Technical field

The present invention relates to a process for disinfecting seawater, specifically a process for disinfection using pure oxygen. The invention further relates to the use of pure oxygen for disinfecting seawater as well as apparatus arranged to perform such a process.

Background of the invention

Offshore seawater is generally disinfected using electro-chlorination. Electrochlorination involves passing an electric current through the seawater, resulting in the production of sodium hypochlorite. However, there are several challenges and problems associated with this method of disinfection. These include safety concerns associated with the production of hydrogen and the use of chlorine, which is a red chemical, as well as the labour intensive nature of the process, both in terms of operation and maintenance. Thus, alternative methods for disinfecting seawater are sought which address one or more of these issues. In particular, a more environmentally friendly and/or cost effective disinfection solution is desirable.

The present inventors have unexpectedly found that a process for disinfecting seawater using pure oxygen offers an attractive solution. In particular, they have developed a process in which disinfection is carried out as part of a cyclic system together with water electrolysis.

Summary of the Invention

In a first aspect, the invention provides a process for disinfecting seawater, said process comprising the steps:

(i) contacting the seawater with pure oxygen so as to produce purified seawater; and

(ii) electrolysing said purified seawater to produce hydrogen and oxygen; wherein the oxygen produced in step (ii) is at least partially recycled for use in step (i).

In a further aspect, the invention provides the use of pure oxygen for disinfecting seawater. In another aspect, the invention provides apparatus arranged to perform a process as hereinbefore described, comprising:

(i) A contactor configured to receive a seawater stream via at least one first inlet and a gas stream comprising pure oxygen via at least one second inlet so as to produce a purified seawater stream, wherein said purified seawater stream exits the contactor via a liquid phase conduit; and

(ii) An electrolyser configured to receive said purified seawater stream via at least one inlet so as to produce hydrogen gas and oxygen gas, wherein the hydrogen gas exits the electrolyser via a first gas phase conduit and the oxygen gas exits the electrolyser via a second gas phase conduit and wherein the second gas phase conduit is in fluid communication with the at least one second inlet of the contactor so as to allow at least a portion of the oxygen gas to flow from the electrolyser to the contactor.

Detailed Description of the Invention

The present invention describes a process for disinfecting seawater comprising the steps:

(i) contacting the seawater with pure oxygen so as to produce purified seawater; and

(ii) electrolysing said purified water to produce hydrogen and oxygen; wherein the oxygen produced in step (ii) is at least partially recycled for use in step (i).

In the context of the invention, the term “disinfecting” is intended to cover at least the partial removal of substances such as sediment, suspended solids, colloids, microorganisms, organic matter, pathogenic bacteria, viruses or dissolved gases, which have harmful effects on seawater utilization processes. In particular, the “disinfecting” process should achieve at least partial removal of algae and other sea life which could lead to biofouling and/or contamination.

Ultimately, following disinfection, the seawater should be of suitable purity for use in an electrolysis process for producing hydrogen gas and oxygen gas (i.e. step (ii) of the process of the invention). Those skilled in the art will be familiar with the necessary levels of purity for such applications. Typically, the purified seawater produced in step (i) undergoes a desalination step prior to its use in step (ii). Such desalination processes, which remove dissolved salts, are well known to the skilled person. Step (i) of the process involves contacting the seawater with pure oxygen. By “contacting” we mean bringing the seawater into contact, either directly or indirectly, with the oxygen. Any suitable contacting method may be employed. For example, the seawater may be flushed with a cross-current flow of pure oxygen. In such processes, it is typical to then pass the flushed seawater through a filter. This filter (or screen) will trap contaminant particles and allow the purified seawater to pass through.

Typical flow rates for oxygen in step (i) are 5 to 30 Nm ³ O ₂ per m ³ seawater.

To the extent that the oxygen used in step (i) is oxygen which has been recycled from the electrolysis process, the pressure of the supplied oxygen is generally in the range 30 to 40 bar.

The oxygen employed in step (i) may originate from several sources. As hereinbefore described, the process of the invention involves a second step in which the purified seawater is electrolysed to produce hydrogen and oxygen. The oxygen produced by the electrolysis step is at least partially recycled to step (i). Thus, a loop process results. The oxygen produced from the electrolysis may be combined with purchased O ₂ . Alternatively, once operating, the loop process may produce sufficient O ₂ from the electrolysis that O ₂ from other sources is not required.

In one embodiment of the invention, the pure oxygen employed in step (i) is provided as a gas stream consisting of pure oxygen.

In an alternative embodiment, the pure oxygen employed in step (i) is provided as a gas stream consisting of pure oxygen and ozone. The ozone may be provided from external sources or may be generated “in-situ" from the oxygen produced during the electrolysis step.

Those skilled in the art will appreciate that by “pure O ₂ ” we mean 100% oxygen gas.

Step (ii) of the process involves electrolysing the purified seawater produced in step (i) to produce hydrogen gas and oxygen gas, wherein the oxygen produced in step (ii) is at least partially recycled for use in step (i). As mentioned above, the purified seawater produced in step (i) is typically further treated in a desalination step prior to its use in step (ii).

The electrolysis of water is a process which will be well known to those skilled in art. It employs electricity to decompose water into oxygen gas and hydrogen gas. The oxygen is produced at the anode and the hydrogen is produced at the cathode. In the process of the invention, the power required for the electrolysis process may be generated by any suitable method, such as one or more offshore wind turbines. An electrolysis unit may be included in each wind turbine structure to be powered by said wind turbine, or there may be a central electrolysis unit that uses power from multiple wind turbines. Any suitable electrolysis process may be used, such as alkaline water electrolysis or polymer electrolyte membrane (PEM) electrolysis.

The H ₂ which is produced is typically used as a fuel. The oxygen which is produced is at least partially recycled for use in step (i). Before being suitable for recycling to step (i) the oxygen may require to be cooled and/or passed through one or more compressors. In one embodiment, a portion of the oxygen may be converted to ozone, which can then be employed in the disinfection process.

The electrolysis step generally produces 600 to 1200 Nm ³ H ₂ per m ³ water. Typical O ₂ production rates for this step are 300 to 600 Nm ³ O ₂ per m ³ water.

As is typical in electrolysis processes, cooling of the system and/or the produced gases may be required. In the processes of the invention, such cooling may be provided by seawater (purified or unpurified).

The process of the invention may be carried out alone or in combination with other disinfection process, such as electro-chlorination.

In one particularly preferred embodiment, the process of the invention is carried out as follows. This process is illustrated in Figure 1 .

A seawater inlet stream 10 is received via a lift pump 1 to a water treatment facility 2. The seawater inlet stream is, at the treatment facility 2, flushed with an oxygen stream 20, which may consist of pure oxygen or a mixture of pure oxygen and ozone. Within the water treatment facility, the seawater may also undergo a desalination process.

A purified seawater stream 30 exits the water treatment facility 2 and is fed to an electrolyser 3, where it undergoes electrolysis to produce hydrogen gas and oxygen gas. The H ₂ produced is fed via stream 40 for further use, e.g. as a fuel. The O ₂ produced is at least partially recycled to oxygen stream 20 via exit stream 50. A portion of the O ₂ may also be fed via stream 60 to an ozone generator 4. The ozone produced exits the generator 4 via stream 70 to be mixed with oxygen stream 20.

The electrolyser 3 may be cooled using seawater via stream 80, which is then recycled back to the water treatment facility 2 via stream 90.

Reject water from the water treatment facility is discharged for disposal via stream 100.

In a further aspect, the invention relates to the use of pure oxygen for disinfecting seawater. The invention also provides apparatus arranged to perform the processes of the invention as hereinbefore defined, comprising:

(i) A contactor configured to receive a seawater stream via at least one first inlet and a gas stream comprising pure oxygen via at least one second inlet so as to produce a purified seawater stream, wherein said purified seawater stream exits the contactor via a liquid phase conduit; and

(ii) An electrolyser configured to receive said purified seawater stream via at least one inlet so as to produce hydrogen gas and oxygen gas, wherein the hydrogen gas exits the electrolyser via a first gas phase conduit and the oxygen gas exits the electrolyser via a second gas phase conduit and wherein the second gas phase conduit is in fluid communication with the at least one second inlet of the contactor so as to allow at least a portion of the oxygen gas to flow from the electrolyser to the contactor.

It will be appreciated that the electrolyser is in fluid communication with the liquid phase conduit of the contactor, such that the purified seawater stream flows from the contactor to the electrolyser, typically via desalination apparatus.

Preferable aspects discussed in the context of the processes of the invention apply equally to the apparatus embodiments.

The “contactor” may be any suitable apparatus which facilitates the necessary contact between the seawater stream and the pure oxygen. One example of a contractor is illustrated in Figure 2.

A seawater inlet stream 5 is flushed with a cross-current flow of pure oxygen 6 before passing through a filter 7. The purified seawater stream may then pass through a lift pump 8.

Description of Figures

Figure 1 : Schematic illustration of the process of the invention

Figure 2: Schematic illustration of an example water treatment apparatus