Nanocarbon Enhanced Membrane for Purification and Dewatering of Solvents and Fuels

Abstract

A nanocarbon immobilized membrane (NCIM) is disclosed. The nanocarbon immobilized membrane is sized to purify different organic-water mixtures. The nanocarbon immobilized membrane can be used to purify solvents, fuels, and other organic compounds. Data using heptane-water, octane-water, fuel-water, and paint thinner-water show 99.9% separation efficiency. High organic flux is also seen at relatively low pressure. This approach has numerous applications, including fuel purification, oil spills clean-up, separation of commercial emulsions, and solvent purification.

Claims

1. A membrane separation system, comprising: a. a membrane module; and b. a membrane positioned within the membrane module, the membrane defining a membrane surface and pores opening onto the membrane surface, wherein the membrane includes immobilized nanocarbons on the membrane surface and within the pores to define a nanocarbon immobilized membrane.

2. The membrane separation system of claim 1, wherein the nanocarbons immobilized on the membrane surface and within the pores are selected from the group consisting of carbon nanotubes, graphene oxide, reduced graphene oxide, and hybrid nanocarbon combinations thereof.

3. The membrane separation system of claim 1, wherein the nanocarbon immobilized membrane is hydrophobic and functions to reject water.

4. The membrane separation system of claim 1, wherein the nanocarbons are functionalized so as to alter at least one of the hydrophilicity and other chemical interaction properties of the membrane, thereby altering membrane selectivity.

5. The membrane separation system of claim 1, wherein the membrane surface and the pores are chemically modified.

6. The membrane separation system of claim 5, wherein the chemical modification comprises incorporation of fluoroalkylsilane perfluorooctyltriethoxysilane.

7. The membrane separation system of claim 1, wherein the membrane is fabricated from PVDF or PTFE.

8. The membrane separation system of claim 1, wherein the membrane module comprises a feed inlet, a feed outlet, and a permeate outlet.

9. The membrane separation system of claim 1, wherein the nanocarbons have a concentration between 3 to 6 weight percentage.

10. A separation method, comprising: a. providing a membrane module that includes a membrane defining a membrane surface and pores opening into the membrane surface, the membrane including immobilized nanocarbons on the membrane surface and within the pores; and b. feeding a feed mixture to the membrane to separate fluids included within the feed mixture into a permeate and a retentate.

11. The separation method of claim 10, wherein the feed mixture includes contaminated water.

12. The separation method of claim 11, wherein the contaminated water forms an immiscible layer or an emulsion.

13. The separation method of claim 11, wherein the water concentration ranges from a trace amount of water to at least 50% of the feed mixture.

14. The separation method of claim 10, wherein the feed mixture comprises water and at least one organic solvent, and wherein the permeate comprises high purity organic solvent.

15. A separation method, comprising: a. providing a membrane module that includes a membrane defining a membrane surface and pores opening into the membrane surface, the membrane including immobilized nanocarbons on the membrane surface and within the pores; and b. feeding an organic water feed mixture to the membrane; wherein the membrane is effective to remove water from the organic water feed mixture.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0025] 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:

[0026] FIG. 1A is a schematic diagram of an exemplary setup of a filtration process in accordance with embodiment(s) of the present disclosure;

[0027] FIG. 1B is a diagram of the filtration process;

[0028] FIG. 2A is an SEM image of the surface of an unmodified PTFE 0.1-micron membrane;

[0029] FIG. 2B is an SEM image of the surface of a PTFE NCIM membrane;

[0030] FIG. 3A is a graphical depiction of a thermogravimetric analysis (TGA) of a PTFE membrane with a pore size of 0.1 micron;

[0031] FIG. 3B is a graphical depiction of a TGA analysis of a PTFE membrane with a pore size of 0.22 micron;

[0032] FIG. 4A is a graphical depiction showing the effect of transmembrane pressure on octane flux at 500 ppm water in feed for PTFE 0.1 unmodified and NCIM-M membrane;

[0033] FIG. 4B is a graphical depiction showing the effect of transmembrane pressure on water rejection at 500 ppm water in feed for PTFE 0.1 unmodified and NCIM-M membrane;

[0034] FIG. 5A is a graphical depiction showing the effect of transmembrane pressure on octane flux at 500 ppm water in feed for PTFE 0.22 unmodified and NCIM-M membrane;

[0035] FIG. 5B is a graphical depiction showing the effect of transmembrane pressure on water rejection at 500 ppm water in feed for PTFE 0.22 unmodified and NCIM-M membrane;

[0036] FIG. 6A is a graphical depiction showing the effect of transmembrane pressure on octane flux at 500 ppm water in feed for PVDF 0.22 unmodified and NCIM-M membrane;

[0037] FIG. 6B is a graphical depiction showing the effect of transmembrane pressure on water rejection at 500 ppm water in feed for PVDF 0.22 unmodified and NCIM-M membrane;

[0038] FIG. 7A is a graphical depiction showing the effect of CNT loading on membrane on octane flux at 500 ppm water in feed and 10 psig pressure for PTFE 0.1-micron NCIM-3 membrane; and,

[0039] FIG. 7B is a graphical depiction showing the effect of CNT loading on membrane on water rejection at 500 ppm water in feed and 10 psig pressure for PTFE 0.1-micron NCIM-3 membrane.

DETAILED DESCRIPTION

[0040] 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.

[0041] 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.

[0042] 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).

[0043] 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.

[0044] Referring to FIG. 1A, an exemplary embodiment of a filtration system 10 according to the present disclosure includes a NCIM module 12, such as a nanocarbon immobilized membrane module. In this embodiment, a flat membrane module is used. It will be understood that other suitable types of membrane modules could be employed, such as a hollow fiber membrane module or a spiral wound membrane module.

[0045] 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.

[0046] 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.

[0047] 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.

[0048] In one embodiment, the CNTs could be single walled. In one embodiment, the CNTs could be carboxyl functionalized.

[0049] 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.

[0050] 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 %.

[0051] 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).

[0052] The overall filtration process is schematically depicted by system 50 in FIG. 1B. The solvent water feed mixture 52 is pumped by pump 54 to the membrane module 56. A portion of the feed mixture passes through the membrane module as purified solvent 58 and the remaining portion, the retentate 60, is recycled to the feed tank for the solvent water feed mixture 52.

[0053] The SEM images of the unmodified PTFE (0.1-micron PTFE) and the NCIM membrane (0.1-micron PTFE, NCIM) are illustrated in FIGS. 2A and 2B. The uniform distribution of CNTs was observed over the entire membrane surface. The SEM image shows the porous structure of the pristine membranes and presence of CNTs on the membrane.

[0054] 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 FIGS. 3A and 3B.

[0055] 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.

TABLE-US-00001 TABLE 1 Water Contact Angle for prepared membranes Membrane Contact Angle 0.1-micron PTFE 120° 0.22-micron PTFE 109° 0.22-micron PVDF  92° 0.1 PTFE NCIM-3 134° 0.22 PTFE NCIM-6 122° 0.22 PVDF NCIM-6 132°
The percentage rejection of water (R %) is defined as:

R(%)=(1−(C.sub.permeate/C.sub.feed))×100  (1)

[0056] Where, C.sub.permeate and C.sub.feed 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,

[00001]$\begin{array}{cc}\mathrm{Solvent}\mathrm{flux}=\frac{\mathrm{Amount}\mathrm{of}\mathrm{solvent}\mathrm{in}\mathrm{the}\mathrm{permeate}\mathrm{side}}{\mathrm{Membrane}\mathrm{area}\times \mathrm{experiment}\mathrm{time}}& \left(2\right)\end{array}$

[0057] 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.

[0058] FIGS. 4A, 4B, 5A, 5B, 6A and 6B illustrate the effect of transmembrane pressure on octane flux and water rejection, respectively. It can be seen from the figures that solvent flux increases sharply with an increase in transmembrane pressure for all membranes. The water rejection decreased slightly with an increase in transmembrane pressure for unmodified and the NCIM membranes. Similar trends have been observed for all PVDF and PTFE membranes.

[0059] FIGS. 4A and 4B show the octane flux and water rejection for octane-water system with NCIM-M on PVDF and PTFE base membranes. The water rejection for unmodified membranes was much lower compared to NCIM. A significant improvement in water rejection was observed with the addition of CNTs. The water rejection was high at a certain pressure range followed by a decreasing trend after a certain pressure point.

[0060] FIGS. 5A and 5B show the effect of transmembrane pressure on octane flux and water rejection at 500 ppm water in feed for PTFE 0.22 unmodified and NCIM-M membrane. The water rejection for unmodified membranes was much lower compared to NCIM. A significant improvement in water rejection was observed with the addition of CNTs.

[0061] FIGS. 6A and 6B show the effect of transmembrane pressure on octane flux and water rejection at 500 ppm water in feed for PVDF 0.22 unmodified and NCIM-M membrane. The water rejection for unmodified membranes was much lower compared to NCIM. A significant improvement in water rejection was observed with the addition of CNTs.

[0062] FIGS. 7A and 7B show the effect of CNT loading on octane flux and water rejection at 500 ppm water in feed and 10 psig pressure for a PTFE 0.1-micron NCIM membrane. The water rejection for unmodified membranes was much lower compared to NCIM. A significant improvement in water rejection was observed with the addition of CNTs.

Removal of Water from Fuel:

[0063] 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-US-00002 TABLE 1a Effect of heptane flux and water rejection at various water concentration for PTFE 0.1-micron membrane and NCIM-M at 10 psig transmembrane pressure. PTFE 0.1 micron NCIM-3 Water Water Water concentration concentration concentration Flux Permeate Rejection Flux Permeate Rejection (ppm) (kg/m.sup.2 .Math. hr) (ppm) (% R) (kg/m.sup.2 .Math. hr) (ppm) (% R)  50 134.65  9.1 81.75% 53.932 0.6  98.80% 100 133.56 17.9 82.10% 50.78  1.2  98.80% 200 128.8  34.3 82.85% 49.57  2.22 98.89% 500 119.43 85.0  83.0% 45.947 0.1  99.98%

TABLE-US-00003 TABLE 1b Effect of heptane flux and water rejection at various water concentration for PVDF 0.22-micron membrane and NCIM-M at 10 psig transmembrane pressure. PVDF 0.22 micron NCIM-6 Water Water Water concentration Flux concentration Rejection Flux concentration Rejection (ppm) (kg/m.sup.2 .Math. hr) (ppm) (% R) (kg/m.sup.2 .Math. hr) (ppm) (% R)  50 65.08  13.0 74.00% 28.96 0.615  98.77% 100 67.2   24.6 75.45% 25.98 0.97   99.03% 200 68.51  49.0 75.51% 26.75 0.912  99.54% 500 66.53 120.0  76.6% 27.76 0     100.00%

[0064] 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.

TABLE-US-00004 TABLE 2 Effect of MWCNTs concentration on membrane performances for kerosene water PTFE 0.1 micron PVDF 0.22 micron NCIM NCIM NCIM NCIM NCIM NCIM LOW MEDIUM HIGH LOW MEDIUM HIGH Membranes (NCIM-.05) (NCIM-3) (NCIM-5) (NCIM-2) (NCIM-6) (NCIM-8) Flux (kg/m.sup.2 .Math. hr) 56.18 43.221 36.83 37.65 33.12 21.12 Water rejection (% R) 95.34% 99.97% 100% 97% 99% 100%

[0065] 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.

[0066] 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.

TABLE-US-00005 TABLE 3a Flux and rejection performance of a kerosene water system at different concentrations for PTFE 0.1 and NCIM-M at 10 psig transmembrane pressure PTFE 0.1 micron NCIM-3 Water Water Water concentration Flux concentration Rejection Flux concentration Rejection (ppm) (kg/m.sup.2 .Math. hr) (ppm) (% R) (kg/m.sup.2 .Math. hr) (ppm) (% R)  50 120.11  9.0 82.00% 48.987 0.05 99.90% 100 121.56 17.3 82.75% 46.879 1.1  98.90% 200 127.65 34.1 82.97% 45.588 2    99.00% 500 123.48 83.0  83.4% 43.221 0.15 99.97%

TABLE-US-00006 TABLE 3b Flux and rejection performance of a kerosene water system at different concentrations for PTFE 0.22 and NCIM-M at 10 psig transmembrane pressure PTFE 0.22 micron NCIM-6 Water Water Water concentration Flux concentration Rejection Flux concentration Rejection (ppm) (kg/m.sup.2 .Math. hr) (ppm) (% R) (kg/m.sup.2 .Math. hr) (ppm) (% R)  50 130.8  10.5 79.00% 58.103   1.5  97.00% 100 132.64 20.9 79.10% 57.656   2.96 97.04% 200 134.46 39.8 80.12% 52.9864  5.46 97.27% 500 140.56 97.0  80.6% 55.44   13.65 97.27%

[0067] 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:

[0068] 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.

TABLE-US-00007 TABLE 4a Flux and rejection performance of a thinner water system at different concentrations for PTFE 0.22- micron NCIM-M at 10 psig transmembrane pressure NCIM-6 Water Water Water Concentration Flux Concentration in Rejection in feed (wt %) (kg/m2 .Math. hr) permeate (ppm) (%) 0 1368.48 N/A N/A 5 1289.1 109.48 99.73% 10 870.49 135.14 99.83% 20 898.1 219.99 99.86%

[0069] 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-US-00008 TABLE 4b Flux and Rejection Performance of a thinner water system at different pressures for 0.22-micron NCIM-M at the same feed water concentration (10 wt % water) NCIM-6 Water Water Transmembrane Flux Concentration Rejection Pressure (psig) (kg/m2 .Math. hr) (ppm) (%) 5 415.62 399.01 99.50% 10 870.49 135.14 99.83% 15 1572.91 103.1 99.87% 20 1397.03 106.64 99.87%

[0070] 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%.

[0071] 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.

[0072] 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.