Classifications

• • C—CHEMISTRY; METALLURGY
  • C10—PETROLEUM, GAS OR COKE INDUSTRIES; TECHNICAL GASES CONTAINING CARBON MONOXIDE; FUELS; LUBRICANTS; PEAT
  • C10L—FUELS NOT OTHERWISE PROVIDED FOR; NATURAL GAS; SYNTHETIC NATURAL GAS OBTAINED BY PROCESSES NOT COVERED BY SUBCLASSES C10G, C10K; LIQUEFIED PETROLEUM GAS; ADDING MATERIALS TO FUELS OR FIRES TO REDUCE SMOKE OR UNDESIRABLE DEPOSITS OR TO FACILITATE SOOT REMOVAL; FIRELIGHTERS
  • C10L3/00—Gaseous fuels; Natural gas; Synthetic natural gas obtained by processes not covered by subclass C10G, C10K; Liquefied petroleum gas
  • C10L3/06—Natural gas; Synthetic natural gas obtained by processes not covered by C10G, C10K3/02 or C10K3/04
• • C—CHEMISTRY; METALLURGY
  • C10—PETROLEUM, GAS OR COKE INDUSTRIES; TECHNICAL GASES CONTAINING CARBON MONOXIDE; FUELS; LUBRICANTS; PEAT
  • C10L—FUELS NOT OTHERWISE PROVIDED FOR; NATURAL GAS; SYNTHETIC NATURAL GAS OBTAINED BY PROCESSES NOT COVERED BY SUBCLASSES C10G, C10K; LIQUEFIED PETROLEUM GAS; ADDING MATERIALS TO FUELS OR FIRES TO REDUCE SMOKE OR UNDESIRABLE DEPOSITS OR TO FACILITATE SOOT REMOVAL; FIRELIGHTERS
  • C10L3/00—Gaseous fuels; Natural gas; Synthetic natural gas obtained by processes not covered by subclass C10G, C10K; Liquefied petroleum gas
  • C10L3/003—Additives for gaseous fuels
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F17—STORING OR DISTRIBUTING GASES OR LIQUIDS
  • F17D—PIPE-LINE SYSTEMS; PIPE-LINES
  • F17D1/00—Pipe-line systems
  • F17D1/08—Pipe-line systems for liquids or viscous products
  • F17D1/16—Facilitating the conveyance of liquids or effecting the conveyance of viscous products by modification of their viscosity
  • F17D1/17—Facilitating the conveyance of liquids or effecting the conveyance of viscous products by modification of their viscosity by mixing with another liquid, i.e. diluting

Definitions

• This invention relates to the transfer by pipeline of mixtures which contain methane or natural gas.
• 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.
• transmission specification natural gas will be hereinafter called “natural gas”.
• 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:
• T temperature of the gas
• the Ideal Gas Equation does not give exactly correct results in actual practice, because gases are compressible. Gas molecules, 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 which reflects the compressibility of the particular gas being measured, at the particular temperature and pressure conditions.
• 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.
• 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.
• 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.
• T R and P R known as reduced temperature and reduced pressure respectively
• T the temperature of the gas in degrees
• T c the critical temperature of the gas in degrees
• 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.
• 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, U.S.A.) 1987.
• Natural gas like methane, shows z factor changes with pressure. Under about 1000 psia the dominant variable in the power relationship is the molecular weight of the gas. At this pressure level, addition of further amounts of ethane or propane increases the molecular weight of the gas more rapidly than the z factor decreases. Thus, there is an advantage to removing ethane and propane from the gas.
• 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). (All percentages in this document are percentages by volume). Ethane can be added until just before the mixture separates 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 +40° F is preferred.
• 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 of ethane decreasing smoothly with rise in pressure.
• Ethylene may be substituted for all or part of the ethane on a 1 :1 volume basis.
• pressure indicated is the maximum pressure to which the gas is compressed. In such a compression-rarefaction arrangement, it is preferred that the ratio between the most compressed and the most rarefied pressures of the gas not exceed 1.3:1.
• C 3 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 C 3 hydrocarbons may be any of propane, isopropane or propylene, separately or in admixture.
• One or more C 3 hydrocarbons may also be substituted, preferably on a 1 :3.5 volume basis, for C 2 hydrocarbons, 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 C 3 hydrocarbon replaces three and a half standard volumes of C 2 hydrocarbon.
• a liquid phase should not be formed means that not more than about 12% of C 3 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 C 2 or C 3 additives can also be used.
• C 4 hydrocarbons tend to liquify easily at pressures between 1 ,000 psia and 2,200 psia, and more than 1 % C 4 hydrocarbons give rise to increased danger that a liquid phase will separate out.
• C 4 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 4 hydrocarbons are not allowed to decompress to less than 900 psia, and preferably not to less than 1 ,000 psia.
• the z factor falls so much that the zM w product tends to lower values than that of pure methane.
• the z factor continues to get smaller with increased percentages of ethane, bringing with it a lower zM w product 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).
• 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 a decreased zM w product, hence decreased pumping costs and more ability to store within a given volume.
• 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 in low portions of the pipeline or forming liquid slugs which affect pumping efficiency. A few liquid droplets in the line however, can be tolerated.
• 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.
• Figures 1 A to 1 E are plots of capacity gain in percent against the content of C 2 hydrocarbons in a mixture of methane and ethane. Each of the plots shows the results at a different pressure.
• Figure 1 F shows calculations of flow ratios for various mixtures of methane and ethane..
• 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 4 is a plot of the horsepower requirements per thousand cubic feet of gas showing different mixtures of ethane and methane, at different pressures.
• FIG. 1 shows, for various pressures and the same temperature, the effect of the addition of ethane to methane.
• the z factor has been calculated for each percentage of ethane from 0 to 40%.
• the lowest calculated z factor 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 1Ato 1E, 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 pipeline, the best packing occurs when the line is filled with pure methane. As ethane is added, the capacity gain percent decreases until there is about 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.
• 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 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 alone.
• Figure 1 D shows that at 1 ,760 psia, the lowest capacity occurs at about
• Figure 1 F shows the effect of the z factor and its product with the average molecular weight of the gas for gases having various amounts of methane and ethane (the ethane is shown in the table as "C 2 ”)
• the flow ratio (1/the root of the zM w product) is plotted.
• Figure 2 shows how the effect 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 C 2 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, therefore, it is preferred 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.
• 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 horsepower 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.
• Figures are for a pipeline of 36" in diameter and a length of 1 ,785 miles, with pumping stations located every 56 miles.
• a throughput of 2.0 million, standard cubic feet per day a mixture of 98% methane and 2% ethane (which corresponds to ordinary natural gas) would require 812,579 horsepower.
• the same standard volume of gas, but containing 35% C 2 can be moved with only 661 ,860 horsepower, for a saving of over 150,000 horsepower.
• 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 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 not sufficient so that, by the concentration where liquefaction occurs (about 40%) there is any saving of horsepower over pumping ordinary natural gas.
• This energy hill however peaks at decreasing concentrations 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 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).
• compositions with hydrocarbon 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 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 composition of the gaseous phase of the mixture to change, thereby changing the z factor and hence the compressibility of the gaseous phase.

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• Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
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Citations (2)

• 1998
  • 1998-04-16 BR BRPI9809794-6A patent/BR9809794B1/pt not_active IP Right Cessation
  • 1998-04-16 EA EA199901042A patent/EA002238B1/ru not_active IP Right Cessation
  • 1998-04-16 WO PCT/CA1998/000354 patent/WO1998053031A1/en active IP Right Grant
  • 1998-04-16 AU AU70201/98A patent/AU747206B2/en not_active Expired

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