US 2500118 A
Description (OCR text may contain errors)
March 7, 1950 H. c. COOPER NATURAL GAS LIQUEF'ACTION Filed Aug. 18, 1945 INVENTOR. Ham/44 C. COOPER WQQ QQMQRTE Patented Mar. 7, 1950 UNITED STATES PATENT OFFICE 2,500,118 4 Y NATURAL GAS LIQUEFACTION Howell C. Cooper, Sewickley, Pa. Application August 18, 1945, Serial No. 611,279
2 Claims. (Cl. 62-1755) This invention relates to the liquefaction of natural gases for storage or transportation purposes, and more particularly to liquefaction of such gases containing nitrogen in substantial quantities, and constituting in fact a mixture of such nitrogen and the methane which is the principal combustible constituent.
The principal object of the invention is to rid the liquefaction apparatus of the nitrogen necessarily received by it, with a minimum loss .of methane; it having been the practice in the art heretofore to simply .vent the nitrogen under conditions entailing the loss of relatively greater amounts of methane.
Briefly, the invention contemplates in this respect liquefaction of the entire methane and nitrogen contents of the natural gas at a lower pressure than would be necessary for liquefaction of the nitrogen only, so that a refrigerant such as methane is employable, utilizing its latent heat in the usual manner.
Further objects of the invention are to conserve power in the process, as will appear.
Still further objects and advantages will be apparent from the following description taken in connection with the accompanying drawing conventionally and diagrammatically illustrating a circuit embodying the invention.
With reference now to the drawing, I is the incoming natural gas supply line, which leads to the product condenser 2 with pressure determined by the compressor 3 or equivalent pressure regulator.
The product condenser 2 is served with a refrigerant, which may be methane, liquefied in the methane condenser 4.
The methane circuit includes, commencing with the methane condenser 4, a methane heat exchanger 5, expansion valve 8, flash tank 1, expansion valve 8, product condenser 2, first storage compressort, intercooler l0, second stage compressor H with its intercooler l2 and third stage compressor l3 with its intercooler l4, methane precooler l5 and back to the methane condenser 4, it being understood that fiow in this methane circuit is in the direction of the order of the parts recited, by way of the suitable connections indicated in the drawing.
Also, in the methane circuit is a bypass l6 by which gas from the flash tank I is passed through the heat exchanger 4 and returned to' the principal methane circuit ahead of the second stage compressor ll.
from which it emerges at 126, partial expansion in the flash tank 'I from which the liquid emerges at 220, boiling within the product condenser 2 at a temperature of 252", taking up heat therein from which it emerges and enters the first stage compressor, as a gas, at 0, leaving the last cooler M at 100 and re-entering'its condenser 4 at a 8 wherein it is reconverted back from a gas to a liquid. The temperature in the bypasslii is --140.
Through this cycle the methane pressure will vary from 600 pounds, as it leaves its condenser 4 to 17 pounds as it leaves product condenser 2, as will be appreciated.
The temperatures recited are in degrees Fahrenheit and the pressures in pounds per square inch absolute, but it is to be understood that the This methane refrigerant circuit is generally a known one and its operation, briefly, includes liquefaction of the methane in its condenser 4 figures are recited above and hereinafter only by way of exemplification.
Liquefaction of the methane in its condenser 4 is accomplished by the employment of ethylene as a refrigerant, which ethylene is liquefied in the ethylene condenser 20. The ethylene circuit is generally similar to the methane circuit, the ethylene. passing from its condenser 20 through expansion valve 2 I, flash tank 22, expansion valve 23, methane condenser 4, first stage compressor 24 with intercooler 25 and second stage compressor 26 with its aftercooler 21 and back to the ethylene condenser; a bypass 28 being provided from the flash tank 22, as before.
The ethylene feeds its condenser at a -8, enters the methane condenser at a 145, leaves it at a 20 and re-enters its condenser 29 at a+ 100, its pressure running from 355 pounds in its condenser 20, to 17 pounds leaving the methane condenser.
The ethylene in turn is liquefied in its condenser 20 by the employment of ammonia liquefled in the ammonia condenser 30 and employed in a substantially similar circuit including expansion valve 3|, fiash tank 32, expansion valve 33, ethylene condenser 20, first stage compressor 34, intercooler 35, second stage compressor 36, aftercooler 31. However, this ammonia circuit preferably includes a bypass around the ethylene condenser 20 and through the methane precooler l5, controlled by the expansion valve 38. It also includes the bypass 39 leading from the flash tank 32.
The ammonia leaves its condenser 30 at 100, enters the ethylene condenser 20 at 24, leaves the ethylene condenser at reenters the ammonia condenser 30 at varying in pressure from 250 pounds in its condenser to 17 pounds leaving the ethylene condenser.
The ammonia condenser is served by cooling water entering by line 40 and leaving by line 4| and which may be served by a coolingtower not illustrated;
The system described will be recognized as what is known as of the cascade type whereinjs employed another in a lower temperature range or being served by another in a higher temperature range, or both. I
The temperature range available from the methane, for condensation of the natural gas mixture in the product condenser 2 is such that only a relatively low pressure of the natural gas such as175 pounds is necessary entering the condenser.
The methane temperature being fixed, the natural gas mixture pressure may be adjusted dependent upon its nitrogen content so that only liquid, including liquefied nitrogen as well as liquefied methane will emerge from the bottom of the product condenser as through the line 50, where the temperature may be 240 and pressure 175 pounds.
The nitrogen having been liquefied along with the methane, it is separated from the latter in a separator 5| which enters by way of an expansion valve 52.
The nitrogen separator 5| is in the form of a tower having the bubble plates conventionally indicated and its operation depends upon a nitrogen liquefaction circuit whereby a liquid nitrogen spray is provided at the upper part of the tower by which gaseous methane is condensed and thereby rejected from a gaseous nitrogen outlet at the tower top.
The nitrogen liquefaction circuit includes the nitrogen condenser 60 wherein the nitrogen is liquefied, expansion valve 6|, flash tank 62, expansion valve 63, separator 5|, nitrogen precooler 64, interstage cooler 65, compressor 66, intercooler 61, interstage cooler 65, second stage compressor 68, aftercooler 69, precoolers l and 64 and back to the nitrogen condenser 60. A bypass connection 1| leads from the flash tank 62, by way of the precooler l0, and back into the circuit ahead of the interstage cooler 65.
The nitrogen condenser 60 is served by methane from the methane condenser 4 by a circuit which bypasses the product condenser 2 and includes the line 80, expansion valve 8|, nitrogen condenser 60, line 82 and product condenser 2.
In the nitrogen liquefaction circuit the nitrogen leaves its condenser 60 in liquid form at a --240, leaves the flash tank 62 at a -282, leaves the separator 4| at a 310, the precooler 64 at a 70, the interstage cooler 65 at +70, the first stage compressor at +320", the interstage cooler 55 at 50, the second stage compressor 68 at +155", the aftercooler 69 at +100, the precooler 70 at -58 and reenters the condenser 60 at 239. The pressure within the nitrogen separator is 25 pounds, which is maintained substantially up to the first stage compressor 66, leaving the aftercooler 69 at 400 pounds.
Within the separator 5| the temperature of the nitrogen is at 310 which is below methane liquefying temperature so that any methane which emerges from the expansion valve 52 as a gas will be condensed and fall to the bottom of the separator where it will be at 260.
Nitrogen gasified in the separator 5| enters the described nitrogen circuit and its refrigerant capacity is largely conserved in the described heat exchangers 64, 65 and I0. The nitrogen circuit a source of refrigerants, each servin is vented immediately ahead of its first compressor 66 under control of a valve 90 by which the nitrogen beyond that necessary for operation of the circuit, is disposed 01'. Once the system is in operation, the amount of nitrogen thus vented will be substantially the amount which enters the system by way of the supply line it being understood that a slight amount of nitrogen will remain in solution in the liquid methane leaving the nitrogen separator 5|.
From the separator-the liquid methane is withdrawn subject to valve 9| to a storage tank 92 from which in turn it may be withdrawn as desired-through line 93, the storage tank being provided with the usual vent or relief 94 to take care of evaporation losses.
In summary, attention is again called to the fact that the temperatures and pressures herein above recited are by way of example only and may vary somewhat according to various conditions, including. the capacity of the system. In the example shown, the capacity is in the order of four million cubic feet of gas per day deliverable to storage, the entering gas comprising a mixture of about 91% methane and 9% nitrogen, by volume, at a temperature of It may also again be observed that all of the incoming natural gas mixture is liquefied, under conditions very substantially less demanding than would be otherwise necessary for liquefaction of the nitrogen content alone; a common refrigerant source is employed for liquefaction of the mixture, and of the nitrogen necessary for separation; and purge of the nitrogen is had after recovery therefrom of most of its value as a refrigerant.
1. In the production of liquid methane from a natural gas mixture containing a substantial amount of nitrogen: liquefying said mixture by heat exchange with a boiling refrigerant, separating nitrogen as a gas from the liquefied solution by reduction of pressure thereon in the presence of liquid nitrogen, employing previously separated and reliquefied nitrogen for the purpose, and venting the resultant gaseous nitrogen in excess of that required to be reliquefied for continuation of the separation process.
2. In the production of liquid methane from a natural gas mixture containing a substantial amount of nitrogen: liquefying said mixture by heat exchange with a boiling refrigerant, separating nitrogen as a gas from the liquefied solution by reduction of pressure thereon in the presence of liquid nitrogen, employing previously separated and reliquefied nitrogen for the purpose, and employing some of said refrigerant for said nitrogen reliquefaction.
HOWELL C. COOPER.
REFERENCES CITED The following references are of record in the file of this patent:
UNITED STATES PATENTS