Industrial equipment for producing hydrogen and oxygen by electrolysis of water, mainly electrolyzers including separators, scrubbers, alkali filters (currently not required for PEM), pressure regulators (used in pressure systems), as well as gas storage and pressurization equipment. According to the hydrogen purity requirements and the production and sales of hydrogen and oxygen, hydrogen purification devices and hydrogen and oxygen pressurized bottling equipment are also needed. In addition, it is also equipped with a pure water production system and DC power supply equipment. The type and quantity of hydrogen production equipment are determined by the specific conditions and requirements of each enterprise.
The electrolytic cell is the main equipment for the electrolysis of water to produce hydrogen and oxygen. The electrolyte is filled into the cell, and the water is decomposed under the action of direct current. Hydrogen is produced on the cathode surface and oxygen is produced on the anode surface.
1. Basic structure of electrolytic cell
The electrolytic cell is composed of electrode plates, diaphragms, insulating seals, clamping devices and other accessories. Since there are many types of electrolytic cells with different structures and accessories, here we only introduce the structure of common alkaline electrolytic cells for reference.
(1) Electrode plate
1. Type of electrode
The electrolytic cells currently used in industry have various electrode structures, all of which are intended to increase the reaction area, reduce overvoltage, and reduce the gas content of the electrolyte, thereby improving the efficiency of the water electrolytic cell and reducing the interelectrode voltage. Reduce power consumption.
(1) Flat electrode (also the most primitive electrode form, this structure has almost been eliminated at present)
The earliest flat electrodes were made of smooth iron sheets. The current density of the electrolytic cell composed of this kind of electrode is only 200-300A/m2 (the current density is related to other factors in addition to the electrode), and the gas content is very high. Later, after improvements, cast iron electrodes were used, and vertical raised ribs were cast in the middle of the electrode plates, which increased the reaction area and increased the current density to about 800A/m2.
The two round holes in the lower part of the figure are the liquid channel rings that enter the electrolyte, and the two round holes in the upper part are the air channel rings that discharge hydrogen and oxygen respectively. This kind of electrode has a simple structure, low cost, and is relatively easy to install. The disadvantages are that the plates are heavy and require high sand casting, it is also difficult to plate nickel on cast iron, the energy consumption is high, and it is easy to be corroded. It is now very rare in China.
(2)Porous electrode
The bipolar plate of the porous electrode is composed of a main plate (also called a separator) and a cathode, anode and anode plates punched with various hole shapes. Common hole shapes on the secondary plate include round, half-moon, rectangular, olive, etc.
This kind of electrode has many small holes punched on the cathode and anode sub-electrodes. On the surface, it seems to reduce the area of the electrode. However, with proper hole diameter and hole spacing, the punching holes create new side surfaces, which actually increases the working area than before. . In addition, a large amount of gas generated on the secondary electrode during operation can pass through these small holes and enter behind the secondary electrode, which greatly reduces the gas content of the electrolyte between the two adjacent cathodes and anodes, reducing the voltage of the electrolyte. loss, which can further bring the distance between the cathode and the anode closer and reduce the voltage of the electrolysis chamber.
The main and auxiliary poles are fixed with rivets, also called supports. This kind of support leg not only plays the role of fixing the main and secondary plates, but also plays the role of conduction. That is, during operation, the current flows from the cathode side of the bipolar plate to the anode side through the support leg. Therefore, when considering the size and distribution of the support legs, they must have a certain strength and uniform distribution, and must reach the cross-sectional area specified by their rated current. The legs on the cathode side are longer than the anode side. This is because the hydrogen produced by the cathode is twice the oxygen produced by the anode.
The disadvantage of this electrode structure is that it requires two nickel platings during manufacturing, that is, the secondary electrode is electroplated individually first, and then the entire electrode is electroplated after riveting and welding. Once the secondary pole is damaged, it cannot be replaced individually.
The main and secondary poles of some electrodes are fixed with screw nuts. This kind of secondary pole is assembled from several thin plates with many half-moon holes punched out.
Direct spot welding is also used to fix the main and auxiliary electrodes (the anode and auxiliary electrodes are made of pure nickel sheets), and there are 700 welding points per square meter of electrode area. This ensures mechanical strength, balances current distribution, and reduces resistive losses. Although the cost of pure nickel anode is higher, it is reported that it has been used for more than 25 years without corrosion.
(3) Mesh electrode
Directly using metal mesh as the negative and positive electrodes of the electrode has been proven to be an ideal method. Because the meshed sub-electrode not only increases the reaction area, reduces the gas content, but also further reduces the distance between the electrodes, making the electrolytic cell more compact, simple to process and manufacture, and easy to maintain.
From the above data, it can be seen that activated single-layer nickel wire mesh and activated iron wire mesh as cathodes have lower interelectrode voltage. Since the activated wire mesh has poor stability, a single layer of activated nickel wire mesh is used as the cathode and secondary electrode material. In order to prevent the activation layer from falling off easily, the surface roughness treatment should be carried out before the nickel mesh is activated. The anode and auxiliary electrodes are directly made of nickel wire mesh. The main plate of the mesh electrode has many milky protrusions. It is not fixed with the secondary electrode, but is directly assembled into a tank body. The German Lurgi electrolyzer and the DQ pressure electrolyzer produced in my country in recent years both use mesh electrodes.
Regardless of the form, the distance between the main electrode plate and the yin and yang auxiliary electrodes has become closer and closer. (Zero plate gap is the development trend)
2. Electrode materials and surface treatment
The material of the electrode used to be cast iron, but now soft iron is mostly used, and the anode surface of the electrode is plated with metallic nickel (pure nickel is also used) and the cathode surface is activated. The purpose of nickel plating on the anode surface is to protect the anode and reduce the overvoltage of oxygen; the purpose of activation of the cathode surface is to reduce the overvoltage of hydrogen.
(1) Nickel plating of anode
Before nickel plating, the electrode plate should be inspected to see if it meets the design requirements, and there should be no burrs, burrs, indentations, breaks, etc. Then use sandblasting, chemical, electrochemical and flushing methods to remove oil stains and rust on the plates to make the surface meet the requirements before electroplating.
The plates must not use any metal as the bottom layer. This is because the chemical activity of this metal is different from that of iron and nickel. In the presence of electrolyte, galvanic corrosion will occur between them, accelerating the damage to the plates; Additionally, the metal itself may be corroded by the electrolyte.
The electroplating solution for nickel plating is made of nickel sulfate and other reagents. The plate to be plated is used as the cathode and the metal nickel is used as the anode. When direct current is passed, the metal nickel on the anode gradually loses electrons and becomes ions and enters the solution. , the nickel ions in the solution move toward the cathode due to the attraction of the cathode and the repulsion of the anode, and then obtain electrons at the cathode and become metallic nickel attached to the surface of the cathode, which causes the surface of the plate to be plated with a layer of nickel. During electroplating, the appropriate temperature, pH value, current density, voltage, time, etc. must be controlled so that the nickel layer can be evenly and firmly bonded to the plated plate.
The technical requirements for nickel plating are:
1) The coating should be light gray dark nickel.
2) The nickel plating must not have wrinkles, peeling, bubbling, obvious burrs or unplated areas. After plating, it must be strictly protected and the nickel layer must not be scratched, bruised or damaged. Repair plating is allowed for scratches and bumps in individual places, but the firm bonding of the coating and porosity requirements must be ensured.
3) The coating thickness of the anode side of the main plate, the anode auxiliary plate and the rivets or bolts is ≥100μm. You can use a thickness gauge to measure any two points in the center.
4) The bonding strength of the coating is not inspected for plate damage. Other nickel-plated small plates can be used for bending inspection. The bending radius is four times the thickness.
5) The nickel plating layer is required to have no pores or very few pores. Its porosity can be tested with potassium ferricyanide K [Fe(CN)] for the blue point test. The blue point index should not exceed 120 points/100cm2. If the porosity meets the requirements and the thickness of the nickel plating layer is less than 20% of the above indicators, it can still be considered qualified.
6) After nickel plating, sodium carbonate should be used for passivation treatment. Porosity inspection must be performed before passivation treatment. In order to prevent corrosion, other parts of the electrolytic cell must also be nickel plated. The thickness of the plating is: frame, airway tube, airway ring, liquid pipe, liquid ring > 60 μm, rivets, pressure plates, and special washers > 40 μm.
(2) Activation of cathode
In the water electrolysis process, the use of cathode activation can generally reduce power consumption by about 10%. The so-called cathode activation means that the cathode and secondary plates are plated with a layer of nickel bottom layer and then a layer of nickel disulfide activation layer. The method of plating the bottom layer of nickel is the same as that of anode nickel plating, and its thickness is generally about 20 μm. The electroplating solution for plating the active layer is composed of nickel sulfate, sodium thiosulfate (sodium soda), ammonium chloride and other reagents. During the electroplating process, appropriate temperature, pH value, current density, etc. must be controlled. ·
The technical requirements for the activation layer are:
1) When coming out of the tank, the activation layer should be yellow-green and then bronze.
2) There should be no peeling, bubbling, etc. in the activation layer. When doing potential tests, the activation layer must not fall off or fall off slightly in the form of powder.
3) The thickness of the activation layer should be ≥12μ and the minimum should not be less than 5μ. The thickness can be measured with a metallographic microscope.
4) The sulfur and nickel content in the active layer should comply with the ratio of Ni2S2. When doing overpotential testing, the current should not be less than 2000A/m2.
If the nickel plating layer or active layer needs to be re-plated because the quality does not meet the requirements, the original coating should be stripped and then plated again. The stripping solution can be made of sodium cyanide, sodium citrate and sodium nitrobenzene sulfonate.
Plated parts such as plates and frames must be properly kept and placed in a ventilated and dry room to prevent corrosion. Rainwater infiltration must be prevented under any circumstances.
3. Diaphragm
Diaphragm quality requirements
In the electrolytic cell, the cathode produces hydrogen and the anode produces oxygen. If they are not separated, hydrogen and oxygen will be mixed, which will not only fail to achieve the purpose of this production, but will also bring serious dangers. This A diaphragm is required to strictly separate hydrogen and oxygen. The quality of the diaphragm is directly related to the purity of hydrogen and oxygen and power consumption. The requirements for the diaphragm are:
1) Bubbles cannot pass through;
2) Can be wetted by the electrolyte, allowing ions in the solution to pass smoothly;
3) Have sufficient mechanical strength;
4) It will not be corroded by alkali in the electrolyte and has strong chemical stability;
5) Cheap and suitable for industrial use.
In the past, people used nickel foil as a diaphragm. It was made by electroplating and had 800-1400 holes per cm2. Such a diaphragm has high mechanical strength, but is easily damaged by electrochemical action, has a short service life, and is prone to short circuit, and the two poles cannot be as close as possible. At present, separators have basically gone through the process from asbestos cloth separators to PPS, and then to PPS + zirconium dioxide. In the future, there may be the application of inorganic membranes. In fact, there are many possibilities for separator materials on the premise of meeting the above performance requirements.
4. Framework
During the electrolysis process, the hydrogen and oxygen gas produced by the anode and cathode electrodes are separated by a diaphragm. Each chamber is separated by a main plate, so the main plate is also called a partition; the chambers are surrounded by a metal (engineering plastics are also used) frame. The traditional method is that the diaphragm cloth is riveted in the frame, so this kind of frame is also called the diaphragm frame. The current novel structure is that the main plate is welded in the frame, forming a plate-frame combination type. Whether it is a diaphragm frame or a plate frame, its thickness has become thinner and thinner, which means that the distance between the cathode and anode auxiliary poles and the distance between the cathode and anode auxiliary poles has become smaller and smaller.
1. Diaphragm frame
The diaphragms in many electrolysers are riveted into metal frames. The metal frame is made of forged steel or welded by special T-shaped steel. There are sealing lines around both sides so that the electrolyte can be sealed in the tank body. A hole is opened in the upper part of the frame, which is the outlet of hydrogen and oxygen respectively, and the hole in the lower part is the inlet of electrolyte. The surface of the frame is also plated with metallic nickel.
The quality requirements of the diaphragm frame are:
1) There is no accumulation of welding slag at the welding joint of the frame, the surface is smooth, and the sealing line of the welding joint is complete;
2) The sealing line should be undamaged. At least one of the several sealing lines must be intact:
3) The nickel plating layer should be free of defects such as peeling and peeling;
4) The liquid inlet and air outlet should be free of blockage and burrs;
5) The diaphragm should be riveted on the oxygen side and must be tightened during riveting to prevent damage to the diaphragm.
2. Plate frame
The plate frame is a key component of the water electrolyzer. It is formed by welding the main plate in the frame, and the welds must be dense. Since the plates and frames of modern water electrolyzers are relatively thin, require high installation requirements, and operate under high pressure, it is crucial to reduce thermal deformation during plate and frame welding. In terms of processing technology, the tungsten argon arc welding method with high arc heat, concentrated arc column and small heat affected zone is generally used. Two welding guns are used at the same time, and a water-cooled copper pad is applied to the welded parts to accelerate cooling.
The plate and frame form simplifies the tank structure, reduces the number of parts and processing volume, reduces the leakage surface of the tank by 50%, and enhances the sealing of the equipment.
5. Insulating sealing materials and clamping devices
1.Sealing material:
The insulation of the electrolytic cell is divided into two aspects, one is the insulation between the tank body and the ground, and the other is the insulation between the pole pieces. If the tank body is not well insulated from the ground, it will pose an extremely serious threat to the safety of the rectifier equipment and is absolutely not allowed. The insulation resistance value to ground can be calculated based on the required 1000Ω per volt. The insulation between the pole pieces is related to current efficiency and safety issues. Due to poor insulation, leakage will occur, which will prevent this part of the current from producing gas and affect the output. If the leakage is serious, this is a short circuit phenomenon and burnout. Possibility of plates and diaphragms.
The insulator that supports the entire tank body is usually a porcelain insulating seat or electrical insulation board. The insulator that supports the plate and frame is a small porcelain seat or bakelite insulating sleeve. The insulating sealing material between the electrodes (frames) is traditionally made of asbestos rubber sheets, or integrally processed polytetrafluoro gaskets. The latest development is the use of a "cloth-pad-in-one" structure.
2. Locking device
The electrolytic chamber assembly becomes a tank body after being clamped. The clamping device is composed of end plates at both ends, large screws, nuts, spring disks and insulating sleeves. Due to thermal expansion and contraction, the size of the tank body changes from time to time, which relies on the force of the spring plate to keep the tank body in a compressed state. To determine the force of the spring disk, it can be calculated based on the gap between the disks and the deformation curve.
6.Other ancillary parts
1. Airway and fluid channel
The air channels and liquid channels of the electrolytic cell are divided into channels for hydrogen, oxygen and electrolyte inlet. According to their positions in the tank body, they can be divided into two types: external air and liquid channels and internal air and liquid channels.
(1) External air and liquid channels
The air channels and liquid channels of the electrolytic cell located outside the tank are called external air and liquid channels. The air channels and liquid channels installed outside the tank can be divided into two types: ring-shaped and cylindrical.
The annular air channel and liquid channel are composed of steel rings equal to the number of chambers. The steel rings are insulated and sealed with asbestos rubber pads, and the steel rings and the frame are connected with short metal tubes. The disadvantages of this form are that it is difficult to install and has high requirements. It is easy to cause leakage due to thermal expansion and contraction, and it is difficult to repair.
The cylindrical airway is a long steel cylinder with an equal number of short tubes welded to the frame. The pipe head and the frame are connected by bronchial tubes and insulating tubes respectively. The advantage of this form is that the equipment is simple, and installation and maintenance are very convenient. However, if the insulating tube is too short and the tube is filled with electrolyte, and under higher voltage conditions, part of the current will leak from one end of the tank through the electrolyte in the insulating tube to the metal cylinder, and then pass through the cylinder to the other end. One end is passed to the tank body. This situation will not only cause serious loss of current, but also during the conduction process of the electrolyte in the insulating tube, the metal tube heads connecting the two ends of the insulating tube are put into work as the positive and negative electrodes respectively, that is, parasitic electrolysis is generated, causing hydrogen and oxygen gas to be produced at both ends. Causes the total purity of the gas to decrease. If the airway is made into a cylindrical shape, the hydrogen and oxygen branch pipes of each electrolysis chamber must be made into a curved shape, and the elbows must be higher than the airway tube. In this way, the electrolyte in each branch pipe is disconnected and is not easily An electrochemical reaction occurs.
It would be ideal if engineering plastics were used instead of steel to make the air tube and liquid tube, but this plastic must have good alkali resistance, temperature resistance and not easy to age. Flow cylinders can also be made from chlorinated polyether pipes.
(2) Internal air and liquid channels
The air channels and liquid channels of the electrolytic cell are located inside the tank body and are integrated with the tank body, which are called internal air and liquid channels, as shown in the figure below:
This structure moves the gas and liquid channels from outside the tank to the inside of the tank, which better solves the problem of leakage in the external gas and liquid channels due to thermal expansion and contraction.
2.Separator
The hydrogen and oxygen coming out through the airway are accompanied by a large amount of alkali liquid. The function of the separator is to separate the gas and alkali liquid. The separated electrolyte is cooled and filtered and returned to the electrolysis chamber, while hydrogen and oxygen gas enter the scrubber respectively.
The separator is generally made into a cylindrical shape. There is one hydrogen and one oxygen each, and the bottom is connected with a pipe and has a cooling water pipe inside. Therefore, the separator also plays the role of cooling the electrolyte and adjusting the pressure on both sides of the hydrogen and oxygen in the electrolyzer. Some separators stand upright next to the tank as a separate device; some lie horizontally on the tank; some expand the airway tube and also play the role of gas-liquid separation.
3. Scrubber
The hydrogen and oxygen coming out of the separator have relatively high temperatures and contain a lot of water vapor and alkali mist. In order to lower the gas temperature and recover raw material water and alkali, the gas must be cooled and washed. Electrolyzers are generally equipped with two scrubbers, and multiple electrolyzers can share a pair of scrubbers. One is a hydrogen scrubber and the other is an oxygen scrubber, both of which are equipped with cooling water pipes. The pure water supplied to the electrolyzer generally passes through the scrubber first to preheat it. The gas entering the scrubber first flows along the pipe from top to bottom, then turns up from the bottom through the zigzag bubble cap, and then passes through the sieve plate and is washed by pure water. The alkali mist entrained in the gas is washed away, and the water vapor is condensed.
The scrubber must be installed at a certain height so that the washed pure water can flow into the electrolyzer by gravity.
There are two types of scrubbers: vertical and horizontal. In addition to cooling and scrubbing gas and preheating pure water, the scrubber also has the function of adjusting the pressure on both sides of hydrogen and oxygen, because the bottoms of a pair of scrubbers are connected.
4.Filter
In order to eliminate the influence of mechanical impurities (such as iron substances, nickel skin, asbestos fibers, asbestos rubber residues, etc.) in the electrolyte on the electrolysis process, prevent the gas and liquid pipelines of the electrolytic cell from being blocked and avoid causing short circuits and electrolysis in the cell. The tank is generally equipped with an electrolyte filter. The size of the filters, the number of internal layers, and the location of the devices in various electrolytic cells are all determined by specific conditions. There is a filter inside the filter, and the filter generally uses 60-80 mesh nickel wire mesh. The filter needs to be disassembled and cleaned regularly during operation, otherwise excessive accumulation of impurities will clog the filter and affect the circulation of the electrolyte.
There are two types of electrolyte filters: vertical and horizontal. The vertical filter is easy to disassemble and clean, and can also reduce the loss of electrolyte when disassembling and cleaning the filter.
5. Pressure regulator
During the electrolysis process, if the pressure of the electrolytic cell changes greatly, it will often cause the mutual penetration of hydrogen and oxygen. Therefore, the pressure on both sides of the hydrogen and oxygen in the cell must be adjusted at any time during operation. Electrolyzers operating at normal pressure generally rely on wet gas storage cabinets, separators, and scrubbers to adjust the pressure. However, when operating under pressure, a pair of pressure regulators must be installed in the system, one is a hydrogen pressure regulator and the other is a hydrogen pressure regulator. Oxygen pressure regulator, this is because a dry gas storage tank is used to store and regulate the amount of gas during pressure operation, and the pressure in the gas storage tank changes with changes in production and consumption.
There are two types of pressure regulators currently used in China. One is the float regulating valve. The liquids in the hydrogen and oxygen pressure regulators are connected to each other. When the pressure of one gas in the system increases, the liquid in the pressure regulator The water level drops, causing the float ball and valve stem to drop accordingly, and the airway needle valve channel increases accordingly, causing the gas to flow out faster. At the same time, the liquid level of the other pressure regulator rises, and the airway needle valve channel increases accordingly. It shrinks, restricting the outflow of gas until the pressure between the two gases is balanced. The other is a membrane regulating valve. A membrane regulating valve is installed on the outlet pipe of the hydrogen and oxygen separators respectively. The upper end of the diaphragm of the hydrogen membrane regulating valve is connected to the oxygen pressure pipe, and the upper end of the diaphragm of the oxygen membrane regulating valve is connected to the hydrogen pressure pipe. In this way, when the oxygen side pressure decreases, the hydrogen side pressure will push the valve stem of the oxygen membrane regulating valve downward to close the oxygen outlet, and the oxygen side pressure will rise; at the same time, the valve stem of the hydrogen side regulating valve will move upward due to the decrease in oxygen pressure in the upper part of the membrane. Open the hydrogen outlet valve until the pressure of the separator on both sides of the gas and oxygen is balanced. Vice versa is also true.
There are also pneumatic unit combination instruments used to control the hydrogen and oxygen pressure difference.
6.Safe water seal
During the electrolysis process, sometimes the working pressure of the electrolyzer continues to rise due to blockage in the external pipeline. In order to avoid this situation and ensure safe production, a safety water seal should be installed in the system, so that when the pressure in the pipeline exceeds the pressure of the water seal water level, the gas can be automatically discharged into the atmosphere. Small water seals are also made of glass or plexiglass, such as Huafu bottles. The height of the water seal should be more than 50% greater than the gas pressure in the scrubber. If a vent pipe is inserted into the bottom of the water level of the scrubber, it can also serve as a safety water seal. When the pressure on one side of the hydrogen and oxygen increases, the liquid level of the scrubber will drop. When the liquid level drops to a certain value, the gas It will be emptied from the vent pipe to achieve the purpose of safety protection. When the system is operating under pressure, a safety valve is added to the hydrogen and oxygen scrubbers to relieve pressure when overpressure occurs.
