CA2235140A1 - Mixtures for pipeline transport of gases - Google Patents

Abstract

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 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 pump the mixture or to compress it.

Description

CA 0223~140 1998-04-16 MIXTURES FOR PIPELINE TRANSPORT OF GASES

Field of the Invention This invention relates to the ll ~"srer by pipeline of mixtures which contain ~ methane or natural gas.

5 Backqround of the Invention As is weil known, methane is the largest component of natural gas, and usually accounts for at least g5U/o 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 ~altogether10 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 0.01 270 Carbon Dioxide 0.00550 Methane 0.95400 Ethane 0.01 970 Propane 0.00510 i-Butane 0.001 70 n-Butane 0.00080 i-Pentane 0.00020 n-Pentane 0.0001 0 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 CA 0223~140 1998-04-16 R = the universal gas co"-~Lanl (which, as is known, varies 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 absoiute (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, because gases are conlpressibie. Gas rnoiec~les, when cvmpressed, packmore 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 which reflects the compressibility of the particular gas being measured, at the particular temperature and pressure conditions.
At aLmOSPher;C 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 TR and PR (known as reduced temperature and reduced pressure respectively) have been defined, where TR= T
TC
PR P
Pc where, T = the temperature of the gas in degrees R
Tc = the critical temperature of the gas in degrees R
P = the pressure of the gas in psia CA 0223~140 1998-04-16 W O 97/19151 PCTtCA96/00749 Pc = the critical pressure of the gas in psia Criticai pressures and critical temperatures for pure gases have been c~lc~ ed, and are available in most handbooks. Where a mixture of gases of knowncomposition is available, a pseudo critical temperature and pseudo critical pressure 5 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.
~ n~ a pseuclo reduoed temperature alld pseudo reduced pressure are known, the compressibility factor z can be found by use of standard charts. One of 10 these is "Compressibility Factors for Natural Gases" by M.D. Standing and D.L. Katz, published in the En~ineerin~ Data ~ook. Gas Processors Suppliers Association, 1 0th edition (Tulsa, Oklahoma, U.S.A.) 19~7.
When the cor,lpressibility 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.
1~ between about 40~F and 120~F) and that it decreases as the pressure rises or the temperature falls. Therefore, less energy need be used to pump a given volume ofmethane (measured at standard volume) at any given normal temperature than wouldbe 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 20 temperature, more rnethane (measured at standard volume) can be stored in a given volume than would be predicted from the Ideal Gas Equation. "Standard volume" isvolume 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 2~ does for pure methane. Thus, natural gas containing 2% ethane and (~.5% propane cannot be packed as tightly as methane alone at a given pressure, and needs moreenergy 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 30 given pressure as could pure methane. (All percentages in this document are percentages by volume).

CA 0223~140 1998-04-16 It is usual in the gas transpo, laLion and storage industry to try to strip out higher hydrocarbons such as ethane, propane, butane and unsaturated hydrocarbonsfrom natural gas if the gas is to be transmitted through pipelines in which the gas is compressed to 1,000 or higher. This leaves mostly methane (with some traces of nitrogen and carbon dioxide) to be transported by the gas pipeline. The materials which are stripped out are then transported or stored separately, often as liquids.

Summarv of the Invention It has now been found that, 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 additives. Above a lower limit (which varies with the additive being added and the pressure~, this results in a smaller z factor than would exist with methane alone, representing increased packing of molecules, and therefore leading to a decrease in the amount of power needed to pump the mixture or to compress it.

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 +110~F). Ethane can beadded until just before the mixture se~ dles into separate gas and liquid (which occurs at about 40% ethane for a pressure 1,000 psia and a temperature of about 35~F). To reduce the danger of liquefaction if there is inadvertent pressure drop, and to reduce temperature extremes, generally operation at 26-35% ethane and 35~F to +~0~F is preferred.
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 21200 psia the beneficial results occur with less and less ethane. For the most CA 0223~140 1998-04-16 W O 97/19151 PCT/CA9C/0074g 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 35% ethane) at 1,000 psia, and at least 6%
ethane (but preferably 15% ethane) at 2,200 psia, with the minimum percentage ofethane decreasi-,g smoothly with rise in pressure. Ethylene may be substituted for all or part of the ethane on a 1:1 volume basis. Where pressure fll ~ctu~tes, as in a gas pipeiine where the gas is compressed at compressor siatiorls and becomes less compressed as it flows between compressor stations, the pressure indicated is the maximum pressure to which the gas is cor~ ~pressed. In such a cor"pression-rarefaction a, l ~nyer"ent, it is preferred that the ratio between the most compressed and the most rarefied pressures of the gas not exceed 1.25:1.
C3 hydrocarbons alone can also be used as the additive. Minimum useable percentage of the total gas mixture vary from a minimum of 5% at 1,000 psia to about 3% at 2,200 psia. Maximum amounts are those which will not cause separation of a liquid phase at the temperature used. The C3 hydrocarbons may be any of propane, isopropane or propylene, separately or in admixture.
One or more C3 hydrocarbons may also be substituted, preferably on a 1:3.5 volume basis, for C2 hyd, uc~, bcins, but not to a point where they cause separation of a liquid phase at the pressure and temperature of operation. (A 1:3.5 basis means that each standard volume of C3 hydrocarbon replaces three and a half standard volumes of C2 hydrocarbon.) Generally, the limitation that a liquid phase should not be formed means that not more than about 12% of C3 hydrocarbons should be present at 1000 psia and 60~F, and lesser amounts should be used as the pressure or temperature increases.
Two or more of the C2 or C3 additives can also be used. 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 volume of natural gas at the pressure and temperature involved.

CA 0223~140 1998-04-16 At pressures over 1000 psig, C4 hydrocarbons do not contribute much to the improvement of the z factor, because an amount which would be large enough to cause a reduction of the z factor is also large enough to liquify, which is undesirable.
Thus C4 hydrocarbons are not additives contemplated by this invention, however C4 5 hy~, .,ca, L~ons which are already present in the natural gas need not be removed if they are present in insufficient quantity to liquify. The presence of more than 1% C4h)~dloca,bon in the mixture is not prer~"t:d, however, as C4 hydrocarbons tend to liquify easily ai pressures beiween 1,C00 psia and 2,200 psia, ~nd more ~han 1~/'o C4 hydrocarbons give rise to increased danger that a liquid phase will separate out. C4 10 hydrocarbons also have an unfavourable effect on the mixture's z factor at pressures just under 900 psia, so care should be taken that, during transport through a pipeline, that mixtures according to the invention which contain C~ hydrocarbons are not allowed to decompress to less than 900 psia, and preferably not to less than 1,000 psia.The addition of amounts of additive below the lower limit (unless two 15 additives with a sy, l13(yi51iC effect are used) actually increases the z factor over that of methane or natural gas alone, and is thus del, i" ,ental. 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 20 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 25 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 pumping costs and more ability to store within a given volume. At 1,350 30 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 CA 0223~l40 l998-04-l6 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.
By the avoidance of liquid phase in this disclosure is meant the avoidance of enough liquid to provide a coherent liquid phase in the pipeline at the temperatures and pressures used. Such a phase can create pipeline problems through pooling inlow portions of the pipeline or forming liquid slugs which affect pumping efficiency. A
few iiquid droplets in ~he line howevel, can be lviera~ed.
The use of a hydrocarbon as additive has the additional advantage that it permits transport of a mixture of methane or natural gas and the hydrocarbon additive in a single pipeline with less energy expenditure than if the two were transported separately in separate pipelines.

Brief DescriPtion of the Drawin~s The invention wiil be described further in association with the following drawings in which:
Figures 1A to 1 E are plots of capacil~/ 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 pipelines at 800 psia and 1,675 psia respectively.
Figure 3 is a summary of pipeline horsepower requirements for various gas mixes, using a 36" pipeline operating at a maximum operation pressure of 1,740 psia, an inlet temperature of 80~F and a ground temperature of 32~F.
Figure 4iS a plot of the horsepower requi,~r"er~ per thousand cubic feet of gas showing different mixtures of ethane and methane, at different pressures.
Detailed DescriPtion of the Embodiments Shown in the Drawin~s 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 CA 0223~140 1998-04-16 calculated z factor, i.e. the most dense packing, has been arbitrarily defined as 0%
capacity gain. Each of the other results has been plotted as a percentage capacity gain with reference to the 0% capacity gain in order to prepare a curve. Curves developed in this way are given in Figures 1A to 1 E, for different pressures, with each 5 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 pipeline, the best packing occurs when lhe line is filleci wil~ pure methane. As etharle is adc:ied, the packing steadily falls (and the capacity gain percent decreases) until there is about 10 25% ethane in the line. 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 pipeline 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 15 gain is greater than occurred with no ethane at all.
Figure 1C shows that this effect is even more pronounced when the pipeline 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, pipeline or storage capacity) gains unattainable with of natural gas or methane 20 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.
2~ At 2,140 psia (Figure 1E) the ad~ilion 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 30 packing, and hence lower cor,lpression cost and pumping cost for transportation occurs when increased amounts of ethane are added over a minimum amount which -CA 0223~140 1998-04-16 _ g _ decreases with increasing pressure. 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 normalnatural 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 si~ows how the e~Fect changes with temperature. Even at 800 psia (Figure 2A) there is a capacity increase (i.e. better packing) as temperature drops, and the capacity gain is greater the more C2 that is present. However, the effect is not nearly as significant as at higher pressures. 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. Generally, there~ore, it is prere"t:d to operate at a relatively low temperature, such as 70~ to -20~F. Higher temperatures (e.g. up to about 120~F) can be tolerated, but detract from the benefits of the invention.
Figure 3 shows ho,:,epo~ver required for different gas mixes of ethane and methane, through a pipeline at a maximum pressure of 1,740 psia and a minimum pressure of 1,350 psia. ~igures are for a pipeline of 36" in diameter and a length of 1,785 miles, with pumping stations located every 56 miles. At a throughput of 2.0 million, ~Lal ,dard cubic feet per day, a mixture of 98% methane and 2% ethane (which corresponds to ol.linary natural gas) would require 812,579 horsepower. However, the same standard volume of gas, but containing 35% C2, can be moved with only 661,860 horsepower, ~or a saving of over 150,000 horsepower. When the throughput is raised to 2.5 million standard cubic feet, natural gas containing 2% ethane cannot practically be transmitted, because the velocities and temperatures involved are too great.
However, gas with 6% ethane can be transmitted, and gas with 35% ethane shows a saving of over 500,000 horsepower over that which is used to transmit the gas with 6%
ethane.

CA 0223~140 1998-04-16 W O 97/19151 PCT/CA961~0749 Figure 4 shows the effect on horsepower requirements per million cubic feet of gas being pumped through the same pipeline as used in Figure 3 when the pipeline gas contains different concentrations of ethane at 35~F.
Figure 4 also shows the negative effect of adding ethane to a typical 5 pipeline running at about 800 psia pressure and 35~F. Required power for pumping increases until the mix contains 26% ethane and then decreases for higher concentrations approaching the liquid phase limits. However, the decrease is notsufFicient so that, by the concenll alion where iiquefac~ion occurs ~about ~%) there is any saving of l ,o, ~e~ower over pumping ordinary natural gas. This energy hill however 10 peaks at decreasing concenl,alions of ethane as operational pressure increases, e.g., 14% at 1,150 psia, 8% at 1,350 psia, 6% at 1,475 psia. This is due to the rate of decrease in the value of the z factor overcoming the rate of increase in density.
As noted, at 800 psia, increasing the amount of ethane even up to 40%
of the mixture does not produce a saving of horsepower. At 1,150 psia, however, an 15 increase of ethane to over about 24% causes a horsepower saving when compared to the values at 2% ethane (which is approximately the amount of ethane in most natural gas). At 1,350 psia, a decrease in horsepower occurs with mixtures containing more than about 14% ethane. As the pressure increases, the horsepower saving occurs with still less added ethane. As can be seen from Figure 4, the peak horsepower 20 requirement shifts to the left of the graph and becomes smaller as pressure increases.
However, reading Figure 4 with Figure 2A and 2B shows that, as temperature decreases, the curve tends to rotate to the right. Thus, as temperature decreases, less addition of ethane is necessary to obtain the desired additional packing and hence decreased compression and pumping energy requirements. The result is lower 25 compressive power requirements for equivalent volumes of higher B.T.U. gas mixes than can be achieved with conventional pipeline specification natural gas. Higher pipeline throughput are able to be economically achieved with these enhanced gasmixes with corresponding increases in horsepower.
Similar effects are seen when ethylene is used in substitution for all or 30 part of the ethane.

W O 97/19lSl PCT/CA96/00749 When propane, isopropane or propylene, or mixtures of any of these gases, are substituted for ali or part of the ethane, the effects are even more pronounced than with ethane, and occur with a smaller addition of each of the compounds mentioned than was necessary with ethane as noted previously in this 5 disclosure.

The preferred composition of the resulting gas is as follows:

COMPONENT MAXIMUM VALUE MINIMUM VALUE
METHANE AND/OR 92% VOLUME 68%VOLUME

ETHANE AND/OR 35% VOLUME 6% VOLUME

PROPANE 9% VOLUME 0%VOLUME
BUTANES AND OTHER Not required, but 0% VOLUME
COMPONENTS OF THE amount present in NATURAL GAS original natural gas can be tolerated if it does not cause separation of a liquid phase at the pressure and temperature used NITROGEN Not required, but amount 0% VOLUME
present in the original natural gas can be tolerated if below 2% by volume CARBON DIOXIDE Not required, but amount 0% VOLUME
present in the original natural gas can be tolerated if below 1% by volume.
COMPONENTS TOTAL
1 00%
TEMPERATURE 70~ F -20~ F

CA 0223~140 1998-04-16 W O 97/191~,1 PCT/CA96/00749 Compositionswith hyd,~JcG,L,on additives outside these ranges generally are of little economic benefit, or approach the limits at which two-phase or liquid behaviour can be seen. Therefore, if mixtures outside these parameters are used,care should be taken to avoid conditions which might cause liquid to fall out of the 5 mixture. Liquid is generally to be avoided, as it may pool in low areas of a pipeline and be difficult to remove. The preferential liquefaction of some components will also cause the ~rnp~sition of the gase~us phase of the mixture to ch~n~e, thereby changing tne z factor and hence the compressibility of the gaseous phase.

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 described, but rather by the scope of the appended claims.

Claims (10)

What is claimed is:

1. A method of transporting natural gas by pipeline, which comprises:

(a) adding to such natural gas sufficient of at least one C2 or C3 hydrocarbon or a mixture of C2 and C3 hydrocarbons such so the hydrocarbon, together with the C2 and C3 hydrocarbon (if any) originally in the natural gas, forms a resulting mixture with a total C2 or C3 hydrocarbon content which is sufficient, at the pressure and temperature to be used for transporting, to reduce the z factor to a level lower than that of the untreated natural gas, and (b) transporting such resulting mixture by pipeline at a temperature of between -40° and +120° Fahrenheit and pressure greater than 1000 psia, said pressure and temperature being chosen so the resulting mixture has no liquid phase at the temperature and pressure of transmission.

2. A method as claimed in claim 1 where the hydrocarbon is selected from (a) between 26 and 40% of at least one C2 compound if the pressure is about 1000 psia, declining smoothly to about 6% to 15% of said C2 compound if the pressure is about 2200 psia, or (b) between 12% and 4% of a C3 compound, if the pressure is about 1000 psia, declining smoothly to the C3 amount which will not cause liquefaction at the pressure used when the pressure is above 1000 psia.

3. A method as claimed in either claim 1 or claim 2, in which there is not more than 1% by volume of carbon dioxide in the resulting mixture.

4. A method as claimed in claim 1 or claim 2, in which there is not more than 2%nitrogen in the resulting mixture.

5. A method as claimed in any of claims 1-4, in which the temperature at which the resulting mixture is transmitted is between -20° F. and +70°F.

6. A method as claimed in any of claims 1-4, in which the pressure at which the resulting mixture is transmitted is between 2160 psia and 1150 psia.

7. A method as claimed in any of claims 1-6 in which the C2 hydrocarbon added tothe natural gas is ethane.

8. A method as claimed in any of claims 1-7, in which the C3 hydrocarbon added to the natural gas is propane.

9. A gas mixture, for use in a pipeline at a pressure greater than 1,000 psia and a temperature of from -40 degrees F. to +100 degrees F., which comprises:

(a) from 65 to 92% by volume of methane (b) from 6 to 35% by volume of ethane (c) from 0 to 9% by volume of propane (d) from 0% by volume of C4 hydrocarbons to a percentage of Cu hydrocarbons which does not liquify at the pressure used (e) not more than 1% each of carbon dioxide or nitrogen the total being 100%, and such mixture being completely gaseous with no liquid phase at the temperature and pressure of intended operation.

10. A gas mixture as claimed in claim 9, said gas mixture being at a pressure of 1000-2200 psia and a temperature of from -20 degrees F. to +70 degrees F.