Ind. Eng. Chem. Process Des. Dev. 1983, 22, 615-619
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Thermogravimetric Determination of the Coking Kinetics of Arab Heavy Vacuum Residuum Robert C. Schucker Exxon Research and Development Laboratorles, Baton Rouge, Louisiana 7082 1
The progressively heavier nature of available feedstocks has put a premium on efficient, iow-cost refinery processes to convert residuum to lighter products. One such process is fluid coking, and the present study was undertaken to provide information on the coking kinetics of Arab Heavy vacuum residuum-a feed of commercial interest. The feed was first separated by solvent deasphalting and liquid-solid adsorption techniques into four fractionsasphaltenes, polar aromatics, aromatics, and saturates. Each of these fractions and the whole residuum were then subjected to nonisothermal kinetic analysis using thermogravimetry. Both weight loss and its first derivative were monitored as a function of temperature at heating rates ranging from 1 'C/min to 20 OC/min. Activation energies and frequency factors were obtained at various conversion levels and in ail cases were shown to increase with conversion. This strongly suggests the use of an activation energy distribution for future coking kinetic modeling.
Introduction It is estimated that world petroleum production will peak in the mid 1990's even though the projected demand for petroleum products will continue to rise. The shortfall between supply and demand must necessarily by met by conservation and the development of alternative energy sources. Since the large-scale commercial exploitation of coal and shale as sources of liquids is still some years away, it is necessary that we get the most out of current petroleum supplies. Unfortunately, available petroleum feedstocks are becoming progressively heavier; that is, they contain a larger fraction of nondistillable vacuum residuum. As a result, there is a substantially driving force to develop efficient, low-cost processes which convert residuum into lighter products, and it is within this context that the current research work was undertaken. Traditionally the process of choice for such conversion has been coking wherein higher molecular weight species are converted into lighter ones by thermal decomposition a t elevated temperatures. Despite major advances in coking technology in recent years, there is still a need to increase the yield of desirable liquids from fluid cokers. The ability to change yield or selectivity patterns, however, implies an alteration of the basic free-radical pathways by which residuum molecules decompose-pathways which are presently at best poorly understood. Furthermore, attempts to gain kinetic or mechanistic insight into these pathways are hampered by the complexity of the feed. One way to circumvent some of these problems is to divide the feed into a number of subfractions and to study each one individually. A survey of the literature, however, shows that this has been rarely done for petroleum or bitumen feedstocks. Levinter and co-workers (1966) did separate bitumen from deasphaltization into three component groups-oils, resins, and asphaltenes-and carried out coking studies on each fraction. However, the experiments were carried out isothermally and only the results for the oil fraction were reported. In addition, most of the work focused on the mechanism rather than the kinetics of coking to the extent that no kinetic data were reported. Most other coking studies have focused on the behavior of the asphaltene fraction. Bestougeff and Gendrel (1964) reported a study wherein asphaltenes of different sulfur content were subjected to isothermal thermogravimetric analysis at temperatures ranging from 350 to 600 OC. They 0196-4305/83/1122-0615$01.50/0
observed that the transformation of asphaltenes became rather rapid at 375 OC but made no attempt to report kinetic data. Moschopedis and co-workers (1978) studied the thermal decomposition of Athabasca asphaltenes by using a horizontal tube furnace and analyzed only gaseous and liquid producta Ritchie and co-workers (1979) used the comparatively new tool of pyrolysis GC/MS to study the volatiles evolved from Athabasca asphaltenes, while Cotte and Calderon. (1981) used a similar approach to study the pyrolysis of Boscan asphaltenes. It should be clear from these citations that more work has been done to determine the chemical nature of pyrolysis products or to derive the mechanism of coking than has been done in support of coking kinetics. The objectives of this study, therefore, were (1)to determine the coking kinetics of a feed of commercial interest and (2) to attempt to simplify the interpretation of the overall kinetics by first separating the feed into subfractions and studying each fraction individually. Although the major emphasis of this study was on coking kinetics, some attempt was also made to relate the kinetic data to structural/mechanistic concepts in coking. Experimental Section Method. The technique chosen for kinetic measurements in this study was thermogravimetric analysis, which has been used for a number of years in polymer chemistry (Flynn and Wall, 1966), and which has been used previously in coking studies (Bestougeff and Gendrel, 1964; Collett and Rand, 1980; Cotte and Calderon, 1981). As a rule, kinetic investigations are carried out isothermally at various temperatures. However, with complex samples, there are problems associated with the interpretation of chemical changes occurring during the heat-up period. These difficulties can be overcome by the use of nonisothermal techniques such as those used recently in studies of the coking of pitch (Sekhar and Ternan, 1979; Collett and Rand, 1980) and oil shale retorting studies (Campbell et al., 1978; Shih and Sohn, 1980; Rajeshwar, 1981). Data from such nonisothermal studies have been shown to be quite reliable provided the measuremenb are made over a range of heating rates (Flynn and Wall, 1966). All experimental work was carried out in a Cahn 113 thermogravimetric analyzer, a schematic of which is shown in Figure 1. The heart of this system is the Cahn 2000 microbalance with a sensitivity of 0.1 kg and a capacity 0 1983 American Chemical Society
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Ind. Eng. Chem. Process Des. Dev., Vol. 22, No. 4, 1983
Table I. Selected Chemical and Physical Properties of Arab Heavy Vacuum Residuum and Subfractions whole n-C, polar asphaltenes aromatics aromatics saturates fraction resid wt% c, wt % H,wt % 0,wt % N, wt % s, wt %
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(_H/C)AT M,, tol., 50 "C c,, mol %
100.0 83.13 9.79 0.58 0.68 5.71 1.41 2500
23.9 82.65 7.91 0.88 1.18 7 .51 1.15 -7500 48.4
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60.3 82.63 9.92
10.4 84.92 11.91
0.56 5.45 1.44 1900 39.7
