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Production of Ethylene Oxide

Ethylene oxide, C2H4O, is a color-less, flammable gas or liquid.Because of its molecular structureethylene oxide is one of the mostversatile chemical intermediates.Industrial production started in1925 using the chlorohydrin pro-cess and was improved in 1931by introducing the much moreeconomic direct catalytic oxidationmethod. Currently, almost all ethyl-ene oxide production plants arebased on the direct oxidation pro-

cess with air or oxygen.Ethylene oxide itself is an excellentdisinfectant, sterilizing agent, andfumigant, when it is used as a non-explosive mixture with N2 or CO2.However, most of it is convertedinto other products such as fibers,foils, bottles, plasticizers, solvents,antifreezes, cosmetics etc.

Production of Ethylene Oxide

Production process is based on a cat-alytic reaction between ethyleneand oxygen using silver as catalyst.No other metal until now can com-

pete with silver, which, in practice,is deposited on various kinds ofporous support materials. The exactcomposition and structure of thesupport material do strongly influ-ence to process and product quality.It is considered as know how of theproducers and therefore kept secret.

Several, but similar, technologiesexist for ethylene oxide production.Differences depend mainly onwhether air or pure oxygen is usedfor oxidation. Production steps forthe oxygen-based process are

described below (text box).Ethylene oxide can cause heavyaccidents because it may explodeand is both highly flammable andreactive. Furthermore it is toxic.Therefore a great number of safetymeasures must be applied in a plantto prevent

Leakage of liquid or gaseous

ethylene oxide, Entry of air or oxygen into

ethylene oxide containers,

Ignition sources, and

Overheating of ethylene oxide.

Feed of ethylene and oxygen intothe reactor and oxidation of ethyl-ene by passing through the reactor(bundles of tubes packed with thecatalyst material) at 200 to 300 C.Together with ethylene oxide, CO2,H2O, and heat is generated as well.

Cooling of the gas that leaves thereactor by means of steam genera-tion or direct heating of the gas inletof the reactor.

Removal of ethylene oxide and CO2from the gas by scrubbing with firstwater and second an aqueouspotassium carbonate solution.

The ethylene oxide dissolves in thesolution.

Desorption of ethylene oxide fromthe solution in the desorber and

stripping of its low-boiling compo-nents.

Final distillation with separationinto water and ethylene oxide.

The process runs continuously using arecycled gas stream through the reac-tors driven by compressors, andreloaded before entering the reactoragain.

Steps of oxygen-based ethylene oxide processing

Fig. 1 Ethylene Oxide production plant

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Process Analyzers

Process analyzers are a very impor-tant part of an ethylene oxide plantfield instrumentation.

Measuring tasks can be groupedinto

Process control and optimization

Product quality control

Personnel and plant safety control

Environmental compliance con-trol.

To perform these analysis tasks ahigh number of different analyzersis installed in an ethylene oxideplant ranking from simple liquidanalyzers up to process gas chro-matographs and mass spectrome-ters. Measurements include rawmaterial and final products, offgases and waste water, ambient airand, of course, the process gas atvarious locations in the process.

Process analyzers play an importantpart within the field instrumenta-tion of an ethylene oxide plant.Users demand a >99 % device avail-ability and expect respective mea-sures built into the analyzers, suchas status signal capability etc. Theusers, on their side, are also pre-pared to contribute to that demandby providing adequate conditions

for installation and operation of theanalyzers.

Oxygen measurement

The determination of the oxygencontent of the process gas up- anddownstream of the reactor (ca. 8 %resp. 6 % O2, see measuring points 3and 5 in fig. 2) is of particular impor-tance. Plant efficiency increases pro-portionally to the oxygen content ofthe process gas and therefore, foreconomical reasons, the plant shallbe run at oxygen contents as high as

possible, close to the explosionlimit, but still reliably distant fromthat. The distance to the explosionlimit can be kept so smaller and theplant efficiency so higher the moreaccurate and reliable the oxygenmeasurement is performed. 1 %higher oxygen content in the pro-cess gas will easily correspond withan additional production rate of sev-

eral hundreds of tons ethylene oxideper year. Therefore, the use of thebest available oxygen measuringtechnology, as offered by the Sie-mens OXYMAT analyzer series, is avery economic decision.

Usually the oxygen measurementis performed redundantly with, forinstance, three separate analyzersinstalled at one measuring point.

Process Analysis Instrumentationof Ethylene Oxide Plants

Fig. 2 Process flow chart with measuring locationsg

234

3 4 5 7 8 9

6

1

10

5

7 1

12

2

13

6

9

8

14

11

Ambient air

Periphery measurements1 Air filter2 Primary reactor3 Primary absorber4 Secondary reactor

5 Secondary absorber6 Converter7 Desorber

Measuring pointPlant part

8 Stripper9 Refiner

Steam

Wastewater

Wastewater

Steam

Air

Ethylene

Process Measurements

Off gas Off gas Ethylene oxide

Air

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In case of a fault, a 2 out of 3 signalevaluation principle is applied andprocessed for appropriate safetymeasures.

Gas Chromatographs and MassSpectrometers

are both indispensable in a modernethylene oxide plant. Their applica-tion depend on criteria such as suit-ability for certain components,selectivity, response time character-istic, or investment and operation

costs. Both of them have preferableapplication fields. Latest develop-ments, e.g. the Siemens MAXUMedition II Gas Chromatograph or theSiemens Quantra Mass Spectrome-ter perhaps will cause some alter-ations of these application fields inthe future.

Siemens Analyzers and AnalyzerSystems

Siemens Process Analytics is, sincea long time, well known for its widespectrum of gas chromatographs,continuous gas analyzers and liquid

analyzers, which are all highly quali-fied and proven for use in a ethyleneoxide plant, also in case of plantrevamping projects. The productline has been extended during thepast years and comprises to day e.g.also the Quantra Mass Spectrometerwhich offers remarkable technicalfeatures for use in an ethylene oxideplant.

Besides the analyzer product busi-ness, Siemens Process Analytics isone of the worlds leading systemsupplier of analyzer systems withmanufacturing facilities in Europe,America and Asia, with a wide appli-cation know how including in-depthexperience in supplying analyzersystems to ethylene oxide plants.

Some of the Siemens analyzers withtheir technical features and corre-sponding user benefits aredescribed in the following.

Features OXYMAT 6 User Benefits

Simple and robust design without movingparts

High operating reliability, availability, andservice life; very low maintenance andspare part requirements

Str ictly l inear measuring principle High precision and flexibility

Very short signal response time T90 time

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The OXYMAT 6 is a gas analyzerthat operates according to the para-magnetic principle and is designedfor high-precision measurements ofoxygen concentrations in gases. Thepulsating magnetic field createsminute flow pulses detected by theSiemens micro-flow sensor and con-verted into the measuring signal.Thus, the OXYMAT 6 does not con-tain any moving parts. The samplestream gas also does not come intocontact with the microflow sensor,

which ensures an extremely longlife time and high operating stabil-ity. A good example for that is theindependence from humidity whichmay be present in the samplegasand could cover the surface of mir-rors (with negative impact to themeasuring accuracy) that are usedwith other oxygen measuring princi-ples.

The ULTRAMAT 6 is a gas analyzerthat operates according to the non-dispersive infrared principle (NDIR).It is designed to perform highlyselective concentration measure-ments of infrared-sensitive gases.The ULTRAMAT 6 uses the two-beam alternating light principlewith a measurement and compari-son cell, dual-layer detector, andoptical coupler.

This optical bench design producesan extremely narrow absorptioncurve minimizing the influence ofoverlapping spectra. Thus an unpar-alleled analytical precision isobtained. Cross sensitivities (causedby physical facts) are minimized byusing the gas filled beam splitter,the dual layer detector with opticalcoupler (optical extension of thedetector chamber) and additionalfilter.

Features ULTRAMAT 6 User Benefits

Dual-layer detector with variable optical pathlength setting (Optocoupler)

Maximum selectivity and thus measuringprecision.Can be optimized for actual analysis

Detector uses microflow sensor with no mov-ing parts to generate the measuring signal

No microphony: Very low signal noiseHigh measuring accuracy

Extremely stable mechanical designElectronic and physical parts separated gastight in one robust IP 65 housing

Very high operating reliability and lifetime

Easy cleaning of gas cell (on site possible) Minimum maintenance costs

Remote control by SIPROM GA software toolInterface to PROFIBUS PA (Option)

Easily integrated in automated systems

Features Quantra User Benefits

Very high Mass Resolution (20 000 at100 amu)

No interferences of components of simi-lar mass

Exact mass determinationMass Accuracy 0,0004 amu

Correct identification of componentsIdentification of unknown components

Ion Ejection application tool Discharge of high concentrated compo-nents for improved detection capability

Robust design of measurement cell, filament,and vacuum system

Suitable for operation in harsh processenvironment

No moving parts contained Long service life, very low maintenanceneeds

Ion pump Cell vacuum 10-10 Torr

No mechanical vacuum pumps required

Table 4 Quantra, analyzer features and user benefits

Table 3 ULTRAMAT 6, analyzer features and user benefits

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The MAXUM edition IIProcess GasChromatograph (Fig. 4) combinesthe outstanding analytical featuresof the Siemens PGC 302 Process GasChromatograph with the ParallelChromatography(separation ofcomplex analyzing tasks into severalmore simple steps) and the ethernetcommunication technology of theAdvance MAXUM gas chromato-graph of Applied Automation in onenew device. Thus the MAXUM edi-tion II is a gas chromatograph of out-

standing performance and very highflexibility. The single or double ovenconcept, the wide selection of col-umns and detectors including thevalveless column switching technol-ogy, and the modern ethernet com-munication capability allows ahighly selective and sensitive analy-sis of most different process compo-nents.

The Quantra Mass Spectrometerby Siemens(Fig. 5) is the first highresolution mass spectrometer worldwide that is capable to perform on-line applications.

Quantra is well suited for a numberof process applications such as tracegas analysis and ambient air moni-toring in the chemical industry, orresidual gas analysis in the semicon-ductor industry. The ICR (Ion Cyclo-tron Resonance) technology pro-vides an unmatched mass resolution

which allows to reliably identifymolecules of almost the same mass.

Quantra was specifically designedfor use in harsh process environ-ments with easy operating proce-dures and very low maintenancerequirements.

With SIPAN 32/34 Siemens offers aline of high performance liquid ana-lyzers in 2- and 4-wire technology,with and without Ex-protection, infield housing or as plug in unit, tomeasure pH, conductivity, redoxpotential, and oxygen.

An extensive line of sensors andfittings covers the whole range ofapplications and measuring ranges.The analyzers are equipped withHART and PROFIBUS DP/PA inter-faces and are easy to integrate into

automated systems using theSimatic PDM Engineering tool.

ig. 5 Quantra Mass Spectrometer Fig. 6 SIPAN Liquid AnalyzersFig. 4 MAXUM edition II Gas

Chromatograph

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Measuring Locations, Components, Analyzers (1)

MP Measuring point location Process medium Measuring component Measuring range Suitable Siemens analyzer

1 Stripper outlet Waste water Conductivity 0 ... 250 microS/cm SIPAN

2 Refiner outlet Waste waterpH pH 5 .... pH10 SIPAN

Conductivity 0 ... 250 microS/cm SIPAN

3 Primary reactor inletFresh/recycle gas mix nozzle

Process gas

O2C2H4CH4CO2C2H6N2ArH

2O

C2H4OCH3ClC2H5ClVinylClAllylCl

0 ... 10 %0 ... 40 %0 ... 100 %0 ... 10 %0 ... 1 %0 ... 20 %0 ... 10 %0 ... 2 %0 ... 3 ppm0 ... 5 ppm0 ... 5 ppm0 ... 5 ppm0 ... 5 ppm

QuantraMass Spectrometer

O2 + ArC2H4CH4CO2C2H6N2H2OEO

0 ... 25 %0 ... 40 %0 ... 100 %0 ... 10 %0 ... 1 %0 ... 20 %0 ... 2 %0 ... 5 %

MAXUM IIProcess GC

C2H4CO2 0 ... 50 %0 ... 5 % ULTRAMAT 6F 2R

O2 0 ... 10 % OXYMAT 6F Ex

O2 0 ... 10 % OXYMAT 6F Ex

O2 0 ... 10 % OXYMAT 6F Ex

C2H4O 0 ... 50 ppm MAXUM IIProcess GC

4 Secondary reactor inlet Process gas

O2C2H4CH4CO2C2H6N2Ar

H2OC2H4OCH3ClC2H5ClVinylClAllylCl

0 ... 10%0 ... 40%0 ... 100%0 ... 10%0 ... 1%0 ... 20%0 ... 10%

0 ... 2%0 ... 3 ppm0 ... 5 ppm0 ... 5 ppm0 ... 5 ppm0 ... 5 ppm

QuantraMass Spectrometer

CH3ClC2H5ClVinylClAllylCl

0 ... 5 ppm0 ... 5 ppm0 ... 5 ppm0 ... 5 ppm

MAXUM IIProcess GC

Table 5 Measuring locations, components and analyzers (1)

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Measuring Locations, Components, Analyzers (2)

MP Measuring point location Measuring medium Measuring component Measuring range Suitable Siemensanalyzer

5 Secondary reactor outlet Process gas

O2C2H4CH4CO2C2H6N2ArH2OC2H4OCH3ClC

2H

5Cl

VinylClAllylCl

0 ... 10 %0 ... 40 %0 ... 100 %0 ... 10 %0 ... 1 %0 ... 20 %0 ... 10 %0 ... 2 %0 ... 3 ppm0 ... 5 ppm0 ... 5 ppm0 ... 5 ppm0 ... 5 ppm

QuantraMass Spectrometer

O2 + ArC2H4CH4CO2C2H6N2H2OC2H4O

0 ... 25 %0 ... 40 %0 ... 100 %0 ... 10 %0 ... 1 %0 ... 20 %0 ... 2 %0 ... 5 %

MAXUM IIProcess GC

CO2CO

0 ... 5 %0 ... 100 ppm

ULTRAMAT 6F 2R Ex

O2 0 ... 10 % OXYMAT 6F Ex

O2 0 ... 10 % OXYMAT 6F ExO2 0 ... 10 % OXYMAT 6F Ex

6 Refiner inlet SteamDiss. O2 0 ... 100 ppm SIPAN

Conductivity 0 ... 2 microS/cm SIPAN

CH4 0 ... 300 ppm FIDAMAT with stripper

7 Desorber inlet Steam CH4 0 ... 300 ppm FIDAMAT with stripper

8 Converter inlet Off gas secondary absorber C2H4O 0 ... 50 ppm MAXUM IIProcess GC

9 Desorber outlet Off gas desorber C2H4O 0 ... 50 ppm MAXUM IIProcess GC

10 Secondary absorber out-let

Process gas C2H4O 0 ... 50 ppm MAXUM IIProcess GC

11 Plant area Ambient air UEG 0 ... 25 % NN

C2H4O 0 ... 1 ppm Quantra MS12 Stripper outlet Waste water Diss. O2 0 ... 20 microg/l SIPAN

13 Refiner outlet Waste water Diss. O2 0 ... 20 microg/l SIPAN

14 Stripper outlet Waste water pH pH 1 ... pH 14 SIPAN

Table 6 Table 5, continued

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Siemens Aktiengesellschaft

Automation and Drives (A&D)Process Instrumentation and Analytics

Siemens AG 2003Subject to change

User Benefits

Process analyzers in an ethyleneoxide plant are of extreme signifi-cance for correct, safe and efficientplant operation. The analyzers mustbe of very high quality and reliabilityand their installation must be per-formed with a high level of know howand practical engineering expertise.

Siemens Process Analytics

is capable, with respect to bothrequirements, to offer optimal per-formance and related benefits to the

user, because of the respective Siemens analyzers

rank on the top of the list of all ana-lyzers available on the market,

the OXYMAT 6, specifically, pro-vides the outstanding measuringaccuracy, reliability, and stabilityrequired to operate the process asclose as possible to the explosionlimit level for highest plant effi-ciency,

Siemens Process Analytics has per-formed successfully numerousdeliveries of analyzer systems to

ethylene oxide plants includingdetailed engineering of the ana-lyzer shelters,

Siemens Process Analytics is part ofSiemens Process Automationwhich provides a strong positionfor integration of analyzer systemsinto plantwide automation solu-tions,

Siemens Process Analytics world-wide presence with competencecenters, well trained personnel,and technical informations contin-

uously available from the web, forfast and effective customer supportwherever it is required, and

delivery from one hand from firstplanning up to after sales supportover the entire life cycle of theinstallation provides clear responsi-

bilities and cost transparency.

Fig. 7 Analyzer shelter

Case Study

For further questions please contact our local partner or the following addresses:

Siemens AGA&D PI 2M Process Analyticsstliche Rheinbrckenstr. 50D-76187 KarlsruheDeutschland

Tel.: +49 721 595-4234Fax: +49 721 595 6375E-Mail:processanalytics@siemens.com

www.processanalytics.com

Siemens Applied Automation

500 West Highway 60Bartlesville, OK 74003USA

Tel.: +1 918 662 7000Fax: +1 918 662 7052E-Mail: saaisales@sea.siemens.com

www.sea.siemens.com/ia

Siemens Pte. LimitedA&D PI2 Regional Head QuarterThe Siemens Center60 MacPherson RoadSingapore 348615

Tel.: +65 6490 8702Fax: +65 6490 8703E-Mail: splanalytics.sgp@siemens.com

www.processanalytics.com