Optimal gas analysis decisions improve ethylene plant operation - Emerson
←
→
Page content transcription
If your browser does not render page correctly, please read the page content below
Process Controls,
Originally appeared in:
January 2020, pgs 75-79
Instrumentation
Used with permission.
and Automation
M. KAMPHUS, Emerson Automation Solutions,
Hasselroth, Germany; and D. MCMILLEN,
Emerson Automation Solutions, Houston, Texas
Optimal gas analysis decisions improve
ethylene plant operation
One of the most common and impor- measurement technologies such as pho- companies thrive in the competitive pet-
tant building blocks in the petrochemi- tometric nondispersive infrared (NDIR) rochemical industry.
cal industry is ethylene, constituting a and nondispersive ultraviolet (NDUV), The three main gas analysis technolo-
huge and fast-growing worldwide indus- thermal conductivity, paramagnetic and gies will be explained first, and where
try. Ethylene is an intermediate chemical electrochemical oxygen. each should be applied to optimize opera-
used to manufacture many commercial While virtually every ethylene plant tions is then discussed.
products—approximately 200 MMt uses some combination of these tech-
will be produced in 2020. Nearly half of nologies, not all use them optimally. Gas chromatography. The composi-
global ethylene demand is for polyethyl- Applying the right specific gas analysis tion analysis of hydrocarbon gases is im-
ene (PE) production, but it is also used system in each process and unit will help portant in ethylene production as the raw
to make vinyl chloride, ethylbenzene
and many other valuable intermediate
products, such as ethylene oxide (EO),
ethanol EO and ethanol.
Ethylene plant personnel strive to
produce 99.99% pure product. These
stringent purity requirements must be
verified in production and at custody
transfer points because the presence of
impurities can poison catalysts and affect
downstream processes, leading to costly
repairs and downtime. Stringent environ-
mental requirements relating to gas emis-
sions also exist. Since purity is essential,
the precision and reliability of the mea-
surement is of key importance. Speed of
response to any potential issues—along
with cost reduction—are also critical to
prevent potential process upsets and to
ensure optimum throughput.
Many online compositional measure-
ments can be used to improve operations
and profitability. The measurement of
purity and process optimization in ethyl-
ene plants is accomplished by gas analysis
systems, principally gas chromatographs
(GCs), laser spectroscopy analysis sys-
tems and conventional continuous gas
FIG. 1. Gas chromatographs are used to make a wide variety of measurements in ethylene plants.
analyzers. The latter use a variety of
Hydrocarbon Processing | JANUARY 2020 75
Process Controls, Instrumentation and Automation
product is refined and processed, and gas steps of ethylene production. In addition, change, so the measurement is effectively
chromatography is one of the most widely the use of GCs in ethylene production is continuous and obtained in real time.
used techniques for hydrocarbon gas spe- established and proven, so plant person- Conversely, GCs work on the princi-
ciation and composition measurement nel are familiar with this technology. ple of injection and separation followed
(FIG. 1). Advantages include a wide range by analysis, with cycle times ranging
of measurement [from parts per million Laser spectroscopy. Laser spectroscopy from 1 min to more than 15 min depend-
(ppm) levels up to 100%], a high degree of is the newest of the analysis technologies ing on the application. Therefore, the
repeatability and detection of many differ- and has undergone significant develop- measurement data is periodic rather than
ent components in complex gas mixtures. ment in the last decade. Today’s semicon- continuous.
All GCs are composed of the same ductor, laser-based systems incorporate
functional components: the sample han- both quantum cascade laser (QCL) and Conventional continuous analyzers.
dling system, the chromatograph oven and tunable diode laser (TDL) technology, Conventional continuous gas analyzers
the controller electronics. A GC works allowing multiple component measure- utilize familiar sensing technologies such
by physically separating the hydrocarbon ments to be made by a single device. as NDIR, NDUV photometer and ther-
compounds in a column, with different hy- Advantages of laser spectroscopy in mal conductivity detection, along with
drocarbon compounds exiting the column ethylene production include the ability to paramagnetic and electrochemical sen-
at different times, with each then detected. detect, analyze and monitor 6–12 differ- sors. These analyzers are generally the
In an ethylene plant, a key advantage of ent gases in one analyzer, eliminating the choice whenever emissions must be moni-
a GC is its ability to measure both heavy need for multiple analyzers, carrier gases tored, and they can also be used to provide
(beyond C2 ) and light gases, which makes and complex sample handling systems re- the required fast response times in mea-
it the primary, if not only, choice in some quiring coolers or chillers. Multi-compo- surements of light hydrocarbons (FIG. 3).
nent measurements using hybrid QCL/ Conventional continuous gas analyz-
TDL technology (FIG. 2) are especially ers measure extractive single or multiple
valuable in applications such as ethylene molecules (analytes) in process gases,
product certification, which would nor- with the following technologies applied
mally require multiple GCs. depending on the required measurement:
In addition, the laser data spectros- • NDIR for nitric oxide (NO),
copy collection methodology enables carbon monoxide (CO), carbon
many thousands of spectra to be gathered. dioxide (CO2 ), methane (CH4 ),
Each measurement is updated within one ethylene and propene (C3 H6 )
second with a good signal-to-noise ra- • NDUV for sulfur dioxide (SO2 )
tio. Advanced signal processing enables and nitrogen dioxide (NO2 )
real-time validation of measurements • Chemiluminescence for nitrogen
and greatly reduces the need for calibra- oxides (NOx )
tions, minimizing ongoing operational • Electrochemical and paramagnetic
costs. The response time is fast, typically detectors for oxygen
less than 10 sec. to achieve 90% of step • Thermal conductivity for percent
hydrogen
• Flame ionization detectors
for total hydrocarbon.
Up to five measurements using these dif-
ferent technologies can be combined in
a single analyzer to provide continuous,
multi-component gas analysis.
For photometer and oxygen measure-
ments, conventional continuous gas ana-
lyzers and QCL gas analyzers can be used
in similar applications, with technology
selection dependent on the number of
components to be measured, detection
limits and gas composition. For example,
when cross-interference gases are present,
the response of traditional non-dispersive
photometer technology can be influenced,
and a GC or a QCL analyzer should be
considered. However, when measuring
FIG. 2. Hybrid QCL/TDL technology provides FIG. 3. Conventional continuous gas analyzers hydrogen, conventional continuous gas
speed, precision and reliability in critical areas of are often used for process control, product analyzers or GCs are a better choice be-
the purification train and product certification. optimization and emissions monitoring.
cause QCL analyzers are not capable of
76 JANUARY 2020 | HydrocarbonProcessing.com
Process Controls, Instrumentation and Automation
providing hydrogen analysis. These tradi- in each technology. GCs eliminate interfer- to provide backup capability between each
tional continuous gas analyzers are typi- ence issues by physically separating com- other in the event of an analyzer failure.
cally the most cost-effective technology of plex process gases before taking the mea- Conversion control requires the mea-
choice when monitoring one or two com- surement, but this separation takes time. surement of the feed to the furnaces. If
ponents in process gas streams. Conversely, conventional continuous gas the feed is a simple mix of ethane and pro-
analyzers provide fast readings, but must pane, a GC is used. Complex feeds like
Ethylene production. The typical eth- make measurements without the benefit naphtha require a density measurement
ylene plant is divided into two basic sec- of chromatographic separation. These ana- to estimate the composition, typically
tions: the cracking furnaces (hot side) lyzers must also avoid potential interfer- performed not by an analyzer, but by a
and the fractionation train (cold side). ences by other gases in the process stream specialized instrument.
The fractionation train takes the effluent using spectroscopic techniques. Cracking furnaces need to be decoked
from the furnaces and separates it into a One of the most impactful measure- about once every 20 d of operation be-
wide range of chemical products. Stack ments performed in ethylene production cause the furnace coils acquire a coating of
gases from the cracking furnaces and from is feedstock composition—used for op- carbon over time and lose efficiency. This
steam generation are discharged to the en- timization of furnace operation. Due to is done by burning off the coke with air in
vironment and must be monitored. the presence of heavy gases in the stream, a steam atmosphere. The progress of the
The hot side—cracking furnaces. a GC is typically used to measure Btu decoking phase is monitored by measur-
The hot side of ethylene production is (C1–C6+) value to ensure optimal tem- ing the amount of CO2 in the effluent. It
the real moneymaker in an ethylene plant peratures and performance, with a thermal rises as the carbon is burned off and then
because increased throughput achieved conductivity detector on each furnace fuel decreases to zero when the furnaces are
by enhancing cracking furnace opera- line. For Btu firing rate control, the fuel gas clean. CO2 measurement in the effluent is
tion can greatly increase profitability. In to the furnace is normally analyzed for Btu usually made by a conventional continu-
a plant producing 1 MMtpy of ethylene, content. This requires complete analysis ous gas analyzer, which measures CO2 in
a 1% change in throughput can produce of the stream to obtain the most reliable conjunction with removing contaminants
an enormous improvement in the bot- Btu measurement. using an NDIR photometric detector.
tom line. Energy represents about 25% of Another key analysis point for hot side After leaving the radiant coils of the
the cost of ethylene production, and ap- measurements is the exit from the trans- furnace, the product mixtures are cooled
proximately 18% of energy consumed is fer line exchangers. It is common to use instantaneously in transfer line exchang-
associated with heating and losses in the two GC analyzers at this sample point for ers to preserve the gas composition.
cracking furnaces, so furnace control has cracked gas control. For velocity (or resi- This process is followed by oil quench,
a substantial impact on overall plant ef- dence time) control, the cracked gas is ana- gasoline fractionation, water quench and
ficiency. Over-cracking results in catalyst lyzed to determine the molecular weight. gasoline stripping to deliver pyrolysis fuel
deactivation due to coke formation, while The sample point is located on the effluent oil and pyrolysis gasoline. The gas is then
under-cracking reduces production. of the secondary transfer line exchanger. compressed in a multi-stage compressor
Coke formation is undesirable because Since the speed of analysis is critical for train to separate it into various compo-
the layer of coke acts as an insulator on the proper control, two analyzers are recom- nents. Acid gases are removed with caus-
inside of the furnace tubes, reducing heat mended for each sample point. The first tic scrubber, drying and cooling.
transfer from the furnace tubes into the analyzer measures key compounds such To ensure process control, conven-
cracked gases. The reduced heat transfer as C2 , C2=, C3 and C3= to provide the tional continuous gas analyzers are used
necessitates increased tube skin tempera- control system with the information re- to measure CO and CO2 after the caustic
tures to maintain the cracking reactions. quired to calculate an estimated molecular scrubber. The hydrogen-rich tail gas from
Although the furnace tubes are fabricated weight for the velocity control. The sec- the dryer/cooler stage is also analyzed for
from high-temperature alloys, eventually ond analyzer provides a slower but more hydrogen purity using conventional con-
the metallurgical limits of the metal are complete analysis of the cracked gas. The tinuous gas analyzers.
approached. At this point, the furnace analysis from the second analyzer is used Ethylene plants also require special
tubes must be decoked. Failure of a fur- to trim (or update) the estimated molecu- measures for emissions monitoring. NOx,
nace tube would allow flammable cracked lar weight obtained from the first analyzer. CO and SO2 emissions from cracking fur-
gases to meet the hot furnace combustion Since it is not always practical to have naces and steam boilers need to be kept
gases, which could result in an explosion. two analyzers on every furnace, a dedicat- in compliance. NOx limits are, in some
Feedstock and ethane recycle gases fed ed analyzer can be used for each furnace to global areas, less than 50 ppm, and there
into the cracking furnaces require measure- perform the short analysis, with one ana- are often requirements for certified emis-
ment of the various hydrocarbon concen- lyzer configured to provide complete anal- sions monitoring to report values to the
trations for optimal blending. This can be ysis for several furnaces. This is done by authorities.
done with GCs, while conventional contin- routing the sample gases from the furnaces Both QCL analyzers and traditional
uous gas analyzers can also be used to moni- to the analyzer using a header system. technologies can be used for emissions
tor the recycle streams to maintain a consis- Furthermore, since the data from the monitoring, depending on the compo-
tent feed gas quality to the cracking unit. short analysis is so critical for the proper nents and ranges to be measured. NDIR
The choice between the two depends control of the velocity rates, the analyzers or NDUV are typically used to measure
upon the strengths and trade-offs inherent providing fast readings are typically set up CO, CO2 and SO2. Chemiluminescence
Hydrocarbon Processing | JANUARY 2020 77
Process Controls, Instrumentation and Automation
Hydrogen-rich tail gas AX AX can be used, depending on the impurities
AX Methane-rich tail gas 7
Ethylene product 12
Propylene product present and the ranges to be measured.
AX
2
3 Hydrogen Hydrogen The methane-rich tail gas often needs
Acetylene MA/PD to be blended to ensure constant heating
reactor reactor values or gas concentrations. Depending
Cold box on the accuracy needed for the measure-
C splitter C splitter ment and the components to be analyzed,
AX
AX AX AX
15
Mixed olefins a GC or a conventional continuous gas
1 5 10 analysis measurement can be made. GCs
provide a much higher accuracy with the
Feed from
cracking AX
AX
AX
AX ability to measure many more components
6 11
furnace
Deethanizer
8 13 selectively, whereas a conventional contin-
Depropanizer Debutanizer
Ethane recycle Propane uous gas analyzer provides faster response.
Demethanizer to furnace feed An analyzer (AX4 in FIG. 4) monitors
Py-gasoline from compressors the bottom streams of the demethanizer to
C + gasoline measure light gases, such as C1 and CO2.
AX AX AX AX Any gases that get to this point ultimately
4 9 14 16 end up in the final ethylene product stream
FIG. 4. Process flow diagram of a typical ethylene plant fractionation train.
as impurities and should be minimized.
This is done by using measurements of the
C1 /C2 ratio, which are made using a GC.
is used for NOx and paramagnetic is Simultaneously, the remainder of the The next series of analyzers are also
used for O2. QCL analyzers can make furnace effluent leaves the bottom of the GCs and are used to control the purity
all these emissions measurements in one deethanizer and is fed into the depro- of the ethylene product. This starts with
analysis, in ranges from ppm to percent panizer. Any pyrolysis gasoline condensed the measurement (AX5 in FIG. 4) of the
concentrations. in the compression stage after the cracking deethanizer overhead, used to minimize
The cold side—fractionation train. furnaces is also fed to the depropanizer. the amount of C3 in the stream while max-
The separation of the furnace effluent into At the depropanizer, the output is sent imizing the recovery of the C2=.
various products is performed through overhead to a reactor to remove any meth- To monitor the removal of the acety-
a series of fractionator towers that selec- yl acetylene (MA) and propadiene (PD) lene (AX6 in FIG. 4), an analysis is per-
tively separate one chemical group at a from the stream by hydrogenation. This formed in the effluent from the acetylene
time. The effluent stream moves from one reactor’s output is then sent to the C3 split- converter to ensure acetylene levels will
fractionator tower to the next, with the ter fractionator to separate the propylene meet final ethylene product specifications.
remainder after all tower processing sold from the propane. The last of the furnace This is done through the addition of hy-
as gasoline. The order in which the com- effluent enters the final fractionator tower drogen in catalytic beds.
pounds are removed can be seen in FIG. 4. to separate out the C4 olefins, leaving the Two acetylene converter units are em-
After the furnace effluent has been rest to become the pyrolysis gasoline that ployed: one in service and one on stand-
cooled and compressed, it enters the first exits the bottom of the debutanizer. by. Analytical data are required for the
fractionator tower to remove methane Production stages. In the fraction- inlet stream, mid-bed and outlet streams
and hydrogen. These light compounds ation train, the first analyzer (AX1 in of the acetylene converters to optimize
leave the overhead of the demethanizer FIG. 4) monitors the overhead streams of conversion and avoid process excursions.
and enter a cold box, where they are fur- the demethanizer. This measurement can These acetylene converters are fixed-bed
ther separated into a hydrogen-rich tail be used to minimize the loss of ethylene catalytic hydrogenation reactors, and the
gas and a methane-rich tail gas. into this stream, which mainly consists of catalyst has a finite life. Inlet, mid-bed and
Other processes needing hydrogen use methane and hydrogen. A laser-based or outlet measurements indicate acetylene
the hydrogen-rich stream, and the meth- conventional continuous gas analyzer is breakthrough. The standby converter
ane-rich stream is often used as a fuel for used to perform the measurement. should be brought online before the acety-
burners and heaters. The remainder of Two more analyzers (AX2 and AX3 in lene breaks through into the outlet.
the furnace effluent leaves the bottom of FIG. 4) monitor separation into the hydro- While GCs are frequently used for this
the demethanizer and enters the deetha- gen-rich tail gas stream and the methane- stage, laser spectroscopy should also be
nizer. Components exit the overhead of rich tail gas downstream of the cold box. considered since it can provide all criti-
the deethanizer and enter a reactor to re- Hydrogen purity can be measured with cal measurements in a single analyzer in
move any acetylene produced by hydro- a thermal conductivity analyzer utilizing seconds. Fast response is critical because
genation. Once the acetylene has been suppressed ranges. The hydrogen is some- acetylene can poison catalyst and cost mil-
removed, the reactor output is sent to a times further purified by a pressure swing lions of dollars in losses.
C2 splitter fractionator that separates the adsorption (PSA) process. If impurities At the C2 splitter, an analyzer (AX7
ethylene from the ethane. The ethylene is in hydrogen need to be measured down- in FIG. 4) monitors the ethylene product
sold as product, and the ethane is recycled stream of the PSA process, either a QCL stream for impurities, while another (AX8
back to the cracking furnaces as feed. or a conventional continuous gas analyzer in FIG. 4) measures the bottom streams to
78 JANUARY 2020 | HydrocarbonProcessing.com
Process Controls, Instrumentation and Automation
minimize any loss of ethylene in the eth-
TABLE 1. Ethylene plant fractionation train measurement
ane that is being recycled. These measure-
ments can be made using a GC, as well as a Analyzer Component
conventional continuous gas analyzer. no. Stream measured Measurement objective
A series of GCs are used to control the 1 Demethanizer overhead C2= Minimize the loss of ethylene
purity of the propylene product, starting 2 Hydrogen-rich tail gas H2 Determine H2 purity
with the measurement (AX9 in FIG. 4) of 3 Methane-rich tail gas Btu Calculate Btu for use as fuel gas
the stream leaving the bottom of the deeth- 4 Demethanizer bottom streams C1, C2= Control methane in the
anizer. Any C2 components present would ethylene product stream
end up in the propylene product stream, an 5 Deethanizer overhead C3 Control ethylene purity product
undesirable outcome, so these components 6 Acetylene reactor effluent Acetylene Control acetylene impurity
need to be controlled by measuring and in ethylene product
maintaining the optimum C2 /C3= ratio. 7 Ethylene product C1, C2, Measure impurities in
The depropanizer overhead stream is acetylene, CO2 ethylene product
monitored (AX10 in FIG. 4) for C4s, and 8 C2 splitter bottom stream C2= Minimize the loss of ethylene
this measurement is used to minimize in the ethane recycle stream
their presence in the propylene product. 9 Deethanizer bottom stream C2, C3= Control ethane in the
To monitor the removal of MA and PD, propylene product stream
a GC is installed at the exit of the MA/ 10 Depropanizer overhead nC4 Control propylene product purity
PD reactor (AX11 in FIG. 4). C3 splitter
11 MA/PD reactor effluent MA/PD Control MA/PD impurity
measurements can be made using con- in propylene product
ventional continuous gas analyzers, like 12 Propylene product C2, C3, Measure impurities in
the in-tray C2 splitter measurements. MA/PD propylene product
Two analyzers monitor the propylene 13 C3 splitter bottom stream C3= Minimize the loss of propylene
product for purity (AX12 in FIG. 4), and into the propane stream
the propane stream (AX13 in FIG. 4) to 14 Depropanizer bottom streams C3, C4= Control propane in the mixed
minimize the loss of propylene. The final olefins product stream
series of analyzers monitors the final sepa- 15 Debutanizer overhead stream C3, C4, Measure impurities in the
rations of the furnace effluent, beginning iC5, nC5 mixed olefins product stream
with the measurement of the depropaniz- 16 Debutanizer bottom streams C4= Minimize losses of C4 olefins
er bottom streams (AX14 in FIG. 4). This into the C5+ gasoline stream
analyzer monitors the C3 /C4= ratio, used
to control C3 impurity levels in the mixed used whenever heavy gases above C2 are nologies should be considered.
olefins product stream. present in a complex mixture of several hy- With the large number of chemical
Two more analyzers, typically GCs, drocarbons, and when speciation is more separations being performed in an ethyl-
monitor the purity of the mixed olefins important than fast response. For fast, ene plant, gas analysis plays a key role in
(AX15 in FIG. 4), with this measurement continuous measurements of smaller mol- process control by monitoring the vari-
used to minimize the loss of C4 olefins in ecules in process control and emissions ous process streams in ethylene produc-
the gasoline stream (AX16 in FIG. 4). TABLE monitoring applications, laser spectros- tion. Process analytics performed using
1 summarizes the previous text, showing copy, non-dispersive photometry, thermal gas analysis measurement data can be
the components measured in each stream, conductivity and paramagnetic sensing used to maximize yields and ensure prod-
along with the measurement objective. technologies are commonly used. uct quality, thereby increasing profits by
After ethylene leaves the fractionator, Hybrid QCL/TDL analyzers are often improving efficiency.
its purity must be certified by measuring preferred for multi-component measure- MICHAEL KAMPHUS is the Global
certain key contaminants using GC or la- ments when cross-interference gases are Product Manager for Emerson’s
ser spectroscopy. Ethylene plants produce present, which can affect the measure- Rosemount process gas analyzers.
product at 99.99% purity for use as feed ment accuracy of traditional non-disper- He holds a PhD in chemistry from
Bielefeld University in Germany,
to PE, PE glycol and polyvinyl chloride sive photometer technology. However, and has several years of experience
plants, among others. the detection limits of QCL analyzers are as an industrial process gas analysis
Final product ethylene can be trans- slightly lower than non-dispersive pho- application engineer. Dr. Kamphus is a member of the
German section of the Combustion Institute and the
ported in pipelines as a supercritical fluid, tometer-based analyzers. Process Analytics Working Group for the German
or it can be cooled for transport by ship as For applications requiring measure- Chemical Society.
a cryogenic liquid. Certification of ethyl- ment of one or two components in
DAVE MCMILLEN is the North
ene purity can be performed by a GC or process gas streams, the conventional America Business Development
by laser spectroscopy when response time continuous gas analyzers are more cost- Manager for Emerson’s Rosemount
is critical. effective. Individual applications will vary process gas analyzers. Mr. McMillen
has more than 15 yr of experience
significantly depending on the specific in process analytics and is a
Analysis rules of thumb. While every process used for ethylene production, so member of the Analytical Division
application is different, GCs are generally these guidelines only suggest which tech- of the International Society of Automation.
Article copyright © 2020 by Gulf Publishing Company. All rights reserved. Printed in U.S.A.
Hydrocarbon
Not to be distributed in electronic or printed form, or posted on a website, without express written permission Processing
of copyright holder. | JANUARY 2020 79You can also read
