Microporous carbon materials to separate nitrogen in associated and non-associated natural gas streams

Abstract

The present invention relates to a process for the manufacture of microporous carbon materials to perform selective separations of nitrogen in gas mixtures such as hydrogen sulfide, carbon dioxide, methane and C.sub.2, C.sub.3 and C.sub.4.sup.+ hydrocarbons, with high efficiency, shaped of microspheres or cylinders from copolymers of poly (vinylidene chloride-co-methyl acrylate) with density of 1.3 to 1.85 g/cm.sup.3 or poly (vinylidene chloride-co-vinyl chloride) with density of 1.3 to 1.85 g/cm.sup.3, using two stages. The first stage consists of a surface passivation of the material by chemical attack in a highly alkaline alcohol solution, with the aim of effecting a precarbonization on the surface of the copolymer that during the pyrolysis process is not deformed and gradually develops microporosity. The material of the first stage presents, in the layer, percentages between 55% to 85% carbon, between 5% to 20% oxygen, and between 10% to 40% chlorine. The interior of the material presents lower percentages of carbon, between 30% to 65%, oxygen in the amount of between 2% to 6%, and chlorine in the amount of between 30% to 60%. The second stage consists of the gradual pyrolysis of the passivated copolymer, with the aim of developing microporosity and high surface area values; as well as during the melting and gas dehydrohalogenation stages thereof, the deformation of the material is avoided. The morphology of the copolymers are microspheres of 125 to 225 micrometers, or cylinders of 4 mm in height and 3 mm in diameter, which after pyrolysis reduce its size by 35% with respect to the initial one. The material of the second stage, which is already microporous carbon material, presents in the layer percentages between 90% to 100% carbon and between 10% to 0% oxygen.

Claims

1. A carbon material, comprising: a microporous carbon material in the form of a microsphere, wherein the microporous carbon material is prepared from the pyrolysis of an alkaline pre-treated material, wherein the pre-treated material comprises copolymers of polyvinylidene chloride chemically treated with alkali or alkaline earth metal hydroxides or alcohol oxides; wherein the pre-treated material comprises an inner layer having carbon in a percentage range of 30% to 65% by mass, oxygen in a percentage range of 2% to 6% by mass, and chlorine in a percentage range of 30% to 60% by mass; and wherein the microporous carbon material comprises a layer having carbon in a percentage range of 90% to 100% by mass, and oxygen in a percentage range of 10% to 0% by mass.

Description

BRIEF DESCRIPTION OF THE DRAWINGS OF THE INVENTION

(1) In order to have a greater understanding of the process of the present invention for the manufacture of microporous carbon material, reference will be made to the figures that are attached.

(2) FIG. 1 shows a diagram of the process in two consecutive stages, for the manufacture of microporous carbon material of the present invention.

(3) FIG. 2 shows the scanning electron microscopy micrograph of the carbon microporous material, showing the spherical shape of the particles.

(4) FIG. 3 shows the micrograph of scanning electron microscopy of the microporous carbon material prepared by direct calcination by pyrolysis of the polyvinylidene copolymer, which has irregular shape and size.

(5) FIG. 4 shows a diagram where the interpretation that is performed according to the injection of the binary gas mixture is presented graphically, wherein in F.sub.R=1, there is no separation, F.sub.R=0.6, there is little separation, and F.sub.R=0.2 or less, there is a high separation.

(6) FIG. 5 shows the chromatogram for the separation of a mixture of N.sub.2, CH.sub.4, CO.sub.2 and H.sub.2S in the SP-X carbon molecular sieve type material.

(7) FIG. 6 shows the micrograph of scanning electron microscopy for the microsphere of the PVDC (Polyvinylidene chloride) copolymer; the elemental composition obtained by EDS shows a cross section and chemical analysis of both the surface and the interior is as follows: External wall: 56.89% carbon, 5.37% oxygen, 37.73% chlorine, and Internal wall: 55.80% carbon, 3.32% oxygen, and 40.88% chlorine.

(8) FIG. 7 shows the scanning electron microscopy for the microsphere the passive CTQ copolymer (chemically treated carbon), the elemental composition obtained by EDS; a cross section is shown and the chemical analysis of both the surface and of the interior is as follows: External wall: 69.62% carbon, 11.90% oxygen, 19.48% chlorine, and Internal wall: 50.80% carbon, 4.56% oxygen, and 44.64% chlorine.

(9) FIG. 8 shows the scanning electron microscopy for the microporous carbon material SP-X, the elemental composition obtained by EDS; and the chemical analysis of both the surface and the interior is as follows: External wall: 84.0% carbon, 16.0% oxygen, and Internal wall: 84.0% carbon, 16.0% oxygen.

DETAILED DESCRIPTION OF THE INVENTION

(10) The present invention relates to a process of manufacturing microporous carbon material consisting of two consecutive stages. The first stage consists of a surface passivation of copolymers of poly (vinylidene chloride) by chemical attack in a highly alkaline alcohol solution, with the objective to carry out a precarbonization on the surface of the copolymer, so that during the pyrolysis process the microporosity is not deformed, and developed gradually; the second stage consists in the gradual pyrolysis of the passivated copolymer, with the aim of developing high micro surface and surface area values; as well as during the melting and gas dehydrohalogenation stages thereof the deformation of the material is avoided.

(11) Thus, Step MP of FIG. 1 represents the starting materials being subjected to this technique, basically copolymers of polyvinylidene chloride (PVDC) with polyvinyl chloride (PVC) in the form of spheres 60 to 80 mesh, among which are the Saran XU of Dow Co, the Ixan of Solvay Co., the poly (vinylidene chloride-co-methyl acrylate), PVDC-AM, of Aldrich (430404) with density of 1.78 g/cm.sup.3, poly (vinylidene chloride-co-vinyl chloride) from Aldrich (437107) with a density of 1.73 g/cm.sup.3.

(12) Stage 1a of FIG. 1 represents the beginning of the process of the present invention: precarbonization by chemical attack in liquid phase by means of an alkali hydroxide dissolved in an alcohol, which consists of reacting the copolymer with an alkaline hydroxide solution (preferably of potassium), in a C.sub.1-C.sub.4 alcohol (preferably iso-propyl alcohol) inside a reactor with mechanical stirring. The reaction conditions are: temperature from 15 to 80 C., preferably at 30 C., and time from 0.5 to 16 h, preferably from 3 to 7 h.

(13) The material obtained in Step 1a is filtered and washed with distilled water 3 to 10 times for 60 min each at 20-90 C., preferably with 2 washes for 5-40 min each at 25 C. (Stage 1b of the FIG. 1).

(14) The material from Step 1b is dried in a vacuum oven for 6-18 h at 60-90 C., preferably 12-18 h at 70 C. (Step 1c of FIG. 1).

(15) Stage CTQ in FIG. 1 represents the material resulting from the first stage, which is identified as Carbon from Chemical Treatment (CTQ).

(16) Stage 2 in FIG. 1 represents the embodiment of the second stage of the process of the present invention, calcination at temperatures greater than 500 C. in an inert atmosphere. In this stage, the CTQ material of the first stage is subjected to calcining by pyrolysis in a tubular furnace inside a quartz tube and under an inert atmosphere of helium, argon, or nitrogen (preferably of helium), with flows of 30-100 cm.sup.3/min at 500-1200 C. (preferably between 700 and 900 C.), by means of a controlled heating ramp at a heating rate of 5 C./min, for a time of 3 to 12 h (preferably of 6 h).

(17) The CMS Step in FIG. 1 represents the final product of the process of the present invention, identified as Microporous Carbon (CMS) resulting from the second stage. Microporous Carbon retains the original form of the original material: extruded body microspheres with a uniform particle size distribution although smaller than that of the source material, in the form of 100 to 150 micrometer spheres.

(18) In Table 1, examples of the SP-X materials prepared by the process of the present invention are described under different conditions of molar concentration (M) of potassium hydroxide; iso-propanol-H.sub.2O ratio (% volume); attack time chemical; and final density after the pyrolysis process.

(19) TABLE-US-00001 TABLE 1 SP-X prototypes prepared according to Example 1. QUANTITY TIME OF RAW CHEMICAL BULK MATERIAL ISOPROPANOL- ATTACK DENSITY PROTOTYPES (g) KOH (M) H.sub.2O (% vol) (h) (g/cm.sup.3) SP-X1 25 6.0 100 2 0.59 SP-X2 25 6.0 100 16 0.49 SP-X3 25 6.0 90 2 0.60 SP-X4 25 6.0 60 2 0.63 SP-X5 25 4.0 100 2 0.61 SP-X6 25 4.0 100 16 0.50 SP-X7 25 4.0 90 2 0.68 SP-X8 25 4.0 60 2 0.54 SP-X9 25 3.0 100 2 0.61 SP-X10 25 3.0 100 16 0.49 SP-X11 25 3.0 90 2 0.61 SP-X12 25 3.0 60 2 0.57 SP-X13 25 2.0 100 2 0.62 SP-X14 25 2.0 100 16 0.58 SP-X15 25 2.0 90 2 0.68 SP-X16 25 2.0 60 2 0.59 SP-X17 25 1.0 100 2 0.60 SP-X18 25 1.0 90 16 0.59 SP-X19 25 1.0 60 2 0.56 SP-X20 25 1.0 100 2 0.67

(20) According to the above in Example 1, the typical procedure for the preparation of the microporous carbon material motifs of this invention is described, and in Examples 2 and 3 the evaluation of its capacity for separation of nitrogen, methane, carbon dioxide, and hydrogen sulfide, and its quaternary mixture in these materials.

Example 1

(21) Stage 1. Chemical Treatment (Dehydrohalogenation).

(22) Preparation of the Alcoholic Potassium Solution:

(23) 100 g of potassium hydroxide are weighed and introduced into a 2000 cm.sup.3 reactor. 1000 cm.sup.3 of the C.sub.2-C.sub.4 alcohol is added to the reactor with the caustic potash and stirred until the potash is completely dissolved at room temperature.

(24) Dehydrohalogenation:

(25) Saran XU copolymer from Dow Co. (40-60 mesh spheres) is weighed and added to the 2000 cm.sup.3 reactor with the iso-propanol/potash solution. The mixture is stirred for 1-16 h, at 25 C. The dispersion turns dark brown to black. After the reaction, the mixture is decanted and vacuum filtered. The solid material is washed twice with two volumes of iso-propanol within the same funnel. The material is transferred to a 1000 cm.sup.3 beaker, and 800 cm.sup.3 of distilled water are added at 80 C. and stirred for 20 min. The dispersion is filtered with the aid of vacuum. The filtrate is dark brown in color and sent to alkaline aqueous waste. The operation requires at least 2-8 washes until the filtrate is crystalline, has neutral pH, and shows negative test with silver nitrate solution (Negative chloride test). The product obtained is dried in a vacuum oven at 70 C., for 16 h. The cold material is weighed and the apparent density is determined, which should be from 0.7 to 0.9 g/cm.sup.3, according to the standard technique ASTM D 2854. These materials is denoted as CTQX.

(26) Stage 2. Thermal Treatment by Pyrolysis (in Tubular Furnace).

(27) 15 to 20 g of the CTQX material obtained in Step 1 is weighed and placed in a tubular quartz chamber, inside a tubular quartz jacket, and in turn inside a tubular furnace in the corresponding cavity. The flow of inert gas, helium of high purity, is measured at 100 cm.sup.3/s. A washing bottle (scrubber) with sodium hydroxide solution at 30% mass and external vent is adapted to the outlet of the oven. The material is subjected to heat treatment (700 C. for 6 h), using a ramp of 0.04 h for 100 C., 6 h for 100 C. at the calcination temperature (700-1100 C.), 6 h at the temperature of calcination and 4 h from the calcination temperature to cool to 100 C., and at room temperature at a heating rate of 5-15 C./min and cooling of 1-10 C./min. The material obtained with this treatment is denoted as SP-X X (see Table 1). Control tests are made of the developed process, making experimental measurements of material properties, such as bulk density (ASTM D 2854), physical appearance, BET surface area by adsorption-desorption of nitrogen at 196.15 C. (77K), and elemental analysis by energy dispersive spectrometry (EDS) in scanning electron microscopy, as described in Table 2.

(28) As can be seen, the microporous carbon materials SP-X, example of this invention, have surface area values between 895 to 1170 m.sup.2/g, which are characteristic of microporous materials. The majority of the materials obtained by the described steps retains the initial spherical shape and presents a reduction in size from 10% to 45%. The percentage of carbon and oxygen in these materials ranges from 80-97% and 3-20% mass, respectively, which indicates the presence of oxygenated functions on the surface of the carbon material.

(29) FIGS. 2 and 3 show a scanning electron microscopy micrograph of some of the SP-X CMS materials obtained by the process of the present invention, where the spherical shape of the particles FIG. 2 is shown in comparison with that prepared by calcination direct by pyrolysis of the polyvinylidene copolymer, which has irregular shape and size (FIG. 3).

(30) TABLE-US-00002 TABLE 2 Values of surface area, morphology and carbon and oxygen content obtained in the characterization of the various SP-X prototypes. SURFACE AREA Carbon Oxygen PROTOTYPE (m.sup.2/g) (% mass) (% mass) Morphology SP-X1 1,099.31 82 18 microspheres SP-X2 958.28 80 20 microspheres SP-X3 1,135.01 83 17 microspheres SP-X4 1,147.12 97 3 amorphous SP-X5 1,171.27 90 10 microspheres SP-X6 1,018.62 89 11 microspheres SP-X7 1,162.75 92 8 microspheres SP-X8 1,166.47 90 10 amorphous SP-X9 1,140.36 90 10 microspheres SP-X10 895.55 84 16 microspheres SP-X11 1,165.64 84 16 microspheres SP-X12 1,158.88 84 16 amorphous SP-X13 1,160.82 95 5 microspheres SP-X14 1,013.47 94 6 microspheres SP-X15 1,116.32 95 5 amorphous SP-X16 1,055.74 97 3 amorphous SP-X17 1,154.08 96 4 amorphous SP-X18 1,143.15 94 6 amorphous SP-X19 1,131.18 94 6 amorphous SP-X20 1,142.52 95 5 amorphous

Example 2

(31) The microporous carbon materials obtained with the technique described in EXAMPLE 1 (Table 1), were evaluated with the objective of measuring the efficiency of nitrogen separation in binary mixtures of nitrogen-methane, nitrogen-carbon dioxide, and nitrogen-acid hydrogen sulfide, by gas chromatography. This was done using a methodology based on the inverse gas chromatography technique, in which the adsorbent material is the subject of analysis, and gases such as nitrogen, methane, carbon dioxide, and hydrogen sulfide are the reference molecules. The test consists of placing a sample of adsorbent material of known mass (about 0.7 g) inside a stainless steel column 50 cm long by 0.1 cm internal diameter. Prior to the separation test, the packed column is installed in the chromatograph with a thermal conductivity detector and conditioned at 200 C. with a soft flow at 10 cm.sup.3/min of helium for 3 h. Once the conditioning is finished, the chromatograph oven is cooled. Then the evaluation of the separation of the gas mixture by the packed column is carried out using the following conditions: helium carrier gas with flow of 15 cm.sup.3/min, gas flow to be tested in the adsorbent of 35 cm.sup.3/min, injector temperature of 100 C., and detector temperature of 220 C.; the adsorption is carried out in the temperature range of 20-45 C. and with a pressure range of 2-15 kg/cm.sup.2; and the desorption is carried out with a temperature of 135-250 C. and at reduced pressure up to atmospheric pressure. A typical example of the chromatographic program is indicated in Table 3.

(32) Once the program is established, 5 L of a test sample is injected through a gas syringe, the composition of which is preferably equal parts by mass of the following gases: nitrogen, methane, carbon dioxide, and hydrogen sulfide. Once the chromatogram is obtained, the retention times are recorded, and with this data the Retention Factor FR is calculated, which is the ratio between the retention time (T.sub.R1) of the gas with the lowest value in the time scale and the T.sub.R2 of the gas with greater value. The microporous carbon material evaluated that presents smaller F.sub.R values are considered the most selective for the separation of the components of the mixture and consequently, to separate the nitrogen more efficiently. The equation to determine the Retention Factor is described below:

(33) $F_R=\frac{t_{R1}}{t_{R_2}}{t}_{R2}>t_{R1}$

(34) TABLE-US-00003 TABLE 3 Program for the heating ramp used in the gas chromatography. Heating rate Stage ( C./min) Temperature ( C.) Time (min) 1 35 20 2 25 135 1 3 40 175 7 4 70 200 15

(35) In FIG. 4, the interpretation that must be performed according to the injection of a binary gas mixture is presented graphically.

(36) Table 4 shows the results obtained for SP-X materials, prepared according to Example 1. In the same table, the T.sub.R of CO.sub.2 and H.sub.2S are described, as long as these were desorbed from the microporous carbon material at 30 C. The microporous carbon materials evaluated that do not present values in the Table means that the desorption occurs at temperatures higher than 175 C.

(37) TABLE-US-00004 TABLE 4 Retention time values (T.sub.R) and retention factors (F.sub.R) for the separation of binary mixtures of nitrogen-methane, nitrogen-carbon dioxide and nitrogen-hydrogen sulfide, by means of inverse gas chromatography, at 30 C. SEPARATION N.sub.2CH.sub.4 PROTOTYPE F.sub.R T.sub.R CO.sub.2 T.sub.R H.sub.2S SP-X1 0.210 33.08 32.60 SP-X2 0.223 SP-X3 0.201 19.20 SP-X4 0.180 SP-X5 0.200 15.83 SP-X6 0.209 SP-X7 0.198 22.12 SP-X8 0.194 23.36 24.26 SP-X9 0.210 17.08 23.89 SP-X10 0.225 SP-X11 0.197 18.01 18.01 SP-X12 0.211 14.16 14.63 SP-X13 0.209 12.64 12.89 SP-X14 0.212 SP-X15 0.193 SP-X16 0.194 SP-X17 0.196 SP-X18 0.194 SP-X19 0.201 SP-X20 0.184

Example 3

(38) The microporous carbon materials obtained with the technique described in EXAMPLE 1 (Table 1), were evaluated with the objective of measuring the efficiency of nitrogen separation in quaternary mixtures of nitrogen-methane-carbon dioxide and hydrogen sulfide, by means of chromatography of gases according to the technique described in EXAMPLE 2. The chromatogram shown in FIG. 5 describes that the separation is feasible when using as stationary phase the carbon molecular mesh SP-X prepared in EXAMPLE 1. The values of T.sub.R (min) they are 2.95 for nitrogen, 6.29 for methane, 8.35 for carbon dioxide, and 10.13 for hydrogen sulfide.

(39) According to these results, it is possible to say that the microporous carbon materials prepared with this technique present an efficient separation of the nitrogen-methane mixture. In addition, it can remove highly efficient mixtures of nitrogen and carbon dioxide as well as nitrogen-hydrogen sulfide. Some of the microporous carbon materials prepared in this invention retain, in a very important way, carbon dioxide and hydrogen sulfide, requiring the material to be subjected to high temperatures for the desorption thereof, while in other prepared materials they are separated and desorbed at temperatures below 175 C. It should be noted that microporous carbon materials prepared whose chemical attack was carried out at concentrations of 3 to 5 M are those that effect the separation and desorption of carbon dioxide and hydrogen sulfide at temperatures below 175 C.

(40) Another relevant aspect of the present invention is that the material resulting from this process essentially retains the shape of the starting copolymer, with a slight decrease in size, which means a considerable saving in material losses if it is carried out by other treatments necessary to confer it to the product a homogeneous shape and size, which generates a technical advantage over the preparations through the process of calcination by direct pyrolysis. It should also be noted that with the procedure described in this invention and depending on the molar concentration of the base, activation can be generated by introduction of oxygenated groups on the surface of the material, according to the results set forth in Table 2. With this procedure, by passivating the surface of the PVDC-AM copolymer, the percentage of carbon can be increased with respect to the original, reducing the amount of chlorine, causing that during the pyrolysis stage the evolution of the gases is more controlled, avoiding deformations in the final carbon microporous material. FIG. 2 shows the morphology obtained by the method described in this invention, and FIG. 3 shows the morphology obtained with the direct pyrolysis without the chemical treatment described in this document. FIGS. 6, 7, and 8 show the gradual changes in the elemental composition, obtained by EDS in the scanning electron microscopy, for the copolymer of PVDC-AM SX in FIG. 6, the same material passivated with the chemical treatment described here in FIG. 7, and the final SP-X carbon microporous material in FIG. 8. Taking into consideration the methodology of Example 2, which consists in the determination of the chromatographic separation in gas phase with a column packed with the SP-X material, if a quaternary mixture of nitrogen-methane-carbon dioxide-hydrogen sulfide is injected, the obtained chromatographic profile is shown in FIG. 4, a result that is extendable for the use of these subject matter materials of the present invention, in the separation of the components of natural gas, including high concentrations of nitrogen.