Problem 1 :
Use the data in Table 2.2 (ADECH 04 lecture) to calculate molar proportion of hydrogen, methane, and ethylene from ethane cracking. How would this product distribution change if conditions were such that the kinetic chain length of the reaction was reduced (i.e. that the number of propagation steps per initiation step went down)?
Solution :
In table 2.2, the product distribution in steam cracking that uses ethane as the feed : H2 (3.6%wt), CH4 (4.2 %wt) and C2H4 (48.2 %wt)
Molar proportion of H2 and ethylene : (3.6/2.02)/(48.2/28.05)=1.05
Molar proportion of methane and ethylene : (4.2/16.04)/(48.2/28.05)=0.15
Molar propotion of ethylene and itself : 1.0
Initiation :
𝐶
𝐻
3
𝐶
𝐻
3→𝐶
𝐻
3. +𝐶
𝐻
3.
Propagation step of ethane thermal cracking :
𝐶
𝐻
3. +𝐶
𝐻
3
𝐶
𝐻
3→𝐶
𝐻
4+ 𝐶
𝐻
3
𝐶
𝐻
2.
𝐶
𝐻
3
𝐶
𝐻
2.→𝐶
𝐻
2=𝐶
𝐻
2+𝐻.
𝐻. +𝐶
𝐻
3
𝐶
𝐻
3→𝐻. +𝐶
𝐻
3
𝐶
𝐻
2.
5
𝐶
2
𝐻
6→2𝐶
𝐻
3+4
𝐶
2
𝐻
4+3
𝐻
2

If conditions were such that the kinetic chain length of the reaction was reduced, that of methane will increase and the proportion of H2 will be decrease.

Problem 2 : Consider ethylene production from an ethane cracker operating as indicated in Table 2.2 (ADECH 04 lecture), assuming no losses in the separation section (which is of course unrealistic).
a) What is the productivity of ethylene?
b) Calculate the yield of ethylene after 1st ethane recycle? (calculation offered 100 tons feedstock ethane)
c) How many recycle times of ethane to get maximum ethylene amount? (from 100 tons feedstock)
Solution :
All the calculation based on 100 tons of ethane as feedstock.
Productivity of ethylene :
H=(48.2/28.053)/(100/80.069)=51.66%
When ethane is recycle for the 1 st time .
m(C2H4) = 48.2+40x48.2/100=67.48 tons
Y=(67.48/28.053)/(100/30.069)=72.33%
When ethane is recycle for 2 nd times
m(C2H4)=48.2 + 40x 48.2/100+40x40x48.2/100
for n times

𝑚
𝐶
2
𝐻
4=48.2+
𝑖=1
𝑛
40
𝑛
48.2
100

Y=(m(C2H4)/28.053)/(100/80.069)
We try the value of n from 1 until we get the stable and maximum of Y
So the result will be n = 20 and y= 86.1%
To get the maximum yield , ethane has to be recycle 20 times

Problem 3 : Suggest mechanisms for the formation of (i) toluene, and (ii) styrene, in the thermal cracking of 1-phenylbutane.
Solution :
Tolunene

𝐻
2
𝐶
𝐻
2
𝐶
𝐻
2
𝐶
𝐻
3→
𝐻
2
𝐶
𝐻
2
𝐶𝐻.𝐶
𝐻
3+𝐻.


𝐻
2
𝐶
𝐻
2
𝐶𝐻.𝐶
𝐻
3→
𝐻
2.+𝐶
𝐻
2=𝐶𝐻𝐶
𝐻
3



𝐻
2.+𝐻.→
𝐻
3

Styrene

𝐻
2
𝐶
𝐻
2
𝐶
𝐻
2
𝐶
𝐻
3→.𝐶
𝐻
2
𝐶
𝐻
2
𝐶
𝐻
3+𝐻.

.𝐶
𝐻
2
𝐶
𝐻
2
𝐶
𝐻
3→+𝐶
𝐻
2+𝐶
𝐻
3
𝐶
𝐻
2.

𝐶
𝐻
3
𝐶
𝐻
2.+𝐻.→𝐶
𝐻
3
𝐶
𝐻
3



Problem 4 : What would you expect to be the major products formed in the thermal cracking of (i) isobutane, and (ii) 2,2-dimethylpropane?
Solution :
Isobutene
Initiation : 𝐶