US2658360A - Transportation of natural gas - Google Patents

Description

Nov. 10, 1953 B. MILLER 8, 6

TRANSPORTATION OF NATURAL GAS Filed May 8, 1946 MEIHANE ou'r I 27 I NITROGEN OUT I I I REMDVALOFHYDROGEN SULFIDE, l

CARBON DIDXIDEAND WATER I METHANE AND NITROGEN IN I a 3/ L 4 9.7 I l HEAT EXCHANGER I-- I I I a A r 7 I so I l 29. L I 7 L i J l l I ")"""""W HEAT EXCHANGER L26 i 1 26 F B f v 1 1 I I I e I I l 9 I HEAT EXCHANGER c l .N g i 14 i i i l. I I7 15 l l I I l EXPANDER 16 l I L- I I I I I I l /6 I I EXPANDER 9,1 I l 1 I l 1.- -J l INVENTOR. Belg'allzp'lc Miller. t BY 2 Xfionzeg Patented Nov. 10, 1953 TRANSPORTATION OF NATURAL GAS I Benjamin Miller, Ozone Park, N. Y., assignor to The Chemical Foundation, Incorporated, a corporationof New-York ApplicatiomMay s, 1946,-Serial:N.0..668, 094

3 Claims; (c1. 62175.5)

This'invention relates to the transportation of natural gas. The primary object of the invention is a material reductionin the cost of long-distance transportation of natural-gas fuel by removing from thenatural gas the non-combustible nitrogen, this removal being accomplished either in the producingfield or at a convenient inter? mediate'point'intransit'to the point of ultimate use as fuel.

The nitrogen content of natural gases varies over'a wide'range. Some contain so much nitrogen'as to be substantially non-combustible; others contain but a trace. A substantial portion of the natural gas used as fuel in locations hundreds of miles/from the producing wells contains oi the order of"5% to 20% of nitrogen. The treatment of such gas'is the particular object of this invention;

While'natural 'gas containing: 5% to 120% of nitrogen is usually a satisfactory fuel, the re-'- moval of the nitrogen will make it'better, because the'diluent nitrogen increases the heat 'lost by'escape'cf the combustion productsat a'temperature higher than the ambient; Removal of the nitrogen also improves the gas from the stand:- point of cost of distribution, and even more from the standpoint of cost'of transportation.

Where natural gas is used'ias'fuel, and by far the greatest portion of it is so used, the. important thing; is its heating value, the number of heatunits released by the combustion-of a unit quantity? this is usually-expressed in terms of British'thermal units (B.'t. u.) per standardcubic foot. If -a gas has a heating va'rue of 1000 B. t. u. per-standard cubic foot, and contains nitrogen, then removal of the nitrogen will increase the heating value to 1111 B. t. u. per cubic foot. Nine cubic feet of the denitrogenized gas will be required for actual use as fuel to take the place of ten cubic feet of theoriginal; Then the pipes through which the gas flows from. the point at which denitrogenization is accomplished to. the consuming burners can carry 4 more fuel value in the same number of cubic feet. But more importantly the capacity of the pipes will be increased even more. than 3? because the weight of a cubic foot of methane, the principal component of natural gas, is only 57% as great as the Weight of a cubic foot of nitrogen, and the carrying capacity of a long distance gas pipe-line is inversely proportional to the square root of the weight of a cubic foot of the gas flowing. A further advantage of denitrogenization is that the power required to compressthe gas at each compressing station will be lower.

-As a particular example there may beyconsidered a typical natural. gas from, thegreat Hugoton field which is located in the western portions of Kansas and Oklahoma and the'ex'etreme northwestern portion of Texas. After pass-'- ing through a natural gasoline plant for removal of. all hydrocarbons less volatilethan normal butane, a typical gas from the-Hugoton field would have the following analysis:

Component: Percent. ,by;.volume Methane I 75.72 Propane 2.54 Ethane 5.83 Iso-butane 0-;20 Normal butane 0.42 Nitrogen .15.29

A cubic'foot of gas--(measured-at'60 degrees Fahrenheitand- 14.735 pounds per square inch absolute) having this composition would weigh 0.05220 pound, and would have a heating-value of-958 B. t. u. Removal of the nitrogen-would leave a gas weighing only-0.04820 pound per cubic foot,- having a heating value of 1131 B. t: u. per cubic foot. The capacity of'a pipe-line for the denitrogenizedgas would be 23% greater than its capacity for the-=gasbefore denitrogenization, on aheating value basis. This increase in-capacity -of"23%' would be due to an increaseof 18% in theheatingvalue of each cubic foot'and to an increase of over 4% in the capacity on a cubic foot basis;

The method of separation which I employ involves the liquefaction of at least a substantial portion of the natural gas. While a process involving liquefactionof gases has been used extensively to prepare oxygen from air, and a'similar process has been-applied for the extraction of helium from natural gas; such proces'seshave required such costly equipment and-*have -been so costly to operate as to discourage consideration of liquefaction of natural gas as a part" of a process for removing nitrogen therefrom. But-I have found that nitrogen can beremoved' economically from natural gas intended to'be transported. by pipeline for a long distance by conducting the liquefaction and separation steps under a high pressurecomparable to thepressure .at which the gas is to betransported, and then expanding. theseparatedfnitrogen to obtain the necessaryrefrigerating. effect... Frequently the gas is availzibleirom the. producing .wells at a pressure high enough for the process, but in any case the pressure required for economical transportation by pipe-line is suflicient. In some cases the pipe-line pressure is higher than necessary for the separation process, although the pressure at which the gas is available from the wells is not so high as desired for the separation process. In such cases it is desirable to compress the gas to the separation pressure, make the separation, and then compress the denitrogenized gas to the pipeline pressure. While the separated nitrogen is expanded to obtain the refrigerating effect, the pressure of the denitrogenized gas is conserved as fully as possible, and may even be increased in some cases.

The separation of gases by liquefaction and fractionation usually requires considerable power; such power cost is usually the greatest cost involved. However, I may not require any power for the separation, in that my system will require no more powerln some cases less powerthan is required to conduct the transportation according to the method now employed. At present all of the gas must be com pressed to the transportation pressure, and as it travels through the pipe line the pressure is reduced, which means that recompression is required at intervals. For example, under present practice, the gas may come out of the well at a pressure of 425 pounds per square inch, and be allowed to flow through a pipe gathering system which connects many wells to a field station; it may reach that station at a pressure of the order of 350 pounds per square inch. It is then compressed to 750 p. s. i. and put into a pipe line. After traveling 75 miles the pressure may drop to about 600 p. s. i. The gas is then compressed again to about 750 p. s. i., and fed into the next section of line. This process is repeated approx imately every 75 miles, and the gas may be compressed a dozen times before it reaches its destination. In the novel system of the invention the gas would be compressed at the field station to about 450 p. s. i. It would then pass through the nitrogen removal plant and be separated into two partsone containing most of the nitrogen, and the other containing very little nitrogen, but most of the fuel value. The nitrogen-containing portion would be expanded to atmospheric pressure, thereby deriving most of the necessary net refrigerating effect. The fuel fraction would be delivered under pressure of 250-300 p. s. i., the pressure drop being incident to recovery of the regenerated portion of the refrigerating effect. Then only this portion would have to be compressed to 750 p. s. i. for transportation. If the gas contained nitrogen, only 80% 01 it would have to be compressed to 750 p. s. i. At each recompressing station there would be another sav- 111g in power, because the quantity of gas handled would be 20% less for a certain amount of heating eifect. The pressure loss in going through the nitrogen removal plant would require power to make up, but this would be far outweighed by the savings of power at each recompressing station.

The invention will be explained with the aid of the accompanying drawing, which is a flow sheet showing a preferred modification of the process. The natural gas may be first treated for the extracting of natural gasoline, butane, and propane; although it is not necessary that these less volatile hydrocarbons be completely removed, it is desirable that the amount of easily liquefiable material be small. Hydrocarbon extraction need not be more complete than would be practised in any event to avoid condensation in the pipe line. The extraction of natural gasoline may be effected by any conventional method, such as scrubbing the gas with absorption oil. There remains a mixture which is designated for convenience as methane and nitrogen, although minor amounts of ethane, propane, and butane may be present, and in some cases the total percentage of hydrocarbons other than methane may exceed the percentage of nitrogen. The mixture will generally contain water, frequently contain a small amount of carbon dioxide, and occasionally contain a small amount of hydrogen sulfide. The presence of hydrogen sulfide, carbon dioxide, or water is quite undesirable, so that the first step shown on the flow sheet is their removal. The removal may be by an convenient means, such as by scrubbing the gas with a solution of an amine, followed by passing the gas through a mass of activated alumina as carried. out for example in the extraction system l.

The mixture then passes through line 3 and heat exchanger A, where it is cooled. From heat exchanger A the mixture passes through the line 4, reboiler coil 5 of the Iractionator 6, and is further cooled. It then passes through line I and heat exchanger B, where it is further cooled, and partially liquefied. The mixture then passes through line 8, heat exchanger C and line 9, exchanger D, where it is further cooled, and more of it liquefied and thence passes into the fractionator 6 through line [0.

The fractionator is preferably a bubble tower type. At the bottom is a reboiler section II enclosing the reboiler coil 5; in the reboiler section liquid accumulates until it reaches the selected level. Then the liquid level controller I! allows liquid to leave the reboiler section and pass to heat exchanger B. Vapor from the top of the fractionator passes through line l3 and heat exchanger C where it is warmed. It then passes through line H to expander [5 where its pressure decreases, and it gives up mechanical energy to drive compressor IS. The expansion cools the vapor, and part of it may condense, but generally the exit pressure from expander I5 is controlled so that no condensation occurs therein. The cold vapor then passes through line H to reflux coll l8 where it is reheated. It then passes through line l9 and expands again in expander 20 where mechanical energy is abstracted to drive associated compressor 2|. The expansion cools the vapor and again some of the vapor may condense. The cooled vapor then passes through line 22 to the reflux coil 23 located in the upper section of the fractionating column. In passing through the coil the vapor absorbs heat and the heated vapors then pass through line 24 to the heat exchanger D, thence through the line 25 to heat exchanger B and then through line 26 to heat exchanger A. In this series of heat exchanges the vapor is progressively increased in temperature and is discharged through line 21 at approximately atmospheric pressure and at a temperature slightly lower than the temperature of the mixture which enters heat exchanger A through line 3.

The liquid which accumulates in the reboiler section of the fractionator and passes through line 28 to heat exchanger B, evaporates therein under a pressure somewhat lower than the pressure in the fractionator. The vapor formed as a result of this evaporation leaves heat exchanger B through line 29 and passes through heat exby-pass across each expander, so that isenthalpic expansion of part or all of the nitrogen stream may be employed. This is particularly useful when starting up the process, before any liquid has accumulated in the reboiler section, and until the liquid has risen to the selected level.

When starting up the process the stream leaving the top of the fractionator will have the same composition as the incoming mixture. As the gas expands, isenthalpically, isentropically or in an intermediate manner, its temperature drops. The

colder gas passes back in heat exchange relation with the incoming gas, and the temperature level decreases progressively until liquefaction starts, then more slowly until the equilibrium temperature level is reached. It is possible to shorten the starting period by the use of auxiliary refrigeration, as by cooling the incoming mixture by heat exchange with propane boiling at atmospheric pressure.

It will be understood that the fluids in the several parts of the equipment are at various temperatures. The incoming mixture will normally be at a temperature close to the prevailing atmospheric temperature, but in going through the process zones of lower temperatures are encountered. the lowest temperature being at the outlet of expander 20, where it may be of the order of 320 degrees Fahrenheit. It is therefore essential that the equipment be well insulated against heat leakage in from the ambient, and also to prevent heat exchange except where such is desired.

The nitrogen stream will usually be allowed to escape to the atmosphere, although there may be usese developed for it; however, Where the methane content of the nitrogen stream is high, the nitrogen stream may be used as fuel.

It will now be appreciated that the process of the invention insures many positive economical advantages. nitrogen in the gas enriches the gas with respect to its fuel value and concomitantly reduces power requirements for compression to the extent that the heavier nitrogen constituent is eliminated.

This advantage is a cumulative one since such lowered power requirements are reflected at each recompression stage in the entire system. As explained, additional economies may be achieved by utilizing the expansion of the nitrogen fraction to secure at least a part of the desired refrigeration.

I claim:

1. The method of preparing, from natural gas containing a substantial fraction of nitrogen, a gaseous fuel suitable for economical long distance pipe line transportation which comprises cooling a mixture derived from said natural gas and containing nitrogen and methane under a pressure in the range of 400 to 600 pounds per square inch to a temperature at which at least a substantial portion of the methane is liquefied, separating the mixture while substantially maintaining the pressure into a liquid portion in which the ratio of methane to nitrogen is substantially greater than it is in the mixture, and a vapor portion The described denuding of the 1 8 in which the ratio of methane to nitrogen is substantially less than it is in the mixture. evaporating the liquid in heat exchange relationship with another portion of the mixture while maintaining on the liquid a pressure not less than half of that maintained on the mixture and passing the vapor evolved by evaporation of the liquid in heat exchange relationship with another portion of the mixture while maintaining said last recited vapor under substantially the same pressure under which it was evolved, expanding the vapor in which the ratio of methane to nitrogen is substantially less than it is in the mixture under conditions which insure that the temperature of the vapor immediately after expansion is lower than the temperature of the vapor when it left the separation step, and passing the expanded vapor in heat exchange relationship with another portion of the mixture, the cooling oil the mixture taking place in at least three stages of progressively lower temperature, the expanded vapor supplying the cooling effect in the lowest temperature stage and the evaporating liquid supplying at least part of the cooling effect in an intermediate temperature stage.

2. The method of preparing, from natural gas containing a substantial concentration of N2, a gaseous fuel suitable for economical long distance pipe line transportation under substantial pressure which comprises cooling a mixture derived from said natural gas and containing nitrogen and methane while maintaining the pressure in excess of 400 pounds per square inch to liquefy at least a substantial portion of the methane, subjecting the mixture to fractionation under substantial superatmospheric pressure in the range 400-600 pounds per square inch to produce a liquid substantially free from nitrogen and a vapor containing substantially all of the nitrogen, expanding said nitrogen-containing vapor under conditions which insure that the temperature of the vapor immediately after expansion will be lower than the temperature of the vapor leaving the fractionation step. passing the expanded vapor in heat exchange relation with another portion of said nitrogen-containing vapor which has not yet been expanded to liquefy a portion thereof, utilizing the liquid formed in the last-recited step, as reflux in said fractionation step and pass ing the nitrogen-containing vapor separated from the reflux in heat exchanging relationship with another portion of the mixture whereby the said nitrogen-containing vapor is superheated prior to the said expansion.

3. The method of preparing, from natural gas containing a substantial concentration of N: a gaseous fuel suitable for economical long distance pipe line transportation under substantial pressure which comprises cooling a mixture derived from said natural gas and containing nitrogen and methane while maintaining the pressure in excess of 400 pounds per square inch to liquefy at least a substantial portion of the methane, subjecting the mixture to fractionation under substantial superatmospheric pressure in the range 400-600 pounds per square inch to produce a liquid substantially free from nitrogen and a vapor containing substantially all of the nitrogen, expanding said nitrogen-containing vapor under conditions which insure that the temperature of the vapor immediately after expansion will be lower than the temperature of the vapor leaving the fractionation step, passing the expanded vapor in heat exchange relation with another portion of said nitrogen-containing vapor which has not yet been expanded to liquefy a portion thereof, utilizing the liquid formed in the last-recited step, as reflux in said fractionation, expanding the vapor leaving the refluxforming step under conditions which insure that the temperature of the vapor immediately after expansion will be lower than the temperature of the vapor leaving the reflux-forming step, and passing such further expanded vapors in heatexchanging relationship with another portion of said nitrogen-containing vapor, which has not yet been expanded to liquefy a portion thereof, and utilizing the liquid formed in the last recited, step as additional reflux in said fractionation step.

BENJAMIN MILLER.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date Le Seur Feb. 19, 1901 Claude Jan. 16, 1917 Claude et a1 June 10, 1924 Haynes Apr. 3, 1928 Roberts Nov. '27, 1928 Gregory Feb. 13, 1934 Pellitzer June 28, 1938 Brewster Nov. 1, 1938 Barton Oct. 10, 1939 Ward et a1 Dec. 9, 1941 Gilmore July 12, 1949

Claims (1)

1. THE METHOD OF PREPARING, FROM NATURAL GAS CONTAINING A SUBSTANTIAL FRACTION OF NITROGEN, A GASEOUS FUEL SUITABLE FOR ECONOMICAL LONG DISTANCE PIPE LINE TRANSPORTATION WHICH COMPRISES COOLING A MIXTURE DERIVED FROM SAID NATURAL GAS AND CONTAINING NITROGEN AND METHANE UNDER A PRESSURE IN THE RANGE OF 400 TO 600 POUNDS PER SQUARE INCH TO A TEMPERATURE AT WHICH AT LEAST A SUBSTANTIAL PORTION OF THE METHANE IS LIQUEFIED, SEPARATING THE MIXTURE WHILE SUBSTANTIALLY MAINTAINING THE PRESSURE INTO A LIQUID PORTION IN WHICH THE RATIO OF METHANE TO NITROGEN IS SUBSTANTIALLY GREATER THAN IT IS IN THE MIXTURE, AND A VAPOR PORTION IN WHICH THE RATIO OF METHANE TO NITROGEN IS SUBSTANTIALLY LESS THAN IT IS IN THE MIXTURE, EVAPORATING THE LIQUID IN HEAT EXCHANGE RELATIONSHIP WITH ANOTHER PORTION OF THE MIXTURE WHILE MAINTAINING ON THE LIQUID A PRESSURE NOT LESS THAN HALF OF THAT MAINTAINED ON THE MIXTURE AND PASSING THE VAPOR EVOLVED BY EVAPORATION OF THE LIQUID IN

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