Novel, robust and sustainable desalination of seawater

Abstract

As the world becomes short of quality drinking water, the desalination of seawater has become increasingly important. The most common commercial processes of seawater desalination, reverse osmosis (SWRO) and multi-stage flash (MSF) distillation, are both technically difficult and costly, both in terms of equipment and energy demand. In order to address this problem, a novel, and relatively simple, sub-boiling, sparging method is proposed here, which uses a continuous flow of helium gas to remove water vapour from seawater, so that desalinated water can be condensed and collected and the sparging gas, helium, be recycled. Optimum separation conditions are achieved by continuous direct heating of the seawater, during sparging, to maintain an operating temperature of about 80 deg C. This sparging method could also be used to sterilise contaminated wastewater.

Description

[0001] Novel, robust and sustainable desalination of seawater

[0002] Background

[0003] Although the world is becoming increasingly short of quality drinking water, there is a vast reservoir of water in the seas that cover about 70 % of the earth’s surface. The issue, of course, becomes to desalinate the seawater in a low cost, low energy process, which can be applied to all the coastal regions around the globe and wherever ground water is brackish.

[0004] Many poorer countries find seawater reverse osmosis (SWRO) and multi-stage flash (MSF) distillation too complex and expensive to be widely used. SWRO requires the use of expensive membranes, which have a relatively short lifetime and must be protected by expensive precleaning of the seawater. The seawater must be pressured up to 70 atm and the costs involved necessitate the application of sophisticated pressure-recovery systems. The relatively high osmotic pressure of seawater means that the desalination process leads to the production of large volumes of saline waste of about twice the salt concentration of seawater and this saline waste must be pumped far out to sea to reduce environmental damage to marine life. A further concern is the need for a steady power supply , since shutting the SWRO process and re-starting is costly and equipment can easily be damaged.

[0005] The MSF process also requires expensive equipment and sophisticated control systems because of the use of large volumes of superheated seawater and large-scale vacuum sy stems to cause flash boiling of the heated seawater. The system also has to deal with scaling of the heating surfaces and requires access to cheap thermal energy and so MSF plants are normally constructed close to electric power stations which produce large amounts of waste heat.

[0006] There is an increasing need for lower technology solutions which could be applied in all countries with significant coastal resources. One such technology is ion exchange but unfortunately this low temperature, low pressure process is dependent on the use of large volumes of acid and base regenerant chemicals, required to regenerate the resins after depletion from the adsorption of NaCl and other salts in seawater. No practical process has yet been developed to treat seawater salt levels using this relatively simple process.

[0007] Prior Art

[0008] WO 2009 / 103112 describes a method for desalinating water by passing gas bubbles through an aqueous saline solution, extracting water vapour from the aqueous saline solution into the gas bubbles, recovering the water vapour from the gas bubbles and condensing the recovered water vapour thereby desalinating the water. However, the process disclosed therein has several disadvantages. The present invention aims to overcome those disadvantages and discloses a significantly improved process.

[0009] The significantly improved process disclosed in the present invention uses helium gas instead of air used in the prior art for desalination of seawater. The present invention has identified that heated helium gas can assist liquid water to transfer into the vapour phase, well below7the boiling point, hence increasing the potential for using helium as a carrier gas for enhanced water vapour collection. The use of helium gas instead of air provides significant advantages over the prior art disclosed in WO 2009 / 103112 and the process disclosed in the present invention is comparable or better than the production rate achievable using multistage flash distillation processes.

[0010] In contrast to the prior art process, the present invention uses a hollow fibre sparging system for generating bubbles throughout the column eliminating the need for using sinter. The prior art process is limited to a low column solution restricting the effective operating chamber height to less than 30 cm. This means, the process scale up is much more difficult requiring multi-level, cross-flow systems, requiring large surface area sinters for process scale up. The use of hollow fibres to sparge helium gas in accordance with the present invention makes it suitable for use on tall columns.

[0011] Further, in the prior art process the method was based on using hot inlet gases to continuously transfer heat to the column solution . However, thi s has a practical limit due to the high enthalpy of vaporization of water of about 45 C. In the present invention, direct heating of the column solution is used to raise the operating temperature to 80 C, which in combination with the enhanced vapour collection of helium gas, should produce much higher water vapour densities close to those achieved by boiling.

[0012] Summary of the invention

[0013] The vapour pressure of water depends almost entirely on the temperature of the solution, as shown in Figure 1. Hence, collecting water vapour from a solution heated to, say, 80 C is similar to that collected via boiling, at the same temperature but under a reduced pressure. Subboiling systems are much easier to control and do not create scaling deposits. The use of helium carrier gas to collect water vapour from seawater, heated to about 80 C, offers the potential for efficient water vapour collection in a controlled manner. The helium gas used in the present invention can be recycled after condensation of the collected water vapour.

[0014] The use of helium gas in accordance with the present invention makes it ideal for this role because it has a significantly lower heat capacity than air, which assists in reducing heating costs and makes it easier for condensation cooling. It also produces smaller bubbles naturally in seawater and it has been found to have a higher water vapour collection capacity. Helium also has a significantly lower water solubility than air. Additionally, helium gas has a much better effect on breaking up water-water bonds surrounding bubbles which assi st in evaporation of water. It is estimated that helium gas is at least three times more effective in breaking up water-water bonds surrounding bubbles. Use of helium as a carrier gas has the added advantage of producing finer seawater bubbles. In addition, because helium has a low solubility in water, a low vacuum pressure is sufficient to the condense product water to produce de-gassed ‘pure’ water.

[0015] According to the present invention, a sub-boiling, helium or helium / air gas sparging system is used to efficiently and continuously remove water vapour from seawater heated to about 80 C, via a combination of hot inlet sparging gases and direct heat exchange. The inlet sparging gases are heated to, say, between 80 to 150 C and this heat is assisted by direct heating of the solution, through heat exchange pipes filled with either hot water or heated air.

[0016] In another novel aspect of the present invention, the heated sparging helium gases are released throughout the sparging column using an array of freely moving porous hollow-fibres, attached to the base of the sparging column, where they are fixed into a resin, with the gases entering the hollow tubes continuously at the base of the column. This sparging column will efficiently collect water vapour at densities close to that achieved by normal boiling but in a much more controlled, sub-boiling process. This is achieved, in part, because the salt in seawater inhibits the coalescence of bubbles and so maintains relatively small bubble sizes (roughly in the range 1 - 3 mm in diameter), compared with water. The water vapour laden exit gas is then cooled in a condenser to remove the liquid water, desalinated product, and the helium or helium / air gas mixture is re-heated and supplied to the base of the sparging unit, in a continuous recycling process. In this process the seawater can be concentrated as high as 5-6 M, if required, before discharging the concentrate. By comparison, SWRO discharges the concentrate at only I M. In another embodiment of this invention, the porous hollow fibres can be replaced with an array of fine, non-porous hollow fibres, of different lengths designed to release bubbles throughout the sparging unit, at the end of each fibre.

[0017] In further embodiments of this invention, these sparging units could also be used to sterilize contaminated water by passing hot gases, such as air, CO2 or mixtures, through the fibres, without significantly raising the temperature of the sparging solution, as well as remove other contaminants, such as ethanol and PF AS compounds.

[0018] In yet another embodiment of the present invention, the carrier gas helium can be recycled following water vapour condensation via cooling and use seawater feed / heat exchange to cool output gas to recycle thermal energy.

[0019] The use of direct heat in the present invention unlike the gas evaporation limited process disclosed in the prior art, which was limited to a column solution temperature of about 40-45 C, makes it possible to maintain higher temperatures in the column. This makes the present process suitable for sustainable heating using heat sources such as solar energy.

[0020] The ability to maintain higher bubble column operating temperatures in accordance with the present invention enables substantial increase in water collection, while maintaining a subboiling process. Use of helium gas makes it possible to achieve greater than 80 % water vapour content at higher column solution temperature.

[0021] In another aspect of the present invention, it is possible to use helium / air mixture containing 50 - 100 % helium and recycle the gas mixture after condensing the water vapour.

[0022] In yet another aspect of this invention, this process can be used for sterilising contaminated water, through use of hollow fibres and added amino acid to prevent bubble coalescence. In addition, the sparging units of the present invention can be used to remove ethanol from water mixtures and removing PFAS compounds from contaminated water.

[0023] A significant disadvantage of standard commercial desalination processes, such as SWRO and MSF, is that they are unable to remove estrogen present in seawater or wastewater. In contrast, the present invention being a sub-boiling process, there is no aerosol transfer takes place across into the vapour phase. Hence, the process of the present invention is ideal for the removal of estrogen present in seawater. The presence of estrogen in water is an emerging health issue which has not been resolved satisfactorily. Detailed description of the invention

[0024] For helium gas sparging to be effective it is important that the (vapour-free) collector gas be released throughout the solution reactor and this can be conveniently achieved by a delivery system using porous hollow fibres. These are commercially available in many different materials and sizes. For example, for hot helium gas sparging, the fibres could be made from glass, polysulfone, polytetrafluoroethylene, polyvinylidene difluoride (PVDF) or cellulose acetate (CA). The internal tube could have a diameter of between 0.2 mm to 2 mm, with wall thicknesses up to 0.4 mm, and the pores could be in the range of 40 - 100 microns. Since it has been established that seawater inhibits the coalescence of gas bubbles and so helps maintain bubbles in the optimum size range (of 1 - 3 mm diameter), it has been found that, pores of this size produce bubbles in this size range.

[0025] Use of hollow fibres in accordance with the present invention is another significant improvement over the prior art. It should be noted that, a single hollow fibre of internal radius of 1 mm can transfer about 2500 times more gas volume (released in the form of bubbles) than can one 20 Jim pore in a sinter. A 3 mm diameter gas bubble leaving a 40 micron diameter pore in a flat sinter occupies a cross sectional area of 0.07 square centimetres. A porous hollow fibre of I m length has more than 1000 porous channels in its wall through which bubbles are released but occupies the same cross-sectional area as a single active pore in a flat sinter. Hence, only one porous hollow fibre is required to cover a horizontal area of about 70 square centimetres or one fibre in a circle of radius about 5 cm.

[0026] Thus, in accordance with the present invention, the fibres can be fixed at one end into a resin at the base of the sparging column, as illustrated in the schematic diagram in Figure 2, and would be free to move in the column, whilst being made buoyant by the helium or helium / air inside the fibres, which is flowing continuously through their hollow cores. Each of the fibres would be sealed at their top to force the carrier gas to exit via the porous walls of the hollow fibres. It may be necessary to vary' the pore size along the hollow fibres to ensure that there is a uniform release of the carrier gases along the length of the fibres. This may well be needed, especially, for high columns of say 5 m or more in height. For sparge columns of this height, even bubbles formed in the lower half of the column will continue to collect more water vapour, to maintain equilibrium as the bubbles rise and expand in the reduced hydrostatic pressure. In another embodiment of this invention, it would be to replace the porous hollow fibres with much finer, non-porous, hollow fibres, and use a high density of these fibres with a suitable range of lengths, so as to ensure uniform sparging.

[0027] In accordance with the present invention, the hollow fibres are spaced in such a way as to give a high density of gas bubbles throughout the column, for a given inlet gas pressure. The inlet gas pressure can be increased as the column height is increased to maintain bubble production rate.

[0028] Both configurations can be used to efficiently collect water vapour continuously from the heated (to about 80 C) seawater, which would flow continuously into the sparge column, as shown in Figure 2. The outlet gases containing a high density of water vapour wouki then be cooled in a condenser and the desalinated liquid water product collected. It may be useful to use the seawater feed to assist in this cooling / condensation process. The helium or helium / air mixture (containing between 50 and 100 % helium) would then be heated to, say, between 80 to 150 C, and then passed into the open tubes at the base of the sparging unit.

[0029] It should be noted that the method and the apparatus disclosed in the present invention is different to the prior art method in another aspect. As the prior art method was limited only to hot gas inlet heating, which due to the high enthalpy of water vaporization limits the effective operating temperature to about 40-45 C, resulting in a much lower water vapour density (see Figure 1). In contrast, the present invention uses external heating coils, as shown in Figure 2, which assist in maintaining the temperature in the sparging column at about 80 C. This feature in combination with the enhanced vapour collection of helium gas, produces much higher water vapour densities close to those achieved by boiling.

[0030] It has been recently demonstrated that the bubble coalescence inhibition displayed by seawater can be reproduced in low' salt w'ater via the addition of natural amino acids, such as L-tyrosine, even at concentrations as lew as 20 mM, at the isoelectric point (or i.e.p.). This amino acid is environmentally safe and is a food additive. Its use will facilitate further applications of the sparging processes described here, such as for the sterilization of water contaminated with bacteria and viruses. The sterilization process occurs when hot bubbles collide with the microorganisms. So, both sparging processes described here can be used to sterilize a contaminated water supply simply by the continuous flow of hot gases, such as air or CO? or mixtures, into the hollow fibres. In this application there is no need to recycle the gases or heat the contaminated water. A continuous flow process can be used in which the contaminated water flows into the top of the sparge unit and the sterilized water is taken from near the bottom of the unit. This process could also be used to sterilise sewage wastewater.

[0031] The described sparging processes could also be used to remove ethanol from water, via absorption into the bubbles produced throughout the sparging unit. Heated inlet air in the temperature range of, say, greater than 50 C and less than 150 C, in combination with suitable levels of added L-tyrosine, could be used to remove ethanol vapour via collection into the continuous flow of fine bubbles.

[0032] These sparging units could also be used to adsorb PFAS contaminants onto the surface of the bubbles, to be removed at the top of a sparging unit in a foam. The efficiency of this process could also be improved by the addition of L-tyrosine, to reduce bubble size, and suitable environmentally acceptable co-surfactants, such as Na+ octanoy 1 cysteine, to enhance PFAS collection.

[0033] While various embodiments of the present invention have been described above, it should be understood that they have been presented by way of example only, and not by way of limitation. It wall be apparent to a person skilled in the relevant art that various changes in form and details can be made therein to suit different situations without departing from the spirit and scope of the present invention. Thus, the present invention should not be limited by any of the abovedescribed exemplary embodiments.

Claims

The claims defining the invention are as follows1. A sub-boiling sparging process to produce desalinated water from seawater, the process comprising the steps of sparging helium gas or helium gas and air mixture bubbles continuously through the seawater using a sparging device, extracting water vapour from seawater into helium gas bubbles, recovering water vapour from the helium gas bubbles and condensing the recovered water vapour to produce desalinated water.

2. A process according to claim 1 where seawater is heated to around 80 C.

3. A process as defined in claim 2 sparging of helium gas is carried out by a device using porous hollow fibres designed to release bubbles of sparging gases, throughout the device, in the size range of greater than 0.5 mm diameter and less than 5 mm diameter.

4. A process as defined in claim 3 wherein the hollow fibres are fixed in a resin at the base of the sparging unit, with the centre hollows remaining accessible, but are sealed at the other end.

5. A process as defined in claim 2 wherein sparging of helium gas is carried out by a device using a high density of fine, open-ended, non-porous, hollow fibres of different lengths, designed to release bubbles of greater than 0.5 mm diameter and less than 5 mm diameter, throughout the seawater solution being sparged.

6. A process as defined in claim 5 wherein the hollow fibre units are designed to release bubbles in the size range of 1 --- 3 mm in diameter.

7. A process as defined in any one of claims 3 to 6 where hollow fibres are be made from glass, polysulfone, polytetrafluoroethylene, polyvinylidene difluoride (PVDF) or cellulose acetate (CA).

8. A process as defined in claims 1 to 7 where sparging gas is pre-heated to temperatures between 80 and 150 C.

9. A process as defined in claim 8 where the exit gases, after water condensation, are heated and recycled as inlet gas to the sparging unit.

10. A process as defined in any of claims 1-9, wherein a natural amino acid is added to improve bubble coalescence inhibition when using low salt water.1 1. A process as defined in claim 10 where the natural amino acid is L-tyrosine.

12. A process as defined in claim 11, where it is used for removing ethanol from aqueous solutions.

13. A process as defined in claim 12, where it is used for the removal of perfluoroalkyl and polyfluoroalkyl substances (PFAS) from contaminated water.

14. A process as defined in any one of claims 1-13 where it is used for the removal of estrogen present in seawater.

15. An apparatus for the desalination of seawater, the apparatus comprising a sparging column to receive heated seawater, porous hollow fibres to sparge helium gas through seawater, a means for collecting the water vapour carried by the helium gas bubbles, a heat exchanger to condense the water vapour collected in the bubbles, a means for recycling the helium gas used in the sparging process and a means for maintaining the seawater temperature around 80 C within the sparging column.

16. An apparatus as defined in claim 15 wfierein the hollow fibres are fixed in a resin at the base of the sparging unit, with the centre hollows remaining accessible, but are sealed at the other end and designed to release bubbles of sparging gases, throughout the device, in the size range of greater than 0.5 mm diameter and less than 5 mm diameter.

17. An apparatus as defined in claim 15 wherein sparging of helium gas is carried out by a device using a high density of fine, open-ended, non-porous, hollow fibres of different lengths, designed to release bubbles of greater than 0.5 mm diameter and less than 5 mm diameter, throughout the seawater solution being sparged.

8. An apparatus as defined in any of claims 15-17 where hollow fibres are be made from glass, polysulfone, polytetrafluoroethylene, polyvinylidene difluoride (PVDF) or cellulose acetate (CA).

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