Chemical Engineering April 2023 - 29

worse, out of balance? If steam,
cooling water or natural gas supply
are limited in the facility, it may not
be possible to expand or carry out
other projects without doing some
electrification projects. If the cost
to purchase or produce electricity
is lower than importing natural
gas, then electrification can reduce
operating costs.
Is there a need to re-balance the
steam and electricity usage, but
there are concerns about what to
do with excess fuel gas? There are
potential synergies in executing an
electrification project coupled with
a project to increase liquefied petroleum
gas (LPG) recovery from the
fuel-gas system. Burning less fuel
translates into less CO2 generation,
which is a strategic driver for many
companies. By recovering more
LPG and heavier material from the
fuel-gas system and replacing it with
natural gas, CO2 emissions will be
further reduced, even if the fired duty
does not decrease. In most markets,
the revenue from the sale of LPG
products and other liquid products
is more than the cost of additional
import of natural gas, so there can
be financial benefits in addition to
CO2 reduction.
Does a facility-wide energy optimization
sound intriguing, but
also intimidating? If the owner is
planning a strategic reconfiguration
based on market-driven feed
or product changes, that may be an
opportunity to replace aging steamdriven
compressors with new motor-driven
compressors that have a
fit-for-purpose duty specification.
ELECTRIFICATION
OPTIONS
Once these questions have been
considered, operators can begin
to weigh their options. For industrial
processing facilities, there are
three primary electrification options,
described below:
* Heat pumps replacing reboilers
or
condensers
for
distillation
columns
*
Electric heating or electric boilers
replacing steam or fuel-fired heaters
* Electric motors replacing steamturbine
drivers
These options can have significant
impacts in the right situation. Table 1
summarizes the potential impact of
the different options to a facility. They
are discussed individually
in the
following sections.
TABLE 1. IMPACT OF ELECTRIFICATION ON
PROCESS OPERATIONS
IMPACTS
Heat pumps
Heat pumps use
a compressor to
compress a lowertemperature
vapor
stream to a highenough
pressure
that the condensing
temperature
is above the temperature
required to recover the heat
in a traditional heat exchanger. Historically,
this design has been called a
heat pump, but has also been called
mechanical vapor recompression
(MVR). This scheme can be used for
distillation columns where the column
overhead is compressed to a high
enough pressure to provide the heat
to reboil the bottom of the column. It
is also possible to design a distillation
heat pump with a separate fluid in a
closed loop that is compressed, condensed
in the reboiler, further cooled
by dropping the pressure across a
valve, and then used to cool the overhead
condenser. Most examples in
the industry use the overhead vapor
as the heat-pump fluid, as shown in
Figure 2.
Fuel consumption
Emissions
Water consumption
Steam requirement
Equipment efficiency
Electricity demand
Temperature controllability
Heat pumps can have significantly
better energy efficiency than traditional
reboiler or condenser systems,
with savings on both the reboiler
side (which often contains steam)
and the condenser side (which often
contains cooling water). Energy-intensity
reductions of up to 90% are
reported in the literature.
Heat pumps work very well for columns
that do not have a large temperature
difference between the top
and bottom of the column. Propane/
propylene splitters, ethane/ethylene
splitters (such as in steam-cracker
units), and isobutane/n-butane splitters
are good candidates for heat
pumps.
There
are
also
applications
in aromatics complexes and
methanol production.
For most other distillation separations
used in refineries and petrochemical
complexes, the much
larger temperature change across
the column significantly reduces the
value of the heat-pump approach
and makes it impractical.
Heat pumps are best suited for
a new design, as it would require a
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major revamp to retrofit an existing
column with a heat pump. This option
can reduce consumption of fuel,
steam and water, reduce emissions
and improve efficiency at the cost of
a higher electricity demand.
Electric heaters
There are two ways to electrify steam
or fuel-fired heaters, as follows:
1. Replace a steam heater with an
electrically heated exchanger
2. Replace a fuel-fired steam boiler
with an electric boiler
To convert a steam-heated exchanger
to an electric heater, the
steam and condensate piping and
controls are replaced with electrical
supply, which may reduce maintenance
costs and capital costs for a
new facility. An advantage of a steam
exchanger is that the maximum
temperature is self-limiting, meaning
that the process fluid cannot be
heated to a temperature higher than
the condensing temperature of the
steam. An electric heater will not
have the same intrinsic feature, and
controls must be designed to provide
protection against overheating
the process fluid for upset, low-flow
and blocked-in scenarios.
Like the heat pump, this option
can reduce fuel, steam and
water consumption, reduce emissions
and improve efficiency, while
increasing power consumption
and potentially reducing processtemperature
controllability.
Another way to electrify heaters is
to replace a fuel-fired steam boiler
with an electric boiler. This may be
a good option for a revamp of an
existing
facility because this option
reduces emissions from steam
generation while still supporting existing
process-steam demand, including
steam-injection and steamheating
needs.
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Chemical Engineering April 2023

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