WO1997019152A1

WO1997019152A1 - Mixtures for storage and pipeline transport of gases

Info

Publication number: WO1997019152A1
Application number: PCT/CA1996/000750
Authority: WO
Inventors: Ian Morris; Glen Perry
Original assignee: 665976 Alberta Ltd.
Priority date: 1995-11-17
Filing date: 1996-11-18
Publication date: 1997-05-29
Also published as: CA2235140A1; AU7488596A; WO1997019151A1; AU7488496A

Abstract

For storage of natural gas at pressures over 1,000 psia, it is advantageous to add to natural gas an additive which is a C2 or C3 hydrocarbon compound, or a mixture of such hydrocarbon compounds. Above a lower limit (which varies with the additive being added and the pressure), this results in a smaller z factor, representing increased packing of molecules, and therefore leading to a decrease in the amount of power needed to compress the mixture. For storage or pipeline transportation of natural gas at pressures over 800 psia, it is advantageous to add ammonia to the natural gas, in an amount such that the ammonia does not create a liquid phase at the temperature and pressure used. The ammonia-natural gas mixture can be compressed or pumped with a lower energy expenditure than would be needed for an equivalent volume of natural gas alone. When more than 4 % by volume of ammonia is present, the pumping through pipelines is also aided by the refrigerant effect of the ammonia, which reduces the temperature of the gas being transported.

Description

MIXTURES FOR STORAGE AND PIPELINE TRANSPORT OF GASES

Field of the Invention

This invention relates to the storage or transfer, as by pipeline or tanker, of mixtures which contain methane or natural gas

Background of the Invention

As is well known, methane is the largest component of natural gas, and usually accounts for at least 95% by volume of what is known as "transmission specification" natural gas Other usual components are ethane (usually about 2%), propane (usually about 0 5%), butanes, pentanes and possibly hexanes (altogether amounting to less than about 0 3%), with the balance being nitrogen and carbon dioxide In this disclosure, transmission specification natural gas will be hereinafter called "natural gas" For example, the natural gas as transmitted through the pipelines of TransCanada Pipeline Limited from Alberta, Canada to Ontario, Canada has typically the following percentage composition by volume

Component Feed

Nitrogen 0 01270

Carbon Dioxide 0 00550

Methane 0 95400

Ethane 0 01970 Propane 0.00510 i-Butane 0 00170 n-Butane 0 00080 i-Pentane 0 00020 n-Pentane 0 00010 n-Hexane 0 00020

The relation between pressure, volume and temperature of a gas can be expressed by the Ideal Gas Law, which is stated as PV = nRT, where P = pressure of gas V = volume of gas n = number of moles of the gas R = the universal gas constant (which, as is known, vanes somewhat depending on volume and temperature)

T = temperature of the gas

If the equation is expressed in English units, the pressure is in pounds per square inch absolute (psia), the volume is in cubic feet, and temperature is in degrees R (degrees

Fahrenheit plus 460)

The Ideal Gas Equation does not give exactly correct results in actual practice, Decause gases are compressible Gas rrioiecuies, when compressed, pacK more tightly together than would be predicted by the Ideal Gas Equation, because of intermolecular forces and molecular shape To correct for this, an added term, the compressibility factor z, can be added to the Ideal Gas Equation This is a dimensionless factor whic reflects the compressibility of the particular gas being measured, at the particular temperature and pressure conditions

At atmospheric pressure or gage pressures of a few hundred pounds the compressibility factor is sufficiently close to 1 0 so that it can be ignored for most gases, and so that the Ideal Gas Law can be used without the added term z However, where pressures of more than a few hundred pounds exist, the z term can be different enough from 1 0 so that it must be included in order for the Ideal Gas Equation to give correct results According to the van der Waals theorem, the deviation of a natural gas from the Ideal Gas Law depends on how far the gas is from its critical temperature and critical pressure Thus, the terms T_R and P_R (known as reduced temperature and reduced pressure respectively) have been defined, where

T_R = J T_c

P_c where,

T = the temperature of the gas in degrees R T_c = the critical temperature of the gas in degrees R

P = the pressure of the gas in psia P_c = the critical pressure of the gas in psia

Critical pressures and critical temperatures for pure gases have been calculated, and are available in most handbooks Where a mixture of gases of known composition is available, a pseudo critical temperature and pseudo critical pressure which apply to the mixture can be obtained by using the averages of the critical temperatures and critical pressures of the pure gases in the mixture, weighted according the percentage of each pure gas present

Once a pseuαo reduced temperature and pseudo reduced pressure are known, the compressibility factor z can be found by use of standard charts One of these is "Compressibility Factors for Natural Gases" by M D Standing and D L Katz, published in the Engineering Data Book, Gas Processors Suppliers Association, 10th edition (Tulsa, Oklahoma, Li S A ) 1987

When the compressibility factor z of methane is read from the charts, it is found that the factor z is always less than 1 0 in normal temperature ranges (i e between about -40°F and 110°F) and that it decreases as the pressure rises or the temperature falls Therefore, less energy need be used to pump a given volume of methane (measured at standard volume) at any given normal temperature than would be expected at that temperature if the methane were an ideal gas This effect is more marked at higher pressures Similarly, as the pressure is increased at a constant temperature, more methane (measured at standard volume) can be stored in a given volume than would be predicted from the Ideal Gas Equation "Standard volume" is volume measured at standard pressure and temperature (STP))

Natural gas, like methane, shows z factor changes with pressure However, the z factor does not decrease as much with pressure for natural gas as it does for pure methane Thus, natural gas containing 2% ethane and 0 5% propane cannot be packed as tightly as methane alone at a given pressure, and needs more energy to compress or pump than methane alone If the amount of ethane in the natural gas is increased to 4%, the z factor drops still less with pressure, so that the gas is still more difficult to compress or pump and cannot be packed as tightly at a given pressure as could pure methane (All percentages in this document are percentages by volume) It is usual in the gas transportation and storage industry to try to strip out higher hydrocarbons such as ethane, propane, butane and unsaturated hydrocarbons from natural gas if the gas is to be stored at pressures of 1 ,000 or above This leaves mostly methane (with some traces of nitrogen and carbon dioxide) to be stored The materials which are stripped out are then stored separately, often as liquids

Summary of the Invention it has now been found that, for natural gas storage at pressures over 1 ,000 psia, it is advantageous to add to the natural gas an additive which is a C₂ or C₃ hydrocarbon compound, ammonia or a mixture of such additives Ammonia alone is useful as an additive for gas storage at pressures down to about 800 psia Above a lower limit (which vanes with the additive being added and the pressure), this results in a smaller z factor, representing increased packing of molecules, and therefore leading to a decrease in the amount of power needed to compress the mixture for storage and to keep it compressed It is also advantageous to add ammonia to natural gas to be transmitted through pipelines at pressures above 800 psia

Detailed Description of the Invention

If ethane is the additive, enough ethane must be added to methane or natural gas to give a gas composition having a minimum of about 26% ethane for operation at 1 ,000 psia and normal temperatures (-40°F to +120°F) Ethane can be added until just before the mixture separates into separate gas and liquid (which occurs at about 40% ethane, at 1 ,000 psia and temperatures of about 35° F, and at a lesser amount of ethane for iower temperatures and higher pressures) Generally operation at 26-35% ethane and 35°F to +40°F is preferred when the pressure is at or just above

When the pressure is raised to 2,200 psia, the addition of enough ethane to natural gas to give a gas composition having more than 6% ethane gives some beneficial results Thus, as pressure increases, in the range from 1 ,000 psia to about 2,200 psia the beneficial results occur with less and less ethane For the most beneficial results, however, an addition of enough ethane to give at least 15% ethane is preferred at pressures of 2,200 psia Thus for ethane as an additive, an amount is added to give a gas which has at least 26% ethane (but preferably up to 35% ethane) at 1 ,000 psia, and at least 6% ethane (but preferably 15% ethane) at 2,200 psia, with the minimum percentage of eihane decreasing smoothly with rise in pressure Ethyiene may De substituteα for all or part of the ethane on a 1 1 volume basis One or more C₃ hydrocarbons may also be substituted for ali or part of the C₂ hydrocarbons Where C₃ hydrocarbons alone are used as the additive, their minimum useable percentage of the total gas mixture can vary from 5% at 1 ,000 psia to about 3% at 2,200 psia The C₃ hydrocarbons may be any of propane, isopropane or propylene, separately or in admixture The use of C₃ hydrocarbons as the additive decreases the z factor, and hence the packing of molecules, right up until a separate liquid phase separates out For storage purposes, a liquid phase can be tolerated, as the methane of the natural gas is highly soluble in C₃ hydrocarbons Thus, although a liquid phase separates out at a volume concentration of 12% to 9% or so or propane, for example, as the pressure increases from 1000 psia to 2000 psia, this is not critical Percentages of propane and/or other C₃ hydrocarbons of up to 40% by volume can be present The gaseous phase which is present when a two-phase mixture is present still benefits from a high z factor, and the liquid phase dissolves appreciable methane

C₄ hydrocarbons (eg butanes and butylenes) and other components of the natural gas need not be separated out before storing gases according to this invention Of course, it may be desired to separate out corrosive materials so that they will not damage the compressors

Ammonia can also be used as the additive, either in substitution for or in admixture with the hydrocarbon additives Approximately 10-12 % by volume NH₃ causes separation of a liquid phase, depending on the pressure and temperature, so the amount added should be below the amount which causes separation of a liquid phase. Any amount of ammonia gives some benefit at the pressures of this invention, but a minimum of 0.5% is preferred in order to get appreciable advantages. Ammonia gives a beneficial effect at even a lower pressure than the other additives, and can be added when storage will take place at any pressure above 800 psia. Two or more of these additives can be used together. The use of two or more additives has a synergistic effect in many cases, so that less than the minimum amount of each is needed than would be needed if only one were present, in order to produce the z factor over that of an equivalent standard voiume of natural gas at the pressure and temperature involved. The addition of amounts of additive below the lower limit (unless two additives with a synergistic effect are used) actually increases the z factor over that of methane or natural gas alone, and is thus detrimental. For example, when the pressure is 1 ,000 psia and the temperature is 35° F, mixtures of methane and ethane having less than about 26% ethane have a z factor greater than methane alone (all percentages are based on volumes at standard pressures and temperatures). Adding ethane to increase the percentage of ethane from 2% to, for example 12% at this pressure and temperature is therefore counterproductive, as it increases the z factor and therefore increases the energy required to pump or compress for storage a given standard volume of gas. However, when more than 26% of ethane is present, the z factor becomes lower than that of methane. The z factor continues to get smaller with increased percentages of ethane, to the point where further increase of ethane causes separation of a liquid phase (at about 40% ethane at 1 ,000 psia and 35°F). Thus, adding ethane to natural gas so that there is a mixture containing more than 26% ethane at 1 ,000 psia and 35 °F leads to increased packing of molecules and hence decreased compression costs and more ability to store within a given volume. At 1 ,350 psia and 85°F, improved results over methane alone are obtained when only 17% of ethane is present in the mixture. Where the pressure is increased to 1 ,675 psia at 35°F, mixtures with 13% or more ethane, and the balance methane give better packing and a lower z factor than methane alone. At 2,140 psia and 35° F, the improved effect is shown in mixtures of 6% ethane and the balance methane. In this disclosure gas "storage" includes the holding of gas in a tank or subterranean cavern or the like, and also the holding of gas in an endless loop of pipeline All of these types of storage are collectively referred to as a "storage chamber"

Hydrocarbon additives are particularly advantageous for storage as they permit storage of a mixture of methane or natural gas and the hydrocarbon additive in the same storage chamber with less energy expenditure than if the two were stored separately Ammonia is also a useful additive, as it can be made easily and cheaply from waste hydrogen or natural gas Thus, a ready supply of ammonia can be iri de available wherever there is natural gas to store for transport

The addition of any amount of ammonia to natural gas decreases z factor significantly from that of natural gas alone at pressures over 800 psia and at temperatures from -40°F to +110°F Depending on the pressure and temperature, up to about 10-12% ammonia can be added before the mixture begins to separate out into separate liquid and gas phases Increasing the percentage by volume of ammonia, right up until a liquid phase separates, steadily increases the benefit obtained Thus, there is an advantage to adding ammonia to natural gas which is to be compressed and stored (as in a tanker or storage tank), as less energy is needed for compression than would be needed for an equivalent standard volume of methane or natural gas alone

However, the economics of whether it is worthwhile to add ammonia for storage may vary, depending upon the costs of making the ammonia and separating the natural gas from the ammonia when the two are removed from storage for use

Where the mixture is to be pumped through a pipeline however, an unexpected and beneficial further effect of using ammonia occurs In a pipeline, there are pumping stations at intervals along the pipeline In each pumping station, the gas is compressed As the gas moves toward the next pumping station, it gradually loses pressure and expands The compression during passage through the compressor station heats the gas, and it cools while passing through the pipeline, passing some of its heat to the surrounding soil through the pipeline wall

Ammonia has the property of being a refrigerant, which absorbs heat as it expands Thus, when an ammonia/natural gas or ammonia/methane mixture is compressed and then subsequently allowed to flow through a gas pipeline, the ammonia cools the mixture as it expands To get significant cooling, enough ammonia has to be present so that the cooling effect is relatively large relative to other heat losses and gains within the gas mixture A minimum of about 4% by volume of ammonia is therefore necessary before the cooling becomes significant This cooling reduces even further the cost of pumping arising from the effect of ammonia on the z factor Thus, for pipeline use, more than 4% by volume of ammonia is preferred At the delivery point for the pipeline, the ammonia can be separated from the natural gas and can be sold, as ammonia is a commercially valuable product

By adjusting the amount of ammonia added to the gas mixture ana by spacing the pumping stations so that a desired temperature drop occurs between stations because of the refrigerant effect of ammonia, the gas mixture can be made to flow through the pipeline at temperatures not exceeding a particular desired temperature In the case of pipelines flowing through permafrost (as in the Arctic or Antarctica), the line can be designed and the added amount of ammonia adjusted so that the pipeline temperature never exceeds the melting temperature of the permafrost This makes feasible uninsulated or only slightly insulated pipelines through permafrost, which pipelines do not damage their environment by melting it

Brief Description of the Drawings

The invention will be described further in association with the following drawings in which Figures 1A to 1E are plots of capacity gain in percent against the content of C₂ hydrocarbons in a mixture of methane and ethane Each of the plots shows the results at a different pressure

Figures 2A and 2B are plots of capacity gain versus temperature (in degrees Fahrenheit) for the same gas mixtures at 800 psia and 1 ,675 psia respectively Figure 3 is a plot of the z factor for a mixture of methane and various concentrations by volume of ammonia, at different pressures and the same temperature

Figures 4A and 4B are plots of the horsepower needed to compress natural gas in an idealized pipeline with various amounts of added ammonia Figure 4A shows the effect of addition of ammonia on gas initially at 1100 psia, while Figure 4B shows the effect on gas initially at 1900 psia

Detailed Description of the Embodiments Shown in the Drawings

Dealing first with Figure 1 , this shows, for various pressures and the same temperature, the effect of the addition of ethane to methane In each case, the z factor has been calculated for each percentage of ethane from 0 to 40% Then, the lowest caicuiated z factor, i e the most dense pacKing, nas been aroitraniy defined as 0% capacity gain Each of the other results has been plotted as a percentage capacity gam with reference to the 0% capacity gam in order to prepare a curve Curves developed in this way are given in Figures 1 A to 1 E, for different pressures, with each curve representing the situation for one pressure The temperature represented by each curve is 35° F

Looking at Figure 1A, it is seen that, for an 800 psia storage chamber, the best packing occurs when the chamber is filled with pure methane As ethane is added, the packing steadily falls (and the capacity gain percent decreases) until there is about 25% ethane in the chamber After this, the capacity gain begins to increase again, but it does not reach the levels obtained for pure methane

Figure 1 B shows the effect of addition to methane of ethane for a storage chamber at 1 ,150 psia Here, the capacity gain steadily decreases from 0% ethane to about 12% ethane, and then increases again After approximately 25% ethane, the capacity gain is greater than occurred with no ethane at all

Figure 1 C shows that this effect is even more pronounced when the chamber pressure is increased to 1 ,350 psia The lowest capacity now occurs at approximately 7%, and mixtures with more than 17% ethane exhibit packing (and hence, storage capacity) gains unattainable with of natural gas or methane alone

Figure 1 D shows that at 1 ,760 psia, the lowest capacity occurs at about 5%, and anything over 12% ethane gives a better capacity gain than is attainable with natural gas or methane alone For best results, however, at least 15% ethane should be present At 2,140 psia (Figure 1E) the addition of about 4% ethane gives a benefit, and the benefit steadily increases all the way up to the point at which the ethane begins to separate out in a liquid phase For best results, however, at least 12% ethane and preferably 15% ethane should be present Thus, it will be seen that for pressures above about 1 ,000 psia better packing, and hence lower compression cost occurs when increased amounts of ethane are added At 1 ,150 psia (Figure 1 B) about 24% ethane must be added to get the same packing effect as the approximately 2% ethane in normal natural gas if more than this amount is added, however, better packing occurs with each addition As the pressure increases, successively less ethane need be added to give better packing (and hence lower transmission costs and compression costs) than with natural gas Indeed, at 2,410 psia, even about 4% ethane shows some advantage, although the advantage is of course greater as more ethane is added

Figure 2 shows how the effect changes with temperature Even at 800 psia (Figure 2A) there is a capacity increase as temperature drops, and the capacity gain is greater the more C₂ that is present With higher pressure (Figure 2B) the capacity gain is much greater with temperature, and the improvement in capacity gain becomes still greater as increased amounts of ethane are added

Figure 3 shows the effect of 2%, 6% and 10% percentages by volume of ammonia on the z factor, at 30°F It will be noted that ammonia has a positive effect even at 800 psia, and that increasing percentages of ammonia increase the effect Increasing pressure also increases the effect, to somewhere between 2000-2200 psia depending on percentage of ammonia, after which further pressure increases do not provide increased benefits Figure 4 plots, for temperatures of 30 °F and 60° F, the calculated effect of different amounts of added ammonia to transmission specification natural gas for an idealized 50 mile long pipeline on flat land The pipeline is assumed to be 36 inches in diameter and to be filled at the input end with gas pressurized to the input pressure Gas mixtures flow isothermally through the pipeline, and sufficient pumping energy is applied at the input end to give a pressure drop of 150 psia In Figure 3A, the gas is initially at 1100 psia, and in Figure 3B, the gas is initially at 1900 psia It will be noted that, in each case, the addition of any amount of ammonia decreases the total horsepower needed for operation of the pipeline at all temperatures and pressures shown However, when operation is carried out at lower temperature and at higher pressure, the effect is more pronounced

The foregoing has illustrated certain specific embodiments of the invention, but other embodiments will of course be evident to those skilled in the art Therefore it is intended that the scope of the invention not be limited by the embodiments αescriDeα, but ratner Dy the scope of the appended claims

Claims

We claim as our invention

A method of storing natural gas, which comprises

(a) admixing the natural gas with one or more hydrocarbons selected from the group of saturated and unsaturated C₂ and C₃ hydrocarbons, to form a resulting mixture, the amount of said one or more hydrocarbons being sufficient so mat the z factor of tne resulting mixture is lower than the z factor for natural gas alone at the pressure at which the mixture will be stored, and

(b) storing said resulting mixture at a temperature not exceeding 110°F and a pressure or at least 1000 psia

2 A method as claimed in claim 1 , where the hydrocarbon comprises one or more C₂ hydrocarbons, in an amount which depends on the pressure at which the storage takes place, said amount being sufficient to give the mixture a total C₂ hydrocarbon content which ranges smoothly from a minimum percentage by volume of not less than 26% when the storage is at a pressure of 1000psιa , to a minimum percentage is about 15% when the pressure is 2200 psia

3 A method as claimed in claim 1 or 2 , in which the hydrocarbon is ethane

4 A method as claimed in claim 1 , where the hydrocarbon is one or more C₃ hydrocarbons in an amount which depends on the pressure at which the storage takes place, said amount being sufficient to give the mixture a total C₃ hydrocarbon content which ranges smoothly from a minimum percentage by volume of not less than about 5% when the storage is at a pressure of 1000 psia , to a minimum percentage of about 5% when the pressure is 2200 psia 5 A method as claimed in claims 1 or 4, in which the hydrocarbon is propane or a mixture of both

6 A method as claimed in any of claims 1-5, in which the hydrocarbon is present in insufficient quantity for the formation of a liquid phase at the temperature and pressure of storage

7 A method of storing or transporting natural gas, which comprises,

(a) admixing the natural gas with ammonia to form a resulting mixture, and

(b) storing said resulting mixture or transmitting it by pipeline at a temperature not exceeding 110°F and a pressure of at least 800 psia

8 A method as claimed in claim 7, in which the resulting mixture is stored, and the ammonia is present in an amount of at least 0 5% by volume, but an amount insufficient to form a liquid phase at the temperature and pressure of storage

9 A method as claimed in claim 7, in which the resulting mixture is transmitted by pipeline, and the ammonia is present in an amount of at least 4% by volume, but an amount insufficient to form a liquid phase at the temperature and pressure of such transmission