CA2052762C - Advanced synergistic system for heavy production - Google Patents

Abstract

An improved dual temperature hydrogen sulphide exchange process substantially cost reduced over processes specified heretofore for the production of large scale volumes of heavy water (D2O) in which the method of contacting natural heavy water by the H2S gas for the purpose of recovering deuterium is changed from a sieve tray tower means to a gas diffusion cell means and in a preferred simplified process flow arrangement, low level D enrichment by H2S
exchange is closely integrated with a CECE assisted electrolysis process, with or without final vacuum distillation processing, to produce 99.75% pure heavy water at a breakthrough level of cost reduction.

Description

Title: Advanced Syneraistic System For Heavy Production FIELD OF THE INVENTION
This invention provides for the production of heavy water (deuterium oxide, D20) with advances in processing technology directed toward reducing the cost of production very substantially below that of present methods.
BACKGROUND OF THE INVENTION
Heavy water's principal non-weapon usage is as a moderator in Canada's Candu natural uranium reactor system. Other nuclear reactor systems such as the U.S. light water water moderated system use enriched uranium as fuel. Any comparison of the cost of power produced by heavy water moderated and light water moderated nuclear reactor systems may, in a final analysis, be primarily decided by the cost of production of heavy water versus the cost of enriching uranium.
Heavy water cost as produced in present plants, as a component of a Candu reactor's total capital cost, has reached 15% to 20% compared to an original target of 5%. The cost of enriching uranium is not readily available.
However it would appear safe to assume that since the cost of the U.S. light water moderated reactor system has escalated beyond normal inflation, mainly due to attenuated periods of construction due to regulatory interference and high interest costs attendant thereto, that uranium enrichment costs from older written down U.S. facilities have no worse than maintained their original relationship in overall system cost. In addition, improved processes for enrichment have been developed.
However enriching costs may have fared, it is the purpose of the invention described in outline hereunder, to bring heavy water production cost back to, or close to, its original target of 5% of total capital cost of a Candu reactor.
The value to Canada of a Candu system which has a substantial margin of cost advantage over competing systems will be very great; when measured in terms of lower domestic nuclear power costs, increased exports of reactor components and Candu Technology worldwide and, possibly, exports of power to the U.S.
Over the past twenty years the Candu reactor system, has fallen behind with respect to the promise originally held for it as a world competitive nuclear reactor system by Canadian nuclear scientists, such as the brilliant Benton Lewis, who saw its many advantages over the other systems in preponderant use today.
A number of reasons for the failure of the Candu natural uranium heavy water moderated system's failure to come up to its potential, in terms of worldwide acceptance, may be given. Some of these reasons are well recognized. One in particular is that the light water moderated system fathered by the U.S. had a substantial head start. With large capacity for uranium enrichment in place and vast financial resources of the U.S. behind it, Candu was faced at best with a difficult uphill struggle to find acceptance.
Another disadvantage came later when peaceful nuclear power systems fell into disfavour because of their association in the public mind with weapons of destruction. This unwarranted disfavour became most deeply rooted in the U.S. where it remains today. Although Canadian fear reaction to peaceful nuclear power has not been as great, there has been an inevitable spill over effect which slowed down Canada's domestic reactor programme considerably which in turn lessened the opportunities to maintain momentum for the build up of a cost improved and more competitive Candu component supply system.
Less well recognized, however, is the loss of competitiveness with respect to system cost which has resulted first from runaway inflation in the capital cost of the Candu reactor system and second from inflation's aftermath of extremely high interest rates which may, in the eighties, have had an even more damaging effect than high inflation. This is especially indicated since high interest cost has compounded astronomically into nuclear station capital costs as the fear of nuclear power stretched out the cycle of regulatory approval, through engineering, through construction and commissioning by several years.
Even less perceived or perhaps forgotten, is that Candu, pits a higher, heavy water included capital cost plus lower natural, uranium operating cost against the light water moderated reactors lower non-heavy water included capital cost plus higher enriched uranium operating cost, when all economic factors have worked against Candu's higher front end capital cost, as it has unfortunately turned out.
As will be seen from a further discussion of heavy water cost of production by present plants, heavy water may have become Candu's Achilles heel although the more general background of broader economic factors briefly discussed above have also been prime contributors.
Deuterium, as an isotope of hydrogen, occurs in minute quantities in all hydrocarbons. As the earth's most plentiful hydrocarbon, ordinary water is overwhelmingly the greatest storehouse of deuterium. Twice as heavy as hydrogen, when deuterium is combined with oxygen to form D20 or heavy water, D20 is roughly 11 % heavier, than H20. This difference in weight between deuterium and hydrogen is a principal factor on which processes for the recovery of deuterium have been based.
It has been reported that about 100 methods have been used for the recovery of deuterium. However, to date only four processes or combination of the four have been used to produce large quantities of D20 such as the 400 to 500 tons required for the initial charge to a 500-megawatt Candu reactor.
Vacuum distillation, extraction from hydrogen produced by the electrolysis of water and the H2S dual temperature exchanger, or GS process as it is commonly called, are the three most used processes. Extraction from hydrogen as a by-product of ammonia production is a fourth method. Of the four, the GS process is by far the preponderant contributor to heavy water production today. This is because it has been shown to be the least costly method by which to recover the minute quantities of deuterium found in the abundant natural water. Although the least costly method of recovery of large quantities of deuterium, the cost of recovery is still very high, substantially higher than originally hoped for when heavy water moderation was planned into the Candu nuclear reactor.
The GS process is based on the principle that the deuterium in water at hot temperatures is stripped out of the water when it is contacted by hot hydrogen sulphide (H2S) gas in an exchange with hydrogen and this exchange of deuterium into H2S gas is reversed when water and H2S gas contact one another at cold temperature. It has been postulated that when the sulphur in the HZS gas is heated, the energy generated separates the deuterium atom from the hot water in exchange for a hydrogen atom with the H2S gas and that at cold temperatures, the sulphur becomes passive and releases the deuterium atom from the H2S gas returning it to its natural habitat, so to speak, while returning a hydrogen atom to the H2S gas.
One of the dilemma's of heavy water production is that if large quantities of H20 are needed, at the present time no better method than .. j~ .
hydrogen sulphide exchange has been identified although this method, at least in the case of the present apparatus used to contact water and gas, has, as noted, proved much higher in cost than expected. This excess over the anticipated cost for producing heavy water may have become a major reason for the Candu reactor's failure to gain the worldwide acceptance as a nuclear reactor system that its many other attributes merit.
Deuterium and tritium, a second still heavier isotope of hydrogen, which occurs in much more minute quantities in nature even than deuterium, are key isotopic elements with which scientists are working as potential fuels for the fusion process.
In the event that fusion power is harnessed, with deuterium and/or tritium as its fuel, the world will be well supplied because, white these isotopes exist in relatively very small quantities in hydrocarbons such as H20, natural water is so plentiful that it represents a vast source for any foreseeable needs of deuterium and tritium for such uses as a moderator in natural uranium reactors or as a fuel for fusion. No other source of deuterium or tritium exists of remotely comparable size to natural water.
Regardless of end use, the challenge of economic recovery of deuterium from water for its present usage remains. This challenge is strongly related to the very low percentage content of deuterium in the world's waters.
The content of D20 in the world's waters ranges from a low of perhaps 135 ppm in fresh water to a high of 160 ppm in certain sea waters such as the Gulf of Mexico. Again, there is some uncertainty as to a complete explanation for content differences. However, it is, almost certain that they are related to the effects of the hydrologic cycle in hot or cold climates and the tendency of D to accumulate by the process of solar evaporation and rainfall confined to large bodies of warm water, but to be partly lost in dispersion by rainfall over large land masses.
The effect of such a relatively small content of deuterium is that if it is to be recovered from water, when substantial amounts of D2~ are required, extremely large volumes of water must be processed. For example, in the case of present Canadian heavy water plants, roughly 10 million gallons per day of H20 must be processed to produce approximately 250 gallons of D20 per day. More to the point, such a large quantity of water requires extremely large and complex apparatus (probably the largest pressure vessels and gas blowers in the world) to handle counter current flows of water and H2S moving at precise velocities, highly controlled temperatures and flow paths seeking to separate out minute quantities of deuterium atoms.
Heavy Water Production History A brief history of the development of heavy water production processes is relevant to an understanding of the significance of the advances in processing proposed by the present invention.
Until after the Second World War, heavy water was produced by the electrolysis of water and by water distillation. A combined electrolysis hydrogen-water exchange plant at Trail, B.C. and three distillation plants in the U.S.A., along with an earlier electrolysis plant in Norway, produced relatively small quantities of D20. After a period of about ten years work on the hydrogen sulphide (H2S) dual temperature exchange process, this method for deuterium recovery from water became the basis for two large scale heavy water plants at Savannah River, Georgia and at Dana, near Chicago.
Becoming known as the SRP and Dana plants, they were built under extreme pressure of the hydrogen bomb programme, for which heavy water was a strategically critical material. No costs or effort were spared to obtain a secure supply. However, in a relatively short time the Dana facility was found to be superfluous to U.S. needs for heavy water and not long after SRP's production was cut back to one third of its capacity. This history points to a fail safe approach to assured supply of heavy water and appears to suggest some initial uncertainty about the production capability of the world's first large scale GS plants.
In the early sixties, after the adoption by Canada of the Candu natural uranium heavy water moderated reactor system had been officially decided, the construction of Canada's own facilities for heavy water production was mandated. No nation other than the U.S. had ever considered producing heavy water in large volumes and the U.S. had cut back its total capacity to one-sixth that of the SRP and Dana plants by that time.
After some delay in procurement and fears of failure of Canada's first plant constructed at Glace Bay, Nova Scotia, a secure supply of heavy water was obtained by a crash programme to build a second plant at Port Hawkesbury, Nova Scotia on the Strait of Canso. This plant became the model for an ensuing rapidly expanded heavy water plant construction programme to meet what was seen by the early seventies to be needed for a burgeoning Candu reactor programme.
In short, at a different time and place, heavy water production history had repeated itself. In the early 1950's a crash programme of heavy water production proceeded in the U.S. in response to the hydrogen bomb scare. Fifteen years later a crash programme of heavy water production proceeded in Canada in response to the Candu nuclear plant programme.
The urgency for heavy water production in Canada became acute from a combination of initial delay in procurement, including the failure of Canada's first production unit at Glace Bay and an acceleration in Candu reactor construction.
The significance of these crash programmes is, to this day, largely lost with respect to their effect on the cost of heavy water produced by facilities built with compelling urgency. And yet, the significance is both clear and great.
The first effect is that all heavy water plants built in the Canadian programme have suffered from the pressures of failsafe construction and operation with little regard for the cost improvement phases large chemical and other production facilities sooner or later pass through. Even more important, the pressures for first charge reactor supply tended to force such rapid building of heavy water plant capacity that any serious consideration of a third generation process (SRP modular design plant being #1, Canada's scaled up design being #2) could not realistically be given. And now that surplus capacity exists, there is a danger that consideration of a cost improvement redesign will not be pursued as vigorously as is probably justified having regard to heavy water's high cost relationship to total reactor cost.
The original AECL target for the heavy water share of total capital cost of a Candu reactor was 5°!° or about $35.00 per kilogram. At today's production cost of $350.00 to $375.00 per kilogram in heavy water plants costing $400 million to $500 million for 400 ton/year capacity, the percentage of heavy water cost in a Candu reactor would seem to have grown to more like 15% or even 20%. It is evident that the combination of a three to fourfold miss in targeted heavy water cost and the obvious challenge in building and operating the large and complex facilities today's heavy water plants represent, must raise the question as noted above, of whether heavy water has become the Achilles heel of the Candu reactor programme, especially when the Candu reactor system rates so highly in other respects including on-line capability and inherent safety characteristics but is still not well accepted worldwide.
The foregoing comments on the history of heavy water production with particular reference to high cost and complexity are confined to the hydrogen sulphide dual temperature exchange process, commonly known as the GS process. This process represents about 90% of the cost of heavy water production by present plants. The remaining process costs which take D20 enrichment from about 20% to 99.75% purity is represented by vacuum distillation. The finishing stage is not in need of any significant cost improvement emphasis by process redesign.
Another process step which may be employed (and in fact was originally used at SRP) is recovery of deuterium from electrolytically produced hydrogen which, in turn, has been produced from water partially enriched in deuterium by means of a GS process. Work by AECL has continued to develop an improved catalyst-assisted higher yield of recovery of deuterium from electrolytic hydrogen with what is understood to be considerable success. Thus the need for a breakthrough in cost improvement is confined largely to the hydrogen sulphide exchange or GS process which performs the work of achieving first recovery of the minute quantities of deuterium as found in natural water and bringing the deuterium recovered forward to a level of enrichment at which the now mature and cost improved electrolytic and vacuum distillation processes may take over to achieve a 99.75°l° pure D20 product.
In the development of the processing advances proposed by this invention to reduce the cost of production of the present GS process, answers to the question "what is wrong with the present process?" proved extremely instructive and a key to a cost improvement solution.
A long catalogue of process complexities leading to high capital and operating cost of the present GS system was readily developed. However, it was soon evident that one basic feature of the present system is the root cause from which almost all process difficulties, with attendant high cost, devolve. This basic feature is the sieve tray-tower means of contacting large volumes of water with H2S gas.

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The principal difficulties, rising from this basic feature, from which a complex amalgam of processing challenges expand and grow to reach almost overwhelming proportions in their effect on the cost of production, are five in number:
(1 ) The sieve tray-tower means of counter current processing of water and H2S gas in the large volumes required to expose the minute quantities of deuterium in water to recovery by water and HZS
gas contact is an inherently high cost-low recovery rate process method.
The high cost is due to a number of factors such as low space utilization for the water and gas throughput, the low level of gas-liquid contact achieved and the limitations placed on process flows, temperatures and pressures by the sensitivity to process upsets by tray weeping, foaming, vapour carryover and others, caused by the relationship of a large number of sieve trays arranged in series from top to bottom of tall towers.
With respect to the recovery rate which is dependent upon the H2S and water contact, recovery is limited by the need for the gas to both perform the function of contacting the deuterium in water and to sustain by the gas flow upward, an even flow of water across the trays without weeping through tray perforations, or being otherwise upset as by excessive foaming.
(2) The factors in (1 ) bring about the need for the use of very large pressure vessels, or, alternatively, with a modular design, a larger number of smaller vessels still of very substantial size, each equipped with complex piping systems, heat exchangers, gas compressor blowers, pumps and valves designed to handle large flows of corrosive materials and hazardous gas.
(3) The present process system requires continuous refluxing of enriched fractions through very large and complex equipment. The flows in which deuterium exchange takes place are, in effect, constantly reworked in a manner requiring rigid control of liquid to gas ratios to avoid process upsets, the smallest of which may seriously penalize output and cause loss of enrichment to effluent. The system of refluxing which is required in order to provide for enrichment accumulations within the vessels of the system, establishes enrichment _g_ profiles throughout the trays in the towers which sub-optimize the already low net recovery of the gas-water contact system. This is reflected in a content of deuterium higher than the stripping capability of H2S on a "once through" basis which is shown when a first stage tower is started up.

(4) Despite costly heat recovery systems, net energy costs are high because of the large liquid throughput and recirculated gas flows, and substantial heat unrecovered in effluent water and in a cooling tower system.

(5) The sum effect of the sheer size and complexity of the present sieve tray-tower process, operating with hazardous materials, adds cost to almost every component, large or small, including the operating control system.
SUMMARY OF THE INVENTION
This invention, therefore, provides a process for producing enriched heavy water, comprising flowing a stream of water containing a natural level of deuterium through at least one gas diffusion cell, flowing a stream of H2S
gas at a different temperature than the stream of water through the at least one gas diffusion cell counter-currently to the general direction of the stream of water so that the H2S gas contacts the water in the at least one gas diffusion cell to exchange deuterium between the HzS gas and the water, and transferring at least a portion of at least one of the H2S gas and the water from the at least one gas diffusion cell for further processing to obtain an enriched D20 fraction.
In particular, the H2S gas is blown through at least one gas injection tube and into the water stream to create gas bubbles through the water stream outside of the gas injection tube. When the H2S gas leaves the water stream it is directed, preferably to a further gas diffusion cell, generally counter-current to the direction of the water stream.
In the preferred embodiment of the invention a plurality of gas diffusion cells are provided and organized into a first section of at least one cold diffusion cell and a second section of at least one hot diffusion cell. The stream of water flows through the first section and then the second section.
The HZS gas flows through the second section and then through the first section. The HZS gas is preferably heated and humidified before flowing to the second section, and preferably cooled and dehumidified before flowing to the first section.
The transfer of at least one of the H2S gas and the water occurs at points of generally maximum enrichment of the D20 fractions in at least one of the first section and the second section. In a preferred embodiment the transfer is a cascade flow of at least one of the H2S gas and the water. In particular the transfer of the water occurs at or near the exit of the first section for the water stream, and the transfer of the H2S gas occurs at or near the exit of the second section for the H2S gas stream. The transfer of the H2S gas and the water is to a second stage comprising a first section of at least one cold diffusion cell, a second section of at least one hot diffusion cell, a recirculating water stream, and a recirculating HZS gas stream, and wherein the transferred water and H2S gas are mixed with the respective recirculating water stream and HzS gas stream for further processing to obtain enriched D20 fractions. It can be appreciated that at least a portion of at least one of the H2S gas and the water can be transferred from the second stage for further processing in further like stages until the desired enriched D20 fraction is achieved. The final processing of the deuterium enriched water can be through use of a vacuum distillation.
A second process according to the invention has the at least one gas diffusion cell comprised of a first section of at least one hot diffusion cell, and with the stream of H2S gas recirculated through a second section of at least one cold diffusion cell. Here the H2S gas is heated and humidified before flowing to the first section, and cooled and dehumidified before flowing to the second section. A stream of recirculating water is flowed through the second section counter-currently to the stream of H2S gas.
In this embodiment the minimum deuterium content in the stream of water is at or near the exit of the first section for the stream of water, and wherein the minimum deuterium content in the H2S gas is at or near the entrance to the first section for the stream of H2S gas. The maximum deuterium content in the stream of recirculating water is at or near the exit of the second section for the stream of recirculating water. At least a portion of the water from the stream of recirculating water is transferred after exiting from the second section to a reservoir. The partially enriched deuterium content water is transferred from the reservoir for further processing, such as, for example, by at least one of an electrolysis process and vacuum distillation.

Make-up water containing a natural level of deuterium is introduced into the stream of recirculating water after the transfer of the portion to the reservoir. Make-up water containing a natural level of deuterium is also introduced into the reservoir.
A further process according to the invention has the stream of H2S gas recirculated through at least one accumulator, wherein the accumulator comprises at least one gas diffusion cell and a reservoir of cold water. Here the H2S gas is heated and humidified before flowing to the first section, and cooled and dehumidified before flowing to the accumulator. As before, the minimum deuterium content in the stream of water is at or near the exit of the first section for the stream of water, and the minimum deuterium content in the H2S gas is at or near the entrance to the first section for the stream of H2S
gas.
In a preferred embodiment of this process a series of accumulators are provided and the H2S gas flows consecutively through the series of accumulators. Here the maximum deuterium content in the reservoir water is in the water in the reservoir of the first accumulator in the series.
Partially enriched deuterium content water is transferred from the reservoir of the first accumulator for further processing, again by, for example, at least one of an electrolysis process and vacuum distillation.
A make-up draw of water can be transferred from each successive accumulator in the series of accumulators to the one immediately preceding it.
Preferably, the make-up draw from each successive accumulator is taken from the bottom of the reservoir of that accumulator.
This invention also provides for an apparatus for producing enriched heavy water, comprising a body defining at least one gas diffusion cell therein and a flow path for a stream of water containing a natural level of deuterium through the gas diffusion cell, at least one gas injection tube presented within the gas diffusion cell and disposed so that one end thereof extends into the stream of water, wherein the gas injection tube directs H2S gas into the water stream to contact the water and create bubbles through the water stream outside the gas injection tube, and at least one baffle presented within the gas diffusion cell above the stream of water to direct the H2S gas above the stream of water through the body. The injection tube and baffle define a flow path for the H2S gas that is generally counter-current to the flow path of the stream of water.

In a preferred embodiment the apparatus further comprises a blower means provided in each gas diffusion cell to blow the H2S gas through the gas injection tube and a funnel to direct the H2S gas into the gas injection tube.
The gas injection tube can also be provided on the one end thereof with a gas bubble reducer sheet.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a simplified schematic diagram illustrating the means by which feedwater at natural deuterium level concentration is fed through a plurality of gas diffusion cells in a single elongated channel in counter current contact with H2S gas blown down to the channel's bed through gas injection tubes then up through the flowing water and over to the next and further cells outside of its tubes to the remainder in a series of cells to the channel's feedwater entrance. Similar means are employed for dual temperature exchange of deuterium "D" in hot and cold series of gas diffusion cells.
Fig. 2 is a simplified schematic diagram illustrating a dual temperature hydrogen sulphide exchange arrangement in which pairs of cold and hot series of gas diffusion cells in three stages provide for deuterium "D"
enrichment to a level of 10% to 20% with finishing enrichment to 99.75% pure heavy water D20.
Fig. 3 is a simplified schematic diagram illustrating a dual temperature hydrogen sulphide exchange arrangement using a single series of hot gas diffusion cells with a 'once through' flow of feedwater and a separate single series of cold gas diffusion cells served by a recirculating stream of cold water interconnected to an enrichment reservoir and then to a CECE equipped electrolyzer for producing 99.75% heavy water from water enriched to a low level e.g.: <.25%.
Fig. 4 is a simplified schematic diagram illustrating a dual temperature hydrogen sulphide exchange using a single series of hot gas cells with a once through flow of feedwater and a separate single series of cold water "D"
enrichment accumulators for a low level e.g.: <.25% "D" enrichment water as the feed to a CECE equipped electrolyzer for producing 99.75% heavy water.
DETAILED DESCRIPTION OF THE INVENTION
With the objective of a substantial reduction in the cost of heavy water production, by specifically eliminating as many high cost and low yield recovery problems of present plants, apparatus has been invented which eliminates the sieve tray-tower means of water-gas contact in the hydrogen sulphide exchange process of an overall system which may additionally comprise either or both electrolytic and vacuum distillation process, each process playing a part in producing heavy water (D20) at 99.75% purity.
Apparatus has been invented replacing the sieve tray-tower method of contacting water and H2S gas. This consists of what has been called gas diffusion cells (Figure 1). Through these cells, a broad, high head (6" to 12"
deep) stream of feedstock natural water (1) is passed in a horizontal pressure vessel (12) (see arrows 59). Within each cell at its bottom the flowing water is contacted by H2S continuously blown down (see arrows 55) through gas injector tubes (7) immersed in the stream of water to within a short distance of the water stream's bed (53). The gas is continuously diffused in the water, then bubbles rapidly up through it (see arrows 57), outside the gas injector tubes (7), to the water's surface. The H2S gas rising up through the water stream from its bed will likely have its gas bubbles reduced in size by means of gas bubble reducer (11) sheets having closely spaced perforations to foster larger number of smaller and discrete perpendicular streams of H2S gas to achieve increased contact with deuterium in the water.
Gas diffusion cells will be used in both a hot and cold stage of water flow and the H2S gas contact: the hot stage for extracting or stripping deuterium from the continuously flowing stream of hot water when contacted by upwardly flowing H2S gas after it has been blown through gas injection tubes (7) to the bottom of the water stream; and the cold stage in which the stripped deuterium in the H2S gas is returned to a cold stream of water also flowing through a set of gas diffusion cells. The gas diffusion cells in the hot and cold stages may be of identical design or may be modified where advantageous to suit any differences between the hot and cold deuterium exchanges.
As many gas diffusion exchange cells will be provided, arranged in series in each of the hot and cold vessels, as are needed to ensure a high degree of gas-water contact and a high rate of deuterium recovery. Gas diffusion cells may vary in size, but a number of cells 10' x 10' square are indicated as a guide to rough dimensioning of the size of the apparatus in each of the series of hot and cold vessels. As noted in the Figure 1 the number and diameter of gas injection tubes (7) will vary with the gas and water flow velocities, system pressure, area of cell water bed, water depth, etc. Again, as a rough approximation, the total number and diameter of gas injection tubes at their gas exit ends is expected to represent a square area in a relation to the area of the water bed in each cell, in a range of 1:4 to 1:8.
All gas injector tubes will be funnelled at their gas entrance end and welded together to form a seal so as to direct an even flow of gas down through the tubes which will be of subtantially smaller diameter from the bottom end of their funnels (8) to their bottom or gas exit end. The number of tubes in each cell and their diameter will be designed to provide a wide spread diffusion of gas bubbles up through the water stream outside of the gas injector tubes to the surface of the water and thence out of the cells to the next in a series of cells. All gas diffusion cells will be equipped with inclined baffles (15) to direct the H2S leaving the water stream's surface to the next cell (as indicated by arrows 61 ). As will be discussed hereunder, the gas and water flow arrangement between hot and cold cells and between two or more stages of enrichment accumulation will depend upon the overall process system configuration selected for accumulation and transport of enriched fractions of deuterium as described hereunder.
In detail and in accordance with the present invention a dual temperature hydrogen sulphide exchange process is provided in which water is contacted in a series of hot and cold gas diffusion cells in which H2S gas is blown down tubes (7) to the bed (53) of flowing water (1) and up to the surface of the water outside of the tubes as means of creating an exchange of D and H in favour of HzS gas in a hot series of gas diffusion cells (3(a) to 3Q) with 3(a) and 3(b) illustrated in Figure 1) and an exchange of D and H in favour of water in a cold series of gas diffusion cells (not illustrated). The process of "down and up" contacting of H2S gas and water is continuously counter current, that is the water flows horizontally in a channel in one direction (59) and a single stream of H2S gas in the generally opposite direction while being blown down to the bottom or bed of the flowing water through tubes and up to the surface of the water with the down and up flow of gas being repeated through series of gas diffusion cells.
As will be seen from Fig. 2, Fig. 3 and Fig. 4 different process arrangements may be provided for when gas diffusion cells are used to contact water with H2S gas. The choice of arrangement will be substantially dependent upon the level of D enrichment of water to which it is desired to carry the dual temperature hydrogen exchange process before completing the enrichment to 99.75% D purity by second or third processes such as vacuum distillation (known as the DW process) or electrolysis where substantial improvement in efficiency have been made by Atomic Energy Canada's CECE process in recent years.
Fig. 1 is a simplified schematic of water and H2S contacting for purposes of deuterium exchange. A hot cell series has been used for purposes of illustrating the gas diffusion cell method of contacting water and H2S gas.
As shown by Fig. 1 natural D content level feedwater (1), after being heated by steam (as at 1A), is passed into and through a horizontal channel (71) served by a plurality of diffusion cells (3). Each of the gas diffusion cells (3) is equipped with its own booster compressor blower means (for example, 4a, for cell 3b) or in the case of the first cell of the series by a main compressor blower (4) for passing a circulating stream of H2S gas (5) down through a gas disperses (14) into injection tubes (7) funnelled and sealed (8) to direct the H2S down into the aforesaid flowing heated feedwater to within a short distance from the aforesaid channel bed (53) and then up through the flowing water to the inclined gas forwarding and foam suppression baffle (15) where the gas is directed over to the gas entrance and down through a gas disperses (14a) to the next in the series of cells consecutively by booster compressor blowers (4a) described above. As indicated at (77) the gas will flow to additional diffusion cells 3(c) to 3Q), as required, and then, where required, to the cold series of cells (2). In the case of a cold cell series means for cooling the H2S gas before it flows through the cold series will be provided, rather than for heating by steam as shown at (1A) Figure 1. Also means for passing some of the partially enriched cold water will be provided at the end of the last cell in the cold series of cells to a next enrichment stage pair of cold-hot series of cells with the remaining water, after heating, being fed into the channel of the hot series of cells.
It will be noted that when a cold and hot series of cells are paired in stages as described below for Fig. 2 (where a dual temperature hydrogen sulphide exchange process provides for taking the level of enrichment to a level of 10% or considerably higher if desired) the gas diffusion cell method of contacting a horizontal flow of water as differentiated from today's conventional sieve tray method of contacting, will enable the use of less symmetry in gas and liquid flows between the cold and hot series where it is advantageous to do so. The use of asymmetry enabled by gas diffusion cells will be directed toward increasing the net recovery of D by two or three percentage points over today's sieve tray tower systems i.e. from 19% to 21 or 22%.
Each gas diffusion cell in the series will be separated by walls (10) (see Figure 1) to direct the counter currently flowing H2S gas down to the channel's bed and up through the flowing water. Each cell will also be equipped with a perforated gas bubble reducer sheet (11 ) to reduce the size of the H2S gas bubbles causing them to rise in thin streams of gas up to the water surface and on to the next cell. The water flow down through the channel will be controlled to regulate flow velocity by means of a series of weirs and gates designed to sustain a generally even head of water at a desired height throughout the channel's length.
In the schematic of Fig. 1 a hot stage series of gas diffusion cells is illustrated. At the exit from the pressure vessel (12) containing the plurality of hot gas diffusion cells is shown for illustration the 10t" cell of a total of 10 cells numbered 3(a) to 3Q). At the exit of cell (3a) "D" depleted water (13) is discharged to effluent, or used for other processes.
It wilt be noted that where a cold and hot series of gas diffusion cells are paired as in Fig. 2, the cold series of cells will follow essentially the same design arrangement as the hot series.
Process Flow Alternatives The invented process provides for two basic cascade flow arrangements for achieving enriched D20 fractions, concentrating such enriched fractions and transporting same to exit of a hydrogen sulphide exchange process to the final processes) of electrolysis and/or vacuum distillation.
(1) High D level Enrichment H2S Dual Temperature Exchange Process Arrangement The first alternative flow arrangement is similar to that of present heavy water plants with differences related to the contacting of water with being made by means of the gas diffusion cells described above and as shown in Figure 1, instead of the sieve tray tower method used today. This flow arrangement will be used where it is found economic to enrich natural feedwater to 10%-20% levels by the GS hydrogen sulphide dual temperature exchange process and thereafter to 99.75% enrichment by distillation or electrolysis.
As shown in Figure 2, feedwater (1) will enter a first stage cold section of cold gas diffusion cells (2) and flow on into a first stage hot section of hot gas diffusion cells (3). The cells in the cold and hot section, may match one another both with respect to the number of cells and in the detailed designs of the cells, or may if advantageous be arranged asymmetrically with respect to the number of cells, or with respect to the number of gas injector tubes per cell and their size and spacing factors.
H2S gas will be blawn by compressor type blowers (4 and 4a, in Figure 1 ) counter current to the water flow (except down into the bottom of the water stream almost to its bed 53 and up through the water to its surface and beyond) first through the first stage hot section series of hot gas diffusion cells (3) in Figures 1 and 2 to and through the first stage cold section series of cold gas diffusion cells (2) in Figure 2 and recirculated (5) (as shown in Figure 1 ).
As with present heavy water plants, gas humidification apparatus (6) in Figures 1 and 2, and dehumidification apparatus (16) in Figure 2, will be provided at the entry of the HzS gas to the hot section and prior to entry to the cold section, respectively.
Humidification and dehumidification apparatus will be of simpler design than present plants and arranged to suit the gas diffusion cell horizontal pressure vessel arrangement. Direct steam injection to each of the hot cells, individually will be a feature of the invented system directed toward maximizing the stripping performance of the H2S gas in the hot section. This is something present sieve tray-tower processes may not do, either as readily or with the same benefits.
As with present plants maximum enrichment of deuterium in the water stream (1) will occur at or near the cold section exit immediately prior to dehumidification and maximum enrichment of deuterium in the H2S gas stream at or near the exit of the gas from the hot section and before the entry of the gas to the dehumidification apparatus.
Water depleted in deuterium ((13) from Figures 1 and 2) from passing through the hot section series of cells will proceed to effluent after recovery of heat in the water stream.

As with present plants, accumulated enriched deuterium fractions will be created at points of maximum enrichment in the first stage (18), as noted above for transfer to a second stage (19), for further enrichment concentration. Transfer of enrichment from stage to stage may be made as present plants may do, either by water cascade, for example, at 50, or by gas cascade, for example, at 52. These cascades can be alone or in combination, such as, for example, the water and gas cascades as illustrated in Figure 2.
Or, in a dual cascade system (Canadian Patent No. 911,137), water alone may be cascaded from one of a paralleled pair of cold and hot stages and by gas alone from a second pair.
Second (19) and third stage (20) series of cold and hot gas diffusion cells may be used, or a third or even forth stage, substantially reduced in size, may revert to sieve tray-tower hydrogen sulphide exchange, if advantageous.
In detail, Fig. 2 illustrates a simplified schematic of a process arrangement for carrying out by means of dual temperature hydrogen sulphide exchange the enrichment of deuterium content in water to a level of 10% or substantially higher if desired. In this arrangement three stages of cold-hot pairs of series of gas diffusion cells are provided. Feedwater (1) at natural level of D enrichment of 140 ppm is passed through a cold cell series of gas diffusion cells (2) and after heating by hot gas from hot cell series (3) is passed through said hot cell series and depleted in deuterium then to effluent (13) with "D" content approximating 110 ppm. The recirculating gas stream (5) flows counter currently to the water after being heated and humidified in H2S
gas heater humidifier (6) through hot cell series (3) and after being cooled and dehumidified (16), through cold cell series (2) and then recirculated (5) to continuously repeat its counter flow against new feedwater. In this process maximum enriched water is achieved and available for cascade (as at 50) from first stage vessel system (18) to second stage vessel system (19) for recirculation as at (93). At the same time a maximum level of "D" enriched H2S gas is established at (52) for purposes of cascade from the first stage vessel system (18) to the second stage vessel system (19). Cascades from second stage pairs of cold and hot series (19) to third stage pairs of cold and hot series (20) will follow in a similar manner to provide recirculated water as at (97) and recirculated gas as at (99) in third stage (20).
Stage to stage D enrichment cascade may be made with equal effect by means of water only, or gas only, or gas and water. An additional cascade arrangement may be used where two hot-cold first stage vessel pairs are provided by which enriched water only is cascaded from one of the cold-hot vessel pairs and H2S gas only is cascaded from the other. Again the same enrichment level in the second stage cold-hot vessel pair will result, however the size of the second stage cold-hot vessel pair may be substantially reduced using this cascade arrangement.
Third stage vessel system (20) may complete the dual temperature H2S exchange process to achieve a level of 10% to 20% or additional smaller stages may take the "D" enrichment level still higher, theoretically to the 99.75% level, However, vacuum distillation (22) will normally be used for final finishing of the maximum enriched water (101) to 99.75°!o purity heavy water (as at 91) although, CECE assisted electrolysis process may also be used and the vacuum distillation process made smaller. Any reflux from the vacuum distillation (22) can be returned to the recirculated units (97) as at (103).
It will be noted that an arrangement whereby a high level of enrichment is accumulated and constantly refluxed, using stages of diffusion cells within the hydrogen exchange step, poses the same basic challenge faced by sieve tray-tower processing arrangements.
The first difficulty will be the existence of a rising cell-to-cell increase in concentration in the cold section as water proceeds horizontally to the end the this section and a cell-to-cell decrease in enrichment as the water further proceeds in a hot section and flows to effluent, with H2S flowing counter current, having a matching profile. Both the water and gas profiles in a gas diffusion cell process will be similar to the profiles in a sieve tray-tower process, although enrichments will differ. Both systems will send to water effluent a higher content of deuterium than would be the case if the separation apparatus were not performing the duty of accumulating enrichment profiles to make available some part of such enrichment at its highest level for transfer to a second and third stage. Any lost recovery resulting from the existence of enrichment profiles which are "attenuated" to levels of enrichment by several times is, in effect, the price the system pays for accumulating a high level of deuterium enrichment concentration within the hydrogen sulphide exchange process overall. The difficulty may be seen as analogous to running on a treadmill. All Canadian heavy water plants sieve tray tower designs when operated in the past or as may be operated today by Ontario Hydro, actually achieve close to a 65% theoretical hot water H2S stripping yield of deuterium but because of the refluxing inherent in the hot-cold pair arrangement of each stage only a net final recovery of about 19°l° is achieved thus in effect taking three strides forward and somewhat more than two strides back, a high price indeed to pay for the refluxing arrangement needed to achieve a high 10% to 20% H2S exchange recovery level in the GS process.
A second difficulty is that plant shutdowns (whether planned or not) or process upsets will cause a loss to effluent of some previously recovered deuterium not yet carried forward from the hydrogen sulphide exchange process. In all cases, production time is lost bringing enrichments of the three stages back into planned operating profiles required for an efficiently producing dual temperature hydrogen exchange process having hot and cold pairs arranged in stages.
Despite this common difficulty, substantially greater flexibility will exist with gas diffusion cell operations to minimize losses to effluent of deuterium already recovered, by means of an asymmetrical hot section series of cells versus the cold section series of cells. For instance, direct steam injection may be used to maximize the stripping capability of H2S gas by increasing the temperature at which the cells closest to effluent are operated.
It is also considered that gas diffusion cell apparatus will provide opportunities to "pull through" higher levels of enriched fraction extraction to succeeding H2S exchange stages and final finishing process such as electrolysis or vacuum distillation by operating the enrichment accumulation and refluxing system by "stealing" some enriched water as a side-stream to that transferred. Operating the hot section asymmetrically with respect to water flows may be done with gas diffusion cells while extremely difficult to do with sieve tray-tower operation which must maintain equilibrium between hot and cold pair flows of water and gas to avoid tower upsets.
In a system where the rate of deuterium separation may be substantially greater than present sieve tray-tower separation, i.e. up to 80%
vs. 65%, it will be especially important to minimize the system's exposure to effluent loss due to the existence of an enrichment accumulation arrangement. Gas diffusion cells arranged in hot and cold pairs and stages of these will indeed create greater levels of enrichment exposure to loss in effluent. Greater flexibilities in their operation in horizontal pressure towers will provide opportunities to minimize such losses. However, as will be seen hereunder such losses, may be entirely avoided.

(2) Low D level Enrichment H2S Dual temperature Exchange Process Arrangement The second alternative cascade flow arrangement (Figure 3) is a radical departure from the cascade flow arrangement of present sieve tray-tower H2S exchange processes, a principal objective of this arrangement is to reduce to the very minimum any loss of deuterium stripped from natural feedwater (1) in a series of hot diffusion cells, indicated generally by reference character (3) but can comprise cells (3a-3j), as discussed above for Figure 1, by passing hot feedwater directly into and through the hot cells to effluent, depleted as to deuterium to the extent of the maximum achievable by contact with hot H2S gas flowing counter currently through the series of hot diffusion cells for deposit of the deuterium stripped out of the hot water in a separate cold water deuterium accumulator system designed to achieve a low level of D enrichment water, i.e. 1% to 2% D content (33) in Figure 3. This low level of enrichment compares to the 10% to 20% to which today's large scale heavy water plants normally take their H2S exchange process.
A low level 1 % to 2% D enrichment H2S dual temperature exchange process with a <70% net recovery of .0145 natural D content water compared to about 19% net recovery for present plants, will integrate extremely well with a CECE assisted electrolysis process (34) in Figure 3 with 70% net recover and provide a reduction in heavy water cost of breakthrough dimensions.
Where for illustration, 50 tons of hydrogen and 400 tons of oxygen per day are produced by electrolysis to be used in a modest scale size companion process plant co-producing synthetic oil and electric power (36 representing the hydrogen and oxygen produced) in Figure 3 (as in Canadian patent #1265760) a cost for 200 tons per year of by-product heavy water of about $100 per kilogram or 30% of reported present costs of $350 per kilogram, is virtually assured. For a larger scale size companion process co-producing 150 tons of hydrogen and 1200 tons of oxygen electrolytically the heavy water cost may be expected to drop to $70 per kilogram bringing heavy water cost down to the original Candu programme target of 5% of total reactor capital cost. As one example of the synergy involved in producing heavy water with the companion processes described above the heated effluent D depleted water of the "once through" feedwater (13) in Figure 3 to the heavy water process, while still hot could substantially meet all or part of the requirements for water of the companion process.
In a low D level enrichment process arrangement, natural feedwater (1) will be pre-heated to a temperature of 80°C or higher (if desired by heat exchange with hot effluent water) and fed directly to a hot series of gas diffusion cells (3) (without first passing through any interconnected cold section series of cells) and immediately further heated by direct steam injection to each cell to a temperature at which the H2S gas performs its deuterium stripping function most effectively. As noted, problems of hydraulic instability, foaming and vapour carryover of existing plants, are eliminated or minimized by the gas diffusion cell method of contacting water with H2S gas.
It is expected that the temperature of the hot section may be increased by at least 10°C over the top temperature of 140°C at which existing hot towers with sieve trays operate, thus further increasing the recovery rate over that provided by the increased contact of water and H2S gas enabled by gas diffusion cells. Pressure may also be increased to a still higher level of differential over the pressure at which present plants operate of about 300 PSI if this is found to result in a higher yield of recovery.
In one arrangement H2S gas containing deuterium stripped from natural feedwater at the higher rate provided for by the gas diffusion method of contact will proceed to a cold section series of gas diffusion cells, indicated generally by reference character (2), but can comprise cells (2a-2j), similar to cells (3a-3j), as discussed for Figure 1, through which a stream of constantly recirculating cold water (25) is flowing. It will be noted that this recirculating cold water stream is not in any way connected to the main stream of feedwater fed directly into the hot section series of gas diffusion cells, and after which, being substantially stripped of deuterium in the hot cells, proceeds to effluent (13). It will be noted that cold temperature gas diffusion cell operations may be carried out at lower temperature levels than sieve tray tower operation for greater efficiency in transfer of D from HZS gas to cold water because diffusion cell operations may permit a closer approach to the temperature at which hydrate formation occurs with less concern about upsets to water and gas flows.
The constantly recirculating stream of cold water (25) collects, and accumulates deuterium by its contact with the dehumidified and cooled H2S
gas carrying the deuterium stripped from the water in the hot section which is flowing counter current to the cold water flowing through a series of gas diffusion cells (2) in a cold section. This cold section functions to both release the enriched deuterium fractions from the H2S gas and to gradually build an inventory of deuterium concentration within a recirculating stream of cold water which is supported by a substantial reservoir of water (29), maintained probably at a level of about 10% of the volume of daily feedwater throughput.
As previously noted the level of deuterium enrichment concentration built up in the cold water reservoir is normally expected to be substantially lower than levels to which present plants take their hydrogen, sulphide exchange steps before switching to vacuum distillation, i.e. to about 1 % to 2% compared to 10% to 20%.
When a desired level of deuterium concentration is reached in the cold water reservoir, low level enriched water will be drawn from the reservoir (as at 33) to a next succeeding step of electrolytic production (34) and a anal vacuum distillation production step if desired, to a final D20 product of 99.75%
purity. It will be noted that input (30) of enriched deuterium fractions to the reservoir (29) or draws (as at 33) from the reservoir (29) will be made so as to take advantage of any preferential migratory concentration tendencies of heavier D20 in a large tall H20 reservoir tower whether naturally occurring or as may be induced by mechanical or catalytic means. Water drawn from the reservoir to be used as feedwater for electrolytic production (as at 35) of hydrogen and oxygen which has achieved a 1 % to 2% level of D20 enrichment will normally be mixed with additional natural D level water to reduce its percentage D level enrichment to a lower level but still high enough for the efficient production of large scale volumes of CECE electrolytically produced heavy water (37) as a by-product of electrolytic H2 and 02 production (36). Draws from the reservoir of D enriched water (33) will normally be made from the reservoir's bottom where the D content is highest.
Although as noted a high level D enrichment arrangement to the 10%
or greater level of enrichment using gas diffusion cells will enable a higher net recovery of D from feedwater, the principle advantage over existing sieve tray tower systems will be the lower cost of the plant and its apparatus.
As seen from this preferred embodiment of the instant invention, where hydrogen and oxygen are required in large quantities for other processes, major saving in heavy water production cost can be realized by using a dual temperature H2S exchange process with gas diffusion cell water and HZ

contacting to only a low level of enrichment followed by electrolysis to achieve all or most of the remaining enrichment.
In detail, the embodiment of Figure 3 is the first of two alternative process arrangements for low level dual temperature HZS exchange processing followed by an electrolysis step employing AECL's CECE process to finishing or near finishing 99.75% purity heavy water, with or without a final small vacuum distillation step. In this arrangement natural "D" content 140 ppm feedwater (1 ) is fed directly to and through a hot series gas diffusion cells (3) in pressure vessel (26). Passing directly through a plurality of diffusion cells (3) (for example 3(a) to 3(j)) after being heated to a level of 140°C, the once through flow of feedwater may be relieved of its D content by <70% by counter current contact with HZS gas (23) thus retaining within the low level dual temperature H2S exchange process approximately 100 of the 140 ppm "D" control of natural water, sending only 40 ppm to effluent (13). Thus, as wilt be seen by a comparison with the effluent "D" contents of dual temperature HZS exchange process which is taken to a level 10% or greater as described in Figs. 1 and 2 which is normally the objective of today's H2S exchange processes, the low level enrichment process of the instant invention may retain downstream in the process 70 ppm's of "D" enrichment more than present H2S processes i.e. 100 ppm versus 30 ppm. Stated in the converse, the "D" content of the once through basic feedwater to the process will be only 40 ppm versus 110 ppm for present H2S exchange processes.
It is important that it be understood that this large difference is not in any way related to the separation efficiency of sieve tray tower water method of contacting water and H2S gas presently in general use and the instant inventions gas diffusion cell method. The sieve tray tower theoretical separation is given as 65% not much lower than <70% expected for gas diffusion cell theoretical separation. As stated earlier in the description of the instant invention present H2S exchange processes, in electing to use stages of cold and hot pair series of sieve trays in towers to reach a 10% to 20%
level of "D" enrichment within the H2S exchange process, "pay the price" of reducing net actual recovery from 65% theoretical separation to about 19%.
The key to the ability of the low level enrichment arrangement to achieve a net recovery close to 70% are the provision for a "once through" flow of hot feedwater with such feedwater flow being disengaged from a separated flow of cold recirculated water, while H2S gas is circulated through the hot water to strip out D from the feedwater and carry it at a maximum HZS enrichment level (21) to HZO recirculation (25) for purposes of depositing the "D" picked up in the H2S recirculating stream (23) after achieving maximum enrichment (21 ) and then cooled for its travel through the cold gas diffusion cell series (2).
It will be seen from Figure 3 that the cold series of gas diffusion cells (2) of cold pressure vessel (27) performs the function of establishing a profile of enrichment in the water of the cold series of cells ranging from, minimum "D" enrichment value at cell (2) of the series of cells at the point where make-up water (28) for draws made to reservoir (29) enters the recirculating cold H20 stream, to a maximum "D" enrichment value in the recirculating cold H20 stream (24) from which draws (30) to reservoir (29) will be made.
It will be seen that the recirculating H2S gas stream (23) will have its minimum D content (31) where it has left cell (2) of the series of cold gas diffusion cells after contacting make-up water (28) at natural D content level and after being heated in the H2S gas heater humidifier (6), will return to repeat its counter flow contact of continuously flowing feedwater which will also reach its minimum D content (32) at the point of its exit to effluent as D
depleted water (13). Achieving minimum D level contents in both the H2S gas and feedwater at expressly points (31) and (32) in the process is a key objective of the invented process arrangement.
H20 draw or transfer (30) to the bottom of reservoir (29) will be made of maximum D content water (24) and partially enriched D content water (33) to a level of 1 % to 2% will be transferred from the reservoir to the electrolysis CECE process (34). Normally this 1 % to 2% D content draw will have its D
content diluted by the addition of natural D level water to produce an electrolyser D content feed in the range of .25% to .50% enrichment where substantial amounts of hydrogen and oxygen are needed for process use or to be sold. 99.75% pure heavy water (37) will be produced by the electrolyser CECE assisted process or a final small vacuum distillation be used for final enrichment. Make-up water for the draw to the electrolyser (35) from the bottom of the reservoir (29) will be provided by natural D content water to the top of the reservoir (29). "D" depleted hydrogen and oxygen products of water electrolysis (36) will proceed to process or market.
In a preferred embodiment, for example where large quantities of high purity electrolytic hydrogen and oxygen are desired for a companion process such as syncrude and electric power production by coal and heavy oil co-processing, the feedstock to an electrolysis step would be supplied in part from the cold water reservoir at a deuterium concentration level of from 1 %
to 2% and part from natural level feedwater for an average D20 content of .25%
to .50%. The electrolysis step would provide for the use of AECL's CECE
catalyst assisted process for deuterium extraction from electrolytic hydrogen.
Final 99.75% purity D20 may be produced by the electrolysis step or a vacuum distillation step added. With electric power produced for the companion process and for a utility grid such as by coal gasification combined cycles at a net improvement in power cost, a further credit to the cost of electrolysis can be expected by operating electrolytic cells only with or partly with off-peak power to make more power available for peak demand of the utility grid.
It will be noted that with the above arrangement no refluxing of enriched fractions will proceed back beyond the cold water reservoir. Make-up for water feed (28) from the cold water low level enrichment reservoir and converted to hydrogen and oxygen will be obtained from water vapour carryover from the hot section and condensate from the hot H2S gas, any reflux from electrolysis or vacuum distillation steps and the balance by means of water at natural deuterium level.
It will also be noted that where uses for electrolytic hydrogen and oxygen do not exist, the low concentration level water of the reservoir may make available an attractive feedstock for a hydrogen-water exchange step.
In an alternative low D level enrichment H2S dual temperature exchange process arrangement (Figure 4) deuterium, stripped from a flow of natural level D content feedwater through a single stage series of hot diffusion cells (3) (as also shown in Figure 3), will be carried by the recirculation stream of HZS gas to and through a series of smaller cold water reservoirs (29) each about one tenth the size of the reservoir of Figure 3 above. Each of these smaller cold water reservoirs (29a, 29b, 29c), acting as accumulators of D
enrichment, will be provided with gas diffusion cell means to carry the cooled de-humidified H2S gas with the D picked up in the hot series of diffusion cells, down to the bottom of the reservoirs and back up to the surface of their water, and being interconnected for H2S gas flow from one to the other, for repeated down and up flows of the gas through the series. The gas will exit at the top of the final reservoir in the series for return to be reheated and humidified (6) to carry out its hot diffusion cell stripping continuously by a single recirculating stream of H2S.
The series of cold water reservoir D enrichment accumulators (29a, 29b, 29c) will be designed to establish a profile of D enrichment for the series of reservoirs as a whole which has its greatest D enrichment at the bottom of the first reservoir until, natural water level D content is reached at the top of the last reservoir. Here the H2S gas recirculating stream exits for its return to hot diffusion cell stripping of the basic feedwater stream with its H to D
exchange capacity fully restored.
The desired profile of D enrichment for the series of reservoirs as a whole is provided by making the draws 33 of enriched water to the electrolysis (34) process from the bottom of the first reservoir of the series and the make up water for this draw being made from the bottom of the second of the series of reservoirs to the top of the first series, with this procedure being followed from reservoir to reservoir throughout the series, the last of the reservoirs then receiving the make up for its transfer to the next to last reservoir at its top by the addition of natural D content water. With this system any reflux of water from the electrolysis step at a higher D enrichment level than the natural water D content will be returned (121) to the most advantageous point in the series of reservoirs and the intra series flows of water between reservoirs adjusted accordingly.
~In detail, then, the second of two alternative process arrangements for low level dual temperature H2S exchange followed by an electrolysis process and if desired a small final vacuum distillation process, as shown by simplified schematic Fig. 4, feedwater 1 at 140 ppm D content is fed directly to and through a hot series of gas diffusion cells (3) in pressure vessel (12). As in the case of the first alternative process arrangement illustrated by Fig. 3 and discussed in more detail, the second alternative arrangement provides for a separated once through hot water H2S gas contacting system for recovery of <70% of the 140 ppm of deuterium in natural water to send D depleted water to the level of 40 ppm to effluent (13), or used in a companion process.
As with the Fig. 3 arrangement HZS gas is recirculated counter currently through the hot series of gas diffusion cells (e.g. 3(a) to 3(j)) and then through a cold series of gas diffusion cell D enrichment accumulators (29a, 29b, 29c). As with the Fig. 3 arrangement the H2S gas will be heated and humidified (6) and after being circulated through the hot series of gas diffusion cells (3) will be cooled in H2S cooler (16) and then circulated through the cold water D enrichment accumulators (29a, 29b, 29c).
With this cold water D enrichment accumulation system a series of D
enrichment accumulators (29a, 29b, 29c) each with its own gas diffusion cell and individual reservoir of cold water will be arranged in series, e.g. from i to x, (where 29a in Figure 4 represents i, 29b represents ii to ix, and 29c represents x) through which the recirculation H2S gas stream will flow down through tubes of each to the bottom of a substantial head of water and then up through the water outside of the tubes to the surface of the accumulators with the H2S gas stream being blown down and up through the series of accumulators consecutively and continuously.
With this arrangement the first enrichment accumulator through the last in the series of accumulators will provide a maximum D enrichment (33) in the first accumulator of 1 % to 2% and reducing enrichment in each accumulator through the last accumulator which will contain the least or minimum D
enrichment.
When cold accumulator (i) reaches a high enough level of D
accumulation draws or transfers (33) of partially enriched water will be made to a CECE assisted electrolysis process (34) for the production of 99.75%
pure heavy water (as at 37) with or without a final small vacuum distillation process.
As draws from the cold accumulator (i) are made 10 others in the series of accumulators will forward a "make up" transfer to the next lower numbered accumulator as at (107a, 107b, and 107c). All draws or transfers to the electrolysis process and between accumulators will be made from the bottom of the accumulators to the top of the next lower numbered accumulator in the series to take advantage of the tendency of enrichments of each accumulator to be highest at its bottom because of the direction of the flows of H2S gas up through higher levels of water and also because of the 11 % higher specific gravity of heavy water.
As with process arrangements of Fig. 3 "D" depleted hydrogen and oxygen produced by water electrolysis (36) will proceed to process use (as heated if advantageous) or sold.
It is important to the understanding of the low level D enrichment process arrangements illustrated by Figs. 3 and 4 that water flows of the arrangement by comparison with cold-hot pairs of vessels in stages _29_ arrangement used to achieve a tenfold greater D enrichment within the dual temperature H2S exchange process are substantially simplified and in a sense presented "more passively" for the extraction and accumulation of "D"
delivered to the process in the feedwater, than the H2S gas which is worked harder to strip D out of the hot feedwater and to deposit and accumulate D in a separate system of cold water. It will be seen that in the low level D
enrichment level arrangements higher heads of water flow are utilized, the flows are more direct and less refluxing of water is required. However when the recirculating stream of H2S gas serving the low level of D enrichment process arrangements is worked harder a greater capacity of compressor-blower may be required for, given throughput of feedwater. Any increase in cost for H2S handling should be more than offset by reduced water handling cost. Far more important, however, will be the net D recovery of 70% of the D
in the feedwater from present sieve tray tower processes of 19% in the low level enrichment process.
Great importance, therefore, must be attached to finding a "partnership" for low level D enrichment process arrangements with the efficient CECE electrolysis process and high demand for hydrogen and oxygen for other companion processes or markets.
It will be seen that except for differences in the specific arrangement for D enrichment accumulation as described above and shown by Figures 3 and 4, both of these low D level enrichment H2S dual temperature exchange processes meet the same fundamental objectives of eliminating staged hot and cold pairs of dual temperature exchange that result in the loss of about two thirds of the deuterium that may be transferred from natural feed water to H2S gas at temperatures of about 140°C when hot and cold exchange pairs are interconnected for the purposes of achieving a 10% to 20% level of D
enrichment within three hot and cold pair stages before changing to an electrolysis and/or vacuum distillation process to achieve 99.75°!° pure heavy water. The advent of a high 70% plus recovery CECE electrolysis process is of course a key factor making economic the truncation of the dual temperature exchange process to a 1 % to 2% level of enrichment by the means described above where large quantities of H2 and 02 are produced for other purposes.
From the foregoing outline of the principal features of the invented process it will be seen that the change from the sieve tray-tower means of contacting water and H2S gas, to the system of gas diffusion cells with water and gas flows as shown in Figures 1, 2, 3 and 4 and as described heretofore in a more detailed description referenced to the Figures, provides an answer to the dilemma of continuing to use nature's large stores of deuterium which exist in natural water without being required to pay a price for production which severely penalizes the Candu system.
The runaway costs of sieve tray-tower heavy water processes with low net yield of recovery and extreme challenges to maintain controlled and safe operation in huge apparatus is reversed by the gas diffusion cell process method. So much cost improvement opportunity appears to flow out of this single change that the original 5% targeted heavy water component of total Candu reactor capital cost may be brought within reach.
The extreme constraints of sieve tray-tower water and gas flows are largely removed. The gross and net separation and recovery rates will be substantially increased. Space utilization will be increased and the size of apparatus correspondingly reduced. Net energy cost per unit of production will be reduced and the cost of heat recovery systems also.
The elimination of towers and their replacement by horizontal pressure vessels by itself is a case in point. In addition to improving the space utilization factors of the deuterium separation apparatus by as much as 50%, the smaller horizontal pressure vessels may be constructed from reinforced concrete which will provide both pressure containment and heat insulation at less cost than the large steel pressure vessel towers with conventional insulation. All equipment, i.e. piping, valves, blowers, etc. will be correspondingly reduced in size and cost.
When such significant reductions in the size and hence cost of apparatus are taken together with the increased recovery rate provided by gas diffusion cells, the opportunity provided for reduced D20 unit cost is substantial.
It is not within the scope of this invention to attempt to project anything like a complete set of details for process redesign which arise from a change as fundamental as the use of gas diffusion cells instead of the past practice of using the sieve tray-tower method. The invented process will undoubtedly create a wide ranging new set of process engineering challenges and opportunities in H2S exchange processing. With fewer constraints from water flowing back and forth across a large number of perforated trays, H2S
can do its work of separation and transport much more effectively.

When the benefits of diffusion cell means of contacting water by H2S are combined with the benefits provided by the preferred embodiment whereby low D level H2S dual temperature exchange process is followed by a CECE electrolysis process and the heavy water produced is a synergistic by-product of syncrude and electric power production using substantial volumes of electrolytic hydrogen and oxygen a breakthrough level of heavy water cost reduction is virtually assured.
It is difficult to assess the value to Canada of the improved worldwide acceptance which should result from a substantial heavy water cost improvement. One must assume it would be very significant, especially if the cost improvement resulted from the use of apparatus greatly simplified and reduced in size to those nations planning large nuclear reactor systems whom might logically wish to produce their own heavy water.

Claims (40)

1. A process for producing enriched heavy water, comprising:
a) flowing a stream of water containing a natural level of deuterium through at least one gas diffusion cell;
b) flowing a stream of H2S gas at a different temperature than the stream of water through the at least one gas diffusion cell counter-currently to the general direction of the stream of water so that the H2S gas contacts the water in the at least one gas diffusion cell to exchange deuterium between the H2S gas and the water; and c) transferring at least a portion of at least one of the H2S gas and the water from the at least one gas diffusion cell for further processing to obtain an enriched D2O fraction.

2. A process according to claim 1 wherein the H2S gas is blown through at least one gas injection tube and into the water stream to create gas bubbles through the water stream outside of the gas injection tube.

3. A process according to claim 2 wherein the H2S gas leaving the water stream is directed generally counter-current to the direction of the water stream.

4. A process according to claim 3 wherein the H2S gas is directed to a further gas diffusion cell and blown through a further at least one gas injection tube into the water stream.

5. A process according to any one of claims 1 to 4, wherein the at least one gas diffusion cell comprises a first section of at feast one cold diffusion cell and a second section of at least one hot diffusion cell, and wherein the stream of water flows through the first section and then the second section.

6. A process according to claim 5 wherein the H2S gas flows through the second section and then through the first section.

7. A process according to claim 6 wherein the H2S gas is heated and humidified before flowing to the second section.

8. A process according to any one of claims 6 to 7, wherein the H2S
gas is cooled and dehumidified before flowing to the first section.

9. A process according to any one of claims 5 to 8, wherein the transfer of at least one of the H2S gas and the water occurs at points of generally maximum enrichment of the D2O fractions in at least one of the first section and the second section.

10. A process according to claim 9 wherein the transfer is a cascade flow of at feast one of the H2S gas and the water.

11. A process according to any one of claims 9 to 10, wherein the transfer of the water occurs at or near the exit of the first section for the water stream.

12. A process according to any one of claims 9 to 11, wherein the transfer of the H2S gas occurs at or near the exit of the second section for the H2S gas stream.

13. A process according to any one of claims 9 to 12, wherein the transfer of the H2S gas and the water is to a second stage comprising a first section of at least one cold diffusion cell, a second section of at least one hot diffusion cell, a recirculating water stream, and a recirculating H2S gas stream, and wherein the transferred water and H2S gas are mixed with the respective recirculating water stream and H2S gas stream for further processing to obtain enriched D2O fractions.

14. A process according to claim 13 wherein at least a portion of at least one of the H2S gas and the water is transferred from the second stage for further processing in further like stages until the desired enriched D2O fraction is achieved.

15. A process according to claim 14 wherein the final processing of the deuterium enriched water occurs using vacuum distillation.

16. A process according to any one of claims 1 to 4, wherein the at least one gas diffusion cell is comprised of a first section of at least one hot diffusion cell.

17. A process according to claim 16 wherein the stream of H2S gas is recirculated through a second section of at least one cold diffusion cell.

18. A process according to claim 17 wherein the H2S gas is heated and humidified before flowing to the first section.

19. A process according to any one of claims 17 to 18, wherein the H2S
gas is cooled and dehumidified before flowing to the second section.

20. A process according to any one of claims 17 to 19, wherein a stream of recirculating water is flowed through the second section counter-currently to the stream of H2S gas.

21. A process according to claim 20 wherein the minimum deuterium content in the stream of water is at or near the exit of the first section for the stream of water, and wherein the minimum deuterium content in the H2S gas is at or near the entrance to the first section for the stream of H2S gas.

22. A process according to any one of claims 20 to 21 wherein the maximum deuterium content in the stream of recirculating water is at or near the exit of the second section for the stream of recirculating water.

23. A process according to claim 22 wherein at least a portion of the water from the stream of recirculating water is transferred after exiting from the second section to a reservoir.

24. A process according to claim 23 wherein partially enriched deuterium content water is transferred from the reservoir for further processing.

25. A process according to claim 24 wherein the further processing is at least one of an electrolysis process and vacuum distillation.

26. A process according to any one of claims 23 to 25, wherein make-up water containing a natural level of deuterium is introduced into the stream of recirculating water after the transfer of the portion to the reservoir.

27. A process according to any one of claims 23 to 26, wherein make-up water containing a natural level of deuterium is introduced into the reservoir.

28. A process according to claim 16 where the stream of H2S gas is recirculated through at least one accumulator, wherein the accumulator comprises at least one gas diffusion cell and a reservoir of cold water.

29. A process according to claim 28 wherein the H2S gas is heated and humidified before flowing to the first section.

30. A process according to claim 29 wherein the H2S gas is cooled and dehumidified before flowing to the accumulator.

31. A process according to any one of claims 28 to 30, wherein the minimum deuterium content in the stream of water is at or near the exit of the first section for the stream of water, and wherein the minimum deuterium content in the H2S gas is at or near the entrance to the first section for the stream of H2S gas.

32. A process according to any one of claims 28 to 31, wherein a series of accumulators are provided and the H2S gas flows consecutively through the series of accumulators.

33. A process according to claim 32 wherein the maximum deuterium content is in the water in the reservoir of the first accumulator in the series.

34. A process according to claim 33 wherein partially enriched deuterium content water is transferred from the reservoir of the first accumulator for further processing.

35. A process according to claim 34 wherein the further processing is at least one of an electrolysis process and vacuum distillation.

36. A process according to any one of claims 32 to 35, wherein a make-up draw of water is transferred from each successive accumulator in the series of accumulators to the one immediately preceding it.

37. A process according to claim 36 wherein the make-up draw from each successive accumulator is taken from the bottom of the reservoir of that accumulator.

38. An apparatus for producing enriched heavy water, comprising:
a body defining at least one gas diffusion cell therein and a flow path for a stream of water containing a natural level of deuterium through the gas diffusion cell;

at least one gas injection tube presented within the gas diffusion cell and disposed so that one end thereof extends into the stream of water, wherein the gas injection tube directs H2S gas into the water stream to contact the water and create bubbles through the water stream outside the gas injection tube; and at least one baffle presented within the gas diffusion cell above the stream of water to direct the H2S gas above the stream of water through the body, wherein the injection tube and baffle define a flow path for the H2S
gas that is generally counter-current to the flow path of the stream of water.

39. Apparatus according to claim 38 further comprising a blower means provided in each gas diffusion cell to blow the H2S gas through the gas injection tube.

40. Apparatus according to claim 39 wherein the gas injection tube comprises a funnel to direct the H2S gas into the gas injection tube.

Apparatus according to any one of claims 38 to 40, wherein the gas injection tube comprises on the one end thereof a gas bubble reducer sheet.