000.270.CSE-163.1 Introduction to Process Analysers

7/27/2019 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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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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Siemens Maxum GC

H2S in waste water

(see previous slide)


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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

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000.270.CSE-163.1 Introduction to Process Analysers