Process and apparatus for the storage and transportation of liquefied gases
United States Patent Kniel et al.  
 PROCESS AND APPARATUS FOR THE- STORAGE AND TRANSPORTATION 0F LIQUEFIED GASES Inventors: Ludwig Kniel, Scarsdale; Frederick Fussman, Bronx, both of NY.  
 Assignee: The Lummus Company, Bloomfield,  
 Filed: Apr. 27, 1973 Appl. No.2 354,925  
 US. Cl. 62/50; 62/54; 62/55; 62/240 Int. Cl. Fl7c 7/02 Field of Search 62/45, 47, 50. 51, 54, 62/55, 240; 114/74 A References Cited UNITED STATES PATENTS H1968 Vanklccf 62/55 9/1968 Williams et al. 62/55 10/1970 Frijlink et a1. 62/55 X Battey 62/54 Becker 62/55 X Primary Examiner-Meyer Perlin Assistant E.\&#39;aminerRonald C. Capossela Attorney, Agent, or FirmMarn &amp; Jangarathis [57] ABSTRACT There is disclosed a vessel having a plurality of cryogenic tanks for the storage of liquefied gases wherein the cryogenic tanks are provided with conduit means and a heat exchange means whereby the unavoidable boil-Off from a lower boiling liquefied gas stored in a first cryogenic tank means, is passed in indirect heat transfer relationship with the unavoidable boil-off from a higher boiling liquefied gas stored in a second cryogenic tank means, to re-liquefy the vapors Of the higher boiling liquefied gas. Additional conduit means are provided to permit alteration of the gaseous atmosphere in the second cryogenic tank means (after unloading its content) to the atmosphere of the liquefied gas to be subsequently stored and transported therein.  
 39 Claims, 2 Drawing Figures PATENTEDAPRISIFRS 3,877. 240  
 Fig. I  
 Fig. 2  
 PROCESS AND APPARATUS FOR THE STORAGE AND TRANSPORTATION OF LIQUEFIED GASES This invention relates to the storage of liquefied gases, and more particularly to novel processes and apparatus for the storage and transportation of liquefied gases at about atmospheric pressure in a vessel, such as a ship. barge or the like.  
 BACKGROUND OF THE INVENTION In the transportation of liquefied gases, such as natural gas, ethylene, ethane or like liquefied hydrocarbons, and ammonia or like inorganic compounds, which require either high pressures for normal ambient temperatures or low temperatures to maintain the liquid state at about atmospheric pressure, problems could arise especially in the co-transportation of two or more such liquefied gases in a vessel, such as a ship, barge or the like. Generally, it has been found desirable to store and transport liquefied gases, e.g., ethylene, on a ship at about atmospheric pressure in tanks properly insulated (cryogenic tanks) and associated with a refrigeration plant for condensing vapors which normally evolve from the liquefied gas during transportation. In the transportation of liquefied natural gas (LNG), it is common to utilize vaporizing LNG, which results, inter alia, from conductive heat transfer, as a fuel for the vessels propulsion system, such as disclosed in U.S. Pat. No. 2,938,359 to Cobb et al. The use of vapors from other liquefied gases, e.g. ethylene, is either impracticable or uneconomic-al.  
  In U.S. Pat. No. 2,795,937 to Sattler et al. there is described a process and apparatus for the storage and transportation of volatile liquids, in particular, the transportation of LNG and a second liquefied gas, e.g., ethylene, in heat insulated tanks wherein vaporized LNG is employed as fuel to the propulsion system of the vessel. The second liquefied gas is maintained at a temperature low enough to eliminate the necessity of venting the liquefied gas containing tank by expanding LNG into heat transfer equipment positioned within the tank containing the second liquefied gas. Such process and apparatus is uneconomic since the use of LNG as a liquid refrigerant by expanding same and the subsequent use thereof as a fuel is costly. Additionally, the positioning of heat transfer equipment within the tank containing the second liquefied gas renders such equipment inaccessible for repairs.  
 OBJECTS OF THE INVENTION An object of the present invention is to provide novel processes and apparatus for the storage of liquefied gases.  
  Another object of the present invention is to provide novel processes and apparatus for the storage and transportation of liquefied gases by a vessel.  
  A further object of the present invention is to provide a novel process and apparatus for the storage and transportation of liquefied gases by a vessel wherein the vapors evolving from the lower boiling liquefied gas being transported as a result mainly of heat leakage and varying sea and weather conditions (hereinafter sometimes referred to as the unavoidable boil-off&#34;), is utilized as a means to liquefy the vapors (i.e., unavoidable boiloff) evolving from the higher boiling liquefied gas being transported.  
  A still further object of the present invention is to provide novel processes and apparatus for the en route preparation of a cryogenic tank of any such vessel for the storage and transportation of a liquefied gas different than the liquefied gas previously transported therein.  
 Another object of the present invention is to provide a novel process and apparatus for modifying an existing liquefied natural gas ship having cryogenic tanks for the storage and transportation of LNG and a liquefied gas having a higher boiling point than LNG.  
 &#39; Still another object of the present invention is to provide a novel process and apparatus to be utilized on new liquefied gas ships having cryogenic tanks for the storage and transportation at about atmospheric pressure of diverse liquefied gases.  
  A still further object of the present invention is to provide a novel system for transporting liquefied natural gas and ethylene from a source thereof to a user location.  
 SUMMARY OF THE INVENTION These and other objects of the present invention are achieved on a vessel having a plurality of cryogenic tanks for the storage of liquefied gases by providing the cryogenic tanks with conduit means and heat exchange means whereby the unavoidable boil-off from the lower boiling liquefied gas stored in a first cryogenic tank means thereof, is passed in indirect heat transfer relationship with the unavoidable boil-off from the higher boiling liquefied gas stored in a second cryogenic tank means, to re-liquefy the vapors of the higher boiling liquefied gas. Additional conduit means and other apparatus are provided to permit alteration of the atmosphere in the cryogenic tank (after un-loading its contents) to the atmosphere of the liquefied gas to be subsequently stored and transported therein (as more fully hereinafter discussed). The vessels in service at the present day for transportation of liquefied gases are generally designed to carry from 5 to 7 similarly sized tanks of the spherical or prismatic type, although other shapes of cryogenic tank may be employed. To facilitate an understanding of the present invention, a preferred emstoring the liquefied ethylene regardless of the size and number of such tanks, it being understood that volumetric ratios will vary depending upon the liquefied gases being transported. While the drawing includes fluid communication equipment, such as valves, pumps, and the like, it is understood that additional such equipment has been omitted from the drawing to facilitate the description thereof and the placing of such equipment at appropriate places are deemed to be within the scopeof those skilled in the art.  
 BRIEF DESCRIPTION OF THE DRAWING The invention will now be described with reference to the accompanying drawings wherein like numerals are used throughout and wherein:  
  FIG. 1 is a schematic elevational view of a vessel illustrating the general arrangement of the various tanks and associated apparatus; and  
  FIG. 2 is a schematic flow diagram of a preferred embodiment of the invention.  
 7 DESCRIPTION OF THE INVENTION Referring now to FIG. 1, there is illustrated a vessel, generally indicated as 10, provided with five spherical cryogenic tanks T including a re-liquefaction assembly, a cargo control station, and an off-loading station, generally indicated as 12, 14 and 16, respectively, with the reliquefaction assembly 12 being disposed between the first and second tank proximate to the super structure of the vessel 10. Such a vessel may have a capacity of, for example, 125,0Om of LNG, with principal characteristics being an overall length of about 900-1,000 feet, a draft of 36 feet or more, and a displacement of 94,600 long tons at speeds of about 20 knots or more. In accordance with the present invention, one tank T(LE) to store liquefied ethylene and the remaining tanks T(LNG) to store liquefied natural gas will be provided with piping or conduit configuration in fluid communication with the re-liquefaction apparatus 12 as more fully hereinafter described. A preferred placement of the liquefied ethylene storage tank T(LE) to minimize piping expenses is to position the liquefied ethylene tank proximate to the stern of the ship, however it is understood that other consideration, e.g., partial loading conditions, order of loading and unloading of liquefied ethylene and LNG, etc., may make it desirable to position the liquefied ethylene storage tank T(LE) amidships. All of the cryogenic tanks of the vessel 10 are usually of the same constructions and heavily insulated.  
  Referring now to FIG. 2, there is illustrated the reliquefaction apparatus 12 with a piping configuration associated with the tank T( LE) for the storage of liquefied ethylene. A line for the loading and off-loading of liquefied ethylene is provided with a suitable fitting (not shown) for connecting the line 20 to an appropriate dock side facilities (not shown). The line 20 is in fluid communication with loading line 22 under the control of valve 24, and with an unloading pump 28 by line 30 under the control valve 32. Unloading pump 28 may be located inside the tank, i.e., submerged, or externally of the tank.  
  A line 34 is provided on the top portion of the liquefied ethylene tank T( LE) under the control of valve 36 in fluid communication via lines 38 and 40 under the control of valves 42 and 44 with a fractionating condenser, generally indicated as 46. The tank T(LE) is provided with safety vent line 26 under the control of a safety relief valve 18 by line 34. Line 34 is in fluid communication with a line 48 under the control of valve 50 with a compressor 52 and thence with line 40 by line 54 under the control of valve 56. A line 58 under the control of valve 60 is in fluid communication with a reboiler coil 63 positioned in the lower portion of the fractionating condenser 46 (as more fully hereinafter discussed) and with line 40 by line 62 under the control of valve 64.  
  A vapor header or manifold, generally indicated as 66, is in fluid communication by conduits 68 with the remaining tanks of the vessel 10 containing LNG. The header 66 is in fluid communication by line 70 under the control of valve 72 with a heat exchange coil 74 disposed within the upper portion of the fractionating condenser 46; The outlet of the coil 74 is in fluid communication by line 76 under the control of valve 78 via a booster compressor 80 with a fuel manifold 82. The top of the condenser 46 is in fluid communication by line 84 under the control of valve 86 with a heat exchange coil 88 disposed within the upper portion of the fractionating condenser 46 proximate to the coil 74 and thence with line 76 by line 90 under the control of valve 92.  
  The header 66 is in fluid communication by line 94 under the control of valve 96 with the suction side of a compressor 98 with the outlet therefrom being in fluid communication with line 100. Line 100 is in fluid communication with line 102 and 104 under the control of valves 106 and 108, respectively. Line 102 is in fluid communication with line 70, and line 104 is in fluid communication with lines 110 and 112 under the control of valves 114 and 116, respectively, with line 110 being in fluid communication with the liquefied ethylene tank T(LE).  
  The bottom of the fractionating tower 46 is in fluid communication by line via pump 122 with lines 124 and 126 under the control of valves 128 and 130, respectively. Line 124 is in fluid communication with the liquefied ethylene tank T(LE) whereas the line 126 is in fluid communication with liquefied ethylene holding tank 132. The liquefied ethylene holding tank 132 is provided with a line 134 under the control of valve 136 for the fluid communication with dock side facilities, as more fully hereinafter discussed. The bottom of the ethylene holding tank 132 is provided with a conduit 138 in fluid communication by a pump 140 with line 142 under the control of valve 144. Line 142 is in fluid communication with line 146 and 148 under the control of valves 150 and 152, respectively. Line 146 is in fluid communication with a line 154 positioned within the tank T(LE) and through line 156 under the control of valve 158 with line 34. Line 148 is in fluid communication with tank T(LE) via heat exchanger 160 through line 162 under the control of valve 164.  
  As hereinabove discussed, the processes and apparatus of the present invention will be described with reference to the storage and transportation of LNG and liquefied ethylene, although it is understood that the invention is also applicable to the handling of LNG and other diverse liquefied gases. It is understood in the shipment of a liquefied gas that vapors will evolve as a result of the heat leakage into the cryogenic tanks from the surrounding environment as well as from the motion of the vessel in the sea (converted kinetic energy). It will be appreciated that a vessel is subjected to movement in heavy seas and will evolve a greater proportion of vapors due to kinetic energy input into the cargo, than a vessel passing through a calm sea. As hereinabove disclosed, this invention relates to the storage of liquefied gases in a plurality of cryogenic tanks positioned on a vessel, however, for the transportation of LNG and liquefied ethylene the volumetric capacity of the tank(s) storing LNG is in a ratio of at least 4:1 with respect to the volumetric capacity of the tank(s) storing liquefied ethylene. Therefore, with regard to the vessel of FIG. 1, one tank of the five cryogenic tanks would be provided with conduit means and ancillary equipment necessary to permit the storage of the liquefied ethylene, and re-liquefaction of the unavoidable boil-off, as hereinabove discussed.  
 TRANSPORTATION OF LNG AND LIQUEFIED ETHYLENE In operation, the tank T(LE) of the vessel is filled with liquefied ethylene and the remaining tanks T(LNG) of the vessel 10 are filled with LNG at pressures slightly above atmospheric. The ethylene vapors resulting from unavoidable boil-off are withdrawn from tank T( LE) by line 34 and passed by line 38 and 40 into the fractionating condenser 46. In fractionating condenser 46, the ethylene vapors are passed in indirect heat transfer relationship to natural gas (unavoidable boil-off) introduced into the coil 74 of the condenser 46 from line 70 collected in manifold 66 from the tanks T( LNG) by lines 68. It will be understood that valve 72 will be opened at this time whereas the valves 96, 106 and 116 will be closed. The amount of unavoidable boil-off from the LNG in the tanks T( LNG) is sufficient to provide the cooling requirements to re-liquefy the unavoidable boil-off from the tank T( LE) in fractionating condenser 46 with liquefied ethylene being returned by line 124 to the tank T(LE) by pump 122 in fluid communication with the bottom of the fractionating condenser 46 through line 120. The natural gas withdrawn from the coil by line 76 is passed as fuel by booster compressor 80 through line 82 to the propulsion system of the vessel (not shown).  
  Should the liquefied gases be at ambient pressures or lower vice slightly above atmospheric pressure (which will generally be the case when prismatic tanks are utilized, or when a tank is incompletely filled with liquefied gas), it is necessary to utilize the compressors 52 and 98 to increase the pressure of the respective gaseous streams to overcome frictional resistance in the various lines, valves, and associated apparatus. Utilization of the compressors will generally not be necessary when the tanks are of the spherical type. The compressor S2 is preferably of the non-lubricated reciprocating type with the suction volume thereof being regulated either by a variable speed transmission or by clearance pockets and valve lifters, or through a combination of such devices actuated by a pressure controller 170 in communication with the tank T(LE) by line 172. The compressor 98 may be a compressor similar to the compressor 52. It is contemplated that in some instances it may be necessary to utilize a plurality of such compressors in parallel. It is noted that in this operation the ethylene vapors are not passed by line 58 through the reboiler coil 63 in the fractionating condenser 46, the use thereof being hereinafter discussed. It will be further understood that pump 122 may not be necessary since the liquefied ethylene may usually be readily passed by gravity to the liquefied ethylene tank T( LE). Upon the vessel reaching its destination, the liquefied ethylene and/or LNG are unloaded in a manner known to those skilled in the art. Liquefied ethylene, for example, is withdrawn by the pump 28 from the tank T(LE) by line 30 and is passed through lines 30 and to dock side facilities (not shown) with lines 34 and 174 being in fluid communication with the facilities to provide for pressure equalization.  
  It is desirable to maintain the tankers in continuous service with minimal time for layovers or delays (outage time). Thus, if sufficient liquefied ethylene to fill the tank T(LE) is not available at the loading terminal, rather than permit the vessel to ship out with an empty or partially filled tank, it is desirable to take advantage of such space by filling the tank T(LE) with LNG. In order to fill the tank T(LE) with LNG it is necessary to prepare the tank for such usage (by precooling and by removal of the ethylene atmosphere), and this may be conveniently accomplished while the vessel is en route to the loading terminal since in port preparation is wasteful of LNG. Preparation of the tank T(LE) for the cargo switch, either from liquefied ethylene to LNG or vice versa, involves two operations, i.e., (a) the replacement of the tanks atmosphere with an atmosphere of the material to be transported, and (b) the cooling or heating of the tank to a temperature compatible with that of the boiling point of the material to be stored to thereby reduce or avoid the so-called load flash or a negative pressure which arises when the cryogenic tank is either too warm to too cold, respectively, to receive the new material.  
 CONVERSION OF THE LIQUEFIED ETHYLENE TANK TO LNG STORAGE Unloading liquefied ethylene in a manner known to those skilled in the art includes the introduction of gaseous ethylene from a shore side source in fluid communication with storage tanks into the space in the tank T(LE) created by the withdrawn liquefied ethylene. After unloading liquefied ethylene from, for example, a 25,000 cubic meter tank, there will remain about 50 metric tons of ethylene vapors at a temperature of about l40F, at a pressure of about 0.5 psig. as well as a residual amount of liquefied ethylene. A predetermined amount of LNG remains in the LNG tanks to maintain the tanks at proper pressure and temperature conditions during the return voyage, as is known to those skilled in the art.  
 - On the return voyage, the unavoidable boil-off from the LNG tanks is collected in the manifold 66 and a minor portion thereof is introduced into tank T(LE) through line 112 and under the control of valves 116 and 114, respectively, or alternately, through line 94, 100, 104 and 110 via the compressor 98 if the pressure in T(LNG). is insufficient. The remaining portion of the unavoidable boil-off from the LNG tanks is passed to the coil 74 in the fractionating condenser 46 through line 70 under the control of valve 72. Ethylene vapors are gradually downwardly displaced by the natural gas introduced into thetank T(LE) with only limited amount&#39;of cross mixing effected at the interface since natural gas which is essentially methane (with limited amounts of nitrogen) is considerably lighter than the ethylene vapors contained in the tank especially upon being warmed-up by the walls of the tank. The displaced ethylene vapors are withdrawn from tank T(LE) through line 154 under the control of valve 158, line 156, line 34 and line 48 under the control of valve 50 by the compressor 52. Compressed ethylene vapors are then introduced by lines 54 and 40 into the condenser 46 wherein the ethylene vapors are passed in indirect heat transfer relationship with natural gas in coil 74 to liquefy thereby the ethylene vapors. Liquefied ethylene collected in the bottom of the condenser 46 is withdrawn from condenser 46 by pump 122 through line and is passed by line 126 under the control of valve 130 to the liquefied ethylene holding tank 132. Ethylene vapors&#39;in the liquefied ethylene holding tank 132 are returned to the compressor 52 through lines 134, 34 and 48.  
  During the initial phases of the purge procedure, as hereinabove discussed, essentially pure ethylene vapors from the liquefied ethylene tank T(LE) are introduced into the fractionating condenser 46. Trace impurities, such as methane and hydrogen in the ethylene vapors are withdrawn as condenser overhead in line 84 and are passed through coil 88 in condenser 46 to heat the gas prior to introduction into the fuel gas header 82. After purge procedure has progressed to a point where about 75% of the volume of tank T(LE) includes natural gas, the gaseous stream in line 154 will contain increasing amounts of methane which necessitates the use of the reboiler coil 63 in condenser 46 to strip the methane from condensed liquefied ethylene accumulating in the bottom of the condenser 46.  
  The purge of the tank T(LE) with cold natural gas will concomitantly cool the tank and by the time a volume of natural gas at a temperature of 235F equal to the volume of the tank has been introduced into the tank the temperature of the walls of the tank will have dropped by some 6 to 8F depending of course on the heat capacity of the material of construction of the tank walls and the amount of insulation. In this regard, spherical tanks which are constructed for a slight pressure above atmospheric will take a considerably longer time to cool down than do membrane type of tanks. To purge a tank of its ethylene vapors under atmospheric pressure will take from about 60 to 80 hours in which time some 92 to 95% of the ethylene vapors will have been condensed into the liquid phase. The liquefied ethylene stored in liquefied ethylene. holding tank 132 will be contaminated with a little methane which is of no consequence in view of the subsequent use of the liquefied ethylene in the tank 132, as more fully hereinafter discussed.  
  It will be obvious to one skilled in the art that in accordance with this hereinabove discussed purge procedure that a tank may be cooled only a limited number of degrees regardless of the length of time of such procedure. If there were no heat losses, the tank could be cooled to within a few degrees of the temperature of the natural gas introduced into the coil 74 in the condenser 46. However, in accordance with the purge procedure of the present invention, the temperature of the tank T( LE) is lowered to a temperature of about 95 to 2()()F. To further reduce the temperature of the tank to about the temperature of the boiling point of LNG would require the introduction of LNG at a controlled rate of injection such as is the normal practice when cooling LNG tanks at the loading terminal. Without the purge procedure of the present invention, the amount of LNG required to cool the tank is in the order of about 26 metric tons depending upon the heat capacity of the materials of construction of the tank walls and the amount of insulation. Such an amount of LNG to be vaporized is equal to the propulsion fuel requirement of about 2 V2 to 3 hours of a tank ship designed in accordance with the above specification. Consequently, it is thus advantageous to effect such purge procedure during the return voyage of the vessel since vaporizing LNG may be passed to the propulsion unit of the vessel rather than effecting the purge procedure at the loading terminal and burning the purged vapors in a flare stack included in the loading terminal facilities, thereby losing the fuel value thereof.  
 CONVERSlON FROM LNG STORAGE TO LlQUEFlED ETHYLENE STORAGE Should a tank of vessel previously have been used to transport LNG and is to be readied for liquefied ethylene transport, it is necessary to purge the tank T(LE) of the natural gas and LNG as well as to heat the walls thereof. Accordingly, liquefied ethylene from the holding tank 132 is withdrawn through line 138 by pump 140 and is passed through line 142 and 148 to heat exchanger 160 for vaporization of the liquefied ethylene. Gaseous ethylene is withdrawn from heat exchanger 160 and is passed through line 162 under the control of valve 164 to the liquefied ethylene tank T( LE). Upon introduction into the tank T(LE), gaseous ethylene is condensed in the colder atmosphere and against the colder tank walls thereby warming up both the atmosphere and tank walls. As the pressure tends to increase, gases are withdrawn by line 34 and passed to the fuel gas header 82 via the condenser 46 and the coil 88 through lines 38, 40, 84, 90 and 76. Upon reaching a temperature of about l70F, introduction of gaseous ethylene in line 162 is discontinued and the tank T(LE) permitted to warm up further by natural conduction (&#39;i.e., normal heat leakage).  
  Upon reaching a temperature of about l50F, the remaining portion of liquefied ethylene in the holding tank 132 is withdrawn at a controlled rate through line 138 by pump 140 and passed through line 142 and 146 under the control of valves 144 and 150, respectively, and by line 154 into the tank T(LE). As the tank T(LE) becomes further warmed by heat transfer from the surrounding environment, a portion of the liquefied ethylene in the tank T(LE) becomes vaporized thereby gradually driving-off the residual amounts of natural gas through line 34 and into the fuel gas header 82, as hereinabove discussed. Minor quantities of gaseous ethylene will be contained in the natural gas withdrawn from the tank T(LE) through line 34 and such minor quantities of gaseous ethylene are condensed in the fractionating condenser 46 and returned to the tank T( LE) through line 124 by pump 122.  
  Liquefied ethylene is introduced into the liquefied ethylene holding tank 132 at the outset of the contemplation of ethylene transportation in an amount or inventory sufficient to replace the natural gas atmosphere in the tank T( LE) by an ethylene atmosphere of relativelyhigh purity. Moreover, the atmosphere will constantly be further purified by continued operation of compressor 52 to maintain the pressure in the tank T( LE) at a constant value upon arrival of the vessel at the on-loading facilities whereat the vapor space of the tank T( LE) is placed in fluid communication by lines 34 and 174 with shore side liquefied ethylene storage tanks without endangering or impairing the quality of liquefied ethylene stored therein. Thus, liquefied ethylene may be taken on board without any appreciable load flash loss&#34; occasioned by the tank T(LE) not being in temperature equilibrium with the liquefied ethylene being loaded.  
  it will be appreciated that the modifications of the pressure maintenance or re-liquefaction system to eliminate any loss of a more valuable cargo, e.&#39;g., ethylene in the simultaneous transportation of LNG and liquefied ethylene or other higher boiling hydrocarbons and to create a capability while the vessel is at sea to prepare for a change in liquefied gas as hereinabove described, eliminates costly delays of the vessel in port and saves valuable cargo by a very modicum of additional equipment.  
 SYSTEM FOR FLEET TRANSPORTATION OF LIQUEFIED GASES It will be understood that a viable size LNG route would require a plurality of such vessels handling between about 3 million to 5 million tons of LNG annually for the transportation of LNG from the source port to the terminal port. Liquefied ethylene has been shipped in small refrigerated tank ships (up to about 4,000 cubic meters) in mostly coastwise traffic over distances in the order of 1,000 nautical miles. The exportation of LNG together with the liquefied product of an ethylene plant would effect economies in the transportation of liquefied ethylene as compared with tank ship transportation of liquefied ethylene per se. Thus, if it were necessary to transport about 3 million metric tons per year of LNG and about 450,000 metric tons of liquefied ethylene per year between terminals about 7,000 nautical miles apart, a system would require a fleet of six tank ships having the specifications hereinabove described with at least three tank ships modified with the hereinabove discussed reliquefaction system, although it is contemplated that all of the tank ships of the fleet may be so equipped. Thus, the ships could carry LNG alone, or LNG and liquefied ethylene depending upon the quantity of liquefied ethylene available to be transported upon arrival of any one of the ships of the fleet at the loading terminal. Thus, if the expected quantity of liquefied ethylene at the loading terminal would be insufficient to fill the tank at the estimated time of arrival, the captain of the vessel would be accordingly notified and the tank T( LE) would be subjected during the return voyage to the hereinabove discussed purge procedure to change the atmosphere in the tank T(LE) to permit LNG loading. Alternately, if the tank T(LE) had stored LNG and a sufficient quantity of liquefied ethylene is available to fill the tank, the tank T(LE) could be converted to liquefied ethylene handling duty as also hereinabove discussed.  
  The liquefied ethylene would be transported without loss thereof by utilizing the cold potential in the unavoidable boil-off from the LNG tanks during transportation as a result of heat leakage as well as the effects of sea conditions.  
  The storage facilities at the terminals would depend on the capacity of the vessels, and would be nominally sized to store a multiple of about 1.2 to 2.0 times the volume of a vessel. Thus, it is possible in accordance with the system of the present invention to provide a fleet of vessels capable of handling LNG and liquefied ethylene, and to more frequently transport the liquefied ethylene in smaller volumes thereby reducing the size of the liquefied ethylene storage facilities at the terminals to a fraction of that required if a liquefied ethylene tank ship of optimal size were employed. Thus, the system of the present invention would be to employ a fleet of vessels equipped with the hereinabove disclosed re-liquefaction facilities, which vessels are in continuous operation between terminals wherein one cryogenic tank of each vessel is capable of handling LNG or liquefied ethylene with provisions to eliminate or minimize ethylene losses and wherein the tank so equipped may be converted to the storage of another liquefied gas during transit of the vessel.  
 EXAMPLE The following table is illustrative of the operating characteristics of the process of the present invention for the re-liquefaction of ethylene vapors utilizing the unavoidable boil-off from the LNG tanks of a tanker having a capacity of 125,000 cubic meters:  
  As hereinabove mentioned, the present invention is applicable to the transportation of other combinations of liquefied gas, e.g., LNG-ethane; LNG-LPG; LNG- propane; LNG-ammonia, etc., wherein the liquefied gases have substantially different boiling points. It is particularly advantageously applicable to the transportation of a combination of liquefied gases wherein the lower boiling liquefied gas may be used as fuel for the vessel. The lower boiling gas should be present in an amount to provide the refrigeration potential to condense the unavoidable boil-off of the higher boiling liquefied gas. With regard to the transportation of LNG- liquefied ethylene the respective volumetric capacity of the tank(s) should be in a ratio of at least 4:1 as hereinabove discussed, whereas with other combinations of liquefied gas, the ratios will differ. For example, the transportation of LNG-liquefied ethane would require volumetric ratios of between about 3.0:1 to 3.511 whereas for an LNG-LPG system, a volumetric ratio of as low as 2:1 would be satisfactory.  
  While the invention has been described in connection with several exemplary embodiments thereof, it will be understood that many modifications will be apparent to those of ordinary skill in the art; and that this application is intended to cover any adaptations or variations thereof. Therefore, it is manifestly intended that this invention be only limited by the claims and the equivalents thereof.  
 What is claimed is:  
  l. A process for the re-liquefaction of a gas evolved from a first liquefied gas stored at about atmospheric pressure in a first cryogenic tank means proximate to a second cryogenic tank means storing a second lique- 3. The process as defined in claim 2 wherein said higher boiling liquefied gas is returned to said cryogenic tank means storing said higher boiling liquefied gas.  
  4. The process as defined in claim 2 wherein said liquefied gases withdrawn from said respective cryogenic tank means are compressed prior to step (c).  
  5. The process as defined in claim 2 wherein the gas from said second cryogenic tank means recovered from step (c) is passed to a propulsion unit of said vessel.  
  6. The process as defined in claim 2 wherein said second liquefied gas is LNG.  
  7. The process as defined in claim 6 wherein said first liquefied gas is liquefied ethylene and wherein the ratio of the volumetric capacity of said second cryogenic tank means to the volumetric capacity of said first cryogenic tank means is at least about 4:1.  
  8. The process as defined in claim 6 wherein said first liquefied gas is liquefied ethane and wherein the ratio of the volumetric capacity of said second cryogenic tank means to the volumetric capacity of said first cryogenic tank means is between about 3.0:] to 3.5: l.  
  9. The process as defined in claim 6 wherein said first liquefied gas is LPG and wherein the ratio of the volumetric capacity of said second cryogenic tank means to the volumetric capacity of said first cryogenic tank means is about 2:1. I  
  10. Apparatus for the re-liquefaction of a gas evolved from a first liquefied gas stored in first cryogenic tank means proximate to a second cryogenic tank means storing a second liquefied gas wherein said liquefied gases are stored at about atmospheric pressure and wherein said liquefied gas have different boiling points which comprises:  
 a. first conduit means for withdrawing a gas from said first cryogenic tank means;  
 b. a second conduit means for withdrawing a gas from said second cryogenic tank means; and  
 c. heat transfer means for passing said gases in indirect heat transfer relationship to re-liquefy the gas withdrawn from the cryogenic tank means storing the higher boiling liquefied gas.  
  11. The apparatus as defined in claim 10 including a third conduit means for passing said liquefied gas from said heat transfer means to said cryogenic tank means storing said higher boiling liquefied gas.  
  12. The apparatus as defined in claim 10 wherein said re-liquefaction apparatus is disposed on a vessel.  
  13. The apparatus as defined in claim 12 including compressor means disposed in said first and second conduit means.  
  14. The apparatus as defined in claim 12 including a fourth conduit means for the gas withdrawn from said cryogenic tank means storing a lower boiling liquefied gas to .propulsion means for said vessel.  
  15. Apparatus for modifying a cryogenic tank on a vessel having a plurality of cryogenic tanks for storing at about atmospheric pressure liquefied gases having different boiling points wherein said modified tank is to store a higher boiling liquefied gas which comprises:  
 a. heat transfer means positioned on said vessel;  
 b. a first conduit means disposed between the vapor space of said one of said plurality of cryogenic tanks and said heat transfer means; and  
 c. a second conduit means disposed between the vapor space of said remaining tanks and said heat transfer means, said heat transfer means providing for the passage in indirect heat transfer relationship of vapors in each of said conduit means.  
  16. The apparatus as defined in claim 15 including a third conduit means disposed between said heat transfer means and said one of said plurality of tanks.  
  17. The apparatus as defined in claim 16 including a fourth conduit means disposed between said heat transfer means and a liquefied gas holding tank.  
 18. The apparatus as defined in claim 17 including a fifth conduit means disposed between said holding tank and said one of said plurality of cryogenic tanks, and wherein a second heat transfer means is provided in said fifth conduit means.  
 19. A method for modifying a cryogenic tank on a vessel having a plurality of cryogenic tanks to store at about atmospheric pressure liquefied gases having different boiling points wherein said modified tank is to store a higher boiling liquefied gas of said liquefied gases which comprises:  
 a. positioning a heat transfer means on said vessel;  
 b. providing a first conduit means from the vapor space of said one of said plurality of tanks to said heat transfer means; and  
 c. providing a second conduit means between the vapor space of said remaining tanks and said heat transfer means. said heat transfer means providing for the passage of said gases in each of said conduit means in indirect heat transfer relationship therebetween.  
  20. The method as defined in claim 19 wherein a third conduit means is provided between said heat transfer means and said one of said plurality of tanks.  
  21. The method as defined in claim 20 wherein a fourth conduit means is provided between said heat transfer means and a liquefied gas holding tank.  
  22. The method as defined in claim 21 wherein a fifth conduit means is provided between said holding tank and said one of said plurality of tanks and wherein a second heat transfer means is included in said fifth conduit means.  
 . 23. The method defined in claim 19 wherein a compressor means is provided in said first and second conduit means.  
  24. A process for the preparation of a first cryogenic tank means for the storage of a first liquefied gas on a vessel having a second cryogenic tank means for the storage of said first liquefied gas wherein said first cryogenic tank means previously stored a second liquefied gas having a higher boiling point and wherein residual quantities of said second liquefied gas remain in said first cryogenic tank means after unloading said second liquefied gas therefrom, which comprises:  
 a. withdrawing a gas evolved from the residual portions of gas in said second cryogenic tank means;  
 b. introducing a portion of said evolved gas of step (a) into said first cryogenic tank means;  
 c. withdrawing a gas from said first cryogenic tank means;  
 (1. passing said gas of step (c) in indirect heat transfer relationship with the remaining portion of gas from step (a) to liquefy said gas of step (c); and  
 e. passing said liquefied gas recovered from step (d) to a storage holding zone.  
 25. The process as defined in claim 24 wherein said first liquefied gas is LNG.  
  26. The process as defined in claim wherein said second liquefied gas is liquefied ethylene. and wherein the ratio of the volumetric capacity of said second cryogenic tank means to thevolumetric capacity of said first cryogenic tank means is&#39;at least about 4:1. a  
  27. The process as defined in claim 24 wherein said preparation is effected during transit of said. vessel;  
  28. Apparatus for the preparation of a first cryogenic means for the storage of a first liquefied gas on a vessel having a second cryogenic tank means for the storage of said first liquefied gas wherein said first cryogenic tank means previously stored a second liquefied gas having a boiling point higher than said first liquefied gas which comprises:  
 a. a first conduit means for withdrawing a gas from said first cryogenic tank means;  
 b. a second conduit means for withdrawing a gas from said second cryogenic tank means;  
 c. a third conduit means for passing a portion of said gas withdrawn from said second cryogenic tank means to said first cryogenic tank means; and  
 d. heat transfer means for passing in indirect heat transfer relationship the gas in said first conduit means and the remaining portion of gas in said second conduit means to liquefy said gas in said first conduit means.  
  29. The apparatus as defined in claim 28 including a fourth conduit means for passing liquefied gas to a storage tank.  
  30. The apparatus as defined in claim 28 wherein the ratio of the volumetric capacity of said second cryogenic tank means to the volumetric capacity of said first cryogenic tank means is at least about 4:1 and wherein said first and second liquefied gases are LNG and liquefied ethylene, respectively.  
  31. A process for the preparation of a first cryogenic tank means for the storage of a first liquefied gas on a vessel having a second cryogenic tank means for the storage of a second liquefied gas having a boiling point lower than said first liquefied gas wherein said first cryogenic tank means previously stored said first liquefied gas, which comprises:  
 a. introducing into said first cryogenic tank means a gas formed from said first liquefied gas;  
 b. withdrawing a gas from said first cryogenic tank means and passing said gas in indirect heat transfer relationship with a gas evolved from said second liquefied gas in said second cryogenic tank means to condense higher boiling gas contained therein; and  
 c. discontinuing the flow of said gas into said first cryogenic tank means after a predetermined period of time and thereafter introducing a first portion of said first liquefied gas into said first cryogenic tank means.  
  32. The process as defined in claim 31 wherein said first and second liquefied gases are liquefied ethylene and LNG, respectively, and wherein the ratio of the volumetric capacity of said second cryogenic tank means to the volumetric capacity of said first cryogenic tank means is at least about 4:1.  
  33. Apparatus for the preparation of a first cryogenic tank means for the storage of a first liquefied gas on a vessel having a second cryogenic tank means for the storage of a second liquefied gas having a lower boiling point than said first liquefied gas and wherein said first cryogenic tank means previously stored said second liquefied gas which comprises:  
 a. a storage tank on said vessel for said first liquefied gas; .1  
 b. a first conduit means for introducing into said first cryogenic tank means a gas from said storage tank;  
 c. a second conduit means for withdrawing gas from 7 said first cryogenic tank means;  
 d. a third conduit means for withdrawing a gas from said second cryogenic tank means; and  
 e. a heat transfer means for passing in indirect heat transfer relationship the gas in each of said second and third conduit means to condense any higher boiling gas in said second conduit means.  
  34. The apparatus as defined in claim 33 wherein the ratio of the volumetric capacity of said second cryogenic tank means to the volumetric capacity of said first cryogenic tank means is at least about 4:1 and wherein said first and second liquefied gases are liquefied ethylene and LNG, respectively.  
  35. A method of transporting a first liquefied gas from a loading terminal to an unloading terminal utilizing a plurality of vessels having a first cryogenic tank means for said first liquefied gas and a second cryogenic tank means for a second liquefied gas having a boiling point higher than said first liquefied gas which comprises:  
 a. determining the quantity of said second liquefied gas to be stored at said loading terminal at the estimated time of arrival of said vessel; and  
 b. preparing en route said second cryogenic tank means for the loading of said first liquefied gas should the amount of said second liquefied gas be insufficient to fill said second cryogenic tank means, the preparation of said second cryogenic tank means including the introduction of a portion of natural gas in said first cryogenic tank means into said second cryogenic tank means and the reliquefaction of higher boiling gas withdrawn from said second cryogenic tank means by the passage of the gas withdrawn from said second cryogenic tank means in indirect heat transfer relationship to a portion of natural gas withdrawn from said second cryogenic tank means.  
  36. The method as defined in claim 35 wherein said first and second liquefied gases are LNG and liquefied ethylene, and wherein the ratio of the volumetric capacity of said first cryogenic tank means to the volumetric capacity of said second cryogenic tank means is at least about 4:1.  
  37. A method of transporting a first liquefied gas and a second liquefied gas having a boiling point higher than said first liquefied gas from a loading terminal to an unloading terminal utilizing a plurality of vessels having a first cryogenic tank means for said first liquefied gas and a second cryogenic tank means for said second liquefied gas wherein said cryogenic tank means previously stored said first liquefied gas which comprises:  
 a. determining the quantity of said second liquefied gas to be stored at said loading terminal at the estimated time of arrival of said vessel; and  
 b. preparing en route said second cryogenic tank means for the loading of said second liquefied gas should the amount be sufficient to fill said second cryogenic tank means, the preparation of said second cryogenic tank means including the introduction of a higher boiling gas formed from said second liquefied gas into said second cryogenic tank 39&#39;. The method as defined in claim 38 wherein said first and second liquefied gases are LNG and liquefied ethylene. and wherein the ratio of the volumetric capacity of said first cryogenic tank means to the volumetric capacity of said second cryogenic tank means is at least about 4:1.