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4.2.1. Raw C
3
 cut 
The feed of the C
3
selective hydrogenation units is a blend of different C
3
compounds with a 
major percentage of propane and propylene. Sulfur, nitrogen, arsenic and mercury can be 
present in trace amounts. 
The following table indicates the typical proportion of the raw C
3
cut:
Compound Quantity 
Propane + propylene 
Methylacetylene (MA) 
Propadiene (PD) 
(C
2
-
) + (C
4
+
) 
90 to 95% wt. 
1.5 to 4 % wt. 
1.5 to 3 % wt. 
0.25 % wt. 
Table B-4-1: Typical composition of a raw C
3
 cut entering a selective hydrogenation unit 
4.2.2. Raw C
4
 cut 
In addition to C
4
components, raw C
4
cut contains C
3
and C
5
components in minority 
proportions. It can also contain impurities in trace amount such as sulphur, nitrogen, C
8+
and 
polymers, carbonyl compounds, acetone, mercury or arsenic. 
4.2.3. Raw pyrolysis gasoline 
Raw pyrolysis gasoline has a high aromatics content, about half of this figure being benzene. 
It contains also diolefins, olefins and sulphur in variable amounts. Impurities such as nitrogen, 
arsenic, antimony, or mercury can also be present. 
 
4.2.4. Hydrogen make-up 
 
The hydrogen make-up can contain trace amounts of light hydrocarbons (CH
4
, C
2
H
6
), carbon 
oxides, chlorhydric and sulfuric acids and mercury. These compounds are in very little 
quantity but contaminate the catalyst. They can be consequently present in effluents from 
regeneration. 
 
4.2.5. Catalyst 
These hydrogenations are promoted with a palladium or nickel-on-alumina catalyst. 
4.3. Products characteristics 
Hydrogenated C
3
cut is mainly composed of propylene (85%) and propane (15%). It shall 
have an MAPD content inferior to 500 ppm (vol). It is routed to the next unit of the olefin 
plant. There is no by-product from this unit. 
C
4
product is mainly composed of butenes and routed to the next unit of the olefin plant. It 
contains also butane and traces amount of C
3
and lighter, C
5
dienes and acetylenes, 
acetonitrile, sulfur and water. Waste effluents are produced: water and purge gas from recycle 
and reflux drums. 

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44
The hydrogenated raw pyrolysis gasoline should be olefin, diene and sulphur free. 
4.4. Normal operations 
Simple flow sheets of each process are represented below.