chemical physics of discharges - Argonne National Laboratory

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The phenomena ir. t:te pias::.& ::et experiments which convert less nitrogen than
the ,;axinurn pred3ct.Yd 6). the ?'re:czilig rnuuel my well be l.ir,ked to a [nixing or diff'L;sian
rate (CHq a:. i 32 were !~dt prcfliixta) or' tc temperatures 4nsur'ficient to reach
Thi;.qsesticn is ~ i ~ sub,).:et l : 12 a separ-te theoretical analysis now in progress
Tf .
by the author .
THE REACTION SEQUENCE
The experimental data support the fallowing description of the overall reac-
tion seqdence:
i. Plasma Reactic;;w. ' Feed reagerits. are dissociated to their atomic constitu-
ents in a therim, plasma. Some Tthermai ioiiizatiQn accurs at sufficiently
high ternperatu.-s.
CF& i 92 - c + H + N + C+ + H+ + N+ + e-
2. Initial Coohnt Plasma taken inco a small-diameter cold tube rapidly
cools witn chemical reactio:is following equlibriwn.
C + H + N + ions - H + CIV + C2H + N2 - HCN + @ + N2 + C2w
3. Frozen Reaction Emetics. Rapid cooling continues, but at a rate much
greater than the progress of the apparently complex reaction sequence
necessary to des%roy trle species HCN, C2H2, and %. Hence the composi-
tion of the cooiliig gas is frozen at the end of Step.2.
Evidence fyr the Freezing llbdel in Other Reacting Systems. The freezing model ,
likely is applicable to a wide variety of reacting thermal plasma systems which '
emplcy a rapid quench. hmnann and. Timiinsl5 found a mechanism involving the freezin&
of equilibratiag reactiuns 'ta be applicable to their study of the quenching of \
nitrogen-oxygetl plasmc. Ahed by a wealth of published rate data on chemical reactiotis
in air, they were ~ble to model thc time-temperature-composition history of
M-0 plasma cooling within mail-diameter tubes. A quite similar freezing tempera- 1
ture, Tf , was found at 35Oi;'K 1'c.r that system. The equilibrium composition of nitric
uxi?e, NO, at Tf was preserved during continued rapid coolfng.
\
CONCLUSIONS ' . , . . \
Mixtures of mechane and nitrogen can be fed continuously to a thermal argon
indluctiori plasma maintaified abcve 13,300 K. .A rapid quer.ch of the heated plasma
kxture produces HCN, C2@, anc &.
the nitrogen converted, range between 3 and 70 percent, a function of the input
,
Final yields of HCN, expressed'as a fraction Of I
stoicbiometry . i
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