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Reverse osmosis desalination plant

reverse osmosis desalination plant; from James Grellier

Desalination

Desalination is necessary in a world becoming short of fresh water.

 

link Hard water
link Water softening
link Magnetic descaling

V Desalination.

V Reverse osmosis.

V Electrodialysis.

 

      "seawater is rendered potable by evaporation " 

Aristotle, c. 350 B.C.

Desalination

Removal of salt from seawater and brackish water is important for society. The minimum energy required, using reverse osmosis, can be easily calculated from the osmotic pressure of the water [2535] (the osmotic pressure is the pressure required to remove the salt by pushing cleaned water through the semipermeable membrane). The osmotic pressure of 'standard' seawater (~35 g ˣ kg-1 ) is 2.576 MPa. This is equal to 2.576 106 N ˣ m-2 = 2.576 106 kg ˣ m-1 ˣ s-2 = 2.576 106 J ˣ m-3 = 0.716 kWh ˣ m-3. In terms of energy cost, this is rather low for the provision of a metric ton of drinking water. The actual energetic cost of desalination will be somewhat greater than this (2x - 3x) due to inefficiency, including material losses and fouling [2535] The consequential higher pressure generally used is ~ 6 MPa with the lowest achievable energy consumption of reverse osmosis being ~2 kWh ˣ m-3 [3190] (36% efficiency).

 

At low temperature, water evaporates in inverse proportion to the relative humidity (RH), being maximal at zero RH and zero at 100% RH (-5 °C, [2694] ). The 'simple' way to desalinate water is to boil followed by condensation of the steam. This is inefficient as all the liquid has to reach its boiling point before further water is evaporated. To evaporate 1 m3 of water will take about 625 kWh plus ~ 88 kWh wasted in heating it up to the boiling point. This is a great energy cost, far greater than that theoretically necessary. This gives a maximum efficiency of about 0.3 - 0.4 %; a very inefficient process that is only used where there is plentiful waste heat available. It becomes relatively less costly where the liquid is highly contaminated (raising the osmotic pressure) or where the heat for evaporation is low cost. This energy cost may be greatly reduced by the use of porous and large-scale graphene aerogels with efficient solar steam generation (54-83%) [2945]. Low-cost evaporation may also be achieved using ambient temperature (or warm waste air) to bubble through the liquid [2186]. The air in the bubbles efficiently becomes saturated with water so removing it, with little wasted heat transfer to the bulk liquid, and the water can then be recovered later from the water-saturated air by condensation. The liquid cools when evaporating but may make use of ambient or waste heat to recover. At the steady state, the volumetric balance is

ΔP + ΔT ˣ CpTe = ρTe ˣ ΔHTe

where ΔP and ΔT are the hydrostatic differential pressure (Pa) and temperature difference, respectively, between the gas inlet and outlet, CpTe is the specific heat per unit volume of the flowing gas (J ˣ m-3 ˣ T-1), Te is the steady state temperature near the surface, ρTe is the water vapor density at Te (kg ˣ m-3) and ΔHTe is the enthalpy of vaporization at Te (J ˣ kg-1) [2650].

Outline reverse osmosis setup

 

Outline reverse osmosis setup

Reverse Osmosis

Where there is plentiful salty or brackish water but fresh water is scarce, the salt can be removed from the water by reverse osmosis [2177]. The energy cost for this process is high but has been substantially reduced over the years. This energy (~ 2 kWh ˣ m-3) and financial (~ 1-2 US $ ˣ m-3) cost may be raised if less seawater is used but lowering water preparation costs, or reduced if lower salt-content water is used, like waste-water (~20% used) or brackish water (~20% used). The pressure applied is about twice the osmotic pressure of the feed solution and increases slightly with temperature. This is a high-growth area with most of the world's desalination produced by reverse osmosis and the remainder by energetically-more expensive distillation. London has a reverse osmosis plant using the brackish Thames estuary water and capable of producing about 145,000 m3 ˣ day-1 using renewable energy from restaurant waste fat and oil. Pretreatment of the water feed is very important and this includes filtration and chemical treatment.

 

Elements of graphene oxide

Representative elements of graphene oxide

BNNT (7,7)

from [3103]

Boron nitride nanotube (7,7), from [3103]

 

Reverse osmosis membranes usually consist of a thin (~1 µ) separation film attached to a macroporous supporting material. The functional separation layer, separates the solutes from the feed water, is often made of crosslinked aromatic polyamide.The hydration structure of such membranes has been studied [3192].

 

Much work has concerned the development of improved membranes [3231] . A nano-porous carbon composite membrane has been found to display high water flux due to exceptionally high surface diffusion, together with an excellent salt rejection [2616, 2958]. Both factors can be explained in terms of the hydrophobic surface interaction with water. A great advance in this area is the production of graphene oxide membranes (see left) with controllable interlayer spacing (< 1 nm) [2880]. Laminated graphene oxide has a surprisingly high water uptake capacity (0.58 g ˣ g-1 graphene oxide) and very high adsorption/ desorption rates, attributed to the high capillary pressures plus µ-sized surface capillaries [2991]. The graphene oxide pores are effective in blocking hydrated Na+ and Cl- ions with large energy barriers [2978]. Alternative methods utilize vertically aligned graphene sheets bridged by twisted carbon fibers [2981] and boron nitride nanotubes (BNNT) [3103]

 

Outline forward osmosis setup

 

Outline forward osmosis setup

Magnetic and electric fields may be arranged to affect forward osmosis. The flow rate may be reduced due to increased hydrogen bonding within a magnetic field. Also, applying an electric or magnetic field may cause the rejection the ions ) [3150].

 

Recent developments include forward osmosis, where a solution, with very high osmotic pressure, draws water through the semipermeable membrane without any external pressure (so lowering any risk of membrane plugging). The draw solution is chosen such that the high osmotic material is easily removed (e.g. by filtration) so releasing the pure water.

 

 

 

 

Outline of electrodialysis desalination, from [3208]

 

Outline of electrodialysis desalination, from [3208]

Electrodialysis

Shown right is an outline of the electrodialysis desalination process [3208]; CEM = cation-exchange membrane, e.g. Nafion®, AEM = anion-exchange membrane, e.g. quaternary ammonium. The thickness of the cells are generally much thinner than shown (< mm). The electrodes cause the ions to move towards their opposite charges.The cation exchange membrane contains fixed negative charges allowing positive ions to move across but block anions. Conversely, the anion exchange membrane contains fixed positive charges blocking cations but allowing the anions to cross .

 

Electrodialysis has a limitation compared with reverse osmosis as it cannot remove uncharged or higher molecular weight ionic components from the feed stream.

 

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This page was established in 2002 and last updated by Martin Chaplin on 28 March, 2018


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