Reaction time in the dark zone

Fig. 6.10 Reaction time in the dark zone decreases with increasing (NO) at constant pressure.

Table 6.11 Dark zone lengths and reaction times in the dark zone.

Table 6-7. Dark zone length and reaction time in the dark zone for noncatalyzed and catalyzed propellants.

In the dark zone, the temperature increases relatively slowly and so for the most part the temperature gradient is much less steep than that in the fizz zone. However, the temperature increases rapidly at about 50 pm from where the flame reaction starts to produce the luminous flame zone. The gas flow velocity increases with increasing distance due to the increase in temperature. The mole fractions of NO, CO, and Hj decrease and those of N2, CO2, and H2O increase with increasing distance in the dark zone. The results imply that the overall reaction in the dark zone is highly exothermic and that the order of reaction is higher than second order because of the reduction reaction involving NO. The derivative of temperature with respect to time t in the dark zone is expressed empirically by the formulal =l... 

Since the final gas-phase reaction to produce a luminous flame zone is initiated by the reaction in the dark zone, the reaction time is determined by the dark zone length, L4, i. e., the flame stand-off distance. Fig. 6.8 and 6.9 show data for the dark zone length and the dark zone temperature, T, respectively, for the propellants listed in Table 6.3. The luminous flame front approaches the burning surface and... 

The dark zone length of liF-catalyzed propellants is increased by the addition of LiF in the region of super-rate burning, similar to the case of Pb-catalyzed propellants, as shown in Fig. 6.28. Table 6-11 shows the dark zone lengths and reaction times Xg in the dark zone producing the luminous flame at two different pressures,... 

Since the final gas phase reaction to produce a luminous flame zone is initiated by the reaction in the dark zone, the reaction time is determined from the dark zone length Ld, i.e., the flame standoff distance. Figures 6-7 and 6-8 show the results for the dark zone length and dark zone temperature, Td, of the propellants listed in Table 6-1, respectively. The luminous flame front approaches the burning surface and the dark zone length decreases as pressure increases for the propellants. There is no clear difference between the propellants with respect to the dark zone length and the pressure exponent of the dark zone, d = n - m, defined in Eq. (3.70) is determined to be approximately -2.0. The overall order of the reaction in the dark zone is also determined to be m= 2.6 for all the propellants. However, the dark zone temperature increases as pressure increases at constant (N02) and also increases as (N02) increases at constant pressure. 

MPa and 3.0 MPa. The reaction time defined in Eq. (6.3) remains relatively unchanged and no significant effect on the dark zone reaction is seen by the addition of LiF and C. 

A solution of Pd(dba)2 (1.75 g, 3.0 mmol), t-Bu(i-Pr)2P (560 mg, 3.0 mmol), methylenecyclopropane(43.3 g, 0.8 mol) and diethyl fumarate (135.2 g, 0.786 mol) in anhyd toluene (100 mL), was pumped through a stainless steel capillary tube heated to I50°C (temperature control). The pumping rate, as well as length and diameter of the tube, was adjusted in order to achieve a reaction time of 27 min in the hot zone of the tube. Subsequent fractionating distillation of the dark-red mixture yielded a forerun of toluene, followed by a main fraction (172.3 g bp 60-65 °C/10 Torr) consisting of toluene (0.9%), diethyl fumarate (1.1%), trans-S (R = Et 96.5%), diethyl tranj-4-methylcyclopent-3-ene-l,2-dicarboxylate (0.5%) and cw-8 (R = Et 1.0%), as determined by GC analysis. In order to prevent an increase of isomerization of the exocyclic double bond to yield the cyclopentene product, the bath temperature during distillation should not rise above 90°C. The yield of trans-S is 166.3 g (93.6%). 

Fig. 6 Schematic drawing of ZSM5 catalyst bed deactivation. View of the fused silica reaction tube at about 40 % of catalyst life time. Black zone (I) of deactivated catalyst particles covered with coke ("methanol coke"). Small dark reaction zone (II) in which methanol conversion to 100 % occurs. Blue/grey zone (III) of active catalyst on which a small amount of "olefin coke" produced by the olefinic hydrocarbon product mixture has been deposited on the crystallite surfaces. The quartz particles before and behind the catalyst bed (zones 0) remain essentially white.

The zones containing VBra are heated, one after another, to dark-red heat while a Hg stream is passed through. The heating is best accomplished in a tubular electric furnace, in which heating is more uniform than with open flames with such a furnace, pronounced local overheating and reaction rate differentials are prevented and the total reduction time is short. The reduction of 2 g. of VBrg takes 1-1.5 hours. 

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