Separation of gases via carbonized vinylidene chloride copolymer gas separation membranes and processes therefor

Abstract

A process for separating hydrogen from a gas mixture having hydrogen and a larger gas molecule is comprised of flowing the gas mixture through a carbonized polyvinylidene chloride (PVDC) copolymer membrane having a hydrogen permeance in combination with a hydrogen/methane selectivity, wherein the combination of hydrogen permeance and hydrogen/methane selectivity is (i) at least 30 GPU hydrogen permeance and at least 200 hydrogen/methane selectivity or (ii) at least 10 GPU hydrogen permeance and at least 700 hydrogen/methane selectivity. The carbonized PVDC copolymer may be made by heating and restraining a polyvinylidene chloride copolymer film or hollow fiber having a thickness of 1 micrometer to 250 micrometers to a pretreatment temperature of 100? C. to 180? C. to form a pretreated polyvinylidene chloride copolymer film and then heating and restraining the pretreated polyvinylidene chloride copolymer film to a maximum pyrolysis temperature from 350? C. to 750? C.

Claims

1. A method of making a carbonized polyvinylidene chloride copolymer film or a carbonized hollow fiber, the method comprising, (a) providing a polyvinylidene chloride copolymer film having a thickness of 1 micrometer to 250 micrometers or a hollow fiber having a diameter of 1 micrometer to 250 micrometers, (b) heating the polyvinylidene chloride copolymer film or the hollow fiber to a pretreatment temperature of 100? C. to 180? C. to form a pretreated polyvinylidene chloride copolymer film or a pretreated hollow fiber while restraining the polyvinylidene chloride copolymer film or the hollow fiber, and (c) heating the pretreated polyvinylidene chloride copolymer film or the pretreated hollow fiber to a maximum pyrolysis temperature from 350? C. to 750? C. to produce the carbonized polyvinylidene chloride copolymer or the carbonized hollow fiber while restraining the pretreated polyvinylidene chloride copolymer film or the pretreated hollow fiber, wherein the restraining of steps (b) and (c) is conducted by applying a compressive force, wherein the carbonized polyvinylidene chloride copolymer film or the carbonized hollow fiber has an average pore size greater than a diameter of a large gas molecule larger than hydrogen gas determined by gas adsorption employing gas probe molecules of differing sizes, and wherein the carbonized polyvinylidene chloride copolymer film or the carbonized hollow fiber has a hydrogen permeance in combination with a hydrogen/methane selectivity, wherein the combination of hydrogen permeance and hydrogen/methane selectivity is (i) at least 30 Gas Permeation Unit (GPU) hydrogen permeance and at least 200 hydrogen/methane selectivity or (ii) at least 10 GPU hydrogen permeance and at least 700 hydrogen/methane selectivity.

2. The method of claim 1, wherein the maximum pyrolysis temperature is at most 650? C.

3. The method of claim 1, wherein the polyvinylidene chloride copolymer film is a polyvinylidene chloride copolymer comprised of vinylidene chloride and at least one of the following: a vinyl monomer, a vinyl chloride monomer, an acrylate monomer, a methacrylate monomer, a styrenic monomer, acrylonitrile, methacrylonitrile, itaconic acid, and chlorotrifluoroethylene.

4. The method of claim 1, wherein the polyvinylidene chloride copolymer film has a crystallinity percentage of 25% to 75% determined by differential scanning calorimetry.

5. The method of claim 1, wherein the polyvinylidene chloride copolymer film during step (b) is dehydrochlorinated by at least 10%, but not fully dehydrochlorinated.

6. The method of claim 1, wherein the thickness is from 10 micrometers to 150 micrometers.

7. The method of claim 1, wherein the polyvinylidene chloride copolymer film is formed by melt-extrusion at a stretch ratio from 1 to 8.

8. The method of claim 1, wherein the large gas molecule comprises olefin, paraffin, or both.

Description

EXAMPLES

(1) PVDC Copolymer Film Preparation:

(2) Melt Extruded Films of MA 4.8 wt % Copolymer

(3) Base resin XUS32904.01 available from The Dow Chemical Company, Midland, MI (PVDC copolymer with 4.8 wt % methyl acrylate (MA) comonomer, Mw=96,000) was blended with 2 wt % epoxidized soybean oil (based on total amount of blend), 4 wt % dibutyl sebacate, and 2 wt % PLASTISTRENGTH L-1000 an acrylic lubricant available from Arkema PLC, France. The blend was extruded through a 1.75 inch width film die (controlled at 174? C.) followed by water quench and stretch winding. The wind rate was controlled to obtain films of different thicknesses: 2, 4, 8, and 12 mil (1 mil=25.4 micrometer). The films after winding were cut into approximately 12 inch wide and 2 feet length pieces and laid on flat desktop for about one week. Coupons of ? inch diameter were cut for carbonization as described below.

(4) Melt Extruded Films of MA 8.5 wt % Copolymer

(5) Base resin SARAN 806 available from The Dow Chemical Company (PVDC copolymer with 8.5 wt % methyl acrylate comonomer, Mw=85,000) was blended with 2 wt % epoxidized soybean oil and 2 wt % PLASTISTRENGTH L-1000. The blend was extruded in the same manner as above. The wind rate was controlled to obtain films of different thicknesses: 2, 4, 8, and 12 mil. The films after winding were cut into approximately 12 inch wide and 2 feet length pieces and laid on flat desktop for about one week.

(6) Solution Cast Films of MA 4.8% Resin

(7) Base resin XUS 32904.01 was dissolved in tetrahydrofuran (THF) to realize a 15 wt % polymer solution. The solution was poured onto a flat glass plate and cast using a knife having a 28 mil clearance. Approximately 2 mil films were obtained after the THF evaporated in air.

(8) Solution Cast Films of VC 17.6% Resin

(9) Base resin XUS 32061.01 was dissolved in tetrahydrofuran to make a 15 wt % polymer solution. The solution is poured onto a flat glass plate and cast using a 28 mil clearance knife. Approximately 2 mil film was obtained after solvent evaporation.

(10) Latex Cast Films of DARAN SL158

(11) Latex dispersion of DARAN SL158 (Owensboro Specialty Polymers, Inc.) was poured onto glass plate and cast using a knife having a 4 mil clearance. The cast films were dried at 75? C. in an air purged oven for about 2 hours. Approximately 1.5 mil films were obtained. Table 1 shows a summary of information on all eleven precursor films.

(12) TABLE-US-00001 TABLE 1 Precursor Films Film Precursor Preparation thickness Film # method Base resin [mil] 1 Melt extrusion XUS32904.01 2 2 Melt extrusion XUS32904.01 4 3 Melt extrusion XUS32904.01 8 4 Melt extrusion XUS32904.01 12 5 Melt extrusion SARAN 806 2 6 Melt extrusion SARAN 806 4 7 Melt extrusion SARAN 806 8 8 Melt extrusion SARAN 806 12 9 Solution casting XUS 32904.01 2 10 Solution casting XUS 32061.01 2 11 Latex casting DARAN SL158 1.5
Carbon Membrane Formation

(13) A two-step pyrolysis approach was used. The precursor films were heated to a first temperature of 130-150? C. for 24 hours in a low temperature oven purged by 2 L/min of air (pretreated films), which was followed by further heating to pyrolyze the pretreated films to temperatures in the range of 350-950? C. in a 6 ID quartz tube furnace purged by 5 L/min of nitrogen.

(14) For the initial low temperature pretreatment, 12 disks (? inch diameter) sandwiched between graphite plates, with two pieces of 10 mil TEFLON sheets being used to separate the two sides of the membrane from the graphite plates. The weight of graphite plates are about 0.2-0.8 kg. Alternatively, the graphite plates and TEFLON sheets were replaced with porous ceramic honeycomb plates, through which HCl generated should be transported out swiftly. A scrubber connected to this oven contained a 10 wt % sodium hydroxide aqueous solution. A loaded oven was heated at 1? C./min to 130, 140, or 150? C. and held for 24 hour under 2 L/min of air purge.

(15) For the second heating step, the 12 pretreated disks were sandwiched between the graphite plates without the Teflon sheets or honeycomb plates were loaded into a 6 ID quartz tube furnace. A scrubber connected to this furnace contained a 10 wt % sodium hydroxide aqueous solution. The furnace was raised to different final temperatures ranging from 350-950? C. at various ramp rates (1, 3, 5? C./min), and held for 30 minutes at the final temperature and then cooled down to room temperature (?25? C.). After cooling down, the carbon membranes were put into a storage box continuously purged with dry nitrogen at a flow rate of 5 Liter/min.

(16) Carbon Membrane Permeation Test Protocol

(17) The carbon membranes were masked onto a standard 25 mm filter holder (Millipore #4502500, EMD Millipore Corp., Germany) using an impermeable aluminum tape, leaving an open defined permeation area. A two-part epoxy (J-B Weld twin tube) was then applied along the interface of the tape and the carbon membranes. Single gas permeation tests of several gas species were conducted at 20? C. with a continuous upstream feed (25 sccm, 1 atm) and downstream He purge (2.0 sccm, 1 atm). The permeate carried by the He purge gas was analyzed by a GC (gas chromatograph) with a TCD (thermal conductivity detector for H.sub.2 and CO.sub.2) and FID (flame ionization detector for CH.sub.4). The concentrations in all gases were lower than 5%, so the gas flow rate in downstream was considered the same as the He flow rate. The membrane permeate rate was calculated using the He purge flow rate times the permeate concentrations measured by GC. The single gas permeation tests were conducted in the following orderH.sub.2, CO.sub.2, and CH.sub.4. The tests were run for several hours to days until the permeate concentrations were steady. The parameters to make the carbon membranes and the resulting permeation results are shown in Table 2.

(18) TABLE-US-00002 TABLE 2 Final Pyrolysis Hydrogen Methane CMS film Pre-treatment Temp. Ramp rate Precursor Sandwich Permeance Permeance H.sub.2/CH.sub.4 Example Temp. [? C.] [? C.]* [? C./-min] film # Plates (GPU) (GPU) Selectivity Comp Ex 1 140 350 1 3 Graphite 0.7 0.026 28 Ex 1 130 950 1 4 Graphite 11.1 0.003* 4269* Ex 2 130 650 1 1 Graphite 53.0 0.058 914 Ex 3 140 950 3 2 Graphite 10.7 0.008* 1390* Comp Ex 2 150 350 3 1 Graphite 3.7 0.083 45 Ex 4 150 800 1 2 Graphite 17.5 0.017 1007 Ex 5 150 650 3 2 Graphite 49.5 0.064 773 Ex 6 130 800 3 4 Graphite 17.3 0.018 979 Ex 7 130 500 3 3 Graphite 65.4 0.274 239 Ex 8 140 650 5 4 Graphite 37.6 0.044 863 Comp Ex 3 150 500 1 4 Graphite 37.4 0.105 356 Comp Ex 4 130 350 5 2 Graphite 1.3 0.049 27 Ex 9 140 500 5 1 Graphite 35.0 0.017 2120 Ex 10 150 950 5 3 Graphite 19.8 0.011 1850 Ex 11 130 500 3 1 Honeycomb 169.0 0.487 350 Ex 12 130 500 3 2 Honeycomb 96.9 0.479 246 Ex 13 130 500 3 3 Honeycomb 66.4 0.483 149 Ex 14 130 500 3 4 Honeycomb 46.2 0.257 204 Ex 15 130 500 3 5 Honeycomb 151.8 1.598 95 Ex 16 130 500 3 6 Honeycomb 92.3 0.322 286 Ex 17 130 500 3 7 Honeycomb 65.7 0.614 107 Ex 18 130 500 3 8 Honeycomb 47.7 0.171 279 Ex 19 130 500 3 9 Honeycomb 90.5 0.135 670 Ex 20 130 500 3 10 Honeycomb 125.5 0.756 166 Ex 21 130 500 3 11 Honeycomb 98.6 0.206 478 Ex 22** 150 700 1 2 Honeycomb 78.0 0.26 385 *Very close to detection limit **Pretreatment for 150? C. for 60 hours, then pyrolysis with 1? C./min to 700? C., hold 0 minute

(19) From the results shown in Table 2, the final pyrolysis temperature has the most effect on the H.sub.2 permeance and H.sub.2/CH.sub.4 selectivity. The membranes with final temperature around 500-650? C. showed the best combination of H.sub.2 permeance and H.sub.2/CH.sub.4 selectivities. In Examples 11-14, which were made of different thicknesses of melt extruded PVDC-MA 4.8% resin, both H.sub.2 permeance and H.sub.2/CH.sub.4 selectivity increase as the precursor film thickness decreases from 12 mil to 2 mil. Examples 15-18, which were made of different thicknesses of melt extruded PVDC-MA 8.5% resin, the H.sub.2 permeance increases continuously as the precursor film thickness decreases from 12 mil to 2 mil, while the H.sub.2/CH.sub.4 selectivity peaks at 4 mil. Therefore, the optimum thickness of precursor film for the combination of H.sub.2 permeance and H.sub.2/CH.sub.4 selectivity is somewhat dependent on the copolymer composition.

(20) Examples 19 and 20 (solution cast precursor films) as well as Example 21 (latex precursor film) show similar high H.sub.2 permeance and H.sub.2/CH.sub.4 selectivities as those made of melt extruded films.

(21) Carbon Membrane Adsorption

(22) Gas adsorption was used to measure the average pore size of Example 13. The adsorption was performed using a Micromeritics ASAP 2020 instrument at 20? C. Carbon membranes were broken into pieces (?2-5 mm) and loaded into a quartz sample holder. Each sample was degassed at 100? C. for 12 hours before each gas adsorption was performed in the sequence of CH.sub.4, CO.sub.2, C.sub.2H.sub.4, C.sub.3H.sub.6, and iC.sub.4H.sub.10.

(23) All the gases, except iC.sub.4H.sub.10, adsorb in large amounts (greater than about 10 cc (STD)/g) at ?600 mmHg: C.sub.3H.sub.6 (83.3 cc(STD)/g) and iC.sub.4H.sub.10 (4.1 cc(STD)/g). From these results, the average micropore size was considered to be between the molecular size of C.sub.3H.sub.6 and iC.sub.4H.sub.10, 4.0-5.0 ?. Therefore, it is expected that the Example 13 carbon membrane would permeate CH.sub.4 (3.8 ? molecular size) at a high rate. However, surprisingly, it was found that CH.sub.4 gas essentially does not permeate through the Example 13 carbon membrane.