Hydrocarbon Processing - February 2022 - 23

Special Focus Digital Technologies
Z. HOU, S. WANG, D. CAMPBELL and W. CHAN,
Aspen Technology, Bedford, Massachusetts
Optimize refining operations using
plant digital twin based on molecular modeling
A new era of the energy revolution has arrived with the rapid
growth of innovative technologies that utilize alternative energy
resources, such as electricity, solar, wind and hydrogen, and an
increasing demand for sustainable use of natural resources.
Refiners are facing unprecedented challenges in maintaining
their profit margins. For example, the demand for fuels, especially
gasoline, is decreasing significantly due to the widespread
adoption of new vehicles that run on various fuels. In the next
decade, the refining industry is expected to shift from a fueloriented
industry to a raw material-oriented industry; therefore,
crude-to-chemicals is one of the major technology paths
for refining operations. Moreover, strict governmental and societal
regulations on carbon dioxide (CO2
sulfur oxides (SOx
) and nitrogen oxides (NOx
), methane (CH4
) emissions are
),
pushing refiners to reduce carbon emissions and upgrade to alternative
feedstocks (e.g., biomass, pyrolysis oils) together with
petroleum fractions. FIG. 1 shows a typical refining flowsheet for
crude-to-chemicals.
This article will describe how molecule-based process simulation
can help address the challenges of achieving crude-tochemicals
process benefits, developing refining processes to
process alternative feedstocks, such as biofuels, and more.
Process description. As shown in FIG. 1, crude oil and biofuels
both feed into a refinery. Crude oil first passes through a crude
distillation unit (CDU), where it gets separated into various
boiling fractions: straight-run naphtha, distillate, gasoil and atmospheric
residue (AR). Biofuels are upgraded via a hydro-deoxygenation
(HDO) unit and converted to paraffinic distillate
materials. The effluent of the HDO is mixed with the petroleum
distillate from the CDU and upgraded via a hydrotreater unit
to produce high-quality diesel. Alternatively, the product of the
HDO can be combined with the distillate and/or gasoil from
the CDU and cracked through reactors (e.g., a hydrocracker
or HCR) to produce HCR naphtha. Moreover, naphthas from
different plants (e.g., straight-run naphtha and HCR naphtha)
can be further upgraded through reactors (e.g., reformer, steam
cracker) to produce more desirable products.
Challenges with the crude-to-chemicals process. Unlike
the traditional refinery, the products of the crude-to-chemicals
refinery are not solely fuels (e.g., gasoline, diesel, jet fuel)-the
naphtha fractions are the feedstocks to petrochemical plants
(e.g., ethane cracker and aromatics production). The typical
scope of traditional refinery optimization is a single unit (e.g.,
CDU, reformer, hydrocracker) or several units. For the crudeto-chemical
scope, the operator must move beyond the unit
level optimization to consider optimization across both the refinery
and the chemical plant.
However, the traditional lumped model approach cannot
solve the optimization of the process simulation flowsheet shown
in FIG. 1. The lumped models used in refinery process simulations
(TABLE 1) usually define their species by physical properties, such
as boiling point, specific gravity or solubility; however the species
used to model a petrochemical plant are molecular components.
For example, a detailed paraffin-isoparaffin-olefin-naphthenes-aromatics
(PIONA) carbon number breakdown is required
to model ethylene cracking and/or aromatic chemicals.
The lumped model cannot provide functions to precisely propagate
the molecules from the refining units to the chemical units.
Furthermore, the new feedstocks like the biofuel feeds shown in
FIG. 1 are beyond the definition of the lumped model and cannot
be described using the existing lumps. It is also necessary to
describe refining upgrading processes at a highly granular level
to get a better understanding of the chemistries, and provide
optimal operations that comply with regulations on carbon and
other emissions. This motivated the development of a novel approach
to address this issue: molecular level modeling.
Molecular level modeling. Because molecules are the fundamental
elements in any refining and chemical process-and
can reveal the nature of chemical conversions-this novel
molecular modeling solution approach provides refiners an
optimal solution to address the aforementioned challenges. It
begins with the company's molecular characterization (MC)
technology,1,2
which has established a molecular library conFIG.
1. Representative refinery models for crude-to-chemicals.
Hydrocarbon Processing | FEBRUARY 2022 23
Hydrocarbon Processing - February 2022

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