Delgado, M. Khvastunkov, K. Marwan and A. Marri, Saudi Aramco, Dhahran, Saudi
The disposition of the crude C4
stream from a steam cracker is a project configuration topic that deserves some
discussion. Depending on the business case supporting the project, this stream
may be sold as crude C4s into the market; processed into valuable
components, including 1,3-butadiene, isobutylene and normal butenes for further
processing; or it may be fully or partially saturated, hydrogenated and
recycled back to the ethane cracker. Ultimately, economics and strategic goals will
dictate which of these options, or combination thereof, is to be incorporated
as the basis of the project for an optimum configuration. This decision should
take place in the early project development stages to minimize the amount of
rework. This article discusses some of these options, compares recycle-cracking
crude C4s vs. not recycle-cracking, and highlights the expected
impact on overall steam cracker process yields.
Crude C4’s value chain. Crude
C4s are a byproduct of steam crackers, primarily ones that process
heavy feeds, such as naphtha. Steam crackers that process lighter feeds are
lower in C4 yields (FIG. 1); therefore, have a lesser need for crude C4
processing facilities. The feed to the steam cracker greatly defines not just
the steam cracker scope, but also the downstream scope necessary to handle the
volumes of C4+ coproducts that are produced.
The composition of the crude C4
stream from a steam cracker contains several key components with value-added
potential. These are 1,3-butadiene, isobutylene, butene-1, c-2-butene and t-2-butene.
Extraction of these necessitates the addition of scope items that need to be
considered in a project. FIG.
2 is indicative of a representative C4 value chain.
Economic motive. A major motive for recycling co-cracking is to
take advantage of the lower C4 prices and avoid logistic and
marketing challenges involved in C4 derivatives selling. There are various
approaches in recycling butadiene (BD), which includes selective hydrogenation that
targets BDs and full hydrogenation that targets all C4 olefins—selective
hydrogenation yields a higher level of propylene. Depending on the project
scope, operations can target the preferred yield (FIG. 3).
As shown in FIG. 4, the capital
investment for a total hydrogenation unit (THU) + BD + methyl tertiary-butyl
ether (MTBE) case is higher than that of the THU for the same plant capacity. However,
the product value of THU + BD + MTBE case potentially offsets the capital
investment difference due to the higher value of the extracted products. A net
present value (NPV) assessment would need to be made for the specific project
location and market conditions.
hydrogenation and co-cracking of crude C4s. This
is the simplest configuration using a light naphtha feed. The full crude C4
stream is hydrogenated, and the effluent is recycled back to the steam cracker
The olefins, diolefins and acetylenes are fully saturated. Note: This
option will greatly impact the steam cracker unit (SCU) yields, and the SCU
licensor needs be involved for this option to be included in the design. One of
the hydrogenation technology companies should be consulted regarding this
option, as well. This applies to any case where the hydrogenated C4s
are recycled back to the SCU. If recycled cracking is not preferred, the C4
co-product can could be sold as crude C4s. No 1,3-butadiene nor any
other value-added products are produced.
This option would require a THU. Depending on
the amount of acetylenes, diolefins and olefins, this unit will require up to
three hydrogenation reactors to ensure full hydrogenation of the targeted
species. This would be the only scope to handle the C4 stream in
this case. However, if the C4s
will be at least partially sold as crude C4s, then the necessary
export facilities will be needed to export into the market. A cost/benefit
analysis would be needed to compare both options. This analysis would examine
differences in scope and capital expenditure (CAPEX), steam cracker yield, cost
of production and a NPV comparison to determine whether the additional CAPEX
for export facilities is justified.
Full extraction alternative. One
alternative to the basic configuration with a light naphtha feed results in
decreased raffinate-3 material to be co-cracked in the steam cracker, which
impacts the overall yield basis from the steam cracker. This may be
accomplished by adding an olefins conversion process (OCP) technology and
supporting units, such as a BD selective hydrogenation unit (BSHU) and a
1,3-butadiene extraction unit (BDU). The olefins conversion technology makes
use of metathesis and isomerization reactions to produce additional propylene.
This is accomplished by reacting ethylene with C4 olefins. If
propylene is profitable and ethylene is in excess supply, this is an attractive
option. The employment of a BSHU converts BD and acetylenes into butenes, which
could then be fed to the OCP unit, as well.
the configuration shown in FIG. 6, the BSHU converts BD into butenes, which can then be
sold as a product stream for further processing.
There are many alternatives available in
processing the crude C4 stream, such as using a skeletal
isomerization unit—which can convert normal butenes into isobutylene—in
conjunction with a BDU and an MTBE unit. The additional isobutylene from the
skeletal isomerization unit can then then be converted into MTBE to boost its
production. Some of the isobutylene might also be used in the production of
polyisobutylene as a value-added product. However, the differences in scope and
CAPEX, steam cracker yield and differential NPV calculations must be analyzed
to choose the optimum configuration that best supports the business case.
The sample configuration shown in FIG. 7 is a typical
layout that extracts 1,3-butadiene, converts isobutylene into MTBE and fully
saturates the olefins in the remaining raffinate-2 stream.
C4 value chain configuration chosen for a new petrochemical complex
greatly depends on key project business case drivers, such as steam cracker
feed slate, commercial and market conditions, and product value-uplift
potential. Several basic configurations have been presented here which can be
used as a starting point in the support of future greenfield petrochemical projects
and the objective of maximizing chemicals production. These basic layouts may
be used for further configuration evaluations and definition to tailor-fit the
individual project’s business objectives and help in securing a successful C4
component management system and positive economic return to the business. HP