Anti-fouling materials and methods for identifying same

Abstract

In general, the present invention relates to the use of materials that resist fouling on their surfaces. Such materials can be used in the construction of various process equipment having surfaces exposed to a given fluid that contains a foulant or chemical that can deposit or adhere on the surface and thereafter continue to grow on the surface causing a decrease in the performance of the equipment and process. The materials are selected by properties using a method that takes into account the dielectric spectrum of the fluid in which the surface is exposed and the dielectric spectrum of the surface itself. It was unexpectedly found that if the dielectric spectra of the surface and the fluid match to within a relative consistency, then the foulant should not adhere to the surface. In some embodiments, the dielectric spectra to be matched include intrinsic refractive index.

Description

[0001] This application is a divisional application of the invention patent application No. 201880078234.4 entitled "Anti-fouling material and method for identifying the same", filed on October 1, 2018. Background of the Invention Invention Field This invention relates to methods for reducing or eliminating fouling on the surface of materials or components. More specifically, this invention relates to methods for identifying a construction material (either as a coating or as the material itself) for such a surface, wherein the dielectric spectrum of the material matches or is similar to the dielectric spectrum of a fluid exposed thereto.

[0002] Related technical descriptions Surface fouling, or undesirable deposits of material on a given surface, is a problem affecting a wide range of fields, from energy and transportation to medicine. Energy production and distribution systems are particularly vulnerable to fouling-related damage because they rely on the continuous cleanliness of their functional surfaces for purposes such as heat transfer, catalytic chemical reactions, and corrosion resistance. Some degree of particulate fouling or particle adhesion to surfaces is present in almost all geothermal power plants, oil refineries, nuclear power plants, chemical processing, and marine systems. Fouling on internal piping surfaces increases pressure drop across components, reduces heat transfer efficiency, and can completely block coolant passages, leading to decreased process efficiency and potentially requiring component replacement. Considering the increased energy consumption costs, reduced output, and fouling-related maintenance, the economic impact is estimated to be in the billions of dollars, with an estimated cost of $15 billion in 2013 alone.

[0003] Teflon or other smooth polymers can be used in certain situations to reduce or minimize fouling because they resist the buildup of almost all potential contaminants. However, many energy systems that experience fouling operate under conditions too harsh for Teflon or any organic material to remain stable. Therefore, a more general solution to the fouling problem is needed. Specifically, a solution is needed to address fouling in harsh environments, such as those in various energy systems. Brief Overview of the Invention The initial deposition of fouling or dirt on a surface may be caused by van der Waals forces, which are often the dominant forces determining the adhesion of fouling in energy systems. Therefore, preventing such forces should reduce or eliminate any initial deposition of fouling caused by van der Waals forces, which in turn will lead to a reduction or elimination of fouling. The theory of van der Waals forces suggests that a significant reduction or elimination of van der Waals forces will result from achieving or designing a close match between the dielectric spectrum of the fouled surface or dirt and the dielectric spectrum of the fluid. Of course, for a given system, the potential fouling and fluid are essentially determined by the system and may not be variable. In other words, fouling cannot be prevented simply by altering the composition of the fouling and fluid without first sacrificing the primary purpose of the system. Therefore, by selecting the structural materials (either as a coating or as the material itself) for surfaces exposed to potential fouling, whose dielectric spectrum matches or approximates the dielectric spectrum of the fluid exposed therein, it should be possible to reduce or eliminate van der Waals forces, thereby preventing fouling deposition.

[0004] In one embodiment, the present invention provides a method for selecting a construction material for exposure to a surface containing a contaminant fluid, comprising selecting a construction material for exposure to a surface containing a contaminant fluid during use, wherein the construction material (or a coating on a material or the material itself, regardless of which is exposed to the fluid) has a refractive index in the full refractive index spectrum that matches or approximates the refractive index in the full refractive index spectrum of the fluid.

[0005] In one embodiment, the present invention provides a method for selecting a structural material for a surface exposed to a fluid containing contaminants, comprising selecting a structural material for use on a surface exposed to a fluid containing contaminants during use, wherein the refractive index of the structural material in the full refractive index spectrum is within 20% of the refractive index of the fluid in the full refractive index spectrum. It should be understood that, in one embodiment, at least one value in the entire spectrum needs to match to within 20%.

[0006] In another embodiment, the present invention provides a method for selecting a construction material for a surface exposed to a fluid containing contaminants, comprising selecting a coating for use on a surface exposed to a fluid containing contaminants during use, wherein the refractive index of the coating in the full refractive index spectrum is less than 20% of the refractive index of the fluid in the full refractive index spectrum.

[0007] In another embodiment, the present invention provides a construction material comprising a body having a surface exposed to a fluid containing contaminants during use; and a refractive index within the total refractive index spectrum, which is within 20% of the refractive index within the total refractive index spectrum of the fluid.

[0008] In another embodiment, the present invention provides a coating for a surface, including a coating for a surface exposed to a fluid containing contaminants during use; and a refractive index within the total refractive index spectrum, which is within 20% of the refractive index within the total refractive index spectrum of the fluid. In one embodiment, the coating is disposed on the surface of a material. Brief description of the attached diagram Figure 1 The process of scale buildup is illustrated, starting from a clean surface, then a single layer of particles adheres, followed by the continuous growth of scale. Figure 2 Examples of EELS plots and typical VEELS calculated and measured spectra are shown; Figure 3 The Hamaker constant for various materials is shown using full spectrum and Tabor-Winterton calculations. Figure 4 An example of a cantilever beam used for measuring adhesion forces using AFM-FS is illustrated; Figure 5 An example is a vacuum inert AFM chamber; and Figure 6 The use of the USPEX algorithm is illustrated schematically. Invention Details Various embodiments of the present invention are described below with reference to the accompanying drawings. However, this description should not be construed as limiting the scope of the invention or merely illustrating embodiments thereof. Rather, it should be considered as examples of various embodiments of the invention, as the invention encompasses other embodiments not specifically listed herein, such as substitutions, modifications, and equivalents within the spirit and scope of the invention and defined by the claims. Therefore, it should be understood that references to "invention" or "this invention" should not be interpreted as meaning that the invention is limited to one embodiment, or that each embodiment must include a given feature described in conjunction with another embodiment, or that the phrases are used in combination for description.

[0009] In general, the present invention relates to materials that resist fouling on their surfaces during use. Such materials can be used in the construction of various process equipment having surfaces exposed to a given fluid containing contaminants or chemicals that may deposit or adhere to the surface and subsequently continue to grow there, leading to a degradation in the performance of the equipment and / or the process. Materials are specifically selected using a method that takes into account the dielectric / refractive spectrum (directly correlated) of the fluid to which the surface is exposed. It has been unexpectedly found that contaminants should not adhere to the surface if the dielectric spectra of the surface and the fluid match or are similar. In some embodiments, the dielectric spectrum to be matched includes the intrinsic refractive index.

[0010] More specifically, and without being limited by theory, the initial deposition or adhesion of dirt or dirt particles on a surface is considered to be attributable to van der Waals forces between the surface and the dirt. According to Lifshitz's theory of van der Waals forces, it has been determined that van der Waals forces should not exist if the dielectric spectrum of the dirt or surface material is the same as that of the fluid carrying the dirt and exposed to the surface. By eliminating this force, dirt should not adhere to the surface, or an initial monolayer of dirt particles should not form on the surface; therefore, no dirt continues to grow on the surface. Figure 1 The scaling process is illustrated, starting with a clean surface, followed by the adhesion of partial or complete monolayer particles, and then the continuous growth of scale. The focus of this invention is to reduce or eliminate the initial deposition of monolayer particles.

[0011] It should be understood that matching the dielectric spectrum of the contaminant to that of the fluid will produce the same result; however, in most practical applications, the contaminant and fluid cannot be “selected” because these are determined by the process used. Therefore, the only practical approach is to select a suitable construction material for the surface exposed to the fluid. It should be understood that construction materials include materials used for the surface exposed to the fluid, and in some embodiments, include coatings on the underlying material used. Therefore, it should be understood that if a given surface is coated, the coating can be similarly selected based on matching the dielectric spectrum of the coating to that of the fluid. It should also be understood that “matching” dielectric spectra does not necessarily mean that the dielectric spectra of the surface and the fluid must be precisely matched. In some embodiments, “matching” can be within ±0-20% of each other. In some embodiments, “matching” can be within ±5%, ±10%, or ±20%, and these “matches” can be wavelength-dependent or frequency-dependent, tuned to the wavelength or frequency that most significantly determines the van der Waals forces.

[0012] Without being bound by theory, the following analysis presents an explanation of the finding that van der Waals (vdW) forces decrease or are eliminated by matching dielectric spectra. (Fluid) f Two materials a and b The force and spacing between vdW r The square is inversely proportional, similar to the Coulomb interaction: This force and (This is called the Hamek constant) is proportional. This constant defines the magnitude of the force and whether it is an attractive or repulsive force (the latter is less common). Since the vdW force is generated by induced dipoles produced by the motion of coupled electrons, the Hamek constant is calculated by considering the material's response to an oscillating electric field—in other words, an optical property like refractive index or reflectivity. This is derived from the material's imaginary frequency dielectric response. Calculating the Hamek constant is the most straightforward method, but this elusive quantity is directly related to more conventional measurement methods, such as reflectance or ellipsometrics. It can also be obtained with high accuracy using more advanced measurement methods, such as valence electron energy loss spectroscopy (VEELS). The formula for the full-spectrum relativistic Hamek constant (with the fewest assumptions and restrictions, but also the most complex) is: in yes Boltzmann constant in the figure, where T is the Kelvin temperature. It is the optical hindrance factor (which explains that photons with different polarizations have different propagation path lengths). At the virtual (composite) frequency The difference in dielectric response to virtual photons, where n It is a discrete energy level from 0 to ∞. The variables can be considered as the contribution of different polarizabilities at different frequencies or differences in electronic vibrations at different frequencies to the adhesion energy. Each of these can be expressed as follows: Here It is a material j At imaginary frequency The dielectric response function under these conditions. Finally, each of these can be represented by its measurable real component by applying the Kramers-Kronig relation, which links the real and imaginary components of any causal function: Where ω is the real frequency. In this way, we can transform the tabulated optical data into the Hamek constant, or measure it via adhesive force using atomic force microscopy (AFM-FS). Using this knowledge, we have a deterministic angle of attack: the difference term in Equation 3. If the material... a and b either and fluid f If the two imaginary dielectric spectra are equal, then theoretically adhesion should not occur. Since the real and imaginary components of the dielectric spectrum are related by Equation 4, finding a match between the dielectric spectrum of the fluid and any material should result in little / no adhesion generated by the vdW force, sufficient to significantly reduce or eliminate fouling.

[0013] A method is needed to determine the full dielectric spectrum of existing off-surface materials. These spectra can be determined directly using a capacitance bridge setup or ellipsometric measurement, and indirectly via reflectance spectroscopy and VEELS. Alternatively, spectra can be obtained through ab initio band structure calculations (e.g., density functional theory (DFT) simulations). For accurate calculation of the Hamek coefficient, it is important to obtain experimental dielectric response data over a wide frequency range, from static to IR (important for water) to UV. VEELS enables experimental testing over a very wide frequency range (0–100 eV is typical). However, optical reflectance measurements remain the simplest experimental method.

[0014] Figure 2 The diagram shows a graph of EELS, along with typical VEELS calculated and measured spectra. VEELS is an electron energy loss technique that measures the energy lost by electrons as they pass through a very thin sample in a single scattering collision. The sample thickness is chosen to be much smaller than the electron range in the medium, ensuring almost all single scattering events in the non-transmittent portion of the electron beam. This provides an additional safeguard in the measurement data because methods such as DFT directly predict the energy levels of valence electrons (which are detected using VEELS). Here, the number of electrons that lose a certain amount of energy is measured and listed as arbitrary units of intensity. This is normalized to known absolute values ​​at certain wavelengths (possibly obtained via reflectance spectroscopy) to produce the dielectric spectrum of the material.

[0015] In all practical experiments, there are limitations on the upper and lower frequencies that can be tested; however, the KK transform needs to be measured over an infinite frequency range (as shown by the limit of the integral in Equation 4). Therefore, experimental data are usually supplemented with frequency approximations below and above the measured frequency. A typical choice for such approximations is the power law. The indices of the high side and the low side β The values ​​differ. This power-law approximation can also be used for interpolation to frequencies between measured data points. Another option for the approximation is a constant value corresponding to the low-frequency range of the minimum measured frequency. and the results of free electrons in the high-frequency range (in It is the plasma frequency of the free electron gas.

[0016] Methods for measuring material reflectivity and obtaining dielectric spectrum transformations are well-established. After determining the frequency-dependent reflectivity, the KK transform is applied to obtain the frequency-dependent phase measurement. Reflectivity and phase are combined in a linear system of formulas to obtain the frequency-dependent real and imaginary dielectric responses. Then, substitute it directly into Formula 4 to obtain the result for the imaginary frequency. The dielectric response is then determined. The forces resulting from van der Waals interactions are then determined according to equations 3, 2, and 1.

[0017] It should be understood that for those with similar absorption frequencies ω and low refractive indices in the visible spectrum... n The effective Tabor-Winterton approximation (TWA) method can be used for initial candidate material searches. The TWA is: Using this method, the refractive index of the visible spectrum n Furthermore, the zero-frequency dielectric constant (polarizability) can be used to initially guess which materials should be smooth or resistant to fouling. Since the properties of both fluids and fouling particles are already given, the goal of seeking materials to reduce adhesion can be systematically viewed as finding materials with specific optical properties suitable for the system. The relatively cumbersome experimental measurement method can be significantly extended through computational analysis of literature containing measured spectra of materials (applying Equations 2 and 3) and modeling of new materials, where the spectra are based on band structure calculations using DFT simulations. Due to the large number of sample material arrangements to be simulated and the complex dependence of adhesion on material properties, genetic algorithms are well-suited for exploring this computational design space.

[0018] According to TWA, one way to minimize vdW interactions is to minimize the difference term in the numerator of Equation 5. Replacing the running conditions with actual values, the second term in Equation 5 exceeds the first. Therefore, the initial candidate antifouling material should have a refractive index close to the fluid refractive index. Furthermore, for materials with unknown dielectric properties, it is reasonable to ignore the first term of Equation 5.

[0019] In one embodiment, a system of interest includes a water-insoluble fluoride that tends to be very hard, soil-rich, and insoluble in water. Figure 3 The Hameke constants for various materials are shown, calculated using full spectrum and Tabor-Winterton methods. As indicated, many fluorides exhibit low Hameke constants and refractive indices.

[0020] In one embodiment, where geothermal power plants primarily use high-temperature water as the fluid and are subject to SiO2 particle fouling, the data listed in the table can be used to identify water-insoluble, relatively hard (to avoid SiO2 corrosion), and low-refractive-index minerals. The TWA (Total Water Surface Area) can be applied to calculate the expected vdW force in water at room temperature. Table 1 summarizes the materials identified so far.

[0021] Table 1: Candidates and refractive indices of smooth surface coatings compared to water and typical passivated oxide scale. (Parentheses indicate that dielectric constant data are not applicable approximations.) Referring to Table 1, note how each of these materials possesses a visible refractive index very close to that of water, compared to passivation oxide layers typically formed on carbon steel (Fe3O4) and stainless steel or nickel-based superalloys (Cr2O3). As can be seen in Table 1, a considerable number of materials have a refractive index close enough to that of water to produce a significant improvement over passivation oxides, which naturally grow on structural materials within, for example, geothermal fluids or nuclear reactors.

[0022] Figure 4 An example of a cantilever beam used for measuring adhesion forces using AFM-FS is shown. The adhesion model is confirmed by directly measuring the adhesion force using AFM-FS. In this technique, particles of dirt / scale material (in this case, SiO2) are fixed to the end of a thin SiN cantilever beam. The cantilever beam is brought into contact with the surface to be measured, and then a laser beam bounces off the back of the cantilever beam, as... Figure 4 As shown. The deflection of the laser is proportional to the bending of the cantilever beam, thus generating the force required to bend the cantilever beam (in accordance with its spring constant). k (Then). The cantilever beam is then pulled off the surface, and the force required to remove the particles is measured using the same technique. It should be understood that to obtain accurate AFM-FS data, contamination from airborne moisture, water ions, and surrounding hydrocarbons must be taken into account and subtracted.

[0023] In one embodiment, a vacuum inert AFM chamber can be used. Figure 5 A vacuum inert AFM chamber is shown. As illustrated, the vacuum chamber allows the entire system to be evacuated to 10⁻⁶ using a turbomolecular pump. -3 Torr, while a differentially pumped argon ion sputtering gun is used to sputter-clean the surface of the material to remove oxides and organic contaminants. AFM-FS can be performed on fresh surfaces in a vacuum or dry inert gas atmosphere without exposure to air.

[0024] Although the present invention provides a method for reducing or eliminating fouling on a material or surface by using the optical properties of a given material, and in particular using the Hameke constant as a numerical measure of fouling tendency, an evolutionary algorithm method (such as USPEX), which can be optimized to explore the multidimensional space of possible crystal structures and find those that are most energy-favorable, can be used to calculate the Hameke constant of the crystal structure.

[0025] Density functional theory (DFT) software packages (such as VASP (Vienna ab initio simulation package)) can be used to find the frequency-dependent dielectric response supported by earlier work. The script then solves for the full spectrum and (Equation 2) to derive the calculated Hamek constant. DFT calculations also provide measurements of the crystal structure energy, indicating whether the structure is practically stable under operating conditions.

[0026] Figure 6 The use of the USPEX algorithm is illustrated schematically. In the USPEX algorithm, crystal structures are optimized in an evolutionary manner. The algorithm begins by forming random structures that satisfy initial "hard" constraints (e.g., number of atoms and atom types). These structures are then evaluated to obtain a fitness value for each structure, which determines whether the structure will persist in subsequent generations. In the standard usage of USPEX, this fitness value is the total energy of the structure, which results in the chemically most stable (lowest energy) crystal. For this invention, the fitness value replaces the Hamek constant, favoring structures with the lowest adhesion. In this case, local energy minimization (still needed to find practical materials) is implemented as relaxation of the structure in the DFT simulation. The remaining structures are then modified by three operators: inheritance, mutation, and permutation. Some of the resulting structures will have more favorable fitness values, and the process is repeated until a satisfactory number of high-performance structures are found.

[0027] Based on the foregoing, it should be understood that many materials can be selected to provide a “smooth” surface to which dirt or dirt particles will not adhere, thus preventing the formation of an initial monolayer, which in turn prevents further growth of dirt or scale. It should be further understood that, in most cases, the characteristics or chemical composition of the dirt are not important in determining which construction material or coating to use for a given fluid in which the material or its surface is exposed. In other words, the selection of the material or construction material or coating is general for each pair of material / coating and working fluid, regardless of the large amount of potentially depositing material in a given system. Furthermore, since the visible refractive index of all working fluids in an energy system ranges from 1.2 to 1.7, TWA is a valid and accurate estimate of the total vdW force between the dirt and the material surface immersed in a non-vacuum working fluid. Moreover, since the molecule in TWA (Equation 5) contains multiple differences ( (item), therefore only the working surface (material) a ) and fluid ( f An approximate match between the working surface and the fluid(s) or between the fluid and the contaminant(s) is required. Because controlling the working surface is easier than controlling the contaminant, only an approximate match between the working surface and the fluid is needed by simply matching the inherent refractive index in the visible or UV spectrum.

[0028] In some embodiments, the selected material is matched to the visible refractive index of the surface or the fluid in which it is exposed within a portion of the entire spectrum within ±20%. In some embodiments, the fluid may be a gas or liquid and may have a visible refractive index ranging from 1.00 to 1.76. In some embodiments, the visible refractive index of the material may be in the range of 1.00 to 1.76 or 1.06 to 1.60. In some embodiments, the material may be a solid, a bulk material, or a coating applied to another material such that the coating alters the surface chemistry of the underlying material. In some embodiments, the material may more generally be a surface modified by processes such as oxidation, fluorination, surface nitriding, carbon penetration, etc. In some embodiments, the material may be a crystalline material selected or designed by characteristics to match the visible refractive index of the surrounding fluid within a relative consistency of ±20% of the visible refractive index of a portion of the entire spectrum. In some embodiments, the fluid may be water and may not contain any contaminants. In some embodiments, the fluid may be water and may contain more than one type of contaminant. In some embodiments, the fluid may be water, and the material may have an inherent visible refractive index in the range of 1.06 to 1.60. In some embodiments, the fluid may be water, and the material may contain fluorine. In some embodiments, the fluid may be water, and the material may contain a fluorine-containing crystalline solid. In some embodiments, the fluid may be water, and the material may contain fluorite (CaF2), cryolite (Na3AlF6), albite, or microcline (KAlSi3O8). In some embodiments, the material may be an amorphous material selected or designed by characteristics to match the visible refractive index of the surrounding fluid within a relative consistency of ±20% across a portion of the entire spectrum. While the entire spectrum (including both visible and invisible ranges) may be important, in some embodiments, a match is determined within the 400–800 nm range of the visible spectrum.

[0029] It should be understood that, in some embodiments, the matching between the material's refractive index and the surrounding fluid can be a matching of the ultraviolet (UV) refractive index, which is opposite to the visible refractive index, or a spectral portion of the UV range. While the entire UV spectrum may be important, in some embodiments, the matching is determined in the 10-400 nm range of the UV spectrum.

[0030] In some embodiments, the material may be subjected to radiation, for example, in a nuclear power plant. In this case, corrosion deposits (crud) are a form of fouling that degrades the nuclear fuel cladding. Therefore, it is essential to select materials that provide radiation resistance during use to maintain their properties. The aforementioned fluorine-based minerals not only resist radiation damage, but the atomic defects generated by radiation actually improve their wettability and thermal transfer properties over time without sacrificing their resistance to fouling. In some embodiments, ZrN and TiC are resistant to corrosion deposit formation in PWR environments. In some embodiments, TiN is also resistant to corrosion deposit formation in PWR environments.

[0031] It should be understood that the selection of certain materials for constructing surfaces exposed to fluids carrying fouling is broadly applicable. For example, in 2013, fouling alone cost the United States $15 billion in lost revenue. Every major, energy-intensive heat transfer, energy production, and chemical processing system containing impurities is subject to fouling, and each of these systems constitutes a potential revenue stream. For example, the present invention can be used to reduce the accumulation of corrosion deposits on nuclear fuel, sludge in the secondary heat transfer cycle of nuclear power plants, silica in deep geothermal boreholes, iron and steel-based corrosion products and “black powder” in refinery heat exchangers, and hard water deposits that occur virtually everywhere, including commercial and residential water piping systems.

Claims

1. A method for selecting a structural material for a surface exposed to a fluid containing contaminants, comprising: The construction material is selected for use on surfaces exposed to a fluid containing contaminants during use, wherein the refractive index of the construction material is within 20% of the refractive index of the fluid in a portion of the full spectrum.

2. A method for selecting a structural material for a surface exposed to a fluid containing contaminants, comprising: Select a coating for surfaces exposed to a fluid containing contaminants during use, wherein the refractive index of the coating is within 20% of the refractive index of the fluid in a portion of the full spectrum.

3. A structural material comprising: A body having a surface exposed to a fluid containing dirt during use; and The refractive index is within 20% of the refractive index of the fluid in a certain part of the full spectrum.

4. A coating for a surface, comprising: Coatings for surfaces exposed to fluids containing contaminants during use; and The refractive index is within 20% of the refractive index of the fluid in a certain part of the full spectrum.

5. The coating according to claim 4, wherein the coating is disposed on the surface.

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