Hydrocarbon Processing - June 2021 - 37

Special Focus Process Optimization
A. K. COKER, A.K.C. Technology; and A. ALSUHAIBANI,
Texas A & M University, College Station, Texas
Optimize product blending using Excel spreadsheets
and LINGO software-Part 1
Linear programming (LP) is an optimization modelling
technique in which a linear function is maximized or minimized
when subjected to various constraints. This technique has been
useful for guiding quantitative decisions in business planning
in industrial engineering, refineries and chemical plants. Petroleum
refining involving gasoline blending bears a different cost
of manufacturing; the proper allocation for each component into
its optimal disposition is of major economic importance. To address
this problem, most refiners employ LP that permits the rapid
selection of an optimal solution from multiple feasible alternative
solutions, as each component is characterized by its gasoline
blending in petroleum refining. Examples provided in this article
are the solver from Microsoft Excel and LINGO software.
Product blending plays a key role in preparing refinery products
for the market to satisfy product specifications and environmental
regulations. The objective of product blending is
to assign all available blend components to satisfy product demand
and specification to minimize cost and maximize overall
profit. Almost all refinery products are blended for the optimal
use of all the intermediate product streams for the most efficient
and profitable conversion of petroleum to marketable products.
For example, typical motor gasolines may consist of straightrun
naphtha from distillation, crackate [from fluidized catalytic
cracking (FCC)], reformate, alkylate, isomerate and polymerate,
in proportions to make the desired grades of gasoline and
the specifications.
Basic intermediate streams can be blended into different
finished products. For example, naphthas can be blended into
gasoline or jet fuel streams, depending on the demand. On-line
blending is more prevalent than batch through optimization
programs. In most cases, the component blend nonlinearly for
a given property (e.g., vapor pressure, octane number cetane
number, viscosity, pour point) and correlations and programming
are required for reliable predictions of the specified properties
in the blend.
One critical economic issue for refineries is selecting the
optimal combination of components for the products. Refining
does not provide commercially viable products directly, but
rather semi-finished products, which must be blended to meet
the correct customers' specifications. Blending is an important
operation within the refinery-it is a physical process in which
accurately weighed quantities of two or more components are
mixed thoroughly to form a homogeneous phase, which can be
either similar or dissimilar in nature.
Blending specifications. Most products obtained from distillation/fractionation
columns are blended with fractions obtained
from other units to help minimize waste and invariably
increase the quantity of the products. Almost all products-
from gas to lube oil-are not only blended with fractions, but
also with additives. All such blends are formulated to have the
required properties conforming to the correct specifications.
Blended products include gasoline, jet fuels, heating oils and
diesel fuels. The objective of product blending is to allocate the
available blending components so that demands and specifications
are met at the least cost and to provide products that maximize
overall profit.
Gasoline blending is much more complicated than a simple
mixing of the components. A typical refinery may have as many
as eight to 15 hydrocarbon streams to consider as blendstocks.
These may range from butane (the most volatile component) to
a heavy naphtha and can include several gasoline naphthas from
crude distillation, catalytic cracking and thermal processing
units, in addition to alkylate, polymer and reformate. Modern
gasoline may be blended to simultaneously meet 10-15 different
quality specifications, including:
* Vapor pressure
* Initial, intermediate and final boiling points
* Sulfur content
* Color
* Stability
* Aromatic content
* Olefin content
* Octane measurements for several different portions of
the blend
* Local governmental or market restrictions.
Since each individual component contributes uniquely in
each of these quality areas and each bears a different manufacturing
cost, the proper allocation of each component into its
optimal disposition is of major economic importance.
Different component streams are blended into various grades
of gasoline, including 83 octane (blended with an oxygenated
fuel such as ethanol), regular 87 octane and premium 92 octane.
The Reid vapor pressure (RVP) is set depending on the average
temperature of the location the gasoline will be used (cold temperatures
require higher RVP than warmer climates). These two
specifications are the most significant, and they are documented
with each blend to minimize the potential of octane giveaways.
If the octane specification is 87, then each 0.1 octane over
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Hydrocarbon Processing - June 2021

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