1080
Ind. Eng. Chem. Process Des. Dev.
Fasesan, S. 0. Ph.D. Thesis, University of Manchester Institute of Science and Technology. Manchester, England, 1980. Grocott, G. J., Masters (Technical) Thesis, University of Manchester Institute of Science and Technology, Manchester, England, 1961. Kharbanda, 0. P.: Chu, J. C. Br. Chem. Eng. 1970, 15 (6), 792. Kupferberg, A.; Jameson, G. J. Trans. Inst. Chem. Eng. 1970, 4 8 , T140. Lemieux, E. J.: Scottl, L. J. Chem. Eng. Prog. 1969, 65(3),52. Porter, K. E.; Jenkins, J. D. Inst. Chem. Eng. Symp. Ser 1979, No. 56, 3.2/21.
1985, 2 4 , 1080-1087
Raper, J. A.; Phuong, T. V.; Fell, C. J. D. Paper presented at the 5th Australian Chemical Engineering Conference, Canberra, Sept 14-16, 1977. Todd, W. G.; Van Winkle, M. Ind. Eng. Chem. Process Des,Dev. 1972, 7 7 , 578.
Received f o r review October 13, 1983 Accepted October 29, 1984
Pyrolysis of Volatile Aromatic Hydrocarbons and n-Heptane over Calcium Oxide and Quartz Danlel L. Elllg, Chlu K. Lal, David W. Mead, John P. Longwell,' and Wllllam A. Peters' Energy Laboratory and Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02 139
Benzene, toluene, 1-methylnaphthalene, and n-heptane pyrolysis was studied over the temperature range 550-950 OC, by passing the vapor of the pure compound (initial concentration 2.4-3.9 mol % in helium) through -5.5 cm deep packed beds of calcium oxide/quartz mixtures, or of quartz in control experiments. The total pressure was 1 atm, and contact times were 0.9-1.3 s. The calcium oxide significantly increased the rates and extents of pyrolysis of the aromatics, reducing the temperature for a given percentage conversion by around 140, 140, and 170 OC for benzene, toluene, and 1-methylnaphthalene, respectively. In contrast, CaO decreased the comparable temperature for the aliphatic n-heptane by only -40 'C. Coke was the major product from pyrolysis of the aromatics over both beds. Minor amounts of coke deposition slightly increased the CaO activity for benzene and toluene pyrolysis, but continued coking produced a strong acthmy decay that was fitted to an Elovich model. Oxygen burn-off regenerated 75% and 100% of the initial CaO activity for these two compounds, respectively. A generic stone chemical property, rather than specific BET surface area, is believed responsible for the CaO pyrolysis activity.
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Coal pyrolysis or gasification frequently generates coproduct tars. Opinions differ on the industrial value of these products. They are usually rich in aromatic compounds, phenols, mutagens, and carcinogens and, depending on operating conditions, can account for 5 to 20 wt 5% of the coal. They thus can place significant burdens on product recovery and waste stream clean-up equipment as well as reduce carbon utilization and hence degrade process thermal efficiency. Poor handling and storage behavior including viscosity increases, phase separations, and incomplete miscibility with petroleum products are related problems. On the other hand, these products are high-energy content liquids (typically 16 000-18 000 Btu/lb) readily and economically recoverable from coal. Redemption of these values, by controlled handling procedures such as in-plant or over-the-fence combustion or refining, is therefore worthy of consideration. The technical and economic feasibility of such utilization strategies, as well as the design and operating requirements of waste stream clean-up equipment, will depend on the amounts and composition of the tars exiting the primary conversion reactor. Previous work in this laboratory (Yeboah et al., 1980) demonstrated yield reductions and quality improvement when fresh coal pyrolysis tars were treated with calcium oxide (CaO or calcined dolomite), at temperatures above 400 "C at contact times of about 1s. Pyrolysis of Illinois No. 6 bituminous coal or a 5545 (w/w) mixture of Texas lignite and Illinois No. 6 coal over CaO and a temperature range of 425-760 "C produced tars lower in yield and oxygen content and higher in H/C ratio than those from pyrolysis in the presence of sand. Major
reductions in the evolution of vapor-phase sulfur compounds including thiophenic species were also observed (Yeboah et al., 1982). These results were interpreted in terms of CaO-enhanced cracking of aromatic compounds, and similar effects are believed to be attainable under gasification conditions. Literature data to assess the role of CaO were unavailable, although there have been several studies of the homogeneous and heterogeneous cracking of aromatics (see reviews by: Madison and Roberts, 1958; Johns et al., 1962; Fitzer et al., 1971) and of CaO-facilitated reactions of aliphatics below 400 "C. A brief review of the latter is given by Mead (1979). Research was therefore undertaken to determine the effect of CaO on pyrolysis of aromatics. The present paper presents results for low boiling pure aromatic compounds. Results for phenolic compounds, less volatile aromatics, and fresh coal pyrolysis tars will be reported on later. Specific questions here addressed include the following: (1)What are the effects of calcium oxide on the rates and extents of thermal cracking of pure aromatic compounds and on the yields and compositions of the resulting products at temperatures from 550 to 950 "C? (2) How does CaO activity depend on its extent of utilization, and can deactivated CaO be conveniently regenerated? (3) Are the observed CaO effects primarily due to its moderately high surface area? Experimental Section Apparatus and Procedure. Pyrolysis of n-heptane, benzene, toluene, and 1-methylnaphthalene over CaO was
0196-4305/85/1124-1080$01.50/00 1985 American Chemical Society
Ind. Eng. Chem. Process Des. Dev., Vol. 24, No. 4, 1985 1081 Table I. Experimental Conditions for Pyrolysis of High Volatile Aromatics and n -Heptane over Quartz and CaO init concn of total wt of hydrocarbon fed, compd,bmol contact time, s bed temp, O C % (at temp, mg hydrocarbon bed matern CaOd 545-667 2.4 3.7 n-heptane quartz 585-667 CaO 715-822 3.9 1.1 (700)-0.9 (900) 5.1 benzene quartz 846-944 CaO 574-767 3.3 1.3 (600)-0.9 (900) 5.0 toluene quartz 764-877 1-methylnaphthalene CaO 553-813 2.5 1.3 (600)-0.9 (900) 5.4 quartz 748-881 See text for particle sizes. Balance is high purity helium. 120 s. dl:6.6 (w/w) mixture with quartz.
Based on V/F, and a bed void fraction of 0.5. Feed time is approximately
D r y Ice-Alcohol
Both
GOI Prehcolcr
U Liquid N8lrogm Both
Figure 1. Schematic of apparatus.
studied for the temperature range from 550 to 950 "C at a totalpressure of about 1atm. The experimenta required controlled contacting of the vapors of each compound with CaO (or quartz in control experiments) at short residence times and high temperatures. A quartz tubular reactor, packed with shallow beds of stationary, particulate solids and operated in a semicontinuous mode, was therefore employed. The apparatus is shown schematically in Figure 1. The cylindrical reaction chamber (0.9 cm 0.d. X 0.7 cm i.d. X 50 cm long) was mounted vertically in an electrical tube furnace fitted with alumina end plugs and supplemental heaters to reduce axial temperature variations to f 2 OC. The beds (-5.5 cm deep by 0.7 cm i.d.) were held in the reaction chamber between two plugs of quartz wool. The reaction temperature was interpolated from thermocouple measurements of the bed temperature before and after each run. The organic reactants (all liquids at room temperature) were fed just upstream of the reaction chamber, using a Sage Instruments Model 341 syringe pump. They were vaporized by the preheated carrier gas (helium, stated purity 99.9999% ) to give initial concentrations ranging from 2.4 to 3.9 mol % and, after rapid transport to the reaction zone, vapor-solid contact times (V/F, assuming a bed void fraction of 0.5) of 0.9-1.3 s. In a typical run, about 4-5 mg of compound were fed in about 2 min. Upon exiting the reador, products and unconverted feed were conveyed to a stainless steel U-tube (6.4 mm 0.d. and 38 cm long) filled with Porapak QS chromatographic packing and immersed in liquid nitrogen (-196 "C), where all reactor effluents, except hydrogen and helium, were trapped quantitatively. Two additional traps were inserted upstream of the Porapak QS trap in the runs with 1methylnaphthalene. An unpacked stainless steel U-tube in an ice bath was located directly downstream of the reaction chamber for collection of higher molecular weight products. For capture of 1-methylnaphthalene and poly-
cyclic aromatic compounds, it was followed by a straight tube packed with XAD-2 resin, pretreated by 24-h Soxhlet extraction in nanograde methylene chloride. In all runs the trap(s) effluent was vented to a hood. When all the hydrocarbon had been fed, the reactor and traps were flushed with helium for about 15 min, sealed, and removed for analysis. The contents of the Porapak trap were analyzed on a Perkin-Elmer Model 3920 gas chromatograph by using a 3.66 m X 3.2 mm Porapak QS column, temperature programmed from -80 to 220 OC at a rate of 16 OC/min, and helium carrier gas at a flow rate of 30 cm3/min. This provided good air/CO separations and elution of benzene and toluene in about 20 min. In the 1-methylnaphthaleneruns, unreacted feed and higher boiling products were recovered by washing the ice bath trap with CH2C1, and Soxhlet-extracting the XAD-2 resin for 24 h with CH2C12. The resulting solutions were combined and analyzed for naphthalene and l-methylnaphthalene on a Perkin-Elmer Sigma 1 chromatograph using a 15 m SE 52 capillary column (J & W Scientific, Rancho Cordova, CA), temperature programmed from 45 to 240 "C at 3 "C/min, and helium carrier gas at a flow rate of 25 cm3/min. Reagents and CaO Preparation. Runs with ground quartz provided base-line (control) data for the reactor. Test data were obtained with packed beds of ground quartz and powdered CaO. Calcium oxide (
