Hydrocarbon Processing - March 2021 - 46

Petrochemical Technology
ecules from pygas value to monomer value, there are other benefits connected to extracting styrene out of pygas. Styrene saturation to EB contributes a large portion of the exothermic heat
in the first stage hydrotreating. With styrene removed from the
feed to the hydrotreater, the heat released in the reactor decreases, the tendency of polymerization for styrene and other
diolefins lessens, fouling problems become less severe and the
pressure across the first-stage hydrotreater drops. With less
heat released, the outlet temperature decreases, which means
fewer hydrocarbons vaporize, and hydrogen partial pressure in
the reactor outlet increases. To maintain the same partial hydrogen pressure in the vapor phase at the reactor outlet, the
hydrogen feed flowrate can be reduced to save hydrogen consumption. With a lower vaporization ratio inside the catalyst
bed, better distribution and diffusion of hydrogen inside the
liquid hydrocarbon can be achieved. The selectivity of diolefins and acetylenes to mono-olefins is improved. Both catalyst
regeneration cycle length and service life will be prolonged. All
these advantages result in improved operation of the trickle bed
reactor. TABLE 5 summarizes the impact on pygas hydrotreating
with and without styrene extraction.
After applying styrene extraction to pygas processing, the
pygas-based mixed xylenes product has a different EB content.
In general, pygas-based mixed xylenes are not considered as a
TABLE 5. Summary of the impact on pygas hydrotreating
with and without styrene extraction
Before proprietary
styrene processa

After proprietary
styrene processa

RIT/ROT for ﬁrst-stage
reactor, °C

55/102

62/85

RIP/ROP for ﬁrst-stage
reactor, kg/cm2g

27/26.2

27/26.5

Recycle/fresh ratio
for ﬁrst-stage reactor

2.8

2.3

H2 consumption
over feed, kg/t

6.5

Catalyst regeneration
cycle, yr

Catalyst life, yr

preferred feed for PX production, as the EB content is much
higher than reformate-based mixed xylenes. This relegates the
pygas-based mixed xylenes to solvent use or gasoline blending.
After incorporating styrene extraction within the pygas processing scheme, the EB content in pygas-based mixed xylenes
drops significantly as a result of avoiding the styrene conversion into EB in the hydrotreater. Typically, the EB content in
pygas-based mixed xylenes will be in the range of 20%-30%,
with styrene extraction compared with 50%-60% when
styrene is not extracted. EB is a molecule that imposes challenges for PX production. Although EB can be reformed to PX
through isomerization, the capital expenditure and operating
expenditure associated are significantly higher. Any decrease
of EB content in the feed to the PX complex means a higher
efficiency in terms of per ton of PX manufactured. This is the
reason that pygas-based, EB-rich mixed xylenes are not a preferred feedstock for a PX complex.
The integrated refining/petrochemical complex consistently shows higher profitability than the corresponding standalone refinery and ethylene cracker. The refinery sends gasoil,
naphtha and LPG to the steam cracker as the feedstock for light
olefins production, and receives pyrolysis fuel oil, pygas, C4 raffinate and hydrogen for transportation fuel production. Pygasderived styrene and the resultant xylenes add another level
to improve the integration of processing facilities. Under this
scenario, pygas-derived BTX will be supplemental to reformerderived BTX feeding the aromatics complex for PX production. With the proprietary styrene technology, an integrated
refinery/petrochemical complex can produce SM in addition
to p-xylene and benzene as aromatic products.
At present, seven proprietary styrene processing units have
been licensed, with the largest having a capacity of 80,000 tpy
(FIG. 5).
a

NOTES
Sulzer GTC Technology's GT-Styrenesm process

ZHEPENG LIU is Director of Global Engineering for Sulzer GTC Technology,
based in Houston, Texas. With more than 25 yr of experience in the refining
and petrochemical industries, his background covers catalyst and process
development, process design, technical services and business management.
He studied chemical engineering at Tianjin University, the National University
of Singapore and Delft University of Technology. He also holds an MBA degree
from the University of Houston-Victoria.
SACHIN JOSHI is the Business Segment Leader (GT-Styrene, pygas and
solvents) at Sulzer GTC Technology's Houston headquarters. He earned an MS
degree in chemical engineering from the Institute of Chemical Technology,
University of Mumbai in India. He has more than 18 yr of experience in the
refining and petrochemical industries, with expertise in technology and process
development, process engineering and process simulations. While at Sulzer
GTC, Mr. Joshi has helped develop new patented technologies in aromatics
production. Previously, he worked with Lummus Technology, Ingenious Inc.,
Reliance Industries Ltd. (Jamnagar Refinery) and Gharda Chemicals Ltd.
HAIYAN MA is a Senior Process Engineer for Sulzer GTC Technology based
in Beijing, China. She received her MS degree in chemical engineering from
Beijing University of Chemical Technology. Ms. Ma has more than 8 yr of
experience in process engineering design in the petrochemical industry.

FIG. 5. Commercial styrene unit-utilizing the proprietary styrene
process-operating in China.

MARCH 2021 | HydrocarbonProcessing.com

RONGRONG ZHANG is a Principal Engineer for Sulzer GTC Technology based
in Beijing, China. He received his MS degree in chemical engineering from
Beijing University of Chemical Technology. Mr. Zhang has more than 10 yr
of experience in product manufacturing and process engineering design
in the refining and petrochemical industries.

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Hydrocarbon Processing - March 2021

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