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TitleEthylene Production
TagsCracking (Chemistry) Distillation Valve Gas Compressor Boiler
File Size2.6 MB
Total Pages28
Document Text Contents
Page 1

Chapter 1

Ethylene Production

NOTE: The following chapter is the
first of many to be released as part
of a Chemical sourcebook. These
chapters will be released to eDocs
as they are completed and when
fully developed, compiled into one

Ethylene is one of the most important
petrochemical intermediates and is a feedstock for
many various products. End products made with
ethylene include food packaging, film, toys, food
containers, bottles, pipes, antifreeze, carpets,
insulation, housewares, etc. Chemicals that are
made from ethylene in order to produce these end
products are polyethylene, ethylene dichloride,
ethylene oxide, ethylbenzene, and vinyl acetate,
just to name a few.

Global ethylene capacity utilization has remained
above 90% since 2004 until 2008's economic
meltdown. In 2007, 2 million tonnes per year (tpy)
of ethylene capacity was added, according to the
Oil & Gas Journal. As of January 1, 2009, global
capacity was 126.7 million tpy. Capacity has been
added in recent years due to expansions and

debottlenecking at existing plants, as well as
greenfield plants being built in the Middle East and
Asia. Due to the change in market conditions and
the economy, there is an over‐supply of ethylene
capacity. Many plants have been taken offline in
this time period, are operating at reduced rates, or
are undergoing turnarounds. As the ethylene
market rebounds, capacity will increase. In fact,
based on new capacities announced and plants
that are under construction, global ethylene
capacity is expected to be at 162 million tpy by
2012, ahead of the demand growth.

There are five major licensors of ethylene plants:
KBR; Technip; Linde; Shaw, Stone & Webster;
and Lummus. While ethylene production differs
slightly by licensor, the overall process is fairly
similar (see Figure 1‐1). There are also some
differences in the process coming from the type of
feedstock being used. Some of these differences
will be highlighted. This chapter will cover the
general steps in ethylene production and will
discuss the critical valve applications within an
ethylene plant, what valve challenges those
applications present, and the recommended
Emerson solutions.

Figure 1‐1. General ethylene process (naptha fed cracker)

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Figure 1‐2. General ethylene furnace schematic

I. Furnace
The two primary feedstocks for ethylene
production are naphtha and natural gas (ethane,
propane, butane, etc.). The first step in the
production of ethylene is to take the feedstock and
crack it into ethylene and other various products in
a furnace. This process is called pyrolysis.
Pyrolysis is the thermal cracking of petroleum
hydrocarbons with steam, also called steam
cracking. The main types of commercial furnaces
are the ABB Lummus Global furnace, Millisecond
furnace (KBR), Shaw� furnace (ultraselective
cracking furnace), Technip furnace, and the Linde
PYROCRACK� furnace. See Figure 1‐2 for a
general schematic of an ethylene furnace.

The feed hydrocarbon stream is pre‐heated by a
heat exchanger, mixed with steam, and then
further heated to its incipient cracking temperature
(932�F to 1256�F or 500�C to 680�C depending
upon the feedstock). At this point, it enters a
reactor (typically, a fired tubular reactor) where it is
heated to cracking temperatures (1382�F to
1607�F or 750�C to 875�C). During this reaction,
hydrocarbons in the feed are cracked into smaller
molecules, producing ethylene and co‐products.

The cracking reaction is highly endothermic,
therefore, high energy rates are needed. The
cracking coils are designed to optimize the
temperature and pressure profiles in order to
maximize the yield of desired or value products.
Short residence times in the furnace are also
important as they increase the yields of primary

products such as ethylene and propylene. Long
residence times will favor the secondary reactions.

Table 1‐1. Furnace Reactions

Primary Reactions


Ethylene C4 products

Propylene C5 products

Acetylene C6 products

Hydrogen Aromatics

Methane C7 products


Maximum ethylene production requires a highly
saturated feedstock, high coil outlet temperature,
low hydrocarbon partial pressure, short residence
time in the radiant coil, and rapid quenching of the
cracked gas. Valves in the furnace section play a
critical role in maximizing ethylene production and

There are three critical control valve applications in
the furnace area: dilution steam ratio control, feed
gas control, and fuel gas control. Each will be
discussed in further detail in the subsequent text.

Dilution Steam Ratio Control
The quantity of steam used (steam ratio) varies
with feedstock, cracking severity, and design of
the cracking coil. Steam dilution lowers the
hydrocarbon partial pressure, thereby enhancing
the olefin yield. Because of this, it is important to
obtain the appropriate ratio and maintain proper

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During drum fill operation, the boiler is under
minimal pressure. This causes the entire pressure
drop to be taken across the feedwater startup
valve. Because of this, the formation of cavitation
becomes a concern. Sizing of the startup valve
must be done in combination with the feedwater
regulator valve. This is to ensure that the
feedwater regulator valve does not experience any
service conditions that lead to damaging
cavitation. The most common split is that 80%
capacity in the startup valve is equal to 20%
capacity in the regulator valve. Once the transition
to the regulator valve has begun, the startup valve
closes. Improper use is one of the main issues
surrounding two valve feedwater systems. For
example, the startup valve is not being used at all
and the regulator valve is being used to perform
both functions. This can be a major problem if the
boiler feedwater regulator was not sized or
selected to perform both functions. There can also
be an issue if switching between the startup and
the regulator valve is happening too quickly.

Because of the cavitation concerns and taking the
full pressure drop, the startup valve should utilize
some form of anti‐cavitation trim. Typically, in
process plants, since the pressures are not as
high as power plants, Cavitrol III trim is selected.
440C trim is recommended for the case of treated
feedwater. For cases where one valve is
performing the startup and regulator duties,
characterized Cavitrol trim can be designed to
handle the cavitating conditions at startup and
then standard equal percentage or linear
characteristic for steady‐state conditions to
maximize capacity. Another common issue in both
the startup and regulator valves is to see them
operated below the minimum operating point. This
can cause “gear‐toothing” damage on the plug.
Damage can be limited by utilizing a lower metal
piston ring or matched plug/cage combination.
This limits the amount of clearance flow between
the plug and cage thus minimizing erosion effects.
Another solution is to use the protected inside seat
technology with Cavitrol III trim. This technology is
designed so that the shutoff surface is not
exposed to potential erosion. Protecting the
shutoff surface will extend the sealing life of the
trim. Using the low travel cutoff feature of the
FIELDVUE digital valve controller is ideal. The
instrument can be setup so that the valves do not
throttle below a minimum point.

The continuous blowdown application is constantly
removing concentrated water from the drums and
removes a significant level of suspended solids.
Typically, this application is flashing. Flashing is a
system phenomenon and, therefore, cannot be

Figure 1‐18. Gear‐toothing


prevented. The best way to handle it is to minimize
the amount of damage being caused by flashing.
Use of an angle valve with a downstream liner is
recommended to minimize the amount of damage
caused by flashing. It is much more economical to
replace a liner than to replace an entire valve.

The condensate recirculation valve is similar to the
feed pump recirculation valve in that it also
protects the pump from cavitation. Inlet pressure
and temperature differ from the feedwater system.
The dissimilarities from the feedwater system
include the inlet pressure and temperature. Inlet
sizing often indicates that flashing is occurring.
The end user needs to ensure that there is not a
sparger or diffuser downstream emitting back
pressure on the valve. This will cause cavitation
rather than flashing. Cavitation can also cause
noise and vibration. Tight shutoff is needed on this
application because it prevents loss of condenser
vacuum, loss of condensate pressure and flow to
the deaerator, and saves money in terms of
wasted pump horsepower. Valve selection is
typically an EWT valve with Cavitrol III trim. Flow is
usually 25‐35% of the condensate pump's full
capacity. Class V shutoff is highly recommended
to minimize leakage past the seat because it can
cause damage.

X. Flare System
In an ethylene plant, vent to flare systems are on
several of the unit operations such as the quench
tower, distillation columns, steam systems, etc.
Vent valves are used to depressurize the unit for
safe shutdown and, possibly, for startup as well.
Due to the high pressure drop and high mass flow,
they are severe service applications. They are also
critical reliability applications as part of the safety
shutdown systems. These valves are closed
except in flare scenarios. Plant personnel need to
ensure that the valves move when the process
requires flaring.

Page 15


The first challenge for vent valves is to have
optimum sizing and selection of the valve with a
silencer/diffuser. It is a balancing act when sizing
this system as a result of optimizing the noise
attenuation by adjusting how much pressure drop
each component is taking. Because they directly
affect each other, they should be considered an
engineered solution and sized together as a
system. Tight shutoff is a major concern as any
leakage causes a loss in plant efficiency. Class V
seat load is recommended as it will minimize
leakage past the vent valve while in the closed
position. This is usually a high noise application
due to the pressure drop and high mass flow.
Operation may be intermittent and for a short
duration so high noise may be tolerable.
Awareness and concern for structurally intolerable
noise levels are necessary. Noise requirements
are likely to be driven by plant and regulatory
noise requirements.

Globe or angle valves with a Whisper Trim III or
WhisperFlo trim for noise attenuation, are the
typical recommendation for this application. There
are some exceptions to this. Vent valves on the
quench tower may be able to use butterfly valves
with diffusers due to the low pressures in this
particular application. Also, cryogenic valves may
be needed on some of the distillation column vent

valves. Because this is a valve that normally sits
closed but needs to move when called upon, the
FIELDVUE DVC6000 SIS (Safety Instrumented
Systems) is an optimized solution. As plants are
paying more attention to their safety loops and
performing safety evaluations, these are being
tagged as SIS applications because they are key
to ensuring that the process can flare in the event
they are needed (shutdown, startup, and
emergency event). Partial stroke testing can be
performed using the DVC6000 SIS. This can be
done without interrupting normal operation and
requires the valve to move from 1 to 30% of its
total travel.

XI. Conclusion
Ethylene plants use hundreds of control valves
throughout the entire production process and
understanding the various applications is essential
in order to apply an engineered solution towards
them. With the selection of an appropriate Fisher
control valve solution, plant performance will
improve as a result of enhanced reliability,
variability, safety, etc.

Page 27


Figure 2‐12. 24000 with DVC2000


Figure 2‐13. GX with ENVIRO‐SEAL Bellows


environments. The cleaning process required
varies from one polysilicon producer to the next.
Emerson has created and utilizes standard
degreasing levels for Fisher control valves; Class
A for non‐oxygen fluids and Class AAA for oxygen
applications. Typically, polysilicon projects have

required some form of Class A degreasing. This
may be modified to specify a certain cleaner or a
non‐phosphate rinse at the end of the cleaning
process. Emerson has also worked with polysilicon
manufacturers to tailor a cleaning process to meet
their specific needs.

Safety Instrumented Systems
Due to the nature of the process fluids involvement
and the reactions taking place, safety is an
important and critical part of the production
process. Safety instrumented systems (SIS) are
put in place to ensure protection against accidents
such as toxic chemical releases, facility
explosions, or fires. The SIS valve is present to
take the process to a safe state when specified
(dangerous) conditions are violated. Loops are
evaluated by plants to determine what safety
integrity level (SIL) level is needed for that safety
application. Several Fisher products have been
certified and are suitable for use in SIS
applications. These products include the
FIELDVUE DVC6000 SIS, GX, Vee‐ball, HP,
easy‐e, 8580, A81, and the Control‐Disk along
with the respective actuators for the valves.
Typically, safety valves operate in one static
position and only move upon an emergency
situation. Without mechanical movement for long
periods of time, unreliability inherently increases.
To ensure availability on demand, SIS valves must
undergo regular testing. This includes not only full
stroke tests, but also partial stroke tests that can
be used to increase the time between full proof
tests. The DVC6000 SIS can be utilized to perform
partial stroke testing. It can move the valve
between 1 and 30% from its original position.
Partial stroke tests can be scheduled via the
automatic test scheduler. If an emergency demand
occurs, the response will override the testing.
Using smart positioner technology with partial
stroke testing provides for predictive maintenance
through diagnostic information.

V. Conclusion:
As technical uses for polysilicon grow along with
the demand for solar power, the polysilicon market
will continue to expand. Reliability, safety, control,
purity levels, and variability are all affected by the
control valves used in the process. It is important
to understand the polysilicon process in order to
apply the proper control valve solution for various

Page 28


Emerson Process Management
Marshalltown, Iowa 50158 USA
Sorocaba, 18087 Brazil
Chatham, Kent ME4 4QZ UK
Dubai, United Arab Emirates
Singapore 128461 Singapore

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