Method for purifying water using low energy consumption

The present invention relates to a method for cleaning water with low consumption of energy.

It is necessary to clean water for a number of purposes. It may be a question of cleaning water for household use, desalting seawater for a purpose, cleaning water for use within several industrial fields, or concentrating undesired substances, i.e a by-product, to as small a volume as possible, or indeed to a solid material.

A number of arrangements are currently available for achieving these purposes. A common arrangement is a boiler arrangement in which the temperature of the water is allowed to rise until the water boils away, leaving an essentially solid material. Furthermore, the water that has boiled off can condense and be collected to be used as clean water.

A further arrangement to achieve these purposes is to make use of a method before the boiling procedure is carried out, such as membrane distillation, abbreviated as "MD". This has been known since the 1980s as a method of cleaning water.

Membrane distillation is an arrangement for allowing a first liquid to pass close to a second liquid, while not mixing with it, which arrangement comprises a number of flat sheets. A first sheet is provided with a membrane through which water can pass only when it is in the gaseous phase. Temperature differences between sheets cause the water to vaporise, to pass through the membrane and to condense onto a second wall, which is colder than the water that is to be cleaned. The surface tension of the water ensures that the water cannot pass through the membrane. The term "water passes through the

membrane" is used to denote that the water is in its vapour phase .

MD can be used in many fields. Seawater, for example, can be allowed to flow along the membrane, and the water that has vaporised to pass through the membrane and to be collected on the other side of the membrane and in order to be used as clean water. In this way, all substances, except for pure water, remain in the water that is to be cleaned, denoted hereunder as a "water residual". There are several areas of use for membrane distillation. It may conceivably be waste water that is to be cleaned, not only to clean the water in order to use clean water in industries that require clean water, but also to clean the water such that an essentially solid by-product of the undesired substances remains.

There is, however, one problem with cleaning water using MD followed by a boiling procedure, namely that the consumption of energy is high. A great deal of energy is consumed when boiling the water since the boiling point of the water must be reached and maintained.

The present invention solves this problem.

The present invention thus relates to a method for cleaning water comprising membrane distillation, abbreviated as "MD", in a membrane distillation arrangement, which distillation is caused to use differences in partial pressure with the aid of a hydrophobic membrane, through which only clean water in a gaseous state is caused to pass, whereby a water residual with an elevated level of contaminants does not pass through the membrane, and where the water residual is caused to be transferred to a boiler arrangement, and is characterised in that steam that is formed during the boiling procedure is caused to be led to a heat exchanger where the steam is

caused to condense, and in that condensed steam that is released during the condensation process in the heat exchanger is caused to be added to the water that is to be cleaned in the membrane distillation arrangement.

The invention is described in more detail below, partially in association with embodiments of the invention shown in the attached drawings, where

- Figure 1 shows schematically a method comprising membrane distillation, boiling and a heat exchanger from which heated water and condensed steam are caused to be added to the water that is to be cleaned;

- Figure 2 shows schematically a method comprising two membrane distillation arrangements, one boiler arrangement and three heat exchangers from which heated water is caused to be added to the water that is to be cleaned;

- Figure 3 shows schematically a method comprising two membrane distillation arrangements, and two heat exchangers from which heated water is caused to be added to the water that is to be cleaned;

- Figure 4 shows schematically a method comprising membrane distillation arrangements in a system, to which system steam from a boiler arrangement is caused to be added, where a heat exchanger is located such that clean water is led to it, from which heat exchanger heat from heated water is caused to be added to unclean water.

The present invention thus relates to a method for cleaning water comprising membrane distillation 1, abbreviated as "MD", in a membrane distillation arrangement 1. The distillation 1 is caused to use differences in partial pressure with

the aid of a hydrophobic membrane through which membrane only- clean water 2 in a gaseous state is caused to pass. A water residual 4, which contains an elevated content of contaminants, does not pass through the membrane. The water residual s 4 is caused to be transferred to a boiler arrangement 3.

Figure 1 shows according to the invention that steam 8 that is formed during the boiling procedure 3 is caused to be transferred to a heat exchanger 6 in order to heat unclean water, 10. The heat exchanger 6 is of a suitable known type. W The steam 8 is there caused to condense. Condensed steam 7 that is released during the condensation in the heat exchanger 6 is caused to be added to the water 10 that is to be cleaned in the membrane distillation arrangement 1.

Figure 2 shows according to the invention that several mem- 15 brane distillation arrangements Ia, Ib are placed in series, and where heat exchangers 61, 62 are located upstream of the membrane distillation arrangements Ia, Ib, through which heat exchangers 61, 62 condensed steam 7a, 7b from the heat exchanger 6 is caused to pass. The condensed steam 7a, 7b is 0 caused to be used for heating of the water that is caused to be added to the membrane distillation arrangements Ia, Ib. It is appropriate that the heating medium is water. After the condensed steam 7a, 7b has passed the heat exchangers 61, 62, what is now cold, clean water 100 is caused to be led and 5 collected as clean water 2 without contaminants. Furthermore, condensed steam 7c from the heat exchanger 6 can be caused to be led directly to clean water 2 without contaminants.

Figure 3 shows according to the invention that unclean cold water 11 is caused to be added to the heat exchanger 6 in 0 order to cool the steam 8, from which heat exchanger 6 hot water 12 is caused to be obtained. The steam 8 is in this way

caused to be cooled by the cold water 11, and hot water 12 is obtained. The hot water 12 is caused to constitute unclean water 10, which is caused to be added to the membrane distillation arrangement Ia.

Water is heated during industrial processes in a number of industries. The industry may be the paper industry, the steel industry, the foodstuffs industry, etc. Water from these industries can be cleaned according to the present invention. When it is a matter of, for example, the industrial fields named above, the water already has an elevated temperature before the cleaning process begins.

The membrane distillation 1, the boiling procedure 3 and the heat exchanger 6 may be comprised within a closed system. Unclean water 10 can in this way be caused to be added to the system, and not only clean water 2 but also a water residual 4 can be caused to be obtained from the system.

According to one embodiment, the boiler arrangement 3 can be heated with the aid of, for example, solar radiation. A reflector, for example (not shown in the drawings), directs the sunlight onto the boiler arrangement 3.

According to a further embodiment, the heat exchanger 61 can be placed before the membrane distillation arrangement 1. The water 10 that is to be cleaned is thus caused to be heated to a temperature, such as 90-95 °C. Thus the water is caused to pass the membrane distillation arrangement 1 or several membrane distillation arrangements 1 in series two or more times without it being necessary to heat the water 10 that is to be cleaned between these. The number of times that the water 10 that is to be cleaned is caused to pass the membrane distil- lation arrangement 1 depends on the quantity of water that passes through the membrane. When the quantity of water that

passes through the membrane per unit time is not sufficiently efficient, it is required that the water 10 that is to be cleaned is reheated.

According to a further embodiment, unclean water 10 is caused to be added to a pretreatment stage 5, see Figure 1. The unclean water 10 is caused during the pretreatment 5 to be degassed or treated in another suitable manner such that the properties of the water 10 are suitable for the membrane distillation 1. Agents that increase the surface tension, for example, may be added to the waste water that is to be cleaned. Waste water normally retains residues of washing agents and thus it has a level of surface tension for the water that has been reduced. The water 10 after the pretreatment 5 is caused to be added to the membrane distillation 1. The water 10 is caused to pass the membrane in its gaseous phase in the distillation arrangement 1, and clean water 2 in its gaseous phase passes through the membrane. The clean water 2 is caused to be separated from the closed system. The water residual 4 is subsequently caused to be transferred to the boiler arrangement 3 where the water residual 4 is caused to boil in order that additional steam 8 is to be caused to be released from the water residual 4. Steam 8 from the boiler arrangement 3 is caused to be transferred to the heat exchanger 6 at which the steam 8 condenses, and condensed steam 7 from the condensation is collected and caused to be added to the water 10 that is to be cleaned in the membrane distillation arrangement 1. In this manner a method is obtained that is caused to clean water with a low consumption of energy.

According to a further embodiment, water that has passed the membrane distillation 1 is caused to pass the membrane distillation 1 again. A number of cycles comprising this distil-

lation 1 are caused to continue until a desired viscosity of or concentration level of undesired substances in the water 10, known as "water residual" 4, is caused to be obtained. The water residual 4 is subsequently caused to be transferred onwards to the boiler arrangement 3.

According to a further preferred embodiment, the cleaning is caused to take place as has been described above. Figure 2 shows that a further membrane distillation arrangement Ib is in addition caused to be placed in series with the first membrane distillation arrangement Ia. Unclean water 10 is in this case caused to be added to the membrane distillation arrangement Ia. The water residual 4 from the membrane distillation arrangement Ia is caused to be added to the membrane distillation arrangement Ib that has been placed in series. Thus unclean water 10 is caused to pass more than one membrane distillation arrangement 1 before the water residual 4 is caused to be transferred to the boiler arrangement 3.

According to a further preferred embodiment, the cleaning is caused to take place as has been described above, see Figure 2. When steam 8 from the boiler arrangement 3 is caused to be transferred to the heat exchanger 6, the steam 8 will condense and condensed steam 7b will be caused to be transferred to the heat exchanger 62 in order to heat the water residual 4 that has been obtained after the membrane distillation 1. The condensed steam 7a that has been formed during the condensation is caused to be added to a heat exchanger 61, through which heat exchanger 61 unclean water 10 is caused to pass before the membrane distillation arrangement Ia. Unclean water 10 is caused to be heated in the heat exchanger 61. The water residual 4 that has been caused to pass the heat exchanger 62 is caused to be added to the membrane distillation arrangement Ib. There are in this example two membrane dis-

tillation arrangements Ia, Ib, both of which are caused to be preceded by a heat exchanger 61, 62.

According to a further preferred embodiment, the cleaning is caused to take place as has been described above, see Figure 3. In addition to this, steam 8 from the boiler arrangement 3 is caused to be transferred to the heat exchanger 6. Cold water 11 is caused to be added to the heat exchanger 6 in order to cause the steam 8 to condense. Given that the steam 8 is caused to condense, the temperature of the cold water 11 is raised to give hot water 12. The hot water 12 constitutes the unclean water 10 that is subsequently caused to be added directly to the membrane distillation arrangement Ia. The condensed steam from the steam 8 that has been condensed at the heat exchanger 6 is caused to be led to the clean water 2 with no contaminants. Furthermore, condensed steam 7b can be caused to be led through the heat exchanger 62 in order to heat further the water residual 4 before the membrane distillation Ib. What has now become cold, clean water 100 is subsequently caused to be led to clean water 2 without contami- nants. In addition, clean water is caused to be released to clean water 2 without contaminants after the membrane distillation procedures Ia, Ib.

Figure 4 shows, according to a further embodiment, how steam 8a-8d that is formed at the boiler arrangement 3 can be added to the water in the system. A mixture of steam 8a-8d and water in its liquid state has in this case obtained a density that is lower than that of water alone in its liquid state. Furthermore, at least a part of the system is extended in the vertical direction. The term "vertical" also means that an angle to a plumbline may be present. The purpose of the said mixture is to achieve circulation in that the mixture with lower density rises in a first vertical part of the system.

The steam 8a-8d condenses as the circulation proceeds and is converted to water in its liquid form again. The density falls gradually as the steam 8a-8d condenses. The circulatory water accompanies the flow in the system and falls in a sec- ond vertical part of the system. A circulation is in this way obtained in a system that thus does not require a pump to pump the water in the system around.

The steam 8a-8d can be added at any position throughout the system, such as, for example, before heating of the water 10 that is to be cleaned, before the membrane distillation Ia, or after the membrane distillation Ia but before the subsequent membrane distillation Ib. Thus a circulation of the water in the system is obtained, which circulation is described in the paragraph immediately preceding this.

According to one preferred embodiment steam 8d is added to the cleaned water 2 since the steam 8 is free of contaminants. If the steam 8a-8c is added to the system before the cleaning process of the water has been completed, the clean steam 8 will produce circulation of the water in the system.

In the case in which the steam 8 is added to the system, the clean water 2 can, for example, maintain a higher temperature, approximately 50-90 ⁰ C, than in the case in which the steam is not added to the system at all. In this case, the temperature would instead lie at around approximately 40 ⁰ C. Since the clean water without contaminants is given a higher temperature than that of the uncleaned water 10, the clean water 2 is caused thereafter to pass the heat exchanger 61, through which process the uncleaned water 10 is heated.

According to one preferred embodiment, the clean water 2 without contaminants that maintains a higher temperature than the water residual 4 is caused to oass the heat exchanσer 62

in order to heat the water residual 4 before the membrane distillation Ib.

According to a further preferred embodiment, the membrane distillation is of the type of membrane distillation that has an air gap, known as "air gap membrane distillation (AGMD)".

A number of embodiment procedures and uses have been described above. The membrane distillation Ia, Ib, the boiler arrangement 3 and the heat exchangers 6, 61, 62 can, however, be designed in any other suitable manner without deviation from the fundamental concept of the invention.

The present invention is thus not limited to the embodiment procedures described above, since it can be varied within the scope defined by the attached patent claims.