1 General introduction
Carbon molecular sieve (CMS) as a new type of adsorbent has developed rapidly since it was industrialized in the late 1960s. CMS is a special activated carbon, mainly composed of micropores below 1nm and a few large pores. Due to its special microporous structure, it can be adsorbed according to the size and shape of the molecule, thus having the ability to adsorb molecules. Since EmmettL discovered in 1948 that the carbonization of Saran resin (a polymer of ethylene vinylidene and vinylidene chloride) has a sieve effect, a lot of work has been carried out in various countries. In recent years, research in this area has also been carried out in Western Europe, Japan and China. CMS is mainly used in the field of adsorption separation, it has been maturely applied to pressure swing adsorption separation of N2 and O2 in the air. At present, the major companies producing CMS in the world are mainly Kuraray, Takeda, and some Chinese companies, among them, Gamma Gas CMS are widely used on our own nitrogen generators, also export for overseas nitrogen generator manufacturing and equipment updating.
2 Production process
Main production of CMS is as below:
Coconut shell primary carbonization → ball milling → kneading and kneading → extruded strip molding → broken strip granulation → secondary carbonization → activation → aperture adjustment → detection → finished product
Primary carbonization: refers to the method of carbonizing raw materials under suitable pyrolysis conditions under inert atmosphere. Under pyrolysis conditions, various groups, bridge bonds, free radicals and aromatic rings in the raw material molecules have complexly decomposed and polycondensed reaction, which leads to the formation of carbon pores, expansion and shrinkage of pore size. It is suitable for resins with high volatile substances in fruit shells, such as apricot kernel shell, coconut shell, peach kernel shell, etc.
Kneading and extruding strips: After the primary carbonized material is grinded to the required particle size by ball mill, polyethylene glycol is used as an auxiliary agent, phenol resin is a binder, and water is mixed in a kneading machine evenly according to a certain ratio, and the strip is extruded on the pellet making machine. The purpose of kneading is to make the primary carbonized material have a certain viscosity, which is helpful for forming during the extrusion process.
Broken bar granulation: The extruded molding material is dried and sent to the broken bar device to break the bar to the desired particle size. The purpose of the broken bar is to make the length of the particles uniform, so that the particles are activated in the same way, and the product obtained by post carbon deposition has consistent performance.
Secondary carbonization: The secondary carbonization process is to place the dried coconut shell extruded molding material in the atmosphere and prepare the carbonized product under appropriate pyrolysis conditions. During the pyrolysis process, each group, bridge bond, free radical and aromatic rings and other complex decomposition and polymerization reactions occur, and geothermally unstable components are released in the form of volatiles. The purpose is to develop the pores of carbonized products, expand or shrink the pore size. Conditions that affect the performance of secondary carbonized products include carbonization constant temperature time, carbonization final temperature and carbonization heating rate.
Activation: refers to the method of slowly heating treatment under active media conditions on the basis of carbonized land to further increase the surface area, and develop its pore structure. The purpose of activation is to make the active agent react with part of the carbon in the carbonaceous material and the carbon generated by the volatility decomposition of the carbonization process, so that the closed pores and blocked pores can be opened, so that the prepared activated carbon not only has a higher adsorption capacity, but also has higher micropore volume number.
Pore size adjustment: using carbon deposition method, on the basis of gas activation, using hydrocarbons or polymer compounds as pore plugging agent to crack it at high temperature, after gas activation, carbon deposits in the pores of porous materials to plug holes, adjust Pore.
Pore diameter adjustment: Using carbon deposition method, on the basis of gas activation, hydrocarbons or polymer compounds are used as pore plugging agents to crack them at high temperature. After gas activation, carbon deposits in the pores of the porous material to block pores and adjust hole.
3 Key technologies for carbon molecular sieve production line
The indexes of carbon molecular sieve mainly include strength and nitrogen production rate.
Strength index, which has two main factors: a) the particle size of the raw material crushed, b) the extrusion strength of the molding process. It must be ensured that the particle size can be stabilized at 20 microns-this requires precise control of the raw material crushing process, and the requirements for extrusion strength are also very strict, as stable as possible at 8MPa.
Nitrogen production rate, which is the key index of carbon molecular sieve. It is quite complicated to control, and the carbonization temperature, temperature rise rate, deposition time, activation time and other parameters must be accurately controlled. The first problem that must be solved is the thermal insulation of the equipment. If this is not well controlled, it is difficult to provide a stable processing temperature.
Maintaining temperature is just the first step. The key technology is the coherent and continuous combination of each process. Because the temperature control of carbonization, deposition and activation is different, it is necessary to considering the temperature conversion of its automatic transition process, and it is necessary to set up a processing transition space between different programs and realize the conversion of different temperatures in the transition space to ensure the smooth completion of each process.
In addition, solving the stability of carbon molecular sieve processing is also a difficult problem in the production of carbon molecular sieve. This needs to focus on the transport stability, deposition concentration stability, activation condition stability, treatment time stability, stability of the molecular sieve and the pore-filling agent or activator, etc., only in every step when it is stable and reliable can the stability of the quality of the carbon powder sieve be finally guaranteed.
To achieve these, the following key technologies must be solved: the raw material extrusion molding speed is uniform and continuous, and the particle size is uniform; after the molding, the pellet material is transported uniformly and stable; the carbonization, deposition, and activation stations require uniform mixing of the particles and need to be controlled. The vibration frequency need be controlled well, which enables the raw materials to evenly contact the pore-filling agent or activator during the treatment process, so as to achieve the purpose of uniform treatment and stable index. This process also needs to fully deal with the reasonable relationship between longitudinal vibration and lateral transmission, so that the raw materials can ensure both stability and continuity during the processing. The above-mentioned key technologies are interrelated and mutually restrictive. Only when every step is handled well can the final desired performance be achieved. The controlling process of carbon molecular sieve is a comprehensive multi-disciplinary knowledge and extremely complex process. Every step must be carefully designed, and the connection of each step needs to be meticulous. Only in this way can the produced carbon molecular sieve have stable and reliable performance. This is also why many manufacturers even after many years production, the performance of their CMS is still not stable.