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 arossiter@putman.net.

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 arossiter@putman.net.

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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