My new hydrogen packaging technology classifies well with special gas packages or receptacles having abosorbents, adsorbents, or solvents. It more particularly concerns room temperature packaging of hydrogen in a receptacle containing a solvent comprising a special blend of hydrocarbons, of which at least one solvent component will be at or near its critical region state under the temperature and pressure conditions prescribed for practice of the invention.
Any method of room temperature hydrogen packaging, whether using highly compressed pure gas or my present invention, incurs less need for elaborate apparatus than does cryogenic storage. I consider a low cost container to be highly desirable in connection with providing a hydrogen source for electric vehicle fuel cells, or internal or external combustion engines, er for any hydrogen-consuming application where production of large numbers of portable, refillable containers is contemplated. Portability is not, however, essential in all possible application areas; for example, in certain load-levelling schemes for large-scale electrical power systems, storing hydrogen produced by electrolysis of water during periods of off-peak consumption would employ a large stationary container, operated on the same principles of smaller packaging systems embodying my invention.
Recent developments in supercritical fluids used as solvents seem to point somewhat generally in the direction of the invention. xe2x80x98SFExe2x80x99, or ie., supercritical fluid extraction, is known to include mixing hydrogen and supercritical carbon dioxide for a variety of processes, such as hydrogenating oils, isomerization, polymerization, and conducting certain chemical syntheses. The enhancement of solvation accounting for increasing utilization of supercritical fluids in a solvent role has not been directed to enhanced dissolution of low molecular weight, highly volatile gases like hydrogen, however, but has mainly been directed to dissolving high molecular weight, low volatility solids and liquids like fats and oils. My packaging of hydrogen in solution with below-specified hydrocarbons is thought to exploit critical region phenomena in a manner unanticipated by any known proposal to admix a supercritical fluid and hydrogen.
On a terminological issue, attention is drawn to use by D. Dixon and K. Johnston in their reference entry for xe2x80x9cSUPERCRITICAL FLUIDSxe2x80x9d, KIRK-OTHMER Encyclopedia of Chemical Technology, of the term xe2x80x98compressed fluidxe2x80x99 to cover a co-extensive meaning I encompass hereinafter using xe2x80x98dense-phase fluidxe2x80x99 as a more recently fashionable term of jargon. The term xe2x80x98compressed fluidxe2x80x99 was used by Dixon and Johnston to encompass xe2x80x9ceither a supercritical fluid, a near-critical fluid, an expanded liquid, or a highly compressed gas, depending on temperature, pressure, and composition.xe2x80x9d In disclosing my present invention, I employ xe2x80x98dense-phase fluidxe2x80x99 synonymously, as covering the same four forms of xe2x80x98compressed fluidxe2x80x99 mentioned by Dixon and Johnston. These forms of fluid do not correspond to sharply distinct states of matter, which are not found in the thermodynamic vicinity where critical region phenomena occur, causing many substances to manifest appreciable departures from classical expected behaviour of ideal liquids, ideal gases, ideal solutions or ideal mixtures. For example, where ordinary engineering practice commonly regards liquids as so negligibly compressible as to be practically incompressible, this is not the case for expanded liquids, which manifest both gas-like compressibility and diffusivity even though at liquid-like densities. In or near critical regions, fluid viscosities often are intermediate between what is usual for gases on the one hand and liquids on the other. It is thought that such concurrence in dense-phase fluids of both gas-like and liquid-like properties underlies their recognized utility for replacement of many traditional organic liquid solvents.
Rather than concern with use of solvents for extractive processes, for cleaning in general, or for thinning of viscous resins, glues, or paints to promote handling ease, the present invention concerns solvation as a technique for storing a solute gas, viz., dissolved hydrogen, the gas being thereby packaged for subsequent use in fuel cells or combustion apparatus, eg., engines, torches, and the like. There are at least two old familar examples of storing combustible gases as solutes in liquids: acetone has long been known to store dissolved acetylene; and propane condensed under pressure to the liquid state is known capable of storing dissolved methanee.
Early in 1990, in SAE Technical Paper 900586 entitled xe2x80x9cMethane Solubility and Methane Storage in Suitable Liquid Hydrocarbon Mixturesxe2x80x9d, I and B. D. Turnham reported our investigation into possible advantages of fueling combustion engines powering road vehicles with methane stored by dissolution in propane or other hydrocarbons, experimentally blending some mixtures intended to make improved methane packaging solvents. We reported that about 70% more methane could be packaged in a given tank filled with an appropriate mixture of liquid hydrocarbons than by storing the methane alone in the same tank at the same temperature and pressure. In some compositions we made and tested, reduced solvent mixture densities obtained by selective blending of different hydrocarbons produced effective methane-packaging solvents which made for lighter weight packaging than pure propane. Ethane containing blends in particular seemed to hold promise and in the SAE paper we stated: xe2x80x9cIn fact ethane itself with a critical temperature of 305 K was tested as a possible solvent but good data could not be obtained.xe2x80x9d
Retrospectively, I consider that our data collection difficulties pertaining to some aspects of the methane storage research are attributable at least in part to critical region phenomena, eg., critical opalescence. The methane storage research in a sense predisposed me when subsequently turning to hydrogen storage to revisit ethane and other light hydrocarbons as possible storage media, ie., for making hydrogen packaging solvents.
In the vast accumulation of background data accessible to workers in the field are calculations of the mole fraction of dissolved hydrogen when in solution with condensed ethane at very high pressures and temperatures far below ethane""s critical region, which R. J. Sadus has supplied in a Table in High Pressure Phase Behaviour of Multicomponent Fluid Mixtures, Elsevier Science Publishers, 1992. For example, the mole fraction of hydrogen of 0.728, equivalent to approximately 10% by weight, is listed for a binary ethane and hydrogen mixture at 175.2xc2x0 K (ie., xe2x88x9297.8xc2x0 C.) and a pressure of 233 MPa (ie., approximately 2,299 international standard atmospheres). Although 10% by weight of hydrogen in solution is better than the amount achieved by a number of known hydrogen storage methods utilizing packaging media, it is my opinion that the magnitudes of refrigeration and high pressure involved make the Sadus thermodynamic data merely citable as pertinent and of interest, rather than as disclosing a practical new method of hydrogen storage. Such extreme thermodynamic conditions require costly and elaborate apparatus and receptacles both to produce and to maintain.
Important objects of the present hydrogen packaging method include: 1. a less energy-consuming manner of forcing a given amount of hydrogen at room temperature into the volume of a given receptacle than by forcing hydrogen alone into an empty receptacle; 2. a less energy consuming manner of packaging hydrogen than by its cryogenic liquifaction or other process requiring a great magnitude of refrigeration; 3. reduced packaging system weight by comparison to metal hydride type systems; 4. cheaper receptacle filling material to act as a hydrogen storing medium at less expense than costly to manufacture nanoscale particulate absorbents based on carbon allotrophs; and, 5. reduced need for exceptionally strongly built pressure vessels rated considerably above fifty atmospheres to store hydrogen non-cryogenically.
I have discovered how to utilize, as effective hydrogen packaging media, relatively inexpensive and readily obtained alkanes or paraffins, also called saturated aliphatic hydrocarbons, processable using simple apparatus to procure a solvent with capacity to store hydrogen as a solute at room temperature and at from at least about twenty atmospheres up to fifty atmospheres of pressure, wherein, when a solvent component is in or near its critical region and forms a dense-phase fluid, a much greater amount of molecular hydrogen per litre of containerized solution is dissolved than would otherwise be storable by itself in the same size container at the same conditions, as merely a compressed gas. The essential concept involves that the volume of a suitable container at those conditions should be shared by the hydrogen with a dense-phase fluid solvent component of the solvent comprising blended aliphatic hydrocarbons with which the hydrogen is in solution, forming a single phase that fills the container. Preferred as the hydrogen-packaging solvent in carrying out the invention is a mixture of nine parts by weight ethane and one part by weight hexane, but I expect that minor proportioning adjustments and substitutions are within the skill of those in the art, who if desiring to do so could readily substitute one specific aliphatic hydrocarbon component in place of another without truly departing from the principles and spirit of the invention. For example, if butane with its lower carbon number were to be substituted in place of hexane as the minor component, more would be used, whereas if octane with its higher carbon number were the substitute, then less would be used.
According to projections, when one litre of preferably constituted solution weighs about 150 grams, from about 10% and up of that will be the dissolved hydrogen. That is, I suggest storing at least about 15 grams of hydrogen per litre by its dissolution in the solvent. If hydrogen were stored by itself at room temperature and twenty atmospheres, merely as a compressed gas, the weight of hydrogen per litre would not be above about 1.65 grams. Thus it is apparent that a single container filled with a solution applying my invention can package as much hydrogen as can be stored unmixed in nine identical containers at the same conditions.
In certain circumstances as discussed below, for some but not all application end-uses, there will be a need to separate the hydrogen for use. That need can be met by resort to one or another of at least two well known types of separation process, the best known two being separation by selective diffusion using a palladium membrane or the like, and by so changing thermodynamic conditions as to reduce solubility for the solute of the solvent portion of the solution and incur a phase separation liberating an excess of hydrogen from that previously dissolved. I consider the state of the art respecting such separations and related separation technologies to be within the skill of those skilled in the art, without undue experimentation, who will doubtlessly work out many details of separating schemes which are properly regarded as outside the scope of the present invention.
Without wishing to be bound by any theory, I assume that in some way generally resembling solubility enhancement phenomena encountered in the near-critical conditions for ethane in my previous research, ie., with dissolved methane, there here likely occurs a conferral of short-range order that spaces hydrogen molecules closer together for the given pressure than in typical unreactive gaseous mixtures wherein the usual assumption is that co-presence of non-reacting components is without their having significant effect on one another. Not to my knowledge mentioned anywhere in the field as applying to resolution of room temperature hydrogen packaging problems without need for high pressures above fifty atmospheres is myessentially simple to carry out suggestion to employ as a storage medium, co-occupying a receptacle with hydrogen, a substantially binary fluid solvent comprising suitably blended aliphatic hydrocarbons, at least one of which, the major component, should be a dense-phase fluid at or near critical region conditions when packaging the solute (hydrogen), for example: ethane. I prefer a blend of ethane and hexane respectively in a nine-to-one parts by weight ratio to form the solvent. Using xe2x80x9csubstantiallyxe2x80x9d to qualify xe2x80x9cbinary fluid solventxe2x80x9d is appropriate here because if ordinary technical grade hexane is employedxe2x80x94which it can bexe2x80x94there will generally be contained therewith, without detrimental effect, minor amounts of other n-alkanes besides ethane, meaning in that case, if very strictly speaking, that the solvent body is actually a multi-component type at least ternary or higher rather than perfectly binary, although being substantially binary insofar as having one significant major component together with a significant minor one, as the two important members of a substantially binary solvent system.
Although the invention per se relates to subject matter which might well be conveyable without a drawing, as it has been above, its manner of being put to use in technical applications contexts will be more readily understood by having reference below to schematic figures of drawing which illustrate prophetic examples of filling and using receptacles and adjunct apparatus in practice of the new hydrogen packaging method of my invention.