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4
 
raw C
4
cut
hydrogenated C
4 
reactor
1
column
reflux 
drum
static
mixer
pump
cooler
purge 
gas
WLD
WLD
purge 
gas
static
mixer
reactor
2
cooler
cooler
pump
recycle 
drum
steam
Figure B-4-2: Flow sheet of the C
4
 selective hydrogenation process 
Feed section 
The feed to the reactor consists of both fresh and recycle C
4
cut streams. The recycle stream 
comes from the recycle drum. The recycle stream and the fresh stream are first mixed together 
before being mixed with the hydrogen make-up. 
 
Reactor section 
The total stream enters the first reactor top and flows downwards through the fixed catalyst 
bed. The hydrogenation of butadiene occurs selectively according to the following reaction: 
CH
2
═ CH ─ CH ═ CH
2
+ H
2
→ CH
2
═ CH ─ CH
2
─ CH
3
 
The reactor effluent is then sent in the recycle drum where free water is purged if necessary. 
In case of two phase mixture, vapor shall be partially condensed in the condenser. The 
resulting liquid falls back to the condenser and the purge gas is sent to the flare. Liquid from 

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the recycle drum is then pumped, cooled split into two streams, the recycle C
4
cut and the 
stage 2 feed, that goes to the second reactor. 
The stage 2 feed is mixed with a second hydrogen make-up and enters the second reactor 
bottom and flows upwards through the catalytic bed. The remaining butadiene is 
hydrogenates selectively and the effluent is sent to the C
4
stabilizer. 
Stabilization section 
The stage 2 effluent is preheated in a heat exchanger with the stabilizer bottom and feed the 
stabilizer. In the stabilizer, light compounds of the products due to the H
2
make-up quality are 
removed. The column overhead is partially condensed and collected in the reflux drum where 
vapor is purged and water drained. The liquid returns back to the column as reflux.
The column bottom stream constitutes the final product. This stream is cooled down and 
routed to product storage.