Patent ID: 12239940

DETAILED DESCRIPTION

FIG.1schematically illustrates a typical solution mining process (one not in accordance with the present invention), where a suitable low concentration solution100is injected using a pump120to a subsurface mineral formation130such as a subsurface mineral ore to create a high concentration solution110that is sent to further processing140. The mineral ore may comprise sugar or salts for example sodium chloride, potassium chloride, calcium chloride or other salts.

Because the density increases as minerals dissolve in the low concentration solution100, pumping energy is required to lift solution with a mass equal to the density difference from the formation to the surface. If the density difference is 200 kg/m3and the solution is extracted from a depth of 2 km, an injection pressure of about 39 bar is required (not including pressure losses in the system).

FIG.2shows an example solution mining process in accordance with the invention in which a surplus of high concentration solution110is extracted from the substance concentration130and recirculated between the mineral formation130and a suitable PRO system200, the remaining high concentration solution110is sent for further processing140. A low concentration solution100is fed by a feed pump120under low pressure to the PRO system200where it mixes with high concentration solution110to produce a dilute solution150. The entire dilute solution150mixture is injected into the mineral formation130to dissolve additional minerals. The extracted volume of high concentration solution110and the reinjected dilute solution150must be of equal volume to maintain a constant volume in the mineral formation150. In this way, the mixing of the low concentration solution100and the high concentration solution110, is moved from taking place in the mineral formation130to the osmotic power system200, where the energy can be harvested, allowing the extraction to be driven by the spontaneous mixing of low concentration100and high concentration solutions110.

FIG.3shows a variation of the process shown inFIG.2. Here the entire volume of high concentration solution110extracted from the formation130is sent to the PRO system200, where it mixes with low concentration solution100. Part of the resulting dilute solution150is sent back the formation130, while the remaining part of the dilute solution160is sent for further processing140. In this setup the dilute stream160sent for further processing140will be lower in concentration than the extracted high concentration stream110. This version of the invention is thus useful for scenarios where the further processing does not rely on and prefers lower concentrations. An example could be discharge of dilute formation water as part of excavation of a cavern for gas or other storage, where disposal of water with high concentrations of minerals may be difficult. It is possible to obtain the same end result from the layout given inFIG.2but this requires the further processing step to comprise of an additional osmotic power unit. In the layout inFIG.3, this can be accomplished in one system using fewer components.

FIG.4shows an example PRO type osmotic power unit200, suitable for use in the systems ofFIG.2or3. The high concentration solution110is pressurized in a pressure exchanger210(e.g. a heat exchanger, a rotary pressure exchanger etc.) at a pressure below the difference in osmotic pressure between the high concentration110and the low concentration100solutions. The pressurized high concentration solution is then sent to one side of a semi-permeable membrane220, while the low concentration solution100is sent to the other side of the membrane220. The low concentration solution is pressurized using a feed pump230prior to being sent to the membrane220. Due to the difference in osmotic pressure, solvent will spontaneously move from the low concentration side to the high concentration side to equalize the chemical potential across the membrane220. This creates a dilute solution150the total pressure of which is higher than the total pressure of the high concentration stream110on input to the semi-permeable membrane220. A first fraction of this dilute solution150is directed to a power generating device250such as a turbine to produce electricity. A second part of the dilute solution150is passed to the pressure exchanger210where pressure from the dilute solution150is transferred to the high concentration solution110.

Passage through the power generating device250reduces the total pressure of the first fraction of the dilute solution150to the injection pressure. The dilute solution150can then be passed to the mineral formation130without the need for any additional mechanical pumping. This may provide a particularly efficient solution mining process, in particular in comparison to those in which the dilute solution150is pressurized using a pump driven using electricity generated in the osmotic power unit200.

In some embodiments the second fraction of dilute solution150output from the pressure exchanger210is not reinjected into the mineral formation130. In the process ofFIG.2, it may be combined with the high concentration solution110sent for further processing140or disposed of as appropriate, for example into a nearby watercourse. In the process ofFIG.3, the second fraction of dilute solution150may be sent for further processing as stream160.

In some embodiments, the entire stream110going to the pressure exchanger210is reinjected into the formation130. In this case the second fraction of dilute solution150output from the pressure exchanger210and first fraction of dilute solution150output from the power generation device250must be combined and reinjected. A pump (not shown) is used to pressurize the second fraction after passage through the pressure exchanger210to the injection pressure before it is recombined with the first fraction (which is at the injection pressure already).

In some embodiments the pressure exchanger is absent. In the same or yet further embodiments a pump is used to pressurize the high concentration solution110prior to passing over the membrane220. This makes all the pressurized dilute solution150available for passing through the power generation device250(by which passage the pressure of the dilute solution150is reduced to the injection pressure) and thereby allows the entire dilute solution150to be sent directly for injection.

Only the high concentration solution110must be pressurized at the high pressure (>30 bar) required for injection, whereas the low concentration solution100can be pumped to the membrane using a low pressure (<15 bar). Power is needed to drive the pump or pressure exchanger for the high concentration solution110and the low pressure pump230for the low concentration solution100, and by operating the PRO process at a pressure higher than the injection pressure, the power generating device can utilize the pressure gradient for energy generation (to power the high pressure pump and the low pressure pump) while the diluted solution150can be sent directly for injection.

It is also possible to use several osmotic power units200in combination to enhance the efficiency of the process.FIG.5comprises an example of such a system which comprises two osmotic power units (A, B) (though the system could include any number of stages (A, B) in succession of each other) of the type shown inFIG.4. The dilute solution150coming from the prior stage A is used as the high concentration solution110for the subsequent stage B. The dilute solution150from the subsequent stage B passes through the power generation system250to have a pressure on exit equal to the injection pressure. The dilute solution is then reinjected into the mineral formation130. The two stages A, B can operate at different pressures, with the pressure in the prior stage A being higher than in the subsequent stage B. To maximize energy generation, it is desirable to operate the PRO process at high pressures, but as the pressure is increased, the degree of dilution of the dilute solution150that can be obtained is lowered because the osmotic pressure difference decreases as solvent crosses the membrane220. Operating with dual stages as illustrated thus allows for a greater energy generation and dilution to lower concentrations. This may mean that less additional brine (high concentration solution110) needs to be extracted from the substance concentration130to run the PRO process.

In a variation of the process ofFIG.5, the pressure exchanger210is omitted from both stages A, B. Instead, the high concentration solution110is pressurized using a pump and then passes over the membrane220in prior stage A to produce dilute solution150. After passage through the power generation device250of prior stage A the dilute solution150is used as the high concentration solution110of subsequent stage B. In some embodiments, passage through power generation device250of prior stage A reduces the pressure of dilute solution150to the operating pressure for the membrane220of subsequent stage B. That is to say, the dilute solution from prior stage A can be passed directly to the membrane of subsequent stage B without the need for any pumping and thereby removing the need for any additional pump. In this way, there is no need for an additional pump or pressure exchanger to pressurize the solution before it enters the subsequent stage B membrane220.

FIG.6shows a schematic diagram of a mobile production unit350for use with a salt formation130. Injection well310and extraction well315extend from the surface to a salt cavern330located within the salt formation130. An outflow port340of production unit300is connected to injection well310and an inflow port345connected to extraction well315(these connections being shown with dashed lines inFIG.6). The mobile unit350comprises an osmotic power unit200, a control system (not shown) and other elements of a solution mining system not shown here for clarity. The mobile unit350further comprises an inflow port360and output flow port365, both connected to a water source (not shown). Within mobile unit350a hydraulic system connects the osmotic power unit200to the various ports as follows (shown by dashed lines inFIG.6); inflow port360is connected to the low-salinity input of the osmotic power unit, outflow port365with the waste (low-salinity) output of osmotic power unit200, outflow port340with the osmotic power unit output for the stream derived from the high-salinity input, and inflow port345with the high-salinity input of osmotic power unit200. The total pressure of the osmotic power unit output for the stream derived from the high-salinity input is substantially equal (barring minor pipe flow losses etc.) to the total pressure of the stream at the outflow port340and the head of the injection well310. Accordingly, there is no pump situated between the output from the osmotic power unit200and the head of the injection well310. A portion (not shown) of the high-salinity stream from extraction well315is split off upstream of the mobile production unit350and sent for further processing, for example use in an industrial process. Once the cavern330has been excavated fuel, for example hydrogen, biogas, natural gas, methanol and/or ammonia, may be pumped into the cavern for storage.

In a variation of the process shown inFIG.6, the entire high-salinity stream from extraction well315is sent to the osmotic power unit200and part of the stream derived from the high-salinity input after passage through the osmotic power unit200can be discharged through outflow port365with the waste stream. In this way the volumetric balance in the cavern330can be maintained.

It will be appreciated that the apparatus ofFIG.6can be used with other minerals in place of salt.

The impact of the present invention on the efficiency of the solution mining process can be seen in the consideration of the following systems (all of which produce 100 m3 saturated brine per hour).

A traditional solution mining process uses an injection pump to pressurize the fluid for injection into the salt formation. As shown below, such a process requires an energy input of 163 kW/hour to operate.

ProcessInjection pumpTotalFlowm3/h103.09Pressurebar40Efficiency0.7EnergykW−163−163

The energy requirements for a solution mining process that uses electricity from an osmotic power unit including a turbine to power an injection pump that pressurizes the fluid for injection into the salt formation is shown below. The feed and draw pumps are used to pressurize the low and high concentration flows respectively prior to passage over the semi-permeable membrane, use of such pumps increasing the efficiency of the osmotic power unit and balancing the flow either side of the membrane. The ERD is an energy recovery device that transfers pressure from the reduced concentration output stream to the high concentration input stream. It is fed by the draw pump. In the example process below the injection pump that returns all diluted saltwater to the salt formation. Such a process requires an energy input of 43 kW/hour to operate.

FeedDrawInjectionProcesspumppumpERDTurbinepumpTotalFlowm3/h1296262102165Pressurebar8.92808024.5Efficiency0.70.630.950.840.7EnergykW−47.5−5.7−8.3185.5−167−43

The energy requirements for an example process in accordance with the present invention is shown below. No injection pump is needed as the turbine lowers the pressure to the injection pressure. Again, all the diluted saltwater is returned to the salt formation. Such a process requires an energy input of 4 kW/hour to operate. Further, this efficiency may be achieved without a pressure exchanger, thereby reducing the number of components required in the system.

FeedDrawInjectionProcesspumppumpTurbinepumpTotalFlowm3/h12963.5167—Pressurebar9.28024.2—Efficiency0.70.90.840.7EnergykW−49−1632080−4

Whilst the present invention has been described and illustrated with reference to particular embodiments, it will be appreciated by those of ordinary skill in the art that the invention lends itself to many different variations not specifically illustrated herein.

Where in the foregoing description, integers or elements are mentioned which have known, obvious or foreseeable equivalents, then such equivalents are herein incorporated as if individually set forth. Reference should be made to the claims for determining the true scope of the present invention, which should be construed so as to encompass any such equivalents. It will also be appreciated by the reader that integers or features of the invention that are described as preferable, advantageous, convenient or the like are optional and do not limit the scope of the independent claims. Moreover, it is to be understood that such optional integers or features, whilst of possible benefit in some embodiments of the invention, may not be desirable, and may therefore be absent, in other embodiments.