RefineryWide Sim HEMag March PDF
RefineryWide Sim HEMag March PDF
RefineryWide Sim HEMag March PDF
Refinery wide
simulation
Denis Westphalen and Hiren Shethna,Aspen
Technology, Inc., Canada, explore how a rigorous
refinery wide simulation tool can be used to analyse
different strategies for benzene control in gasoline.
rocess simulation has been widely used in refineries for investment decisions. This article analyses different strategies
its feedstocks. Understanding the true economic potential of a Analysis of benzene control
refinery is a complex challenge, due to the intricate interde-
pendency of its processing units. Linear programming (LP)
strategies
To demonstrate how refinery wide modelling can help compa-
models have been successfully used in the decision making
nies optimise their operational performance, a typical scenario
process for many years; however, refiners should be aware of
in which a refinery needs to identify the optimum benzene con-
the limitations of this approach. The LP model is an excellent
trol strategy for a particular set
economic evaluation tool, but
of operating conditions has
because the model itself is built Table 1. Properties of crude oils A and B
Property Crude A Crude B been examined. The details of
with parameters that are not
API 41.28 29.78 the refinery and its operational
always kept up-to-date to truly
Paraffins by volume (%) 55.92 36.99 status are outlined below.
reflect the refinery capabilities, Naphthenes by volume (%) 21.42 34.37 Figure 1 is a simplified
or is kept up-to-date with simpli- Aromatics by volume (%) 22.66 28.64 process flow diagram for the
fying assumptions, it does not
refinery model. Crude oil is fed
deliver the optimum economic
Table 2. Reformer feed stream to the crude distillation unit that was modelled
answer and cannot adequately resolve ques-
Property Value as a rigorous column with four side-strippers
tions related to operating constraints and new Aromatics by volume (%) 14.42
and four pump-arounds. The atmospheric
paradigms. Due to these limitations, the incor- Benzene (volume %) 0.01 residue is sent to the vacuum distillation unit
poration of rigorous models into existing work RON 58.3 (VDU), where VGO is obtained and used as
processes can help to ensure that daily opera- D86 IBP 93.53
the feedstock to the FCC unit. The FCC unit
tional decision making is based on more com- D86 10% 102.92
D86 30% 106.18
was rigorously modelled including the reactor
plete and accurate information.
(riser, stripping and regeneration), the FCC
A refinery wide simulation tool can therefore D86 50% 114.12
D86 70% 121.73 main fractionator and the gas plant. The HF
provide a range of important benefits, including
D86 90% 145.82 alkylation unit was also part of the simulation.
the ability to:
D86 FBP 175.1 Naphtha is drawn from the top of the CDU and
Understand the impact of sent to the naphtha stabiliser
changes in unit operations. Table 3. Gasoline pool composition where light components are
Find the best set of Source Volume % RON Aromatics Benzene removed. Stabilised naphtha is
operating conditions. (vol %) (vol %)
hydrotreated (not rigorously
Quickly test the LP model Reformate 41.7 101.3 76.7 4.92
modelled in this work) and sent
Isomerate 13.4 80.2 0 0
accuracy. to the naphtha complex (Figure
Alkylate 12.4 91.8 0 0
Update the LP vectors in a FCC naphtha 32.5 97.5 17.9 0.96 2). The naphtha complex com-
timely manner. Total 100 96.5 19.7 2.36 prises the reforming and the
Respond quickly to isomerisation units.
operational changes/upsets and new regulations. These two units play an important role in the benzene contents
Identify process bottlenecks. and final quality of gasoline. The reformer is a CCR platform-
Evaluate different process configurations. ing unit operating at 500 kPa (72.5 psia) and all four reactors
at 510 C (950 F). The use of a refinery wide simulation tool
Maximise the efficiency of existing equipment.
allows the study of the interdependency of those two units, the
Components of a refinery wide interactions with the different crude blending options, the influ-
ence of the operation of the CDU and the connection with the
simulation tool other units that produce gasoline.
To provide companies with an effective and easy to use
Two different crude oils are blended before being fed to the
application, a refinery wide simulation tool based on rigorous
CDU. Table 1 shows some properties of crude oils A and B.
models needs to incorporate a number of key components;
these include:
Base case
Thermodynamic engine: phase equilibria and the On the base case, crude oils A and B are blended using a
calculation of thermodynamic properties are necessary to 1:1 ratio on volume basis. Table 2 shows the properties of the
any rigorous simulation. feed stream to the reformer unit and Table 3 shows the gaso-
Assay management system: the access to assay libraries line pool composition.
and to assay characterisation tools are necessary to
represent the crude oils. Influence of pre-fractionation
Calculation of petroleum properties based on assay One of the strategies employed to control benzene in gasoline
information: petroleum properties such as octane number, is the prefractionation of naphtha so that benzene and ben-
pour point, cloud point, flash point must be calculated at zene precursors are removed from the reformer feed. Once
any point of the flowsheet. Special property blending cal- those components are removed, they are sent to the
culations are essential. Isomerisation unit, where benzene is saturated. Choosing the
Library of standard unit operations: the user must be able right benzene reduction strategy requires an understanding of
to build a simulation model using pre-defined unit the impact that each strategy has on such factors as overall
operations as distillation columns, heat exchangers, etc. gasoline production, refinery balance and economics. The rig-
Refinery reactors: first principle models for refinery orous simulation model of the refinery makes it possible to
reactors must be available for the user. These models analyse different alternatives and process conditions and iden-
must be tuneable using plant data. Kinetic models must tify the optimum approach.
be developed from real pilot/plant data. Benzene concentrations in the reformate depend on the
Flowsheeting environment: unit operations and reactors amount of benzene and benzene precursors in the feed to the
can be linked together in a flowsheeting environment. The reformer. Benzene formation occurs by conversion of cyclo-
flowsheeting environment holds the model topology and hexane and methylcyclopentane to benzene and through the
invokes all model calculations. dealkylation of heavy aromatics to benzene.
Using the simulation model, the influence of the initial boil- This could include using model to examine the detailed
ing point of the reformer feed on some key properties was impact of different feedstocks on the downstream units,
analysed. Since all operations (distillation columns, reactors, including the reactor units. For instance, the blending ratios of
heat exchangers and others) are part of the same simulation crudes A and B can easily be changed and those changes
model, this study can be performed in a straightforward way. are automatically propagated through the flowsheet. The opti-
Figures 3, 4, 5, and 6 depict the influence of the D86 IBP of the mum reforming temperature and initial boiling point of the
reformer feed on the reformate research octane number, ben- reformer feed stream can thus be determined for a different
zene concentration, hydrogen yield, and reformate, isomerate blending ratio. In this way, it is possible to view the model as a
and gasoline yields, respectively. desktop virtual refinery that allows refiners to gain a much bet-
It can be concluded that the benzene concentration on ter understanding of their complex operations. This greater
reformate, and therefore on gasoline, can be significantly understanding can help to identify improvement opportunities,
reduced if the initial boiling point of the reformer feed is and can ultimately lead to more efficient and profitable refinery
increased. The decrease in the hydrogen and reformate yields operations.
must be analysed in the context of the overall refinery, taking
into account the hydrogen demand in other processing units.
The increase in the reformate octane number suggests that References
the severity of the reformer could be reduced. 1. Aspen RefSYS 1.0, Users Manual, Aspen Technology, Inc..
2. KEESOM, W. H., KUCHAR, P. C., Penex and Platforming Synery for
Efficient Naphtha Processing and Benzene Control, UOP, LLC.
Further opportunities 3. WIER, M. J., UTLEY J., ELSTEIN, J., SCHWAKE, D., Strategies for
Whilst this analysis has identified the potential for significant Maximizing Profits from Reforming Units, NPRA Annual Meeting, San
Francisco, California (1998).
performance improvements by making minor alterations to the 4. SUDKAMP, R., Linking Crude Units with UOP Reactor Models in a Multi-
operating strategy, the refinery wide simulation model also Plant Refinery HYSYS Model, Presented at the AspenTech Users Group
makes it possible to evaluate the impact of a broader range of Meeting, Paris (2003).
5. FORREST, J., REYES, E., Refinery-Wide Rigorous Simulation Solution
potential changes to explore whether they could offer greater Drives New Value for Refiners, NPRA Plant Automation and Decision
benefits. Support, San Antonio, Texas (2003).