Refrigeration – Storage of solidified or liquified gas – Cryogen stored in both phases
Reexamination Certificate
2001-08-31
2003-07-01
Doerrler, William C. (Department: 3744)
Refrigeration
Storage of solidified or liquified gas
Cryogen stored in both phases
C062S049100, C062S053200, C053S403000
Reexamination Certificate
active
06584781
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to the storage and transportation of compressed gases. In particular, the present invention includes methods and apparatus for storing and transporting compressed gas, a marine vessel for transporting the compressed gas and storage components for the gas, a method for loading and unloading the gas, and an overall method for the transfer of gas, or liquid, from one location to another using the marine vessel. More particularly, the present invention relates to a compressed natural gas transportation system specifically optimized and configured to a gas of a particular composition.
The need for transportation of gas has increased as gas resources have been established around the globe. Traditionally, only a few methods have proved viable in transporting gas from these remote locations to places where the gas can be used directly or refined into commercial products. The typical method is to simply build a pipeline and “pipe” the gas directly to a desired location. However, building a pipeline across international borders is sometimes too political to be practical, and in many cases is not economically viable, e.g. where the gas must be transported across water, because deep water pipelines are extremely expensive to build and maintain. For example, in 1997, the proposed 750 mile pipeline linking Russia and Turkey via the Black Sea, was estimated to have an initial cost of 3 billion dollars, without any consideration for maintenance. In addition, costs are also increased because both construction and maintenance are treacherous and require extremely skilled workers. Similarly, transoceanic pipelines are not an option in certain circumstances due to their limitations regarding depth and bottom conditions.
Due to the limitations of pipelines, other transportation methods have emerged. The most readily apparent problem with transporting gas is that in the gas phase, even below ambient temperature, a small amount of gas occupies a large amount of space. Transporting material at that volume is often not economically feasible. The answer lies in reducing the space that the gas occupies. Initially, it would seem intuitive that condensing the gas to a liquid is the most logical solution. A typical natural gas (approximately 90% CH
4
) can be reduced to 1/600
th
of its gaseous volume when it is compressed to a liquid. Gaseous hydrocarbons that are in the liquid state are known in the art as liquefied natural gas, more commonly known as LNG.
As indicated by the name, LNG involves liquefaction of the natural gas and normally includes transportation of the natural gas in the liquid phase. Although liquefaction would seem the solution to the transportation problems, the drawbacks quickly become apparent. First, in order to liquefy natural gas, it must be cooled to approximately −260° F., at atmospheric pressure, before it will liquefy. Second, LNG tends to warm during transport and therefore will not stay at that low temperature so as to remain in the liquefied state. Cryogenic methods must be used in order to keep the LNG at the proper temperature during transport. Thus, the cargo containment systems used to transport LNG must be truly cryogenic. Third, the LNG must be re-gasified at its destination before it can be used. This type of cryogenic process requires a large initial cost for LNG facilities at both the loading and unloading ports. The ships require exotic metals to hold LNG at −260° F. The cost is generally in excess of one billion dollars for a full scale facility for one particular route for loading and unloading the LNG which often makes the method uneconomical for universal application. Liquefied natural gas can also be transported at higher temperatures than −260° F. by raising the pressure, however the cryogenic problems still remain and the tanks now must be pressure vessels. This too can be an expensive alternative.
In response to the technical problems of a pipeline and the extreme costs and temperatures of LNG, the method of transporting natural gas in a compressed state was developed. The natural gas is compressed or pressurized to higher pressures, which may be chilled to lower than ambient temperatures, but without reaching the liquid phase. This is what is commonly referred to as compressed natural gas, or CNG.
Several methods have been proposed heretofore that are related to the transportation of compressed gases, such as natural gas, in pressurized vessels, either by marine or overland carriers. The gas is typically transported at high pressure and low temperature to maximize the amount of gas contained in each gas storage system. For example, the compressed gas may be in a dense single-fluid (“supercritical”) state.
The transportation of CNG by marine vessels typically employs barges or ships. The marine vessels include in their holds, a multiplicity of closely stacked storage containers, such as metal pressure bottle containers. These storage containers are resistant internally to the high pressure and low temperature conditions under which the CNG is stored. The holds are also internally insulated throughout to keep the CNG and its storage containers at approximately the loading temperature throughout the delivery voyage and also to keep the substantially empty containers near that temperature during the return voyage.
Before the CNG is transported, it is first brought to the desired operating state, e.g. by compressing it to a high pressure and refrigerating it to a low temperature. For example, U.S. Pat. No. 3,232,725, hereby incorporated herein by reference for all purposes, discloses the preparation of natural gas to conditions suitable for marine transportation. After compression and refrigeration, the CNG is loaded into the storage containers of the marine vessels. The CNG is then transported to its destination. A small amount of the loaded CNG may be consumed as fuel for the transporting vessel during the voyage to its destination.
When reaching its destination, the CNG must be unloaded, typically at a terminal comprising a number of high pressure storage containers or an inlet to a high pressure turbine. If the terminal is at a pressure of, for example, 1000 pounds per square inch (“psi”) and the marine vessel storage containers are at 2000 psi, valves may be opened and the gas expanded into the terminal until the pressure in the marine vessel storage containers drops to some final pressure between 2000 psi and 1000 psi. If the volume of the terminal is very much larger than the combined volume of all the marine vessel storage containers together, the final pressure will be about 1000 psi.
Using conventional procedures, the transported CNG remaining in the marine vessel storage containers (the “residual gas”) is then compressed into the terminal storage container using compressors. Compressors are expensive and increase the capital cost of the unloading facilities. Additionally, the temperature of the residual gas is increased by the heat of compression. This increases the required storage volume unless the heat is removed and raises the overall cost of transporting the CNG.
Previous efforts to reduce the expense and complexity of unloading CNG, and the residual gas in particular, have introduced problems of their own. For example, U.S. Pat. No. 2,972,873, hereby incorporated herein by reference for all purposes, discloses heating the residual gas to increase its pressure, thereby driving it out of the marine vessel storage containers. Such a scheme simply replaces the additional operating cost associated with operating the compressors with an operating cost for supplying heat to the storage containers and residual gas. Further, the design of the piping and valve arrangements for such a system is necessarily extremely complex. This is because the system must accommodate the introduction of heating devices or heating elements into the marine vessel storage containers.
In summary, although CNG transportation reduces the capital costs associated with LNG, the costs are still high due to a
Bishop William M.
Pemberton David J.
White Charles N.
Conley & Rose, P.C.
Doerrler William C.
Enersea Transport, LLC
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