Method of extracting components of gas mixtures by pertraction on nanoporous membranes

Abstract

The invention relates to the field of membrane gas separation. A method of removing components of gas mixtures which is based on passing the components of a gas mixture through a nanoporous membrane and subsequently selectively absorbing them with a liquid absorbent that is in contact with the nanoporous membrane, wherein to prevent the gas from getting into the liquid phase of the absorbent and the liquid phase of the absorbent from getting into the gas phase, a nanoporous membrane with homogeneous porosity (size distribution less than 50%) and a pore diameter in the range of 5-500 nm is used, and the pressure differential between the gas phase and the liquid absorbent is kept below the membrane bubble point pressure. An acid gas removal performance of more than 0.3 nm.sup.3/(m.sup.2 hour) in terms of CO.sub.2 is achieved at a hollow-fiber membrane packing density of up to 3200 m.sup.2/m.sup.3, which corresponds to a specific volumetric performance of acid gas removal of up to 1000 nm.sup.3 (m.sup.3 hour). The technical result is that of providing effective extraction of undesirable components from natural and process gas mixtures.

Claims

1. A method of extracting components of natural, process, or associated petroleum gas mixtures by pertraction, the method comprising: passing a feed mixture in a gas phase on one side of a nanoporous membrane and an absorbent on the opposite side of the membrane, wherein the membrane includes pores with an average pore diameter in a range between 5 nm and 500 nm and a dispersion of a pore size distribution that does not exceed 50%, and providing the diffusion of the gas components through the pores of the nanoporous membrane and their absorption with a liquid absorbent stream while generating a pressure difference between the gas phase and the liquid absorbent, which is below the nanoporous membrane bubble point pressure to prevent penetration of the gas into the absorbent liquid phase and the absorbent liquid phase into the gas phase; wherein the final extraction rate of the extracted components is greater than or equal to 0.3 nm.sup.3/m.sup.2h and the specific volumetric performance of pertraction is 1000 nm.sup.3/m.sup.3h.

2. The method according to claim 1, wherein the pressure difference between the gas phase and the liquid absorbent is maintained constant by using an automatic pressure maintaining system and by means of a liquid absorbent column pressure.

3. The method according to claim 2, wherein the nanoporous membrane bubble point pressure is up to 10 bar.

4. The method according to claim 1, wherein the nanoporous membrane is made in a flat-frame or tubular geometry, or in the form of hollow fibers.

5. The method according to claim 4, wherein the outer surface of the nanoporous membrane is chemically modified to provide a higher affinity for a solvent.

6. The method according to claim 4, wherein the nanoporous membrane is made in the form of an asymmetric membrane containing a selective nanoporous layer on a large pore substrate.

7. The method according to claim 1, wherein an increase in the extraction extent of absorption is achieved by using a nanoporous membrane characterized by a packing density of up to 3200 m.sup.2/m.sup.3.

8. The method according to claim 1, wherein a constant gas/liquid phase contact area is provided.

9. The method according to claim 7, wherein the extracted components are acid gases; and natural, process, or associated petroleum gases are used as the feed mixture.

10. The method according to claim 8, wherein solutions of amines are used as the liquid absorbent.

11. The method according to claim 9, wherein materials resistant to the action of amine solution are used as the material of the nanoporous membrane.

12. The method according to claim 1, wherein a regeneration of the absorbent, which has passed through the nanoporous membrane, is further performed by passing the absorbent through a separate nanoporous membrane by providing a pressure difference between the gas phase and the liquid absorbent below the nanoporous membrane bubble point pressure and by using a stripping gas not containing absorbed components, wherein the purified absorbent is reused for extraction of components of natural and process gas mixtures, thus providing a closed process cycle.

13. The method according to claim 9, wherein the acid gases are selected from a group consisting of CO.sub.2, H.sub.2S, SO.sub.2, CH.sub.3SH, C.sub.2H.sub.5SH and (CH.sub.3).sub.2S.

14. The method according to claim 10, wherein the amines are selected from a group consisting of monoethanolamine, diethanolamine, and methyldiethanolamine.

15. The method according to claim 11, wherein the materials resistant to the action of amine solutions are selected from the group consisting of polytetrafluoroethylene, polypropylene, polysulfone, polyethersulfone, polyether ether ketone, polyvinylidene fluoride, and alumina.

Description

BRIEF DESCRIPTION OF FIGURES

(1) The invention is illustrated by the following figures, wherein:

(2) FIG. 1 shows the principle of extraction of components of natural and process gas mixtures by petraction on nanoporous membranes.

(3) FIG. 2 shows typical micrographs of the microstructure of the nanoporous membrane surface used to carry out the method.

(4) FIG. 3 shows typical chromatograms of a test raw mixture consisting of 94.5% CH.sub.4, 5% CO.sub.2, and 0.5% H.sub.2S and a processed gas, which are prepared using the claimed method (specific flow rate of feed gas is 1 nm.sup.3/m.sup.2/h).

(5) FIG. 4 shows dependence of the extraction extent of acidic components on the feed mixture flow rate, obtained by the claimed method.

(6) FIG. 5 shows dependence of the extraction extent of C.sub.2H.sub.6S on the feed mixture flow rate, obtained by the claimed method.

EMBODIMENTS OF THE INVENTION

(7) The present invention is illustrated in relationship to specific embodiments, which are not intended to limit the scope of the invention.

Examples 1 to 6. Extraction of CO.SUB.2 .and H.SUB.2.S from Mixtures Simulating the Composition of Natural Gas

(8) The process of extracting acidic components is carried out as follows. A feed gas is fed to the acid gas absorber of a pertraction module. The contact between an absorbent and the gas occurs at a temperature of 30 C., and the regeneration of an absorbent solution occurs at 80-130 C. For regeneration, the absorbent solution is cyclically fed to a regenerator where desorption of absorbed CO.sub.2 and H.sub.2S occurs.

(9) The pertraction process is intensified by using a module designed for countercurrent mass exchange. The membrane pertraction module includes an absorber body with a cartridge of hollow polymer fibers and a regenerator. The module provides for the installation and preservation of hollow fiber membrane elements that provide a possible contact between liquid and gaseous media under a process pressure of up to 10 atm, a gas flow rate of up to 10 nm.sup.3/h, and a liquid flow rate of up to 0.1 nm.sup.3/h. The hollow fiber membrane module with a diameter of up to 150 mm is configured to be installed and replaced. The absorber body has a tubular shape with a length of 900 mm, an inner diameter of 160 mm, and a wall thickness of 3 mm. The housing of the regenerator completely repeats a similar element of the absorber. The system for supplying gas and liquid phases is configured to maintain a constant transmembrane pressure between the gas phase and a liquid absorbent by using an automatic pressure maintaining system and by means of a liquid absorbent column pressure.

(10) An aqueous solution of monoethanolamine (25%) is used as the absorbent for carrying out the method. Hollow fibers based on polyvinylidene difluoride (PVDF), polyethersulfone, polysulfone and polypropylene are used as the nanoporous membrane. The main characteristics of the membranes are given in Table 1, and typical micrographs of the used membranes are shown in FIG. 2.

(11) The method was tested using a gas mixture consisting of 94.5% CH.sub.4, 5% CO.sub.2, and 0.5% H.sub.2S, and the flow rate of the feed mixture was ranged from 0.5 to 10 nm.sup.3/hour. The content of acid gases in the mixture was determined by chromatography. FIG. 3. shows chromatograms of the test feed mixture consisting of 94.5% CH.sub.4, 5% CO.sub.2, and 0.5% H.sub.2S and a retentate at a specific flow rate of the feed mixture of 1 nm.sup.3/m.sup.2/h.

(12) It can be seen that the implementation of the method leads to almost complete removal of both CO.sub.2 and H.sub.2S (the detection limit of hydrogen sulfide by this method is 0.005 vol. % of H.sub.2S). FIG. 4 shows dependence of the extraction extent of acidic components on the flow rate of the feed mixture. Throughout the entire range of the flow, hydrogen sulfide is completely removed from the mixture, while the extraction extent for carbon dioxide decreases with increasing the flow rate of the feed stream.

(13) At the same time, a 90% extraction extent for CO.sub.2 provides processing a gas mixture with a content of carbon dioxide of up to 20% to the extent, which meets the requirements of the STO Gazprom 089-2010. Thus, the membrane pertraction module can be used for pre-conditioning of associated petroleum gas in regard to acidic components, with a specific acid gas extraction rate for CO.sub.2 over 0.3 nm.sup.3/(m.sup.2.Math.h). At a hollow fiber membrane packing density of up to 3200 m.sup.2/m.sup.3, this corresponds to a specific volumetric performance of acid gas removal of up to 1000 nm.sup.3/(m.sup.3.Math.hour).

(14) The method for removing mercaptans was tested using a gas mixture consisting of 1.3% N.sub.2, 67.2% CH.sub.4, 4.5% CO.sub.2, 7.8% C.sub.2H.sub.6, 5.1% C.sub.3H.sub.8, 4.6% i-C.sub.4H.sub.10, 7.8% n-C.sub.4H.sub.10, 1.0% i-C.sub.5H.sub.12, 0.6% n-C.sub.5H.sub.12, and 0.1% C.sub.6H.sub.14, with a content of C.sub.2H.sub.6S of 54 mg/m.sup.3. The flow rate of the feed mixture ranged from 0.05 to 0.25 nm.sup.3/h. The content of mercaptans was determined by chromatography-mass spectrometry. Dependence of the extraction extent of mercaptans on the flow rate of the feed mixture is shown in FIG. 5. It can be seen that the proposed method provides a reduced concentration of mercaptans in the gas mixture; however, their extraction extent is less (10-45%) than the extraction extents for hydrogen sulfide and carbon dioxide.

(15) Thus, according to the obtained data, the claimed method allows effective extraction of undesirable components of natural and process gas mixtures and a significant reduction in the size of the absorption modules, which significantly reduces the capital investment and operating costs of gas processing facilities.

(16) TABLE-US-00001 TABLE 1 The main characteristics of the membranes used as examples in the method for extracting components from natural and process gas mixtures by pertraction on nanoporous membranes Example 1 2 3 4 5 6 Membrane material PVDF polyethersulfone polysulfone polypropylene Surface type Hydrophilic Hydrophobic Hydrophilic Hydrophobic Hydrophilic Hydrophobic Average pore size, nm 10 10 10 10 10-30 100 500 Bubble point, bar 0.5 0.2 >2 0.2 0.3 0.2 Permeability for 0.42 0.62 0.61 10 40 CO.sub.2, m.sup.3/(m.sup.2 .Math. atm .Math. h) CO.sub.2 selection rate, 0.03 0.02 0.035 0.0068 0.08 0.36 nm.sup.3/(m.sup.2 .Math. h) Membrane packing 1000 1000 1000 1000 1000 3200 density, m.sup.2/m.sup.3 Specific volumetric 30 20 35 6.8 80 >1000 performance, nm.sup.3/ (m.sup.3 .Math. h)