Power up Your Energy
Efficiency Efforts
Energy Efficiency eHANDBOOK
TABLE OF CONTENTSCombat Low Rate, Low Efficiency 4
Production cutbacks in continuous processes boost energy consumption
Double Up on Cogeneration 8
Consider tradeoffs and operator training when looking at cogeneration opportunities
Improve Efficiency with Direct Steam Injection 10
Technology offers notable cost savings for high-pressure applications
Additional Resources 13
AD INDEXPick Heaters • www.pickheaters.com 2
Victory Energy • victoryenergy.com 7
Energy Efficiency eHANDBOOK: Power up Your Energy Efficiency Efforts 3
www.ChemicalProcessing.com
The coronavirus-induced economic
slowdown and the recent gyrations
of oil prices have many refineries
and chemical plants running at reduced
throughputs. The impact on profitability
and employment has become headline
news. However, much less has been said
about the impact on energy efficiency.
Energy intensity is the amount of energy
used per unit of production — i.e.,
Energy Intensity = Energy Consumption/
Production.
Low energy intensity corresponds to
high energy efficiency; as the equation
makes clear, this is achieved with low
energy consumption and high production
rates. This simple fact produces many
ramifications — one of the most obvious is
the adverse impact of cuts in production
rate.
Chemical plants and refineries are designed
to run at maximum efficiency at their
nameplate capacity. As production falls in
continuous processes, energy consumption
doesn’t drop proportionately. Many rea-
sons for this exist, with the majority linked
to control methods, equipment constraints,
and leaks and losses.
Most flow control systems are inherently
inefficient. Two common examples involv-
ing centrifugal pumps illustrate this point.
In so-called “bypass control,” the flow rate
to the downstream consumer is regulated
by recycling fluid from the pump discharge
Combat Low Rate, Low EfficiencyProduction cutbacks in continuous processes boost energy consumptionBy Alan Rossiter, Energy Columnist
Energy Efficiency eHANDBOOK: Power up Your Energy Efficiency Efforts 4
www.ChemicalProcessing.com
either to the pump suction or to a feed
drum ahead of the pump. When the flow
required by the downstream consumer
declines, the recycle flow increases. How-
ever, the flow through the pump and, hence,
the pump’s energy consumption remain
essentially constant. Because energy con-
sumption stays the same while production
goes down, energy intensity increases.
In the second example, “throttle control,” a
valve in the pump discharge line adjusts the
flow. The valve closes to reduce the flow.
This imposes backpressure on the pump,
which has to deliver a higher discharge
pressure. This, in turn, demands more
energy per unit of flow. In addition, both
the pump and its driver (usually an electric
motor) move away from their design points
to new operating points, which are invari-
ably less efficient. The result, once again, is
an increase in energy intensity — although
the increase isn’t usually as large as it would
be with bypass control.
In contrast to these examples, variable
speed control can, in some cases, reduce
energy intensity as flow rate goes down.
However, this is typically more expensive
to implement.
Control systems also can cause a rise in
energy intensity as throughput drops in dis-
tillation columns. The flows of reflux streams
and stripping steam often are set based on
nameplate throughput, then held constant.
Consequently, when feed rates drop, there
isn’t a commensurate fall in energy con-
sumption. Modifying flow control systems to
maintain a constant reflux ratio or stripping
steam ratio — or, better, to keep product
specifications constant using online chemical
analysis — can correct this problem.
To overcome minimum turndown limits
in distillation columns, boilers, furnaces,
and other equipment requires energy and
minimum flow restrictions in piping. For
example, as a distillation column reaches
its turndown limit, it may make sense to
increase its reflux ratio to maintain liquid
and vapor traffic instead of reducing it, as
discussed in the previous paragraph. When
boilers reach their turndown limits, many
sites either vent steam or, alternatively,
deliberately use steam inefficiently within
their processes to avoid a visible vent.
When a flow rate approaches the minimum
limit in a pipe, prudence may dictate recy-
cling fluids, which increases pumping costs.
Heat losses through piping and vessel
walls, steam leaks, and condensate are
Most flow control systems are inherently inefficient.
www.ChemicalProcessing.com
Energy Efficiency eHANDBOOK: Power up Your Energy Efficiency Efforts 5
insensitive to throughput, so they become
a larger percentage of energy consumption
as production falls, causing energy inten-
sity to rise.
With the exception of some of the sim-
pler control issues, resolving most of
these problems usually requires signifi-
cant investment. However, in some cases,
operating changes also can provide
improvements, especially where multiple
pieces of equipment run in parallel. For
example, it may be possible to shut down
one process train in a multi-train plant,
one or two pumps or fans in a large cool-
ing water system, or one or two boilers in
a large steam system. However:
• The operating changes must not compro-
mise safety or reliability.
• You must consider system interactions.
For example, shutting down a cooling
water pump eliminates the energy use in
that pump, but the reduction in flow may
adversely affect the energy intensity of
equipment that uses the cooling water
(e.g., refrigeration units).
ALAN ROSSITER is Chemical Processing’s Energy Col-
umnist. Email him at [email protected].
When feed rates drop, there isn’t a commensurate fall in energy consumption.
www.ChemicalProcessing.com
Energy Efficiency eHANDBOOK: Power up Your Energy Efficiency Efforts 6
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Double Up on CogenerationConsider tradeoffs and operator training when looking at cogeneration opportunities
By Alan Rossiter, Energy Columnist
I discussed aspects of cogeneration in a
few earlier columns (May 2019’s “Get All
Steamed Up,” http://bit.ly/3aIkIxU and
March’s “Take a Closer Look at Cascaded
Efficiency,” https://bit.ly/2R0yYd5). This
is a huge subject, and I am returning to it
again both to present some basic principles
and to provide a cautionary tale.
Cogeneration is the sequential produc-
tion of two distinct forms of useful energy
from a single primary energy source. Most
often, the two different forms of energy are
heat and power (i.e., “combined heat and
power” or “CHP”). In large-scale industrial
applications, the heat typically is deliv-
ered via steam, and the power generally
is either electric or shaft power delivered
directly from a steam turbine to a pump or
a compressor.
Most power in the United States comes
from heat engines (e.g., gas turbines, steam
turbines, and internal combustion engines)
using fossil fuels and operated in a cycle.
These devices must adhere to one of the
most fundamental principles of physics: the
second law of thermodynamics — which can
be stated as thus: “Cyclical heat engines
can never convert all of the incoming heat
to power — they always reject some of it.”
Raising the temperature of the hot portion
of the cycle and lowering the tempera-
ture of the cold portion can improve the
efficiency; but for any given hot and cold
temperatures, the efficiency never can
exceed that of an ideal “Carnot cycle” oper-
ating between the same temperatures.
Commercial gas turbines and steam tur-
bines typically reject at least 60% of their
Energy Efficiency eHANDBOOK: Power up Your Energy Efficiency Efforts 8
www.ChemicalProcessing.com
incoming heat, so their standalone energy
efficiency is less than 40%. However, if
the exhaust is hot enough, its heat can be
recovered for process heating or other
purposes. In such arrangements, it’s pos-
sible sometimes to make beneficial use of
80% or more of the primary energy, so the
overall energy efficiency can reach 80% or
higher. There is a tradeoff though: making
the exhaust hot enough for use reduces the
standalone efficiency of the heat engine,
so it produces less power per unit of
heat supplied.
DON’T FORGET TRAININGA number of years ago, I was on the design
team for a major petrochemical plant. The
preliminary design included a large (20-
MW) compressor driven by a condensing
steam turbine. The steam entered at 600
psig and exhausted below atmospheric
pressure, at 3.7 psia (7.5 in Hg). The exhaust
steam, at 150°F, wasn’t hot enough for
beneficial use, so it went to a water-cooled
condenser and the heat was rejected to
ambient through the cooling water system.
An adjacent plant consumed 100,000 lb/h
of 150-psig steam, obtained by passing
steam from the 600-psig header through
a letdown valve. I saw an opportunity for
cogeneration: replace the condensing steam
turbine with an extraction/condensing tur-
bine. In this design, all the steam passes
through the front end of the turbine, after
which 100,000 lb/h is withdrawn through
the extraction port at 150 psig for use at the
adjacent plant. The remaining steam passes
through the back end of the turbine and
then goes at 3.7 psia to the condenser. The
steam that passes through the extraction
port sequentially is used to produce power
in the turbine and to deliver heat to the
adjacent plant. The design change reduces
both the condenser duty and the overall
steam requirement, saving the site more
than $1,500,000/yr.
Several years later, I returned to the plant
with a team to conduct a site-wide energy
assessment. We found it was still producing
large amounts of 150-psig steam from 600-
psig steam with the letdown valve; I assumed
the site had decided against my extraction
turbine recommendation. Not so! The
machine had been installed but the operators
didn’t know how to operate the extraction
port. Consequently, the turbine was running
in condenser-only mode. Simply recom-
mending the extraction/condensing turbine
wasn’t sufficient; operator training also was
required. After the assessment, the operators
were trained, the extraction port was com-
missioned, and the site benefitted from this
cogeneration opportunity.
Have any of your improvement projects been
hampered by lack of operator training? Let
me know if you can share examples.
ALAN ROSSITER is Chemical Processing’s Energy Col-
umnist. Email him at [email protected].
www.ChemicalProcessing.com
Energy Efficiency eHANDBOOK: Power up Your Energy Efficiency Efforts 9
The most common method of trans-
ferring energy with steam is indirect
heat exchange, which is used in
familiar applications including process, plant
sanitation and reactor vessels. Condensing
the steam releases latent heat, and a mem-
brane, such as a tube or plate, transfers that
heat into a fluid. The process generates a
byproduct condensate that is discharged
through a trap and returned to its source,
typically a boiler, where it continues to pro-
duce steam.
This tried-and-true method, however, has
a drawback. Because of the pressure drop
as the condensate exits the trap, some
portion inevitably is lost to flash evapora-
tion. To keep the system functional, cold
replacement water must be added. As
the condensate is lost, system efficiency
is impacted. The level of impact varies in
accordance with the steam supply’s pres-
sure — the higher the pressure, the less
efficient the system (Figure 1).
Yet an alternative method exists that is
ideal for high-pressure systems: direct
steam injection (DSI). Here, the steam is not
held within a membrane to keep it separate
from the process fluid but rather is blended
directly into it. The need to recover conden-
sate is thereby eliminated, and, instead of
being lost to flashing, it is used fully. As a
result, the system achieves 100% heat trans-
fer efficiency.
DSI’S ADVANTAGESThe DSI approach offers several advan-
tages — chief among them is cost savings.
The boiler used in a DSI system is fed by
Improve Efficiency with Direct Steam InjectionTechnology offers notable cost savings for high-pressure applicationsBy Tony Pallone, Pick Heaters, Inc.
Energy Efficiency eHANDBOOK: Power up Your Energy Efficiency Efforts 10
www.ChemicalProcessing.com
the same cold replacement
water used in indirect heat
exchangers and requires
greater heat input to con-
vert this water to steam.
However, this is more than
offset by reduced steam
demand at the use point,
yielding a net reduction in
fuel consumption and cost
savings for the end user. A
DSI system can save up to
28% of the fuel required for
indirect heat exchangers.
DSI also offers more pre-
cise temperature control
because of its rapid-re-
sponse adaptation to load
changes. Condensate is
not recovered, eliminating
the need for a flash tank or
condensate return system
(Figure 2). Finally, surface
area is not required to
effect heat transfer, making
for a more compact device
that is easier both to house
and to maintain.
INDUSTRIAL APPLICATIONSDSI systems are well-suited
to a variety of industrial
applications that can benefit
from a steady supply of
on-demand, precisely
controlled hot water. One
system option is a constant
flow heater, which serves
the cross-industry trend
of shifting from steam to
hot water for jacketed
CONDENSATE LOSSFigure 1. In indirect heat exchange, a portion of the condensate is lost due to flashing and must be replaced with cold water. Flash losses vary with steam supply pressure. Source: Pick Heaters Inc.
DIRECT STEAM INJECTIONFigure 2. With direct steam injection, steam is completely con-sumed and no condensate is returned. Flash losses are eliminat-ed. Source: Pick Heaters Inc.
www.ChemicalProcessing.com
Energy Efficiency eHANDBOOK: Power up Your Energy Efficiency Efforts 11
heating, eliminating the potential for hot
spots, burn-on and thermal shock. A variable
flow heater allows for frequent start-stop
applications, making it a natural fit for plant
sanitation and clean up.
The food and beverage industry also has
employed DSI systems for in-line product
cooking, clean-in-place (CIP) heating and
nitrogen gas injection. A sanitary jet cooker
can heat, cook or sterilize water and slur-
ry-type food products on a continuous,
straight-through basis. Some models feature
low-velocity mixing and a nonshearing design
to handle small food pieces without damage.
In the chemical processing industry, DSI
supports automated systems with precise
temperature control that ensures optimal
effectiveness of jacketed reactors and elimi-
nates the potential for destruction and waste
of heat-sensitive products. Other applica-
tions include charging reactor vessels, tank
cleaning and CIP and smooth blending of
condensate streams.
Additional industries that have realized
efficiency improvements by deploying DSI
include pulp and paper, energy and power
and pharmaceutical.
THINKING BEYOND THE TRADITIONALThe biggest obstacle to wider DSI deploy-
ment is insufficient understanding of the
technology. Process engineers long have
been focused on strategies to minimize the
condensate lost in indirect heat exchange
systems. DSI removes condensate from
the equation in a way that may seem too
good to be true. To help determine if a DSI
system is right for your application, a DSI
vendor should provide data and case stud-
ies to back up efficiency and cost-savings
claims, and conduct a customized energy
comparison study.
Although DSI is inappropriate for a few
applications — low-pressure systems, for
instance, or systems processing liquids that
must be kept separate from steam — the
technology represents a giant leap forward
for a range of industry needs.
TONY PALLONE is a writer for Pick Heaters, Inc. For
more information visit www.pickheaters.com.
www.ChemicalProcessing.com
Energy Efficiency eHANDBOOK: Power up Your Energy Efficiency Efforts 12
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