AU2005100689A4 - Process for desalination of seawater with zero effluent and zero greenhouse gas emission - Google Patents

Description

Title of the invention Process for desalination of seawater with zero effluent and Zero greenhouse gas emission SDescription ;Background of the invention Increasing deforestation and unabated emission of greenhouse gases have contributed directly to global 00 00warming. This has accelerated the melting of ice in arctic and sharply affected precipitation of rain in n many parts of the world. Increasing usage of water by industries, lack of water recycling facilities and poor management of water facilities have contributed for worldwide shortage of drinking water. Salinity and draught are major problems in many countries including Australia. In order to overcome the water problems many desalination plants have been set up all over the world, invariably using RO (reverse Osmosis) and thermal processes such as MFD or MVR etc. Such desalination facilities desalinate the seawater with TDS ranging from 35,000 ppm to 60,000ppm thereby recovering fresh water with TDS less than 500 ppm, ranging from 1 0/oto 60% of the feed and discharging the balance highly concentrated effluent with TDS ranging from 55000ppm to 150,000 ppm.back into the sea, causing irreparable damage to marine life. Recent studies show that the salinity of seawater has increased over a period of time. It is a known fact that increasing salinity of seawater will in turn sharply reduce rain precipitation (increasing salinity will increase the boiling point and decrease the freezing point of seawater) and

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Oenhance the melting of ice in the arctic. This will contribute for further global warming and draught.

Increasing salinity of seawater will increase the Iosmotic pressure of seawater, which in turn will also greatly decrease the efficiency of the existing, operating desalination plants around the world due

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0to dramatic change in the water chemistry of the sea. Desalination of seawater is an energy intensive process and is directly responsible for increasing greenhouse emission.

Therefore it becomes imperative to design a new process that will overcome the above deficiencies and curtail the global warming .A novel process of seawater desalination by integrating various existing, commercially proven technologies and systems has been designed.

A semipermeable membrane is a selective barrier that permits the separation of certain species in a fluid by a combination of sieving and sorption diffusion mechanisms. In terms of energy, membrane separations have an important advantage in that, unlike evaporation and distillation, no change of phase is involved in the process, thus avoiding latent heat requirements. No heat is required with membranes, thus it is possible to produce products with functional properties superior in some respects to those produced by conventional processes. Membrane technology also V enables to simultaneously concentrate, fractionate

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Sand purify the products.

Membranes can selectively separate components \0 over a wide range of particle sizes and molecular weights, from macromolecular such as starch and protein to monovalent ions. Membrane should be oO \0 selected such that the size of the pores is smaller than the size of the smallest particle in the feed t n stream that is to be retained by the membrane.

Membranes are available in several different configurations such as tubular, hollow-fiber, plate and frame and spiral wound. Some of the design work better than others for a particular application, depending upon viscosity, and concentration of suspended solids, particle size and temperature.

The membrane processes are classified according to the driving force used in the process shown below.

DrivingForce Process 1.Pressure Microfiltration, Utrafiltration, Nanofiltration Reverse osmosis 2.ElectricalPotential Electrodialysis 3.Partial Pressure Per evaporation 4Concentration gradient Dialysis.

The process of cross-flow pressure driven membrane filtration is very simple, requiring only 00 00 the pumping of the feed stream tangentially across

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the appropriate membrane, that is parallel to the Smembrane surface. The membrane splits the feed stream into two streams: one stream is the permeate, consisting of components small enough to pass through the membrane pores: the other stream is the concentrate (retentate) consisting of components large enough to be retained by the membrane. The retentate stream is usually recirculated through the membrane module because one passage through the membrane may not deplete the feed significantly.

Important operating variables are applied transmembrane pressure and cross flow velocity through the membrane module. Cross flow velocity is the average rate at which the process fluid flows parallel to the membrane surface. Velocity has a major effect on the permeate flux. The permeate flux depends on the applied transmembrane pressure for a given surface area unto a threshold transmembrane pressure. Above this pressure, which is experimentally determined for each application, higher pressures have little or no effect.

In fact, too high a pressure may aggravate fouling of the membrane.

The seawater is pre-treated by Micro filtration (MF) and Ultrafiltration (UF) so that dissolved solutes Of 0.005-0.1 microns are separated, this corresponds to INO a molecular weight cut-off of about 1,000 to 500,000.Depending upon the molecular weight cutoff selected, the membrane will concentrate higher

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00 molecular weight materials species while allowing dissolved salts and lower molecular weight to pass t through the membranes.

After this pre-treatment, the seawater is subject to Nanofiltration Nanofiltration membrane displays excellent rejection of divalent ions while allowing majority of monovalent ions to pass.

Organic molecules in the range 200-300 molecular weight ranges are also highly rejected. The unique separation capability of NF provides the opportunity to selectively concentrate either valuable or undesirable substance the process stream with greater effectiveness, consistency, reliability and economy. Thus valuable products such as Magnesium, Calcium and sulphate salts are retained by NF from the seawater while sodium, potassium and chloride ions allowed to pass through, thus effectively separating the hard ions and soft ions.

NF is a membrane technology on the rise because it is more cost effective than RO(reverse osmosis) in certain applications. In most water resources, there will be a single charged ions such as Sodium ions(Na+)and Chloride ions (C1-)and double charged ions such as (Ca++)and Sulphate(S04- )ions. The single charged,postive ions do not generally participate in scale formation, so they may be considered as soft ions. The double charged positive ions such as Calcium and Magnesium, in conjunction with certain negatively charged ions can form hard scales within water treatment

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00 equipments and piping and are called hardness ions.

NF membranes also remove large colour molecules ~ti and many of the trihalomethane (THM) precursors.

Nanofiltration can treat the same amount of water, or more, at half the pressure. Since NF membrane can remove the double charged scaling ions like Calcium and Magnesium ions, another term for NF is membrane softening. The largest users of the NF technology are municipal drinking water plants. A future trend is for NF to replace lime softening to achieve an industry standard of 50 parts per million of alkalinity plus meet the federal THM limits.

Description of the invention tbThis novel process involves recovery of valuable ;Z chemicals such as Magnesium from seawater in the O form of Magnesium Chloride after the pre-treatment of the seawater such as MF and UF and by passing through NF membrane at a pressure less than 00 00 atm. and treating the retentate with chemicals such

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as Lime/Dolime to precipitate Magnesium and n Calcium salts for further processing. The permeate is passed through an RO membrane to obtain the required quality of water and discharging the renetate into NF retentate for further processing .The rentate from NF and RO after chemical treatment are subject to evaporation for recovery of balance quantity of water and Sodium chloride, thus eliminating effluent discharge back into the sea.

Further embodiment of this process involves treating the discharge from seawater desalination plants using membrane separation process such as NF and RO and to generate green and renewable energy such as Hydrogen from deionised water recovered by desalination and wind to operate the process described above. The major advantage of this invention is to facilitate the successful integration of existing, commercially proven and operating technologies and systems in different parts of the world to achieve zero effluent discharge into sea and achieving zero greenhouse gas emission for such desalination applications.

The process involves, 1. Separation of divalent ions such as Calcium, magnesium and Sulphate present in the seawater by N 'membrane softening' using Nanofilteration at a pressure less than 35 atm, 2. The permeate resulting by the above NF separation 00 00is further subject to RO(reverse osmosis) separation at a higher pressures not exceeding 40 atm in a i single or multiple staging to recover upto 60% of the initial seawater feed.

3. The concentrate from the first NF separation and the second RO separation are combined and 4. Subjecting the above concentrates to chemical treatment to separate Calcium, Magnesium and Sulphate salts by precipitation and filtration and Subjecting the filtrate to further evaporation and crystallization of Sodium Chloride thereby recovering further quantity of water with TDS less than lppm.

6. Alternatively by evaporating the filtrate containing predominantly Sodium Chloride and relatively free from Calcium, Magnesium and Sulphate ions to the saturation point, which can be further electrolysed for production of caustic soda, Chlorine and Hydrogen as by products S7. Alternatively the recovered water by evaporation 0 Swith TDS less than 10ppm is further electrolysed to Sgenerate Hydrogen by known prior art such as electrolysis, thermolysis, plasma splitting etc and use such Hydrogen for power generation using Fuel cell such as PEMFC (proton exchange membrane 0 fuel cell), Solid oxide fuel cell etc to supplement the 00 \O wind energy which can be used as the prime source of renewable energy for this process.

Pressure driven membrane process such as MF(Microfilteration),UF(ultrafiltration) NF(nanofilteration) and RO (reverse osmosis) are commonly employed techniques in desalination of seawater/brackish water and other water treatments.

These membranes can be designed depending upon their mechanism of separation such as capillary or diffusion, mechanical strength, resistance to corrosion and temperature., cost and economy of mass production etc.'Permselectivity'is the term used for specific separation of an ion using such membrane and they are called'permselect membrane'.

With this background it is now possible to design a suitable or use an existing commercially available 'perm select membrane' to separate the 'hard ions' or divalent ions such as Mg++ and S04from 'soft ions' or monovalent ions such as Na and Cl- from seawater or brackish water or any other source of effluent containing such ions.

t Seawater is first subject to MF and UF to separate 0 Sforeign matters, suspended matters, organics and Scolours from the seawater prior to NF and RO separation.

N The clear filtered seawater is then subject to NF filtration consisting of'perselect membranes' that selectively retains the divalent ions such as 00 0 Calcium, Magnesium and Sulphate and allow monovalent ions such as Sodium, Pottasium and tin Chloride ions to pass through under a pressure less Sthan 35 atm. The pore size of this membrane is designed in such a way that it will retain at least of divalent ions such as Ca, Mg, Al, Zn, SO4, OH, C104, I and allow other monovalent ions such as Na, Li, Ba, K, NH4, Cs, N03, Br,Cl and H20 to pass through.

Claims (3)

1. 2.The retentate mixture from the above mentioned NF and RO separation streams are subject to chemical treatment such as precipitation using lime/dolime etc using prior art to separate Magnesium and Calcium salts for further processing and recovery of chemicals, more specifically Magnesium Chloride for further recovery of metallic Magnesium using known electrolysis process currently used.
2. 3.After separation of Calcium, magnesium and sulphate ions the retentate is further evaporated to recover balance quantity of water and sodium chloride, thereby eliminating any discharge of effluent back into the sea from such desalination plants.The brine can be concentrated to the saturation point of sodium chloride,that can be t directly fed into an electrolyser for the production 0 O of Caustic soda,Chlorine and Hydrogen. The depleted brine from the electrolytic cell can be recycled for further concentration to saturation and Sreturned to the cell for further electrolysis.This step will effectively eliminate dissolution of common salt and preperation of brine currently practised in 00 \1 caustic soda production.Such brine will be relatively free from Calcium,Magnesium and t sulphate ion which are otherwise removed by precipitation in brine treatment section of the caustic production.
3. 4.The recovered water from the above retentate with TDS less than lOppm is subject to further processing such as electrolysis, thermolysis or plasma splitting etc to generate green and renewable and purified source of Hydrogen for fuel cell applications to generate power and heat that can be used as a supplementary source of energy for the above processing. integrated four step processes mentioned above to achieve zero effluent discharge and zero greenhouse emission from seawater desalination plants which are currently in operation or being designed for future usage by suitably modifying the process design by retrofitting the above mentioned systems.