thermal cracking of ethylene, propylene and light hydrocarbon

acetylene then acetylene.
B. :+Xl_gne crac+g
In CoWete amlogy with propylene cracking, the two major initial
reactions are :
%
C2fijo+C 2FI 4 -+ C~EI?O -+ c 4M 6 +HO
~ ( rno1-lsec-l) 1
7 107
~(~ccal/mol)
7.3 8)
c 2H 3 o+i:l--tC k 9 2H 2 +Ho+l,i 5 loq1 31.5 9)
The vinylradical acts as a p radical. in 8) and as a
9). The kinetic parameters of Z;'nese reactions were
extensive literature survey ( 1 1,15,16,17).
The border line between the two zrea's is now given hy :
k8(C21130)(C2H4) = k (C H O )(l.i)
9 2 3
This line is the flit1 line in Figure 18. ';;itti a second orcier
initiation 1i.ke 2C H 422I.i- "+C21150 and a teir;iinatj.on involving
two vinylradicals ;i2&Ii3 -+ .? , !C4Hg), an order of 2 is predicted
for Conditions in the polymerization zone and a.n order of 1 for
conditions in the dccoi~iposition zone.
Si1:lon and Iijaclc (1 5) and Silcocks (1 8) observed seconci order
kinetics, while l.:olera and Stuhbs (16) found second order 1cineti.c.s
in the region 5 of Fi,mre 18 and first order kinetics in region TI,
in agreement with tie proposed border line. R.m.uzi's data (11 )
ivhi.cli arc located in the transition area lead to an order of 312
for tiic ~:lolial. disa?xarrr.nc:e reaction.
The pilot plant experiinento cover the transition zone. The order
!ea.s shown in 'I'al:~lea 4 and 6 to evolve fron! 1 .6 to 1 .2 as the
temperature is increased and the activation eiiergy from 40 to
2.boIit 75 kce?.l/iilol.
I '
J?'Iie pol.yilir~i.iarttion can also lie of a J!lOleCILlar nature :
.A(I mol-' sec-l) :,'(1cca,l/rnQl) iter
C,ii +I ,'jC I L ---> polymer
r 4 4
3 107 27 19 IC)
Th.e ratio of the ra.c?ica.l. reaction and molecular condensation rates
was ca3.cula.ted fron 2.n isotlierraal. non cteady state radi.ca1 siimlation
of the ethylene experiinents by Sunderan & PToment (IO), to be
71 at 80GV and 1.; conversion, 15 at OOO°C and 3:~ conversion ana
10 at 825°C and 8 ;i conversion.
It follows that molecular condensation cannot be neglected a.t
8CO°C and higher. This explains vlhy the experi.inenta1 Cg+ frsction
was SO stroiicly correla.ted aitli the temperature. In propylene
crrtcitiiie the ratios of the rates of the radical po1yl;ierisation and
the moleculcir conderisation at OGCV would Le of the ordcr of IC00
because the allyl rzdical is hich and butadiene is low.
It is clear also froiii Piwre 16 for ethylene ax1 from Figure 17
for propylene that tile influence of the pnr-tial and total pressure
on fne order anci activation encrt27 vrill be inore or less pronounced
depending u?on the distance frnrn the border line. The higher acti-
vation enerzies Given in lnbles 6 and 7 for the conditions of the
classes 2, 4 and, 5 may be related to t
Pinally, il; ShOii.ld be added that even
less tila (1.2 wt ,:, in the etiiyleiie feed, the initiation reaction
C 11 i2CI11° cannot lje ne!:lecterl, as concluded already by
2 6
1:m~ugi (I I ) unc~ BY yowell and liartin (20) .
141