Hydrocarbon Processing - March 2022 - 30

Petrochemical Technology
Lastly, the authors will demonstrate
the successful implementation of a heat
pump equipped with proprietary reboiler
technologies for the upgrade of Borealis'
C3
splitter unit in Porvoo.
Heat pump principle. The heat pump
principle in the frame of a distillation column
(FIG. 1) targets the recovery of a part
of the condensation duty released in the
overhead condenser to be used for the
boiling service at the bottom of the column.5
The
overhead vapor will be compressed
at a pressure level allowing the
overhead vapor to be condensed in the
column reboiler. This requires products
having close boiling temperatures (typically
less than 45°C) and column bottom
temperatures below 100°C. A prime example
widely used in refining and petrochemical
operations is the propane-propylene
splitter column.
Energy savings are expected on a heating
medium for the reboiler and in a reduction
of the cooling water flowrate for
the condenser. However, additional power
is required to drive the compressor by
using either an electric motor or a steam
gas turbine. This additional power demand
should be considered in the energy
balance and carbon footprint.
The vapor compressor discharge pressure
is directly linked to the reboiler boiling
temperature, where the minimization
of the temperature approach in the reboiler
will allow for the minimization of the
compressor work.
The major part of the gas overhead
stream is routed to the reboiler. A trim condenser
is required to condense the remaining
vapor for reflux and other purposes.
FIG. 2. View of two types of proprietary heat
exchanger tubes: Enhanced boiling tubesa
(left) and enhanced condensing tubesb
(right).
Condenser and reboiler of a distillation
column. If we focus on the heat
exchangers associated with the distillation
column, we must manage-on one
side-the overhead condenser, which is
generally water- or air-cooled, depending
on the operating pressure of the distillation
column. On the other side, the
reboiler is generally heated by steam, but
other sources can be used, such as a liquid
heating medium (e.g., hot oil, quench
water or hot process stream) or even
electric heaters.
The most common exchanger type
for these two services in the energy industry
remains the shell-and-tube heat
exchanger. Depending on the service, its
orientation and fluid side allocation can
differ. An overview of a typical condenser
and reboiler arrangement is detailed in
TABLE 1. The fouling propensity of each
stream will play a key role in finalizing
side allocation. Tube-side allocation for
cooling water from the open loop is common
to manage a minimum water velocity
in tubes and to maintain a good shear
rate to minimize fouling.
Depending on fouling and the associated
cleaning requirement, the TEMA
type of shell-and-tube heat exchanger
will be defined. Horizontal NXN-type
exchangers will perform well with a clean
overhead vapor to be condensed, while
horizontal AES-type exchangers will be
recommended in the case of a fouling
liquid boiling on the shell side heated by
quench water, with both sides requiring
mechanical cleaning.
The heat duties can be large in this
FIG. 3. View of Borealis' petrochemicals plant in Porvoo, Finland. Photo courtesy of Borealis.
TABLE 1. Typical condenser and reboiler arrangement
Service
Hot fluid
Condenser
Reboiler
Reboiler
Overhead vapor
Steam, hot oil, quench water
or process stream
Steam, hot oil, quench water
or process stream
30 MARCH 2022 | HydrocarbonProcessing.com
service (several tens of MW are common),
and several shells operating in parallel
are usually required for the reboiler.
Therefore, technologies that can improve
the efficiency of the exchanger are of high
value. Plain tubes can be replaced by externally
finned tubes, which will help to
increase the heat transfer area by a ratio
of 2-3. Ultimately, dual enhanced tubes
will provide the necessary heat transfer
intensification, along with an increase in
surface to maximize the heat transfer for
both sides of the tube.
Cold fluid
Cooling water
Bottom liquids
Bottom liquids
Installation
Horizontal
Horizontal
Vertical
Process stream
allocation
Shell side
Shell side
Tube side
Remarks
-
Typically, hydrocarbon maximum
vaporization rate of 50 wt%
Typically, hy-drocarbon maximum
vaporization rate of 35 wt%

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Hydrocarbon Processing - March 2022

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