The present invention relates to an electrolyser system integrated into an industrial process such as secondary wastewater treatment or sulfur recovery units and into refineries or processing plants utilising either or both of such processes, for example for processing wastewater containing oil and grease or toxic pollutants, or exhaust gases containing hydrogen sulfide.

Secondary wastewater treatment is the removal of organic matter from sewage or other wastewater. It typically occurs after a primary treatment step, which can involve physical phase separation of settleable solids. During secondary wastewater treatment, biological processes are used to remove dissolved and suspended organic matter from the wastewater. The process can be an aerobic or anaerobic process, depending on the treatment technology or biological process/microorganisms being used. The first aspect of the present invention concerns aerobic processes where oxygen is required for the process.

Sulfur recovery units utilise a chemical process for the removal of sulfur from a material. There are various processes that can be used, but the most common is the Claus process. It recovers elemental sulfur from gaseous hydrogen sulfide by performing a combustion of a hydrogen sulfide laden gas, usually at temperatures above 850 °C, whereby the following reactions occur:

2 H ₂ S + 3 0 ₂ 2 SO? + 2 H ₂ O

2 H ₂ S + SO ₂ 3 S + 2 H ₂ O

2 H ₂ S + O ₂ 2 S + 2 H ₂ O

Combusting the hydrogen sulfide in air thus can break down the hydrogen sulfide into Sulfur and water.

A common bottleneck in refineries and other processing plants is the processing of wastewater or the processing of exhaust gases containing hydrogen sulfide. When the output of wastewater and exhaust gases containing hydrogen sulfide hits that bottleneck, the refinery or processing plant needs to slow down or cease further operations to allow the secondary wastewater treatment or sulfur recovery units to catch up. Alternatively, the wastewater and exhaust gases need to be stored for processing later or elsewhere. These alternative options, however, are themselves problematic. It would therefore be advantageous to increase the efficiency of secondary wastewater treatment plants or sulfur recovery units, such as Claus process plants, to “de-bottleneck” such refineries or processing plants, or at least to allow higher throughputs of wastewater and exhaust gases within the refinery or the processing plant before a bottleneck occurs.

According to a first aspect of the present invention there is provided a secondary wastewater treatment system comprising an aerobic bioreactor, aeration blowers and at least one solid oxide electrolyser cell (SOEC), wherein: the aeration blowers are arranged to blow an oxygen containing gas through wastewater in the bioreactor; and the at least one solid oxide electrolyser cell comprises an anode, a cathode and an electrolyte, a steam input and an oxygen enriched gas output, wherein the oxygen enriched gas output connects to a gas infeed for the aeration blowers.

With this arrangement, an oxygen enriched gas from the at least one SOEC can be fed into the bioreactor during operation of the at least one solid oxide electrolyser cell and the bioreactor. If the oxygen enriched gas has a percentage of oxygen higher than ambient air (i.e. higher than 21 % by volume), and it usually will do when the at least one SOEC is operating, then the bioreactor will be fed a higher volume percentage of oxygen, which in turn will increase the efficiency of the aerobic process in the bioreactor. With increased efficiency, more wastewater can be treated.

In some embodiments the oxygen containing gas for the bioreactor comprises ambient air mixed with oxygen enriched gas from the at least one solid oxide electrolyser cell.

In some embodiments the oxygen containing gas for the bioreactor comprises just oxygen enriched gas from the at least one solid oxide electrolyser cell.

In some embodiments the oxygen containing gas for the bioreactor will have an oxygen content greater than that of ambient air, i.e. greater than 21 % by volume. Preferably it will be 23 to 24% by volume or higher than 24% by volume. In some embodiments, an oxygen enriched gas stream from the oxygen enriched gas output from the at least one SOEC is thermally connected across a heat exchanger to a gas infeed of the at least one solid oxide electrolyser cell, such as a steam stream for the steam input of the at least one solid oxide electrolyser cell, or an ambient air or nitrogen rich gas infeed for the at least one SOEC. In other words, an oxygen enriched gas stream from the oxygen enriched gas output from the at least one SOEC and a gas infeed of the at least one solid oxide electrolyser cell are in fluid flow communication with a heat exchanger for exchanging heat between the oxygen enriched gas stream and a feed gas for the gas infeed. This allows the oxygen enriched gas stream exiting the oxygen enriched gas output during operation of the at least one solid oxide electrolyser cell, which will be hot from the at least one solid oxide electrolyser cell, to be cooled by the feed gas for the gas infeed prior to entering the aerobic bioreactor, and for the feed gas for the gas infeed to be heated before being fed into the at least one SOEC. This then further improves the efficiency of the system.

In some embodiments, a buffer tank may be provided between the aeration blowers and the at least one SOEC to allow a buffer for the oxygen enriched gas stream from the at least one SOEC.

The at least one solid oxide electrolyser cell usually has a hydrogen output. A hydrogen stream will exit the hydrogen output during use of the at least one solid oxide electrolyser cell. A buffer tank may be provided for it.

In some embodiments the hydrogen stream can be thermally connected across a gas infeed (e.g. ambient air, a nitrogen enriched gas feed instead of the ambient air, or a steam stream, or both) for the at least one solid oxide electrolyser cell via a heat exchanger. In other words, a hydrogen stream from the hydrogen output and a gas infeed of the at least one solid oxide electrolyser cell are in fluid flow communication with a heat exchanger for exchanging heat between the hydrogen stream and a feed gas for the gas infeed. This allows the hydrogen stream exiting the at least one solid oxide electrolyser cell, which will be hot from the at least one solid oxide electrolyser cell (i.e. relatively hotter than the gas infeed), to be cooled, and for the feed gas for the gas infeed into the at least one SOEC, which will be relatively colder than the hydrogen stream, to be heated. This then further improves the efficiency of the system. According to a second aspect of the present invention there is provided a method for secondary treatment of wastewater, comprising providing a secondary wastewater treatment system comprising an aerobic bioreactor, aeration blowers and at least one solid oxide electrolyser cell (SOEC), wherein: the aeration blowers blow an oxygen containing gas through wastewater in the bioreactor; and the at least one solid oxide electrolyser cell comprises an anode, a cathode and an electrolyte, a steam input and an oxygen enriched gas output, the oxygen enriched gas output being connected to a gas infeed for the aeration blowers and feeds an oxygen enriched gas stream from the at least one solid oxide electrolyser cell to the aeration blowers.

With this arrangement, as with the system of the first aspect, an oxygen enriched gas from the at least one SOEC is fed into the bioreactor during operation of the at least one solid oxide electrolyser cell and the bioreactor. If the oxygen enriched gas has a percentage of oxygen higher than ambient air (i.e. higher than 21% by volume), and it usually will do when the at least one SOEC is operating, then the bioreactor will be fed a higher volume percentage of oxygen, which in turn increases the efficiency of the aerobic process in the bioreactor. With increased efficiency, more wastewater can be treated.

In some embodiments the oxygen containing gas for the bioreactor comprises ambient air mixed with oxygen enriched gas from the at least one solid oxide electrolyser cell.

In some embodiments the oxygen containing gas for the bioreactor comprises just oxygen enriched gas from the at least one solid oxide electrolyser cell.

In some embodiments the oxygen containing gas for the bioreactor will have an oxygen content greater than that of ambient air, i.e. greater than 21% by volume. Preferably it will be 23 to 24% by volume or higher than 24% by volume.

In some embodiments, an oxygen enriched gas stream from the oxygen enriched gas output from the at least one SOEC is thermally connected across a heat exchanger to a gas infeed (e.g. ambient air, a nitrogen enriched gas feed instead of the ambient air, or a steam stream, or both) of the at least one solid oxide electrolyser cell and the oxygen enriched gas stream exiting the oxygen enriched gas output during operation of the at least one solid oxide electrolyser cell is cooled by the gas infeed prior to entering the aerobic bioreactor, and the gas infeed is heated before being fed into the at least one SOEC. In other words, an oxygen enriched gas stream from the oxygen enriched gas output from the at least one SOEC and a gas infeed of the at least one solid oxide electrolyser cell are in fluid flow communication with a heat exchanger for exchanging heat between the oxygen enriched gas stream and a feed gas for the gas infeed. This then further improves the efficiency of the system.

In some embodiments, a buffer tank is provided between the aeration blowers and the at least one SOEC which buffers the oxygen enriched gas stream from the at least one SOEC. The buffered gas stream may further cool in the buffer tank prior to being fed into the aerobic bioreactor.

The at least one solid oxide electrolyser cell usually has a hydrogen output. A hydrogen stream will exit the hydrogen output during use of the at least one solid oxide electrolyser cell. A buffer tank may be provided for it.

In some embodiments the hydrogen stream can be thermally connected across a gas infeed (e.g. ambient air, a nitrogen enriched gas feed instead of the ambient air, or a steam stream, or both) for the at least one solid oxide electrolyser cell via a heat exchanger. In other words, a hydrogen stream from the hydrogen output and a gas infeed of the at least one solid oxide electrolyser cell are in fluid flow communication with a heat exchanger for exchanging heat between the hydrogen stream and a feed gas for the gas infeed. This allows the hydrogen stream exiting the at least one solid oxide electrolyser cell, which will be hot from the at least one solid oxide electrolyser cell (i.e. relatively hotter than the gas infeed), to be cooled, and for the gas infeed into the at least one SOEC, which will be relatively colder than the hydrogen stream, to be heated. This then further improves the efficiency of the system.

According to a third aspect of the present invention there is provided a sulfur recovery system comprising a sulfur recovery unit and at least one solid oxide electrolyser cell (SOEC), wherein: the sulfur recovery unit has a sulfur dioxide containing gas input and an oxygen containing gas input; and the at least one solid oxide electrolyser cell comprises an anode, a cathode and an electrolyte, a steam input and an oxygen enriched gas output, wherein the oxygen enriched gas output connects to the oxygen containing gas input.

With this arrangement, an oxygen enriched gas from the at least one SOEC can be fed into the sulfur recovery unit during operation of the at least one solid oxide electrolyser cell and the sulfur recovery unit. If the oxygen enriched gas has a percentage of oxygen higher than ambient air (i.e. higher than 21% by volume), and it usually will do when the at least one SOEC is operating, then the sulfur recovery unit will be fed a higher volume percentage of oxygen, which in turn will increase the efficiency of the sulfur recovery unit’s sulfur recovery process. With increased efficiency, more sulfur dioxide containing gas can be treated.

In some embodiments the sulfur recovery unit receives an oxygen containing gas for the sulfur recovery process that comprises ambient air mixed with oxygen enriched gas from the at least one solid oxide electrolyser cell.

In some embodiments the oxygen containing gas for the sulfur recovery unit instead comprises just oxygen enriched gas from the at least one solid oxide electrolyser cell.

In some embodiments the oxygen containing gas for the sulfur recovery unit will have an oxygen content greater than that of ambient air, i.e. greater than 21 % by volume. Preferably it will be 23 to 24% by volume or higher than 24% by volume.

In some embodiments, an oxygen enriched gas stream from the oxygen enriched gas output of the at least one SOEC is thermally connected across a heat exchanger to a gas infeed (e.g. ambient air, a nitrogen enriched gas feed instead of the ambient air, or a steam stream, or both) of the at least one solid oxide electrolyser cell and the oxygen enriched gas stream exiting the oxygen enriched gas output during operation of the at least one solid oxide electrolyser cell is cooled by the gas infeed prior to entering the sulfur recovery unit, and the gas infeed is heated before being fed into the at least one SOEC. In other words, an oxygen enriched gas stream from the oxygen enriched gas output from the at least one SOEC and a gas infeed of the at least one solid oxide electrolyser cell are in fluid flow communication with a heat exchanger for exchanging heat between the oxygen enriched gas stream and a feed gas for the gas infeed. This then further improves the efficiency of the system.

In some embodiments, a buffer tank may be provided between the at least one SOEC and the sulfur recovery unit to allow a buffer for the oxygen enriched gas stream from the at least one SOEC.

In some embodiments, hot gas venting from the sulfur recovery unit is thermally connected across a heat exchanger to a gas infeed (e.g. ambient air, a nitrogen enriched gas feed instead of the ambient air, or a steam stream, or both) of the at least one solid oxide electrolyser cell and a hot gas stream exiting the sulfur recovery unit during operation of the sulfur recovery unit is cooled by the gas infeed, and the gas infeed is heated before being fed into the at least one SOEC. In other words, hot gas venting from the sulfur recovery unit and a gas infeed of the at least one solid oxide electrolyser cell are in fluid flow communication with a heat exchanger for exchanging heat between the hot gas venting from the sulfur recovery unit and a feed gas for the gas infeed. This then further improves the efficiency of the system.

In some embodiments, a buffer tank may be provided between the sulfur recovery unit and the gas infeed to allow a buffer for the hot gas venting from the sulfur recovery unit.

The at least one solid oxide electrolyser cell usually has a hydrogen output. A hydrogen stream will exit the hydrogen output during use of the at least one solid oxide electrolyser cell. A buffer tank may be provided for it.

In some embodiments the hydrogen stream can be thermally connected across a gas infeed (e.g. ambient air, a nitrogen enriched gas feed instead of the ambient air, or a steam stream, or both) for the at least one solid oxide electrolyser cell via a heat exchanger. In other words, a hydrogen stream from the hydrogen output and a gas infeed of the at least one solid oxide electrolyser cell are in fluid flow communication with a heat exchanger for exchanging heat between the hydrogen stream and a feed gas for the gas infeed. This allows the hydrogen stream exiting the at least one solid oxide electrolyser cell, which will be hot from the at least one solid oxide electrolyser cell (i.e. relatively hotter than the gas infeed), to be cooled, and for the gas infeed into the at least one SOEC, which will be relatively colder than the hydrogen stream, to be heated. This then further improves the efficiency of the system.

According to a fourth aspect of the present invention there is provided a method for sulfur recovery comprising providing a sulfur recovery unit and at least one solid oxide electrolyser cell (SOEC), wherein: the sulfur recovery unit has a sulfur dioxide containing gas input and an oxygen containing gas input; and the at least one solid oxide electrolyser cell comprises an anode, a cathode and an electrolyte, a steam input and an oxygen enriched gas output, wherein the oxygen enriched gas output connects to the oxygen containing gas input and feeds an oxygen enriched gas stream from the at least one solid oxide electrolyser cell to the sulfur recovery unit .

With this arrangement, an oxygen enriched gas from the at least one SOEC is fed into the sulfur recovery unit during operation of the at least one solid oxide electrolyser cell and the sulfur recovery unit. If the oxygen enriched gas has a percentage of oxygen higher than ambient air (i.e. higher than 21 % by volume), and it usually will do when the at least one SOEC is operating, then the sulfur recovery unit is fed a higher volume percentage of oxygen than just ambient air would provide, which in turn increases the efficiency of the sulfur recovery unit’s sulfur recovery process. With increased efficiency, more sulfur dioxide containing gas can be treated.

In some embodiments the sulfur recovery unit receives an oxygen containing gas for the sulfur recovery process that comprises ambient air mixed with oxygen enriched gas from the at least one solid oxide electrolyser cell.

In some embodiments the oxygen containing gas for the sulfur recovery unit instead comprises just oxygen enriched gas from the at least one solid oxide electrolyser cell.

In some embodiments the oxygen containing gas for the sulfur recovery unit has an oxygen content greater than that of ambient air, i.e. greater than 21 % by volume. Preferably it will be 23 to 24% by volume or higher than 24% by volume. In some embodiments, an oxygen enriched gas stream from the oxygen enriched gas output of the at least one SOEC is thermally connected across a heat exchanger to a gas infeed (e.g. ambient air, a nitrogen enriched gas feed instead of the ambient air, or a steam stream, or both) of the at least one solid oxide electrolyser cell and the oxygen enriched gas stream exiting the oxygen enriched gas output during operation of the at least one solid oxide electrolyser cell is cooled by the gas infeed prior to entering the sulfur recovery unit, and the gas infeed is heated before being fed into the at least one SOEC. In other words, an oxygen enriched gas stream from the oxygen enriched gas output from the at least one SOEC and a gas infeed of the at least one solid oxide electrolyser cell are in fluid flow communication with a heat exchanger for exchanging heat between the oxygen enriched gas stream and a feed gas for the gas infeed. This then further improves the efficiency of the system.

In some embodiments, a buffer tank is provided between the at least one SOEC and the sulfur recovery unit to buffer the oxygen enriched gas stream from the at least one SOEC.

In some embodiments, hot gas venting from the sulfur recovery unit is thermally connected across a heat exchanger to a gas infeed (e.g. ambient air, a nitrogen enriched gas feed instead of the ambient air, or a steam stream, or both) of the at least one solid oxide electrolyser cell and a hot gas stream exiting the sulfur recovery unit during operation of the sulfur recovery unit is cooled by the gas infeed, and the gas infeed is heated before being fed into the at least one SOEC. In other words, hot gas venting from the sulfur recovery unit and a gas infeed of the at least one solid oxide electrolyser cell are in fluid flow communication with a heat exchanger for exchanging heat between the hot gas venting from the sulfur recovery unit and a feed gas for the gas infeed. This then further improves the efficiency of the system.

In some embodiments, a buffer tank is provided between the sulfur recovery unit and the gas infeed to buffer the hot gas venting from the sulfur recovery unit.

In some embodiments, steam generated by the sulfur recovery unit is used to generate electricity for use by the SOEC.

The at least one solid oxide electrolyser cell usually has a hydrogen output. A hydrogen stream then exits the hydrogen output during use of the at least one solid oxide electrolyser cell. In some embodiments the hydrogen stream is thermally connected across a gas infeed (e.g. ambient air, a nitrogen enriched gas feed instead of the ambient air, or a steam stream, or both) for the at least one solid oxide electrolyser cell via a heat exchanger. In other words, a hydrogen stream from the hydrogen output and a gas infeed of the at least one solid oxide electrolyser cell are in fluid flow communication with a heat exchanger for exchanging heat between the hydrogen stream and a feed gas for the gas infeed. This allows the hydrogen stream exiting the at least one solid oxide electrolyser cell, which will be hot from the at least one solid oxide electrolyser cell (i.e. relatively hotter than the gas infeed), to be cooled, and for the gas infeed into the at least one SOEC, which will be relatively colder than the hydrogen stream, to be heated. This then further improves the efficiency of the system.

In each aspect of the present invention, in some embodiments the at least one solid oxide electrolyser cell operates at a temperature of between 400 and 1000 degrees centigrade. More typically it operates at a temperature of between 450 and 650 degrees centigrade.

For each aspect of the present invention, in some embodiments the at least one solid oxide electrolyser cell is part of a stack of such cells.

According to a fifth aspect of the present invention there is provided a chemical processing plant comprising a secondary wastewater treatment system and/or a sulfur recovery system as defined above.

In some embodiments the chemical processing plant is a refinery and comprises both a secondary wastewater treatment system and a sulfur recovery system as defined above.

With these arrangements the secondary wastewater treatment system and/or the sulfur recovery system can process wastewater and/or sulfur dioxide containing gas from other processing units in the plant or refinery, such as a distillation or fractionation unit, a fluid catalytic cracking unit, a hydrotreater, a hydrocracker, a gas plant (for example for producing LPG or fuel gas), a visbreaker or a coker. A further benefit from such an arrangement is that the hydrogen output from the at least one solid oxide electrolyser cell can be utilised elsewhere in the plant, for example in a hydrotreater, a hydrocracker or an isomerisation unit.

BRIEF DESCRIPTION OF DRAWINGS

These and other features of the present invention will now be described in further detail, purely by way of example, with reference to the accompanying drawings, in which:

Figure 1 shows a prior art solid oxide electrolyser cell;

Figure 2 shows a prior art secondary wastewater treatment plant for the removal of organic matter from sewage or other wastewater using an aerobic process;

Figure 3 shows an embodiment of the present invention in which the solid oxide electrolyser cell is incorporated with a secondary wastewater treatment plant;

Figure 4 shows a prior art sulfur recovery unit for the treatment of a hydrogen sulfide laden gas;

Figure 5 shows an embodiment of the present invention in which the solid oxide electrolyser cell is incorporated with a sulfur recovery unit;

Figure 6 shows a flow diagram of various processes in a refinery in which a solid oxide electrolyser cell is incorporated with various processes thereof to improve the efficiency of those processes;

Figure 7 shows a modified embodiment of secondary wastewater treatment, similar to that of Figure 3, but with the hydrogen output from the SOEC being used to pre-heat the steam and air for the SOEC;

Figure 8 shows a modified embodiment of sulfur recovery unit, similar to that of Figure 5, but with the hydrogen output from the SOEC being used to re-heat the oxygen enriched gas for the sulfur recovery unit and with steam from a waste heat boiler of the sulfur recovery unit being used to generate electricity, the electricity and the steam feeding to the SOEC;

Figure 9 shows a modified embodiment of sulfur recovery unit, similar to that of Figure 5, but with the hydrogen output from the SOEC being used to re-heat the oxygen enriched gas for the sulfur recovery unit and preheat the air for the SOEC, with steam from a waste heat boiler of the sulfur recovery unit being used to preheat steam feeding to the SOEC;

Figure 10 shows a modified embodiment of sulfur recovery unit, similar to that of Figure 5, but with the hydrogen output from the SOEC being used to preheat the air and steam for the SOEC, and with steam from a waste heat boiler of the sulfur recovery unit being used to generate electricity, the electricity and the steam feeding to the SOEC; and

Figure 11 shows a modified embodiment of sulfur recovery unit, similar to that of Figure 5, but with the steam from a waste heat boiler of the sulfur recovery unit being used to supplement the steam fed to the SOEC and being used also to preheat the air for the SOEC.

SPECIFIC DESCRIPTION

Referring first of all to Figure 1 , the operation of a solid oxide electrolyser cell, or SOEC/SOEL 36 is schematically illustrated. SOEC and SOEL are interchangeable terms as a solid oxide electrolyser or solid oxide electrolyser cell.

As shown in Figure 1 , there is a single solid oxide electrolyser cell 36 in this example, but commonly multiple cells are stacked in series to form a stack of cells, and multiple stacks (or cells) might be mounted in parallel too.

The SOEC 36 comprises an anode 48, a cathode 50, an electrolyte 52, a hydrogen output 40, a steam input 42, plus also a hot gas (usually nitrogen rich) input 54 and a hot gas (oxygen enriched) output 56. To operate the SOEC 36, a current source 44 is applied across the SOEC 36, which then, through electrolysis, separates from the steam some of its components by splitting the water molecules, thus producing an output stream of hydrogen and oxygen, with the hydrogen venting through the hydrogen output 40, along with unconverted steam, and the oxygen enriching a hot gas output at the hot gas output 56. For this to occur, a steam stream is fed into the cathode at the steam input 42, which cathode is porous. When the current and thus a voltage is applied, the steam at the cathode-electrolyte interface is reduced to form pure H2 and oxygen ions. The hydrogen gas then diffuses back up through the cathode 50 and is collected at its surface as hydrogen fuel, along with unconverted steam, whereby the hydrogen outputted is wet hydrogen. The oxygen ions are instead conducted through the dense electrolyte. At the electrolyte-anode interface, the oxygen ions are oxidized to form pure oxygen gas, which is collected at the surface of the anode by the hot gas, thus oxygen enriching that hot gas.

The electrolyte 52 must be dense enough that the steam and hydrogen gas cannot diffuse through it as that could lead to a recombination of the H2 and O2 in the SOEC.

Referring next to Figure 2, an example of an aerobic secondary wastewater treatment unit 10 is schematically illustrated. It enables the removal of organic matter from sewage or other wastewater 12.

The secondary wastewater treatment process typically occurs after a primary treatment step, which can involve physical phase separation of settleable solids, including filtration or cyclonic separation, or some other primary clarifying step. The part treated wastewater 12 is then passed into a bioreactor 14, which is a tank for containing the wastewater, in which microorganisms are introduced, along with a supply of oxygen for ensuring the survival of the microorganisms. The microorganisms perform a biological process on the wastewater 12 to remove dissolved and suspended organic matter from the wastewater 12. The processing in the bioreactor is usually the most energy intensive portion of the process taking up 45 to 75% of the energy footprint due to the constant need to run the aeration blowers to maintain the viability of the microorganisms, and to keep the wastewater moving and fluid throughout the tank.

Although the biological process can be an aerobic or anaerobic process, more commonly an aerobic process is utilised, although it depends on the treatment technology or biological process/microorganisms being used as to whether the process is aerobic or anaerobic. The first aspect of the present invention concerns aerobic processes where oxygen is required for the process. After treatment in the bioreactor 14, the secondarily treated wastewater is passed to a second stage clarifier 16 which can attempt to remove remaining nitrogen, phosphorus, heavy metals, pathogens and bacteria. Treated wastewater can then exit the system if it passes testing at an analyser station 18, for example with an NH3 analyser 20 and an NO _X analyser 22, or it can recycle around through the bioreactor 14 again.

During the process, dissolved oxygen (DO) levels are monitored - for example in the bioreactor using a DO probe 28 and perhaps at the analyser station 18, which allows an aeration control valve 24 to be controlled - controlling the amount of air, and thus oxygen, fed into the bioreactor 14 by an aeration blower 26. The DO level needs to be maintained above a threshold for sustaining the activity of the microorganisms. A common difficulty with these secondary treatment units 10, however, is that they have an absolute water treatment capacity for a given size of bioreactor and only with increased oxygen levels versus air can that capacity be increased.

Referring next to Figure 3, a first embodiment of the first aspect of the present invention is shown. This figure again shows schematically the aerobic secondary treatment process from Figure 2, but the secondary wastewater treatment unit 10 is now combined with a solid oxide electrolyser cell 36, so that oxygen enriched gas/air 46 venting from an anode side of the SOEC 36 at the hot gas output 56 can be fed into a gas infeed 58 of the secondary wastewater treatment unit 10. This can then enrich the oxygen levels in the bioreactor 14 to increase the wastewater treatment capacity of the secondary wastewater treatment unit 10.

In a preferred level the gas bubbling through the bioreactor is enriched with oxygen from the SOEC 36 so that it is supplied with more than 21% oxygen by volume, and more preferably at least 22 to 24% by volume or more than 24% by volume. In a most preferred configuration the oxygen is at about 28% by volume.

As the oxygen enriched gas 46 from the SOEC 36 will be hotter than ambient air when it exits the SOEC, it is passed through at least one heat exchanger 60, 62 before passing into the aeration blowers 26 of the secondary wastewater treatment unit 10. In this example, the hot oxygen enriched gas 46 passes through two heat exchangers 60, 62. The first is an air heat exchanger 60 which allows the hot oxygen enriched gas 46 to heat ambient air 64 that is being fed into the hot gas input 54 on the anode side of the SOEC 36. The air 64 being fed into the SOEC will thus be able to be adequately close to the required operating temperature of the SOEC 36 when it enters the SOEC, thus maintaining operational efficiency of the SOEC 36. This also part cools the hot oxygen enriched gas 46. The second heat exchanger 62 is then a steam heat exchanger 62 which then allows that oxygen enriched gas to heat a steam stream 66 for a steam input 42 of the SOEC prior to its entrance into the SOEC on the cathode side of the SOEC 36, thus further enabling the maintenance of the operational efficiency of the SOEC 36. Yet further, this further reduces the temperature of the oxygen enriched gas 46 to a suitable temperature for passing through the aeration blowers 26.

A buffer tank 140 may be positioned along this oxygen enriched gas supply line to buffer the supply of oxygen enriched gas to the aeration blowers, and further cooling might occur in that buffer tank 140.

An added benefit of the combination of the SOEC 36 with the secondary wastewater treatment unit 10 is that there is a hydrogen stream 68 outputted from the SOEC 36, which hydrogen stream can be utilised elsewhere in the plant, or collected, for example in a buffer tank 150, and sold. It can even be used to help heat the steam stream 66 or the ambient air 64 to further improve the operational efficiency of the system. Such an arrangement is shown in Figure 7. In Figure 7, .rather than storing the hydrogen in a buffer tank 150, the hydrogen is fed through two heat exchangers 146, 148, the first being across the steam stream 66 and the other being across the ambient air feed 64. The hydrogen stream, which will be hot upon exiting the SOEC can thus heat the steam before it enters the SOEC 36, and likewise can heat the air 64 before it feeds into the SOEC. In this embodiment, the initial heating is of the steam to achieve a higher temperature for the steam, whereas the second heat exchanger 148 instead pre-heats the ambient air 64 prior to heat exchangers 54 and 60.

This combination of the SOEC 36 and the secondary wastewater treatment unit 10 technologies thus can improve efficiencies of the SOEC 36 and the secondary wastewater treatment unit 10 primarily as an oxygen enriched gas is supplied to the secondary wastewater treatment unit 10, thus debottlenecking the secondary wastewater treatment unit 10 and the plant - as commonly the throughput of the secondary wastewater treatment unit 10 dictates the amount of other processing that the plant can perform. Secondly, however, the use of heat exchangers allow a better thermal balance across the plant. The combination thus is a cost-effective way of meeting or increasing permit limits (as only processed wastewater can be disposed of), of allowing higher production rates by the plant, such as refined oils in a refinery, of lowering costs as the SOEC is a cost effective addition to the plant given its oxygen and hydrogen outputs, and simple air (or nitrogen), steam and electricity inputs. It also can help to avoid a larger capital expenditure of increasing the size or number of bioreactors, as the increased oxygen percentage by volume can increase the throughput capacity of the existing bioreactor(s).

In brief, therefore, the combination provides:

Debottlenecking of a wastewater plant without a need to upgrade or replace the bioreactor(s); An ability to meet permit limits without substantial upgrades to the entire plant - the SOEC 36 can be retrofitted into the air supply for the secondary wastewater treatment unit 10 to either supplement the air supply for the bioreactor or to replace the air supply; Potential debottlenecking of an entire Refinery/Petrochemical Site, especially where a hydrogen supply thereto is also an area of bottlenecking as the SOEC also outputs hydrogen;

A potential for reducing operating costs of an existing system as there will be reduced blower demand due to a given volume of blown gas containing a higher percentage by volume of oxygen - the only gas needed from the air by the bioreactor’s micro-organisims; and

A potential for reducing operating costs as there are fewer heating requirements - the gas infeeds (steam and air) to the SOEC can be heated by its outputs and there will be no need to heat (only cool) the gas infed into the secondary wastewater treatment unit 10.

Referring next to Figure 4, an example of a prior art sulfur recovery unit 70 is schematically illustrated. It utilises a chemical process for the removal of sulfur, known as the Claus process, whereby elemental sulfur is recovered from gaseous hydrogen sulfide by performing a combustion of the gas. In this example there are four stages. These are an initial “thermal stage” at a temperature above 850 °C - ideally between 950 degrees and 1200 degrees centigrade, followed by three consecutive “catalytic stages” at temperatures from 350 degrees to 180 degrees centigrade.

During the process, the following reactions occur, which break down the hydrogen sulfide (H2S) into sulfur (S) and water (H2O), with sulfur dioxide (SO2) as an occasional intermediary:

2 H ₂ S + 3 O ₂ 2 SO ₂ + 2 H ₂ O

2 H ₂ S + SO ₂ 3 S + 2 H ₂ O

2 H ₂ S + O ₂ 2 S + 2 H ₂ O

The sulfur recovery unit 70 comprises three main components in the thermal stage. There is the burner 72, where air 30 is fed (containing oxygen) along with acid gas 32 (containing H2S). Gaseous oxygen 34 can also be added to enrich the air 30 that is fed to the burner 72. That gas and air 30, 32, 34 then feeds through to a combustion chamber 74, at which further gaseous oxygen can be added (or the previous gaseous oxygen 34 can instead be fed directly to the combustion chamber 74). There is then a waste heat boiler 76 where the combusted gas products (process gas 38) is cooled in a process in which it turns boiler feed water (BFW) 78 into steam 80, thus recovering energy from that process gas. The subsequently cooler process gas 38 then exits that thermal stage and passes to a separator 82 for recovering some of the sulfur contained therein as liquid sulfur 84 - for example it may condense in the waste heat boiler or the separator.

Thereafter, the process gas 38 is sent through three catalytic stages using Claus reactors 90, 92, 94.

During the first catalytic stage, the process gas 38 is reheated by a first heater 86, and passed to a first Claus reactor 90. The output from that is then cooled by a first process gas cooler 88 to extract further liquid sulfur 84 therefrom as further condensate. The process gas 38 then moves to a second catalytic stage in which it is passed through a second heater 96, a second Claus reactor 92 and a second process gas cooler 100 to extract even further liquid sulfur 84 therefrom (again as a condensate). The process gas 38 then goes through a third catalytic stage in which it is passed through a third heater 98, a third Claus reactor 94 and a third process gas cooler 102 to extract even further liquid sulfur 84 therefrom (again as a condensate). Although three such cycles are illustrated, there may be just one such cycle, two such cycles or more than three such cycles.

Finally the process gas 38 is analysed by an analyser 118 for remaining hydrogen sulfide (H2S) and sulfur dioxide (SO2) to check the concentration ratios are suitable for a subsequent Claus tail gas processes. For example, the concentration ratio of these gases should usually be between 2:1 and 10:1 , depending upon the Claus tail gas treatment process installed in the plant, and if analysed and found to be suitable, the process gas 38 is passed on to that Claus tail gas treatment process through a tail vent 104. Otherwise the process gas 38 is returned for further catalytic stage cycles, and possibly a further pass through the thermal stage. The collected liquid sulfur 84 is meanwhile drained off through pipework 106.

Usually in the first catalytic stage cycle the process gas 38 is passed to the first Claus reactor 90 at about 350 degrees centigrade, and to the third Claus reactor 94 at around 180 degrees centigrade, with the gas 38 being fed into the second Claus reactor 92 at a temperature between these temperatures, such that the temperature of the first Claus reactor is higher than that of the second and the third is lower than the second.

Referring next to Figure 5, a first embodiment of the third aspect of the present invention is shown. In this embodiment, the prior art sulfur recovery unit 70 is again schematically illustrated, but there is incorporated therewith a solid oxide electrolyser cell 36, so that oxygen enriched gas/air 46 venting from an anode side of the SOEC 36 at the hot gas output 56 thereof can be fed into the burner 72 of the sulfur recovery unit 70 either to supplement ambient air or to replace ambient air. In this embodiment a buffer tank 140 is provided in the feed line between the SOEC and the sulfur recovery unit 70.

With this arrangement, the process air 30 that it is supplied to the sulfur recovery unit 70 can be enriched with oxygen so that it has more than 21 % oxygen by volume (21 % being the usual volume percentage in ambient air). More preferably the process air 30 will be at least 22 to 24% by volume of oxygen, or more preferably more than 24% by volume. In a most preferred configuration the oxygen percentage per volume is at about 28% to 45% by volume. In some embodiments the oxygen enriched gas/air from the SOEC may be passed into the process air and the gaseous oxygen feed 34, either or both to the burner 72 or the combustion chamber 74 of the sulfur recovery unit 70.

By increasing the volume percentage of oxygen in the gas feeds to the thermal stage of the sulfur recovery unit, the efficiency of the sulfur recovery unit can be improved as the combustion of the hydrogen sulfide and any resulting sulfur dioxide can be more complete, with stoichiometric volumes being easier to achieve, with less flue gases. Achieving the desired higher combustion temperatures thus also becomes easier to maintain.

The integration of the SOEC 36 with the sulfur recovery unit 70 can also utilise the steam from the waste heat boiler 76 as that steam can be used by the SOEC as the steam stream 66 for the steam input 42 on the cathode side of the SOEC 36.

An added benefit of the combination of the SOEC 36 with the sulfur recovery unit 70 is that there is a hydrogen stream 68 outputted from the SOEC 36, which hydrogen stream can be utilised elsewhere in the plant, or collected, for example in a buffer tank 150, and sold

Furthermore, to maintain good efficiencies in the SOEC 36, the outputted oxygen enriched gas 46 from the SOEC can pass across the ambient air input (hot gas input 54) of the SOEC 36 to heat that input air via a heat exchanger 60, as shown in Figure 5. The hydrogen output can likewise do so. See, for example, the modified arrangement in Figure 9 and 10. In each case, this is achieved across one or more heat exchanger. In Figure 9, for example, the hydrogen stream 68, rather than being stored in a buffer tank 150, is fed through two heat exchangers 146, 148, the first being across the oxygen enriched gas 46 from the SOEC 36 and the second 148 being across the ambient air feed 64 for the SOEC 36. The hydrogen stream, which will be hot upon exiting the SOEC can thus reheat the oxygen enriched gas 46 (cooled by a preceding heat exchanger 60 across the ambient air feed for the SOEC) before it enters the sulfur recovery unit 70, and likewise can heat the ambient air 64 before it feeds into the SOEC 36. In this embodiment, the initial heating is of the oxygen enriched gas 46 to reheat it, whereas the second heat exchanger 148 instead pre-heats the ambient air 64 prior to heat exchangers 54 and 60. In Figure 10, the first heat exchanger 146 is instead across the steam feed 66 for the SOEC, so the initial heating is instead of the steam 66, whereas the second heat exchanger 148 still pre-heats the ambient air 64 prior to heat exchangers 54 and 60.

Referring back to Figure 5, hot gas 116 venting from the combustion chamber, may pass across a heat exchanger 112 to further heat up the steam before the steam stream 66 enters the SOEC and/or across a further heat exchanger 114 to further heat up the ambient air 64 for the SOEC 36.

Note too that the steam 80 from the waste heat boiler 76 may be used in the SOEC. See the optional connection line 142 in Figure 5, along which a buffer tank 144 may be provided. Likewise see a similar arrangement in Figure 8, where additionally the hydrogen 68 is fed across the oxygen enriched gas via a further heat exchanger 146 to reheat it, much like in Figures 9 and 10, albeit here without the further heat exchanger 148 across the ambient air feed 64.

Still referring to the arrangement in Figure 8, in this embodiment, as also in the embodiment of Figure 10, the steam 80 from the waste heat boiler 76 is also used to drive an electricity generator 152, which generates electricity for powering - partially of fully - the SOEC 36. In Figure 8 this is done with the steam from the generator 152 being the sole supply of steam 66 to the SOEC 36 so the steam from the waste heat boiler 76 both powers the SOEC 36 and provides all the steam 66 for the SOEC. In Figure 10, the steam 80 from the waste heat boiler 76 instead supplements the steam 66 for the SOEC 36.

Referring next again to Figure 9, this arrangement does not use an electrical generator, but instead uses the steam 80 from the waste heat boiler to preheat the steam 66 for the SOEC. This may be useful if the steam from the waste heat boiler has too many impurities in it to feed through the SOEC.

Referring finally to Figure 11 , a further embodiment of the present invention is shown. Although largely similar to that of Figure 5, in this embodiment the steam 80 from the waste heat boiler 76 is used both the preheat the ambient air feed to the SOEC 36 across a heat exchanger 54, but also to supplement the steam feed 66 for the SOEC 36. The oxygen enriched gas 46 also preheats the ambient air feed 64 to the SOEC 36 across a heat exchanger 60. The hydrogen feed 68, however, is stored in a buffer tank 150 for later use, as in Figure 5, rather than being used to preheat or reheat the feeds to the SOEC 36.

Integrating the SOEC 36 with a sulfur recovery plant thus can make use of the output oxygen enriched air, or even pure oxygen produced by an SOEL, if separated from the hot air inputted into the SOEC, plus also, the high grade heat contained in that output from the SOEC (or in the hydrogen output from the SOEC or the heat in the output from the thermal stage of the sulfur recovery unit 70. There are thus many possible ways to increase the overall efficiency of the combined system.

The improved efficiency of the sulfur recovery unit 70 also allows a greater volume of acid gas to be processed as there will be less hot flue gas (as less nitrogen) and a more efficient combustion, which in turn can offer substantial debottlenecking of a plant, such as a refinery, that uses the sulfur recovery unit, which this also being achievable as a retrofit to existing sulfur recovery units, and thus at a comparably low investment level versus building a custom sulfur recovery unit.

There is therefore a higher overall plant energy efficiency.

There can also be a decrease in residual ammonia (NH3) content as there will be less nitrogen in the process air 30 fed into the sulfur recovery unit 70.

Referring finally to Figure 6, a plant 108 incorporating both a sulfur recovery unit 70 and a secondary wastewater treatment unit 10 is shown.

As can be seen, in the plant 108 (a chemical processing plant in the form of a refinery in this example) there are multiple different forms of fluid processing equipment, with a variety of industrially useful outputs, including isobutane (for alkylation), fuel gas, LPG, gasoline, jet fuel, diesel, petrolcoke, fuel oil, bitumen and sulfur.

In many of the processes being carried out there is a waste product of hydrogen sulfide (H2S), and a number of the processes require or produce hydrogen, air (or oxygen) and steam. Integration of an SOEC 36 into the operations of the plant provides a highly valuable improvement in many of the processes, since the oxygen enriched gas exiting the SOEC is useful in many of these processes, such as the sulfur recovery unit, the secondary wastewater treatment unit 10, as per the above aspects of the present invention, but also in fluid catalytic cracking. Yet further the hydrogen from the SOEC 36 is useful for the hydrotreater, hydrocracking and isomerisation processes. The retrofitting of the SOEC with the processes within the plant thus provides highly beneficial benefits in terms of operational efficiencies and cross-process integration, and in particular thermal efficiencies where heat from one process can be utilised to heat source gases in other processes where previously a heater might have been needed.

These and other features of the present invention have been described above purely by way of example. Modifications in detail may be made to the invention within the scope of the claims as appended hereto.