000.270.CSE-163.1 Introduction to Process Analysers
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Transcript of 000.270.CSE-163.1 Introduction to Process Analysers
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Copyright 2007 Fluor Corporation
Process Analysers000.270.CSE-163.1
Introduction to Process Analysers
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Discussion Points
1. What is an Analyser and what makes it a ProcessAnalyser?
2. Why do we need this?
3. Overview of some common Analyser types and howthey work
4. Positives & Negatives
5. Sample Conditioning
6. Analyser Shelters and Cabinets
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What is an Analyser?
Most of the Instrumentation discussed elsewhere deals with thephysical attributes of the process (e.g. temperature, pressure
and flow) or of the well-being of the plant itself (e.g. vibration,
current and valve position). To understand Analysers we needto define
Analysis .. The determination of how much of agiven component is in a sample
This is achieved by again using the physical attributes of these
components in instrumentation that allows them to bemeasured and quantified .. this is an Analyser
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What make a Process Analyser?
This is usually an automated Analyser complete with safe and effective
sample collection, conditioning and disposal
The results produced by the Process Analyser may be required to automaticallychange the configuration of or even shutdown the process plant. Thereforecritical aspects affecting Analyser choice will be:
Sensitivity of the Analyser and accuracy of the result H2S (safety issue) is often measured in the range 0 10 ppm
Time taken to present the sample to the Analyser transport time through a 30m sample line is typically 30 - 60 seconds
Time taken to analyse and report the result from continuous monitoring to a new result every few minutes (varies
according to project specification)
Time taken to react to the result
a 24 ball valve may take every few minutes to fully close (varies according toproject specification)
Availability of the Analyser when an Analyser is calibrating itself it is not reporting new results
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Why do we need an Analyser?
Almost all products are produced and sold against a specification which
will usually include a minimum purity and maximum allowable amounts ofimpurities. Any material not meeting this specification cannot be sold andwill incur additional costs for storage and reprocessing (or disposal).
An Analyser can often al low th e detect ion of out-of-sp ecif icat ion (OOS)pro cessin g at an ear ly stage. Ear ly intervent ion wil l minim ise thequant i ty of OOS produ ced. Early intervent ion can also prevent the
OOS mater ial from contamin at ing an even greater quant i ty o f in-specif icat ion mater ial .
Many processes in the chemical. Biochemical, nuclear, Oil & Gas andpower industries can produce or use materials that can cause OOS productor which can be independently released and cause harm to people or theenvironment. Often small process changes can enhance the formation ofthese materials.
An A nalyser can often detect the format ion o f unw anted mater ials atsou rce again al lowin g ear ly intervent ion (examp les of the detect ion o fsulph ur impur i t ies in the Oi l & Gas indus try wi l l be shown )
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Analyser Types UV/Vis Spectroscopy
Continuous monitoring of asample stream passing througha glass cell using ultraviolet(usually) or visible light. The
Analyser measures thedifference in intensity betweenlight before and after it haspassed through the sample
Different types of chemicalstructure will absorb light atspecific frequencies. Analysis atthese frequencies allows us tomeasure and quantify materialswith a specific chemical structure
0
0.2
0.4
0.6
0.8
1
1.2
1.4
220 240 260 280 300 320
Wavelength (nm)
Absorbance(AU)
1% SO2
1% SO2 + 1% H2S
1% H2S
Example: mixture of SulphurDioxide and Hydrogen Sulphide in
Air. Immediate action may be
needed to increase introduction of
air to reduce the H2S component
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UV/Vis Spectroscopy (2)
Ametek 900 UVSpectrophotometer
2 x UV lamps plus
filter wheel allows
measurement at
multiple wavelengths.
Note long light path for
greater accuracy. The
calculation is carried
out within the analyser
and the result (in ppm)
is available as 4 - 20
mA or Modbus
1. Analysis at 285 nm allows quantification of sulphur dioxide (SO2)
2. Analysis at 235 nm and subtraction of the now known SO2 component will allows
quantification of hydrogen sulphide (H2Svery toxic!)
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UV/Vis Spectroscopy (3)
Advantages
Continuous process immediate result reporting Robust process the same components will always absorb at the same
frequencies. Linear response at low concentrations
Easy maintenance - UV lamp replaced at scheduled intervals & optical bench canbe replaced for maintenance off-line
Quick calibration this is a function of the immediate reporting of results. Thetime taken is however long it takes to pass the calibration sample into the cell and
then remove it again, Disadvantages Complicated optics are temperature and humidity dependent Sample impurities can coat the light cell affecting the measurement Glass components cannot take pressure changes Process impurities will often have a similar absorbance spectrum to the products.
An unexpected impurity may not be detected
Wide range of responses for different components (in the example almost anyinstrument could be used to quantify SO2 but H2S in a mixture required multi-wavelength detection and complex calculations (if only H2S had been present asimpler instrument could have been used)
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Analyser Types Gas Chromatograph
The sample is injected into a
constant flow of carrier gas(usually hydrogen). As it passes
along the heated quartz glass
column which has been coated
with a high boiling gum different
components of the sample
repeatedly dissolve in the gum
and are then re-vaporised at
different rates. This process
separates components which
are each measured in a detector
as they emerge from the column
Sample injection uses micro flow-switching valves and is controlled by the
Analyser. The most common form of detector uses flame ionisation the carrier
gas is burnt in air and a potential applied across the flame. As components
emerge from the column this electrical current changes and these changes are
measured and amplified
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Gas Chromatograph (2)
Separation of C4 to C6 hydrocarbons by GC
The area of each peakis proportional to the
quantity of that
component present
although the response
of each peak may
differ.
Note this separation
took 5 minutes to
complete. If there
were any higher boiling
components present
these would interfere
with the next sample
unless the analytical
sequence cleared them
from the column
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Gas Chromatograph (3)
Analysis of H2S in a waste water streamN2 gas is bubbled through the waste water sample in the Sample
Conditioning Cabinet and the extracted gas injected into the GC
Very accuratedetermination this
peak is for 10.15 ppm
H2S.
Note this separation
took 3 minutes to
complete. A muchquicker response
would be needed if this
was a safety system
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Gas Chromatograph (4)
Siemens Maxum GC
H2S in waste water
(see previous slide)
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Gas Chromatograph (5)
Advantages
Very sensitive process the method can work at very low componentconcentration (< 1 ppm) Wide range of detectors, columns and sampling techniques to suit most
applications
Easy maintenance very little to go wrong (only moving parts are the oven fanand sample injection) and robust components
Process impurities can be separately detected and quantified
Disadvantages Batch-wise process result reporting after the event (5 to 10 minutes later). Not
good for plant control or safety systems where a quick response is required.
The quality of the separation is totally temperature dependent. Generally thecolumn oven ensure this is the case. However for accurate or complicateddeterminations a constant temperature environment is needed.
Sample impurities can block the column affecting or even completely stopping the
detection. Late running peaks need to be removed before next injection. Long calibration time a calibration run takes the same time a sample as the
whole point is to replicate a normal injection but with a known componentconcentration. For multiple components it may be necessary to calibrate againstseparate calibration samples,
Live flame in the detector requires good separation when used in non-flammableenvironments
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Analyser Types Paramagnetic O2 Analyser
The paramagnetic oxygen sensor consists of a cylindrical shaped containerinside of which is placed a small glass dumbbell. The dumbbell is filled withan inert gas such as nitrogen and suspended on a taut platinum wire within anon-uniform magnetic field. The dumbbell is designed to move freely as it issuspended from the wire. When a sample gas containing oxygen isprocessed through the sensor, the oxygen molecules are attracted to thestronger of the two magnetic fields. This causes a displacement of the
dumbbell which results in the dumbbell rotating.
An opposing current is applied to restore the dumbbell to its normal position.The current required to maintain the dumbbell in its normal state is directlyproportional to the partial pressure of oxygen and is representedelectronically in percent oxygen.
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Paramagnetic O2 Analyser (2)
Advantages Continuous process with fast response time. Ideal for process control or safety
monitoring.
Easy, quick and infrequent calibration uses nitrogen (0%) and ambient air(21%)
Very specific - impurities (even corrosive components) do not affect detection as
the paramagnetic effect is peculiar to oxygen and a few oxygen containingmaterials
Disadvantages
Poor sensitivity (measurement range is from 1 to 100%). Notrecommended for trace oxygen measurements
Very sensitive to vibration or other detector movement
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Analyser Types Infra-red (IR or FTIR) Analyser
Instrument similar to UV absorption.An Infrared (IR) Analyser measure
changes in light absorption at a
particular IR wavelength as it is
affected by the sample. Note it is
more complicated than UV
absorption in that IR light isabsorbed and then re-emitted at
different wavelengths as the
molecular structure is first excited
and then relaxes back to its original
energy level.
The signals are relatively weak and
noisy. Fourier Transform (FT-IR) is
an electronic technique to
manipulate and enhance this signal.
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Infrared Analyser (2)
Advantages Continuous process with fast response time. Ideal for process control or
safety monitoring.
Very specific can be used for components that are very similar instructure or are too inert to have an effect by other techniques
Disadvantages Poor sensitivity without FT High noise to signal ratio Very sensitive to vibration or other detector movement
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Example - Paramagnetic and IR Analysers
ABB Magnos O2 and Uras IR detectors in Analyser Cabinet
Analysis of O2 and SO2 in flue gas
(high water vapour content)
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Analyser Types Total Organic Carbon
Acid is added to the aqueous sample to convert inorganic carbon to CO2which is sparged out of the system with an inert gas. The remaining organiccarbon is then oxidised to CO2 (various methods) and this is detected andquantified
Organic carbon occurs in municipal water supplies and comes from bothdecaying organic matter and from organic impurities (such as pesticides)
that are eluted into the source water
Normally specified to maximum allowable levels in high purity watersystems (pharmaceutical / biotechnology / nuclear industries) as well as
being monitored in drinking water
Used as a test to continuously monitor pharmaceutical cleaning method
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Analysers not normally requiring a Shelter
Humidity
Gaseous systems only Detector measures change in conductivity across an absorbent polymer betweenelectrodes
In-situ continuously monitoring in-line meter and used for process control or alarmmonitoring
Conductivity and pH Aqueous systems only Both can be in-situ continuously monitoring in-line meters Both are normally used for process control or alarm monitoring Both are susceptible to interference by entrained impurities pH meters have thin membranes often on glass and need near-ambient pressure pH meters will transfer tiny amounts of brine into the test solution
Gas Chromatography (new)
GCs are now available in EEx enclosures and can be used in hazardous zones Thermal conductivity detection not as sensitive as flame ionisation but allows use
of non-flammable gases
Currently only available for simple separations
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Sample Probe
The probe must be designed to sample
near the middle of the pipe away fromareas of turbulent flow or where particlesmight be picked up (probe in left handdiagram is too long)
If removal of the probe is required (eitherby specification or design) a double
block/bleed arrangement may be needed. In some cases the sample is returned
back to the process at the sample point.Some form of sample pump would beneeded within the loop.
Where sample probes and lines are
required to be heated this would usuallybe controlled from I/O at the analyser
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Sample Conditioning
Reasons for Sample Conditioning
Small volume
Many analysers require a very small sample (e.g. 1 l typical for GC) Fast loops will be required
Residues
High boiling components will take a long time to clear GC column Tars will block columns and coat optics Even water will block sample lines and fog optics if it condenses at the
wrong time.
Initial separation
Condensation prior to analysis Change of phase (e.g. extraction of component from an aqueous
stream to either a gas or organic solvent prior to GC)
Chemical changes to the sample to allow better quantification.
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Sample Conditioning
Example SCS CabinetHGCE Project
Sample stream is waste water containing a
demulsifier with potential to contain up to 10 ppm
H2S contamination. GC was selected as analyser
and extraction of H2S from solution into gas
prevents the demusifier interfering with the
analysis.
N2 is sparged through the sample and this
extracted vapour passed to the GC analyser
Other operations being carried out are:
N2 flow rate control Sample flow rate control with excess
bypassing via fast loop
SCS cabinet heated to constant 50C
Valve time controlled from Analyser to switch
between sample and calibration gas
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The Shelter - Analyser Environment
The provision and design of an Analytical Shelter will depend on the requirements of
the Analyser instrument. This in turn will generate a number of issues for which thecriticality and priority will need to be assessed and mitigated.
Changes in the elution time of GC peaks can mean wrong identification and/orchanged peak size, More frequent calibration would be required which equates to
the analyser being off-line. Therefore constant temperature preferred.
Most analyses are greatly effected by temperature and humidity (e.g. precisionoptics in spectrophotometers). Again constant temperature preferred.
Length of sample line? This should be minimised. However there may be a play-off between line length and ideal shelter positioning.
Is there a suitable pressure drop to drive the sample to the Analyser? Vendor willneed to supply transport calculations
Will the automatic calibration span gases be easily available onsite.
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Shelter Environment
Does the sample line require trace heating (e.g. to prevent condensation)?Are power and/or instrument air required to drive the sample probe? If so
are they controlled from the shelter?
Disposal of waste sample and Analyser effluent. Can it go back into theprocess line (preferred)? Is there a back pressure on the disposal line that
could damage the Analyser optics (e.g. a flare header may often have a 3
bar backpressure)? Assume worst case design criteria.
Consider the routing of utility pipework, power cables, signal cables,drains and HVAC air intake. These may have to come considerable
distances from suitable safe areas via underground trenches or overhead
trays and supports
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Shelter Safety
Zoning issues. Interior of Shelter may be different from the outsideenvironment. Where is the safest place for the fresh air intake?
Safe disposal of toxic and/or flammable process and calibration streams. Safehandling of flammable carrier gases using flow restrictors. Vendor to provide
calculations to show maximum possible concentration of toxic and flammable
material can never exceed specified limits (not above the TLV (TWA) or 20%
of the Lower Explosive Limit see Project and Fluor design criteria)
Isolation of Flame Ionisation Detectors. Where the Analyser or other electricalcertification does not meet the shelter zone requirement additional certified
isolation such as differential pressure enclosures may be required
Safe area for instrument engineer attention for maintenance and calibration ofthe Analysers. Safe and easy access for changing of gas cylinders and
maintenance of Sample Conditioning Systems There will be a requirement for a number of safety sensors both in the Shelter
and air intake (e.g. smoke detector, flammable, toxic and asphyxiant gases,
shelter differential pressure). A Cause & Effect chart should cover each safety
eventuality and define the outputs to local and site alarm systems.
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Analyser Shelters
Exterior layout detail for Analyser
Shelter on Habshan Gas Expansion
Project in Abu Dhabi. This sheltercontains 4 Ametek UV
Spectrophotometers used for H2S
determination in various plant locations.
HVAC units are to the right hand side
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Analyser Shelters
Interior layout drawing of same
Analyser Shelter. The entries for the
heated sample lines to each of the 4
Analysers can be seen to the left of
each instrument on the drawing
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Introduction to Process Analysers
Any Questions ?