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Abstract
SUMMARY
The thesis can be divided into three parts: (I) an informative section throwing hight on the target material (activated carbon) and decolorization of the sugarcane liquor, (II) an experimental section describing the production of activated carbons and techniques utilized for its characterization and testing, and (III) the bulk of investigation displaying the results of the physico-chemical, adsorptive and decolorization properties of the derived activated carbons.
In section (I), the origin and development of color in sugar syrup was outlined where some coloring materials are inherited from the sugarcane, and others are developed through the various processing stages (phosphatation, carbonation, heating ….etc). Emphasis on the current decolorization procedures followed to obtain the final customer white sugar were outlined. The role of adsorbents, especially bone char and activated carbons (powdered or granular), in the refining process was mentioned. Recent literature on activated carbon was reviewed with respect to its main specific, identification, and characteristic properties. Essential factors determining the properties are mostly: the type of precursor, scheme of activation, and sometimes after-treatment procedures. Three main feedstocks, are the most popularly and commonly employed, on the industrial scale: wood, coal, and coconut shells. These are not economically available in most countries, which directed attention to locally abundant carbonaceous feedstocks; among which agricultural by-products present the most feasible. These are renewable, low east, low ash, accumulating in vast amounts, and exhibit mostly good mechanical properties. Meanwhile, they lost their long accepted central role as low cost fuel materials, and consequently became an environmental nuisance in most countries.
Production of activated carbons has traditionally followed one of two major schemes: physical (or recently ”thermal”) and chemical, activation. The former involves a carbonization step (devolatalization) followed by an activation (gasification) step under oxidizing gas atmospheres. A recent scheme was developed which involves only one step combining both carbonization and activation in a one uninterrupted process (genially denoted by ”steam pyrolysis”). In the other chemical activation scheme, the primary raw material (mostly a lignocellulose) is mixed with a chemical (activant or impregnant) which generally reacts with the precursor to inflict physical and chemical changes, and usually achieved at lower temperatures, with a resulting higher carbon yield.
In the experimental section (II), three sugarcane bagasse fractions [whole bagasse, bagasse fiber, and bagasse pith] were studied to obtain activated carbons by appling the two, single-step activation techniques: the ”chemical route” utilizing phosphoric acid (groups I&II) and the ”steam pyrolysis route (group III). In the ”chemical activation” scheme, the main variables were: precursor, activant concentration, duration, temperature and atmosphere of carbonization. For group III, the factors under consideration were: the precursor and temperature of pyrolysis. Several physico-chemical properties were evaluated, these include yield and ash content, thermogravimetry, elemental atom% (EDX), X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier Transform Infrared Spectroscopy (FTIR), and surface acidity. Adsorptive properties of the developed activated carbons were evaluated by determining the porosity characteristics by the standard N2 adsorption at 77K, adsorption from solution using probe molecules (iodine, p-nitrophenol, methylene blue and congo red), and color removal capacity of factory-grade sugar liquor as well as of molasses solution.
In chapter III, of the results section, it was demonstrated that chemical activation, with H3PO4, resulted in considerably higher carbon yield (36-44%) that surpassed the yield under the steam activation scheme (12-17%). The three groups of carbon adsorbents present typically two different categories: acidic carbons (pH=3.0-5.4) and basic carbons (pH=8.4-9.6). This was ascribed to the presence of surface oxygen functional groups (and/or phosphates) of acidic or basic nature, respectively. Chemically-activated carbons contain low inorganic matter (6-10%) whereas the steam-activated carbons contain much higher ash (13-32%) due to the excessive carbon gasification under the oxidizing atmosphere at the higher temperatures, in case of the latter scheme. Thermogravimetry proved that impregnation with H3PO4 brought about considerable effects on the pyrolysis course of raw bagasse. This was described and explained in light of the developed mechanisms of activation by H3PO4 in case of the lignocellulosic materials. Thermal profiles of the derived activated carbons indicate that they are thermally stable up to 500°C, and the main loss beyond is associated with decomposition of the surface-oxygen functionalities followed by controlled carbon gasification under flow of air in the TG cell. However, this process is very slow and complete combustion, at 800°C, to carbon oxides was not attained.
XRD profiles of the produced activated carbons exhibited only low broad and diffuse diffraction bands within the ranges of 2θ=23-26 and 43-48°, which was associated with the ”turbostratic” pseudo-graphitic structure. The former represents the height of the disorted graphite, Lc, and the latter due to the lateral dimensions of the ”crystals, La. These features were more distinctly differentiated in case of the steam-activated carbons, but exhibited differently separated peak maxima, for the H3PO4-derived carbons, indicating much degradation and fragmentation under action of the acidic activant. Such trend was confirmed by the SEM-micrographs, showing small corpuscular particles for the latter category and fibrous structures retaining the original botanical shape and morphology, in the S-group.
Chemical analysis determined by (EDX), indicated that silicon is the most prevalent in the S-group, whereas the H3PO4-activated carbons showed the prevalent existence of P due to the formation of organic and inorganic phosphorus compounds. Iron and chromium content was increased in the latter carbons probably due to chemical action on the SS-reactor tube. FTIR-spectra indicated absorption bands ascribed to the presence of: free or H-bonded water and OH groups, symmetric and asymmetric CH3-, CH2- and C-H groups, some few oxygen functionalities (C=O) and C=C aromatic skeletal bonds. Very small absorption due to the characteristic carboxyl groups was observed, supporting the recent postulations that acidity in the H3PO4-activated carbons is mostly associated with phosphorus compounds.
In chapter IV, a detailed description of the porous structures was outlined, by analysis of the low temperature N2 adsorption isotherms at 77K. The classical BET equation, the theory of volume filling of micropores (Dubinin - Raduskevich), and the comparison to non-porous carbons-method (αS-plot), were applied so as to get several porosity parameters: the specific surface area (total, mesoporous and microporous) and pore volume (total, microporous and mesoporous), the detailed and complementary internal structure. Development in these texture characteristics (and their modification) was followed as function of many variables of preparation.
Under chemical activated with H3PO4, high quality adsorbing carbons were obtained, with specific surface areas (SA) of 620-1040 m2/g total pore volumes (VP) of 0.50-0.82 ml/g, and average pore dimensions ( ) of 16.0 up to 19.0 Å. They were also characterized by well-developed microprous / mesoporous porosity in appreciable contents. Impregnation with 30-40% H3PO4, followed by carbonization at 500°C for 3h, present the most recommended condition to obtain the developed structures. Flowing air, during carbonization, generally enhanced the pore structure. The steam-pyrolysis scheme was restricted to the currently reported temperature range of 600-800°C. Pyrolysis at 600°, under flowing steam, resulted in low adsorbing carbons: SA=180-370 m2/g, VP= 0.09-0.19 ml/g with around 10 A. Mean values of these properties were enhanced by raising the temperature: at 700°C =630 m2/g, = 0.34 ml/g and = 11.7 Å and at 800°C: =620 m2/g, = 0.43ml/g and = 13.9 Å. The 800°C-derived activated carbons exhibit surface properties that approach those of the H3PO4-series, but with characteristically higher content of microporosity as estim