Process Analytical Technology - ETH Z · sample gas and auxiliary gas (N 2) are injected and divided into two streams sample gas and auxiliary meet in ring-shape path at B side, a

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How much oxygen is inside ?

27.10.2015 Jacqueline Waldvogel, Yves Wittwer 1

Process Analytical Technology

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monitoring of process parameters to ensure:

cost-effective production

product quality

efficient planning of maintenances

safe plant operation

two important concepts related to Process Analytics:

feed-forward: “proactive”, avoid errors before they occur

feed-back: “reactive”, correct errors after they already occurred


Process Analytics

W. Kessler. Prozessanalytik: Strategien und Fallbeispiele aus der industriellen Praxis. 2006.

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for both correction mechanism, online monitoring of

certain process parameters is necessary


Process Analytics

http://cse.csusb.edu/dick/cs557/a1.html, 21.10.2015

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Pro:

analysis is performed by

experts

flexible

cheap

appropriate surrounding

Contra:

slow

«ownership of data» not

guaranteed

direct process control is not

possible


Offline Analysis

sample is transported to a

laboratory with highly-skilled

staff


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Pro:

relatively fast

Contra:

usually without qualified

personal

instruments have to be more

solid


Atline Analysis (exline)

manually or (half-) automatized sampling

and analysis near by the process


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Pro:

fast

highly specific analyser

feedforward and feedback

control possible

Contra:

expensive

calibration more difficult

sampling is accident-sensitive


Online Analysis

usually analysis in a bypass

condition: tR (investigated property) > tR (sensor) The time needed for a investigated property to change must be smaller than the time required for a

complete measurement (including data evaluation).


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Pro:

direct information obtained

no sampling needed (therefore

less accident-sensitive)

Pro

expense calibration

no sample-pre-treatement

possible

high requirements for

instruments


Inline Analysis

similarities to online analysis, but

without sample-taking.


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large interest in measuring oxygen quantitatively

variety of different methods available

oxygen is important in many fields, e.g:

medicine

biology

industry


Why monitoring oxygen?

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production of:

sulfuric acid from sulfur

S + O2 SO2

2SO2 + O2 SO3 H2SO4

nitric acid (Ostwald process)

ethylene oxide

hydrogen peroxide (Anthraquinone process)


Industrial usage of O2: A few examples

Goor, G.; Glenneberg, J.; Jacobi, S. Ullmann’s Encyclopedia of Industrial Chemistry, 18, 393-427.

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continuous process

direct oxidation

silver-based catalyst

prevent further oxidation to CO2 and H2O

oxygen concentration as crucial parameter

-> monitoring of oxygen


Considered reaction

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typical concentration of oxygen: 6 – 8 %

temperature range: 200 – 280 °C

pressure range: 10 – 20 bar

high reliability of the analytical method/ instrument

gas is consisting different components (organic compounds)

fluctuations of gas pressure might be possible

installation of the instrument in explosive protected area

(ATEX zone 2)


Needs in this case:

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Atmosphères Explosibles

ATEX 95 – 2014/34/EU: equipment directive

ATEX 137 – 1999/92/EG: workplace directive

explosion protection

high-risk areas are divided in zones by ATEX

frequency

duration of dangerous explosive atmosphere


ATEX

http://www.swissts.ch/de/produkt-und-sicherheitstechnische-

pruefungen/konformitaetsbewertungen/explosionsschutz-atex/, 23.10.2015

| | 27.10.2015 Jacqueline Waldvogel, Yves Wittwer 13

ATEX

www.atexloadcell.com, 17.10.2015

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avoid ignition sources

self ignition, static electricity, ultrasonic, hot surfaces, sparks, …

-> electrical parts are especially critical

ignition protection

flush critical components with inert gas (electrical)

separate electrical parts in case of emergency

protect electrical components from explosion by stable covers


ATEX


http://artidor.com/en/products/explosion-protection.html, 20.10.2015

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Chemical Reactions

Winkler method

Chromogenic reactions

Electrochemical

Lambda electrode

Clark electrode

Paramagnetic sensors

Gas chromatography

Optical

Absorption spectroscopy

Quenching based


Major methods for oxygen determination

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developed 1888 by Ludwig W. Winkler

only for dissolved oxygen

easy to use due to commercially available kits


Chemical methods: Winkler method

http://www.coleparmer.com/Product/LaMotte_Dissolved_Oxygen_Test_Kit_Refill_Winkler_Titration_Met

hod_50_tests_kit/EW-53003-05, 22.10.2015

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Step 1: fixation of dissolved oxygen

2 Mn2+ + O2 + 4 OH- 2 MnO(OH)2

Step 2: Use Mn(IV) to generate I2 in an amount, which is

proportional to the original concentration of O2

MnO(OH)2 + 2I- + 4H+ Mn2+ + I2 + 3H2O

Step 3: determine amount of I2 by titration (eventually with

the help of an indicator)

2S2O32- + I2 2S4O6

2- + 2I- respectively with I3-


Chemical methods: Winkler Method

Winkler, L.W. “Die Bestimmung des im Wasser gelösten Sauerstoffes.” (1888).

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colour change of dye due to reaction with oxygen

example: “Ageless Eye”

reduction usually done chemically (e.g. by ascorbic acid)


Chemical methods: Chromogenic Method

http://pubs.rsc.org/en/content/articlehtml/2010/an/c0an00049c; 27.10.2015

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application in food packaging

other examples for chromogenic reactions:

haemoglobin based

based on oxide formation of nickel


Chemical methods: “Ageless Eye”

Wang X., Wolfbeis O.S., Chem. Soc. Rev., 2014, 43, 3666ff.

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Pros Cons

- easy to use - no continuous online measurements

- often cheap

(Winkler Kit: 41.50$ for 50 tests)

- time intensive methods

- readout often directly by eye, but… - … subjective readout

- very low lifetimes

conclusions concerning our case:

no online measurements possible

no real-time correction of process parameters possible

difficult implementation into the plant

not suitable for our case


Chemical methods: Pros and Cons

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example: Lambda-type

electrode

ZrO2 behaves like a solid

electrolyte for oxygen (needs

high T >650 °C)

ZrO2 pumping disc pumps

oxygen from p1 p2

generation of different oxygen

concentrations in p2 and p1

according to the Nernst

equation a voltage is

generated at the ZrO2

sensing disc 27.10.2015 Jacqueline Waldvogel, Yves Wittwer 21

Electrochemical methods

http://www.first-sensor.com/cms/upload/appnotes/AN_XYA-O2_E_11154.pdf, 23.10.2015

https://www.crystec.com/staoxye.htm, 26.10.2015

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Nernst equation

measured voltage is compared

to reference voltages V1 and V5

as V1 is reached, chamber (p2)

is evacuated, as V5 is reached

it is pressurized

time needed for one cycle is

proportional to sample oxygen

concentration 27.10.2015 Jacqueline Waldvogel, Yves Wittwer 22


http://www.first-sensor.com/cms/upload/appnotes/AN_XYA-O2_E_11154.pdf, 23.10.2015

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example: Clark electrode

O2 enters the cell through a barrier (e.g. a

membrane)

cathode: O2 + 2e- + 2H2O H2O2 + 2OH-

H2O2 + 2e- 2 OH-

anode: 4Ag 4Ag+ + 4e-

measure current, which is proportional to

the amount of O2 entering the cell

many other cell types available as well



picture: https://www.quora.com/How-do-DO-probes-work, 21.10.2015

http://hansatech-instruments.com/products/introduction-to-oxygen-measurements/general-oxygen-

electrode-measurement-principles/, 23.10.2015

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Pros Cons

- fast - lifetime: 1-2 years

- easy to use - aging requires regular calibration

- inline analysis possible - temperature dependence

- pressure dependence

- interferences possible


inline/ online measurements possible

real-time measurements possible

possible problems because of process conditions and ATEX guidelines

in principle suitable for our case


Electrochemical methods: Pros and Cons

http://www.brandtinst.com/biosystems/appnotes/Downloads/7.%20How_Electrochemical_Sensors_Work

.pdf, 23.10.2015

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based on paramagnetic properties of triplet oxygen.

working principle:

sample gas and auxiliary gas (N2) are

injected and divided into two streams

sample gas and auxiliary meet in

ring-shape path

at B side, a magnetic field is created,

which draws the oxygen of the

sample gas into.

the flow rate at B decreases in comparison with the one at A.

thermistors at point A and B determine the flowrate, which is

converted into an electrical signal.

its difference is proportional to the amount of oxygen in the sample

gas.


Paramagnetic sensors

http://www.yokogawa.com, 22.10.2015

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Pros Cons

- long-term stable measurements - interferences with other paramagnetic

compounds

- high resistance to vibrations - only works at low temperatures

- high sensitivity (0-1 vol-%) - no moisture allowed

- fast response (≈3s) - only works at low pressure

- easy calibration


Suitable for online analysis

Process temperature too high

Process pressure too high

not suitable for our case


Paramagnetic sensors: Pros and Cons

http://www.yokogawa.com/an/download/bulletin/Bulletin11P03A01-01E.pdf, 22.10.2015

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separation of gases with

GC possible

example: Analysis of

power plant flue gases

carrier Gas: H2

pressure: 2.2 bar

two columns

Porapak Q (apolar polymer)

molecular sieve 5A

detector: TCD


Gas Chromatography – An example

http://http://www.thermoscientific.com/content/dam/tfs/ATG/CMD/CMD%20Documents/Application%20&

%20Technical%20Notes/Chromatography/Gas%20Chromatography/AN-10351-GC-Flue%20Gases-

TRACE%201110-AN10351-EN.pdf, 23.10.2015

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Detection ?

GC-MS: may have some problems with low mass of

oxygen

GC-IR: does not work since O2 is IR-inactive

TCD (thermal conductivity detector)


Gas Chromatography

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measuring thermal conductivity

of the sample and a reference

electrically heated filament surrounded

by gas (sample or reference)

heat is conducted by gas to detector

block

conductivity is dependant of the gas

(composition)

reference and gas cell form a

wheatstone bridge

record a voltage difference


Thermal Conductivity Detector, TCD

https://de.wikipedia.org/wiki/Wärmeleitfähigkeitsdetektor#/media/File:Thermal_Conductivity_Detector_1.

svg, 23.10.2015

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Pros Cons

- high precision - slow

- case specific

- potential use of explosive gases


no online measurements possible

no real-time correction of process parameters possible

process pressure is too high

problems with ATEX guidelines ?

Not suitable for our case


Gas Chromatography: Pros and Cons

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absorption bands of O2: 760 nm, one deep in the UV

760 nm: b1Σ+g←Χ3Σ-

g (triplet singlet)

emission peak: 1270 nm

UV/Vis absorption

continuous measurement




https://www.piketech.com/files/images/metal_short-path.png, 23.10.2015

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TDLAS: Tuneable diode laser absorption spectroscopy

excitation by tuneable laser

measure absorption to determine concentration,

temperature or pressure

Pro:

high sensitivity

wide temperature range (up to 1000 °C)



http://www.lasercomponents.com/fileadmin/user_upload/home/Datasheets/lc/veroeffentlichung/diodenla

ser-absorption-photonik-03-02.pdf, 23.10.2015

http://ac.els-cdn.com/S0925400513012562/1-s2.0-S0925400513012562-main.pdf?_tid=ded467ca-7987-

11e5-a82a-00000aacb35e&acdnat=1445606197_ea0dfdd993ab1a9f0bbc1f7178af5dd6, 23.10.2015

http://www.ltt.uni-erlangen.de/inhalt/pdfs/praktmt/Anleitung.pdf, 23.10.2015

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Pros Cons

- easy to handle - expensive instrument (TDLAS)

- fast (TDLAS) - bulky instrumentation

- interferences (H2O, CO2)


online measurement possible

real-time measurement possible

problems with interferences?

in principle suitable for our case


Absorption spectroscopy: Pros and Cons


http://www.lasercomponents.com/fileadmin/user_upload/home/Datasheets/lc/veroeffentlichung/diodenla

ser-absorption-photonik-03-02.pdf, 23.10.2015

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Jablonski – diagram

dynamic quenching: collision induced energy transfer


Quenching based methods

https://www.picoquant.com/applications/category/life-science/singlet-oxygen, 23.10.2015

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Stern-Volmer equation describes the relation between the

luminescence intensity and the oxygen concentration

Stern-Volmer equation:

F0/ F: fluorescence without/ with oxygen, KSV: Stern-Volmer

constant (function of probe lifetime and polymeric solvent)




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in reality luminescence intensity

is often not linearly dependent

on oxygen concentration

this is caused by different

surroundings of the fluorophore

by the polymer (see next slide)

this can be corrected by using

e.g. multiple Stern-Volmer

constants representing the

different environments



f: fraction of total emission corresponding to each component


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optical isolation

non-transparent layer

avoid interferences e.g. by ambient light

oxygen sensing layer

luminophore

solid support

polymer matrix/ layer




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in principle, every luminophore interacting with oxygen

can be used

properties of the luminophore (e.g. stability, lifetime of

excited states,…) mainly (but not only) determine

properties of sensor

suitable luminophores are for example:

fullerenes

diverse metal-ligand complexes

porphyrins


Oxygen sensing layer


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either protection layer or host matrix for luminophore

requirements

optically transparent

oxygen permeability

long-term stability

if used as host matrix, solid support and luminophore must be

compatible

diverse functions

protection of luminophore

modify luminophore properties, e.g. lifetime of excited state

selective diffusion of oxygen

for example: silicones


Solid support


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Pros Cons

- highly adjustable - possible interferences with other

quenchers, e.g. SO2, NOx, many olefins,

halogenated organic species, humidity,...

- high sensitivity possible - not many sensors working at high

temperatures

- fast - extremely case specific


online/ inline measurements possible

real-time measurements possible

difficult to find proper sensor, e.g. Mo6Cl12 sensor can be used at high

temperature (> 600 °C)

In principle suitable for our case


Quenching based methods: Pros and Cons


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suitable not suitable

electrochemical methods chemical methods

absorption spectroscopy paramagnetic sensors

quenching based methods gas chromatography


Conclusion

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LMB: leucomethylene blue

MB: methylene blue


Abbreviations