Tubular reverse polarity self-cleaning tank

Abstract

A process for self-cleaning an electrolytic cell involves introducing a flow of seawater into an electrolytic cell having one or more cathodes and anodes. The cathodes and anodes are substantially completely coated with a coating composition. As seawater flows between the electrodes, a forward bias is applied between the anode and the cathode at a first current density. A reverse bias is then provided at the cathode. The reverse bias is provided at a second current density that is lower than the first current density. When the reverse bias is applied, the polarity of the cathode is reversed for a short period of time. This produces a small amount of hydrochloric acid on the surface that was the cathode, causing dissolution of calcium, magnesium, or other deposits on the surface of the electrode without damaging the coating composition on the electrode.

Description

[Technical field]

[0001] The present invention relates to electrolytic cells, in particular for the biocidal treatment of seawater in offshore, coastal and shore installations. [Background technology]

[0002] Electrolysis of seawater or other dilute aqueous sodium chloride solutions with the production of active chlorine, a mixture of hypochlorite and other oxidizing species, has found several industrial applications that exploit the biocidal and disinfecting properties of the product. An application of particular interest is the biocidal treatment of seawater for cooling, fire water, utility water, desalination, and other land and sea applications, where circulating seawater needs to be treated with a biocide to prevent fouling and clogging of pipes, vessels, and channels by marine organism growth. Preventive biocidal treatment can involve in-situ generation of hypochlorite by using an electrolytic cell. Electrochlorination of seawater eliminates the storage, handling, and purchase of harmful chemicals.

[0003] During the electrolysis process, seawater passes through the electrolysis cell and leaves the cell as a sodium hypochlorite solution and by-product hydrogen gas. The solution is piped to a tank or cyclone where hydrogen can be removed from the solution. The resulting solution leaving the cell is a mixture of seawater, hypochlorite, and hypochlorous acid. Electrolysis of sodium chloride solution (seawater) involves passing a direct current between the anode (positive pole) and the cathode (negative pole) to separate salt and water into their basic elements. Chlorine produced at the anode immediately undergoes a chemical reaction to form hypochlorite and hypochlorous acid. Hydrogen and hydroxide are formed at the cathode, with the hydrogen forming gas and the hydroxide helping to form hypochlorite, raising the pH of the outlet stream to about 8.5.

[0004] The overall chemical reaction can be shown as follows: Salt + Water + Energy → Sodium Hypochlorite + Hydrogen NaCl+H2O+2e →NaClO+H2 The electrolytic chlorine generation process can cause fouling of the electrolytic cells due to the build-up of scale, which is hardened calcium and magnesium deposits. In the absence of an active method to remove these deposits, periodic chemical-based cleaning is required, which may also involve physically removing the cell for cleaning, thus necessitating neutralization of the acid solutions used.

[0005] A common technique for cleaning electrolytic cells involves periodically rinsing the electrodes with hydrochloric acid. Although this is a simple procedure for a trained technician or operator, the entire process is very difficult and costly, not only in terms of equipment downtime, but also in terms of the operational and environmental considerations associated with procuring, storing, and safely disposing of the chemicals at offshore installations.

[0006] Several companies offer "self-cleaning" bath technology. This process is based on the principle of high velocity seawater flow, which can cause a "scouring" effect over the electrode surface, physically removing calcium and magnesium scale buildup. However, it has been found that periodic scouring actions only wash away small amounts of deposits. Unfortunately, such methods do not eliminate hardness buildup over time, which eventually leads to blockage of the bath annulus and irreversible damage to the bath.

[0007] For these reasons, there is a need for a technically and economically viable solution that minimizes scale build-up on electrolyzers while reducing logistical maintenance related downtime. [Brief description of the drawings]

[0008] [Figure 1] FIG. 2 is a cross-sectional view of a tubular reverse polarity vessel, according to one embodiment. [Diagram 2] FIG. 2 is a schematic diagram of a tubular reverse polarity cell assembly, according to one embodiment. [Diagram 3] FIG. 13 illustrates dissociation of deposits on a reverse polarity vessel by reverse polarity, according to one embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0009] An embodiment of the present invention relates to a controllable process for self-cleaning of a seawater electrolyser or "electrolyser". The process relies on the local seawater conductivity, which varies with temperature and salinity. The process involves automatic regulation or modulation of the reverse-biased power supplied to the electrolyser.

[0010] According to one embodiment, a process for self-cleaning an electrolytic cell is disclosed. The electrolytic cell comprises cathode and anode electrodes. The electrodes are substantially completely coated with a coating composition. A seawater flow is introduced between the electrodes. A forward bias is applied between the anode and the cathode as the seawater flow passes between the electrodes. The forward bias is applied at a relatively high first current density. The self-cleaning process further involves applying a reverse bias at a variable potential (relative to a fixed potential) to the cathode to achieve a reduced second current density. In one embodiment, the second current density is 5% or more of the predetermined forward bias current. The reverse bias is applied at a regular, predetermined frequency. For example, the reverse bias is provided for a predetermined period within / every 24 hours. The current can be alternated in an internal power supply. When the reverse bias is applied, the polarity of the cathode is reversed for a short period so that the cathode functions as the anode. This produces a small amount of hydrochloric acid (HCl) on the surface that was the cathode. This short-term "in situ" production of HCl at low current density and given normal frequency causes dissolution of calcium, magnesium, or any other scale that has built up on the surface of the electrode. It can also prevent further scale build-up and permanent damage to the electrode and the coating composition thereon. This ensures optimal forward bias operation with high production of HCl.

[0011] According to another embodiment, a tubular reverse polarity ("TRP") or reverse bias electrolytic cell comprises an electrically conductive outer tubular sleeve formed from (A) a tubular terminal anode and (B) a tubular terminal cathode, and a bipolar tubular electrode having a cathode end and an anode end. The terminal electrodes (anode / cathode) have a slightly larger diameter than the bipolar tubular electrodes. This allows each terminal electrode to be fitted over the bipolar electrodes. The terminal anode and terminal cathode are separated by a central coupling mechanism. In addition, the ends of the terminal electrodes also include coupling mechanisms. An annular space separates the terminal electrode and the bipolar electrode. The terminal electrode and the bipolar electrode are substantially completely coated with a coating composition configured to withstand periodic changes in forward and reverse current bias on the electrodes. According to another embodiment, a TRP electrolyzer system includes two or more TRP electrolyzers, a casing enclosing the TRP electrolyzers, and a control panel for automatically providing forward and reverse bias currents to the TRP electrolyzers.

[0012] The present invention may be described in further detail below and with reference to the accompanying drawings, all of which illustrate or relate to the apparatus, systems, and methods of the present invention. In the drawings, which are not intended to be drawn to scale, each like component shown in the various figures is represented by a like numeral.

[0013] Depending on the context, all following references to the "invention" may in some cases refer to certain particular embodiments only. In other cases, it can be recognized that references to the "invention" may refer to the subject matter set forth in one or more, but not necessarily all, of the claims.

[0014] FIG. 1 illustrates an example of a TRP electrolytic cell (or, equivalently, a "TRP cell" or simply "cell") 100. The TRP cell 100 includes an elongated, electrically conductive, outer tubular sleeve 110. The tubular sleeve 110 includes tubular terminal anode and cathode electrodes 115A and 115C (collectively "terminal electrodes 115"). Enclosed within the tubular sleeve 110 is a bipolar tubular electrode 120. The bipolar electrode 120 includes a cathode end and an opposing anode end.

[0015] The terminal electrode 115 has a slightly larger diameter than the bipolar tubular electrode 120 such that an annular space is formed between the inner surface of the terminal electrode 115 and the outer surface of the bipolar electrode 120. This allows the terminal electrode 115 to be fitted over the bipolar electrode 120. The terminal anode 115A and the terminal cathode 115C are separated by a central coupling mechanism 115B. In addition, the ends of the terminal electrode also include coupling mechanisms 115D, 115E. The coupling mechanisms may include seals. The seals may include one or more seal rings.

[0016] The TRP chamber 100 includes an inlet and an outlet and is configured with connection nodes to the electrodes to allow for fluid flow of liquid throughout the chamber. The bipolar electrode 120 and the terminal electrode 115 are substantially completely coated with a suitable composition capable of withstanding reversals of electrode polarity. In one embodiment, the coating composition / coating can include one or more metals selected from the group of platinum group metals (e.g., ruthenium and / or iridium). In another embodiment, the coating can also include one or more metals selected from the group of valve metals. In yet another embodiment, the coating can include one or more elements selected from the group including nickel, iron, and cobalt, alone or in combination. However, it is understood that the coating can include other suitable and stable mixtures capable of withstanding periodic current reversals.

[0017] The TRP tank 100 is configured to be self-cleaning, thus eliminating the need for external acid cleaning of the TRP tank 100, which also eliminates any associated logistical issues and operational downtime.

[0018] According to one embodiment, as shown in FIG. 2, TRP tank system 200 includes a casing 210 that encloses a plurality of TRP tanks 100A-100D ("100") as described with respect to FIG. 1. In one embodiment, casing 210 can be made of stainless steel. Casing 210 can be configured to enclose between 2 and 16 tanks. However, one of ordinary skill in the art will understand that the number of tanks can vary as needed. In one or more embodiments, tank system 200 can be skid mounted.

[0019] The vessel system 200 further includes a transformer 230 configured to provide power to a control board 240. The control board 240 can include a current reversal device or mechanism 245.

[0020] During normal operation, a predetermined forward bias (DC current) is applied to the tank 100. The forward bias involves the application of a high density current. For example, the current density may be 0.5-4 kA / m when seawater flows through the tank 100. 2 The electrolysis produces a large amount of sodium hypochlorite. During this process, deposits form on the cathode surface of the cell due to the specific nature of the source feed water and the electrolytic action. The deposits can include, but are not limited to, calcium and magnesium deposits. The accumulation of deposits reduces the performance of the electrode.

[0021] According to another embodiment, a process for self-cleaning the tank 100 involves applying a reverse bias at a low current density (e.g., 5% or more of the forward bias) to generate small amounts of HCl that can be used to dissociate deposits.

[0022] The reverse bias can be applied for a predetermined period of time so that the deposits dissolve without damaging the coating applied to the electrodes. In one embodiment, the reverse bias is applied for a predetermined period of time each 24 hours at a predetermined low current density. This can be followed by alternating bias / DC current in an internal power supply in the same electrical connection to the bath. Such a self-cleaning process can be automated to eliminate the possibility of operator error. The self-cleaning process ensures a long operational life of the TRP bath and minimizes operational downtime.

[0023] An illustrative example of deposit dissociation by reverse polarity, with the cathodic surface acting as the anode, is shown in Figure 3. When the polarity is reversed, both the cathodic end of the bipolar electrode and the terminal cathode act as anodes for short periods of time. The polarity can be reversed at a frequency and duration appropriate to generate small amounts of HCl on the surface that was the cathode of the electrolytic cell.

[0024] Such short-term "in situ" generation of HCl at low current density and appropriate frequency dissolves deposits or scale buildup on the electrodes before they become too thick and hard to dissolve, preventing permanent damage to the respective electrodes to ensure optimal forward bias operation. This can facilitate continuous free-flow operation of the electrolytic cell. In addition, lower current densities in reverse bias for short periods of time eliminate built-up hardness while not affecting electrode life or performance.

[0025] In one or more embodiments, the self-cleaning process described herein can be applied to an electrolyzer with one or more electrolytic cells. For example, the electrolyzer can include multiple cathode and anode electrodes.

[0026] The TRP tank can be used on large installations, both onshore and offshore, or on other smaller installations. System embodiments can also be used in floodwater, cooling water, and fire water loops. The TRP tank can also be used in industrial power and shore biofouling control applications. The TRP tank has an optimized design that requires minimal operation and maintenance requirements. It has a once-through flow design that eliminates recirculation requirements. The TRP tank is constructed of corrosion resistant materials that make it durable. The TRP tank assembly, including the number of TRP tanks enclosed in a casing, can be customized to meet site-specific requirements. The TRP tank is configured to consume minimal power.

[0027] Thus, the present invention may be well adapted to achieve the objects and advantages mentioned, as well as those inherent therein. The specific embodiments disclosed above are illustrative only, as the invention may be modified and practiced in different but equivalent manners apparent to those skilled in the art having the benefit of the teachings herein. For example, the self-cleaning process, as disclosed herein, can be used to remove accumulated deposits from any electrolytic cell having a fully coated cathode and anode. The process does not depend on the tubular shape or size of the electrolytic cell. In addition, the process does not depend on the presence of a bipolar electrode positioned within a sleeve comprising a monopolar electrode. For example, according to one embodiment, a process for self-cleaning an electrolytic cell comprises: (A) providing an electrolytic cell comprising (i) one or more cathode electrodes and (ii) one or more anode electrodes, the electrodes being substantially completely coated with a coating; and (B) (i) applying a reverse bias to the cathode electrodes at a predetermined frequency and (ii) for a predetermined period of time within a 24-hour period. The process involves applying a reverse bias at 5% or more of a given forward bias, which causes the cathode electrode to act as the anode for a short period of time, and produces hydrochloric acid (HCl) to dissolve scale buildup on the electrode.

[0028] All ranges described herein include endpoints, including those that describe a range "between" two values. Terms such as "about," "generally," "approximately," and the like should be construed as modifying a term or value such that it is not absolute. Such terms may be defined by the context, and the terms that the context modifies as those terms are understood by those of ordinary skill in the art. The term "approximately" and variations thereof are defined as most, but not necessarily entirely, of what is specified as understood by those of ordinary skill in the art.

[0029] Furthermore, no limitations are intended to the details of construction or design shown herein. It is therefore apparent that the specific exemplary embodiments disclosed above may be altered or modified and all such variations are considered within the scope and spirit of the present invention. While systems and methods have been described as "comprising" or "containing" various components or steps, the systems and methods may also "consist essentially of" or "consist of" various components and steps.

Claims

1. 1. A process for self-cleaning an electrolytic cell, comprising: (i) introducing a seawater stream into the electrolytic cell, the electrolytic cell being configured for offshore, nearshore and shore installations, the electrolytic cell comprising: (i) one or more cathode electrodes; (ii) one or more anode electrodes; the cathode electrode being substantially completely coated with a coating composition and the anode electrode being substantially completely coated with the coating composition; (ii) applying a forward bias at a first current density between the anode electrode and the cathode electrode when seawater flows between the anode electrode and the cathode electrode; (iii) providing a reverse bias at the cathode electrode, the reverse bias being provided at a second current density less than the first current density, the reverse bias being provided at a periodic predetermined frequency.

2. The first current density is 0.5-4 kA / m ２ 2. The process of claim 1 , wherein

3. 2. The process of claim 1, wherein the second current density is greater than or equal to about 5% of the first current density.

4. 3. The process of claim 2, wherein the reverse bias is provided at a variable potential to achieve a substantially reduced second current density.

5. 5. The process of claim 4, wherein the reverse bias produces a smaller amount of hydrochloric acid compared to the amount of hydrochloric acid produced in the forward bias.

6. 6. The process of claim 5, wherein the hydrochloric acid generated during the reverse bias causes dissolution of calcium deposits, magnesium deposits, or both, that accumulate on the anode and cathode electrodes without damaging the coating composition on those electrodes.

7. The process of claim 6 , wherein the predetermined frequency is configured to prevent long term build-up of the deposits.

8. 8. The process of claim 7, wherein the reverse bias is provided for a predetermined period of time within each 24 hour period.

9. The process described in claim 1, wherein the seawater flowing between the anode electrode and the cathode electrode has a fixed salinity / conductivity.

10. 2. The process of claim 1, wherein the electrolytic cell is a tubular cell.

11. 1. A tubular reverse polarity ("TRP") electrolyser comprising: (i) a terminal cathode electrode; (ii) a terminal anode electrode; and an electrically conductive outer tubular sleeve formed from an inner tubular bipolar electrode having a cathode end and an anode end; The TRP electrolyzer, wherein the terminal cathode electrode, the terminal anode electrode, and the bipolar electrode are substantially completely coated with a coating composition configured to withstand periodic reversals of polarity.

12. 12. The TRP electrolyzer of claim 11, wherein the terminal cathode electrode and the terminal anode electrode have a slightly larger diameter than the bipolar electrode.

13. 13. The TRP electrolyzer of claim 12, wherein an annular space separates the terminal cathode and anode electrodes from the bipolar electrode.

14. 12. The TRP electrolyzer of claim 11, wherein the terminal anode electrode and the terminal cathode electrode are separated by a center seal.

15. 12. The TRP electrolyzer of claim 11, wherein each opposing end face of the terminal cathode electrode and the terminal anode electrode is provided with a seal.

16. 1. A TRP electrolyzer system comprising: Two or more TRP electrolysers according to claim 11; a casing for enclosing the TRP electrolytic cell; and a control board for automatically providing forward and reverse bias currents to the TRP electrolyzer.

17. 10. The process of claim 1, wherein the electrolytic cell is skid mounted.

18. 11. The process of claim 10, wherein the tubular cell comprises an electrically conductive outer tubular sleeve formed from a terminal cathode electrode, a terminal anode electrode, and an inner tubular bipolar electrode having a cathode end and an anode end, the terminal cathode electrode, the terminal anode electrode, and the bipolar electrode being substantially completely coated with a coating composition configured to withstand periodic reversals of polarity.

19. 20. The process of claim 18, wherein the terminal cathode electrode and the terminal anode electrode have a slightly larger diameter than the bipolar electrode.

20. 20. The process of claim 18, wherein an annular space separates the terminal cathodic and anodic electrodes from the bipolar electrode.

21. 20. The process of claim 18, wherein the terminal anode electrode and the terminal cathode electrode are separated by a center seal.

22. 20. The process of claim 18, wherein each opposing end face of the terminal cathode electrode and the terminal anode electrode is provided with a seal.

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