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SOLVENT REMOVAL PROCESS
This application is a 371 of PCT/GB04/03242, filed on Jul. 28, 2004, which claims priority of uk 0317839.9 filed on Jul. 30,2003. 5
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a process for removing a 10 solvent from a solution. In particular, although not exclusively, the present invention relates to a process for removing water from an aqueous solution, such as seawater.
2. The Prior Art
Various methods for removing solvents from solutions are 15 known. For example, water may be extracted from seawater by distillation methods such as multi-stage flash distillation. In a multi-stage flash distillation process, seawater is introduced into a series of tubes and heated to an elevated temperature. The heated seawater is then introduced into an 20 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 seawater streams. A series of evaporation chambers 25 are employed. Thus, the evaporation or flashing step occurs in multiple stages.
Water may also be separated from seawater by reverse osmosis. In reverse osmosis, seawater is placed on one side of a semi-permeable membrane and subjected to pressures of 5 to 8 MPa. The other side of the membrane is maintained at atmospheric pressure. The resulting pressure differential causes water to flow across the membrane, leaving a salty concentrate on the pressurized side of the membrane. Typically, the semi-permeable membranes have an average pore size of, for example, 1 to 5 Angstroms.
After a period of operation, the pores of the semi-permeable membrane may become obstructed by deposited salts, biological contaminants and suspended particles in the seawater. Thus, higher pressures may be required to maintain the desired level of flow across the membrane. The increased pressure differential may encourage further clogging to occur. Thus, the membranes must be cleaned and replaced at regular intervals, interrupting the continuity of the process and increasing operational costs.
Attempts have been made to reduce the level of clogging of the membrane. For example, the seawater may be pretreated to remove suspended particles and biological matter. Alternatively or additionally, the residual solution on the high- 5Q pressure side of the membrane may be discharged at regular intervals to prevent the osmotic pressure from exceeding a predetermined threshold.
According to a first aspect of the present invention, there is provided a process for removing a solvent from a first solution, said process comprising
positioning a selective membrane between the first solu- 60 tion and a second solution having a higher osmotic potential than the first solution, such that solvent from the first solution passes across the membrane to dilute the second solution, and
extracting solvent from the second solution, 65
wherein the membrane has an average pore size of at least 10 Angstroms,
wherein the second solution contains solute species that are too large to pass through the pores of the membrane.
Preferably, the membrane has an average pore size of from 10 to 80 Angstroms, more preferably, 15 to 50 Angstroms. In a preferred embodiment, the membrane has an average pore size of from 20 to 30 Angstroms. The pore size of the membrane may be selected depending on the size of the solvent molecules that require separation. In general, the larger the pore size, the greater the flux through the membrane.
Any suitable selective membrane may be used in the process of the present invention. An array of membranes may be employed. 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.
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.
In one embodiment, one or more tubular membranes may be disposed within a housing or shell. The first solution may be introduced into the housing, whilst the second solution may be introduced into the tubes. As the osmotic potential of the first solution is lower than that of the second solution, solvent will diffuse across the membrane from the first solution into the second solution. Thus, the second solution will become increasingly diluted and the first solution, increasingly concentrated. The diluted second solution may be recovered from the interior of the tubes, whilst the concentrated first solution may be removed from the housing.
When a planar membrane is employed, the sheet may be rolled such that it defines a spiral in cross-section.
One or more solutes may be present in each of the solutions. In a preferred embodiment, the first solution comprises a plurality of solutes, whilst the second solution is formed by dissolving one or more known solutes in a solvent.
In the process of the present invention, the first solution is placed on one side of a selective membrane. A second solution having a higher osmotic potential is placed on the opposite side of the membrane. Typically, although not exclusively, the second solution has a higher solute concentration (and therefore lower solvent concentration) than first solution.
As a result of the difference in osmotic potential between the first solution and the second solution, solvent passes across the membrane from the side of low osmotic potential to the side of high osmotic potential. The flow occurs with a high flux due to the large average pore size of the membrane. High pressures are not required to induce solvent flow. However, a pressure differential across the membrane may be applied, for example, to enhance the speed of the separation process.
Although the solute species in the first solution may be sufficiently small to pass through the pores of the membrane, they are prevented from doing so at least initially because of the high osmotic potential on the other side of the membrane. The flow of these solute species across the membrane is only possible once the osmotic potential is equal on both sides of the membrane or the osmotic potential is higher in the first solution.
The second solution contains solute species that are too large to pass through the pores of the membrane. As a result, solvent from the first solution will diffuse into the second solution at a high rate, whilst the passage of solute between the two solutions is restricted or prevented.
Optionally, the second solution may also contain solute species that are sufficiently small to pass through the pores of
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the membrane. These small species will not pass across the membrane if their concentration in the second solution is below their concentration in the first solution. Thus, in a preferred embodiment, the second solution optionally contains at least one solute species that is sufficiently small to 5 pass through the pores of the membrane in a concentration that is less than the concentration of the corresponding species in the first solution.
As solvent passes from the first solution into the second solution, the first solution becomes increasingly concen- 10 trated. Once the concentration of the first solution reaches a certain threshold, the solution may be recovered or discarded. Thus, the process of the present invention may be used to convert the first solution into a concentrated form for disposal. Alternatively, further solvent may be removed from the 15 concentrated first solution by repeating the initial membrane separation step. Specifically, further solvent may be removed from the concentrated first solution by placing this solution on one side of a semi-permeable membrane. A further solution having an osmotic potential that is higher than that of the 20 concentrated solution may be placed on the opposite side of the membrane, such that solvent from the concentrated first solution passes across the membrane into the further solution. The further solution may contain the same solute(s) and solvents) as the second solution. Alternatively, the further solu- 25 tion may contain different components.
After solvent from the first solution has passed into the second solution, the second solution may be recovered. The second solution may be at an elevated pressure, even when a pressure is not applied to induce solvent flow from the first 30 solution to the second solution. This is because the flow of solvent from the first solution into the second solution occurs along a concentration gradient. This pressure may be used to aid the subsequent extraction of solvent from the second solution. For example, when solvent is extracted from the 35 second solution by thermal methods, such as multi-stage flash distillation (MSF), the pressure of the second solution may be used to supplement the pumping of the second solution to the multi-stage flash distillation unit. When solvent is extracted from the second solution by membrane methods, such as 40 nanofiltration and reverse osmosis, the pressure of the second solution may be used to supplement the pressure applied to the second solution to induce solvent flow from the second solution across the selectively permeable membrane. Valves and other pressure regulating devices may be used to control 45 the pressure accordingly. One or more pumps may also be used to supplement the pressure of the process streams if necessary.
The initial flux of solvent across the membrane may be 2 to 5Q 80 lm~2hr_1, preferably, 5 to 40 lm~2hr_1, for example, 15 to 201m~2hr_1, even in the absence of an applied pressure on the first solution. However, the flux may vary depending on a number of factors such as the concentration gradient of the two solutions across the membrane. 55
The fluid velocity across the surface of the membrane may be varied as required to reduce the risk of fouling of the membrane. Generally, the greater the fluid velocity across the surface of the membrane, the lower the risk of fouling.
Solvent may be extracted from the second solution using 60 any suitable method. For example, the solvent may be extracted by thermal/pressure methods (e.g. crystallization and distillation) or using a membrane. Suitable membrane methods include reverse osmosis, nanofiltration, electrodialysis reversal and ion exchange. When reverse osmosis is 65 employed, the same type of membrane employed in the direct osmosis step may be used in the reverse osmosis step. Solvent
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may be extracted from the second solution using hybrid methods combining, for example, thermal and membrane methods of separation.
In a preferred embodiment, nanofiltration membranes are employed to extract solvent from the second solution.
Nanofiltration is particularly suitable for separating the large solute species of the second solution from the remainder of the solution.
Suitable nanofiltration membranes include crosslinked polyamide membranes, such as crosslinked aromatic polyamide membranes. The membranes may be cast as a "skin layer" on top of a support formed, for example, of a microporous polymer sheet. The resulting membrane has a composite structure (e.g. a thin-film composite structure). Typically, the separation properties of the membrane are controlled by the pore size and electrical charge of the "skin layer". The membranes may be suitable for the separation of components that are 0.01 to 0.001 microns in size and molecular weights of 100 gmol-1 or above, for example, 200 gmol-1 and above.
As well as filtering particles according to size, nanofiltration membranes can also filter particles according to their electrostatic properties. For example, in certain embodiments, the surface charge of the nanofiltration membrane may be controlled to provide desired filtration properties. For example, the inside of at least some of the pores of the nanofiltration membrane may be negatively charged, restricting or preventing the passage of anionic species, particularly multivalent anions.
Examples of suitable nanofiltration membranes include Desal-5 (Desalination Systems, Escondido, Calif), NF 70, NF 50, NF 40 and NF 40 HF membranes (FilmTech Corp., Minneapolis, Minn.), SU 600 membrane (Toray, Japan) and NRT 7450 and NTR 7250 membranes (Nitto Electric, Japan).
The nanofiltration membranes may be packed as membrane modules. Spiral wound membranes, and tubular membranes, for example, enclosed in a shell may be employed.
Alternatively, the membranes may be provided as a plate or in a frame.
A multi-stage flash distillation method (MSF) may also be employed to extract solvent from the second solution. For example, the second solution may be heated and introduced into an evaporation chamber, where it is subjected to a pressure below its vapour pressure. The sudden reduction in pressure causes boiling or flashing to occur. The flashed vapours may be separated from the remainder of the solution by condensation. A series of evaporation chambers are preferably employed. Thus, the evaporation or flashing step can take place in multiple stages. In a preferred embodiment, heat energy from the flashed vapours is transferred to the incoming solution by heat exchange. As a result of this transfer of heat, the vapours are condensed and the temperature of the incoming solution increased.
Multiple effect distillation (MED) may also be employed to extract solvent from the second solution. 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 second solution to boil in a series of stages without the need for additional heat to be supplied after the first effect.
In multiple effect distillation, the second 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. Thus, 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
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