Carbon attrition

Arena, U., D Amore, M., and Massimilla, L., Carbon Attrition during the Fluidized Combustion of CoalAIChEJ., 29 40 (1983)... 

The above calculations show a carbon loss of about 4 percent of the coal feed, primarily as fines produced by carbon attrition or by the shrinkage of the coal feed. As coal particle feed size increases the attritted carbon increases (note t in Eq. 21 is proportional to d ) but the elutriated carbon (Eq. 19) decreases. Carbon losses can therefore be minimized by the judicious choice of coal feed size. The simplified model presented above yields the following expression for the optimum size ... 

To design and implement systems for water treatment, it is not only the adsorption charaeteristics of the activated carbon that must be considered but also the effect that the carbon may have on the practical operation of the unit. In this context, the pressure drop generated across a GAC bed is one of the most important factors, as it also is for gas phase applications. The particle size distribution should be optimized to attain an acceptable pressure drop commensurate with the desired rate of adsorption. The carbon attrition resistance is another important parameter. Part of the operating cost of adsorbers is due to the loss of carbon fines during transport, handling, and regeneration. 

It is useful in checking cafoon production. It is a measure for checking v ether purchased carbons meet the standard specification it is also used m. evaluating the effect of plant carbon handling procedures on carbon attrition. 

Donsi et al. [21] derived the following equation for carbon attrition in a bubbling fluidized bed combustor ... 

Arena U, D Amore M, Massimilla L. Carbon attrition during the fluidized combustion of coal. AlChE J 29 40-48, 1983. 

Attrition of activated carbon can be minimized by proper design of the adsorption vessels. The air-flow rate should be below 100 ft/min., preferably below 60 ft/min., and good air distribution to the top of the bed should be provided. Plugging of activated carbon beds can be prevented by eliminating any sources of carbon attrition and providing an adequate screen or filter ahead of the adsorption vessel. 

Operating experience with the Couitaulds plant has shown the recovery efficiency to be at least as high as for a fixed-bed plant and steam consumption a little lower. Carbon attrition has proven to be somewhat of a problem as the accumulation of flnes in the bed increases the residence time, which in turn leads to a greater adsorption of water at the expense of CS2. This problem can be partially resolved by drawing off fines collected from the exit gas instead of returning them to the bottom adsorber tray as shown in the flow diagram. However, this. solution leads to an increased requirement for makeup carbon. 

Physical Properties. Physical properties of importance include particle size, density, volume fraction of intraparticle and extraparticle voids when packed into adsorbent beds, strength, attrition resistance, and dustiness. These properties can be varied intentionally to tailor adsorbents to specific apphcations (See Adsorption liquid separation Aluminum compounds, aluminum oxide (alumna) Carbon, activated carbon Ion exchange Molecular sieves and Silicon compounds, synthetic inorganic silicates). 

The commercialization by Kureha Chemical Co. of Japan of a new, highly attrition-resistant, activated-carbon adsorbent as Beaded Activated Carbon (BAC) allowed development of a process employing fluidized-bed adsorption and moving-bed desorption for removal of volatile organic carbon compounds from air. The process has been marketed as GASTAK in Japan and as PURASIV HR (91) in the United States, and is now marketed as SOLD ACS by Daikin Industries, Ltd. 

A fluidi2ed-bed catalytic reactor system developed by C. E. Lummus (323) offers several advantages over fixed-bed systems ia temperature control, heat and mass transfer, and continuity of operation. Higher catalyst activity levels and higher ethylene yields (99% compared to 94—96% with fixed-bed systems) are accompHshed by continuous circulation of catalyst between reactor and regenerator for carbon bum-off and continuous replacement of catalyst through attrition. 

Thermal oxidizers must be built to provide the residence time and temperatures to achieve the desired destruction efficiency (DE). As such, thermal oxidizers are comparatively larger than catalytic oxidizers since their residence time is two to four times greater. Historical designs of thermal oxidizers were comprised of carbon steel for the outer shell and castable refractory or brick as the thermal liner (a refractory is like a cement, which is put on the inside of the rector shell to act as a thermal insulation barrier). Modern units are designed and built using ceramic fiber insulation on the inside, which is a lightweight material, and has a relatively long life. Old refractory would tend to fail over a period of years by attrition of expansion and contraction. 

Characteristics of attrition and adsorption were investigated to remove CO2 in fluidized hed using activated carhon, activated alumina, molecular sieve 5 A and molecular sieve 13X. For every dry sorbent, attrition mainly still occurs in the early stage of fluidization and attrition indexs(AI) of molecular sieve 5A and molecular sieve 13X were higher than those of activated carbon and activated alumina. Percentage loss of adsorption capacity of molecular sieve 5A and molecular 13X were 14.5% and 13.5%, but that of activated carbon and activated alumina were 8.3% and 8.1%, respectively. Overall attrition rate constant (Ka) of activated alumina and activated carbon were lower than other sorbents. 

Therefore, in this study, activated carbon, activated alumina, molecular sieve 5A, and molecular sieve 13X were used as dry sorbents to control carbon dioxide in a fluidized bed. In addition, the attrition and percentage loss of adsorption capacity of the dry sorbents were investigated. 

Table 2 summaries overall attrition rate constants (Ka) and physical properties for each dry sorbent. As shown in Table 2, Ka of activated alumina was the lower than any other sorbent, but was similar to activated carbon. However, we used activated carbon as dry sorbent to control CO2 because it is the most cost-effective among others.

Table 3. Summary of attrition rate constants for activated carbon at different gas velocities.

Cleaver, J. A. S., and Ghadiri, M., Impact Attrition of Sodium Carbonate Monohydrate Crystals, Powder Tech., 76 15 (1993)... 

Hypersorption A continuous chromatographic separation process using a moving bed. Invented in 1919 by F. D. Soddy (famed for his work on isotopes) at Oxford and developed commercially for petroleum refinery separations by the Union Oil Company of California in 1946. Six plants were built in the late 1940s, using activated carbon as the adsorbent. The process was abandoned because attrition of the bed particles proved uneconomic. 

Strength against attrition is particularly important for catalysts in slurry-bed reactors, where physical breakage of the catalyst particles, ultimately to fines, can prevent their use for those reactions. The strength of the high surface area skeletal structures can be contrasted against activated carbon, which readily breaks down due to attrition in these types of environments. For the few environments where attrition is still a problem (e.g., oxidative dehydrogenation of alcohols), the skeletal catalytic material... 

In a continuous reformer, some particulate and dust matter can be generated as the catalyst moves from reactor to reactor and is subject to attrition. However, due to catalyst design little attrition occurs, and the only outlet to the atmosphere is the regeneration vent, which is most often scrubbed with a caustic to prevent emission of hydrochloric acid (this also removes particulate matter). Emissions of carbon monoxide and hydrogen sulfide may occur during regeneration of catalyst. 

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