US 3298805 A
Description (OCR text may contain errors)
Jan. 17, 1967 H. c, sEcoRD ET AL 3,298,805
NATURADGBS FOR TRANSPORT Filed Aug. 10, 1965 Tmpercnure "amssaJ INVENTOR HERBERT CAMPBELL SECORD ATTORNEYS United States Patent This application is a continuation-in-part of our copending application Serial No. 297,522, filed July 25, 1963, now Patent No. 3,232,725.
This invention relates to the storage and transportation of natural hydrocarbon gas mixtures and, more particularly, to a method whereby a natural gas mixture is contained in a dense 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. Certain heavier gas mixtures rich in propane and/ or butane sometimes are recovered and transported as liquids (LPG), but the lighter natural gases consisting principally of methane are often flashed oif and burned or vented at the wellhead. It is the broad purpose of this invention to provide a new and improved method for storing and transporting these lighter hydrocarbon gases which are rich in methane and thus make their enormous energy potential 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 have been proposed by others heretofore for the storage and shipment of light hydrocarbon gas mixturesrich in methane but none of them has been entirely satisfactory. The socalled 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 disadvan tages because of the enormous cost involved in achieving and maintaining such extremely low cryogenic temperatures. By turning instead to certain combinations of moderate compression and refrigeration as described in our above-identified prior application, 'we have provided a method of transport of methane-rich natural gas mixtures which is often vastly superior economically to LNG or any other conventional process. It involves temperatures no less than about the critical temperature of methane (116" F.) and pressures no less than the bubb le point-dew point pressure of the gas so that the contained mixture is always in a single-phase state. While its container costs are greater than with LNG because the container must withstand pressure, the savings in refrigeration are so great that our prior method is notably more economical overall.
However, recent experience with our improved method has led to the discovery that lesser operating temperatures (still well above the cryogenic temperatures of LNG) and reduced operating pressures closer to and even below the bubble point-dew point pressure of the gas mixture result in yet greater cost savings when, for example, a relatively leaner mixture is to be transported and/ or the distances involved are comparatively greater. In this new mode of operation, neither refrigeration expense nor container costs far outweighs the other.
Accordingly, the present method ofstoring a natural gas mixture preferably for transport contemplates mixtures containing at least 60 mol percent methane and at least mol percent methane-plus-ethane, the remainder being heavier hydrocarbons and up to 10 mol percent inert constituents, and having a gross calorific value of 800 B.t.u./s.c.f. to 1600 B.t.u./s.c.f. The method provides that the temperature and pressure of the gas mixture be established within an operating state wherein the maximum operating temperature is immediately below about the critical temperature of methane, the minimum operating temperature is about 200 F., the maximum operating pressure is 300 p.s.i. above the bubble point-dew point pressure of the gas mixture at the operating temperature, and the minimum operating pressure is 15 p.s.i. below the bubble point-dew point pressure of the gas mixture at the operating temperature. Having achieved piessures and temperatures within these parameters, the gas mixture is contained in the operating state to prevent expansion and is thermally insulated against substantial heat leakage into the gas mixture so that it remains in the operating state throughout the duration of its containment. In this fashion the gas mixture is maintained in a dense state suitable for storage and transport at minimal 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, but it excludes propane-butane mixtures conventionally handled in the liquid state as LPG and artificially prepared solutions of pure methane dissolved in a heavier carrier such as ethane. If the mixture originally contains more than the specified amount of inert constituents (up to 10 mol percent), they should be reduced accordingly; not only will this increase the heating value of the cargo but in the case of excess carbon dioxide it will avoid solidification and in the case of excess nitrogen it will lower the vapor pressure of the mixture. The contemplated gas compositions are somewhat leaner overall as compared to the range of compositions applicable to our prior method, since in no case here will they include less than 60 mol percent methane and 80 mol percent methane-plus-ethane. One Of the most important features of the invention is that these lighter mixtures are precisely the Methane Ethane 5 .02
Propane 2.71 Butane 2.43
Nitrogen .3 4 Carbon dioxide .66
The method of the invention may be better under stood 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 liquefication temperature of the gas mixture at atmospheric pressure and in absolute terms it might be apprOXimately -25 8 F. Point B is the true critical point of the gas mixture at which the various lines of uni-form liquid and vapor concentrations within the twophase 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. Critical points of representative natural gas mixtures contemplated for use in this method are at pressures of about 675 p.s.i.a. to 1800 p.s.i.a. and temperatures of about -l30 F. to +75 F.
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 since its physical state varies primarily with respect to density. Thus, if the gas mixture is compressed from the point X to Y and then cooled to Z, its density would gradually change without a distinct change in phase. Only when changes in temperature and pressure are carried out through the two-phase envelope, for example directly between X and Z, can the creation of a part liquid and part vapor condition be distinctly noted. Therefore, the behavior of natural gas is referred to herein as that of a fluid whenever it is outside the two-phase region of the envelope, and by this is meant a compressible single-phase fluid.
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 1-3, 3-4, 4-5 and 5-1 on the diagram. Thus, the gas mixture is brought to an operating temperature below the dotted line connecting the points 3 and 4 in the diagram, which is immediately below about the critical temperature of methane (1l6 F.). In every instance, therefore, the operating temperature defined herein is necessarily less than the minimum operating temperature called for in applicants aforementioned Patent No. 3,232,725. Above that maximum temperature illustrated by the line 3 in the accompanying diagram, the absolute pressures required render the containment costs disproportionately large. The diagram also indicates the minimum operating temperature by the dotted line connecting the points 1 and 5, which is about 200 F. More refrigeration is therefore necessary in all forms of the contemplated method than in our prior method, but the gas mixture is not chilled to the low cryogenic temperatures of LNG be cause below about 200 F. refrigeration costs begin to rise steeply, the gain in density decreases, and a point of diminishing returns is reached in the economics of container costs as explained hereinafter.
In the diagram the dotted line connecting the points 4 and 5 indicates the maximum operating pressure of 300 p.s.i. above the bubble point-dew point pressure of the gas mixture. For the gas compositions contemplated in this method, the maximum operating pressure at the warmest condition of operation (point 4) may be in the order about 1000 p.s.i.a., while at the coldest condition of operation (point 5) it may be in the order of about 500 p.s.i.a. Since the intended temperature conditions are in most instances less than the critical temperature B of the contemplated mixtures, this definition of maximum operating pressure necessarily results in absolute compression of a relatively modest degree. Consequently, the bottles or containers in which the gas mixture is disposed during the practice of the method may be particularly large and constructed of a material (e.g., high nickel content steel or a high strength aluminum alloy) chosen more for its resistance to low temperatures than for its resistance to greatly elevated pressures.
The minimum operating pressure in accordance with the method is shown by the dotted line connecting the points 1 and 3 in the diagram, which throughout the contemplated temperature range is 15 psi. below the phase boundary of the gas mixture. For virtually all of the gas mixtures intended for the practice of this method, the minimum operating pressure under conditions of least refrigeration (point 3) will be in the order of about 500 p.s.i.a., and for conditions of greatest refrigeration (point 1) it will be in the order of about p.s.i.a. Below this limit of minimum pressure, the average density of the contained mixture becomes too low for economical operation.
It will be noted that practically all of the contemplated operating region on the phase diagram is above the bubble point-dew point line and thus in the single-phase condition of a dense fluid without ullage or other evidence of the coexistence of liquid and vapor. However, since the minimum operating pressure is below the bubble point-dew point line, the presence of an observable interface between separate liquid and vapor phases i not excluded from the contemplated operating conditions. Most of the gas compositions applicable to this method have a critical temperature greater than the critical temperature of methane and hence this narrow two-phase region included in the operating state is beneath the bubble point portion of the phase boundary where a slight amount of vapor exists with a considerably larger volume of liquid. In almost all cases, no more than 10 percent by volume of the gas composition contained in the operating state will be vapor even at the lowest operating pressure and temperature at the point 1 in phase diaphragm. One of the principal reasons why this narrow region of two-phase conditions is contemplated by the present invention is to provide a slight ullage space within the containers so that in the event of rapid heating of the containers under emergency conditions (such as flooding of seawater around the containers in the hold of a ship) the resulting increase in pressure will not occur too rapidly. Somewhat less cargo is transported per container when this allowance for ullage is made. However, safety require ments for single-phase operation may necessitate the addition of empty surge chambers in the ship to accommodate expansion of the cargo and this extra cost may well outweigh the economic disadvantage of lesser net cargo in the two-phase state.
In the commercial practice of this invention, optimum cost savings are present at temperatures well below the critical temperature of methane. Thi is one reason why the maximum operating temperature is defined as immediately below rather than at, the critical temperature of methane and few if any circumstances will require the choice of an operating temperature at that limit.
The present method is less expensive than our prior method as a mode of static storage since its operating pressures are considerably lower. Nonetheless it may be desirable under some circumstances of marine transport to avoid static storage at the points of loading and unloading so that the gas mixture can be prepared at a relatively constant rate for shipment and delivered with similar uniform flow rates to consumers. To do this one ship is made available for loading at all times while another is unloading and the remainder of the fleet plies between the two terminal ports. Thus, at least four ships are usually required. This avoids the costs of double loading and unloading operations from static storage tanks which otherwise would be required at both ports.
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, and this working range is defined on the phase diagram by the dotted line connecting the points 1, 2, 7 and 6. Here the maximum operating temperature (the line 2-7) is -130 F. and the minimum operating temperature (the line 1-6) is as defined above, which is to say about -200 F., well below the critical temperature of gas mixtures intended for this method. The maximum operating pressure (the line 6-7) is 100 p.s.i. above the bubble point pressure of the gas mixture at the operating temperature. Finally, the minimum operating pressure (the line 1-2) again as defined above, 15 p.s.i. below the bubble point pressure of the mixture at the operating temperature. In this region of operation, the gas mixture can be contained at densities from 400 to 575 times its normal density of atmospheric pressure and temperature. The practice of the invention can be illustrated by a typical transport of the Sahara gas mixture defined previously which has a specific gravity relative to air of .648. This gas mixture may be piped from a wellhead along with its associated heavier hydrocarbons and delivered to separator facilities where the gas is separated from the associated liquid and dehydrated. The mixture is then piped overland under pressure and at ambient temperature to dockside where it is to be loaded on board ship. At that point its pressure and temperature may be brought to the chosen operating condition, for example 170 F. and 224 p.s.i.a. (essentially at its bubble point pressure at that temperature) by cooling at high pressure to 150 F. and then expanding to the operating pressure and temperature. This condition is maintained as the gas mixture is delivered into containers within the hold of the ship where about one percent by volume is ullage which provides the requisite expansion space for safety purposes. Alternatively, the gas mixture may be expanded into the containers in a fashion such that its pressure and temperature vary through the two-phase region of the diagram before the final operating state is achieved.
The containers may be elongated bottles of a material such as 9% nickel content steel or a high strength aluminum alloy located in a thermally insulated hold. They may be about ten feet in diameter and perhaps fifty feet long vertically arranged in a multiplicity of suitably interconnected batteries. The density of this cargo in the operating state described above (-170 F. and 224 p.s.i.a.) is about 24 lbs/ft. and this is about 485 times its normal density at atmospheric temperature and pressure. Taking into account all the cost factors in a shipment as described above over a trans-Mediterranean route of about 500 miles, from the beginning of loading at embarkation to the end of unloading at the destination, the gas mixture is transported at significantly less cost per unit weight by the present method than by the highertemperature method we have disclosed previously. As compared to LNG, of course, the unit cost advantage is even more impressive.
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 piped 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 pipe-line gas supply.
1. A method of storing for transport a natural gas mixture containing at least 60 mol percent methane and at least mol percent methane-plus-ethane, the remainder being heavier hydrocarbons and up to 10 mol percent inert constituents, and having a gross calorific value of from 800 B.t.u./s.c.f. to 1600 B.t.u./s.c.f., which comprises:
(a) establishing the pressure and temperature of the gas mixture within an operating state wherein (i) the maximum operating temperature is immediately below about the critical temperature of methane,
(ii) the mini-mum operating temperature is about (iii) the maximum operating pressure is 300 psi. above the bubble point-dew point pressure of the gas mixture at the operating temperature, and
(iv) the minimum operating pressure is 15 psi. below the bubble point-dew point pressure of the gas mixture at the operating temperature,
(b) containing said gas in the operating state to prevent expansion of said gas mixture; and
(c) thermally insulating the contained gas mixture against substantial heat leakage into said gas mixture so that it remains in said operating state throughout the duration of its containment;
((1) whereby the gas mixture is maintained in a dense state suitable for storage and transport at minimal compression, refrigeration and containment costs per unit weight of gas mixture.
2. A method of storing a natural gas mixture according to claim 1 wherein the maximum operating temperature is -130 F. and the maximum operating pressure is p.s.i. above the bubble point pressure of the gas mixture at the operating temperature.
3. A method of storing a natural gas mixture according to claim 2 wherein the density of the gas mixture in the operating state is 400 to 5 75 times greater than its density at atmospheric pressure and temperature.
4. A method of storing natural gas mixture according to claim 1 wherein said gas mixture is contained in said operating state in a multiplicityof containers resistant to the chosen operating temperature and pressure, and said containers are surrounded by thermal insulation and transported by ship.
5. A method of storing natural gas mixture according to claim 4 wherein said gas mixture is loaded in and unloaded from at least four of said ships sequentially at a substantially uniform rate without static storage at the points of loading and unloading.
References Cited by the Examiner UNITED STATES PATENTS 2,217,678 10/1940 Goosmann. 2,231,500 2/ 1941 Harlow.
OTHER REFERENCES Katz et al.: Industrial and Engineering Chemistry, vol. 32, No. 6, pp. 817-827 (June 1940).
MORRIS O. WOLK, Primary Examiner.
JOSE-PH SCOVRONEK, Examiner.
Dedication 3,298,805.Herbert Campbell Sec-0rd, Markyate, England and Bernard J. Clarke, Columbus, Ohio. NATURAL GAS FOR TRANSPORT. Patent dated Jan. 17 1967. Dedication filed Sept. 16, 1971, by the assignee, Vehoc Corporation. Hereby dedicates to the Public the entire remaining term of said patent.
[Oflicz'al Gazette December 28, 1.971.]