Ethylene tars

The principal sources of feedstocks in the United States are the decant oils from petroleum refining operations. These are clarified heavy distillates from the catalytic cracking of gas oils. About 95% of U.S. feedstock use is decant oil. Another source of feedstock is ethylene process tars obtained as the heavy byproducts from the production of ethylene by steam cracking of alkanes, naphthas, and gas oils. There is a wide use of these feedstocks in European production. European and Asian operations also use significant quantities of coal tars, creosote oils, and anthracene oils, the distillates from the high temperature coking of coal. European feedstock sources are 50% decant oils and 50% ethylene tars and creosote oils. 

An important clue to the relevance of radical chemistry prior to mesophase formation comes from the study of Singer and Lewis (51) who observed that, in the carbonization of ethylene tar pitch (which leads to a smaller size of optical texture in resultant coke), both the concentration and rate of build-up of radicals are larger than for a petroleum-derived pitch. As discussed below, lower concentrations of radicals lead to larger sized optical textures. The relationships between transient and stable radicals have yet to be established. 

Miyazawa et al. (92) related rates of decrease of aliphatic hydrogen protons during pyrolysis of ethylene tar pitch to formation of mesophase. Yokono et al, (93) used the model compound anthracene to monitor the availability of transferable hydrogen. Co-carboniza-tions of pitches with anthracene suggested that extents of formation of 9,10-dihydroanthracene could be correlated with size of optical texture. The method was then applied to the carbonization behaviour of hydrogenated ethylene tar pitch (94). This pitch, hydrogenated at 573 K, had a pronounced proton donor ability and produced, on carbonization, a coke of flow-type anisotropy compared with the coarse-grained mosaics (<10 ym dia) of coke from untreated pitch. 

Figure 4 shows the relation between the Tj of ethylene tar pitch and the inverse of temperature. The Tj of ethylene tar pitch has the value of 400 msec at room temperature. It decreases with increasing temperature, and reaches its minimum value at about 420 K. As is shown in the figure, it has its maximum value of 190 msec at about 600 K, and the value decreases rapidly with increasing temperature. 

Resolved NMR Spectra and Hydrogen Aromaticity. Typical spectra for ethylene tar pitch are shown in Figure 5. On heating up to about 357 K, a broad resonance with no resolved structure becomes observable. At about 396 K the NMR spectrum shows two discrete lines, 200 Hz apart, which correspond to aromatic and aliphatic protons. 

Figure 4. Temperature dependence of proton spin-lattice relaxation time(Ti) of ethylene tar pitch during heating at 2 K-min ...

Figure 6 shows the relation between of ethylene tar pitch and the elevating temperature with the range of 1 K/min. It is clear that increases drastically after the temperature exceeds about 593 K. Hence Tq and to in Equation 4 can be taken as 593 K and 300 min, respectively. Based on Equation 6, observed values of In [(1 - /jjJ (df Jdt)] for ethylene tar pitch were plotted against t - to). A typical example of the results is shown in Figure 7, with q 4.17 x 10 K min, which corresponds to the heating rate of about 1 K/min. As expected, a... 

Figure 6. Temperature dependence of hydrogen aromaticity (fa J for ethylene tar pitch (heating rate is 1 K min )...

After the measurement of the NMR spectra of Kureha pitch at 723 K and ethylene tar pitch at 743 K, the samples were immediately quenched to room temperature and observed by polarized-light microscopy. It was confirmed that the bulk mesophase was produced. 

Figure 9. A NMR spectrum (a) from ethylene tar pitch at 743 K during heating at 1 K min and (h) a comparison of the experimentally observed spectrum with a computer-simulated spectrum (1) total simulated curve (2) Gaussian component with T2 = 30 Lsec (3) Gaussian component with T2 = 70 p ec (4) Lorentzian component with T2 = 260 pLsec and (5) Lorentzian component with T2 = 970 pisec.

Generally the sweep width of broad-line NMR is about 10 to 10 Hz, while that of high-resolution NMR is about 10 Hz at 36.4 MHz for a proton. But the H NMR spectra of the mesophase for Kureha pitch (Figure 8) and ethylene tar pitch (Figure 9) were swept over 2.5 x 10" Hz. Thus the sweep width of the H NMR spectrum for the carbonaceous mesophase is intermediate between those of broad-line H NMR... 

By means of computer simulation we found that the NMR lines for the mesophase include several components. The results with computer simulation are summarized in Table III. From the viev oint of general NMR behavior (20), Kureha pitch and ethylene tar pitch at the early stages of carbonization contain about 85% and 68% rigid structures, respectively. 

Alternatives are coal tars and ethylene tars, but may produce a different quality of carbon black. 

Coal tar is the distant third most likely feedstock used to produce furnace carbon blacks (next to cat cracker bottoms and ethylene tars ). There is some use of coal tar in China to manufacture carbon black for rubber applications. 

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