Upgrading carbonization properties of naphtha tar with aluminum chloride

The tar produced in the naphtha cracking for ethylene production (ETP) is a prominent resource of carbon precursors, although it may require some chemical treatment because of its low coking value. In this paper, its acid-catalyzed treatment using a minimal amount (3 or 5 wt%) of AlCl₃ was examined aiming at its conversion into promising feedstocks for needle coke and coking additives. ETP was effectively converted with 5 wt% of AlCl₃ at 225°C for 24hr to increase the coke yield by 13% and to develop a flow domain texture as shown in Fig. 1-b. Raising the treatment temperature up to 300°C further increased the coke yield, at the same time, maintaining the flow texture development (Fig. 1-c). Modification with the same amount of catalyst at 320°C deteriorated anisotropic development, allowing some elongated mosaic texture development (Fig. 1-d). The amount of catalyst as low as 3wt% was still effective to modify the ETP at this temperature as shown by Fig. 1-e. More severe treatment conditions, however, led to a larger area of elongated mosaic texture in the resultant coke. Increasing the amount of benzene insoluble matter (BI) might disturb the smooth coalescence of anisotropic spheres since the benzene soluble component (BS) fractionated from the same pitch developed a good flow texture (Fig. 1-g). Reflux during the treatment was effective for increasing the yield of treated pitch and for developing an excellent flow domain texture in the resultant coke (Fig. 1-h). The optical textures developed in the resultant cokes obtained by co-carbonization of the pitches with fusible, slightly-fusible, and non-fusible coals (refer to Table 1 and Fig. 2) are illustrated in Figs. 3, 4 and summarized in Table 4. As shown in Fig. 3-a, the original ETP showed essentially no modification activity for cocarbonization with any of the coals. The pitches, which were treated with 5wt% AlCl3 at 280°C for a certain defined period, exhibited considerable activities for the coals studied in the present study to develop anisotropic textures in the major area of the resultant cokes. The treatment with 3wt% of AlCl₃ required more severe conditions to prepare active pitches. The optical micrographs of cokes obtained from a non-fusible coal (Taiheiyo coal) by co-carbonization with pyridine-soluble (PS) and benzene-soluble (BS) fractions of 3D pitch are shown in Fig. 5-b and c, respectively. Both PS and BS fractions showed considerable modifying activities for the coal to give a more homogeneous appearance of anisotropic development than their parent pitch. The structural indices of the pitches such as f_a (aromaticity) and R_nus (naphthenic ring number in the unit structure) calculated according to the modified Brown-Ladner method are summarized in Table 2. As shown in Fig. 6, such structural characteristics could be built in effectively by heat treatment with the catalyst through a series of acid-catalyzed reactions such as dealkylation, ring closure of alkyl side chains, aromatic condensation reactions, and thermal dehydrogenation. The structural characteristics, f_a and R_nus, of the pitches are correlated with their modifying activities for the coals as illustrated in Fig. 8. Larger values of both f_a·Y and R_nus·Y/R_tus (Y: coke yield, R_tus: total ring number in the unit) are favorable for developing anisotropy from non-fusible as well as fusible coals in co-carbonization. Such characteristics of the pitches may be associated with their hydrogen donating and dissolving abilities for the coals during co-carbonization.