Salting and Destruction of PFAS Using Reverse Osmosis and Salt Separation

Abstract

Per- and polyfluoroalkyl substances (PFAS) are destroyed by oxidation under supercritical conditions. PFAS in water are concentrated by reverse osmosis, and the resulting solution is desalted under supercritical conditions, after which the PFAS are destroyed under supercritical conditions.

Description

[Technical Field]

[0001] This application claims the benefit of priority to U.S. Provisional Patent Application No. 63 / 062,251, filed August 6, 2021.

[0002] Per- and polyfluoroalkyl substances (PFAS), including perfluorooctanesulfonic acid (PFOS), perfluorooctanoic acid (PFOA), and hundreds of other similar compounds, are widely used in the United States for many applications, and there is significant associated concern due to widespread contamination, along with uncertainty regarding risks to human health and the environment. PFAS are molecules with chains of carbon atoms surrounded by fluorine atoms. The C—F bond is highly stable, and the compounds persist in the natural environment. Some PFAS contain hydrogen, oxygen, sulfur, phosphorus, and / or nitrogen atoms. One example is PFOS: [ka]

[0003] Although several PFAS compounds (PFOA and PFOS) with known human health risks have been voluntarily phased out, legacy contamination remains. Alternative PFAS compounds have been introduced without a full understanding of their health risks. Currently, only PFOA and PFOS are covered by federal lifetime health advisories, and no maximum contaminant levels (MCLs) have been established to regulate acceptable levels of these and other PFAS compounds in drinking water. As of May 2020, PFAS contamination has been identified in 1,582 drinking water sources in 49 states. Current PFAS-contaminated water treatment technologies are expensive, and managing spent media can be costly and result in long-term liability.

[0004] Numerous methods have been developed to remediate PFAS in the environment, for example, Oberle et al., US2019 / 0314876, describes a method and system for remediating PFAS-containing soil that involves heating the soil to volatilize the PFAS, capturing and condensing them, adding steam, and subjecting the concentrated PFAS solution to electro-oxidation.

[0005] The application of supercritical water oxidation (SCWO) to PFAS is relatively new and presents new challenges. SCWO of organic compounds has been known for a long time and has been described in numerous papers and patents. For example, U.S. Patent No. 4,861,497 to Welch et al. describes the use of liquid-phase oxidizing agents, such as hydrogen peroxide or ozone, in supercritical water for the destruction of organic compounds. Tests of propylene glycol at 750–860°F and 5,000 psia (pounds per square inch) achieved approximately 98% destruction. U.S. Patent No. 5,232,604 to Swallow et al. describes the SCWO of organic compounds using oxidizing agents, such as hydrogen peroxide, and reaction rate enhancers, such as nitric oxide. In one example, sodium hydroxide and sodium nitrate were used to neutralize the hydrochloric acid produced by the oxidation of methylene chloride. Aquarden Technologies' U.S. Published Patent Application No. 2019 / 0185361 focuses on the SCWO process, where precipitation occurs in the zone where the fluid goes from subcritical to supercritical, and designs a reactor with a residue outlet connection near this zone. Miller et al. reported on the SCWO reaction of a fecal simulant in the presence of 48% excess oxygen in "Supercritical Water Oxidation of a Model Fecal Sludge with the Use of a Co-Fuel," Chemosphere 141 (2015) 189-196. Auxiliary fuel can be used to generate a hydrothermal flame in the SCWO reactor, which is characterized by high temperatures typically exceeding 1000°C. See "Supercritical Water Oxidation," in Advanced Oxidation Processes for Wastewater Treatment, (2018), 333-353.

[0006] Despite the development of systems to destroy PFAS, there is still a need for an efficient system to treat PFAS compositions and completely destroy PFAS. Summary of the Invention [Means for solving the problem]

[0007] First, water or soil samples are treated to concentrate PFASs in a substantially reduced volume. In some instances, the PFAS-containing media is stored in a concentrated form and does not require additional processing for concentration. The concentrated PFAS mixture is then containerized and transported to a centralized location for PFAS destruction. Alternatively, in some preferred embodiments, the concentrated PFAS mixture is treated at the site where it is generated. The concentrated PFAS solution is destroyed by supercritical water oxidation (SCWO), which we have found can rapidly reduce PFAS concentrations by more than 100,000 times, e.g., PFOA from 1700 ppm (parts per million) by mass to less than 5 ppt (parts per trillion). For efficient destruction with little or no external heat during steady-state operation, fuel can be added to provide some or all of the heat required for oxidation (or, in some cases, PFAS may be present with sufficient organic matter to serve as fuel). The resulting wastewater is confirmed to contain little or no PFASs, typically less than 5 ppt, and is returned to the environment as safe, clean water.

[0008] In one embodiment, the PFAS-containing aqueous solution is subjected to one or preferably multiple reverse osmosis cascade steps to form a brine and desalted solution. Prior to reverse osmosis (RO), the PFAS-containing aqueous solution is generally subjected to an optional filtration step and one or more water softening steps in which ions such as Ca and Mn are replaced with Na ions. The end result of the cascade is a fraction volume reduction, preferably at least 60%, more preferably at least 80%, and in some embodiments, 80-95%, prior to treatment with SCWO to destroy the PFAS. The PFAS is partitioned so that at least 80%, preferably at least 90%, or at least 95%, or at least about 99% of the PFAS in the initial aqueous mixture is partitioned into the brine fraction resulting from the reverse osmosis process.

[0009] In another aspect, the invention provides a method for destroying PFAS, comprising the steps of providing water and an aqueous solution comprising PFAS; subjecting the aqueous solution to reverse osmosis to produce a clean water fraction and a brine-enriched fraction, the PFAS concentration being at least 50% higher than the aqueous solution; preheating the brine-enriched fraction in a heat exchanger to form a preheated enriched fraction at a subcritical state; and passing the preheated enriched fraction through a heated pre-reactor to convert the brine-enriched fraction to a supercritical state at a first temperature and precipitate sodium chloride. removing at least a portion of the sodium chloride to produce a brine-reduced fraction; passing the brine-reduced fraction through a reactor and oxidizing the fraction under supercritical conditions, wherein the oxidant concentration and / or temperature is higher than in the pre-reactor; producing a clean, warm aqueous solution having a PFAS concentration that is at least 90% lower than in the aqueous solution; and optionally, transferring heat from the clean, warm aqueous solution to the aqueous solution in the heat exchanger in the pre-heating step.

[0010] The method is further characterized by one or any combination of the following features: the aqueous solution is filtered prior to reverse osmosis; the effluent passes through a post-heat exchanger located downstream of the pre-reactor and then through a pre-heat exchanger located upstream of the pre-reactor (flow direction (upstream or downstream) refers to the flow of the aqueous solution being subjected to PFAS destruction); the brine-enriched fraction resulting from reverse osmosis and having a PFAS concentration at least 50% higher prior to the step of removing at least a portion of the sodium chloride comprises precipitate; and the method further comprises the step of adding an acid to dissolve the precipitate; the pre-reactor comprises a first tube leading to a collection vessel and a second tube leading from the collection vessel, the collection vessel having an inner diameter at least twice as large as the inner diameter of the first tube; and optionally, a fuel or an oxidizer is added to the pre-reactor; and the pre-reactor comprises a transcritical hydrocyclone, preferably the transcritical hydrocyclone is of the type described herein.

[0011] The present invention is optionally characterized by an energy-efficient method for destroying PFAS, optionally characterized by steps of reverse osmosis and salt separation followed by oxidation in an SCWO reactor, and optionally characterized by the aqueous solution comprising water and PFAS having a first volume, wherein 10% or less (or 5% or less, or 1% or less, or 0.1% or less) of the first volume is subjected to supercritical conditions, and wherein the method destroys at least 95% (or at least 98%, or at least 99%) of the PFAS in the first volume under supercritical conditions.

[0012] In another aspect, the invention includes a system for destroying PFAS, the system including: a reverse osmosis system; a conduit from the reverse osmosis system to a salt separator, the salt separator including a first tube leading to a collection vessel and a second tube exiting the collection vessel, the collection vessel having an inner diameter at least two times larger than the inner diameter of the first tube; and a conduit from the salt separator to a supercritical reactor.

[0013] The system may be further characterized by one or any combination of the following features: the collection vessel contains a supercritical fluid; the first tube protrudes at least 5 cm into the collection vessel; the second tube does not protrude into the collection tube; and a first heat exchanger disposed in the conduit between the reverse osmosis system and the salt separator, wherein during operation the first heat exchanger exchanges heat between the subcritical PFAS-containing water stream and the PFAS-free water return from the supercritical reactor, the first tube protruding at least 5 cm into the collection vessel beyond the second tube; the heat exchanger is a tube-in-tube heat exchanger; and the salt separator includes a plurality of collection vessels, each collection vessel including a set of inlet and outlet tubes, each set at a higher temperature than the previous set in the direction of flow.

[0014] In a further aspect, the present invention provides a transcritical hydrocyclone comprising: a conical chamber including an inlet for introducing a supercritical fluid into the conical chamber in a tangential direction along an inner wall of the cyclone, an upper outlet adapted for flow of supercritical fluid, and an outlet pipe adapted for flow of liquid; and a cone disposed in the conical chamber adapted to form a flow path between the inner wall of the cyclone and an outer wall of the cone, the cone including a bottom opening.

[0015] The hydrocyclone may be further characterized by one or any combination of the following features: the bottom opening includes a diffuser, the diffuser includes a plurality of openings, the plurality of openings include an open area at least two or at least three times larger than the cross-sectional area of ​​the outlet pipe, the openings are inclined from a central axis to an inner wall of the lower pipe (and are not in a straight line (e.g., not the shortest distance)), and further includes a mechanism adapted to move the cone along the central axis of the conical chamber relative to the inner wall of the conical chamber, and includes a heat exchanger thermally connected to an outer wall of the conical chamber, the heat exchanger including a jacket for fluid flow.

[0016] The present invention also provides an apparatus having any of the passivating coatings described herein.The present invention further provides a method for destroying PFAS, comprising the steps of providing an aqueous solution containing water and PFAS, preheating the aqueous solution, providing a reactor having reactor walls coated with a ceramic coating that is resistant to hydrofluoric acid, treating the solution in the reactor with an oxidizing agent while the water is in the supercritical phase to produce a clean, warm aqueous solution having a PFAS concentration that is at least 90% lower than the aqueous solution, and transferring heat from the clean, warm aqueous solution to the aqueous solution in a heat exchanger in the preheating step. Preferably, the ceramic comprises BC (boron carbide), SiC (silicon carbide), TaC (tantalum carbide), WC (tungsten carbide), a metal fluoride such as YF3 (yttrium fluoride), YN (yttrium nitride), LaF3 (lanthanum fluoride), LaN (lanthanum nitride), YbN, YbF3, or any lanthanide nitride or lanthanide fluoride, HfN (hafnium nitride), CeN (cerium nitride), CeF3 (cerium fluoride), TaN (tantalum nitride), Ta (tantalum), TaF (tantalum fluoride), ZrN (zirconium nitride), ZrF (zirconium fluoride), WN (tungsten nitride), chromium oxide, or a combination thereof.

[0017] Any of the embodiments of the present invention are further defined by one or any combination of the following: the method is carried out in a mobile trailer, the PFAS-containing aqueous mixture contains at least 100 ppm PFAS, and the method reduces the PFAS concentration to at least 10 6 or 10 7 or 10 8The method reduces PFAS concentrations to 1 ppm or less, or 0.1 ppm or less, or 0.01 ppm or less, or 1.0 ppb or less, or by treating the effluent after exiting the reactor with a solution comprising NaOH, LiOH, or KOH to produce a neutralized solution that is discharged or reused to neutralize additional effluent, and at least partially evaporating the neutralized effluent into the atmosphere, such that the PFAS-containing aqueous mixture contains at least 100 ppm PFAS by weight (in some embodiments, at least 500 ppm or at least 1000 ppm PFAS). A method for converting effluent containing PFAS to 0.1 ppb or less, or 0.01 ppb or less, wherein the PFAS-containing solution is mixed with a solution containing 30 to 50 wt% H2O2, preferably at a weight ratio of 30:1 to 70:1 wt% PFAS solution:H2O2, and the PFAS-containing solution is passed through an SCWO reactor with a residence time of 20 seconds or less, preferably 10 seconds or less, or 5 seconds or less, or 0.5 to 5 seconds, and the PFAS-containing solution is added at a controlled rate of 50 to 150 mL / min (standard conditions), and no external heating is required after startup. The PFAS-containing aqueous mixture contains at least 100 ppm PFOA, and the method reduces the PFOA concentration to at least 10 6 or 10 7 or 10 8 In some embodiments, the decrease is up to about 10 9 The method is performed in a mobile trailer at a PFAS-contaminated site.

[0018] Any of the methods of the invention are further defined by converting PFAS concentrations of at least 100 ppb by mass of PFAS (in some embodiments at least 500 ppb or at least 1000 ppb of PFAS) to 1 ppb or less, or 0.1 ppb or less, or 0.01 ppm or less, or 7 ppt or less, throughout the entire process or in the SCWO portion of the process, or by converting PFOA concentrations of at least 100 ppb by mass of PFOA (in some embodiments at least 500 ppb or at least 1000 ppb of PFOA) to 1 ppb or less, or 0.1 ppb or less, or 0.01 ppb or less, or 5.0 ppt or less of PFOA, in some embodiments in the range of 1 ppm to 5 ppt (parts per trillion) of PFOA. Alternatively, it is defined by converting a PFOS concentration of at least 100 ppb by mass of PFOS (in some embodiments, at least 500 ppb or at least 1000 ppb of PFOS) to 1 ppb or less, or 0.1 ppb or less, or 0.01 ppb or less, or 5.0 ppt or less of PFOS, in some embodiments, in the range of 1 ppm to 5 ppt (parts per trillion) of PFOS. The process is also characterized by the same level of destruction starting at PFAS concentrations of 1 ppm or greater. In some embodiments, PFAS-contaminated water containing at least 1000 ppt of at least one (or at least three, or at least four, or at least five, or at least six) compounds selected from the group consisting of PFHxA (perfluorohexanoic acid), PFHpA (perfluoroheptanoic acid), PFOA, PFBS (perfluorobutanesulfonic acid), PFHxS (perfluorohexanesulfonic acid), PFHpS (perfluoroheptanesulfonic acid), and PFOS, and combinations thereof, is reduced by at least two (or at least three, or at least four, or at least five) orders of magnitude when treated by the process.In some embodiments, PFAS-contaminated water containing at least 100 ppt of at least one (or at least three, or at least four, or at least five, or at least six) compounds selected from the group consisting of PFBA (perfluorobutanoic acid), PFPeA (perfluoropentanoic acid), PFHxA, PFHpA, PFOA, 6:2FTS (6:2 fluorotelomer sulfonic acid), and 8:2FTS (8:2 fluorotelomer sulfonic acid), and combinations thereof, is treated by the process to reduce the concentration by at least two (or at least three, or at least four, or at least five) orders of magnitude and / or to 5 ppt (or 1 ppt) or less.

[0019] The invention also includes a PFAS destruction device comprising an SCWO reactor and any of the components described herein. The invention also includes a PFAS destruction system comprising an SCWO reactor containing a PFAS-containing aqueous mixture, the system comprising any of the conditions and / or fluids described herein.

[0020] Removal of salts helps prevent clogging of SCWO reactors and provides additional benefits, such as reduced corrosion and the production of a PFAS-free aqueous effluent with fewer contaminants.

[0021] The resulting clean water (non-brine water) fraction exiting the SCWO reactor can be optionally passed through an adsorbent medium such as activated carbon or an ion exchange resin and returned to the environment. As with any embodiment described herein, this pretreatment method can be used by itself or in combination with any other embodiment or technique described herein.

[0022] Various aspects of the present invention are described using the term "comprising," although in narrower embodiments the invention can alternatively be described using the term "consisting essentially of" or the narrower "consisting of." [Brief explanation of the drawings]

[0023] [Figure 1] Figure 1 is a schematic diagram of a water pretreatment system for treating PFAS-contaminated water before passing it through the SCWO reactor. [Figure 2] Figure 2 shows the salt separator with the inlet and outlet tubes connected to the collection tube. [Figure 3A] Figure 3a shows a transcritical hydrocyclone with an open flow passage. [Figure 3B] Figure 3b shows the transcritical hydrocyclone when not in operation with the flow passage closed. [Figure 3C] Figure 3c shows the diffuser of a transcritical hydrocyclone. [Figure 3D] Figure 3d is a cross-sectional view of a transcritical hydrocyclone. DETAILED DESCRIPTION OF THE INVENTION

[0024] According to the present invention, PFAS-contaminated water has its conventional meaning. Because the method of the present invention is capable of reducing PFAS concentrations to less than 5 ppt, it can be applied to solutions containing 5 ppt or more, more typically at least 1 ppm. The source of PFAS-contaminated water can be from soil or surface or groundwater in areas exposed to PFAS contamination. These areas are industrial areas, such as those in the electronics industry (e.g., wire / cable coating and semiconductor substrate manufacturing), particularly where waterproof or non-stick coatings are applied. Another common source of PFAS-contaminated water is areas around airports and firefighting training areas exposed to aqueous film-forming foam (AFFF). Another source is storage containers, which are typically accumulated for future disposal or disposal. Non-fluorinated organic compounds are generally present in PFAS-contaminated water, and chlorinated or brominated compounds may be present, particularly in AFFF residues.

[0025] Pretreatment Trash and other solids can be removed from PFAS-contaminated water prior to PFAS destruction. Typically, this can be accomplished through one or more filtration steps. In some embodiments, multiple filtration steps can be used to remove smaller particles. For example, filters can be valved to allow only one filter or series of filters to be used, so that one filter or set of filters can be cleaned or replaced while another filter or set of filters continues to operate. The filters can be any type of filter known for filtering water, such as a bag filter, cartridge filter, metal screen, or sand (preferably silica sand). Alternatively, or additionally, centrifugation can be used to remove solids.

[0026] Because undesirable counterions (typically Ca and Mg) foul RO membranes, PFAS-contaminated water can be subjected to softening treatments. These softening treatments include one or any combination of ion exchange resins, lime softening (precipitation of solids with aqueous calcium hydroxide), chelating agents (e.g., treatment with EDTA), and reverse osmosis. In any pretreatment, the capture of PFAS in the pretreatment medium must be considered. Alternatively, or in addition, compounds such as organics can be removed by passing the water through a hydrophobic clay to separate and / or remove emulsified hydrocarbons.

[0027] Reverse osmosis (RO) systems can remove or concentrate PFAS from a water stream. PFAS-free (or PFAS-reduced) water passes through the membrane, while PFAS and salts are directed to a saline stream. PFAS removal efficiency and capacity can be improved by using a cascade of RO membranes. In some embodiments, RO is used to increase the concentration of PFAS by at least 5-fold or at least 10-fold, and in some embodiments, by 5-30-fold, 5-20-fold, or 10-40-fold. In some preferred embodiments, the influent to the RO preferably has a total dissolved solids content of 1200 ppm or less. However, other systems including larger pumps and tighter wound membranes can handle much higher TDS and achieve effective concentrations in accordance with the present invention, with chlorine levels of 0.5 ppm or less, more preferably 0.1 ppm or less, pH 1-12, more preferably 2-11, substantially no oil or grease, very low levels of Ba and Si (which, if initially present, can be removed in a water softening step), and flow rates depending on the scale required; in some embodiments, RO is performed in the range of about 3-5 gallons (11 L-19 L) / minute.

[0028] A preferred embodiment of the present invention is illustrated schematically in FIG. 1. PFAS-contaminated water entering the system can be subjected to a number of optional pretreatments, including one or more of filtration storage in tank 102 (not shown), water softening pretreatment 104, and a feed tank 106 connected to a reverse osmosis system 108. Water softening to replace other cations with sodium cations can be accomplished by conventional means, such as passing through an ion exchange resin. The reverse osmosis treatment (described above) produces a permeate 100 having a PFAS concentration that is reduced by 10, 100, 1000, 10,000, or more times compared to the PFAS-contaminated water entering the system. In some cases, particularly with relatively concentrated PFAS solutions entering the system, the permeate can be subjected to additional RO treatment to reduce the PFAS levels in the permeate to low levels, such as below 70 ppt, so that the water can be released into the environment. The concentrate from the additional treatment can be combined with the concentrate solution or can be combined with the incoming PFAS-contaminated water, such as in tank 102.

[0029] The concentrate 112 typically comprises an aqueous solution having a PFAS concentration 10 times, 100 times, 1000 times, or 10,000 times or more greater than the PFAS-contaminated water entering the system. Accordingly, the present invention provides an energy-efficient system in which PFASs in the PFAS-contaminated water are completely destroyed by 90%, 99%, or 99.9% or more. Thus, in a preferred PFAS destruction process, only 10%, 1%, 0.1%, or 0.01% or less of the water in the PFAS-contaminated water is heated to SCWO conditions.

[0030] The concentrated PFAS water can optionally be passed through a heat exchanger 114, which can be a tube-in-tube heat exchanger.

[0031] The concentrated PFAS water 112 passes through the salt separator 116. The salt separator can have multiple zones operating at different temperatures or pressures. The tubes can be heated by a tube furnace surrounding the tubes. Multiple vertical tubes (six in Figure 1, three facing upwards and three facing downwards) can have a relatively large inner diameter to prevent clogging, for example, at least 1.5 cm, at least 2.0 cm, or at least 1 inch (2.4 cm). At the bottom (relative to gravity) of each salt separation tube is a larger-diameter container (collection container 220), preferably with an inner diameter of at least 5 cm, at least 10 cm, or in the range of 5-20 cm. Preferably, the collection container has a diameter at least two or at least four times larger than the inlet tube. The collection container can be heated, for example, with electrical tape. The collection container is connected to the inlet and outlet and preferably has a depth of at least 20 cm, at least 30 cm, or at least 40 cm, and in some embodiments, a depth of 25-75 cm. Salts formed in the inlet tube fall into a collection vessel, where they are continuously, or more typically, periodically removed and treated to remove PFAS or other contaminants, as needed. Toward the bottom of the collection vessel, there is preferably a valve leading to a drain outlet for removing brine or brine slurry that collects at the bottom of the collection vessel. Optionally, a pump assembly can be used to evacuate the contents of the collection vessel at high pressure during operation. In some preferred embodiments, the salt separation tube inlet 222 (which carries fluid to the collection tube) extends at least 5 cm or at least 10 cm into the collection vessel (relative to the outlet to the upward-flowing tube), enhancing the downward flow of the denser salt fraction to the bottom of the collection tube, which forces the lighter fraction out of the outlet 224. The collection vessel contains baffles to minimize turbulence and mixing near the bottom of the collection vessel. Typically, the bottom of the collection vessel is subcritical.

[0032] The concentrated PFAS water 112 typically enters the salt separator in a subcritical, preferably near-supercritical, state to fully dissolve the salts in the water, allowing for a long residence time before the salts drop out of solution and fall into a collection vessel. Alternatively, the water 112 can enter the salt separator in a supercritical state. In the salt separator, the temperature is raised to bring the solution to a supercritical state, causing sodium chloride and other salts to precipitate from solution. The conditions (typically temperature) of successive zones in the salt separator can be controlled so that the salts become increasingly insoluble as they pass through the salt separator. In some embodiments, the solution entering the salt separator is below 370°C and can be raised to a temperature in the range of 375-450°C within the salt separator. Optionally, a fuel such as alcohol can be added before or during the salt separation process to increase the temperature. This is particularly desirable because it has been unexpectedly found that heat transfer from the tube furnace to the aqueous composition is less than predicted by calculations.

[0033] Contact between the subcritical saltwater phase and the supercritical saltwater phase results in preferential separation of PFASs into the supercritical saltwater phase. Preferably, the PFAS concentration in the saltwater phase is at least 20% by mass or at least 50% by mass less than that in the supercritical saltwater phase. The salt separation step can remove more than 90% by mass or more than 95% by mass of NaCl (or other salts insoluble in supercritical water), while only 5% by mass or less, or 2% by mass or less, or 1% by mass or less, or 0.5% by mass of PFASs (or organic degradation products) is removed from the saltwater phase. Different salts precipitate at different temperatures and can be removed in different salt separation steps.

[0034] After the salt separator, the desalinated water can pass through a heat exchanger 118 and then be combined with an oxidizer 120, typically hydrogen peroxide, before being introduced into the SCWO reactor 144, where any remaining PFASs are destroyed. In the diagram presented, the peroxide (or other oxidizer) is added immediately before the reactor, but there may be options for adding the peroxide at various locations, such as upstream of the salt separator. The advantages of adding the oxidizer at multiple locations include: 1) minimizing the possibility of hot spots where the peroxide is added, and 2) facilitating the destruction of PFASs in the salt separator. However, adding peroxide upstream of the salt separator has the disadvantage of potentially exacerbating corrosion. The PFAS-free effluent can be passed through a heat exchanger, such as 118 or 114, to recover heat and then stored or exit the system as PFAS-free effluent 124.

[0035] The clean effluent preferably passes back through the second and first heat exchangers. At any point after the SCWO reactor, the clean water is preferably neutralized, such as by the addition of sodium hydroxide. Optionally, the clean water can also be treated (e.g., to remove Cr or other metals) before disposal or return to the environment.

[0036] Transcritical Hydrocyclone In another embodiment, salts can be removed using a transcritical hydrocyclone. The transcritical hydrocyclone 30 includes a conical chamber 32 including an inlet 34 for introducing a supercritical fluid into the conical chamber tangentially along the inner wall 33 of the cyclone, a top outlet 35 adapted for the flow of the supercritical fluid, an outlet pipe 36 adapted for the flow of liquid, and a cone 37 disposed in the conical chamber adapted to form a flow passage 38 between the inner wall of the cyclone and the outer wall of the cone. The cone has a bottom opening, which is preferably a diffuser. The diffuser has a plurality of openings 41. The openings preferably have a combined open area at least two or at least three times the cross-sectional area of ​​the outlet pipe. In some preferred embodiments, the holes in the diffuser can be oriented at an angle opposite to the flow direction of the cyclone; for example, if the cyclone is oriented clockwise, the holes are oriented at a counterclockwise angle. Preferably, the diffuser extends into the outlet pipe.

[0037] Typically, a hydrocyclone includes a fluid heat exchanger (not shown) that cools the outer wall 42 of the conical chamber, usually water as the coolant. The heat exchanger is located on the conical section and extends as far down as the separator to the outlet pipe.

[0038] During operation, the salt-containing PFAS aqueous supercritical composition enters through the inlet 34 and flows tangentially along the inner wall of the conical chamber. The cone forms a conical flow path 38. Preferably, the cone is adjustable to vary the cross-sectional area of ​​the conical flow path. For example, the cone can be attached to the outlet pipe 35 via a threaded connection with a lock nut and / or washer. Reducing the flow path area increases the velocity of the fluid through the flow path.

[0039] The inner wall 33 is sufficiently cooled to form a small subcritical phase adjacent to the wall 33. The supercritical salt is relatively concentrated, and centrifugal force drives the salt toward the inner wall 33, passing through the subcritical phase on the wall. In some cases, the subcritical phase forms droplets on the wall. The subcritical brine phase is discharged downward into the outlet pipe. A two-phase gravity separator with a level transmitter controller and control valve can be installed at the bottom of the outlet pipe.

[0040] Supercritical hydroxide method (SCWO) The PFAS-containing water is preferably heated before (usually immediately before) entering the reactor. Heat from the reactor is used to heat the water entering the reactor. The use of a heat exchanger makes the process more energy efficient, compact, and extends the useful life of the reactor. A tube-in-tube heat exchanger is particularly desirable. PFAS are destroyed and converted to carbonate, fluoride, and sulfate salts. The equipment can be designed as either 1) a stationary unit or 2) a transportable unit. Stationary units can be employed at permanent treatment plants, such as municipal water treatment systems. Portable units can be used in areas with low load requirements where temporary installation is sufficient. Portable units are sized to be transported in smaller enclosed spaces, such as semi-trucks or trailers, or shipping containers. This design can be adapted to treat other organic contaminants by changing operating parameters without equipment modifications.

[0041] A preferred SCWO reactor design is a continuous or semi-continuous system in which a (usually pretreated) PFAS-containing aqueous solution is passed through the SCWO reactor. Because solids can form in the SCWO reactor, it is desirable to have the reactor sloped downward so that the solids are pulled downward by gravity and exit the reactor. In some embodiments, the flow path is straight and perpendicular (0°) to gravity. In some embodiments, the reactor is sloped relative to gravity, e.g., at an angle of 5-70°, 10-50°, 10-30°, or 10-20° (from vertical), and can have bends to allow flow to move in the reverse direction to provide a compact device with constant downward flow relative to gravity. Preferably, the reactor vessel is a cylindrical pipe formed of a corrosion-resistant material. Desirably, the pipe has an internal diameter of at least 1 cm, preferably at least 2 cm, and in some embodiments, up to about 5 cm.

[0042] The flow through the SCWO equipment components under supercritical conditions should be turbulent (Re at least 2000, preferably in the range of 2500-6000). The effluent from the SCWO reactor can enter the salt separator under supercritical conditions.

[0043] SCWO systems operate by increasing the feed temperature and increasing the feed pressure. The pressure can be increased by heating alone (preferred), or by adding additional heat via a compressor or high-pressure (reciprocating) pump. The temperature can be increased by adding heat through a conduit (in the case of a continuous reactor) or through the reaction chamber (in the case of a batch reactor) and / or by adding a fuel, such as an alcohol or hydrocarbon, to generate heat by oxidation in solution. Supercritical conditions are maintained for oxidation, and conditions within the reaction conduit or chamber are preferably in the range of 374°C to 700°C and at least 220 bar, more preferably 221 to 300 bar. In some embodiments, the temperature within the SCWO reactor is maintained above 500°C, or above 600°C, and within the range of 500 to 650°C, or 600 to 675°C.

[0044] oxidizing agent Two tested reactive oxygen sources used in supercritical water oxidation for the destruction of PFAS are oxygen gas (O2) and hydrogen peroxide (H2O2). In addition to or instead of these two species, other reactive oxygen sources or oxidizing agents can be added to destroy PFAS in the oxidation reactor. Other oxidizing agents include oxyanion species, ozone, and peroxyacids.

[0045] The preferred oxidizing agent is hydrogen peroxide, which can be added in excess (e.g., at least 50% excess, or at least 100% excess, or in the range of 50% to 300% excess), where the excess hydrogen peroxide reacts to form dioxygen and water.

[0046] fuel SCWO devices require heating, such as an external flame or resistive heating, during start-up. External heating is also required during operation unless the reaction solution contains high concentrations of PFAS or other organics. Alternatively, or in addition to external heating of the SCWO device, heat can be provided by the oxidation of a fuel, such as alcohol. Preferred fuels include methanol, ethanol, propanol (usually isopropanol), or combinations of these.

[0047] Handling of fluorine by-products from PFAS destruction The corrosive effluent from the SCWO reactor, containing an aqueous HF solution at high temperatures (e.g., about 600°C), can enter the mixing pipe. Cooling water, usually containing hydroxy salts, is sent to the mixing tube where it mixes with the corrosive effluent. The cooled effluent contains dissolved fluoride salts, such as NaF.

[0048] Post-processing Because the SCWO process destroys essentially all PFAS, the treated effluent can be safely returned to the environment. In some embodiments, at least a portion of the effluent is evaporated into the air. The generated vapor is cooled and in equilibrium with the aqueous phase, typically resulting in 100% humidity. However, the vapor stream exists due to carbon dioxide formed as a reaction by-product and excess oxygen to ensure complete oxidation. Feeds with relatively low organic vapor content (such as PFAS-spiked distilled water samples) generate very little (sometimes unmeasurable) vapor. This is safe because the PFASs are destroyed and remaining contaminants (metals, NaF, etc.) have very high vapor pressures and tend not to evaporate with the water. Precipitates, such as fluoride salts, can be filtered or centrifuged from the effluent. The PFAS-free effluent can be passed through a heat exchanger, where the effluent is cooled by the PFAS-contaminated water flowing into the reactor. If desired, the effluent can be subjected to reverse osmosis and / or other treatments, such as ion exchange resins and other adsorption media (Metsorb™), to remove metals or other contaminants before being discharged or disposed of.

[0049] Passivation treatment of the internal surfaces of salt separators or SCWO reactors The interior surfaces of the device can be coated with a corrosion resistant material such as platinum aluminide, BC (boron carbide), SiC (silicon carbide), TaC (tantalum carbide), WC (tungsten carbide), metal fluorides such as YF3 (yttrium fluoride), YN (yttrium nitride), LaF3 (lanthanum fluoride), LaN (lanthanum nitride), YbN, YbF3, or any lanthanide nitride or lanthanide fluoride, HfN (hafnium nitride), CeN (cerium nitride), CeF3 (cerium fluoride), TaN (tantalum nitride), Ta (tantalum), TaF (tantalum fluoride), ZrN (zirconium nitride), ZrF (zirconium fluoride), WN (tungsten nitride), or combinations thereof. The coating material resists corrosion and protects the underlying metal under the process conditions of supercritical water, typically HF, an oxidizing agent. A preferred coating material is a chromium oxide coating (preferably containing at least 80 wt. % or at least 90 or 95 wt. % chromium oxide, compatible with fluids with pH levels of 0.1 to 11, and stable up to at least 2,300°F (1,260°C)). This coating is not affected by the low levels of HF content anticipated in our process. This coating can be formed, for example, by washcoating a chromium oxide slurry onto the equipment walls, followed by baking at 752°F to 975°F (400°C to 525°C). Multiple cycles of coating and / or baking are desirable to minimize to zero porosity.

[0050] Alternatively, or in addition, corrosion can be reduced by sacrificial electrodes or externally powered cathodic protection.

[0051] Mobile Unit An example of a mobile unit is one that is transportable on (and preferably operable within) a trailer. For example, the system can be transported (and optionally operated) on a trailer having dimensions of 29 feet (8.8 m) or less in length, 8 feet 6 inches (2.6 m) or less in width, and 13 feet 6 inches (4.1 m) or less in height. While these dimensions define the preferred size of the mobile system, one skilled in the art will recognize that other dimensions for the mobile unit are available.

[0052] Example Reverse Osmosis - 364 kg of well water was spiked with PFAS at a concentration of 2400 μg / L, and the spiked sample was passed through a water softener (PFAS concentrations were measured and showed no change during the softening process). The softened PFAS water was then processed in a reverse osmosis system for 160 minutes until the concentration reached 38.5 kg. The results were as follows: %efficiency = (C feed -C perm / C feed )x100, as shown in the table below. [Table 1]

[0053] The test results showed that PFASs were not removed by filtration or water softening. While achieving high PFAS removal efficiency, the PFAS concentration in the concentrate was increased tenfold. Surprisingly, precipitates were observed in the concentrate, but they could be dissolved by adding acid to lower the pH.

Claims

1. 1. A method for destroying PFAS, comprising: providing an aqueous solution comprising water and PFAS; subjecting said aqueous solution to reverse osmosis to produce a clean water fraction and a saltwater enriched fraction having a concentration of said PFAS that is at least 50% greater than said aqueous solution; preheating the brine enriched fraction in a heat exchanger to form a preheated enriched fraction at a subcritical state; passing the preheated concentrated fraction through a heated pre-reactor to convert the brine concentrated fraction to a supercritical state at a first temperature and precipitate sodium chloride; removing at least a portion of said sodium chloride to produce a saline-reduced fraction; passing said salt-reduced fraction into a reactor and oxidizing said fraction under supercritical conditions, said oxidizing having a higher oxidant concentration and / or temperature than in said pre-reactor; producing a clean, warm aqueous solution having a PFAS concentration that is at least 90% lower than that of the aqueous solution; and Fuel or oxidant is added to the pre-reactor; and the preheating step further comprises transferring heat from the hot clean aqueous solution to the aqueous solution in the heat exchanger. method.

2. The method of claim 1, wherein the fuel or oxidizer comprises an alcohol.

3. The method of claim 1, wherein the pre-reactor comprises a transcritical liquid cyclone.

4. The method of claim 1, wherein the salt-reduced fraction passes through a heat exchanger and then combines with hydrogen peroxide before being introduced into the SCWO reactor.

5. The method of claim 1, wherein the aqueous solution comprising water and PFAS comprises a first volume, A method wherein no more than 10% of said first volume is subjected to supercritical conditions, and wherein at least 95% of the PFAS in said first volume is destroyed in the supercritical conditions.

6. The method of claim 1, wherein the method is carried out in a trailer or shipping container.

7. A method for destroying PFAS, comprising: providing an aqueous solution comprising water and PFAS; treating the aqueous solution to reduce its volume to form a reduced volume PFAS solution having a first concentration of PFAS; adding hydrogen peroxide to the reduced volume of PFAS solution, the hydrogen peroxide being added in excess of the amount needed to destroy PFAS; passing the reduced volume of the PFAS solution through a SCWO reactor to subject the reduced volume of the PFAS solution to supercritical water oxidation; and producing a clean effluent having a concentration of PFAS that is at least 100,000 times less than said first concentration of PFAS; Including, The flow through the SCWO reactor is turbulent having an Re of at least 2000. method.

8. The method of claim 7, wherein the clean effluent contains 5 ppt or less of PFAS.

9. The method of claim 7, comprising producing a clean effluent having a concentration of PFAS that is at least 1,000,000 times lower than the first concentration of PFAS.

10. The method of claim 7, comprising producing a clean effluent having a concentration of PFAS that is at least 10,000,000 times lower than the first concentration of PFAS.

11. The method of claim 7, wherein the clean effluent is treated with a solution containing NaOH, LiOH, or KOH.

12. A method for destroying PFAS, comprising: providing an aqueous solution comprising water and PFAS; treating the aqueous solution to reduce its volume to form a reduced volume PFAS solution having a first concentration of PFAS; adding hydrogen peroxide to the reduced volume of PFAS solution, the hydrogen peroxide being added in excess of the amount needed to destroy PFAS; passing the reduced volume of the PFAS solution through a SCWO reactor to subject the reduced volume of the PFAS solution to supercritical water oxidation; and producing a clean effluent having a concentration of PFAS that is at least 100,000 times less than said first concentration of PFAS; Including, The reduced volume of PFAS solution is mixed with a solution containing 30-50 wt% H 2 O 2 in a weight ratio of 30:1-70:1 wt% PFAS solution; method.

13. The method of claim 7, wherein the reduced volume of PFAS solution is passed through the SCWO reactor with a residence time of 20 seconds or less.

14. The method of claim 7, wherein after start-up, no external heating is required for the SCWO reactor.

15. The method of claim 7, wherein the method is carried out in a mobile trailer at a PFAS-contaminated site.

16. The method of claim 7, wherein the SCWO reactor has an internal ceramic coating, the ceramic comprising B4C (boron carbide), SiC (silicon carbide), TaC (tantalum carbide), WC (tungsten carbide), a metal fluoride such as YF3 (yttrium fluoride), YN (yttrium nitride), LaF3 (lanthanum fluoride), LaN (lanthanum nitride), YbN, YbF3, or any lanthanide nitride or lanthanide fluoride, HfN (hafnium nitride), CeN (cerium nitride), CeF3 (cerium fluoride), TaN (tantalum nitride), Ta (tantalum), TaF (tantalum fluoride), ZrN (zirconium nitride), ZrF (zirconium fluoride), WN (tungsten nitride), or a combination thereof.

17. A method for destroying PFAS, comprising: providing an aqueous solution comprising water and PFAS; treating the aqueous solution to reduce its volume to form a reduced volume PFAS solution having a first concentration of PFAS; adding hydrogen peroxide to the reduced volume of PFAS solution, the hydrogen peroxide being added in excess of the amount needed to destroy PFAS; passing the reduced volume of the PFAS solution through a SCWO reactor to subject the reduced volume of the PFAS solution to supercritical water oxidation; and producing a clean effluent having a concentration of PFAS that is at least 100,000 times less than said first concentration of PFAS; Including, The SCWO reactor comprises a flow channel having an inner diameter of at least 1 cm, the flow channel being inclined downward with respect to gravity. method.

18. The method of claim 17, wherein the flow through the SCWO reactor is turbulent having an Re of at least 2000.

19. The method of claim 7, wherein the hydrogen peroxide is added in at least a 50% excess.

20. A method for destroying PFAS, comprising: providing an aqueous solution comprising water and PFAS; subjecting said aqueous solution to reverse osmosis to produce a clean water fraction and a saltwater enriched fraction having a concentration of said PFAS that is at least 50% greater than said aqueous solution; preheating the brine enriched fraction in a heat exchanger to form a preheated enriched fraction at a subcritical state; passing the preheated concentrated fraction through a heated pre-reactor to convert the brine concentrated fraction to a supercritical state at a first temperature and precipitate sodium chloride; removing at least a portion of said sodium chloride to produce a saline-reduced fraction; passing said salt-reduced fraction into a reactor and oxidizing said fraction under supercritical conditions, said oxidizing having a higher oxidant concentration and / or temperature than in said pre-reactor; producing a clean, warm aqueous solution having a PFAS concentration that is at least 90% lower than that of the aqueous solution; and Optionally, transferring heat from the hot clean aqueous solution to the aqueous solution in the heat exchanger during the preheating step; Including, the clean hot aqueous solution passes through a post-heat exchanger located downstream of the pre-reactor and then through a pre-heat exchanger located upstream of the pre-reactor; method.

21. The method of claim 20, wherein the aqueous solution is filtered prior to reverse osmosis.

22. The method of claim 20, wherein the aqueous solution comprising water and PFAS comprises a first volume; 10% or less of the first volume is placed in a supercritical state; and The method, wherein at least 95% of the PFAS in the first volume is destroyed under supercritical conditions.

23. A method for destroying PFAS, comprising: providing an aqueous solution comprising water and PFAS; subjecting said aqueous solution to reverse osmosis to produce a clean water fraction and a saltwater enriched fraction having a concentration of said PFAS that is at least 50% greater than said aqueous solution; preheating the brine enriched fraction in a heat exchanger to form a preheated enriched fraction at a subcritical state; passing the preheated concentrated fraction through a heated pre-reactor to convert the brine concentrated fraction to a supercritical state at a first temperature and precipitate sodium chloride; removing at least a portion of said sodium chloride to produce a saline-reduced fraction; passing said salt-reduced fraction into a reactor and oxidizing said fraction under supercritical conditions, said oxidizing having a higher oxidant concentration and / or temperature than in said pre-reactor; producing a clean, warm aqueous solution having a PFAS concentration that is at least 90% lower than that of the aqueous solution; and Optionally, transferring heat from the hot clean aqueous solution to the aqueous solution in the heat exchanger during the preheating step; Including, The method, wherein the brine enriched fraction resulting from the reverse osmosis and having a PFAS concentration at least 50% higher prior to the step of removing at least a portion of the sodium chloride, contains a precipitate, and further comprising the step of adding an acid to dissolve the precipitate.

24. The method of claim 20, wherein the pre-reactor includes a first tube entering a collection vessel and a second tube exiting the collection vessel, the collection vessel having an inner diameter at least twice as large as the inner diameter of the first tube, and optionally, fuel or oxidizer is added to the pre-reactor.

25. The method of claim 20, wherein the prereactor comprises a transcritical hydrocyclone; The transcritical hydrocyclone has a conical chamber, an inlet for introducing a supercritical fluid into the conical chamber in a tangential direction along an inner wall of the cyclone; an upper outlet adapted for flow of a supercritical fluid; an outlet pipe adapted for liquid flow; a cone disposed in the conical chamber adapted to form a flow passage between an inner wall of the cyclone and an outer wall of the cone, the cone including a bottom opening; A method comprising:

26. A system for destroying PFAS, comprising: A reverse osmosis system; a conduit from the reverse osmosis system to a salt separator, the salt separator includes a first tube entering a collection vessel and a second tube exiting the collection vessel, the collection vessel having an inner diameter at least twice as large as an inner diameter of the first tube; the first tube protruding at least 5 cm into the collection container beyond the second tube; the conduit; and a conduit from the salt separator to a supercritical reactor; Including, the system.

27. The system of claim 26, wherein the collection vessel contains a supercritical fluid.

28. The system of claim 26, wherein the first tube protrudes at least 5 cm into the interior of the collection container and the second tube does not protrude into the interior of the collection container.

29. A system for destroying PFAS, comprising: A reverse osmosis system; a conduit from the reverse osmosis system to a salt separator, the salt separator includes a first tube entering a collection vessel and a second tube exiting the collection vessel, the collection vessel including an inner diameter at least twice as large as an inner diameter of the first tube; and a conduit from the salt separator to a supercritical reactor; a first heat exchanger disposed in the conduit between the reverse osmosis system and the salt separator; Including, During operation, the first heat exchanger exchanges heat between a subcritical PFAS-containing water stream and a PFAS-free water return stream from the supercritical reactor. system.

30. The system of claim 29, wherein the heat exchanger is a tube-in-tube type heat exchanger.

31. The system of claim 29, wherein the salt separator includes a plurality of collection vessels, each collection vessel including a set of inlet and outlet tubes, each set being at a higher temperature than the previous set in the flow direction.

32. A transcritical hydrocyclone comprising: A conical chamber, an inlet for introducing a supercritical fluid into the conical chamber in a tangential direction along an inner wall of the cyclone; an upper outlet adapted for flow of a supercritical fluid; an outlet pipe adapted for liquid flow; a cone disposed in the conical chamber adapted to form a flow passage between an inner wall of the cyclone and an outer wall of the cone, the cone including a bottom opening, the bottom opening including a diffuser extending downwardly from the cone, the diffuser including a plurality of openings; A transcritical hydrocyclone comprising:

33. A transcritical liquid cyclone as described in claim 32, wherein the plurality of openings include an open area at least two times or at least three times larger than the cross-sectional area of ​​the outlet pipe.

34. A transcritical liquid cyclone as described in claim 32, wherein the multiple openings are inclined from the central axis toward the inner wall of the lower pipe.

35. A transcritical liquid cyclone as described in claim 32, further comprising a mechanism adapted to move the cone along a central axis of the conical chamber relative to the inner wall of the conical chamber.

36. The transcritical liquid cyclone of claim 32, comprising a heat exchanger thermally connected to an outer wall of the conical chamber.

37. The transcritical liquid cyclone of claim 36, wherein the heat exchanger includes a coating for fluid flow.

38. A method for destroying PFAS, comprising: providing an aqueous solution comprising water and PFAS; subjecting said aqueous solution to reverse osmosis to produce a clean water fraction and a saltwater enriched fraction having a concentration of said PFAS that is at least 50% greater than said aqueous solution; preheating the brine enriched fraction in a heat exchanger to form a preheated enriched fraction at a subcritical state; passing the preheated concentrated fraction through a heated pre-reactor to convert the brine concentrated fraction to a supercritical state at a first temperature and precipitate sodium chloride; removing at least a portion of said sodium chloride to produce a saline-reduced fraction; passing said salt-reduced fraction into a reactor and oxidizing said fraction under supercritical conditions, said oxidizing having a higher oxidant concentration and / or temperature than in said pre-reactor; producing a clean, warm aqueous solution having a PFAS concentration that is at least 90% lower than that of the aqueous solution; and Optionally, transferring heat from the hot clean aqueous solution to the aqueous solution in the heat exchanger during the preheating step; Including, the pre-reactor and / or the reactor have reactor walls coated with a ceramic coating that is resistant to hydrofluoric acid, the ceramic comprising at least 80 wt. % chromium oxide; method.

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