WO1993024412A1

WO1993024412A1 - Electrochemical process and apparatus for deoxygenation of water or aqueous solutions

Info

Publication number: WO1993024412A1
Application number: PCT/FI1993/000229
Authority: WO
Inventors: Seppo Tapio YLÄSAARI; Ilkka Kai Juhani Vuorilehto; Olof Berndt Wilhelm FORSÈN
Original assignee: Ylaesaari Seppo Tapio; Ilkka Kai Juhani Vuorilehto; Forsen Olof Berndt Wilhelm
Priority date: 1992-05-29
Filing date: 1993-05-27
Publication date: 1993-12-09
Also published as: AU4072293A; FI922485A0; FI922485A

Abstract

The invention relates to an electrochemical process and an apparatus for a deoxygenation of water or aqueous solutions. In this electrochemical process, the oxygen dissolved in water or an aqueous solution is removed electrochemically in a three-dimensional electrically conductive cathode particle bed of a cell, which bed consists of mechanically wear-resistant particles, which are smaller than 5 mm and which are metal particles or metal, ceramic or polymeric particles coated with a sufficiently thick metal layer, and through which bed the water or the aqueous solution to be treated is led. In the apparatus, by means of which an electrochemical deoxygenation of water or aqueous solutions is performed and which comprises a flow-through cell (1) divided into one or several anode chambers (5) and into one cathode chamber (7) filled with a particle bed (8), the cathode chamber extends in the flow direction of the flow-through cell further than the anode chambers and is filled with an electrically conductive cathode particle bed, which consists of mechanically wear-resistant particles, which are smaller than 5 mm and which are metal particles or metal, ceramic or polymeric particles coated with a sufficiently thick metal layer.

Description

Electrochemical process and apparatus for deoxygenation of water or aqueous solutions

The invention relates to an electrochemical process and an apparatus for a deoxygenation of water or aqueous solutions.

Oxygen dissolved in water and aqueous solutions causes corrosion in several technical systems, such as conventional power plants and nuclear power plants, heating, plumbing and air conditioning tech¬ niques as well as in cooling and heating systems. For this reason, dissolved oxygen has to be removed from water as accurately as possible. In the manufacture of soft drinks, for instance, the aim is to minimize the presence of oxygen.

Up till now, deoxygenation of water and aqueous solutions has mainly been carried out physically or chemically.

A condition of physical deoxygenation is that the solubility equilibrium will be disturbed. Into contact with an aqueous solution is brought a gas mixture, in which the partial pressure of the oxygen to be removed is much lower than the equilibrium pressure. Then the solution-gas system begins to move towards a new equilibrium state, i.e. the oxygen flows from the aqueous solution into a gas phase. As a gas phase is generally used water vapour. Physical deoxidizing processes are degasification under over¬ pressure and degasification in vacuum. By means of these processes also other gases dissolved in water can be removed.

In a degasification under overpressure, i.e. in a thermal degasification, the water is made to boil, whereby the escaping vapour takes along the gases. Degasification under overpressure requires a tempera- ture of at least 102 °C and a corresponding pressure of saturated vapour.

In a vacuum degasification, the water is made to boil under a pressure lower than the normal atmo- spheric pressure by reducing the pressure e.g. by means of a vacuum water pump and by heating the water to a boiling point corresponding to said pressure.

By means of degasification under overpressure, moderately small oxygen contents ( 10 to 20 μg 0₂/kg H₂0) are achieved already, but oxygen contents, how¬ ever small they may be, may cause corrosion problems.

For removing the residual oxygen as well as for eliminating small oxygen leaks, chemicals are used the influence of which is based on reactions with oxygen. By means of chemical deoxygenation, oxygen contents lower than 5 to 10 μg/kg are achieved.

The most used deoxidizing chemical is hydra- zine, NH₂-NH₂. It is a colourless liquid fuming in the air and mixing with water under all circumstances. The aqueous solution is basic and corrosive. Hydra- zine has been classified as a carcinogen and a toxin of Class II.

Sodium sulphite is also a very usual deoxidiz¬ ing chemical. It is a solid salt dissolving well in water. The aqueous solution is basic.

Other deoxidizing chemicals are e.g. erythorbic acid and tannins, (see: Korroosiokasikirja, pages 295 to 299).

Electrochemical removal of oxygen dissolved in water and apparatuses for this deoxygenation are de¬ scribed in some patent specifications and applica¬ tions (see e.g. Norwegian Published Specifications 159 847 and 165 068, U.S. Patent 4 830 721 and USSR Patent 966 026). In most above processes and/or appa- ratuses, oxygenous water flows through a net or a perforated or designed metal plate acting as a cath¬ ode and is reduced or reacts with the hydrogen gener¬ ated on the cathode.

The physical processes have a drawback of high investment costs, especially in cases when vapour is not easily available. Moreover, plenty of energy is required for heating the water.

The chemical processes have problems with en¬ vironmental and work safety. Hydrazine must not come into contact with the skin and it must not be in¬ haled. A use of sodium sulphite is not to be recom¬ mended either. In spite of attempts, it has not been possible to develop a non-toxic and efficient deoxi¬ dizing chemical. Net-shaped or plate electrodes used in most electrochemical deoxygenation processes and/or ap¬ paratuses mentioned above are not suitable for a reduction of oxygen, since the electrode shall have plenty of surface against which the oxygen shall be caused to impact.

USSR Patent 966 926 discloses a process for deoxygenation of water in a cathode chamber of a cell by utilizing an insoluble electrode, whereby the cathode is an activated carbon dust cathode provided with a catalyst for ionization of oxygen.

In practice, several district heating and heat¬ ing, plumbing and air conditioning systems are not deoxidized at all, because of the lack of a suitable process. Therefore, the pipes begin to rust rapidly. Under some circumstances, however, corrosion products can form a metal protecting layer, which delays the rusting.

The object of the present invention is to avoid the drawbacks presented above and to provide a pro- cess and an apparatus by means of which a substanti- ally complete deoxygenation of water or aqueous solu¬ tions is easy to perform in practice.

An essential feature of the process and appa¬ ratus of the invention is a threedimensional cathode, which cathode has a very large surface, consists of a packed bed and is manufactured of a mechanically dur¬ able material.

The invention thus relates to an electrochemi¬ cal process for a deoxygenation of water or aqueous solutions, which process is characterized in that the oxygen dissolved in water or an aqueous solution is removed electrochemically in a threedimensional elec¬ trically conductive cathode particle bed of a cell, the cell voltage of which is 1.9 to 2.0 volts, which bed consists of mechanically wear-resistant parti¬ cles, which are smaller than 5 mm, preferably about 0.2 to 1.2 mm, and which are metal particles or met¬ al, ceramic or polymeric particles coated with a suf¬ ficiently thick metal layer, through which bed the water or the aqueous solution to be treated is led and which bed is separated by means of an ion-ex¬ change membrane from all anode chambers provided with insoluble anodes.

The particle bed can consist of particles manu- factured of a metal or a metal alloy or of particles in which a suitable core is entirely coated with a sufficiently thick layer of a metal or a metal alloy. The most suitable metal for the particles of the bed is copper. Metal alloys, such as copper alloys and silver bronze, are conceivable. The coating metal of the particles of the bed could be for instance sil¬ ver. The particle core to be coated could be a glass, polymeric, ceramic or metal particle or bead, for instance. The particle bed consists each time of par- tides of a rather identical size. The size of the particles may vary. With a decreasing particle size the surface area of the cathode increases, which in¬ tensifies the deoxygenation at the same time. The particle size is suitably smaller than 5 mm, prefer¬ ably about 0.2 to 1.2 mm.

In the cathode the particles form a bed filling the cathode chamber substantially entirely in a di¬ rection perpendicular to the water flow direction and a large part of the cathode chamber or preferably substantially the entire cathode chamber in the water flow direction.

The surface area of the particle bed is suitab¬ ly over 5 m²/100 cm³ of packed bed measured by BET method. An especially suitable surface area of the particle bed is about 35 m²/100 cm³ of packed bed.

The cathode chamber is separated by means of an ion-exchange membrane from the anode chambers, which may be one or several in number. A suitable membrane is for instance a Nafion-417 cation-exchange mem¬ brane.

As an anode is used an insoluble anode. A suit¬ able anode material is for instance a titan plate coated with an iridium-based oxide. The cell voltage is 1.5 to 2.5 volts, preferab¬ ly 1.9 to 2.0 volts.

The flow rate of water may vary. A suitable flow is about (2 cm³/s)/(100 cm³ of packed bed).

The water or the aqueous solution to be treated may be led through several successive and/or parallel cells, if needed.

The oxygen content in the water treated by the electrochemical process according to the invention is less than 3 μg/kg water only. Compared with traditional processes, advantages of the present invention are efficient deoxygenation and low power consumption. As far as work safety and environment are concerned, it is also preferable to avoid using toxic deoxidizing chemicals. Compared with the majority of the electrochemi¬ cal processes presented above, an advantage is the very large surface area of the cathode. This compells almost each oxygen molecule to impact against the cathode and to be reduced. An additional advantage is the mechanical dura¬ bility of a cathode with a large surface area; the bed used as a cathode material in the present inven¬ tion, manufactured of metal particles or particles coated with metal, endures mechanically much better than active carbon the water flow and various pheno¬ mena associated with the water flow, such as turbu¬ lences and vibrations. However, the flow resistance of the particle bed is not remarkable.

An apparatus according to the invention, by means of which an electrochemical deoxygenation of water or aqueous solutions is performed, comprising a flow-through cell divided by means of an ion-exchange membrane into one or several anode chambers provided with a degassing orifice and insoluble anodes and into one cathode chamber filled substantially entire¬ ly with a particle bed in a direction perpendicular to the water flow direction and in large part or pre¬ ferably substantially entirely in the water flow di¬ rection, and a filter screen in front of an output of the cathode chamber to prevent an outflow of bed ma¬ terial, is characterized in that the cathode chamber extends in the flow direction of the flow-through cell further than the anode chambers and is filled with an electrically conductive cathode particle bed, which consists of mechanically wear-resistant parti- cles, which are smaller than 5 mm, preferably about 0.2 to 1.2 mm, and which are metal particles or met¬ al, ceramic or polymeric particles coated with a suf¬ ficiently thick metal layer. The cathode particle bed usable in this appara¬ tus, the ion-exchange membrane for separating the bed from the anode chambers and the anodes have been de¬ scribed above in connection with the process accord¬ ing to the invention. The invention will be explained in the follow¬ ing in more detail with reference to the attached drawing, in which a figure shows schematically one preferred embodiment of the apparatus.

According to the figure of the drawing, a pre- ferred embodiment of the apparatus comprises a flow- through cell 1 divided by means of an ion-exchange membrane 2 and 3 into two anode chambers 5 provided with a degassing orifice 4 and insoluble anodes 6 and into one cathode chamber 7 filled substantially en- tirely with a particle bed 8 in a direction perpendi¬ cular to the water flow direction and in large part or preferably substantially entirely in the water flow direction. In this embodiment of the apparatus, the cathode chamber extends in the flow-through di- rection of the cell further than the anode chambers to guarantee that the oxygen seeping from the anode chamber through the membrane into the cathode chamber will be finally removed. In front of an output 9 of the cathode chamber, there is a filter screen 10 pre- venting an outflow of bed material.

The cathode and anode materials as well as the ion-exchange membranes have been discussed in connec¬ tion with the description of the process already earlier. The following examples describe the invention more specificly. Example 1

A deoxygenation on a laboratory scale was per¬ formed as follows. A flow-through cell of acrylic resin was con¬ structed for experiments. The cell was divided by means of a membrane into a cathode chamber and two anode chambers in such a way that water was flowing only through the cathode chamber. The volume of the cathode used was 100 cm³ of packed bed, which consisted of copper granules of 0.2 to 1.2 mm (Outokumpu Oy). The total surface area of the copper granules was about 35 m². The bed was very compact. The pressure loss was nevertheless only 0.02 bar.

As an anode were used two titan plates of 70 cm² coated with an iridium-based oxide. The anodes were placed in two separate anode chambers at a distance of 0.5 cm from the cathode. A Nafion-417 cation-exchange membrane was used for the separation of the cathode and anode chambers. In addition to the cations, also hydroxyl ions were capable of penetrating said membrane.

In all experiments, the cell was filled with water in such a way that no air was left therein. A water flow of 2 cm³/s (=2 g/s) and a cell voltage of about 2 V were adjusted. The oxygen content in the oxygen-saturated water entering the cell was about 8500 μg/kg water. The oxygen-saturated water was manufactured by mixing distilled water in an open vessel. To improve the electrical conductivity, 0.05 M of Na₂S0₄ was added to the water.

Experiments were performed on the cell, in which the cell voltage was varied and the oxygen con- tent in the water flowing out of the cell was measur¬ ed. At the same time, the part of secondary reactions was examined. The following results were obtained:

At the measurement of the oxygen content, the limit of observation was 3 μg/kg, and at the measure¬ ment of hydrogen peroxide, the limit of observation was 100 μg/kg. The copper content in the water flown through the cell was also measured. It did not exceed the limit of observation 100 μg/kg in any experiment.

From the results is seen that the cell removes over 99.95 % of the oxygen dissolved in the water. The amount of hydrogen peroxide generated is negli¬ gible. At low voltages, a generation of hydrogen is also rather insignificant. At too low voltages, the oxidation of copper will be a problem. A suitable operating voltage is 1.9 to 2.0 volts. Example 2

The experiment of Example 1 was repeated, but a slightly coarser bed was used for forming the cath¬ ode, which bed consisted of copper wire cut to pieces (length 2 to 5 mm and diameter 0.5 mm; Outokumpu Oy). The total surface area of the copper particles was about 5 m². The following results were obtained:

By using this coarser copper bed, the deoxyge¬ nation was not quite as efficient as by using the finer copper bed according to Example 1.

Example 3

The cell described in Example 1 was used also in long-term experiments. During four days, oxygen- saturated water was brought into the cell (about 8300 μg oxygen/kg water) . The oxygen content in the water flowing out of the cell continued to be less than 3 μg/kg. The cell voltage was 2.0 volts.

Reference example 1

The experiment of Example 1 was repeated, but carbon in the form of crushed graphite was used for forming the cathode bed, the particle size of which carbon was 1 to 2 mm (ElectroCell AB) . The following results were obtained:

Graphite tends to generate hydrogen peroxide, which is the reason why it cannot be used for deoxy¬ genation, for hydrogen peroxide is also a strong oxi- dizer and a strong cause for corrosion. The possibilities of using electrochemical de¬ oxygenation are not restricted to the laboratory scale. Electrochemical deoxygenation can best be ap¬ plied to heating, plumbing and air conditioning tech¬ niques and to district heating networks. Other appli- cations are water circulations of the industry and power plants and the manufacture of soft drinks.

Claims

Claims :

1. An electrochemical process for a deoxygena¬ tion of water or aqueous solutions, c h a r a c - t e r i z e d in that the oxygen dissolved in water or an aqueous solution is removed electrochemically in a threedimensional electrically conductive cathode particle bed of a cell, the cell voltage of which is 1.9 to 2.0 volts, which bed consists of mechanically wear-resistant particles, which are smaller than 5 mm, preferably about 0.2 to 1.2 mm, and which are metal particles or metal, ceramic or polymeric parti¬ cles coated with a sufficiently thick metal layer, through which bed the water or the aqueous solution to be treated is led and which bed is separated by means of an ion-exchange membrane from all anode chambers provided with insoluble anodes.

2. A process according to claim 1, c h a r ¬ a c t e r i z e d in that the water or the aqueous solution to be treated is led through a particle bed consisting of copper particles.

3. A process according to claim 1 or 2, c h a r a c t e r i z e d in that the water or the aqueous solution to be treated is led through a par- ticle bed the surface area of which is more than

5 m²/100 cm³ of packed bed, preferably about 35 m²/100 cm³ of packed bed.

4. An apparatus, by means of which an electro¬ chemical deoxygenation of water or aqueous solutions is performed, comprising a flow-through cell (1) di¬ vided by means of an ion-exchange membrane (2, 3) into one or several anode chambers ( 5) provided with a degassing orifice (4) and insoluble anodes (6) and into one cathode chamber (7) filled substantially en- tirely with a particle bed (8) in a direction perpen- dicular to the water flow direction and in large part or preferably substantially entirely in the water flow direction, and a filter screen (10) in front of an output (9) of the cathode chamber to prevent an outflow of bed material, c h a r a c t e r i z e d in that the cathode chamber extends in the flow di¬ rection of the flow-through cell further than the anode chambers and is filled with an electrically conductive cathode particle bed, which consists of mechanically wear-resistant particles, which are smaller than 5 mm, preferably about 0.2 to 1.2 mm, and which are metal particles or metal, ceramic or polymeric particles coated with a sufficiently thick metal layer.

5. An apparatus according to claim 4, c h a r a c t e r i z e d in that the particle bed consists of copper particles.

6. An apparatus according to claim 4 or 5, c h a r a c t e r i z e d in that the surface area of the particle bed is more than 5 m²/100 cm³ of packed bed, preferably about 35 m /100 cm³ of packed bed.