Field of the invention

This invention relates generally to novel heat exchange processes in which an expanded gas is reheated by exchange with another process stream. In one aspect, the expanded gas is oxygen, and the heat exchange step is employed in a stepwise process for extracting work from the gas, which is initially at elevated pressure. In another aspect, a process is disclosed for extracting work from carbon dioxide. In a still further aspect, the process stream cooled in the heat exchange is warm or hot saline water and the invention provides a mechanism for recovering desalinated and preferably potable water therefrom.

By "elevated pressure" herein is meant a pressure greater than atmospheric pressure. By "heat exchange" herein is meant any arrangement for passing heat between process streams, whether they are in close proximity in respective passageways of a heat exchanger or remote from each other with the heat transferred indirectly via intermediate media and/or processes.

Background of the invention

It has been proposed to combust carbonaceous, typically fossil, fuels in a controlled atmosphere continuously supplied with fresh oxygen rather than with air. This has the advantage that the absence of nitrogen substantially eliminates problematic NO _x emissions in the off-gases, which thereby primarily comprise carbon dioxide, water vapour, excess oxygen and contaminants derived from the fuel. Compression and cooling of the off-gases allows recovery of the water, while contaminants (sulphur compounds would be typical) can be removed by well-known processes. Raising the pressure of the residual gas mixture above the critical point for carbon dioxide liquefies the latter, allowing it to be separated from the oxygen. The carbon dioxide is then in a form suitable for geosequestration or as a feed to a bioreaction process in which the carbon dioxide is consumed by suitable photosynthetic organisms such as certain green algae that can produce high protein animal food, fertiliser and/or biodiesel. The critical pressure for liquefaction of the carbon dioxide in the dewatered off-gases is about 90 bar. After separation of the liquefied carbon dioxide, the residual off-gas product is a highly compressed (90+ bar) oxygen.

In a first aspect, the present invention is directed to the optimal utilisation and processing of oxygen that is at an elevated pressure.

British patent 669759 discloses an arrangement in which producer gas from a gas producer is treated in a cooler/precipitator/purifier and burnt in a combustion chamber. The off-gases are cooled and directed in succession to a pair of turbines to produce power, with reheating between the turbines in a heat exchanger. It is stated that the temperature of the gases passing through the turbines may for example be from 400°C to 1,000°C.

US patents 3,724,229 and 3,892,103 disclose a desalination system in which saline water is cooled to form an ice slurry that concentrates the non-saline water and is then separated to provide potable water. In US 3,724,229, the heat extracted from the saline water is used to heat LNG, which is then expanded in a turbine to produce power. The saline water is pre-cooled in a heat exchanger and further cooled by heat exchange with the LNG in a further heat exchanger. From here the cooled saline water passes to a spray-type vaporiser, causing the formation of small crystals of ice. This ice is in turn separated from the brine in a washer and extracted as water in a condenser. Reference to any prior art in this specification is not, and should not be taken as, an acknowledgment or any form of suggestion that this prior art forms part of the common general knowledge in Australia or any other jurisdiction or that this prior art could reasonably be expected to be ascertained, understood and regarded as relevant by a person skilled in the art. ' Summary of the invention

In its first aspect, the invention provides a process for utilising oxygen gas at elevated pressure, comprising extracting work from the oxygen gas by stepwise expanding the gas with a corresponding reduction in its temperature to a temperature substantially below 0°C, and between each expansion reheating the gas substantially without altering its pressure. Because the expanded gas is at a temperature substantially below 0°C, it is possible according to the invention to utilise the latent heat of freezing of water, more specifically saline water, for the interstage reheating. In another aspect, the invention adapts the first aspect to optimally utilise and process oxygen derived from bioreaction processes and usually therefore from a carbon dioxide feed.

In its first aspect, the invention therefore extends to a process comprising: providing oxygen gas at an elevated pressure; extracting work from the oxygen gas by stepwise expanding the oxygen gas with a corresponding reduction of its temperature to substantially below 0°C, reheating the oxygen between each expansion substantially without altering the pressure of the oxygen gas, by heat recovered from warm or hot saline water, whereby the water is converted to an ice slurry from which desalinated water is recoverable; and optionally, recovering the oxygen resulting from the last expansion.

In an embodiment, said oxygen gas is provided by separation from combustion off- gases that also include carbon dioxide. The abovementioned recovered oxygen may be recycled as a fuel to a combustion process from which the off-gases are generated.

Advantageously, the oxygen gas is recovered from a combustion process in which carbonaceous fuel is combusted in an atmosphere fed with substantially pure oxygen rather than with air whereby the off-gases are primarily carbon dioxide, water vapour, excess oxygen and non-nitrogenous contaminants derived from the fuel. Particularly advantageously, the oxygen recovered from the process according to the first aspect of the invention is recycled to this combustion process. In another embodiment, the oxygen is provided from a bioreactor vessel in which the oxygen is produced by carbon-fixing organisims therein.

In a second aspect, the invention provides a method of recovering energy from carbon dioxide, comprising: delivering the carbon dioxide to carbon fixing organisms in a bioreactor vessel to sustain the organisms therein, wherein the organisms therein are of a kind that thrive in a medium at a pressure substantially greater than atmospheric pressure, and the bioreactor vessel is a closed chamber operable at a pressure substantially greater than atmospheric pressure; recovering, from the bioreactor vessel, oxygen gas produced by said organisms, which oxygen gas is at an elevated pressure; extracting work from the oxygen gas by stepwise expanding the oxygen gas with a corresponding reduction of its temperature to substantially below 0°C, and between each said stepwise expansion reheating the oxygen gas, substantially without altering the pressure of the oxygen gas by heat recovered from warm or hot saline water, whereby the water is converted to an ice slurry from which desalinated water is recoverable.

The carbon dioxide may be derived from the combustion of a carbonaceous, typically fossil, fuel. Alternatively, the carbon dioxide may be derived from any of a variety of processes that output carbon dioxide, e.g. heating of calcium carbonate whether in limestone form or otherwise, or a sewerage treatment plant, or a cement production plant.

Oxygen may be recovered from the last expansion and recycled to a combustion process from which the carbon dioxide is generated. In both aspects of the invention, the elevated pressure of the oxygen gas is preferably 90 bar or greater. The temperature of the provided oxygen gas in the first aspect of the invention, or of the oxygen gas recovered from the bioreactor vessel in the second aspect, is preferably 5°C or less, more preferably 0°C or less.

In both aspects of the invention, the step of extracting work from the oxygen gas is carried out advantageously by effecting each of the stepwise expansions in a gas turbine device of a gas expansion type, and still more advantageously by employing the gas turbine devices to drive electricity generation plant.

The saline water may be at a temperature in the range 30° to 60°C: a particularly suitable temperature would be in the region of 40°C. Sources of such water include subterranean or artesian reservoirs generally. A convenient source may be a subterranean source currently removed for specific purposes, for example the subterranean water recovered from below coal reserves being mined by open cut methods in order to prevent upward movement of the coal body.

The invention in either of its aspects may further include recovering desalinated water from the ice slurry. The residual higher salinity water may be delivered to a bioreactor that produces oxygen at elevated pressure for the process of the invention. This bioreactor may also consume carbon dioxide in a process according to the second aspect of the invention.

In an embodiment, the oxygen recovered from the last expansion is also subjected to said reheating. Alternatively, the oxygen recovered from the last expansion is liquid oxygen.

As used herein, except where the context requires otherwise, the term "comprise" and variations of the term, such as "comprising", "comprises" and "comprised", are not intended to exclude further additives, components, integers or steps.

Brief description of the drawing

The invention will now be further described, by way of example only, with reference to the accompanying schematic diagram, designated Figure 1, of a combined power generation and desalination plant incorporating the first and second aspect of the invention.

Detailed description of the embodiments

The illustrated combined power generation and desalination plant 10 includes a train 20 of gas expansion turbines 21 , 22, 23, 24 that jointly drive conventional electricity . generation plant indicated generally at 30. There are two feed streams to plant 10: a substantially pure oxygen gas stream 12 at elevated pressure, for example 100 bar or higher, and a stream of warm medium salinity water 14. Oxygen gas stream 12 would typically be at a temperature below ambient and perhaps well below 0°C but for the purposes of the present example is an oxygen stream at about 2°C that has either been separated from a carbon dioxide/oxygen mixture in gas separation plant 100 or is an output oxygen stream from a bioreactor 200 operating at elevated pressures, e.g. a bioreactor employing algae grown in conditions equivalent to those found in deep polar waters. In the former case, the carbon dioxide would typically have been separated by increasing the pressure of the gas mixture above the critical point for liquefaction of the carbon dioxide, thereby allowing it to be separated as liquid carbon dioxide.

The medium salinity water 14 may typically be at a temperature in the region of 40°C and for the purposes of this example is assumed to be from a subterranean source as earlier discussed. Oxygen stream 12 passes in series through turbines 21-24, being stepwise expanded as it successively passes through the turbines to thereby drive the generator. The pressure reduction at each step is about 25 bar and this results in a significant reduction in the temperature of the gas, for example of the order of 230°C, to substantially below 0°C. After each expansion, the gas is reheated in a respective heat exchanger 41-44 by heat exchange with a low grade heat source, in this case the warm medium salinity water 14, which is fed in parallel to the four heat exchangers, and also to a fifth heat exchanger 45 employed to pre-heat the oxygen gas feed stream 12.

In an exemplary operating mode, the feed oxygen stream 12 is at 2°C, the pressurised gas streams fed by conduits 51-54 to the turbines are at 30°C, the expanded oxygen streams delivered by conduits 61-64 to the succeeding heat exchangers are at -200°C, and the final recovered oxygen gas stream 13 is at 30°C and approximately atmospheric pressure (1 bar).

One end use for the oxygen gas stream 13, especially where stream 12 is derived from combustion off-gases from which carbon dioxide and water vapour have been separated, is to recycle the recovered oxygen as the combustion gas for the combustion system producing those off-gases. In an alternative end use, there is no final heat exchanger 44 and the final expansion stage is a liquid turbine that is allowed to lower the temperature of the oxygen to the boiling point of oxygen so that the recovered oxygen is liquid oxygen.

Turning to the saline water side of the heat exchangers 41-44, the flow rate and flow volume of the saline water delivered via conduits 84-81 are managed so that the cooled streams recovered in parallel via conduits 71-74 are at about -1.8°C and consist of an ice slurry, i.e. a mixture of non-saline ice particles and liquid water having a higher salinity than the stream 14. These components are separated in an ice decanter 75 to produce a substantially non-saline and indeed potable ice slurry stream 76 and a higher salinity liquid water stream 77 at -1.8°C. The latter may serve as a feed to a bioreactor, e.g. bioreactor 200, utilising salt consuming or salt tolerant organisms or algae (which may in turn generate high pressure oxygen stream 12). As earlier discussed, the reheating of the expanded gas in heat exchangers 41-44 can be achieved with low grade heat sources generally, but the illustrated configuration makes use in particular of warm medium salinity water. In the heat exchangers 41-44, the heat employed to reheat the oxygen then includes the latent heat of freezing of the water as it produces the ice slurry. In the case of pre-heat exchanger 45, because the temperature transition of the oxygen stream is less than in the reheating heat exchangers 41-44, the cooled saline water delivered via conduit 85 and recovered from heat exchanger 45 via conduit 78 will typically not be the ice slurry recovered from the other heat exchangers. This stream will not therefore be fed to the ice decanter and will be put to other purposes.