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发表于 2008-7-17 14:56:28
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Ash content
It reduces the overall activity of activated carbon. It reduces the efficiency of reactivation. The metals (Fe2O3) can leach out of activated carbon resulting in discoloration. Acid/water soluble ash content is more significant than total ash content. Soluble ash content can be very important for aquarists, as ferric oxide can promote algal growths, a carbon with a low soluble ash content should be used for marine, freshwater fish and reef tanks to avoid heavy metal poisoning and excess plant/algal growth.
Carbon tetrachloride activity
Measurement of the porosity of an activated carbon by the adsorption of saturated carbon tetrachloride vapour.
Particle size distribution
The finer the particle size of an activated carbon, the better the access to the surface area and the faster the rate of adsorption kinetics. In vapour phase systems this needs to be considered against pressure drop, which will affect energy cost. Careful consideration of particle size distribution can provide significant operating benefits.
Examples of adsorption
Heterogeneous catalysis
The most commonly encountered form of chemisorption in industry, occurs when a solid catalyst interacts with a gaseous feedstock, the reactant/s. The adsorption of reactant/s to the catalyst surface creates a chemical bond, altering the electron density around the reactant molecule and allowing it to undergo reactions that would not normally be available to it.
Adsorption refrigeration
Adsorption refrigeration and heat pump cycles rely on the adsorption of a refrigerant gas into an adsorbent at low pressure and subsequent desorption by heating. The adsorbent acts as a "chemical compressor" driven by heat and is, from this point of view, the "pump" of the system. It consists of a solar collector, a condenser or heat-exchanger and an evaporator that is placed in a refrigerator box. The inside of the collector is lined with an adsorption bed packed with activated carbon absorbed with methanol. The refrigerator box is insulated filled with water. The activated carbon can adsorb a large amount of methanol vapours in ambient temperature and desorb it at a higher temperature (around 100 degrees Celsius). During the daytime, the sunshine irradiates the collector, so the collector is heated up and the methanol is desorbed from the activated carbon. In desorption, the liquid methanol adsorbed in the charcoal heats up and vaporizes. The methanol vapour condenses and is stored in the evaporator.
At night, the collector temperature decreases to the ambient temperature, and the charcoal adsorbs the methanol from the evaporator. The liquid methanol in the evaporator vaporizes and adsorbs the heat from the water contained in the trays. Since adsorption is a process of releasing heat, the collector must be cooled efficiently at night. As mentioned above, the adsorption refrigeration system operates in an intermittent way to produce the refrigerating effect.
Helium gas can also be 'pumped' by thermally cycling activated carbon 'sorption pumps' between 4 kelvins and higher temperatures. An example of this is to provide the cooling power for the Oxford Instruments AST series dilution refrigerators. 3He vapour is pumped from the surface of the dilute phase of a mixture of liquid 4He and its isotope 3He. The 3He is adsorbed onto the surfaces of the carbon at low temperature (typically <4K), the regeneration of the pump between 20 and 40 K returns the 3He to the concentrated phase of the liquid mixture. Cooling occurs at the interface between the two liquid phases as 3He 'evaporates' across the phase boundary. If more than one pump is present in the system a continuous flow of gas and hence constant cooling power can be obtained, by having one sorption pump regenerating while the other is pumping. Systems such as this allow temperatures as low as 10 mK (0.01 kelvin) to be obtained with very few moving parts.
Applications
Activated carbon is used in gas purification, gold purification, metal extraction, water purification, medicine, sewage treatment, air filters in gas masks and filter masks, filters in compressed air and many other applications.
One major industrial application involves use of activated carbon in metal finishing field. It is very widely employed for purification of electroplating solutions. For example, it is a main purification technique for removing organic impurities from bright nickel plating solutions. A variety of organic chemicals are added to plating solutions for improving their deposit qualities and for enhancing properties like brightness, smoothness, ductility, etc. Due to passage of direct current and electrolytic reactions of anodic oxidation and cathodic reduction, organic additives generate unwanted break down products in solution. Their excessive build up can adversely affect the plating quality and physical properties of deposited metal. Activated carbon treatment removes such impurities and restores plating performance to the desired level.
Environmental applications
Carbon adsorption has numerous applications in removing pollutants from air or water streams both in the field and in industrial processes such as:
Spill cleanup
Groundwater remediation
Drinking water filtration
Air purification
Volatile organic compounds capture from painting, dry cleaning, gasoline dispensing operations, and other processes
Medical applications
Activated carbon is used to treat poisonings and overdoses following oral ingestion.
It is thought to bind to poison and prevent its absorption by the gastrointestinal tract. In cases of suspected poisoning, medical personnel either administer activated charcoal on the scene or at a hospital's emergency department. Dosing is usually empirical at 1 gram/kg of body weight, usually given only once. Depending on the drug taken, it may be given more than once. In rare situations activated charcoal is used in Intensive Care to filter out harmful drugs from the blood stream of poisoned patients. Activated carbon has become the treatment of choice for many poisonings, and other decontamination methods such as ipecac-induced emesis or stomach pumps are now used rarely.
While activated carbon is useful in an acute poisoning situation, it has been shown to not be effective in long term accumulation of toxins, such as with the use of toxic herbicides.[2]
Mechanisms of action:
Binding of the toxin to prevent stomach and intestinal absorption. Binding is reversible so a cathartic such as sorbitol may be added as well.
It interrupts the enterohepatic circulation of some drugs/toxins and their metabolites
Allows certain drugs/toxins to be drawn out of the blood and bind to the charcoal in the intestine - a kind of "gut dialysis"
Incorrect application (e.g. into the lungs) results in pulmonary aspiration which can sometimes be fatal if immediate medical treatment is not initiated.[3] The use of activated charcoal is contraindicated when the ingested substance is an acid, an alkali, or a petroleum product.
For pre-hospital use, it comes in plastic tubes or bottles, commonly 12.5 or 25 grams, pre-mixed with water. The trade names include InstaChar, SuperChar, Actidose, and Liqui-Char, but it is commonly called simply Activated Charcoal.
As an over-the-counter drug, it is often used to treat mild diarrhea.
Gas purification
Filters with activated carbon are usually used in compressed air and gas purification to remove oil vapours, odours, and other hydrocarbons from the air. The most common designs use a 1 stage or 2 stage filtration principle where activated carbon is embedded inside the filter media. Activated charcoal is also used in spacesuit Primary Life Support Systems.
Distilled alcoholic beverage purification
Activated carbon filters can be used to filter vodka and whiskey of organic impurities. Since the activated carbon does not bind well to alcohols, the percentage of ethanol is not significantly affected, but the carbon will bind to and remove many organic impurities which can affect color, taste, and odor. Passing an organically impure vodka through an activated carbon filter at the proper flow rate will result in vodka with an identical alcohol content and significantly increased organic purity, as judged by odor and taste.[4]
Scrubbing Mercury from Stack Gas
Activated carbon, often impregnated with iodine or sulfur, is widely used to trap mercury emissions from coal fired power stations, medical incinerators, and from natural gas at the wellhead. This carbon is a specialty product costing more that $2.00 per pound. However, it is often not recycled.
The mercury laden activated carbon presents a disposal dilemma. If the activated carbon contains less than 260 ppm mercury, Federal regulations allow it to be stabilized (for example, trapped in concrete) for landfilling. However, waste containing greater than 260 ppm is considered to be in the high mercury subcategory and is banned from landfilling (Land-Ban Rule). It is this material which is now accumulating in warehouses and in deep abandoned mines at an estimated rate of 1000 tons per year.
The problem of disposal of mercury laden activated carbon is not unique to the U.S. In the Netherlands this mercury is largely recovered[5] and the activated carbon is disposed by complete burning. |
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