Hydrocarbon Processing - March 2021 - 43

Special Focus

Petrochemical Technology
Z. LIU and S. JOSHI, Sulzer GTC Technology, Houston, Texas;
and H. MA and R. ZHANG, Sulzer GTC Technology,
Beijing, China

Best practices for pygas-based styrene extraction
Pyrolysis gasoline (pygas) is a by-produced fraction of hydrocarbons generated from a steam cracker. Rich in aromatics
content, particularly for benzene and toluene, pygas was historically blended into the gasoline pool as a high-octane number component. In this modern era, due to stringent benzene
specifications in the gasoline pool and growing demand for the
aromatic molecules from petrochemicals and chemicals manufacturing, the majority of pygas is fed to aromatics extraction
units for benzene, toluene and mixed xylenes (BTX) recovery.
How much pygas can be by-produced from a steam cracker
primarily depends on the type of feed to the cracker and the
cracking severity. TABLE 1 lists the yield of pygas, which is typically defined as the hydrocarbon fraction from C5s to 204°C
(399.2°F) end point, from the various cracker feedstocks under
relatively high cracking severity.
The composition of pygas consists of more than 200 species,
including paraffins, naphthenes, olefins, diolefins, cyclo-olefins,
acetylenes, aromatics, alkenyl aromatics and multi-ring aromatics, along with impurities, such as sulfur, nitrogen and chloride.
TABLE 2 indicates a typical hydrocarbon PONA breakdown of
pygas when cracking naphtha feed.
To prepare the feed for aromatics extraction, mono-olefins
and diolefins in the raw pygas need to be saturated, and sulfur,
nitrogen and chloride need to be removed. The practical approach is to hydrotreat the raw pygas in two stages. First-stage
hydrotreating saturates diolefins to mono-olefins, and secondstage hydrotreating saturates the mono-olefins and removes
sulfur, nitrogen, chloride and other impurities. While the twostage hydrotreating scheme is widely accepted in the industry,
the fractionation scheme for the hydrotreated pygas varies from
case-to-case. Fractionation is optimized based on the evaluaPrefractionation
C5
Pygas feed

DeC5

Two-stage
hydrotreating
C6 - C8

DeC8

Aromatics extractive
distillation
C6-C8
non-aromatic
raffinate

1st-stage
hydrotreater

EDC

BTX
extract
SRC

C9+
2nd-stage
hydrotreater

FIG. 1. Typical pygas processing scheme.

Post-fractionation

tion of process technology and economic benefits of separating
various products. Typically, for the mega-sized liquid-fed steam
crackers, the C5 cut and C9+ cut are segregated from the C6-C8
heart-cut since both the C5 and C9+ cuts contain many valuable
unsaturated monomers that can be utilized for manufacturing of
high-value derivative products. The C6-C8 cut is hydrotreated
in two stages and sent for the extraction of benzene, toluene and
xylenes. FIG. 1 presents the integrated process configuration of
pygas pre-fractionation, heart-cut two-stage hydrotreating, aromatics extractive distillation and aromatics post-fractionation.
Steam cracking of liquids such as naphtha, diesel and gasoils
co-produces 25 kg-35 kg of styrene monomer (SM) for every
ton of ethylene production. This corresponds to about 5 wt%
SM in the raw pygas and about 40 wt% SM in a raw C8-rich cut
when such a stream is separated. TABLE 3 provides pygas C8 cut
compositions. For a steam cracker producing 1 MMtpy of ethylene, there is the potential to recover up to 35,000 tpy of SM.
Historically, styrene recovery from pygas was not considered
because the right technology was not available and ethylene
crackers were not large enough for recovering styrene at an ecoTABLE 1. Cracker yields

Cracker feedstock

Amount of feedstock
Amount of pygas byconsumed when 1 t of
produced when 1 t of
ethylene is produced, t ethylene is produced, t

Ethane

1.246

0.25

Butanes

2.47

0.162

Light-naphtha
Full-range naphtha

3.18

0.48

3.423

0.764

Atmospheric gasoil

3.938

0.716

Vacuum gasoil

4.282

0.687

TABLE 2. Typical pygas composition
Benzene

Wt%
Toluene

Benzene
column

Saturates

Olefins

Diolefins

Aromatics

Total

0.1

0.2

0.5

0.3

3.4

11.8

15.5

Toluene
column

0.7

0.9

31.2

36.8

0.6

0.7

2.5

16.3

20.1

Xylenes

0.3

0.4

1.5

9.6

11.9

C9+

1.2

0.3

3.1

10.8

15.3

Total, wt%

3.3

5.9

67.9

100

Hydrocarbon Processing | MARCH 2021

Hydrocarbon Processing - March 2021

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