Activated carbon is an amorphous structural material based on graphite crystallites. Its main element is carbon, and some non-carbon elements such as oxygen, hydrogen, nitrogen, sulfur, etc., which are mostly combined at the edge of the carbon network to form surface compounds. . The most important feature of activated carbon is its well-developed pore structure, large specific surface area and adsorption performance, and its specific surface area can be as high as 1000-3000 m2/g. These characteristics of activated carbon determine its strong adsorption capacity for gases, inorganic or organic substances in solution, and colloidal particles. As an excellent adsorbent, activated carbon has unique pore structure and surface active functional groups, stable chemical properties, high mechanical strength, acid, alkali, heat, insoluble in water and organic solvents, and can be regenerated after failure. For pollution control (such as gas purification and water treatment), food processing, chemical, military chemical protection. In addition, activated carbon can also act as a catalyst and catalyst support.
At present, most of the applications are powdered, granular activated carbon and activated carbon fibers. The former is mainly made of natural raw materials such as coal, wood, nut shells, and the like. Although these materials can be successfully used to produce activated carbon with high performance and low cost, these products have many problems. First, due to the structural differences in the batch of natural raw materials, it is difficult to control the structural characteristics of the activated carbon to obtain the same quality product. In addition, when powdered activated carbon and granular activated carbon are used in the packed bed, there are disadvantages such as large pressure drop, channel effect, particle wear, particle entrainment, etc. [6]. For activated carbon fiber, a certain structure and performance can be obtained by controlling the processing conditions, but the application of the activated carbon fiber is limited in application.
In environmental management applications, the carbon materials required must contain a certain pore structure, and the pollutants can be effectively absorbed and removed as they pass through the pores. In general, the higher the utilization rate of carbon materials, the better the treatment effect on pollutants. For example, compared with activated carbon, activated carbon fiber has many advantages. Its specific surface area is larger than that of activated carbon by weight, and its adsorption performance is better than that of activated carbon. However, the adsorption performance of activated carbon fiber is not superior to that of activated carbon because of volume, because the body of activated carbon fiber The density is 0.1~0.2g/cm3, which is less than 0.5g/cm3 of activated carbon. Similarly, when powder or granular activated carbon is used in a packed bed, the adsorption performance of activated carbon in the packed bed is difficult to be fully utilized due to the lower packing density of the activated carbon and the resulting diffusion resistance.
Due to the above problems in the production and application of powdered, granular activated carbon and activated carbon fibers, a new type of high-density carbon material capable of solving these problems has been sought. Activated carbon monolith is a new type of carbon material developed in response to this demand. The bulk activated carbon has reduced pressure, large surface area, rich microporous structure, high density, excellent mass transfer performance, high mechanical strength, good wear resistance, good adsorption performance and fast adsorption and desorption rate. This material is one of the most ideal materials for environmental management.
2 Structural properties and adsorption properties of bulk activated carbon
2.1 Structural characteristics
A carbon block structure material having a high density and a high porosity, which has a high ratio of surface area to volume. Bulk activated carbon can be made into various sizes and shapes according to needs and designs. It mainly has three shapes of cylindrical, elliptical cylindrical and square, and its interior is composed of square, circular, hexagonal and triangular carbon channel structures. . The bulk activated carbon has a large surface area and a rich microporous structure. The surface area and pore volume vary greatly depending on the starting materials and preparation conditions. It has been reported that the maximum surface area per unit support volume of a block-like activated carbon with a ceramic support structure can reach 90,000 m 2 /L, which is 10,000 to 40000 m 2 / of the carbon-ceramic composite structure reported as a catalytic converter for a motor vehicle. L is still much higher. By using a porous ceramic block as a carrier, a bulk activated carbon having a surface area of ​​700 to 1000 m 2 /g can be obtained by immersing in a resin. Coal, wood, coconut shell, peat, etc. can be used as raw materials to prepare a bulk activated carbon with a surface area of ​​400-800 m2/g and a total pore volume of 0.5-1.3 cm3/g. The surface area of ​​the bulk activated carbon prepared by using the cellulose microcrystals as raw materials can reach 1000~2000 m2/g, and the total pore volume and micropore volume can reach 0.4~1.0 cm3/g and 0.4~0.9 cm3/g, respectively.
Under normal circumstances, the surface area of ​​the block activated carbon fiber can reach 500~2000m2/g, but it has been found that the bitumen activated carbon fiber with a surface area of ​​2600m2/g and a micropore volume of 1.23cm3/g can be prepared by using asphalt as a raw material.
The bulk activated carbon with a density of 0.56~0.99g/cm3 can be obtained by changing the pressing force and heat treatment conditions of the raw materials, and the density of 0.6, 0.79, 0.88g/cm3 can be obtained by using coconut shell, peach kernel and coffee beans respectively. Activated carbon block.
Generally, the density of the activated carbon fiber is <0.2 g/cm3, and the bulk activated carbon fiber having a density of 0.2 to 0.86 g/cm3 can be prepared by changing the pressure during the hot pressing of the fiber.
2.2 Adsorption performance
In the adsorption process, the main role is micropores. The adsorption of gas by micropores can be divided into three steps: firstly, the adsorption of the corresponding pore walls at low pressure produces a strong adsorption; secondly, the molecular diffusion caused by the internal shrinkage of the microporous network. The effect; finally, the selectivity of the size and shape of the molecule, preferentially adsorbing the single molecule. The adsorption properties of activated carbon are closely related to its surface area and microporous structure. The bulky activated carbon (fiber) has a large surface area and a rich microporous structure, which makes it have excellent adsorption properties. Even under high flow rate operating conditions, the pressure drop of bulk activated carbon is very low, which is one of the reasons for its good adsorption performance.
In general, the adsorption capacity of the bulk activated carbon (fiber) containing the binder is smaller than that of the bulk activated carbon (fiber) containing no binder. This is because the presence of the binder reduces the carbon content in the activated carbon block and also blocks a portion of the micropores. Therefore, in the case of preparing a bulk activated carbon (fiber) having a binder, it should be considered from the viewpoint of the adsorption capacity, and it is preferable to select an adhesive which does not have a clogging effect on the micropores or has a minimum clogging effect on the pore structure.
The adsorption performance of bulk activated carbon mainly depends on structural parameters and processing parameters. Structural parameters include carbon adsorption porosity, cell wall thickness and carbon content; processing parameters include fluid flow rate, adsorbate concentration, and adsorption potential (the adsorption potential depends on the carbon structure and the molecular weight of the adsorbate). There is also a certain relationship between the amount of adsorption and the rate of loss of ignition. In the case of a high concentration of pollutants, the amount of adsorption increases with the increase of the loss rate, while the bulk of activated carbon with a high rate of loss at a low concentration is a pollutant. The adsorption amount is lower than the loss rate, which is caused by a change in the pore diameter of the micropore. The pores formed at low burnout rates are generally small in pore size and thus exhibit high adsorption performance at low adsorbate concentrations.
At the same porosity, increasing the wall thickness of the pores can increase the carbon content per unit volume of the bulk structure, thereby increasing the adsorption capacity. It was found that [6], when the fluid flow rate was 15000 cm3/min butane concentration was 80×10-6, the adsorption amount of butadiene on the thin-walled (0.19 nm) bulk activated carbon was 127.9 mg, and under the same conditions. The bulky activated carbon with a thick wall (0.29 nm) can absorb 185 mg of butane. At the same fluid flow rate, when the concentration of toluene is 80×10-6, the adsorption amount of toluene on the thin-walled (0.19 nm) bulk structure is 427 mg, and the adsorption amount can be increased when the wall thickness is increased to 0.29 nm. Up to 579mg. Therefore, it is possible to increase the adsorption efficiency of the bulk activated carbon by changing its wall thickness.
3 Preparation of bulk activated carbon
There are four main methods for the preparation of bulk activated carbon: the first direct extrusion molding method, that is, direct extrusion molding of carbonaceous materials; the second hot pressing molding method; the third is carbon coating method, which is made of ceramics as a carrier. A carbonaceous material is applied to the surface to produce a bulk activated carbon; the last method is a sol-gel method.
3.1 Direct extrusion molding
Directly extruded block activated carbon can be divided into two types: binderless and binder. The binderless bulk activated carbon is prepared by first mixing a thermosetting resin (which may be a solid or a liquid) with a carbon-containing organic and inorganic additive, and then extruding through a mold to dry the molded body at a certain temperature. Curing to stabilize the block structure, then carbonizing and activating. Carbonization is mainly carried out in an inert environment. The gas used for carbonization is nitrogen. The activation of bulk carbon can be carried out by physical activation or chemical activation. The common physical activators are air, carbon dioxide and water vapor. The chemical activators are available. Phosphoric acid, zinc chloride and potassium hydroxide. Inorganic and organic additives containing carbon in the preparation process not only facilitate the extrusion molding of the resin, but also can form numerous tiny channels, so that by-products generated during high-temperature carbonization can be sufficiently removed through small holes, thereby avoiding The rupture of the block structure. In addition, activators can also enter the interior of the material through these small holes, allowing the entire structure to be fully activated. Studies have shown that by controlling the material composition of the extruded substrate, activated carbon having a bulk structure of 5% to 100% carbon can be prepared.
The preparation method of the bulk activated carbon with binder is: firstly carbonizing the raw material, grinding the powder into a powder and thoroughly mixing the binder, then extruding and molding, and then pyrolyzing the extruded carbon block at a high temperature to Improve the performance of the binder while reducing the content of the binder in the carbon block, and finally carbonization, activation. Although the binder increases the density of the activated carbon, since the presence of the binder blocks a part of the pores of the activated carbon, it is necessary to select the best binder for the different applications in the preparation of the bulk activated carbon having the binder.
3.2 Hot pressing method
This type of method is mainly for the production of bulk activated carbon fibers. For the bulk activated carbon fiber with binder, the preparation method is as follows: firstly, carbon fiber (usually made of homogeneous asphalt, polyacrylonitrile, rayon and heavy oil) is cut into small sections having a length of less than 1 mm. , and organic binder (usually homogeneous asphalt, phenolic resin, thermosetting resin, etc.) is uniformly mixed in a certain ratio; then pressed into a block under a certain degree and pressure, and finally the carbon fiber block is dried and solidified. , carbonization, activation to obtain the finished product. The disadvantage of this method is that the activated carbon fiber loses its fiber structure characteristics (because the fiber must be shortened), and the manufacturing process is complicated, resulting in high manufacturing cost and low yield.
The binderless activated carbon fiber is usually prepared by directly hot-forming the fiber strand at a certain temperature and pressure; then, the formed fiber block is stabilized at a certain temperature; finally, carbonization and activation are formed. Activated carbon fiber block. The fiber block is simple to prepare and can effectively increase the density of the activated carbon fiber. K. Miura et al. have used bitumen as a raw material, and a high-density activated carbon fiber block having good adsorption properties is obtained by this method.
3.3 Carbon coating method
This method is mainly applicable to the preparation of bulk activated carbon using ceramics as a carrier. The preparation process can be divided into two steps: one is the preparation of ceramic blocks. Materials which can be used as ceramics include cordierite, alumina, clay, and the like. The ceramic material is first thoroughly mixed with the temporary binder, and then dissolved in water or other solvent to make it a plastic body to facilitate extrusion molding; and these plastic compounds are extruded in a mold. Usually, in order to ensure that the moisture in the ceramic block is removed without breaking it, the ceramic block is subjected to a special drying treatment, and finally the dried ceramic block is calcined to a porous structure at a high temperature.
The second is the immersion of carbonaceous materials. The fired porous ceramic block is immersed in a solution containing a carbon material such as resin, polyethylene, coal tar or the like, and dried, solidified, charred, and activated to obtain a bulk structure of activated carbon. The activated structure of the block structure obtained by this method has high strength, good durability, and good adsorption performance. The disadvantage of this method is that the carbon content is limited by the number and structure of the ceramic pores. Even in ceramic structures with a large amount of pores, the carbon content is still small.
In addition, from the cost point of view, the cost of this manufacturing process is also high. This is because this method involves two high-temperature treatment processes: one is to treat the ceramic into a porous structure at a high temperature, and the other is to treat the carbonaceous material again and then treat it at a high temperature into a massive porous Activated carbon. It is necessary to find a preparation method of a bulk activated carbon with high carbon content and low cost, and at the same time, use waste resources such as paper and plastic as much as possible. It has been reported that a low-cost bulk activated carbon having a honeycomb structure based on ceramics has been prepared. The activated carbon of the honeycomb structure is obtained by using fiber paper as a raw material, forming a honeycomb structure, and then modifying it by immersing in asphalt. The honeycomb structure of the activated carbon block obtained by this method is not only low in cost, high in carbon content (nearly 85%), but also has unique properties: rich microporous structure, molecular sieve effect, oxidation resistance, relatively high The pore structure is very stable under the heat treatment conditions.
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