IEEE Robotics & Automation Magazine - June 2015 - 65

Oil Refinery and Short-Term Scheduling
The Refinery Process
An oil refinery is composed of various production units such
as tanks for material storage, a complex pipeline system, utility system, and so on. The overview of a refinery in China is
shown in Figure 1.
Roughly speaking, the operational process of an oil refinery has three stages: 1) crude-oil operations, 2) production, and 3) final product distribution. At the first stage,
crude oil is carried to the port near the refinery plant by
crude-oil tankers and unloaded into storage tanks. The
crude oil in the storage tanks is then moved into charging
tanks in the plant using a pipeline. Finally, the charging
tanks feed oil into distillers for processing, which ends the
first stage. At the second stage, the middle products after
distillation are further processed by various production
units until a variety of components are obtained. These
components are then mixed to form the final products. The
final products are distributed to markets by different means
at the third stage.
With intensive market competition and sustainable development requirements, extensive modifications have to
be made to the operations of the process industry. Profits
can be increased significantly by using advanced information technology to improve the operations of a process
plant [16]. In recent years, great attention has been paid to
process engineering tools to enable factory automation. A
plant is operated in a hierarchical way with three levels: 1)
production planning at the upper level, 2) production
scheduling at the middle level, and 3) process control at the
lower level. Presently, at the upper level, commercial software that is developed based on linear programming techniques is widely applied to generate production plans for a
whole refinery. At the lower level, advanced control systems
have been installed for unit control in most oil refineries to
optimize various production objectives to maximize productivity gains.
In this process, the area that needs to be improved is the
method and software tool at the middle level, or the so-called
short-term production scheduling [16], [24]. This process is
still being done manually by planners in today's practice.
Therefore, there is an urgent need to develop effective methodologies for the middle level so that full automation can be
implemented in all three levels to achieve sharp productivity
increases for the whole plant.

It is known that the STSP of crude-oil operations is one of
the most difficult scheduling problems in a refinery, and this
article attempts to solve this problem.
The Short-Term Scheduling Problem
To address this issue, one has to realize various constraints
and requirements. Before giving the constraints for the system, we first present a brief introduction to the process of an
oil refinery.
To be profitable and to meet global market demands, a
refinery should process a variety of crude-oil types. A distiller is designed to process some types of oil, but not all
types of oil can be processed by every distiller. This in turn
means that a storage tank or charging tank can only hold
one type of oil at a time. Thus, when a tank is charged, it
should be empty or the oil type that is charged into it must
be same as that in the tank. Before crude oil can be processed, the brine should be separated from it, which requires that the oil must sit in the tank after the tank is filled
and before it can be discharged. This time delay is called oil
residency time constraints. Furthermore, a tank cannot be
charged and discharged at the same time, i.e., an overlap for
charging and discharging a tank is not allowed.
A pipeline is used to transport crude oil from storage tanks
to charging tanks, and there is always crude oil in the pipeline
that cannot be emptied. Since a pipeline is tens of kilometers
long and various crude-oil types are processed, there may be
multiple oil segments in the pipeline with a different type of
oil in each segment.
In distiller feeding, a charging tank is required to feed a
distiller at any time and, therefore, poses the following constraint. Assume that a charging tank is feeding a distiller and
expected to be emptied at time x. To make the distillation
process stable, the successor tank must start to feed the distiller at time x 1 < x, such that during the time interval 6x 1, x@,
two tanks are feeding this distiller simultaneously. This is
called a charging-tank-switch-overlap constraint. Furthermore,
a distiller should work continuously without being interrupted and can only be stopped for maintenance. Based on this
analysis, we face the following resource constraints:
● the limitation of the number of storage and charging tanks
and their capacity
● the limitation of flow rate for oil unloading from tankers
and oil transportation via the pipeline
● the availability of various crude-oil types and their amount
in the system.

Figure 1. An overview of an oil refinery.

JUNE 2015

IEEE ROBOTICS & AUTOMATION MAGAZINE

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