Hydrocarbon Processing - July 2021 - 66

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quantities of the additives becomes more difficult in non-linear
properties cases. Determining the amounts of these blends is
based on solving two or more equations with two or more unknowns,
depending on the property equations.
A typical refinery optimization problem can be split into
sub-problems using a spatial decomposition scheme proposed
by Jia and Ierapetritou.4
These sub-problems are solved as independent
optimization problems, and the combined results
are considered an approximate solution to the overall refinery
optimization problem.
Several commercial tools are available for solving standalone
refinery optimization problems, including software products
for online and offline blending optimization problemsa,b
products for online and offline blending optimizationc,d,e
;
; and
online optimization solutions for primary and secondary process
unitsf,g
.
FIG. 6 shows a schematic decomposition of the refinery optimization
problem, and FIG. 7 shows an overview of petroleum
refining and blending processes.
Mathematical formulation. The problem formulation includes
multiple blenders and storage tanks, as shown in FIG. 6.
Properties of the final products are modelled using blend laws
that relate the final product property to the quantities and properties
of feed components. Blend product properties are either
linearly or non-linearly blended based on the volume or mass
of the feed components. For example, the RON of the blend
is modelled by linear blend laws, while the RVP of the blend
is modelled by non-linear blend laws. The blend laws used to
estimate product property based on volume/mass fraction or
volume/mass flow and property of feed components are:
1. Linearly blended by component fractions in tanks:
the property of product blend is a linear function
Fractionation and
reaction processes
Crude oil
unloading
Crude oil
blending
where Fi is the volume/mass flowrate of component i
to the blend header.
Blend optimization is subjected to different constraints
types, including operational (equipment limits on component
flow), inventory (volume limits on feed components) and
quality (analyzer limits and tank property specification).
Solving the problem. Solving the proposed problem in real
time is one of the main requirements; however, if this is impossible,
then the intermediate iteration result after a fixed time interval
should be feasible and better than the previous iteration
results. This requirement ensures that a feasible solution is provided
for cases where the solver cannot converge in the available
time for a real-time optimization. The proposed problem
is solved using a developed solverh
due to its feasible region
search robust nonlinear programming (NLP) solver and meets
all solution requirements.4
Gasoline blending
Product
shipping
of component volume/mass fractions and properties
(Eq. 46):
PEj = XiBVi,j
i
∑
(46)
where Xi is the volume/mass fraction of component i
in the product tank.
2. Linearly blended by component flows in blend headers:
the instantaneous property of the product blend is a
linear function of component volume/mass flowrates
and properties (Eq. 47):
PEj = i
∑ FiBVi,j
∑Fi
i
(47)
Crude-oil
marine
vessels
Storage
tanks
Charging
tanks
Crude
dist.
units
Primary
units
Crude oil unloading and blending
FIG. 6. Schematic division of refinery-wide operation.
66 JULY 2021 | HydrocarbonProcessing.com
Other
prod
units
Secondary
units
Production unit scheduling
Comp
stock
tanks
Blend
headers
Finished
product
tanks
Blending
Product blending and shipping
Shipping
points
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