Xylose Decolorizing Activated Carbon Thermal Regeneration Process And Pore Structure Characterization

Xylitol is being widely used in areas such as: medicine, chemical engineering, foodstuff, tan, fuel. Decoloring via activated carbon plays a most important role in its production. However, while in service, the activated carbon will eventually become exhausted in its capacity to adsorb organic compounds. Once exhausted of their adsorption capacity, spent carbons can be landfilled, incinerated, thermally regenerated for reuse or replaced by fresh carbon. If discarded, the landfilled carbon potentially creates a hazardous waste problem and cradle-to-grave responsibility. Use of activated carbon only once is not only inefficient use of carbon but generates excessive waste to the environment. Regeneration refers to removal of the adsorbate and restoration of previous adsorptive capacity of the original activated carbon. Regeneration restores the activated carbon to a state where it is virtually identical in properties to the virgin precursor. Regeneration has become widely adopted in the last decade as it represents the most cost effective and environmentally sound option.The present thesis analyzed the advantages and disadvantages of regeneration methods of spent activated carbons, and thermal regeneration including conventional heating and microwave heating were carried out and compared. Spent activated carbon used in xylose decolorization using neutralization method(raw material NO.1) and ion-exchange method(raw material NO.2) were used as raw materials to carry out regeneration experiments. The influences of (1) heating rate, regenerating temperature and regenerating time; (2) microwave power, microwave heating time on activated carbon adsorption capacities were investigated.The effects of above mentioned parameters on pore structure of regenerated activated carbons were studied by nitrogen adsorption isotherms. The specific surface areas of regenerated activated carbons, spent carbon and fresh commercial activated carbon were calculated using the BET equation, while the micropore and mesopore distributions of carbons were characterized by H-K and BJH methods, respectively, and the degree of surface roughness or irregularities of regenerated activated carbons were measured by fractal dimension.The results of conditional experiments by conventional heating show that regenerated activated carbons from two kinds of raw materials present the same trends as following: the adsorption capacity of regenerated activated carbons increases with increasing heating rates at first, and then tends to slow. However, it increases with increasing temperature. The adsorption capacity of carbon was enhanced when the regeneration temperature was higher than 1073 K, and reached the maximum value at temperature of 1223 K. Higher regeneration temperature results in the elimination of any volatile compounds adsorbed in the carbon porosity, including residual moisture, and the thermal decomposition of other less volatile compounds, the blocked pores were opened and restored, giving rise to an increase in adsorption capacity of regenerated activated carbon. However, if regeneration temperature was higher than a desired value, it would result in damage to the original carbon pore structure. The best adsorption capacity of regenerated activated carbons was obtained at a regeneration time of 90 min, indicating that the adsorption capacity of spent carbon could be restored under a relatively regeneration temperature. Further increasing regeneration time would result in damage to the original carbon pore structure, giving rise to a decrease in adsorption capacity of regenerated carbons.With increasing heating rates, the BET surface area, micropore volume, mesopore volume and total volume were increased to some certain values and then decreased with further increasing heating rates. With increasing regeneration temperature, the BET surface area, micropore volume, mesopore volume and total volume were also enhanced greatly, the maximum BET surface area of regenerated carbon at temperature of 1223 K was 1062 m~2/kg. Further increasing regeneration temperature would result in damage to the original carbon pore structure, lowering the adsorption capacity. As regeneration process proceeded to certain values, most spent carbons experienced a progressive loss of microporosity and a rapid development of their external surface area which usually exceeded the levels in the virgin carbon. These effects have been associated with the conversion of micropores into mesopores due to the progressive burn out of pore walls. In any case, the characteristics of the virgin carbon in terms of BET surface area and micropore volume could not be restored completely by means of steam regeneration alone.Regeneration by microwave heating shows that the adsorption capacities of regenerated activated carbons increase with increasing microwave power and microwave heating time. Pore structures of regenerated activated carbons by microwave heating were investigated. The results show that the higher the microwave power is, the better the effect of regeneration is. Regenerated activated carbon using time of 20min and power of 800 W has the biggest specific surface area and pore volume, the micropore of it is also well developed. While the pore structure of carbons is seriously destroyed when the regenerating time was 35min, and the pore size was enlarged obviously.The caramel adsorbtion experiments showed that the amount of caramel adsorbed onto reactivated carbons derived from the first batch material by conventional heating increased at first and then decreased with increasing regenerating temperatures. The amount of caramel adsorbed was more than 90%, when the regenerating temperature was 1223K, the caramel adsorbtion of regenerated activated carbons were worse when the regenerating temperature was too low or too high. It was unfit to decolor in sugar liquid. Conditional experiments showed that carbons regenerated in short times were poor in caramel decolorization. It isn't suitable for sugar decolorization. Prolong regenerating time, the amount of caramel adsorbed increases. The amount of caramel adsorbed was more than 90% when the regenerating time was 120min., the amount of caramel absorbed onto regenerated activated carbons had no changes with further increasing in the regenerating time to 180minThe adsorption capabilities of activated carbons regenerated for 15 to 35min from the first batch materials by microwave heating were excellent when the microwave power was 800W. Results showed that carbons regenerated by microwave heating had a high amount of caramel adsorption of more than 90%, and high methylene blue adsorption of more than 130ml/g, which met the requirements of industrial production. In particular, the caramel adsorption of carbons was 100% which reached the first grade standards in the GB/T13803.1-1999 when heating time was 20min.From the comparisons of different materials and heating methods, we concluded that regenerated activated carbons by microwave heating were suitable for sugar decolorization. Comparatively, regeneration by microwave heating was easier to industrialization which ascribed to advantages such as: shorter time, better efficiency, convenient operation and simple equipment. The regenerating process of microwave heating proposed in present thesis could efficiently reactivate and reuse spent activated carbons in great quantity. It can not only reduce environment pollution and save resources, but also create certain economy value.