Patent Grant
11874057
The present invention relates to a process to separate nitrogen from methane by permeation and cryogenic distillation.
In particular it relates to a method for separation of nitrogen from a methane containing stream, which may for example be natural gas or biogas.
Other components that can be present, in addition to CH4 and N2, include higher hydrocarbons (e.g., benzene, ethane, propane, butane, pentane, hexane, heptane, toluene, etc.), carbon dioxide (CO2), carbon monoxide (CO), hydrogen gas (H2), helium (He), hydrogen sulfide (H2S), ammonia (NH3), water vapor, etc. Water can be present in any amounts. Feeds that contain particulate matter can be purified using a suitable filtration device, for example.
Specific examples of mixtures that comprise CH4 and N2 include natural gas (such as but not limited to traditional natural gas, shale gas, associated gas) and biogas (such as but not limited to gas from digesters, landfills, etc.). In biogas, N2/CH4 ratio can range typically from 0-1% mol. (in which case no particular nitrogen removal treatment is required) to 10% mol. or more. Natural gas usually contains very small amounts of nitrogen compatible with pipeline specifications but some natural gas fields contain higher amount of nitrogen ranging from a few percent up to close to 100% in some extreme cases. In some cases, the biogas and natural gas field contain only a limited amount of nitrogen (typically from 3-4% mol. up to 10-15% mol.).
The membranes are selected based on their performance for the desired separation, that of CH4 and N2, for instance. Possible membranes that can be employed are described in U.S. Pat. Nos. 5,669,958 and 6,630,011B1. Membranes having the potential to effect the CH4-N2 separation often include rubbery membranes such as those having a rubbery separation layer. Some potential examples of materials that can be employed for the separation layer include poly(dimethyl siloxane) (PDMS), e.g., homopolymers of dimethylsiloxane, and copolymers of dimethyl siloxane with methylethyl siloxane, methyl propyl siloxane, methyl butyl siloxane, methyl pentylsiloxane, methyl hexyl siloxane, methyloxtyl siloxane, methyl phenyl siloxane. The rubbery material can include block copolymers of dimethylsiloxane or methyloctylsiloxane with polyarylethers, polyamides, polyesters, polyketones, polyimides or block copolymers of dimethyl siloxanes or methyl octyl siloxane with silicates. Another possible material is a ladder-type silicone block copolymer with a general formula of:
HO{[C6H5SiO1.5]n[Si(CH3)2O]m}H, where n=30-60 and m=80-130.
The stream preferably contains less than 13% mol, nitrogen, possibly less than 10% mol. nitrogen and even less than 7% mol. nitrogen.
Nitrogen can often be found in biogas or in natural gas. While not presenting a major problem for some applications, nitrogen generally reduces the heating value of natural gas. Although small amounts of this inert gas can often be tolerated, natural gas containing levels higher than 4-5% vol. of N2 is typically unacceptable.
Various approaches can be employed to reduce nitrogen levels. The most common rejection technology relies on cryogenic separation. While relatively efficient, the cryogenic removal of N2 can requires large equipment and balance of plant, rendering this approach uneconomical in some situations, particularly for small flow rates.
Membrane separation is a very cost effective and simple way to separate gases. Separating CH4 and N2, however, has proved to be challenging. Some rubbery membranes such as poly(dimethylsiloxane) and derivatives, poly(methyloctylsiloxane, and polyamide-polyether copolymer can achieve a CH4/N2 selectivity of 2 to 4. Generally, this is not found satisfactory for generating a high product purity and good product recovery.
It is known that the CH4/N2 selectivity can be increased at low temperatures (below 0° C.). U.S. Pat. No. 5,669,958 to Baker et al., for example, describes operating poly-siloxane membranes at temperatures as low as −50° C., for a CH4/N2 selectivity of up to 6, to remove N2 and generate pipeline quality gas with high methane recovery. The method described in this patent utilizes a turbo-expander to supply the cooling required by the process.
In U.S. Pat. No. 6,425,267 to Baker et al., a two- or three-stage membrane process for CH4/N2 separation is conducted at an intermediate low temperature such that high CH4 recovery is achieved without the use of external refrigeration or turbo-expansion. The incoming feed gas is cooled to a sub-ambient temperature by a combination of residue and permeate streams; the cooling is generated by the Joule-Thomson effect of the membranes.
U.S. Pat. No. 6,630,011 B1 to Baker et al. describes a separation of CH4 and N2 that uses a multi-stage membrane process to achieve high methane recovery. The process is optionally operated fully or partially at low temperature for enhanced performance.
In a publication by K. A. Lokhandwala et al, several two stage and three stage arrangement membrane processes for nitrogen removal were discussed, see K. A.
Lokhandwala et al., J. Membrane Sci., vol. 346, page 270-279, titled “Membrane separation of nitrogen from natural gas: A case study from membrane synthesis to commercial deployment”.
In a publication by R. Pathare and R. Agrawal., all five possible different membrane arrangements for two stage or three stage design are listed based on membrane cascade schemes, see R. Pathare and R. Agrawal, J. Membrane Sci., vol. 364, page 263-267, titled “Design of membrane cascades for gas separation”.
Both of the following techniques are known to separate methane and nitrogen:
Generally, cryogenics works well but investment costs are usually high and a single column process is not very efficient.
Generally, membranes can work only in complex multi-stage and energy intensive schemes. This invention relates to a method for separating nitrogen from a methane-containing feed stream using membranes and cryogenics. The membrane is used to bridge the gap between the feed stream composition, which is not suitable for a very efficient cryogenic NRU process, and the composition required to run such efficient cryogenic process. In an example of such method (see mass balance) a high-pressure (˜70 bar) feed stream containing ˜6% mol nitrogen, must be enriched to approximately 13% mol. nitrogen to be treated in an efficient cryogenic NRU.
The cryogenic unit will produce medium to high-pressure, methane-rich products with various nitrogen contents, which can be combined with the membrane permeate and compressed to the final product pressure in a single or multiple compressor(s). The nitrogen content of each of the methane-rich products from the cryogenic NRU can be adjusted so that the final methane-rich product meets the required specification regarding the nitrogen or total inert content.
In order to optimize the operation of the membrane to maximize the methane recovery while maintaining a target product concentration and a high methane recovery for the combined methane product, certain embodiments of the invention provide a control process.
By this process, the pressure of the membrane permeate is controlled using a control valve with a pressure indicator and controller (PIC) detecting the pressure of the permeate. The higher the pressure of permeate, the lower the flow rate of the permeate and the lower the nitrogen concentration in the permeate. If the permeate flowrate decreases, the non-permeate flowrate necessarily increases and thus the flow rate sent to the cryogenic unit increases, at the same time as the nitrogen concentration in the non-permeate feeding the cryogenic unit decreases. This should generally have the effect of reducing the overall methane recovery for the entire process (permeation and cryogenic separation) by increasing the content of methane in the nitrogen rejected from the cryogenic separation. The pressure level of the permeate is optimized, for example manually, to maximize that methane recovery which can be monitored with an analyzer on the waste stream. Alternatively an automatic control/cascade can be installed with AIC on stream W05 in cascade of the PIC on stream P01.
According to the present invention, there is provided a process for the separation of nitrogen from a feed stream containing at least methane and nitrogen, with a methane content between 4 and 12% mol., preferably between 5 and 10 mol. %, consisting of at least the following steps:
According to optional features:
The process may further comprise
The process may comprise at least one of the following steps:
The invention now will be described more fully hereinafter with reference to the accompanying drawing, in which illustrative embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art.
As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items. Further, the singular forms and the articles “a”, “an” and “the” are intended to include the plural forms as well, unless expressly stated otherwise. It will be further understood that the terms: includes, comprises, including and/or comprising, when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. Further, it will be understood that when an element, including component or subsystem, is referred to and/or shown as being connected or coupled to another element, it can be directly connected or coupled to the other element or intervening elements may be present.
It will be understood that although terms such as “first” and “second” are used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another element. Thus, an element discussed below could be termed a second element, and similarly, a second element may be termed a first element without departing from the teachings of the present invention.
Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein.
The invention will now be described in greater detail, referring to the FIGURE.
All described and/or depicted features on their own or in any desired combination form the subject matter of the invention, irrespective of the way in which they are combined in the claims or the way in which said claims refer back to one another
The process including a permeation step in block M and a further cryogenic distillation step involving three distillation columns K01, K02, K03, columns K02 and K03 being thermally linked to form a double column, with the top gas of column K02 being condensed in a bottom condenser of column K03 and thereby evaporating the bottom liquid of column K03.
A gaseous stream 1 containing at least nitrogen and methane preferably contains less than 13% mol, nitrogen, possibly less than 10% mol nitrogen and even less than 7% mol nitrogen. The stream may contain at least one component from the following list: water, carbon dioxide, benzene, ethane, propane, butane, pentane, heptane, toluene, carbon monoxide.
Gaseous stream 1 preferably contains at least 80% mol methane, still more preferably at least 90% mol methane.
The feed stream 1 may contain between 2% mol and 10% mol nitrogen.
Alternatively the feed stream 1 may contain between 10% mol and 40% mol nitrogen.
The gaseous stream 1 is cooled in a heat exchanger E05 and separated in a rubbery type membrane M to form a permeate 4 enriched in methane at at least 3 bars and a non-permeate enriched in nitrogen. The permeate 4 is warmed in heat exchanger E05.
Preferably the non-permeate 3 is at a pressure of at least 3 bara and contains at least 1.1 times more nitrogen than the feed stream 1. The non-permeate 3 may contain at most 80% mol nitrogen or at most 50% mol nitrogen.
The membrane operates at less than 10° C.
The non-permeate enriched in nitrogen is also warmed in heat exchanger E05 and then optionally purified in purification unit P to remove any components which would freeze at temperatures encountered during the distillation process, such as water, carbon dioxide to form gaseous stream 7. Stream 7 is cooled in heat exchanger E01 and divided in two, one part 11 being cooled in heat exchanger E01 and the rest being used to control the temperature of stream 9 downstream of E01. Stream 9 is sent to reboiler E04 in which bottom liquid 13, 15 from column K01 is vaporised to form a stream sent to the bottom of first distillation column K01, as reboil.
Stream 9 is condensed in heat exchanger E04 and sent to an intermediate level of column K01 as feed.
Stream 11 is the top feed for first distillation column K01.
The top stream 19 from column K01, enriched in nitrogen, is cooled in heat exchanger E01 (not at the warm end, the drawing is schematic) and is sent as gaseous feed to the bottom of column K02. Heat exchanger E01 is typically composed of several heat exchangers.
The bottom stream 13 from first distillation column K01 is enriched in methane and is divided in three. One part 15 is previously described, part 17 is warmed in exchanger E04 and the rest is pumped in pump P02, vaporised in exchanger E01 and forms stream 6.
The operating pressure of column K01 is at least 5 bars greater than that of column K02.
The bottom liquid of column K02 is cooled in subcooler E03, expanded and sent as feed to column K03. Top gas from column K02 is sent to the condenser E02 at the bottom of column K03 and part of the condensed liquid formed is sent as stream 23 to the top of the column K03.
Bottom liquid 25 is removed from column K03, pressurized in pump P01 and then warmed in subcooler E03. It Is then mixed with liquid 17 and the mixture is vaporized in exchanger E01 to form gaseous stream 5. Stream 5 is expanded in a valve, is mixed with the permeate 4 following valve expansion of permeate 4 and the two phase mixture is separated in phase separator S1. The gas from phase separator S1 is compressed in compressor C1, cooled in cooler R1, separated again in phase separator S2, compressed in compressor C2 and cooled in cooler R1 and then mixed with stream 6 to form treated methane rich gas 3.
In an alternative example, the phase separators S1 and/or S2 are not present and the mixture formed by mixing stream 4 and stream 5 remains entirely gaseous.
Top gas 2 from column K03 is a nitrogen-enriched gas which is warmed in subcooler E03 and then in exchanger E01.
The purity of the stream 2 is measured downstream of heat exchanger E01 using an analyzer AIC.
A pressure indicator and controller PIC measures the pressure of the permeate 4 and controls a valve to increase or reduce the flowrate of stream 4. The pressure indicator and controller is itself controlled by the analyzer AIC. The higher the pressure of the permeate 4, the lower the flowrate of the permeate 4 and the lower the nitrogen content in the permeate 4. If the permeate flowrate 4 decreases, the non-permeate flowrate 3 necessarily increases and thus the flowrate 7 sent to the cryogenic unit K01, K02, K03 increases, at the same time as the nitrogen concentration in the non-permeate 7 feeding the cryogenic unit decreases.
This should generally have the effect of reducing the overall methane recovery for the entire process (permeation and cryogenic separation) by increasing the content of methane in the nitrogen 2 rejected from the cryogenic separation. The pressure level of the permeate 4 is optimized, for example manually, to maximize that methane recovery which can be monitored with an analyzer AIC on the waste stream 2.
Alternatively an automatic control/cascade can be installed with the controller AIC on stream in cascade of the PIC on stream 4.
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.
The singular forms “a”, “an” and “the” include plural referents, unless the context clearly dictates otherwise.
“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” as used herein may be replaced by the more limited transitional terms “consisting essentially of” and “consisting of” unless otherwise indicated herein.
“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.
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.
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.
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.
| Number | Name | Date | Kind |
|---|---|---|---|
| 5647227 | Lokhandwala | Jul 1997 | A |
| 5669958 | Baker et al. | Sep 1997 | A |
| 6425267 | Baker et al. | Jul 2002 | B1 |
| 6630011 | Baker et al. | Oct 2003 | B1 |
| Entry |
|---|
| Lokhandwala, et al., “Membrance Separattion of Nitrogen from Natural Gas: A Case Study from Membrane Synthesis to Commercial Deployment,” J. Membrane Sci., vol. 346, p. 270-279. |
| Pathare, et al., “Design of Membrane Cascades for Gas Separation,” J. Membrane Sci., vol. 364, p. 263-267. |
| Number | Date | Country | |
|---|---|---|---|
| 20230022033 A1 | Jan 2023 | US |
