US3057706A

US3057706A - Adjusting the heating value and specific gravity of natural gas

Info

Publication number: US3057706A
Application number: US744595A
Authority: US
Inventors: Jr Walton H Marshall; Wilfred C Gains
Original assignee: Conch International Methane Ltd
Current assignee: Conch International Methane Ltd
Priority date: 1958-06-25
Filing date: 1958-06-25
Publication date: 1962-10-09
Anticipated expiration: 1979-10-09
Also published as: DE1089918B; NL240564A; ES250320A1; FR1230750A; BE579952A

Description

Oct- 9, 1 62 w. H. MARSHALL, JR, ETAL 3,057,706

ADJUSTING THE HEATING VALUE AND SPECIFIC GRAVITY OF NATURAL GAS Filed June 25, 1958 2 Sheets-Sheet l MATCH L/A/ifie. 1

INVENTORS ZlJaIian iifilm-sh alz l'n BY wiI/red 0. Gums 2 Sheets-Sheet 2 INVENTORS walion ff, 77Zars/zc ZZ,Jr. Z01 lfrcgfpains 0 z! WM YZZm K I fliprneys GRAVITY OF NATURAL GAS Oct. 9, 1962 w. H. MARSHALL, JR., ETAL ADJUSTING THE HEATING VALUE AND SPECIFIC Filed June 25, 1958 MATCH L MA.

United States Patent f 3,057,706 ADJUSTING TIE HEATING VALUE AND SPE- CIFIC GRAVITY OF NATURAL GAS Walton H. Marshall, Jr., Downings, Va., and Wilfred C.

Gains, Kansas City, Mo., assignors, by mesne assignments, to Conch International Methane Limited, Nassau, Bahamas, a corporation of the Bahamas Filed June 25, 1958, Ser. No. 744,595 16 Claims. (Cl. 48-196) This invention relates generally to improvements in the art of reforming natural gas, and more particularly, but not by way of limitation, to an improved method of adjusting the heating value and specific gravity of a natural gas containing a wide range of hydrocarbons.

As it is well known in the art, some localities have an abundant supply of natural gas, whereas other localities have very little or no natural gas resources. When such localities are reasonably close to one another, the natural gas may be transported from one locality to the other by means of a pipeline. However, when such localities are separated by long distances, or by substantial bodies of water, transportation by pipeline becomes impractical. A new industry is in the stage of development for the primary purpose of liquefying natural gas in a locality having an abundant supply and transporting the natural gas in liquefied form to a remote locality having a natural gas shortage. The liquefied natural gas occupies about the space required for natural gas in gaseous form, thereby making transportation over long distances practical. At the remote locality, the liquefied natural gas is revaporized and used as a fuel. In some of these remote localities, the natural gas can be revaporized and used directly as a fuel. In other localities, the gas specifications for existing equipment therein are different from the specifications of the revaporized natural gas, thereby requiring that the revaporized natural gas be reformed to a lower heating value and new specific gravity before use.

As it is also known in the art, natural gas usually contains a rather wide range of hydrocarbons when produced. Ordinarily, methane comprises the major proportion of the natural gas, with the heavier hydrocarbons, such as ethane, propane, butane and the like being present in the gas in minor proportions. Since the heating value and specific gravity of methane are nearer to the usual specifications for a gaseous fuel in the remote locality (compared with the heating value and specific gravity of the heavier hydrocarbons) it is ordinarily considered most desirable to remove the heavier hydrocarbons before liquefying and shipping the natural gas. However, the separation of the heavier hydrocarbons prior to liquefaction complicates the liquefaction operations. Also, we have found that the heavier hydrocarbons may be easily and economically separated when the liquefied natural gas is revaporized, and that the heavier hydrocarbons may be most economically utilized in the remote locality having a natural gas shortage.

The present invention contemplates a novel method of adjusting the heating value and specific gravity of a natural gas containing a wide range of hydrocarbons, wherein the heavier hydrocarbons are used in the formation of a carrier gas for subsequent blending with the methane component to produce a gaseous fuel having the required heating value and specific gravity. In the preferred embodiment, the heavier hydrocarbons used for forming the carrier gas are first oxidized by partial combustion with air. The products of combustion are subsequently reacted with water to form a carrier gas having a low 3,057,706 Patented Oct. 9, 1962 heating value and containing an appreciable proportion of carbon dioxide. The reactions are carried out in such a manner that the heat generated by the reactions is effectively used to preheat the reactants and provide an economical process. The carbon dioxide may be subsequently removed from the carrier gas prior to blending of the gas with the methane when it is desired to reduce the specific gravity of the resulting fuel. This invention further contemplates the formation of a carrier gas in combination with the separation of the heavier hydrocarbons from the methane in a liquefied natural gas feed stream, such that heat made available in the formation of the carrier gas may be effectively utilized in the separation of the heavier hydrocarbons and methane, and wherein work may be recovered from the revaporized methane.

An important object of this invention is to economize the liquefaction and subsequent conversion of natural gas to a gaseous fuel having a modified heating value and specific gravity.

Another object of this invention is to eificiently and economically adjust the heating value and specific gravity of natural gas.

A further object of this invention is to utilize at least a portion of the heavier hydrocarbons in a natural gas containing a wide range of hydrocarbons to form a carrier gas, which in turn may be used for adjusting the heating value and specific gravity of the methane component of the natural gas.

Another object of this invention is to provide an economical and efficient method of forming a carrier gas having a low heating value and an easily adjustable specific gravity.

Another object of this invention is to utilize the heat generated in the formation of a carrier gas for facilitating the separation of the heavier hydrocarbons from the methane of a liquefied natural gas feed stream, and for the revaporization of the methane component.

A still further object of this invention is to recover work in the revaporization and reforming of a liquefied natural gas to a lower heating value.

Other objects and advantages of the invention will be evident from the following detailed description, when read in conjunction with-the accompanying drawings which illustrate our invention.

In the drawings:

FIGURE 1 is a part of a flow diagram illustrating a' FIG. 1, reference character 4 designates a line leading from a suitable insulated storage tank (not shown) for transferring liquefied natural gas to a pump 6. The liquefied natural gas will normally be stored at about atmospheric pressure, or slightly above, and have a temperature of from 240 to 258 F., depending upon the composition of the gas. .For the purpose of the present description, it will be assumed that the liquefied natural gas being fed to the pump 6 has a temperature of -246 F. and the following analysis, although it will be understood that the cornposition' will vary, depending upon where the natural gas was produced,, and the following analysis is included herein only as an example.

The pump 6 forces the liquefied natural gas through a line 8 at increased pressure to the medial portion of a fractionating tower 10. The tower 10 may be of any desired construction which provides a vertically extending fractionating zone for separating the heavier hydrocarbons from the methane of the liquefied gas fed to the tower, and which will provide a revaporization of the methane, as will be more fully hereinafter set forth. The pump 6 substantially increases the pressure of the liquefied natural gas being fed to the tower 10, and preferably increases this pressure to about 585 p.s.i.g. Also, a pair of heat exchangers 12 and 14 are interposed in the line 8 between the pump 6 and the tower 10 for heating the liquefied natural gas being fed to the fractionating tower. The heat exchangers 12 and 14 receive suitable heating mediums, as will be hereinafter described, having sufficient heat to warm the liquefied natural gas to approximately its bubble point temperature prior to injection of the liquefied natural gas into the tower 10.

The liquefied natural gas is preferably slightly reduced in pressure as it enters the medial portion of the fractionating tower 10, as from 585 p.s.i.g. to 535 p.s.i.g., to facilitate the revaporization of the methane component and provide an upward fiow of the methane vapors into the upper end of the tower 10, with a simultaneous downward fiow of the heavier hydrocarbons into the lower portion of the tower 10. The contents of the lower end portion of the tower 10 are circulated through a reboiler 16, heated as will be hereinafter set forth, a maintain the contents of the lower portion of the tower 10 at a temperature which will induce the revaporization of any methane in the lower portion of the tower, with a consequent upward flow of the methane vapors into the upper portion of the fractionating tower. In the example being described, the reboiler 16 is heated to such an extent that the contents in the lower end portion of the tower 10 will be maintained at a temperature of about 135 F.

Simultaneously with the heating of the contents in the lower portion of the tower 10, the vapors collecting in the upper end portion of the tower are directed through a line 18 to the heat exchanger 12 interposed in the liquefied natural gas feed line 8. The temperature of the overhead vapors discharging from the tower 10 will be about 118 F. to provide a warming of the liquefied natural gas being passed through the heat exchanger 12. Also, of course, the methane enriched vapors being passed through the line 18 will be cooled by passage through the heat exchanger 12, with the temperature of these vapors being reduced to about 120 F. This cooling of the methane vapors will condense a portion of the vapors; therefore, the vapors passed through the heat exchanger 12 are directed through a line 20 to an accumulator 22 which also acts as a separator for collecting the condensate in the lower portion thereof. These condensates are in turn refluxed through a line 24 back to the upper section of the tower 10 to maintain the desired temperature in the upper portion of the tower 10 and facilitate the condensation of heavier hydrocarbons which may have been vaporized and gravitated unwardlv throu h the tower 10.

It should also be noted that the overhead vapors passed through the line 18 will be substantially all methane and contain only a minor proportion of the heavier hydrocarbons. With a feed gas composition as previously described, the overhead vapors discharging through the line 18 will be approximately comprised of 98.45 mol percent of CH; and 1.55 mol percent C H The condensate refluxed through the line 24 to the upper end of the tower 10 will have a slightly higher percentage of the heavier hydrocarbons, with a typical analysis being 97.81 mol percent CH6 and 2.19 mol percent C H On the other hand, the condensate in the lower end portion of the tower 10 will be substantially all heavier hydrocarbons, with a typical analysis being:

Composition: Mol. percent CH 1.0 C H 45.7 C H 32.1 iC H 7.7 1'1C H1o iC H 2.4 I1C5H12 c rt 1.2 C7H16 0.8

The heavier hydrocarbons in the lower end portion of the tower 10 are discharged from the tower through a line 26 for the formation of a carrier gas, as will be hereinafter described.

As previously indicated, the overhead methane enriched vapors from the tower 10 will be cooled to about -l20 F. by passage thereof in heat exchange relation with the liquefied natural gas being fed to the tower 10. After removal of the condensates from this vapor in the accumulator 22, the vapor is directed through a line 28 to a suitable expander 30 for recovering work from the revaporized methane. The expander 30 may take any desired form, such as a turbine, which will provide a workproducing zone through which the revaporized methane may be expanded to recover energy therefrom. The methane being fed to the expander 30 is passed through four heat exchangers 32 through 35 to raise the temperature of the methane vapors and increase the energy recovered by expansion of the vapors through the expander 30. Suitable heating mediums made available by the formation of the carrier gas, as will be hereinafter described, are passed through the

heat exchangers

32, 34 and 35; whereas the expanded methane vapors discharging from the expander 30 may be directed through a line 36 back through the heat exchanger 33.

The heat exchangers 32 through 35 are operated at such temperatures as to raise the temperature of the methane vapors to about 700 F., with the pressure of the methane vapors being about 525 p.s.i.g. prior to expansion thereof. The pressure of the methane vapors is decreased to about p.s.i.g. by passage through the expander 30, with the temperature being decreased proportionately. However, the temperature of the expanded vapors discharging from the expander 30 through the line 36 will be at a higher temperature than the methane vapors as they are passed from line 28 through the heat exchanger 33, such that the expanded vapors may be used as a heating medium for the methane vapors prior to expansion. The expanded methane vapors discharging through the line 36 from the heat exchanger 33 are at a temperature of about 100 F. and a pressure of about 100 p.s.i.g. for subsequent use thereof as a fuel.

The liquefied heavier hydrocarbons discharging from the lower end of the fractionating tower 10 are expanded through a suitable expansion valve 38 to at least partially revaporize the heavier hydrocarbons. The combined vapors and liquid are then directed through a line 40 to a heat exchanger 42 as illustrated in FIG. 2. It should also be noted that the expansion valve 38 reduces the pressure of the heavier hydrocarbons to about 185 p.s.i.g. The heavier hydrocarbons passing through the heat exchanger 42 are further heated, preferably to a temperature of about 147 F., and will have a pressure of about p.s.i.g. The revaporized heavier hydrocarbons may then all be used to form the carrier gas, or a portion thereof may be blended back with the revaporized methane in the line 36 through a by-pass line 44.

i As will be apparent to those skilled in the art, the amount of the heavier hydrocarbons needed for forming the carrier gas will depend upon the ultimate desired characteristics of the fuel gas. Therefore, a variable proportion, from Zero to the major portion of the heavier hydrocarbons, may be by-passed through the line 44 for mixing with the methane in the line 36, depending upon the initial composition of the liquefied natural gas, as Well as the desired final characteristics of the fuel gas. In the example being described, slightly more than one-half of the heavier hydrocarbons are by-passed through the line 44 back to the line 36. With 255,398 pounds per hr. of liquefied natural gas having the previously described analysis being fed to the tower 10, and with the ultimate heating value of the fuel gas being set at 540 B.t.u. per cu. ft., about 67,037 lbs. per hr. of the heavier hydrocarbons are by-passed through the line 44; Whereas about 61,979 lbs. per hr. of the heavier hydrocarbons are fed on through the line 40 for use in forming the carrier gas.

The revaporized heavier hydrocarbons to be used in formation of the carrier gas are passed from the line 40 through a preheater 46 for raising the temperature of these vapors and facilitating subsequent oxidation of the vapors with air in suitable partial oxidation furnaces 48. In the example being considered, the revaporized heavier hydrocarbons discharging through the line 50 from the preheater 46 to the furnaces 48 is at a temperature of about 1000 F. The preheater 46 may conveniently be in the form of a gas heater to utilize a portion of the natural gas being handled.

The air being used for combustion in the furnaces 48 is provided by a suitable compressor 52 having a suitable filter 54 on the intake thereof as illustrated in FIG. 1. The air discharging through the line 56 from the compressor 52 is preferably at a pressure only slightly above atmospheric, such as 2 p.s.i.g., with a temperature of about 100 F., depending somewhat upon atmospheric conditions. In order to reduce the horsepower required in further compressing the air, we prefer to remove moisture and cool the air. Therefore, the air is directed by the line 56 into the lower section of a glycol contactor 58. The contactor 58 may be of any desired construction which will provide an intimate contact of the air flowing upwardly through the tower with cold glycol flowing downwardly through the tower to provide a substantial drying and cooling of the air passed through the contactor. The air discharging from the upper end of the contactor 58 is directed through another line 60 to the lower end of a second glycol contactor 62. An intermediate stage compressor 64 is interposed in the line 60 to increase the pressure of the air entering the second contactor 62. Glycol is circulated through the

contactors

58 and 62 as will be described below.

The second contactor 62 completes the further removal of Water from the air and discharges the essentially moisture-free air through a line 66 for further compression by a compressor 68 before being fed to the partial oxidation furnaces 48. The air is first, however, preferably preheated to the same temperature as the heavier hydrocarbons fed to the furnaces 48. Therefore, the air in the line 66 is forced by the compressor 68 partially through a line 70 to a heat exchanger 72 heated by the products of combustion, as Will be described. However, the major portion of the air may be by-passed through a line 74 around the heat exchanger 72 to combine with the partially preheated air at the inlet of a preheater 76. The preheater 76 may be of any desired type, such as a gas fired heater, to heat the air to a temperature corresponding to the temperature of the heavier hydrocarbon vapors in the line 50, with the heated air being discharged through a line 78 to combine with the revaporized heavier hydrocarbons in the line 50 upstream of the furnaces 48. As previously noted, the temperature Composition: Mol. percent CH 0.07 H 24.10 CO 20.06 H O 4.51 N 50.17 CO 1.09

The intermediate gas (the products of combustion) discharging from the furnaces 48 will have a temperature of about 2500 F. and is directed through a line 80 to the heat exchanger '72 for preheating a portion of the air as previously described. However, prior to flowing through the heat exchanger 72, the intermediate gas in the line 80 is quenched with water directed through a line 82 into the line 80. This water may be obtained from any desired source, and is forced through the line 82 by a pump 84 at a pressure of about 200 p.s.i.g. Also, it is preferred that the water pumped through the line 82 have a temperature of about 235 F. After passage through the heat exchanger 72, the intermediate gas is further quenched with water from a line 86 leading from the line 82, before the intermediate gas is fed into the lower section of a gas scrubber 88. An excess of water is mixed with the intermediate gas in the line 80 to substantially cool the intermediate gas being fed to the scrubber 88.

Water is recirculated through the gas scrubber 88 by means of a line 90 and pump 92 to further cool the intermediate gas and remove a portion of the water which was used to quench the intermediate gas. The excess water obtained by operation of the scrubber 88 may be drained through a line 94 to a suitable disposal point (not shown). The intermediate gas discharging through a line 96 from the top of the gas scrubber 88 is at about 135 p.s.i.g. and 300 F., and has the following analysis.

From the above analysis it will be observed that the intermediate gas in discharging from the gas scrubber has substantial proportions of carbon monoxide and water.

The intermediate gas in the line 96 is fed to a series of shift converters 98, as illustrated in FIG. 1, to produce the carrier gas. The major portion of the intermediate gas in the line 96 is directed *by a by-pass line 100 through a heat exchanger 101 for heating this portion of the intermediate gas before the gas is fed to the shift converters 98. The heat exchanger 101 is heated by the products discharging from the shift converters 98, such that a substantial proportion of the intermediate gas is prelgesated prior to reaction thereof in the shift converters In the shift converters 98, a catalytic reaction takes place, principally between carbon monoxide and water, to produce carbon dioxide and hydrogen in accordance with the reaction: CO+H O CO +H The reaction is an exothermic reaction which raises the temperature of the gases in the converters to about 840 F., and it is promoted by the use of such catalysts as brown iron oxide. The following is a typical analysis of the composition of the gas leaving the shift converters 98 through the line 102.

Composition: Mol. percent CH 0.04 H 22.07 CO 1.05

H O 40.55 N 26.27 co 10.02

through the heat exchanger 35 is at a sutficiently high heat level to heat the methane vapors to about 700 F. for expansion through the expander 30, as previously described. After passage through the heat exchangers 101 and 35, the carrier gas is directed through a line 108 to the heat exchanger 34, which is also used to heat the methane vapors flowing to the expander 30. The carrier gas directed through the heat exchanger 34 will be at a temperature level between the carrier gas directed through the heat exchanger 35 and the expanded methane vapors being directed through the heat exchanger 33 to provide a progressive heating of the methane vapors being fed to the expander 30. The temperature of the carrier gas being directed through the heat exchanger 34 will be at about 405 F.

The carrier gas discharging from the heat exchanger 34 is directed (see FIG. 2) into the lower section of a secondary gas scrubber 110. The scrubber 110 may be any desired construction which will provide a removal of water and a reduction in temperature of the carrier gas flowing upwardly through the scrubber by water flowing downwardly through the scrubber. The water used in the scrubber 110 may be fed to the upper section of the scrubber by a line 112 leading from the water line 82 previously described. The water collecting in the lower end of the scrubber 119 is discharged through a line 114 to a steam drum 116. This water will be at a temperature of about 275 F. and a pressure of about 110 p.s.i.g. upon being flashed into the steam drum 116. The steam drum 116 is preferably operated at a pressure of about 8 p.s.i.g. to provide the conversion of a portion of the water to steam in the steam drum. The condensate in the drum 116 remaining after flashing is directed through a. line 118 to the inlet of the pump 84 which supplies water to the water line 82. The pump 84 increases the pressure of the water fed through the line 118 to about 200 p.s.i.g. for recirculation through the scrubber 110, as well as for quenching the intermediate gas in the line 80 as previously described. In the example disclosed, the major portion of the water flowing through the line 82 is obtained from the line 118, such that the temperature of the water in the line 82 may be easily retained at about 235 F. Such make-up water as is necessary is fed to the pump 84 through another line 120 from a suitable source of supply (not shown).

The carrier gas discharging from the upper end of the scrubber 110 through a line 122 has a low heating value by comparison with the methane gas and may (after removing the remaining water therefrom) be used for blending with the methane and heavier hydrocarbon vapors in the line 36 to form a fuel gas having a heating value lower than the heating value of the original natural gas. The carrier gas in the line 122 contains a substantial amount of carbon dioxide, which provides the carrier gas with a specific gravity normally higher than the specific gravity of the original natural gas and which, in some cases, would provide the resulting fuel gas with a specific gravity higher than the specifications for the equipment in the locality where the fuel gas is to be burned. Therefore, when a lower specific gravity is required, we prefer to direct the carrier gas from the line 122 into the lower section of a hot carbonate absorption tower 124 for the removal of carbon dioxide by a hot carbonate system, as will be hereinafter described. It will be understood, how ever, that the CO may be removed by use of an amine system, water scrubbing, or the like. The carrier gas discharging from the upper end of the absorption tower 124 has a temperature of about 235 F. and is directed through a line 126 back to the reboiler 16 of the fractionating tower 10, as illustrated in FIG. 1. As previously described, this hot gas forms the heating medium for the reboiler 16 to maintain the desired temperature for the contents in the lower end of the fractionating tower 10. The carrier gas is directed from the reboiler 16 back through a line 128 to the heat exchanger 42 (FIG. 2) utilized for revaporizing the heavier hydrocarbons prior to utilization of the heavier hydrocarbons in the formation of the carrier gas.

Cooling of the carrier gas in the reboiler 16 and heat exchanger 42 condenses at least a portion of the H 0 therein. Therefore, the carrier gas is then directed on through the line 128 to a separator 130. The collected water is discharged from the lower end of the separator 130 through a line 132 to a suitable disposal point (not shown). The remaining vapor passing through the separator 130 is discharged through a line 134 to a second separator 136 for a substantially complete removal of water from the gas. It is also desirable to interpose a heat exchanger 138 in the line 134 to cool the gas being fed to the second separator 136 and assure the removal of a substantial portion of the water from the gas. The heat exchanger 138 may be cooled by water, since the carrier gas being fed to the second separator 136 is at a temperature of about 200 F. The water separated in the separator 136 is discharged through a line 140 to a suitable disposal point (not shown).

The gas remaining after separation of the water is directed through a line 142 to the line 36 for blending with the revaporizcd methane (and heavier hydrocarbons previously mixed with the methane vapors) to form the final fuel gas. In a typical system, the fuel gas in the line 36 downstream of the connection with the line 142 will have a heating value of about 540 B.t.u. per cu. ft. and a specific gravity of 0.6, which are the gas specifications for the majority of existing gas fired appliances in areas formerly served by manufactured gas.

As previously indicated, the glycol contactors 58 and 62 (FIG. 1) are operated by a closed glycol cycle to provide an eflicient removal of moisture from the air being used to oxidize the heavier hydrocarbons in the formation of the carrier gas. The glycol is stored in a surge drum 144 and is pumped into the upper sections of the

containers

58 and 62 by suitable pumps 146 and 148. The major portion of the glycol is pumped through a line 150 by the pump 146 into the upper section of the glycol contactor 58 at a pressure of about 30 p.s.i.g. to provide a low pressure operation for the contactor 58 and the removal of the major portion of the moisture from the air while the air is passing through the contactor 58. A minor portion of the glycol is pumped through a line 152 by the pump 146 at a pressure of about 80 p.s.i.g. into the upper section of the higher pressure contactor 62.

It may also be noted that the glycol being fed to both of the

contactors

58 and 62 is at a temperature of about 25 R, such that the air passing through the contactors will heat the glycol to about 80 F., and the air will be cooled to reduce the horsepower required to compress the air in the compressors 64 and 68, as previously described. The glycol leaving the lower end of the contactor 58 is forced by a pump 154 through a line 156 back to the heat exchanger 32 used in initially heating the methane vapors being fed through the line 28 to the expander 30. Also, the glycol leaving the lower end of the contactor 62 is fed through a line 158 into the line 156 to provide a passage of the major portion of the glycol through the heat exchanger 32. As previously noted, the glycol at this point in the system will be at a temperature of about 80 F. to provide the initial warming of the methane vapors flowing to the expander 30. The glycol is then directed on through the line 156 for passage through the heat exchanger 14 utilized in warming the liquefied natural gas being fed through the line 8 to the fractionating tower It). The temperature of the glycol entering the heat exchanger 14 will be at about 62 F. to provide a substantial transfer of heat to the liquefied natural gas feed stream, such that the liquefied natural gas will be raised to approximately its bubble point temperature before being fed to the fractionating tower 10 as previously described. The glycol leaving the heat exchanger 14 will be at a temperature of about 25 F. and is directed on through the line 156 back to the glycol surge drum 144 to complete the cycle.

As will be apparent, the glycol will pick up a substantial amount of moisture by passage through the

contactors

58 and 62. Therefore, a minor portion of the glycol being pumped through the line 156 is by-passed through a line 160 to a glycol still 162. The glycol flowing through the line 169 is preheated in a heat exchanger 164 by the relatively pure glycol discharging from the still 162, as will be described. The still 162 operates in the usual manner to provide an upward flow of water vapor to the upper section of the still and the downward flow of relatively pure glycol into the lower section of the still. Vapor is Withdrawn from the upper section of the still through a line 166 and passed through a water cooler 168. A portion of the cooled H O directed through the cooler 168 is refluxed back to the upper section of the still for maintaining the upper section of the still at the desired temperature, while the remaining cooled H O is directed to a suitable disposal point (not shown).

The contents in the lower end of the still 162 are circulated by a pump 170 partially through a line 172 into a reboiler 174, and partially through a line 176 back through the heat exchanger 164 to join with the glycol in the line 156. The glycol being directed through the reboiler 174 is maintained at a temperature of about 285 F. and is recirculated to the lower section of the still to maintain the lower section of the still at the desired temperature. It will be apparent that the glycol forced by the pump 170 through

lines

172 and 176 will be substantially pure, such that the glycol cycle will not become over-saturated with water.

As previously indicated, carbon dioxide is removed from the carrier gas flowing through the absorption tower 124 (FIG. 2) by a hot carbonate cycle. The carbonate leaving the lower end of the absorption tower 124 is directed through a line 178 to the upper section of a hot carbonate stripper 180 for removal of carbon dioxide from the hot carbonate. Steam generated in the steam drum 116 is directed through a line 182 to the lower section of the stripper 180 for counter-flow with the hot carbonate fed to the upper section of the stripper. The hot carbonate accumulating in the lower section of the stripper 180 is forced by a pump 184 through a line 186 back to the upper section of the absorption tower 124 to complete the hot carbonate cycle.

The steam and separated carbon dioxide accumulating in the upper end of the stripper 180 are directed through a line 138 to a suitable separator 190. The carbon dioxide and steam being fed to the separator 190 are preferably cooled by a heat exchanger 192 to facilitate the condensation of the steam and the efficient separation of the CO and H in the separator 190. The heat exchanger 192 may be cooled by water, since the temperature of the carbon dioxide and steam fed to the exchanger 192 is at a temperature of about 235 F. Condensate collecting in the lower end of the separator is discharged through a line 194 to a suitable disposal point (not shown). The carbon dioxide collecting in the upper end of the separator 190 is discharged through a line 196 to a suitable receiver (not shown) for use in any desired manner. Utilization of the separated carbon dioxide forms no part of the present invention and will therefore not be described herein.

From the foregoing it will be apparent that the present invention provides a novel method of adjusting the heating value and specific gravity of a natural gas containing a range of hydrocarbons. The heavier of the hydrocarbons are separated from the remaining gas and reformed to a carrier gas having a low heating value and substantially any desired specific gravity. The reforming of the heavier hydrocarbons is accomplished by first partially oxidizing the heavier hydrocarbons to an intermediate gas, and then the intermediate gas is reacted with water to provide a carrier gas having a drastically reduced heating value. The oxidation and reaction of the intermediate gas is accomplished by the utilization of the maximum amount of heat in the products for preheating the reactants in both steps of the reforming operation.

It will also be apparent that the present invention provides a novel method of separating the heavier hydrocarbons from methane in a liquefied natural gas as in a simple and economic manner, such that a portion of the heavier hydrocarbons may 'be efliciently utilized to form a carrier gas for blending with the remaining natural gas in the formation of a fuel gas having the desired heating value and specific gravity. The separation of the heavier hydrocarbons from the methane in the liquefied natural gas is facilitated by utilizing the heat generated in the conversion of the heavier hydrocarbons to a carrier gas, and the revaporized methane may be used in the recovery of work in a system utilizing the present invention, with heat generated in the conversion of the heavier hydrocarbons being used to heat the revaporized methane and increasing the amount of work which may be recovered from the revaporized methane. It will be further apparent that economic utilization of the heavier hydrocarbons in a liquefied natural gas will simplify the liquefaction of the gas to provide the maximum in economy of liquefaction, transportation and reforming of natural gas for use in remote localities having a deficient natural gas supply.

Changes may be made in the combination and arrangement of steps and procedures, as well as apparatus, as heretofore set forth in the specification and shown inthe drawings, it being understood that changes may be made in the precise embodiment disclosed without departing from the scope and spirit of the invention as defined in the appended claims. For example, the separation of the heavier hydrocarbons from the methane in a natural gas may be accomplished separately from the conversion of the heavier hydrocarbons to a carrier gas. In other words, a liquefied natural gas may be revaporized and/ or separated into heavier and lighter hydrocarbons in one area, and then transported in gaseous form by pipeline or the like to a separate locality Where the reforming of the natural gas to a lower heating value is accomplished. Also, the liquefied natural gas may be revaporized and separated into heavier and lighter hydrocarbons in any desired manner, as by successively flashing the natural gas in such a manner that the components of the gas are progressively separated. Furthermore, when the natural gas contains an insufficient amount of heavier hydrocarbons for the formation of the carrier gas, a portion of the methane may be by-passed and used with the heavier hydrocarbons for this purpose.

We claim:

1. In a method of reducing the heating value and adjusting the specific gravity of a natural gas composed mostly l l of methane and containing a minor proportion of heavier hydrocarbons, the steps of:

(a) separating at least a portion of the heavier hydrocarbons from the remainder of the natural gas,

(b) partially burning the separated heavier hydrocarbons with air to produce an intermediate gas comprising, principally, H C and N and minor proportions of the heavier hydrocarbons, H 0 and C02:

(0) reacting the intermediate gas with H O to produce a carrier gas comprising, principally, H H O, N and CO and minor proportions of the heavier hydrocarbons and CO,

(d) separating at least the major portion of the H 0 from the carrier gas, and

(e) blending the remaining carrier gas with said remainder of the natural gas.

2. The method defined in claim 1 characterized further in separating the CO from the carrier gas before blending said remaining carrier gas with said remainder of the natural gas.

3. The method defined in claim 1 characterized further in drying, cooling and then compressing the air prior to combustion thereof with the heavier hydrocarbons.

4. The method defined in claim 1 characterized further in preheating the air and heavier hydrocarbons prior to combustion thereof.

5. The method defined in claim 1 characterized further in that at least part of the air is preheated prior to combustion thereof by passage in heat exchange relation with the intermediate gas.

6. The method defined in claim 1 characterized further in that the heavier hydrocarbons are at least partially preheated prior to combustion thereof by passage in heat exchange relation with the carrier gas.

7. The method defined in claim 1 characterized further in that the air and heavier hydrocarbons are preheated to about 1000 F. prior to combustion thereof, and said combustion is continued until the temperature of the intermediate gas is about 2500 F.

8. The method defined in claim 1 characterized further in that the intermediate gas is quenched with H 0 and cooled to about 300 F. prior to the reaction of the intermediate gas with the H 0.

9. In a method of preparing a liquefied natural gas composed mostly of methane and containing a minor proportion of heavier hydrocarbons for use as a fuel, the steps of:

(a) revaporizing the liquefied natural gas,

(b) separating at least a portion of the heavier hydrocarbons from the remainder of the natural gas,

(c) partially burning the separated heavier hydrocarbon with air to produce an intermediate gas comprising, pricipally, H CO and N and minor proportions of the heavier hydrocarbons and CO 12 (d) reacting the intermediate gas with H O to produce a carrier gas comprising, principally, H H O, N and CO and minor proportions of the heavier hydrocarbons and CO,

(e) separating at least the major portion of the H 0 from the carrier gas, and

(f) blending the remaining carrier gas with the remainder of the natural gas.

10. The method defined in claim 9 characterized further in feeding the liquefied natural gas into a fractionating tower for separation of the heavier hydrocarbons and simultaneously revaporizing the remainder of the natural gas.

11. The method defined in claim 10 characterized further in that the liquefied natural gas is fed into the central portion of the fractionating tower, and reboiling the contents of the lower portion of the tower by passage thereof in heat exchange relation with the carrier gas.

12. The method defined in claim 10 characterized further in passing the overhead vapors from the fractionating tower in heat exchange relation with the liquefied natural gas being fed to the tower, removing condensates from said overhead vapors and refluxing said condensates to the upper portion of the fractionating tower.

13. The method defined in claim 10 characterized further in compressing the liquefied natural gas upstream of the fractionating tower.

14. The method defined in claim 13.characterized further in expanding the overhead vapors from the fractionating tower through a work-producing zone.

15. The method defined in claim 14 characterized further in drying the air by contact with glycol, heating the glycol to remove moisture therefrom, and passing the heated glycol in heat exchange relation with the overhead vapors from the fractionating tower prior to expansion thereof, and then passing the heated glycol in heat exchange relation with the compressed liquefied natural gas.

16. The method defined in claim 14 characterized further in heating the overhead vapors before expansion thereof by passing said vapors in heat exchange relation with the carrier gas.

References Cited in the file of this patent UNITED STATES PATENTS 1,918,254 Faber July 18, 1933 1,926,170 Oberfell et al Sept. 12, 1933 2,177,068 Hutchinson Oct. 24, 1939 2,383,715 De Jahn Aug. 28, 1945 2,465,235 Kubicek Mar. 22, 1949 2,537,708 Scharmann Ian. 9, 1951 2,568,351 Milbourne Sept. 18, 1951 2,671,718 De Coriolis Mar. 9, 1954 2,795,559 Whaley June 11, 1957

Claims

1. IN A METHOD OF REDUCING THE HEATING VALUE AND ADJUSTING THE SPECIFIC GRAVITY OF A NATURAL GAS COMPOSED MOSTLY OF METHANE AND CONTAINING A MINOR PROPORTION OF HEAVIER HYDROCARBONS, THE STEPS OF: (A) SEPARATING AT LEAST A PORTION OF THE HEAVIER HYDROCARBONS FROM THE REMAINDER OF THE NATURAL GAS, (B) PARTIALLY BURNING THE SEPARATED HEAVIER HYDROCARBONS WITH AIR TO PRODUCE AN INTERMEDIATE GAS COMPRISING, PRINCIPALLY, H2, CO AND N2 AND MINOR PROPORTIONS OF THE HEAVIER HYDROCARBONS, H2O AND CO2, (C) REACTING THE INTERMEDIATE GAS WITH H2O TO PRODUCE A CARRIER GAS COMPRISING, PRINCIPALLY, H2, H2O, N2 AND CO2, AND MINOR PROPORTIONS OF THE HEAVIER HYDROCARBONS AND CO, (D) SEPARATING AT LEAST THE MAJOR PORTION OF THE H2O FROM THE CARRIER GAS, AND (E) BLENDING THE REMAINING CARRIER GAS WITH SAID REMAINDER OF THE NATURAL GAS.