US 3150495 A
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Description (OCR text may contain errors)
Sept. '29, 1964 D 3,150,495
E. E. REE STORAGE AND PRESSURE CONTROL OF 'REFRIGERATED LIQUEFIED GASES Filed Aug. 9, 1962 INVENTOR. E.E. R EED A T TORNEYS United States Patent 3,150,495 STORAGE AND ERESSURE CGNTROL OF REFRIGERATED LIQUEFIED GASES Edwin E. Reed, Bartlesville, Okla, assignor to Phillips Petroleum Company, a corporation of Delaware Filed Aug. 9, 1962, Ser. No. 215,913 5 Claims. (Cl. 6254) This invention relates to storage of liquefied gases and pressure maintenance thereof. In one aspect it relates to storage of such liquefied gases as ammonia and liquefied petroleum gases at atmospheric or substantially at atmospheric pressure.
The term liquefied petroleum gases includes such material as liquefied propane, liquefied butane, mixtures of these two hydrocarbons, and liquefied propane and liquefied butane or mixtures thereof with minor amounts of such other materials as ethane, ethylene, propylene, butylene, isobutane, isobutylene, pentane and amylene. Included with these minor amount constituents are such gases as nitrogen, carbon dioxide, and even traces of oxygen.
The materials ammonia, liquid ammonia, or liquefied ammonia as referred to throughout this specification and claims embody a major proportion of ammonia, such as 98 to 99 percent or more by gaseous volume, and such minor amounts of one or more of hydrogen, nitrogen, argon, methane, or helium when combined so as to make up the diiference, that is, 2 to 1 percent or less by gaseous volume.
In the production of liquid ammonia and liquefied petroleum gases, retention of the liquefied material in closed vessels for storage and/ or transportation presents many problems. Since these materials are normally gases, that is, they are gases at ordinary atmospheric temperature and pressure, retention at atmospheric temperature and pressure requires uneconornically large vessels. For this reason such materials are stored as liquids in containers at subatmospheric temperatures or at superatmospheric pressures, or both.
For storing these materials at subatmospheric temperatures provision must be made for cooling the materials and for maintaining them at the desired temperatures.
For storage at superatmospheric pressures, it is merely necessary to provide vessels having suitable wall thickness coupled with suitable fabrication. However, pressure vessels are expensive. Likewise, maintaining materials at subatmospheric temperatures is also expensive. In many instances, a compromise is made as regards temperature and pressure for storage of these materials.
According to this invent-ion, I propose to store such material as ammonia and liquefied petroleum gases in vessels at atmospheric or substantially atmospheric pressure and at subatmospheric temperature. I have a method and means for storing these materials in the least expensive vessels, that is, at atmospheric or substantially atmospheric pressure, with expenditure of only a minimum for utilities to maintain suitable subatmospheric temperature.
One important consideration in pressure and temperature maintenance in the storage of these materials is the presence of small amounts of gases having higher vapor pressures than the vapor pressure of the material being stored. Such gases are sometimes known as uncondensable gases, uncondensables, etc. One method employed involves withdrawal of uncondensable rich gas from above the surface of the stored liquid containing such gases in solution, compressing the withdrawn gases, cooling for condensation, and merely returning the condensate and uncondensed gas to the storage vessels. Another method employed is merely to vent the undesired gases. This latter method obviously is to be avoided since valuable gases are lost as well as the uncondensable gas or gases.
I have discovered an improvement over the just stated method of operation whereby I am able to maintain liquid ammonia or liquefied petroleum gas at suitable subatmospheric temperature for storage under atmospheric or substantially atmospheric pressure at less cost than conventional. In this storage operation I withdraw vapor from the storage vessel, compress the vapor and cool to produce condensate. During this condensation the condensate dissolves at least some of the difficultly condensable normally gaseous impurities. I carry out a phase separation between condensate and uncondensed, com pressed gases. I reduce the pressure of the condensate and gas phase separately in refrigeration steps. Some vaporization with simultaneous chilling occurs in the pressure reduction of the condensate. Following the pressure reductions of the condensate and of the uncondensed compressed gases 1 admix the chilled expanded gases with the pressure reduced, partly vaporized and chilled condensate. This admixing operation further dissolves a portion of the difiicultly condensable material in the pressure reduced and chilled liquid. Some pressure reduced gases still remain as a gas phase and this gas and all liquid are introduced into the liquid in the storage vessel at least near the bottom thereof so that the introduced gas is permitted to bubble upwardly through the tank liquid to dissolve a further portion of the gas. In this manner a maximum amount of the difi'icultly condensable, high vapor pressure material is returned as liquid, i.e., in solution in the liquid contents of the vessel, thereby minimizing the temperature reduction required for storage at atmospheric pressure.
An object of this invention is to provide a method and apparatus for storage of liquefied normally gaseous materials containing minor amounts of diflicultly condensable, normally gaseous materials at the minimum over-all cost. Another object of this invention is to provide a method and an apparatus for storing such liquefied gaseous material containing minor amounts of diificultly condensable normally gaseous material at a relatively high temperature and yet sufficiently low for storage at atmospheric or at substantially atmospheric pressure. Still other objects and advantages of this invention will be realized upon reading the following description which, taken with the attached drawing, forms a part of this specification.
The drawing illustrates, in diagrammatic form, an arrangement of apparatus parts for carrying out the process of this invention.
In the drawinng, reference numeral 11 identifies a tank which can be a storage tank in a tank farm, a cargo tank on a barge or in an ocean-going vessel. Reference numeral 13 identifies a dome or an upper gas containing portion of vessel 11. A conduit 15 communicates with dome 13 for passage of gas to a compressor 17 powered by a prime mover or engine 19. The compressor outlet connects through a conduit 21 to a condenser 23 and the condensate therefrom passes on through a conduit 29 into a phase separation vessel 31. Coolant or refrigerant enters condenser 23 by way of a conduit 25 and leaves by way of conduit 27. Condensate leaves phase separating tank 31 by Way of a conduit 33 and passes through expansion motor valve 37 and on through a conduit 35 and ultimately is returned to the lower portion of the liquid phase 51 in tank 11. The flow of condensate through conduits 33 and 35 is regulated by the motor valve 37. A float apparatus 39 senses liquid level in separator tank 31 and a signal from the float apparatus is passed to controller 55 which emits a signal responsive to the signal from float 39 to manipulate the motor valve 37. Upon rise of liquid level in separator 31 the controller emits a signal to open somewhat further the motor valve 37, and vice versa. In this manner a reasonably constant liquid level is maintained in separator 31. The gas phase from separator 31 is passed through a conduit 41 and a motor valve 43 and is introduced into the liquid phase flowing through conduit 35. The flow of this gas phase is regulated by a pressure conntroller 47 which communicates by way of a pressure tap 45 with the gas-containing space of separator 31 and with the motor of the motor valve 43. Thus, upon an increase of pressure in separator 31, motor valve 43 opens to pass an increased quantity of gas.
Upon passage of liquid phase to motor valve 37, a reduction of pressure to about atmospheric occurs accompa nied by some vaporization of the condensate and proportionate cooling. Thus, the fluid flowing through conduit 35 is at a lower pressure and at a lower temperature than the condensate in separator vessel 31. Similarly, vapor flowing through valve 43 is reduced in pressure to about atmospheric and proportionately chilled and this chilled vapor dissolves to a marked extent in the chilled liquid flowing through conduit 35. Valve 49 is positioned in conduit 35 at a point relatively close to vessel 11. This valve 49 is normally fully open and is used as a manual valve in case it is desired to close off conduit 35. The tank storage pressure is intended to be at least approximately atmospheric pressure. The gas phase in the dome 13 is identified by reference numeral 53.
In one instance the liquefied petroleum gas comprising about 6.2 percent by volume ethane (C hydrocarbon) and 93.8 percent propane (C hydrocarbon) is stored in vessel 11. Storage pressure is approximately 14.7 p.s.i.a. (pounds per sq. in. absolute) at approximately 53 F. The vapor in dome 13 in equilibrium with this liquid phase contains 27.8 percent C hydrocarbon and 72.2 mole percent C hydrocarbon. These percentage compositions of the LPG as described herein are given in terms of mol percent. A compressor corresponding to compressor 17 withdraws 875 cubic feet per minute of vapor from the upper portion of tank 11 and compresses this vapor to a pressure of about 60 p.s.i.a. at which pressure the temperature of the gas phase is increased from -53 F. to about +50 F. A 125 horsepower compressor is required for compression to the mentioned 60 p.s.i.a. A conventional Freon refrigeration system is used for providing a coolant for use in a condenser corresponding to condenser 23. The compressed gas is thus chilled by Freon refrigeration and the condensate and uncondensed gases pass on into a separator vessel corresponding to vessel 31. The liquid phase passing through expansion valve 37 and the gas phase passing through a valve corresponding to valve 43 were pressure reduced, resulting in temperatures of approximately 53 F. At these temperatures the remaining condensate, the gas produced by vaporization of a portion of the condensate and the gas from expansion valve 43 are intermingled and a considerable proportion of the gas from conduit 41 is dissolved in the condensate. The mixed condensate and undissolved gas pass on through conduit 35 into the bottom portion of liquid 51 in tank 11 at about atmospheric pressure. By being introduced into the liquid phase at the bottom thereof the gas has an additional opportunity on bubbling upwardly through the liquid to dissolve in the liquid thereby further reducing the uncondensable gas which will ultimately be recycled through the compressor and condenser system. The liquid and gas issuing from the separator vessel 31 have pressures of approximately 55 p.s.i.a.
While the above example illustrates the operation of this invention by the use of refrigerant Freon it is not essential that this particular refrigerant be used. Conventional cooling water can, if desired, be passed through condenser 23 by way of the conduit 25 and withdrawn from the condenser by way of the conduit 27. In this case, however, the compressor corresponding to compressor 17 will be required to compress the withdraw gas to a pressure of about 380 p.s.i.a. at which pressure the compressed gas will have a temperature of about 190 F. This latter compression requires the expenditure of approximately 615 horsepower to compress the 875 cu. ft. per minute of gas to the latter mentioned pressure. Water will enter condenser 23 at a temperature of about F. and leaves the condenser at about F. The temperature of the condensate and uncondensed gas in separator 31 is about F. at a pressure of about 360 p.s.i.a.
When using Freon as refrigerant in condenser 23 for refrigerating the 875 cu. ft. of gas per minute after compression to the hereinbefore mentioned 60 p.s.i.a., the Freon system requires about 336 horsepower. This 336 horsepower plus the horsepower required for compression of the 875 cubic ft. of gas per minute to 60 p.s.i.a. gives a total power requirement of 461 horsepower. This power requirement therefore possesses an advantage of 154 horsepower per hour in favor of the use of the Freon for coolant in condenser 23 over the use of water as a coolant.
While I have hereinabove described the operation of the apparatus of my invention for cooling and maintaining at a subatmospheric temperature and at atmospheric pressure, an LPG comprising C and C hydrocarbons the method and apparatus can be used equally well for refrigeration of liquid ammonia. Liquid ammonia produced along the Gulf Coast contains as inert and difficulty condensable gases hydrogen, nitrogen, methane and argon. Ammonia produced in this area contains from about 2 to about 5 milliliters of these inert gases in each gram of liquid ammonia. In one instance liquid ammonia produced in that area was stored at a pressure of approximately 45 p.s.i.g. Ammonia stored at said 45 p.s.i.g. pressure contained in the vapor phase above the liquid ammonia about 1.2 volume percent of these inert gases. This proportion of inert gas corresponded to approximtely 15.78 milliliters of inert gas per gram of ammonia. The liquid phase at this 45 p.s.ig. pressure contains approximately 0.15 milliliters of inert gas per gram of liquid ammonia.
In the Texas panhandle area ammonia produced from local gas contains from about 1 milliliter to about 2 /2 milliliters of inert gases per gram of liquid ammonia. In this particular instance the liquid ammonia was stored r in at 18 p.s.i.g. (pounds per sq. in. gauge) pressure. The
inert gas in the ammonia comprised hydrogen, nitrogen, methane, argon and helium.
Portions of apparatus disclosed herein are thermally insulated, as for example, tank 11, conduit 29, separator vessel 31 and all conduits leading from vessel 31 back to tank 11.
The apparatus herein described withdraws gas from ammonia storage containing along with ammonia one or more of the above-mentioned inert gases, compresses the withdrawn gases, and condenses a portion of the compressed gases. The condensate is largely liquid ammonia. Upon phase separating the condensate and uncondensed gas, then pressure reducing them separately for chilling, and admixing the pressure reduced gas with the remainder of the condensate and its produced gas, a substantial portion of the inert gases are redissolved in the liquid. The final stream of liquid and gas are introduced into the bottom of the liquid ammonia in its storage tank.
By continuous withdrawal of the gas phase over liquefied petroleum gas, or the gas phase over liquid ammonia, compressing, condensing, separating condensate from gas, separately reducing pressure on each with concomitant chilling, then mixing phases and introducing the mixed phases to the bottom of the stored liquid for a period of time, substantially all of the inert gases or high vapor pressure gases are returned to the liquid phase from which they came. Thus, storage under atmospheric pressure is easily achieved.
While certain embodiments of the invention have been described for illustrative purposes, the invention obviously is not limited thereto.
That which is claimed is:
1. A method of maintaining a liquefied gas comprising a major proportion of a readily condensable normally gaseous material and a minor proportion of a more difiicultly condensable normally gaseous material under regulated storage pressure in a closed storage vessel which comprises the steps of:
(1) withdrawing vapor of said materials from the vapor space of said vessel;
(2) compressing withdrawn vapor of step (1);
(3) cooling the compressed vapor of step (2) so as to condense at least a substantial proportion of the readily condensable material;
(4) passing the material from step (3) to an enclosed separation chamber to separate same into condensate comprising principally said condensable material and vapor comprising principally said more difficultly condensable material;
(5) passing the condensate of step (4) thru an expansion zone to vaporize a substantial proportion of same and provide a cool stream thereof;
(6) passing the vapor of step (4) thru a separate expansion zone to provide a cool stream thereof;
(7) passing the cool stream of step (6) directly from its expansion zone into the cool stream of step (5) to form a mixed cool stream of condensate and vapor and cause a portion of said more difiicultly condensable material to go into solution; and
(8) passing the mixed cool stream of step (7) directly into the lower section of the storage vessel so as to bubble the vapor fraction thereof upwardly thru stored liquid to eifect further solution thereof in said stored liquid.
2. The method of claim 1 wherein the pressure in said storage vessel is maintained at substantially atmospheric pressure and the expansion in steps (5) an (6) is to a pressure just above the pressure in said vessel.
3. The method of claim 1 wherein said readily condensable material is principally propane and said more diificultly condensable material is principally ethane.
4. The method of claim 1 wherein said readily condensable material is principally ammonia and said more difficultly condensable material comprises at least one gas selected from the group consisting of hydrogen, nitrogen, methane, argon, and helium.
5. A system comprising in combination:
(1) a vapor-tight storage vessel having a vapor space in its upper section;
(2) a vapor-liquid separation vessel;
(3) a first conduit having means therein for compressing vapor from said vessel connecting the vapor space of (1) with the separation vessel of (2);
(4) a second conduit having an expansion motor valve therein conecting the lower liquid section of the separation vessel of (2) directly with the lower section of the storage vessel of (1);
(5) a third conduit having an expansion motor valve therein connecting the upper vapor section of the separation vessel of (2) directly with the second conduit of (4) downstream of the expansion valve therein;
(6) a pressure controller sensitive to vapor pressure in the vessel of (2) and in actuating control of the motor valve of (5); and
(7) a liquid level controller sensitive to liquid level in the vessel of (2) and in actuating control of the motor valve of (4).
References Cited in the file of this patent UNITED STATES PATENTS 1,371,427 Kerr Mar. 15, 1921 2,001,996 Whitman May 21, 1935 2,059,942 Gibson Nov. 3, 1936 2,550,886 Thompson May 1, 1951 2,887,850 Adams May 26, 1959