Gas separation membrane

Abstract

A membrane suitable for separating a gas from a gas mixture comprising a non cross-linked PVAm having a molecular weight of at least Mw 100,000 carried on a support wherein after casting onto the support, said PVAm has been heated to a temperature in the range 50 to 150 C., e.g. 80 to 120 C.

Claims

1. A process for the formation of a composite membrane suitable for separating a gas from a gas mixture comprising: (I) obtaining a polyvinylamine (PVAm); (II) hydrolyzing said PVAm under conditions of acid to form a pre-treated PVAm; (III) forming a solution of said pre-treated PVAm in a solvent; (IV) casting said solution onto a support to form the composite membrane comprising the support and the solution of pre-treated PVAm in solvent; and optionally (V) thermal treating said composite membrane, wherein the PVAm in the composite membrane is not crosslinked.

2. A process as claimed in claim 1 wherein said support is porous.

3. A process as claimed in claim 1 wherein said solvent comprises methanol, ethylene glycol, formamide or mixtures of one or more of said solvents with water.

4. A process as claimed in claim 1 wherein the pH of the solution is at least 6.

5. A process as claimed in claim 1 wherein said PVAm in the composite membrane has a molecular weight of at least Mw 50,000 and wherein the composite membrane is heated to a temperature in the range 50 to 150 C. in step (V).

6. A process as claimed in claim 1 wherein said PVAm in the composite membrane has a molecular weight of at least Mw 50,000 and wherein the membrane is heated to a temperature in the range 80 to 120 C. in step (V).

7. A process as claimed in claim 1 wherein said PVAm in the composite membrane has a molecular weight of at least Mw 50,000 and is non-crosslinked.

8. A process as claimed in claim 1 wherein said PVAm in the composite membrane has a molecular weight of at least Mw 50,000 and wherein the support has a molecular weight of at least 20,000.

9. A process as claimed in claim 1 wherein said PVAm in the composite membrane has a molecular weight of at least Mw 50,000 and said support is microporous.

10. A process as claimed in claim 1 wherein the PVAm in the composite membrane has a Mw of at least 100,000.

11. A process as claimed in claim 1, wherein said support is polytetrafluoroethylene (PTFE), polypropylene (PP), sulphonated polysulfone (PSf), polyvinylidene fluoride (PVDF), polyimide (PI), polyether imide (PEI), aliphatic polyamide, polyetheretherketone (PEEK), or polyphenylene oxide (PPO).

12. A process as claimed in claim 1, wherein a carbon dioxide permeable layer separates the PVAm in the composite membrane from the support.

13. A process as claimed in claim 12, wherein said carbon dioxide permeable layer is polydimethylsiloxane (PDMS), PVA (polyvinylalcohol) or chitosan.

14. A process as claimed in claim 1 wherein the pretreated PVAm comprises less than 5 wt % polyvinylamide.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) FIG. 1 is a diagram of the experimental set up used to measure permeance.

(2) FIG. 2 shows permeance variation with feed pressure for cross-linked vs thermally treated membranes.

(3) FIG. 3 shows selectivity variation with feed pressure for cross-linked vs thermally treated membranes.

(4) FIG. 4 shows permeance variation of the composite membranes of the invention with pressure.

(5) FIG. 5 shows selectivity variation of composite membranes of the invention with pressure.

(6) FIG. 6 shows permeance variation of the support.

(7) FIG. 7 shows permeance variation of the composite membranes of the invention using thermally treated membranes vs non strengthened membranes.

(8) FIG. 8 shows selectivity variation of the composite membranes of the invention using thermally treated membranes vs non strengthened membranes.

(9) FIG. 9 shows the water uptake contrast between high and low Mw PVAm membranes.

PERMEATION TESTING

(10) Permeance of the membranes was measured with an apparatus equipped with a humidifier, see FIG. 1. FIG. 1 shows an experimental setup for gas permeation measurements. The chosen gases may be mixed in any ratios in a gas flow line A, in which flow, pressures and temperature are controlled. The gas mixture is lead to humidifiers in tanks 1 where it bubbles through water, and then to a membrane separation cell 2. Either the retentate stream, or the permeate stream may be lead to a gas chromatograph (GC) 4 for analysis of the composition. The water excess is removed by a liquid separator 6 before going to the GC.

(11) The various gas flows are controlled by valves and flow controllers. The abbreviations FI, FC, PI and PC in circles are flow indicator (FI), flow controller (FC), pressure indicator (PI) and pressure controller (PC), respectively. The use of this equipment will be familiar to the skilled person.

(12) Premixed gas with a molar composition of 10% CO.sub.2-90% N.sub.2 was used as feed and methane was used as a sweep gas, both feed and sweep gases being humidified by passage through two water bubblers. The permeate side was maintained at atmospheric pressure and the total flow (permeate plus sweep) was measured with a soap bubble meter. The RH % was controlled by two bypass valves controlling the ratio of dry/wet gas (V.sub.4 and V.sub.14). The compositions of the permeate and feed were analysed continuously by a micro GC (gas chromatograph) Agilent 3000 equipped with two thermal conductivity detectors (TCDs) and two columns, Molecularsieve and Plot Q. A liquid separator was installed before GC sampling valve in order to prevent the moisture penetration inside GC. The permeance of CO.sub.2 and N.sub.2 was calculated using complete mixing model, from the total permeate flow rate measured with a soap bubble meter, feed and permeate pressure and the gas compositions of feed and permeate gas measured by GC.

(13) $Q_i=\frac{J_i}{A\left(x_{r,i}{p}_h-x_{p,i}{p}_1\right)}$
where Q.sub.i represents the permeance (m.sup.3(STP)/(m.sup.2 bar h) of component i (CO.sub.2 or N.sub.2), Ji represent the flux m.sup.3(STP)/h, A the membrane area (m.sup.2), x.sub.r,i and x.sub.p,i molar concentration on feed and permeate side respectively (mol %) and p.sub.h and p.sub.i absolute pressure on feed and permeate side (bar).

(14) The selectivity of CO.sub.2 over N.sub.2 was calculated using the permeance ratio of the two gases when permeating together in a mixture (10% CO.sub.2-90% N.sub.2) without excluding the reciprocal coupling effect between gases.

(15) All experiments were conducted at a constant temperature of 25 C. and the pressure difference between the feed and the permeate sides was 1-15 bar.

(16) Results for the membranes of the invention with a 10% CO.sub.2/N.sub.2 mixture are presented in the figures.

Example 1

PVAm Purification (Pre-Treatment)

(17) PVAm with a molecular weight of 340 000 was purchased from BASF and is a linear polymer (Scheme 1) having more than 90% of the amide groups of polyvinylformamide (PVAF) hydrolyzed to amino groupsmore than 90% is in PVAm and 10% is in PVAF form. The polymer is obtained by polymerization of the vinylformamide (VFA) monomer to polyvinylformamide and consequently hydrolyzed under acid or basic conditions to form polyvinylamine.

(18) ##STR00002##

(19) The PVAm polymer was provided in the form of aqueous solution with pH=8. Further purification of the polymer was carried out in order to remove possible traces of vinylformamide monomer or other impurities and to maximize the hydrolysis degree.

(20) The solution was purified in successive steps:

(21) (I) re-precipitation in acetone and/or acetone and ethanol blend:

(22) (II) washing with acetone

(23) (III) filtration

(24) (IV) drying until constant weight and

(25) (V) re-dissolution in distilled water.

(26) This procedure was repeated several times and the resulting polymer solution was completely hydrolysed by acidic hydrolyse in presence of HCl 5M solution and was re-precipitated in the form of PVAm.HCl (protonic form).

Example 2

(27) Comparative PVAm (Mw 80,000) was prepared by the Hofmann reaction of polyacrylamide based on Hiroo Tanaka and Ryoichi Senju, Preparation of polyvinylamine by the Hofmann degradation of polyacrylamide, Bulletin of the chemical society of Japan, 49, 10 (1976) 2821.

Example 3

Flat Sheet Membrane Preparation

(28) Composite asymmetric membranes were prepared by casting a solution of PVAm onto asymmetric porous polysulfone supports. The polysulfone porous support was either MWCO 20000 or 50000 respectively. The supports were washed in advance with large amounts of distillate water. The desired thickness of the PVAm layer was controlled by pouring a known volume of solution into a confined circular surface with known area on the support. The calculated and resulted PVAm membrane thickness was 1.2 m on dry basis for all membranes (unless otherwise mentioned). The thickness of the dry membranes was confirmed from Scanning Electron Microscopy (SEM) cross section pictures.

(29) The membranes were dried at 45 C. for 90 minutes and then kept at room temperature for 24 hours. Membranes were used as such or treated as described in Example 4.

Example 4

Cross-Linking/Thermal Treatment

(30) For the cross linking experiments, ammonium fluoride solution was used. The membranes were cross linked with a known volume of aqueous solutions of NH.sub.4F having concentrations of 1M to 3M. Subsequently the membranes with NH.sub.4F were heat treated at 90 C. for one hour and kept for 24 hours at room temperature before permeation tests.

(31) Thermal treatment was carried out by annealing the obtained membranes at 90 C.-125 C. for one hour in absence of ammonium fluoride.

Example 5

Hollow Fibre Type Membrane Preparation

(32) The membranes were prepared using hollow fibre membrane porous supports formed from polysulfone (PSf) or polyphenylene oxide (PPO). Two procedures for coating outside and respective inside hollow fibre were developed. For outside coating, the membranes were prepared by dip coating technique: the porous support was immersed in PVAm aqueous solution followed by drying at 45 C. The procedure was repeated until a defect free film of PVAm was formed on the surface of the porous support. The resulted hollow fibres were thermally treated (annealed) at 90-125 C.

(33) For inside coating, aqueous PVAm was circulated for 30 minutes inside the hollow fibre lumen, followed by drying as in the case of outside dip coating.

(34) In more detail, supports were mounted in a module and coating inside was performed in steps:

(35) A vacuum pump was connected to the module, evacuating the outside of the hollow fibre from atmospheric pressure to 30 mbar (see T. Kouketsu et al, supra).

(36) The coating solution was circulated for 30 minutes into the bore side of hollow fibres

(37) The excess solution was removed by nitrogen purge

(38) The coated hollow fibres were dried and the temperature and the drying time were dependent on the boiling point of the solvents used

(39) More than one layer of PVAm can be applied

(40) The following day, heat treatment of the membrane was performed as per outside coating.

Example 6

Membrane Testing: Water Uptake (Swelling Degree) Experiments were Measured on the Non Strengthened Flat Membranes

(41) Maximum water uptake capacity of PVAm by weight was investigated using fully humidified nitrogen. Water uptake by membranes represents a key parameter for facilitated transport where the reaction between CO.sub.2 and amino groups takes place in presence of water. High water uptake capacity is directly related to density of polar groups (amino groups) per volume. The degree of polymeric chains entanglement will dictate the water holding capacity of polymer. Water has a positive effect by catalysing the reaction between amino groups and the CO.sub.2, provides a transport medium for the CO.sub.2 to the amino group reaction sites and increases polymeric chains mobility. Water uptake could have had the negative effect of loosening the PVAm structure to the point of disrupting completely the chain packing but that is surprisingly not observed. The effect of swelling on the mechanical stability of the PVAmHM membrane is counter-balanced by the dense chain packing.

(42) The Mw influence on water uptake was investigated by comparing the water uptake of PVAmHM (340000 Mw) to that of PVAmLM (80000 Mw). As can be seen from FIG. 9 the water uptake (SD %) increases fast with time, approaching a plateau after 6 hours. The experiments were carried out until no change in the mass of swollen polymer was observed (22-24 h). PVAmHM presented a maximum water uptake at equilibrium of 31.4% and PVAmLM 24.7% by weight. From repeated measurements PVAmHM presents in average 23% higher water uptake than PVAmLM. This fact can be explained by the PVAmHM's higher Mw (longer polymer chains), providing a more entangled structure and higher density of polar sites per polymer volume. The higher water uptake of PVAmHM in comparison with PVAmLM can be interpreted in terms of better structural film integrity in absence of crosslinker under humid conditions.

Example 7

Effect of Molecular Weight on NH4F Cross-Linked Flat Membranes

(43) The higher molecular weight PVAm ensures a denser packing of the amino groups and higher water uptake leading to higher selectivity and permeance compared to low molecular weight PVAm.

(44) Four times higher Mw of PVAm (340 000) allows the use of a porous support with higher MWCO that increases permeance without affecting the mechanical strength of the composite membranes.

(45) Using larger pore size not only decreases the mass transfer resistance towards CO.sub.2 but it changes the support separation mechanism itself. An ultrafiltration membrane with low pore size (MWCO) may present selectivity towards the N.sub.2 via Knudsen diffusion and not towards CO.sub.2.

Example 8

Effect of Support

(46) We investigated the influence of the support as a separate membrane under normal operating conditions in order to clearly accesses its contribution to the overall membrane performance especially in the humid conditions. FIG. 6 show the separation performance of the porous supports tested with mix gas feed. As can be seen the dry PSf has a very low CO.sub.2 permeance and selectivity. The porous supports act as membranes presenting selectivity when using humidified gases. It is clear from FIG. 6 that high MWCO support presents higher CO.sub.2 permeance especially at lower pressure ranges.

Example 9

Crosslinking

(47) FIGS. 2 and 3 show the effect of the NH.sub.4F solution concentration used for crosslinking at 90 C. High Mw provides a more entangled structure of the polymeric chains (equivalent of a crosslinking) providing better film mechanical properties in absence of a crosslinker.

Example 10

Effect of Thermal Treatment

(48) FIGS. 7 and 8 show the effect of thermal treatment on one of the flat membranes of the invention. The permeance and selectivity are only slightly better than for the non strengthened analogues suggesting this step is barely necessary but for pressures above 10 bars, the non strengthened membranes lack sufficient mechanical strength,

Example 11

Effect of Feed Pressure

(49) FIGS. 4 and 5 shows the effect of the applied feed pressure on PVAmHM (340000 Mw) composite membrane CO.sub.2 permeance and selectivity. The decrease in CO.sub.2 permeance and CO.sub.2/N.sub.2 selectivity with the increase in feed pressure (CO.sub.2 partial pressure) correlates with the CO.sub.2 facilitated transport mechanism and it is explained by the saturation of fixed site carrier amino groups. It can be observed that the PVAmHM cast on more porous PSf support presents higher permeance up until 10 bars due to the larger pore size of the support. The difference in permeance is attributed to the effect of the porous support acting as a resistance in series with the selective PVAm layer. The CO.sub.2 permeance of both membranes becomes similar at pressure higher than 10 bar due to a possible compression of the more porous support. The higher CO.sub.2/N.sub.2 selectivity up until 3 bar is explained by the higher CO.sub.2 permeance of the 50000 MWCO support.

Example 12

(50) Each flat membrane was continuously operated between 30 and 60 days showing stable performances by swinging feed relative humidity (RH %) between 0% to 100%, the pressure between 1 bar and 15 bar. A maximum permeance of 0.6 m.sup.3 (STP)/(m.sup.2 bar h) and selectivity of 200 at 1.1 bar feed pressure were obtained for PVAmHM 1.2 m thickness membranes casted on 50 000 MWCO PSf without crosslinking.

Example 13

Effect of Solvent

(51) Different solvents provide different volatilities and the quality of very thin films can be affected by drying conditions, especially for hollow fiber coating or large flat membranes. Maximum permeance and selectivity using different solvents, for a flat composite membrane of 1.2 m thickness without crosslinking cast on PSf 50000 MWCO as support is presented in Table 1. The operating conditions were 10% CO.sub.2-90% N.sub.2 gas mixture at 2 bar as a feed. Using different solvents not only as a mean of casting a polymer in a shape of a film but mainly as an agent modifying the structure of the PVAm film provides different separation properties. The PVAm film structure and separation properties changes function of the PVAm solubility in different solvents, evaporating rate of the solvents, pH of the solvent (changing the character of PVAm from acid to base and implying the reactivity change of NH2 groups) and interaction with the porous support (hydrophobic-hydrophilic character).

(52) TABLE-US-00001 TABLE 1 Effect of casting solvent 10% water- Type of solvent Water 90% formamide Permeance 0.6 2.1 m.sup.3(STP)/m.sup.2 bar hr CO.sub.2/N.sub.2 selectivity 204 388

Example 14

Gas Permeation Results for Hollow Fibre Type of Membrane

(53) Table 2 presents the results of gas permeations using hollow fibre (HF) type membrane with a selective separating layer of PVAm of approx. 1 m. If CO.sub.2/N.sub.2 selectivity does not differ to a great extent it may be attributed to the PVAm selective layer, while CO.sub.2 permeance is affected to a great extent by the porous support. A porous support with a nanometer dense top layer such as polyphenylene oxide represents a more convenient choice in terms of gas separation and mechanical strength due to the intrinsic material properties.

(54) TABLE-US-00002 TABLE 2 Effect of different HF support Type of HF PSf outside PPO membrane coated PSf inside coated outside coated OD/ID mm 0.6/0.3 1/0.6 0.5/0.3 Permeance 0.1 0.016 1.46 m.sup.3 (STP)/m.sup.2 bar hr CO.sub.2/N.sub.2 selectivity 140 157 191 OD outer diameter, ID inner diameter