SOUR GAS FEED SEPARATIONS AND HELIUM RECOVERY FROM NATURAL GAS USING BLOCK CO-POLYIMIDE MEMBRANES

Abstract

Compositions of and methods for separating components of a natural gas stream are disclosed. In one embodiment, the method includes receiving an inlet stream comprising natural gas, the inlet stream having an inlet pressure, and the inlet stream further comprising methane, helium, and an impurity. The method includes allowing the inlet stream to contact a block co-polyimide membrane, the block co-polyimide membrane exhibiting both higher permeability for and higher selectivity for the helium and the impurity than for the methane at the inlet pressure of the inlet stream and separating the methane from the helium and the impurity to create a retentate stream, the retentate stream comprising an increased concentration of methane relative to the inlet stream. The method also includes creating a permeate stream comprising the helium and the impurity at an increased concentration of helium and impurity relative to a concentration of helium and impurity in the inlet stream.

Claims

1. A method for separating components of a natural gas stream, the method comprising the steps of: receiving an inlet stream comprising natural gas, the inlet stream having an inlet pressure, and the inlet stream further comprising methane, helium, and an impurity; allowing the inlet stream to contact a block co-polyimide membrane, the block co-polyimide membrane exhibiting both higher permeability for and higher selectivity for the helium and the impurity than for the methane at the inlet pressure of the inlet stream; separating the methane from the helium and the impurity to create a retentate stream, the retentate stream comprising an increased concentration of methane relative to the inlet stream; and creating a permeate stream comprising the helium and the impurity at an increased concentration of helium and impurity relative to a concentration of helium and impurity in the inlet stream.

2. The method according to claim 1, further comprising the step of: separating the helium from the impurity using a helium-separation block co-polyimide membrane, the helium-separation block co-polyimide membrane exhibiting both higher permeability for and higher selectivity for the helium than for the impurity.

3. The method according to claim 2, wherein the helium-separation block co-polyimide membrane is substantially the same as the block co-polyimide membrane.

4. The method according to claim 1, wherein the block co-polyimide membrane is selected from the group consisting of: {(6-FDA-mPDA)-(6-FDA-durene)}; {6-FDA-PTCDA-FDA}; {6-FDA-TBB-FDA}; {6-FDA-BAPT-FDA}; {(PTCDA-FDA)-(PMDA-mPDA)}; {(PMDA-FDA)-(PTCDA-mPDA)}; {(ODA-FDA)-(PTCDA-mPDA)}; {(6-FDA-BAPT)-(6-FDA-FDA)}; {(PTCDA-mPDA)-(6-FDA-FDA)}; {(PTCDA-FDA)-(ODA-mPDA)}; {(PTCDA-FDA)-(6-FDA-FDA)}; {(6-FDA-TBB)-(6-FDA-FDA)}; {(6-FDA-TBB)-(6-FDA-durene)}; {(6-FDA-mPDA)-(6-FDA-BAPT)}; {(PTCDA-mPDA)-(6-FDA-FDA)}; {6-FDA-mPDA-BAPT}; and {6-FDA-FDA-mPDA}.

5. The method according to claim 1, wherein the block co-polyimide membrane is {(6-FDA-mPDA)-(6-FDA-durene)}.

6. The method according to claim 1, wherein the impurity comprises more than one component selected from the group consisting of: CO.sub.2, N.sub.2, and H.sub.2S.

7. The method according to claim 6, wherein the inlet stream comprises H.sub.2S between about 1 volume percent concentration and about 20 volume percent concentration.

8. The method according to claim 6, wherein the inlet steam comprises H.sub.2S between about 10 volume percent concentration and about 20 volume percent concentration.

9. The method according to claim 1, wherein the inlet pressure of the inlet stream is between about 200 psia and about 1,000 psia.

10. The method according to claim 1, wherein the inlet pressure of the inlet stream is between about 500 psia and about 1000 psia.

11. The method according to claim 1, wherein the inlet pressure of the inlet stream is between about 900 psia and about 1,000 psia.

12. The method according to claim 1, further comprising the steps of: combining more than one monomer in a mixture of monomers; creating a block co-polyimide polymer; and forming the block co-polyimide membrane from the block co-polyimide polymer by applying a solution casting method to the block co-polyimide polymer.

13. The method according to claim 12, wherein the more than one monomer is selected from the group consisting of: 6-FDA; mPDA; durene diamine; PTCDA; PMDA; BAPT; TBB; FDA; and ODA.

14. The method according to claim 13, wherein the mixture of monomers comprises 6-FDA, mPDA, and durene diamine.

15. The method according to claim 1, wherein the method further comprises the step of: adjusting operating conditions of a system, the system comprising the block co-polyimide membrane, such that pure gas selectivity of the block co-polyimide membrane to helium relative to the methane is between about 50 and about 150.

16. The method according to claim 1, wherein the method further comprises the step of: adjusting operating conditions of a system, the system comprising the block co-polyimide membrane, such that pure gas selectivity of the block co-polyimide membrane to CO.sub.2 relative to the methane is between about 30 and about 60.

17. The method according to claim 15, wherein operating conditions of a system comprise the inlet pressure of the inlet stream.

18. The method according to claim 16, wherein the operating conditions of a system comprise the inlet pressure of the inlet stream.

19. The method according to claim 1, further comprising the step of improving performance of the block co-polyimide membrane with a chemical modification selected from the group consisting of: bromination of the block co-polyimide membrane; molecular weight increase of the block co-polyimide membrane; and modification with bulky diamine groups including 9,9-bis(4-aminophenyl)fluorine, 9,9-bis(4-aminophenyl-3-isopropyl-5-methyl-phenyl)fluorine, and 4,4-methylene bis(2,6-diisopropylaniline).

20. The method according to claim 5, wherein a co-polyimide (6-FDA-mPDA)-(6-FDA-durene) block ratio is selected from the block ratios consisting of: (2500/15000); (15000/2500); (2500/2500); (5000/5000); (7500/7500); (10000/10000); (12500/12500); (15000/15000); and (20000/20000).

21. The method according to claim 5, wherein co-polyimide (6-FDA-mPDA)-(6-FDA-durene) block ratios comprise (5000/5000) and (15000/15000).

22. The method according to claim 19, wherein the method further comprises the step of: adjusting operating conditions of a system, the system comprising a brominated block co-polyimide membrane, such that pure gas permeability of the brominated block co-polyimide membrane to CO.sub.2 is about 115 barrers.

23. The method according to claim 19, wherein the method further comprises the step of: adjusting operating conditions of a system, the system comprising a brominated block co-polyimide membrane, such that the pure gas permeability of the block co-polyimide membrane to He is about 110 barrers.

24. The method according to claim 1, wherein the method further comprises the step of: adjusting operating conditions of a system, the system comprising a block co-polyimide membrane, such that mixed gas selectivity of the block co-polyimide membrane to H.sub.2S and CO.sub.2 relative to methane are about 23 and 27, respectively.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0026] These and other features, aspects, and advantages of the present disclosure will become better understood with regard to the following descriptions, claims, and accompanying drawings. It is to be noted, however, that the drawings illustrate only several embodiments of the disclosure and are therefore not to be considered limiting of the disclosure's scope as it can admit to other equally effective embodiments.

[0027] FIG. 1 is a schematic representation of one embodiment of a 2,2-bis-(3,4-dicarboxyphenyl) hexafluoropropane dianhydride (also known as 6-FDA) 1,3-phenylenediamine (also known as mPDA) 1,2,4,5-tetramethylbenzene (also known as durene) block co-polyimide (also referred to throughout the disclosure as (6-FDA-mPDA)-(6-FDA-durene) block co-polyimide or block co-polymer).

[0028] FIG. 2 is a graph showing the CO.sub.2/CH.sub.4 permeability-selectivity trade-off for a (6-FDA-mPDA)-(6-FDA-durene) block co-polyimide membrane of the present disclosure.

[0029] FIG. 3 is a graph showing the H.sub.2S/CH.sub.4 permeability-selectivity trade-off for a (6-FDA-mPDA)-(6-FDA-durene) block co-polyimide membrane of the present disclosure.

[0030] FIG. 4 is a graph showing the overlay of the .sup.1H NMR spectrum of (6FDA-mPDA)-(6FDA-durene) synthesized using three different pathways.

[0031] FIG. 5 is a graph showing the thermogravimetric analysis (TGA) trace of (6FDA-mPDA)-(6FDA-durene) (15000/15000) block co-polyimide.

DETAILED DESCRIPTION

[0032] So that the manner in which the features and advantages of the embodiments of compositions and methods for gas component separation of raw natural gas, as well as others, which will become apparent, may be understood in more detail, a more particular description of the embodiments of the present disclosure briefly summarized previously may be had by reference to the embodiments thereof, which are illustrated in the appended drawings, which form a part of this specification. It is to be noted, however, that the drawings illustrate only various embodiments of the disclosure and are therefore not to be considered limiting of the present disclosure's scope, as it may include other effective embodiments as well.

[0033] Embodiments of the present disclosure teach membrane gas separation applications particularly for sour gas feed separations and helium recovery from natural gas using aromatic block co-polyimide membranes that exhibit high gas permeabilities and selectivities in both pure and mixed gas streams. Embodiments of these aromatic block co-polyimide membranes can be developed from a wide range of monomers. One such monomer includes 2,2-bis-(3,4-dicarboxyphenyl) hexafluoropropane dianhydride, also known as 4,4-(hexafluoroisopropylidene) diphthalic dianhydride, also known as 6-FDA.

[0034] Some embodiments of the present disclosure use monomers such as 1,3-phenylenediamine, also known as mPDA. Some embodiments of the present disclosure use monomers such as 2,3,5,6-tetramethyl-1,4-phenylenediamine, also known as durene diamine. Such exemplary monomers are used in combination to form different block lengths of (6-FDA-mPDA)-(6-FDA-durene) block co-polyimides. The chemical structure of certain exemplary monomers are pictured in Table 1.

TABLE-US-00001 TABLE 1 Chemical structures of exemplary monomers for use in embodiments of the present disclosure. 6-FDA mPDA Durene diamene [00001] [00002] [00003]

[0035] FIG. 1 is a schematic representation of one embodiment of a (6-FDA-mPDA)-(6-FDA-durene) block co-polyimide or block co-polymer. In certain embodiments of the disclosure, in order to improve the performance of block co-polyimides and produce less rigid polymers that are mechanically workable as membranes, chemical modifications such as bromination of the synthesized co-polyimides and molecular weight increase of the co-polyimides are used. Modifications with other suitable reagents such as bulky diamine groups including 9,9-bis(4-aminophenyl)fluorine (also known as CARDO), 9,9-bis(4-aminophenyl-3-isopropyl-5-methyl-phenyl)fluorine (also known as CARDOS), and 4,4-methylene bis(2,6-diisopropylaniline) (also known as MDIPA) are used.

[0036] These modification steps can significantly improve the performance of the co-polyimides, in some embodiments, as to one or more of permeability, selectivity, and sensitivity to molecular size of individual gas components in a gaseous mixture. In some embodiments, the block co-polyimide membranes of the present disclosure exhibit substantial cross-linking. In other embodiments, substantially no cross-linking is observed, and the membranes function in the absence of cross-linking.

[0037] In certain examples of the present disclosure, n=1; however, other suitable values for n can be conceived by one of ordinary skill in the art depending on the application. Suitable values for (Um) in FIG. 1 include, but are not limited to, (2500/15000), (15000/2500), (2500/2500), (5000/5000), (7500/7500), (10000/10000), (15000/15000), (20000/20000).

[0038] In addition, development of aromatic block co-polyimides of the present disclosure can be carried out using other monomers including: 3,4,9,10-Perylentetracarbonsauredianhydrid, also known as PTCDA; Pyromellitic dianhydride, also known as PMDA; 1,4-bis(4-aminophenoxy)triptycene, also known as BAPT; 4,5,6,7-Tetrabromo-2-azabenzimidazole, also known as TBB; 4,4-(9-Fluorenylidene)dianiline, also known as FDA; and 4,4-Oxydiphthalic anhydride, also known as ODA.

[0039] Such example monomers can form example block co-polymers including for example: {6-FDA-PTCDA-FDA}; {6-FDA-TBB-FDA}; {6-FDA-BAPT-FDA}; {(PTCDA-FDA)-(PMDA-mPDA)}; {(PMDA-FDA)-(PTCDA-mPDA)}; {(ODA-FDA)-(PTCDA-mPDA)}; {(6-FDA-BAPT)-(6-FDA-FDA)}; {(PTCDA-mPDA)-(6-FDA-FDA)}; {(PTCDA-FDA)-(ODA-mPDA)}; {(PTCDA-FDA)-(6-FDA-FDA)}; {(6-FDA-TBB)-(6-FDA-FDA)}; {(6-FDA-TBB)-(6-FDA-Durene)}; {(6-FDA-mPDA)-(6-FDA-BAPT)}; {(PTCDA-mPDA)-(6-FDA-FDA)}; {6-FDA-mPDA-BAPT}; and {6-FDA-FDA-mPDA}.

[0040] The process of gas permeation through polymeric membranes is predominantly modeled by the solution-diffusion mechanism. The transport of a penetrant through a nonporous film involves three steps: (1) first, the dissolution of the penetrant in the film; (2) followed by a transfer of the penetrant across the membrane due to a concentration gradient; and (3) desorption of the penetrant to the permeate side. The relative affinity and transfer rate of each penetrant in the polymer define the transport and separation of the gases. The dissolution of the penetrant in its simplest mathematical form is represented by Henry's law shown as Equation 1:

C=SpEquation 1.

[0041] The diffusion rate across the membrane is modeled by Fick's law of diffusion shown as Equation 2:

[00001]$\begin{array}{cc}J=-D.Math.\frac{C}{X}.& \mathrm{Equation}.Math..Math.2\end{array}$

[0042] In Equations 1 and 2, S is the solubility constant, D is the diffusion coefficient, J is the penetrant flux,

[00002]$\frac{C}{X}$

is the concentration gradient across the membrane, and C is the concentration of dissolved species in equilibrium with a gas at partial pressure p. Assuming that the diffusion and solubility coefficients are independent of concentration, the permeation rate per unit area of membrane, through thickness l, is then expressed as Equation 3:

[00003]$\begin{array}{cc}{j}_i=\frac{D_i.Math.S_i\left(p_{\mathrm{if}}-p_{\mathrm{ip}}\right)}{l}=\frac{p_i\left(p_{\mathrm{if}}-p_{\mathrm{ip}}\right)}{l}.& \mathrm{Equation}.Math..Math.3\end{array}$

[0043] J.sub.i is the molar flux (expressed in terms of cm.sup.3 (at standard temperature and pressure (STP))/cm.sup.2.Math.s), p.sub.if is the partial pressure of component i on the feed side, and p.sub.ip the partial pressure of component i on the permeate side. The diffusion coefficient, D.sub.i, is an indication of the mobility of the individual molecules in the membrane material, and the gas sorption coefficient (S.sub.i, with units of cm.sup.3 (STP) of component i/cm.sup.3 of polymer per pressure) is an indication of the volume of molecules dissolved in the membrane material. The product D.sub.iS.sub.i can be defined as P.sub.i, which is called the membrane permeability; and this is a measure of the membrane's ability to permeate gas. The conventional unit for expressing permeability is the Barrer, where 1 Barrer=10.sup.10 (cm.sup.3 (STP).Math.cm)/(cm.sup.2.Math.s.Math.cmHg). An accurate measure of a membrane's ability to separate two gases, i and j, is the ratio of their permeabilities, .sub.i/j; this parameter is called the membrane selectivity, and it can be written as Equation 4:

[00004]$\begin{array}{cc}{}_{i/j}=\frac{P_i}{P_j}=\left[\frac{D_i}{D_j}\right]\left[\frac{S_i}{S_j}\right].& \mathrm{Equation}.Math..Math.4\end{array}$

[0044] The ratio

[00005]$\left[\frac{D_i}{D_j}\right]$

is the ratio of the diffusion coefficients of two gases and can be viewed as the mobility selectivity, which indicates the relative diffusion of individual molecules of two gases i and j. Mobility selectivity is proportional to the ratio of the molecular size of the two gases. The ratio of the sorption coefficients,

[00006]$\left[\frac{S_i}{S_j}\right],$

indicates the relative concentration of gases i and j in the membrane material. The sorption of a component increases with the condensability of gas.

[0045] The separation factor, *.sub.i/j, is often used as a measure of efficiency or selectivity for mixed gas permeation. This is conventionally given as Equation 5:

[00007]$\begin{array}{cc}{}_{i,j}^{*}=\frac{y_i/y_j}{x_i/x_j}.& \mathrm{Equation}.Math..Math.5\end{array}$

[0046] In Equation 5, y.sub.i and y.sub.j are the mole fraction of components i and j on the permeate side, and x.sub.i and x.sub.j are the mole fraction of components i and j on feed side of membrane, respectively.

[0047] Example embodiments of the disclosure provided as follows show the permeation behavior of pure gas, and gas mixtures consisting of He, CO.sub.2, H.sub.2S, CH.sub.4, N.sub.2 and C.sub.2H.sub.6, through dense film membranes of the block co-polyimide (6-FDA-mPDA)-(6-FDA-durene) for simultaneous separation of CO.sub.2, H.sub.2S, N.sub.2 and other contaminants from sour gas streams, and for helium recovery from natural gas.

Examples

Example 1: Preparation of Block-Co-Polyimide Dense Film Membrane (6-FDA-mPDA)-(6-FDA-durene)

[0048] Aromatic (6-FDA-mPDA)-(6-FDA-durene) co-polyimide (FIG. 1) was synthesized according to the following procedure from 2,2-bis-(3,4-dicarboxyphenyl) hexafluoropropane dianhydride, also known as 6-FDA (recrystallized from acetic anhydride), 2,3,5,6-tetramethyl-1,4-,phenylenediamine, also known as durene diamine, and 1,3-phenylenediamine, also known as mPDA.

[0049] The block co-polyimide (6-FDA-mPDA)-(6-FDA-durene) (5000/5000) was synthesized as follows. First, mPDA (10 mmol) was dissolved in m-Cresol (17 mL). Then, 6-FDA (8.9974 mmol) was added to the mPDA and m-Cresol, and the mixture was stirred at 180 C. under N.sub.2 for 8 hours. The reaction was cooled to room temperature, and durene diamine (7.4778 mmol), 6-FDA (8.4805 mmol) and m-Cresol (17 mL) were added to the mixture. The resulting solution was stirred at 180 C. under N.sub.2 for 8 hours. The reaction was cooled to room temperature and diluted with 10 mL m-Cresol. The mixture was poured into methanol (250 mL). The resulting polymer was filtered, washed with methanol, crushed and extracted with methanol (by a soxhlet extractor) to remove any remaining trace of m-Cresol. The polymer was then dried at 150 C. under vacuum overnight.

[0050] NMR identification data for the block co-polyimide (6-FDA-mPDA)-(6-FDA-durene) (5000/5000) is presented as follows: .sup.1H NMR (400 MHz, DMSO-d6, , ppm): 8.17 (d, 4.72H, 6FDA), 7.96-7.87 (m, 6.08H, 6FDA), 7.75-7.64 (m, 4.08H, 6FDA/mPDA), 7.55-7.52 (m, 3.97H, mPDA), 2.06 (s, 12H, durene). Tg=349 C., Td=507 C., d=1.39 g/cm.sup.3. The data obtained are discussed further herein.

[0051] A large block length and a large ratio of different blocks in co-polyimides can greatly diminish selectivity in some embodiments. However, permeability is much higher for higher block length co-polyimides (such as, for example, (15000/15000)) than for lower block length polymers (such as, for example, (5000/5000)). In the case of large blocks, only relatively few of each block can be incorporated in one polymer chain, and thus the excess disrupts polymer chain packing in the membrane creating domains containing mostly 6FDA-mPDA and others containing mostly 6FDA-durene. CH.sub.3 moieties of durene diamine increase inter-chain spacing thus diminishing the membrane's ability at discriminating gases (selectivity) in a way that normally cannot be compensated by mPDA moieties.

[0052] However, in the case of small length blocks, such as, for example, (5000/5000), the blocks are small enough for the main polymer chain to organize in a way that the resulting membrane benefits from the properties of the parent homo-polymers. The increase in inter-chain spacing allowing for faster transport does not diminish greatly the ability of 6FDA-mPDA sections to discriminate gases based on their size. This results in a material with a good balance between the distribution of free volume generated by CH.sub.3 moieties of durene diamine and the good packing induced by the mPDA moieties.

[0053] Block co-polyimide (6-FDA-mPDA)-(6-FDA-durene) (15000/15000) was synthesized as follows. First, mPDA (10 mmol) was dissolved in m-Cresol (18 mL). Then, 6-FDA (9.6592 mmol) was added, and the mixture was stirred at 180 C. under N.sub.2 for 8 hours. The reaction was cooled to room temperature and durene diamine (8.4958 mmol), 6-FDA (8.8366 mmol), and m-Cresol (18 mL) were added to the mixture. The resulting solution was stirred at 180 C. under N.sub.2 for 8 hours. The reaction was cooled to room temperature and diluted with 10 mL m-Cresol. The mixture was poured into methanol (250 mL). The resulting polymer was filtered, washed with methanol, crushed and extracted with methanol (by a soxhlet extractor) to remove any remaining trace of m-Cresol. The polymer was then dried at 150 C. under vacuum overnight.

[0054] NMR identification data for the block co-polyimide (6-FDA-mPDA)-(6-FDA-durene) (15000/15000) is presented as follows: .sup.1H NMR (400 MHz, DMSO-d6, , ppm): 8.19 (m, 6FDA), 7.98 (m, 6FDA), 7.78 (s, 6FDA), 7.70 (m, mPDA), 7.58 (m, mPDA), 2.09 (s, durene). Tg=344 C., Td=507 C., d=1.54 g/cm.sup.3.

[0055] Brominated block co-polyimide (6FDA-mPDA)-(6FDA-durene) (15000/15000) was synthesized as follow: 1 to 5 gram of the co-polyimide was dissolved in 100 mL of chloroform and stirred overnight for complete dissolution. Then, the solution was transferred to a three-necked reactor fixed with a mechanical stirrer and an N.sub.2 gas inlet. Stirring was started at 400 rpm.

[0056] In a separate beaker, 50 mL of chloroform solvent was taken and added carefully to 6 mL of liquid bromine, and this bromine solution was transferred to a separating funnel fixed with the three-necked reactor. Next, the bromine solution was slowly and drop-wisely added from the separating funnel to the polyimide solution under constant stirring. The reactor was covered with aluminum foil. The separating funnel was then removed after complete transfer of bromine solution and the reaction was allowed for more than about 6 hours under constant stirring. The brominated co-polyimide was precipitated by slowly and carefully adding a required amount of methanol. The solution was then stirred for 30 minutes, and then the brominated polymer was filtered out. The polymer was then dried at room temperature overnight followed by oven drying at 60 C. under vacuum for another overnight period.

[0057] A block co-polyimide dense film membrane was prepared as follows. Dense films were prepared by a solution casting method. An N-methyl-2-pyrrolidone (NMP) solution containing about 2-5 wt. % of polymer was filtered through a 45-m filter, then a 10-m filter and finally a 1-m filter to remove non-dissolved materials and dust particles. The solution was then cast on a dry and clean petri dish. The dish was slowly heated in an oven to 50 C. for about 24 hr, then to 100 C. for another 24 hr, and finally to 150 C. for 24 hr to allow for film formation. The resulting film was finally dried in an oven at 180 C. overnight to remove residual solvent.

Example 2: Evaluation of the CO.SUB.2./CH.SUB.4 .and He/CH.SUB.4 .Pure Gas Separation Performance of (6-FDA-mPDA)-(6-FDA-Durene) Membrane Prepared in Example 1

[0058] The permeability coefficients of pure gases CO.sub.2, CH.sub.4, and He, and selectivities for CO.sub.2/CH.sub.4 and He/CH.sub.4 through the block co-polyimide (6-FDA-mPDA)-(6-FDA-durene) tested at various upstream pressures (200-400 psia) and 35 C. are shown in Tables 2 and 3 as follows. The co-polyimide membrane (5000/5000) has pure gas permeabilities of about 37 and 93 barrers for CO.sub.2 and He, respectively, and the pure gas CO.sub.2/CH.sub.4 and He/CH.sub.4 selectivities are about 61 and 155, respectively, as shown in Table 2.

[0059] However the block co-polyimide with (15000/15000) block length exhibited pure gas permeabilities of about 44 and 70 barrers for CO.sub.2 and He, respectively, and the pure gas CO.sub.2/CH.sub.4 and He/CH.sub.4 selectivities are about 30 and 48, respectively as shown in Table 3. Modification of the membrane by bromination greatly improves the membrane performance as the pure gas permeabilities of the brominated membrane (15000/15000) shown in Table 3 significantly increases to 115 and 110 barrers for CO.sub.2 and He, respectively. Pure gas CO.sub.2/CH.sub.4 and He/CH4 selectivities are about 31 and 30 respectively, which are similar or insignificantly changed as compared to an unbrominated membrane.

TABLE-US-00002 TABLE 2 Pure gas permeation properties of block co-polyimide (6-FDA-mPDA)- (6-FDA-durene) (5000/5000) membrane at 35 C. Pressure Permeability (Barrer) Selectivity (psia) CO.sub.2 CH.sub.4 N.sub.2 He CO.sub.2/CH.sub.4 N.sub.2/CH.sub.4 He/CH.sub.4 200 37.44 0.60 1.55 93 62.40 2.58 155.0 300 36.00 0.59 1.54 91 61.00 2.61 154.5 400 37.73 0.62 1.56 93 60.85 2.51 150.1

[0060] Data point B in FIG. 2 is a data point of CO.sub.2 permeability and CO.sub.2/CH.sub.4 selectivity obtained at 200 psia for the block co-polyimide (6-FDA-mPDA)-(6-FDA-durene) (15000/15000) membrane at 35 C. shown in Table 3 below.

TABLE-US-00003 TABLE 3 Pressure Permeability (Barrer) Selectivity (psia) CO.sub.2 CH.sub.4 N.sub.2 He CO.sub.2/CH.sub.4 N.sub.2/CH.sub.4 He/CH.sub.4 Pure gas permeation properties of unbrominated and brominated block co-polyimide (6-FDA-mPDA)-(6-FDA-durene) (15000/15000) membrane at 35 C. 100 43.5 1.47 2.30 70.1 29.7 1.57 47.8 200 43.1 1.60 2.43 69.6 27.0 1.52 43.6 300 44.8 1.63 2.53 69.0 27.4 1.55 42.2 Pure gas permeation properties of the brominated block co-polyimide (15000/15000) 100 115.0 3.7 4.8 110 31.1 1.30 29.7

Example 3: Evaluation of the CO.SUB.2./CH.SUB.4 .Mixed Gas Separation Performance of the (6-FDA-mPDA)-(6-FDA-Durene) Block Co Polyimide Membrane Prepared in Example 1

[0061] The permeability properties of quaternary gas mixtures consisting of 10, 59, 30 and 1 vol. % concentration CO.sub.2, CH.sub.4, N.sub.2 and C.sub.2H.sub.6, respectively, through the block co-polyimide membranes were tested at different upstream pressures. The results are summarized in Tables 4 and 5. The permeability values of CO.sub.2 decrease with increasing feed pressure, and CO.sub.2/CH.sub.4 selectivity also declines to about 32, as shown in Table 5.

TABLE-US-00004 TABLE 4 Mixed gas permeation properties of the block co-polyimide (6-FDA-mPDA)-(6-FDA-durene) (15000/15000) membrane as a function of feed pressure at 22 C. using a gas mixture containing 10, 59, 30 and 1 vol. % concentration of CO.sub.2, CH.sub.4, N.sub.2 and C.sub.2H.sub.6 respectively. Pressure Permeability (Barrer) Separation factor (psia) CO.sub.2 CH.sub.4 N.sub.2 C.sub.2H.sub.6 CO.sub.2/CH.sub.4 N.sub.2/CH.sub.4 C.sub.2H.sub.6/CH.sub.4 200 26.68 1.42 1.67 0.66 18.78 1.18 0.46 300 26.45 1.79 2.12 1.08 14.82 1.19 0.60 400 24.38 2.16 2.46 1.49 11.31 1.14 0.69 500 23.09 2.66 2.94 2.05 8.67 1.10 0.77

TABLE-US-00005 TABLE 5 Mixed gas permeation properties of the block co-polyimide (6-FDA-mPDA)-(6-FDA-durene) (5000/5000) membrane as function of feed pressure at 22 C. using a gas mixture containing 10, 59, 30 and 1 vol. % concentration of CO.sub.2, CH.sub.4, N.sub.2 and C.sub.2H.sub.6 respectively. Pressure Permeability (Barrer) Separation factor (psia) CO.sub.2 CH.sub.4 N.sub.2 C.sub.2H.sub.6 CO.sub.2/CH.sub.4 N.sub.2/CH.sub.4 C.sub.2H.sub.6/CH.sub.4 300 18.54 0.53 0.82 0.21 34.84 1.54 0.38 400 16.92 0.49 0.76 0.19 34.33 1.54 0.39 500 15.54 0.48 0.73 0.19 32.10 1.51 0.39

Example 4: Evaluation of the CO.SUB.2./CH.SUB.4 .and H.SUB.2.S/CH.SUB.4 .Sour Quaternary Mixed Gas Separation Performance of the (6-FDA-mPDA)-(6-FDA-Durene) Membrane Prepared in Example 1

[0062] The permeability properties of a simulated sour gas mixture consisting of 10, 59-60, 10-30 and 1-20 vol. % concentration of CO.sub.2, CH.sub.4, N.sub.2 and H.sub.2S, respectively, through the membrane of block size (5000/5000) were tested at different acid gas (CO.sub.2+H.sub.2S) partial pressures and different H.sub.2S concentrations. These results are shown in Table 6.

TABLE-US-00006 TABLE 6 Mixed gas permeation properties of the block co-polyimide (6-FDA-mPDA)-(6-FDA-durene) (5000/5000) membrane as a function of H.sub.2S composition in the feed gas (with total pressure of 500 psia) and acid gas (CO.sub.2 + H.sub.2S) partial pressure at 22 C. using a gas mixture containing 10, 59-60, 10-30 and 1-20 vol. % concentration of CO.sub.2, CH.sub.4, N.sub.2 and H.sub.2S, respectively. H.sub.2S Partial Separation factor conc., pres. Permeability (Barrer) CO.sub.2/ H.sub.2S/ (vol. %) (psia) CO.sub.2 CH.sub.4 N.sub.2 H.sub.2S CH.sub.4 N.sub.2/CH.sub.4 CH.sub.4 1 55 18.72 0.51 0.77 7.74 36.93 1.51 15.27 10 100 9.494 0.28 0.39 5.18 33.55 1.38 18.29 20 150 13.38 0.49 0.44 11.2 27.25 0.89 22.85

[0063] The membrane was subjected to feed gas compositions with up to a maximum of 20 vol. % concentration H.sub.2S. As shown in Table 6, permeability coefficients of all the penetrants, CO.sub.2, CH.sub.4, N.sub.2 and H.sub.2S stay relatively constant or slightly decrease with the increase in pressure. The H.sub.2S/CH.sub.4 separation factor increases as the partial pressure and H.sub.2S concentration in the feed are increased. The value was about 23 at the maximum total pressure of 500 psia, H.sub.2S concentration of 20 vol. %, and H.sub.2S partial pressure of 150 psia. Additionally, the CO.sub.2/CH.sub.4 separation factor decreases with increasing partial pressure and H.sub.2S concentration, and the selectivity at low partial pressure was as high as about 37, and it was about 27 at high pressure.

[0064] Importantly, at a moderate feed pressure of 500 psia and 20 vol. % concentration H.sub.2S in the feed gas mixture, H.sub.2S/CH.sub.4 and CO.sub.2/CH.sub.4 selectivities are still about 23 and 27, respectively, in the block co-polyimide membrane. Moreover, the CO.sub.2/CH.sub.4 selectivity of the co-polyimide does not degrade to the same extent as was reported for cellulose acetate (CA), even under the much more aggressive conditions tested here. This stability at moderate pressures and high H.sub.2S concentration is surprising and unexpected, and is shown in FIGS. 2 and 3

[0065] Referring now to FIG. 2, a graph is provided showing the CO.sub.2/CH.sub.4 permeability-selectivity trade-off for a (6-FDA-mPDA)-(6-FDA-durene) block co-polyimide membrane of the present disclosure. Point (A) shows the selectivity ratio of CO.sub.2/CH.sub.4 versus the permeability of CO.sub.2 in barrer for a (6-FDA-mPDA)-(6-FDA-durene) membrane of the present disclosure at 500 psia total pressure, 20 vol. % concentration H.sub.2S, and 150 psia H.sub.2S partial pressure. These results are shown in Table 6. Point (B) shows the selectivity ratio of CO.sub.2/CH.sub.4 versus the permeability of CO.sub.2 in barrer for a (6-FDA-mPDA)-(6-FDA-durene) membrane of the present disclosure at about 200 psia, when pure gases including CO.sub.2, CH.sub.4, and He were tested across the membrane, as described in Example 2. These results are shown in Table 2. Point (C) in FIG. 2 shows the selectivity ratio of CO.sub.2/CH.sub.4 versus the permeability of CO.sub.2 in barrer for a brominated (6-FDA-mPDA)-(6-FDA-durene) membrane of the present disclosure at 100 psia, when pure gases including CO.sub.2, CH.sub.4, and He were tested across the membrane, as described in Example 2. These results are shown in Table 3.

[0066] Referring now to FIG. 3, a graph is provided showing the H.sub.2S/CH.sub.4 permeability-selectivity trade-off for a (6-FDA-mPDA)-(6-FDA-durene) block co-polyimide membrane of the present disclosure. Point (A) shows the selectivity ratio of H.sub.2S/CH.sub.4 vs. the permeability of H.sub.2S in barrer for the (6-FDA-mPDA)-(6-FDA-durene) membrane of the present disclosure at 500 psia total pressure, 20 vol. % concentration H.sub.2S, and 150 psia H.sub.2S partial pressure. These results are shown in Table 6.

[0067] FIG. 4 shows an .sup.1H NMR spectrum of the obtained co-polyimides. The peaks in the range of 8.17-7.52 ppm are assigned to the hydrogen atoms of 6FDA and mPDA, and the peak at 2.06 ppm is assigned to the CH.sub.3 groups of durene. This confirms the incorporation of all units in the co-polyimides.

[0068] In order to make the target copolymers, two different synthetic pathways were considered; sequential and parallel synthesis. The first synthetic pathway was the sequential synthesis, in which different blocks were built one after another in the same vessel. Without being bound to any theory or principle, it is believed that when the components of the second block are added to the first block they will react preferentially with each other before reacting with the first block.

[0069] The second synthetic pathway used in order to obtain block copolymers was the parallel synthesis. In this case, the pathway starts with making both block components of the final copolymer separately, at the same time, hence controlling their respective length and size distribution. In this way, there are truly two types of blocks that can be reacted onto one another. Another step in the process is the addition of the more soluble block to the less soluble block in m-cresol, followed by heating and mechanical stirring at 180 C. under N.sub.2 overnight. The structure of the copolymers obtained via the different synthetic pathways was checked by .sup.1H NMR. In FIG. 4, an overlay of the .sup.1H NMR spectra of block copolymers of (6FDA-mPDA)-(6FDA-durene) can be observed, and the three spectra are substantially identical. It could thus be concluded that the two pathways are equivalent as far as the overall structure of the block copolymers is concerned.

[0070] Referring now to FIG. 5, a graph is provided showing the thermogravimetric analysis (TGA) trace of (6FDA-mPDA)-(6FDA-durene) (15000/15000) block co-polyimide. Table 7 displays properties of the synthesized polyimides: (A) (6FDA-mPDA)-(6FDA-durene) (5000/5000) and (B) (6FDA-mPDA)-(6FDA-durene) (15000/15000).

TABLE-US-00007 TABLE 7 Properties of the synthesized polyimides: (A) (6FDA-mPDA)-(6FDA- durene) (5000/5000) and (B) (6FDA-mPDA)-(6FDA-durene) (15000/15000). T.sub.d at 5% weight Polymer T.sub.g ( C.) loss (N.sub.2) ( C.) Density (g/cm.sup.3) A 349 507 1.39 B 344 507 1.54

[0071] Thermogravimetric analysis was used to study the thermal stability of the prepared polyimides. FIG. 5 shows a typical TGA curve obtained for the synthesized polymers. The temperatures (T.sub.d) at 5% weight loss in nitrogen are shown in Table 7. The results show that all synthesized co-polyimides have a very good thermal stability of about 500 C. The glass transition temperature (T.sub.g) of the synthesized polymers was determined by differential scanning calorimetry (DSC) and is also shown in Table 7. T.sub.g is an indicator of the cooperative motion of polymeric chains. The densities of the prepared polymers are shown in Table 7. Densities were determined by floatation on small pieces of pre-dried membranes using hexane and tetracholoroethane as solvents.

[0072] The results described in Tables 2-6 and FIGS. 2-3 uniquely show greater selectivities and permeabilities for CO.sub.2, H.sub.2S, and He. The results are surprisingly better than those obtained when using many industrial glassy polymers and other high-performance rubbery and glassy polymer membranes. For example, at a relatively high feed pressure of 500 psia and a relatively high 20 vol. % concentration H.sub.2S in a feed gas mixture comprising CO.sub.2, N.sub.2, CH.sub.4, and H.sub.2S, the H.sub.2S/CH.sub.4 and CO.sub.2/CH.sub.4 ideal selectivities exhibited by membranes of the present disclosure are much greater than those obtained when using standard industrial membranes. Additionally, the CO.sub.2/CH.sub.4 selectivity does not degrade or suffer significant loss to the same extent as is reported for cellulose acetate (CA), even under much more aggressive conditions tested here.

[0073] Another advantage exhibited by embodiments of membranes in the present disclosure is that the co-polyimide membranes are not only acid gas selective, but also more selective to N.sub.2, as compared to CH.sub.4. In other words, the permeation of N.sub.2 in aromatic polyimides is greater than CH.sub.4. In this way, energy is saved as embodiments of the membrane of the present disclosure simultaneously permeate both acid gas and N.sub.2, while keeping CH.sub.4 on the high pressure side of the membrane, also known as the retentate side.

[0074] Another advantage exhibited by embodiments of membranes in the present disclosure is that the modification of the membrane by bromination greatly improve the membrane performance, as the pure gas permeabilities of the brominated membrane significantly increase for both CO.sub.2 and He respectively with no or insignificant change in pure gas CO.sub.2/CH.sub.4 and He/CH.sub.4 selectivities, when compared to unbrominated membrane.

[0075] Current commercial membranes exhibit selectivity for CO.sub.2/CH.sub.4 and H.sub.2S/CH.sub.4 in the range 20 to 26 and permeance of 60 to 80 GPU for CO.sub.2 and H.sub.2S. These membranes require stringent pretreatment for water and heavy hydrocarbons content, as the membranes are very susceptible to swelling and plasticization in the presence of heavy hydrocarbons, benzene, toluene, and xylene (BTX), water and other condensable gases. A typical natural gas composition includes about 1-20 mol. % H.sub.2S; 2-7 mol % CO.sub.2; 10-36 mol % inert gases (that include N.sub.2, He etc.,); 0.2 mol. % water; 0.2-3.0 mol % C.sub.2+; and up to 1000 ppm BTX at total operating pressure in the range 800-1000 psi. For further improvement in the economics and minimizing methane slippage, membranes need to exhibit consistent selectivity of CO.sub.2/CH.sub.4 and H.sub.2S/CH.sub.4 in the range 30 to 40 in wet sour gas with H.sub.2S concentration up to 20 mol. % at total pressure in the range 900-1000 psi; and in the presence of C.sub.3+ heavy hydrocarbons (about 3%) and benzene, toluene, and xylene (BTX) (about 1,000 ppm); and exhibit consistent permeances of 100.sup.+ GPU for CO.sub.2 and H.sub.2S in the aforementioned mentioned conditions.

[0076] Permeance (expressed in GPU, gas permeation unit) is another parameter often used in industry to express the membrane performance. It is a pressure normalized flux, and it is related to permeability by: Permeability units: 1 Barrer=110.sup.10 (cm.sup.3(STP).Math.cm)/(cm.sup.2.Math.s.Math.cmHg). For permeance units: 1 GPU=1 Barrer/1 micron (10.sup.6 m).

[0077] The singular forms a, an, and the include plural referents, unless the context clearly dictates otherwise.

[0078] In the drawings and specification, there have been disclosed embodiments of compositions and methods for separating the components of raw natural gas, and although specific terms are employed, the terms are used in a descriptive sense only and not for purposes of limitation. The embodiments of the present disclosure have been described in considerable detail with specific reference to these illustrated embodiments. It will be apparent, however, that various modifications and changes can be made within the spirit and scope of the disclosure as described in the foregoing specification, and such modifications and changes are to be considered equivalents and part of this disclosure.