Patent Application
20240190727
Per- and poly-fluoroalkyl substances (PFAS) are a class of man-made amphophilic compounds (i.e., surfactants with a chemical structure that includes a hydrophilic “head” group and a fluorinated “tail”) that have been used to manufacture consumer products and industrial chemicals, including, inter alia, aqueous film forming foams (AFFFs). AFFFs have been the product of choice for firefighting at military and municipal fire training sites around the world. AFFFs have also been used extensively at oil and gas refineries for both fire training and firefighting exercises. AFFFs work by blanketing spilled oil/fuel, cooling the surface, and preventing re-ignition. PFAS in AFFFs have contaminated the groundwater at many of these sites and refineries, including more than 100 U.S. Air Force sites.
PFAS are very difficult to treat largely because they are extremely stable compounds which include carbon-fluorine bonds. Carbon-fluorine (C—F) bonds are among the strongest known bonds in nature and are highly resistant to breakdown.
Due to PFAS contamination in groundwater, surface water, agriculture, and drinking water and their associated health risks, there has been a great focus on developing practical and effective water treatment technologies. Treatment technologies developed so far have included adsorptive and destructive methods. Some PFAS can associate strongly to proteins through either electrostatic or non-electrostatic (hydrophobic/hydrophilic) physisorption as well as site specific chemisorption. Based off these properties, adsorptive methods have been developed to remove these specific compounds from water, but are not effective for all PFAS compounds. Adsorptive methods to date include granular activated carbon (GAC), ion exchange (IX), polymers, and protein addition. These methods show promise in effectively removing some of the compounds from contaminated water, but do not destroy the PFAS compound, leading to the generation of PFAS contaminated residues and concentrates.
Due to the recalcitrant nature of PFAS and the C—F bond, destructive methods development has come with difficulties. Destructive methods of PFAS in contaminated water that have been studied for their treatment ability include sonolysis, thermal degradation, photocatalytic ozonation, electrochemical oxidation, persulfate, alkaline hydrothermal treatment, microwave/persulfate, UV, ionizing radiation electron beam, gamma-irradiation, boron-doped diamond film electrode oxidation, electrical discharge plasma, and biodegradation. Novel reductive processes such as photogenerated hydrated electrons (e−aq) are also being developed as a treatment technology. The methods described above have been demonstrated to be effective in degradation and lead to varying degrees of destruction involving reduction in chain length, cleaving of the C—F bond, and removal of the head group.
A promising treatment technology, non-equilibrium plasma discharge, is not in thermodynamic equilibrium and only the electron temperature is much hotter than the rest of the gas. Non-equilibrium plasma can generate a reactive environment of heat, ultraviolet (UV) radiation, and highly reactive chemical species such as electrons, ions, and reactive neutral species. At the gas-liquid interface, non-equilibrium plasma generates diverse reactive environments containing a variety of reactive chemical species like reactive oxygen species (ROS, such as 1O2, H2O2, O3, etc.), reactive nitrogen species (RNS, such as peroxynitrite, ONOO−, and peroxynitrate, ONOOO−), radicals (H−, O−, OH−, NO−, NO2−), as well as hydrated electrons (e−aq). These reactive species are responsible for targeting anything dissolved or suspended, including contaminants in the water and potentially degrading them in the plasma water treatment system. Plasma water treatment applications include a multitude of different discharge types, such as pulsed corona/streamer/spark discharge, DC pulseless corona discharge, dielectric barrier discharge, gliding arc discharge, DC glow discharge, and DC arc discharge, all leading to different reactive environments with different treatment potentials.
A recently developed enhanced contact plasma reactor for the destruction of PFAS featured a high-voltage electrode in the gas, just above the liquid surface, and a grounded ring electrode submerged just beneath the liquid surface to achieve contact between plasma streamers and the entire reactor volume. Plasma was formed by applying a sufficiently high electrical potential between the high-voltage and grounded electrodes via an external plasma-generating network. Argon gas was pumped through submerged diffusers to produce bubbles and form a layer of foam on the liquid surface. This foam concentrated surfactant-like contaminants (e.g., PFAS) and enhanced the contact between the liquid and the plasma, exposing the contaminant at the gas-liquid interface to reactive oxidative and reductive species in the plasma. See Singh et al., Environ. Sci. Technol., vol. 53, pp. 2731-2738 (2019). The amount of processed liquid was limited by the reactor volume, however, to a static volume of a few liters.
When any PFAS destruction method is applied to a large volume (e. g., 1 gallon or more) of water, such as municipal drinking water, the treatment method must be robust enough to break down the carbon-fluorine bond, and achieve a low target contaminant concentration, on the order of nanograms per liter in the resulting treated water.
Therefore, there is a need for a high-throughput fluid treatment system for generating a plasma discharge in a fluid that concentrates surfactant-like contaminants.
Various embodiments disclosed herein relate to methods and apparatus for fluid treatment with plasma discharges. In accordance with one or more embodiments, a flow-through fluid treatment system for generating a plasma discharge in a fluid includes a high-voltage electrode forming a fluid inlet into a cylindrical flow-through reactor, the fluid inlet having an inlet inner diameter, a ground electrode forming a fluid outlet out of the cylindrical flow-through reactor, the ground electrode and the high-voltage electrode disposed coaxially across a gap between the electrodes in a cylindrical flow-through reactor space. The fluid treatment system further includes a gas inlet into the cylindrical flow-through reactor, disposed tangentially in an interior wall of the cylindrical flow-through reactor to generate a vortex gas flow within the cylindrical flow-through reactor space, thereby generating a negative gauge pressure within the fluid inlet, and a high-voltage power supply electrically connected to the high-voltage electrode for generating a plasma discharge across the gap, thereby producing plasma treated fluid. In some embodiments, the fluid outlet can have an outlet inner diameter that is larger than the inlet inner diameter. In certain embodiments, the gap can be in a range of between about 1 mm and about 8 mm. In some embodiments, the fluid can be a foam. In some of these embodiments, the flow-through fluid treatment system can further include a foam fractionation system in fluid communication with the fluid inlet. In certain of these embodiments, the foam, that is, the foamy fluid processed by the foam fractionation-based water treatment system, can comprise amphophilic compounds, such as per- and poly-fluoroalkyl substances (PFAS). In some embodiments, the fluid can be a liquid. In certain of these embodiments, the liquid can include a surfactant.
In accordance with one or more embodiments, a method of plasma treating fluid includes flowing a fluid through a fluid inlet having an inlet inner diameter into a cylindrical flow-through reactor, the fluid inlet forming a high-voltage electrode, flowing the fluid out of a fluid outlet, the fluid outlet forming a ground electrode disposed coaxially across a gap between the electrodes in a cylindrical flow-through reactor space. The method further includes flowing a gas through a gas inlet into the cylindrical flow-through reactor tangentially along an interior wall of the cylindrical flow-through reactor, thereby generating a vortex gas flow within the cylindrical flow-through reactor space and a negative gauge pressure within the fluid inlet, and generating a plasma discharge across the gap, thereby producing plasma treated fluid. In certain embodiments, the fluid outlet can have an outlet inner diameter that is larger than the inlet inner diameter. In some embodiments, the gap can be in a range of between about 1 mm and about 8 mm. In certain embodiments, the fluid can be a foam that is drawn into the cylindrical flow-through reactor by the negative pressure within the fluid inlet. In some of these embodiments, the method can further include flowing the fluid through a foam fractionation system in fluid communication with the fluid inlet. In certain of these embodiments, the foam can comprise amphophilic compounds, such as per- and poly-fluoroalkyl substances (PFAS). In some embodiments, the fluid can be a liquid. In certain of these embodiments, the liquid can include a surfactant.
The water treatment systems and methods described herein have many advantages, including enabling a high-throughput fluid treatment system for generating a plasma discharge in a fluid with concentrated surfactant-like contaminants.
The foregoing purposes and features, as well as other purposes and features, will become apparent with reference to the description and accompanying figures below, which are included to provide an understanding of the invention and constitute a part of the specification, in which like numerals represent like elements. The figures are not necessarily drawn to scale, emphasis instead being placed upon illustrating embodiments.
It is to be understood that the figures and descriptions of the present invention have been simplified to illustrate elements that are relevant for a clearer comprehension of the present invention, while eliminating, for the purpose of clarity, many other elements found in systems and methods of plasma discharge in fluid. Those of ordinary skill in the art may recognize that other elements and/or steps are desirable and/or required in implementing the present invention. However, because such elements and steps are well known in the art, and because they do not facilitate a better understanding of the present invention, a discussion of such elements and steps is not provided herein. The disclosure herein is directed to all such variations and modifications to such elements and methods known to those skilled in the art.
Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Although any methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present invention, the preferred methods and materials are described.
As used herein, each of the following terms has the meaning associated with it in this section.
The articles “a” and “an” are used herein to refer to one or to more than one (i.e., to at least one) of the grammatical object of the article. By way of example, “an element” means one element or more than one element.
“About” as used herein when referring to a measurable value such as an amount, a temporal duration, and the like, is meant to encompass variations of +20%, +10%, +5%, +1%, and +0.1% from the specified value, as such variations are appropriate.
“HV” as used herein means high-voltage, such as a voltage in excess of 1,000 V.
Ranges: throughout this disclosure, various aspects of the invention can be presented in a range format. It should be understood that the description in range format is merely for convenience and brevity and should not be construed as an inflexible limitation on the scope of the invention. Where appropriate, the description of a range should be considered to have specifically disclosed all the possible subranges as well as individual numerical values within that range. For example, description of a range such as from 1 to 6 should be considered to have specifically disclosed subranges such as from 1 to 3, from 1 to 4, from 1 to 5, from 2 to 4, from 2 to 6, from 3 to 6, etc., as well as individual numbers within that range, for example, 1, 2, 2.7, 3, 4, 5, 5.3, and 6. This applies regardless of the breadth of the range.
The objective of the systems and methods described herein is to remove contaminants from various types of liquids, including groundwater, surface water, agriculture and drinking water, industrial or municipal wastewater, and industrial process water, and oxidize/reduce and destroy them without leaving any treatment gap or secondary waste problems. Various embodiments disclosed herein relate to methods and apparatus for fluid treatment with plasma discharges. In accordance with one or more embodiments, as shown in
In accordance with one or more embodiments, as shown in
where U is the bubble rising velocity, ρwater and ρair are the density of water and air, respectively, d is the bubble diameter, and u is the water viscosity.
As gas bubbles 191 rise in water, PFAS molecules from water are adsorbed on the surface of air bubbles. This is because PFAS behaves as a surfactant as a result of its hydrophobic tail and a hydrophilic head. The hydrophobic tail of a PFAS chemical is adsorbed to the surface of air bubbles, changing gas bubbles into concentrated PFAS-adsorbed foam 192. Foam typically rises through the water volume in the FF tank 190.
When compressed gas is employed to boost foam in the FF tank 190 on a continuous basis and foam fills the head space 193 in the FF tank 190, the pressure in the head space 193 eventually reaches pressure P1. Note that pressure regulator 171 is set to P1, whereas pressure regulator 172 is set to P2. P1 is greater than P2 by at least 1-5 psig. (Note: P1 also needs to be slightly larger than P2 for the optional gas recirculation 173 to compressor 175 shown in
At this point, valve 121 opens, allowing foam 192 to move from the head space 193 in FF tank 190 to a gliding arc reactor (GAR) 130. When there is little or no foam 192 in the head space 193 of the FF tank 190, valve 121 is closed.
As described above and shown in
The compressed gas 172 entering the GAR 130 through gas inlet 170 tangentially enters the circular-gap space 135 between the two tubular electrodes 110 and 140. As a plasma arc is discharged between the two electrodes 110 and 140, the compressed gas pushes the arc along the perimeter of the two tubular electrode edges 110 and 140. As the arc continuously glides along the circumferential direction, significant cooling takes place at the two tubular electrode edges, preventing both electrodes from overheating, burning or melting. The compressed gas 170 pushes a part of arc into the circular-tube ground electrode 140, which is connected to the outlet pipe 150 of GAR 130, stretching the arc along the flow direction of the compressed gas.
As shown in
In accordance with one or more embodiments, as shown in
In another embodiment, as shown in
As shown in
The key idea is to utilize gas plasma 230 (i.e., gliding arc discharge, GAD) between the head space 293 in FF tank 290 and foam collection tank 205 such that foam 292 enters the outlet 250 of the ground electrode 240 in GAD 230, thus making direct contact with arc plasma, whereby PFAS molecules adsorbed on foam surface 292 are destroyed by solvated electrons and other active plasma species. As some foam is converted to liquid, it is collected at the bottom of foam-collection tank 205 and returns to FF tank 290 by gravity.
As also shown in
Moisture-trapping membranes 207, such as a mesh with small pores, installed within head space 206 of the foam collection tank 205 upstream of scroll compressor 275, allow clean and dry gas to flow to GAD 230 through gas inlet 270. As the scroll compressor 275 continuously injects gas to both MBG 297 and GAD 230, foam 292 is continuously generated in FF tank 290, while PFAS molecules adsorbed on foam 292 are continuously destroyed in a closed-loop gas recirculation system.
Operating the FF tank 290 and GAD 230 in vacuum yields several advantages. First, the footprint of the entire system will be relatively small (e. g., 4 ft×6 ft). Second, the use of vacuum reduces the energy requirement for the complete PFAS mineralization to approximately 10,000 kWh/kg PFAS, a level of performance that is consistent with low and competitive operating and total costs for the PFAS destruction process, as the plasma breakdown voltage in vacuum will be significantly less compared to that at the atmospheric pressure according to the Paschen curve. As shown in
The PFAS destruction system described herein is built around a gliding arc discharge (GAD), which is a stable gas plasma as long as GAD operates with dry compressed gas. GAD is constructed with two coaxially positioned circular tubes: the inner tube with a smaller diameter is the high-voltage electrode, whereas the outer tube is the ground, through which compressed gas together with active plasma species exits. The axial gap between the two electrodes is maintained relatively small in a range of between about 1 mm and about 8 mm, for effective plasma breakdown. Compressed gas tangentially enters the cylindrical gap between the two electrodes, forcing plasma arc to glide along the perimeters of the two electrodes and thus minimizing electrode-overheating problem. When the arc is created between the HV and ground electrodes, the vortex gas flow in GAD pushes and stretches the arc by approximately one inch toward the exit of the ground electrode (i.e., the only outlet of gas).
As shown in
The moisture-trapping membrane described herein effectively separates gas from foam. The separation performance of membranes depends on several parameters, including the pore size and material of membranes, the gas flowrate through the membrane, the number of membranes, etc.
As described above, in some embodiments, dry compressed gas is injected into the GAD. In some of these embodiments, the compressed gas containing PFAS-laden foam is injected with the use of a scroll compressor over several hours.
In an exemplary embodiment, a PFAS-destruction skid based on a foam and gas closed-loop recirculation system with no PFAS emission to the atmosphere has a footprint of 4 ft×6 ft with a 10-gallon-foam-fractionation water tank and a 20-gallon-foam-collection gas tank.
In accordance with one or more embodiments, as shown in
In some embodiments, the fluid to be treated can be a liquid instead of a foam. For example, the liquid can be a leachate from a landfill containing contaminants, such as PFAS, polychlorinated biphenyls (PCBs), polychlorinated dibenzodioxins, other dioxins, etc. In certain of these embodiments, the liquid can include a surfactant, such as cetrimonium bromide (CTAB), hydroxypropyl methylcellulose (HPMC), or saponin, a plant-based biodegradable chemical for increasing foaming, or other such additives which increase the concentrations of contaminants in foamate.
In accordance with one or more embodiments, as shown in
The disclosures of each and every patent, patent application, and publication cited herein are hereby incorporated herein by reference in their entirety. While this invention has been disclosed with reference to specific embodiments, it is apparent that other embodiments and variations of this invention may be devised by others skilled in the art without departing from the true spirit and scope of the invention.
This application claims the benefit of U.S. Provisional Application No. 63/386,535, filed Dec. 8, 2022, which is hereby incorporated by reference in its entirety.
| Number | Date | Country | |
|---|---|---|---|
| 63386535 | Dec 2022 | US |
