Abstract:
A method and apparatus is described for transporting a natural gas in the form of a hydrate. The method utilizes the pressure and temperature conditions of a submerged vessel to facilitate the formation and maintenance of natural gas as a hydrate during the subsea voyage and the subsequent reconversion of the hydrate to a natural gas when the destination of the vessel is reached. 
     The submarine vessel can have: supplementary refrigeration, a hold or void in which a natural gas is hydrated, and a membrane pervious to gas and water within the hold. In the vicinity of the hold bottom are situated gas conductors with spargers through which the natural gas is pumped into the hold. Adjacent to them are cold-water distributor pipes through which water is pumped into the hold for forming a gas hydrate and for removing the heat of formation. 
     The membrane is spaced from the interior wall of the hold so that a gap around and within the hold is formed. This gap provides a path for the water and gas to travel upward to an exit conduit located near the top of the vessel. From this conduit, the gas and water is recycled to form a hydrate.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method and apparatus for hydrating and transporting natural gas in the form of a hydrate. More particularly, the method and apparatus utilize the pressure and temperature conditions surrounding a submerged vessel to facilitate, maintain and reconvert a natural gas to and from a hydrate. 
     2. Description of the Prior Art 
     The knowledge that a natural gas is usable as a fuel has been known, as well as used as such, for a long time. A problem associated with its use is transporting if from the source to another place, since it ordinarily occupies a larger volume per available Btu than other fuels that are solid or liquid. One manner of overcoming this problem is to increase it pressure in the transfer container. Alternatively, the natural gas industry has utilized the process of reducing the temperature until the gas becomes a liquid at a predetermined pressure. This method results in liquid natural gas, LNG. Another way that is being considered by the natural gas industry is to partially oxidize methane, the major constituent of natural gas, to methanol in order to produce an easily transportable liquid. 
     In the first method--utilizing increased pressure--heavy containment devices are necessary. They are expensive to construct and, in the case of marine transport, difficult to maintain the requisite pressure. Likewise, if this method is used in the form of a cross-country pipeline, large compressors are necessary. This arrangement results in a large consumption of the natural gas itself to provide the pressure differential that moves the gas through the pipeline. 
     In the second method--manufacturing LNG--there is a very high investment required for the processing plant as well as the tankship, because of the equipment necessary to liquefy methane at low temperatures. As a result, large refrigeration loads utilize much of the gas sought to be transported. For instance, to convert a natural gas to a liquid consumes 12% to 14% of the gas. Another 6% to 8% of the source gas boils off during the sea transportation, though it is possible to utilize a portion of this boiled-off gas as a boiler fuel. 
     The third process--converting natural gas to methanol--also requires capital investments similar to those of LNG manufacture. Noteworthy of the conversion to methanol is that the conversion destroys up to 47% of the calorific value of the source gas. 
     SUMMARY OF THE INVENTION 
     According to the present invention, a method is devised whereby the amount of energy and equipment required to hydrate and dehydrate a natural gas is significantly reduced. Consequently, not only does the invention eliminate considerable amount of costly equipment, but there are also substantial savings in energy costs. 
     The invention utilizes the effects of pressure and temperature of the water of the particular depth of the vessel&#39;s submergence. Specifically, a vessel is lowered to a depth at which the pressure and the temperature is suitable, or nearly so, for hydrating natural gas. Then the natural gas is transferred aboard the ship from a reservoir or a well located nearby and it is sparged with cooling water as required or otherwise intimately contacted with water to form a hydrate. Once the hydrate is formed, the vessel is relocated while keeping the natural gas in a hydrate form to the desired site. Upon its arrival there, the vessel can be raised in the water to decrease the pressure and increase the temperature to facilitate reconverting the hydrate to the natural gas it originally was. During and after such reconversion, the natural gas is transferred to the marketing site or in some cases a storage area. 
     Alternatively, some natural gas may be transferred aboard the vessel prior to lowering. This natural gas may be compressed or liquefied by conventional methods. 
     In case the water elevation of the vessel does not provide adequate pressure and temperature to carry out the conversion or reconversion of the gas to the required form, supplementary means for appropriately varying them can be provided aboard the transporting vessel. Such means may be as insignificant as insulating the submerged vessel either internally or externally of the holding tank for the natural gas. Or it may be as sophisticated as circulating the surrounding waters through the holding tank in appropriate conduits to transfer the heat given off by the exothermic reaction of hydration, or adding heat required by the endothermic reaction or dehydration. 
     The submarine vessel mentioned above can have: supplementary refrigeration, a hold or void (a fluid-tight compartment) in which a natural gas is hydrated, and a membrane pervious to gas and water within the hold. In the vicinity of the hold bottom are situated gas conductors with spargers through which the natural gas is pumped into the hold. Adjacent to them are cold-water distributor pipes through which water is pumped into the hold for forming a gas hydrate and for removing the heat of formation. The membrane is spaced from the interior wall of the hold so that a gap around and within the hold is formed. This gap provides a path for the water and gas to travel upward to an exit conduit located near the top of the vessel. From here, the gas and water is recycled to form a hydrate. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is a schematic illustration of a submarine vessel located beneath the water surface and interconnected by a pipeline to an onshore gas processng plant. 
     FIG. 2 is a simplified flow schematic of the hydrate process cycle in water depth of 1500 feet. A cutaway plan view of the vessel is also illustrated. 
     FIG. 3 is a schematic illustration of a cross section of the vessel taken along section line 3--3, FIG. 2. 
     FIG. 4 graphically illustrates the energy requirements for a liquid gas system and a hydrate system at the conditions of FIG. 2. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     An initial step of the preferred method is to lower the vessel 100 to a point D, FIG. 1, where sufficient pressure from the water is achieved. Simultaneously, at such a depth it is desirable that the temperature of the surrounding water be suitable for the hydration process. Examples of the desirable temperature and pressure variations are given and elaborated on below. 
     For example, a gas of 90% methane, 9.5% ethane, and 0.5% propane may be transferred through pipeline 109, pump 125 and pipeline 120 to the submerged vessel 100 moored to an offshore terminal 123 from an onshore gas processing plant 110. A thermodynamic derivation indicates that hydration of this gas will start when the temperature is 40°F and the pressure is at 267 psia. Alternatively, if the pressure is 600 psia, hydration starts at 52°F, but a temperature of 42.5°F is required to solidify the total gas stream. The final pressure and temperature conditions, however, should be respectively higher and lower to provide a driving force for the conversion. Thus, 600 psi and 40°F is recommended, though the entire process could be conducted in the temperature range of 35°F to 125°F and a pressure range of 150 psia to 1038 psia (alternatively 1300 feet to 2000 feet). 
     With respect to the temperature, it is noteworthy that the contacting temperature for making the natural gas hydrate is desirably near or above the freezing point of water, e.g., temperature of about 35°F is satisfactory. This is achievable at a depth of 2000 feet (approximately 880 psig). Ordinarily at 2000 feet the temperature rarely exceeds 50°F and is more likely around 40°F. Such conditions are available on voyages between Seward, Alaska, and Los Angeles, Calif. This route provides sufficient water temperature and depths to preserve a hydrate at 800 psia and 46°F or even at 600 psia and 41°F. 
     The components of the natural gas are also an important consideration. For instance, if nitrogen is present, an increased pressure is required for a given quantity of solidification since this component is detrimental to complete solidification. The quantity of residue gas that cannot be solidified is approximately 3 times the nitrogen gas content. Additionally, it is preferable that the natural gas be free of CO 2 , H 2  S, butanes, most of the propane, as well as most hydrocarbons heavier than normal butane prior to processing. The reason is that these compounds, which have different properties, are likely to form a hydrate yielding unpredictable results owing to the possible coexistance of 3 phases. In this respect, preprocessing for the subsequent hydrate conversion is similar to LNG requirements. 
     Furthermore, at a given pressure, ethane hydrates at a higher temperature than methane, while requiring one and a half times more water. But these requirements are substantially offset by having approximately one and three quarters more heating or calorific value. Thus, ethane actually improves the economic advantages of the present invention. On the other hand, the presence of propane is not as favorable because it requires nearly three and a half times as much water with only two and a half times the calorific value of methane. Yet the presence of some propane is tolerable. 
     Another factor controlling the rate of reaction is the film resistance of water to hydrocarbon gases. Hydrocarbon solubility in water is very low. But by continuously removing the hydrocarbon molecules from the water phase by forming hydrates, the equilibrium is shifted to favor hydration. Thus, both large mass-transfer areas [finely divided water phase] and agitation of the aqueous phase to bring hydrocarbon molecules in contact with hydrate crystal structures being formed are beneficial for hydrate formation. 
     In short, sparging of the gas stream to the liquid phase is an effective way of accomplishing the goal of forming a hydrate quickly. Excess water, however, is necessary for this to be a satisfactory solution. Other alternatives that facilitate hydration are small amounts of alcohol or soaps in the water, or the utilization of an emulsion of water in mineral oil. 
     Referring specifically to FIGS. 2 and 3, an outline of the submarine vessel, which may be self-propelled or towable, is indicated by number 100. Gas is introduced from the surface through intake 114 through valve 158 into conductor 109. From here it is sparged through spargers 131 into the cold water 130 contained in the vessel 100. This water may be there from a prior run or added at the loading dock, point D, FIG. 1. 
     Gas hydrate crystals are formed upon contact of the gas with the cold water. Additional cold water is jetted into the region of formation of the hydrate in order to remove heat of formation. the hydrate crystals 168 by virtue of their low specific gravity relative to water float upward. They are retained at the upper part of the vessel by a porous membrane 110 (pervious to water and gas). In this location they form a semi-solid mass 167 which builds downward as the vessel is loaded. The lower limit of this mass is approximated by dashed line 163. 
     Within the submarine 100, FIGS. 2 and 3, are three systems for producing and maintaining the low temperatures required for producing and preserving the gas hydrate. The first of these can be used at the point of loading. In this case, valves 154 and 155 are opened, and cold water is pumped from surface refrigeration facilities to the vessel through pipeline 107 and cold water water distributors 108. The water is jetted through nozzles 132 on the distributors 108 (opposite the gas spargers 131) into the region of hydrate formation. It then passes upward with the hydrate (indicated by arrows numbered 160, FIG. 3) and passes through permeable membrane 110 (indicated by arrows numbered 161, FIG. 3). From here it returns through the collector pipe 111 at the top of the vessel. Then it passes through gas separator 171 which separates out any excess gas from the water. The gas is returned through pipe 170 for recycling and the water is returned to onshore facilities through pipe 134. 
     The second system can be used in transit through sea water cold enough to prevent melting of the hydrate at the ambient pressure, such as 42°F and 600 psia. Valves 156 and 157 are opened and sea water is induced through intake 112 and pumped (by pump 104) through cold water distributors 108 out collector 111 and returned to the sea through outlet 113. 
     The third system can be used in transit through sea water too warm to prevent melting of the hydrate. For instance, a simple closed-circuit vapor-compression system can be used for refrigeration--though other refrigeration systems are usable. 
     In the closed-circuit system, FIG. 2, the refrigerant may be propane while the coolant is water at a water depth of 1500 feet (pressure of approximately 650 psia in fresh water). The coolant has an entrance temperature of 55°F at valve 150. And at valve 151 it has an exit temperature of 65°F after passing through a condenser 101. The condenser condenses the refrigerant before it is throttled through expansion valve 105 where its temperature is reduced to 29°F. While changing from a liquid to a vapor, the refrigerant is utilized to cool down the water passing through chiller 102 from 42.8°F to 34°F. The vaporized refrigerant is then passed through the compressor 106 at a higher pressure and temperature so that it will liquefy at atmospheric pressure. With valve 153 closed and valve 152 open, water is induced through intake 112 into chiller 102. From here, the newly chilled water increases in temperature one degree due to heat transfer gains to the water from its surroundings as it flows from the exit of the chiller 102 into cold water distributors 108. As before, this water moves through membrane 110 to collector pipe 111 and outlet 113. 
     As an alternative to using refrigeration described above, an external water layer (not illustrated) between the vessel hull and the cargo hold is chilled as low as possible, e.g., 32°F, to form slush ice before leaving the loading point D, FIG. 1. With proper insulation in combinaton with the slush layer, hydrate reconversion during a voyage is minimized because heat gains of the hydrate are reduced. 
     When the vessel 100 is at the desired site (Point E, FIG. 1), it is raised to a depth where the water pressure is less than required to preserve the hydrate. Thus the hydrate melts. During this stage, the reconversion process is slowed down unless more heat is added, since the reverse process is endothermic. To provide this heat, the warmer water the vessel is in is circulated through the water or ice layer formed between the cargo and vessel hull 100. With the hydrate converted to a natural gas, the gas is pumped through pipelines 121, 126 by pump 127 mounted on the offshore platform 122 to a tank 124. 
     The theoretical energy requirements for the liquefaction, based on a modified ideal cascade system, as compared to hydration of a methane flow of one million standard cubic feet per day (MMSCFD) as a function of initial condensing temperature is graphically shown in FIG. 4. Along a horizontal axis is plotted the condensing temperature and along a vertical axis is plotted theoretical brake horsepower per MMSCFD, where ##EQU1## 
     In the equation, Q 2  is the heat-extracted Btu/hr/MMSCFD. T 2  is the absolute temperature of the boiling refrigerant, °R. And T 1  is the condensing temperature of the refrigerant, °R. In both curves on the graph of FIG. 4 and as noted above, the only variable is condensing temperature; all other conditions are as set out in the preceding paragraphs. 
     The theoretical energy requirements for the liquefaction cycle are based on a cascade process using 3 trains having one or more stages in combination with after-cooling. There are other known systems, such as either the Hampson-Linde or the Claude System to achieve liquefaction of a natural gas. They were not chosen, however, because the cascade process expands only liquids; thus it is less irreversible with consequent greater power economy. 
     The following briefly describes derivation of the point on the graph for liquefaction whose abscissa, condensing temperature, is 70°F. In the first train, the methane stream is initially cooled down by evaporating propane, which is in turn compressed and liquefied in an after-cooler using water as a coolant. The stream then passes to the second train, where the stream is additionally cooled by evaporating ethylene, in turn liquefied by evaporating propane, followed by the propane being liquefied by water cooling. The methane stream finally becomes a liquid at 201°F and 14.7 psia. This is achieved in the third train by the evaporation of methane, which is liquefied by evaporating ethylene, in turn liquefied by evaporating propane. The propane in turn is liquefied by water cooling. A summary of the conditions at the various points in each stage is shown below: 
     
         SUMMARY OF CASCADE PROCESS    Train 1         Train 2         Train 3          ΔH=Q.sub.2                          ΔH=Q.sub.2                                          ΔH=Q.sub.2    Tin,       Tout,          MBH  TBHP Tin,                       Tout,                          MBH  TBHP Tin,                                       Tout,                                          MBH  TBHP    °R       °R          MMSCFD               MMSCFD                    °R                       °R                          MMSCFD                               MMSCFD                                    °R                                       °R                                          MMSCFD                                               MMSCFD__________________________________________________________________________Methane Stream    520       426          183.2               --   426                       310                          348.8                               --   310                                       201                                          129.5                                               --AftercoolerCoolant--water    515       525          --   --   515                       525                          --   --   515                                       525                                          --   --    Refrig-Stage    erant1   Propane    530       416          --   13.9 530                       416                          --   37.5 530                                       416                                          --   19.72   Ethylene    -- -- --   --   421                       305                          --   52.1 421                                       305                                          --   27.43   Methane    -- -- --   --   -- -- --   --   310                                       196                                          --   41.8Subtotal TBHP       13.9            89.6            88.9TOTAL TBHP                                               192.4__________________________________________________________________________Abbreviations:   Tin = Temperature in, T.sub.1   Tout = Temperature out, T.sub.2   ΔH = Change in Ethalphy from a Mollier diagram with Btu/Hr   converted      to MBH/MMSCFD   °R = Degrees Rankine   MBH/MMSCFD = Thousands Btu per hour per million standard cubic   feet     per day. For methane, this is derived as follows:    1    (1,000,000 SCF)                      1    16.04 lbs.    1000 (379.5 SCF/mole) day                      24-hr/day  mole    where there are 16.04 lbs. of methane per mole and SCF is a    standard cubic foot                        MBH    1000   TBHP = Theoretical brake horepower =                       MMSCFD  2547 Btu/Hp 
    
     The conditions for the closed-circuit vapor-compression cycle whose ordinate is also 70°F (530°R) are as follows: 
     Water depth, 1500 feet 
     Pressure, 650 psia 
     Hydrate temperature, 502°R 
     Sensible heat, 16.275 MBH/MMSCFD 
     Heat of hydration, 2687.7 MBH/MMSCFD 
     Q 2  total, 89.7 HP/MMSCFD 
     Q 2  total becomes 104.7 HP/MMSCFD when pumping power is added for circulating water. Pressure drop for water circulation is 1 psi per 100 feet of line, plus a fixed value of 20 psi for pressure drop through the condenser and through the spray header. 
     The conclusions drawn from the graph, FIG. 4, follow. The hydrate cycle when at 1500 feet requires 46.6% of the theoretical refrigeration energy required by an LNG cycle; see Curves A and C, FIG. 4. If pumping power for water circulation is added, the value increases to 54.4%, Curve B, FIG. 4. 
     Another way of comparing a hydrate process and a liquid natural gas process is to compare total Btu requirements. For the hydrate process, they are about 5 times greater than for the manufacture of liquid natural gas. But, since the cooling for the hydrate is accomplished between 35°F and 125°F rather than between 100°F and -268°F, the refrigeration horsepower required to make a hydrate is 1/3 that required to make the liquid natural gas. Power requirements are even further reduced if the cooling water is at a lower temperature than the assumed conditions. 
     Contrasted to the above, a few subsea routes may not provide the requisite temperature and pressure. In this case, a supplemental means for regulating the pressure and temperature is used. For example, the hull of the submarine may have more or less insulation to allow for the needed heat transfer between the hold and the environment surrounding it. Likewise, an additional cooling or heating unit may be utilized to aid in hydrating and dehydrating the natural gas. A compressor 200 can also be operatively connected to the holding tank to supplement the pressure due to the water depth of the vessel. 
     Further, the particular equipment available for shipping the natural gas as a hydrate may make it desirable to ship the hydrate as a slurry rather than as a solid. For example, see U.S. Pat. No. 3,514,274, which deals with water-surface transporation of a hydrate slurry. The method disclosed can be made applicable with the present invention. In this method the natural gas is contacted with a C 3  -C 5  hydrate within a range of 25°to 40°F at a pressure above 80 psia. The crystals formed by this contact are carried in a menstruum of liquid C 3  -C 5  under controlled pressures and temperatures. The hydrate of natural gas is returned to its original state by contacting the slurry C 3  -C 5  vapor at less than 80 psia at a temperature between 25° to 40°F. By utilizing this procedure with a submerged vessel to achieve the necessary pressures and temperatures, great economies result. 
     Although only selected embodiments of the present invention have been described in detail, the invention is not to be limited to any specific embodiment, but rather only by the scope of the appended claims.