US2716332A - Systems for separating nitrogen from natural gas - Google Patents

Description

30, 1955' P. E. HAYNES 2,716,332

SYSTEM FOR SEPARATING NITROGEN FROM NATURAL GAS Filed April 20, 1950 HYDROGEN SULFIDE, CARBON DDXIDE WATER REMOVE IN V EN TOR. Page: .6 M4 rlv-t his flTTORNE).

United States Patent SYSTEMS FOR SEPARATING NITROGEN FROM NATURAL GAS Pierre E. Haynes, Indianapolis, Ind., assignor to Koppers Company, Inc., a corporation of Delaware Application April 20, 1950, Serial No. 157,141

8 Claims. (Cl. 62-123) The present invention relates to the removal of nitrogen from natural gas and more particularly to improvements in processes of reducing the nitrogen content of natural gas in the course of recompression during transmission from its source to the place of its consumption.

Extensive facilities have been provided, and additions thereto have been contemplated, for transporting natural gas over long distances, particularly by way of long cross-country pipe lines. Since the value of natural gas for use as fuel and for other purposes depends mainly on its hydrocarbonaceous constituents, including principally methane, and smaller proportions of other alkanes such as ethane, propane, butane and others, the removal of an incombustible constituent such as nitrogen, particularly before transmission of nitrogenbearing natural gases for long distances, has become an important consideration.

Besides hydrocarbons and nitrogen, natural gas coming from existing wells is known to contain hydrogen sulfide, carbon dioxide, helium and water vapor. It is customary to remove hydrogen sulfide and water from natural gas in the vicinity of its source, in view of the respective corrosiveness and hydrate-forming tendencies of these constituents. It is also common practice to remove and recover heavy and more readily condensable hydrocarbons. It has not been customary in actual practice to remove nitrogen, carbon dioxide and helium from natural gas before transmission through pipe lines to distant places of utilization.

Of the aforementioned latter three gases, nitrogen is found present in natural gas in much larger proporn'ons than either of the others. In some cases, the proportion of nitrogen is as high as eighty percent, depending on the location of a reserve. It has been recognized that the removal from natural gas of excess nitrogen (excess is considered by some authorities to be ten percent or more) should result in important benefits, and that its presence has certain objectionable features among which are the following:

A. It causes a reduction in heating value and may limit the marketability of the gas if the heating value is less than accepted standards for such fuel.

B. It may limit the separation and recovery of the heavier hydrocarbons from the gas for separate sale as natural gasolines and low pressure gas, because the removal of these heavier hydrocarbons causes a further reduction in heating value of the residual gas.

C. It imposes a burden on the gas transportation and distribution facilities by reducing the capacity of such facilities to transport maximum amounts of combustible gas.

D. It may reduce the efliciencies of operations such as carbon black production and liquefied gas storage for peak load demand.

The present invention includes improvements in processes whereby nitrogen is removed from nitrogenbearing natural gas with efiiciency and economy on a commercial scale. The invention further includes processes and means whereby advantages are gained which result from overcoming any one or more of the abovementioned objections. Not only does the removal of nitrogen result in an increase in fuel value of natural gas, there is also an improvement in the flow characteristics of the gas as a result of an appreciable reduction in the specific gravity and a decrease in viscosity. The net effect of removing nitrogen from natural gas is to increase substantially the volumetric deliverability, and consequently the B. t. u. deliverability, of a transmission system. Furthermore, it can be demonstrated that the economic benefits from nitrogen removal increase in nearly direct proportion to the percentage of nitrogen removed and the distance the gas is transported.

For the removal of nitrogen from nitrogen-bearing natural gas, nitrogen separated from the gas, in the present process, is effectively employed as a cooling medium for condensing out methane, and other hydrocarbons if present. During the condensation step which is conducted while maintaining the natural gas under pressure, uncondensed nitrogen separated from liquefied methane is expanded with performance of external work, and the required cooling is performed with the thus expanded nitrogen. Under conditions hereinafter indicated, no other cooling means and no extraneous refrigerants are found necessary. Furthermore the number of heat exchangers in the system may be reduced to a minimum.

In a more specific application of the invention to removal of nitrogen while natural gas is in transit in long pipe lines, advantage is taken of the use of the pressure employed to send the gas to its destination, as well as of the nitrogen content of the gas. Provision is made for adjustment of the proportion of nitrogen and for recycling thereof as needed while maintaining the pressure of the natural gas during separation of nitrogen therefrom. Not only is an appreciable saving of energy thereby realized but also a saving in the cost of equipment.

The nature of the invention is further described in detail hereinbelow by way of exemplification, and with reference to the accompanying drawing in which is shown diagrammatically a preferred arrangement of apparatus suitable for carrying out steps in the process claimed.

In the figure shown in the drawing, apparatus for continuous removal of nitrogen from natural gas is illustrated which comprises a heat exchanger 1, a condenser 2 provided with a liquid receiver 3, a turbine 4, and piping for the system described hereinbelow.

Gas purification means, represented by the rectangle 5, are provided for the pretreatment of natural gas, when necessary, for the removal of such constituents as carbon dioxide, water vapor, hydrogen sulphide, and the heavier hydrocarbons. Such constituents, when present, are each removed in a well-known or appropriate manner including physical and/ or chemical methods.

A pipe 6 is provided for conducting nitrogen-bearing natural gas under pressure from a source to the aforesaid pretreatment means 5. From thence, a pipe 7 conducts the pretreated gas to the warm end of the heat-exchanger 1. A pipe 8 and valves 9 and 10 are provided for bypassing the pretreatment means 5.

Pipes 7 and 8 are connected to piping 11 in the heatexchanger 1. A pipe 12 connects piping 11 to the liquid receiver 3 of the condenser 2. A pipe 13 is provided for conducting uncondensed gas out of the condenser tubes 14 of the condenser 2. The pipe 13 is connected to a pipe 15 provided with a valve 16, and to a pipe 17 provided with a valve 18. The pipe 15 is connected to piping 19 in the heat-exchanger 1. Pipe 17 and piping 19 are connected to a pipe20 which leads to the turbine 4. Expanded gas from the turbine 4 is conducted through a pipe 2.1ainto ,the cooling space .22 surrounding the condenser tubes 14. Apipe 23 connects the space 22 to apipe 24 provided with a valve 25, and to a pipe 26 provided with a valve 27. The pipe 26 connects the pipe 23 to the heatexchanger ll, :and ithe pipe .24 rconnects :the :pipe 23 ;to a heat-exhangerlfi. Azpipe I29 connects the exchanger .28 tothe. pipe 26.

.A (pipe 5.0 is provided for conducting condensate .from the receiver .8 of :the condenser 2 through tthe heatexchanger 28 to a pump :31. 'Ifhe delivery pipe .32 :ofthe pump 51 is connected to'piping 33 in theheat-exchanger 1 which 'in turn is connected to :a .pipe .54 ifOl' remitting punificd methanetrom :the; system.

pipe 4.0 connected to :the heat-exchanger 1 serves to conduct separated nitrogen :to recycling :means, and to .a pipe 41 :provided with a valve 4 2 which is opened when nitrogen siszemittedrfizomithe system. For recycling :nitrogen .through the system,;a :pipe 431havingra valve 44 .connects1the .pipe- 4t) =1t0 a compressor 45, and apipe 46 connests thecornpressor-ds rto'thezpipen. Apipe =47 leading to the .compressorliS and having a valve 48, .is provided for introducing nitrogen into the system :wh en needed for thepurposes set forth hereinbelow.

The above apparatus is PIOVidGd tO serve primarily as an auxiliary system for the treatment of natural gas to remove nitrogen. The system -is1brought into operation by :connecting :the .pipe *6 :to a source of nitrogen-bearing natural :gas iundersufiiciently 'high pressure. When coming direct from a well or reserve, the gas :usually requires a preliminary purification in which case it is subjected to treatment in purification means 5, under the incoming pressure, for the removal of such impurities as hydrogen sulphide, carbon :dioxide, and water, and for the separation-otheavier hydrocarbons.

In natural gas transmission systemsnow in use, the .gas arrives .at a recompression plant at a pressure of about 500 to 550 pounds per square inch and is recompressed to apressure ofabout 700-to 750 pounds per square inch. Prior :to recompressionin such transmission systems, the above-describedapparatus performs its function in a manner to beiexplained below.

:If the gas contains any one or more of the above-mentioned constituents which are to be removed first, if desired, the valve 9 is kept open and the valve 1 is shut. If the treatment of the'gas for the removal of such constituents is not considered necessary, the valve 9 is shut and the valve 10 in the pipe 8 is opened. Natural gas, preferably substantially free of the above-mentioned impurities and heavierhydrocarbons, passes at the incoming pressure through the piping 11 in the heat-exchanger 1 andsthrough-the pipe '12 to the receiver 3 of the condenser 2 wherein cooling is'efiected to condense preferably substantially all of the methane and other low-boiling hydrocarbons while under pressure of the incoming natural gas.

The nitrogengas whichseparates from liquefied methane passes out of the condenser tubes 14 and is conducted through the pipe '13. Part of the nitrogen passes successively-"through-pipes '15 and '19 to'pipe2tl, and partthrough pipe 1 7 topipe which leads to-theturbine 4 Where it is expanded to about atmospheric pressure with performance of external work. The resulting --cooled nitrogen passes through pipe 21 into the space 22 around the condenser tubes 14 to serve as the cooling medium for the natural gas. The valves i6 and 18 are adjusted so that the gas receives =t-he desired superheat in the heat-exchanger 1. It is preferred that the nitrogen remain in gaseous state in the ooolin-g'space in the condenser 2. It is also preferred that the temperature of the nitrogen be sufiiciently lowered not-onlytorthepurpose of eliminating practically all the --metha-ne from the gas in the condenser, but also to subcool liquid methane in the heatexchanger 28 and to maintain it in liquid 'form as it passes from the receiver 3 to thepurnp 31.

The subcooled methane, -or purified natural gas, is pumped by the pump 31 through the pipe'32 and piping 33 in heat-exchanger 1 from which it emerges in pipe 34 at about atmospheric temperature and is delivered to a transmission line.

The expanded nitrogen passing from the space 22 through pipe 23 is preferably divided into two streams one of which passes through the valve 25 in pipe 24 to the heat-exchanger 28 for the purpose indicated above, and the other of which passes through pipe 26. The part of the nitrogen serving to 'subcool the methane passes from the heat-exchanger 28 through the pipe 29 to join with the balance of expanded nitrogen from pipe .23 .passing through the valve '27. The valves 25 and 27 are adjusted to apportion the nitrogen for obtaining the desired srihcooling .inathe zheataexchanger 28.

The nitrogen from pipe 26 is brought into indirect heatexchange relationship with the nitrogen passing to the turbine 4 in piping 19; with the incoming natural gas in piping 11; and with the outgoing natural gas in piping 33. The nitrogen emerges from the exchanger lrinto the pipe 40 at about atmospheric temperature. Thenitrogenmay be recycled, :when required :in the system, through the valve 44 in :the rpipe=43 rconnectedto the compressor 45 which raises the ,pressure of the recycled nitrogen and feeds it through the pipe 46 into the pipe 6 at'substantially the tpressureof the incoming natural gas. if nitrogen .=is needed .in the system eat commencementof operations or during operation, :it :can .be admitted through pipe 47.. Nitrogen :that is .removed from the ;gas and that .is .not needed in ithe system can be withdrawn from the system through )the valve 42 in :pipe 41 and can be used in ammonia synthesis :and 'for other purposes.

.It is seen from the.:above:descriptionthat adjustment of the proportion of nitrogen in :the natural gas brought to the system .-can be readily made to .a preselected percent when necessary, or when desired, and for obtaining substantially complete removal of nitrogen. A maximum proportion of 35 percent nitrogen in'natural gas is .found to render theprocess selt-sufiicient. When .the nitrogen in the natural gas under treatment is present in .a :proportion .above the selected maximum, nitrogen that is separated from the ,gas in .the system .is not recycled, but is withdrawn from the system through the pipe 41. When the proportion of nitrogen in natural gas from a given source is less .-than the selected maximum .propor.tion,.adjustrnent .of the nitrogen content of such igasis made no this latter proportion as the gas :enters .thesystern at the .pipe :6. .Such .selected proportion .is substantially continuoushnrnaintainedifdesired, by regulation of valve 48 when necessary, and byregulation of the valves 42 and 44. Samples of the incoming natural gas .may be taken .at suitable intervals to ascertain the nitrogen content .of the :gas. The volumetric addition of nitrogen to be made to the gas :can :be determined with meters, as is well understood.

By way of further illustration, the .following specific example of lthe process is set .forth:

.Nitrogembearin-g natural .gas which has been preferably pretreated to remove, substantially completely, undesirable constituents other than .nitrogen .as referred to.above,.is passed into the .piping 111 in heat-exchanger 1 at a temperatureof about 127 C. and -.a:pressure of 500 to 550 pounds per square inch. ,If .thezincoming gas contains 15 per cent nitrogen, .additional nitrogen is mixed with .the gas to bring the nitrogen content :up to, for instance, 35 percent, .while maintaining .thetlattenpressure. The gas .at such pressure is cooled by.nitrogen entering .the condenser 2 at about .83 K. This nitrogen is the nitrogen vthatvis separated from .natural gas in the condenser .2, and that .is conducted through pipe 13, is provided with controlled superheat, and arrives at the turbine 4 at about .K. and about 500 to 550 pounds pressure, and is expanded to lower its temperature to the aforesaid'83' K. or preferably a little vaboveits boiling point, or substantially or materially below .the boiling point of .methane at the pressure of the natural The natural gas from which nitrogen is thus removed, passes out of the receiver 3'at about 180 K. under the initial pressure and in liquid form. This liquid is further cooled to about 165 K. by expanded nitrogen from the cooling space 22 of the condenser 2. The processed natural gas leaves the heat-exchanger 1 through pipe 34 at about 27 C. When the system is installed in a recompression plant, the pressure of the processed gas needs only to be raised from the pressure in the systern up to or preferably slightly above outgoing transmission line pressure.

The expanded nitrogen from the condenser 2 and heat exchanger 28 brought together in heat-exchanger 1, emerges from the latter exchanger at about 27 C. and is recompressed to the pressure of the incoming natural gas and recycled through the system, as it is needed, to bring the nitrogen content of the gas up to the preselected proportion or, as in this example, up to about 35 per cent by volume, or so that substantially one-third of the volume of the resulting mixed gases is nitrogen.

The invention hereinabove set forth is embodied in particular form and manner but may be variously embodied within the scope of the claims hereinafter made.

I claim:

1. In a process of removing nitrogen from nitrogenbearing natural gas, steps comprising while passing said natural gas under pipeline pressure through a gas separation system, adding nitrogen to said natural gas at said pressure to raise its nitrogen content from a given lower proportion to a higher preselected proportion to obtain upon subsequent separation and expansion of said nitrogen a lowering of its temperature materially below the boiling point of methane at said pressure, cooling the resulting mixture in said system at said pressure to liquefy substantially completely all methane while separating uncondensed nitrogen from said liquefied methane,-expanding said uncondensed separated nitrogen with performance of external work to lower the temperature of said separated nitrogen materially below the boiling point of methane as aforesaid and passing the expanded nitrogen into indirect heat-exchange relationship with said mixture to effect said cooling to liquefy said methane as aforesaid, and withdrawing substantially nitrogen-free natural gas from said system.

2. In a process of removing nitrogen from nitrogenbearing natural gas, steps comprising while passing said natural gas under pipeline pressure through a gas separation system, adding nitrogen to said natural gas at said pressure to raise its nitrogen content from a given lower proportion to a higher preselected proportion to obtain upon subsequent separation and expansion of said nitrogen a lowering of its temperature materially below the boiling point of methane at said pressure, cooling the resulting mixture in said system at said pressure to liquefy substantially completely all methane While separating uncondensed nitrogen from said liquefied methane, expanding said uncondensed separated nitrogen with performance of external work to lower the temperature of said separated nitrogen materially below the boiling point of methane as aforesaid and passing the expanded nitrogen into indirect heat-exchange relationship with said mixture to effect said cooling to liquefy said methane as aforesaid, compressing to said pipeline pressure expanded nitrogen coming from the aforesaid cooling step and adding it to natural gas entering said system to adjust its nitrogen content in the manner and for the purpose aforesaid, and withdrawing substantially nitrogen-free natural gas from said system.

3. In apparatus for separating nitrogen from nitrogenbearing natural gas, a heat-exchanger for precooling natural gas under superatmospheric pressure with expanded nitrogen separated from said gas, a condenser for separating said gas into liquefied methane and uncondensed nitrogen under said pressure, means for conducting natural gas from said heat-exchanger into said condenser, means for expanding said uncondensed nitrogen with production of external work, means for conducting said uncondensed nitrogen under said pressure from said condenser to said expanding means, said last-named conducting means including control means. for adjustably providing said uncondensed nitrogen with superheat, the said expanding means having gas-passage connection to the cooling space of said condenser for exclusive introduc-' tion of expanded gas from said expanding means into said cooling space, means for conducting expanded nitrogen from said cooling space of said condenser to said heatexchanger for precooling said natural gas, and means for withdrawing liquefied methane from said condenser.

4. In apparatus for separating nitrogen from nitrogen.- bearing natural gas, a heat-exchanger for precooling natural gas under superatmospheric pressure with expanded nitrogen separated from said gas, a condenser for separating saidgas into liquefied methane and uncondensed nitrogen under said pressure, means for conducting natural gas from said heat-exchanger into said condenser, means for expanding said uncondensed nitrogen with production of external work, means for conducting said uncondensed nitrogen under said pressure from said a condenser to said expanding means, said last-named conducting means including control means for adjustably providing said uncondensed nitrogen with superheat, said control means comprising a valve in said last-named conducting means for controlling flow of nitrogen gas directly to said expanding means and a valve-controlled pipe sec tion in said heat-exchangerfOr'bypassing a controlled portion of nitrogen through said heat-exchanger to said expanding means; means for conducting the expanded nitrogen from said expanding means into the cooling space of said condenser, the said cooling space having gas-entry connection exclusively for said expanded nitrogen; means for conducting expanded nitrogen from said cooling space of said condenser to said heat-exchanger for precooling said natural gas, means for withdrawing liquefied methane from said condenser, means for bringing a portion of said expanded nitrogen from said condenser into indirect heatexchange relationship with said liquefied methane from said condenser to subcool said liquefied methane, and means for conducting said liquefied methane from said last-named means through said first-named heat exchanger in indirect heat-exchange relationship with said expanded, nitrogen from said condenser.

5. In apparatus for separating nitrogen from nitrogenbearing natural gas, a heat-exchanger for precooling natural gas under superatmospheric pressure with expanded nitrogen separated from said gas, a condenser for separating said gas into liquefied methane and uncondensed nitrogen under said pressure, means for conducting natural gas from said heat-exchanger into said condenser, means for expanding said uncondensed nitrogen with production of external work, means for conducting said uncondensed nitrogen under said pressure from said condenser to said expanding means, said last-named conducting means including control means for adjustably providing said uncondensed nitrogen with superheat, means for conducting the expanded nitrogen from said expanding means into the cooling space of said condenser, the said cooling space having gas-entry connection exclusively for said expanded nitrogen; means for conducting expanded nitrogen from said cooling space of said condenser to said heat-exchanger for precooling said natural gas, means for withdrawing liquefied methane from said condenser, means for compressing expanded nitrogen to substantially the pressure of said natural gas entering said first-named heat-exchanger, and means for selectively adding the recompressed nitrogen to said natural gas for raising the nitrogen content of said gas to a preselected proportion.

6. In a process of removing nitrogen from nitrogenbearing natural gas at pipe line pressures, steps comprising while introducing into a condensing zone at substantially pipe line pressure, natural gas having less than about One thild *by volume of nitrogen, adding enough nitrogen at said pressure to increase the nitrogen content to substantially one-third; while substantially avoiding a drop -1npressure of the resulting mixture of gases separately removing liquid methane and nitrogen gas from said condensing zone and expanding said separated nitrogen With5performance-of external work to 'lower'the temperature of separated nitrogen below the 'boilingpoint of methane at said pressure, and passing'the 'expande'd'nitrogen into indirect heat-exchange relationship with -rri-ixed natural 'gas and nitrogen in said 'zoneto effect'liquefaction of methane-in themixtureg'thereby obtaining substantialiy nitrogen-free natural gas for transmission at "pipe line pressure.

"7. Apparatus for removing nitrogen from precompressed nitrogen-'bearingnaturalgas, said apparatus comprising a gas separation system, means for introducing said nitrogen-bearing natural gas into said system under incoming pipe line-pressure, said system including a condenser for separatingliquid methanefrom said gas at said incoming pipeline pressure, said condenser having a cooling space exclusively for cooling with expanded nitrogen separated from said gas; a pump for pumping separated methane at outgoing pipe line pressure, means for withdrawing iseparated nitrogen from said system, and a pump for separately returning separated nitrogen to said system at said incoming pipe line pressure for raising the nitrogen content of said gas to a preselected proportion.

8. Apparatus for removing "nitrogen from precompressed nitrogembearing natural gas in transmission for long distances from its source to a remote destination, said-apparatus comprising a gas'separationsystem, means for introducing said nitrogen-bearing natural gas into said system under pressure employed 'in 'said transmission, adjustable .means."for introducing nitrogen at said pressure for1admixture1with said gas 'to'adjust the proportion of said nitrogen to obtain upon'subsequent separation :and expansion of said nitrogen 'a lowering of its temperature below the boiling point of methane at said pressure, said system including a condenser "for separating liquid methane from s'aid gas at said pressure, said-'condenserhaving a cooling space 'forcoolin'g with expanded nitrogen separated from said gas, and means for Withdrawing separated methane fromv said system for delivery under pressure employed in said transmission.

References ,Cited inthe file. of this patent UNITED STATES PATENTS 1,212,455 Claude Jan. 16, 1917 15664412 Haynes Apr. 3, 1928 1,804,432 Pollitzer May "12, 1931 2,048,076 Linde July 21, 1936 2,209,748 Schlitt July 30,, T940 2,495,549 Roberts "Jan. '24, 1950 2,500,118 Cooper Mar. 7, 1950 2,500,129 Lavel'ty Mar. 7, 1950 2,504,051 "S'chiebel Apr. 11, 1950 '2,5'19;9'5'5 Deming Aug. 22, 1950 2,524,397 Roberts "Oct. '3, 1950 2,534,903 Etienne Dec. 19, i) 2,548,377 *Kapit-za Apr. 10, 1951 2,557,171 .Bodle June 19, 1951 2,658,360 Miller Nov. 10, 1953 677,945 Miller May 11, 1954

Claims (1)

1. IN A PROCESS OF REMOVING NITROGEN FROM NITRGENBEARING NATURAL GAS, STEPS COMPRISING WHILE PASSING SAID NATURAL GAS UNDER PIPELINE PRESSURE THROUGH A GAS SEPARATION SYSTEM, ADDING NITROGEN TO SAID NATURAL GAS AT SAID PRESSURE TO RAISE ITS NITROGEN CONTENT FROM A GIVEN LOWER PROPORTION TO A HIGHER PRESELECTED PROPORTION TO OBTAIN UPON SUBSEQUENT SEPARATION AND EXPANSION OF SAID NITROGEN A LOWERING OF ITS TEMPERATURE MATERIALLY BELOW THE BOILING POINT OF METHANE AT SAID PRESSURE, COOLING THE RESULTING MIXTURE IN SAID SYSTEM AT SAID PRESSURE TO LIQUEFY SUBSTANTIALLY COMPLETELY ALL METHANE WHILE SEPARATING UNCONDENSED NITROGEN FROM SAID LIQUEFIED METHANE, EXPANDING SAID UNCONDENSED SEPARATED NITROGEN WITH PERFORMANCE OF EXTERNAL WORK TO LOWER THE TEMPERATURE OF SAID SEPARATED NITROGEN MATERIALLY BELOW THE BOILING POINT OF METHANE AS AFORESAID AND PASSING THE EXPANDED NITROGEN INTO INDIRECT HEAT-EXCHANGE RELATIONSHIP WITH SAID MIXTURE TO EFFECT SAID COOLING TO LIQUEFY SAID METHANE AS AFORESAID, AND WITHDRAWING SUBSTANTIALLY NITROGEN-FREE NATURAL GAS FROM SAID SYSTEM.

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