WO2002032534A1

WO2002032534A1 - Method and apparatus for purifying liquid

Info

Publication number: WO2002032534A1
Application number: PCT/DK2001/000684
Authority: WO
Inventors: Gunnar Minds; Hans MADSBØLL
Original assignee: Teknologisk Institut
Priority date: 2000-10-18
Filing date: 2001-10-16
Publication date: 2002-04-25
Also published as: AU2002210392A1

Abstract

The invention is a method and apparatus for distillation of impure liquids, wherein a first amount of impure liquid is heated in a solar collector. The heating causes at least part of said impure liquid to evaporate at a pressure below atmospheric pressure. The vapour is passed through a turbine expander doing work on that to produce shaft energy. The shaft energy is transferred to a turbo compressor, the operation of which causes evaporation of a second amount of impure liquid in a second evaporator to form a second vapour. The second vapour is passed through said turbo compressor, whereby said second vapour is mixed with said first vapour. The vapour mixture is cooled in heat transfer relation to condense the vapour mixture, before it is collected.

Description

Method and apparatus for purifying liquid

Background of the invention

The present invention relates to a method and apparatus for purifying liquids, prefera- bly water.

Facing the need for producing clean water in areas where the amount of ground water is low, different methods have been proposed to purify sea water or waste water by distillation. Many of these areas with a high need but low supply of purified water could use solar collectors as a necessary energy source to drive the water purification process. Using solar collectors is a great advantage because this energy source does not imply pollution and the costs for establishment and maintenance are low.

A method and apparatus to purify water is known from US patent no. 4,186,060, where impure liquid is evaporated in a boiler under reduced pressure. The resulting vapour is adiabatically compressed to a pressure in excess of the vaporisation pressure. A first portion of the compressed vapour is directed through a turbine, and a second portion is bypassing the turbine, after which the two portions are admixed and pass at ambient pressure through a condenser, such as the condenser side of the boiler, wherein the vapour will, upon condensing, give up at least enough thermal energy to vaporise the feed liquid. In order to fulfil the total need of energy for this process, some work is added by directly mixing the compressed vapour with a volume of hot combustion gas. An alternative for the added work is by directly driving the turbine with an externally powered engine.

The method described in the above mentioned patent seems promising at first sight, especially because stated figures have been calculated very optimistically by not taking any energy losses into consideration. However, this method, is not suited in connection with solar collectors, as the supplied hot gases for the necessary added work need a temperature of more than 120°C and a pressure of more than two atmospheres. A desalination plant for the production of fresh water has been proposed in Japanese patent JP53061565. Hot, high pressure steam is produced in an evaporator with a solar energy absorber, fed into a steam ejector, mixed with steam from the heater tank and condensed into hot water after heating the sea water in the tank. The hot water is fed back to the evaporator. Part of the hot water is fed into the heat exchanger with the cold sea water and removed as plain water. This kind of system has a very low efficiency, because the expansion of the steam through the ejector causes a high increase of the entropy.

In US patent no. 3 928 145, a process for production of power, fresh water and food from the sea and the sun is disclosed. Because this process is designed to produce power and, the production of fresh water is relatively small. It would be desirable to have a process optimised for fresh water production.

It is a purpose of the present invention to provide an improved method and apparatus for water purification.

This object is achieved with a method for distillation of impure liquids, wherein a first amount of impure liquid is heated by an external heating source in a heating unit. The heating causes at least part of said impure liquid to evaporate at a first temperature and a first pressure to form a first vapour, wherein the first pressure is below atmospheric pressure. The first vapour is passed through a turbine expander doing work on that to produce shaft energy, whereby said first vapour expands to a second pressure and cools to a second temperature. The shaft energy is transferred to a turbo compressor, the operation of which causes a third pressure to be produced in the second evaporator, whereby at least part of a second amount of impure liquid in a second evaporator to evaporate at a third temperature to form a second vapour. Preferably, the third temperature is below the first temperature and the third pressure is below the first pressure. The second vapour is passed through said turbo compressor, whereby said sec- ond vapour achieves a fourth pressure and a fourth temperature, and whereby said second vapour is mixed with said first vapour after said first vapour has passed said expander, wherein the pressure of the vapour mixture is below atmospheric pressure. The vapour mixture is cooled in heat transfer relation to condense the vapour mixture, before it is collected.

It is characteristic for the method according to the invention that the pressures in the evaporators and in the heating unit are below atmospheric pressure. The advantage is that the external heat can be supplied by solar radiation, because the evaporation temperature is lowered when the pressure is lowered. Being able to exploit solar heat is one of the intentions of this invention.

Description of the drawing

More advantages of the method according to the invention and the apparatus according to the invention will be readily understood from the detailed description in the following with reference to the drawing, where

FIG. 1 is a diagram of a first embodiment of an apparatus according to the invention,

FIG. 2 is a diagram of a second embodiment of an apparatus according to the invention, FIG. 3 is a schematic drawing of the rotor and stator arrangement in the turbine expander, FIG. 4 is a schematic drawing of the rotor and stator arrangement in the turbo compressor.

FIG. 1 is a diagram of a first embodiment of an apparatus according to the invention. In an external heating unit 2, a first amount of impure liquid is heated. As external heating unit, in principle any heating device can be used, but it is preferred to use a solar collector.

An example of impure liquid is sea-water, which will be used in the following as example, but it is understood, that the apparatus and the method according to the inven- tion can be used to purify other impure liquids, for example waste water, especially from fishing industries. Sea water is collected and enters the apparatus through conduct 100 and 102. In the external heating unit 2, the sea water is heated to a first temperature and transferred through a conduct 104 where it starts evaporating in a first evaporator 4. The boiling temperature of the liquid depends on the temperature, and might be chosen for in- stance to 50°C at a pressure of 0.12 atmospheres, or to 85°C at 0.6 atmospheres.

There are in principle no limits for pressure or temperature, though the preferred pressures are well below atmospheric pressure. If the external heating unit 2 is a solar collector, a temperature of 50°C is preferred, because higher temperatures imply un- wanted precipitation, for example of salt.

The formed first vapour from the first evaporator 4 is conducted through conduct 106 to the entrance side 10 of a turbine expander 8, passing through it, and by expansion doing work on the turbine expander 8 to produce shaft energy. Thereby, the first va- pour expands to a second pressure and cools to a second temperature at the exit side

12 of the turbine expander 8.

In the evaporator 4, salt and impurities of the water are gathered with some not evaporated water. This impurity enriched liquid is led into a second evaporator 6 through conducts 108 and 110.

Energy from rotation of the turbine in the turbine expander 8 is transferred to the rotors of a turbo compressor 10 through a coupling 14, for example through a gearing box with suitable, rotating axles.

As the turbo compressor 10 operates, a third pressure is produced in the second evaporator 6 connected to the entrance side 16 of the turbo compressor 10, whereby at least part of a second amount of impure liquid evaporates at a third temperature to form a second vapour. The third temperature is below the first temperature and the third pres- sure is below the first pressure. The produced second vapour passes through said turbo compressor 10, whereby said second vapour achieves a fourth pressure and a fourth temperature at the exit side 18 of the compressor 10. The ratio between the fourth pressure at the exit 18 of the compressor 10 and the third pressure is, for example 2, and preferably between 1.4 and 1.6.

In a conduct 132, the second vapour and first vapour are mixed and transported to a condenser 20 with heat exchanger 22 in order to be cooled and condensed. The pure liquid, for example water, is then collected after a pump 136 at the exit conduct 134.

In the condenser, heat from the vapour mixture is transferred to a cooling liquid, which preferably is impure liquid which is supplied through conduct 128 from sea water supply 100. This way, supply of impure liquid to the system implies preheating of the supplied impure liquid, which is a way to optimise the efficiency of the system.

The preheated impure liquid is supplied to the second evaporator 6 through conduct

118, 120, and 110. This supply of liquid preferably is of a much larger amount than the supply to the second evaporator 6 from the first evaporator 4.

In order to counteract an accumulation of impurities as salt in the system, part of the impure liquid is released from the system through pump 122 and conduct 124 into release conduct 126.

Additional external energy can be added to the system by supplying additional shaft energy to the expander 8, for example by a connected motor 24 as indicated in FIG. 1. This additional energy increases the production rate of destilled liquid. This engine can be electrically or fuel driven.

In table 1 below, pressures, temperatures, heat energies and flows are shown in relation to positions as indicated by numbers on FIG. 1.

In FIG. 2, a diagram of a second embodiment of an apparatus according to the invention is shown.

The second embodiment has a number of similarities as compared to the first em- bodiment. Impure liquid enters the system from liquid supply 100 through conduct

110, is heated externally 2 and evaporated in first evaporator 4. Generated first vapour transfers energy to an expander 8, whereby shaft energy is created to drive turbo compressor 10.

In the first evaporator 4, salt and impurities of the water are gathered with some not evaporated water. This impurity enriched liquid is led into a second evaporator 6 through conducts 108 and 110. Through conduct 116, impure liquid is supplied to the second evaporator 6. To reduce the amount of salt and impuritues in the liquid, part of the liquid in the second evaporator 6 can be released through conduct 118, pump 122, and conduct 124.

In analogy to the embodiment explained in FIG. 1, the operation of the turbo compressor 10 causes a third pressure in the second evaporator 6, whereby at least part of a second amount of impure liquid evaporates at a third temperature to form a second vapour, which passes through said turbo compressor 10.

In a conduct 112, the second vapour and first vapour are mixed and transported to a heat exchanger 20 in order to be cooled and condensed. The pure liquid, for example water, is then collected after a pump 136 at the exit conduct 134.

The construction of the heat exchanger and the second evaporator implies a temperature gradient from the bottom of the second evaporator 6 to the top, such that the high- est temperature will be at the top of the liquid. Thereby, the work to be supplied by the turbo compressor is minimised. The consequence is a higher efficiency of this second embodiment as compared with the first embodiment. In addition, the system is easier in construction, smaller and cheaper to produce.

In table 2 below, pressures, temperatures, heat energies and flows are shown in relation to positions as indicated by numbers on FIG. 2.

As compared to an apparatus comprising a steam ejector, as mentioned above in Japa- nese patent JP53061565, the efficiency is higher in an apparatus according to the invention, because a better optimisation can be achieved.

An example of a low pressure, high efficiency turbine 30 is shown in FIG. 3, which is a schematic drawing of the cross section of the rotor and stator arrangement in the turbine expander 8. It comprises 4 rotors, 34, 34', 34", 34'", which are connected to the same rotation axle 32 with stators 36, 36', 36", 36"' between the rotors. In table 3, parameters are presented for the different stators 36, 36', 36", 36'" and rotors 34, 34', 34", 34'". The parameters are inlet and exit radius for the hub and the tip of the rotors and stators and, furthermore, the blade angle at the inlet and the exit of each of the rotors 34, 34', 34", 34'" or stators 36, 36', 36", 36'". The position for parameters for inlet hub 38, exit hub 40, inlet tip 42, exit tip 44 are for reason of illustration indicated on FIG. 3 for the fourth rotor 34'".

Examples of parameters for the blade angle, the tip and the hub of the blades for the low pressure turbine are indicated table 3 below.

Table 3. Parameters for the turbine expander, at vapor inlet temperature of 55°C, von- densation temperature of 27°C, mass flow 1.25 kg/sec, 8000 rpm.

36 34 36' 34' 36" 34" 36'" 34 '"

Radius hub inlet [m] 0.197 0.189 0.180 0.171 0.160 0.149 0.136 0.128

Radius hub inlet [m] 0.189 0.180 0.171 0.160 0.149 0.136 0.128 0.120

Radius hub inlet [m] 0.245 0.250 0.257 0.268 0.281 0.299 0.320 0.340

Radius hub inlet [m] 0.250 0.257 0.268 0.281 0.299 0.320 0.340 0.360

Inlet blade angle [°] 0 27 0 32 0 31 0 31

Exit blade angle [°] 51 -47 57 -53 61 -58 61 -57

The turbo compressor 8 can be of axial or radial type, or a combination thereof. It is known to use radial compressors in connection with evaporation of liquids. However, an axial compressor has never before been used for water vapor at the above stated pressures as in an apparatus according to the invention. The reason for this is a general lack of knowledge and technology. Through intense study of this problem, a principle has been obtained for an axial compressor suitable for this kind of apparatus. The benefit is a higher efficiency than known aparatus for destination of impure liquids. For example, compared to the apparatus as described in Japanese patent JP53061565 and mentioned before, the efficiency is a factor of 2.5 to 3 times higher. FIG. 4 is a schematic drawing of the cross section of the rotor and stator arrangement for a turbo compressor 10 in an apparatus according to the invention. It comprises two rotors, 46, 46', which are connected to the same rotation axle 50 and with two stators. In table 4, parameters are presented for the different stators 48, 48 and rotors 46, 46'. The parameters are inlet and exit radius for the hub and the tip of the rotors and stators and, furthermore, the blade angle at the inlet and the exit of each of the rotors 46, 46' or stators 48, 48'. The position for parameters for inlet hub 52, exit hub 54, inlet tip 56, exit tip 58 are for reason of illustration indicated on FIG. 4 for the first rotor 46.

Examples of parameters for the blade angle, the tip and the hub of the blades for the low pressure turbo compressor are indicated in table 4 below, where all listed angles are root-mean-square (RMS) values.

Table 4. Parameters for the turbine expander, at vapor inlet temperature of 24°C, von- densation temperature of 29°C, mass flow 3.0 kg/sec, 8000 rpm, power 250kW.

46 48 46' 48'

Radius hub inlet [m] 0.150 0.200 0.230 0.257

Radius hub exit [m] 0.200 0.230 0.257 0.280

Radius tip inlet [m] 0.500 0.500 0.500 0.500

Radius tip exit [m] 0.500 0.500 0.500 0.500

Inlet blade angle [°] 45 -22 42 -23

Exit blade angle [°] 31 6 35 6

The low pressure turbine expander and also the turbo compressor may have a different number of rotors and stators than shown in FIG. 3 and FIG. 4.

Claims

1. Method for distillation of impure liquids, wherein

- a first amount of impure liquid is heated by an external heating source in a heating unit, said heating causing at least part of said impure liquid to evaporate at a first temperature and a first pressure to form a first vapour, wherein the first pressure is below atmospheric pressure,

- said first vapour is passed through a turbine expander, said first vapour doing work on that turbine expander to produce shaft energy, whereby said first vapour expands to a second pressure and cools to a second temperature,

- said shaft energy is transferred to a turbo compressor, the operation of which causes a third pressure to be produced in the second evaporator, whereby at least part of a second amount of impure liquid in a second evaporator evaporates at a third temperature to form a second vapour, where the third temperature is below the first tempera- ture and the third pressure is below the first pressure,

- said second vapour is passed through said turbo compressor, whereby said second vapour achieves a fourth pressure and a fourth temperature, and whereby said second vapour is mixed with said first vapour after said first vapour has passed said expander, wherein the pressure of the vapour mixture is below atmospheric pressure, - said vapour mixture is cooled in heat transfer relation to condense said vapour mixture,

- said condensed vapour mixture is collected.

2. Method according to claim 1, c h a r a c t e r i s e d in that said heat transfer rela- tion comprises heat transfer from said gas mixture to impure liquid.

3. Method according to claim 1 or 2, c h a r a c t e r i s e d in that the ratio between the fourth pressure and the third pressure is less than 2, and preferably between 1.4 and 1.6.

4. Method according to claim 1 - 3, c h a r a c t e r i s e d in that said first amount of liquid is heated by absorption of solar energy.

5. Method according to claim 1-4, characterised in that said turbo compressor comprises at least one turbine from the group consisting of a radial turbine and an axial turbine.

6. Apparatus for distillation of impure liquids comprising

- a heating unit for heating by an external heat source a first amount of impure liquid,

- a first evaporator for forming a first vapour at a first temperature and at a first pressure of at least part of said first amount of impure liquid, wherein said first pressure is below atmospheric pressure,

- a turbine expander with an entrance side and an exit side, where the entrance side is connected to said first evaporator and configured to be driven by expansion of the first vapour during passage of said first vapour from said entrance side to said exit side of said turbine expander, - a second evaporator for evaporation of a second amount of impure liquid at a third temperature and third pressure to form a second vapour,

- a turbo compressor with an entrance side connected to said second evaporator operable to cause passing of said second vapour from said entrance side of said to said exit side of said turbo compressor, - a coupling between said turbine expander and a turbo compressor for transfer of shaft energy between said turbine expander and said turbo compressor,

- a gas conductor connected to said exit of said turbine expander, to said exit of said turbo compressor and to an entrance of a condenser arranged to cause condensation by heat transfer from a mixture of said first and said second vapour.

7. Apparatus according to claim 6, characterised in that said condenser is configured to transfer heat to impure.

8. Apparatus according to claim 6 or 7, characterised in that said heating unit is a solar collector.

9. Apparatus according to claim 6-8, characterised in that turbo compressor is of the axial type.

10. Apparatus according to claim 6-9, characterised in that said apparatus comprises a motor to transfer mechanical energy to said turbine in said turbine expander.