Inventor: AL-MAYAHI ABDULSALAM [GB] ; SHARIF ADEL
Classification: - international: F03G7/00;
F03G7/00; - European: F03G7/00C
Also published as:WO2005017352 // US2006225420 //
JP2007533884 // ES2288261 // EP1660772
Abstract -- A process for
driving a prime mover, said process comprising a) positioning a
selective membrane between a liquid and a solution having a higher
osmotic potential than the liquid, such that the solution becomes
pressurised by the influx of liquid across the membrane, b) using
the pressure generated in the solution to drive a prime mover, c)
recovering the solution, d) separating at least some of the
solvent from the solution to form a residual product, and e)
recycling the separated solvent and/or the residual product of
step d) to step a).
Description
[0001] The present invention relates to a process for driving a
prime mover. The present invention also relates to an apparatus
for driving a prime mover, for example, for generating hydraulic
power.
[0002] When a dilute aqueous solution (low osmotic potential, low
entropy) is separated from a concentrated aqueous solution (high
osmotic potential, high entropy) by a semi-permeable membrane,
water from the dilute aqueous solution will flow across the
semi-permeable membrane to dilute the concentrated aqueous
solution. The phenomenon is known as osmosis.
[0003] U.S. Pat. No. 3,978,344 describes a process in which
osmotic energy is converted into other forms of energy.
Specifically, this reference describes a process in which a
semi-permeable membrane is used to separate a body of freshwater
from a body of seawater. The difference in osmotic potential
between the freshwater and the seawater causes water to pass
through the membrane into the seawater, resulting in an increase
in hydrostatic pressure on the seawater side of the membrane. The
seawater may be ejected as a pressurised stream, which may be used
to rotate a turbine coupled to an electrical generator.
[0004] The passage of freshwater through the semi-permeable
membrane results in dilution of the seawater, which eventually
limits the production of energy from a given body of seawater.
Thus, the diluted seawater must be replenished periodically or
continuously with a fresh seawater.
[0005] According to the present invention, there is provided a
process for driving a prime mover, said process comprising
[0006]a) positioning a selective membrane between a liquid and a
solution having a higher osmotic potential, than the liquid, such
that the solution becomes pressurised by the influx of liquid
across the membrane, [0007]b) using the pressure generated in the
solution to drive a prime mover, [0008]c) recovering the solution,
[0009]d) separating at least some of the solvent from the solution
to form a residual product, and [0010]e) recycling the separated
solvent and/or the residual product of step d) to step a).
[0011] In one embodiment, the residual product of step d) is
recycled to the solution of step a). Alternatively or
additionally, the separated solvent may be recycled to the liquid
side of the membrane in step a).
[0012] The influx of liquid across the selective membrane
generates pressure (e.g. hydrostatic pressure) the solution. The
pressurised solution from step a) may be used directly to. drive
the prime mover. Alternatively, the solution from step a) may be
recovered and some of the solvent of the recovered solution may be
removed. The resulting concentrated solution may be used to drive
the prime mover before being recycled to step a). Alternatively or
additionally, the solvent separated from the solution may be used
to drive the prime mover. In all instances, at least some of the
pressure generated in the solution in step a) is used to drive the
prime mover. Optionally, the generated pressure in the solution
may be used to pump the solution for subsequent processing.
[0013] Any suitable selective membrane may be used in step a). The
membrane may have an average pore size of 1 to 60 Angstroms,
preferably, 2 to 50 Angstroms, more preferably, 5 to 40 Angstroms,
for example, 10 to 30 Angstroms. In one embodiment, the membrane
has an average pore size of 12 to 25 Angstroms.
[0014] The average pore size of the membrane is preferably smaller
than the size of the solutes in the solution. Advantageously, this
prevents or reduces the flow of solute across membrane by
diffusion, allowing liquid to flow across the membrane along the
osmotic (entropy) gradient. The flux of liquid across the membrane
is influenced by the pore size of the membrane. Generally, the
larger the pore size, the greater the flux.
[0015] Suitable selective membranes include integral membranes and
composite membranes. Specific examples of suitable membranes
include membranes formed of cellulose acetate (CA) and membranes
formed of polyamide (PA). Preferably, the membrane is an
ion-selective membrane. Conventional semi-permeable membranes may
also be employed.
[0016] The membrane may be planar or take the form of a tube or
hollow fibre. If desired, the membrane may be supported on a
supporting structure, such as a mesh support. The membrane may be
corrugated or of a tortuous configuration.
[0017] The prime mover may be any suitable device, which is
suitable for converting energy in the solution into mechanical
power. Suitable prime movers include rotary prime movers, such as
turbines. Thus, the prime mover may be used to generate power.
[0018] Alternatively, the prime mover may be or form part of a
pressure exchange system. Thus, the prime mover may also be used
to transfer energy from the pressurised solution to another fluid.
Examples of suitable pressure exchange systems are described in
U.S. Pat. No. 4,887,942, U.S. Pat. No. 5,338,158, U.S. Pat. No.
5,988,993 and U.S. Pat. No. 6,540,487. The pressure exchange
system may comprise a housing having a body portion with end
elements at opposite ends of the body portion. A rotor may be
positioned in the body portion in substantially sealing contact
with the end plates. The rotor may be provided with at least one
channel extending longitudinal from one end of the rotor to the
opposite end of the rotor with an opening at each end. In use, the
channel(s) provide alternate hydraulic communication between a
high pressure liquid, such as the pressurised solution from step
a), and a low pressure liquid to transfer pressure between the
liquids.
[0019] In the process of the present invention, a selective
membrane is positioned between a liquid and a solution. The
solution has a higher osmotic potential than the liquid. Thus, the
total dissolved salt (TDS) concentration of the solution is
typically higher than that of the liquid.
[0020] The difference in osmotic potential between the liquid and
the solution causes solvent to pass across the selective membrane
from the side of low osmotic potential (i.e. low solute
concentration or low entropy or high solvent concentration) to the
side of high osmotic potential (i.e. high solute concentration or
high entropy or low solvent concentration). This influx of liquid
leads to an increase in pressure in the solution. For example, the
pressure of the solution may be increased from 10.sup.5 to
10.sup.7 Pa to a pressure of 1.1.times.10.sup.5 to
5.0.times.10.sup.7 Pa due to the influx of liquid. In a preferred
embodiment, the pressure of the solution may be increased from
10.sup.5 to 10.sup.7 Pa to a pressure of 1.5.times.10.sup.5 to
2.5.times.10.sup.7 Pa due to the influx of liquid.
[0021] Step a) of the process of the present invention may be
carried out in a housing. The housing is preferably provided with
an inlet for introducing the liquid to one side of the selective
membrane and an outlet for removing pressurised solution from the
opposite side of the membrane. In a preferred embodiment, the
influx of liquid into the solution is sufficient to eject the
solution from the housing at a pressure of 1.1.times.10.sup.5 to
5.0.times.10.sup.7 Pa, preferably, 1.5.times.10.sup.5 to
2.5.times.10.sup.7 Pa. The outlet of the housing may be sized to
ensure that the solution is ejected at appropriate pressures.
Additionally or alternatively, a nozzle (a pressure regulator) may
be coupled to the outlet to adjust the pressure of the solution
accordingly.
[0022] The pressurised solution may be removed from the housing,
for example, as a pressurised stream. The pressure generated in
the solution may be used to drive a prime mover. The solution may
be introduced directly to the prime mover to drive the prime
mover, for example, to generate power. Alternatively, the pressure
generated in the solution may be transferred to another liquid via
a pressure exchange system. The pressurised solution from step a)
may be used directly in step b) to drive the prime mover.
Alternatively, it may be possible to carry out steps c) and d) of
the process prior to step b).
[0023] The pressure generated in the solution may be sufficient to
eject the solution from the housing to an elevated height of, for
example, 10 to 2500 m, preferably, 50 to 1500 m. Thus, the
solution may be introduced to the prime mover from an elevated
height. In this way, at least part of the potential energy of the
solution is converted into mechanical energy. The mechanical
energy of the prime mover may be subsequently converted into other
forms of energy, such as electricity and/or heat. In certain
embodiments, it may be desirable to store the solution at an
elevated location, prior to introducing the solution to the prime
mover.
[0024] In step c), the solution is recovered. Solvent is then
removed from the recovered solution (step d). This solvent removal
step may be carried out by any suitable solvent removal/separation
method. Thermal and/or membrane separation steps may be employed.
A combination of one or more thermal separation steps and/or one
or more membrane separation steps may be employed.
[0025] Examples of suitable thermal separation techniques include
evaporation, distillation and crystallization. Evaporation may be
carried out naturally, for example, by allowing the solvent to
evaporate in air under ambient conditions. Alternatively,
evaporation may be carried out in a cooling tower. Suitable
distillation methods include multi-stage flash distillation (MSF),
multi-effect distillation (MED) mechanical vapour compression
(MVC) and rapid spray desalination.
[0026] In multi-stage flash distillation, the solution is
introduced into a series of tubes and heated to an elevated
temperature. The heated solution is then introduced into an
evaporation chamber and subjected to a pressure below its vapour
pressure. The sudden reduction in pressure causes boiling or
flashing to occur. The flashed vapours are separated from the
salty residue by condensation on the tubes of the incoming
solution streams. A series of evaporation chambers is typically
employed. Thus, the evaporation or flashing step occurs in
multiple stages.
[0027] Multiple effect distillation takes place in a series of
effects and uses the principle of reducing the ambient pressure in
the various effects. This permits the solution to boil in a series
of stages without the need for additional heat to be supplied
after the first effect.
[0028] In multiple effect distillation, the solution may be
preheated and sprayed onto the surface of evaporator tubes as a
thin film of liquid. The tubes are heated by passing a steam
through the tubes. On coming into contact with the heated surface
of the tubes, the sprayed liquid evaporates. This vapour is used
to heat the evaporator tubes of the next effect and the transfer
of heat causes the vapour in the tubes to condense. By evaporating
and condensing the solution in this manner, the solvent from the
solution may be recovered.
[0029] The efficiency of the multiple effect distillation step may
be increased by compressing the vapour of at least one of the
effects. The combination of multiple effect distillation and
compression is known as MED-thermo compression.
[0030] Mechanical vapour compression (MVD) may also be used to
remove solvent from the solution. In mechanical vapour
compression, vapour from a vessel is typically extracted and then
condensed by compression in a tube located within the vessel. The
compression and condensation step generates heat, which heats the
walls of the tube. When solution is sprayed onto the surface of
the tube, it evaporates generating more vapour. By repeating the
extraction, compression and condensation steps, further solvent
may be recovered from the solution.
[0031] Rapid spray desalination (RSD) may also be used to remove
solvent from the solution. In a typical rapid spray desalination
process, air is blown across a heating element into an evaporation
chamber. As the heated air moves along the evaporation chamber, a
nebulized solution of, for example, brine, is injected into the
evaporation chamber. The moving vapour and brine droplets pass
through a mechanical filter, which traps the brine droplets,
allowing the pure vapour phase to pass on towards a condenser. The
brine droplets may be periodically flushed from the filter.
[0032] As mentioned above, crystallization methods may also be
employed to separate solvent from the solution. Crystallization
may be affected to crystallize the solvent or solute out of
solution.
[0033] Crystallization may be carried out by cooling the solution
to, for example, the freezing point of the solvent. This causes at
least some of the solvent in the solution to crystallize. This
crystallized solvent may then be removed. Crystallization may be
preferred in cool climates, where the low ambient temperatures may
be used to reduce the temperature of the solution to effect
crystallization.
[0034] Alternatively, a thermal separation column may be employed
to affect crystallization. For example, the solution may be cooled
in a thermal separation column such that at least some of the
dissolved solutes precipitate out of solution. These precipitates
may collect at the bottom of the column and recovered, leaving the
solution at the top of the column with a reduced solute
concentration. Advantageously, the solution may be formed using a
salt having a solubility that is sensitive to temperature
variations. Preferably, such salts readily precipitate out of
solution at low temperatures. Examples of such salts include
hydrogenphosphates such as disodium hydrogenphosphate
(Na.sub.2HPO.sub.4.12H.sub.2O).
[0035] In one embodiment, the solution from step a) may be
transferred to an elevated height (e.g. top of a mountain) where
the ambient temperature is i) low enough to crystallize the
solutes species in the solution or ii) below the freezing point of
the solution to crystallize the solvent. This causes separation of
the solution into two portions. One portion has a low solute
concentration, whilst the other portion has a higher solute
concentration. Each of these solutions may be returned to ground
level so that the potential energy of the solutions may be used to
drive the prime mover. These solutions may be recycled to step a).
[0036] Suitable membrane methods for separating solvent from the
solution include ion-exchange, electro-dialysis, electro-dialysis
reversal, nanofiltration and reverse osmosis. When membranes are
used, they should be able to withstand the high pressures
generated in the system.
[0037] The thermal energy required to drive the solvent removal
step may be provided by a number of sources. For example, the
thermal energy may be provided by the surroundings (e.g.
evaporation at ambient temperature), geothermal sources and/or
solar energy. Evaporation at ambient temperature may be favoured
in hot climates. It may also be possible to induce solvent removal
by passing air, for example, dry warm air, over the solution, so
as to effect evaporation of the solvent. Alternatively or
additionally, the excess heat from an industrial process (e.g. a
power station, a refinery, chemical plant) may be used to drive
the solvent removal step. In other words, the solvent removal step
of the present invention may be used to remove excess heat from an
industrial process.
[0038] In a further embodiment, the thermal energy required to
drive the solvent removal step may be provided by the combustion
of a fuel, such as oil, wood, peat, bushes, grass, straw, natural
gas and coal. Waste products may also be incinerated to provide
the thermal energy required for the solvent removal step.
[0039] In yet a further embodiment, the thermal energy required to
drive the solvent removal step may be provided by biological
processes, such as thermogensis and fermentation.
[0040] In another embodiment, the thermal energy required to drive
the solvent removal step may be provided by the compression and
decompression of gas (e.g. air). When a gas expands isotropically
at a given temperature, its final temperature at the new pressure
is much lower. The resulting cold gas, can be used as a
refrigerant, either directly in an open system, or indirectly by
means of a heat exchanger in a closed system. Conversely, the
compression of gas causes the temperature of the gas to increase.
The heat of compression can be used to heat the solution and/or
evaporate the solvent.
[0041] In yet another embodiment, the thermal energy required to
drive the solvent removal step may be provided by wind power. Wind
power can be used to compress air and the heat of compression can
be used to heat the solution and/or evaporate the solvent. The air
may then be decompressed and the cooling effect of the
decompression can be used to cool the solution and/or condense the
vapour. The use of air as a coolant is based on the principle
that, when a gas expands isotropically at a given temperature, its
final temperature at the new pressure is much lower. The resulting
cold gas, in this case air, can then be used as a refrigerant,
either directly in an open system, or indirectly by means of a
heat exchanger in a closed system.
[0042] The sources of thermal energy discussed above may be
particularly useful for removing solvent by
evaporation/distillation.
[0043] Once solvent is removed from the solution, a residual
product is produced. This residual product is preferably recycled
to a solution that is suitable for use in step a). For example,
the residual product produced in step d) of the process may be
recycled to the solution of step a) of that process.
Alternatively, when more than one of the processes of the present
invention are carried out (e.g. concurrently), it is possible to
recycle the residual product of one of the processes to step a) of
another of the processes.
[0044] The process of the present invention may be carried out
continuously, reducing or eliminating the need for replacing or
replenishing the solution of step a) with fresh solution. It is
also not necessary to add fresh solute to the solution, although
this may be desirable in some instances.
[0045] The residual product may take the form of a solid product
or a concentrated solution. Where the residual product is a solid
product, the solid product may be added to the solution of step a)
to increase the solute concentration of the solution of step a).
This can help to maintain the difference in solute concentration
between the two sides of the membrane, and ensure that the flow of
liquid across the membrane occurs at a sufficient rate.
[0046] Where the residual product is a solid product, the solid
product may also be diluted with solvent to produce a concentrated
solution. This concentrated solution can be introduced to the
solution of step a). By adjusting the concentration of this
solution accordingly, the solute concentration of the solution of
step a) may be maintained at desired levels. This can help to
maintain the difference in solute concentration between the two
sides of the membrane, and ensure that the flow of liquid across
the membrane occurs at a sufficient rate.
[0047] When the residual product is a concentrated solution, the
concentrated solution may be introduced to the solution side of
the selective membrane of step a). In certain embodiments, it may
be necessary to alter the concentration of the concentrated
solution prior to use, for example, by adding more solvent or
solute to the solution. By adjusting the concentration of the
solution accordingly, the solute concentration of the solution may
be maintained at desired levels. This can help to maintain the
difference in solute concentration between the two sides of the
membrane, and ensure that the flow of water across the membrane
occurs at a sufficient rate.
[0048] The solvent removed in the solvent removal step may be
recovered and recycled, for example, to a liquid that is suitable
for use in step a). The solvent removed in step d) of the process
may be recycled to step a) of the process, or, alternatively, when
a plurality of processes of the present invention are carried out
(e.g. concurrently), the solvent removed in step d) of one
particular process may be recycled to step a) of another process.
[0049] Alternatively or additionally, the removed solvent may be
discarded or used for other purposes. In one embodiment, the
liquid is seawater and the solution is an aqueous solution. Thus,
the solvent removed from the solution in step d) is water. This
water may be used for a number of applications, including
agricultural, industrial and domestic applications (e.g. as
drinking water). Thus, in this embodiment of the present
invention, the process of the present invention may be used to
desalinate seawater.
[0050] It may be possible to replenish or to replace the liquid of
step a) with fresh liquid, for example, periodically or
continuously.
[0051] The liquid employed in step a) of the process of the
present invention is preferably water or an aqueous solution. For
example, the liquid may be seawater, freshwater (e.g. from rivers,
lakes and underground sources) and brackish water. Grey water
streams, for example, waste washing water (e.g. laundry) and
streams from gullies, may also be used. Thus, the liquid may
contain impurities that are typically found in water from these
sources. For example, the liquid may contain dissolved salts, such
as metal or ammonium salts. Examples of salts that may be present
include fluorides, chlorides, bromides, iodides, sulphates,
sulphites, sulphides, carbonates, hydrogencarbonates, nitrates,
nitrites, nitrides, phosphates, aluminates, borates, bromates,
carbides, chlorides, perchlorates, hypochlorates, chromates,
fluorosilicates, fluorosilicates, fluorosulphates, silicates,
cyanides and cyanates. Preferably, salts of alkali and/or alkali
earth metals are employed. Examples of such metals include, but
are not limited to, lithium, sodium, potassium, magnesium, calcium
and strontium. In one embodiment, the liquid is seawater and,
therefore, includes sodium chloride in a concentration of at least
3 weight %.
[0052] In an alternative embodiment, the liquid may be an effluent
from an industrial or agricultural process.
[0053] The solute concentration (i.e. TDS) of the liquid may be 0
to 40 weight %, preferably, 0.0 to 6 weight %.
[0054] During osmosis, at least some of dissolved solutes and
suspended impurities in the liquid will be prevented from flowing
across the membrane. Preferably, all dissolved solutes/impurities
will remain on the liquid-side of the membrane, allowing the
liquid to flow across the membrane to dilute the solution on the
other side of the membrane. Thus, where the liquid is a solution
of a solute dissolved in a solvent, the solute is preferably
prevented from flowing across the membrane, allowing the solvent
to flow across the membrane to dilute the solution on the other
side. Specifically, where the liquid is an aqueous solution,
dissolved solutes and/or suspended impurities in the solution are
preferably prevented from flowing across the membrane, allowing
water to flow across the membrane to dilute the solution on the
other side.
[0055] The solution may be formed of an organic and/or inorganic
solvent. Suitable organic solvents include hydrocarbons, such as
aliphatic and aromatic hydrocarbons. Mixtures of organic solvents
may be employed. The hydrocarbons may be straight chain, branched
and/or cyclic. Examples include, but are not limited to, alkanes,
alkenes and alkynes. The hydrocarbons may be substituted with one
or more heteroatoms, for example, fluorine, chlorine, bromine,
iodine, oxygen, sulphur, nitrogen, and/or phosphorus atoms. In one
embodiment, oxygenated hydrocarbons, such as aldehydes, ketones,
carboxylic acids, ethers, esters, alcohols and/or their
derivatives may be employed. For example, glycol ethers and glycol
ether esters may also be employed. Alternatively or additionally,
halogenated solvents, such as chlorinated, brominated and/or
fluorinated hydrocarbons may be employed.
[0056] Suitable inorganic solvents include acidic solvents,
alkaline solvents and/or water. Water is preferably employed as
solvent in the solution.
[0057] The solution is preferably an aqueous solution.
[0058] Suitable solutes for the solution include organic
compounds, biological compounds and/or inorganic compounds.
[0059] Suitable organic compounds include hydrocarbons, such as
aliphatic and aromatic hydrocarbons. Mixtures of two or more
organic compounds may be employed. The hydrocarbons may be
straight chain, branched and/or cyclic. Examples of suitable
hydrocarbons include, but are not limited to, alkanes, alkenes and
alkynes. The hydrocarbons may be substituted with one or more
heteroatoms, for example, fluorine, chlorine, bromine, iodine,
oxygen, sulphur, nitrogen, and/or phosphorus atoms. In one
embodiment, oxygenated hydrocarbons, such as aldehydes, ketones,
carboxylic acids, ethers, esters, alcohols and/or their
derivatives may be employed. The organic solute species may have a
molecular weight of from 100 to 10000 gmol.sup.-1, preferably, 300
to 5000 gmol.sup.-1, more preferably, 400 to 2000 gmol.sup.-1 and,
even more preferably, 500 to 1000 gmol.sup.-1.
[0060] Suitable biological compounds include proteins, amino
acids, nucleic acids, carbohydrates and lipids. Mixtures of two or
more biological compounds may be employed. Preferred biological
solutes include sugars, such as cane sugar and/or beet sugar.
Glucose, fructose and sucrose may also be employed. The biological
solute species may have a molecular weight of from 100 to 10000
gmol.sup.-1, preferably, 300 to 5000 gmol.sup.-1, more preferably,
400 to 2000 gmol.sup.-1 and, even more preferably, 500 to 1000
gmol.sup.-1.
[0061] Preferably, the solution is a solution of one or more
inorganic compounds, such as inorganic salts. Suitable salts
include metal or ammonium salts. Mixtures of two or more salts may
be employed. Examples include, but are not limited to, fluorides,
chlorides, bromides, iodides, sulphates, sulphites, sulphides,
carbonates, hydrogencarbonates, nitrates, nitrites, nitrides,
hydrogenphosphates, phosphates, aluminates, borates, bromates,
carbides, chlorides, perchlorates, hypochlorates, chromates,
fluorosilicates, fluorosilicates, fluorosulphates, silicates,
cyanides and cyanates. Preferably, salts of alkali and/or alkali
earth metals are employed. Examples of such metals include, but
are not limited to, lithium, sodium, potassium, magnesium, calcium
and strontium.
[0062] Preferably, the solution is an aqueous solution of at least
one salt selected from sodium chloride, potassium chloride,
potassium nitrate, magnesium sulfate (e.g. MgSO.sub.4.6H.sub.2O or
MgSO.sub.4.7H.sub.2O), magnesium chloride (e.g.
MgCl.sub.2.6H.sub.2O), sodium sulfate (e.g.
Na.sub.2SO.sub.4.10H.sub.2O), calcium chloride (e.g.
CaCl.sub.2.2H.sub.2O or CaCl.sub.2.6H.sub.2O), sodium carbonate,
disodium hydrogenphosphate (Na.sub.2HPO.sub.4.12H.sub.2O) and
potassium alum (24H.sub.2O). In a preferred embodiment, the
solution is an aqueous solution of sodium chloride.
[0063] Preferably, the solution is formed by introducing a known
quantity of a solute into a known quantity of solvent. Preferably,
the solution consists essentially of a selected solute dissolved
in a selected solvent. For example, in one embodiment, the process
of the present invention further comprises the step of dissolving
a selected solute in a selected solvent. In one embodiment, the
solution is formed by mixing ammonia and carbon dioxide in water.
The resulting solution may contain a concentrated solution of
ammonia, carbon dioxide, ammonium carbonate, ammonium bicarbonate
and ammonium carbamates as described in WO 02/060825.
[0064] Alternatively, the solution may be derived from an existing
stream such as a waste stream from an industrial process. For
example, the solution may be a cooling tower blowdown effluent,
seawater, a water desalination effluent or an effluent from an oil
extraction process.
[0065] In one embodiment, the solution has a solute (e.g. salt)
concentration of 1 to 400 weight %, preferably, 2 to 100 weight %,
more preferably, 5 to 80 weight %, for example, 10 to 50 weight %.
The solute may be one or more of the solutes mentioned above. For
example, the solute may be a salt selected from sodium chloride,
potassium chloride, potassium nitrate, magnesium sulfate (e.g.
MgSO.sub.4.6H.sub.2O or MgSO.sub.4.7H.sub.2O), magnesium chloride
(e.g. MgCl.sub.2.6H.sub.2O), sodium sulfate (e.g.
Na.sub.2SO.sub.4.10H.sub.2O), calcium chloride (e.g.
CaCl.sub.2.2H.sub.2O or CaCl.sub.2.6H.sub.2O), sodium carbonate,
disodium hydrogenphosphate (Na.sub.2HPO.sub.4.12H.sub.2O) and
potassium alum (24H.sub.2O). Alternatively, the solute may be a
sugar.
[0066] In one embodiment, the solution is a sodium chloride
solution having a sodium chloride concentration of 2 to 39 weight
%, preferably, 5 to 35 weight %, more preferably, 10 to 30 weight
%. In another embodiment, the solution is a potassium chloride
solution having a potassium chloride concentration of 5 to 50
weight %, preferably, 10 to 45 weight %, more preferably, 15 to 35
weight %. In another embodiment, the solution is a potassium
nitrate solution having a potassium nitrate concentration of 5 to
80 weight %, preferably, 10 to 60 weight %, more preferably, 15 to
45 weight %. In yet another embodiment, the solution is a calcium
chloride solution having a calcium chloride concentration of 5 to
120 weight %, preferably, 10 to 100 weight %, more preferably, 15
to 80 weight %. In an alternative embodiment, the solution is a
sodium carbonate solution having a sodium carbonate concentration
of 5 to 45 weight %, preferably, 10 to 35 weight i, more
preferably, 15 to 30 weight %. In a further embodiment, the
solution is a disodium hydrogenphosphate solution having a
disodium hydrogenphosphate concentration of 5 to 39 weight %,
preferably, 10 to 35 weight %, more preferably, 15 to 30 weight %.
In another embodiment, the solution is a sodium sulphate solution
having a salt concentration of 5 to 45 weight %, preferably, 10 to
40 weight %, more preferably, 15 to 39 weight %. In yet another
embodiment, the solution is a magnesium sulphate solution having a
magnesium sulphate concentration of 5 to 100 weight %, preferably,
10 to 80 weight %, more preferably, 15 to 75 weight %.
[0067] The difference in TDS of the liquid and the solution may be
at least 1 weight %, for example, from 1 to 39 weight %,
preferably, 5 to 35 weight %.
[0068] Any suitable selective membrane may be used in the process
of the present invention. An array of membranes may be employed.
Suitable membranes include cellulose acetate (CA) and polyamide
(PA) membranes. The membrane may be planar or take the form of a
tube or a hollow fibre. Thin membranes may be employed. If
desired, the membrane may be supported on a supporting structure,
such as a mesh support.
[0069] In one embodiment, one or more tubular membranes may be
disposed within a housing. The liquid may be introduced into the
housing, whilst the solution may be introduced into the tubular
membranes. As the solvent concentration of the liquid is higher
than that of the solution, liquid will diffuse across the membrane
into the solution. Thus, the solution will become increasingly
diluted with liquid. The diluted solution may be recovered from
the interior of the tubular membranes, whilst the liquid may be
removed from the housing.
[0070] When a planar membrane is employed, the sheet may be rolled
such that it defines a spiral in cross-section.
[0071] The pore size of the membrane may be selected depending on
the size of the solvent molecules that require separation. The
membrane may have an average pore size of 1 to 60 Angstroms,
preferably, 2 to 50 Angstroms, more preferably, 5 to 40 Angstroms,
for example, 10 to 30 Angstroms. In one embodiment, the membrane
has an average pore size of 12 to 25 Angstroms.
[0072] It may be possible to use a membrane having a pore size
that allows two or more different types of solvent molecules to
pass through the membrane. Conventional semi-permeable membranes
may be employed. Typically, such semi-permeable membranes have an
average pore size of, for example, 1 to 5 Angstroms.
[0073] The flow of solvent across a selective membrane is
generally influenced by thermal conditions. Thus, the liquid and
solution on respective sides of the membrane may be heated or
cooled, if desired. Preferably, the solution is heated to a
temperature of 30 to 90.degree. C., preferably, 50 to 70.degree.
C. The liquid may be cooled, for example, to -20.degree. C. to
20.degree. C., for example, 7 to 12.degree. C. Chemical reactions
may also be carried out on either side of the membrane, if
desired. In one embodiment, the solution and/or liquid may be
agitated. In another embodiment, the solution and/or liquid may be
subjected to an external field, such as an electrical, microwave
and/or laser field, to enhance the osmotic potential difference
between the two solutions.
[0074] The process of the present invention may further comprise a
pre-treatment step of removing contaminants, such as suspended
particles and biological matter, from the liquid (e.g. a waste
stream, seawater, river water, lake water or brackish water).
Additionally or alternatively, a threshold inhibitor to control
scaling may be added to the liquid. Pre-treatment steps to alter
the pH of the liquid may also be employed.
[0075] Optionally, the solution may also be treated to remove
contaminants, such as suspended particles and biological matter.
Additionally or alternatively, a threshold inhibitor to control
scaling may be added to the solution. Pre-treatment steps to alter
the pH of the solution may also be employed.
[0076] Optionally, step a) of the process may be repeated one or
more times. Thus, the pressurised solution from step a) may be
positioned on one side of a further selective membrane and a
further solution may be placed on the other side of the membrane.
The further solution has a higher osmotic potential than the
solution on the other side of the membrane, such that the further
solution becomes pressurised by the influx of liquid across the
membrane. The pressure of the further solution may be used to
drive the prime mover.
[0077] According to a further aspect of the present invention,
there is provided an apparatus for driving a prime mover, said
apparatus comprising [0078]a prime mover, [0079]a housing
comprising a selective membrane for separating a liquid from a
solution having a higher solute concentration than the liquid and
configured such that liquid passing through the membrane
pressurises the solution, [0080]means for transmitting the
pressure generated in the solution to the prime mover, [0081]means
for recovering the solution, [0082]means for separating solvent
from the solution to produce a residual product, and [0083]means
for recycling the residual product and/or the separated solvent to
the housing.
[0084] The residual product may be recycled to solution contained
in the housing of the apparatus. Alternatively, the residual
product may be recycled to solution contained in the housing of
another apparatus according to the present invention.
[0085] The prime mover may be any suitable device which is
suitable for converting energy in the solution into mechanical
power. Suitable prime movers include rotary prime movers, such as
turbines. Thus, the prime mover may be used to generate power.
[0086] Alternatively, the prime mover may be or form part of a
pressure exchange system. Thus, the prime mover may also be used
to transfer energy from the pressurised solution to another fluid.
Examples of suitable pressure exchange systems are described in
U.S. Pat. No. 4,887,942, U.S. Pat. No. 5,338,158, U.S. Pat. No.
5,988,993 and U.S. Pat. No. 6,540,487.
[0087] These and other aspects of the present invention will now
be described with reference to the accompanying drawings, in which
[0088]
FIG. 1 is a
schematic flow diagram of a process according to a first
embodiment of the present invention,
[0089]
FIG. 2 is a
schematic flow diagram of a process according to a second
embodiment of the present invention, and
[0090]
FIG. 3 is a
schematic flow diagram of a process according to a third
embodiment of the present invention.
[0091] Reference is first made to FIG. 1 of the drawings. This
Figure depicts a process according to a first embodiment of the
present invention. The process is performed using an apparatus 10
comprising an osmotic cell 12, a prime mover 14 (e.g. turbine
coupled to an electrical generator) and a separator 16. The
osmotic cell 12 comprises a semi-permeable membrane 18.
[0092] In use, water 11 (e.g. seawater) is introduced to one side
of the membrane 18. A 30 weight % solution of sodium chloride 13
is introduced to the opposite side of the membrane 18. As the
sodium chloride solution has a sodium chloride concentration that
is higher than the total dissolved salt (TDS) concentration of
seawater, water flows across the membrane 18 by osmosis. The
influx of water across the membrane 18 increases the pressure of
the sodium chloride solution.
[0093] The pressurised sodium chloride solution is removed from
the osmotic cell 12 and introduced to the prime mover 14. It is
not necessary to pump the sodium chloride solution as the solution
is pressurised by the osmosis step. The pressurised sodium
chloride solution is used to drive the prime mover 14. The
mechanical energy produced may be converted to other forms of
energy, such as electrical energy.
[0094] The sodium chloride solution may then be removed from the
prime mover 14 and introduced into the separator 16. In the
separator 16, water is removed from the sodium chloride solution
by evaporation. Once water is removed from the sodium chloride
solution, the sodium chloride solution is recycled to the osmotic
cell 12 for re-use. Thus, fresh sodium chloride solution is not
required to replenish or replace the sodium chloride solution in
the osmosis step.
[0095] The water removed by the evaporation step may be recovered
and used, for example, as drinking water. Thus, this embodiment of
the present invention may be used to desalinate seawater.
[0096] The apparatus 10 is located in close proximity to a
conventional power station 22. The power station 22 comprises a
boiler 24, a prime mover 26 (steam turbine) and a thermal unit 28
(condenser in power plant).
[0097] In use, water is introduced into the boiler 24 via a pump
30. The water is heated in the boiler 24 by the combustion of fuel
32 to produce superheated steam. The superheated steam is then
introduced at high pressure to the prime mover (steam turbine) 26,
and is used to drive the prime mover 26 to generate mechanical
energy. The mechanical energy of the rotating prime mover 26 may
be converted into other forms of energy, such as electrical
energy.
[0098] Saturated or superheated steam is then recovered from the
prime mover 26 and introduced into the thermal unit 28. In the
thermal unit, the steam is condensed to water. The excess heat
from the steam is used to evaporate water from the sodium chloride
solution of apparatus 10. Thus, the sodium chloride solution from
the prime mover 14 of apparatus 10 is used as a coolant in the
thermal unit 28 of the power station 22. The separator 16 of
apparatus 10, therefore, is effectively the same as the thermal
unit 28 of the power station 22.
[0099] Once cooled, the condensed steam of the power plant is
recycled to the boiler 24 via the pump 30.
[0100] Removed steam or water from the sodium chloride solution by
the separator 16 can be used as a pure water product or recycled
to unit 10.
[0101] Reference is now made to FIG. 2 of the drawings. This
figure depicts a process according to a second embodiment of the
present invention. The process is performed using an apparatus
100. Apparatus 100 is similar to the apparatus 10 of FIG. 1 and
like components of the apparatus have been labelled with like
numerals. Apparatus 100, however, is adapted for use in cold
climates. Thus, unlike the apparatus 10 of FIG. 1, the apparatus
100 comprises a separator 116 that is a crystallizer. In use,
solution emerging from the prime mover 14 is introduced into the
separator 116 and cooled by the ambient temperature to produce ice
and a concentrated sodium chloride solution. The former is removed
and discarded, whilst the latter is recycled to the osmotic cell
12.
[0102] Reference is now made to FIG. 3 of the drawings. This
figure depicts a process according to a third embodiment of the
present invention. The process is performed using an apparatus
200. Apparatus 200 is similar to the apparatus 10 of FIG. 1 and
like components of the apparatus have been labelled with like
numerals. Apparatus 200, however, is adapted for use in warm dry
climates. Thus, unlike the apparatus 10 of FIG. 1, the apparatus
100 comprises a separator 216 that relies on natural or effective
evaporation and/or solar energy to remove solvent from the
solution emerging from the prime mover 14.
