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Aqueous solution electrolyzer
The form of aqueous solution electrolytic cells can be divided into two categories: diaphragm electrolytic cells and non-diaphragm electrolytic cells. Diaphragm electrolytic cells can be divided into homogeneous membranes (asbestos wool), ionic membranes and solid electrolyte membranes (such as β-Al2O3); non-diaphragm electrolytic cells are further divided into mercury electrolytic cells and oxidation electrolytic cells.
When different electrolytes are used, the structure of the electrolytic cell is also different.
Aqueous solution electrolyzers are divided into two categories: diaphragm and non-diaphragm. Diaphragm electrolytic cells are generally used. Diaphragmless electrolyzers are used in chlorate production and in the production of chlorine and caustic soda by the mercury method. Maximizing the electrode surface area per unit volume can improve the production strength of the electrolytic cell. Therefore, the electrodes in modern diaphragm electrolyzers are mostly vertical. Electrolyzers show different performance and characteristics due to different internal components, such as material, structure, and installation.
Molten Salt Electrolyzer
It is mostly used for the production of low melting point metals. It is characterized by running at high temperature, and should try to prevent the entry of water and prevent the reduction of hydrogen ions on the cathode. For example, when sodium metal is produced, the reduction potential of sodium ions is very negative and the reduction is very difficult. Anhydrous molten salt or molten hydroxide without hydrogen ions must be used to avoid hydrogen evolution at the cathode. For this reason, the electrolysis process needs to be carried out at a high temperature, such as 310 °C when electrolyzing molten sodium hydroxide, and when it contains sodium chloride as a mixed electrolyte, the electrolysis temperature is about 650 °C.
The high temperature of the electrolytic cell can be achieved by changing the electrode spacing to convert the electrical energy consumed by the ohmic voltage drop into thermal energy. When electrolyzing molten sodium hydroxide, iron or nickel can be used for the tank body. Electrolyzing molten electrolyte containing chloride often causes the anode to generate wet chlorine gas due to the inevitable introduction of a small amount of water into the raw material, which has a strong corrosive effect on the electrolytic tank. Electrolyzers for electrolyzing molten chloride generally use ceramic or phosphate materials, and iron can be used for parts not affected by chlorine. The cathode and anode products in the molten salt electrolysis cell also need to be properly separated, and should be drawn out of the cell as soon as possible to prevent the cathode product metal sodium from floating on the surface of the electrolyte for a long time, which will further interact with anode products or oxygen in the air .
Non-aqueous electrolytic cell
Because the non-aqueous solution electrolyzer is often accompanied by various complex chemical reactions when producing organic products or electrolyzing organic substances, its application is limited, and its industrialization is not much. The organic electrolyte generally used has low conductivity and low reaction speed. Therefore, a lower current density must be used, and the electrode spacing should be minimized. Electrode structures using fixed or fluidized beds have a larger electrode surface area, which increases cell productivity.
Classification by electrode connection
Electrolyzers can be divided into unipolar and bipolar electrolyzers according to the connection method of electrodes. The electrodes of the same polarity in the unipolar electrolytic cell are connected in parallel with the DC power supply, and the polarities of the two sides of the electrodes are the same, that is, they are both anodes or cathodes at the same time. The electrodes at both ends of the bipolar electrolytic cell are respectively connected with the positive and negative electrodes of the DC power supply to become the anode or the cathode. When the current flows through the electrolytic cell through the electrodes in series, one side of each electrode in the middle is the anode and the other side is the cathode, so it has bipolarity. When the total electrode area is the same, the current of the bipolar electrolytic cell is smaller, the voltage is higher, and the investment of the required DC power supply is less than that of the unipolar type. The bipolar type generally adopts the structure of a filter press, which is relatively compact. However, it is easy to leak and short circuit, and the slot structure and operation management are more complicated than the unipolar type. The cross-section of the unipolar electrolytic cell is generally rectangular or square, the cylindrical shape occupies a large area, the space utilization rate is low, and the use is less.
The distance between the cathode and anode is one of the important factors affecting the cell voltage. As the electrode 
spacing increases, the ohmic voltage drop in the slot increases, and the slot voltage increases. Especially when working with high current, this voltage loss is more serious. Modern electrolytic cells use various measures to reduce the pole spacing, such as the use of diffusion anodes and modified diaphragms to make zero pole spacing electrolytic cell structures.
The residence time of the electrolyte in the electrolytic cell not only affects the production capacity of the equipment, but also affects the current efficiency of the electrolysis process in some cases, such as the production of sodium chlorate by electrolysis, due to the intermediate products of hypochlorous acid (HClO) and secondary The chemical reaction speed between chlorate ions (ClO3) is very slow. If it stays in the electrolytic cell for a long time, it will not only reduce the utilization rate of the electrolytic cell, but also the hypochlorite ions will oxidize on the anode surface or reduce on the cathode surface, reducing the current efficiency. . As a result, modern cell designs seek to reduce the volume and allow the electrolyte to flow rapidly along the electrodes. If further reactions are required, a separate chemical reactor can be installed outside the electrolysis cell.
The electrodes in the electrolytic cell are compactly installed vertically, the conductive plates are easy to connect, and it is beneficial to reduce the bubble effect. Because gas bubbles are often attached to the surface of the electrode with gas evolution, the working surface area of the electrode will be reduced. In addition, the solution near the electrode will also be filled with bubbles, increasing the resistance of the solution, a phenomenon called "bubble effect". However, in the vicinity of the vertical electrode surface, the characteristics of high aeration, low solution density and fast rising speed in the solution can be used to form a natural circulation of the electrolyte, so that the bubbles can accelerate away from the electrode surface and reduce the bubble effect. When the vertical electrode is used as a gas electrode, the shape of the electrode is mostly mesh, which not only increases the working surface area, but also facilitates the escape of air bubbles.
Electrolyzer materials can be steel, cement, ceramics, etc. Steel is alkali resistant and is the most widely used. For highly corrosive electrolytes, the inside of the steel tank is lined with lead, synthetic resin or rubber.
At present, electrolyzers are developing in the direction of large capacity and low energy consumption. The bipolar electrolyzer is suitable for large-scale production, and has been used in the electrolyzed water and chlor-alkali industries successively.
Water electrolysis hydrogen production electrolyzers mostly use iron as the cathode side and nickel as the anode side of the series electrolyzers (like a filter press) to electrolyze the aqueous solution of caustic potash or caustic soda. Oxygen from the anode and hydrogen from the cathode. The cost of this method is high, but the product purity is high, and hydrogen with a purity of more than 99.7% can be directly produced.
