US3535885A

US3535885A - Method of transporting natural gas

Info

Publication number: US3535885A
Application number: US720797A
Authority: US
Inventors: Willem Frijlink; Alfred L Van Kleef
Original assignee: Shell Oil Co
Current assignee: Shell USA Inc
Priority date: 1965-02-05
Filing date: 1968-04-12
Publication date: 1970-10-27
Anticipated expiration: 1987-10-27
Also published as: DE1501748A1; GB1084622A; NL6501473A; ES322562A1

Description

OC- 27, 1970 w. FRIJLINK ETAL METHOD OF TRANSPORTING NATURAL GAS Filed April 12, 1968 N .GE

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INVENTORS:

WILLEM FRIJLINK ALFRED L. VAN KLEEF BY: ,zo

THEIR ATTORNEY United States Patent O Int. Cl. FZSj 1/02 U.S. Cl. 62-9 6 Claims ABSTRACT OF THE DISCLOSURE A method of transporting natural gas in the liquid state between a production point and a consumption point. The natural gas is liquefied at the production point by cooling and the liquefied material is passed into a reservoir. The reservoir is transported to the consumption point where the liquefied material is converted back into natural gas by bringing it into heat exchange with a cold-carrier consisting essentially of water mixed with at least one freezing-point depressant, the coldecarrier having a condensation temperature approximately equal to the condensation temperature of the natural gas. The cold-carrier is cooled down as a result of contact with the liquefied material and passed into a reservoir which is transported back to the production point where the cooled cold-carrier is brought into heat exchange with the natural gas at the production point.

CROSS-REFERENCES TO RELATED APPLICATIONS This application is a continuation-in-part of application Ser. No. 523,877, filed Feb. l, 1966, now abandoned.

BACKGROUND OF THE INVENTION Field of the invention The invention relates to a method of transporting normally gaseous material; in particular a natural gas such as methane, in the liquid state.

Description of the prior art To facilitate .gas transport, it is known to liquefy the gasat a production point by cooling it. With gases having a relatively low atmospheric boiling point, such as a natural gas, for example, the amounts of energy required for the liquefaction, are relatively large. Accordingly, the necessary capital outlay in the cooling plant is also relatively high. It is, therefore, of great importance to find means for reducing the amount of energy required to liquefy the gas at the production point.

With this object in mind a method of transporting normally gaseous material in the liquid state has been proposed. At a production point, the gas is liquefied by cooling, and passed in the liquid state into a reservoir. When the reservoir is filled, it is transported to a consumption point where the liquefied gas is converted to the gaseous state by bringing it into heat exchange with a cold-carriet. The cold-carrier, which is now cooled down significantly, is passed into a reservoir which, when filled Patented Oct. 27, 1970 ICC with the cooled cold-carrier, is transported to the production point. At the production point, additional gas is liquefied as mentioned earlier by bringing it into heat exchange with the cooled cold-carrier.

It should be noted that 'by production point is meant the place where the gas is liquefied for dispatch, while by consumption point is meant the place where the liquefied gas in converted, on arrival, to the gaseous state for consumption.

In the above-mentioned method, the cold is thus passed back to the production point by means of a cold-carrier. It is desirable for the cold-carrier to be able to absorb a large amount of cold per unit of volume, so that the reservoir space required for transporting the cold-carrier will be as small as possible. To this end the cold-carrier should possess a relatively high specific heat and a relatively high specific gravity. Other requirements which the cold-carrier must meet are that it must be easily manageable and transportable, be easily obtainable and low in price and have satisfactory freezing and boiling points.

It has been known for many years that the freezing point of water can be lowered by using a freezing-point depressant (brine and antifreeze are well-known examples). Furthermore, it has been known in the transport or peakshaving of gas, wherein the gas is temporarily liquefied and thereafter regasified, to store the cold obtained during the regasification temporarily in a coldcarrier to be used again at a later stage for the liquefaction of a new batch of gas. The cold-carriers used in this process, which has been known for many years, have always been substances passing through a phase change, e.g., from the liquid to the gaseous phase and vice versa, these substances being oxygen, air, ammonia or, as in a U.S. Pat. No. 3,034,309 to Muck, nitrogen. The disadvantages of the use of these substances in a process of transporting or peakshaving gas using a cold-carrier are disclosed in detail in a U.S. Pat. No. 3,324,670 to Van Kleef.

SUMMARY OF THE INVENTION It is an object of this invention to provide an improved method for transporting a normally gaseous material in the liquid state.

It is a further object of this invention to provide a coldcarrier for transporting natural gas in the liquid state which is able to absorb a large amount of cold per unit of volume.

It is a still further object of this invention to provide cold-carrier for transporting natural gas in the liquid state lwhich always remains in the liquid phase during the process of liquefying and regasifying the gas.

These objects are carried out by liquefying natural gas at a production point by cooling and passing the liquefied material into a reservoir. The reservoir is transported to a consumption point where the liquefied material is converted back into natural gas by bringing it into heat exchange with a coldcarrier consisting essentially of water mixed with at least one freezing-point depressant and the cold-carrier having a condensation temperature approximately equal to the condensation temperature of the natural gas. The cold-carrier is cooled down as a result of contact with the liquefied material and passed into a reservoir which is transported back to the production point where the cooled cold-carrier is brought into heat exchange with the natural gas at the production point.

Suitable cold-carriers are, by way of example, water mixed with ammonia (NH3); water mixed with at least one alcohol, for example, water mixed with methanol (CH3OH) or water mixed with ethanol (C2H5OH); or water mixed with two or more alcohols or other substances. Water mixed with a glycol, for example, diethylene glycol C2H4(OH)2, can also be used as a cold-carrier. One example of a suitable cold-carrier is a mixture of 66 mole percent NH3 having a freezing point of l00 C. This is approximately the condensation temperature of common, natural gases at 30-40 ata., such as those produced in Sahara, Libya and Groningen.

The liquefied gas can be transported in the reservoir at a pressure of 1 atm., but in many cases it may be desirable to use higher pressures.

BRIEF DESCRIPTION OF THE DRAWING FIG. l is a diagrammatic view of an arrangement according to the present invention; and

FIG. 2 is a diagrammatic view of the arrangement of FIG. l showing another feature of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawings and particularly to FIG. l, it may be seen how natural gas may be transported in the liquid state between a production point and a consumption point. The gas is introduced into a heat exchanger at the gas producing area where it is cooled by contact with a cooled cold-carrier consisting essentially of water mixed with at least one freezing point depressant (FPD) and having a condensation temperature approximately equal to the condensation temperature of the natural gas. Vapors may be given off and the liquefied gas is passed into a reservoir which is transported to the gas consuming area. At this point, the liquefied gas is converted back to the gaseous state by bringing it into contact with a cold-carrier in a heat exchanger at the gas consumingr area. The gas is removed from the heat exchanger. The cold-carrier, cooled down as a result of the foregoing, is passed into a reservoir for transportation back to the producing area where the cooled cold-carrier is brought into contact with the natural gas introduced into the heat exchanger at the producing area.

Referring now to the diagram of FIG. 2, the arrangement is essentially the same as that of FIG. 1. However, it can be seen that the cold-carrier at the producing area, warmed as a result of contact with the gas introduced into the heat exchanger at the producing area, may be passed to a reservoir and transported back to the consuming area where it is used as the cold-carrier for the heat exchanger in the consuming area.

The cold-carrier is used at the production point to liquefy a quantity of gas by causing the cold from the cold-carrier to cool down the gas to be liquefied. As a result, therefore, the cold-carrier rises in temperature, or is warmed up, for example to the ambient temperature. If desired the cold-carrier can now be consumed at the production point, for example, in processes of the chemical industry carried out at the production point. If ammonia is used as the freezing point depressant, the mixture of water and ammonia can be used, for example, in the manufacture of fertilizers. It is also possible to separate the ammonia, the alcohols or any other freezing point depressants present, from the water at the production point. These separated freezing point depressants can either be used up on the spot or passed back to the consumption point. In the latter case at the consumption point, these separated freezing point depressants are mixed again with an amount of water to make the necessary cold-carrier. Then, on arrival of a quantity of liquefied gas at the consumption point, the cold-carrier is used to gasify this liquefied gas, whereupon the cooled coldcarrier is transported to the production point.

It is, of course, also possible not to separate the freezing point depressants at the production point, but to pass the warmed up cold-carrier as a whole into a reservoir as illustrated in FIG. 2 and to transport the latter from the production point to the consumption point. In most cases it is preferable to transport the cold-carrier to the consumption point at atmospheric or approximately atmospheric pressure, although it is also possible to use higher pressures, if desired. In many cases the cold-carrier which has been cooled down at the consumption point can be advantageously transported to the production point in the same reservoir as used for transporting the liquefied gas from the production point to the consumption point as illustrated in FIG. 2. This has the advantage that the reservoir is invariably kept at a low temperature and therefore need not be cooled at the production point. Due to the high specific gravity and the high specific heat of the proposed cold-carrier, the reservoir space of the reservoir in which the liquefied gas has been transported from the production point to the consumption point is in many cases large enough for transporting the cold cold-carrier from the consumption point to the production point.

It will be obvious that the reservoirs for the transport of the liqueed gas and/or of the cold-carrier can be built into, or form a part of, a ship, railway wagon, lorry or other vehicle.

The temperature ranges contemplated for the coldcarries can best be defined as follows:

(a) Tmm: some degrees centigrade above the (eutectic) freezing point at a pressure of l atm. absolute.

(b) Tmx: about l0 degrees centigrade below the boiling point of the cold-carrier. In this case the pressure has considerable influence on this boiling point.

In general, the cold-carrier will remain in the liquid phase. An exception is the case in which in the gasproducing location water is removed from a cold-carrier comprising H2O and NH3. The NH3 can then be sent back to the gas-consuming location in gaseous condition, if desired.

It should be clear that it is necessary to produce additional cold to effect the described method of natural gas transport. This extra cold usually has to be produced at the lowest temperature considered. This can be done at the gas-producing location in the following ways:

(a) Gas is cooled down as much as possible by the cold-carrier and is finally cooled down by lowering the pressure and by flashing to the desired temperature for transport. Thus, the vapor flashed off is recirculated or is used as fuel.

(b) Gas is cooled down as much as possible by the cold-carrier and then to the transport temperature by means of a nitrogen compression-expansion cycle.

In the above cases additional cold is supplied to the natural gas. If desired the additional cold can also be supplied to the cold-carrier, whereby it is sometimes preferred to do this in the gas-consuming location:

(a) The liquefied natural gas is flashed off to a lower temperature. The remaining liquid is gasied by heatexchange with the cold-carrier; the vapor can also be made to take up heat from the cold-carrier, if desired.

(b) The liquefied gas is gasified by heat-exchange with the cold-carrier. The cold-carrier is additionally cooled down in a scheme in which additional cold is produced in a nitrogen compression-expansion cycle.

In the above, four methods have been mentioned for producing additional cold. Other methods exist moreover, which have of course to be used in such a Way that the natural gas transport takes place in the most efiicient manner.

Throughout this specification, the term natural gas is used to include those hydrocarbons which are the components of the combustible mixture which is withdrawn from the earth. Thus, the method described herein can be used in the transport of liquefied natural gases,

such as methane, ethane, propane, butane or other liquefied gases.

By way of example only, the following survey has been made of the transport of natural gas using various cold-carriers.

QUANTITY OF NATURAL GAS TO BE TRANSPORTED: 6,000 TONS PER PRODUCTION DAY Mixture of Mixture methanol of ammonia Q Isopentane, and Water and water Cold-earner: 05H12 CHQOH and H2O NH3 and H2O I Conditions of natural gas:

Temperature in C -160 -82 -90 Pressure in atm. abs 1 40 30 Volume in cu. LiL/day 13, 000 17, 600 16, 900

1I Conditions of warmed up co1dcarrier:

Temperature in C +20 +20 +20 Pressure in atm. abs. 1 l 1 Volume in cu. m./day 21, 700 13, 600 7, 500

III Conditions of cold cold-carrier:

Temperature in C -157 -85 -95 Pressure in atm. abs. 1 1 1 Volume in cu. :1L/day.. 17, 300 12, 500 7, 000

Case A Case B Case C Volume of I -l- II in cu. m./day 34, 700 31, 200 24, 400

In case A, using isopentane as cold-carrier, 13,000

cu. m. of liquefied natural gas will be supplied to the consumption point. The quantity of cold cold-carrier which has to be transported to the production point is 17,300 cu. m., of which 13,000 cu. m. can be transported to the production point in the reservoir in which the liquefied natural gas has been supplied. Extra reservoir space will be required for the difference of 4,300 cu. m. of cold cold-carrier, in order to transport the cold coldcarrier from the consumption point to the production point. In the transport of the warmed cold-carrier and the liquefied natural gas from the production point to the consumption point 21,700 cu. m. of reservoir space is needed for the warmed cold-carrier and 13,000 cu. m. for the liquefied natural gas. The total reservoir space required is, therefore, 34,700 cu. m.; it should, however, be noted that no heat-insulated or pressure reservoir is necessary for the transport of the warmed cold-carrier.

In case B, using a mixture of methanol and Water as cold-carrier, 17,600 cu. 1n. of liquefied natural gas will be supplied to the consumption point. The quantity of cold cold-carrier which has to be transported to the production point is 12,500 cu. m., which, being a smaller quantity than 17,600 cu. rn., can therefore be wholly transported in the reservoir in which the liquefied natural gas has been supplied. Extra reservoir space is therefore not required for transporting the cold cold-carrier from the consumption point to the production point. In the transport of the warmed cold-carrier and the natural gas from the production point to the consumption point 13,600 cu. m. of reservoir space is needed for the warmed cold-carrier and 17,600 cu. m. for the liquefied natural gas. The total reservoir space required is therefore 31,200 cu. m.; it should, however, be noted that no heat-insulated or pressure reservoir is necessary for the transport of the warmed cold-carrier.

In case C, using a mixture of ammonia and water as cold-carrier, 16,900 cu. m. of liquefied natural gas will be supplied to the consumption point. The quantity of cold cold-carrier which has to be transported to the production point is 7,000 cu. m., which being a smaller quantity than 16,900 cu. m. can be wholly transported in the reservoir in which the liquefied natural gas has been supplied. Extra reservoir space is therefore not recarrier and 16,900 cu. In. for the liquefied natural gas. The total reservoir space required is therefore 24,400 cu. m.; it should, however, be noted that no heat-insulated or pressure reservoir is necessary for the transport of the warmed cold-carrier.

It appears, therefore, from the above that, as far as the required reservoir space is concerned, case B is more favorable than case A and case C is more favorable than case B.

The cold-carriers mentioned may also be used in storage of methane or natural gas for economy purposes. During periods of small demand for natural gas, this can be liquefied by passing it in heat exchange with any one of the cold cold-carriers according to the invention. The natural gas thus liquefied can then be stored in suitable reservoirs and in periods of large demand for natural gas the latter is gasiiied by passing it in heat-exchange with the warmed cold-carrier. 'Ihe cooled down cold-carrier is then stored until a period of small demand for natural gas arrives again. Then the cold cold-carrier is used for liquefying a quantity of natural gas which is stored until the demand for natural gas rises again.

The following table shows the various freezing points that may be obtained where ammonia (NH3), methanol (CHgOH) and ethyl alcohol (C2H5OH) are used as freezing point depressants in cold-carriers in accordance with the teachings of the invention.

Thus it can be seen from the foregoing table that Various weight percentages of Water mixed with a freezing point depressant in accordance with this invention give different freezing points enabling one to choose the most desirable cold-carrier for the gas to be liquefied.

What has been described is a novel method for facilitating the transport of liquefied gas, particularly nautral gas, such as methane, from a production point to a consump- 7 tion point utilizing a cold-carrier at atmospheric pressure consisting of water mixed at least one freezing point depressant, the cold-carrier having a condensation temperature approximately equal to the condensation temperature of the liquefied gas. Thus the main object of this invention is to transport (or store for peakshaving purposes) liquefied gas by means of a cold-carrier which is able to absorb a large quantity of cold per unit of volume. In this manner, the storage or transporting volume of the cold cold-carrier is as small as possible thus enabling substantial savings to be obtained on the investment in heat-insulated tanks for the cold cold-carrier. Furthermore, such a cold-carrier must be as inexpensive as possible and easily obtainable. Thus, the cold-carrier must possess high specific gravity and heat, such as water. However, a disadvantage of water is that its freezing point is relatively high. Thus, it is proposed by this invention to add to the water a freezing point depressant such as alcohols, glycol or ammonia. The quantity `of such depressant to be used depends entirely on the gas which is to be liquefied or regasified. For example, in the case of propane which has a boiling point of -40 centigrade, only a small quantity of ammonia or alcohol needs to be added t-o the water. In the case of butane which has a boiling point of -0.6 centigrade, a still smaller quantity of ammonia or alcohol needs to be added to the water to keep it in pumpable condition during the whole process. In the latter example, 99 mole percent water and 1 mole percent ammonia would be sufficient.

From the above, it should be clear that the quantity of freezing point depressant added to the water depends entirely on the boiling point of the gas to be liquefied and regasified in the process. Thus, only so much freezingpoint depressant has to be added to the water so that it remains pumpable during the process. In each case, depending upon the gas to be liquefied and regasied, and on the pressures at which the liquefaction takes place, one skilled in the art is readily able to decide the right quantity of freezing-point depressant to be added to the water.

Various methods of carrying out the concepts of this invention may become apparent to one skilled in the art, and it is to be understood that such modifications fall within the spirit and scope of the appended claims.

We claim as our invention:

1. A method of transporting natural gas in the liquid state between a production point and a consumption point comprising the following steps:

(a) liquefying said natural gas at a production point by cooling;

(b) passing the liquefied natural gas into a reservoir;

(c) transporting the reservoir containing the liquefied natural gas to a consumption point;

(d) converting the liquefied natural gas to the gaseous state at the consumption point by bringing it into 8 heat-exchange with a cold-carrier consisting essentially of water mixed with ammonia as a freezingpoint depressant in the proportion of 66 rrole percent water and 34 mole percent ammonia;

(e) passing the cold-carrier cooled down as a result of step (d) into a reservoir;

(f) transporting the reservoir containing the cooled cold-carrier to the production point; and

(g) bringing the cooled cold-carrier into heat-exchange with the natural gas at the production point to cool the latter as described in step (a).

2. The method of claim 1 including the step of transporting said liquefied natural gas in the reservoir at a pressure greater than one atmosphere.

3. The method of claim 1 including the additional steps of passing the cold-carrier which has been warmed up as described in step (g) into a reservoir and transporting the reservoir to the consumption point where the warmed coldcarrier is utilized to convert the liquefied natural gas into the gaseous state.

4. The method of claim 1 including the step vof transporting the warmed cold-carrier to the consumption point under a pressure of at least one atmosphere.

5. The method of claim 1 including the step of transporting the cooled cold-carrier from the consumption point to the production point in the same reservoir in which the liquefied natural gas has been transported from the prdouction point to the consumption point.

6. The method of claim 1 wherein the step of bringing the liquefied gas into heat-exchange with a cold-carrier includes the step of bringing the liquefied gas into heatexchange with a cold-carrier at approximately atmospheric pressure.

References Cited UNITED STATES PATENTS 2,884,763 5/1959 Faulk 62-55 2,975,604 3/1961 McMahon 62-9 3,018,632 1/1962 Keith 62--9 FOREIGN PATENTS 1,233,474 5 1960 France.

OTHER REFERENCES Section S2(d) of Patents in 69 Corpus Juris Secundum, American Law Book Co., 1951, pp. 374-375.

NORMAN YUDKOF, Primary Examiner A. F. PURCELL, Assistant Examiner U.S. Cl. X.R.