Based on the products formed and the rates observed, a Rice-type, free-radical mechanism was proposed for the thermal decomposition of n-butane. Extensive studies of the thermal decomposition of paraffinic hydrocarbons, especially lighter ones, have been made, and a comprehensive review of the work has been presented by Steacie (1). Just as for other paraffins, the relative amounts of various products in the case of the fully inhibited decomposition of.
Part of the variability in results is probably attributable to the chain nature of the reaction. One possible contributor to the spread of results was the difficulty in extrapolation.
APPARATUS
Part of the preheater at the end next to the mixing chamber was housed in a stainless steel guide tube with a 0. When the preheater was connected to the reactor, part of the preheater together with the mixing chamber was mounted inside the reactor. The lower end of the nozzle fit snugly on the outer wall of the mixing chamber, as shown in Figure 2.
A pitot tube probe was placed in one of the grooves and a thermocouple probe in the other. The pitot tube is connected by a tee both to the kinetic side of the micromanometer and to a sample line.
In the simplified approach, the reactor is considered to be isothermal as well as isobaric, and the composition of the reacting mixture is assumed to be uniform with respect to radius. The order of the overall reaction thus depends directly on the ratio of the butane concentration to the free radical concentrations. Surface effects of the reactor tube wall, both in catalyzing the reaction and in terminating the free radical chains, have not been investigated experimentally.
The temperature of the reactor wall, as well as the inlet gas temperature, was continuously recorded on a Micromax recording potentiometer. The velocity measurements at any traverse position in the reactor required measurement of the static and velocity heads.
RESULTS
Gas temperature at the inlet of the reactor was controlled by the preheaters and the heaters on the reactor wall below the inlet nozzle. The pressure drop, although unknown, was assumed to be independent of the pitot tube position for each run. The difference between the pitot tube reading and the static head reading therefore represented a relative measure of the linear gas velocity at the pitot tube.
The accuracy of the chromatographic analysis was demonstrated indirectly by analyzing duplicate samples taken from the same sample bulb. The composition of the samples taken from the reactor via the Pitot tube are listed in Appendix B.
CORRELATION OF DATA Temperature
Further detailed discussion of reducing the pressure data to obtain velocities is given in Appendix Eo. Under such conditions, the concentration of butane varied slightly both within a run and from run to run. Thus, the effect of concentration on reaction rate was not readily detectable. On the other hand, the agreement indicates that the order of the reaction lies somewhere between 1 and 2, as predicted. This shows that more reactions took place near the wall than at the center, even at low conversions, probably due to the limited heat transfer rate, although the difference is very small.
The rates of formation of the other three products, i.e., ethylene, propylene, and butylene, can now be predicted from equations 67, 68, and 69 respectively. A further test of the mechanism is provided by calculating the rates of formation and disappearance of H·, CI~:J·, and.
DISCUSSION Apparatus
Since the flow at the exit of the reactor was Poiseuille, the axis of the tube should have been parallel to the axis of the reactor tube. However, the location of the static pressure outlet was not critical due to the method used to correlate the velocity data. It was thus concluded that the actual temperature of the reaction was not within reasonable limits affected by the gas inlet temperature.
Thus, the portion of the reactor length where the gas temperature could have been lower than the wall temperature was less than one inch. Experimental results here refer to the temperature and. velocity profiles as well as the composition of the reaction mixture at the exit of the reactor. Samples of the reaction mixtures were also taken through the sampling port in the reactor head.
Since natural convection caused circulation at the outlet end of the reactor, the flow pattern there was probably distorted. This is expected due to the endothermic nature of the reaction as well as the shorter retention time of the gas molecules flowing into the center of the reactor. The composition of the reaction mixture is fairly uniform in the radial direction, as illustrated in Appendix B.
Most previous investigations reported the rate constant corresponding to a first-order rate expression. At low conversions, Case II should still approximate the actual behavior of the reaction mixture reasonably well. The velocity at the inlet of the reactor was flat along the tube and was found by measurement to be parabolic at the outlet.
PEBBLES
CHAMBER
TANK 1 Runs
TANK 3
SUMMARY OF REACTOR OPERATING CONDITIONS FOR 460°C RUNS Run Average Wall Inlet Gas Butane Feed Reactor Number Velocity No. SUMMARY OF REACTOR OPERATING CONDITIONS FOR 510°C RUNS Run Average Wall Inlet Gas Butane Feed Reactor SUMMER Velocity No. CONDITIONS FOR 560°C RUNS Run Average Wall Inlet Gas Butane Feed Reactor Number Velocity No.
SUMMARY OF CONVERSION AND MATERIAL BALANCE FOR 460°C RUNS Mole c 4H 10 Converted mole ratio of C to H Moles of products in the product stream formed per l Mole of run Quantity. SUMMARY OF CONVERSION AND MATERIAL BALANCE FOR 510°C RUNS Mole c 4H 10 Converted mole ratio of C to H Moles of products in the product stream formed per l Mole of run No. SUMMARY OF CONVERSION AND MATERIAL BALANCE FOR 560°C RUNS Mole c 4H 10 Converted mole ratio of C to H Moles of products in the product stream formed per 1 mole of run no.
A least-squares analysis of the experimental data was used to determine the value of N for each component. Dalton's law was used in combination with individual compressibility factors in calculating the composition of the gas in the bottle. In calibrating the chromatographic system, four or more runs were made for each mixture.
The average values of N for the first nine components shown in Table Alii were found to be constant over varying amounts of the components. Although the minimum amount of the analyzed calibration samples corresponded to 0.7% by volume for isobutane in a 1 cc. Due to changes over time in the nature of the constituent parts of the chromatographic system, daily calibration is required for quantitative studies.
Since steel and iron are excellent thermal conductors, the reactor head temperature remained significantly lower than the wall temperatures as the reactor heated up. Additionally, the effect of the reactor head shall be considered to be equal to an imaginary boundary four inches above the upper thermocouple on the reactor wall. However, he notes the importance of natural convection in heat transfer at the reactor exit.
Because the gas at x = 0 is hot, the reactor head is cold, and the reactor wall is relatively warm, some circulation of the gas in a pattern as shown in Figure C3 will be induced due to natural convection. The temperature of the gas therefore increases as the gas moves further away from r. The absorption and radiation of gases are caused by changes in the energy level of the.
Theoretically, a~ for any gas can be estimated as a function of temperature, pressure, and wavelength from the molecular structure of the gas. The second part consists of the rest of the reactor and is at a temperature of 510°C.
REACTOR HEAD
WALL
RUN 34
- INTRODUCTION
From the compositions, the conversions in terms of number of moles of butane converted per 100 moles of feed were calculated for samples at r/r0 - 0.0 and 0.70. The basis of calculation is 100 mol of feed with 6 mol of components other than butane. The number of moles of butane equivalent to Y moles of products was therefore ~C/4 and the value of Z was 4Y/~C.
After a careful literature search, the following empirical formulas were chosen to be used in the evaluation of the physical properties of n-butane and its main decomposition products, i.e. The activation energy for the reverse reactions involving the joining of two free radicals is zero. A number of approximate solutions have been obtained by neglecting various terms in the equations, such as those obtained using boundary layer theory, and the Stokes equation for very slow motion.
In the first case, potential flow conditions were assumed to exist at the inlet of the plates. In the second case, these conditions were assumed to exist only well upstream of the inlet. Due to viscous effects, boundary layers formed on both walls, and their thickness increased in the downstream direction.
At small distances downstream from the inlet, the boundary layers grew in the same way as those along single plates, with a flat velocity distribution in the central core. To satisfy the continuity equation, the velocity at the center had to increase at such a rate as to compensate for the decrease in the flow velocity in the boundary layers. Downstream, the assumption of a flat velocity distribution in the central core no longer holds.
