WO2012011821A1

WO2012011821A1 - Method and device for low energy purification of water

Info

Publication number: WO2012011821A1
Application number: PCT/NO2011/000211
Authority: WO
Inventors: Tore Skjetne; Sten-Roger Karlsen; Bjørn ARSTAD
Original assignee: Ecowat As
Priority date: 2010-07-23
Filing date: 2011-07-22
Publication date: 2012-01-26
Also published as: NO20101059A1; NO332199B1

Abstract

The invention relates to a method for purifying polluted water comprising supplying the polluted water. The pressure and temperature conditions are controlled in the hydrate forming reactor so as for forming hydrates. The hydrates and thereby the enriched polluted water are transported to a separator for separating the hydrates from the polluted water. The pure water is conducted to a pure water reservoir wherein the enriched polluted water is transported from the separator to a high osmotic potential side of at least one saline energy cell and water of lower osmotic potential is transported to a low osmotic potential side of at least one saline cell and converting energy represented by osmotic pressure arising due to the difference in osmotic potential in the saline cell to mechanical energy.

Description

Method and device for low energy purification of water.

The present invention concerns a method and a device for low energy purification of water. The need for purification of water can arise in different connections . The purpose can be to provide pure drinking water for consumption, to prevent spill of water-soluble or water transported contaminants, to get legally acceptable low concentration of chemical components in effluents from an industrial process, or to obtain desired components contained in the water, but in concentrations that are too low for general recovery. Primarily there are two main reasons for conducting water purification; either for production of pure water or for recovery of substances that are dissolved in the water. Different methods exist for purification of water;

filtration, distillation, centrifugation, etc. Many methods are excellent for certain contaminants, but are inefficient for others. Few methods are good for all types of water- soluble contents .

Many of these efficient water purification processes may be high energy demanding .

Prior art

Different processes for purifying water are discussed in the inventors patented invention NO321097, which describes equipment and a method for treating polluted water by forming hydrates and draw out pure water after dissociation of the formed hydrates, the so-called Ecowat process. The process results in pure water and a brine that may be disposed or, when the Ecowat process is used within the oil production process, re-injected to the well. When the brine should be reinjected it has to be mixed with sea water to balance the ions in the brine and precipitate potentially problematic low- soluble scale-forming salts like BaS04 , CaSo4 etc before reinjection takes place.

NO321097 uses separate heat exchangers to handle the energy from the exotherm process of hydrate forming.

US4781837 relates to concentration of liquid utilizing the difference of osmotic pressure between to liquids and to recover a solvent from a solution using the opposite process. A process is also provided to recover part of the energy available between the two fluids.

N0314575 concerns a semi-permeable membrane consisting of one thin layer of a non-porous material (the diffusion skin) , and one or more layers of a porous support material (the porous layer) . Further a method for providing elevated pressure by osmosis as well as a device for providing an elevated osmotic pressure and electric power are described.

Problems to be solved

Effective water purification processes are energy demanding processes. For the Ecowat process the energy consumption will be about 1-3 kWh/m3 of pure water to run pumps and motors. Recompression of the hydrate forming compound will need another 6-9 kWh/m3 pure water. Pure water and pure emission to the lowest cost will always be a demand.

Especially, by purification of water in connection with petroleum exploitation, the energy to run the process

generally may be produced in a gas turbine with increasing emission of hydrocarbons to the air. One solves the problem with hydrocarbons in the outlet by the purification of the water, but contaminates the air using the gas turbine. Injection of brine to maintain hydraulic pressure inside reservoirs is commonly used for enhanced oil and gas recovery. Typically the injection water is pumped into the lower parts of the reservoir into the water zone. If untreated seawater is used for pressure support, there is an increased risk for permeability changes in the porous structures in the reservoir formation. The permeability change occurs due to ionic conditions. Seawater contains sulphate ions, while the water in the reservoir (the formation water) often contains cations like barium, strontium and calcium. In combination these ions form precipitates, sealing off pore throats whereby further injections may be difficult, if not impossible. One solution to this challenge is desulphuring plants where sulphate is removed from seawater prior to injection.

During petroleum production water is co-produced. The ionic composition of the produced water is almost identical to the reservoir water. By treating produced water in the Ecowat process the concentration of ions in the concentrate will be increased multiple times. When mixing the concentrate with seawater some salts will rapidly form precipitate and become solids (like SrS04, BaS0 , CaS04) . The precipitates may be extracted from the injection water before injection and thereby removing the risk of downhole permeability changes. In addition, other ions from the concentrated formation brine still in solution will help stabilizing clays and shales in the reservoir, thus preventing flow problems during water inj ection . When running the Ecowat process, for any purification purpose, the concentrated brine holds a potential energy by having a high concentration of ions related to other nearby available liquid sources as sea water, river water, fresh water, formation water etc. At a given temperature the osmotic pressure in a system is given by the molality the salt in water has . When gas hydrate forms the remaining water becomes more concentrated, which means the the molality rises. Hence, during a treatment of e.g. seawater or some other source of water, the Ecowat process can rise the concentration of salts, some five times or more, depending on the original

concentration of salts in the water. Then the osmotic pressure is rising accordingly. Energy can be collected when this pressure is released. For a rise of five times the potential energy is around 5 kw per liter released concentrated water. This is valuable energy which is desirable to maintain in the system .

Short summary of the invention

A solution for the above mentioned problems is the present invention. The present invention is a method for recapturing energy while purifying polluted water (3, 31) , comprising the steps of :

- supplying the polluted water (3, 31) from an inlet for such polluted water andproviding a hydrate forming compound (8, 36) and hydrate seeds (32) to the polluted water (3, 31) in the hydrate forming reactor (C, T) ,

- controlling pressure and temperature conditions in the hydrate forming reactor (C, T) so as for forming hydrates (4, 33) ;

- transporting the hydrates (4, 33) and thereby the enriched polluted water (10, 38) to a separator (D, U) ;

- separating the hydrates (4, 33) from the enriched polluted water (10, 38) in a separator (D,U) ;

- transporting the resulting separated hydrates (6, 34) to a dissociation tank (F, W) , and

- dissociating the transported, separated hydrates (6, 34) so as for separating pure water (9, 37) and the hydrate forming compound (8, 36) with consecutive return of the hydrate forming compound (8, 36) via a pump (P) to the hydrate forming reactor (C, T) , and conducting the pure water (9, 37) to a pure water reservoir (K, X) wherein the novel features is -the enriched polluted water (10, 38) is transported from the separator (D, U) to a high osmotic potential side (RH) of at least one saline energy cell (R) and water of lower osmotic potential is transported to a low osmotic potential side (RL) of at least one saline cell (R) ;

- converting energy represented by osmotic pressure arising due to the difference in osmotic potential in the saline cell (R) to mechanical energy.

Another aspect of the invention is an apparatus for purifying polluted water (3, 31) from an inlet for the polluted water (3, 31), comprising

- a hydrate forming reactor (C, T) arranged for forming hydrates (4, 33) from the polluted water (3, 31),

- a hydrate forming compound (8, 36) for being added to the polluted water (3, 31),

- a control system (CS) for sensing and controlling pressure and temperature conditions in the hydrate forming reactor (C, T) so as for forming hydrates (4, 33);

- a separator (D, U) for receiving and separating the hydrates (4, 33) and enriched polluted water (10, 38) ;

- a dissociation tank (F, W) for receiving and dissociating resulting separated hydrates (6, 34) for separating pure water (9, 37) and the hydrate forming compound (8, 36),

- a return line (L8, L36) for returning the hydrate forming compound (8, 36) via a pump (P) to the hydrate forming reactor (C, T) , and

- a pure water outlet line (L) for the pure water (9, 37) to a pure water reservoir (K, X) wherein the novel features are - a mixer (IM) on the return line (L8, L36) between the pump (P) and the hydrate forming reactor (C, T) , the mixer (IM) arranged for injecting hydrate seeds (32) to the polluted water (3, 31) in the hydrate forming reactor (C, T) for improving hydrate formation,

- one or more saline cells (R) with a high osmotic potential side (RH) for receiving the enriched polluted water (10, 38) from the separator (D, U) , and a low osmotic potential side (RL) for receiving water of lower osmotic potential;

- a converter (RT) for converting osmotic pressure arising due to the difference in osmotic potential in the saline cell (R) to mechanical energy.

Additional specific features to the invention are given in the attached dependent claims.

Advantages

Recapture of part of energy

A first advantage with the present invention is that the process utilizes the osmotic potential which is latent in the concentrated brine resulting from the purifying process. This energy may be recovered and may be returned into the process as either electrical power or direct mechanical energy. When transforming this energy by a generator into electrical power it becomes easy to control and adjust the power required by any desired power consuming parts internally as for running pumps, valves etc, or for other process consumptions. This reduces the need for externally supplied energy. The present invention thus provides a contribution to clean-energy technologies by saving energy while reducing water pollution and also indirectly reducing C02 emission due to reduced gas turbine consumption. Integrated precipitation

Another advantage is that for the case of reinjection into oil reservoirs, the precipitation and the energy extraction may take place in the same process step. The low salinity flow must be carefully monitored to make sure that the precipitates are formed downstream of the osmotic barrier.

Reduced loss of energy

A third advantage of the invention is the combined heat exhanger-reactor tank (Q) . When build the dissociation reactor tank together with the hydratisation tank there will be a reduced loss in energy due to shorter energy transport .

Improved heat exchange

A fourth advantage is the form of the pipes used in at least one of the reactortanks i.e. at least the one part of the heatexchanger . The special formed pipes are multiple direction corrugated to increase the contact surface and the turbulence of the fluid and/or hydrate flow on both sides of the pipe wall. The heat transfer may be doubled or tripled when using corrugated pipes . Accelerated hydrate formation

A fifth advantage of the invention is the way of mixing the hydrate forming agent (a gas) with the polluted water. The mixing takes place in a mixing unit that produce small gas bubbles and water droplets. This increases the reaction speed with a minimum of power input.

Improved hydrate forming gasses

A sixth advantage is the use of "design gasses" as hydrate forming agent. Hydrate forming compounds are characterized by the fact that they are relatively small, non-polar molecules, primary lower hydrocarbons , e . g . methane , ethane and propane , but also Carbon dioxide, Nitrogen, Oxygen, dihydrogen

sulphide, halogenated hydrocarbons wherein halogen is selected among Fluorine and Chlorine, Tetrahydrofuran, ethylene oxide, noble gases, such as Helium, Neon, Argon, Xenon, Krypton, sulphur hexafluoride and dinitrogen oxide may be used.

Due to favourable pressure and temperature properties of Xenon this might be a suitable gas to use to minimize the energy consumption of the process. However Xenon is rare and

expensive.

Many natural gasses are well suited for the purpose of forming hydrates but due to the explosion risk they are less

favourable. When designing a composition of natural gas and C02 as a "cover" gas one may reduce the explosion risk but keep the good properties that save power input. Propane and iso-butane are such favourable natural gasses well suited for designing. Blending 4% propane with 96% C02 keeps the pressure properties of propane and the delta T properties of C02.

Higher amounts of propane may improve the hydrate forming process but gives a higher explosion risk.

Short figure captions

Figure 1 shows a flow sheet of an embodiment of the background art, the so-called Ecowat process for purification of produced water.

Figure 2 shows a flow sheet of an embodiment of the background art Ecowat process for purification of water from a water source to drinking water.

Figure 3 shows a flow sheet of an embodiment of the background art Ecowat process for purification of gas/air. Figure 4 shows pressure/temperature relations for dissociation of hydrates, at different salt concentrations, with C02 as hydrate forming compound. Hydrate formation is kinetically controlled and takes place at conditions above/to the left of the respective curves .

Figure 5 shows pressure/temperature relations for dissociation of hydrates, at different salt concentrations, as for Fig. 4, but with CH4 as hydrate forming compound.

Figure 6 shows pressure/temperature relations for dissociation of hydrates, at different salt concentrations, similar to Fig. 5, but with C2H6 as hydrate forming compound. Figure 7 shows a flow sheet for a low energy purification of water according to one embodiment of the invention.

Figure 8 shows parts of Fig. 7 with illustrations of details in inserts 8a and 8b

Figure 8a shows an illustration of corrugated pipes in the heat exchanger (Q)

Figure 8b shows a longitudinal section of the mixer (IM)

Figure 9 shows a flow sheet of an alternative embodiment of the purification process ahead of the osmotic energy

recovering step with hydrate forming unit and dissociation unit separated instead of integrated in the heat exchanger (Q) and additional degassers for the different process steps.

Description of embodiments of the invention

The present invention relates to a low energy method for purifying polluted water and a device arranged for this process. According to the invention the polluted water (3, 31) is lead trough pipes from an inlet for polluted water then supplying the polluted water (3, 31) to a hydrate forming reactor (C, T) . A hydrate forming compound (8, 36) and hydrate seeds (32) are introduced to the polluted water (3, 31) in the hydrate forming reactor (C, T) . The pressure and temperature conditions in the hydrate forming reactor (C, T) are

controlled so as for forming hydrates (4, 33) . The so formed hydrates (4, 33) and thereby the enriched polluted water (10, 38) are transported to a separator (D, U) which separates the hydrates (4, 33) from the polluted water (10, 38) . The resulting separated hydrates (6, 34) are then transported to a dissociation tank (F, W) . Here occurs the dissociating of the transported, separated hydrates (6, 34) so as for separating pure water (9, 37) and the hydrate forming agent (8, 36) .

Subsequently the hydrate forming agent (8, 36) is returned via a pump (P) to the hydrate forming reactor (C, T) . The pure water (9, 37) is conducted to a pure water reservoir (K, X) . The enriched polluted water (10, 38) is transported from the separator (D, U) to a high osmotic potential side (RH) of at least one saline energy cell (R) . Water of lower osmotic potential is transported to a low osmotic potential side (RL) of the at least one saline cell (R) . Energy represented by osmotic pressure arising due to the difference in osmotic potential in the saline cell (R) is converted to a mechanical energy .

In an embodiment of the invention the energy arising due to the difference in osmotic potential in the saline cell (R) is used for running at least the pump (P) either directly, mechanically via axles, belts or gears, or indirectly

In an embodiment of the invention the osmotic pressure formed in the saline cell (R) is utilized in a turbine (RT) or motor running a generator (RG) for producing electrical energy. The convertor (RT) may in an embodiment be a fluid driven motor.

In an embodiment of the invention the heat generated during hydrate forming (4, 33) is heat exchanged with the

dissociation process of said separated hydrates (6, 34) in the heat exchanger (Q) . The heat exchanger (Q) comprises the hydrate forming tank (C, T) and the dissociation tank (F, W) . In an embodiment of the invention the heat exchanger (Q) is a tube -in- tube or a tube- in- shell heat exchanger wherein one or more of the tubes are corrugated, which will increase the turbulence and the heat transfer. Such special formed pipes are multiple directions corrugated to increase the contact surface and the turbulence of the fluid and/or hydrate flow on either sides, both internally and externally related to the pipe wall.

In an embodiment of the invention the water of lower osmotic potential comprises said polluted water (3, 31) .

In an embodiment of the invention the polluted water (3, 31) is produced water from a petroleum production well. Such polluted water is not allowed to be discharged directly to the sea.

In an embodiment of the invention the water of lower osmotic potential comprises a portion of fresh water, sea water or the polluted water (3, 31) fed through an inlet line (SW, FW, CW) . Which type of low osmotic potential water used, will be decided depending on the reason for purifying the polluted water. During a petroleum production formation water treatment at an offshore platform it will be natural to use seawater. In that case the precipitation step (SP) will follow. Often the produced water is richer on Barium, Strontium, and Calcium ions than sea water, and sea water is much richer on sulphates. If those two types of brine are mixed, the sulphate salts of the mentioned ions precipitate and form insoluble solids. By mixing the two brines, the sulphate ions are removed and the remaining brine is good for reinjection into the reservoir as pressure support.

In an embodiment of the invention the polluted water (3, 31) is heat exchanged with the pure resulting water (9,37) in a heat exchanger (LH) if the polluted water (3,31) has a higher temperature than preferred for an efficient hydrate producing process. An inlet temperature of the water to be treated which is closer to the process temperature for the hydrate formation will lower the need of energy input. Correspondingly the pure water (9,37) temperature will increase from a typical process temperature, about -1,5 degrees C, may be better for the surroundings and prevent freezing of the resulting pure water (9,37) .

In an embodiment of the invention the hydrate forming compound (8, 38) is a designed gas comprising a mix of C02 and a light hydrocarbon compound. The light Hydrocarbon compounds may be primary lower hydrocarbons, e. g. methane, ethane and propane, but also carbon dioxide, Nitrogen, dihydrogen sulphide, halogenated hydrocarbons wherein halogen is selected among Fluorine and Chlorine, Tetrahydrofuran, ethylene oxide, noble gases, such as Helium, Neon, Argon, Xenon, Krypton, sulphur hexafluoride and dinitrogen oxide.

In an embodiment of the invention the enriched polluted water (10, 38) is fed through an ion exchanger for exchanging poisonous ions with non poisonous ions for precipitating before disposal of the resulting polluted water.

Claims

1. A method for recapturing energy while purifying polluted water (3, 31) from an inlet for polluted water, comprising the steps of:

- supplying said polluted water (3, 31) to a hydrate forming reactor (C, T) and providing a hydrate forming compound (8, 36) and hydrate seeds (32) to said polluted water (3, 31),

- controlling pressure and temperature conditions in said hydrate forming reactor (C, T) so as for forming hydrates (4, 33) ;

- transporting said hydrates (4, 33) and thereby enriched polluted water (10, 38) to a separator (D, U) separating said hydrates (4, 33) from said enriched polluted water (10, 38);

- transporting resulting separated hydrates (6, 34) to a dissociation tank (F, W) , and

- dissociating said transported, separated hydrates (6, 34) so as for separating pure water (9, 37) and said hydrate forming compound (8, 36) with consecutive return of said hydrate forming compound (8, 36) via a pump (P) to said hydrate forming reactor (C, T) , and conducting said pure water (9, 37) to a pure water reservoir (K, X) ;

characterized by

-said enriched polluted water (10, 38) is transported from said separator (D, U) to a high osmotic potential side (RH) of at least one saline energy cell (R) and water of lower osmotic potential is transported to a low osmotic potential side (RL) of at least said saline cell (R) ;

- converting energy represented by osmotic pressure arising due to the difference in osmotic potential in said saline cell (R) to a mechanical energy.

2. The method of claim 1, wherein said energy is used for running at least said pump (P) .

3. The method of claim 1 or 2 , wherein said osmotic pressure formed in said saline cell (R) is utilized in a turbine (RT) or motor running a generator (RG) for producing electrical energy .

4. The method of any of the preceding claims, wherein the heat generated during forming said hydrates (4, 33) is heat exchanged with said dissociation process of said separated hydrates (6, 34) in the heat exchanger (Q) .

5. The method of any of the preceding claims, wherein said water of lower osmotic potential comprise a portion of said polluted water (3, 31) .

6. The method of any of the preceding claims, wherein said water of lower osmotic potential comprises sea water.

7. The method of any of the preceding claims, wherein said water of lower osmotic potential comprises fresh water.

8. The method of any of the preceding claims, wherein said water of lower osmotic potential comprise said polluted water (3, 31) .

9. The method of any of the preceding claims, wherein said polluted water (3, 31) is produced water from a petroleum production we11.

10. The method of any of the preceding claims, wherein said polluted water (3, 31) is heat exchanged with said pure water (9, 37)in a heat exchanger (LH) .

11. The method of any of the preceding claims, wherein said hydrate forming compound (8, 38) is a designed gas comprising a mix of C02 and a light hydrocarbon compound.

12. An apparatus for recapturing energy while purifying polluted water (3, 31) from an inlet for said polluted water (3, 31), comprising

- a hydrate forming reactor (C, T) arranged for forming hydrates (4, 33) from said polluted water (3, 31),

- a hydrate forming compound (8, 36) for being added to said polluted water (3, 31) - a control system (CS) for sensing and controlling pressure and temperature conditions in said hydrate forming reactor (C, T) so as for forming hydrates (4, 33) ;

- a separator (D, U) for receiving and separating said hydrates (4, 33) and enriched polluted water (10, 38);

- a dissociation tank (F, W) for receiving and dissociating resulting separated hydrates (6, 34) for separating pure water (9, 37) and said hydrate forming compound (8, 36),

- a return line (L8, L36) for returning said hydrate forming compound (8, 36) via a pump (P) to said hydrate forming reactor (C, T) , and

- a pure water outlet line (L) for said pure water (9, 37) to a pure water reservoir (K, X) ;

characterized by

- a mixer on said return line (L8, L36) between said pump (P) and said hydrate forming reactor (C, T) , said mixer (IM) arranged for injecting or recycling hydrate seeds (32) to said polluted water (3, 31) in said hydrate forming reactor (C, T) for improving hydrate formation, - one or more saline cells (R) with a high osmotic potential side (RH) for receiving said enriched polluted water (10, 38) from said separator (D, U) , and a low osmotic potential side (RL) for receiving water of lower osmotic potential;

- a converter (RT) for converting osmotic pressure arising due to the difference in osmotic potential in said saline cell (R) to mechanical energy.

13. The apparatus of claim 12, wherein said hydrate forming tank (C, T) and said dissociation tank (F, W) forms the heat exchanger (Q) .

14. The apparatus of claim 13, wherein the heat exchanger (Q) is a tube-in-tube or a tube- in-shell heat exchanger wherein at least one or more of the tubes are corrugated.

15. The apparatus of any of the preceding claims 12 to 14, wherein said convertor (RT) is arranged for providing

mechanical energy, to at least said pump (P) either directly or indirectly.

16. The apparatus of any of the preceding claims 12 to 15, wherein said convertor (RT) is a fluid driven motor or a turbine (RT) .

17. The apparatus of any of the preceding claims 12 to 16, wherein said low osmotic potential side (RL) is provided with an inlet line (SW, FW, CW) for receiving water of lower osmotic potential such as fresh water, sea water or said contaminated water (3,31) .

18. The apparatus of any of the preceding claims 12 to 17, wherein a heat exchanger (LH) is arranged for heat exchanging said polluted water (3, 31) with said pure water (9, 37) .