US006161386A
`6,161,386
`[11] Patent Number:
`(15
`United States Patent
`Lokhandwala
`[45] Date of Patent:
`Dec. 19, 2000
`
`
`4,857,078
`[54] POWER GENERATION METHOD
`8/1989 Watler oe ee ececteeteneeneeneee 55/16
`
`INCLUDING MEMBRANESEPARATION 10/1990 Blumeet al.wn.eeeceeeseseeeees 55/164,963,165
`
`
`5,040,370
`8/1991 Rathbone....
`60/649 X
`5,089,033
`2/1992 Wijmams «seers 55/16
`Inventor: Kaaeid A. Lokhandwala, UnionCity,
`[75]
`
`Calif 4/1993) Wijmans oo...eeeseeeteneeeeeees 55/165,199,962
`
`oe 55/16
`,
`5,205,843
`4/1993 Kashemekatet al.
`ale cascsssscsscsssssssesssesnseesees 95/50
`5,281,255
`.
`1/1994 Toy
`et
`
`7/1994 Rao etal, 95/47
`[73] Assignee: Membrane Technology and Research,
`5,332,424
`5,374,300 12/1994 Kaschemekat et al. cece. 95/39
`Inc., Menlo Park, Calif.
`
`4/1995 Lokhandwala et al. oe 95/49
`5,407,467
`
`3/1996 Toy et ale eceeceesseccecsseeeeseseneeeee 95/50
`5,501,722
`9/1998 Avakovet al.eee 60/649
`5,806,316
`
`
`
`[21] Appl. No.: 09/220,971
`
`[22]
`Filed:
`Dec. 23, 1998
`[SD] Ute C07 eeeececcccsseececcsssseecesesssseecessessees FO1K 25/06
`
`[52] US. C1. ec ceceeeteeeeeseeeneeeee 60/649; 60/651; 60/671;
`95/50; 95/96; 95/143; 95/144; 95/237; 95/240
`[58] Field of Search... ee 60/649, 651, 671,
`60/679; 55/16, 23, 31, 33, 68, 158; 95/39,
`95, 48, 50, 54
`
`[56]
`
`.
`References Cited
`U.S. PATENT DOCUMENTS
`
`4,370,150
`4,685,940
`
`1/1983 Fenstermaker 0.0... eee eeneeee 55/16
`8/1987 Soffer et al.
`..cecccsseseseeeeees 55/158
`
`:
`:
`Primary Examiner—Hoang Nguyen
`Attorney, Agent, or Firm—J. Farrant
`<
`7]
`ABSTRACT
`A method for generating electric power, such asat, or close
`to, natural gas fields. The method includes conditioning
`natural gas containing C3, hydrocarbons and/or acid gas by
`means of a membrane separation step. This step creates a
`leaner, sweeter, drier gas, which is then used as combustion
`fuel
`to run a turbine, which is in turn used for power
`generation.
`
`21 Claims, 8 Drawing Sheets
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`102
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`105
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`109
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`CRUSOE-1021
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`CRUSOE-1021
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`1
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`

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`U.S. Patent
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`Dec. 19, 2000
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`Sheet 1 of 8
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`6,161,386
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`102
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`105
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`109
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`FIG. 1
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`2
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`

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`U.S. Patent
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`Dec. 19, 2000
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`Sheet 2 of 8
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`6,161,386
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`FIG. 2
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`3
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`U.S. Patent
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`Dec. 19, 2000
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`Sheet 3 of 8
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`6,161,386
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`301
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`RRA
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`FIG.
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`317
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`l Y
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`I
`!
`v=,
`x 318
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`al!|
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`4
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`U.S. Patent
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`Dec. 19, 2000
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`Sheet 4 of 8
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`6,161,386
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`401 PC
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`407
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`FIG. 4
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`5
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`U.S. Patent
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`Dec. 19, 2000
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`Sheet 5 of 8
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`6,161,386
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`504
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`510 RASA
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`FIG. 5
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`6
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`U.S. Patent
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`Dec. 19, 2000
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`Sheet 6 of 8
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`6,161,386
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`601
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`FIG. 6
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`7
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`U.S. Patent
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`Dec. 19, 2000
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`Sheet 7 of 8
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`6,161,386
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`710
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`709
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`FIG. 7
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`U.S. Patent
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`Dec. 19, 2000
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`Sheet 8 of 8
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`6,161,386
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`810
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`809 LLLALALLALALa
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`801
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`811
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`FIG. 8
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`6,161,386
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`1
`POWER GENERATION METHOD
`INCLUDING MEMBRANE SEPARATION
`
`This invention was made in part with Government sup-
`port under Contract No. DE-FG03-95ER82022 awarded by
`the Department of Energy. The Government has certain
`rights in this invention.
`FIELD OF THE INVENTION
`
`The invention relates to generation of power by means of
`gas-fired turbines. More particularly, the invention concerns
`the usc of a membranc scparation step to condition raw
`natural gas to render it suitable for use as turbine fuel,
`burning the fuel to power the turbine, and using the turbine
`to drive an electricity generator.
`BACKGROUND OF THE INVENTION
`
`Natural gas is the most important fuel gas in the United
`States and provides more than one-fifth of all the primary
`energy used in the United States. Natural gas is also used
`extensively as a basic raw material in the petrochemical and
`other chemical process industries. The composition of natu-
`ral gas varies widely from field to field. For example, a raw
`gas stream may contain as much as 95% methane, with only
`minor amounts of other hydrocarbons, nitrogen, carbon
`dioxide, hydrogen sulfide or water vapor. On the other hand,
`streams that contain relatively large proportions of heavier
`hydrocarbons and/or other contaminants are common.
`Before the raw gas can be sentto the supply pipeline, it must
`usually be treated to remove at least one of these contami-
`nants.
`
`As it travels from the wellhead to the processing plant and
`ultimately to the supply pipeline, gas may pass through
`compressors or other field equipment. These units require
`power, and it is desirable to run them using gas enginesfired
`by natural gas from the field. Since the gas has not yet been
`brought to specification, however, this practice may expose
`the engineto fuel that is of overly high Btu value, low octane
`number, or corrosive.
`Arelated operation is to use field gas as combustion fuel
`for turbines, which are then used to drive other equipment,
`for example, electric power generators and compressors. In
`this case, the water and hydrocarbon dewpoints of the gas
`should be below the lowest temperature likely to be encoun-
`tered en route to the turbine. If this is not done, the feed
`stream may contain entrained liquid water and hydrocar-
`bons. These do not burn completely when introduced inta
`the turbine firing chamber, and can lead to nozzle flow
`distribution problems, collection of liquid pools and other
`reliability issues. Additionally high concentrations of heavy
`hydrocarbonstend to make the fucl burn poorly, resulting in
`coke formation and deposition of carbon in the fuel path-
`ways and on the turbine blades. These deposits reduce
`turbine performance and affect reliability.
`There is a need, therefore, for a process that can be used
`in the field to lower to an appropriate level the dewpoint of
`gas destined for turbine fuel. The process should employ
`simple, robust equipment that can operate under field con-
`ditions without
`the need for sophisticated controls and
`frequent operator attendance or maintenance. The gas thus
`treated could then be used more reliably as turbine fuel.
`That membranes can separate C,, hydrocarbons from gas
`mixtures, such as natural gas, is known, for example from
`US. Pat. Nos. 4,857,078,5,281,255 and 5,501,722. Separa-
`tion of acid gases from other gases is taught, for example, in
`US. Pat. No. 4,963,165. It has also been recognized that
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`compression/condensation and membrane separation may
`be combined, as is shown in U.S. Pat. Nos. 5,089,033;
`5,199,962; 5,205,843 and 5,374,300.
`The problem of upgrading raw gasin the field, such as to
`sweeten sour gas, is addressed specifically in U.S. Pat. No.
`4,370,150, to Fenstermaker. In this patent, Fenstermaker
`teaches a process that uses a membrane, selective for hydro-
`gen sulfide and/or heavier hydrocarbons over methane, to
`treat a side stream of raw gas. The process produces a
`membrane residue stream of quality appropriate for engine
`fuel. The contaminants pass preferentially through the mem-
`brane to form a low-pressure permeate stream, which is
`returned to the main gasline upstream ofthe field compres-
`SOr.
`
`U.S. Pat. No. 6,053,965,relates to the use of a separation
`membrane in conjunction with cooling to achieve upgrading
`of raw natural gas to run field engines.
`USS. Pat. No. 6,035,641, relates to the use of a membrane
`to upgrade gas containing large amounts of nitrogen, fol-
`lowedbyuse of that gas as combustionfuelfor a turbinethat
`generates electric power.
`
`SUMMARY OF THE INVENTION
`
`The invention is a method for generating electric power.
`The invention is particularly useful in generation ofelectric
`poweron-site at, or close to, a gas field. Advances in gas
`turbine technology, such as combined cycle processes and
`development of mini-turbines, are beginning to make such
`electric power generation attractive. Under some conditions,
`the economics of using the gas directly to make electricity
`are more favorable than piping the gas itself to distant
`locations. Not only is electric power useful to a wider base
`of industries than natural gas, but the costs of electricity
`transmission across long distances are usually significantly
`lower than the corresponding gas transport costs.
`The methodof the invention includes conditioning natural
`gas containing C,, hydrocarbons and/or acid gas by means
`of a membrane separation step, so that the gas can be used
`as combustion fuel to run a turbine, which is then used for
`power generation. The membrane separation step can be
`used to provide some dewpoint control and/or sweetening of
`the raw gas, thereby rendering it suitable for turbine fuel.
`The process is carried out using al least part of a natural gas
`stream that is at high pressure, commonly, although not
`necessarily, after it has passed through a compressor.
`In a basic embodiment, the present invention comprises:
`(a) performing a membrane separation step, comprising:
`(i) providing a membrane having a feed side and a
`permeate side and beingselective for C,, hydrocar-
`bons over methane;
`(ii) passing a high-pressure gas stream comprising C,,
`hydrocarbons and methaneacrossthe feed side of the
`membrane;
`(iii) withdrawing from the feed side a residue stream
`depleted in C,, hydrocarbons and enriched in meth-
`ane compared with the gas stream;
`(iv) withdrawing from the permeate side a permeate
`stream enriched in C,, hydrocarbons anddepleted in
`methane compared with the gas stream;
`(b) using at
`least a portion of the residue stream as
`combustion fuel in a turbine;
`(c) using the turbine to drive an electric power generator.
`The method removes C,, hydrocarbons and/or acid gas
`from the raw gas. The methane-enriched residue stream is
`used to provide all or part of the combustion fuel for a
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`6,161,386
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`3
`turbine. The shaft power provided by the turbine is used at
`least in part to power an electric generator.
`The method of the invention can be carried out with
`
`several types of membranes. Thefirst is rubbery membranes.
`The second is membranes made from the so-called “super-
`glassy” polymers, defined and described in the Detailed
`Description of the Invention below, that exhibit anomalous
`behavior for glassy materials in that
`they preferentially
`permeate larger, more condensable molecules over smaller,
`less condensable molecules. A third alternative is inorganic
`membranes, such as microporous carbon or ceramic mem-
`branes.
`
`The C,, hydrocarbons/methane membrane separation
`step may be carried out in one or multiple membranestages.
`Turbine fuel frequently, although not necessarily, enters
`the combustion chamberofthe turbine at a pressure of about
`150-350 psia. Commonly, therefore, the gas must be com-
`pressed in one or multiple compressionstages asit passes to
`the turbine. In this situation, the membrane separation step
`can be incorporated conveniently to follow the compression
`stages, and the invention then includes the following steps:
`(a) compressing and then cooling a gas stream comprising
`C,, hydrocarbons and methane, resulting in the forma-
`tion of a high-pressure gas stream depleted in C3,
`hydrocarbons and a liquid condensate enriched in C3,
`hydrocarbons;
`(b) performing a membrane separation step, comprising:
`(i) providing a membrane having a feed side and a
`permeate side and being selective for C,, hydrocar-
`bons over methane;
`(ii) passing the high-pressure gas stream across the feed
`side of the membrane;
`(iti) withdrawing from the feed side a residue stream
`depleted in C,, hydrocarbons and enriched in meth-
`ane compared with the gas stream;
`(iv) withdrawing from the permeate side a permeate
`stream enriched in C,, hydrocarbons and depleted in
`methane compared with the gas stream;
`(c) using at
`least a portion of the residue stream as
`combustion fuel in a turbine;
`(d) using the turbine to drive an electric power generator.
`The compressionstep is typically carried out using a fixed
`speed compressor or compressors, including a return loop
`for passing gas back from the exhaust to the inlet side of the
`compressor train to accommodate fluctuations in gas flow.
`Optionally, the membrane separation unit may be positioned
`so that
`the permeate stream forms a return line to the
`compressorinlet. This increases the amount of hydrocarbon
`liquids recovered in the aftercooler section of the compres-
`SOT.
`
`Gasstreamsto be treated by and used in the method of the
`invention may, and frequently do,
`include multiple
`contaminants, such as C3, hydrocarbons, hydrogen sulfide
`and water. Since the membranes used in the invention are
`selectively permeable not only to C3, hydrocarbons,butalso
`to hydrogen sulfide and water vapor, the membrane residue
`stream exiting the membrane separation step is leaner,
`sweeter and drier than the membrane feed stream. This
`
`leaner, sweeter, drier siream is usually suitable to be [ed
`directly to the combustor without any additional treatment.
`The membraneseparation step of the invention is essen-
`tially passive, and in many cases can be incorporated into the
`power generation train without requiring any additional
`rotating equipment beyond what would alreadybe provided
`in a prior art compressor/turbine/generator configuration.
`
`4
`In summary, the invention provides the following ben-
`efits:
`
`1. Enables turbines to be run using otherwise sub-quality
`gas.
`
`2. Avoids damage to turbines by out-of-spec gas.
`3. Can produce additional NGL for sale if desired.
`It is to be understood that the above summary and the
`following detailed description are intended to explain and
`illustrate the invention without restricting its scope.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`FIG. 1 is a schematic drawing of a basic embodiment of
`the invention.
`
`FIG. 2 is a schematic drawing of an alternative embodi-
`ment of the invention.
`
`FIG. 3 is a schematic drawing of an embodiment of the
`invention in which the permeate stream is returned to the
`inlet side of the compressor.
`FIG. 4 is a schematic drawing of an embodiment of the
`invention in which additional poweris generated by a steam
`turbine.
`
`FIG. 5 is a schematic drawing of an embodiment of the
`invention in which power for compression of the raw gas is
`provided bya turbine.
`FIG. 6 is a schematic drawing of three-stage compression,
`not in accordance with the invention.
`
`FIG. 7 is a schematic drawing of a preferred embodiment
`in which the membrane separation step forms part of the
`compressor return loop.
`FIG. 8 is a schematic drawing of a preferred embodiment
`of the invention in which the permeate stream is not returned
`to the compressionstep.
`
`DETAILED DESCRIPTION OF THE
`INVENTION
`
`The term gas as used herein means a gas or a vapor.
`The terms C3, hydrocarbon and heavier hydrocarbon
`mean a hydrocarbon having at least three carbon atoms.
`The term high-pressure gas stream meansa gas stream at
`a pressure of at least 100 psia.
`The terms lighter and leaner mean reduced in C,, hydro-
`carbons content.
`
`The term sweeter means reduced in hydrogen sulfide
`content.
`
`The terms two-step and multistep as used herein mean an
`arrangement of membrane modules or banks of membrane
`modules connected together such that the residue stream
`from one module or bank of modules becomes the feed-
`stream for the next.
`
`The terms two-stage and multistage as used herein mean
`an arrangement of membrane modules or banks of mem-
`brane modules connected together such that the permeate
`stream from one module or bank of modules becomesthe
`feedstream for the next.
`
`The term membrane array means a set of membrane
`modules or banks of modules connected in multistep
`arrangement, multistage arrangement, or mixtures or com-
`binations of these.
`
`The term product residue stream meansthe residue stream
`exiting a membrane array when the membrane separation
`process is complete. This stream may be derived from one
`membrane bank, or may be the pooled residue streams from
`several membrane banks.
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`The term product permeate stream means the permeate
`stream exiting a membrane array when the membrane sepa-
`ration process is complete. This stream may be derived from
`one membrane bank, or may be the pooled permeate streams
`from several membrane banks.
`
`Percentages herein are by volume unless otherwise stated.
`The invention is an improved method for generating
`electric power. The method includes conditioning natural
`gas containing C,, hydrocarbons and/or acid gas by means
`of a membrane separation step, so that the gas can be used
`as combustion fuel to run a turbine, which is then used for
`power generation.
`they can deliver
`A feature of modem turbines is that
`electricity at high efficiency, while utilizing fuel gas that is
`not of pipeline quality composition, such as in regard to
`water content, hydrocarbon dewpoint and so on. For
`example, pipeline specification is typically no more than 4
`ppm hydrogen sulfide, no more than 1-3% carbon dioxide,
`no more than about 140 ppm water vapor, and a hydrocarbon
`dewpoint below0° C. at 1,000 psia, whichtranslates roughly
`to a total C3, hydrocarbon content of no more than about
`5%, of which no more than about 1-2% is C,, hydrocar-
`bons. In contrast, a turbine may be able to operate satisfac-
`torily on a gas that contains, for example, as much as 1,000
`ppm of hydrogen sulfide and/or 10% total C,, hydrocarbons
`or more.
`
`For turbines, the most important issue is to reduce the
`water and hydrocarbon dewpoints below the lowest tem-
`perature likely to be encountered en route to the turbine.
`Depending on the environmentof use, this generally means
`water and hydrocarbon dewpoints below about 25° C. at 300
`psia, but can mean a much lower dewpoint, such as below
`0° C. at 300 psia.It is also desirable to remove benzene and
`other aromatic compoundsthat are typically present in raw
`gas at the level of about 1,000 ppm,since these compounds
`contribute disproportionately to carbon deposition on tur-
`bine component surfaces.
`Although these requirements are less demanding than
`typical pipeline specifications, many or most raw gas
`streams do not meet them. The streams that may be treated
`by the process of the invention are diverse and include,
`without
`limitation,
`those that contain excess C3,
`hydrocarbons,
`large amounts of acid gases, specifically
`hydrogen sulfide and/or carbon dioxide, and/or large
`amounts of water vapor. The methane contentof the raw gas
`may be any value, but commonly will be in the range
`50-99% methane, and most typically will be in the range
`60-95% methane.
`The invention in its most basic form is shown schemati-
`
`cally in FIG. 1. Except as explicitly set forth otherwise, all
`of the considerations and preferences expressed above with
`respect to FIG. 1 apply also to the embodimentsof the other
`figures. Referring now to FIG. 1, stream 101is the stream to
`be treated by and then used in the method of the invention.
`The first step or steps of the invention involves removing
`contaminants from raw gas to meet engine or turbine fuel
`specification by passing a stream of the raw gas across a
`contaminant-selective membrane. For the process to provide
`a useful separation between the C,, hydrocarbons and
`methane or between the acid gas components and methane,
`the pressure of stream 101 should beatleast about 100 psia.
`If the rawgasis not at a sufficiently high pressure to provide
`adequate transmembrane driving force,
`it may be com-
`pressed before passing to the membrane separation step as
`described for other embodiments below.
`
`Gas stream 101 passes to the membrane separation unit
`102. This unit contains a membranethatis selective for C3,
`
`6
`hydrocarbons over methane. To provide suchselectivity, the
`membrane is preferably made from an elastomeric or rub-
`bery polymer. Examples of polymers that can be used to
`make elastomeric membranes, include, but are not limited
`to, nitrile rubber, neoprene, polydimethylsiloxane (silicone
`rubber), chlorosulfonated polyethylene, polysilicone-
`carbonate copolymers,
`fluoroelastomers, plasticized
`polyvinylchloride, polyurethane, cis-polybutadiene, cis-
`polyisoprene, poly(butene-1), polystyrene-butadiene
`copolymers, styrene/butadiene/styrene block copolymers,
`styrene/ethylene/butylene block copolymers, thermoplastic
`polyolefin elastomers, and block copolymers of polyethers,
`polyamides and polyesters. Silicone rubber is the most
`preferred material for separating C,, hydrocarbons from
`methane. Another type of preferred membrane, if the con-
`taminant of primary concern is hydrogen sulfide, is one in
`which the selective layer is a polyamide-polyether block
`copolymer having the general formula
`
`HO—¢C—PA—C—O—PE— 04H
`oO
`oO
`
`where PA is a polyamide segment, PE is a polyether segment
`and n is a positive integer. Such polymers are available
`commercially as Pecbax® (Atochem Inc., Glen Rock, NJ.) or
`as Vestamid® (NuodexInc., Piscataway, N.J.). These types
`of materials are described in detail in U.S. Pat. No. 4,963,
`165, and their use in treating gas streams laden with acid
`gascs is described, for example, in U.S. Pat. No. 5,407,467.
`These materials also exhibit selectivity in favor of C;,,
`hydrocarbons over methane, but are generally slightly less
`selective in that regard than silicone rubber.
`Altermatively, the membrane can be made from a super-
`glassy polymer. Super-glassy polymers have a rigid
`structure, high glass transition temperatures, typically above
`100° C., 200° C. or higher, and have unusually high free
`volume within the polymer material. These materials have
`been found to exhibit anomalous behavior for glassy
`polymers, in that they preferentially permeate larger, more
`condensable, organic molecules over smaller inorganic or
`less condensable organic molecules. The best known super-
`glassy polymeris poly(trimethylsilylpropyne) [PTMSP], the
`use of which to preferentially separate condensable compo-
`nents from lower-boiling, less condensable components is
`described in U.S. Pat. No. 5,281,255, for example. This type
`of membrane would be useful in the present invention as an
`organic-component selective membrane.
`Yet anotheralternative is to use finely microporous inor-
`ganic membranes, such as the adsorbent carbon membranes
`described in U.S. Pat. No. 5,332,424, the pyrolysed carbon
`membranes described in U.S. Pat. No. 4,685,940, or ccrtain
`ceramic membranes. These membranesare, in general, more
`difficult to make and less readily available than polymeric
`membranes, and are less preferred, although they may be
`useful in some circumstances.
`
`The membrane maytake the form of a homogeneousfilm,
`an integral asymmetric membrane, a multilayer composite
`membrane, a membrane incorporating a gel or liquid layer
`or particulates, or any other form known in the art. If
`elastomeric membranes are used, the preferred form is a
`composite membrane including a microporous support layer
`for mechanical strength and a rubbery coating layer that is
`responsible for the separation properties. If super-glassy
`membranes are used, they may be formed as integral asym-
`metric or composite membranes.
`The membranes may be manufacturedas flat sheets or as
`fibers and housed in any convenient module form, including
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`6,161,386
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`7
`spiral-wound modules, plate-and-frame modules and potted
`hollow-fiber modules.
`‘The making of all
`these types of
`membranes and modulesis well knownin theart. Flat-sheet
`
`membranes in spiral-wound modules is the most preferred
`choice.
`
`Membrane unit 102 may contain a single membrane
`module or bank of membrane modules or an array of
`modules. A single-stage membrane separation operation is
`adequate for many applications.
`If the residue stream
`requires further purification, it may be passed to a second
`bank of membrane modules for a second processingstep. If
`the permeate stream requires further concentration, it may be
`passed to a second bank of membrane modules for a
`second-stage treatment. Such multi-stage or multi-step
`processes, and variants thereof, will be familiar to those of
`skill
`in the art, who will appreciate that the membrane
`separation step may be configured in many possible ways,
`including single-stage, multistage, multistep, or more com-
`plicated arrays of two or more units in serial or cascade
`arrangements.
`In cases where substantial amounts of both C,, hydro-
`carbons and hydrogen sulfide must be removed, an optional
`configuration is two-step, with one bank of membrane
`modules containing silicone rubber membranes tor better
`C,, hydrocarbon removal and the other bank containing
`polyamide-polyether block copolymer membranesfor better
`hydrogen sulfide removal.
`High-pressure stream 101 flows across the membrane
`surface. The permeate side of the membranc is maintained at
`lowerpressure to provide a driving force for transmembrane
`permeation. C,, hydrocarbons, including benzene and other
`aromatics, if present, acid gases and water vaporall perme-
`ate the membrane preferentially, resulting in contaminant-
`enriched permeate stream 103 and contaminant-depleted
`residue stream 104.
`
`As is familiar to those of skill in the art, the separation
`performance achieved by the membrane depends on such
`factors as the membrane selectivity,
`the pressure ratio
`between feed and permeate sides, and the membrane area.
`The transmembrane flux depends on the permeability of the
`membrane material, the pressure difference across the mem-
`brane and the membrane thickness.
`Oneparticular advantage is the ability of the membranes
`to operate in the presence of high concentrations of water
`and hydrocarbons. Water is always present in raw natural gas
`streams to some extent, as vapor, entrained liquid, or both.
`The presenceof this water has little or no adverse effects on
`the types of membranes used in the invention, but will
`simply permeate the membrane along with the higher hydro-
`carbons. Even if the membrane separation is performed at
`close to the dew point for these components, any condcn-
`sation of liquid will not damage the membrane.
`Stream 103 is withdrawn from the membrane permeate
`side and may be directed to any appropriate destination, such
`as mixing with any other stream of similar pressure or
`compositionthat is available, or may be further processed or
`disposed of as a discrete stream.
`The second step in the method of the invention is to use
`the treated gas as fuel for a turbine that delivers mechanical
`power. The fuel gas may be passed directly from the
`membrane unil to the combustor of the turbine, as indicated
`in FIG. 1, or may be subjected to temperature, pressure or
`further composition adjustment as required. Whether indi-
`rectly or directly, stream 104 passes into the fuel inlet line
`of combustor 105. Air stream 106 also passes into the
`combustor. Optionally stream 106 may be compressed using
`shaft power generated by the turbine, as illustrated with
`
`10
`
`30
`
`35
`
`40
`
`45
`
`50
`
`55
`
`60
`
`65
`
`8
`respect to FIG. 4, below. Hot gas 107 from the combustorts
`used to drive turbine 108 and exits as exhaust stream 109.
`
`Turbine 108 may be of any convenient type. Most types
`of turbines knowninthe art can be used in the method of the
`
`including, but not limited to, single-shaft and
`invention,
`turbines, single-stage and multi-stage turbines,
`split-shaft
`and turbines cmploying simple open cycle, regenerated
`Brayton cycle, combined cycle and other configurations.
`The design and operation of such turbines is well known in
`the art. As discussed in more detail below with respect to
`FIG. 4,
`it
`is most preferred to use a combined cycle
`arrangement, in which the hot exhaust gas from the primary
`turbine is used to generate steam, which can then be used to
`generate additional power by expansion through a steam
`turbine. This tends to provide higher overall thermal effi-
`ciency than other turbine configurations.
`The third step in the process is electric power generation.
`Shaft power developed by the turbine is deployed through
`shaft 110 to drive electricity generator 111. The generator
`may be of any type, bulis typically a three-phase synchro-
`nous generator, as is well known in the art. The power
`generated may be used on-site, or may be fed to a power
`grid.
`‘lypical on-site power generation and cogeneration facili-
`ties can produce between 1 and 500 MW of power, with
`most installations in the 10-100 MW range. At an energy
`conversion efficiency of 50%,
`the natural gas volumes
`required to produce this amountof powerare between 2 and
`20 MMscfd. Streams of this size arc well suited for treat-
`
`ment by membraneseparation. For example, assuming that
`the membrane has a propane/methaneselectivity of at least
`about 3 or above, a stream of 5-10 MMscfd can typically be
`processed using a membrane area of about 500-800 m? of
`membrane, housed in 20-40 membrane modules.
`Electric power produced by the method of the invention
`can be used for a diversity of power needs at remote sites.
`Representative uses include, but are not limited to, running
`of manufacturing or processing facilities for agricultural
`crops, for example sugar mills, or other raw materials. As yet
`another example, cogeneration is used to provide a very
`economical source of power and heat for some cities. The
`power is sold to the grid and the waste heat from the
`generators is used for building heating.
`An embodiment using a combined cycle turbine configu-
`ration is shown in FIG. 4. Referring to this figure, feed
`stream 401 passes to membraneseparation step or unit 402.
`Here the stream is divided into residue stream 404, enriched
`in methane and depleted in C,, hydrocarbons compared
`with stream 401, and permeate stream 403, enriched in C,,
`hydrocarbons and depleted in methane compared with
`stream 401. Stream 403 may be subjected to further
`treatment, mixed with other gases to form a fuel supplyfor
`use elsewhere in the plant, or sent to any other desired
`destination. Stream 404 passes into the fuel inlet line of
`combustor 406. This figure includes the option to use the
`primary turbine to drive the compressor that compresses
`incoming combustion air. Air stream 407 passes through
`compressor 408 and passes as compressed air stream 409 to
`the combustor. Hot gas 410 from the combustor is used to
`drive primary turbine 411 and exits as exhaust stream 413.
`Shaft power developed by the turbine is deployed through
`shaft 412 to drive compressor 408 and through shaft 414 to
`drive electricity generator 415. The hot exhaust gas stream
`413 from the primary turbine enters steam generator 416,
`which will typically contain economizer, evaporator and
`superheater heat exchange zones. The cooled exhaust gas
`exits the process or apparatus as stream 417. Steam pro-
`
`13
`
`13
`
`

`

`6,161,386
`
`9
`duced in the steam generator enters steam turbine 420 as
`stream 419. ‘The steam turbine may also be of any conve-
`nient design and type, such as single-stage or multi-stage,
`Rateau or Curtis design, condensing or non-condensing,
`extraction or admission, and impulse or reaction. FIG. 4
`shows a closed Rankine cycle, in which the exhaust steam
`cxits as stream 418 and is condensed and pumpedbackto the
`steam generator by a condenser and pump (not shown for
`simplicity). Shaft power developed bythe steam turbine is
`deployed through shaft 421 to drive an additionalelectricity
`generator 422.
`A preferred embodiment of the invention for use in
`situations in which the incoming raw gas must be com-
`pressed is shownin FIG. 2. Referring to this figure, raw gas
`stream 201 is passed into compressor 202, where it
`is
`compressedto any desired pressure. The compressionstep is
`generally, although not necessarily, carried out using a fixed
`speed compressor or compressors. Such compressors
`include a loop, not shown, for passing gas back from the
`exhaust to the inlet side of the compressor train to accom-
`modate fluctuations in gas flow. Typically the preferred
`pressure to which the gas should be compressed is in the
`range 150-350 psia. Compression may be carried out in a
`single stage or in multiple stages. As just two examples, raw
`gas at a pressure of about 50 psia might be raised in pressure
`through two stages to 200 psia, or raw gas at 30 psia might
`be raised in pressure through three stages to 350 psia.
`Compressed stream 203 passes to aftercooler/separator
`section 204, resulting in the formation ofliquid hydrocarbon
`stream 205, which is withdrawn and sent to any desired
`destination or treatment. In some areas of the country and
`the world, natural gas liquids (NGL), such as those recov-
`ered in stream 205, represent a useful additional by-product
`of the methodof the invention, either because the NGL has
`value in its own right, or as a feedstock for production of
`other petrochemicals, for example.
`Remaining gas stream 206 passes to the membrane sepa-
`ration unit 207. C,, hydrocarbons, acid gases and water
`vapor permeate the membrane preferentially, to form C3,
`hydrocarbon-enriched permeate stream 208, which is with-
`drawn and sent to any desired destination or treatment, and
`leaner, sweeter, d