The atoms of activated carbon have a large specific surface area, making them exhibit a strong attraction to the outside. These are called van der Waals forces that attract molecules around the gas or liquid. The combination of these attractive forces and the forces between the molecules in the surrounding medium gives the activated carbon a surface adsorption force. Some molecules have structures that make them easier to absorb than others, and we can separate different molecules based on this principle.
Physical adsorption occurs during the removal of contaminants from gas streams and liquid streams. The porous structure provides a large specific surface area for the activated carbon, making it easy for contaminants to accumulate in the activated carbon. This attraction exists among all the molecules. In this way, the surface molecules of the pore walls have a strong attraction and attract the molecules of the contaminants through the channels of the pores. It must be pointed out that the molecules of the adsorbed contaminants must be smaller than the size of the open pores so that they can pass through the pores and be accumulated. Now, you can understand why we use different raw materials and activation conditions to produce different types of activated carbon with different pore structures, the purpose is to make our products suitable for different purposes.
In addition to physical adsorption, chemical reactions also occur on the surface of carbon. Activated carbon contains not only carbon but also a small amount of hydrogen and oxygen on its surface. These components are in the form of various compounds and functional substances, including carbonyl, hydroxyl, phenol, esters, and benzoquinone. These oxidizing agents and complexes on the carbon surface are capable of chemically reacting with substances adsorbed by the activated carbon. Here is a typical example: in water treatment, activated carbon reacts with chlorine in water to convert chlorine to chloride. In this way, the chlorine is removed, and the unpleasant taste and odor in the water are gone.
Traditional activated carbon regeneration method
1.1 Thermal regeneration method
The thermal regeneration method is currently the most widely used and the most mature activated carbon regeneration method in the industry [2, 3]. The activated carbon after treatment of organic wastewater is generally divided into three stages of drying, high temperature carbonization and activation according to the change of organic matter when heated to different temperatures. In the drying stage, the volatile components on the activated carbon are mainly removed. In the high-temperature carbonization stage, a part of the organic matter adsorbed on the activated carbon is boiled, vaporized and desorbed, and a part of the organic matter is decomposed, and a small molecule hydrocarbon is desorbed, and the residual component remains in the pore of the activated carbon to become a "fixed carbon". At this stage, the temperature will reach 800-900 ?° C, in order to avoid oxidation of activated carbon, generally under vacuum or inert atmosphere. In the next activation stage, a gas such as CO2, CO, H2 or water vapor is introduced into the reaction vessel to clean the activated carbon micropores to restore the adsorption performance, and the activation phase is the key to the entire regeneration process. Although the thermal regeneration method has the characteristics of high regeneration efficiency and wide application range, in the regeneration process, additional energy heating is required, and the investment and operation cost are high.
1.2 Biological regeneration method
The biological regeneration method utilizes domesticated bacteria to analyze the organic matter adsorbed on the activated carbon and further digest and decompose into H2O and CO2 [1, 2]. The biological regeneration method is similar to the biological method in sewage treatment, and there are also aerobic methods and anaerobic methods. Since the pore size of activated carbon itself is very small, and some are only a few nanometers, microorganisms cannot enter such pores. It is generally believed that cell autolysis occurs during regeneration, that is, cell enzymes flow to the extracellular, and activated carbon adsorbs enzymes. Therefore, an enzymatic center is formed on the surface of the carbon, thereby promoting the decomposition of the pollutants and achieving the purpose of regeneration.
The biological method is simple and easy to operate, and the investment and operating costs are low, but it takes a long time and is greatly affected by water quality and temperature. Microbial treatment of pollutants is highly targeted and needs to be specifically domesticated for specific substances. In the degradation process, it is generally impossible to completely decompose all the organic matter into CO2 and H2O, and the intermediate product remains on the activated carbon and accumulates in the micropores, and the regeneration efficiency is significantly reduced after repeated cycles. This limits the industrial application of the biological regeneration method.
1.3 wet oxidation regeneration method
Under high temperature and high pressure conditions, oxygen or air is used as an oxidant to oxidize and decompose organic matter adsorbed on activated carbon in a liquid phase into a small molecule, which is called a wet oxidation regeneration method . The regeneration conditions are generally 200 to 250 ?° C, 3 to 7 MPa, and the regeneration time is usually within 60 min. The wet oxidation regeneration method has a wide range of treatment targets, a short reaction time, and stable regeneration efficiency, and no additional heating is required after the start of regeneration. However, for some refractory organics, more toxic intermediates may be produced.
The Environmental College of Tongji University takes the change of phenol adsorption isotherm as the evaluation standard, systematically studies the main influencing factors in the process of activated carbon wet oxidation regeneration, and discusses its regularity in theory; discusses the synergy between the main factors. The possibility of multiple cycles of saturated carbon regeneration was investigated. The changes of the structure of activated carbon in the wet oxidation process were studied. The optimal regeneration conditions of the activated carbon obtained by the experiment were as follows: regeneration temperature 230 ?° C, regeneration time 1 h, oxygenation pO20.6 MPa, carbon addition 15 g, water addition 300 mL. The regeneration efficiency reached (45 ?± 5)%, and the regeneration efficiency decreased by only 3% after 5 cycles of regeneration. Partial oxidation of micropores on the surface of activated carbon is the main reason for the decrease in regeneration efficiency.
In addition to their respective drawbacks, the traditional activated carbon regeneration technology usually has three common defects: (1) the loss of activated carbon tends to be large during the regeneration process; (2) the adsorption capacity of activated carbon after regeneration is significantly reduced; (3) the regeneration occurs. The exhaust will cause secondary pollution of the air. Therefore, people can either improve on traditional regeneration techniques or explore new regeneration technologies.
Production process and properties of wood activated carbon:
1. The external form of charcoal: The high-quality charcoal section has a black luster, and it emits a loud and crisp metallic sound when struck. The charcoal fired at different temperatures has different external forms. Charcoal fired at less than 250 degrees Celsius, the surface is brown, not easy to break, there is a flame when burning; the surface of charcoal fired at 300-350 degrees Celsius is black, when the firing temperature reaches 500 degrees Celsius, when the tapping, charcoal is issued Loud metal sound.
2. Fixed carbon for charcoal: Fixed carbon is a hypothetical concept. It is ash-free charcoal at the specified high temperature, generally 850-950 ?° C, without air being introduced for calcination. General charcoal may contain 70% to 80% of fixed carbon. As the calcination temperature increases, the relative amount of fixed carbon in the charcoal increases.
3. Volatile matter of charcoal: When charcoal is calcined at high temperature, gaseous products such as carbon monoxide, carbon dioxide, hydrogen, methane and other hydrocarbons are released as volatiles. When the temperature of firing charcoal is within 300-700 degrees Celsius, the composition of volatiles separated during charcoal calcination changes with the following changes: the content of carbon dioxide, carbon monoxide and methane gradually decreases, while the content of hydrogen gradually decreases. increase. When the temperature of the charcoal is increased, the calorific value of the charcoal is increased, and the calorific value of the gas is lowered.
[Method for determination of carbon tetrachloride by activated carbon]
This standard specifies the determination of the adsorption rate (activity) of wood activated carbon carbon tetrachloride.
This standard applies to wood activated carbon.
2, activated carbon method summary
The carbon tetrachloride adsorption rate (activity) of activated carbon is determined by passing carbon tetrachloride-laden air through a sample of activated carbon of known quality under the specified conditions until the mass of the carbon sample is no longer increased, and then measuring the carbon sample. The quality of carbon tetrachloride. The test equipment required for this test consists mainly of a control system that supplies air pressure, a device that removes gaseous and liquid oil and water from the air, and a specific concentration of tetrachloride in the gas stream flowing through the activated carbon sample. The carbon preparation system and the control air mixture (air + quadruple carbon) are composed of a system of carbon sample flow rates.
3, the meaning and application of activated carbon
The activated carbon measured by this method is actually a measure of the pore volume of the activated carbon sample. This method is a means of determining the degree of activation of activated carbon. Therefore, it is also an effective method for quality control of gas phase activated carbon. This activity value is not necessarily a measure of the adsorption efficiency of the activated carbon for other adsorbates or the effectiveness of the other operating conditions.
4, activated carbon device
4.1 Rotameter, LZB-4.
4.2 adsorption tube (see Figure 1)
4.2.1 Perforated plate: The number of holes is 20-24, and the diameter of the hole is 0.3-0.4mm.
4.2.2 Adsorption tube: made of industrial white glass, the appearance requires no bubbles and other obvious problems. There should be no more welds and wrinkles in the glass weld, and the milled end of the pipe should be melted. The inner diameter of the tube inside the scale is in the range of (20?0.3)mm.
4.2.3 Grinding plug: no air leakage is allowed, and the density is good.
4.3 CTC vapor generation bottle (see Figure 2).
4.4 Pressure gauge: Model Y-60Z M10?1, accuracy 0.25, range 0-0.16Mpa.
4.5 Gas drying tower: 250mL.
4.6 porous gas washing bottle; 250mL.
4.7 constant temperature water bath.
5.1 Install the components of the instrument as shown in Figure 3. Install the number of adsorption tubes as needed, but ensure that the flow distribution is even.
5.2 Process Description
Connect the instrument to the compressed air switch. After the compressed air is opened, the air first enters the air purifying bottle filled with activated carbon, and enters the buffer bottle through a dry bottle containing silica gel and a purification bottle containing molecular sieve, and then carbon tetrachloride vapor is generated. The bottle is then passed through a rotameter and a serpentine tube (wound with a proportional tube of 1 m or more) into the adsorption tube.
5.3 Airtight inspection
All components of the instrument and the installed instrument are inspected for air tightness before use. The steps are: a) the relationship cock K3; b) the compressed air is introduced to generate a pressure of 60 Kpa in the system; C) the piston K2 is closed, and the gas drop within 1 min is not more than 2.6 kPa. If it is unqualified, check the cause and repair or replace the non-hermetic parts.
6, activated carbon test conditions
6.1 Carbon layer height: (10?0.2) cm.
6.2 Gas flow rate: (1 670?70) mL/min.
6.3 Adsorption temperature: (25?1) ?°C
6.4 Carbon tetrachloride vapor: (250?10) mg/L.
7, activated carbon test procedures
7.1 Sample preparation
The sample is baked at 105-110 ?° C to constant weight, placed in a desiccator for use. Wipe the adsorption tube and weigh it to m1 (along with the plug, accurate to 1mg). Divide the prepared sample two or three times. Installed into the adsorption tube, the carbon layer is high (10?2) cm (height after tapping). After filling, the tube plug is coated with Vaseline and then weighed to m 2 (accurate to 1 mg). Weighing is completed. Insert the adsorption tube vertically into the constant temperature water bath.
7.2 Carbon tetrachloride vapor generation bottle preparation
Carbon tetrachloride was added from the inlet tube to the carbon tetrachloride generation bottle, and the height of the carbonized carbon in the bottle was about 8 cm. Will GB/T12496.5 -1999
The carbon tetrachloride vapor generating bottle is connected to the system and placed vertically in an ice water bath (0-1 ?° C).
7.3 Process operation
When everything is ready, open the cocks K1, K2, K3, k4, close K5, turn on the compressed air, let the clean and dry air pass through the CTC vapor generation bottle, adjust K1 until the total airflow is stable (1 670?70 )mL/min. After the parameters are stable, open K5, turn off K4, let the carbon tetrachloride-laden air pass through the adsorption tube, and press the stopwatch to ensure the airflow passes at a flow rate of (1 670?70) mL/min. Sample 1h. Then open K4 and close K5. Remove the adsorption tube and wipe the net and weigh m3 (accurate to 1mg). Put the adsorption tube into the device, open K5, close K4, and let the air flow pass through the sample for another 10 min. Repeat the above operation until the adsorption is saturated (the difference between the two weighings is not more than 10 mg). The compressed air is then turned off and the test is over.
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