the front side to transmit solar radiation. At the top section of the envelope, the vapour flow is diverted to the backside (see arrow 5), where vapour condenses against a condenser 2 (which also can be of tubular or bag-wise shape). As the vapour cools down, it descend to the lower part of the device where is it is diverted to the front section again, thereby closing the vapour circulation loop. Counter-current wise, the contaminated solution is fed to the bottom part of the condenser. In the condenser 2, the solution is pre-heated by the condensing vapour. At the top-part of the condenser, the solution is diverted to the front-side of the device, by means of a line 4, where it enters the membrane absorber 1. At this station, the solvent starts to evaporate, thereby cooling down the solution inside the membrane envelope. At the bottom outlet of the membrane envelope, the concentrated solution either can be re-diverted to the condenser by means of line 6 and valve 7, or discharged. Optionally, the sensible heat contents of the concentrated solution can be transferred to the solution feed by means of a heat exchanger 8. By condensing the vapour against the feed flow and transferring the heat of the concentrated solution to the feed, this device utilises recuperation of latent heat. As an example, this device according to fig. 4 is a tubular variant of the continuously regenerative device shown in. fig. 3. To enhance the solar radiation heat flux, the backside of the evaporation section can be equipped with a concentrating mirror 16, which focuses solar heat onto the membrane absorber envelope 1. Furthermore the envelope air is partially removed trom the envelope casing to ennance vapour transport. The presence of the wall member 16 underneath the membrane 1 divides the space inside the device 3 in a first area, containing the tubular membrane 1, and a second area underneath the element 1. In use the first area experiences direct solar heating. Therefore this first area will be relatively warm. The second area is protected against direct solar heating by means of the wall element 16. Therefore in use the second area will be completely or partly shaded. Because of the separation of the space in the said first and second area, a temperature difference will be formed between the said areas. This temperature difference is needed in order to enable condensation of water vapour inside the second area. Condensation preferably takes place at the circumference of the feed line 2, which runs through the second area. In this way the latent heat is effectively used in order to preheat the liquid that is transported towards the membrane 1. It should be noted that the membrane 1 according to figures 3 and 4 could either be a micro-porous membrane as suggested in WO 0072947 or a continuous membrane as disclosed in WO00/28807. Optionally, the casing can be insulated from ambient condition by means of a Dewar-type of (vacuum) insulation. The device according to fig 5 functions on the basis of the counter-current vapour and solution feed flow as outlined in fig. 3. However, in this embodiment the membrane envelope 1 does not function as a solar absorber. The casing can be entirely non-transparent. Heat is supplied to the solution flow by means of a separate heat exchanger inside or outside the casing. The heat supply can be of any source (solar, waste heat, fossil fuel). The device according to fig, 6 also functions on the basis of the counter-current vapour and solution feed flow as outlined in fig. 3 (solar heat supply) or fig. 6 (internal heat exchanger). In this embodiment, however, the vapour flow is propelled by means of a fan or compressor. With respect to the evaporation /condensation cycle, the liquid flow does not need to be propelled. However, when system contents needs frequent refreshment to avoid crystallisation, scaling or sedimentation of the contaminant, the liquid flow also may be circulated through the system, thereby transferring heat from the bleed to the feed flow as outlined previously. As in this embodiment the vapour flow is propelled, the vapour can be condensed directly to the backside of the evaporator envelope, as shown in the figure. In case a fan is used, it is only intended to propel the vapour re-circulation, without affecting the evaporator and condensation temperatures. A compressor can be used to achieve a substantial downward shift in condensation temperature. By means of the compressor and a restriction orifice or valve, the casing envelope is partitioned in a high pressure (condenser tube) and low-pressure (evaporator) section. As a result of the reduced pressure at the evaporator and the increased pressure at the condenser section, the evaporation /condensation process takes place at a substantially lower temperatures thereby reducing the thermal losses of the evaporator through the transparent section of the casing. In the embodiment of fig. 7, an external heat source, stepwise regenerative membrane evaporator/condenser device is directly coupled to a solar collector. The evaporator/condenser unit can be placed along side, on top, at the bottom or behind the solar collector. The solar collector can be of any kind. The device according to fig. 8 uses the regeneration principle according to fig. 2. It comprises a repetitive structure of a membrane, a carrier gas/vapour gap, a liquid and vapour impermeable foil or sheet and a liquid layer. The vapour, which is generated at a membrane surface, is transported either by convection or diffusion to the impermeable foil or sheet where it condenses, thereby releasing its condensation heat. The condensation heat is accumulated in the adjacent, next lower temperature liquid layer and re-used for evaporation. The membrane can be of the porous and non-porous type and mechanically stabilised either by being laminated onto a substrate or supported by means of reinforcement layers at one or both sides of the membrane. The liquid and air/vapour gaps can be maintained by means of a spacing material. The impermeable layer of the top section is replaced by a solar collector absorber sheet The top part of the system is equipped with a transparent cover plate to provide an air gap and thereby insulate the solar absorber from the ambient. At the backside of the cassette, a liquid impermeable foil, sheet or plate is present to enable the lowest temperature step condensation. The internal pressure of the system acts on the front absorber plate and backside condenser plate. The pressure is balanced by means of a casing, firmly connecting the front and backside plate by means of strips or tensions bars or the like. The supply and discharge flows of feed, brine and product is controlled by meansrof inlet-outlet port incorporated in the casing, which will be elaborated further in the sequel. The condenser reject heat can be used for domestic hot water heating. The device according to this embodiment uses the regeneration principle according to fig. 2 and fig. 4, comprising a repetitive structure of a membrane, an air/vapour gap, a liquid impermeable foil or sheet and a liquid layer. However, in this embodiment, the device is not integrated with a solar collector. Instead, the outer liquid layer is heated by means of an external heat source, to provide the energy for evaporation. This device according to this embodiment uses the regeneration principle according to fig. 10. In this embodiment, however, the membrane-impermeable-sheet sandwich is spirally wound such as to provide condensation against the backside of a liquid gap. The solution feed flow is fed into the inner or outer winding. While spiralling outwards or inwards, the temperature of the liquid decreases as the solute evaporates via the membrane Fig. 11 shows an embodiment wherein the evaporation compartment is defined by means of a first hollow member, such as a cylinder, and a second hollow member, such as a second cylinder, positioned inside the first member. The interior of the first member and the exterior of the second member define a space for containing the aqueous liquid. According to fig 11a the exterior of the first member provides an evaporation compartment. The interior of the second member provides a condensation compartment. That means that the exterior of the larger cilinder forms the evaporation surface, wherein the interior of the smaller cilinder forms the condensation surface. According to fig lib interior of the second member provides an evaporation ■ compartment. The exterior of the first member provides a condensation compartment. That means that the interior of the smaller cilinder forms the evaporation surface, wherein the exterior of the larger cilinder forms the condensation surface. In figs. 12a and 12b two possible embodiments are shows of a multi stage system wherein a first device according to the invention is coupled to a further device. Each ) device will have a specific vapour pressure. According to fig 12a the feed line of a first device is coupled to a feed line of a second device. In fig 12b an embodiment is shown wherein the discharge line of a first device is coupled to a feed line of a second device. In figure 12b a heat exchanger 8 is shown. It Is optional to remove the heat exchanger. In that case ,fpre-heated" solution will be forwarded to the feed line of the second device. Claims 1. Device for evaporation and condensation of water from an aqueous liquid by means of a temperature difference and/or concentration difference, which device comprises: - a vapour chamber; - an evaporation compartment located inside the vapour chamber, the evaporation compartment having at least one wall comprising a closed, hollow and water permeable membrane having an outer surface contacting the vapour chamber so that water from the aqueous liquid inside the evaporation compartment can evaporate from the membrane into the vapour chamber; - a condensation compartment in communication with the vapour chamber for condensing at least part of said evaporated water, characterized in that the vapour chamber is adapted to transmit direct solar radiation from outside the vapour chamber into the inside of the vapour chamber, in that the evaporation compartment is adapted to receive heat by means of said direct solar radiation transmitted into the vapour chamber, and in that the device comprises means for regeneration of the latent heat, wherein latent heat released during condensation of water vapour is used to heat the aqueous liquid. 2. Device according to claim 1, wherein the device comprises a feed line for feeding the aqueous liquid towards the evaporation compartment, wherein at least the end part of the feed line extends through the condensation compartment, part of the exterior of the feed line providing a condensation surface. 3. Device according to claim 2, wherein heating means are provided for supplying heat to the feed line, the heating means being positioned downstream of the condensation surface, 4. Device according to one of the preceding claims, wherein the device is enclosed by means of a vapour-tight enclosure, the enclosure being made of a material transparent for solar radiation, 5. Device according to claim 4, wherein a separation wall is provided inside the enclosure, the separation wall separating the enclosure into said vapour chamber and said condensation compartment, 6. Device according to claim 5, wherein the separation wall is formed as a concentrating mirror, adapted to focus solar heat onto the evaporation compartment. 7. Device according to one of the preceding claims, wherein the device comprises ventilation means, such as a fan, in order to urge vapour in the vapour room towards the condensation compartment, 8. Device according to one of the preceding claims, comprising removal means for removing non-condensable gasses from the vapour chamber. 9. Device according to one of the preceding claims, comprising at least a first and a second evaporation compartment, wherein a first part of the exterior of the second evaporation compartment provides a condensation surface in order to condense vapour evaporated by means of the first evaporation compartment 10. Device according to one of the preceding claims, wherein the membrane material is spectral selective. 11. Device according to claim 9, wherein the device comprises a layer-structure, each layer comprising a liquid channel bounded of an water impermeable foil at a first side thereof and by means of a water permeable membrane at the other side thereof and a vapour channel bounded at a first side thereof by means of said water permeable membrane and bounded at the other side of the vapour channel by means of a water impermeable foil of an adjacent layer, 12. Device according to claim 1, wherein the evaporation compartment is defined by means of a first hollow member, such as a cylinder, and a second hollow member, such as a second cylinder, positioned inside the first member, wherein the interior of the first member and the exterior of the second member define a space for containing the aqueous liquid, and wherein a first element of the exterior of the first member and the interior of the second member provides a vapour chamber, the second element of the exterior of the first member and the interior of the second member providing a condensation compartment. 13. Assembly of a first and a second device according to one of the preceding claims, wherein the devices are coupled in order to form a multi stage system, 14, Method for evaporation and condensation of water from an aqueous liquid by means of a temperature difference and/or concentration difference, o wherein one uses a device comprising - a vapour chamber; - an evaporation, compartment located inside the vapour chamber, the evaporation compartment having at least one wall comprising a closed, hollow and water permeable membrane having an outer surface contacting the vapour chamber so that water from the aqueous liquid inside the evaporation compartment can evaporate from the membrane into the vapour chamber, - a condensation compartment in communication with the vapour chamber for condensing at least part of said water evaporated from the membrane into the vapour chamber, characterized in that the vapour chamber is adapted to transmit direct solar radiation from outside the vapour chamber into the inside of the vapour chamber, aud that evaporation compartment is adapted to receive heat by means of said direct solar radiation transmitted into the vapour chamber, o wherein the aqueous liquid is fed through the evaporation compartment; o wherein the evaporation compartment is heated by direct solar radiation received by the evaporation compartment. 15. Device for evaporation and condensation of water from an aqueous liquid substantially as herein above described with reference to the accompanying drawings. 16. Method for evaporation and condensation of water from an aqueous liquid substantially as herein above described with

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