Hydrocarbon Processing - March 2022 - 33

Petrochemical Technology
by the co-authors' companies (FIG. 7).
When in operation, the heat pump system
was reported to show savings in steam
production of about 6 MW. This satisfied
the goal of minimizing the heating
of quench water and to avoid any investment
in additional steam usage. Regarding
cooling water, no extra capacity was
required-thus saving some investment.
In the balance, the compressor required
about 1.3 MW of electricity.
Shifting energy input from steam to
electricity helps in sourcing energy from
renewable sources. Borealis will purchase
more than 20 MW of wind power over the
next decade, enabling a 13% increase in
renewable power in its overall electricity
consumption at its production facilities
in Porvoo.
What would have been the final
scheme without the co-authors'
companies technology? The following
will provide a comparison between standard
tubes and the proprietary enhanced
nucleate boiling tubesa
.
Usually, approved technology for
this service is standard low-finned tube
technology. This technology requires
an increase of the heat transfer area of
the tube, typically by a factor of 2-3 vs. a
plain tube. If this technology is considered
vs. plain tube technology, it is not necessarily
better for this application for the
following reasons:
* The shell-side heat transfer
coefficient is controlled by the
heat flux (i.e., the difference of
temperature between the cold and
the hot stream). The enhanced
nucleate boiling tubea
was specially
developed for light hydrocarbon
nucleate boiling to generate
bubbles at very small temperature
superheat, with minimal heat
flux. The resulting heat transfer
coefficient is 2× higher than the
one for a low-finned tube under the
conditions of this case.
* The tube-side heat transfer
coefficient is the governing
coefficient for this application.
The enhanced nucleate boiling
tubea
proposes an inner groove
that increases turbulences to help
disrupt the liquid film at the wall,
making the area available for new
vapor to be condensed. It also
provides an additional heat transfer
FIG. 5. New compressor housing.
Photo courtesy of Borealis.
FIG. 7. Heat exchanger leaving the factory
in Austria.
Hydrocarbon Processing | MARCH 2022 33
area compared to the smooth
internal surface of a low-finned tube.
Because the enhanced nucleate boiling
tube technology is more efficient, a
design based on low-finned tubes cannot
meet the same targets, making it necessary
to increase the size of the equipment.
TABLE 2 details two design scenarios compared
to the installed one. Scenario 1 (iso
duty) is a design based on 30 fins per
inch (fpi) low-finned tubes, maintaining
the same LMTD compared to the actual
design. Scenario 2 (iso size) maintains
the actual size, replacing each enhanced
nucleate boiling tubea
by a 30-fpi lowfinned
tube and a larger LMTD. For
Scenario 2, the lower heat duty must be
compensated by the increase of condensing
vapor pressure by +1.8 bar, resulting
in a larger compression power.
Scenario 1 (iso duty): Larger size
means larger CO2
footprint. In the first
case (low-finned design meeting the same
heat duty), a 75% increase in tube length
is required with the low-fin technology
to perform the same duty. As the original
solution weight is already heavy (60 t), it
was mandatory to add a second shell. For
some cases with multiple shell designs,
the co-authors' companies' solutions can
help reduce the number of shells, saving
on capital expenditures and meeting
space limitations.
For the low fin design, the authors calculated
a total dry weight that was 87%
higher than the actual design. As these
exchangers are made of steel, the manufacturing
weight is directly related to the
carbon emissions during the manufacturing.
The publicly available French database
managed by ADEME-the French
Agency for Ecological Transition-gathers
several pieces of information about
carbon emissions. ADEME differentiates
between recycled and new carbon steel.
The CO2
pending on the final mix of steel sources.
In this case, the enhanced nucleate boiling
tubea
solution saves 87% of CO2
emissions are shown in FIG. 8.
emissions by 1 t of steel ranges
equivalent emitted, deemissions.
The impact of tube technology-
the enhanced nucleate boiling tubea
vs. a
low-finned tube-and the origin of steel
on CO2
Besides capital expenditure savings
and minimizing CO2
footprint, additional
savings in installation costs should also be
considered. A larger number of shells results
in more piping (for the distribution
of vapor from the compressor and condensate
reflux to the column), structures
and foundation concrete. In addition, for
thermosiphon-based heat exchanger units
with multiple shells, particular care in designing
the cold-side piping to and from
the column must be taken to ensure good
fluid balance between the shells operating
in parallel. This case also assumes that the
required space for multiple shells would
have been available around the column.
Scenario 2 (iso size): Energy efficiency
supports carbon footprint reduction.
The second case maintains the
existing space, replacing each enhanced
nucleate boiling tubea
solution with a 30fpi
low-finned tube. As the efficiency of
the low-finned tube is lower, to reach the
from 0.938 (recycled steel) to 2.211 (new
steel) t of CO2
FIG. 6. Reboiler/condenser installation at
the bottom of the column. Photo courtesy
of Borealis.
Hydrocarbon Processing - March 2022

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