Optimised Method For The Bulk Storage and Transport of Natural Gas

Abstract

This invention relates to the pressurized and chilled containment of natural gas contained within a liquid state of light hydrocarbon matrix mixture. It specifically yields the optimal storage net-density of natural gas within a propane-based solvent that is possible to attain, relative to that of Compressed Natural Gas under identical storage conditions. It further outlines how such mixtures can be advantageously handled for the bulk transport of natural gas. 3232725 February 1966 Secord et al. 3407613 October 1968 Muller et al. 5315054 May 1994 Teel 5900515 May 1999 Mallinson et al. 6111154 August 2000 Mallinson et al. 6201163 March 2001 Morris et al. 6217626 April 2001 Morris et al. 6725671 April 2004 Bishop 7137260 November 2006 Perry

Description

FIELD OF THE INVENTION

This invention enables the collection of natural gas to be efficiently rendered within a liquid rich hydrocarbon matrix which is subsequently carried on board a transportation mode. The transportation mode is then used to convey the product to market. On arrival at its destination, the natural gas is released from the matrix and offloaded either in its original field composition, or in an enhanced form to meet particular market specifications.

The invention permits the acquisition of natural gas from so called “stranded” reserves not served by LNG technology, on or offshore, to be delivered to a market's storage and transmission pipeline systems. Notwithstanding, the gas can be conveyed to its destination by land modes such as truck and rail, or by marine mode on barges and ships. Given the lighter containment vessels required for this invention, the justification of air transportation is not ruled out.

BACKGROUND INFORMATION

Natural gas is primarily moved in a gaseous form by pipeline. U.S. Pat. Nos. 6,217,626-B1, and 6201163-B1 show that improved density of the packing of natural gas constituents can be achieved under certain storage conditions through the addition of light-hydrocarbon and other solvents.

The light-hydrocarbons referred to here are ethane, propane and butane. This tighter packing of molecules of constituents of the natural gas mix, referred to in these patents, takes place in a gaseous or dense phase state.

The increased densities for the gas mixes are achieved by addressing their properties and seeking lower levels of compressibility (Z) factor associated in the Ideal Gas Equation. Within limits of the process, the objective is to move the value of Z of the hydrocarbon rich storage mix to its lowest possible value. This value is predicated on attaining storage conditions at or near the inflection points of graphic representations of the hydrocarbon compressibility factor carried in GPA and physics literature.

Moving the dense packed storage mixes into the liquid phase by adjustments of temperature and pressure reveals that greater net density values can be achieved for the natural gas component in a liquid matrix mixture created by increasing the content of lower mol percent light-hydrocarbons.

BRIEF DESCRIPTION OF THE FIGURES

Reference is made, in the detailed description of the invention, to the accompanying illustrations:

FIG. 1. Phase Envelope for Matrix Mix with Trinidad Gas and 8% Propane Based Solvent

FIG. 2. Optimal Density and Compressive Ratio Summary for Compressed and Matrix Storage of Natural Gas.

SUMMARY OF THE INVENTION

Work has been undertaken, prior to and subsequent to the issue of those patents, by various entities to improve the storage densities of their modes of transport of compressed natural gas (CNG) and liquid phase gas fuel mixtures. Referenced patents by Secord, Muller, Teel, Mallinson and Bishop have a common thread in seeking to increase heat content of fuels or improving contained densities. All consider the augmentation of the of light hydrocarbon component to achieve their ends.

These methods then aim to reduce the energy requirement and material intensity and capital needs of the objectives by means of compression or cooling of a natural gas matrix in various phases from gaseous through liquid forms. All bulk conditioning modes proposed for natural gas, with the exception of atmospheric LNG, aim towards efficient storage modes above 1000 psig in pressure.

This invention seeks to achieve in an energy efficient manner, at pressures below 850 psig using a hydrocarbon solvent to achieve packing densities of natural gas that are an order of magnitude just below two thirds that of LNG. (This yields a net storage density gain of the order 360:1 compared to 600:1 for LNG). This technology does not require the capital, process intensity, and long lead times of LNG or Compressed LNG (CLNG) systems to materialize, and can therefore cater to smaller, marginal, remote reserves, not yet served by existing technologies.

After claiming benefits only for Carbon Dioxide as an additive, U.S. Pat. No. 7,137,260 undertakes a dissertation on the mode of obtaining denser natural gas storage through increased concentrations of light hydrocarbon additives. This derives from work undertaken in the abovementioned dense-phase patents of Item (002). The description outlines the continued effect of obtaining denser natural gas storage densities by moving into the liquid phase. The addition of light hydrocarbons to a natural gas mix can, under defined storage conditions of pressure and temperature, yield a greater net density of the natural gas component within the mix than would be found in the natural gas mix alone under these same conditions.

By adjustments of additive, pressure or temperature (individually or selectively) continued gains can be achieved by storing the natural gas mix in a liquid phase matrix maintained at temperature and pressure conditions close to the trace of the Bubble Point curve of the phase diagram of that particular matrix mixture.

Published figures in US Patent Application No. 20060042273—“Storage of Natural Gas in Liquid Solvents: Methods to Absorb and Segregate Natural Gas Into and Out of Liquid Solvents” show practical and commercial limits for 3 solvents within the bounds of claimed storage condition. Coverage for ethane, propane and butane are claimed to yield effective net densities under storage conditions of pressure up to a maximum of 1400 psig, at temperatures between −40 F and −80 F. (Net-density gains in storage within the claimed region when compared to LNG translate to the order of 253:1 to 294:1 at the 1400 psig condition). No claims are made for hydrocarbon solvents heavier than butane.

The common 1400 psig, −40 F storage condition in documents described in (005) and (006) above, show respective mol percentages of ethane, propane and butane to attain effective storage densities as (35%, 17.5%, and 11%) and (25%, 18%, and 15%). The declining percentage with increased carbon numbers of the solvent is consistent with the mathematical requirement that the denser solvents are to assume smaller percentages of hydrocarbons in order to permit the natural gas content of these mixes to be as great as possible.

This invention now shows that beyond a certain point of concentration of these solvents, their addition becomes ineffective in improving the yield of natural gas from the gas matrix. The maximum density of this matrix mix that yields gains in improved storage packing beyond that of simple natural gas lies in the region of 21 lb/ft3.

High density mixes resulting from a high concentration of heavier hydrocarbons do not yield optimal net-densities of the natural gas component when compared to equivalent CNG densities.

There are liquid compression constraints at temperatures below −120 F (attributable to the critical temperature of methane, the primary constituent of natural gas), that restrict further net-density gains in pressurized (i.e., above 500 psig) matrix mixes.

This invention establishes a storage region, using a low mol percentage of propane based solvent outside of existing claim areas. The base natural gas mixes used are consistent with practical clean burning levels promoted by North American specifications. The method is equally applicable to leaner and richer base mixes.

A higher net-density for the natural gas content at lowest percentage solvent was tracked. The optimal storage conditions of temperature and pressure for the matrix mix were found to lie along the liquid side of the locus of the rotation points of the “S” profile of successive bubble point curves. These curves relate to successive phase diagrams of mixes having decreasing solvent concentration. (The rotation point is the pressure/temperature condition where the upper convex curve of the bubble point trace assumes a concave profile.)

DETAILED DESCRIPTION OF THE INVENTION

Increasingly accurate Equation of State methodology has become available in recent years to determine the behavior of hydrocarbons in under-investigated storage regions as detailed for this invention. The VMG APRD model was used to generate properties for this invention.

Furthermore, developments in low temperature steels, fiber composite technology, and aluminum alloys for use at temperatures of −100 F, coupled with advances in low temperature fluids in, for example, the silicone family make this invention feasible.

The invention stems from the dense phase technology of natural gas achieved by an increase in light hydrocarbon constituents. This is achieved by the addition of the light hydrocarbons or reduction of natural gas methane concentration employing commonly used industry technologies. By adjustment of storage conditions of temperature/pressure it is possible to position the gas mix at a point where its compression factor “Z” is decreased, resulting in a more densely packed arrangement of molecules than occurs in the compressed natural gas alone.

Further cooling at decreased pressures causes the same mix to move into the liquid phase. At locations of temperature close to the bubble point curve the liquid is still compressible and can be packed to greater net-densities with moderate increase in pressure and a small temperature decrease.

Beyond this optimal point there is little benefit in increasing the relatively incompressible density with greater expenditure of work.

It has been found that this point of maximum gain in density for minimum expenditure of energy lies just outside of the rotation point of the convex and concave sections of the bubble point curve of the phase envelope of the gas mixture. A series of superimposed phase envelopes of decreasing mol percentages of the solvent shows an arced locus of these rotation points, along which the points of maximum density for minimal energy lie. Either side of this locus, at adjacent pressure/temperature points, the net-densities relative to the CNG density exhibit a fall off in gain.

For a specific component composition of solvent and a specific component composition of natural gas there is a particular trace of these rotation points. This trace drops in temperature with decreasing percentages of the solvent. As the approach is made to critical temperature of methane, further decreases in mol % solvent lose the ability to gain net-density with minimal work. The mixture essentially behaves as a straight CNG/LNG blend with the added liability of a minor amount of solvent working against the net-density of the natural gas component.

FIG. 2 illustrates the trend for solvent mol percentage at best value of net densities for individual pressure/temperature points for two specific solvent and natural gas component mixes. Best values trend as follows:

Pressure: 900 psig	Temp: −80 F.	Net-Density: 14.46 lb/ft3 @
		12% solvent
		CNG Density: 9.11 lb/ft3
Pressure: 800 psig	Temp: −90 F.	Net-Density: 15.38 lb/ft3 @
		12% solvent
		CNG Density: 8.93 lb/ft3
Pressure: 700 psig	Temp: −100 F.	Net-Density: 16.04 lb/ft3 @
		8% solvent

The final point of 16.04 lb/ft3 is equivalent to a Compressive Ratio of 363 ft3/ft3 for the natural gas component of the matrix mixture. This is double the 180 ft3/ft3 achieved for the natural gas alone treated as a chilled CNG. It will also be noted that there is a sharp cut off in gain for all pressures at a temperature of −120, where the best matrix gas mix is found to have a 6% solvent content and have a net-density value that is marginally better than its CNG counterpart. Moving up or down the −100 F column at pressures above and below the 700 psig Best Point there is also a marked drop off in the gain of the net-density of matrix mix over that of the corresponding CNG value.

We have thus established an optimum net-density of natural gas that it is possible to store at 700 psig. At the same temperature of −100 F the table shows that CNG would have to be at 1000 psig to achieve a similar value. For storage in a pressure vessel of common external dimensions and material, it is clear that the wall thickness would be proportionate to the containment pressure. Lighter, less expensive storage vessels are therefore possible with this invention.

To take the wall thickness benefit one stage further, the containment system could be comprised of sections of pipe. Treated under design codes for hydrocarbon pipelines, such as those issued by the Canadian Standards Association, the matrix mixture is handled as a petroleum liquid and is given a larger design factor than the pure gas CNG case. Wall thickness divided by the design factor in each scenario is used to establish a wall thickness with built-in safety allowances.

This would result in at least a 2:1 benefit of mass of natural gas to mass of containment pipe of a design for the matrix mixture over that for CNG. This benefit is directly reflected in capital costs and represents a substantial saving for bulk transport of large amounts of natural gas using this invention.

Translating this invention into a bulk mode of transport can apply to marine, land, and air freight forms of transport. The basic requirements of the transport system are described here for marine transport, adaptations of which can be designed by those skilled in the art for other modes mentioned here.

• • At sea, a collection point on a buoy for marine pipelines from source points (wells, process skids) for loading the natural gas is required.
  • A matrix preparation station is provided where injection of liquid solvent can be made into the natural gas stream, and pressurized and cooled to the storage conditions. According to scale of operation, the station can mounted on a platform, tethered barge or even on board the transport ship.
  • A storage system is now required, residing in a cooled, insulated enclosure with an inert atmosphere. Re-circulated nitrogen would provide both a cooling heat transfer path, and the desired inert properties.
  • The storage system can be of fiber wrap, aluminum or steel fabrication in the form of a pressure vessel or piping network. According to shipping bureau requirements for hold sizes, the containment system can be fitted to each hold of the vessel or run full or partial length of the vessel. In either case, it can protrude upwards into the allowable height of the cooled enclosure.
  • The vessel can be a motorized ship or moveable barge.
  • A loading system to move the matrix mixture into and out of the vessels at storage pressures is of critical importance. The mixture has to be maintained at liquid conditions while in the storage vessels during all stages of loading, transit and unloading. Pressure reduction, in particular, would cause the liquid to flash into the vapor or gas state with an accompanying drop in temperature, possibly beyond the specifications of the container materials.
  • Moving the liquid into storage against a back pressure of a denser displacement fluid is required. Once the vessel is full, the valves can be closed and the fluid recycled for use in other vessels yet to be filled. The reverse applies on off-loading, the denser displacement fluid is used to push the matrix mixture out of the container and a valve closed once the container is filled with displacement fluid. The fluid, in turn, is de-pressured and drained for reuse, while the containment system is filled with inert gas.
  • Flashing of the liquid phase into a vapor/gas mix takes place outside of the containment system, in the associated process plant.

Offloading is designed such that the solvent can be recycled to storage for use on an additional voyage. Market conditions could dictate that all, or part, of the solvent is delivered. The off-loading system requires a fractionation process train to take required constituents from the matrix mix. This train comprises a de-ethanizer module to first segregate methane and ethane fraction from the stream. The residual product from this tower then passes through a de-propanizer where the solvent is separated from the heavy hydrocarbon stream. Again, prevailing market conditions can dictate if this equipment is installed on the ship, a tender barge, or the shoreline.

Market needs now determine the recombination of the hydrocarbon fractions to suit local specifications. Normally, the methane/ethane stream flow would be recombined with the heavier hydrocarbons and heat value adjustment made with selected addition of propane. The bulk of propane is recycled to the ship.

When the propane base mix is off-loaded, the ship can pick up a solvent cargo at/or en route to the natural gas loading point.

This system is devised to fill the void in gaining access to the gas reserves deemed too small for LNG to be considered serviceable, that is to say, the majority of offshore reserves. All process needs are proven technologies in common use in the industry. This invention combines them in a unique manner to attain a means of handling natural gas for transport within a liquid matrix, under conditions not claimed in prior art.

The economics derive from the fact that billions of dollars of front-end investment and long-term planning need not be spent on any distant offshore LNG style production facilities. Instead, a buoy or minor process platform, tied into seabed facilities or a shore-bound export point, are all the access features that are required to pick up the gas.

Similarly, a receiving buoy or near-shore connection to transmission pipelines or storage removes the need for an LNG terminal at the market end of the voyage.

The forgoing description has described particular embodiments of the invention. Other embodiments will be evident to those skilled in the art. These embodiments are not limiting, and are covered by the scope of the appended claims that define the upper limits of efficiency of this mode of storage.

OTHER REFERENCES

Application No 20060042273	March 2006	Publication of US Patent Application:
		Storage of Natural Gas in Liquid Solvents, and Methods to
		Absorb and Segregate Natural Gas into and out of Liquid
		Solvents: Morris: Ian: et al.
LNG Inside Upstream	Issue of Dec. 1, 2006	Article on Compressed LNG
(Publication of Lloyds, London)		“Exxon Mobil Weighs Up Putting Pressure on LNG”
LNG Inside Upstream	Issue of Dec. 1, 2006	Article on ‘LNG-lite’
(Publication of Lloyds, London)		“SeaOne Uncoils its ‘LNG-lite’ Concept”

Claims

1. A method of creating optimal density of liquid mixtures of propane and other natural gas constituents for transport—The propane content having a mol percentage between 5.5 and 10.9, and the natural gas component having maximum Wobbe Index values of 1350 to 1475 in U.S. units.

2. A method of storing natural gas in a liquid matrix maintained by conditions of pressure (650 psig to 1000 psig) and temperature (−81 to −115 F). The method utilizes a minimum amount of work expenditure to yield a maximum net-density gain for the natural gas component relative to that of compressed natural gas alone under the same conditions. The operative condition is achieved in the liquid phase vicinity of the convex/concave point of rotation of the bubble point curve of the phase envelope attributed to the optimal specified mol percentage mixture.

3. A plurality of process and containment equipment specifically required for loading and unloading the claimed liquid matrix such that it can be transported in bulk, by land, sea or air.