Processing liquefied natural gas

Sept. 20, 1960

W. H. MARSHALL, JR

2,952,984

PROCESSING LIQUEFIED NATURAL GAS

Filed June 23, 1958

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ATTORNEYS

2,952,984

Patented Sept. 20, 1960

United States Patent Office

2,952,984

PROCESSING LIQUEFIED NATURAL GAS Walton H. Marshall, Jr., Downings, Va., assignor, by mesne assignments, to Conch International Methane Limited, a corporation of the Bahamas

Filed June 23,1958, Ser. No. 743,732

Claims. (Cl. 62—27)

This invention relates generally to improvements in the art of distributing and processing natural gas, and more particularly, but not by way of limitation, to an improved method of preparing liquefied natural gas for use as a fuel, or partly for use as a fuel and partly for use as raw material in the petrochemical industry.

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 pipeine. 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 %oo 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. 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. At least some of these heavier hydrocarbons can be separated before the natural gas is liquefied and transported to the remote localities. However, it is sometimes desirable to liquefy the heavier hydrocarbons along with the methane and transport the entire natural gas composition to the remote locality in liquid form. These heavier hydrocarbons have two important uses at the remote localities. Firstly, when the natural gas must be reformed to a lower heating value and new specific gravity to meet existing equipment specifications, the heavier hydrocarbons may be conveniently used in the formation of a carrier gas having a lower heating value and new specific gravity, with the carrier gas being subsequently mixed with methane to form the gaseous fuel having the desired heat content and specific gravity specificaitons. Secondly, the heavier hydrocarbons may be economically used as a raw material in the petrochemical industry in the remote locality.

The present invention contemplates a novel method of separating the methane and heavier hydrocarbons in liquefied natural gas, with a simultaneous revaporization of the methane. The revaporization and separation are accomplished in a fractionating zone in such a manner that a portion of the heat in vapors withdrawn from the fractionating zone may be efficiently utilized in heating the liquefied natural gas fed to the fractionating zone, and the resulting vapors will be extremely rich in methane, i.e., containing a very minor proportion of the heavier hydrocarbons. Condensate formed in said vapors by passage in heat exchange relation with the liquefied natural gas are refluxed to the fractionating zone to enhance the separation of methane in the fractionating zone. The present method also contemplates the recovery of work from the separated methane, as well as the utilization of 5 heat from the expanded methane vapors for warming methane vapors being fed to the work-producing zone, such that the maximum energy of the separated methane may be utilized. The separated heavier hydrocarbons may be used either in forming a carrier gas or as a raw ma10 terial in the petrochemical industry, as previously indicated.

An important object of this invention is to provide an efficient method of separating methane from a liquefied natural gas with a minimum of equipment and cost.

Another object of this invention is to simultaneously separate methane from liquefied natural gas and revaporize the methane.

A further object of this invention is to provide a novel method of fractionating liquefied natural gas, wherein 20 the overhead from the fractionating zone is utilized to heat the liquefied natural gas being fed to the fractionating zone.

A still further object of this invention is to recover power in a process of revaporizing liquefied natural gas.

Other objects and advantages of the invention will be evident from the following detailed description, when read in conjunction with the accompanying drawing which illustrates this invention.

The single figure of the drawing is a flow diagram il30 lustrating a practice of this invention.

As previously indicated, natural gas ordinarily comprises a rather wide range of hydrocarbons, and it is desirable that all of the hydrocarbons be liquefied prior to transporting the composition to a remote locality. To 35 illustrate the present invention, it will be assumed that the liquefied naural gas to be separated and revaporized has the following analysis, although it will be understood that this composition is merely an example and. that the invention may be practiced with a liquefied natural gas.hav40 ing substantially any composition:

Component: Methane . Ethane Propane Isobutane n-Butane . Isopentane n-Pentane Benzene n-Heptane

Mol. percent

------------------------------71.23

------------------------------ 13,65

------------------------------ 9.10

----------------------------- 2.20

------------------------------- 2.25

----------------------------- 0.68

----------------------------- 0.33

------------------------------ 0.34

----------------------------- 0.22

Referring to the drawing in detail, reference character 10 designates a line leading from a source (not shown) of the liquefied natural gas, which will ordinarily be a 5₅ suitable insulated storage tank located in a coastal region where the liquefied natural gas may be received from similar insulated containers mounted aboard a ship used in transporting the gas to the remote locality. The liquefied natural gas is ordinarily stored at about atmospheric pressure, or slightly above, with a temperature of about —246° F. to —258° F., depending upon the precise composition of the gas. This low pressure and cold liquefied gas is fed to a pump 12 for pumping the liquefied natural gas through another line 14 to a fractionat55 ing tower 16 at increased pressure. It is preferred that ⁰ the pump 12 provide a substantial increase in the pressure of the liquefied natural gas, with the pressure being raised to about 585 p.s.i.g. to facilitate fractionation and subsequent power recovery, as will be more fully κθ hereinafter set forth. Also, a pair of heat exchangers 18 and 20 are interposed in the line 14 to heat the liquefied natural gas being fed to the fractionating tower 16

2,962,984 until approximately the bubble point temperature of the liquefied natural gas is reached. In other words, the Equefied natural gas being fed to the tower 16 is pressurized and then heated to approximately its bubble point. The detailed operations of the heat exchangers 18 and 20 are described in detail below.

The fractionating tower 16 may be of any desired construction which provides a vertically extended fractionating zone. The Equefied natural gas is fed to the medial portion of the tower 16 and is shghtly expanded, as from 585 p.s.i.g. to 535 p.s.i.g. into the tower 16 to facilitate separation of the methane from the heavier hydrocarbons. The liquid accumulating in the lower end portion of the tower 16 is circulated through a re-boiler 22 for heating ffie tower to such a temperature that methane enriched vapors will be directed into the upper portion of ffie tower, and ffie Equid in ffie lower portion of the tower will be enriched with the heavier hydrocarbons. In ffie example used for illustration, the liquid in the lower portion of ffie tower 16 is heated to a temperature of about 135° F. by the re-boiler 22. The reboner 22 derives its heat from any suitable heating medium, such as a gaseous product from a reforming operation, to maintain ffie desired temperature of ffie Equid in ffie lower portion of the tower 16. The liquid accumulating in ffie lower portion of ffie tower 16 is selectively withdrawn through a line 24 to maintain ffie desired liquid level in ffie tower. A typical analysis of liquid withdrawn through ffie line 24 is as follows:

Component: Mol. percent

Methane----- 1.0

Ethane------------- 45.7

Propane--------------- 32.1

Isobutane 7.7 n-Butane 7.9

Isopentane------------- 2.4 n-Pentane------------ 1.2

Benzene------------- 1.2 n-Heptane 0.8

This Equid withdrawn through line 24 will, of course, be at the same pressure as ffie tower 16 and at a temperature of about 135° F. It is preferred ffiat ffie Equid be passed through a suitable expansion valve 26 for at least partiaUy vaporizing ffie Equid and reducing ffie pressure thereof to a pressure suitable for transportation through a pipeline 28 to ffie point of use. As previously indicated, these heavier hydrocarbons may be conveniently used in forming a carrier gas by a reforming operation or used as a raw material in a petrochemical industry.

The methane enriched vapors rising to ffie upper portion of the fractionating tower 16 are withdrawn through a line 30 and passed through the heat exchanger 18 interposed in the Equefied natural gas feed line 14. A typical analysis of the vapors withdrawn through ffie line 30 is as foUows:

Component: Mol. percent

Methane---------------.---------------- 98.45

Ethane________________________________. 1.55

Also, with ffie temperature and pressure conditions previously indicated in ffie tower 16, ffie vapors withdrawn through ffie line 30 wiU be at about 535 p.s.i.g. and —118° F. When these vapors are passed through the heat exchanger 18, ffie vapors will be cooled to about —120° F., whereas the Equefied natural gas wEl be slightly warmed to enhance ffie subsequent fractionation thereof. Cooling of ffie methane enriched vapors directed through ffie line 30 condenses a portion of ffie vapors. Therefore, ffie vapors are directed through a Ene 32 from ffie heat exchanger 18 into an accumulator 34 which also functions as a separator for collecting ffie condensates in the lower portion thereof. These condensates are refluxed to ffie upper portion of ffie tower 16 through a line 35. The refluxed condensates wEl be at about —120°

F. and, as it is weU known in ffie art, wiU fadEtate ffie condensation of heavier- hydrocarbon vapors passing upwardly through the tower 16, as well as ffie vaporization of ffie methane component in ffie condensate, to 5 facilitate a more complete separation of the methane from the heavier hydrocarbons in the tower 16. The condensates being refluxed to the tower 16 wiU be enriched with methane, but will contain an appreciable proportion of heavier hydrocarbons. A typical analysis is: ¹⁰ Component: Mol. percent

Methane________________________________97.81

Ethane________________________________ 2.19

On the other hand, ffie vapors withdrawn through a 15 line 36 from the top of the reflux accumulator 34 will have a smaller percentage of heavier hydrocarbons, with a typical analysis thereof being as follows: Component: Mol. percent

Methane _________________________:______99.05

Ethane _________________________________ 0.95

It may also be noted that the vapors discharged through ffie line 36 will be at about 535 p.s.i.g. and about —120° F. These vapors are passed through a series of heat ex25 changers 38, 40 and 42 to provide a substantial wanning of ffie vapors. It is preferred ffiat the methane vapors be warmed by passage through ffie heat exchangers 38, 40 and 42 to a temperature of about 700° F. Whereupon, the gas is expanded through a suitable expander 44 down 30 to about 100 p.s.i.g. for ffie recovery of a substantial amount of work from ffie revaporized methane. The device 44 may be of any suitable form, such as a turbine, which wiU form a work-producing zone through which the methane may be expanded to produce work. In a 35 typical installation, where about 255,398 pounds per/hr. of Equefied natural gas are fed to ffie tower 16, and having a composition as previously described, the expander 44 should produce about 7400 B.H.P.; whereas the pump 12 for increasing the pressure of ffie Equefield natural gas 40 will require only about 790 B.H.P., thereby resulting in a substantial power recovery from ffie system.

The exhaust from the expander 44 is fed through a line 46 to ffie heat exchanger 40 to provide heating of the feed to ffie expander, as previously described. After 45 the revaporized methane has been passed through ffie heat exchanger 40, it may be used directly as a fuel, or reformed to a lower heat content and new specific gravity, as previously indicated. In ffie example disclosed, the revaporized methane discharging from the heat exchanger 50 40 wiU be at about 100 p.s.i.g. and 100° F.

The methane vapors passing from ffie line. 36 to ffie heat exchanger 38 on their way to ffie expander 44 are initially heated by a heating medium passed through ffie heat exchanger 38. The heating medium passed through 55 the heat exchanger 38 may be of any suitable form having a temperature higher than ffie temperature of ffie methane vapors in the line 36. When ffie present method is being used in combination with a reforming process, ffie heating medium for ffie heat exchanger 38 may con60 veniently be a hot fluid from the reforming operation. For the temperature and pressure conditions previously described, the heating medium passed through ffie heat exchanger 38 should have a temperature of about 81° F. This heating medium may then be transferred through a 65 Ene 48 to the heat exchanger 20 used for heating the Equefied natural gas being fed to ffie fractionating tower 16. It will be understood, however, ffiat separate heating mediums may be used in ffie exchangers 20 and 38, it being only necessary that ffie Equefield natural gas being 70 fed to the tower 16 be raised approximately to its bubble point temperature, and ffie revaporized methane in the line 36 be raised to a temperature for efficient heat transfer with the expanded methane vapors in ffie line 46.

When it is desired to heat the revaporized methane in 75 the line 36 to a temperature which will provide the most

2,952,984 efficient expansion of the methane through the expander 44 and permit use of this expanded methane for heat exchange relation with the feed to the expander, it is required that an additional heating medium of rather high temperature be passed through the heat exchanger 42. Any desired heating medium may be used with the heat exchanger 42, it only being necessary that the revaporized methane being fed to the expander 44 be increased in temperature for an efficient expansion. When the present method is being used in combination with a reforming operation, a hot gas from the reforming operation at about 840° F. may be passed through the heat exchanger 42. Thus, the revaporized methane may be raised to a temperature of about 700° F. before passing through the expander 44.

From the foregoing it will be apparent that the present invention provides an efficient method for separating methane from liquefied natural gas by using the minimum of apparatus, and with a minimum of cost. It will be further apparent that the separation of the methane from the heavier hydrocarbon components of a liquefied natural gas stream is accomplished simultaneously with revaporization of the methane, such that the separted components will be in proper form for use as a fuel or for further processing. The method may be performed in such a manner that a substantial amount of work may be recovered from the revaporized methane. It will also be apparent that the present invention provides a novel method of fractionating liquefield gases, wherein the overhead vapors from the fractionating zone are passed in heat exchange relation with the liquefied gases being fed to the fractionating zone, and wherein the resulting condensates are refluxed to the fractionating zone, to provide an efficient fractionation without the necessity of using a separate reflux condenser and refrigerant.

Changes may be made in the combination and arrangement of steps and procedures, as well as the form and arrangement of apparatus, heretofore set forth in specification and shown in the drawing, it being understood that changes may be made in the precise embodiment disclosed without departing from the spirit and scope of the invention as defined in the following claims.