Hydrocarbon Processing - July 2021 - 47

Process
Optimization
G. R. MARTIN, Sulzer GTC Technology, Irving, Texas
Advances in light ends processing units using DWCs
For those who have been in the refining
industry for many years, it is obvious
that significant changes are occurring.
Refining is not going away, but refineries
built for long-term existence will not
be built like those of the past. Forces and
needs are dictating changes in the refining
business climate.
Dividing wall columns (DWCs) are
a useful tool for helping meet these demands.
Using DWCs in the design of complex
light ends processing units can provide
a lower-CAPEX design over that of a
conventional gas plant. DWC technology
has already proved its value through numerous
applications. However, it must be
used in the appropriate application to yield
a successful outcome. This article addresses
the use of DWCs to fractionate light
hydrocarbon streams. While the applications
shown are common for the refining
industry, this technology also can be used
in other types of processing plants where
light hydrocarbon streams are processed.
DWCs reduce the amount of materials
of construction and, in many applications,
reduce the energy required to produce
end products. In other applications
DWCs may only reduce the materials of
construction. The reduction in energy
consumption has a continuous benefit,
with DWCs often saving 10%-30% over
conventional distillation configurations.
This varies depending on the process, and
comparisons should always be evaluated
to justify the design selection.
In the past, CAPEX savings in materials
of construction were the primary and
often only interest, but changing business
dynamics are forcing businesses to consider
even nonprofit issues, such as carbon
footprint. With the use of DWCs it is
common for the materials of construction
of the major processing equipment to be
reduced by 30%. Not only is CAPEX reduced,
but this reduction in materials of
construction also results in energy savings
and less impact on the environment.
Pressure on corporations to reduce
fossil fuel consumption, reduce carbon
emissions and make less of an impact on
the environment have made these considerations
a real factor in business decisions.
In simple terms, this is the concept of doing
more with less. DWCs not only give
processing plants a lower-CAPEX means
of providing the distillation tools to meet
a plant's processing objectives, but they
also provide improvement on the environmental
impact. These advancements
make sense for the business world-i.e.,
they allow changes that improve business
economics while simultaneously enabling
operators to do more with less, thereby reducing
their impact on the environment.
As previously noted, it is common to
reduce construction materials of the major
processing equipment by 30% when
using a DWC design over a conventional
one. If the volume of metal and other
construction materials needed to build a
plant is reduced, then the mining, shipping
and processing of the metal ore could
be reduced, which would reduce the shipping
of this metal to fabrication shops,
the fabrication of finished products and
the shipping of the products, all of which
could potentially lead to a savings of 30%
in environmental impact. If all businesses
could similarly reduce their environmental
impact in building new distillation
columns, buildings and other businessrelated
assets, then this would have a significant
environmental impact.
Light ends processing. Processing of
light ends is commonly required in many
different processing units. Common applications
include:
* Saturated gas plants (SGP)
* FCC gas concentration plants
* Coker gas plants
* NGL gas plants
* Flare gas recovery.
DWCs can be used in these applications,
as well as others, to lower CAPEX and
OPEX and to reduce the impact on the
environment.
Saturated gas plants. A simplified process
flow diagram (PFD) of a common
conventional SGP configuration is shown
in FIG. 1. Numerous deviations have been
used in the flow scheme for SGPs over
the years. Designs include complex configurations
with refrigeration units; more
moderate designs (such as that shown in
FIG. 1); lower-cost, simple designs with
low LPG recovery; or no SGP where the
FCC gas concentration unit processes the
saturated gas streams.
Within the design of SGPs such as that
shown in FIG. 1, many variations have been
used. Debutanizers have been installed
upstream of the deethanizer, the primary
absorber has been separated from the
deethanizer, sponge absorbers have been
omitted, intercoolers have been used and
not used on the primary absorber, refrigeration
systems to supply cold absorption
oil have been included, and other deviations
have resulted in variations of this
basic flow scheme.
Regardless of which conventional PFD
has been implemented, the conventional
SGP design requires numerous columns
to provide quality molecular management
and component recoveries. In FIG. 1, the
seven product streams (offgas, propane,
isobutane, normal butane,
isopentane,
normal pentane and heavy naphtha) require
seven to eight columns to fractionate
the crude unit naphtha and lighter
compounds into the streams shown in
FIG. 1 for sale or further processing. One
Hydrocarbon Processing | JULY 2021 47
Hydrocarbon Processing - July 2021

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