ES2473190B1 - Multi-Effect Distillation Device - Google Patents

Abstract

A high efficiency MED multi-effect distillation device that includes a high thermal conductivity wall to transfer heat between the two external faces of the high thermal conductivity wall, formed by a structure that supports at least one heat pipe, reducing substantially the heat exchange surface between two effects of the multi-effect distillation device and the necessary temperature jump between effects.

Description

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01-20-2014

DESCRIPTION

Multi-Effect Distillation Device

OBJECT

The present invention relates to a high efficiency device for the desalination of large scale salt water.

STATE OF THE TECHNIQUE

The water desalination method called Multi-Effect Distillation (MED), hereinafter MED, is currently the most efficient large-scale thermal desalination method from the thermodynamic point. It is based on a sequence of 10 cycles of evaporation of salt water and condensation of the resulting water vapor, called effects. In each of these effects, evaporation and condensation of water occurs at a pressure substantially equivalent to the vapor pressure of water at that temperature. So that the pressure and temperature are less and less from effect to effect, and the latent heat released in the condensation of an effect is used

15 as latent heat for evaporation in the following effect. In the current design of the MED, the contact walls between two effects are the walls of a long network of tubes to achieve a large heat transfer surface. The wall of these tubes acts as a condenser on its inner face, which is part of a certain effect, and as an evaporator on its outer face, which is part of the following effect.

In the MED method, latent heat is transferred through a heat conduction surface between two consecutive effects, the first effect being at a higher temperature and higher vapor pressure, than the temperature and vapor pressure of the next effect.

This heat conduction surface is normally the wall of a tube, through which the water vapor generated in the first effect circulates and is projected on this tube

25 salt water to evaporate in the second effect.

Since the water vapor in the first effect is substantially at the steam pressure of its interior temperature, the water vapor condenses on the inner side of the tube, effect condenser 1, and the latent heat released, by the change of vapor phase to water, is transferred through the tube wall being captured by the water that evaporates on

30 the outer surface of the tube, evaporator of the second effect.

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The current MED devices contain a large network of tubes inside that act as a condenser on its inner face, a condenser of the n effect, and as an evaporator on its outer face, an evaporator of the n + 1 effect.

MED devices can now transfer a large amount of latent heat between the condensation and evaporation phases that occur on the inner and outer sides of the condenser / evaporator tube. These tubes are usually aluminum alloys.

According to Fourier's law, heat transfer by conduction through a material is a function of:

Heat conduction surface

The thermal conductivity of the material

The temperature difference on both sides of the material

Material thickness

Aluminum alloys have a thermal conductivity between 200 and 250 W / (m.K). Given this conductivity, MED devices require a large heat conduction surface to achieve the necessary heat transfer in the water distillation process. Therefore, the inside of a MED device contains a huge beam of tubes. The size of the tubes requires a certain wall thickness to support the structural requirements. The result of these factors is that current MED devices require a thermal jump of about 2ºC per effect, so that since they work from a heat source of less than 70º C and a heat sink between 30º and 25ºC, temperature from the sea in warm areas, the maximum number of effects that can be inserted in practice is between 10 and 16 effects.

A heat pipe, or heat pipe, is a heat transfer device consisting of a tube closed at both ends inside which there is a working fluid, at a pressure substantially equivalent to the vapor pressure of this fluid. When the ends of this tube are exposed to two zones with different temperatures, the working fluid evaporates at the end at a higher temperature, heat source, by absorbing the heat necessary to change its phase, and the steam travels at high speed until the cold end where it condenses releasing the heat of phase change to the outside of the cold end, heat sink. The return of the condensed liquid to the hot end is done by capillarity or gravity. The heat pipe is the most efficient device for heat transfer between two points at different temperatures.

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While the thermal conductivity of aluminum and its alloys is 200 W / (mK) 250 W / (mK), and that of other metals such as copper is 400 W / (mK), heat pipes with conductivities can be constructed Effective millions of W / (mK). That is, the exchange of heat by means of a heat pipe requires an exchange surface significantly smaller than that required using aluminum alloys.

The substitution of aluminum alloys or other metal alloys as a heat conduction surface within the MED device, by a wall between effects formed by a beam of heat pipes acting as a heat conduction surface in a desalination device , it allows us to reduce the heat conduction surface needs thousands of times, it allows us to eliminate the enormous network of pipes whose walls currently act as a heat conduction surface and, in addition, it provides three important added advantages:

The possibility of working with temperature differentials between effects that are less than the current 2 degrees. In other words, more effects can be inserted between a temperature gradient and, consequently, multiply the number of effects and the production of distilled water.

Allow water vapor to flow without physical obstacles in its path between the evaporator and the condenser. Currently the water vapor must pass through the forest of tubes that form the evaporator of the MED device and its passage must be forced into other tubes to achieve its condensation.

It allows to drastically reduce the maintenance cost of the device thanks to the simplicity of the design that can be applied, by eliminating the complexity of the tube bundles.

Water has a thermal conductivity of 0.58 W / (m.K), so it is essential to reduce the layers of passive water accumulated on the surfaces of the condenser or evaporator. For this, the wall of heat pipes allows to apply constructive solutions to properly channel the water, since the ends of the heat pipes can have shapes and be covered by finishes that allow to minimize the problems of thermal resistance by unwanted accumulation of water on the faces of the heat pipes that act as an evaporator and as a condenser.

SUMMARY

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The present invention seeks to solve one or more of the drawbacks set forth above by means of a high efficiency MED multi-effect distillation device as defined in the claims.

One aspect is to insert a wall formed by at least one heat pipe containing a biphasic working fluid, namely PACT high thermal conductivity wall, as a heat transfer surface between two consecutive effects of the high efficiency MED device, to through which the heat of condensation of one effect is transferred to the heat of evaporation of the following effect; and as a heat transfer surface between the exterior and the first and last effects, Since the thermal conductivity of a wall of high thermal conductivity PACT is superior to the thermal conductivity of the current walls of aluminum alloy tubes that act as Heat transfer surface, we can reduce the heat conduction surface several times within each effect of the high efficiency MED device.

A wall of high thermal conductivity PACT comprises at least one heat pipe and a support structure for mechanically supporting the heat pipes.

The heat transfer between each effect of the high efficiency MED device occurs through the high thermal conductivity wall PACT, so that a face of the high thermal conductivity wall PACT acts as a condenser of an effect, in which It releases the heat of condensation from the phase change from vapor to liquid water, and the other side of the wall of high thermal conductivity PACT acts as an evaporator of the following effect on which salt water is evaporated.

The placement of a PACT high thermal conductivity wall between two effects of the high efficiency MED device causes the heat pipes included in the PACT high thermal conductivity wall to capture the heat released in the condensation of the first effect and transfer it to its end exposed to a lower temperature, where it is captured by the salt water that is provided on its outer face to change from the liquid phase to gas vapor, generating water vapor.

Another aspect is the terminations of the PACT high heat conductivity wall heat pipes that allow to reduce unwanted water accumulations.

Another aspect is the application of the PACT high thermal conductivity wall as a heat exchange surface between the liquids extracted from each effect and the salt water to be provided within the effects.

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So that the introduction of walls of high thermal conductivity PACT in the MED device of high efficiency allows to improve the following thermal resistance:

The resistance to thermal conduction through the entrance wall to the effect.

This resistance is improved by the high conductivity of a wall consisting of heat pipes placed on a support structure, which allows the ends of the heat pipes to not perform any structural function and can be formed by material of the minimum thickness necessary to maximize its thermal conductivity.

The thermal resistance generated by the water layer on the evaporator face.

This thermal resistance is improved by reducing the thickness of the water layer and the breakage of surface tension that is applied on the face of the ends of the heat pipes that act as evaporator of the PACT high thermal conductivity wall. These ends of the heat pipes can take forms designed to facilitate the flow of water over them and with a large heat exchange surface to facilitate evaporation. Additionally, this thermal resistance generated by the water layer on the evaporator face is improved by placing an integrated felt structure on the face of the PACT high thermal conductivity wall, which acts as an effect evaporator, so that the salt water is pour over this felt structure. The felt type structure is formed by high thermal conductivity material, such as aluminum alloys, so that it allows to multiply the heat transmitting surface that is in contact with water to evaporate and allows to break the surface tension of a layer of water, so that the drops of water become in contact with the structure of felt heat transmitting, multiplying the efficiency of the evaporator within the effect.

These solutions allow much lower resistance than those that occur when the evaporator is the surface of a bundle of tubes on which water flows forming films of a thickness that is difficult to control, which generates resistance to heat transmission.

3- Resistances in the interior trip as steam.

This resistance is improved by the simplification of the interior design of the effect because so much material surface is no longer needed. The elimination of hundreds or thousands of tubes inside each effect allows to eliminate these obstacles to the flow of

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vapor in the evaporation phase and avoids entropy increases that are also caused by forcing the circulation of steam within a tube bundle of the current MED device.

4- The resistance generated by the water layer on the condensing face.

This thermal resistance is improved by increasing the heat transfer surface that is free of water, which may come into contact with the water vapor to condense. This is achieved by making the ends of the heat pipes protrude from the support structure of the PACT high thermal conductivity wall, so that the water slides over the surface of the PACT high thermal conductivity wall and leaves the ends of the Free heat pipe to condense the next water vapor. The support surface may be designed to efficiently channel the distilled water flows produced, keeping the cold ends of the heat pipes free of layers of water that would limit heat exchange.

5- The thermal resistance generated by the tubes inside a traditional heat exchanger.

This thermal resistance is improved by inserting the wall of high thermal conductivity PACT between a tube containing a liquid from which heat is to be extracted, a tube that conducts the brine extracted from an effect or from distilled water, and a tube containing a liquid at You want to provide heat, salt water.

BRIEF DESCRIPTION OF THE FIGURES

A more detailed explanation of the device according to embodiments of the invention is given in the following description based on the attached figures in which:

Figure 1 shows a diagram of the interior of an intermediate effect different from the first and last of a current MED device, with the tube inside acting as a condenser of an effect and outside acts as an evaporator of the next effect.

Figure 2 shows a high efficiency MED device including walls of high PACT thermal conductivity between the effects, which allows to eliminate physical obstacles in the vapor path between evaporator and condenser.

Figure 3 shows the scheme of the vertical cut of a wall of high thermal conductivity PACT.

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Figure 4 shows a section of the high efficiency MED device with a wall of high thermal conductivity PACT supported by a structure that adopts a cylindrical shape on which the heat pipes are inserted, to increase the heat conduction surface.

Figure 5 shows a diagram of the PACT high thermal conductivity wall as a heat transfer surface between a tube containing a liquid from which heat is to be extracted and another tube containing a liquid to which heat is to be provided. In other words, a heat exchanger by means of the PACT high thermal conductivity wall, either current or countercurrent, to improve the heat transfer between liquids within a MED.

Figure 6 shows an integration scheme of a high efficiency MED device within a stage of an MSF desalination device.

Figure 7 shows a diagram of a wall of high thermal conductivity PACT in which, on its evaporating face, a felt structure is integrated.

DESCRIPTION OF AN EMBODIMENT

Figure 1 illustrates the part of the evaporator and condenser of two intermediate effects of a current MED device in which basically, the water vapor generated in an effect 1 is channeled into a tube 2. Said tube circulates within the following effect 3 subjected to a vapor pressure and temperature lower than the previous effect. Saltwater 4 is projected on the tube 2. Given the difference in temperature between the inside and outside of the tube 2, the wall acts as a latent heat exchanger between the steam that condenses on the inner face of the tube 5 and the seawater that evaporates on the outer face of the tube 6.

A diagram of a wall of high thermal conductivity PACT is illustrated in Figure 3 comprising a support structure 17 in which at least one heat pipe is installed. The shape and arrangement of these heat pipes can be very varied depending on the properties that you want to give the PACT high thermal conductivity wall. So that you can transfer more or less heat power, that you can channel the water generated on the face that acts as a condenser, or create a better film of water on the face that acts as an evaporator. For example, heat pipes may have the classic cylindrical or flat tube shape 19 or may have other shapes with different sections at their two ends 18; the ends of the heat pipes can be arranged so that they touch and cover the entire face of the wall of high thermal conductivity PACT 23 or they can

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be arranged so that they leave a part of the structure uncovered and only cover part of the face of the wall 22; The ends of the tubes can be flattened 21 or they can protrude from the structure 20, being able to become part of a felt-like structure, to increase the heat transfer surface and offer surfaces free of water accumulations. The PACT high thermal conductivity wall as described can have an effective thermal conductivity superior to that of aluminum alloy tubes, so it can perform the same heat transfer function with a lower surface than what is required with tubes Aluminum alloy

The structure 17 supports the forces acting between two effects and the forces acting on the evaporator and the condenser, allowing the faces of the external walls of the heat pipes, at their ends, should not exert structural functions and may have thicknesses minimums that offer minimal resistance to thermal conductivity. In the absence of this support structure, if the outer walls of the heat pipe had to have an area equivalent to the surface of the section of the MED device, then the separation walls should withstand mechanical loads, requiring a greater thickness. Thickness that would decrease the thermal conductivity of the wall and lead to the need for a larger surface of thermal exchange. The problem of the thickness of the walls of separation between effects is solved with the wall of high thermal conductivity PACT, thanks to the incorporation of a support structure, on which a series of heat pipes are housed whose ends can have reduced walls thickness because they do not need to withstand structural forces.

A diagram of the high efficiency MED device is illustrated in Figure 2 in which, for the purpose of illustration, we only reproduce three effects, the two extreme effects and an intermediate effect. The first effect, effect 1, receives heat from a heat source and the last effect, effect 3, releases it in a heat sink. The second effect represents an intermediate effect and the high efficiency MED device can contain as many intermediate effects similar to effect 2, as the thermal jump between the heat source and the heat sink allows. At the ends of the high efficiency MED device, and between each effect, a wall of high thermal conductivity PACT is sandwiched. In this figure there are a total of four walls of high thermal conductivity PACT, so that:

The first effect receives heat from an external heat source (t1), through the high thermal conductivity wall PACT 7,

The internal temperature of the first effect (t2). is higher than the temperature of the second effect (t3).

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In the first effect

Deaerated seawater is introduced into the effect, by means of a valve 9, or another mechanism that allows to provide a controlled flow so that it forms a thin layer of water at a temperature t2, on the inner face 10 of the high wall PACT 7 thermal conductivity, which acts as an evaporator of the first effect

This high thermal conductivity wall PACT 7 is formed by a support structure that incorporates at least one closed heat pipe that provides the high conductivity of the high thermal conductivity wall PACT to transmit the heat released on the face 11 towards the face 10 of the high thermal conductivity wall PACT.

So that by dosing the seawater from the valve 9 on the face 10, and since the internal pressure of the effect first is substantially the vapor pressure at the temperature t2, the seawater absorbs the heat flux transmitted through the wall 7 PACT for its change of state to water vapor 12, without change of temperature.

The water vapor 12, at the moment in which it is generated, expands and creates a pressure wave that runs unimpeded through the interior space of the first effect that is now free of physical obstacles, since the lattice of tubes

Water vapor 12 reaches the wall of high thermal conductivity PACT 13 through which heat is transferred between effect 1 and effect 2,

Effect 2 is at a temperature t3, less than t2. So that the water vapor 12 that reaches the inner face 14 of the high thermal conductivity wall PACT 13 will condense, releasing latent heat of condensation through the high thermal conductivity wall PACT 13 to the inner face 15 of the effect 2. So that face 14 is the condenser of effect 1 and the high thermal conductivity wall PACT 13 transfers the latent heat to its face 15 which is the evaporator of effect 2.

The steam generated within effect 2 travels to the wall of

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condensation and so on for all intermediate effects. In figure 2 an intermediate effect - effect 2- has been represented, and can be repeated n times until the last effect is reached, in which the heat of condensation is exchanged with an external heat sink.

The increase in the conductivity of the wall of high thermal conductivity PACT allows to eliminate the need for large surfaces of tubes with a lower conductivity, aluminum alloy tubes, and the following effects are achieved in the high efficiency MED device:

The elimination of physical barriers in the path of displacement of water vapor, especially those caused by horizontal, vertical or plate pipes, thereby eliminating the consequent inefficiencies of steam displacement.

The reduction of the generation of layers of water that act as an insulator, typical of the displacement of a vapor that condenses inside a tube and the elimination of water and steam plugs that form inside a condensing tube.

The reduction of the generation of layers of water that act as an insulator on the outer face of the pipe network. These layers of water act as thermal insulator and limit heat conduction.

It allows the application, on the external faces of the heat pipes and of the exposed part of the structure of the wall of high thermal conductivity PACT, of all the knowledge about surface treatment and textures to avoid the formation of layers of water that act as an insulator on the condenser and minimize its thickness on the evaporator.

In case the heat pipes necessary for a certain heat transfer did not fit on a flat wall separating effects, in Figure 4, an example is illustrated in which the heat pipes can be arranged on a polymorphic structure that offers a larger heat conduction surface, even forming a cylindrical structure 16, which supports a series of heat pipes, within which the water vapor to condense circulates.

In the high efficiency MED device that includes high conductivity walls PACT for the exchange of heat between fluids of different temperatures, for example, flowing against the current or current, see figure 5. That is, to transfer the heat of the liquids

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extracted from each effect, brine and distilled water, to the feed liquid of the following effect, salt water, so that the total heat losses of the high efficiency MED device are reduced.

The thermal resistance generated by the water layer on the evaporating face of a wall of high thermal conductivity PACT, is also improved by placing an integrated felt structure on the ends of the heat pipes. Figure 7 illustrates how a felt structure 27 is placed on the face of the high thermal conductivity wall PACT that acts as an evaporation surface of an effect, so that this felt contains salt water that is provided in a controlled manner by one end. 28 and whose excess is collected at another end 29, so that the felt-like structure 27 is maintained with the ideal salt water level to facilitate evaporation and salt water with higher concentration accumulates at the bottom of the collection container 29, from where it is extracted in a controlled way. The felt type structure 27 is formed by thermal conductivity material, such as aluminum alloys, so that it allows multiplying the heat transmitting surface that is in contact with water to evaporate and allows the surface tension of a water layer to break, so that the drops of water become in contact with the structure of felt heat transmitter, multiplying the efficiency of the evaporator within the effect.

The high conductivity of the wall of high thermal conductivity PACT allows to work with lower temperature differential. The thermal efficiency that is achieved with a wall of high thermal conductivity PACT as a heat conduction surface, allows to produce distilled water from a heat transfer between a heat source and a heat sink with small temperature differentials and from heat source temperatures lower than those required in current MED devices. As for example, from warm seas water whose temperature is between 25º to 30ºC as a heat source and use as a sink a cold source such as a liquid gas vaporizer.

The high conductivity of the PACT high thermal conductivity wall allows to work with a lower temperature differential and this also allows the integration of a high efficiency MED device with high PACT thermal conductivity walls between thermal jumps of existing industrial processes. Figure 6 illustrates the case of integration of a high efficiency MED device 26 with high thermal conductivity walls PACT between the heat source constituted by the steam 24 produced in a stage of an internationally known multi-stage evaporation distillation device such as Multistage flash distillation MSF, and the heat sink currently constituted by the tubes in which salt water circulates at a lower temperature in order to condense the water vapor

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over said tubes and heat the sea water that circulates inside. At each stage of the MSF device, the temperature jump between the water vapor and the salt water currently circulating in the tubes is around 10 ° C. So that a high efficiency MED device with high thermal conductivity walls 5 PACT 26 could be inserted between the heat source 24 constituted by the water vapor within a stage of an MSF device at a temperature t1 and the heat sink 25 constituted by salt water at a temperature t5 So that the network of pipes that MSFs currently need is eliminated, the water film on the condensation surface of an MSF stage is minimized using the improvements that a high wall allows

10 PACT thermal conductivity, the increase in entropy associated with the travel of steam within an MSF stage is reduced and with this, the distilled water generation capacity of the current MSF devices is multiplied several times, since each effect of the MED device Highly efficient interleaving manages to generate a similar amount of distilled water than that achieved in a stage of the MSF device.

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Claims (9)

one.

A high efficiency MED multi-effect distillation device; characterized in that the multi-effect distillation device includes at least one wall of high thermal conductivity to transfer heat between the two external faces of this wall of high thermal conductivity, formed by a structure that supports at least one heat pipe to which it is mounted a felt structure (27) on the outside of the ends of the heat pipe.

2.

Device according to claim 1; characterized in that a high thermal conductivity wall of the high efficiency MED device is configured to transfer incoming heat to a first effect, such that the outer face of this high thermal conductivity wall is in thermal contact with a heat source and the Inner side of this high thermal conductivity wall supplies heat to act as an evaporator of the first effect.

3.

Device according to claim 1 or 2; characterized in that a wall of high thermal conductivity is configured to transfer heat of the last effect, so that the outer face of this wall of high thermal conductivity is in thermal contact with a heat sink and the inner face captures heat to act as a condenser.

Four.

Device according to claim 1 or 3; characterized in that at least one wall of high thermal conductivity is adapted to work as a heat conduction surface between consecutive effects, so that in the face located in an effect it acts as a condenser and in the face located in the next effect it acts as an evaporator, so that the latent heat released in the condensation of the steam in one effect, is transferred to the consumption of latent heat in the evaporation of the next effect.

5.

Device according to claim 1 or 4; characterized in that the end of at least one heat pipe that is part of the wall of high thermal conductivity, located on the face of the wall that acts as a condenser, protrudes from the support structure, being able to become part of the type structure felt, to increase the heat transfer surface and offer surfaces free of water accumulations.

6.

Device according to claim 4 or 5; characterized in that the support structure of the heat pipe adopts a different type of shape to a vertical flat surface, an oval or cylindrical shape creating a large tube inside which the water vapor generated in an effect circulates, so that can increase the number of tubes

14 of heat supported to increase the heat conduction surface with the following effect. 7. Device according to claim 4 or 6; characterized in that the heat source has the ambient temperature of the sea water and the heat sink has a 5 lower temperature, substantially the temperature of a liquefied gas reevaporization plant.

8.

Device according to claim 1 or 7; characterized in that distilled water is generated from an aqueous solution other than seawater.

9.

Device according to any claim 1 to 6; characterized in that the

10 high efficiency MED device with at least one wall of high thermal conductivity is configured to be intercalated in an MSF multi-stage evaporation distillation device, the high efficiency MED device having the water vapor of a stadium of initial heat source as the initial heat source an MSF multistate evaporation distillation device, and the final heat sink of the high efficiency MED device with high walls 15 thermal conductivity is salt water, which is heated before being introduced into the MSF multi-stage evaporation distillation device. fifteen DRAWINGS FIG. 1 image 1 FIG. 2 17