Charcoal and Activation

Activation. Activation is a process that increases the surtece area of charcoal and other porous carbon materials. These materials, as produced, have a relatively low porosity. Their structure consists of elementary graphitic crystallites with a large number of free interstices between them. However, these interstices tend to fill with tar-like substances which, on carbonization, block the pore entrances. Opening these pores is accomplished by activation. 

These reactions are endothermic and it is necessary to supply heat to maintain isothermal equilibrium. This is achieved by burning the byproducts, either CO or H2, in situ in air. 

The H2O molecule is smaller than the CO2 molecule and diffuses faster into the pores of the carbon. Consequently, the reaction rate in Eq. (1) is greater than the rate in Eq (2) and steam activation is the more effective (and the more common) process. 

The properties of atypical activated carbon are listed in Table 10.7. In this case, activation was obtained in steam at 57 - 82 kPa for 10 -12 h.f l 

Activation Temp.,°C C/H Mole Reitio Weight % Nitrogen Density g/cm F d (nm) 

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Charcoal and Activated Carbon Tiselius used charcoal for the frontal analysis of sugars, amino acids and other substances. Charcoal absorbs strongly aromatic substances, such as amino acids, which may be explained by virtue of the fact that the carbon-carbon spacings in graphite are almost of the same order as those present in benzene. Charcoal is also employed for the adsorption of fatty acids. 

On July 7, 1855, Michael Faraday wrote to The Times of London to complain that the river Thames was a real sewer and that the whole of the river was an opaque pale brown fluid. He argued that [i]f we neglect this subject, we cannot expect to do so with impunity nor ought we to be surprised if, ere many years are over, a hot season give us sad proof of the folly of our carelessness (http //dbhs.wvusd.k 12.ca.us/Chem-History/Faraday-Letter.html, viewed 4/19/ 98). Society has made much progress since then, but many concerns remain. The use of charcoals and activated carbons in water treatment is probably one of the oldest chemical technologies, and a vast literature has accumulated on this subject [1-6]. 

This takes place in the iiqtnd phase, at 80°C, under 3.106 Pa absolute, in the presence of a palladium- and teUarinm-based catalyst deposited on activated charcoal and activated by nitric arid. Butadiene and make-up and recycle acetic arid are introduced at the same time as compressed air at the base of the reaction system. The freshly prepared catalyst is kept in suspension in recycle acetic acid in a separate agitated vessel. It is continuously added near the top of the reactor. Residence time is about 2 h. 

Adsorption from liquid phase can take place at any of the three interfaces liquid-solid, liquid-liquid, or liquid-vapor. In practice, however, more attention has been directed and more is known about the liquid-solid interface. This is due to the fact that purification of liquids such as water, wine, and oils, and their decolorization and detoxification have been carried out for centuries using charcoals and active carbons. With the expansion of chemical, pharmaceutical and food industries the range of substances to be purified by carbons has increased enormously. 

Carbon in its various forms such as pyrolytic carbon, glassy carbon, carbon fiber, carbon fiber reinforced composites, and activated carbon has been a potential and important material in medical science and in medicinal applications. This is due to its properties such as good biocompatibiUty, nontoxicity, no immune reaction with the body, low density, chemical inertness, low coefficient of friction, elastic modulus similar to that of bone, and high adsorption capacity. Some of these applications of carbon materials have been discussed elsewhere" and are beyond the scope of this book. In this section we shall look into some of the medicinal applications based on adsorption by active carbons. Adsorbent carbons in the form of charcoal and activated carbons have been in use for medicinal and health applications for centuries and have been listed in the pharmacopoeia as antidotes and intestinal adsorbents. 

The two major support materials are activated carbons (commonly called activated charcoal) and activated alumina. Activated carbons impregnated with palladium, platinum, or other metal salts are common in most liquid-phase reactions. 

With good fuels (such as charcoal and active metals), potassium nitrate will react well. Potassium nitrate has the additional property of not undergoing an explosion by itself, even when very strong initiating modes are used. ... 

Describe the difference between regular charcoal and activated charcoal. 

Occupational inhalation exposures occur during the handling of coke, charcoal, and activated earbon, but there is little information on concentrations and sizes of the suspended particles. There are no exposure standards for these materials, aside from nonspecific dust standards, although there are standards for coal tar pitch (10). Similar to other dusts, these materials may cause transient reductions of lung function or aggravation of other respiratory symptoms on brief, high-level exposure. Pneumoconiosis from coke or charcoal dust is possible with heavy exposures, but little information has been reported. Pneumoconiosis has been reported in workers manufacturing carbon electrodes from coke (77) and from occupational exposures to activated carbon (78). 

Fig. 5.14 Adsorption isotherms of water on carbon in (a) to f) with corresponding isotherms of nitrogen in (a), (c) and (J), and of benzene in (f>). (a) Charcoal (b) active carbon AY8 (c) charcoal A (J) charcoal (e) a coal tar pitch kilned at 1200°C (/) a charcoal (S600H). (Redrawn from the diagrams in the original papers.)...

Potassium Graphite. Potassium, mbidium, and cesium react with graphite and activated charcoal to form intercalation compounds CgM, C24M, C gM, C gM, and C qM (61,62). Potassium graphite [12081 -88-8] 8 P gold-colored flakes, is prepared by mixing molten potassium with graphite at 120—150°C. 

Manufacture. Trichloromethanesulfenyl chloride is made commercially by chlorination of carbon disulfide with the careful exclusion of iron or other metals, which cataly2e the chlorinolysis of the C—S bond to produce carbon tetrachloride. Various catalysts, notably iodine and activated carbon, are effective. The product is purified by fractional distillation to a minimum purity of 95%. Continuous processes have been described wherein carbon disulfide chlorination takes place on a granular charcoal column (59,60). A series of patents describes means for yield improvement by chlorination in the presence of dihinctional carbonyl compounds, phosphonates, phosphonites, phosphites, phosphates, or lead acetate (61). 

The refining process most commonly used involves treatment with hot aqueous alkaH to convert free fatty acids to soaps, followed by bleaching, usually with hydrogen peroxide, although sodium chlorite, sodium hypochlorite, and ozone have also been used. Other techniques include distillation, steam stripping, neutralization by alkaH, Hquid thermal diffusion, and the use of active adsorbents, eg, charcoal and bentonite, and solvent fractionation... 

Various types of detector tubes have been devised. The NIOSH standard number S-311 employs a tube filled with 420—840 p.m (20/40 mesh) activated charcoal. A known volume of air is passed through the tube by either a handheld or vacuum pump. Carbon disulfide is used as the desorbing solvent and the solution is then analyzed by gc using a flame-ionization detector (88). Other adsorbents such as siUca gel and desorbents such as acetone have been employed. Passive (diffuse samplers) have also been developed. Passive samplers are useful for determining the time-weighted average (TWA) concentration of benzene vapor (89). Passive dosimeters allow permeation or diffusion-controlled mass transport across a membrane or adsorbent bed, ie, activated charcoal. The activated charcoal is removed, extracted with solvent, and analyzed by gc. Passive dosimeters with instant readout capabiUty have also been devised (85). 

Ethyl Acetate. Catalysts proposed for the vapor-phase production of ethyl acetate include siUca gel, zirconium dioxide, activated charcoal, and potassium hydrogen sulfate. More recendy, phosphoric-acid-treated coal (65) and calcium phosphate (66) catalysts have been described. 

Charcoal is generally satisfactorily activated by heating gently to red heat in a crucible or quartz beaker in a muffle furnace, finally allowing to cool under an inert atmosphere in a desiccator. Good commercial activated charcoal is made from wood, e.g. Norit (from Birch wood), Darco and Nuchar. If the cost is important then the cheaper animal charcoal (bone charcoal) can be used. However, this charcoal contains calcium phosphate and other calcium salts and cannot be used with acidic materials. In this case the charcoal is boiled with dilute hydrochloric acid (1 1 by volume) for 2-3h, diluted with distilled water and filtered through a fine grade paper on a Buchner flask, washed with distilled water until the filtrate is almost neutral, and dried first in air then in a vacuum, and activated as above. To improve the porosity, charcoal columns are usually prepared in admixture with diatomaceous earth. 

Dimethylhexane-2,5-diol [110-03-2] M 146.2, m 88-90. Purified by fractional crystn. Then the diol was dissolved in hot acetone, treated with activated charcoal, and filtered while hot. The soln was cooled and the diol was filtered off and washed well with cold acetone. The crystn process was repeated several times and the crystals were dried under a vac in a freeze-drying apparatus [Goates et al. J Chem Soc, Faraday Trans 1 78 3045 1982]. 

Hydroxy-1-naphthaleneacetic acid [10441-45-9] M 202.2, pK )-4.2, pKej,(2) -8.3. Treated with activated charcoal and crystd from EtOH/water (1 9, v/v). Dried under vacuum, over silica gel, in the dark. Stored in the dark at -20° [Gafni, Modlin and Brand J Phys Chem 80 898 1976. Forms a lactone (m 107°) readily. 

Hydrogen chloride [7647-01-0] M 36.5. Passed through cone H2SO4, then over activated charcoal and silica gel. Fumes in moist air. Hydrogen chloride in gas cylinder include ethylene, 1,1-dichloroethane and ethyl chloride. The latter two may be removed by fractionating the HCl through a trap cooled to -112°. Ethylene is difficult to remove. Fumes in moist air. HARMFUL VAPOURS. 

Potassium hydrogen malate [4675-64-3] M 172.2. A saturated aqueous solution at 60° was decolorised with activated charcoal, and filtered. The filtrate was cooled in water-ice bath and the salt was ppted by addition of EtOH. After being crystallised five times from ethanol-water mixtures, it was dried overnight at 130° in zir [Edenand Bales J Res Nat Bur Stand 62 161 1959],... 

The arrangement and distribution of anodes in gravel and activated charcoal filters is different. Cathodic protection of activated charcoal filters is basically feasible but requires a large number of electrodes and high protection current densities that are twice those for gravel bed filters, so that an electrically insulating layer can be deposited on the steel wall. 

Starek, J., Zukal, A. and Rathousky, J., Comparison of the adsorption of humic acids from aqueous solutions on active carbon and activated charcoal cloths. Carbon, 1994, 32(2), 207 211. 

Filter aids may be applied in one of two ways. The first method involves the use of a precoat filter aid, which can be applied as a thin layer over the filter before the suspension is pumped to the apparatus. A precoat prevents fine suspension particles from becoming so entangled in the filter medium that its resistance becomes exces-sive. In addition it facilitates the removal of filter cake at the end of the filtration cycle. The second application method involves incorporation of a certain amount of the material with the suspension before introducing it to the filter. The addition of filter aids increases the porosity of the sludge, decreases its compressibility, and reduces the resistance of the cake. In some cases the filter aid displays an adsorption action, which results in particle separation of sizes down to 0.1 /i. The adsorption ability of certain filter aids, such as bleached earth and activated charcoals, is manifest by a decoloring of the suspension s liquid phase. This practice is widely used for treating fats and oils. The properties of these additives are determined by the characteristics... 

Certain highly porous solid materials selectively adsorb certain molecules. Examples are silica gel for separation of aromatics from other hydrocarbons, and activated charcoal for removing liquid components from gases. Adsorption is analogous to absorption, but the principles are different. Layers of adsorbed material, only a few molecules thick, are formed on the extensive interior area of the adsorbent - possibly as large as 50,000 sq. ft./lb of material. 

Adsorptive Properties. Substances such as silica gel and activated charcoal can be used to collect (adsorb) certain solids from solution. The adsorber bed may be discarded when depleted or recycled by washing and heating. 

In view of the enthalpy and activation energy (see Section II, B, 1) of the decomposition of arylpentazoles the activation energy for the reversal of the decomposition, the 1,3-addition of elementary nitrogen to arylazides, can be estimated to be 25-30 kcal/mole, an amount which does not exclude the reaction. To ascertain whether the decomposition of arylpentazoles is a reversible reaction, p-ethoxyphenylazide-[j8-N ] (see Section II, B, 3) adsorbed on charcoal was exposed to unlabeled nitrogen (45-50°, 380 atm, 100 hr), but the anticipated exchange of between the reactants was not detected. ... 

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