CA2236738A1 - Process for the production of hydrogen by solar decomposition of water - Google Patents

Abstract

The invention provides a process whereby water vapour at high temperatures is subjected to a high gravity field in a vortex tube reactor such that there is stratification of the reaction mixture leading to further decomposition of water vapour to hydrogen and oxygen.

Description

Background to the Invention H ydrogen has long been viewed as the ideal energy fuel since the product of conventional combustion is water although there may be traces of oxides of nitrogen. There is no carbon dioxide, the major greenhouse gas, that figures prominently in concerns about the deteriorating state of the environment, particularly in urban areas. With the recent developments in fuel cell technology and the high overall energy efficiency of these units, about 2 to 3 times that of hydrocarbon fuels used in internal combustion engines and steam turbine,, the conditions are ideal for the use of hydrogen fuel.
Production of hydrogen is now mainly carried out be reforming hydrocarbons, such as methane, and 'to a smaller extent, by electrolysis of water. Undesirable hydrocarbon consumption at thermal power stations is part of the electrolysis production chain in most instances. Therefore, a method of hydrogen production that is non-polluting is needed.
There have been concerted efforts since the energy crisis of the seventies to transform water, either directly or indirectly, inl:o hydrogen, using solar energy.
There are generally three types of solar based production processes for hydrogen from water. Photov~oltaic cells are simple in principle but high cost and low efficiencies have bc;en barriers. In the so-called water splitting approach, intermediate chemical agents such as Fe0 are reacted with water, at low temperature, to produce hydrogen.
Regeneration of the resultant ferric oxide complex requires very high temperatures. Despite the plethora of systems, striking a balance between ease of production and ease of regeneration appears elusive.
The final type of process is the direct decomposition of water in solar furnaces at very high temperatures. Temperatures required are in the range of 2200-2500C to produce small equilibrium amounts of 10-15°ro hydrogen. The very high temperatures needed prises severe 1 imitations on materials and equipment that might be used to economical ly c~ury out this transformation. However, in contrast to the other approaches, the problems with this approach, though challenging, are fairly well defined.
The objects oi-'this invention are to address the problems of materials, temperature, yield, and process unit operations and lay out a process that can operate under these conditions and is economic as well.

SUMMARY' OF INVENTION
In this invention, art is disclosed whereby water can be transformed into constituent elements of h:~drogen and oxygen, in good yield, by appropriate process design. In particular, a method is presented for raking superheated steam at high vapor velocity, which has been heated in a solar reactor, to cause decomposition of water to the thermodynamic limits, under operating temperature and pressure of the solar reactor. The high velocity exit stream from the solar reactor is then passed into a final reactor system and subjected to centrifuging or stratification methods such as a vortex tube that enable separation an<i collection of components through appropriate exit ports with the lighter fraction containing hydrogen and the heavier fraction containing oxygen.
By means of removing one of the reactants within the secondary reactor, hydrogen, for e:~ample, the reaction can be run under upset equilibria conditions and driven further to completion by this well known technique. Additionally, in the case of a dynamic clhemical equilibrium, the stratification by a vortex tube of the heavy and light material, oxygen and hydrogen respectively, produces a transition zone containing water and hydroxyl species that has a degree of dynamically instability due to a deficiency of hydrogen and oxygen. By Le Chatelier's principle, water and hydroxyl in this transition zone would decompose to hydrogen .and oxygen. This contributes to further production of hydrogen and oxygen. Operating temperatures in the final reactor need to be maintained at levels as close to the primary reactor to maintain. rapid kinetics of equilibration.
V~Jith inadequate residence time or fall off in temperature of the final reactor, any residual material from the transition zone remaining in the secondary reactor can be separated at a heavy ends exit port and sent for recycle. With a well insulated system, the oxygen exiting the secondary reactor can be sent to one or more additional polishing stratification stages before being heat exchanged with feed water. The same can be done with hydrogen as needed.

OVERALL PROCESS DESCRIPTION
Solar energy is provided to the chemical process section by means of many banks of computer controlled mirrors to produce temperatures of about 2500 C required to partially decompose water into hydrogen and oxygen. The land area required is in the range of 100 Acres or about 1/6th of a square mile. This would provide enough energy for a 100 MW plant that would require 380 liters per minute or about 100 gpm of feed water.
Feed water is purified by filtration by known means before passing to the 1 S' heat exchanger. This preheated water is then piped to a 2~ combination heat exchanger solar distillation unit where the water is further purified by a low energy distillation process. This water is then cooled in by the 15' heat exchanger in which thermal energy is transferred to the in-feed water. The water is further purified in a 3rd stage such as reverse osmosis. It is then sent to the radiant energy capture system which captures a major portion of the energy which would otherwise be lost from thermal radiation. The heated water from the recapture unit is then piped into a 3rd specially designed heat exchanger which heats the water to super heated steam from the final stage of the solar reactor. This steam continues through the ls' stage solar reactor under pressure of about 10 atmospheres. This piping leads to the orifice within the reactor which allows the pressure to build. This high pressure allows better thermal transfer and keeps the water from splitting due to its higher pressure.
The output of the orifice leads to the 2°d stage solar reactor. As the steam enters this 2'~ stage of the solar reactor, vapor velocity is increased due to the vacuum on the output of the orifice. This in effect lowers the equilibrium temperatures for the water splitting reaction to occur. The solar furnace needs a residence time of about of a second (Lede et al, International Chemical Engineering, 25(2)246, 1985) for operations at 2500C. As the preheated steam enters the 2~d stage solar reactor, it further expands and decomposes to equilibrium concentrations of hydrogen, oxygen and intermediates according to the work of Ihara (Pergamon Press, 1979) and Lede.
A catalyst such as platinum or a platinum group element can be introduced in the 2~d stage solar reactor, lowering the equilibrium temperature for the water to split.
In one variation of the design of the solar reactor, steam is introduced into a cylindrical vessel that has an internal spiral flight. This sets the steam in spiral motion which prevents channeling and ensures a rapid temperature rise due to increases pressure of the vessel walls caused by centrifugal force. The flight has an open central core. Other arrangements to ensure adequate heat absorption will be evident.

At the exit of the 2~ solar reactor, the hot stream is directed towards the 3'd stage solar reactor which contains one or more vortex tube(s). The vapor stream is injected perpendicularly and on tangent, to the manifold of the vortex tube. This manifold, and its inner conical shape, acts to initial the formation of a vortex. This causes an increase in velocity leading to high gravity of the gas swirl in the vortex tube. Effects of intense gravity cause a mass action equilibrium shift yielding oxygen and hydrogen typically at outside circumference and center of the vortex tube. As well, there is a beneficial drop in pressure throughout the system. It is evident that this drop in pressure drives the decomposition favorably towards the products, as indicated by Ihara. The presence of a catalyst such as platinum or a platinum group element can further decrease equilibrium temperature.

DETAILED DESCRIPTION OF THE INVENTION
The major difficulty in the direct thermal decomposition of water is finding a means to cope with a reaction that is only about 15-20 % complete at achievable reactor temperatures of 2500 C (Etievant, Solar Materials, 24 (1991) 443-440, Elsevier).
This equilibrium is established in the primary reactor upon being exposed to the solar beam. Equilibrium time at 2500 C is rapid, being of the order of exp minus 4 seconds, (Etievant).
The challenge is to separate out the primary reaction products of oxygen and hydrogen and devise an energy effective means of reacting the remaining portion of the reaction product. This section will deal with are dedicated to separating products and improving yields.
Much work regarding separations has been carried out with membranes that diffuse oxygen away from the reaction mixture . There have been some claims for a hydrogen diffusion process.
Separation by diffusion can be difficult from an operating perspective since certain concentration gradients appear necessary which require large surface areas and low pressures, (Etievant, EP o572 944 A1, FR 2 366 216). Therefore, although challenging, low use of gravity or centrifuge based techniques has potential merit since the equilibrium mixture has widely different masses. Etievant has addressed the fundamentals of this approach but does not teach a specific manner of application.

Other means such as quenching the hot exhaust products at a very high speed will prevent the products from recombining to water. This can be achieved with water injection or an inert gas such; as argon. The problem. with this approach is that efficiencies of only 2 to 3 °'o can be achieves since the energy in the hot products can not be recovered effectively.
The major equilibrium reaction products of water at 2500 C include hydrogen, oxygen, steam and hydroxyl species. Since there is no possibility that equipment with moving parts could bc~ used, consideration must be given to static devices that achieve high centrifugal forces due to gas velocit5r. Devices included in this category are; a) F3ecker/Laval nozzles, b) spiral or cane centrifuges and c) Ranque-Hilsch vortex tubes.
The main criteria for functionality in gravity based separation is that peripheral speed, kinetic energy and centrifugal force of a component of the mixture, say oxygen, should be significantly large enough in relation to the kinetic energy or thermal velocity.
'l~his would lead to stratification of the reaction mixture within the centrifuge system. It has been found that this is possible vvith high gas velocities, according to the criteria of Etievant.
As well, if one of the reaction products, such as hydrogen, can be removed from the centrifuge system shortly after translFer from the solar reactor, the reaction can be under upset equilib~ria conditions, driving the reaction further towards completion.
This increases the efficiency of the transformation and reduces the need for costly recycle ~,treams.
'Chis is best seen with a modified Ranque-Hilsch (RH) vortex centrifuge system, the preferred embodiment of the invention. This centrifuge system draws its name from the inventor, G. Ranque (J. Phys. Et Rad., 14 ( 1933) 1125) and R. Hilsch (Rev.
Sci. Ins.
18( 1947) 108), who further investigated the vortex. tube. Lindstrom-Lang also investigated I;as separation (J. Heat Mass Trans., 7,2295( 1964)).
(Jas entering a vortex tube at a tangent develops into a vortex. There is some evidence that RH tubes operate because there is an inner and conventional vortex that spins tightly at the core, and a normal outer vortex that spins rapidly at the periphery.
Tube design to rapidly develop a stable vortex is essential to the operation of RH tubes (Newman Tools, Dartford Corm, USA and Lindstrom-Lange).
Interest in th~~ RH vortex tube has bE:en almost exclusively confined to the primary observation by Ranque that when a ,gas, such as air, is introduced perpendicularly at a tangent to a tube, one outlet of the tube exhausts gas at a lower temperature and the other outlet gas at a higher temperature. With conditions of under 100 psi gas inlet pressure, this device crf~ates a swirl of gas in the range of 1,000,000 rpm (Newman Tools) which translates into angular velocities well into the supersonic range.
Using air, there has been a report of enrichment of oxygen (CU Lindstrom-Lang) and helium/oxygen separation has also been studied. However, this work, nor any other art, teaches that the RH vortex tube can tie used to beneficially carry out equilibrium type chemical tran;~formations by the use of gravity, as is claimed in this work.
Nlore specifically, exit gases from a primary solar reactor can be directed to one or more RH vortex tubes wherein the hot gas is introduced to a RH tube, in a tangential manner, thereby generating large centrifugal forces, due to high gas velocity. After the vortex stabilizes, the heavier oxygen molecules are on the periphery and the lighter hydrogen molecules are at the core of the vortex tube. Because of separation of species, and mass acaion, water decomposes. Oxygen is. drawn off at the periphery of one end of the vortex tube while hydrogen is drawn off from the core at the other end of the tube.
With a proper separating radial gas manifold, both l;ases could be drawn off one end (Newman Tools).
The fact that hydrogen can be separated from oxygen within the vortex reactor, and drawn out of t:he vortex reactor, means that it is possible to achieve upset equilibrium conditions that favor further decomposition of water in response to the removal of hydrogen.
Hydrogen carp be removed from a point near the stabilization zone of the vortex by a smaller diameter inner core. This step may be assisted by the use of a comparative vacuum that is not so strong that it disrupts gas dynamics at intermediate and peripheral zones.

Lindstrom-Lang used internal tubing in the RH vortex tube to effect gas separation. In this instance, inner core diameter can. be about 20-30% of vortex tube diameter. An effective single point of removal has been found to be about 25% of the distance down the core from the tangential inflow. The core may be porous and of extended length in order to promote removal of hydrogen from the vortex tube.
The high centrifugal forces due to gas velocities of 1-2 million rpm, appropriate for this case, cause a radial distribution of m;~ss across the vortex tube. At the center, is hydrogen.
A.t the periphf~ry is oxygen. There is .a transition zone of intermediate between these species consi~;ting of water vapor and hydroxyl groups. The transition zone is chemically unstable since it is deficient in oxygen and hydrogen. Separation is favored by larger diameter vortex tubes operating at higher gas velocities and rpm's. Under these conditions, the mechanical kinetic energy generated by the gas swirl does not have to exceeds the kinetic energy of the gas molecules at a given temperature, and effective stratification occurs.
T'he transition zone strives to reach "regional" equilibrium by forming additional hydrogen and oxygen that move to the core and periphery, respectively.
Therefore, within the final stage reactor, the system is driven to the products of oxygen and hydrogen.
There is a need to maintain temperatures in this system so that the reaction does not become subject to kinetic control. If adequate gas velocities are not achieved, stratification will not take place and the above phenomena will not occur. The vortex tube can then only be used to strip off the equilibrium concentrations of the gases and several additional stages would be required to allow re-equilibration and gas stripping. Other separation means may be just as effective in these circumstances, recognizing that heat and momentum loss would be major limiting factors in this approach.
It will be evident that this approach permits the near or full dissociation of water at temperatures significantly below that needed for ordinary full dissociation which is about 3500C. Additionally, while an operating temperature of 2500C has been cited as desirable, it will be evident that lower temperatures, such as 2200C, can be employed provided that sufficient residence time is allowed for equilibration (Etievant). The benefit of lower temperature operation is ea:>e of operation and the reduction in difficulty of fabricating process equipment for operation at reduced temperatures.
This system i ~ an application of gravity forces to produce useful chemical change that cannot easily be produced by other nneans. The decomposition of water is an ideal case since the reactant, water, is of interrr.~ediate mass between the products of hydrogen and oxygen. It is Evident that with other .equilibrium reactions where this type of distribution prevails, gravity accelerated or catalyzed reactions can be similarly carried out. Ammonia could be made to decompose in a similar manner. Methane reforming may be considered.
Other systems are possible if the appropriate mass distribution applies to reactants and products.

The RH vortex tube might be usefully employed in another manner to improve the process of oxygen diffusion investigated by some, (Etievant). Diffusion depends, in part, upon high partial pressures of oxygen on one side of a membrane material and lower partial pressures on the other side. With the vortex tube, oxygen concentration is at a comparatively high level on the perm>hery.
If the vortex tube was constructed of zirconia oxide material that is know to permit oxygen diffusion, (Etievant), oxygen would be removed as the front traveled down the riibe. In order to assist the overall process, a gentle cone shaped vortex tube would be preferred. Hydrogen would be withdrawn at the apex of the cone. A container for the vortex tube would collect the oxygen under reduced pressure.
E~RIEF DESCRIPTION OF THE DRAWINGS
F'IG. 1 shouts a schematic view of the preferred embodiment of this invention, fir use as a solar hydrogen production process.
F'IG. 2 shouts a cross sectional pictorial view of the preferred embodiment of this invention which closely following the FIG 1 process.
F'IG. 3 shouts an outside view o:f the solar hydrogen tower indicating where the incident solar radiation bombards the tower.
FIG. 4 shouts the side view and general placement of the strut and one way window design for the radiant energy capture system of the solar hydrogen generator.
F'IG. 5 shoves a top down view detail of the one way window and strut design of the radiant energy capture unit.
FIG. 6 shoves the side view detail of the one way window and strut design of the radiant energy capture unit.
F~ IG. 7 shoves the top down viev~ of the solar hydrogen tower with the top dome removed. The center support structure, reactor core, and solar target are visible in the center. Surrounding this reactor assembly is one embodiment of the one way window and strut design based on flat glass components.
FIG. 8 shoves the same view as hIG. 7 but shows another embodiment of the glass structures in which struts a.re not required and the tapered one way glass cones form the structural members as well as radiant energy capture windows.
FIG. 9 shows the helical channel which represents the path that the water and its products take within the heat exchanger of FIG 10.
FIG. 10 shows preferred embodiment of the construction of the high temperature exchanger which has the helical water feed channel feeding up the center o f the exchanger, and the counter-flow smaller helixes feeding the products of hydrogen and oxygew down. Also shown is the thermal break preventing a thermal short circuit.
FIG. 11 shows a detail of anothE;r embodiment of the heat exchanger. A
section of the helix of FIG 10 in which thermal brakes are evident along the path of the helix.
FIG. 12 shows a line path repres>entation of the FIG 10 heat exchanger. The wavy lines represent the helixes as shown in FIG 10 with the center line being the water in-feed line feeling up and the four lines on each side being the hydrogen and oxygen feeding down.
FIG. 13 show another embodiment of the heat exchanger of FIG 10 in which a double he:~ix out-feeding hydrogen and oxygen, helix around the water in-feed helix, forming a compound helix.
F'IG. 14 shows another embodiment of the heat exchanger of FIG 10 in which a triple heli:~ exists and this triple helix, helixes around the exchanger, forming a compound helix.
F'IG. 15 shows a top down view of another embodiment of the heat exchanger of FIG 10 in which the thermal transfer channels are on one flat layer as opposed to being helical.
FIG. 16 shows a side view of FIG 15. Each vat layer is connected to another later separated by a thermal break layer thus preventing a thermal short circuit.
FIG. 17 shows the helical path v~rithin the 15' stage solar reactor shown in FIG
18.
FIG. 18 shows a cutaway view of the 1 S' stage solar reactor.
FIG. 19 shows the helical path within the 2"'~ stage solar reactor shown in FIG
20.
FIG. 20 shows a cutaway view of the 2"d stage solar reactor.
F'IG. 21 shows the pictorial view of the outside of the vortex tune 3 'd stage reactor.
F'IG. 22 shows a cross sectional view of the vortex tube 3 ~d stage solar reactor with the in-weed manifold.
F'IG. 23 shows the detail of an injector nozzle of the vortex tube 3~d stage solar reactor.
FIG. 24 shows detail view of another embodiment of the vortex tube 3 'd stage reactor showing more than one injector an showing the in-feed manifold.
F'IG. 25 shows the preferred embodiment of the detail construction of the 3~a stage solar reactor which incorporates the vortex tube.
F'ig. 26 shows a cutaway view of a plurality of vortex tubes assemblies with the 3~d stages solar reactor and mounted on top of the 2"d stage solar reactor.
FIG. 27 shows another embodiment of the 3 rd stage solar reactor positioned in relation to the 2~d stage solar reactor helix.
FIG. 28 shows a close up side view of another embodiment of the 3 ~d stage solar reactor shown in FIG. 27.
FIG. 29 shows a top down view of the embodiment as described in FIG. 28.
FIG. 30 shows another embodiment of the 3'~d stage solar reactor.
FIG. 31 shows the side view of a~ typical solar hydrogen station which would be geographically located in the latitudes not far from the equator, where the sun is high in the sky.
FIG. 32 shows the top down view of FIG 31 "
FIG. 33 shows the side view of a typical solar hydrogen station which would be geographiically located in the latitudes far from the equator, where the angle of the sun in low on the horizon.
F1:G. 34 shows the top down vievv of FIG 33.
FIG. 35 shoves a small scale solar hydrogen station which uses a specially shaped reflective dish and tracks the sun directly.

Claims

1) A process whereby water vapor at high temperatures, in the range of 1800C
to 3000C
is subjected to a high gravity field due to supersonic swirl velocity in a vortex tube reactor such that then. is stratification of the reaction mixture leading to further and essentially complete decomposition of water vapor to hydrogen and oxygen by mass action.