METHOD FOR GAS SEPARATION

Abstract

A method for separating at least one hydrocarbon from a feed containing a mixture of at least one hydrocarbon and nitrogen, comprising contacting the feed with an adsorbent comprising a porous support wherein the porous support comprises exchangeable cations and at least a portion of the exchangeable cations are organic cations.

Claims

1. A method for separating at least one hydrocarbon from a feed containing a mixture of at least one hydrocarbon and nitrogen, comprising contacting the feed with an adsorbent comprising a porous support wherein the porous support comprises exchangeable cations and at least a portion of the exchangeable cations are organic cations, wherein the porous support has pore diameters large enough to adsorb a hydrocarbon of interest.

2. A method for separating at least one hydrocarbon from a feed containing a mixture of at least one hydrocarbon and nitrogen of claim 1, wherein one of the at least one hydrocarbons is methane.

3. A method for separating at least one hydrocarbon from a feed containing a mixture of at least one hydrocarbon and nitrogen according to claim 1, wherein the porous support is selected from coordinated polymeric materials, including metal organic frameworks, aluminosilicates, zeolites, zeolite-like metal-organic frameworks, molecular sieves, titanosilicates, layered hydroxides or hydrotalcites.

4. (canceled)

5. A method for separating at least one hydrocarbon from a feed containing a mixture of at least one hydrocarbon and nitrogen according to claim 1, wherein the porous support is a zeolite, where the zeolite is selected from the group consisting of ferrierite, brewsterite, stilbite, dachiardite, epistilbite, heulandite and clinoptilolite, and where the zeolite has faujasite, Linde type A or chabazite topology.

6-7. (canceled)

8. A method for separating at least one hydrocarbon from a feed containing a mixture of at least one hydrocarbon and nitrogen according to claim 5, wherein the zeolite has a Si/Al ratio of 2-20.

9-10. (canceled)

11. A method for separating at least one hydrocarbon from a feed containing a mixture of at least one hydrocarbon and nitrogen according to claim 1, wherein the organic cation is from an ionic liquid, including organic cations such as a substituted ammonium cation, a substituted phosphonium cation or an organic nitrogen-containing cation, or where the organic cation is selected from the group consisting of monomethylammonium, dimethylammoinium, trimethylammonium, tetramethylammonium, monoethylammonium, diethylammonium, triethylammonium, tetraethylammonium, monopropylammonium, dipropylammonium, tripropylammonium, tetrapropylammonium, monobutylammonium, dibutylammonium, tributylammonium and tetrabutylammonium.

12-13. (canceled)

14. A method for separating at least one hydrocarbon from a feed containing a mixture of at least one hydrocarbon and nitrogen according to claim 1, wherein the organic cation content of the porous support is at least 5% of the ion-exchangeable cations in the aluminosilicate; at least 10% of the ion-exchangeable cations in the aluminosilicate; at least 20% of the ion-exchangeable cations in the aluminosilicate; at least 30% of the ion-exchangeable cations in the aluminosilicate; at least 40% of the ion-exchangeable cations in the aluminosilicate; at least 50% of the ion-exchangeable cations in the aluminosilicate; at least 60% of the ion-exchangeable cations in the aluminosilicate; at least 70% of the ion-exchangeable cations in the aluminosilicate; at least 80% of the ion-exchangeable cations in the aluminosilicate at least 90% of the ion-exchangeable cations in the aluminosilicate.

15. A method for separating at least one hydrocarbon from a feed containing a mixture of at least one hydrocarbon and nitrogen according to claim 1, wherein, the selectivity for methane over nitrogen is at least 5.

16. A method for separating at least one hydrocarbon from a feed containing a mixture of at least one hydrocarbon and nitrogen according to claim 1, wherein the feed includes coal mining gas, biogas and LNG vent gas.

17. A method for separating at least one hydrocarbon from a feed containing a mixture of at least one hydrocarbon and nitrogen according to claim 1, wherein the methane content of the feed is from 1% to 50%.

18. An adsorbent comprising a porous support wherein the porous support comprises exchangeable cations and at least a portion of the exchangeable cations are organic cations, wherein the porous support has pore diameters large enough to adsorb a hydrocarbon of interest.

19. An adsorbent according to claim 18, wherein the porous support is a silicate such as an aluminosilicate, or a zeolite, a zeolite-like metal-organic framework, a molecular sieve, a layered hydroxide or a hydrotalcite.

20. (canceled)

21. An adsorbent according to claim 18, wherein the porous support is a zeolite.

22. An adsorbent according to claim 21, wherein the zeolite is selected from the group consisting of ferrierite, brewsterite, stilbite, dachiardite, epistilbite, heulandite, chabazite and clinoptilolite.

23. An adsorbent according to claim 21, wherein the zeolite is an X zeolite of a Y zeolite.

24. An adsorbent according to claim 21, wherein the zeolite has a Si/Al ratio of 2-20.

25-26. (canceled)

27. An adsorbent according to claim 18, wherein the organic cation is from an ionic liquid.

28. An adsorbent according to claim 18, wherein the organic cation is a substituted ammonium cation, a substituted phosphonium cation or an organic nitrogen-containing cation.

29. An adsorbent according to claim 18, wherein the organic cation is selected from the following: monomethylammonium, dimethylammoinium, trimethylammonium, tetramethylammonium, monoethylammonium, diethylammonium, triethylammonium, tetraethylammonium, monopropylammonium, dipropylammonium, tripropylammonium, tetrapropylammonium, monobutylammonium, dibutylammonium, tributylammonium and tetrabutylammonium.

30. An adsorbent according to claim 18, wherein the organic cation content of the porous support is at least 5% of the ion-exchangeable cations in the aluminosilicate; at least 10% of the ion-exchangeable cations in the aluminosilicate; at least 20% of the ion-exchangeable cations in the aluminosilicate; at least 30% of the ion-exchangeable cations in the aluminosilicate; at least 40% of the ion-exchangeable cations in the aluminosilicate; at least 50% of the ion-exchangeable cations in the aluminosilicate; at least 60% of the ion-exchangeable cations in the aluminosilicate; at least 70% of the ion-exchangeable cations in the aluminosilicate; at least 80% of the ion-exchangeable cations in the aluminosilicate at least 90% of the ion-exchangeable cations in the aluminosilicate.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0073] Further features of the present invention are more fully described in the following description of several non-limiting embodiments thereof. This description is included solely for the purposes of exemplifying the present invention. It should not be understood as a restriction on the broad summary, disclosure or description of the invention as set out above. The description will be made with reference to the accompanying drawings in which:

[0074] FIG. 1 shows a representation of ab initio DFT calculations used to estimate interaction energies;

[0075] FIG. 2 is an FTIR plot demonstrating the existence of TMA+ in the zeolite;

[0076] FIG. 3 is a TGA plot;

[0077] FIG. 4 is a plot of the change in parent NaY zeolite (CBV100) upon 31% exchange of its Na.sup.+ ions with TMA.sup.+ ions, showing the zeolite's cumulative pore volume as a function of pore size before and after ion exchange;

[0078] FIG. 5 is a plot of single component (CH.sub.4 or N.sub.2) adsorption isotherms on parent zeolite NaY (a) and ion exchanged TMAY (b, c, d);

[0079] FIG. 6 is a plot of CH.sub.4/N.sub.2 selectivity for their equimolar binary mixture on ionic liquidic zeolite TMA-Y at 248 K;

[0080] FIG. 7 shows a binary isotherm of equimolar CH.sub.4+N.sub.2 on TMA-Y at 248 K;

[0081] FIG. 8 shows binary isotherms of equimolar CH.sub.4+N.sub.2 on TMA-Y at 273 K;

[0082] FIG. 9 shows binary isotherms of equimolar CH.sub.4+N.sub.2 on TMA-Y at 303 K;

[0083] FIG. 10 is a plot of CH.sub.4/N.sub.2 selectivity as a function of composition at a fixed total pressure of 2 bar and 248 K;

[0084] FIG. 11 is a plot of CH.sub.4/N.sub.2 selectivity as a function of composition at a fixed total pressure of 4 bar and 248 K;

[0085] FIG. 12 is a plot of the binary breakthrough of 10% CH.sub.4+90% N.sub.2 at 2 bar pressure and 248 K temperature from a bed of TMA-Y: (a) outlet mass flows, and (b) normalized molar flows;

[0086] FIG. 13 is a binary isotherm (a) and selectivity (b) with varying composition and fixed total pressure of 2 bar; and

[0087] FIG. 14 is a schematic representation of a PSA process; and

[0088] FIG. 15 is a schematic of the dual-reflux PL-A cycle. The second half of the cycle is the same but with the adsorption bed numbering reversed. The combined pressurization and blowdown step is called the pressure reversal step.

DESCRIPTION OF EMBODIMENTS

[0089] Ab initio density functional theory (DFT) calculations show a CH.sub.4 molecule (left panel of FIG. 1) binds much more strongly on TMAY (with 3 TMA+ per zeolite supercavity) than a N.sub.2 molecule (right panel) does. Charge difference analysis of the DFT calculation shows much a greater charge redistribution for the adsorbed CH.sub.4 molecule on TMAY than for N.sub.2: charges accumulate (electron cloud 1) at the end of the gas molecules closest to the TMA.sup.+ ion and deplete (electron could 2) from the gas molecule's other end.

[0090] It will be appreciated that higher degrees of TMA.sup.+ saturation will substantially suppress the adsorption of N.sub.2. Theoretical studies by DFT calculation show the energy of adsorption of CH.sub.4 in a typical TMAY zeolite increased from 22.92 kJ/mol with one TMA.sup.+ per supercavity, to 27.20 kJ/mol with two TMA.sup.+, and eventually to 29.57 kJ with three TMA.sup.+ per supercavity. By contrast, the energy of N.sub.2 adsorption was not sensitive to the number of TMA.sup.+ in the zeolite supercavity, with a change of less than 1.5 kJ/mol. Generally, the larger the difference in adsorption energy, the higher the adsorption selectivity in the low pressure region.

[0091] The pore volume of zeolites is normally between 9-30%, as shown in Table 1. A number of zeolite candidates (Table 2) were selected as potential parent frameworks for preparing adsorbents with organic cations, which may be referred to as Ionic Liquidic Zeolites (ILZ). However, this does not exclude the suitability of other medium/large pore zeolites.

TABLE-US-00001 TABLE 1 List of selected zeolites and their pore dimensions (reference: http://izasc.ethz.ch/fmi/xsl/IZA-SC/ft.xsl). Max diffusible Accessible Zeolite type Max radius (Å) diameter (Å) volume FAU 11.24 7.35 28% LTA 11.05 4.21 21% AFR 8.36 6.97 20% AFS 9.51 6.01 22% AFY 7.82 5.9 22% ATS 7.3 6.82 16% BEA 6.68 5.95 23% BEC 6.95 6.09 21% BOF 5.58 4.67  9% BOG 8.05 6.88 18% BOZ 8.71 4.92 23% BHP 9.51 6.01 22% CGS 5.86 4.01 11% -CLO 15.72 6.31 34% CON 7.45 5.6 19% DFO 11.29 7.19 22% DON 8.79 8.07 16% EMT 11.55 7.37 28% EON 7.83 6.79 13% EZT 6.57 6.13 13% GME 7.76 7.11 17.3%   IFR 7.24 6.38 16% IMF 7.34 5.44 12% IRR 14.46 12.12 38% ISV 7.01 6.32 21% ITR 6.36 5.12 12% -ITV 9.32 6.98 38% IWR 7.48 5.91 19% IWS 8.25 6.66 23% IWV 8.54 7.03 22% IWW 7.07 6.25 15% LTF 8.16 7.5 11.6%   LTL 10.01 7.5 15.4%   MAZ 8.09 7.5 13.2%   MEI 8.06 6.9 21.6%   MOZ 10.03 7.54 13.1%   MOR 6.7 6.45 12.6%   OFF 7 6.61 15.1%  

TABLE-US-00002 TABLE 2 Examples of zeolites with large diffusible diameter and accessible pore volumes. Zeolite Zeolite pore size Max diffusible Accessible name (Å) diameter (Å) volume FAU 11.24 7.35 28% -CLO 15.72 6.31 34% EMT 11.55 7.37 28% -ITV 9.32 6.98 38% MEI 8.06 6.9 21.6%   DFO 11.29 7.19 22% LTL 10.01 7.5 15.4%   MOZ 10.03 7.54 13.1%   GME 7.76 7.11 17.3%  

[0092] The zeolites of the present invention can be prepared by ion exchanging the existing cations in the aluminosilicate frameworks with organic cations or by synthesizing the aluminosilicate frameworks with organic cation templates.

[0093] Commercial NaY (CBV100 and CBV712) and NaX and laboratory made sodium chabazite (CHA) were tested for TMA exchange. Both the Y and X zeolites having the same crystal structure and topology, belong to the FAU family with a pore aperture diameter of 7.4 Å, whereas the chabazite zeolite belongs to the CHA family with a much smaller pore aperture, 3.8 Å in diameter.

[0094] Ion exchange of tetramethylammonium was conducted by shaking tetramethylammonium chloride and zeolite in a water bath at 40-70° C. for 0.5-15 hr. The mixture was centrifuged at 3000 rpm for 5 min and the solid component washed with deionized water. The centrifuge and washing steps were repeated. The solid component was shaken in a water bath with further tetramethylammonium chloride at 40-70° C. for at least 0.5-15 hr and centrifuged and washed as before two further times. The solid component was dried at no higher than 250° C.

[0095] The chemical compositions of the prepared TMAY and TMAX were confirmed by ICP-MS (inductively-coupled plasma mass spectrometry). The unit cell formula for TMAY and TMAX was [C.sub.4H.sub.12N].sub.8.1Na.sub.18.9Al.sub.27SI.sub.69O.sub.192 and [C.sub.4H.sub.12N].sub.3.7Na.sub.38Al.sub.41.7Si.sub.54.3O.sub.192, respectively, indicating that the corresponding degree of TMA saturation (i.e. ion exchange rate) was 31% and 9%, respectively. As discussed previously, lower TMA saturation in the X zeolite is believed to be a result of the cation density being too high to allow for higher degree of cation exchange due to space hindrance and diffusion limitation. This results in excessive metal cations sitting unexchanged in the cavity of the zeolites.

[0096] The crystal structure of the materials was verified by powder XRD. There was no change for the positions of the main peaks after ion exchange.

[0097] The existence of TMA.sup.+ in the zeolites was confirmed by FTIR as shown in FIG. 2 by the presence of the N.sup.+ and CH.sub.3 peaks.

[0098] The thermal stability of the organic cation exchanged zeolites was verified by Thermal Gravimetric Analysis (TGA), which suggests the TMA-zeolites are stable up to 573 K as shown in FIG. 3.

[0099] The synthesized TMAY (Si/Al=2.55) with 31% TMA.sup.+ exchange contained 8 TMA.sup.+ per unit cell, which is equivalent to no more than 4 TMA.sup.+ per supercavity. This admits the possibility of having TMA.sup.+ sitting in the passage connecting two supercavities. Though full saturation of TMA.sup.+ in FAU is desired, it is unlikely achievable in practice because of (1) the lack of space for diffusion of the large organic cations inside some cavities and (2) the maximum exchange rate diminishes with a decrease in the Si/Al ratio, i.e. increase of cation density. The adsorption equilibrium experiments of TMAX and TMAY suggest FAU zeolites with a higher degree of TMA saturation have a higher CH.sub.4-to-N.sub.2 selectivity.

[0100] TMAY powder was pressurized into 1-2 mm pellets with a 50 ton high pressure pelletizer (XRF Scientific Instruments). The TMAY pellets (5.36 g dry base) were preactivated at 593 K under vacuum on a Micromeritics ASAP2020 for 24 hr.

[0101] Single-component isotherms of N.sub.2 and CH.sub.4 adsorption on TMAY were collected at temperatures ranging from 243 K to 323 K and pressures up to 120 kPa using a standard volumetric method on a Micromeritics ASAP2020 accelerated surface area and porosity analyzer. The surface area and DFT pore size distribution of the prepared samples were measured by N.sub.2 adsorption at 77 K. Prior to each measurement, the samples were thoroughly dehydrated and degassed on a Micromeritics ASAP2020 analyzer by stepwise heating (1 K/min) up to 593 K and held at 593 K under high vacuum for 300 min and then cooled to 295 K followed by backfill with helium.

[0102] Larger accessible pore volumes also allow for higher adsorption capacity at very high pressures. As shown in FIG. 4, the pore volume of FAU zeolite is reduced by 60% after exchanging 30% of the original Na.sup.+ by TMA.sup.+. However, this should not affect the loading of CH.sub.4 at low and medium pressures. For example, experimental data in FIG. 5 suggests CH.sub.4 capacity was even improved by 15% at 1 bar pressure after TMA.sup.+ exchange.

[0103] FIG. 5 shows a plot of single component (CH4 or N2) adsorption isotherms on parent zeolite NaY (a) and ion exchanged TMAY (b, c, d). The calculated CH.sub.4/N.sub.2 selectivity is improved by 300% in TMAY compared with parent NaY; CH.sub.4 capacity is improved by 15.

[0104] Following preactivation, TMAY pellets were transferred into a stainless-steel adsorption column (⅜ in. diameter, 16 cm long) and flushed with helium for 1 hr. Binary CH.sub.4/N.sub.2 adsorption isotherms on TMAY were measured with a dynamic column breakthrough (DCB) apparatus as known in the art. Binary breakthrough experiments were carried out at 248 K, 273 K and 303 K respectively in the pressure range of 103.8-902.8 kPa by feeding CH.sub.4/N.sub.2 gas mixtures (with CH.sub.4 mole fraction of 0.064-0.914) at a flow rate of 50 mL/min at STP set by mass flow controllers. All component gases used were supplied by BOC with the flowing fractional purities: He 99.999%, CH.sub.4 99.995%, and N.sub.2 99.999%. FIG. 6 shows that tetramethylammonium-Y zeolite, has a CH.sub.4/N.sub.2 selectivity constantly between 6 and 8 for binary equimolar mixtures of CH.sub.4+N.sub.2 at 248 K up to the maximum tested pressure of 9 bar. Importantly this selectivity was achieved without loss of CH.sub.4 capacity. That the TMAY maintained high CH.sub.4/N.sub.2 selectivity even at high pressures is particularly advantageous for the gas industry for two reasons: (1) most of the upstream and downstream gas processes are operated at elevated pressures and thus a high pressure CH.sub.4/N.sub.2 separation will help to retain the energy; (2) high pressure CH.sub.4/N.sub.2 separation process will also reduce the column size and the cost.

[0105] FIG. 7 shows the CH.sub.4 capacity at 248 K reaches as high as 2.1 mmol/g in the binary equimolar mixture of CH.sub.4 and N.sub.2.

[0106] FIGS. 8 and 9 present the results of binary adsorption isotherms at 273 K and 303 K respectively.

[0107] FIG. 10 presents the results of CH.sub.4/N.sub.2 selectivity at 2 bar and 248 K as a function of CH.sub.4 mole fraction. FIG. 11 presents the results of CH.sub.4/N.sub.2 selectivity at 4 bar and 248 K as a function of CH.sub.4 mole fraction.

[0108] FIG. 12 is a plot of the binary breakthrough of 10% CH.sub.4+90% N.sub.2 at 2 bar pressure and 248 K temperature from a bed of TMA-Y: (a) outlet mass flows, and (b) normalized molar flows. The data in FIG. 12 can be used to show that the rate of adsorption by either N.sub.2 or CH.sub.4 is fast (similar to the parent zeolite), which means that the selectivities being observed here are equilibrium properties of the material.

[0109] FIG. 13 shows the CH.sub.4 capacity at 248 K reaches 2.0 mmol/g at 2 bar.

[0110] A simulation of a 10-step two column PSA process using TMAY to treat an equimolar binary mixture of CH.sub.4+N.sub.2 using TMAY achieved 94.1% CH.sub.4 product purity and 94.4% recovery with the following conditions [0111] Column physical size: ID=100 mm L=1.8 m [0112] Adsorbent: TMAY, 2 mm pellet [0113] Feed flow rate: 130.5 litre per minute (standard) [0114] Pressure: 3.0 bar [0115] Temperature: 30° C.

[0116] The ten step process is represented schematically at FIG. 14 wherein the steps are as follows: [0117] Step 1: adsorption [0118] Step 2: blow down to 1 bar [0119] Step 3: pressure equalization [0120] Step 4: depressurization/purge to the other column [0121] Step 5, 6 and 7: desorption/evacuation [0122] Step 8 and 9: pressurization/equalization from the other column [0123] Step 10: re-pressurization

[0124] Experiments using a four-step dual-reflux pressure swing adsorption process schematically represented in FIG. 15 to treat a 2.6% dilute CH.sub.4 achieved 22 times enrichment of methane with 99.9% recovery, with the following conditions:

TABLE-US-00003 TABLE 3 The operating parameters that were held constant across all experiments. The valve positions in FIG. 15 correspond to Step 1 (left) and Step 2 (right). Cycle Parameters Feed/Purge Time 120 s Pressurisation/Blowdown Time 90 s High Pressure 5.0 bar Low Pressure 1.4 bar Pressure Ratio 3.6 Column 1 Column 2 Step 1 HP Heavy Purge LP Feed/Light Purge Step 2 Blowdown Pressurisation Step 3 LP Feed/Light Purge HP Heavy Purge Step 4 Pressurisation Blowdown Feed Parameters Flowrate 1.25 SLPM Pressure 1.4 bar Temperature (atmospheric) 20-25° C. Fractional Axial Feed Position 0.5

[0125] Isothermal equilibrium adsorption for single component gas of O.sub.2, CO, C.sub.2H.sub.6 and C.sub.3H.sub.8 was tested on TMAY, respectively. At 298 K and 100 kPa total pressure, the determined selectivity (based on direct comparison between the uptakes of two gases at same partial pressure) for the above gases are CH.sub.4/O.sub.2=5.28, CH.sub.4/CO=2.97, C.sub.2H.sub.6/CH.sub.4=4.66, C.sub.3H.sub.8/CH.sub.4=2.71.

[0126] Other ionic liquidic zeolites (FAU framework) containing organic cations of dimethylamine and trimethylamine have been prepared as described above. Elemental analysis of the resultant products shows the DMA-Y has a cation exchange rate of 60% and the TriMA-Y has 42%, in comparison with 30% for TMA-Y, which is consistent with the size of the organic cations, i.e. smaller organic cations allows for higher exchange rate.

[0127] FIG. 16 shows the CH.sub.4 capacity on three of the FAU type ILZ materials, namely DMA-Y, TriMA-Y, and TMA-Y. It is noticeable that TMA-Y has the highest capacity for CH.sub.4 adsorption. Further study suggests the CH.sub.4/N.sub.2 selectivity on TMA-Y remains the highest among the three samples.

[0128] Throughout this specification, unless the context requires otherwise, the word “comprise” or variations such as “comprises” or “comprising”, will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers.