The inventions disclosed herein relate to desalination or purification of saline or otherwise polluted water using gas hydrates (referred to herein as xe2x80x9cdesalinationxe2x80x9d for simplicity but to be understood as encompassing other types of water purification). More particularly, the inventions relate to open-water systems (or partially open-water systems) for hydrate-based desalination or purification (although certain methodologies disclosed herein may also be used in fully land-based installations) and, in another aspect, to vaporization of Liquified Natural Gas (LNG),.
Desalination or purification of saltwater or otherwise polluted water using hydrates is known in the art. For example, as illustrated in Max et al., U.S. Pat. No. 5,873,262, a hydrate-based fractionation column can be located in the open ocean. The column extends downward far enough for the lower portion of the column to be located at a depth where the water pressure is high enough and the water temperature is low enough for methane hydrate to form spontaneously (and remain stable) upon introducing a hydrate-forming substance (e.g., methane gas) into the lower portion of the column. The hydrate-forming substance combines with seawater, which enters the column freely through its open lower end, and forms naturally buoyant methane hydrate. The methane hydrate rises naturally within the column into a region where pressure and temperature conditions are such that the hydrate no longer is stable, and the hydrate dissociates (xe2x80x9cmeltsxe2x80x9d) naturally to release the hydrate-forming substance (e.g., methane gas) and fresh water which has been extracted from the saltwater via the hydrate. The fresh water is collected, e.g., for drinking or other purposes, and the hydrate-forming substance is captured and recycled for reuse in the process.
A variety of gas hydrate-forming substances are known in the art and include hydrocarbons besides methane (including but not limited to ethane, propane, butane, cyclopropane, cyclobutane, and mixtures thereof), carbon dioxide, and various mixtures of substances including one or more hydrate-forming substances. It is also known in the art that certain gas hydrate-forming substances can be mixed with the water to be treated in liquid form. (Therefore, unless otherwise specified, the inventions claimed herein in terms of a xe2x80x9chydrate-forming substancexe2x80x9d are to be interpreted as encompassing any and all such species of gas hydrate-forming substances.)
The temperature of the water to be treated is an important parameter which bears on the formation of hydrate. The colder the water used for hydrate formation, the lower the pressure at which the hydrate will form for a given hydrate-forming substance. Additionally, for a given volume of water to be treated, using colder water allows more hydrate to be produced before the temperature rises (as a result of exothermic formation of the hydrate) to a level where hydrate will no longer form spontaneously at a given depth.
In some restricted bodies of marine and other water such as the Mediterranean Sea, surface waterxe2x80x94particularly during the winter and springxe2x80x94may be cooler than water at depth. In other areas where stratified watermasses or upwelling or downwelling watermasses occur, the coldest water in a local sea area may be found at a horizontal level somewhere between the surface and a water depth which provides the requisite pressure for hydrate formation.
Furthermore, depending on the hydrate-forming substance being used, the fractionation column may have to extend relatively deep into the body of water in order for the pressure at the depth where the hydrate-forming substance is introduced into the water to be treated is suitably high for hydrate formation. For example, U.S. Pat. No. 5,873,262 refers to feeding methane into seawater at depths exceeding 100 metersxe2x80x94a depth at which pressure exceeds 11 times atmospheric pressure (ten meters of water depth for each additional atmosphere of pressure). Therefore, the energy costs associated with pumping the methane gas (or other gaseous hydrate-forming substance) which has been released upon dissociation of the hydrate back down to depth represents a significant cost of operation.
With respect to vaporization of Liquified Natural Gas, the LNG industry is based on the formation of LNG as a means of transporting it to markets from a producer country, where transport of the gas by pipeline or other means is not possible or is commercially of less benefit. LNG is a man-made substance that is made by liquification of natural gas by a process of refrigeration and compression. It is commonly stored at its boiling temperature of about xe2x88x92162 degrees C. in highly insulated vessels that are pressurized slightly above atmospheric pressure (e.g., approximately 5 psig above atmospheric pressure). Vaporization of LNG during transport absorbs heat, which acts to keep the liquid cold. The vapor is vented, usually to fuel a combustion engine of some type, and the pressure and temperature in the vessel remains constant.
LNG is imported into the United States to make up a shortfall in North American natural gas production. In part, the transition from liquid petroleum fuels to natural gas is being driven by the fact that for a given unit of energy, natural gas is a cleaner source of fuel for combustion. Methane has the highest hydrogen-to-carbon ratio of any hydrocarbon, and it bums cleaner with fewer toxic emissions, which has caused it to be considered the combustible fuel xe2x80x9cof the future.xe2x80x9d This has led to environmental legislation requiring transition to gas fuels.
Because many natural gas pipelines in the United States are old and have bottlenecks that restrict transport of gas at peak periods, the import of natural gas, as LNG, directly to the target market without the construction of pipelines is an advantage over building new or expanding old pipelines.
When the LNG reaches its destination, it must be vaporized or converted back to a gaseous form in order to be distributed as gas and used as a combustion fuel. Presently, the energy requirement for the vaporization process is achieved by a process of xe2x80x9csubmerged combustion.xe2x80x9d Energy must be expended to compensate for both the simple warming of the LNG/gas and the endothermic change of state from liquid to gas. In this method, the LNG is passed through a large assembly of pipes that are submerged in a water bath that is heated by combustion of some fuel (usually part of the gas load itself). The vaporization achieved using this process consumes fuel in relatively large and expensive apparatus that demands special maintenance, especially with respect to unwanted biosystems in the water bath.
As a byproduct of the combustion process that heats the vaporization water bath, exhaust is created, which contributes to pollution. Emissions are mitigated by bubbling exhaust through the water bath. However, this results in some of the combustion products becoming dissolved in the water, which commonly renders it acidic and produces a potential for corrosion and for a situation where the water bath may become a hazardous material.
No commercial products (of which I am aware) are derived from the existing thermal vaporization process during the vaporization of LNG.
The present inventions provide a number of different means by which to increase the efficiency and benefits of gas hydrate-based desalination or water purification. According to one aspect, the invention recognizes that in some instances, water that is the coldestxe2x80x94and hence most conducive to formation of gas hydratexe2x80x94in a naturally occurring body of water may be found at depths other than that at which the hydrate formation region of an open-water (e.g., marine) hydrate fractionation desalination installation will be located. In particular, the coldest water may be found at depths above (i.e., less than) those which yield pressures necessary to support hydrate formation (and hence above the depths at which the hydrate formation region of the installation should be located). Alternatively, the coldest water may be located below the depths at which the hydrate formation region of the installation is located, e.g., where the economics of construction limit the depth to which the installation extends.
Therefore, according to the invention, water is drawn from a depth yielding water to be treated that is colder than, and hence preferable to, ambient water found at the level of the hydrate formation region, and that colder water is brought to the formation region. In the illustrated embodiment, water to be treated is drawn from above the hydrate formation region, but the claimed invention embraces drawing water to be treated from below the formation region, as well. Moreover, in the illustrated embodiment, the water intake system is configured to selectively access water to be treated from a number of different levels so that the level from which water to be treated is drawn may be varied depending on where the coldest water is to be found.
Preferably, the water to be treated is brought to low enough pressure depths before being introduced into the hydrate formation region of the fractionation column that non-hydrate-forming gas that is dissolved in the water to be treated is exsolved. Additionally, the water to be treated preferably is pre-treated by having an initial, pre-treatment amount of hydrate-forming substance mixed with it to promote hydrate formation. The water to be treated may also be cooled by supplemental means before being delivered to the hydrate formation region, either by supplemental cooling or refrigeration means or by being passed in heat-exchanging relationship with hydrate that is dissociating endothermically in the dissociation region of the installation.
The invention further recognizes that significant savings can be realized by using a pre-pressurized source of gas hydrate-forming substance and, upon the subsequent dissociation of hydrate formed from the hydrate-forming substance, passing the substance downstream for use in a downstream application instead of reprocessing it and pumping it back down to depth for further use in the hydrate fractionation desalination process. Thus, in accordance with this aspect of the invention, pre-pressurized hydrate-forming substance may be drawn from undersea deposits, e.g., of methane (and possibly other hydrocarbon gases) and injected directly into the hydrate formation region of a hydrate fraction installation with little or no additional pumping or pressurization. Upon dissociation of the hydrate, the hydrate-forming substance, e.g., methane (and other gases, as appropriate), is passed downstream for appropriate use thereof.
Alternatively, Liquified Natural Gas (LNG) may be used as the hydrate-forming substance. In that regard, in another aspect, the invention provides a means for vaporizing LNG using hydrate thereof to do so, with the LNG being passed downstream (e.g., ashore) after the hydrate has dissociated. If the LNG is injected into saltwater, e.g., in an open-water marine installation, fresh water can be obtained from the saltwater as a highly desirable xe2x80x9cby-productxe2x80x9d of the LNG vaporization.
Moreover, using LNG as the gas hydrate-forming substance has an attendant effect that significantly increases efficiency of the desalination fraction process. In particular, when it is injected into the water to be treated, the LNG rapidly vaporizes underwater, and vaporization is an endothermic process. At the same time, the LNG forms hydrate, which is an exothermic process. The heat released upon hydrate formation is substantially consumed by the LNG vaporization process and, as a result, residual brines (saline enriched fluid xe2x80x9cleft overxe2x80x9d after hydrate formation) are heated to less extent by the heat of hydrate formation than would be the case where such heat of hydrate formation is not consumed. Because elevating the temperature of the water in which hydrate forms reduces the efficiency of the process in terms of hydrate-forming capacity of a given volume of water, consuming the heat of hydrate formation in the LNG vaporization process limits temperature rise of the residual brines and therefore increases the efficiency of the hydrate fractionation desalination process.
In another aspect, the invention provides a hydrate fractionation desalination installationxe2x80x94e.g., an open-water or marine installationxe2x80x94in which multiple fractionation columns are provided with a common water collection section into which they all release fresh or purified water. The fractionation columns and the common water collection section are cooperatively configured so that the columns can be removed, e.g., for servicing, and the associated fitment xe2x80x9ccappedxe2x80x9d so that the desalination process can continue without the removed column.
In another aspect, the invention provides a hybrid hydrate fractionation desalination installation having a subsea hydrate formation and thermal/salinity equilibration installation that is located on the seafloor, generally close to land, and a hydrate dissociation installation that is located on land. A connector assembly provides communication between the two installations, so that hydrate formed in the subsea installation can pass up and into the land-based dissociation installation.
Such a hybrid configuration maximizes the cooling/refrigerant potential of the hydrate, which dissociates endothermically, since heat released by exothermic formation of the hydrate is dissipated into the ocean via the equilibration section. Preferably, a xe2x80x9cslugxe2x80x9d of hydrate-rich slurry is formed in the connector assembly and then released to flow into the land-based dissociation installation. Preferably, the land-based installation is a pressurized installation, i.e., one which is configured to process hydrate in a batch mode.