RAPID THERMAL CRACKING OF wHEXADECANE A T ELEVATED PRESSURES B. M . FABUSS, J. 0. S M I T H , R.
I. L A I T , A N D A . S .
BORSANYI
.Monsanto Research Cor$., Everett, iMass. C. N. S A T T E R F I E L D Department of Chemical Engineering, .tc[assachusetis Institute of Technology, Cambridge, .%lass.
The thermal cracking of pure cetane (n-hexadecane) was studied in a flow reactor at 1 100" to 1300" F. and pressures of 200 to 1000 p.s.i. The product was analyzed for individual paraffins and olefins, carbonaceous deposits, and diolefins and aromatics as a group. Initially polymerization reactions predominate, so that the liquid fraction of the product ( C b and higher) has an average molecular weight at low per cent conversions several times that of the initial cetane. Average molecular weight steadily drops with increased per cent cracking. The amounts of carbonaceous deposits were far less than those previously reported for paraffin cracking, probably because of high flow rates. The rate of reaction was substantially limited b y the rate of heat transfer, The first-order rate constants at a specified temperature were independent of pressure. The over-all results provide a quantitative picture of the complex interaction of polymerization and cracking phenomena of a typical pure high molecular weight paraffin at elevated pressures.
HE thermal cracking reactions of paraffin hydrocarbons Thave been estensively studied, but little work has been done on pure hydrocarbons containing more than about eight carbon atoms, particularly a t elevated pressures. Voge and Good (72) reported studies on cetane a t 1- and 21-atm. pressure. Hepp and Frey ( d ) studied the cracking of propane and butane a t 1500 and 2500 p.s.i.g. and give direction to the earlier literature on py-olysis reactions of propane, butane, and hesane under pressure. Sachanen ( 8 ) discusses cracking in general and gives extensive references to the literature. In the present studies a three-variable experimental program [vas designed to study the effects of pressure, temperature, and residence time on the cracking of cetane. Pressures were 200. 500. and 1000 p.s.i.g.; temperatures were llOOo>1200', and 1300' F. : and feed rates icere 1 2, 4, and 8 liters of liquid per hour. (Corresponding residence times were 0.25 to 10 seconds.) These conditions include higher temperatures and shorter reaction times than those previously reported in the literature for any hydrocarbon of comparable molecular weight. The investigation was undertaken in connection with a study of the feasibility of cooling very high speed aircraft by means of an endothermic reaction of the fuel prior to combustion. Cetane (ti-hesadecane) is a good model for a saturated paraffin jet fuel. and use of a pure compound avoids many of the difficulties of interpretation of results that occur when dealing ivith the complex mistures that comprise an actual fuel.
REACTOR
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Figure 1 .
Experimental apparatus
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Experimental Apparatus. Figure 1.
T h e experimental apparatus is shown in
Cetane \\as pumped from a pressurized fuel container by a Hills-11cCanna liquid feed pump to a preheater in which it \$as heated to the highest temperature a t which no substantial amount of cracking occurred. T h e preheater consisted of two coaxial steel tubes, and the feedstock was heated in the annulus by a n internal and a n external electric heater. T h e volume of the preheater was 87 ml. Pressure gages were located as shoivn on Figure 1.
The reactor consisted of a 347 stainless steel tube: 51 inches long, '/s-inch I.D., Vs-inch O.D., 48 inches of which was heated. T h e corresponding total volume was 10.25 ml. T h e reactor furnace was controlled in three sections by thermocouples placed in the reactor tube wall. T h e reactor wall temperature was measured a t three points, 6 : 24: and 41 inches from the reactor inlet, by thermocouples screwed into wells in the wall of the tube. The ends of the thermocouples were l / 3 2 inch from the inside wall of the reactor tube. Heat input rates could be adjusted so that the reactor wall was held isothermal within 10" F., as indicated by these three thermocouples. The inlet and outlet fluid temperatures were measured by thermocouples placed in two small steel blocks fastened onto the two ends of the reactor tube and heated by cartridge heaters to the measured temperature. By controlling the cartridge heater so as to equalize the temperatures recorded a t the thermocouple measuring the fluid temperature and a t that measuring the block temperature, heat losses were minimized. Reaction products were expanded to atmospheric pressure by a n automatically operated reducing valve and were then led to two condensers in series, a gas-sampling outlet, and a gas flowmeter. The first condenser was cooled with tap water, and the second with ice water. Experimental Procedure. The reactor was heated to the desired temperature. T h e inlet section of the reactor was overheated by approximately 40" to 50" F., since a temperaVOL. 1
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ture drop of this order was generally experienced after the flow started. The preheater was heated to 950' F.: since preliminary experiments showed that a t this temperature cracking even a t the lowest flow rates remained below 1%. The feed container was filled with a measured volume of cetane and pressurized with nitrogen to 15 p.s.i.g. Then the liquid pump was set to the desired feed rate, and, using the bypass line, the flow rate was checked. The reactor and preheater were pressurized with nitrogen to the desired pressure in order to avoid excessive residence time of the feed in the reactor during pressure buildup. After the conditions in the system reached the preset values, the feed pump was started. During the experimental run the pressure was kept constant by a pneumatically operated pressure valve controlled electronically by the pointer of the pressure gage. Steady state was usually achieved in the first 1 to 2 minutes. After the experimental run was completed, simultaneously the feed pump was stopped, pressure was reduced to atmospheric, and the preheater and reactor furnaces were opened up to cool them rapidly below cracking levels. The complete system was then flushed with approximately 10 liters of nitrogen to carry out small amounts of holdup from the system. Gas samples were collected during the runs, and the liquid products collected in the two condensers were united. T h e volume of feed remaining in the feed container was established after each run.
A total of 40 useful runs were made, from two to five for each combination of the three variables. The conversions obtained varied from 1 to 97% of the inlet cetane. Each run lasted for about 30 minutes after steady state was achieved, but three runs were extended for about 6 hours to establish that the 30-minute runs were truly representative and to obtain more accurate quantitative information on the amount of carbonaceous deposit formed in the reactor. Some kinetic studies at 500 p.s.i.g. were also made in a micro flow reactor that consisted of a hypodermic tube 114 cm. long and 0.02 inch in inside diameter. T h e tube was coiled: heated electrically, and held inside a Dewar flask for heat insulation. Cetane was fed at room temperature, and it is estimated that approximately the first 20% of the coil acted as a preheater. Further kinetic studies were performed in batch reactors at 800' F. One and a half milliliters of cetane was sealed into a glass tube of approximately 4-ml. volume, frozen, and evacuated before sealing. The tube was heated in a large aluminum block furnace for 2 to 6 hours. Feedstock. The cetane was ASTM grade n-hexadecane from Humphrey-Wilkinson, Inc., North Haven, Conn. (m.p. 18.2' C., b.p. 286.5' C., n'," 1.4344, d:' 0.7733, minimum purity 99%). Analytical Procedure. The analytical procedure has been described in detail (7). The product composition studies reported here all came from runs in the */*-inch I.D. reactor. The analyses and measurements included: (on gases) total volume and composition by gas chromatography; (on liquids) total volume, density, molecular weight by freezing point depression, cetane content by gas chromatography, and distillation into
