Patent Application
20220323914
The present disclosure relates to nanocarbon immobilized membranes (NCIMs) that may be used in various applications, e.g., to purify different organic-water mixtures.
The separation of water from fluid systems, including systems that include only trace amounts of water, is critical to generate high purity solvents, including alkanes, paint thinners, alcohols, and fuels, such as gasoline and kerosene. Dewatering of solvents is important in various contexts, e.g., to generate high purity solvents and represents an important issue in recycling. The contaminated water that may be present in solvents can form an immiscible layer or an emulsion. For example, water in gasoline can affect engine performance. High purity fuels, gasoline, kerosene, paint thinners, and biofuels are important industrial products.
In a typical immiscible organic-water mixture, as many as four different phases, including organics, organic-in-water or water-in-organic emulsions and water, may co-exist. [See, e.g., Korotney, D., Water Phase Separation in Oxygenated Gasoline, Recuperado el, 1995. 24; Tharwat, T., et al., Formation and stability of nano-emulsions, Advances in Colloid and Interface Science, 2004. 9: p. 108-109.] An efficient and economic separation process to remove the water is highly desired. Conventional techniques include gravity separators, adsorbents, and skimming techniques. [See, e.g., Lu, D. and Z. Liu, Hyperlenses and metalenses for far-field super-resolution imaging, Nature communications, 2012. 3(1): p. 1-9.]
Membrane-based technologies are becoming attractive alternatives due to relatively less energy requirement, cost effectiveness, custom fabrication, and wide application range. Conventional hydrophilic membranes can be employed for gravity-driven separations, but would not be effective to restrict the permeation of free organic-water mixture or water in organic emulsion. [See, e.g., Lu, D. and Z. Liu, Hyperlenses and metalenses for far-field super-resolution imaging, Nature communications, 2012, 3(1): p. 1-9.] Typically, hydrophobic and oleophilic (contact angle with organic solvents is <90°) membranes are used in a crossflow filtration system to avoid any formation of a barrier layer that prevents permeation of organic solvents. However, these membranes are prone to fouling by organic solvents during demulsification. [See, e.g., Hu, B. and K. Scott, Influence of membrane material and corrugation and process conditions on emulsion microfiltration. Journal of membrane science, 2007, 294 (1-2): p. 30-39.]
Gasoline and kerosene are petroleum-based fuels. Petroleum refineries and blending facilities produce motor gasoline for sale at retail gasoline fueling stations. Most of the gasoline that petroleum refineries produce is unfinished gasoline or gasoline blend stocks. Gasoline blend stocks require blending with other liquids to make finished motor gasoline, which meets the basic requirements for fuel that is suitable for use in spark ignition engines.
Kerosene is mainly composed of a mixture of hydrocarbons, typically 10-16 carbon atoms per molecule, miscible with petroleum solvents but immiscible in water. One of the most important and the largest uses of kerosene is as fuel for aircraft jet engines and some rocket engines. In jet airlines, the fuel is used quickly and, as a safety measure, the fuel tanks may be slumped on a regular basis, which helps to remove any water present in the tank. Further, if the fuel is stored for a longer period, condensation may occur on the fuel due to temperature changes, which causes expansion and contraction. The water phase separation becomes more important with the introduction of oxygenated gasoline, which has more affinity toward water. [See, e.g., Korotney, D., Water Phase Separation in Oxygenated Gasoline, Recuperado el, 1995. 24.] The contaminated water not only reduces its efficiency and uniformity, but also lead to mechanical damage. Maintaining high purity and recovering kerosene from waste slumped product is highly important from both an economic and ecological point of view.
Removing water from solvents is equally important. Organic solvents, such as benzene, toluene, and xylenes (BTX), are used in a wide range of manufacturing processes and commercial products. Octane and heptane are hydrocarbons with the chemical formula C8H18 and C7H16, and are components of gasoline (petrol). The water present in gasoline affects engine performance and durability. Heptane or n-heptane, a straight chain alkane (C7H16), is widely used in laboratories as a non-polar solvent. Other important solvents include alcohol, acetone, and methyl ethyl ketone.
The solvents have wide industrial use to produce plastic, paints, glues, solvents, and serve as intermediates to produce other chemical substances. Paint thinners are mixtures of volatile organic compounds that are commonly used to dilute paints, inks, adhesives, and as cleaning agents for different purposes. Paint thinners can also be used to ‘thin’ or reduce the viscosity of paint so they can be used in sprayer applicators. Release of volatile organic solvents to atmosphere can increase the level of ozone and photochemical pollutants leading to adverse effects in public health. Equipment, process modification, improved operating practices, and recycling can lead to lower emission rates of organic solvents.
For at least the foregoing reasons, it would be desirable to enhance the separation of water contamination from fuels and solvents.
Pervaporation (PV) has been extensively used in solvent separation, where a dense nonporous membrane is used. In PV, the membrane preferentially absorbs a particular component from a miscible liquid mixture, and the adsorbed component diffuses selectively across the membrane due to the concentration gradient between the feed and the permeate side. The permeate side is often kept under vacuum, so the component permeates as a vapor.
PV is used extensively on systems which are difficult to separate by existing separation processes, such as distillation, adsorption, and extraction. PV has been used in applications such as the removal of water from a water-organic solution at low water concentrations, separation of organics from its aqueous solution when organic concentration is low, and the separation of an organic-organic miscible system. PV works with miscible systems.
However, PV has limitations, such as low permeation rate, limited feed concentration range, and lack of membrane stability as the membrane absorbs the permeate.
It would be desirable to improve PV-related performance, particularly in view of the limitations noted above.
In accordance with embodiments of the present disclosure, an advantageous nanocarbon membrane and membrane separation system are disclosed, as well as a process that utilizes a nanocarbon membrane module to effect desired separations. In one embodiment, a nanocarbon is immobilized on a membrane to selectively remove water. The nanocarbon is sized to prevent water from passing through the membrane by serving as a selective filter. The nanocarbon is configured such that the membrane exhibits super hydrophobic properties.
For purposes of the present disclosure, nanocarbons of all types are included and may be used, such as carbon nanotubes (CNTs), graphene oxide (GO), and reduced graphene oxide (r-GO). A hybrid nanocarbon could be employed according to exemplary embodiments of the present disclosure. For example, the hybrid nanocarbon could include CNTs and r-GO or CNTs and GO. It will be understood by persons skilled in the art that other suitable membranes, including other hybrid membranes, may be employed based on the disclosure herein.
In another embodiment of the present disclosure, the membrane surface and pores associated with the membrane surface (i.e., opening onto the surface of the membrane) are chemically modified with fluoroalkylsilane perfluorooctyltriethoxysilane (FAS), e.g., through a dip-coating process. The incorporation of FAS on the membrane surface and within its pores improved the hydrophobicity of the membrane. Water contact angle was observed to increase significantly. It will be understood that other compounds could be used to improve the hydrophobicity of the membrane. The separation performance of the membrane is further improved by introduction of nanocarbons (such as CNTs, GOs, rGOs and hybrids thereof) during a phase inversion process or after surface modification, e.g., with FAS.
In one embodiment, a membrane separation system includes a membrane module and a nanocarbon immobilized membrane used to separate an organic solvent.
In exemplary embodiments of the present disclosure, a porous membrane is employed in a selective filtration mode that is applicable with immiscible systems. In advantageous implementations of the disclosed porous membrane, there are no concentration-related limitations on system operations. The disclosed process has a very high permeation rate, and the permeation could take place in a liquid phase.
Immobilized nanocarbons (NCs) in different types of membranes are disclosed herein, e.g., a NCIM, and the disclosed membranes alter the solute-membrane interactions. The disclosed membranes may be used in a variety of applications, such as liquid-liquid extraction, pervaporation, nanofiltration, and removing volatile organics from air. The NCs, especially CNTs, are known to have rapid sorption and desorption capacities leading to rapid mass transfer. According to the present disclosure, advantageous NC-related contributions are noted relative to improving potential fouling behavior due to a screening effect and enhanced electrostatic repulsion between the membrane and potential foulant(s).
Immobilizing NCs, such as CNTs, in the pores may beneficially alter the solvent or fuel-membrane interactions, which is one of the major physicochemical factors affecting the permeability and selectivity of membrane(s). The NCs serve as paths for organic solvent transport and high flux can result from transport along the smooth walls of CNTs as well as molecular ordering inside the nano pores. The highly hydrophobic nature of CNTs also decreases the tendency of pores to become wet with aqueous solution so that higher transport of pure organic solvent can occur.
In one embodiment, a filtering process includes at least one NCIM module, such as a nanocarbon immobilized membrane module. The filtering process may advantageously include a membrane module containing the membrane, feed and permeate inlets and outlets, and one or more flow pumps, such as a feed pump.
Any combination and/or permutation of the embodiments disclosed herein is envisioned. Other objects and features will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are provided as an illustration only and not as a definition of the limits of the present disclosure.
To assist those of skill in the art in making and using the disclosed nanocarbon immobilized membrane and associated systems and methods, reference is made to the accompanying figures, wherein:
The present disclosure now will be described more fully hereinafter with reference to the accompanying drawings, in which illustrative embodiments of the invention are shown. In the drawings, the relative sizes of regions or features may be exaggerated for clarity. This present disclosure may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Well-known functions or constructions may not be described in detail for brevity and/or clarity.
The terminology used herein is to describe particular embodiments only and is not intended to limit the scope of the invention. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. Unless otherwise defined, all terms (including 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. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein.
Exemplary embodiments are directed to a nanocarbon immobilized membrane (NCIM). Although discussed herein with respect to a carbon nanotube, it should be understood that embodiments can generally be applied to other nanocarbons, such as graphene oxide (GO) and reduced graphene oxide (r-GO).
A first exemplary embodiment of a filtration system according to the present disclosure is disclosed below. The system includes subsystems and components to measure and control process variables, such as pressure, as required for effective performance. The system could employ sensors or other condition detection and control subsystems or components that might be required to process at a particular rate or at a particular scale, as will be readily apparent to persons skilled in the art.
Referring to
The setup could include one or more pumps, and a feed mixture 14, such as a solvent water feed mixture or a fuel water feed mixture. A pump 16 is used to pump the feed mixture 14 through the membrane module 12, a portion of which is recirculated and collected as the retentate 18 while the permeate 20 is recovered from the system 10. Pressure gauges 22, 24, 26 are positioned at various points in the system 10 to monitor pressure conditions.
The materials and the methods of the present disclosure used in one embodiment will now be further described below. While the noted embodiment discusses the use of specific compounds and materials, it is to be understood that the present disclosure could employ other suitable compounds or materials. Similar quantities or measurements may be substituted without altering the method embodied below.
Heptane and octane are used as surrogate solvents as well as fuels in these exemplary implementations of the disclosed method. Materials include heptane, octane (Fisher Scientific Ltd.), kerosene (Fisher Scientific Ltd), paint thinner (Klean-Strip® Paint Thinner, Home Depot, Newark, N.J.), deionized water (Barnstead 5023, Dubuque, Iowa), and multi-walled CNTS (MWCNTs) (Cheap Tubes Inc., Brattleboro, Vt.). The average diameter and length of the CNTs is generally ˜30 nm and 15 μm, respectively, although the present disclosure is not limited by or to such exemplary dimensional properties. A porous composite polytetrafluoroethylene (PTFE) membrane on a polypropylene (PP) support layer and polyvinylidene difluoride (PVDF) may be used in exemplary implementations of the disclosed methods.
In one embodiment, the CNTs could be single walled. In one embodiment, the CNTs could be carboxyl functionalized.
In one embodiment, CNTs were dispersed in a solution containing acetone along with a small amount of polyvinylidene difluoride (PVDF) and sonicated for four hours. The PVDF solution acted as a binder during nanomaterial immobilization. The PVDF-nanomaterial dispersion was thereafter coated uniformly over the membrane surface to incorporate the CNTs, and then allowed to dry overnight under the hood to allow the acetone to evaporate. Different amounts of CNTs have been used to fabricate the NCIM and an optimized NC concentration has been determined. Three membranes were fabricated with low concentration, medium, and higher concentrations of CNTs and are referred to as NCIM-X, where X is the percentage of CNTs. For example, NCIM-6 contains six percent CNTs.
In this embodiment, a digital gear pump (Cole Parmer) was used to pump the solvent-water or fuel-water feed mixture through the membrane module and was recirculated and collected as the retentate. Different feed concentrations were prepared by adding water to octane from 50 ppm to 500 ppm, heptane from 50 ppm to 500 ppm, kerosene from 50 ppm to 500 ppm, and paint thinner from 5 to 20 wt %.
The pressure of the solvent-water or kerosene-water feed mixture was controlled using a pressure controller valve and measured by a pressure gauge. The feed pressure varied between 6 to 20 psig. The feed flow rate was 40 mL/min as measured and was monitored by a flowmeter (Cole Parmer).
The overall filtration process is schematically depicted by system 50 in
The SEM images of the unmodified PTFE (0.1-micron PTFE) and the NCIM membrane (0.1-micron PTFE, NCIM) are illustrated in
Thermal stability of the unmodified and NCIM membrane was studied by thermogravimetric analysis (TGA). The thermal stability of the membranes in presence of CNTs increased, as shown in
The water contact angles of the unmodified membranes and NCIM are shown in Table 1. A droplet size of 4 mm was used to measure contact angles. The presence of CNTs dramatically altered the contact angle. The water contact angle for NCIM was higher than the unmodified membranes, which demonstrates the water repelling ability of NCIM.
The percentage rejection of water (R %) is defined as:
R(%)=(1−(Cpermeate/Cfeed))×100 (1)
Where, Cpermeate and Cfeed are the concentration of water in permeate and feed, respectively. The transmembrane pressure (ΔP) is defined as the pressure difference across the membrane. The solvent flux at a particular ΔP is obtained from,
The membranes prepared using different MWCNTs concentrations of 5 mg, 10 mg, and 15 mg in PTFE and PVDF membrane with different pore sizes are termed as NCIM LOW (NCIM-L), NCIM MEDIUM (NCIM-M), and NCIM HIGH (NCIM-H), as shown in Table 2 below.
Removal of Water from Fuel:
It can be seen from Table 1a and Table 1b that the NCIM significantly improved the water rejection for both membranes. The heptane flux remains comparable for both unmodified membrane and NCIM. Between the PTFE and PVDF membranes, the PVDF membrane exhibits better water rejection with low heptane flux, which may be due to the high hydrophobicity of PVDF membrane and denser membrane compared to PTFE membrane.
Table 2 demonstrates the effect of CNTs concentrations on kerosene flux and water rejection at 500 ppm water in feed and 10 psig transmembrane pressure. The membranes prepared using different CNTs concentrations was conducted on a trial-and-error basis and an optimum concentration was chosen (NCIM-M) for separation studies. The membranes fabricated with the lower CNT concentrations and higher concentrations than the optimum value is designed as NCIM LOW (NCIM-L) and NCIM HIGH (NCIM-H), respectively. Accordingly, the low, optimum, and high concentrations of CNT are determined based on the weight percentages.
From Table 2, it is apparent that the MWCNTs concentration affects the separation performances significantly. With increased concentration, the water rejection increases dramatically. However, after reaching an optimized concentration, the kerosene flux started reducing, possibly due to the partial blockage of the membrane pores. Similar trends were observed for octane-water and heptane-water system. Tables 1a & 1b show the flux and separation performance at various water concentrations with different membranes at 10 psig transmembrane pressure for a heptane water system.
Table 3a and 3b show the flux and separation performance of various membranes for water in kerosene system at a transmembrane pressure of 10 psig with different concentrations.
It can be seen from Tables 3a and 3b that NCIM-M significantly improved the water rejection for both membranes. Between these two membranes, the PTFE 0.1 micron exhibits better water rejection due to the high hydrophobicity and PTFE 0.22 exhibits high water flux due to larger porosity. The water rejection was found much higher for NCIM compared to unmodified commercial membrane. Incorporation of MWCNTs on the membrane surface did not affect the kerosene flux significantly.
Removal of Water from Paint Thinners:
Different water-thinner mixtures (5, 10, and 20 wt % water) were prepared by adding distilled water into the thinner. The water-thinner mixtures were then stirred in a closed vessel for 3 hours to prepare a water-thinner dispersion. The water-thinner immiscible mixture was then passed through the membrane module system under a vigorous stirring condition. The permeate was collected at a certain feed pressure. The flux was calculated and the rejection (%) of water was evaluated using the GC-MS analysis data. Table 4a demonstrates the separation performances of NCIM-6 under different conditions. The GC-MS analysis of the Klean-Strip® Paint Thinner revels the presence of 0% water in the permeated paint thinner.
It is clear from Table 4a that NCIM-6 successfully separates the water from the water-thinner dispersion and a rejection of >99.7% was achieved. The thinner flux was quite high at only 10 psig transmembrane feed pressure. The solvent flux although reduced and the water content in the permeate increased slightly with increasing feed water content.
Table 4b demonstrates the effect of transmembrane pressure at 10 wt % water in feed. The solvent flux increased with an increase in transmembrane pressure as expected. However, the water rejections were maintained constant over 99.5%.
The incorporation of an optimized amount of CNTs on the membrane surface significantly enhanced the water rejection rate for all solvent-water systems including heptane-water, kerosene-water, octane-water, and paint thinner-water systems, while maintaining reasonable solvent flux. The increase in transmembrane pressure increased the organic solvent flux. At a higher transmembrane pressure, the plain membrane and NCIM membrane showed a gradual decrease in water rejection, while the rejection through NCIM optimum remained almost unchanged up to a certain pressure range. Between the two unmodified membranes of PVDF and PTFE, the PVDF membrane exhibited lower water rejection and flux compared to the unmodified PTFE membrane. The high water content in the paint thinner was successfully retained in the feed side and maintained a high water rejection >99.5% under all conditions.
The present disclosure successfully demonstrates a system where the solvent can be freed from its water content (from a trace amount to a large concentration) continuously at very low pressure, eliminating the difficulties in conventional desiccation, absorption or extraction techniques.
The present application claims priority benefit to a provisional patent application entitled “Nanocarbon Enhanced Membrane for Purification and Dewatering of Solvents and Fuels,” which was filed on Mar. 31, 2021, and assigned Ser. No. 63/168,390. The entire content of the foregoing provisional application is incorporated herein by reference.
This invention was made with government support under Agreement No. 1603314 awarded by the NSF. The government has certain rights in the invention.
| Number | Date | Country | |
|---|---|---|---|
| 63168390 | Mar 2021 | US |
