Method and arrangement for purifying water by membrane distillation.

The present invention relates to a method comprising a membrane distillation arrangement in association with the cleaning of water.

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 byproduct, 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 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.

The Swedish patent number 8002233-8 reveals a further arrangement to achieve these purposes, which 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 Swedish patent number SE 507728 reveals a system for heat transport comprising a first heat exchanger, a supply line that is intended to lead a heat-bearing fluid in two parallel columns of fluid to the first heat exchanger in order there to absorb heat. Furthermore, SE 507728 reveals a second heat exchanger that has been arranged to vaporise fluid in a partial flow from the first column of fluid to the second column of fluid, whereby gas bubbles flow up through the first column of fluid and achieve flow of the fluid.

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. 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 byproduct of undesired substances remains .

A pump is normally used in the arrangements described above in order to pump the water around.

One problem with using pumps in order to move the water in the arrangements is, in the first place, that electrical energy is required in order to operate the pump. Cleaning of water is needed in countries, for example, in which the electricity supply network has not been extensively developed and in countries in which electricity is expensive. The pump, in the second place, constitutes a risk factor in the cleaning system. The cleaning system will cease to function in the

event that the pump breaks down, whereby fresh water cannot be produced. Thus, regular maintenance of the pump is required in order to avoid lengthy and unnecessary interruptions in its operation. The maintenance constitutes also an expense in the system.

The present invention solves these problems.

The present invention thus relates to a method for cleaning water comprising membrane distillation, abbreviated as "MD", which distillation is caused to use differences in partial pressure with the aid of a hydrophobic membrane through which membrane only clean water in a gaseous state is caused to pass, whereby a water residual, which contains an elevated content of contaminants, does not pass through the membrane, and it is characterised in that the water is caused to flow in a first circuit comprising a membrane distillation arrangement, in that the water in the first circuit is caused to pass a first heating arrangement, which is caused to heat the water in the first circuit, in that at least a portion of the first circuit is extended in the vertical direction, in that a closed additional second circuit is connected to the vertical part of the first circuit and in that the water in the additional second circuit is caused to be heated to a temperature that is higher than that of the water in the first circuit. The term "vertical" as used above and throughout this document includes that case in which an angle with a plumbline is formed.

The invention relates furthermore to an arrangement of the type and having the principal characteristics that are specified in claim 9.

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 a membrane distillation arrangement and a heating arrangement through which a first circuit of water is arranged, and it shows furthermore a second heating arrangement through which a second circuit of water is arranged;

Figure 2 shows schematically a membrane distillation arrangement, a boiler arrangement, and a heat exchanger; and it shows how heat from the heat exchanger is caused to be transferred to the water that is to be cleaned in the membrane distillation arrangement;

Figure 3 shows in detail the second circuit connected to the first circuit.

Figure 4 shows a membrane distillation arrangement and a heating arrangement through which a first circuit of water is arranged, and it shows furthermore a second heating arrangement through which a second circuit of water is arranged, and it shows a boiler arrangement from which steam is supplied to at least one of the first and the second circuits;

Figure 5 shows a membrane distillation arrangement and a heating arrangement through which a first circuit of water is arranged, and it shows furthermore a second heating arrangement through which a second circuit of water is arranged, and comprises a heat exchanger located such that clean water and steam from a boiler arrangement can pass the heat exchanger, from which heat exchanger heat can be supplied to the membrane distillation arrangement, and at least one of the first and the second heating arrangements.

The present invention thus relates to a method for cleaning water comprising membrane distillation, abbreviated as "MD". The distillation is caused to use differences in partial pressure with the aid of a hydrophobic membrane through which membrane only clean water lβ in a gaseous state is caused to pass. A water residual 15, which contains an elevated content of contaminants, does not pass through the membrane (not shown in the drawings) .

Figure 1 shows, according to the invention, that the water is caused to flow in a first circuit 2 comprising a membrane distillation arrangement 1. The water in the first circuit 2 is caused to pass a first heating arrangement 4, which is caused to heat the water in the first circuit 2. At least a portion of the first circuit 2 is extended in the vertical direction VL. A closed, additional, second circuit 3 is connected to the vertical part of the first circuit, and the water in the additional, second circuit 3 is caused to be heated to a temperature that is higher than that of the water in the first circuit 2.

According to a preferred embodiment, the additional, second circuit 3 comprises a separate, second heating arrangement 5.

According to a further preferred embodiment, at least one of the first heating arrangement 4 and the second heating arrangement 5 is a solar collector.

Figure 2 shows, according to a further preferred embodiment, that at least one of the first heating arrangement 4 and the second heating arrangement 5 in Figure 1 is caused to be heated by heat 7 formed in a heat exchanger 6, in which steam 8 from a boiler arrangement 9 is arranged to condense. In the design shown in Figure 2, the purpose of the boiler arrangement 9, which may be of any known suitable type, is to vapor-

ise the water, whereby the concentration of the contaminants increases in the remaining water. The vaporised water 8 is caused to condense in a heat exchanger 6 that is separated from the boiler arrangement 9. The condensed water can be caused to be added to the unclean water 10, or it can be added directly to the clean water 16. Thus, at least one of the first heating arrangement 4 and the second heating arrangement 5, shown in Figure 1, is arranged to be heated by heat 7 formed in the heat exchanger 6, see Figure 2.

According to a further additional preferred embodiment, two or more circuits 2, 3 are caused to be connected together in series between two or more membrane distillation arrangements 1. Two or more heating arrangements 4, 5 are caused to be located along the circuits 2, 3. Thus, the circuits 2, 3 are arranged connected in series between two or more membrane distillation arrangements 1 with two or more heating arrangements 4, 5 located along the circuits 2, 3.

According to a further preferred embodiment, unclean water 10 is caused to flow into the membrane distillation arrangement 1 in which clean water 16 free of contaminants is caused to be withdrawn out from the membrane distillation arrangement 1. The water residual 15 is caused, instead, to flow in the first circuit 2. A first heating arrangement 4 is located along the vertical part VL of the first circuit 2, through which heating arrangement 4 a portion of the first circuit 2 is caused to be located. The first circuit 2 is caused to be extended in the vertical direction VL. A second heating arrangement 5 is located from a part of the vertical part VL of the first circuit 2, through which heating arrangement 5 a portion of the second circuit 3 is located. The water in the first circuit 2 is caused to flow into the second circuit 3 due to a difference in density at a combined inlet to and

outlet from the second circuit 3 relative to the first circuit 2. The water is in this way caused to flow into the second circuit 3, which water is caused to be heated in the second heating arrangement 5 to a temperature that is higher than the temperature of the water in the first circuit 2, such that water or a mixture of gas 12 and water 11 in its liquid form is obtained, see Figures 1 and 3. This mixture has a density that is lower than the density of the water in the first circuit 2. A flow is in this way established. The mixture flows out from the second circuit 3 and back to the first circuit 2, in which circuit 2 the mixture rises. The gas 12 condenses and is converted back to water 11 in its liquid form and accompanies the circulating water 11 in its liquid form. Thus the gas 12 does not leave the system.

Furthermore, a flow of water is caused to arise from the left column 13 to the right column 14 in Figure 1, and thus from the bottom upwards in the left column 13 and from the top downwards in the right column 14. An important parameter in this context is the volume fraction of gas 12, since this part affects significantly the density, and thus the difference in density, of the water along the first circuit 2. Furthermore, the possible pumping height is determined by differences in density in the first circuit 2 along the water in the first circuit 2.

In addition to this, the water in the first circuit 2 is caused to be heated in the first heating arrangement 4 such that it at this early stage not only preheats the water before the onwards flow of the water in the second circuit 3, but also reduces the density sufficiently for the water that is caused to pass the first heating arrangement 4 to rise in the left column 13.

Thus, the water in the first circuit 2 is caused to be pumped around with the aid of a difference in density of the water, without a pump being required. Thus, no supplied electrical energy is required to pump the water around.

Water that is pumped around in the first circuit 2 is caused to be cleaned in the membrane distillation arrangement 1 either once, twice or more times. When the distillation takes place two or more times in the arrangement 1 and when pumping of water takes place in the first circuit 2, equally large fractions of clean water 16 and of water residual 15 are caused to leave the circuits 2 and 3 through the membrane distillation arrangement 1. Unclean water 10 is added to the first circuit 2 preferably at approximately the same magnitude of fraction as that caused to leave the circuits 2 and 3 and the membrane distillation arrangement 1 in the form of clean water 16 and water residual 15.

Figure 4 shows, according to a further preferred embodiment, that steam 8 from a boiler arrangement 9 is caused to be added to the water in at least one of the first circuit 2 and the second circuit 3. Thus, water is obtained in the circuits 2, 3 that has a lower density, also in this alternative.

Figure 5 shows, according to a further preferred embodiment, that at least one of clean water 16 and steam 8 from the boiler arrangement 9 can be led to a heat exchanger 63. Hot water 59, 60, 61, 62 is led from the heat exchanger 63 back to the system. Hot water 59 can be led to the unclean water 10, hot water 60 can be led directly into the circuit 2, hot water 61 can be led to the first heating arrangement 4 and hot water 62 can be led to the second heating arrangement 5. Any or all of these uses of the hot water may be applied together. The number of locations to which hot water can be

led may vary. Furthermore, clean water 17 that is cold, at least relative to the clean water 16, is obtained.

Through the present invention, thus, can, for example, fresh water be produced from seawater without any other energy than heat energy from the sun being supplied to the system.

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 1, the first circuit 2, the second circuit 3, the first heating arrangement 4 and the second heating arrangement 5 may, however, be designed in another suitable manner without deviating 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.