Article pubs.acs.org/IECR
Electrochemical Oxygen Removal from Seawater in Industrial Scale Using Silver Cathode Utsav R. Dotel,†,‡ Kai Vuorilehto,§ Magne O. Sydnes,† Hans Urkedal,‡ and Tor Hemmingsen*,† †
Department of Mathematics and Natural Science, University of Stavanger, NO-4036 Stavanger, Norway Deox AS, Professor Olav Hanssens vei 7A, NO-4021 Stavanger, Norway § Department of Chemistry, Aalto University, 02150 Espoo, Finland ‡
S Supporting Information *
ABSTRACT: In this study, electrochemical removal of oxygen from seawater in industrial scale is demonstrated. A test rig with an industrial scale electrochemical cell has been constructed and tested with filtrated oxygen rich seawater. The electrochemical cell was comprised of a silver mesh cathode and an iridium oxide anode with a cation exchange membrane. The effects of flow rate, pressure, and applied voltage on oxygen removal efficiency and resulting current were studied. Also, the differential pressure between the anode and cathode chambers affected the performance of the cell, and an overpressure of 0.20−0.30 bar in the anode chamber was optimal in order to obtain maximum oxygen removal. It was possible to achieve an oxygen concentration lower than 5 ppb in seawater at a flow rate of 5 L min−1. No scaling or biofilm problems were observed during a 200 h test period.
1. INTRODUCTION Seawater is pumped at offshore platforms into injection wells to enhance the oil recovery from reservoirs. The oxygen concentration in injected seawater has to be reduced to ppb concentrations in order to avoid corrosion problems in the pipelines.1 Oxygen is normally removed by the use of physical methods, such as gas purging (normally nitrogen) or vacuum towers. Chemical methods, such as addition of oxygen scavengers (sulfites and metasulfites), are mainly used as a backup for the physical process in case of malfunction or in order to remove the last traces of oxygen. Installation of vacuum towers or gas purging facilities has a high capital cost and a large footprint, while the use of sulfite based oxygen scavengers may have a negative impact on the environment and, if overdozed, also gives a high risk of sulfide corrosion of the piplines.2 The use of an electrochemical process for the removal of oxygen from seawater is an alternative method that avoids the disadvantages described for the other methods.3−5 The oxidation of water to yield hydroxium ion and oxygen, and the reduction of oxygen to water are the basis for deoxygenation of water. The main reactions at the two electrodes are two opposite directed reactions: Oxidation reaction at anode:
The cation exchange membrane (CEM) allows the passage of cation from the anode compartment to the cathode compartment. In our recent study, the effect of salt concentration, flow rate, and voltage was examined with respect to oxygen removal in a laboratory scale cell (L: 51 cm, D: 3 cm, W: 31 cm).5 Different electrode materials, such as copper, graphite, and nickel, have been tested in laboratory scale as the cathode material for oxygen reduction; however, associated problems, such as corrosion and hydrogen peroxide production, limit their use for oxygen removal.3 Platinum is the best candidate for oxygen removal; however, the higher cost and the lower availibity are major reasons for not using platinum in large scale. In the laboratory scale cell, the cathode material was a packed bed made of silver spheres. In that cell, seawater was deoxygenated, giving oxygen concentration lower than 5 ppb at flow rates of up to 750 mL min−1. The silver spheres (silver plated on brass spheres) were found to act as an effective cathode material in the laboratory scale cell. However, due to the large mass to size ratio of the cathode material, 159 mg cm−2, the silver spheres are not a suitable material for large scale cells. The use of numerous silver spheres also makes the operation and maintenance of the cell cost and time demanding, which is a very important issue to keep in mind for industrial applications. In addition, the potential difference between brass and silver is more than 0.15 V in seawater, which
2H 2O → 4H+ + O2 + 4e−
Received: Revised: Accepted: Published:
Reduction reaction at cathode: 4H+ + O2 + 4e− → 2H 2O © 2017 American Chemical Society
8954
May 15, 2017 July 12, 2017 July 13, 2017 July 13, 2017 DOI: 10.1021/acs.iecr.7b02004 Ind. Eng. Chem. Res. 2017, 56, 8954−8960
Article
Industrial & Engineering Chemistry Research
anode grid was 50 cm × 100 cm with a thickness of 1 mm. In our previous study a similar anode was applied, and it was found to be an efficient material for oxygen removal.5 Furthermore, the high corrosion resistance of this material makes it suitable as a durable anode.7 A mesh of Monel Alloy 400 (UNS N04400) electroplated with silver was used as cathode (Figure 2). The size of the mesh was 90, with a wire
might result in galvanic corrosion in the case of silver erosion.6 Considering these issues, a different type of cathode material is tested in this study. Thus, this study has focused on a scale up version based on the existing laboratory test cell for industrial use. The current tests were performed under realistic conditions onshore using seawater as both anolyte and catholyte, and using silver plated Monel mesh as the cathode material.
2. MATERIALS AND METHODS 2.1. Electrochemical Oxygen Removal Cell. The design of the electrochemical cell is based on experiences from the lab scale setup in our previous work.5 The cell was designed in such a way that a uniform flow of water was ensured throughout the cathode section. The cell has a flat plate rectangular construction with two side plates and two membranes making up three chambers (Figure 1). The chambers between the side
Figure 2. SEM image of the silver plated Monel mesh.
Figure 1. (a) Dimensions of the electrochemical cell and (b) side-view of the electrochemical cell.
diameter of 0.08 mm. The thickness of electroplated silver on the Monel was 1 μm. The electrochemical characterization of the silver mesh using cyclic voltammetry (CV), linear sweep voltammetry (LSV), and chronoamperometry in a threeelectrode cell is presented in the Supporting Information (SI). Lewatit MC-3470 was used as CEM in order to allow positive ions to pass from the anode to the cathode chamber. According to the specification for the membrane, the membrane stability was in the pH range from 1 to 10 and at maximum temperature of up to 80 °C. The maximum water permeability through the membrane was 269 mL h−1 m−2 at 0.34 bar.8 The membrane was immersed in seawater for at least 2 h before being installed in the cell. 2.2. Experimental Setup and Procedure. The setup for the electrochemical cell and the testing rig is shown in Figure 3. A Manson HCS 3300 power supply was used to regulate and maintain the voltage between the anode and the cathode. The KNICK oxygen sensor (detection limit 1 ppb) was used to monitor the inlet and outlet oxygen concentration at the cathode. The flow rate was measured using a Gems sensor with a measurement range of 2 L min−1 to 30 L min−1. The pressure was measured using a Gems sensors pressure transducer. JUMO tecLine Cl2 sensors (measurement range 1−10 ppm) were used for measuring free chlorine. Seawater from 10 m depth in Risavika, Norway, was used as both anolyte and catholyte. The seawater was filtered through a 5 μm filter prior to entering the cell. The temperature and conductivity of the water were recorded throughout the experiments. The flow rates of the electrolytes were regulated with the valves in front of the inlet to the cell. Similarly, valves after the outlet of the cell were used in order to control the flow rate and the pressure in the anode and cathode chambers. A scheme of the rig is given in Figure 3.
plate and the membrane were the anode chambers, and the chamber between the two membranes was the cathode chamber. The side plates of the cell were made up of polyvinyl chloride (PVC). The PVC surface was smooth in order to allow uniform water flow through the cell. Two cation exchange membranes (CEMs) were placed between the PVC plates, and the anode gasket made up of rubber was used between the CEM and the PVC. The frame and frame gasket made up of polycarbonate/acrylonitrile butadiene styrene (PC/ABS) and rubber, respectively, were used between two membranes as spacer. The dimension of the cell is presented in Table 1. Table 1. Dimension of the Electrochemical Cell (cm) Cell dimension (L × W × D) Cathode length (Lc) Cathode width (Wc) Cathode depth (Dc)
133 × 62 × 13.6 100 50 0.8
The inlets for electrolytes (anolyte and catholyte) in the cell were at the bottom, and the outlets were at the top as shown in Figure 1. Two plates of stainless steel were installed as support walls for the cell. The present cell is provided with only one cathode chamber, and the design is such that there is a possibility of having multiple stacks of anode and cathode chambers with the same side plates and the same inlet and outlet. The total mass of the cell, without electrolyte and electrode materials, is approximately 147 kg. Titanium grade 1 alloy coated with iridium−tantalum based coating was used as the anode material. The dimension of the 8955
DOI: 10.1021/acs.iecr.7b02004 Ind. Eng. Chem. Res. 2017, 56, 8954−8960
Article
Industrial & Engineering Chemistry Research
study the corrosion and erosion of the silver plated Monel mesh. In addition, the CEM was analyzed before and after the test in order to observe changes on the surface or detect biofilm formation.
3. RESULTS AND DISCUSSION In our former study, it was found that the depth of the cathode was an important parameter in order to optimize the cell, and as small as possible depth gave the best results.5 A cathode depth of 8 mm between the two anodes was found to be an optimal depth, and the same depth was used for the cell described herein. Silver plated Monel mesh was used as cathode material in this test, since it is easier to handle than silver spheres and makes the operation and maintenance work more convenient. It was also found economically benificial to use electroplated mesh compared to the numerous electroplated spheres or silver particles. The electrochemical characterization of silver plated Monel mesh in 0.1 M NaOH (see Figure S1 in SI) showed the onset potential for oxygen reduction to be 0.83 V vs reversible hydrogen electrode (RHE). The chronoamperometry test for 15 h demonstrated the stability of the silver mesh. Sodium sulfate or nitric acid was used as anolyte in our previous study,5 but due to the requirement of large volumes of anolyte and easy handling, seawater was used as anolyte in the industrial scale cell. The temperature of the seawater used during the operational period was in the range of 12 ± 1 °C. 3.1. Anodic and Cathodic Reactions. At the anode chamber, chloride is oxidized to chlorine and water is oxidized to oxygen, giving an acidic environment. The charge gradient formed is compensated by migration of hydroxium and sodium cations through the membrane. Theoretically, in a nonchloride solution, the amount of oxygen evolved in the anode will be equal to the amount of oxygen reduced in the cathode. However, in a real experiment using seawater, both oxygen and chlorine gas may be formed at the anode as shown in eqs 2 and 5. At an applied voltage of 3 V between the anode and cathode, chlorine and oxygen were formed in the anode chamber. There have been several theoretical studies based on density functional theory (DFT) calculations for oxygen and chlorine evolution on metal oxides.9−14 The studies show that the electrochemical potential for oxygen evolution is always higher than the potential for chlorine evolution for the oxides demonstrating oxygen adsorption energies,9 and this is also the case for iridium oxide.15 The possible reactions in the deoxygenation cell with standard reduction potentials (SHE) are shown in eqs 2−6.
Figure 3. Schematic outline of the process diagram for the test rig.
The theoretical current (Itheor) for the cell at different flow rates was calculated using eq 1, where CO2 is the mass fraction of dissolved oxygen in the water (g g−1), ZO2 is the number of electrons required per oxygen molecule (4 e−), F is Faraday’s constant (96500 A s mol−1), m is the mass flow rate (g s−1), and MO2 is the molecular weight of oxygen (g mol−1). The obtained theoretical current for different flow rates is presented in the SI (see Table S1). Itheor =
CO2ZO2Fm MO2
(1)
The voltage was regulated to obtain the desired current in the cell. The voltage was chosen as control parameter in order to avoid unncessary side reactions. All voltages reported here are uncompensated for IR-drop. The voltage drop during the cell operation is calculated and presented in SI (Table S2). Seawater was pumped through the anode and cathode chambers of the cell at different flow rates, and the effect of the various flow rates on the oxygen removal efficiency was studied. The current and final oxygen were monitored as a function of the flow rate and applied voltage. Similarly, the effect of anode and cathode pressure at the outlet cell in terms of cell performance for oxygen removal was also registered. Seven or ten sheets of silver mesh were used as cathode in the cell during the experiments. The number of sheets in the cathode were varied in order to find the optimum number silver mesh required to obtain an efficient oxygen removal. The silver mesh was studied before and after the experiments in order to evaluate the performance of the mesh. The mass loss per unit area of the mesh was measured after the experiment. The mesh was also analyzed by SEM in order to
Cl 2 + 2e− ⇌ Cl−
E° = 1.358 V
2H+ + O2 + 2e− ⇌ H 2O2 H 2O2 + 2H+ + 2e− ⇌ 2H 2O
4H+ + O2 + 4e− ⇌ 2H 2O
2H+ + 2e− ⇌ H 2
E° = 0.695 V E° = 1.776 V
E° = 1.229 V
E° = 0.000 V
(2) (3) (4) (5) (6)
Oxygen is removed by being reduced to water in the cathode chamber. The reduction of oxygen on silver follows either a four-electron pathway (eq 5), and has a similar oxygen reduction mechanism and kinetics as found on platinum (see SI for the elementary oxygen reduction reaction pathways),16−18 or a 2 + 2 electrons pathway (eqs 3 and 4), forming first hydrogen peroxide followed by further reduction 8956
DOI: 10.1021/acs.iecr.7b02004 Ind. Eng. Chem. Res. 2017, 56, 8954−8960
Article
Industrial & Engineering Chemistry Research
Figure 4. Current and oxygen concentration as a function of (a) voltage and (b) catholyte flow rate.
Table 2. Effect of Anode Flow Rate on Oxygen Removal Anode flow (L min−1)
Cathode flow (L min−1)
Anode pressure (bar)
Cathode pressure (bar)
Voltage (V)
Current (A)
Final oxygen (ppb)
2.70 9.18
2.50 2.70
0.27 0.47
0.07 0.11
2.80 2.80
3.83 4.30
