GB1585167A - Pressurized gas storage and transport system - Google Patents

Description

(54) PRESSURIZED GAS STORAGE AND TRANSPORT SYSTEM (71) We, OCEAN PHOENIX HOLDINGS N.V. a Company incorporated in the Netherlands Antilles, of Box 564 Willemstad, Curacao, Netherlands Antilles (formerly of Shottegatweg Oost 102 PO Box 507 Curacao), do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention relates to the storage of natural hydrocarbon gas mixtures and, more particularly, to a method whereby a natural gas mixture is stored in a dense operating state suitable for transport, particularly by ship, at minimal compression, refrigeration and containment costs per unit weight of the mixture.

Vast amounts of hydrocarbon gases are available from gas and oil fields in regions so far removed or separated by water from sources of demand that much of it has not heretofore been put to commercially profitable use. It is the broad purpose of this invention to provide a new and improved method for storing and transporting these hydrocarbon gases and thus make their energy available in all parts of the world.

In particular, the new method is intended to make use of ships by which gas mixtures can be transported in bulk.

Several methods are already in use for the storage and shipment of light hydrocarbon gas mixtures rich in methane but none of them has been entirely satisfactory. The so-called LNG process, in which methane-rich natural gas is contained at its liquefication temperature (--258"F) and atmospheric pressure, has shown more promise than most, but as a mode of transport for commercial trades it has distinct disadvantages because of the cost involved in achieveing and maintaining such extremely low cryogenic temperatures. By turning instead to certain combinations of moderate compression and refrigeration, for example as described in U.S. Patent Specifications Nos. 3 232 725 and 3 298 805, a method of transport of methane-rich natural gas mixtures which is superior economically to LNG or any other convenient process can be achieved. In the U.S.Patent Specification No. 3 232 725 temperatures below the critical temperature of methane (- 116 F) and pressures no less than the bubble point pressure of the gas are claimed so that the contained mixture is always in a singlephase state during transport. While its container costs may be greater than with LNG because the container must withstand pressure, the savings in refrigeration are so great that this prior method is notably more economical overall.

In a development of this method, as described in U.S. Patent Specification No.

3 298 805, lesser operating temperatures (still well above the cryogenic temperatures of LNG) and reduced operating pressures closer to and even below the bubble point pressure of the gas are preferred particularly for relatively lean mixture and/or short trades.

Further development of this technology has led to the discovery that even greater reductions in operating temperature and a generally lower and far narrower range of operating pressure can result in still further operational and economic advantages for natural gas mixtures comprising essentially C1 up to C hydrocarbons and having a higher calorific value of from 1000 to 2000 B.t.u./S.c.f. The operational advantages are set out hereinafter.

According to the present invention, a method of storing such a natural gas mixture comprises establishing and maintaining the pressure and temperature of the gas mixture within a dense operating state wherein the maximum operating temperature is below --200"F and the minimum operating temperature corresponds to a bubble point pressure of 25 p.s.i.a for the particular mixture and wherein the maximum operating pressure is 25 p.s.i. above the bubble point pressure and the minimum pressure in the phase boundary of the particular mixture.

In this fashion the gas mixture is maintained in a dense operating state suitable for loading, storage and transport at mini mal compression, refrigeration and containment costs per unit weight of gas.

The foregoing definition of natural gas mixtures suitable for containment in accordance with the present method includes wellhead gases, gases separated from crude oil at a wellhead and tail gases from oil refineries and other processing plants. The natural gas mixture as defined does not include propane-butane mixtures conventionally handled in the liquid state as "LPG".

Among the various natural gase mixtures appropriate to the practice of this invention is a typical solution gas with a calorific value of 1527 B.t.u./s.c.f., and which has the following composition on a mol percent basis.

Methane 60.0 Ethane 23.0 Propane 8.0 Butane 6.5 Pentane 2.5 The method of the invention may be better understood by referring to the accompanying drawing, which is a pressure temperature diagram (not drawn to scale) of a representative natural gas mixture showing the contemplated operating state.

Absolute values are not given in this diagram but the shape of the various curves is illustrative of a typical natural gas composition of the type described previously.

The curve ABC defines the envelope wherein the gas mixture exists in a two-phase state, part liquid and part vapor. Point A indicates the liquefacation temperature of the gas mixture at atmospheric pressure and in absolute terms the gas specified above might be approximately -2500F.

Point B is the true critical point of the gas mixture at which the various lines of uniform liquid and vapor concentrations within the two-phase region of the envelope converge. From A to B the envelope curve is generally referred to as the bubble point line since it marks those definite equilibrium states where vapor will begin to appear, for example, during isothermal expansion of the gas mixture. From the critical point B to the point C on the envelope, the curve is commonly referred to as the dew point line at which liquid begins to condense, for example, during isobaric cooling of the gas mixture.

Within the two-phase envelope ABC, it can properly be said that the gas mixture exists as a "liquid" and a "vapor" but outside the envelope it is best thought of as a compressible "fluid" regardless of pressure and temperature.

In the broadest form of the present method, the gas mixture is compressed and refrigerated to an operating state circumscribed by the dotted lines connecting points 1 to 4 on the diagram. Thus, the gas mixture is brought to a maximum operating temperature ie, a maximum temperature for loading, storage and transport below --200"F indicated by the dotted line connecting the points 2 and 3 in the diagram.

In every instance, therefore, the operating temperature defined herein is necessarily less than the minimum operating temperature called for in both the U.S. Patents Nos.

3,232,725 and 3,298,805.

More refrigeration is therefore necessary in all forms of the contemplated method than the prior methods described in said U.S. Patent Specifications.

In the diagram, the dotted line connecting the points 3 and 4 indicates the maximum operating pressure of 25 p.s.i. above the bubble point pressure of the gas mixture. For the gas compositions contemplated in this method, the maximum operating pressure at the warmest condition (maximum operating temperature) (point 3) may be of the order of about 100 p.s.i.a, whilst the operating pressure of the coldest condition (minimum operating temperature) (point 4) may be of the order of about 50 p.s.i.a. The containment tank in which the gas mixture is disposed in the operation of the method may be particularly large and constructed of a material (e.g. high nickel content steel or a high strength aluminium alloy) chosen more for its resistance to low temperatures than for its resistance to greatly elevated pressures.A suitable containment tank and support system is described and claimed in our British Patent Specification No. 1 522 609.

The minimum operating pressure, ie the minimum loading, storage and transport pressure, in accordance with the method, is shown by the dotted line connecting the points 1 and 2 in the diagram, which throughout the contemplated temperature range is on the phase boundary of the gas mixture. For virtually all of the gas mixtures intended for storage and transport by this method, the minimum operating pressure under conditions of least refrigeration (point 2) will be in the order of about 75 p.s.i.a., and for the minimum operating pressure for conditions of greatest refrigeration (point 1) will be in the order of about 25 p.s.i.a.

It will be noted that the dense operating state is the region on the phase diagram which is on the bubble point line at temperatures below the true critica point B of the mixture, ie, in the singe-phase condition of a dense fluid. However, as discussed hereinafter, a gas "ullage" space may be left at the top of the containment tank on loading, and hence an observable interface between separate liquid and vapor phases will be present in the contemplated operating conditions. Approximately 1 to 2 percent of the volume of the filled container tanks will be vapor even at the lowest operating pressure and temperature at the point 1 in phase diagram.One of the principal reasons why this narrow region of two-phase conditions is contemplated by the present invention is to provide the said ullage space within the containment tanks so that with gradual heating of the cargo en route and/or rapid heating of the tanks under emergency conditions (such as flooding of seawater around the tanks in the hold of a ship), the resulting increase in pressure will not exceed the design pressure of said tanks and the decrease of cargo density will not make the tanks liquid-full. Somewhat less cargo is transported per tank when this allowance for ullage is made. However, surge chambers to accommodate expansion of the cargo will generally cost more than the economic disadvantage of lesser net cargo with ullage in the tanks.

Depending upon such factors as the particular composition of the mixture to be contained, the distance of the trade, and so on, there is a working range of optimum or preferred conditions within the overall limits of pressure and temperature described above.

Natural gas mixtures transported in accordance with this invention may be separated at the point of destination essentially to methane for continuous supply into a transmission system and heavier ends such as ethane, LPG, and natural gasoline which may be fed separately to areas of use. The heavy ends may alternatively be converted mainly to methane by exothermic reaction with steam over a nickel-containing catalyst to augment further the pipeline gas supply.

The operational advantages of the present method include the following: (i) The higher liquefaction temperature achieved by the pressurised system permits virtually all hydrocarbons in the Cl to Cs rainge to be shipped together, with a higher heating value up to about 2000 B.t.u./s.c.f.

compared with a maximum near 1350 B.t.u./s.c.f. at atmospheric pressure. As a result, the "whole gas" from oil-fields and condensate-fields can be transported by a simple single system, avoiding duplication throughout by a parallel LPG system which is required at atmospheric pressure.

(ii) The ability to handle rich gas further reduces liquefaction horsepower; and by avoiding intermediate separation of C4- plus gives a simple straight-through high pressure circuit, with a higher flash component and better heat exchange.

(iii) Avoiding separation at the export terminal permits it to be used whole on delivery as boiler fuel, or to be separated into an optimal range of products according to market conditions . . . pipeline gas, ethane, LPG's, natural gasoline, olefins, storable winter-peak components, etc.

(iv) Pressure makes boil-off on ships unnecessary, avoiding a dual-fuel firing system and the penalty of using gas cargo worth substantially more than Bunker C fuel. It also avoids boil-off in terminal storage. These effects reduce the liquefaction capacity required for a given total trade delivery, by 5% or more on a typical long trade. Ship deliverability is also increased, due to zero boil-off and a lower ullage requirement.

(v) Gas displaced ashore during ship loading can be used to replace cargo removed from storage; and as fuel without much compression, and most of the gas evacuated after unloading likewise.

(vi) Pressurized ships have a higher deliverability in Btu's per cubic meter of gross capacity, for the following reasons: Calorific density of rich pressure cargo is up to 20% more than normal LNG Elimination of boil-off increases delivery by 3.6% on a Gulf-Japan trade.

Ullage is typically 1% for pressure, 2% for atmospheric.

Return "heel" is typically 0.5% for pres sure, 1.0% for atmospheric.

Turn-round time is reduced by easier cargo handling under pressure.

Together, these effects give 15% to 25% greater deliverability under pressure.

(vii) Pressure makes gas ships safer. A tank fracture (due for example to a collision) releasing cargo flashes much of it to gas, and drops out a smaller pool of liquid onto the surface of the sea.

(viii) The operating pressure can be arranged to be so far below the tank design pressure that the cargo can absorb a heat leak far in excess of normal voyage requirements. Hence it can be designed to provide the capacity to absorb excessive heat due, for example, to long port delays, or flooding of a hold, without requiring discharge of cargo.

(ix) The method enables containment tanks to be designed and operated such that a costly secondary barrier is not required, and dry-air can be used instead of inert gas in the cargo holds. Pressure makes unstable cargo conditions generally referred to as "rollover" insignificant.

(x) Cargo handling is simplified by pressure.

Very rapid flash re-cooling provides a gas cushion to accept cargo loading.

Pressure makes cargo transfer operations insensitive to cargo temperature pressure density composition etc. (which can be critical near atmospheric pressure).

(xii) Pressure permits cargo temperature/ pressure to be optimized for minimum costs according to trade length and size and gas composition.

WHAT WE CLAIM IS: 1. A method of storing a natural gas mixture comprising essentially C1 up to C4 hydrocarbons and having a higher calorific value of from 1000 to 2000 B.t.u./S.c.f, said method comprising establishing and maintaining the pressure and temperature of the gas mixture within a dense operating state wherein the maximum operating temperature is below --200"F and the minimum operating temperature corresponds to a bubble point pressure of 25 p.s.i.a for the particular mixture, and wherein the maximum operating pressure is 25 p.s.i above the bubble point pressure and the minimum pressure is on the phase boundary of the particular mixture.

2. A method according to Claim 1, wherein the operating pressure at the maximum operating temperature is between 75 and 100 p.s.i.a, whilst the operating pressure at the minimum operating temperature is between 25 and 50 p.s.i.a.

3. A method according to Claim 1 or 2, wherein at least during loading into containment tanks, at least 1 % of the mixture is left as vapor so as to provide a gas "ullage" space within each tank, and wherein the design pressure of the tanks is selected such that, should a gradual or rapid heating of the stored mixture take place for any reason, the resulting increase in pressure will not exceed said design pressure, and the corresponding decrease in density of the mixture will not make the tanks liquid-full, thereby enabling eliminating the need to provide for liquid-boil-off.

4. A method of storing a natural gas mixture substantially as hereinbefore described with reference to the accompanying drawings.

5. A storage system for natural gas mixtures when used to carry out the method according to Claim 3, or 4, said system including at least one containment tank having a design pressure selected to allow for operation of said method.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (5)

**WARNING** start of CLMS field may overlap end of DESC **. (xii) Pressure permits cargo temperature/ pressure to be optimized for minimum costs according to trade length and size and gas composition. WHAT WE CLAIM IS:

1. A method of storing a natural gas mixture comprising essentially C1 up to C4 hydrocarbons and having a higher calorific value of from 1000 to 2000 B.t.u./S.c.f, said method comprising establishing and maintaining the pressure and temperature of the gas mixture within a dense operating state wherein the maximum operating temperature is below --200"F and the minimum operating temperature corresponds to a bubble point pressure of 25 p.s.i.a for the particular mixture, and wherein the maximum operating pressure is 25 p.s.i above the bubble point pressure and the minimum pressure is on the phase boundary of the particular mixture.

2. A method according to Claim 1, wherein the operating pressure at the maximum operating temperature is between 75 and 100 p.s.i.a, whilst the operating pressure at the minimum operating temperature is between 25 and 50 p.s.i.a.

3. A method according to Claim 1 or 2, wherein at least during loading into containment tanks, at least 1 % of the mixture is left as vapor so as to provide a gas "ullage" space within each tank, and wherein the design pressure of the tanks is selected such that, should a gradual or rapid heating of the stored mixture take place for any reason, the resulting increase in pressure will not exceed said design pressure, and the corresponding decrease in density of the mixture will not make the tanks liquid-full, thereby enabling eliminating the need to provide for liquid-boil-off.

4. A method of storing a natural gas mixture substantially as hereinbefore described with reference to the accompanying drawings.

5. A storage system for natural gas mixtures when used to carry out the method according to Claim 3, or 4, said system including at least one containment tank having a design pressure selected to allow for operation of said method.