HELIUM RECOVERY FROM NATURAL GAS

Abstract

Helium-containing natural gas is processed with three gas separation stages to produce a natural gas product and a Helium-containing gas that may be injected into the reservoir from which the Helium-containing natural gas is obtained. A permeate from the first gas separation membrane stage is compressed and fed to the second gas membrane stage. The permeate from the second gas separation membrane stage is recovered as the Helium-containing gas that may be injected into the reservoir. The non-permeate from the second gas separation membrane stage is fed to the third gas separation membrane stage. The non-permeate from the first gas separation stage is a natural gas product. A permeate from the third gas separation membrane stage is combined with a non-permeate from the first gas separation membrane stage before it is compressed and fed to the second gas separation membrane stage. A non-permeate from the third gas separation membrane stage is fed to the first gas separation membrane stage along with the Helium-containing natural gas.

Claims

1. A method of separating natural gas and Helium from a gas mixture, comprising the steps of: producing a first permeate stream and a first non-permeate stream at a first gas separation membrane that is selective for Helium over methane, each of the first permeate stream and first non-permeate stream comprising Helium and methane, the first permeate stream being Helium enriched in comparison to the first non-permeate stream, the first non-permeate stream being a product natural gas stream; compressing the first permeate stream to provide a compressed first permeate stream; separating the compressed first permeate stream at a second gas separation membrane into a second permeate stream and a second non-permeate stream, the second gas separation membrane being selective for Helium over methane; separating the second non-permeate stream at a third gas separation membrane into a third permeate stream and a third non-permeate stream, the third gas separation membrane being selective for Helium over methane; and feeding a combination of the third non-permeate stream and a Helium-containing natural gas feed stream to the first gas separation membrane, the first gas separation membrane separating the combination into the first permeate stream and the first non-permeate stream.

2. The method of claim 1, further comprising the step of injecting the second permeate into a natural gas reservoir from which the feed gas is ultimately obtained.

3. The method of claim 1, wherein Helium is present in the Helium-containing natural gas at a concentration of less than 0.5 mol %.

4. The method of claim 1, wherein a stream of the second permeate has a mass flow rate of no more than 3% of a mass flow rate of a stream of the Helium-containing natural gas separated at the first gas separation membrane.

5. The method of claim 1, further comprising the step of purifying the second permeate stream to provide a Helium product gas having a Helium concentration of at least 99 mol %.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0016] For a further understanding of the nature and objects of the present invention, reference should be made to the following detailed description, taken in conjunction with the accompanying drawings, in which like elements are given the same or analogous reference numbers and wherein:

[0017] The FIGURE is an elevation schematic view of the method and system for separating Helium from natural gas using three gas separation membrane stages.

DESCRIPTION OF PREFERRED EMBODIMENTS

[0018] As best illustrated by the FIGURE, a feed gas stream 1 is fed to a first gas separation membrane 3. The feed gas stream 1 is ultimately obtained from a natural gas reservoir that also contains Helium. By ultimately obtained, I mean that the raw natural gas extracted from the reservoir may be processed to remove one or more contaminants to render it more suitable for processing in the first gas separation membrane 3. While the feed gas stream 1 may contain a higher concentration of Helium, it typically comprises no more than about 0.5 mol % Helium. The balance of the feed gas 1 is predominantly made up of hydrocarbons with the large majority being methane. While the method of the invention may be performed using a feed gas 1 within a relatively wide range of pressures, typically it is at 30-100 bar. Similarly, while the feed gas 1 may be within a relative wide range of temperatures, typically it is at about 50 C.

[0019] The first gas separation membrane 3 separates the feed gas 1 into a first permeate stream 5 and a first non-permeate stream 7. The first permeate stream 5 is combined with a third permeate stream 9 upstream of an inlet side of a compressor 13. In this manner, the combined flow 11 is compressed by the compressor 13 and the compressed flow 15 is fed to a second gas separation membrane 17. The second gas separation membrane 17 separates the compressed flow 15 into a second permeate stream 19 and a second non-permeate stream 21. The second non-permeate stream 21 is fed to a third gas separation membrane 23 which separates it into the third permeate stream 9 and a third non-permeate stream 25.

[0020] The second permeate stream 19 contains Helium at concentrations many times higher than that of the feed gas 1. Typically, it contains around 30 mol % Helium. The second permeate stream 19 may be further purified to provide product Helium at high purity according to any of the well-known techniques for purification of Helium from natural gas. Preferably, the second permeate stream 19 is instead injected back into the reservoir. In this manner, excess Helium in the raw natural gas extracted from the reservoir need not be processed, separately stored, or used immediately. Rather, the Helium may be stored indefinitely until there is a demand for purified Helium.

[0021] The first non-permeate stream 7 is a product natural gas stream. The product natural gas stream 27 may be introduced into a natural gas pipeline, liquefied, and/or additionally processed to remove one or more contaminants. The product natural gas stream 27 typically is of pipeline grade and contains 97% or more of hydrocarbons.

[0022] The overall Helium recovery can be increased by directing third non-permeate stream 25 to the feed of the first gas separation membrane 3 so that the combination of the feed gas 1 and the third non-permeate stream 25 are separated into the Helium-rich first permeate gas stream 5 and the first non-permeate gas stream 7.

[0023] Suitable materials for use in the separating layer of the gas separation membranes 3, 17, 23 preferentially permeate Helium over the non-Helium constituents of natural gas. Such membranes can be configured in a variety of ways, such as a sheet, tube, or hollow fiber. One of ordinary skill in the art will recognize that the permeate side of a membrane does not necessarily mean one and only one side of a membrane. Rather, in the case of membranes made up of a plurality of hollow fibers, the permeate side actually is considered to be the plurality of sides of the individual hollow fibers that are opposite to the sides to which the relevant feed gas is introduced. Preferably, each of the gas separation membranes 3, 17, 23 is made up of a plurality of hollow fibers. Typically, the membrane is made of a polymeric material such as a polysulfone, a polyether sulfone, a polyimide, a polyaramide, a polyamide-imide, and blends thereof. Particularly suitable polymeric materials for use in the gas separation membranes 3, 17, 23 are described in WO 2009/087520.

[0024] One of the polymeric materials described by WO 2009/087520 and which is useful for practice with the invention is a polyimide containing the repeating units shown in the following formula (I):

##STR00001##

in which R.sub.1 of formula (I) is a moiety having a composition selected from the group consisting of formula (A), formula (B), formula (C), and mixtures thereof, and

##STR00002##

in which R.sub.4 of formula (I) is a moiety having a composition selected from the group consisting of formula (Q), formula (S), formula (T) and mixtures thereof,

##STR00003##

in which Z of formula (T) is a moiety selected from the group consisting of formula (L), formula (M), formula (N) and mixtures thereof.

##STR00004##

In one preferred embodiment, the polyimide component of the blend that forms the selective layer of the membrane has repeating units as shown in the following formula (Ia):

##STR00005##

In this embodiment, moiety R.sub.1 of formula (Ia) is of formula (A) in 0-100% of the repeating units, of formula (B) in 0-100% of the repeating units, and of formula (C) in a complementary amount totaling 100% of the repeating units. A polymer of this structure is available from HP Polymer GmbH under the trade name P84. P84 is believed to have repeating units according to formula (Ia) in which R.sub.1 is formula (A) in about 16% of the repeating units, formula (B) in about 64% of the repeating units and formula (C) in about 20% of the repeating units. P84 is believed to be derived from the condensation reaction of benzophenone tetracarboxylic dianhydride (BTDA, 100 mole %), with a mixture of 2,4-toluene diisocyanate (2,4-TDI, 64 mole %), 2,6-toluene diisocyanate (2,6-TDI, 16 mole %) and 4,4-methylene-bis(phenylisocyanate) (MDI, 20 mole %).

[0025] The polyimide (that is preferably formed in a known way to provide an outer selective layer) comprises repeating units of formula (Ib):

##STR00006##

[0026] In one preferred embodiment, the polyimide is of formula (Ib) and R.sub.1 of formula (Ib) is a composition of formula (A) in about 0-100% of the repeating units, and of formula (B) in a complementary amount totaling 100% of the repeating units.

[0027] In yet another embodiment, the polyimide is a copolymer comprising repeating units of both formula (Ia) and (Ib) in which units of formula (Ib) constitute about 1-99% of the total repeating units of formulas (Ia) and (Ib). A polymer of this structure is available from HP Polymer GmbH under the trade name P84HT. P84HT is believed to have repeating units according to formulas (Ia) and (Ib) in which the moiety R.sub.1 is a composition of formula (A) in about 20% of the repeating units and of formula (B) in about 80% of the repeating units, and, in which repeating units of formula (Ib) constitute about 40% of the total of repeating units of formulas (Ia) and (Ib). P84HT is believed to be derived from the condensation reaction of benzophenone tetracarboxylic dianhydride (BTDA, 60 mole %) and pyromellitic dianhydride (PMDA, 40 mole %) with 2,4-toluene diisocyanate (2,4-TDI, 80 mole %) and 2,6-toluene diisocyanate (2,6-TDI, 20 mole %).

[0028] While the invention has been described in conjunction with specific embodiments thereof, it is evident that many alternatives, modifications, and variations will be apparent to those skilled in the art in light of the foregoing description. Accordingly, it is intended to embrace all such alternatives, modifications, and variations as fall within the spirit and broad scope of the appended claims. The present invention may suitably comprise, consist or consist essentially of the elements disclosed and may be practiced in the absence of an element not disclosed. Furthermore, if there is language referring to order, such as first and second, it should be understood in an exemplary sense and not in a limiting sense. For example, it can be recognized by those skilled in the art that certain steps can be combined into a single step.

[0029] The singular forms a, an and the include plural referents, unless the context clearly dictates otherwise.

[0030] Comprising in a claim is an open transitional term which means the subsequently identified claim elements are a nonexclusive listing i.e. anything else may be additionally included and remain within the scope of comprising. Comprising is defined herein as necessarily encompassing the more limited transitional terms consisting essentially of and consisting of; comprising may therefore be replaced by consisting essentially of or consisting of and remain within the expressly defined scope of comprising.

[0031] Providing in a claim is defined to mean furnishing, supplying, making available, or preparing something. The step may be performed by any actor in the absence of express language in the claim to the contrary.

[0032] Optional or optionally means that the subsequently described event or circumstances may or may not occur. The description includes instances where the event or circumstance occurs and instances where it does not occur.

[0033] Ranges may be expressed herein as from about one particular value, and/or to about another particular value. When such a range is expressed, it is to be understood that another embodiment is from the one particular value and/or to the other particular value, along with all combinations within said range.

[0034] All references identified herein are each hereby incorporated by reference into this application in their entireties, as well as for the specific information for which each is cited.