Method for selecting piping materials

Abstract

One aspect of the present invention is a method for selecting a pipe material to be used for a pipe through which a geothermal fluid containing a monomer or a 2- to 4-mer of ortho-silicic acid is passed in a geothermal power plant. The method includes: comparing the molecular orbital energy levels of at least two resin materials having mutually different compositions or chemical structures to predict the adhesiveness of the resin materials to the monomer or the 2- to 4-mer of ortho-silicic acid; and selecting a resin material, which is selected on the basis of the adhesiveness, as a pipe material with which the adhesion of silica scales precipitated from the geothermal fluid is reduced.

Description

Technical Field

[0001] The present invention relates to a method for selecting pipe materials. In the law

Background Art

[0002] Geothermal power generation involves collecting high-temperature geothermal fluid (geothermal water and geothermal steam) from production wells and generating electricity using the steam separated from the geothermal fluid. The geothermal fluid collected from production wells contains more dissolved silica than well water or river water.

[0003] The dissolved silica in the geothermal water collected from production wells is concentrated by being depressurized in a geothermal power plant, cooled while flowing through the pipes, and its solubility decreases. When the silica contained in the geothermal water becomes supersaturated, it polymerizes into amorphous silica and precipitates as silica scale. Since silica scale may adhere to the inner wall of the pipes and cause blockage of the pipes, the adhesion of silica scale has been a problem in geothermal power plants.

[0004] Conventionally, pipes for reducing the adhesion of silica scale have been studied. For example, Patent Document 1 discloses an inner surface coating for a metal water pipe made of a coating having a thickness of 100 nm or more and 1000 nm or less, containing 80% to 95% by volume of silica, and the balance being an organic substance and pores.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0006] ​However, with the coating for the inner surface of metal water pipes described in Patent Document 1, there was a possibility that geothermal fluid could penetrate through the pores in the coating to the base metal water pipe, potentially causing corrosion of the metal water pipe. Therefore, the application of resin materials as piping materials has been considered, but generally, experimental and empirical methods are used when selecting resin materials with low adhesion to silica scale. As a result, unexpected results can occur, and the selection of resin materials is very time-consuming and labor-intensive.

[0007] One aspect of the present invention provides a method for selecting a suitable resin material for piping that reduces silica scale buildup in geothermal power plants. [Means for solving the problem]

[0008] One aspect of the present invention is a method for selecting piping materials to be used in piping for circulating geothermal fluids containing orthosilicic acid monomers or di-tetramers in a geothermal power plant, wherein the adhesion to orthosilicic acid monomers or di-tetramers is predicted by comparing the molecular orbital energy levels of two or more resin materials with different compositions or chemical structures, and the resin material selected based on the adhesion is selected as a piping material that reduces the adhesion of silica scale precipitated from the geothermal fluid. [Effects of the Invention]

[0009] According to one aspect of the present invention, a suitable resin material can be selected as a piping material that reduces silica scale buildup in geothermal power plants. [Brief explanation of the drawing]

[0010] [Figure 1] This flowchart shows an example of a method for calculating molecular orbital energy levels. [Figure 2] This is a schematic diagram of a geothermal power plant according to one embodiment. [Figure 3]This graph shows the calculated LUMO energy for each resin material. [Figure 4] This is a SEM image showing the surface of a material piece made of PVC. [Modes for carrying out the invention]

[0011] The embodiments for carrying out the present invention will be described below with reference to the drawings.

[0012] The piping material selection method of this embodiment is a method for selecting piping materials used in piping that carries geothermal fluid containing orthosilicic acid monomers or di-tetramers in a geothermal power plant. In a geothermal power plant, the concentration of dissolved silica in the geothermal fluid flowing inside the piping reaches 450 ppm to 900 ppm. In the geothermal fluid, the monomers of orthosilicic acid dissolved at high concentrations undergo dehydration condensation to become dimers, and further dehydration condensation causes them to grow, resulting in the precipitation of silica scale. Furthermore, since the monomers or di-tetramers of orthosilicic acid are particularly reactive, they readily bond with molecules on the inner surface of the piping, that is, they readily adhere to the inner surface of the piping, and it is presumed that this greatly influences the adhesion of silica scale.

[0013] The method for selecting piping materials in this embodiment predicts the adhesion to orthosilicic acid monomers or di-tetramers by comparing the molecular orbital energy levels of two or more resin materials with different compositions or chemical structures. Based on this adhesion, the selected resin material is then chosen as a piping material that reduces the adhesion of silica scale precipitated from geothermal fluids. This allows for the prediction of the adhesion to orthosilicic acid monomers or di-tetramers for multiple candidate resin materials, and enables the selection of a suitable resin material for piping that reduces silica scale adhesion in geothermal power plants. In other words, by using the resin material selected by the method for selecting piping materials in this embodiment as a piping material, the adhesion of silica scale to the inner surface of the pipe can be reduced. Furthermore, by using the resin material selected by the method for selecting piping materials in this embodiment as a piping material, corrosion by geothermal fluids can be suppressed compared to when metal materials are used as piping materials, and installation and replacement of the pipes can be simplified.

[0014] Specifically, examples of two or more resin materials with different chemical structures include two or more resin materials that are structural isomers or stereoisomers. The molecular orbital energy levels of two or more resin materials with different compositions or chemical structures can be obtained by first-principles calculations. First-principles calculations can be performed, for example, using commercially available calculation software such as Gaussian®. Figure 1 is a flowchart showing an example of a method for calculating molecular orbital energy levels. Specifically, the method for obtaining molecular orbital energy levels by first-principles calculations involves setting, for example, the type of calculation, the calculation method, the conditions for initial orbital prediction, the electron density analysis, and the conditions for molecular orbital output. Then, as shown in Figure 1, a molecular model of the molecules constituting the candidate resin materials is created and input (step S1), and basis functions are assigned (step S2). Next, it is determined whether or not this is the first time the basis functions have been assigned (step 3), and if it is determined to be the first time (step S3: Yes), an integral calculation is performed (step S4). Based on the results of the integral calculation, an initial estimation of molecular orbitals is performed (Step S5), followed by a Self-Consistent Field (SCF) calculation (Step S6), and then the forces acting on the atoms are calculated (Step S7). Next, it is determined whether the calculation for structural optimization has converged (Step S8). If it is determined that it has not converged (Step S8: No), the structural optimization calculation is performed again (Step S9), and the process returns to Step 2 to assign the basis functions again. Next, in Step 3, if it is determined that the basis function assignment is not the first time (Step S3: No), the process proceeds to Step 6, followed by Step 7. After that, in Step 8, if it is determined that the calculation for structural optimization has not converged (Step S8: No), the structural optimization calculation is performed again (Step S9), and Steps 2, 3, and 6-9 are repeated until the calculation for structural optimization converges. In step 8, if it is determined that the calculations for structural optimization have converged (step S8: Yes), an electron density analysis (Population analysis) is performed (step S10), and based on the analysis results, molecular orbital information is output (step S11), and the process terminates.

[0015] Figure 1 shows a calculation method using molecular orbital theory as a first-principles calculation, but it is not limited to this. Any calculation method can be used as a first-principles calculation, such as density functional theory (DFT), coupled cluster theory (CCSD, etc.), or Hartree-Fock method (HF).

[0016] In the method for selecting piping materials of this embodiment, the molecular orbital energy level used may be the lowest unoccupied molecular orbital (LUMO) energy level. Since orthosilicic acid monomers or di-tetramers (silica) are electron donors, it is thought that electrons from the orthosilicic acid monomers or di-tetramers move to the LUMO of the resin material, chemically bonding with the molecules constituting the resin material and adhering to the surface of the resin material. Here, the lower the LUMO energy level of the resin material, the easier it is for electrons from the orthosilicic acid monomers or di-tetramers to move to the LUMO of the resin material, and the easier it is for the energy after the move to stabilize. As a result, it is thought that the lower the LUMO energy level of the resin material, the easier it is for the orthosilicic acid monomers or di-tetramers to adhere to the surface of the resin material. From the above, by comparing the LUMO energy levels of several candidate resin materials, the adhesion to orthosilicic acid monomers or di-tetramers can be predicted. Therefore, the method for selecting piping materials in this embodiment allows for the selection of a suitable resin material for piping that reduces silica scale buildup in geothermal power plants.

[0017] In the method for selecting a pipe material according to this embodiment, it is preferable to select a resin material having a LUMO energy level higher than the LUMO energy level of polytetrafluoroethylene (PTFE). Generally, PTFE is known to have low adhesion to many substances among resin materials. However, as a result of intensive studies, the inventor has obtained the finding that PTFE has high adhesion to silica scale. In the method for selecting a pipe material according to this embodiment, by selecting a resin material having a LUMO energy level higher than the LUMO energy level of PTFE, a more suitable resin material can be selected as a pipe material for reducing the adhesion of silica scale in a geothermal power plant.

[0018] Specifically, in the method for selecting a pipe material according to this embodiment, a resin material having a LUMO energy level higher than 3.5 eV, which is the LUMO energy level of PTFE, may be selected.

[0019] Further, in the method for selecting a pipe material according to this embodiment, it is more preferable to select a resin material having the highest LUMO energy level among two or more resin materials having a LUMO energy level higher than the LUMO energy level of polytetrafluoroethylene. Thereby, a more suitable resin material can be selected as a pipe material for reducing the adhesion of silica scale in a geothermal power plant.

[0020] In the method for selecting a pipe material according to this embodiment, a resin material having a LUMO energy level higher than the LUMO energy level of PTFE and an energy difference from the LUMO energy level of PTFE of 0.5 eV or more may be selected. Thereby, a more suitable resin material can be selected as a pipe material for reducing the adhesion of silica scale in a geothermal power plant.

[0021] Specifically, in the method for selecting a pipe material according to this embodiment, since the LUMO energy level of PTFE is 3.5 eV, a resin material having a LUMO energy level of 4.0 eV or more may be selected.

[0022] Figure 2 is a schematic configuration diagram of a geothermal power plant according to an embodiment. The geothermal power plant 100 of the present embodiment has a pipe including a resin material selected by the above-described method for selecting a pipe material at least on the inner peripheral surface. In the method for selecting a pipe material of the present embodiment, a suitable resin material can be selected as a pipe material for reducing the adhesion of silica scale in the geothermal power plant. Therefore, the geothermal power plant of the present embodiment can reduce the adhesion of silica scale to the inner peripheral surface of the pipe.

[0023] The geothermal power plant 100 of the present embodiment preferably has a pipe made of a resin material selected by the above-described method for selecting a pipe material. Generally, in a pipe having a coating layer provided on the inner peripheral surface of the pipe, the coating layer may peel off, and the bonding force between the inner peripheral surface of the pipe and silica scale may increase. In the geothermal power plant 100 of the present embodiment, since the entire pipe is made of the selected resin material, even when the inner peripheral surface of the pipe peels off or is damaged, the bonding force between the inner peripheral surface of the pipe and silica scale can be reduced. Therefore, the geothermal power plant 100 of the present embodiment can further reduce the adhesion of silica scale to the inner peripheral surface of the pipe.

[0024] An example of the configuration of the geothermal power plant 100 will be described with reference to FIG. 2. The geothermal power plant 100 may include a water supply pump 2 that pumps geothermal fluid from a production well 1, a gas-liquid separator 3 that separates the geothermal fluid into geothermal water and geothermal steam, and a turbine 4 that rotates by supplying the geothermal steam separated by the gas-liquid separator 3, which are arranged in order from the upstream side of the geothermal fluid. In the geothermal power plant 100, the region from the production well 1 to the turbine 4 as described above can be defined as a first region 10. That is, the first region 10 includes the production well 1, the water supply pump 2, the gas-liquid separator 3, and the turbine 4. The turbine 4 is connected to a generator 5. The first region is a region in the geothermal power plant 100 where the environment is high temperature and high pressure.

[0025] The geothermal power plant 100 may include piping L1 for introducing geothermal fluid (geothermal water and geothermal steam) pumped from production well 1 to feedwater pump 2, piping L2 for introducing geothermal fluid discharged from feedwater pump 2 to steam-water separator 3, and piping L3 for introducing geothermal steam separated in steam-water separator 3 to turbine 4. That is, the first region 10 may include piping L1 to L3.

[0026] The geothermal power plant 100 may include a condenser 6 that condenses the geothermal steam discharged from the turbine 4, a cooling tower 7 that cools the condensed water in the condenser 6, and a circulation pump 8 that sends the cooled water from the cooling tower 7 back to the condenser 6. In the geothermal power plant 100, the region from the condenser 6 to the point where the condensed water is cooled and returns to the condenser 6 can be designated as the second region 20. That is, the second region 20 may include the condenser 6, the cooling tower 7, and the circulation pump 8. The second region is the region in the geothermal power plant 100 that is under a low-temperature, low-pressure environment.

[0027] The geothermal power plant 100 may include piping L4 for introducing condensed water (hot water) condensed in the condenser 6 to the cooling tower 7, piping L5 for introducing the cooling water cooled in the cooling tower 7 to the circulation pump 8, and piping L6 for returning the cooling water discharged from the circulation pump 8 to the condenser 6. In other words, the second region 20 may include piping L4 to L6.

[0028] The geothermal power plant 100 may include a retention tank 9 located in the flow path through which the geothermal water separated by the steam-water separator 3 flows, and a reduction pump 11 that returns the geothermal water discharged from the retention tank 9 to the reduction well 14. The retention tank 9 allows the polymerization reaction of silica in the geothermal water to proceed and retains the water until the silica-based insoluble components have sufficiently coagulated and settled. In the geothermal power plant 100, the region from the outlet where the geothermal water flows out of the steam-water separator 3 to the reduction well 14 can be designated as a third region 30. That is, the third region 30 may include the retention tank 9, the reduction pump 11, and the reduction well 14. The third region is a region in the geothermal power plant 100 that is under a high temperature and medium pressure or medium temperature and high pressure environment.

[0029] The geothermal power plant 100 may be equipped with piping L7 for introducing geothermal water separated by the steam-water separator 3 into the retention tank 9. That is, the third region 30 may include piping L7.

[0030] The geothermal power plant 100 may have a collector 13 connected to piping L7 that constitutes the flow path between the steam-water separator 3 and the retention tank 9. The collector 13 may be, for example, a container having a wire mesh inside for capturing scale fragments, or a container having a similar function to the retention tank 9. The geothermal power plant 100 may further have a collector 13 connected to piping L6 that constitutes the flow path between the circulation pump 8 and the condenser 6.

[0031] Next, the flow of geothermal fluid in the geothermal power plant 100 will be explained. In Figure 2, the flow of geothermal fluid is shown by solid arrows in each pipe. Production well 1 is a well that brings geothermal water, geothermal steam, or a mixture thereof (geothermal fluid) from the underground geothermal reservoir to the surface. The geothermal fluid pumped up from production well 1 is introduced to the feedwater pump 2 through pipe L1 and sent to the gas-liquid separator 3 through pipe L2. In the gas-liquid separator 3, it is separated into geothermal steam, which is the gaseous component, and geothermal water, which is the liquid component. The separated geothermal steam is sent to the turbine 4 through pipe L3 and used to rotate the turbine 4, which generates electricity with the generator 5.

[0032] The geothermal steam that has passed through turbine 4 is sent to condenser 6 where it is condensed, and the condensed water is further sent to cooling tower 7 via piping L4 for cooling. The cooled water is introduced to circulation pump 8 via piping L5, returned to condenser 6 via piping L6, and used as cooling water for the geothermal steam that has passed through turbine 4.

[0033] The geothermal power plant 100 may have piping in at least the first region 10 and the second region 20 that includes a resin material selected by the piping material selection method described above on at least its inner surface. This makes it possible to reduce the adhesion of silica scale to the inner surface of the piping in regions where silica scale is likely to form, and to more effectively reduce silica scale adhesion as a whole geothermal power plant 100.

[0034] Furthermore, the method for selecting piping materials in this embodiment is not limited to the geothermal power plant with the configuration shown in Figure 2, but can be applied to any geothermal power plant with any configuration. In other words, the geothermal power plant 100 in this embodiment is not limited to the configuration shown in Figure 2, but can be any geothermal power plant. The geothermal power plant 100 may be, for example, a binary cycle type geothermal power plant. [Examples]

[0035] Next, embodiments of the present invention will be described.

[0036] For nine types of resin materials—polytetrafluoroethylene (PTFE), polyvinyl chloride (PVC), ethylenetetrafluoroethylene (ETFE), isotactic polymethyl methacrylate (it-PMMA), syndiotactic polymethyl methacrylate (st-PMMA), polyethylene terephthalate-1 (PET-1), polyethylene terephthalate-2 (PET-2), nylon 6, and polybutylene terephthalate (PBT)—molecular models of 3-5 mers were created for each, and the molecular orbital energy levels were calculated using first-principles calculations. The LUMO energy was then extracted from these molecular orbital energy levels. Note that PET-1 was constructed as a molecular model with two carbonyl groups derived from terephthalic acid positioned on the same side (cis conformation), while PET-2 was constructed as a molecular model with two carbonyl groups derived from terephthalic acid positioned on opposite sides (trans conformation). PBT is constructed as a molecular model (trans conformation) in which two carbonyl groups derived from terephthalic acid are positioned opposite each other.

[0037] Molecular orbital energy levels were calculated using Gaussian®, employing the calculation method shown in Figure 1. The calculation conditions were as follows: structural optimization was selected as the calculation type; second-order perturbation theory (MP2) was selected as the calculation method; the extended Hückel method was selected for initial orbital prediction; and the normal mode was selected for electron density analysis and molecular orbital output.

[0038] Figure 3 is a graph showing the calculated LUMO energy of each resin material. By comparing the calculated LUMO energy of each resin material shown in Figure 3, resin materials with a LUMO energy level higher than that of PTFE were predicted to have low adhesion to orthosilicic acid monomers or di-tetramers. From the nine types of resin materials, ETFE, PVC, nylon 6, it-PMMA, and st-PMMA were selected.

[0039] Furthermore, a sample of PVC material from the selected resin materials was placed in a retention tank at a geothermal power plant. After 107 days, it was removed, ultrasonically cleaned, and its surface was observed at 1000x magnification using a scanning electron microscope (SEM). Figure 4 shows an SEM image of the surface of the PVC material sample. As shown in Figure 4, it was confirmed that the amount of silica scale adhering to the surface of the PVC material sample was small.

[0040] As described above, embodiments have been explained, but these embodiments are presented as examples only, and the present invention is not limited by these embodiments. The above embodiments can be implemented in various other forms, and various combinations, omissions, substitutions, and modifications are possible without departing from the spirit of the invention. These embodiments and their variations are included in the scope and spirit of the invention, as well as in the claims of the invention and its equivalents.

[0041] This international application claims priority under Japanese Patent Application No. 2023-022944, filed on 16 February 2023, which is incorporated herein by reference in its entirety. [Explanation of Symbols]

[0042] 1 Production well 2. Water supply pump 3 Steam water separator 4 Turbines 5 Generators 6. Condenser 7 cooling tower 8. Circulation pump 9 Retention tank 10 First area 11. Reduction pump 13 Collector 14 Reinforcement well 20 Second area 30 Third area 100 Geothermal power plants L1, L2, L3, L4, L5, L6, L7 piping

Claims

1. A method for selecting piping materials used in piping for circulating geothermal fluids containing orthosilicic acid monomers or di-tetramers in a geothermal power plant, A method for selecting piping materials, comprising predicting the adhesion to monomers or di-tetramers of orthosilicic acid by comparing the molecular orbital energy levels of two or more resin materials with different compositions or chemical structures, and selecting the resin material selected based on the adhesion as a piping material that reduces the adhesion of silica scale precipitated from the geothermal fluid.

2. A method for selecting a piping material according to claim 1, wherein the molecular orbital energy levels are obtained by first-principles calculations.

3. The method for selecting a piping material according to claim 2, wherein the molecular orbital energy level is the lowest unoccupied orbital energy level.

4. A method for selecting a piping material according to claim 3, wherein a resin material having a lower air orbital energy level higher than the lower air orbital energy level of polytetrafluoroethylene is selected.

5. A method for selecting a piping material according to claim 4, comprising selecting the resin material having the highest lowest air orbital energy level from among two or more resin materials having a lowest air orbital energy level higher than the lowest air orbital energy level of polytetrafluoroethylene.

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