Hydrocarbon Processing - February 2021 - 44

Process
Optimization
I. S. A. M. MOKHTAR and M. MOSIR,
Petronas, Kuala Lumpur, Malaysia

Optimizing for viability-
VDU revamp in a brownfield project
Revamping an existing column in an
operating plant for higher throughput is
a challenge, especially when the column
is already operating at its rated capacity.
This is particularly true for a vacuum distillation unit (VDU) column, which involves multiple product and pumparound
streams and associated auxiliary units,
such as a steam ejector system.
Optimizing the design and operation
of the column is inevitable to ensure the
viability of a brownfield project, as it
avoids major modifications to the column
and its associated auxiliary systems. This
article discusses how optimization was
carried out on the design and operation
of an existing VDU column in an oil refinery, avoiding major modifications of the
associated ejector, steam, cooling water,
sour water systems and heat exchanger
network and allowing a brownfield project to remain commercially viable.
Case study background. A case study

of an existing VDU column at a refinery
in Malaysia is included in this article.
The VDU column was designed to operate at 9 mmHg operating pressure with a
throughput capacity of 3,500 bpd of lowsulfur waxy residue (LSWR) from an upstream crude distillation unit (CDU).
As part of the refinery's business
growth strategy, the VDU column was
considered for debottlenecking to allow
additional throughput of LSWR, which
is lighter than the existing throughput at
23,000 bpd to the unit. The lighter, higher
LSWR throughput led to higher non-condensable and condensable vapor flows at
the top of the column, thereby massively
increasing the column internals loading

44 FEBRUARY 2021 | HydrocarbonProcessing.com

and producing higher column operating
pressure, which placed additional burden
on the overhead steam ejector system.
Major modifications to the column
shell diameters, steam ejectors and condensers were expected, and the replacement of the equipment would require
massive structural modification within
a congested, operating plant. On top of
that, with the larger steam ejectors and
condensers, major modifications to the
associated steam generation system, cooling water system and sour water treating
unit were expected.
The total cost of the required modifications was estimated at approximately $25
MM, which would render the project uneconomical. Replacing the VDU column
itself was not an option in view of the sheer
complexity of the construction work required in the operating plant, which would
require extended downtime of the refinery. The economic loss associated with the
prolonged downtime would outweigh the
benefit of debottlenecking the column. A
creative solution was needed to economically optimize the column operation.
Solution for brownfield optimization of the VDU. One way to make the

project economically viable is to optimize the column operating conditions
and column internals design efficiencies
and to exhaust all available heat duties
within the pumparound and heat integration network to reduce the impact to the
existing ejector system and minimize the
modifications to the VDU unit. This plan
was outlined using industrially acceptable
process simulators and an in-house sizing
software for column internals.

Optimizing the column operating conditions includes increasing column bed
efficiencies, adjusting the operating pressure, redistributing heat duties in the heat
exchanger network, optimizing pumparound flow, and adjusting the column temperature profile and inlet temperature.
These modifications led to changes for
both the mass and heat transfer profiles
within the column. These changes resulted in the reduction of column internals
liquid and vapor traffic, thereby decreasing the total load at the top of the column
to the overhead steam ejector system.
Debottlenecking a VDU column. Understanding how a VDU column works
and the influence of the auxiliary systems
to the VDU column operation are key to
optimizing both the design and operation
of the column itself. The VDU column
under study was a typical design of five
packed beds, one flashing section and one
stripping section, operated under vacuum
conditions at 9 mmHg. The VDU is designed to fractionate heavy LSWR into
product streams including light vacuum
gasoil (LVGO), medium VGO (MVGO),
heavy VGO (HVGO) and vacuum residue (VR). FIG. 1 shows the schematic of
the VDU under study.
A two-phase feed enters the VDU
column flashing section via an upstream
fired heater. Lighter vapor components
will rise up the column to the wash,
bottom pumparound (BPA), middle
pumparound (MPA), fractionation and
top pumparound (TPA) sections to be
further distillated to product streams
through both the heat recovery pumparound beds and product purification

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