Chemical Engineering December 2014 - 47

Feature Report
ject to significant demand changes,
and they often get out of balance.
This can lead to steam venting or
excessive use of steam letdowns,
or both. Close monitoring, together
with steam models and realtime
optimization, can yield substantial
improvements in energy efficiency
and savings in net energy costs.
Effective maintenance. Another
part of getting the most out of existing
facilities is to ensure that the
equipment is properly maintained.
Our primary focus here is maintenance
of the equipment and systems
that have the largest impact
on energy use - for example, heat
exchangers, especially those in preheat
services.
Heat-exchanger cleaning programs
are an important part of
many energy-management programs.
This area has become quite
sophisticated, with both improved
cleaning techniques (Figure 1) and
better tools for assessing appropriate
cleaning intervals for the heat
exchangers. However, the best
cleaning methods and the most elegant
optimization of cleaning intervals
are of little use when communication
fails [4].
During the course of a crude-unit
preheat-train study in a petroleum
refinery, one of the heat exchangers
was found to be out of service. The
records showed that this particular
heat exchanger had been idle for
more than three months. Further
investigation revealed that the heat
exchanger had been cleaned, and the
work had been completed within a
couple of weeks. The maintenance supervisor
notified the shift supervisor
that the cleaning was complete, but
shift personnel were busy with other
activities, and the heat exchanger
could not be brought back into service
before the shift ended. Unfortunately,
the shift supervisor failed to
advise the next shift about the situation.
There was no follow-up, and the
cleaned exchanger remained out of
service for two and a half months.
When the unit manager was informed,
the heat exchanger was
brought back into service in just a
few hours. The energy loss during
the period that the heat exchanger
had been left idle after the cleaning
was worth over $100,000.
In this case, the key problem was
a breakdown in communication.
Better systems were needed for
tracking the status of maintenance
jobs on the unit. A simple electronic
reminder system, for example, could
have alerted the operators to the
need to bring the heat exchanger
back online.
In addition to heat exchangers,
there are several other key systems
and types of equipment that need
careful attention and preventive
maintenance to maintain energyefficient
operations. These include
furnaces and boilers, steam traps
and insulation, as well as compressors,
pumps and turbines.
Engineered improvements. Additions
and upgrades to plant facilities
can lead to significant improvements
in energy efficiency.
Invariably, these upgrades require
significant input from engineering
personnel to identify, evaluate and
design the projects. Opportunities
can cover a wide range in scale and
type, such as the following:
* Simple piping changes
* Localized insulation projects
(Figure 2)
* Replacements of electric driver
systems (such as installing variable-frequency
electric drives)
* Adding heat exchangers, steam
turbines, distillation columns or
other major equipment items
* New control schemes
A robust energy-management
system tracks opportunities, including
their costs, values and timing
required for implementation, so
that engineered improvements can
be planned, budgeted and executed.
New technologies. Engineered
improvements typically use established
equipment types and apply
proven solutions to identified problems.
In contrast, solutions that
incorporate new ( " breakthrough " )
technologies generally require some
amount of validation through research
and development. Thus, the
amount of time required to implement
new technologies is higher
and their degree of technical and
financial risk greater than for engineered
improvements.
Some of the largest energy-efficiency
improvements in the CPI
have come through technological
breakthroughs. For example, the
development of the low-pressure
polyethylene process in the 1950s
was a major advance over the older
high-pressure process, and the new
process used much less energy per
unit of production. A more familiar
example for most people is the rise
in recent years of compact fluorescent
lights and light-emitting
diodes (LEDs), which provide dramatic
energy savings compared to
the familiar incandescent bulbs.
Technological advances have also
improved some of the key equipment
items that impact energy
usage, such as heat exchangers
and distillation columns, and incorporating
some of the new types
of equipment that are now commercially
available can often lead
to improved engineering solutions.
FIGURE 2. Localized insulation projects, such as the one shown here on the top
head of a distillation column, are examples of engineered upgrades that can lead to
improvements in energy efficiency
Courtesy of Aspen Aerogels
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Chemical Engineering December 2014

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